Arizona Solar Center Blog

Commentary from Arizona Solar Center Board Members and invited contributors.

While blog entries are initiated by the Solar Center, we welcome dialogue around the posted topics. Your expertise and perspective are highly valued -- so if you haven't logged in and contributed, please do so!

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17295 Hits

Happy Holidays Green Reminders

  • Over 60 years of life, it is estimated that the average conifer tree will absorb 1 TON of Carbon Dioxide.
  • Reducing the carbon footprint of an artificial tree to that below a living tree takes 20 years of re-use.
    • Suggestion: buy a potted tree that can be replanted after the holidays; if you do get a cut tree, find your local after Christmas tree recycling facility.
  • The amount of holiday cards sent out every year could fill a ten-story football stadium.
    • Suggestion: send seasonal greetings through e-mail to reduce waste.
  • Most mass-produced holiday wrapping paper CANNOT by recycled.
    • Suggestion: use newspaper or old wallpaper as wrapping, or better yet, give a gift of your time or a donation to a charity, bypassing the gift wrap altogether!
  • One string of LED decorative lights uses on average 90% less energy than traditional lights.
(Submitted December 2009)
1633 Hits

Solar Hot Water: A Primer

HPmag bonus article©2001 This email address is being protected from spambots. You need JavaScript enabled to view it.

Go to Sidebar 1, Maintain Temperature Stratification in Your Tank
Go to Sidebar 2, Rust Never Sleeps: Open Loop vs. Closed Loop

Hot water represents the second largest energy consumer in American households. A typical 80 gallon (300 l) electric hot water tank serving a family of four will consume approximately 150 million BTUs in its seven year lifetime. This will cost approximately US$3,600 (at US$0.08 per KWH), not accounting for fuel cost increases. Then it will be replaced by another one just like it. Hmm. Maybe we should rethink this...

An investment in a solar water heating system will beat the stock market any day, any decade, risk free. Initial return on investment is on the order of 15 percent, tax-free, and goes up as gas and electricity prices climb. Many states have tax credits and other incentives to sweeten those numbers even more. What are we waiting for? Forget the stock market. If you have invested in a house, your next investment should be in solar hot water.

In this article I'm going to cover the most common options for solar water heating, basic principles of operation, and some historical perspective on what has worked and what has not.

Below: A Typical Solar Flat Plate Water Heater.

Solar DHW Panel CutawayA Checkered Past, A Bright Future
Solar thermal's past is a good example of why everyone should be skeptical of government involvement in energy. Lucrative federal and state tax credits for solar energy were initiated under President Jimmy Carter in the '70s, and abruptly eliminated under President Ronald Reagan in 1985. This dealt the solar industry a devastating "one-two punch" from which it still has not recovered.

The intention was to stimulate sales for solar thermal systems. But the tax credits resulted in an aggressive promotion of tax credits rather than solar energy. The infant industry was overwhelmed to meet the demand. The demand vanished when tax credits were eliminated, and a majority of solar thermal companies went out of business. Thousands of orphaned solar thermal systems were left behind looking for a service technician.

The solar thermal industry has been purged of the tax credit telemarketers and overnight experts. Today's solar thermal industry includes reliable, efficient products and well-seasoned professionals who have seen it all. Solar hot water is one of the best investments you can make for your house and for the environment.

Key to illustrationsFirst Things First
The best savings in hot water come from no cost or low cost options. Before you tackle solar hot water, take these steps:

  • Turn the thermostat down. Many water heaters are set to between 140 and 180°F (60 and 82°C). See how low you can go. Try 125°F (52°C) for starters. A hot tub is 106°F (41°C). How much hotter do you need?
  • Wrap the water heater with insulation. Insulated water heater "blankets" are usually available where water heaters are sold. (Be careful with natural gas or propane fired water tanks. They use an open flame to heat the water. You need to provide a space for air at the bottom of the tank, and at the top where the flue exits the tank. Safety comes before efficiency!)
  • Fix those drips. They may not look like much, but they are a constant and persistent drain on your water heating load, and they waste water too.
  • Use flow restrictors and faucet aerators to reduce your hot water consumption.
  • Find other ways to use less hot water. Wash only full loads of clothes and dishes.
  • Insulate your hot water pipes.

How Large a Solar Hot Water System Do You Need?
Hot water usage in the U.S. is typically 15 to 30 gallons (55-110 l) per person per day for home use. This includes primarily bathing, clothes washing, and dishwashing. But your commitment to efficiency has a lot to do with your actual usage.

Below: A 40 gallon batch heater.

Batch Water HeaterThe hot water tank is usually sized to handle one day's worth of consumption. So for a household of four, it would be reasonable to use an 80 gallon (300 l) tank based on daily hot water requirements of 20 gallons (75 l) per person per day.

Smitty and Chuck at AAA Solar in Albuquerque have put forth generally accepted rules of thumb for solar thermal collector sizing based on your climatic region:

  • In the Sunbelt, use 1 square foot (0.09 m2) of collector per 2 gallons (7.6 l) of tank capacity (daily household usage).
  • In the Southeast and mountain states, use 1 square foot of collector per 1.5 gallons (5.7 l) of tank capacity.
  • In the Midwest and Atlantic states, use 1 square foot of collector per 1.0 gallon (3.8 l) of tank capacity.
  • In New England and the Northwest, use 1 square foot of collector per 0.75 gallon (2.8 l) of tank capacity.

Based on these rules of thumb, a household of four with an 80 gallon (300 l) tank will need approximately 40 square feet (3.7 m2) of collector in Arizona, 55 square feet (5.1 m2) of collector in South Carolina, 80 square feet (7.4 m2) of collector in Iowa, and 106 square feet (9.8 m2) of collector in Vermont.

Of course, these are big ballpark calculations that will be affected by your incoming water temperature, hot water temperature setpoint, actual usage, and the intensity of the solar resource at your site. You should generally expect that this will give you 100 percent of your hot water in the summer and about 40 percent of your hot water year-round.

