Page 344 - Handbook of Energy Engineering Calculations

of collector for the 50,000-Btu (52,750-kJ) average daily requirement. Such a
design would provide essentially all the hot-water needs on an average winter
day, but would fall short on days of less than average sunshine. By contrast, a
50 percent recovery of an average summer radiant supply of 2000 Btu/ft 2
2
2
2
(6308 W/m ) would involve the need for only 50 ft (4.6 m ) of collector to
satisfy the average hot-water requirements.
2
2
If a 50-ft (4.6-m ) solar collector were installed, it could supply the major
part of, or perhaps nearly all, the summer hot-water requirements, but it could
2
2
supply less than half the winter needs. And if a 100-ft (9.3-m ) solar
collector were used so that winter needs could be more nearly met, the
system would be oversized for summer operation and excess solar heat would
be wasted. In such circumstances, if an aqueous collection medium were
used, boiling in the system would occur and collector or storage venting of

steam would have to be provided.
The more important disadvantage of the oversized solar collector (fox
summer operation) is the economic penalty associated with investment in a
2
2
collector that is not fully utilized. Although the cost of the 100-ft (9.3-m )
2
2
solar collector system would not be double that of the 50-ft (4.6-m ) unit, its
annual useful heat delivery would be considerably less than double. It would,
of course, deliver about twice as much heat in the winter season, when nearly

all the heat could be used. But, in the other seasons, particularly in summer,
heat overflow would occur. The net effect of these factors is a lower
economic return, per unit of investment, by the larger system. Stated another
way, more Btu (kJ) per dollar of investment (hence cheaper solar heat) can be

delivered by the smaller system.
If it is sized on average daily radiation in the sunniest months, the solar
collector will be slightly oversized and a small amount of heat will be wasted
on days of maximum solar input. On partly cloudy days during the warm

season, some auxiliary heat must be provided. In the month of lowest average
solar energy delivery, typically one-half to one-third as much solar-heated
water can be supplied as during the warm season. Thus, fuel requirements for
increasing the temperature of solar-heated water to the desired (thermostated)

level could involve one-half to two-thirds of the total energy needed for hot-
water heating in a midwinter month.
One disadvantage of solar DHW heating systems is the possibility of the