A Survey of Air-heater Options

Solar-heated air can be used in a wide variety of applications, but it is a limited resource. Only so much sunshine strikes each square foot of the earth, and a well-built collector system will typically deliver about half of this energy to a house in the form of heat. The way this heat is used often means the difference between a useful, cost-effective system and one that never seems to perform properly. Effective use of collected solar heat is especially important in retrofitted systems which, because of space limitations, are often undersized for the houses they are serving. Yet even a small collector used as a supplemental heat source can make a substantial difference in a house's fuel consumption if the heat from it is used effectively. There are also limits to costeffective collector sizing, which are discussed in chapter 3. Even where there is room for a large collector, it is usually not cost-effective to build one so large that it provides 100 percent of a house's spaceor water-heating needs. So before diving into the actual design of your system, it is important to look at the best ways of making full use of the heat a collector delivers. There are a number of factors involved in determining the best ways to "manage" your solar heat, including the temperature of the heated air, the type and size of the collector and the size and layout of your house. These are discussed in the following pages. Technically speaking, the heat generated by a solar air heater is a relatively "low-grade" heat. That's not a disparaging term, for it simply refers to the relatively low temperatures (80 to 140°F) at which air heaters operate. If the airflow rate were reduced, a collector would heat air above this range, but in terms of Btu's delivered, there is a lot more heat in a strong blast of 90°F air than there is in a tiny trickle of 140°F air from the same collector. Why? A collector operating at 90°F has less heat loss than one operating at 140°F and therefore operates more efficiently. In a hotter-running collector, the increased heat losses through the glazing, sides and back of the collector mean that less heat is actually delivered to the living space. This is a very important concept to keep in mind in the design and operation of both small and large systems. This low-grade solar heat is indeed very usable, but it must be handled differently from the 140 to 160°F "highgrade" heat produced by a forced-air furnace. A strong blast of 150°F air from a furnace will feel warm to the occupants of a home, whereas a strong blast of 90°F air from an efficient collector or from rock storage can feel drafty to the occupants, even though it is heating the house. Thus for solar air to heat a living space without drafty discomfort, it must enter the living space slowly and continuously and from several different points. This rule doesn't hold when