Page 350 - Handbook of Energy Engineering Calculations
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3. Compute the area of unshaded collector required
Determine the area of unshaded collector needed to heat this space on an
average clear day in January. An average clear day is chosen because sizing
the collector for extreme or cloudy conditions would cause space overheating
on clear days. January is used because it generally has the highest heating
load of all the months.
To compute the collector area, use the relation A = H /(E)(i ), where E =
D
T
c
a rule of thumb for energy absorptance efficiency of the passive solar-heating
system used, expressed as a decimal. Enter Table 10 for a direct gain system
2
2
to find E = 0.91. Therefore, A = 53,904/(0.91)(1423) = 42 ft (3.9 m ).
c
TABLE 10 Energy Absorptance Efficiency of Passive Solar-Heating
Systems
If the area of unshaded collector computed in this step varies by more than
10 percent from the area of the collector estimated for heat-loss calculations
in step 1, the heat loss should be recomputed with the new areas of collector
and opaque wall. In this example, the computed and estimated collector areas
are within 10 percent of each other, making a second computation of the
collector area unnecessary.
4. Compute the insolation stored for nighttime heating
To compute the insolation to be stored for nighttime heating, the total daily
insolation must be determined. Use the relation i = (A )(i )(E), where i =
D
C
T
D
total daily insolation collected, Btu (J). Therefore, i = (42)(1423)(0.91) =
D
54,387 Btu (57.4 kJ).
Typically 35 percent of the total space heat gain is used to offset daytime
heat losses, requiring 65 percent to be stored for nighttime heating.
Therefore, i = (0.65) i , where i = insolation stored, Btu (J). Thus, i =
S
S
D
S