Batch Water HeaterYour Choices–An Overview
The type of system you choose will depend mostly on your climate. Freeze-free environments allow for simple, low cost designs. A batch heater uses a storage tank as a collector. A direct pump system circulates water from a collector to a storage tank. A thermosiphon system requires no pump for circulation, just the natural flow of gravity.

Most systems will require some measure of freeze protection. Drainback and closed loop systems with antifreeze and heat exchangers are the best choice for freezing locations. The extra parts increase cost and reduce efficiency, but since one frozen moment can turn into a disaster, it's worth the cost.

Direct pump recirculation systems, which circulate hot water through the collector, are often used where freezing is an infrequent occurrence. That's a risky strategy. Draindown systems, designed to drain water from the collectors to avoid freezing, were the most problematic of system designs. Many were removed or converted. Phase change systems, which in theory could collect heat at night using a refrigerant, never made it into the mainstream of commercial viability. Many of the lessons learned in solar hot water are presented in a publication Solar Hot Water Systems: Lessons Learned, by Tom Lane (see Access).

Solar Batch Heaters
The KISS (keep it simple, stupid) rule applies to solar heating. The batch water heater is the simplest of solar hot water systems. Once affectionately referred to as the breadbox water heater by the do-it-yourself (DIY) community, it has become known as the ICS (integrated collector and storage) water heater in the commercial industry. Its simple design consists of a tank of water within a glass-covered insulated enclosure carefully aimed at the sun.

Cold water, which normally goes to the bottom of your conventional water heater, is detoured to the batch heater first. There it bakes in the sun all day long, and is preheated to whatever temperature the sun is able to provide. Water only flows when used. House water pressure causes the supply of new cold water to flow to the inlet of the batch heater, the lower of the two ports.

Simultaneously, the hottest water exits from the higher port. It flows to the input of the existing water heater, which now serves as a backup to finish the heating job as required. Solar preheated water has become the cold water input to the existing water heater. You save whatever the sun is able to provide. And you still get all the hot water you ask for–it's that simple.

Below: Solar Bypass Valve Configurations.

ValvesBypass Valves
A solar bypass is a series of three valves that allow you to bypass the existing water heater. You can shut it down when the solar collector will do the job alone, such as during summer months or utility blackouts. This is a manually operated configuration; just close off the inlet and outlet valve to the existing tank and open the center valve. This allows hot water to pass directly from the solar batch heater to the house.

Caution! These systems produce very hot water! A tempering valve is your protection from being scalded at the tap. You will regularly see temperatures in excess of 160°F (71°C) in summer months, which is much hotter than you are accustomed to getting from your conventional thermostatically controlled water heater. The tempering valve limits the temperature delivered to the tap by mixing in cold water as necessary.

A pressure temperature relief valve (PTRV) must be installed at the hot water outlet of the batch heater in case temperatures or pressures become excessive. You will find one of these valves installed on every conventional hot water tank too. It is a safety measure required by code. This valve only operates in an emergency, and is often replaced if it opens.

Who Can Use a Batch Heater?
Batch heaters are most appropriate for two to four person households (30 to 40 gallon (110-150 l) daily hot water requirement) in climates where freezing is infrequent. Their size is generally limited because the tank is built into the collector.

Multiple collectors can be installed in series for larger capacities. The outlet of the first collector becomes the inlet of the second in order to deliver higher temperatures. Before you put too many on your roof, consider that a 40 gallon (150 l) batch heater will weigh approximately 500 pounds (225 kg).

Some batch heaters have survived the coldest of winters with freeze-free performance because the large mass of the water tank is quite freeze tolerant. But plumbing lines to and from the tank are very vulnerable. You can make it work with a special selective surface on the tank, a well-insulated, double glazed collector, a whole lot of well-sealed pipe insulation (try R-30 or better), heat tape on the pipe, and good karma.

Are you arrogant enough to tempt Mother Nature to turn your water heater into a frozen fountain? Or are you prepared to drain the collector seasonally? If not, this system is not recommended for climates that freeze regularly.

Separate Collector & Storage
The simple design of a batch heater compromises the effectiveness of collector and storage functions. Heating the whole tank of water all at once will take all day to produce useful temperatures. Once hot, you had better use that hot water at the end of the day before the poorly insulated tank loses its precious heat to the cold night sky.

Most solar hot water system designs separate the collector from the storage tank. This can optimize both functions. Why not bring the tank in from the cold, insulate it well, and leave the collectors out in the sun where they belong?

What are the other advantages of separating the collector from the storage tank? Increase the surface area of a collector, compared to the amount of water being heated, and its temperature will rise more quickly. Configure the storage tank to keep the hottest water apart from the coldest water in the tank and you'll have hotter water available sooner. (See sidebar Maintain Temperature Stratification In Your Tank.)

There are also advantages in freezing climates. By separating the collector from the tank, you can put your tank and piping indoors out of a freezing environment, and insulate them better for greater efficiency.

Below: Two roof-mounted flat plate collectors.

Flat Plate CollectorsFlat Plate Collectors
Flat plate collectors are the most common solar thermal collectors. They are most appropriate for low temperature applications (under 140°F; 60°C), such as domestic hot water and space heating.

A flat plate solar thermal collector usually consists of copper tubes fitted to a flat absorber plate. The most common configuration is a series of parallel tubes connected at each end by two pipes, the inlet and outlet manifolds. The flat plate assembly is contained within an insulated box, and covered with low-iron, tempered glass. (See the diagram on page 45.)

The most efficient collector design maximizes solar heat gain, minimizes heat losses, and provides for the most efficient heat transfer from absorber plate to tube. Operating temperatures up to 250°F (121°C) are obtainable, although neither common nor desirable. Remember, you want hot water, not steam.

Selective Surface
A selective surface, often referred to as "black chrome" is far more efficient than a black painted absorber surface. Although a black surface is most efficient at absorbing solar radiation and converting it to heat, it is also highly efficient at re-radiating long wave infrared heat back out. These losses reduce collector efficiency.

