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SolarTown Learning

Solar Thermal Water Heaters: Highly Cost-Effective Renewable Energy

06-03-2009
Category: Solar Water Heaters

The Choices for Heating Your Household Water with the Sun

Americans use a lot of hot water-enough to eat up 15% to 25% of the average energy bill, making it third in the list of what residential energy consumers use most of all. Heating and cooling ranks number one and lighting, cooking, and appliances ranks number two in what energy we use in our homes. Finding ways to reduce the energy you consume for water heating is a highly effective way of reducing your carbon footprint and will give you one of the best returns on your investment.

Types of Solar Water Heaters

The type of solar thermal system you choose for your home will depend on your local climate, your hot water requirements, and how much you're willing to spend for your system. To determine which system is best for your home, there are several basic solar thermal principles you should understand. Let's first look at the major components found in most solar thermal systems:

  • one or more solar collectors to capture solar energy,
  • a circulation system (either active or passive) to move liquid from the collectors to where it is required,
  • an insulated storage tank to keep the water warm, and
  • an optional back-up heating system.

The choices of equipment may seem to confusing since some of these components may not be compatible with each other. Even more importantly, the systems are only suitable for certain climates, making it necessary for you to read and re-read the specifications for the systems to make sure that the system that you want is appropriate for the climate in which you live.

As you search for the right solar thermal system for your home, it is highly recommended that you look for one that is certified by the Solar Rating & Certification Corporation (SRCC). The SRCC is an independent certification organization that tests collectors and systems so that you can buy with confidence. SRCC certification is in most cases required to receive state and federal tax incentives and rebates and ensures that you have chosen a system that meets industry standards for efficiency.

Solar Collectors for Solar Water Heating

There's a range of solar collectors from which to choose: integral collector-storage systems, flat-plate collectors, and evacuated-tube solar collectors. Which system you choose will depend largely on your local climate and the temperatures for which you're aiming. Where temperatures of 200°F and lower are required, flat-plate collectors are generally recommended. Evacuated-tube collectors, on the other hand, are best for areas where temperatures higher than 200°F are needed.

  • Flat-plate collectors: These are the most common type used in solar thermal systems. They have a glazed flat-plate collector mounted on insulated, weatherproofed boxes fitted with a dark absorber plate under one or more plastic or glass covers (known as glazing). Unglazed collectors are also available but since they lack the protection of a glass cover, they are usually located in areas that never freeze. This makes them ideal for solar pool heating installations, whereas glazed flat-plate collectors are better suited for areas where heat is required during winter months.
  • Evacuated-tube solar collectors: These contain rows of tubes which are constructed by encasing metal absorber tubes inside transparent glass tubes. The air in the space between the two tubes is removed creating a vacuum, which prevents convective or conductive heat loss. Although they are extremely efficient when the temperature difference between collector and ambient air temperature is high, they are also quite a bit more expensive than flat-plate collectors. These systems are preferred in applications where very high water temperatures (170°F to 350°F) are desired and are often used for commercial and industrial applications.
  • Integral collector-storage systems (ICS): Unlike the flat-plate and evacuated-tube options above which function only as collectors, ICS systems combine the collector and the storage unit, ICS systems use black tanks which are housed in an insulated glazed box facing the sun. As the sun passes through the glazing, the water is pre-heated. It can be stored there or be moved to the hot water tank and a backup water heater. The ICS unit is very reliable, but since the storage unit is often located outdoors, it is suitable only for mild climates where it never freezes. They can also be used in other climates but only in the summer; the liquid is drained during the colder months. These collectors can weigh as much as 300-400 pounds when full. Consequently, you need to make sure that your roof can withstand the additional weight.

Circulation Systems in Solar Water Heating

Water is heated in the collectors in one of two ways: directly or indirectly. We discuss each of these methods below.

Direct solar thermal systems, sometimes referred to as "open loop" systems, work by circulating household water through the tubes from the water tank to the collectors and back. The water is heated directly by the sun, then stored in an insulated tank until it is used for household purposes. The system is "direct" because the water heated is the water that you actually use in the house. Sometimes the tanks have a supplemental gas or electric back-up heating system for times when hot water use is high or sunshine is scarce. However, since this system uses potable water, there is a risk of water freezing and therefore these are suitable for areas where it rarely freezes.

Indirect solar thermal systems are another kind of circulation system used in solar hot water heaters. Instead of circulating water, these indirect or "closed loop" thermal water heaters circulate an antifreeze liquid used for transferring heat to household water. Sun-warmed fluid moves from the collectors to the heat transfer unit where it warms household water, which is then stored in an insulated tank until it is needed. Since the non-freezing fluid is not as susceptible to freezing as potable water, these systems are well-suited to areas that frequently get below-freezing temperatures. Often called closed loop antifreeze systems, these are some of the most popular because of their climate versatility.

