Solar Hot Water Heaters

Solar water heaters—also called solar domestic hot water systems—can be a cost-effective way to generate hot water for your home. They can be used in any climate, and the fuel they use—sunshine—is free.

How They Work
Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't.

Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.

Three types of solar collectors are used for residential applications:

• Flat-plate collector
  
  Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors—typically used for solar pool heating—have a dark absorber plate, made of metal or polymer, without a cover or enclosure.

• Integral collector-storage systems
  
  Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.

• Evacuated-tube solar collectors
  
  They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for U.S. commercial applications.

There are two types of active solar water heating systems:

• Direct circulation systems
  
  Pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes.

• Indirect circulation systems
  
  Pumps circulate a non-freezing, heat-transfer fluid through the collectors and a heat exchanger. This heats the water that then flows into the home. They are popular in climates prone to freezing temperatures.

Passive solar water heating systems are typically less expensive than active systems, but they're usually not as efficient. However, passive systems can be more reliable and may last longer. There are two basic types of passive systems:

• Integral collector-storage passive systems
  
  These work best in areas where temperatures rarely fall below freezing. They also work well in households with significant daytime and evening hot-water needs.

• Thermosyphon systems
  
  Water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank. These systems are reliable, but contractors must pay careful attention to the roof design because of the heavy storage tank. They are usually more expensive than integral collector-storage passive systems.

Solar water heating systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a demand (tankless or instantaneous) water heater for backup.

For more information about solar water heating system components, see the following information:

• Heat Exchangers for Solar Water Heating Systems
• Heat-Transfer Fluids for Solar Water Heating Systems

Selecting a Solar Water Heater

Before you purchase and install a solar water heating system, you want to do the following:

• Consider the economics of a solar water heating system
• Evaluate your site's solar resource
• Determine the correct system size
• Determine the system's energy efficiency
• Estimate and compare system costs
• Investigate local codes, covenants, and regulations.

For information about specific solar water heater models and systems, see the Product Information resources listed on the right side of this page (or below if you've printed the page).

Installing and Maintaining the System

The proper installation of solar water heaters depends on many factors. These factors include solar resource, climate, local building code requirements, and safety issues; therefore, it's best to have a qualified, solar thermal systems contractor install your system.

After installation, properly maintaining your system will keep it running smoothly. Passive systems don't require much maintenance. For active systems, discuss the maintenance requirements with your system provider, and consult the system's owner's manual. Plumbing and other conventional water heating components require the same maintenance as conventional systems. Glazing may need to be cleaned in dry climates where rainwater doesn't provide a natural rinse.

Regular maintenance on simple systems can be as infrequent as every 3–5 years, preferably by a solar contractor. Systems with electrical components usually require a replacement part or two after 10 years. For more information about system maintenance, see the following:

• Solar Water Heating System Maintenance and Repair
• Solar Water Heating System Freeze Protection
• Scaling and Corrosion in Solar Water Heating Systems

When screening potential contractors for installation and/or maintenance, ask the following questions:

• Does your company have experience installing and maintaining solar water heating systems?
  Choose a company that has experience installing the type of system you want and servicing the applications you select.
• How many years of experience does your company have with solar heating installation and maintenance?
  The more experience the better. Request a list of past customers who can provide references.
• Is your company licensed or certified?
  Having a valid plumber's and/or solar contractor's license is required in some states. Contact your city and county for more information. Confirm licensing with your state's contractor licensing board. The licensing board can also tell you about any complaints against state-licensed contractors.

For contractor information, see the Professional Services resources listed on the right side of this page (or below if you've printed it out).

Improving Energy Efficiency

After your water heater is properly installed and maintained, try some additional energy-saving strategies to help lower your water heating bills, especially if you require a back-up system. Some energy-saving devices and systems are more cost-effective to install with the water heater.

Other Water Heater Options

• Conventional storage water heaters
• Demand water heaters
• Heat pump water heaters
• Tankless coil and indirect water heaters

 

Solar Hot Water Basics By John Patterson

While most people are captivated by the high-tech nature of solar-electric (photovoltaic; PV) systems, in most cases, a solar hot water system will harvest more energy at a substantially lower cost. In fact, compared to PVs, solar hot water (SHW) collectors are more than three times as efficient at producing energy from the sun.

Investing in an SHW system is a smart solar solution for most homeowners. This proven and reliable technology offers long-term performance with low maintenance. And with federal, state, and utility incentives available, these systems offer a quick payback—in some cases, only four to eight years.

A thoughtfully designed SHW system could provide all, or at least a significant amount, of your household hot water needs for some portion of the year. The California Energy Commission estimates that installing an SHW system in a typical household using electric water heating can shave 60 to 70 percent off water heating costs. To get the most for your money, you´ll want a properly sized system that offers the best performance in your climate.

Solar Hot Water System Types

Five main types of solar water heating systems are sold today. These five are a distillation of dozens of types sold over the past 25 years. They are:
• Batch
• Thermosyphon
• Open-loop direct
• Pressurized glycol
• Closed-loop drainback

The proven winners are simple, reliable, and long lasting. Some systems are "open loop" (the domestic water itself is directly heated) and some are "closed loop" (a heat-transfer fluid is heated by the collector and the heat is passed on to the domestic hot water by means of a heat exchanger).

Some systems are "active," using moving parts such as pumps and valves, and others are "passive," using no mechanical or moving parts.

There are many considerations in choosing the best system for a home, but the client and the situation will dictate the right system.

For instance, for a one- to two-person household in a temperate climate where hard freezes rarely occur, you might go with a batch heater, especially if the hot water will be used more at the end of the day rather than first thing in the morning. In a household with three or more people, where aesthetics and weight are not an issue, the thermosyphon system might fit the bill, especially if there´s no room for an additional tank near the existing water heater.

The drainback system, a personal favorite here in the Northwest, requires continuous drop between the solar collector and the solar storage tank. If continuous fall is not possible, there´s always the pressurized glycol system where piping can go up, down, over, and around without concern. Usually more than one option can work for any situation.

The number of people in the household will dictate how large the system will need to be, and which systems are even possible. Rebate and incentive programs may only qualify certain systems in a given area. Some systems are relatively easy to install for do-it-yourselfers, while others most laypeople shouldn´t attempt. See the comparative chart showing features of the different system types. Make your choice, and enjoy using solar energy to heat your water!

Please download the attached PDF for the full article

 

Most efficient-evacuated tubes




Freestanding collector designed for use with flat roof applications.

Welcome to the Solar Savings Evacuated Tube Solar Collector. Whether you have just purchased your collector or are researching before you buy you have taken an important step to reducing pollution and carbon dioxide emission, whilst enjoying piping hot water heated by nature. This ‘solar collector’ has been manufactured to the very highest standards, and will provide you with many years of service, with the minimum of maintenance required. This brochure explains how your collector is intended to work, and provides information to allow you to complete a solar water heating installation. If, after reading this document, you have further questions, please contact your distributor, who will be happy to help you.

Design
Our collectors are suitable for applications where aesthetics as well as efficiency are important. These collectors allow for easy installation and they are suitable for single unit installations or modular large-scale installations for heating or air conditioning projects. The main features are:
• Long service life
• Elegant aesthetically pleasing design
• Easy integration into buildings
• Improved power conversion at low solar irradiation levels

Collector Dimensions
The collector consists of the array of tubes, a heavily insulated manifold header, stainless steel support frame and standard mounting frame package. Each tube is 47mm x 1500mm and the overall dimensions of the panel are 1700x1500x140mm. A 30 tube unit is also available for larger households.




Vacuum Tubes
Unlike cheaper panels, this system does not heat the water directly within the vacuum tubes. Instead, a sealed copper ‘heat pipe’ transfers the heat via convection of its internal heat transfer fluid to a ‘hot bulb’ that indirectly heats a copper manifold within the header. The heat pipes are inserted into curved absorbers forming an assembly which in inserted into the glass tubes. The tubes are made of borosilicate glass, which is strong and has a high transmittance for solar irradiation. In order to reduce the convection heat lost, the glass tubes are evacuated to vacuum pressure or less than 10-3 Pa. Stable vacuum seals are ensured by using a patented technique employing high heat and pressure. In order to keep the stability of the vacuum for a long time, a barium “getter” is used (the silver coating at the tip of the tube). This rare metal coating absorbs any gases that might eventually enter the tube, increasing the lifespan of the vacuum seal. Through evacuating air out of the glass tube the absorber material and selective coating are protected from corrosion and other environmental influences. This ensures a lifetime of at least 15 years without loss of efficiency. The getter also acts as an indicator and will turn white instantly should the tube be broken.

Header Pipe
The manifold has been designed around the use of a small diameter header pipe (28mm ID). This allows for a small manifold casing while still maintaining at least 50mm of insulation. The water volume capacity of the header pipe is less than 1.5 litres for the 30 tube collector, thus allowing fast heating during even overcast conditions. This is important for areas with lower solar irradiation or overcast conditions, as the heat from the manifold can be quickly harnessed, then held in the storage tank.
The header pipe is brazed with Copper-Phosphorus brazing material (BcuP6), giving excellent joint penetration and smooth brazing. This result is a join that is not only strong, but also very neat. As the brazing material is primarily copper (94%), rapid heating and cooling of the header pipe does not compromise the weld integrity.

