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

INTRODUCTION

Sustainable utilization of limited energy resources dictates higher energy efficiency in space heating and cooling applications. A GHP system offers an energy efficient alternative to cooling and heating using either a conventional electric air conditioner/gas furnace combination or an all electric heat pump. In a GHP, the prime mover used to generate power, i.e., the gas engine, is employed to drive a compressor for the heat pump system. GHP is a distributed energy system that offers an excellent energy conservation opportunity by on-site utilization of the waste heat for space heating application. Distributed generation with micro-grid system is also an emerging trend for the alleviation of over-capacity transmission lines as well as utilization of alternative energy sources. Utilization of natural gas for space cooling applications can also help the electrical transmission capacity problems and provide conservation of energy resources.

Most air conditioners and heat pumps in the U.S. are powered by electricity. An alternative source of power to operate the space cooling equipment is desirable to reduce the summertime peak demand experienced by electric utilities and to provide savings in space cooling costs for the consumers. Heat pump systems powered by a small natural gas fired engine have been successfully commercialized in Japan (Takahashi 2006). Takahashi states that in Japan, "... The installed capacity of absorption-type and heat pump-type gas air-conditioning systems was 11.1 million refrigerant ton (RT) in FY 2004, a 5% increase over the previous year, and amounted to a 22.3% share of the entire air-conditioning capacity nationwide excluding residential use. Absorption-type cooling and heating systems came into widespread use in large buildings, and gas engine heat pump-type systems are popular in medium and small buildings." Similar GHP systems are in the process of being introduced in the U.S. market to provide energy efficient space heating and cooling as well as utility cost savings for the consumers.

For a typical military installation, which has thermal loads similar to a typical small city, space cooling is the major contributor to electrical energy consumption and peak electrical demand. For example, a detailed study of end use of electricity at Fort Hood, TX showed that cooling was responsible for 54% of the total peak demand of electricity and 33% of the total electricity consumed on post (Akbari and Konopacki 1995). As a means of reducing the electrical energy requirement for space cooling operations, natural gas powered engine driven chillers were tested in the military facilities during the 1990s. A review of the natural gas cooling systems and natural gas/electric hybrid cooling systems was documented in a report (Sohn and Alvarado 2002).

In 2007, six 10-ton GHP units were installed for the purpose of field testing for performance verification for a full year (from May 2007 to April 2008). The units were installed in six military installations in the southwest United States. Note that the high outdoor temperatures experienced in the southwest present a challenging operating condition for any air-cooled air conditioning system. The objective of the field testing was to collect field operation experience for evaluation of energy conservation and cost savings potential. For the industry, experiences with the prototype operating in this challenging environment would provide valuable lessons for technology development and transfer to the Department of Defense (DOD) and the commercial market. The six demo units were installed during the period of April 17-27, 2007. Collection of cooling performance data from these units began in May 2007. Over 40 data points from each unit were collected at 1-minute intervals. Zaltash et. al. (2007) give a detailed discussion of performance instrumentation. Limited amount of data for the heating performance at the end of the 2007 heating season were collected. Field heating performance, however, for a full heating season is yet to be generated, and will be the subject of a future report.

PRIMARY ENERGY EFFICIENCY CONSIDERATION FOR SPACE COOLING APPLICATION

A diagram of energy flow from the primary energy source to the electricity delivered to the consumer at the end use level showed 32% efficiency including power generation, transmission and distribution (EIA 2006). A breakdown of the EIA's diagram shows that the "Energy Consumed to Generate Electricity" in 2006 was 41,27 Quadrillion BTU, the "Net Generation of Electricity" was 13.83 Quadrillion BTU and the "End Use" after 9% T&D losses was 13.03 Quadrillion BTU; yielding the end use level efficiency of 13.03/41.27 = 0.3157. Therefore, 1 unit of primary energy is converted into 0.32 unit of electricity ready to operate an electric cooling device.

The minimum cooling energy efficiency ratio (EER) required of a unitary heat pump in the range of 5.42 to 11.25 ton capacity is 11.6 EER for the southwest region of the United States (ASHRAE 2004). In terms of cooling coefficient of performance (COP) with a conversion factor of 3.413 between the COP and the energy efficiency ratio (EER),