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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 
Bring Peak Energy Savings


by Danny Parker and Ken Sheinkopf

Danny Parker is a principal research scientist at the Florida Solar Energy Center; Ken Sheinkopf is the center's associate director.


Big energy savings are now within reach with the building of what many have previously considered an idealistic fantasy: a hot-climate home with an energy appetite of less than 10% of typical loads. What's more, this home exerts little net demand on the grid during the utility's peak power times.

 
 
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.
Table 1. Results of Testing Florida Homes
Site Power use kWh Monthly Energy Cost PV Array Output AC kWh Percent PV Output of Total Loads
Test 837 $67 502 60%
Control 1840* $147 None 0%
PV#1 2970 $260 255 8.6%
PV#2  2440 $195 224 9.2%
*This is the power use of the air conditioning only.
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.

Million Solar Roofs Contacts

Million Solar Roofs Initiative
U.S. Department of Energy
1000 Independence Ave., SW
Washington, DC 20585
Tel:(202)586-8779
Web site: www.eren.doe.gov/millionroofs

Florida Solar Energy Center
1679 Clearlake Rd.
Cocoa, FL 32922
Tel:(407)638-1000
Web site: www.fsec.ucf.edu

National Database of State Incentives for Renewable Energy (DSIRE)
Interstate Renewable Energy Council
P.O. Box 1156
Latham, NY 12110-1156
Tel/Fax:(518)459-2601
Web site: www.eren.doe.gov/irec

National Renewable Energy Laboratory
1617 Cole Boulevard
Golden, CO 80401-3393
Tel:(303)275-3000
Web site: www.nrel.gov

To test the feasibility of building new single-family homes that cut air conditioning use to a minimum, researchers here at the Florida Solar Energy Center (FSEC) recently built a 2,425 ft2 home in central Florida using many proven energy-saving building strategies, along with a 4 kW photovoltaic (PV) array that could meet most of the home's daytime electrical demand. We also constructed a similar-sized control house without the energy-saving features.

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;

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  • R-10 exterior insulation over a concrete block wall system, giving the total wall system an R-value of 11;

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  • a 2-ton, 14.4-SEER heat pump with variable-speed air handler;

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  • 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;

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  • a low-friction loss and sealed duct system within the conditioned space;

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  • a programmable thermostat;

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  • a high-efficiency refrigerator; and

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  • high-efficiency compact fluorescent lighting.
The test home's rooftop PV system consists of 2.7 kW of modules facing south and 1.3 kW of modules facing west. The west-facing segment was designed to provide power late in the summer afternoons, when homes typically need it the most. The DC current of the PV system is converted to clean AC current by an inverter that feeds directly into the local utility grid. Lakeland Electric and Water, the local municipal electric utility, owns and operates the PV system.

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.

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  • 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.
Even during the hottest summer days, the occupied test home used 70% less energy for air conditioning than the unoccupied control house used. When PV power production is included, the test home had a net electric demand on the grid of near zero. Current data on the homes can be viewed at http://www.fsec.ucf.edu/~PV/data/lak/. Added Advantages The 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 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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