East Meets West: Gas-Fired Heat Pumps


A Bit of History

As the 1990s unfold, look for a new kind of energy-efficient heating and cooling appliance-a heat pump powered by an internal combustion engine that runs on natural gas. Thanks to efforts on both sides of the Pacific, this new generation of heat pump may become even more affordable and prevalent than its all-electric predecessor.

In 1981, Japan's Ministry of International Trade and Industry (MITI) launched a small-scale gas cooling technology research association, pairing gas engine manufacturers with HVAC equipment manufacturers. As a result, gas engine-driven heat pumps have been on the market in Japan since 1987. And since the late 1970s, the Gas Research Institute (GRI), the U.S. Department of Energy (DOE), and others funded American research and development of this new technology. Engine-driven, absorption (ammonia and water refrigeration cycle), adsorption (solid/vapor refrigeration cycle, with carbon beds absorbing and desorbing refrigerant), and chemisorption gas heat pumps are among technologies under development.

How Gas Heat Pumps Work

Gas engine-driven heat pumps are basically no different than other kinds of heat pumps-they transfer heat from a "source" to a "sink." Most heat pumps do this using a system of heat-bearing refrigerant- filled piping, compressors, and heat exchangers. Normally, the living space serves as a heat source in the cooling season, and a sink in the heating season. (See "," HE, Mar/Apr '89, p. 10; "Ground-Source Heat Pumps," HE, Nov/Dec '90, p. 32; and "Heat Pump Study," HE, Mar/Apr '91, p. 29.)

However gas engine-driven heat pumps differ from other kinds of heat pumps in two fundamental ways. Instead of an electric motor, a natural gas engine powers the heat pump compressor. And in the winter, heat recovered from the engine exhaust (as well as a small optional gas boiler) provides supplemental heating to the indoor space and can even be used to defrost icy outdoor coils. Electricity powers the fans and control circuits.

The gas engine-driven heat pumps discussed in this article are split systems with an outdoor unit housing the engine, compressor, and outside heat exchangers, and an indoor unit housing the air handler and inside heat exchangers. Gas heat pumps use either a two-pipe or four-pipe strategy, referring to the number of pipes running between the indoor and outdoor units (see Fig. 1). Both strategies have a refrigerant loop, like that of an electric heat pump, moving heat from source to sink. The four-pipe system contains an extra route for heat to move, in this case, from the engine itself, the source, to a heat exchanger. The indoor heat exchanger is the sink in winter, and during cooling season, the sink is the outdoor heat exchanger.

The unique qualities of the gas engine create an opportunity for adding the extra pipe system. Since the gas heat pump engine is nothing more than a hybrid auto engine, it needs a radiator for cooling. In winter, the inside unit heat exchanger acts as the radiator. The outside unit heat exchanger acts as the radiator in the summer, merely exhausting heat. The two-pipe system exhausts engine heat by transferring it to the refrigerant loop, which carries it to heat exchangers in the indoor and outdoor units. The four-pipe system has an additional water/glycol coolant loop, carrying recovered engine heat to heat exchangers in the indoor and outdoor units (see Fig. 1).

By recovering engine heat, the system heating output can increase by as much as 25%.(1)(2). This means that even at lower outdoor temperatures, heating capacity is higher than with all-electric heat pumps. Consequently the balance point for gas heat pumps is 10-15 degrees F lower than that of conventional electric heat pumps. (The balance point is the temperature above which the heat pump can provide enough heat for the home without the use of supplemental heating.)

Most gas engine heat pumps feature computer chip-operated, variable-speed engines and fans that match the equipment capacity to the actual building load. With variable speed capabilities, the equipment cycles on and off less frequently, reducing engine wear and tear and extending equipment life. Another benefit of variable speed control is that in the cooling mode, when the engines and indoor blowers run at lower speeds, latent cooling capacity increases, reducing indoor humidity levels more effectively.

Gas heat pump engines are designed to have a lifespan of at least 40,000 hours of operation. (In comparison, automotive engines are designed for a lifespan of 2-3,000 hours-70,000-105,00 miles at 35 mph-and lawn and garden engines 500-750 hours.) For a typical residential application in the United States with up to 4,000 hours of operating time per year, this means an engine life of 10-15 years. As the gas heat pump contains an internal combustion engine, it requires annual maintenance (changing the oil, oil filter, air filter, and spark plug) in addition to typical space conditioning equipment maintenance.

