UFC 3-440-01
14 June 2002
Figure 4-8. Thermal Expansion Versus Temperature Differential
for Copper Pipe
4-3.2
Storage Tank Sizing. The solar storage tank should be specified based on
the sizing criteria that the volume be between 1.5 to 2 gals per square foot (61.1 to 81.5
L per square meter) of total array collector area. This allows considerable flexibility for
finding an off-the-shelf, standard-sized tank that will meet all specifications. Tank
dimensions for the given storage volume and expected floor loads should be noted.
4-3.3
Storage Sub-System Flow Rate. The flow rate in the storage loop depends
on the collector loop flow rate. To ensure that the storage loop can accept the energy
available, the thermal capacity on the storage side of the heat exchanger (the product
of the mass flow rate and constant pressure specific heat) must be greater than or
equal to the thermal capacity on the collector side of the heat exchanger. An
expression relating the volumetric flow rates in the two loops can be determined by
noting that the constant pressure specific heat for propylene glycol is as low as 85
percent of that for water and that the density of water is as low as 95 percent of that for
propylene glycol. Substituting these relationships into the thermal capacities yields the
result that the storage sub-system volumetric flow rate should be at least 0.9 times that
of the total array volumetric flow rate. To be conservative, the flow rate relationship
across the heat exchanger should be determined using Equation 4-1.
Storage Sub-System Flow Rate = 1.25 x AFR
(eq. 4-1)
4-14
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