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Natural gas-driven absorption heat pumps (AHPs) are under renewed scrutiny as a viable technology for space conditioning and water heating for residential and commercial applications because of natural gas production trends, pricing, and the speculation that it might be a “bridge fuel” in the global transition toward energy sustainability. Since any level of natural gas combustion contributes to atmospheric carbon dioxide accumulation, the merits of natural gas–consuming absorption technology are reexamined in this paper from the point of view of expected efficiency as a driver for AHPs throughout the United States using a time-weighted bin temperature analysis. Such analyses are necessary because equipment standards for rated performance are restricted to one set ambient condition; whereas in actual practice, the AHP must perform over a considerably wider range of external conditions in which its efficiency may be vastly different from that at the rated condition. Quantification of variations in efficiency and system performance is imperative to address how to provide the desired application with the least environmental impact. In this paper, we examine limiting features in AHPs and relate them to systemic performances in 16 cities across all 8 climate zones in the United States, each containing 15 bin temperatures. The results indicate that the true expectation for AHP performance is significantly less than what might be optimized for the rated condition. Statistical measures of the variation in water heating COPs show that for most cities, the COP at the rated conditions is outside the 95% Confidence Interval. It is concluded that deployment of AHP water heaters may be restricted geographically by outdoor temperature constraints.

This rebuttal in the form of a letter to the Editor is prepared by the corresponding author of the paper, “Variability of absorption heat pump efficiency for domestic water heating and space heating based on time-weighted bin analysis, Applied Thermal Engineering, Volume 130, 5 February 2018, 515–527. https://orproxy.lib.utk.edu:2113/10.1016/j.applthermaleng.2017.10.142” in response to the “concerns“ sent to the Editor by Dr. Chris Keinath and Mr. Michael Garrabrant of Stone Mountain Technologies, Inc., (SMTI) Johnson City, TN.

Evidence is provided to support the view that 68%–76% of the energy required to produce domestic hot water may be extracted from the ground which serves as a renewable energy resource. The case refers to a 345 m 2 research house located in Oak Ridge, Tennessee, 36.01°N 84.26°W in a mixed-humid climate with HDD of 2218 °C-days (3993 °F-days) and CDD of 723 °C-days (1301 °F-days). The house is operated under simulated occupancy conditions in which the hot water use protocol is based on the Building America Research Benchmark Definition which captures the water consumption lifestyles of the average family in the United States. The 5.3 kW (1.5-ton) water-to-water ground source heat pump (WW-GSHP) shared the same vertical bore with a separate 7.56 KW water-to-air ground source heat pump for space conditioning the same house. Energy and exergy analysis of data collected continuously over a twelve month period provide performance metrics and sources of inherent systemic inefficiencies. Data and analyses are vital to better understand how WW-GSHPs may be further improved to enable the ground to be used as a practical renewable energy resource.