Life Cycle Optimization of Residential Air Conditioner Replacement

Abstract

Utilizing more efficient central air conditioning equipment is one strategy to curb residential energy consumption. The average efficiency of air conditioning units sold tends to increase gradually over time, so replacing old units can reduce energy consumption during operation, but these energy savings must be weighed against the energy associated with the creation of a new unit and disposal of the existing unit. A model based on the lifecycle inventories for each model year of a typical 3 ton central air conditioning unit was developed to find replacement schedules that minimize the (1) energy consumption, (2) greenhouse gas emissions and (3) cost to the homeowner over a period from 1985 thru 2025 for 6 cities across the United States. Dynamic variables such as changes in raw material energy intensity, refrigerant market share, and cost of electricity over time were considered. Over the 41 year time horizon, energy minimization required 7 to 15 units, greenhouse gas (GHG) minimization required 3 to 5 replacements, while cost minimization required 3 units for the various cities examined. The cost of replacing according to an energy optimal schedule was between 11 to 57% more than the cost optimal schedule for the same city. Locations with the hottest climates required the most replacements. Financial incentives were introduced as negative costs into the model to align the optimal cost schedule with energy and GHG optimal schedules. These incentives were substantial. Often, they were in the range of $1000 to $2000 per replacement. The model demonstrates the benefits of optimal replacement under several scenarios including an increase in the federal efficiency standard in 2016, the adoption of a regional efficiency standard, and the replacement of existing central air conditioners with Energy Star units instead of units at the federal minimum efficiency standard. For example, when starting with model years older than 2005, optimal replacement with Energy Star units in Ann Arbor, MI offers the potential for 8% energy savings and 5% GHG savings versus replacing with units at the federal minimum efficiency standard from 2009-2025.