Regardless of circulation type, all systems (other than batch systems) require a storage system of some type. In most cases, solar-heated water is stored in a dedicated tank with a back-up heater for times when solar energy is inadequate or hot water demand is too high. The back-up system can consist either of a standard tank system or a tankless (demand) water heater. In either case, this is referred to as a two-tank system.

For both direct and indirect technologies, there are passive and active solar thermal systems. Passive systems have few if any moving parts and are therefore simpler and less expensive to install and maintain. Passive systems may also have a longer life than active systems. Active systems have moving parts, generally pumps to circulate the liquid through the system. They may be more efficient than passive systems, but they are generally more expensive. We summarize some of the major kinds of passive and active systems below.

Passive Solar Thermal Systems

Relying on the natural buoyancy of warm water and gravity, passive solar thermal systems transfer household water or a heat-transfer liquid through the system without pumps. An added bonus is that they will work even when there is a power outage.

Passive direct solar thermal systems heat the water that you will use in your home. Circulation of household water through a passive direct solar thermal system can occur in one of two ways: using an ICS system or a thermosyphon system. In ICS systems, the water is stored in the collector-storage tanks outdoors, which makes this system suitable for warmer climates where there is no chance of freezing. ICS systems may not be the most appropriate in homes where the greatest hot water consumption takes place in the mornings as most of the heat energy will have been lost overnight.

Thermosyphon (also spelled thermosiphon) systems, on the other hand, do not store water in the collector. They circulate the water (without pumps) to an indoor storage tank that has been installed above the collectors. Water naturally flows through the system-warm water rises to the tank and cool water (which is denser and heavier) falls to the collectors. Thermosyphon systems are more expensive than ICS systems but can be installed where there are somewhat colder temperatures.

Passive indirect solar thermal systems are generally thermosyphon systems. Unlike the thermosyphon systems discussed above, which heat the water that you would use in your home, these systems use an antifreeze fluid, which naturally circulates from the collectors to the tank. From the tank, the heat is transferred to the household water supply.

Active Solar Thermal Systems

Unlike passive solar thermal systems that rely on gravity and natural circulation, active solar thermal systems employ pumps to circulate water or a non-freezing heat-transfer fluid through tubes from the solar collectors to the storage tank. They also regulate the overall process using an electronic control system. As with passive systems, there are two types of active solar thermal systems: direct and indirect.

Active direct solar thermal systems: Household water is circulated through the tubes using energized pumps. Active direct solar thermal systems can employ one of two technologies that activate a pump when the temperature in the collectors is higher than the temperature in the storage tank.

Active direct solar thermal systems can come in one of two forms: the flooded open loop system or the recirculation system. In a flooded open-loop system, the collectors and piping have water in them at all times and the collector is plumbed directly with the storage tank. The pump only runs when solar energy is available for heating. Since there is no freeze protection with these systems, they can be used only where freezing never occurs. Recirculation systems are built nearly the same as a flooded open loop system, except that warm water from the storage tank can be pumped through the system as a method of freeze protection. Obviously these are better suited to areas that experience infrequent freezing, but because they require electricity, they are not approved by the SRCC.

Additional freeze protection can be installed into a solar thermal system with draindown technology. In a draindown system, electric valves are triggered to automatically drain the water from the collector when the temperature drops below freezing. These valves are also activated when the collectors are hotter than the storage tank, allowing the tank to refill and the heating operation to continue.

Active indirect solar thermal systems use an antifreeze fluid and a pumping system to heat the water that you will use in your home. As a safeguard against overheating as well as freezing, the popular drainback systems can be installed. The drainback systems automatically drain the heat transfer fluid (most often water although a mix of water and propylene glycol is sometimes recommended) into a reservoir when the pump shuts off or when the system has met a maximum set temperature in the storage tank. Using a differential controller, these systems help to avoid problems during low hot water use periods (such as vacations) and can be installed in both cold and warm climates.

Choosing a Solar Water Heating System for Your Home

Determining which solar thermal system is a good fit for your household hot water needs depends largely on your climate as well as your budget. Determining the cost of your solar water heating system depends on many factors (average amount of hot water used, the size and type of system you choose, your local climate, etc), but the US Department of Energy gives a good explanation at how to arrive at an estimated cost. Typically you'll need about 20 square feet of collector area for the first two household occupants and an additional 8 square feet of collector area for each extra person.

In warm climates where there is no risk of freezing, virtually any direct system (active or passive) can be used. Direct thermosyphon and active direct systems are well-suited for moderate climates. For areas where freezing is frequent and long-term, either indirect thermosyphon or active indirect drainback systems are essentially required. Solar thermal water heating systems offer the most cost-effective renewable energy possible today, with the potential to reduce your water heating bills by 90% and payback periods of less than five years on average.


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