A highly polished chrome surface would re-radiate the least infrared heat energy, but of course not being black, it would absorb very little. A selective surface combines the best of both worlds; high absorptance with low emittance. Sound high-tech? It's been around since the 1950s, and is used on most commercially available flat plate collectors. Its performance is worth the marginal additional cost, particularly in cold climates where radiant heat loss is greatest.

Below: A Thermomax evacuated tube collector.

Evacuated Tube CollectorEvacuated Tube Collectors
If you want the highest efficiency solar thermal collector, you'll be interested in an evacuated tube collector, such as the one manufactured by Thermomax. Although evacuated tube collectors are more efficient than conventional flat plate collectors, they cost approximately twice as much per square foot.

Each tube and fin of the collector is contained within a glass tube from which all the air has been evacuated. Why? Air carries heat from the hot surface of the tube to the cooler surface of the glass to accelerate heat loss by convection. Eliminate the air and you have eliminated convective heat loss.

To minimize radiant heat loss, the tube is covered with a selective surface. Evacuated tube collectors are most appropriate for high temperature applications (over 140°F; 60°C). They are useful for more common low temperature applications too, such as domestic water and space heating.

Below: Direct Pump Recirculation System.

Direct Pump RecirculationCollector to Tank Interface
With the collector and the storage tank separated, the system design must provide a flow of water (or antifreeze) from tank to collector and return. Small circulating pumps provide the necessary flow with very modest energy requirements. Small hot water systems may use a direct current (DC) circulating pump powered by a single PV module (10 to 30 watts depending upon power requirements). You may be able to do without the pump altogether if you design for natural thermosiphon flow.

Thermosiphon System:
Natural Flow Powered by Gravity

Gravity powers convective flow in a thermosiphon system. Water in the collector becomes buoyant as it is heated, and it rises to an elevated tank. Cooler, heavier water falls from the tank to take its place. For best results, place the top of the collectors at least one foot (30 cm) below the bottom of the tank. Greater height differential will result in greater flow. Larger pipe, shorter runs, and gentle bends will make for an adequate flow rate.

If you require freeze protection, it's not hard to do. The collectors can be filled with an antifreeze solution (propylene glycol is the most common). The heat can be transferred to the domestic water via a heat exchanger.

Direct Pump Recirculation
The direct pump system uses an electric circulating pump to move heat from the collector to the storage tank. This means that you are free from the constraint of placing the collector below the tank, as required for thermosiphon flow. The pump can move heat from the collectors on the roof to a storage tank in the basement. Good sense still calls for minimal length of pipe run for efficiency.

A differential controller turns the circulating pump on or off as required. There are two sensors, one at the outlet of the collectors, and the other at the bottom of the tank. They signal the controller to turn the pump on when the collector outlet is 20°F (11°C) warmer than the bottom of the tank. It shuts off when the temperature differential is reduced to 5°F (2.8°C). Some systems let you adjust this hysteresis.

Below: Closed Loop Antifreeze Heat Exchanger System.

Closed Loop Antifreeze Heat Exchanger SystemIn climates where freezing occurs infrequently, a recirculation-type differential control will turn the circulating pump on when the collector inlet temperature falls to 40°F (4.4°C). The philosophy behind this design is that the cost of heating your collectors with hot water from your tank is low cost freeze protection if only required occasionally.

These systems were commonly used in the sunbelt, and only where freezing is a rare occasion. Recirculation systems are no longer very commonly used due to vulnerability to freezing as a result of power outages, malfunction of sensor or controller, or damaged sensor wires.

Draindown System (Not Recommended)
A draindown system is an open loop system in which the collectors are filled with domestic water under house pressure when there is no danger of freezing. Once the system is filled, a differential controller operates a pump to move water from the tank through the collectors.

A draindown valve, invented in the 1970s exclusively for these systems, provides the freeze protection function. When the collector inlet temperature falls to 40°F (4.4°C), the draindown valve, activated by the controller, isolates the collector inlet and outlet from the tank. It simultaneously opens a valve that allows water in the collector to drain away. A vacuum breaker is always installed at the top of the collectors to allow air to enter the collectors at the top so water can drain out the bottom. Right next to the vacuum breaker, you'll find an automatic air vent to allow air to escape when the system fills again.

Draindown systems have proven to be the most problematic of all freeze protection systems. They are vulnerable to frozen vacuum breakers and air vents, damaged sensors or wiring, lack of proper pipe drainage, and malfunctions with the draindown valve. This type of system is rarely installed new any more, and is not recommended. Many were converted to drainback or closed-loop antifreeze systems.

Closed Loop Antifreeze Heat Exchanger
Closed loop antifreeze systems provide the most reliable protection from freezing. These systems circulate an antifreeze solution through the collectors and a heat exchanger. Propylene glycol is the most common antifreeze solution. Unlike ethylene glycol (used in automobile radiators), propylene glycol is not toxic.

The closed loop antifreeze systems generally have the most parts. You'll find an expansion tank to allow the antifreeze to expand and contract with temperature change. You'll find a pressure relief valve to protect against excessive pressures in the closed loop; a spring-loaded check valve to prevent reverse flow of the closed loop at night so the collectors won't dissipate the heat from the water heater; an air vent and/or air eliminator to help get the air out of the closed loop (air is your enemy–it can block fluid flow through the system); and a pressure gauge so you can tell if your system is still charged. A couple of temperature gauges are a good idea in any system so you can tell how well your system is operating.

There's also one more assembly of fittings. Two boiler drains with a shutoff valve in between will allow you to charge the system with your charging pump. Once ready to charge the closed loop with your antifreeze solution, a charging pump is used to circulate the fluid throughout the loop, expelling all the air in the process.

Closed loop systems like this are quite common, whether they be for solar domestic hot water, radiant floor heating, or hydronic baseboard heating. Despite the many additional parts and fittings, they have a high degree of reliability, and are well understood by heating contractors.

Below: Closed Loop Drainback System

Closed Loop Drainback SystemThere is a downside to the closed loop antifreeze system design. Once a solar water heating system has satisfied its daily responsibilities, the system stops circulating. Without circulation to remove heat from the collectors, temperatures can climb to as high as 400°F (204°C).

