The Heating Energy Demand
The demand of heating energy for an industrial building can be calculated from Eq. (8.25) as
Q = T0(t))dt. (8.28)
Here t is the time and t1 — t2 the time period under consideration (month, year, etc.). The outdoor temperature T0 now depends on time and the indoor temperature is assumed constant.
The quantity m can be considered constant on condition that the flow of outside air qVa is constant within the considered period. Quite often the ventilation is reduced at night and m is then not constant.
If m can be considered constant or if we use an average value of m over
The time range t1 — f2, then we get from Eq. (8.28)
Q = m^2(T,-Tn(t))dt. (8.29)
The quantity J^(Tj — Ta(t))dt is named the “degree day” and is normally calculated for each month but also on a yearly basis. It depends on the climate where the industrial building is situated. This means that different geographic positions have different degree days. From the definition we also see that the degree day depends on the assumed indoor air temperature, which is assumed constant.
How much energy in January is needed for an industrial building if the heat demand is 50 kW at outside temperature -26 °C and indoor temperature 1 8 °C? The average outdoor temperature in January at that place is — 4.7 X’,
Solution From Eq. (8,26) we get
50kW = 1140 W °C 1 .
The average temperature difference between the indoor air and ambient air is
AOM, m = 18 °C — (-4.7 °C) = 22.7 °C.
That means that the average heating power demand becomes
If the room is heated the whole day and night, i. e., mt —31 days = 744 hours, the heating energy demand becomes
Q = ‘Pmean mt = 25.9 kW ■ 744 h = 19.270 kWh.
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