4 tube header pipe.


Close up of brazed header joint
(CuP6 brazing rod)
After brazing, every header pipe is pressure tested to ensure weld integrity. The inlet and outlet are formed in standard 22mm copper to enable the use of conventional compression fittings for the manifold plumbing. The copper manifold is heavily insulated using compressed rock wool. This reduces heat loss to a minimum at night, and during cold weather. In conjunction with our freeze-protection controller. The 50mm thick insulation is been used to protect against heat loss.

The connection between the heat pipe and manifold is critically important to ensure optimal heat transfer. The manifold header pipe is mounted within the manifold casing and is made of 28mm diameter, 1 mm thick copper pipe rated for a maximum pressure of 10 kg/cm2, the standard operational maximum being 6kg/cm2. The ‘hot bulb’ section of the heat pipe fits tightly in the heat pipe port in the manifold. Silicone heat-transfer compound (supplied with each kit) ensures a good transfer between heat pipe and the header pipe in the manifold. Heat transfer is by conduction allowing the manifold to remain fully sealed ensuring water can never leak at the connection.

• Sealed manifolds make collector modules particularly suitable for areas with hard water (limescale)• Sealed manifolds allow the system to operate with high pressures of up to 10 bar, especially useful in large heating or air conditioning projects.• Sealed manifolds eliminate leakages between manifold and vacuum tube.

• Sealed manifolds make it easy to replace collector tubes at any time without interrupting the operation of system.

Rock Wool Insulation
The choice of rock wool insulation is important for a number of reasons:
• Rock wool can handle high temperatures, in fact it is non-flammable
• Provides excellent insulation performance (often used in cavity insulation)
• Is environmentally friendly as it is a natural, recyclable material
Many companies are still using polyurethane, which provides excellent insulation performance, but is far from environmentally friendly. Our collectors are as much as possible, a Green product.
As you can see from the above picture, the rock wool is compressed into blocks. Each block is 73cm long, so 4 are used for a 20 tube collector, 6 in a 30 tube collector. The mold shape fits tightly around the header pipe and tube port shape to ensure maximum insulation performance.

Frame
Each collector is supplied with a stainless steel adjustable width frame. The frame is supplied plain, to match the manifold.

Uprights: Run the full height of the collector and are used for attachment to the mounting surface (roof, wall). Slots are punched out along the length for the attachment of mounting straps (stainless steel roof straps are provided as part of the Solar Savings kit). Additional brackets/holes may be made according to your specific mounting requirements. The width between uprights is adjustable to suit individual installation requirements.

Lower Tube Track: Used for the support and attachment of the evacuated tubes. The cups for the support of each tube are punched out of the stainless steel track with holes provided for the screw clamp to pass through.

Screw Clamps: Because each heat pipes needs to maintain firm contact with the header pipe (for optimal heat transfer) it is important that every evacuated tube is held securely in place along the lower tube track. For this reason instead of plastic or rubber straps, stainless steel screw clamps are used. These clamps provide a convenient and fast attachment method that ensures secure tube attachment for the life of the collector. Installation or removal of a tube is quick and straightforwards, as only a screwdriver is required to loosen the clamp

The key features are as follows:

• High performance, reliable, glass evacuated tubes
• Heat pipe uses non-toxic, in-organic heat transfer compound
• Low heat pipe start up temp (<35deg C)
• Manifold casing available in plain matt finish 304 stainless steel
• Adjustable width frame available in Stainless steel.
• Compressed rock wool insulation (non-flammable, recyclable)
• Copper header pipe – twice pressure tested to 160psi
• ABS plastic (UV stabilized) manifold end caps
• UV stabilized rubber manifold seals and evacuated tube caps
• 8mm ID temperature sensor port
• Screw clamp individual tube attachment
• Compact manifold size HxW of 130x140mm (5.1” x 5.5”)
• Header pipe design enhances heat transfer by creating turbulent water flow.

Model Type
Construction
No. of Collector Pipes
Tube Diameter (OD)
Panel area
Absorber Surface
LxWxH (mm)
1700x1460x140
Weight
55kg
Fluid Content
1.5 l
Pressure Drop@100 l hr-1
10mBar
Angle of Inclination
15-90degrees
Max. Temp (°C)
190°C
Stagnation Temp (°C)
247°C
Heat exchanger material
Copper
Permissible Operating Pressure
6bar
Test Pressure
10bar
Manifold Connection Diameter
22mm
No. of Vacuum Tube Port Diameter
20
Component material specification
Stainless steel / Aluminium header with rockwool insulation
Interconnection Facility for multiple units
yes
Connection Diameter
2 x 22mm

Additional Product Information and Background
Sealed Glass Evacuated Tubes
Evacuated tubes are the key component of the solar collector. The following information will provide you with insight into the history, manufacturing process and general specifications of evacuated tubes.

Evacuated Tube History

The evacuated tube technology was initially developed by Qing Hua University in Beijing in the early eighties, with pilot manufacturing in 1985. By 1988 annual manufacturing volume by Qing Hua had reached 30,000 tubes. By 1996 with the aid of significant financial support from the Chinese government, Qing Hua reached an annual production capacity of 2 million tubes. Continued infrastructure development led to 2.5 million tubes being sold in 1997.

The majority of the tubes were used to supply the local market, with a small percentage (100,000 in 1995) being supplied to Japan, Europe, South America and South-East Asia. The main barrier to large export sales was the technology of the solar system (tank/manifold). Although the tubes performed well, the quality of the storage tanks was average, and did not meet the requirements of the European market. The non-pressure thermosiphon systems did, however, meet the needs of the Chinese market, and therefore sales grew and grew.

In 1998 Qing Hua held 70% of the Chinese solar water heating market. With the breakup of some of the key members of the Qing Hua Solar board members, the patent protection for the tube technology was no longer enforceable, and so other Chinese companies began producing the evacuated tubes. The equipment and machinery used to produce all tubes in China is therefore the same as that developed by Qing Hua. For this reason, if engineering standards are followed, and good quality raw materials use, all tubes manufactured in China should be the same, and provide the same performance. You will find that all Chinese companies provide tubes with the same specifications. Having said this though, there are many companies who use poor quality raw material and make short cuts on engineering requirements. Selection of a professional tube manufacturer is therefore very important.

Solar Savings Product Development

During the development of this collector it became clear that the European and US market needed a solar collector that met the following criteria:

• High performance evacuated tube heat pipe based design
• “Plug and Play” heat pipe system for easy transport, installation and maintenance (changing broken tubes)
• Use of non-toxic heat pipe transfer liquid (not acetone)
• High quality long lasting components (corrosion resistant materials)
• High quality stainless steel finish
• Excellent insulation properties (>50mm thick rock wool)
• Small manifold water volume to ensure fast heating time
• Environmentally friendly through the use of non-polluting, recyclable materials
• Accept mains pressure water supply (6kg/cm2 / 85psi)
• Corrosion resistant manifold header pipe (copper)
• Suitable for open or closed flow operation
• Accept a standard sized temperature sensor
• Compact frame that could be packed with the manifold
• Adjustable width frame to allow for varying installation surfaces
• Quick and simple tube attachment system – permitting easy removal of any one tube
• Compact manifold size
• Cost competitive with high quality flat plate collectors

Certification
Please note that the Solar collector is manufactured in accordance with ISO9002, and it is currently undergoing testing to BS EN 12975 It is from these tests that the absorption (93%) and emission (7%) efficiency values have been verified. The glass manufacturing plant, ensures that quality is controlled throughout every step of the process. They have obtained a wide range of quality management and quality control certificates including the internationally recognized ISO9002 management standard.

EVACUATED TUBE & HEAT PIPE CHARACTERISTICS

The heat pipe and evacuated tube will not get hot after one minute of sitting in the sun – so don’t expect it too. The sealed glass tubes have a short start-up time as the inner glass tube, heat pipe fins and air within the tube must first be heated before the temperature will start to rise considerably. In good conditions it will take less than 5 minutes for the tip of the heat pipe to get too hot to hold (>50deg C). The advantage of the sealed glass evacuated tube is that is acts as a heat store, providing a stable supply of heat to the manifold even during intermittently overcast weather. The tube will continue to provide heat even after the sun has set.

A good test to show the heat storage capacity of the tube is to let the tube heat up outside until the heat pipe tip is hot. Run the tip under cold water for 10 seconds or so to cool it down (drain some of the heat). Stand the tube back up, and within seconds the tip will be red hot again. This can be repeated several times before the heat is “used up”.

Another example of the heat storage is to let a tube heat up outside in the sun, and then bring it inside. You will find after half an hour the tip will still be hot, thus demonstrating the store of heat (energy) inside the tube.