These high stagnation temperatures, as they are called, can cause problems with air pockets and breakdown of glycol antifreeze solutions. Air pockets form because high temperatures drive dissolved gases out of solution. Systems using propylene glycol as the antifreeze may use an inhibitor additive to prolong the life of the glycol. Otherwise, the glycol can break down, resulting in a sludgy deposit. Silicon and hydrocarbon oils have been used to avoid these problems, but they are expensive and are incompatible with seals and gaskets found in most off-the-shelf components.

Drainback: A Simpler Closed Loop
Although similar in name to the draindown type system, the drainback system is far different and much more reliable. It also provides some advantages over the closed loop antifreeze system. Drainback systems may use water as the heat transfer fluid, since the collectors drain when not in operation. Antifreeze provides an extra measure of freeze protection from poor drainage and controller or sensor malfunctions.

A circulating pump operated by a differential control is turned on when the collector outlet is at least 20°F (11°C) warmer than the tank outlet. Water or an antifreeze solution is lifted from a small reservoir tank and circulated through the collectors and back to the tank. Heat is transferred to the domestic water via a heat exchanger in the reservoir tank. The circulation loop through the collectors is a closed loop. The water or antifreeze solution is installed at the time of installation, and does not present a recurring supply of oxygen.

A drainback system requires a larger pump than any of the other systems described here. It must have sufficient capacity to lift the fluid to the highest point in the system. When there is no more heat to be collected, the controller turns the pump off, and all the fluid drains back to the reservoir tank. The collectors are empty. They can't freeze, and they can't overheat the antifreeze. As a DIY homeowner, you won't need a special charging pump either. When it comes time to change the antifreeze, you can just drain and refill the reservoir tank.

Solar Hot Water System Types: Advantages & Disadvantages

System Type Characteristic & Use Advantages Disadvantages
Solar Batch Water Heater Open loop; Integrated collector & storage; Freeze protection generally limited to infrequent or light freeze climates Simple; No moving parts Freeze protection typically poor; Inefficient in cold climates; Small systems only
Thermosiphon Typically open loop; May be closed loop with heat exchanger & antifreeze Simple; Requires no electricity for operation Collector must be located below tank; Inappropriate for use with hard water (open loop system)
Direct Pump System Open loop; Freeze-free climates Flexible placement of tank & collector; can be powered by PV No freeze protection; Inappropriate for use with hard water
Direct Pump Recirculation System Open loop; Climates where freezing is an unexpected occasion Simple; can be powered by PV Freeze protection is limited to infrequent & light freezes; Inappropriate for use with hard water
Draindown Open loop; Designed to drain water when near freezing Can be powered by PV Freeze protection is vulnerable to numerous problems; Collectors & piping must have adequate slope to drain; Inappropriate for use with hard water
Closed Loop Heat Exchanger Closed loop; Cold climates Very good freeze protection; Basic principles well understood by conventional plumbing trades; No problems with hard water; can be powered by PV Most complex of all systems, with many parts; Heat exchanger & antifreeze reduce efficiency; Fluid may break down at high stagnation temperatures
Drainback Closed loop; Cold climates Very good freeze protection if used with antifreeze; No problems with hard water; Simplest of reliable freeze protection systems; Fluid not subject to stagnation temperatures; Simple to homebrew; can be powered by PV Heat exchanger & antifreeze reduce efficiency; Collectors & piping must have adequate slope to drain; Requires larger pump to lift

The Choice is Yours
The system you choose will be determined first by whether you need freeze protection. If you live in a freeze-free climate, choose a batch heater or small thermosiphon unit for small systems serving one to three people. Larger needs can be met with an open loop direct pump system circulating water from storage tank to flat plat collector.

If you need freeze protection or have hard water, choose one of the closed loop systems with antifreeze and a heat exchanger. Either one will heat your water without fear of freezing.

Solar hot water is a good investment. Whether you are a do-it-yourselfer with plumbing skills or want hire a professional installer, I suggest you locate a dealer who serves your area. Ask their professional advice. Find out the products and services they have to offer, and which is the best fit for your needs and climate. Contact the American Solar Energy Society or the Solar Energy Industries Association for assistance in locating a contractor or supplier in your area.

Back to top

Sidebar 1
Maintain Temperature Stratification in Your Tank

Hot water returning from the collector should enter the storage tank about a third of the way down from the top. This water may not be the hottest water you have collected all day, because solar insolation and outside ambient temperatures vary during the day. You don’t want this water to disturb the water at the very top of the storage tank. Draw water for use from the very top of the tank. That is where it’s the hottest.

When hot water is drawn from the tank, it is replaced by new cold water, which should enter at the very bottom. Water circulating to the solar collector should be drawn from the bottom of the tank. Why? Efficiency! Always supply your collector with the coolest water you have available. The cooler a solar collector runs, the less heat it loses to the surrounding environment.

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Sidebar 2
Rust Never Sleeps: Open Loop vs. Closed Loop

A “hydronic” system is one that uses a liquid as its heat transfer medium. The most common alternatives to hydronic systems are air systems. Hydronic systems are nearly always categorized as “open loop” or “closed loop”—often referred to as “direct” or “indirect” respectively. If you are not aware of the difference between these, you run the risk of discovering one day that your system has been eaten alive by a slow yet persistent killer—oxygen.

Open Loop
Open loop systems are subject to a periodic fresh supply of oxygen, ready to trash every bit of cast iron, steel, or other corrodible part in your system. Whenever you draw water at the tap or bath, new water simultaneously moves in to replace it. Along with that new water comes a fresh supply of oxygen.

You have two lines of defense against damage by corrosive oxygen. You can prevent oxygen from entering the system, or you can use materials that are resistant to corrosion. Copper, bronze, brass, stainless steel, plastic, and the glass lining of a hot water tank have no problem with oxygen. Use these materials when dealing with fresh water supplies associated with “open” or “direct” systems.

