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Air systems*.  Since rock has a specific heat of 0.21 Btu/lb-deg.
F, and rock densities (170 lb/ft3) typically contain 20%-40%
voids, then the optimum storage size is 0.8 ft3 per square foot
of collector (range 0.5 to 1.15 ft3 per square foot of
collector).  The range in SI units is 0.15 to 0.35 m3/m2.
Storage volumes in this range will store the equivalent of overnight to
one full day of heating.
In general, for equal storage capacity, the rock pebble bed would have to
occupy a volume 2-1/2 to 3 times larger than a water tank.  Rock storage bins
have higher structural requirements, and tend to lose more heat due to their
greater surface area.  Rock bins generally provide good temperature strati-
fication; contrary to practice in conventional DHW systems, stratification is
desirable in both water and air solar systems.  NCEL has done studies to show
that good stratification can add 5%-10% to overall system performances (Sharp
and Loehrke, 1978).  To achieve this, baffles or modified inlets to the tanks
are used.  However, specially designed tanks with baffles or diffusers are
expensive and not readily available.  To suppress convection warm water
enters and leaves the top of the tank, and cold water, the bottom.  In this
way the hottest water goes to the load and the coldest to the collectors.
A typical DHW system is shown in Figure 2-10.  Use of two tanks insures that
when hot water from the first (tempering) tank is available, the auxiliary
heat will not come on; also less total fuel will be used to bring the smaller
second tank up to temperature.  Single tank arrangements, while possible and
economical, are not recommended due to the fact that they tend to activate
the heating element every time there is a draw of water rather than wait for
the solar collectors to provide additional heated water.  Research is being
done and new tanks designed to overcome this deficiency.  The two-tank
arrangement avoids this control problem.  Two-tank arrangements are suited to
retrofits since the second tank (the water heater) is already there.  A
variation would be to use a heat exchanger (copper coil) in the tempering
tank collector loop for freeze protection.  The tempering tank could then be
an inexpensive unpressurized tank.
Another method of heat storage in air systems that is currently being
investigated is latent heat storage.  Latent heat is stored in a material as
it changes phase from a solid to a liquid.  Materials which have melting
points near the temperatures supplied by solar collectors store heat as they
melt and release it as they resolidify.  The two materials which have
received the most attention are salt hydrates and paraffins.
The advantage of latent heat storage is that it can store very large
quantities of heat per pound of storage material.  Therefore, less volume
should be required for latent heat storage than for heat storage in rock
beds.  However, problems of slow solidification and low heat conductivity
retards effective heat transfer to and from the material.  As a result, a
large surface area-to-volume ratio is required, which significantly increases
the effective volume of latent storage.


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