The sealed glass evacuated tube provides a stable supply of heat even during intermittent weather. There is minimal “peaking and troughing” of heat supply as the clouds intermittently block the sunlight. Heat supply can therefore continue even when there is no sunlight striking the collector, due to the store of heat within the evacuated tube.

Heat Pipes

In addition to the evacuated tubes the copper heat pipe is also vital to the performance of the collector. The heat pipe is an essential link in the heat transfer chain. If this link is poor quality then the efficiency of the whole system will be compromised, regardless of how good the evacuated tubes are.
The key factors to consider when choosing a heat pipe are:

• Operating Temperature Range
• Heat transfer compound
• Heat transfer performance
• Operating life expectancy

NB – DO NOT EXPOSE TUBES TO SUNLIGHT FOR EXTENDED PERIODS WITHOUT COOLING THE TIPS, OR DAMAGE MAY OCCUR. Install header first, and shade tubes from sunlight until the water flow and control is operational

Heat pipes in the Solar Savings collector are custom made using patented inorganic, nontoxic heat transfer compound.

The Inorganic heat pipes have the following features:

• Continuous operating life of more than 110,000 hours (5 year warranty)
• Effective thermal conductance of 25,000 – 30,000 times that of silver.
• Heat flux density of 27.2MW/m2 .
• Heat pipe internal surface is coated with 3 layers, which delay corrosion and oxidation and prevent the production of oxy hydrogen, thus improving the performance stability and operation life of the heat pipe.
• The heat pipe transfers heat along the full length of the heat pipe in a sine wave pattern, with a thermal resistance of almost zero.
• Heat transfer compound is non-toxic if ingested and nonirritant to either eyes or skin.
• Vacuum level of 4x10-6Pa which reduces the boiling temp of the liquid to as low as 25-30deg C

In addition to having a high quality heat pipe, the fins used within the evacuated tube are curved copper fins. We have found a performance increase of 5% using this new fin design when compared to the flat fins previously used.




HEAT PIPE CHARACTERISTICS
The heat pipes used by Solar Savings are different to some other heat pipes, which use acetone as the heat transfer compound. Acetone heat pipes will transfer heat with just the bottom 5 to 10cm placed in a cup of hot water (50deg C). Ours will not. This is not because the performance is poor, but rather because the nature of the heat transfer compound is quite different. Under the vacuum conditions that exist in the heat pipe, and at low heat pipe temperatures (<30oC), this mixture will form a frozen “ball” located in the heat pipe tip. For this reason, when you vigorously shake the heat pipe you will hear a rattling sound and feel an object in the heat pipe tip. If you were to cut the heat pipe open, the vacuum will be lost and you will not find any ball inside, just some orange colored liquid.

The presence of this “ball” indicates the heat pipe has a good vacuum level – although you must consider the ambient air temperature when doing this test. If the ambient temperature is already 30deg the ball may have mostly melted and so no sound will be heard.

As the evacuated tube provides heat along the full length of the heat pipe, rapid “melting” of the ball and subsequent heat transfer will occur at temperatures as low as 30deg C. As you expose the heat pipe to hotter and hotter temperatures, the ball will continue to melt and contribute to the heat transfer process. Once a hot enough temperature is reached the ball will have totally melted and there will be no sound if shaken.

For demonstration purposes, hot water (>45 deg C) can be poured along the bottom two-thirds of the heat pipe. This will ensure rapid melting of the ball and subsequent heat transfer to the tip. Within 60 seconds the tip can achieve a temperature, which is 90-95% of the temperature it is exposed to. The tip can never get hotter than the heat level it is exposed to (not 100% efficient).

If you heat the bottom of the heat pipe with a moderate temperature liquid (50o C), the heat will not be enough to travel to the tip and “melt” the ball. If however you pour that same temperature water along the length of the heat pipe, the heat will quickly melt the ball and heat transfer to the tip will rapidly occur.
Although the heat pipe can transfer heat at temperatures of around 30-35o C, the heat transfer to the tip will only reach 28-32o C, which will not feel hot to the touch. So don’t try and use warm water for demonstration purposes. Use hot water.

NB – DO NOT EXPOSE TUBES TO SUNLIGHT FOR EXTENDED PERIODS WITHOUT COOLING THE TIPS, OR DAMAGE MAY OCCUR. Install header first, and shade tubes from sunlight until the water flow and control is operational

Assembling The Solar Savings Collector

Collector Frame
There are many different types of roofing materials, and solar collectors can be mounted at various angles, either on the surface of the roof or on a framework to achieve the optimum angle on shallow pitch roofs. The mounting frame provided consists of two side rails and a top and bottom support assembly. All frames are made of stainless steel and are designed to be quick and easy to install on all roof types. There are two ways to fix the frame to the roof – either drill directly through the tiles and use coach screws into the rafters (the most popular method among professional installers) – you can then seal the hole with silicone sealant. Alternatively, use builders strap available at any builders’ merchants. Simply slide these up underneath the tiles (fixing directly to the rafters underneath the tiles.

1. Frame Assembly

(1) Locate the Uprights
Nb:The frame should be fully assembled prior to installation of vacuum tubes.

Position the 2 uprights as shown below:

(2) Attach the manifold
Attach the manifold to the uprghts, this is done simply by bolting the upright sections to the manifold using the pre-positioned studs on the underside of the manifold and tightening them until they are secure.



(3) Fix the Lower Tube Track
Attach the tube track to the protrusions of at the base of the uprights, fixing with long screws. (See picture below)

Solar Controller: Essential for efficient use of Solar Heating

The solar controller is an essential part of the solar heating system, in all but gravity-fed systems (Solar Collector is situated lower than the hot water cylinder, and circulation is effected by thermo-syphoning). In all other systems, it will be necessary to use a controller to switch on the pump when the panel is hotter than the hot water storage cylinder. The controller may also be configured to circulate the water to heat the panel in the event that the solar collector becomes dangerously close to freezing. This will only happen in exceptionally cold weather, and will represent a negligible energy drain. Controllers should be fitted that allow the installer to adjust the temperature differential to suit different pipe runs with different heat-losses. More advanced controllers will display the temperature of the collector and of the hot water cylinder, or can control more than one pump or control valve, to allow multiple panels on different roof elevations. We recommend RESOL controllers, as they are the industry-leaders, and produce high quality, reliable equipment. The B1 Controller is the simplest and cheapest unit, but is extremely effective. We recommend this for most installations.

For more than 25 years the controller RESOL B1 leads by its simple and robust concept. Due to its huge adjustment range and its adjustable temperature difference, this low-priced universal differential temperature controller is usually first choice for solar heating systems. • Low-priced differential temperature controller for solar-heating- and air conditioning systems
• Adjustable temperature difference from 2 ... 16 K
• 2 temperature sensors are provided (included in full kit)

Solar Controller: Essential for efficient use of Solar Heating

The solar controller is an essential part of the solar heating system, in all but gravity-fed systems (Solar Collector is situated lower than the hot water cylinder, and circulation is effected by thermo-syphoning). In all other systems, it will be necessary to use a controller to switch on the pump when the panel is hotter than the hot water storage cylinder. The controller may also be configured to circulate the water to heat the panel in the event that the solar collector becomes dangerously close to freezing. This will only happen in exceptionally cold weather, and will represent a negligible energy drain. Controllers should be fitted that allow the installer to adjust the temperature differential to suit different pipe runs with different heat-losses. More advanced controllers will display the temperature of the collector and of the hot water cylinder, or can control more than one pump or control valve, to allow multiple panels on different roof elevations. We recommend RESOL controllers, as they are the industry-leaders, and produce high quality, reliable equipment. The B1 Controller is the simplest and cheapest unit, but is extremely effective. We recommend this for most installations.

For more than 25 years the controller RESOL B1 leads by its simple and robust concept. Due to its huge adjustment range and its adjustable temperature difference, this low-priced universal differential temperature controller is usually first choice for solar heating systems.

• Low-priced differential temperature controller for solar-heating- and air conditioning systems
• Adjustable temperature difference from 2 ... 16 K
• Power supply 230 V (AC)
• 2 temperature sensors are provided (included in full kit)

DeltaSol® B The controller RESOL DeltaSol® B is used for application in standard solar thermal systems as well as in heating and air conditioning systems and persuades by its clear operation concept. A newly developed, multi-functional display enables the user to simultaneously request two temperatures (e.g. collector and store temperature). No annoying switching-over, no guessing but easy pictograms give the user clear information on function and operating status of the controller and the system. The version PG 53.02 is equipped with 2 standard relay outputs, the version PG 51.02 is equipped with 1 standard relay output as well as 3 sensor inputs for Pt1000-sensors, store temperature limitation and manual switch. The central element is the 3-key-field below the display. The newly developed combined LC-
display enables an intuitive and reliable controller configuration as well as a comprehensive visualisation of the system status. Collector cooling and recooling function as well as security switch-off, but also a thermostat function can be easily realised. The controller DeltaSolB is also available as individual OEM-version, so that further system adaptions are possible.

