Exchange Refrigeration System With SteamEPA Grant Number: R828563
Title: Exchange Refrigeration System With Steam
Investigators: Garris Jr., Charles A.
Institution: George Washington University
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 2000 through June 30, 2005
Project Amount: $180,000
RFA: Technology for a Sustainable Environment (2001) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
The objective of this research project was to investigate the potential of developing an environmentally friendly pressure exchange (PE) refrigeration system. There are three basic refrigeration cycles that are suitable for domestic, commercial, and vehicular air conditioning: reverse Rankine cycle, absorption cycle, and ejector refrigeration. The most common conventional technology uses the reverse Rankine cycle, which provides excellent refrigeration but at a substantial cost because it is powered directly by mechanical energy taken from an electric motor or, in the vehicular application, from the internal combustion engine. If electricity is provided to the air conditioning system through fossil fuel generated electrical power plants, which often have efficiencies of less than 40 percent, and transmitted long distances through the power grid with consequent energy dissipation, the cost to the nation in terms of energy use and the emission of global warming effluents is enormous. Furthermore, reverse Rankine cycle refrigeration generally uses hydro-fluorocarbon based refrigerants, which deplete the earth’s ozone layer and further contribute to global warming. In the case of vehicular air conditioning, the reverse Rankine cycle uses valuable mechanical energy taken directly from the output shaft of the internal combustion engine. Because less than 40 percent of the fuel’s thermal energy is converted to mechanical energy and the reverse Rankine cycle uses this precious resource, vehicular air conditioning in its current form is a substantial contributor to energy consumption and to the emission of global warming effluents. Therefore, if a thermally based vehicular refrigeration cycle can capitalize on the 60 percent of the fuel’s thermal energy that is wasted, an enormous benefit would be derived by society in terms of energy saved and reduced emissions to the environment.
The absorption cycle is a thermally energized refrigeration system that is capable of using a vehicle’s waste heat. Modern technology has increased the coefficient of performance (COP) of absorption cycle units for large scale applications to reasonably high levels, particularly in applications where the absorption cycle is integrated with power generation. However, such systems are extremely complex, costly, and space consuming. At the small scale of vehicular air conditioning, the COP tends to decline, and absorption cycle refrigeration for vehicles tends to be expensive, heavy, space consuming, complicated, and generally impractical, as witnessed by its absence from the vehicular market.
Ejector refrigeration is a thermally energized refrigeration system, which has the added potential benefit of being capable of using environmentally friendly refrigerants, such as water. It has been used extensively for commercial air conditioning and refrigeration, particularly in applications where a source of waste heat is readily available. Several automotive manufacturers, such as Toyota, Renault, and Ford Visteon, have shown some interest in it. The advantages of ejector refrigeration systems are that they are very simple and have a low capital cost. For vehicular applications, studies have shown that by using appropriate heat exchangers, there is sufficient energy in the exhaust gas to provide ejector refrigeration for a typical vehicle. This technology can be scaled down to sizes appropriate for vehicles. The main disadvantage of conventional ejector refrigeration systems, however, is that they suffer from a low COP. The low COP is important even if there is abundant waste energy available, for example in vehicular applications, because all of the excess thermal energy used to power the system must be rejected in the condenser (Foa, 1960). If the COP is low, the size of the condenser becomes very large. This, in turn, increases the cost of the system and makes packaging difficult in vehicular applications.
Analysis of the steam-ejector refrigeration cycle reveals that the steam requirement and the COP of the cycle depend upon the efficiency of one key element: the ejector. Nearly a century of research and development on the steady-flow ejectors that have been used in refrigeration systems has brought us near to the pinnacle of this technology. The physical mechanism by which these steady-flow ejectors operate is the turbulent entrainment between the primary (driving) flow and the secondary (driven) flow. This entrainment mechanism is inherently dissipative of energy, and little can be done to improve it. Thus, after more than 100 years of research, conventional ejectors and ejector refrigeration using conventional ejectors probably has attained its ultimate level of performance, which is not adequate for society’s needs and never will be. Thus, even though there is an enormous societal need for an efficient, thermally energized air-conditioning/refrigeration system that uses environmentally benign refrigerants and is low cost, compact, and highly efficient, neither the absorption cycle nor the ejector refrigeration cycle can meet the needs. A comprehensive review of the literature reveals that there is no other technology that appears to be ready to fill the void.
A central hypothesis of our research under the current grant is that, if an ejector with a compressor efficiency on the order of 50 percent or better could be made, the COP of the ejector refrigeration system would increase dramatically, thereby making ejector refrigeration highly competitive with, or superior to, reverse Rankine cycle and absorption cycle systems in terms of efficiency, capital cost, size, weight, and benefit to the environment.
For vehicular applications, such a system would increase vehicular energy efficiency by using waste heat. It would be low in cost, small in size and weight, and would use environmentally friendly refrigerants, such as steam. It would reduce substantially the amount of global warming effluents emitted by vehicles in the summer, and it would take over the vehicular market rapidly.
In commercial and domestic air conditioning applications, because ejector refrigeration is thermally based, the air conditioning system would be amenable to providing air conditioning by burning natural gas on site. This could provide overall improvements in energy efficiency by eliminating electrical transmission losses and losses from power plant machinery, and also would provide reductions in the release of global warming effluents (CO2) by virtue of the use of energy derived from low carbon natural gas (CH4) rather than energy from high carbon crude oil or coal burned at power plants.
