Field-tested cooling performance of gas-engine-driven heat pumps

ASHRAE Transactions, July, 2008 by Chang W. Sohn, Dudley J. Sondeno, Franklin H. Holcomb, James M. Stephens

[FIGURE 6 OMITTED]

The comparison between the laboratory testing and the field testing shows a qualitative agreement in the dependency of COP on the outdoor temperature. In quantitative terms, the comparison showed field overall COP was generally lower than those measured in the laboratory. For the six sites with average outdoor temperatures between 78.2 and 95.4, the measured overall cooling COP ranged 1.06-1.39 while the laboratory measurements ranged 1.3-1.4. It should be noted, however, that a direct comparison of numbers does not provide a clear picture. The laboratory data were generated on a fixed outdoor temperature for each data point reported. Meanwhile, the field measured outdoor temperature reported in Table 5 was based on an average value during the whole period of field testing.

It is reasonably expected that the field overall COP (reported for the 3-month monitoring period) would be lower than those measured at the laboratory (reported at a fixed operating point). For example, the average outdoor temperature represented in the data from Luke AFB (outskirts of Phoenix, AZ) was 95.4 [degrees]F. The typical daytime temperature at Phoenix was above 100 [degrees]F and frequently up to 120 [degrees]F during the monitoring period. A lower COP under such high outdoor temperature was not fully compensated for by the relatively higher COP during the nighttime operation; thereby resulting in lower overall COP for the 3-month period compared to the laboratory measured COP based on a fixed operating condition.

Energy Efficiency of GHP Cooling

For an average temperature of 90.3 [degrees]F for the six sites, the total energy consumed by the six GHP units was 292.9 million BTU and the total cooling effect delivered was 359.4 million BTU. Therefore, the overall COP of the six GHP units during the 3-month period was 1.23:

COP (field) = 1.23

The COP of 1.23 shows one unit of energy in natural gas delivered 1.23 units of cooling. In comparison, the COP of electrical heat pump of 1.09 shows 1.09 units of cooling is delivered at the expense of one unit of energy in natural gas. This field measurement shows GHP heat pumps delivered 13% more cooling than the electrical heat pump using the same amount of primary energy. The favorable primary energy efficiency of GHP heat pump may be attributed to the absence of the 9% transmission and distribution loss (EIA 2006) and variable speed operation of compressor following the load variation. Thermodynamically speaking, the vapor compression refrigeration cycle employed in the GHP unit is the same as the one used in the electric heat pump. The capability of varying energy input, i.e., lower engine speed for reduced cooling load, however, would improve COP compared to a fixed energy input for a compressor at a fixed speed. In this macro-scale analysis, the effect of variable speed operation of compressor in the GHP unit was not clearly distinguished from the other factors. A detailed analysis of dynamic COP as a function of operating conditions (e.g., load, condensing temperature, engine speed, etc.) is currently under study. Note, however, that the primary benefit of the GHP heat pump for space cooling is reduction in peak electrical demand as well as the modest savings in primary energy.