Space cooling is a major contributor to electric utility peak loads in the Southwest. In Arizona, electric utilities typically reach their summer peaks in late afternoon, when temperatures often exceed 100deg.F. Residential conservation measures focused on space cooling can reduce costs associated with these peaks, by allowing utilities to avoid burning fuel in their most expensive units and to defer additions to generation, transmission and distribution capacity.

We conducted a study of 149 houses in the Phoenix, Arizona, area to find out which conservation measures are most effective in reducing space cooling loads.^* We audited the houses, observed hourly whole-house electricity usage for one year, installed one or two space-cooling conservation measures in each study house, and observed electricity usage for another year. We then used regression analysis to study the change in electricity use from the first year to the second, taking into account weather, structures and occupant characteristics.

The conservation measures we installed were: higher-efficiency heat pumps or air conditioners (seasonal energy-efficiency ratio, or SEER about 10) that replaced old models (SEER 6-8), efficient evaporative-cooler motors, pre-coolers to evaporatively cool intake air of existing air conditioners, trees to shade sunstruck sides of the house, increased attic insulation, interior "storm" windows, flat roof reflective coating, and sunscreens. We found the largest savings (in increasing order) resulted from shade trees, attic insulation, more-efficient heat pumps or air conditioners, and pre-coolers.

We also found that power and energy savings could be increased via targeting the types of houses for retrofit. Two factors are important in targeting houses in the desert Southwest: the type of cooling system (an air conditioner or heat pump only, versus dual cooling systems with both evaporative cooling and either an air conditioner or heat pump), and the energy efficiency of the house prior to installing conservation measures.

Adding conservation measures to houses with dual cooling lets the occupants use the evaporative cooler more and the air conditioner or heat pump less while still remaining comfortable, thereby saving significant energy. Savings are greater in houses that are less energy-efficient prior to the addition of conservation measures.

Less-efficient houses can be distinguished by window characteristics. For houses with only an air conditioner or heat pump, the less-efficient ones tend to be those with single-pane windows or with a large south-facing window area. For dual-cooled houses, the less-efficient ones tend to be those with greater window area. In addition, savings from conservation measures to dual cooled houses diminish as the amount of west-facing glass increases.

The figure illustrates the larger savings that can be achieved by targeting houses for retrofits, as compared to installing conservation measures in the average house in the study. An average house has average structural and occupant characteristics. The targeted house is the same as the average house except for the following: the targeted house with an air conditioner or heat pump has single-pane windows and more of its south-facing wall in windows than the average house; and the targeted house with dual cooling has more of its wall area in windows and less of its west-facing wall in windows than the average house.

The savings shown in the figure for shade trees and attic insulation are weighted averages of the savings for houses with dual cooling and houses with air conditioning only, where the weights represent the fraction of the sample with a given cooling type. The savings for pre-coolers and more-efficient heat pumps or air conditioners are for houses with only an air conditioner or heat pump.

Savings can be increased by 0.12-0.20 kW and by 267-348 kWh per summer by targeting houses. Thus, targeting residential retrofit programs, in this case by window characteristics and cooling type, will enable energy program managers to achieve greater savings.

-- Kim Clark and David Berry

Kim Clark and David Berry are economists with the Arizona Corporation Commission's Utilities Division in Phoenix.

Note

* See "Targeting Residential Energy Conservation Measures in the Desert Southwest," by Kim Clark and David Berry, presented at the 1994 Western Regional Science Association Meetings in Tucson, Arizona. For more information, contact the authors at (602)542-5517 or write to: Arizona Corporation Commission, 1200 W. Washington, Phoenix, AZ 85007.

* weighted average dual cooled & air conditioned houses ** air conditioned houses

Figure 1. Weather-normalized power and energy savings per house.