Off-the-shelf solar heating systems don’t always fill the bill. This is where good design and smart planning come in.
Determine the type of heating your system will be supplementing.
Solar thermal can be used for many applications, from water heating to air conditioning to biodiesel processing, but the most common are domestic water heating, pool heating, and space heating. “Combisystems” cover more than one of these applications.Off-the-shelf solar heating systems don’t always fill the bill. This is where good design and smart planning come in.
Figure out what type of backup source will be used.
When your solar heating system can’t take care of the whole load, there are many backup heating choices. The energy source your backup water heater uses will determine the choices you have. For natural gas or propane, traditional tank-type and tankless are the most common. For electricity, you can choose from tank-type, tankless, or heat pump/hybrid water heaters. Gas or electric boilers can also be used to heat water, which is then pumped to an indirect water heater (aka coil tank) or even a plate heater.
For space-heating backup, natural gas, propane, or electric boilers can be used as well as air, ground, or water-sourced heat pumps. The key to selecting is in the heat delivery—the lower the required temperature, the more solar can contribute. Solar thermal is most efficient when the desired water temperature is in the 70°F to 140°F range. Radiant heating works at temperatures from 90°F to 120°F, so this makes it a great match with solar thermal. Next is forced air that works at temperatures from 100°F to 140°F. Last is hot water baseboard heating that works at temperatures between 120°F and 190°F.
In most cases, the solar system ties into the existing space heating system and assists it by preheating the HTF before it gets to the boiler, but after it has left its heat in the floor loop. In this way, solar can contribute the most because the HTF will be at its coldest. If the solar system can produce hotter HTF than comes out of the load, then solar is contributing. If it cannot, then the boiler operates normally with no solar contribution.
Specify the solar loop type that will work best for your application and climate. The main choices are direct, glycol, or drainback systems.
A direct system routes potable water directly through the solar collectors. These systems are only used in year-round warm climates (like Hawaii) where the ambient air temperature never drops below freezing.
In a glycol or pressurized closed-loop system, the solar loop and collectors are filled with a glycol antifreeze solution. The collectors can be mounted in any orientation and be located above or below the solar storage because they require pumps. Some drawbacks are antifreeze maintenance and potential summertime overheating.
A drainback system circulates antifreeze or, more typically, plain water through the collectors only when they are warmer than the load or solar tank. The rest of the time, the collectors and outside pipes are “drained back” and empty—the fluid drains to a holding tank. Advantages are less or no fluid maintenance, higher efficiency, and no possibility of overheating. Drawbacks are that larger circulating pumps are needed, and the orientation of the collectors and pipes must allow unfettered draining back to the holding tank.