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After monitoring energy use in both houses for nearly a year, we determined that the energy-efficient home's total electrical consumption was over 70% less than that of the control house. The home's energy-efficient measures also eliminated the peak afternoon utility load posed by a traditional cooling system. This home is the first residential system in Florida to be a part of the Million Solar Roofs Initiative (MSRI), of which the State of Florida is a partner (see "Solar Power Rising to a Million Roofs in the Millenium," p. 24). The installed system described here is counted toward Florida's goal of 40,000 PV systems by 2010. Record EfficiencyOn June 18, 1998, the hottest daytime temperatures on record were reached in Lakeland, Florida, where both houses are located. During that 24-hour period, the energy-efficient home used only 28% of the air conditioning power needed by the control house to maintain an indoor temperature of 72°F. And when one includes the PV electric generation during the same peak period, the efficient home used 93% less utility energy than the control house used. Remarkably, the energy-efficient home was occupied during this time, while the control house was empty.What made the test home so efficient? A number of energy-saving building features brought about these results. These features were first simulated in our labs and then measured in field tests before being installed in the home. According to the simulation, the most valuable features in terms of energy savings were the spectrally selective low-e double-glazed windows with argon fill, and the white reflective-tile roof with R-30 ceiling insulation. The windows and roof accounted for 35% of the total savings realized by the test house. These features also made it possible to downsize the air conditioner considerably. The high-efficiency air conditioner--described below--produced an additional 29% savings. Other important features included:
The control house, located just a block away, was built like a typical central Florida home. It has a gray-brown asphalt shingle roof, R-30 ceiling insulation, R-4 wall insulation on the interior of the concrete block walls, single-glazed windows, standard appliances and incandescent lights, R-6 ducts in the attic, and a 4-ton, 10-SEER, 7-HSPF heat pump. We also monitored two other homes in the same neighborhood that were constructed by the same builder and were similar in size and features to the control house. Each of these homes has been retrofitted with a rooftop PV array; they were monitored to provide data on PV power and total household electric demand. Because both homes were occupied and neither had any special energy-saving features, we thought it would be interesting to compare their energy use with energy use in the control and test homes. The results for June 1998 are shown in Table 1. Dramatic SavingsBoth of the occupied homes used considerably more energy than the control house uses for air conditioning alone. This suggests that if the control house had been occupied, its total energy use would have been 30%60% greater than it was, making the difference between it and the test home even more dramatic.Several other facts emerged from this study:
Added AdvantagesThe purpose of this project was to use as many energy-saving features as possible and to test their performance, so economic factors were not considered. Several of the features, such as the PV system, were clearly not cost-effective, but several others were. These included the high-efficiency lighting, which had a 4-year payback; the high-efficiency air conditioner (7-year payback); the energy-efficient refrigerator (10-year payback); and the interior duct system (12-year payback). Altogether, the energy-efficient home cost $23,000--or about 17%--more to build than the control house cost.The most valuable features--the high-performance windows, the white tile roof, and the downsized high-efficiency air conditioner--together reduced space-cooling costs by an estimated 64%. However, energy-efficient features shouldn't be judged on economics alone; in addition to cutting energy use, these features often provide other benefits. The more expensive tile roof, for example, will also last much longer than a shingle roof, and the high-performance windows will keep the home's interior much quieter and less prone to uneven temperature swings within the rooms. That's not all. The white roof will tend to make the home's energy system more robust: Return duct leakage from a home that has a cool attic in a hot climate is much less catastrophic than return duct leakage from a home that has a hot attic caused by a dark roof. It is simple to specify these energy-efficient features and to get them installed correctly. The only real difficulty is obtaining the proper coil air flow for the air conditioning system. Contractors do not typically have the equipment necessary to perform this task, and current practice is almost always unsatisfactory. Also, the duct system must be properly sized to achieve proper flow without excessive fan power and noise. Duct slide rules are almost always used to size ducts, and FSEC investigators recommend that a friction-loss coefficient of 0.05 inches water column (IWC) be used with these slide rules, rather than the almost universally used 0.1 IWC. National BenefitsAccording to the U.S. Census Bureau, new construction in hot southern climates totals about 45% of new construction nationwide. Reducing the amount of energy used for air conditioning can thus have a significant effect on the nation's energy consumption. As we have shown, builders today can use current technology and practices to put up new homes that will perform optimally, save the occupants money, and help to meet the national pollution-reduction goals of the Million Solar Roofs Initiative. | Back to Contents Page |
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