Technical data Housing: plastic, PC-ABS and PMMA Protection type: IP 40 / DIN 40050 Size: 172 x 110 x 46 mm Installation: wall mounting, mounting into patch panels is possible Display: LCD, multi-functional combined display with 8 pictograms, two 2-digit text fields and two 4-digit 7-segment displays as well as one 2-coloured luminescent diode Operation: by three pushbuttons in the front of the housing Functions: standard solar controller with adjustable values: minimum- maximum temperature limitation, switch-on and switch-offtemperature difference. Frost protection / cooling function, security
.
Solar Collector Installation
The installation of a Solar Savings collector can be completed in many ways, depending on a number of factors, such as:

• climate (freeze protection, overheating concerns)
• storage tank type (mains pressure, thermal store, gravity fed)
• flow configuration (open flow, closed flow)
• Controller configuration (PV powered pump, Delta T controller)
• Installation location (roof, ground, wall)
• System size (domestic, large scale application)
• System purpose (water heater, central heating, refrigeration)

As a professional solar installer, Solar Savings expects that you will know how to correctly install the collector to ensure efficient performance and system reliability. We can provide you some technical advice as required, but we may not be that familiar with the specifics of your region. When completing a system design the following points should be noted.

1. The heat pipes do not have a temperature cut off like Thermomax, so pressure release valves and/or expansion chambers are required. Pressure should not exceed 85psi under normal use.
2. The system is well insulated, and subzero temperatures will not damage the evacuated tubes or heat pipes, however the header and associated plumbing may be damaged by if the water freezes. Circulation of water through the collector when ambient temperatures are low is suggested as the best “anti-freeze” method. Electrical supply to the pump must be guaranteed, to account for power blackouts (eg DC pump with battery backup).

3. If using a closed system a glycol water mix can be used to provide adequate freeze protection.
4. The manifold is not guarantee against limescale formation, so ensure that water is of suitable quality (closed loop system is suggested for areas with water that is acidic, hard or has high chloride levels)
5. The following is a basic example of a configuration using a thermal store tank, collector and instant gas water heater. This system just supplies domestic hot water, but could easily be configured to also supply heat for infloor/ventilation heating. Thermal stores can be fitted with electric immersion heating as backup, and can accept direct heat input from gas, electric or wood heating sources.

Typical solar integrated heating system and domestic hot water for all year use.




Evacuated Tube Solar Domestic Hot water pre heat installation, gas backup


Thermal Stores
Thermal Stores offer the following key features:

• Mains pressure hot water from an open-vented low-pressure tank (via brazed plate heat-exchanger)
• Light, inexpensive and easy to install
• Excellent corrosion resistance due to anaerobic tank environment
• Provides passive thermal expansion and overheating protection via built in expansion chamber.
• Can accept heat from secondary sources such as wood stove or gas burner.
• Can supply hot water for in-floor heating, air-ventilation heating, spa heating.
• Does not require complicated or expensive plumbing
• Can use a glycol/water mix to provide enhanced freeze protection
• Ideal for use with an “instant” (on demand) gas water heaters, thus ensuring virtually limitless hot water supply (Never run out of hot water again).


What is required for a DIY Installation?

You can install a solar hot water heating system with a minimum of components, but there are many desirable components which improve efficiency and enhance the installation.

ESSENTIAL:
Solar Savings
Solar manifold with 20 Vacuum Solar Heater Tubes
Pipework
Pipe insulation

DESIRABLE

Controller (essential if not using thermo-syphon principle)
Twin coil Solar Hot Water Storage Cylinder
Circulating Pump
Automatic Air Vents

OPTIONAL (depending on installation design):

Expansion vessel and pressurized system kit

Swimming pool kit (Direct Heat)
Solar Savings solar manifold with 20 Vacuum Solar Heater Tubes
Stainless Steel Heat Exchanger (required if you add chlorine to your water – this is because the chlorine will corrode the copper inside the solar collector’s header)
Electronic Controller
Pump

Sample Schematics:

1. ‘Hot Tube’ coil screwed into immersion heater flange
Cheap and easy to install. Ideally requires immersion heater flange to be located in the lower part of the cylinder. Unfortunately, most modern hot water cylinders have top-mounted immersion heaters, which will not allow high efficiency when used to facilitate solar heating

2. Direct heating(simplest method)
Simplest method. Quite efficient, but in areas of hard water, eventually, the solar collector will get ‘furred up’ with limescale, which will reduce efficiency. Easy to retro-fit to an existing direct or indirect hot water cylinder.
[ATTACH=full]22141[/ATTACH]


3. Twin coil hot water tank
This is the best method, but requires the added cost of a twin coil water cylinder. We can supply these at attractive prices, with the added advantage of a double layer of insulation(50mm), keeping heat losses to a minimum. Please contact for details
[ATTACH=full]22142[/ATTACH]

Key to Diagrams:

1. Solar Savings Solar manifold with 20 Vacuum Solar Heater Tubes
2. Pressure Gauge
3. Automatic Air Bleed
4. Drain Cock
5. Expansion Tank
6. Gate Valve
7. Single Check Valve
8. Double Check Valve
9. Filling Loop
10. Circulating Pump
11. Pressure Relief Valve
12. Overflow

 

The second post set up is like the design i have been trying to put togerther on my days off with a simple set up to certain amount of hot water into a small tank to be mixed with cold water and using it during the three season of the area i live in .

The winter time is the only time i'm wondering about the useage of the unit .

 

Are you talking about an evacuated tube or solar panel? If you have not already purchased the parts I would consider the evacuated tubes, one 20 tube manifold is about $900-$1000, not counting all the piping, valves, storage tank, recirculating pump.

What type of hot water heater do you currently have?

I can share information on the evacuated tube hot water heater, as my neighbor has one of these. I have been eyeballing it for awhile and checking up with him on his temperatures. Besides that being a plumber I had to see how this thing worked and what components he had. Very simple system really, the tubes are the only thing that is not a normal plumbing item.

His system is a closed loop pressurized glycol system which uses a water heater that has a coil (heat exchanger) inside the tank. The bottom coil is the solar coil and it has electric elements on the top as a backup when the tubes do not provide enough temperature to the coils. This is all done with one tank.

2 days ago, his hot water temperature on the bottom of the tank was 87 degrees, not bad considering it was 45 degrees outside and heavy overcast. So at times he does need to supplement his evacuated tube system with his backup. On a sunny day he has no problems with hot water. I asked him if had changed his habits on hot water usage, as to time of day he uses hot water. answer: yes.

 

Do you already have a thermal syphon heating loop and are wanting to incorporate evacuated tubes into the system?

Ya, I have been giving some thought into how to incorporate evacuated tubes with an alternative heat source such as coils from a woodstove.

 

I'll have to look up the parabolic reflector system, never heard of it.

 

Water can be a bit trickey as it tends to get real hard in cold weather. Its also expands as it gets hard and anyting that is not expandable water will break.

A black rubber garden will generate enough hot water to take a nice shower in post SHITF, provided you have a way to get the water in the hose

 

Great stuff CC. It looks like you will soon have a new specialty to add to your list of services.

 

Yes sir, you are correct and one needs to incorporate means of freeze protection on the roof, inside the attic space or any other unheated space the piping runs through.

These solar heaters are either a closed loop with glycol antifreeze flowing through the manifold to a coil (heat exchanger) in a water heater tank

or you can flow the domestic cold water directly through the manifold on the roof and into a storage tank. To protect from freezing this way a person could install heat trace wiring.

However, depending on the outside temperature and the amount of flow of the water alone could keep it from freezing. It takes more to freeze flowing water than when it is not in motion. Reason they advise to keep a faucet running in your house when it is below freezing outside. The piping should be insulated and the manifold box comes insulated from the factory.

 

Ill have to read the thred closer,

 

OR

There are also drain back designs, and you use a "Maid O Mist" vent in the top most section of the loop. When the pump quits, water drains back to the tank, the vent opens, allowing air to enter to allow the system to drain.

When the pump starts again ( controlled by a differential thermostat....the panels must be hotter than the tank ), water fills the panels, and rises into the MoM vent, closing it. ( has a float internally ).

 

My bad I overlooked that design method thank you for the correction of another method of freeze protection.

 

How Evacuated Tube Solar Thermal Hot Water Heating works

Evacuated tube collector systems consist of 3 pre-assembled main components:

1. Evacuated tubes
2. CPC mirrors
3. Manifold with heat transfer unit

The Evacuated Tube
Each vacuum tube is composed of two Borosilicate 3.3 glass tubes, one slightly smaller than the other, fused together at the top to make a single tube. The air in the void between the two tubes is then pumped out, creating a vacuum thermal insulation layer.
To convert the daylight into useful solar heat, the internal glass tube is coated with an environmentally friendly, highly “selective” light absorber (Aluminium Nitrite). The absorber converts both direct and diffused light from a large spectrum of wavelengths into heat. The heat produced by the absorber is protected from the atmosphere by the vacuum layer, thereby eliminating thermal-loss. This significantly increases the performance of the solar collector.