Closed Loop
If your system is a “closed system,” you won’t have to worry about oxygen. You will be able to use cast iron components (pumps), which can save you money. Closed systems are charged with fluid at the time of installation. As a permanent part of the installed system, new oxygen is not introduced, and corrosion is not a problem. Read on and you will see several examples of open and closed systems.

Another important consideration with open or direct systems is whether or not you have hard water. Over time, calcium deposits from hard water will clog the collectors, ruining them. These deposits can be removed with periodic use of a descaling solution. But if you have hard water, you’ll be better off with a closed loop system.

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Ken Olson, SoL Energy, PO Box 217, Carbondale, CO 81623 · Fax: 559-751-2001 · This email address is being protected from spambots. You need JavaScript enabled to view it. ·

AAA Solar Supply Inc., 2021 Zearing NW, Albuquerque, NM 87104 · 800-245-0311 or 505 243-4900 · Fax: 505-243-0885 · This email address is being protected from spambots. You need JavaScript enabled to view it. · · For more design and schematic detail click on "Design Guide" then "Hot Water."

Thermo Technologies, 5560 Sterrett Pl., Suite 115, Columbia, MD 21044 · 800-7SOLAR7 or 410-997-0778 · Fax: 410-997-0779 · This email address is being protected from spambots. You need JavaScript enabled to view it. · · Thermomax evacuated tube collector

American Solar Energy Society, 2400 Central Ave. G-1, Boulder, CO 80301· 303-443-3130 · Fax: 303-443-3212 · This email address is being protected from spambots. You need JavaScript enabled to view it. ·

Solar Energy Industries Association, 1616 H St. NW, 8th Floor, Washington, DC 20006 · 202-628-7745 · Fax: 202-628-7779 · This email address is being protected from spambots. You need JavaScript enabled to view it. ·

Tom Lane, Energy Conservation Services of North Florida Inc., 6120 SW 13th St., Gainesville, FL 32608 · 352-377-8866 · This email address is being protected from spambots. You need JavaScript enabled to view it. ·

small HomePower logo graphic©1995 - 2001 Home Power magazine. All rights reserved.

1732 Hits

Solar Hot Water


Hot water.  It is a regular part of our daily lives.  It is used to clean our clothes, wash our dishes...



and to bathe and relax us.  It is used to heat our buildings and even extends use of our swimming pools into the winter months.

But then, hot water doesn't come that way naturally, unless you have a natural hot spring.  Water must be heated in order for us to meet these purposes.


HOT WATER - IMAGE 05 In the past, fires heated water for the variety of cooking, cleaning and bathing uses.

Today we use electricity and/or natural gas to excite water molecules to such a point that it becomes hot. Electricity is generated at some usually distant point source location such as a river whose force spins turbines, or near a coal resource to fire up generators, or even at isolated and highly security conscious nuclear plants.


Natural gas is captured and processed and readied for delivery to the consumer, where it is then turned, creating heat which can be used directly for maintaining comfort, or to transfer that heat energy to another medium - like food, or water HOT WATER - IMAGE 07

The remote site generation of electricity and/or capturing of gas both require transfer to get the product to the consumer. This transfer requires a sophisticated and complex network to assure both quality and quantity needs are met.

Transport of energy always has some losses of product and efficiencies along the way but most arrives ready for use. HOT WATER - IMAGE 10

In the recent past, serious issues and questions have arisen regarding environmental impacts, resource access, energy distribution, and energy cost - issues regarding safety, stability, and security.

Today, some utility companies are incorporating renewable energy systems of wind and sun in their generation of electricity. These renewable energy farms continue the approach of centralized collection, generation, and complex distribution system, but the locations are much closer to the end use consumer, often within the boundaries of communities they serve. In Az. communities like Tucson, Springerville, Prescott, Phoenix and Yuma, utility solar plants are springing up, due in part to Arizona's mandated Energy Portfolio Standard which designates that Arizona utility companies must derive a prescribed about of their energy from solar and renewable energy resources.



The use of the sun to meet people's needs isn't restricted to the actions of large utility companies. More energy, in the form of sunlight, falls upon the roof of a typical house than the entire house uses! Solar energy is the most democratic of energy sources - available to everyone and doesn't require a sophisticated and complex system of extraction, conversion, and transport for people to use. Best of all it is free and directly under your control.

HOT WATER - IMAGE 15 Many Arizonans use the sun's energy to meet daily hot water requirements for bathing, washing, space heating, pool heating, and heating of buildings, and many more are interested.


The benefits of using the sun to heat water include: 
  • Solar water heating reduces the amount of energy required from the utility company thereby reducing monthly bills
  • Less energy demand means less using up of finite oil and gas resources, reduction in the infrastructure required to create and deliver energy to users
  • In replacing other energy resources, it will add to the reduction of pollution, improve air quality, and lessen negative impacts on the environment
less energy... less pollution...
  • Solar water heating is direct, simple, safe and within the individual's direct control
  • Solar water heating will meet all needs when incorporated appropriately
  • Water will be hot, and some even claim it is healthier



  • To quote an Arizona utility - "Just a portion of your house's roof receives more solar energy than you need to heat house hot water all year long. To take advantage of that pollution-free energy you need a solar water heating system."


There’s nothing magic or mysterious about heating water with the sun - a lot of hot water with simple operation and little maintenance, and monthly energy bills will be reduced - a sort of reimbursement for your investment, something a traditional water heating system doesn't provide, and when savings surpass initial investment - it is free!! What more could you ask for?


Using sunlight to heat water is simple and has been done by Arizonans for quite some time. A "batch" water heater was discovered on an outbuilding of the historic Tempe Bakery, and Phoenix's historic Ellis -Shackleford house had a Day/Night solar water heating system, a replica of which remains.  In fact, solar water heaters first appeared in the west around the turn of the century and were heavily used, not only in Arizona but also in Los Angeles, as people heated their water naturally.



