Finding the best value in Thermal Solar
by Rich Schroeher Â Â www.solarsaves.net
Part one of four
There is a lot of interest in solar these days. The recent spike in fuel prices and the desire to be greener are the drivers behind this trend. The government has contributed to the movement with tax credits for installing renewable energy systems. Thatâ€™s the good news. The difficult part is choosing the best system and contractor for your particular situation. There are a lot of companies that have jumped on the bandwagon in renewable energy recently. Where I live there are new ones popping up almost every day. Each has a product line that they sell. Some are good and some not so good. How does the average consumer tell the good from the bad?
This is the first in a four part series. Here we will go through the system types with the pros and cons of each type. In part two we will explain the different types of collectors and their applications. Part three will explain differences in tanks, heat exchangers and controllers. And the last part will be a decision tree and comparison chart so you can compare the different products and make an informed decision based on facts and be able to insure that you are getting the best value.
As far as thermal solar system types go there are basically two, each with several variations.
A direct system circulates the hot water directly through the collectors. This is accomplished by pumping the domestic hot water directly through the collectors and back to the storage tank. This is a very efficient system and works well in areas where there is little chance of freezing and water quality is good. The downside is that they are susceptible to freezing and if the water is hard it can build up on the inside of the collector and reduce performance.
A second variation of this is a thermo siphon system where the tank is required to be above the collectors. The downside again is freezing and the additional roof structure required to hold the weight of a storage tank that can weigh up to 800 pounds or more.
An indirect system uses a separate fluid to bring the heat from the collectors to the storage tank and transfers the heat through a heat exchanger to the water. Of this type there are two variations.
An open loop, or drain back system where the transfer fluid, usually distilled water, drains back to within the heated space when collection shuts down. This requires a separate tank and a larger pump to overcome the gravity of pumping the water up to the roof thus increasing parasitic losses. (Parasitic losses are the energy required to pump the fluid) The higher the drain back tank is to the collectors the smaller the pump that is needed but this may mean installing the drain back tank on a second floor location which may not be convenient or possible. This installation must be done very carefully to ensure that ALL the water drains back within the heated portion of the house. This means setting the collectors so that all water drains to one low point and depending on the roof this could mean the collectors will be set so they appear crooked on the roof. This is normal but may not be the most aesthetically pleasing. These will typically be used for large space heating and intermittent hot water applications such as summer homes where there is not daily hot water use and there is a danger of overheating a glycol solution. These systems require no maintenance but cost more than a glycol system.
The last variant is a closed loop system. This uses a non toxic glycol solution to transfer the heat from the collectors to the heat exchanger. This eliminates the chances for freezing, allows for a smaller pump, allows the collectors to be set straight on the roof and eliminates the need for a separate drain back tank. The down sides are the glycol solution can over time become acidic and will need to be changed about every 5 years. The vast majority of systems are closed loop glycol systems.
Rich Schroeher is a solar energy consultant at Solar Energy Services in Millersville Md.