The CPC Mirror
In order to increase the output of the evacuated tubes, a highly reflecting, weather-proof CPC mirror (Compound Parabolic Concentrator) is placed behind the evacuated tubes. The special mirror geometry ensures that direct and diffuse light, travelling from all orientations, falls onto the absorber. This substantially improves the energy yield of a solar collector in unfavourable conditions, such as Easterly early morning and Westerly late afternoon light. It is also beneficial when it is not possible to install the panels in the ideal south-facing orientation.
Manifold and “U” Tube
Within each glass tube there is a stainless steel “U” tube collection pipe, with a circular metal heat absorber plate. All the “U” tubes are connected to a well insulated manifold, which is designed to allow a direct flow of the heat transfer fluid through the system. The heat transfer fluid collects the heat from the absorber inside the glass tube and transports it to the cylinder. The manifold has a special design which ensures all the tubes exhibit the same hydraulic resistance. The heat transfer fluid collects the heat from the absorber in the glass tube and carries it down to a twin coil cylinder where it heats the bottom part of the cylinder, being the coolest part. The second, upper coil within the cylinder, is connected to the boiler.
A typical installation

How is it controlled?
The whole system is controlled by a differential temperature control unit. The controller has three temperature sensors: Collector, Cylinder and Return. When the temperature of the collector is 8°C hotter than the bottom of the hot water cylinder, the control unit will turn on the pump starting the solar thermal transfer process. The controller ensures the pump is only running when positive solar gain is available.
The control unit also has various safety features, as well as useful solar gain information.

 

7 Solar Water Heating
System Designs

By Michael Hackleman
Issue 65 Sep/Oct 2000

(Rob Harlan is a general and solar contractor with 25 years of experience with solar water heating systems in Mendocino County, California. Rob primarily designs and installs photovoltaic systems today.)
MH: Rob, will you give a brief history of the last 30 years of solar-water heating system design and implementation?

Rob: Solar-water heating systems got a real boost in the 1970s when tax credits were offered by state and federal programs to help folks make the investment. These systems were intended primarily for domestic hot water, i.e., showers, dishwashing, cooking, and clotheswashing. They were also popular for heating the water in pools and hot tubs. This movement slowed to a snail’s pace when the tax credits ran out.
MH: As I recall, a lot of manufacturers also disappeared when the tax credits went away. Of course, some of these systems were poorly designed, used cheap components, or lacked adequate protection against freezing, overheating, or corrosion. I know that you’ve upgraded solar water heating systems over the years, or older systems from homes and businesses in favor of newer designs. What’s your experience of the design and hardware from 30 years ago?

Rob: Some designs were indeed flawed—poorly implemented, overly complex, or incorporating untested ideas. Still, even good designs require some maintenance. The lack of knowledgeable service personnel and parts crippled some systems. The solar collectors from these systems are actually pretty rugged and often find their way back into new installations sold “as is” or used. Today’s manufacturers of solar water heating systems and components have benefited from the lessons learned long ago. Things are back to a steady pace, with a variety of manufactured system types. Most offer good reliability, are warranted, and generally follow time-tested designs.

MH: There are a few parts that are basic to most solar water heating systems (Fig. 2): collector(s), storage tank, heat transfer medium, and interconnecting plumbing. The collector intercepts the sun’s rays and converts it into heat which is transferred to the storage tank using a fluid such as water or antifreeze. An expansion tank is used in closed systems to accommodate the slight changes in volume that result when water or antifreeze is heated and expands. If glycol (a non-toxic antifreeze liquid) is used, a heat exchanger is needed to transfer the heat from the collector to the water that will exit the faucet. A T&P (temperature and pressure) relief valve is a common safety device found at the top of water heaters. If the water gets hotter than it should or the system builds up too much pressure, this valve will open, releasing water until the temperature or pressure drops to safer levels. The simplest control system disables the backup heating system (gas or electricity) during daylight hours, giving the sun a chance to heat all of the water in the storage tank.

Rob: And—on active systems, a controller turns a pump on and off as solar heat is available. Let’s define a few terms used to describe these systems—active vs passive, open vs closed. An active system is one that uses pumps to move the heat about. A passive system is one that contains no pumps, relying instead on natural convection, conduction, or radiation to move heat. An open system means the water circulating through the collector is the same water you’ll use in a shower (Fig. 3). A closed system circulates the separate heated fluid from the collector through a small loop that includes a heat exchanger, usually located in the storage tank (Fig. 4).

MH: I understand why some people choose passive over active designs. Pumps, controls, relays, and motorized valves all require electricity. Electricity is a very specialized and sophisticated form of energy. Folks who live in the country beyond the grid know what a luxury electricity is. We know it’s a luxury because it’s expensive to make. And very expensive to make a lot of it. It’s a shock for folks who have lived most of their life with utility power to move beyond the grid. A passive solar heating design for making domestic hot water or warming a home requires little or no electricity to operate. Fewer parts, less to go wrong, less to take bites out of your pocketbook. With passive, it’s all in the design. Considered experimental in the 1970s, passive solar heating has proven itself worldwide in a wide range of climates. Speaking of climates, why would someone choose a closed system over an open one?

Rob: Freezing protection. If the water in the collector freezes, it will burst a tube or header. It’s messy, it dumps your hot water, and it must be repaired. You don’t have to live in a place with hard freezes. Water in a collector open to a clear sky can actually freeze when the ambient air temperature is as high as 40 degrees F. This condition is called night sky radiation.

MH: Incidently, there are two reasons why water that freezes will burst its plastic, metal, glass, or stone container. Actually, they are simply properties of water. One, water is virtually incompressible. Two, water expands slightly as it changes from a liquid to a solid. Water immobile inside a small tube or pipe and exposed to a freeze, then, will begin to expand as it becomes ice. Unable to compress itself, it makes a bigger volume by breaking whatever contains it.

Rob: True. It’s actually the different strategies used to combat the potential of freezing that define the major types of systems and their relative complexity. I’ve categorized existing systems into seven types: integral collector/storage, thermosiphon, three-season, drain-back, drain-down, re-circulation, and active closed-loop.

MH: Will you describe them all, first generally and then assess their merits and liabilities from your own experience?

Rob: I would be glad to. I must say first that my experience with solar hot water is limited to my service area (coastal northern California) which is a fairly benign climate with occasional light freezes. I ask your readers to keep this in mind as I speak of various systems.
1. The integral collector/storage is the simplest and historically oldest type of solar water heating system. Paint a tank black, put it in a big crate, insulate it on all sides except the one covered by glass or plastic, and point it at the sun. Water in the tank is heated directly by the sun and stored in the same unit. In the trade, this is also know as a breadbox-type system. An example of a manufactured unit of this type is the Servamatic™. Produced in the 1970s, many are still operational today. The same principle can be seen in today’s ProgressiveTube™ unit (Fig. 5). These are also in-line units, positioned between the well and the shower. You get as much hot water as they collect and store.

MH: This is a popular design in homebuilt units, too. Simple, cheap, and often made with recycled materials. I once took a shower at a ranch I was visiting from water heated in a long thin 20-gallon tank inside an old, big refrigerator with a transparent cover pointed south. I had a long, hot shower in the cold night air. Good experience.

Rob: I have very rarely had to service an integral collector/storage type system, which is a testament of their durability.

2. The thermosiphon system is another solar water heating method (Fig. 6). Sunlight strikes tubes and fins inside a collector box through which water or glycol is circulating. The inlet and outlet of the collector are plumbed, respectively, to the inlet and outlet of the storage tank. If we were talking about electricity and polarity, we’d say the collector is in parallel with the storage tank. Still, it forms a loop. The heated fluid moves from the collector to the storage tank and back to the collector through a process called thermosiphon. This is a natural convective action. If you plumbed this as an open system, the storage tank could be your own water heater.

MH: I’d like to elaborate on a few things you’ve said. Thermosiphon results when water heated in the collector expands and rises, pushing cooler water in the rest of the loop into flowing. Cooler water is pushed out of the bottom of the tank and into the bottom of the collector. Once circulation starts, the process continues unabated all day.

Just as the sun heats the water in the collector, the night sky can cool a collector, causing reverse flow. Think about it. Water in the collector is cooled by nighttime stagnation. Cold water is heavier and sinks, pushing the entire loop into reverse flow, moving warmer water from the tank to the collector which is, in turn, cooled. This will quickly give away some of that hard-earned hot water.

The easiest way to avoid this is by positioning the bottom of the tank above the top of the collector (Fig. 6). This is a physics trick that will prevent reverse flow. Sometimes it’s not possible to elevate the tank above your collector. Thermosiphon will work even if the tank is positioned level with or even somewhat below the collector. In this case, the addition of a check valve will prevent reverse flow (Fig. 7). Avoid the standard pressure-type check valve. It’s too resistive to thermosiphon flow. Instead, use a gravity-type check valve. Angle it in with the plumbing for minimal pressure to open, minimum backflow to close.
The solar collector itself is something of a mystery to many folks and I get many questions about it. A common configuration uses a box, a grid of water tubes, insulation, and glass or plastic glazing (Fig. 8). The box is a large shallow pan, with designs varying smaller and larger in width and length than a standard sheet of 4x8-foot plywood and 4-6 inches in depth. Manufactured designs use stainless steel or aluminum for the boxes but most homebuilt units use plywood. If properly glued and screwed and sealed against weather, they are tough.