HOT WATER - IMAGE 22Today, with energy supply, stability, environmental, safety, security and cost issues and concerns, and the desire for energy independence in Arizona, there is increasing incorporation of solar because of product improvements, stable costs, a well defined industry, consumer protections, state oversight, financial incentives in the form of tax credits and even utility rebate programs.

Solar water system - A solar water heater system has a number of component parts:


  • Basically, there is the collector - used to capture the heat in sunlight, and...
  • the water storage tank which is part of both the heat collection loop, storage, and distribution when hot water is needed. A tank is not, nor may no, be necessary for hot water applications as in pool heating and some radiant floor heating installations.
  • Additionally there are elements of a solar water heating system that are applicable. These include an auxiliary heating system used in periods of additional hot water demand; and...
  • a control system for monitoring and coordinating the operation of all a solar system’s components in more sophisticated systems. This controller optimizes heat collection, minimizes heat loss, and provides freeze and over-heating protection to the system.


Solar collector - What is it?  Simply - a container with a glass cover, which allows sunlight to impact the interior surface.


HOT WATER - IMAGE 26Arizonans are very familiar with the direct heating action of sun through windows - an uncovered car in the summer, and even a sunny winter day, can be unbearably hot.  Sunlight moves through the windshield and impacts interior surfaces (seats, dashboard, steering wheel, etc.) and the resulting heat is resisted from escaping by the glass, and the car interior heats up to sometimes intolerable levels.

HOT WATER - IMAGE 27This is the same action that occurs with a solar collector in a solar water heating system.

Solar collectors capture the sun’s light energy conversion into heat, which then heats water or another heat transfer fluid. The collection of the sun’s energy happens at the collector’s dark color interior absorbing surface, under the glazing. As the absorber heats from exposure to sunlight, water moves through the absorber, picking up heat and carrying it to storage or for direct use. There are variations of this basic water heating system utilizing highly efficient heat transfer fluids through the collector then through an exchanger where the heat is transferred to the water to be used. Since the glazing reduces heat loss to the outside air, colder climate conditions may warrant multiple glazings to increase heat retention capabilities.

What About Hot Water Storage?  Like typical water heating systems, the storage tank holds heated water, but in a solar system, water is heated by continued circulation through the collector and is always hot.  Solar hot water system tanks may be integral to the collector element, as in a direct heat batch water system;  mounted in direct connection with the collector...

batch system


HOT WATER - IMAGE 30or separate all-together.  Solar hot water tanks can be a primary means of hot water storage, or used as a preheater, feeding into a regular tank. In all cases, solar tanks are highly efficient, and better  insulated than standard tanks and are usually of larger capacity than regular tanks, in order to provide large hot water storage capacity for nighttime use and days of limited sunlight. Some solar water heating systems can use an existing water heater tank for additional storage. In other, cases existing water heaters should be replaced with a solar tank or a combination of a solar tank and auxiliary storage tank.  Residential solar tanks are commonly available in 3 sizes.  Custom tanks can be provided for special conditions or for larger applications.

Storage tank size is directly related to consumption of hot water and amount of storage desired for days of low or no solar access . Arizona families typically use about 20 gallons per day per person. Once a daily consumption rate is estimated, one can multiply this number by the number of storage days desired - and identify the total amount of storage capacity desired.  This calculation will aid in determining tank size (s) and configuration of the hot water storage system.

Super hot water - Solar water heating systems can generate water much hotter than conventional water heaters so a mixing valve is usually incorporated at the tank area. This is a protective measure of temperature adjustment, by adding cool water to the hot water from the storage tank when necessary during hot water use.

Cold climate impact - Because solar equipment is exposed to outdoor conditions, freezing is an issue which is well mitigated in modern solar water heating systems. When water freezes, it expands - this is why water-filed pipes break during cold weather. Just as in all plumbing exposed to the elements, freeze protection is important, even in areas that experience mild winters with above freezing temperatures. The solar water heater absorber panel is an efficient solar collector and can also be a re-radiator at night. State requirements mandate that safeguards must be built into all solar systems sold Arizona.

HOT WATER - IMAGE 31 A variety of solar hot water approaches are used in Arizona. All have the means of capturing the sun and heating water for use - they vary in the details of solar capture, transport of captured heat, and approach to storage and storage placement. Basically there are 2 fundamental approaches - Direct and Indirect  Direct heat exchange is when the water to be used is heated directly by the solar collector. Indirect heat exchange involves heating an efficient heat transfer medium other than the water, then transferring the gathered heat from the collection medium to the water to be used. Direct heat transfer is highly effective, and even more so when attention is given to water quality. Calcium carbonate residue (scale) can form on the inside of heated containers like tea kettles and in a solar collector. Scale reduces collector performance and may shorten a collector’s life.

Water treatment, a common installation due to Arizona's hard water, often mitigates the condition, and regular care and maintenance is always a good practice. Indirect heat transfer solar systems, beside providing higher heating and lower heat loss, are another approach to dealing with scaling and freezing. Fluid heated in the collector is typically, propylene glycol, a non-toxic antifreeze compound. This heated liquid is in a separate line and loop from the water to be used or stored. The glycol flows through the collector and heats up from the sun's impact, then flows to a heat exchanger where it goes up its captured heat, then goes back to the collector for another round of heat gathering. The heat exchanger transfers the heat collected in the glycol  to the water to be used, which circulates, in a separate piping system loop to the storage tank where it gives up its heat and returns to the heat exchanger for another transfer. As a result, there are two separate fluid loops, one that gathers the sun's heat, and the other which contains water to be used.


Passive Systems

- Batch or Integrated Collector/Storage (ICS) Systems
HOT WATER - IMAGE 33The simplest of systems - water in a dark container exposed to the sun . Contents will get hot, and in an Arizona summer, get very hot. This is the basis of an ICS and batch/ bread box system which combines collector and storage in a single unit. Water flow only occurs when hot water is drawn off.  Direct heating of the tank or tanks, makes this system compact, simple, and effective, and can be used typically as a preheater to a regular water heater or as some Arizonans have done, to meets all needs. These units do not rely on equipment and external energy to work.  Water pressure brings water to the tank. Water sits in the dark colored tank exposed to the sun and gets hot.  When hot water is removed, it is replaced by an equal amount of "new" water.

