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temperature of the rocks.  The conduction between the rocks is small, thus
with no air circulation the rock bet will remain stratified with the top of
the rock bed warmer than the bottom.  Also, limited conduction and convection
in the rock-bed significantly reduces heat loss from the rock-bed.
Heat is drawn from the storage by circulating building air directly through
the rock bed from bottom to top.  The air will then be delivered to the
building at a temperature near the maximum temperature of the collectors.  If
additional heat is required, supplementary heat is added down-stream from the
storage unit.  This system allows the rock bed to deliver useful heat until
all of the rocks are at room temperature.  See Duffie and Beckman (1974) for
design of packed bed rock storage.
A variation is a no-storage air heating system which circulates heated air
when available.  Performance is limited to daytime heating due to the lack of
storage, but such systems are well suited to warehouses and factories which
have mainly daytime operations.
DHW is provided in Figure 2-15 by pumping the water in the preheat tank
through an air-to-water heat exchanger placed in the return air duct from the
collectors.  This is not very efficient and is one of the disadvantages of
the air system.  It would, perhaps, be just as cost effective to have a
separate small liquid system (say two collectors) to provide DHW.

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