This article was originally published in the July/August 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online July/August 1999
Cool Home Features
|The house has a 2-ton, Trane 14.4-SEER heat pump with a variable speed indoor unit. An important part of the high efficiency achieved by the cooling system is a sealed, low-friction-loss duct system inside the conditioned space. The air flow was verified with a flow hood at the time the cooling system was set up.|
|Research engineers John Sherwin and Mike Anello complete instrumentation for the house's meteorological station. When the station measured record high temperatures, the house used only 28% as much air conditioning power as the control house used to keep it a cool 72°F inside.|
|The data acquisition system for the project includes a Campbell CR-10 data logger with power transducers that measure the energy use of each major household appliance (down to the clothes washer). These are installed in both the test house and the control house. Data are collected every 15 minutes, and are transmitted to FSEC nightly over a dedicated phone line.|
|The control home has a net power demand of approximately 4000 watts during peak hours, which is typical of Florida homes. In contrast, the PV house sent electricity back to the utility during parts of this day, including the crucial peak period: During those hours, it produced an average of 1044 watts more than it needed for air conditioning to 74°F inside. Air conditioning was the only load for the demand profiles shown for both houses on this day, since both houses were unoccupied at the time.|
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 Efficiency On 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:
- a solar thermal domestic hot water system with a 40 ft2 solar collector rated at 45,600 BTU/day, run on an electric pump powered by a 10-W PV panel;
- R-10 exterior insulation over a concrete block wall system, giving the total wall system an R-value of 11;
- a 2-ton, 14.4-SEER heat pump with variable-speed air handler;
- a cooling coil air flow that was field-tested using a flow hood to verify that the air flow met manufacturer specifications for the configuration;
- a low-friction loss and sealed duct system within the conditioned space;
- a programmable thermostat;
- a high-efficiency refrigerator; and
- high-efficiency compact fluorescent lighting.
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 Savings Both 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:
- During several hot weeks in May 1998, when both houses were unoccupied, space-cooling energy use was 84% lower in the test home.
- PV power produced from April 22 to May 17 averaged 17.1 kWh per day in the test home. In the control house, this amount of power would have provided less than half the energy used by the cooling system alone. In the test home, however, the PV system produced three times more energy than the cooling system used.
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 Benefits According 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.
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