From Japan, A Track Record

Gas heat pumps have been sold in Japan since 1987. As part of the development process, Tokyo Gas conducted field tests of five gas engine-driven heat pumps (1-2 tons cooling capacity, a typical residential size) produced by five MITI-sponsored partnerships: Sanyo and Kubota, Matsashita (Panasonic) and Kubota, Mitsibushi and Yamaha, Daikin and Yanmar, and Toshiba and Ishikawagima- Shibaura (see Table 1). All the units tested were the two-pipe version. Engines and compressors were automotive or diesel engines and automotive or general-purpose compressors. All units use R-22 refrigerant. Indoor units are the same as those used for electric heat pump applications. The units also heated water for domestic use, stored in auxiliary tanks.

Tokyo Gas installed the units in 15 residential and commercial buildings and evaluated them for up to 18 months between 1983 and 1985. Tokyo Gas monitored:

gas and electric consumption,
operating hours (cycling on/off times, run hours, and defrost time),
engine speed,
indoor and outdoor temperature and humidity,
water temperatures (city water, storage water, and hot water supply), and
refrigerant temperatures.

At the end of the tests, researchers could determine performance characteristics and pinpoint equipment reliability problems which, in turn, indicated the design modifications needed.

All of the units met cooling and water heating loads during the summer, water heating loads in spring and fall, and space heating loads in the winter. Based on the field test data, researchers determined both steady-state and seasonal Coefficients of Performance (COPs) for each of the five units. (The COP, a dimensionless number, is the ratio of total heating or cooling capacity in Btu/hr, to total energy consumption in Btu/hr, under designated operating conditions.) Steady state COPs for heating at 45 degrees F were 1.20-1.40, and for cooling at 95 degrees F, 0.74-.99 (see Table 1).

Researchers compared estimated operating costs (cooling, heating, and water heating) of the Mashista and Daikin gas heat pumps to two conventional electric heat pump scenarios, one with an electric water heater and the other with a gas water heater (3). In both instances, the gas heat pump operating costs were lower. Operating costs for the electric heat pumps were, with a gas water heater, 16-35% higher than that for the gas heat pumps, and with an electric water heater, 29-56% higher.

Several reliability and design problems emerged including noise, vibration-induced piping damage, leaks, control signal errors, starter unreliability, refrigerant pressure problems, engine and valve corrosion, and inadequate oil tank shielding from rain which caused engine burnout. Manufacturers attempted to correct these faults in later models. However, gas heat pumps by design are more likely to leak refrigerant at the shaft seal, which electric heat pumps avoid by keeping both motor and compressor within one case.

In a second study, after gas engine-driven heat pumps had been in the marketplace for four years with 14,000 installed, Tokyo Gas tracked the incidence of problems from April 1990 to March 1991. This study refined the problems into five main areas:

engine-related (39%),
refrigeration cycle-related (14%),
control-related (23%),
unit main body-related (6%), and
other problems (17%).

Tokyo Gas also found that the newer the model, the less frequent these problems appeared, indicating that design was improving. The percentage of units with any kind of trouble at all decreased from 100% for the 1987 model year to 19% for the 1990 model year (4).

U.S. Field Tests

Gas Research Institute has just completed field tests of the York gas engine-driven heat pump. Developed by Battelle Laboratories and GRI, the York unit is a 3-ton capacity, split system gas engine- driven heat pump with a variable speed engine and indoor fan and a two-speed outdoor fan using R-22 refrigerant. Engine heat and an optional natural gas boiler provide supplemental heating and can defrost outside coils (see Table 2).

York dealers trained by Battelle Labs installed gas heat pumps in residences in ten U.S. cities representing a wide variety of climatic conditions (see Table 3 ). Researchers recorded gas and electricity consumption, indoor and outdoor temperatures, and heat pump operating parameters. Researchers evaluated performance, defrosting and cycling, effects of extreme weather conditions, effectiveness of the variable speed control system, and reliability.

"The Honeywell-designed variable speed control system worked well and maintained temperatures to within one-half degree of setpoints," according to GRI's Gary Nowakowski. "Due to speed modulation and accurate compressor sizing, the gas heat pump heated or cooled the houses quickly and then modulated down to lower speeds to maintain temperature and comfort levels." On average, the gas heat pumps cycled once an hour, providing more consistent comfort levels. (Conventional gas furnaces average six cycles per hour. Single- speed electric heat pumps and air conditioners average three cycles per hour.) Heating supply air temperatures for the York test units were 100 degrees F-115 degrees F. By comparison, conventional electric heat pump supply air temperatures are 10 degrees F-15 degrees F lower, and gas furnaces higher at 120 degrees F-130 degrees F). In most summer months during the field test, relative humidity was maintained at close to 50% in all the houses.

