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Forced-Air Solar Heating

There are two types of forced-air systems, the first of which is the most commonly employed heating system in new homes. It relies on a central source of heat that is delivered to various rooms through ducts with the aid of a fan. These systems can be based on solar energy is the heat source is solar.

The second type of forced-air heating systems relies on a heat pump, which can be powered by any hot air source. These systems are somewhat more sophisticated and must be supplemented by a secondary heat source in many cases.

Standard Forced-Air Systems

Forced-air systems are generally used to supplement more traditional air heating mechanisms rather than to supplant them immediately. In the first strategy, solar energy is used to heat water (or air in some cases) in solar collectors. This working fluid is then transferred to the central furnace where a heat exchanger is used to heat the air. The heat from the solar system is generally used to preheat the air entering the furnace, which means that less energy will be needed to heat it to the desired temperature before sending it to the house.

The advantage of forced-air systems is that they can be retrofitted into existing homes and they are relatively simple in terms of the technology involved. These systems are often more affordable and lower maintenance.

There are many disadvantages to such a system. The most obvious is that forced-air is seldom 100% solar. Beyond their reliance on traditional heating for supplementation, these systems also suffer from high rates of inefficiency. A great deal of heat energy is lost in the transfer steps. First, transferring solar heat to the air prior to the central furnace results in heat loss.  Second, heat is lost as the air is circulated through the ducts before reaching its final destination. The longer the duct work, the more air that is lost.

Heat Pumps

The laws of thermodynamics tell us that heat moves from high to low, always. That is to say that heat will always move from a warmer area to a cooler area. The only way to make it move in the reverse direction is through the use of a heat pump and with the expenditure of some energy. The most obvious heat pump is an air conditioner. If run in reverse, an air conditioner can be used to create heat, which is the basic premise of a heat pump.

Air conditioners work through the compression and expansion of a gas. When a gas expands, it absorbs heat and when it is compressed, it releases heat. Thus, by allowing a gas to expand and gain heat and then compressing it elsewhere, it can be used to transfer heat. This is how a heat pump works. In heating mode (they can be used to both heat and cool a space by simply reversing the direction of the cycle), the gas outside of the living space is allowed to evaporate and thus gain heat. Once inside, the gas is compressed and the heat it contains is given off to the ambient environment.

In many cases, heat pumps run on electricity, but they can also run on solar hot water. The principle of heating and cooling through changes in the state of a gas is the same, but the compression and expansion is achieved through a different mechanism. This type of heat pump is called an absorption heat pump and relies on an ammonia-water cycle to operate.

In these pumps, ammonia (or lithium bromide) condenses to release heat into the environment. Then the gas, which is now a liquid, is absorbed by another liquid (usually water). This is also the step that is analogous to the compression step. Finally, the gas-water solution is heated, which is where the solar hot water comes in, causing the gas to evaporate and absorb heat from the water. Now, a low energy heat exchanger can be used to condense the water and start the cycle over again. The heat from the condensing gas can be used to heat air in a central system.

Absorption heat pumps are extremely efficient, but until recently were only available on the industrial scale. They are now available for use in the home. The units are often quite large and, as such, are generally suited to homes over 4,000 square feet or 370 square meters.

The only major drawback to these systems is that ammonia is toxic to humans in high concentration. Thus, it is not recommended that the system be worked on by inexperienced individuals. Overall, however, ammonia dissipates quickly and is of no harm to the ozone layer like many refrigerants. This makes absorption heat pumps one of the most environmentally friendly options for heating a home.

Another tremendous benefit to heat pumps is that they can be run in reverse to cool a home. Until recently, absorption pumps could only be used for one or the other, which meant two units needed to be installed for a single home. Recent advances allow newer models
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