Component sizing key for upscaling high-temperature heat pumps

A European research team has investigated the upscaling effects of high-temperature heat pumps (HTHP) utilizing an R1233zd(E) refrigerant and has found that their size is a key factor for the coefficient of performance (COP).

The group compared, in particular, the performance of two laboratory-scale HTHPs of different capacities to make valid general statements about the challenges of upscaling. “It should be noted that the study focused on reciprocating compressors. The upscaling behavior of other compressors remains to be investigated,” corresponding author Jaromir Jeßberger told pv magazine. 

“Studies commonly focus on a single experimental facility and validate the measured experimental results using simulation models or literature data,” the researchers said. “Nevertheless, given the considerable disparity between these experimental setups and industrial-scale operations, an important research question arises regarding the feasibility of directly upscaling the laboratory results to a larger thermal capacity.”

The research group compared two HTHPs designed in previous literature, one with a thermal capacity of 11 kW (HTHP A) and the other with a thermal capacity of 35 kW (HTHP B). They analyzed both for heat transfer characteristics, heat lost in different components, compressor coefficient, and coefficient of performance (COP).

“The heat pumps under consideration exhibit a high degree of component similarity, such as a reciprocating compressor from Bitzer, plate heat exchangers, and oil separator, which enables a thermodynamic comparison between them,” said the academics. “They both have heat pump cycle with piston compressor and internal heat exchanger (IHX).”

Through their analysis, the scientists found that more extensive setups promise higher efficiency. “The pinch points and temperature glides can be influenced by the relative component sizing; the scaling effects like 15 % higher heat losses in the smaller plant are probably due to a larger ratio of the surface for heat loss to the heating capacity,” they emphasized.

Also, due to a different power draw to friction ratio, the larger HTHP B shows better volumetric efficiency of 0-3.5% higher, as well as better isentropic efficiency of 6%-8.7%. “On the system level, the smaller setups show in most of the operation points a better COP, resulting from the design effects. Considering the COP as a function of the evaporation temperature, the efficiencies are on a similar trendline,” they added.

Finally, to further find a general statement about the upscaling, the scientific group also analyzed the behavior of over 200 data points of industrial HTHP. They also included an analysis of the Carnot-COP of the systems, which is the maximum theoretical efficiency of a heat pump.

“Using these data points, a reasonable assumption for low-fidelity models, for the integration of HTHPs into energy systems or industrial processes, would be to use a COP that is 45% of the Carnot-COP for a set of operating conditions,” they explained.

Their work was presented in “Experimental investigations of upscaling effects of high-temperature heat pumps with R1233zd(E),” published in the International Journal of Refrigeration. The team included scientists from Germany’s University of Bayreuth and Switzerland’s Eastern Switzerland University of Applied Sciences.

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