This article was originally published in the May/June 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online May/June 1996
Cooling By the StarsAn innovative way to cool houses has recently been tried at two sites in California. It's called night evaporative underfloor cooling storage (NEUCS). A NEUCS system takes advantage of the slab floor's thermal mass, which absorbs heat from indoor air during hot summer days. Typically, the slab heats up during the day and then transfers some of the heat to the ground and most of it back to the indoor air during the cooler evening hours. The NEUCS system helps the slab floor remove more heat from the home's air space during the day by circulating water from an evaporative cooler through tubing beneath the slab during the night. The evaporative cooler works like a cooling tower to cool water by as much as 30oF below the outside air temperature. This water is circulated beneath the slab floor, cooling the slab to a lower temperature than it would reach through natural heat transfer without the system.
Davis Energy Group (DEG) developed a NEUCS system appropriate for most dry climates. The evaporative cooler can perform the dual purpose of providing cooled air to the home, while providing slab cooling in the nighttime hours. Operating the slab cooling at night is more effective than during the day, because outdoor wet bulb temperatures are lower at night.
In 1994, Pacific Gas and Electric Company (PG&E) sponsored a house using NEUCS in Rocklin, near Sacramento, California. This stand-alone system has no compressor-based air conditioner installed. The NEUCS system is able to maintain indoor temperatures at or below 78oF, with occasional assistance from the evaporative cooler itself and a fan coil, which circulates cool water under the floor to provide supplemental cooling on the hottest days.
Southern California Edison (SCE) sponsored design of a NEUCS by DEG in a new 2,350 ft2 house in Palm Desert, California. In this hot desert climate (with a humid monsoon season in August), they used an evaporative precooler for the condenser on the home's air conditioning system. Two 500-ft coils of polyethylene tubing were spaced 2 ft on center, 8 inches beneath the surface of the slab floor. The system is controlled to circulate cooled water between 5 pm and 7 am when the outdoor temperature is below 90oF. The slab circulation pump turns off when the air conditioner comes on, and does not come on at all if the daytime temperature stayed below 90oF.
For tubing, the researchers used 1 1/4 inch diameter Driscopipe Series 1000 polyethylene pipe, which cost 44/ft. It took two people three hours to lay out the tubing, plus an additional person-hour to install the manifolds. Other costs beyond the piping included the EvapCon evaporative precooler ($1,750), plumbing, and adding custom controls to the evaporative cooler to run the NEUCS system. Although these costs totalled $2,790, DEG calculated that the costs in a mature market would be closer to $1,500.
Researchers monitored the Palm Desert home in June of 1995 and found that the evaporative precooler lowered water temperature by an average of 14oF. Based on the data they collected, the slab inlet water temperature will be 5oF-6oF higher than wet bulb, when wet bulb temperatures are between 55oF and 60oF. The slab circulating pump and the condenser fan (operating at low speed) used about 0.35 kW with typical slab cooling rates of about 12,000 Btu/h.
DEG used a computer model to predict annual energy savings at the home. They also modeled NEUCS systems in four different Southern California climate zones, and tested various changes to the design. The simulations showed that water flow rate should have the largest effect on system performance, providing greater savings at higher flow rates. The percent of the slab covered by carpeting also should have an effect (with an exposed floor being more efficient). The cost of tubing appears to override the energy saving benefits of narrow tube spacing. Tube diameter (3/4 to 1 1/4 inches) showed little effect on performance, suggesting that lower-cost smaller tube diameters can be used. The simulation found little gain from burying the tubes deeper.
For the home in the arid Palm Desert climate, DEG predicted energy savings of 3,500 kWh per year and peak savings of 0.88 kW, resulting in a simple payback of 2 years. They predicted a payback of 3 years in Fresno, 4 years in Riverside, and 11 years in the coastal climate of El Toro. However, a NEUCS system could eliminate compressor-based air conditioning at the El Toro site, generating an immediate payback.
Although long-term minitoring data have not been collected to demonstrate the continued performance of the system, the greatest applicability of NEUCS seems to be in arid climates where many homes already use evaporative coolers.
DEG's development of the computer model was supported by SCE through the California Institute for Energy Efficiency. For more information, contact DEG at (916)753-1100.
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