Room Air Conditioner Consumption

Room air conditioners tend to be operated not by central thermostatic control, but rather in a manual mode where the room occupant turns the air conditioner on and off depending on room temperature and occupancy. These complex operating patterns are difficult to model with thermal simulation models such as DOE-2. For this reason, we chose to use a simpler method for estimating room air conditioner energy consumption, based on the AHAM (Association of Home Appliance Manufacturers) test procedure. This method is summarized in Equation 1.

                                                             Equation 1

Where

            UEC            =    Unit energy consumption (kWh/year)

days             =     Average annual days of room air conditioner operation (days/year)

hours/day     =    Average daily hours of room air conditioner operation (hours/day)

            capacity    =            Rated capacity of the room air conditioner (Btu/hour)

            EER                         =            Energy-efficiency ratio (Btu/kWh)

Because cooling loads and usage vary with climate, we estimate a default days and hours/day value for each of the cities for which we had weather data (Appendix C-1). We estimate the default daily operating hours using equation 2. These values are rounded to the nearest integer. Climate data used in this equation are drawn from the typical meteorological year (TMY2) weather tapes (Marion and Urban 1995). The first term in equation 2 accounts for the severity of the climate, in terms of dry bulb temperature, while the second term accounts for how humid the climate is. Note that the humidity term is assumed to equal zero for locations above 40ºN latitude. The parameters in equation 2 were estimated heuristically so as to yield results that looked reasonable across a range of climates.

                                                        Equation 2

Where           

tempdb            =            Drybulb temperature at cooling design-day conditions (ºF)

                        tempwb=             Wetbulb temperature at cooling design-day conditions (ºF)

We then derive a value for annual RAC compressor hours from the AHAM test procedure manual (AHAM 1982). We used the value corresponding to 66% of full-load, to account for some cycling that occurs in normal room air conditioner operation (These values are shown in Appendix C-1. Where one of our weather cities was not listed in the AHAM document, the Cooling Load Hour value shown in Appendix C-1 is extrapolated from the geographically closest city, using the TMY2 cooling-degree hours at 74º F as a scaling factor.

Finally, the average days per year of operation is simply the ratio of annual compressor hours to the average daily hours of operation.

Room air conditioner capacity is either input by the user or a national-average typical value is used (12,000 Btu/hour). EER is also either user-entered or drawn from the shipment-weighted average for the year in which the air conditioner was sold (as specified by the user).