## Storage capacity correction

It can be proven that the annual performance of liquid-based solar energy systems is insensitive to the storage capacity, as long as this is more than 50 L of water per square meter of collector area. For the f-chart of Figure 11.2, a standard storage capacity 75 L of stored water per square meter of collector area was considered. Other storage capacities can be used by modifying the factor X by a storage size correction factor Xc/X, given by (Beckman et al., 1977)

where

Mwa = actual storage capacity per square meter of collector area (L/m2). Mw,s = standard storage capacity per square meter of collector area (= 75 L/m2).

FIGURE 11.3 Storage correction factor for liquid-based systems.

FIGURE 11.3 Storage correction factor for liquid-based systems.

Equation (11.14) is applied in the range of 0.5 < (MwJMws) < 4.0 or 37.5 < Mwa < 300 L/m2. The storage correction factor can also be determined from Figure 11.3 directly, obtained by plotting Eq. (11.14) for this range.

Example 11.3

Estimate the solar fraction for the month of March of Example 11.2 if the storage tank capacity is 130 L/m2.

Solution

First the storage correction factor needs to be estimated. By using Eq. (11.14),

0.87

For March, the corrected value of X is then Xc = 0.87 X 2.08 = 1.81. The value of Y remains as estimated before, i.e., Y = 0.68. From the /-chart, f = 0.481 compared to 0.466 before the correction, an increase of about 2%.

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