This article was originally published in the May/June 1997 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 1997

Advanced Two-Stage Design Hits the Market

Until recently, most residential two-stage evaporative coolers were of the down-discharge variety and were generally installed on the roof. Although a rooftop cooler offers certain advantages, including a more central location, installing and maintaining these coolers can be problematic. Some developers also consider the bulky rooftop cooler cabinets unsightly, and have been reluctant to use them.

To help minimize the problems associated with two-stage coolers, the Davis Energy Group (DEG), with funding from the California Energy Commission's Energy Technologies Advancement Program (ETAP), embarked on a program in 1992 to develop an advanced two-stage cooler. In March 1995, DEG completed the project and unveiled its production prototype two-stage cooler.

DEG's advanced two-stage cooler unit improves on prior designs by relying on a compact vertical configuration for ground-, wall-, or attic-mounted residential applications (see figure below). Other unique design features include

·  A single blower upstream of the cooling media to reduce cost, size, and noise.
· Through-the-wall delivery, eliminating duct losses.
· A variable-speed blower motor to increase energy efficiency and to provide quiet, low-speed operation.
· A shared pump and reservoir for both stages.
· An automatic reservoir that purges and drains completely during off cycles to prevent biological growth and odors.
· Reduced maintenance through the use of high-quality components and an improved design. In developing its advanced two-stage cooler, DEG first performed detailed simulations. The simulations indicated that the advanced unit could provide cooling sufficient for a typical 1,700 ft2 new house or a 1,400 ft2 existing house at 70°F peak wet-bulb temperatures.

Following the simulations, six units were field-tested in Northern and Southern California, beginning in the late summer of 1994. The field-test houses ranged in size from 1,000 to 1,600 ft2 and had existing ducted central air conditioning that could be used in lieu of the two-stage cooler. Monitoring results showed the advanced units had average operating efficiencies of 50 Energy Efficiency Ratio (EER), or approximately six times higher than that of conventional cooling systems.

In June 1995, DEG continued performance monitoring on two units that had been installed in homes located near Sacramento. The primary objective of this monitoring was to obtain performance data under hot summer conditions. Other objectives included evaluating indoor air circulation and assessing homeowner satisfaction. Neither home used its existing air conditioner during the study, except when a motor failure put one of the coolers out of commission. This allowed for the collection of conventional cooling data that would otherwise not have been available.

Monitoring results for the two-stage cooler that operated throughout the study showed EERs of 34-38. Average daily energy use of the conventional air conditioner was over five times higher than that of the advanced cooler. Peak condensing unit demand for the air conditioner exceeded 3kW, while peak demand for the advanced cooler was less than 0.7 kW.

A survey conducted among the original six residences revealed generally favorable impressions of the advanced two-stage cooler. Among the more valued features were energy savings, environmental benefits, and improved cooling comfort. Drawbacks cited included unknown maintenance requirements and uncertain equipment life.

Davis Energy Group has licensed the design of the advanced unit to I.D.A.C. Technologies. Production is currendy under way on the first 30 units and installation is expected to begin early this summer. The estimated cost to contractors is $2,250, with the possibility of utility rebates in California. For more information contact I.DA.C. Marketing: HSC, 1051 Folger St., Berkeley, California 94710. Phone: (510) 548-1377; Fax: ( 510 ) 548-7962.

--Leo Rainer
Leo Rainer is a senior engineer with Davis Energy Group in Davis, California.


Figure 1. Diagram of the components of Davis Energy Group's advanced two-stage evaporative cooler. Note that a single blower and water reservoir serve both indi-rect and direct stages.

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