The conversion of chemical or electrical energy to thermal energy in a building constitutes the internal heat gain or internal load of a building. The primary sources of internal heat gain are people, lights, appliances, and miscellaneous equipment such as computers, printers, and copiers (Fig. 26). Internal heat gain is usually ignored in design heating load calculations to ensure that the heating system can do the job even when there is no heat gain, but it is always considered in design cooling load calculations since the internal heat gain usually constitutes a significant fraction of it.
The average amount of heat given off by a person depends on the level of activity, and can range from about 100 W for a resting person to more than 500 W for a physically very active person. Typical rates of heat dissipation by people are given in Table 8 for various activities in various application areas. Note that latent heat constitutes about one-third of the total heat dissipated during resting, but rises to almost two-thirds the level during heavy physical work. Also, about 30 percent of the sensible heat is lost by convection and the remaining 70 percent by radiation. The latent and convective sensible heat losses represent the “instant” cooling load for people since they need to be removed immediately. The radiative sensible heat, on the other hand, is first absorbed by the surrounding surfaces and then released gradually with some delay.
It is interesting to note that an average person dissipates latent heat at a minimum rate of 30 W while resting. Noting that the enthalpy of vaporization of water at 33ºC is 2424 kJ/kg, the amount of water an average person loses a day by evaporation at the skin and the lungs is (Fig. 27)
which justifies the sound advice that a person must drink at least 1 L of water every day. Therefore, a family of four will supply 4 L of water a day to the air in the house while just resting. This amount will be much higher during heavy work.
Heat given off by people usually constitutes a significant fraction of the sensible and latent heat gain of a building, and may dominate the cooling load in high occupancy buildings such as theaters and concert halls. The rate of heat gain from people given in Table 8 is quite accurate, but there is consider-able uncertainty in the internal load due to people because of the difficulty in predicting the number of occupants in a building at any given time. The design cooling load of a building should be determined assuming full occupancy. In the absence of better data, the number of occupants can be estimated on the basis of one occupant per 1 m2 in auditoriums, 2.5 m2 in schools, 3–5 m2 in retail stores, and 10–15 m2 in offices.
Lighting constitutes about 7 percent of the total energy use in residential buildings and 25 percent in commercial buildings. Therefore, lighting can have a significant impact on the heating and cooling loads of a building. Not counting the candle light used for emergencies and romantic settings, and the kerosene lamps used during camping, all modern lighting equipment is powered by electricity. The basic types of electric lighting devices are incandescent, fluorescent, and gaseous discharge lamps.
The amount of heat given off per lux of lighting varies greatly with the type of lighting, and thus we need to know the type of lighting installed in order to predict the lighting internal heat load accurately. The lighting efficacy of common types of lighting is given in Table 9. Note that incandescent lights are the least efficient lighting sources, and thus they will impose the greatest load on cooling systems (Fig. 28). So it is no surprise that practically all office buildings use high-efficiency fluorescent lights despite their higher initial cost. Note that incandescent lights waste energy by (1) consuming more electricity for the same amount of lighting and (2) making the cooling system work harder and longer to remove the heat given off. Office spaces are usually well lit, and the lighting energy consumption in office buildings is about 20 to 30 W/m2 (2 to 3 W/ft2) of floor space.
The energy consumed by the lights is dissipated by convection and radiation. The convection component of the heat constitutes about 40 percent for fluorescent lamps, and it represents the instantaneous part of the cooling load due to lighting. The remaining part is in the form of radiation that is absorbed and reradiated by the walls, floors, ceiling, and the furniture, and thus they affect the cooling load with time delay. Therefore, lighting may continue contributing to the cooling load by reradiation even after the lights have been turned off. Sometimes it may be necessary to consider time lag effects when determining the design cooling load.
The ratio of the lighting wattage in use to the total wattage installed is called the usage factor, and it must be considered when determining the heat gain due to lighting at a given time since installed lighting does not give off heat unless it is on. For commercial applications such as supermarkets and shopping centers, the usage factor is taken to be unity.
Equipment and Appliances
Most equipment and appliances are driven by electric motors, and thus the heat given off by an appliance in steady operation is simply the power consumed by its motor. For a fan, for example, part of the power consumed by the motor is transmitted to the fan to drive it, while the rest is converted to heat because of the inefficiency of the motor. The fan transmits the energy to the air molecules and increases their kinetic energy. But this energy is also converted to heat as the fast-moving molecules are slowed down by other molecules and stopped as a result of friction. Therefore, we can say that the entire energy consumed by the motor of the fan in a room is eventually converted to heat in that room. Of course, if the motor is in one room (say, room A) and the fan is in another (say, room B), then the heat gain of room B will be equal to the power transmitted to the fan only, while the heat gain of room A will be the heat generated by the motor due to its inefficiency (Fig. 29).
The power rating Wmotor on the label of a motor represents the power that the motor will supply under full load conditions. But a motor usually operates at part load, sometimes at as low as 30 to 40 percent, and thus it consumes and delivers much less power than the label indicates. This is characterized by the load factor fload of the motor during operation, which is fload = 1.0 for full load. Also, there is an inefficiency associated with the conversion of electrical energy to rotational mechanical energy. This is characterized by the motor efficiency hmotor, which decreases with decreasing load factor. Therefore, it is not a good idea to oversize the motor since oversized motors operate at a low load factor and thus at a lower efficiency. Another factor that affects the amount of heat generated by a motor is how long a motor actually operates. This is characterized by the usage factor fusage, with fusage = 1.0 for continuous operation. Motors with very low usage factors such as the motors of dock doors can be ignored in calculations. Then the heat gain due to a motor inside a conditioned space can be expressed as
Heat generated in conditioned spaces by electric, gas, and steam appliances such as a range, refrigerator, freezer, TV, dishwasher, clothes washer, drier, computers, printers, and copiers can be significant, and thus must be considered when determining the peak cooling load of a building. There is considerable uncertainty in the estimated heat gain from appliances owing to the variations in appliances and the varying usage schedules. The exhaust hoods in the kitchen complicate things further. Also, some office equipment such as printers and copiers consume considerable power in the standby mode. A 350-W laser printer, for example, may consume 175 W and a 600-W computer may consume 530 W when in standby mode.
The heat gain from office equipment in a typical office with computer terminals on most desks can be up to 47 W/m2. This value can be 10 times as large for computer rooms that house mainframe computers. When the equipment inventory of a building is known, the equipment heat gain can be determined more accurately using the data given in the ASHRAE Handbook of Fundamentals.
The presence of thermostatic controls and typical usage practices make it highly unlikely for all the appliances in a conditioned space to operate at full load. A more realistic approach is to take 50 percent of the total nameplate ratings of the appliances to represent the maximum use. Therefore, the peak heat gain from appliances is taken to be
regardless of the type of energy or fuel used. For cooling load estimate, about 34 percent of heat gain can be assumed to be latent heat, with the remaining 66 percent to be sensible in this case.
In hooded appliances, the air heated by convection and the moisture generated are removed by the hood. Therefore, the only heat gain from hooded appliances is radiation, which constitutes up to 32 percent of the energy consumed by the appliance (Fig. 30). Therefore, the design value of heat gain from hooded electric or steam appliances is simply half of this 32 percent.