Homebuilt designs start with a 4x8-foot sheet of plywood ½ or ¾-inch thick. From it (or another sheet of plywood) cut two 4-6 inch strips from each dimension, supplying the material for the box’s four sides. Large diameter (1½-inch to 2-inch) copper header tubes at the top and bottom of the collector are oriented horizontally and plumbed together with smaller vertical tubes (i.e., ½-inch tubing) spaced 3-6 inches apart. Tin or copper fins or sheet is mechanically and thermally connected in a variety of methods to the tubes. Tubes and fins are blackened with paint or through electrochemical processes. Fittings are added for connection to external plumbing or other collectors. Sheet foam insulation is added behind and to all sides of this assembly when it is mounted in the box.
Glass, greenhouse fiberglass, or some other translucent plastic glazing is added to complete the unit. Glass is available in a range of sizes, particularly if it’s recycled. UV (ultraviolet)-resistant fiberglass is available at local hardware stores in several widths. Don’t burden yourself with plastics that will crystallize in one or two seasons from exposure to the ultraviolet rays of the sun. Select your glazing first. The best economy results when the box is sized to the glass you already have or can get.

Rob: I am reluctant to endorse building one’s own collectors, given the availability of used collectors. If you do build your own, don’t use aluminum absorber plates. They will react adversely with copper tubes. Also, it is best to silver solder any joints within the collector. The collector goes through large temperature swings. This is hard on standard solder joints.

MH: Indeed, the experience of building one’s own collector usually brings about an appreciation for how inexpensive used collectors really are. So, my recommendation to the enthusiastic do-it-yourselfer is: don’t commit to building a whole bunch of collectors without first building one.

Rob: A few more comments on thermosiphoning. If you thermosiphon with water and live in a climate with freezing temperatures, your collector will freeze and burst. Sometimes passive freeze protection valves are installed in such systems. Often called Dole valves, these are designed to open at a preset temperature, 34°F or 45°F. They drip water to create a flow through the collector and, in this way, prevent freezing. In my experience, these valves are not reliable, so I cannot recommend them.
MH: I haven’t used Dole valves personally but I know that some people in the area, including Stephen Heckeroth, do trust and use them. However, it is also my understanding that Dole valves must be periodically inspected and cleaned. If you’re the type of person who isn’t good at regular maintenance, you’d be better off selecting a different system.
Rob: If you live in a climate zone without freezing temperatures, an open thermosiphon system will work well. If not, I still recommend using glycol and a heat exchanger for the thermosiphon loop.
3. The three-season system is another tactic for handling freezing. The idea is to use the solar water heating system for three seasons and drain it for the fourth. It can be a thermosiphon or pumped system and assumes the owner will use another source of energy for heating the water.
4. Drain-back is another type of solar water heating system (Fig. 1). This drains the water in the panels into a tank when there’s no heat available from the sun. The panels are empty of water, then, and cannot freeze. A non-pressurized tank is used to capture this water, and a pump refills the panels when the sun’s warmth is detected.
5. Drain-down is a variation of the drain-back solar water heating system. Here the water is dumped onto the ground. This is a fairly common design, particularly in older systems. It uses a Sunspool™ valve to fill the panels for operation. The same valve, when it reaches a lower temperature, opens to dump the water that’s in the panels onto the ground.
6. Another type of solar water heating system is re-circulation. This method of freeze protection activates a pump to circulate a little bit of hot water from the storage tank back into panels when low ambient temperatures are experienced.
7. Active closed-loop is the final type of solar water heating system on my list (Fig. 9). This design uses any fluid in the collector-to-storage loop that won’t freeze at the low temperatures the system is likely to experience. The heat gathered in the collector is transferred to the water in the storage tank via a heat exchanger. What fluids won’t freeze? I’ve seen systems use glycol, silicon oil, and methanol. Automotive anti-freeze might seem a good candidate, but it’s poisonous. The most popular heat transfer medium is polypropolene glycol, a food-grade dough extender used in the baking industry. It costs about $20 a gallon and is mixed with water. A 10% mixture will protect the collectors down to 20-25°F. The ratio of glycol to water is increased for lower temperatures. I use a 50/50 mixture in my service area.
There’s a lot to be said for using pure water in a solar water heating system. Water is non-toxic, widely available, and cheap. Also, it is the most efficient heat transfer fluid and does not degrade in use. Glycol is also non-toxic but it does break down over time. Exposed to high temperatures, it becomes acidic and will eventually begin to eat your plumbing. So, glycol needs to be checked periodically. I use litmus paper to check its pH. It’s a fairly simple matter to refresh the system with a new glycol-water mix.
Incidentally, there are some types of systems that don’t really fit into any of these seven categories.
The popular Copper-Cricket™ is one example. This system used a 20% methanol mixture under a vacuum to actually “pump” heated fluid down to a storage tank without a pump. It operates on the same principle demonstrated in a coffee percolator to transfer heat. Another is the Sun™-family of solar thermal collectors. These use columns of evacuated tubes to collect and transfer heat.
There’s more basic stuff, too. Some folks just spiral plastic pipe on the ground to pre-heat the water that goes into their standard water heater. It works but if a sudden freeze doesn’t ruin it, long term exposure of the plastic pipe to sunlight will.

MH: The softer, more flexible black plastic tubing you’re referring to is identified as PE, or polyethylene tubing. Ultraviolet radiation from the sun breaks down any kind of plastic, disintegrating the bonds of the polymers and turning the plastic brittle. The black tubing sold in rolls is neither designed to work in direct sunlight nor withstand elevated temperatures. Hot water, particularly with soft water, will leach stabilizers and joint cement from the tubing, too. This is great for showers but you don’t want to drink this water or cook with it.

Rob: If there’s one thing I’ve observed, it’s that most folks who build their own system try to re-invent the wheel, and their designs sometimes reflect a lack of understanding of the basic principles. With good plans, most people could build a good system. Still, many folks don’t want to do it themselves.

MH: I prefer doing my own system yet I have to admit that I have often overrated my ability to be there when the system really needed me. Rob, will you go back through the list of systems and give us your thoughts on the advantages and disadvantages of each type?
Rob: The integral collector/storage system has the advantages of low cost, simplicity, and the lack of pumps or controls. Even homebuilt versions are long-lasting. The tank has enough thermal mass to avoid freezing except in hard-freeze areas. The disadvantages? This design is relatively inefficient and the water often doesn’t reach a very high temperature because the glass-to-mass ratio is small in a breadbox-type system. Heat losses from the collector are high at night, so there is definitely a time of optimal use of the hot water produced, usually afternoons and evenings. The collector/tank combination is heavy, too. Filled, it may reach 650 pounds and tax an unreinforced roof.

The newer ProgressiveTube™ collectors of this type (Fig. 5) are simple and use 4-inch copper tubes and fins with special “selective” surfaces. They extract more of the sun’s energy than blackened surfaces and resist re-radiation of this energy at night. I recommend ProgressiveTube™ systems for my climate zone.
The thermosiphon system has the advantages of simplicity and good efficiency. It doesn’t require electricity and is therefore unaffected by a utility blackout. One disadvantage of thermosiphon flow is that the plumbing must follow strict guidelines—bigger tubing, gentle turns, no low spots, and no restrictive valves—to ensure a smooth, unrestricted flow. An air pocket at a high spot or a large bubble somewhere in the system will stop thermosiphon flow.

MH: I’d like to add to your comments on thermosiphon. I’ve found this to be a neat, natural way to move heat from a collector to storage or use. Water pumping in rural locations can eat a big portion of anybody’s energy pie. Any process that will pump water and the heat it contains through a pipe without external power is a blessing. But—thermosiphon will not tolerate poor planning or a sloppy installation. It wants free, unrestricted motion. Even the check valve must be a gravity-type rather than a pressure-type to avoid becoming restrictive.

Tests have shown that thermosiphon doesn’t start until the collector reaches a critical temperature (Fig.10). Flow commences rapidly, slowing to a more constant rate. A bubble big enough to block a tube will stop flow immediately. The collectors can get hot enough to blow a T&P valve and still no flow. It’s exciting to see water and steam shooting up into the air but, alas, not very productive. Steeply-pitched pipes will ensure a good flow.

I know that in-line, centrifugal-type pumps are used in radiant floor systems to periodically purge the thermosiphon loops of air bubbles. Theoretically, thermosiphon can push water through the pump when it’s off. The pump has another use. It enables the owner to pump more heat into the floor from storage at night.
I added a small purge-pump to one thermosiphon system in the 1970s. I wanted to use primarily thermosiphon but the system included existing plumbing—naturally inaccessible—and the thermosiphon flow kept getting blocked with bubbles. I added a small 12-volt pump in parallel with the check valve (Fig. 11) to occasionally purge the system with a faster flow rate. I used a positive-displacement type to avoid any flow of fluid through the pump when it was off.

