Written by Lisa Walsh

Supercharge your Solar Water Heater

Solar Thermal not Solar PV: When talking about solar and space heating, it’s worth mentioning that the panels used for this type of solar application are not the same panels that power a house, a light, or any other electrically powered appliance. In fact, the only similarity between a PV (photovoltaic/electric) solar panel and a Thermal Solar Panel is that they both absorb solar energy. However, how each panels processes and distributes that energy is entirely different.

Enlarged Domestic Hot Water System: In order to make a solar space heating system cost-effective, most space heating systems are designed to include the home’s domestic hot water supply – thereby offsetting the gas/oil/electric bill and returning the solar system’s investment. Therefore, any space heating system is basically an enlarged solar water heating system. These systems that combine solar water heating and space heating are often referred to as combisystems. This combination is achieved with a solar storage tank that comes equipped with two separate heat exchangers; one for the domestic hot water loop and one for the space heating loop. The cooler water returning from the heating system passes through the upper heat exchanger on it’s way back to the boiler where it does one of two things: It picks up some heat that was generated by the solar system or, if the tank is cooler than the returning water, acts as a buffer tank allowing the boiler to have fewer on / off cycles thereby making it more efficient.

Space Heating Infrastructure: Radiant floor heating systems are highly compatible with solar thermal energy. This is mainly because these systems are designed to operate at low temperatures and thus the solar system can contribute energy more of the time. Hot water baseboards, radiators and other hydronic heaters can also benefit, especially if an outdoor reset control is installed. The outdoor reset control adjusts the boiler’s target temperature according to the outdoor temperature.
Forced air systems can also be modified to accommodate solar by placing a fan coil inside the existing duct work. A controller senses when the fan needs to be activated and, again, a conventional back up system kicks on as needed.

Heat Dissipation in the Summer: Of course here in Maryland/Washington DC,  in our Mid-Atlantic climate, solar energy for space heating is being summoned at a time of year when insolation (sunshine) levels are much lower than the rest of the year. Therefore more solar panels are required to meet the quota. A family of four would realistically need two, 4′ x 8′ flat plate collectors for their home’s hot water supply. Depending on the required space heating square footage, this collector size may be increased anywhere between 30 – 100%. To avoid overheating in the summertime when space heating is no longer required, there needs to be a mechanism to dispose of the excess heat. This can usually be achieved with anti-stagnation functions on the controller or by installing a heat dissipater on the roof. An ideal situation is to redirect this excess heat to a pool or hot tub, thereby creating a year round triple-application system that provides the largest return on investment.