Active Solar

Space Heating and Domestic Hot Water

A flat plate is the most common type of solar thermal collector and is usually used to heat water or propylene glycol. Flat plate collectors generally consists of a black flat plate absorber which intercepts and absorbs the solar energy, a transparent cover such as a low iron glass, that allows solar energy to pass through, but reduces heat loss from the absorber, tubing such as copper to carry the heat transfer medium (water or glycol) that removes heat from the absorber, an insulated backing, and a weather proof frame.

Since we live in NW Wisconsin where winter temperatures regularly fall to -20 F. and below, we use a closed loop system filled with propylene glycol to prevent freezing. Our system consists of 8, 4 foot by 10 foot flat plate collectors connected to 3, five hundred foot loops of 3/4″ diameter Kytec in-floor tubing. The system contains 35 gallons of glycol and is powered by one 40 watt PV panel directly wired to one 12V DC pump. When the sun rises and hits the PV panel, the pump starts to push glycol up the collectors. The glycol collects heat from the flat plate absorber and continues on to the house through insulated tubing buried in the ground.

The hot glycol enters the house and first passes through the 120 gallon DHW storage tank then into the 3 heating zones under the house. We buried the Kytec tubing under 10 inches of sand then poured a 4 inch slab over that, giving us a large “heat bank” that can absorb enough heat to carry us through several weeks of cloudy winter weather. This is called a high mass active solar thermal heating system. Typically, we start to pump glycol under the building in mid-August, this starts to build heat for winter. In mid-May we allow the hot glycol to enter the house pass through our DHW storage tank, then use a shunt loop to divert the glycol back outside to uninsulated tubing buried in the ground. This keeps the house from overheating in the summer months and sufficiently cools the glycol for its return trip to the solar collectors.

Active Solar Heating

There are two basic types of active solar heating systems based on the type of fluid, either liquid or air, that is heated in the solar energy collectors. (The collector is the device in which a fluid is heated by the sun.) Liquid-based systems heat water or an antifreeze solution in a “hydronic” collector, whereas air-based systems heat air in an “air collector.”

Both of these systems collect and absorb solar radiation, then transfer the solar heat directly to the interior space or to a storage system, from which the heat is distributed. If the system cannot provide adequate space heating, an auxiliary or back-up system provides the additional heat. Liquid systems are more often used when storage is included, and are well suited for,radiant heating systems, boilers with hot water radiators. Both air and liquid systems can supplement forced air systems.

Economics and Other Benefits of Active Solar Heating Systems

Active solar heating systems are most cost-effective when they are used for most of the year, that is, in cold climates with good solar resources. They are most economical if they are displacing more expensive heating fuels, such as electricity, propane, and oil heat. Some states offer sales tax exemptions, income tax credits or deductions, and property tax exemptions or deductions for solar energy systems. The cost of an active solar heating system will vary. Commercial systems range from $30 to $80 per square foot of collector area, installed. Usually, the larger the system, the less it costs per unit of collector area. Commercially available collectors come with warranties of 10 years or more, and should easily last decades longer. The economics of an active space heating system improve if it also heats domestic water, because an otherwise idle collector can heat water in the summer. Heating your home with an active solar energy system can significantly reduce your fuel bills in the winter. A solar heating system will also reduce the amount of air pollution and greenhouse gases that result from your use of fossil fuels such as oil, propane, and natural gas for heating or that may be used to generate the electricity that you use.