The "batch" approach has been used in Arizona for quite some time. Evolved improvements in the design have enhanced the effectiveness in water heating and storage. Newer ICS systems incorporate a number of small-diameter storage tanks connected in parallel or series to expose more surface area to the sunlight, heating the water at a faster rate. Improvements in glazings and containers have made the systems more efficient in heat retention and the issue of freezing is mitigated by the pure volume of the water. Some systems use evacuated glass tubes (like a thermos bottle) around the collector to keep heat loss to a minimum.  As a result, ICS systems do not usually operate at temperatures high enough for scale build-up to clog the system.

- Thermosyphon Systems
HOT WATER - IMAGE 34 Hot water  rises and cold water settles. This is because hot water is less dense than cold water due to its molecular "excitement" in being heated. In a standard water heater, colder water is at the bottom of a tank. When heated it becomes less dense and rises to the top of the tank, while being replaced by cooler, upper water, which is, in turn heated, rises, etc.. This cycle is called a convective action.  A thermosyphon solar hot water heating system incorporates natural convection to move fluid heated by the collector to a storage tank. To do this naturally, the storage tank is located at a point higher than the collector. Cool water from the bottom of the tank flows to the bottom of the collector where it is heated making it rise to the top of the storage tank. This process is continuous and occurs whenever there is enough sunlight to warm the liquid in the collector. As a result, thermosyphon systems do not need pumps and for that reason they are considered a passive system .Thermosyphon systems can be used to directly heat and store water, as well as indirectly heat water with the use of an an antifreeze/heat exchanger loop.

- Active Systems/Forced Circulation Systems

HOT WATER - IMAGE 35 These applications, called active systems because a pump is used to move fluid through the solar collector, allow hot water storage to be placed at any convenient location within the building. Forced circulation systems transfer heat either directly by water circulating from the collector to the tank, or indirectly by use of a heat transfer fluid at the collector and transferring that collected heat via a heat exchanger to water in the storage tank. Variations of a forced circulation include Open Loop and Closed Loop systems.

Open loop forced circulation systems transfer heat directly to water to be used. A sensor monitors the storage tank temperature and in the collector loop. When water in the collector loop is hotter than the water in the storage tank, the pump is activated and water from the tank is circulated through the collector. State requirements stipulate provision of equipment to prevent freeze damage, and open loop systems come with recirculation and/or drain down configurations, as well as with freeze plugs or a “dribble” valve.

  • A recirculation system controller activates the pump when collector temperature nears freezing, and storage tank hot water circulates through the collector loop to raise its temperature.
  • A drain down system has a valve located at the bottom of the collector loop which opens when the temperature drops near freezing, and all water in the collector is automatically drained from the collector and piping, into the tank.
  • A freeze plug is simply a valve that opens when the pressure in the collector rises above a certain point. As water changes from liquid to ice, it expands which forces the freeze plug to open and relieve that pressure, thereby avoiding freeze damage to the solar collector and piping.
  • A “ dribble” valve is much like a freeze plug. Composed of a material that shrinks when it gets cold, it opens, allowing water to drain from the collector. Open loop, and closed loop, systems also are installed with a check valve, which allows fluid in the collector loop pipe to move in only one direction in the collector to prevent undesired reverse siphoning and loss of heat when the sun is not available.

- Closed Loop

HOT WATER - IMAGE 36 Closed loop forced circulation systems transfer heat to water to be used in a 2 loop method. In one loop fluid not susceptible to freezing, is heated at the collector and circulated to a heat exchanger which removes the gathered heat and transfers it to a loop containing the water to be used and/or stored, and the collection fluid is circulated back to the collector. There are two separate fluid loops, one for the heat collecting liquid, and the other for the water to be used.  Separately, each moves through the heat exchanger which implements the heat transfer process. A system controller turns the circulating pump on when the collector fluid is hotter than the storage tank water. There are two primary types of closed loop systems: the drain back and the non-freeze.

Drainback forced-circulation systems have an additional tank (drain back tank) for ensuring protection against freezing. When the pump is off, collector fluid drains into the drain back tank. Non-freeze forced circulation systems use an antifreeze-water mixture in the collector loop. The antifreeze mixture provides protection against very high and low collector operating temperatures. An expansion tank is usually included on these systems to allow the collector loop fluid to expand and contract without damaging the pipes.

HOT WATER - IMAGE 37 Arizona is a great location for solar water heaters because of year around bright sunny, cloud-free days. Some installations, still in operation today, have provided up to 100% of the daily hot water requirement and been in constant use for over 20 years. Others have realized energy savings ranging from 75-90% with a modicum of back-up. As a general rule, savings depends on the system, and amount of hot water demand, and timing of use. If large amounts are needed, or lots of early morning hot water is necessary, inclusion of auxiliary heating may be desirable and the amount of electricity or gas used for this is dependent on the capacity and type of storage, and especially amount of and timing of demand.

Seasonal conditions also impact upon the effectiveness of a system.  Summer, spring & fall have more exposure to the sun than winter with its short daylight hours. Summer conditions easily provide 100% of the requirement, while winter, with its less exposure to the sun may necessitate larger storage capacity or ancillary heating and back-up system in colder climates.


- Collector Placement



Collectors are best located in an unshaded area where there is unobstructed access to the sun throughout the year. The ideal location, of course, is the roof. A solar hot water system should be located to minimize piping runs between collector and storage, and, as in all good hot water design, between storage and end use. This reduces materials, and cost, as well as heat loss in the pipes. Collector placement considerations include:

  • A collector facing true south gains equal amounts of sunlight in the morning and the afternoon. If more hot water is desired in the morning, the collector should face somewhat east of true south, and if hot water is more desirable later in the day or early evening, the collector should face west of true south.
  • Collector performance is maximized when placed perpendicular to the sun. Typically, a collector is placed to operate at its optimum during the winter, with its short days of sunlight exposure, lower sun angles, and colder temperatures. For this reason the upright angle of the collector is important in maximizing solar heating of water during wintertime conditions.
  • Optimum collector angle and angle of a roof may not be compatible. This condition may require a support system like a rack or integration into the building form. It is important to note some subdivisions have restrictions regarding equipment on rooftops, and considerations regarding aesthetic integration and maintenance of style. In these cases, rack mounted collectors meet resistance, and resolution may be in the form of collectors placed flush with the roof. While this may reduce optimum performance of a hot water system, it will still provide an extensive amount of solar heated water. It is said that the difference between an ideal angle and a flush roof angle is about $30 per year in savings.