Rob: I’ll go on. The three-season system has the advantages of using the existing water heater as a backup, being inexpensive, and requiring only a small pump. The disadvantages are that it is susceptible to freezing and depends on the owner being there to drain it when the weather is cold. There is an overall limit to the size of this system when it’s plumbed to a water heater of a specific capacity.

The drain-back system (Fig. 1) is relatively simple, versatile, and freeze-proof. The tank used in this type of system is long-lasting and there is little maintenance required. During a blackout (or other loss of electricity to the system), the panels are empty and will not overheat. It’s even possible to set up the system so that thermosiphon will get the heat to your water heater. The disadvantages are most evident in off-grid systems, where the energy used in pumping is relatively high. This is because the pump must be sized to fill the collectors daily rather than just circulate water through them. As well, the tank must be located below the panels so that the water that is drained back will have a place to go. This is my favorite choice of a system for freezing climates.

The drain-down system has the merits of high efficiency and is a freeze-proof system. It uses a small pump with small energy use. The disadvantages? Lots of expensive parts, including a complex controller, and the need for periodic inspection and maintenance. However, in any application with a limited supply of water, the daily dumping of water from the collectors onto the ground will be an issue.

The re-circulation system has the advantage of using a standard hot water heater to double as the storage tank. And it’s freeze-proof if the system is small. It has the disadvantage of wasting a lot of energy. If it’s really cold, the backup heating system, say an electric element, has to heat water that is simply being radiated away from the collector at a significant rate.

The active closed-loop system (Fig. 9) is freeze-proof and contains quality components. One disadvantage is that it is complex, meaning it has pumps, valves, and various controls. The tank with heat exchanger is expensive but adds a lot of useful, well-insulated thermal mass to the system. If utility-powered, the pump won’t work during a blackout.

MH: There’s merit to the idea that if the system depends on electricity, the electricity should be generated from the sun, too. If there’s sun for the collectors, there’s sunlight to make electricity to power the pump and move the heat.

In all of these systems, if the collectors overheat, a T&P relief valve will provide protection. There’s a down side with the T&P valve blowing. First, it gives away a lot of hot water since the valve won’t close until both the temperature and pressure fall. And, second, dumping the heat transfer medium can be expensive—if it’s a glycol/water mixture.

I want to thank you, Rob, for turning me onto the fact that a P-type (pressure-only) relief valve is manufactured. I want to use one of these in my next installation. I suspect it will keep the system from dumping all the hot water since it should close as quickly as the pressure is relieved. The pipes in the collector can take heat, but have a tougher time surviving pressure.

Rob: I guess my critique of the advantages and disadvantages of these systems reveals my bias. Generally, I have found with solar hot water, the simpler the better. The simple systems seem to last longer, as a rule.
MH: Bias? I appreciate your review and advice. I’ve learned a lot. Will you describe how you size a system to the application and match components with each other?

Rob: Almost every hot water system has a backup. I design for 70% solar usage. A four-person family is a good standard. Two 4x8-foot collectors will supply the hot water needs of four people. The tank should be sized to the array. In my climate, I’ve found that 1.8 gallons of fluid per square foot of collector is a good ratio. So, two collectors of 32 square feet each will require a storage tank of 115-gallon capacity. For radiant floors, I’ve found that the collector area should be about 10% of the floor area. The same two 4x8-foot collectors, then, will handle about 650 square feet of radiant floor.

MH: What’s the average cost of water heating with electricity, propane, and natural gas for a 4-person family?

Rob: Yes. Using electricity at 12¢ per kWh, the cost of water heating is about $46 month or $551 annually. Propane at $1.41 per gallon costs about $26 a month or $307 per year. Natural gas and fuel oil are less, as is electricity in other parts of the country. Of course, when a solar water heating system is installed and has returned the investment, the energy from it thereafter is free.

MH: Will you give me an idea of how long it will take to pay off the cost of several of these systems based on these rates?

Rob: I have that information, too. First, let me say that these figures do not include the cost of maintenance, the rise in the cost of utility electricity, the lost interest on the investment, and no tax on the savings. In my experience, these balance each other out.

A new integral collector/storage system using the ProgressiveTube™ design will cost about $2,500 parts and labor to install. After 7.3 years, the system cost will equal the cost of electricity to heat the same water during that time. With propane, it’s about 13 years. If the owner installs the system, the cost is about $1,600. The payback is 4.8 years for the avoided cost of using electricity and 8.7 years if using propane.

A new drain-back system costs $3,500 parts and labor. This is equal to 8.5 years of electricity and 15.2 years for propane for domestic hot water. A system that will heat a hot tub will cost about $4,800. When heated electrically, the payback computes to 7.5 years.
MH: In my experience, folks who install their own solar water heating systems usually begin by putting one collector in a loop to the existing water heater. If you shower in the morning, what’s the conventional method for preventing the water heater from using electricity or propane to reheat this water before the sun gets a chance at the task?

Rob: In an electric heater, it’s easy. A 24-hour timer can be set to lock out the backup heating during daylight hours. The owner can manually override the timer with the flip of a switch during bad weather or unusually high demand. For a propane or natural gas heater, turn the gas valve to the pilot position.

MH: There is a proper way to plumb the solar collector to the standard water heater, too. Today’s water heaters position the cold-water inlet and hot-water outlet at the top of the tank. Cold incoming water to the tank actually drops through a tube inside the water heater which ends just above the bottom of the tank. For thermosiphon flow, this is not a good arrangement; you want the cold water return to the collector to exit directly from the bottom of the tank (Fig. 12). Fortunately, water heaters have a drain valve. There is a way to re-arrange this plumbing (Fig. 13) so that the collector will use this orifice for its thermosiphon loop while you retain the ability to drain the tank.

If someone wanted to assemble their own solar water heating system, what’s a good source of information and parts, beyond the library and internet?

Rob: A wonderfully detailed overview of solar hot water systems, complete with schematics and technical information, is found in the Solar Water and Pool Heating Design and Installation Manual from the Florida Solar Energy Center at (407) 783-6300. Triple A Solar in Albuquerque, NM (800-245-0311) sells used solar-thermal collectors at good rates. Check out local sources of used panels to avoid shipping costs. Six Rivers Solar (816 Broadway, Eureka, CA 95501) at (707) 443-5652 sells a high-quality, rectangular thermal storage tank that integrates the inputs and outputs of collectors, auxiliary heating sources, DHW, radiant floors, and hot tubs (Fig. 1).

Rob Harlan, Mendocino Solar Services, 42451 Road 409, Mendocino, CA 95460
Michael Hackleman, PO Box 327, Willits, CA 95490. E-mail: mhackleman@saber.net

 

Thermosiphon

The principle of the thermosyphon system is that cold water has a higher specific density than warm water, and so being heavier will sink down. Therefore, the collector is always mounted below the water storage tank, so that cold water from the tank reaches the collector via a descending water pipe. If the collector heats up the water, the water rises again and reaches the tank through an ascending water pipe at the upper end of the collector. The cycle of tank -> water pipe -> collector ensures the water is heated up until it achieves an equilibrium temperature. The consumer can then make use of the hot water from the top of the tank, with any water used is replaced by cold water at the bottom. The collector then heats up the cold water again. Due to higher temperature differences at higher solar irradiances, warm water rises faster than it does at lower irradiances. Therefore, the circulation of water adapts itself almost perfectly to the level of solar irradiance. A thermosyphon system's storage tank must be positioned well above the collector, otherwise the cycle can run backwards during the night and all the water will cool down. Furthermore, the cycle does not work properly at very small height differences. In regions with high solar irradiation and flatroof architecture, storage tanks are usually installed on the roof. Thermosyphon systems operate very economically as domestic water heating systems, and the principle is simple, needing neither a pump nor a control. However, thermosyphon systems are usually not suitable for large systems, that is, those with more than 10 m² of collector surface. Furthermore, it is difficult to place the tank above the collector in buildings with sloping roofs, and single-circuit thermosyphon systems are only suitable for frost-free regions.



Also known as: thermosyphoning
Thermosiphoning is considered to be an appropriate technology. This process utilizes natural, renewable resources and the basic laws of thermodynamics to create movement of a heated supply of air or water. The energy source for this process is solar radiation(or any other source of heat): the energy of the sun is captured in a solar collection device and is transferred to either air or water via conduction. The entire process may be explained by the thermosiphoning effect: When air or water is heated, it gains kinetic energy from the heating source and becomes excited. As a result, the water becomes less dense, expands, and thus rises. In contrast, when water or air is cooled, energy is extracted from the molecules and the water becomes less active. It also becomes more dense, and tends to "sink."
Thermosiphoning harnesses the natural density differences between cold and hot fluids, and controls them in a system that produces natural fluid movement. Several systems based on this technology are currently available, and may be read about in greater detail within the following text.