The inspiration for this EPA-supported program was the idea that there is a mechanism for direct flow induction, other than turbulent mixing, that does not rely on highly dissipative turbulent mixing, but rather on the thermodynamically reversible work of interface pressure forces through a mechanism known as pressure exchange (PE). PE was defined by Foa (1960) as “any process whereby a body of fluid is compressed by pressure forces that are exerted on it by another body of fluid which is expanding.” Unlike turbulent mixing, PE relies on the work done by pressure forces exerted by an expanding energetic fluid on a compressed, less energetic fluid across the interface between them. Such energy transfer can be accomplished only in non-steady flow, because pressure forces against stationary interfaces can do no work. Because, in general, the PE process is non-dissipative, an ejector-like device using this operating principle can be highly efficient. Although the overarching goal of this research program was to investigate the potential of obtaining an environmentally friendly PE refrigeration system, the assumption has been that development of such a refrigeration system would be a straightforward engineering exercise if a novel, high-efficiency, ejector-like device could be obtained. The emphasis of our research program, therefore, has been on trying to obtain a fundamental understanding of the PE process in various flow conditions, both subsonic and supersonic, and trying to obtain information needed to design a replacement for a conventional ejector that: (1) is suitable for ejector refrigeration; (2) is of substantially higher efficiency; and (3) can bring on a new era of thermally driven, environmentally friendly, high efficiency refrigeration.
Our literature surveys, before and during our grant period, have shown that there has been a considerable amount of research on two types of PE devices. The first well-known PE device is the “wave-rotor,” which is a fairly complex device that attempts to use PE by filling a relatively long tube with low-energy fluid and then suddenly opening a valve and allowing high-energy fluid to enter. The expansion of the high-energy (primary) fluid compresses and drives the low-energy (secondary) fluid. To achieve these benefits, the wave rotor has a series of tubes arranged on the periphery of a rotor. As the tubes rotate, their relative, instantaneous position causes ports on either end of the tubes to open and close, allowing any one of high-energy primary fluid, low-energy secondary fluid, de-energized primary fluid, or energized secondary fluid to enter or leave the tubes. The device is ingenious and created much excitement in the 1950s and 1960s. Intensive research was done by Brown Boveri, who developed turbochargers, which have been and continue to be used on a number of vehicles. The National Aeronautics and Space Administration (NASA) has been very interested for a number of years in using wave rotors as topping cycles for improving the efficiency of gas turbine engines. Although wave rotors have achieved a good measure of success, they never have attained the high efficiencies sought through PE because of the prevalence of three mechanisms of dissipation, which are unavoidable and inherent in the wave rotor. They are: (1) throttling losses caused by the opening and closing of the ports; (2) friction losses, which result from the rotating dynamic seals around the ports; and (3) the presence of strong normal shock waves. It appears to be well accepted in the field of wave rotors that these dissipation mechanisms are inherent and limit the maximum attainable compressor efficiency of the device. Commercial applications of wave rotors find niches, not as a result of their high efficiencies, but by virtue of other attributes, such as the elimination of “turbo-lag” in superchargers and their ability to directly receive very high temperature products of combustion.
The second well-known PE device is the “crypto-steady pressure-exchange thrust augmenter,” which has been described by Foa (1960). Foa sought to avoid the throttling losses inherent in non-steady flow devices with the hope of attaining the high efficiencies theoretically possible through PE. Foa observed that PE can be created without valving and ports by the use of what he termed a “crypto-steady” flow (i.e., a flow that is steady in a certain moving frame of reference but is non-steady relative to the laboratory). He demonstrated that such flows can be obtained by the use of a free-spinning rotor having canted jets of primary fluid. In such a flow, the flow appears steady relative to the rotor but is non-steady relative to the laboratory. Relative to the laboratory, the primary fluid jets emanating from the rotor form a helical pattern whereby the secondary fluid becomes entrapped in the interstices of the helices and, by the use of an appropriate shroud, is forced into a duct or channel. Thus, work is done by the expanding primary fluid on the compressed secondary fluid by the pressure forces acting across the helical boundary between the two fluids. Much work was done on this concept for aircraft and marine propulsion. All of this work, however, was for applications in which the pressure rise was very small but the ratio of secondary to primary fluid mass flow rates was very high. Under these conditions, high thrust augmentations were obtained, and the value of PE in thrust augmentation was verified. The refrigeration application, however, called for very high pressure rises with small mass flow ratios.
Our research program started by attempting to use Foa’s crypto-steady PE concept under flow conditions appropriate to refrigeration. Armed with the knowledge that crypto-steady PE has the potential of providing society with a highly efficient means of compressing a low-energy fluid through direct contact with a relatively high-energy fluid, thereby avoiding the complexity and the dissipation associated with intervening machinery inherent in conventional compressors, our overarching goal was to determine what configuration such a device might have. To achieve this goal, it was essential to develop an understanding of the underlying physics that control and limit the performance of such a device. It must be understood that, prior to this research program, there never has been an attempt to obtain such a device for high-pressure rise applications, such as refrigeration. The only previous work was done by researchers in the propulsive thrust augmentation area, having an entirely different performance requirement (low-pressure rise/high mass flow ratio.)