  • Energy bills will be lower due to less demand of electricity and/or gas. Savings are directly proportional to efficiency of the system, cost of local gas and electricity, and amount of hot water used.
  • Solar energy replacement for heating water means reduction of gas and/or electricity to be provided by the supplier and avoided new and costly generation and transmission systems.
  • Solar water heating replacement of electricity and gas systems results in avoiding additional pollution created by generating electricity and burning gas - a solar water heater avoids the equivalent pollution of .3 cars per year.
  • Utility providers have various incentive and buy-down programs for solar incorporation and utilization.
  • Local, county, and state government provide incentives for incorporation of solar energy equipment. The State has a tax credit of up to $1000 for the purchase and installation of approved solar water heating systems, and there is no sales tax. The community of Marana waives building permit fees for solar photovoltaic and hot water installations.
  • A solar water heating system is a good investment. Return on this investment will be reduced energy bills and a cleaner environment over the lifetime of the system.



The major elements of a certified solar water heating system should last at least 20 years with proper use, care, and maintenance. Other components, such as vents and mixing valves, may need occasional replacement but are relatively inexpensive and easily replaced. To ensure the best performance, a diligent maintenance practice, like one people use for their cars, is recommended. This includes:
  • Flush all tanks once a year.
  • Annual or bi-annual maintenance check-up by a certified service technician.
  • Keeping the glazing clean and unobstructed.
  • Check for leakage at pipes leading to and from the collector.
  • Check the insulation on the pipes and at all joints.
  • For active systems, allow plenty of space around the pump and other equipment in order to extend its lifetime.
  • Check antifreeze in closed loop systems.
  • If performance drops off, check for consumption pattern changes. If the consumption pattern has not changed, check the system's maintenance manual and/or contact a service representative.

A solar water heater can deliver your hot water all year round - In much of Arizona we are fortunate that we have fewer cloudy and cool days than almost anywhere else in the country, so solar energy can carry much of the load.

  • All system component and system must meet State requirements. Contact the Az. Dept. of Commerce Energy Office for information
  • Az. Registrar of Contractors, the Better Business Bureau and the Az. Solar Energy Industries Association are information sources about solar companies and certified installers.
  • Az. Dept of Revenue and Az. Dept. of Commerce Energy Office are information sources re: approved solar systems and tax benefits
  • A properly installed, approved system must have warranties for the equipment and installation
  • Parts and labor for the entire system for a minimum period of two years from the date of installation.
  • Warranty against freezing for a minimum of five years.
  • Solar Contractors License (State of Az. Registrar of Contractors)
  • Certified Solar Technicians to do the work (Az. Solar Energy Industries Association)
  • Product Meets Az. Dept. of Commerce Energy Office Certifications and Requirements Installation Requirements
  • Meets or exceeds all applicable Codes
  • Conforms to Arizona adopted guidelines and standards that require residential solar water heating systems installed in Arizona to be certified under the Solar Rating and Certification OG-300 rating system.  This certification ensures compatibility of components and provides comparative information for the consumer.  Information can be attained at the State Department of Commerce Energy Office, from solar companies, and at

Arizona Revised statues 33-439 prohibits Homeowner Associations from imposing restrictions that effectively prohibit the installation and use of solar energy.  Recent Arizona court rulings have upheld consumer rights to install solar energy devices, while noting that Homeowner Associations can identify reasonable restrictions as long as they do not significantly increase a solar system's cost or diminish its' efficiency. For further information go to, or contact the Az. Dept. of Commerce Energy Office, or contact the Arizona Solar Energy Industries Association at 888-253-8180.


HOT WATER - IMAGE 44 Conventional water heating uses electric energy or gas. Gas is burned directly in the water heater, but the electric energy released into the water in the form of heat is usually generated by burning a fuel at a central power plant. Burning hydrocarbon-based fuels (such as coal, oil, or natural gas) emits oxides of carbon (Cox), nitrogen (Nox) and sulfur (Sox). Solar water heaters significantly reduce pollutants and contribute to a more clean and healthy environment.

This presentation was constructed by the Arizona Solar Energy Association for the Arizona Solar Center, Inc. under contract with the Az. Dept. of Commerce Energy Office, funded by the Dept. of Energy Million Solar Roofs program. Materials and information were provided by a number of sources, including in big part the Arizona Public Service Consumer’s Guide to Solar Water Heating.

NOTE:  Financial support for this presentation has been provided by the Arizona Department of Commerce (Energy Office) and the U.S. Department of Energy through (DOE) Grant No. DE-FG51-01R021250. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the Energy Office or U.S. DOE. The State of Arizona and U.S. DOE assume no liability for damages arising from errors, omissions or representations contained in this presentation.
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All About Solar Cookers 7


For more information on solar cookers please visit:
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Conservation Programs Credits
A publication of the Arizona Department of Commerce Energy Office
3800 N. Central Ave, Suite 1200
Phoenix, Arizona 85012
(602) 280-1402
or call toll-free in Arizona
(800) 352-5499

Fourth Edition: October 1994

Editor: Jim Arwood
Written by: Jim Arwood and Norma Dulin Gurovich
Layout and Design: Shauna Obergfell
Solar Panel Cooker Layout & Design by: LeAnn Moorehead 

Some of this information was copied from publications by Citizens for Solar, Tucson, Arizona, Solar Box Cookers International, Sacramento California and Solar Box Journal, Seattle Washington.

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