Measured heating seasonal COPs were 1.2-1.5 and cooling seasonal COPs 0.9-1.2 (see Table 4). (Seasonal COPs are calculated over a specified heating and cooling season, and are the ratio of total delivered heating or cooling capacity to total equipment energy input over that time period.) Based on these field tests, GRI estimates energy cost savings of 20%-80% over comparable conventional electric heat pumps and furnace/air conditioners (see Table 4).

According to GRI, the York gas engine heat pump has an equivalent Heating Seasonal Performance Factor (HSPF) of 14.0 and an equivalent Seasonal Energy Efficiency Ratio (SEER) of over 15.0. For comparison, the Trane high-efficiency, variable-speed electric heat pump has an HSPF of 8.40 and a SEER of 14.6-4.8. (HSPF is the total heating output provided by a heat pump during its normal heating season, divided by the total energy consumed by the system. Likewise, SEER is the total cooling output of a heat pump during its normal annual cooling season divided by the total energy input during the same time period (5).)

Beyond Testing-Marketing

In Japan, under the influence of governmental policies geared to improve energy efficiency, gas cooling has been installed in 80% of all new buildings, a small portion of them residential. Equipment first-cost is less important for building owners who otherwise must pay for any additional electric generating capacity they cause. This policy boosts the popularity of gas engine heat pumps, which are produced by Japanese HVAC manufacturers and sold and serviced by the gas utilities.

The majority of Japanese gas heat pumps sold today are split systems or multisplit systems (multiple indoor exchangers connected to a single outdoor heat exchanger) (see Fig. 2). Most are 2-pipe systems (with engine heat recovery on the refrigerant loop) rather than 4- pipe systems, and feature variable-speed controls for the engine with either a single-speed or variable-speed indoor fan.

The Japanese utilities have installed 60,000 units. Gas heat pump manufacturers in Japan include Yamaha (with a 50% market share), Aisin Seiki (mostly larger, commercial size units), Yanmar, Sanyo, Mitsubishi, Nissan, and others. Several Japanese manufacturers, in conjunction with U.S utilities, have been testing their units- prototypes and production models-in various U.S. climates for the past several years.

The York gas engine heat pump is now entering into a field demonstration of 50 units to be sold to consumers in several regions of the country. According to Chuck French of GRI, the demonstration will prime manufacturing and marketing operations in anticipation of a full release of models in 1993. GRI and York expect the unit to be cost-competitive-both in purchase price and operating expenses-with other high-efficiency space-conditioning equipment. It will be sold mostly to the custom housing, replacement, and light commercial markets. As production volume grows and manufacturing costs decrease, the unit will be more competitive with midrange equipment.

York will attempt to advance three environmental selling points to market their gas heat pumps, French believes. Equivalent HSPFs and SEERs exceed the minimum electric heat pump efficiency standards of the National Appliance Energy Conservation Act (NAECA) standards. Also, gas heat pumps, like electric heat pumps, use R-22 refrigerant, an ozone-depleting chlorofluorocarbon gas, but will be able to change over to the same non-CFC refrigerants as electric heat pumps when required by law. (A lingering question is why put CFCs onto the market in the first place along with units that will soon need retrofitting.) According to both the San Francisco Bay Area Air Quality Management District and the South Coast Air Quality District (Los Angeles), the heat pump's 5-hp engine falls below the 50-hp cutoff where Oxides of Nitrogen (NOx) emission regulations come into play. York's NOx emissions have been measured at 300 parts per million.

According to GRI, engine-driven gas heat pumps will benefit homeowners, regulators, utilities, and the environment. They will offer to homeowners the possibility of choosing a cheaper fuel for cooling; to state regulators, a chance to encourage the use of a technology that benefits gas and electric ratepayers, increases efficiency, and fosters environmental quality; to gas utilities, an opportunity to reduce the seasonability of customer gas loads; and to electric utilities, possibly a new demand-side management tool that lowers peak loads and defers new power plant construction.

The Future

In terms of gas heat pump technologies still only on the horizon, the Japan Gas Assoc. is in the process of developing a 1.3-ton residential gas heat pump. MITI has also expanded its gas heat pump program to include absorption units (which use non-CFC, ammonia- based refrigerants) while, in the United States, the DOE and GRI have also been conducting gas heat pump research. For its gas heat pump, York is considering modifications like water heating, generating its own electricity, and a cooling-only use.

The Japanese have proven that gas engine-driven heat pumps are viable. Americans will try to do the same.