Underlying physics

Thermodynamics is the study of energy.

• First law of thermodynamics- States that energy may be changed from one form to another, but cannot be created or destroyed. - Energy is always conserved.

This law may be applied to the movement of water in thermosiphoning system: Energy from the sun is directed and transferred (via conduction and convection) to either water, air, or another medium of choice. This natural process of heating eliminates the need for external energy sources such as fossil fuels or electricity.

• Second law of thermodynamics- States that in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state. - The net return of a system is always less than that of which was initially put in.

Energy is always conserved, however energy (or heat in this case) may often be lost in a given system (thermosiphoning) as heat. Adding insulation with appropriate R values to the system and its plumbing may greatly reduce heat loss, and thus increase efficiency.

• Planck’s Law- the wavelength of radiation emitted from a surface is proportional to the temperature of the surface

Energy transferred as a result of temperature differences between two objects -Dark objects absorb heat, while light objects reflect
Darkly colored collection plates within the solar collector will aid in increasing solar absorption, thus increasing the amount of heat available to heat water or air in thermosiphoning. In contrast, reflective or lightly colored piping and storage tanks should be utilized as the light colors will help to reduce heat radiation out of the system.

Water heating

Passive

The passive thermosiphoning of water is the process of heating and moving water within a system without the need or use of electricity. This process functions by utilizing natual phenomena such as solar energy, gravity, and an available water source. A solar collector, piping, and a water tank are materials required for the heating process. The flow of water is distributed into, within, and out of the solar collector. Cool water enters the bottom of the solar collector where it is then heated via convection by solar radiation. When water is heated it becomes less dense than cooler water, expands, and then rises (flows) through the piping. The heated water exits the top of the solar collector naturally. The cooler and more dense water sinks and remains within the solar collector until it is heated. As the cool water is heated, it expands, rises, is pushed out of the top of solar collector, allowing cool water to flow into the solar collector. This process continues naturally until the temperature of the water reaches an equilibrium with solar radiation input.
Two types of thermosiphon water exchange systems are currently available: the close-coupled system, and the gravity-feed system.

Close-coupled system

Close-coupled systems function on the same principles of passive thermosiphoning mentioned above. The storage tank of these systems must be placed above the solar collector to utilize the water circulation driven by the passive thermosiphoning process.

Schematics

Materials

• Solar Energy
• Solar Collector
• Piping
• Insulation
• Water
• Storage Tank
• Strong roof or other support system

Cost

• Current research (2007) suggests that passive thermosiphon water heaters may range from $500 to $6,500. Pricing may vary due to tank size, solar exposure, and geographical location
• Many countries, states, and utility services provide incentives for renewable energy participation

Pros & cons

Pro's

• Non-polluting
• Energy Savings - No electricity needed for passive thermosiphoning
• Cost Effective
• Space saving - (ie. indoors)

Con's

• Tank exposure to external environmental condition may reduce efficiency, depending on geographical location
• Aesthetics - May be considered visually unpleasing
• Strong support structure needed (i.e. roof)
• Not suitable for extremely cold climates
• Location - must be positioned in an area with suitable solar exposure (i.e. south side of desired area)

Gravity-feed system

Gravity-feed systems utilize the same principals of passive thermosiphoning as does the close-coupled system, however placement of the tank differs. Tanks are installed horizontally into a roof, which is often located directly above the solar collector. Once needed, the heated water within the storage tank takes the path of least resistance and moves via gravity down into the desired location. Gravity-feed systems require more piping/plumbing to distribute the heated water, and this factor should be taken into consideration when installing or purchasing a thermosiphoning system.
Materials


Schematics

• Solar Energy
• Solar Collector
• Piping
• Insulation
• Water
• Storage Tank
• Strong roof or other support system

Cost

• Gravity-feed systems are typically the least expensive passive thermosiphoning water heaters
• Current research (2007) suggests that the cost may range from $400 to $5,500 (Not including the cost-if applicable- of installation). Pricing may vary due to tank size, solar exposure, and geographical location
• Many countries, states, and utility services provide incentives for renewable energy participation

Pros & cons

Pros

• Non-polluting
• Energy Savings - No electricity needed for passive thermosiphoning
• Cost Effective
• Space savings - (ie. indoors)
• Aesthetics - (Horizontal tank placement

Cons

• Plumbing and piping add additional costs to the system
• Aesthetics - May be considered visually unpleasing
• Strong support structure needed (i.e. roof)
• Not suitable for extremely cold climates
• Location - must be positioned in an area with suitable solar exposure (i.e. south side of desired area)

Active

Also known as: pump systems or split systems

Schematics

Active solar heating systems function on the same basis of the thermosiphoning effect, however active systems utilize an energy source other than solar energy to help drive the process. This system installs only the solar collector on the roof, while the storage tank is installed on the ground or anywhere else below. These active water heating units require some external form of energy to pump the water throughout the system. By utilizing additional energy, these active systems are less cost efficient than passive systems.
Materials

• Solar Energy
• Solar Collector
• Electrical energy
• Electrical pump
• Additional piping
• Insulation
• Water
• Storage Tank

Cost

• Current research suggests (2007) that active thermosiphon water heaters may range from $1,200 to $10,500. Pricing may vary due to tank size, internal piping requirements, solar exposure, and geographical location
• Many countries, states, and utility services provide incentives for renewable energy participation

Pros & cons

Pro's

• Money Savings
• Cost Effective
• Aesthetics - Storage tank not placed on the roof
• Greenhouse gas reduction - If insulated properly, it has the potential of polluting as little as passive systems.

Con's

• Uses more energy than a passive system
• Requires more maintenance than a passive system
• Heat loss - during the transfer from the solar collector to the storage tank below
• Pollutes some - from the electrical usage
• Location - must be positioned in an area with suitable solar exposure (ie. south side of desired area)

Passive air exchange


Schematics


An example of a passive solar thermal heating system method is Thermosiphon Heat Exchange. It is based on the principle of natural convection, in which air or water is circulated in a vertical closed-looped circuit without using a pump. Cool air indoors travels through a vent and is directed into an opening in the bottom of a solar collector. The air contained within the solar collector is then heated by the sun via solar radiation. Cool air is dense and will sink, while warm air is less dense and will rise. As the air heats up within the solar collector, it becomes less dense than the cooler air and rises. The warm air rises out of a vent in the top opening of the solar collector, moves into the desired area (i.e. indoors), and is replaced by cooler air. This air exchange process will continue until the indoor air temperature reaches an equilibrium with the temperature outdoors.


Materials

Keep in mind: the bigger the solar collector, the better.
Solar collector
Frame - 6 vertical 2-by-6-inch boards -sideboards - 2-by-6, and a 2-by-8 boards - top sill - lag screws - recommended, but not necessary for attachmant Glaze - corrugated polycarbonate panels - 10 panels - 26 in wide by 8 ft high - Pairs of panels overlapped over 1-by-1-in vertical wood strip - makes 4-foot-wide panels for each bay - ultraviolet-resistant coating - apply to sun-facing side to extend longevity Solar absorption plate - 2 layers black metal window screen - attached across the top and bottom of bays Vents - holes cut through building’s siding Note: - plastic flaps will prevent back flow of air through upper vents at night
Cost

• Current research (2007) suggests that passive heat exchangers may range from $55.00 to $400. Pricing may vary due to size of collector/s, insulation of area to be heated, solar exposure, and geographical location.
• Many countries, states, and utility services provide incentives for renewable energy participation

Pros & cons

Pro's

• Low cost
• Energy saver
• Pollution reduction
• May be used to cool electronics

Con's

• Increased maintainence - (ie. covering during times of low solar radiation)
• Geographical location may alter effectiveness
• Requires manual closing of back draft dampers at night
• South facing installments preferred

References

• National Renewable Energy Laboratory (NREL) Dynamic Maps, GIS Data, and Analysis Tools- Solar Maps (2007)

Available: NREL: Dynamic Maps, GIS Data, and Analysis Tools - Solar Maps

• Citarella, Joe. "Thermosyphons - Better Approach to CPU Cooling?" Overclockers. 5 August 2005. http://www.overclockers.com/articles1246

• Reysa, Gary. "Build a Simple Solar Heater" Mother Earth News. January 2006 Build a Simple Solar Heater

• "Part 2: A Tour of Renewable Energy Applications." UNEP DTIE SCP Branch

• Mirmov, N. I., Belyakova, I. G. "Heat liberation during vapor condensation in a thermosiphon." Journal of Engineering Physics 43(3), pp.970-974, 1982.

• Design and Performance of a Compact Thermosyphon. Aniruddha, P., Yogendra, J., Beitelmal, M,Patel, C., Wenger, T.

Woodruff School of Mechanical Engineering. 2002. http://www.hpl.hp.com/research/papers/2002/thermosyphon.pdf
Interwiki links

• Wikipedia:Thermosiphon