Pump energy type is another factor in solar loop choice. The most common circulating pumps are 120 or 240 VAC. DC pumps are also available that can be energized by PV modules or batteries. AC pumps come in hundreds of sizes and are easy to find at most supply houses or home centers. More expensive DC pumps are available in fewer sizes, but can provide a grid-free solar heating system.
Calculate how much space is available for the solar thermal collectors. For optimum performance, you will need a shade-free south-facing area for your collectors. It is best to have no shading from 9:30 a.m. to 3:30 p.m. throughout the entire year. If you have any doubts, this can be verified by using a shade analysis tool such as a Solar Pathfinder. Also consider future shading from tree growth or new construction.
Facing the collectors south is important but not critical. If your collectors are 30° east or west of true south, there will only be a 5% to 10% performance penalty—though some areas have late or early fog to consider. Keep in mind that the roof is not the only option—collectors can be mounted on the ground, the side of the building (like awnings), or even pole-mounted.
What’s your budget? This is often the biggest limiting factor. Decide what makes financial sense for you, while still meeting your goals, such as specific building standards like Passivhaus or LEED; carbon dioxide offsets; or total energy offset. Utility grants, state rebates, tax incentives, the anticipation of reduced utility bills; and private or government financing options can help you more accurately arrive at a final up-front cost.
Installation costs vary widely from region to region because of differences in labor cost, permit fees, code requirements, climate-specific equipment, and other region-specific factors. Get estimates from reputable solar contractors to gauge what the cost will be. You can dramatically reduce your system costs by taking advantage of local and state government incentives or utility programs. Also, if you have a federal tax liability, you can take a 30% tax credit on the entire installation cost. Check dsireusa.org for available utility, local, state, and federal incentive programs.
Calculate how much solar storage you need—a place to park extra heat until it’s needed. If the sun is shining and there is no immediate load, there is no need to let it go to waste. Water is the most common heat storage medium because of its ease of use and ability to transfer heat easily to a load.
Hot water use is usually sporadic. Solar tanks are usually sized to store a full day’s worth of hot water. A good rule is 1.25 gallons of storage for every square foot of collector in the Northeast and Northwest; 2 gallons per square foot in the Southwest; and 1.5 gallons most everywhere else. Even in locations with excellent winter insolation (like the Southwest), most designers use a 1:2 ratio. Having too much storage can result in the collectors not raising the tank temperature enough each day.
For space heating, it’s a little more complicated. If your building needs a lot of heating even when the sun is out, little storage is needed. But if you also have passive solar heating in your building, you might not need all of the harvested solar hot water until the evening. Storage sizing for space heating systems can be from 1 to 3 gallons per square foot of collector area—it depends on when you will need the heat and when it is available. Thermal mass—such as extra sand under the slab or the concrete slab itself—can also be used for storage by hydronic floor space heating systems.