In Idaho we use the most electrical power in the summer relying on our hydropower resources just when water is typically at its scarcest. A big portion of the peak summer load is due to the mechanical cooling required to keep our homes and businesses at a comfortable temperature. Typical practice in the architectural and engineering design community has been to rely on compression cycle chillers otherwise known as air conditioners. Though this type of system has been easy to implement in any climate with a variety of modifications, it is energy intensive. Now that designers and owners are expecting higher performance systems which will yield cost savings in lower energy bills we look to the past and climate as a resource to achieve better performance.

In the past evaporative cooling was commonly used as a means to condition space. Evaporative cooling takes advantage of the adiabatic cooling process as depicted in the chart on figure 1. This process uses Idaho’s dry climate as a resource to take air that could potentially be 100 degrees and 15% relative humidity with a wet bulb depression of more than 35 degrees and cool the air down to approximately 70 degrees. In building with high internal heat gain loads this technique would only satisfy part of the total load to be removed, which is why a combination system of Direct Evaporative Cooling (DEC) and Cooling Coils (CC) can be used to increase efficiency while maintaining comfort levels.

In the article written by Rick Phillips P.E. for ASHRAE he better defines which combination and arrangement of components will achieve the highest efficiencies. The use of these hybrid cooling systems has many considerations in regards to the way the components are arranged within the air handling units. In this article several arrangements were test with a Typical Metrological Year weather file from Denver Colorado. Each arrangement was tested against a typical non-evaporative cooling system. The components modulated where the supply fan, the chilled coils, and the evaporative membrane. Figure 2 below shows the resultant energy reduction from the various arrangements. Arrangement 1 and 2 both had the supply fan downstream from the CC and DEC components and 3 and 4 put the supply fan in an upstream position. 1 and 3 both place the CC upstream from the DEC and just the opposite in 2 and 4. For a supply air temperature of 55 degrees arrangement 3 showed the highest drop in energy consumption, 67% saving compared to the baseline. It is important to note that these results are indicative of the climate in Denver and for design of similar systems in other locations would require modeling to verify the best arrangement.

SOURCES

Journal Article:
• Phillips, Rick. “Using Direct Evaporative + Chilled Water Cooling”, ASHRAE Journal, V15, N07, (July 2009): 16-19

Publications:
• Kwok, Alison G. and Grondzik, Walter T. “The Green Studio Handbook, Environmental Strategies for Schematic Design”, Architectural Press, London UK, 2007. Pg. 153