Internal Gains

Topics: HVAC, Energy, Thermodynamics Pages: 4 (788 words) Published: December 10, 2010
Energy Efficient Buildings
Internal Heat Gains and Design Heating & Cooling Loads

Internal Heat Gains
People and electrical equipment in buildings give off heat. These internal heat gains must be included in energy balances on the building or zone to determine the net heating or cooling load.

Heat Gain From People
Typical sensible, latent and total heat rates given off by people are shown in the table below (ASHRAE Fundamentals, 2005). The sensible heat gain results from radiation and convection from the human body to the air. The latent heat gain is the energy required to condense the water vapor given off by human respiration and perspiration. The latent heat gain is the product of the mass of water vapor from humans and the enthalpy of evaporation for water vapor at atmospheric pressure.

PersonQtotalQsensibleQlatent
Avg. person, at rest350 [Btu/hr]210 [Btu/hr]140 [Btu/hr]
100 W60 W40 W
Avg. person, light work640 [Btu/hr]315 [Btu/hr]325 [Btu/hr]
185 W90 W95 W
Avg. person, heavy work1600 [Btu/hr]565 [Btu/hr]1035 [Btu/hr]
470 W170 W300 W

Heat Gain From Electricity
All electricity consumed in a building is eventually converted into sensible heat. Thus, the heat gain from electricity is equivalent to the electricity use inside a building or zone. The average American house in uses about 11,000 kWh/yr. To gain intuition about how much heat this is, assume the average house uses 8,760 kWh/year. If so, the average rate of heat gain is about:

Only the electricity consumed indoors becomes heat gain to the space. Since some electricity use may be for outdoor lighting or be consumed by the air conditioner compressor and condenser fan which are located outside, we might estimate that 75% of electricity is used indoors. If so, then:

Thus, the general method for calculating average heat gain from electricity in residences is to find total building electricity use, and multiply by fraction of...
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