The following is an excerpt from Convert Your Home to Solar Energy by Everett M. Barber Jr. and Joseph R. Provey (Taunton Press, 2010). The excerpt is from chapter 6: Active Solar Space Heating.
Types of Active Solar Space Heating Systems
There are a variety of active solar space heating systems and many ways to incorporate them into a home. Some include thermal storage; others don’t. Solar heat distribution can be integrated with a conventional system. Other systems are designed so solar and conventional components supply heat independently. (An industry convention is to call them solar integrated and solar separate.)
Some designs work better than others, and some designs are easier to service than others, as discussed next.
The following five active solar system types are the most common. The systems contribute from less than 5 percent to about 60 percent of the annual space heating load, depending on the size of system and the heating load of the house. The ratios of collector area to heated floor area cited here apply mainly to existing houses with the code-minimum levels of insulation and tightness of 10 years to 15 years ago. Extremely tight and well insulated houses, with moderate to significant passive solar provisions, will require fewer collectors (some none at all).
Small Air-Cooled Array Without Storage
A small air-cooled array without heat storage is low cost and ideal for heating one or two rooms that are used extensively during the day, such as a home office on the north side of the house where there is no passive solar gain. The array is usually small (less than 5 percent of the total heated floor space). Heat gathered by air-cooled collectors is blown directly into the living space. These systems are simple and trouble-free. There are no heat exchangers, heat storage tanks, or pebble bins and no plumbing tie-ins to a solar domestic water heating system. On the downside, the system sits idle for half the year and provides only a small portion of the total space heating load. Attempts to use these systems to heat domestic water during the nonheating season haven’t been very successful.
Small Liquid-Cooled Array Without Storage
Similar in concept to air-cooled arrays are the small liquid-cooled arrays without storage. These systems use liquid-cooled collectors and can be used year-round. During the heating season, the heated liquid, usually a glycol-water solution, is circulated through a small fan-coil unit (a liquid-to-air heat exchanger with a blower) to supply heat to the house. During the nonheating season, the system can be used to heat domestic water. On the upside, this is a simple, easy-to-maintain solar heating system that, if the array is big enough, can supply all of the domestic hot water during the summer. On the downside, the system provides only a small portion of the total space heating.
Medium Liquid-Cooled Array Without Storage
To satisfy a larger share of the heating load, and to help heat domestic water, a medium array (up to 10 percent of the heated floor space) of liquid-cooled collectors can be used. Heated glycol can be circulated through radiant-floor tubing, where some heat storage is provided, or to one or more small fan-coil units. The system doesn’t heat domestic water when it’s supplying space heat. In the off-season, it supplies most if not all of the domestic hot water. These are simple, easily maintained systems, but they still provide a relatively small percentage of the space heating. A drain-back collection loop helps minimize the risk of overheating in summer.
Large Air-Cooled Array With Storage
A large air-cooled array with storage has a relatively large array (up to 20 percent of heated floor space) of air-cooled collectors. Heat distribution is by forced air. Heat can be sent directly to occupied spaces as needed or stored, typically in a bed of pebbles or crushed stone. When the returning air from the collectors is directed to storage, heat is transferred directly to the pebbles. No heat exchanger is required to transfer the heat from the air to the pebbles.
These systems often include a separate, smaller array of liquid-cooled collectors to preheat domestic water year-round, which is better than using an air–water heat-exchange coil in the return air stream from the collector loop to heat domestic water. The reason is that heat exchangers placed there have a strong tendency to suffer freeze damage, and considerable damage can be done to the house by the leaking water.
Large air-cooled arrays can provide as much as 40 percent to 60 percent of space heating. As long as they are used for space heating only, they do not require protection from freezing or overheating, and the systems have proven reliable. On the downside, some system owners complain of a musty odor from the pebble bed, especially when the system starts heating the house in the fall. The ductwork is much larger than the tubing required for a similarly sized array of liquid-cooled collectors. Energy required to run the fan motor is much greater than that required to run circulators in a comparably sized hydronic solar system.
Large Liquid-Cooled Array With Storage
Similar to the air-cooled system is the large liquid-cooled array (up to 20 percent of the heated floor space) with storage. When heat is needed, the array can feed one or more fan-coil units. When there’s no call for space heat, the system stores heat from the array in a tank of water. To minimize collection energy costs, a separate small array of liquid-cooled collectors can be used for domestic water heating year-round.
These systems collect more heat than an array of forced air-cooled collectors of the same area because liquid-cooled collectors are currently more efficient than air-cooled collectors. But provisions must be made to prevent the coolant in the system from freezing or overheating. The storage tank will probably need to be replaced after 25 years to 30 years.
Solar Fraction Goes Up as Heating Load Goes Down
The proportion of space heat a solar array can provide depends, among other things, on how the house is built. At one end of the spectrum are super-insulated houses, built by a very limited number of contractors. At the other end are older, poorly insulated homes with big heating bills. Clearly, the better insulated a house is, the tighter the construction, the less energy of any sort is needed to heat it. The less energy needed to heat the house, the smaller the collector array needed to provide a significant solar fraction. In fact, with good passive design and highly energy efficient construction, there comes a point where an active solar thermal system may not be worth the added cost.
In recent years, some solar installers have been trying to use relatively small arrays, sometimes no larger than would be used to heat domestic water, to provide a significant share of the heating load. In the few cases when house energy consumption and system output have been carefully monitored, the installed solar systems have not come close to their expectations.
*Solar collectors and any supplemental heat sources should have separate distribution systems. This adds to installation costs but greatly simplifies service—and makes emergency service far easier to get.