Energy efficiency tested in special desert homes
Testing, testing …
One of the most interesting home-energy research projects I’ve encountered is the Borrego Springs Zero Energy Home study conducted in 2005-06 by the Building Industry Research Alliance (BIRA), a consortium funded by the Department of Energy’s Building America Program. Working with builder-partner Clarum Homes of Palo Alto, the BIRA team evaluated the comparative performance of three different wall systems in four high-efficiency prototype homes (www.clarumzeroenergy.com) in the desert community of Borrego Springs, Calif.
Testing homes is an integral part of the systems engineering approach used by Building America Research Teams to increase energy efficiency and cost-effectiveness before homes are ready for use in production or community-scale housing.
Unoccupied during testing, each of the four Borrego homes had an identical floor plan of 1,920 square feet, an identical roof system, and a modest 2.4 kilowatt solar photovoltaic (PV) system.
The advanced wall systems tested were: 1) 2-by-6 wood-framed walls with sprayed-in Icynene foam insulation and an additional layer of 1-inch extruded polystyrene; 2) 7-inch structural insulated panel (SIP) walls; and 3) 10-inch T-Mass insulating concrete walls (used in two homes).
All walls ranged from R-26 to R-28 and had identical windows with a U-factor of 0.35. The homes had many additional energy-efficient features specific to hot, dry climates including state-of-the-art cooling systems.
Heating things up
Of all the testing, the “super-cooling” test was probably the most revealing. In this summertime experiment, each home was precooled to 72 degrees by 11 a.m. and then allowed to heat up through the peak air-conditioning (A/C) period of the afternoon. With a maximum ambient temperature of 105 degrees and exterior wall temperatures reaching 120 degrees, the inside air temperature of the 2-by-6-frame home increased 14 degrees in 4.5 hours and triggered the 85-degree A/C setpoint; the SIP home rose 12 degrees. Meanwhile, the T-Mass home rose only 5 degrees!
The results must be interpreted carefully. They clearly show the benefit of thermal mass (concrete) in stabilizing indoor temperature; however, the concrete-wall home required much more energy than the other homes to precool those massive walls.
So what is the moral of this story? We can conclude that off-peak cooling used to “discharge” the internal thermal mass will result in significant cost savings if the electricity rates are higher during peak-use times (i.e. noon to 6 p.m.). The advantage is even greater if a home is located in a climate with large swings in temperature between day and night, so that some discharge occurs at no cost! With PV, this effect can be leveraged even further, because excess electricity generation in the afternoon may be sold back to the utility at the higher peak rate if “time-of-use” metering is in place.
Back to the study …
With energy-efficiency measures and PV in place, the test data were compared to a “virtual” benchmark home that satisfies California’s Title 24 standards. Over the course of a year, all the Borrego homes were pretty similar and reduced whole-house energy use by about 66 percent on average; the estimated energy savings was still about 50 percent on average without PV.
While not achieving zero-energy, this study showed the benefit of incorporating affordable, production-ready energy-efficient features. The T-Mass homes had a slight edge in energy savings but were the most expensive to build. The advanced 2-by-6-frame house was least expensive, and the SIP home cost was near the middle.
More to come
The Borrego study is an example of the type of testing that is needed to establish zero-energy home designs; however, testing needs to be done in each climate zone. Currently, there are some exciting developments that may make testing in our area a reality. Stay tuned!