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 MIL-HDBK-1003/13A
Though slightly lower in performance, the inherent simplicity and implicit
lower cost make these systems an attractive alternative. The user should
insure that the freezing problem is adequately addressed should one of these
models be used.
2.5 Space heating and DHW systems. Space heating systems are a simple
extension of the DHW systems. The collectors and storage tank need to be
resized to provide the greater loads. A heat delivery system is added and
the auxiliary heater (or existing heater) is connected in as backup. The
design of the space heating system, if a retrofit, will depend on the
existing system. Water-to-air heat exchangers may be placed in existing
ductwork, in which case, an unpressurized, unlined tank may be used and
represents a minimum heating system as in Figure 2-12. To provide corrosion
and/or freeze protection the use of a closed collector loop and heat
exchanger are required as in Figure 2-13. The cautions given in Figure 2-5
regarding toxic heat transfer fluids must be observed.
The most versatile system would be as shown in Figure 2-14 in which space
heating and DHW are provided. Remember that 100% backup capability is needed
but that oversizing is not necessary. Auxiliary heat can be used directly
such as a heat pump or separate furnace or it could be added to the main
storage tank using a heat pump, a separate boiler, or electrical resistance
heating.
DHW could be added to Figure 2-12 and 2-13 by adding a preheat coil in the
storage tank. Figure 2-12 has the potential to provide some building cooling
by using the collector at night to radiate heat to the sky and storing cool
water for use during the day. Or a heat pump could be used to cool the
building, reject heat to the storage tank during the day, and then, as
before, cool the tank at night through the solar collectors. Unglazed
collectors are superior to glazed collectors for this application. There are
many variations that could be used with the configurations given in Figures
2-12 through 2-14.
Air type space heating systems are sometimes used and a typical system is
shown in Figure 2-15 (see Table 2-1 for advantages of air versus liquid).
The heat storage tank is replaced by a rock bed (nominally 1-3 inch
diameter). Rock provides very desirable temperature stratification. Designs
should emphasize minimum pressure drop through the rock bed. The rocks
carefully washed and placed can be stored in a bin, which should be
insulated, or beneath the building if this is feasible. Keep dust to a
minimum. Heat collected by the collectors is blown through the rock bed from
top to bottom. Heat is delivered from storage to the building by circulating
air in the reverse direction, bottom to top. Note that in contrast to water
storage, heat cannot be added to and removed from the rocks at the same time.
Hollow core concrete wall panels and decks have been used for solar heat
storage and air ducts. They provide thermal mass and air flow passages in
one design. Attention to proper sealing is necessary to prevent dust
During heat collection, the rocks at the top of the bin will attain a
temperature nearly equal to that of the incoming solar heated air, while the
air leaving the storage will be delivered to the collectors at the minimum
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