Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
This application relates to a heat transfer apparatus and method employing an active regenerative cycle. The invention employs a working fluid and a heat transfer fluid which are physically separated. The working fluid is contained in an array of discrete elements that are distributed over the temperature profile of a regenerative bed located between a thermal load and a heat sink. The work for the cycle and temperature differences for heat transfer are provided by alternating compression and expansion of the working fluid. The heat transfer fluid is circulated relative to the working fluid between the thermal load and the heat sink to enact a regenerative cycle having improved energy efficiency.
A conventional “vapor-compression” refrigeration cycle employs a single refrigerant that is circulated through a conduit between a heat sink and a thermal load. This cycle relies on the thermodynamic principles of adiabatic compression (temperature increase), isenthalpic expansion (temperature decrease) and latent heat of vaporization or condensation of a fluid.
Refrigerants, such as chlorofluorocarbons, hydrochlorofluorocarbons and hydrofluorocarbons, are typically liquids at ambient temperatures. At one stage in the refrigeration cycle, the refrigerant passes through a compressor that increases its pressure and temperature, causing it to release heat as it condenses from a vapor to a liquid form in a condensing heat exchanger. At another stage in the cycle, the liquid refrigerant passes through an expansion valve to reduce its pressure and temperature, creating a two phase fluid. This reduction in temperature causes the refrigerant to absorb heat and evaporate within the evaporative heat exchanger. In this conventional cycle, the “working fluid”, which is compressed and expanded as it circulates, and the “heat transfer fluid”, which accepts heat from the thermal load and rejects heat to the heat sink, are the same thing, namely the volatile refrigerant. The compressor and expansion valve are physically separated, the compressor being at the “hot end” of the cycle and the expansion valve being at the “cold end” of the cycle. The condensing heat exchanger rejects heat to the heat sink while the evaporative heat exchanger absorbs heat from the thermal load.
Regenerative thermodynamic cycles that use regenerators for periodic heat exchange are known in the prior art. In most cases the regenerator is a material which has a large thermal mass and heat transfer surface. In typical regenerative cycles the regenerator is a passive element that is not capable of doing work and whose purpose is to transfer heat back and forth to a working gas periodically during the cycle to enable larger temperature spans to be achieved. The working gas continues to be compressed at the hot end of the cycle and expanded at the cold end of the cycle. Moreover, the working gas is the same gas which is used to transfer heat from the cooled space to the environment via heat exchangers. Stirling, Gifford-McMahon and Orifice Pulse Tube devices are all examples of prior art refrigeration systems employing passive regeneration.
Stirling cycle devices operate on a regenerative thermodynamic cycle, with cyclic isothermal compression and isothermal expansion of the working fluid at different temperature levels, separated by constant volume flow through regenerators with a temperature span from the two different temperatures of compression and expansion. Stirling cycle devices have been used as heat engines, heat pumps, and refrigerators.
In a Stirling cycle machine operating as a prime mover, the working fluid isothermal compression takes place in the hotter chamber, while most of the isothermal expansion takes place in the colder chamber. Some of the heat introduced at the hot chamber is converted to work in the prime mover and the residual heat is rejected at the cold chamber. As will be appreciated by those skilled in the art, when the Stirling cycle is used in a refrigerating machine rather than a prime mover, the working fluid isothermal expansion that absorbs heat occurs in the cold chamber while the isothermal compression of the working fluid, during which heat is rejected, takes place in the hot chamber. In either type of machine the working fluid is shifted between the two chambers through a passive regenerator which is not itself capable of doing work.
In prior art Stirling cycle machines, the “working fluid” which is alternatively compressed and expanded may either be a gas or liquid. For example, U.S. Pat. No. 5,172,554 dated Dec. 22, 1992, Swift et al., discloses a Stirling thermodynamic cycle refrigerator that utilizes a single phase solution of liquid
He as the working fluid. The liquid
He may be present in superfluid
He. As in conventional Stirling cycles, a passive regenerator is employed as a thermal reservoir that maintains a temperature difference between the compressor and expander and functions as a thermal reservoir that cyclically exchanges heat with the working fluid. Work is applied to the working fluid during the Stirling cycle in the compressor and expander rather than within the passive regenerator itself.
U.S. Pat. No. 4,353,218 dated Oct. 12, 1982, Wheatley et al., relates to a heat pump/refrigerator using working fluid that is continuously in a liquid state. The Wheatley apparatus includes a pair of heat exchangers respectively coupled to a thermal load and a heat sink, a displacer forming a pair of reservoirs coupled to the different heat exchangers, a regenerator connecting the heat exchangers, and means for compressing a working fluid that can pass between the reservoirs by way of the regenerator and a heat exchanger. The working fluid may consist of, for example, compressed polypropylene. As in other similar prior art systems, the regenerator is utilized to transfer heat from the working fluid leaving one heat exchanger into fluid leaving the other heat exchanger and does not input work into or remove work from the system.
“Active regenerators” utilize heat transfer materials that not only have large thermal masses and heat transfer surfaces but are also capable of doing work during a thermodynamic cycle. Heretofore active refrigerants have been solids, such as magnetic materials or elastomers. For example, U.S. Pat. No. 4,704,871, Barclay et al., issued Nov. 10, 1987, relates to magnetic refrigerators employing paramagnetic or ferromagnetic materials. When such materials are adiabatically passed into and out of a magnetic field (such as produced by a superconducting magnet) their temperature alternatively increases and decreases. This is referred to as the magnetocaloric effect. By way of example, if Gadolinium at room temperature is adiabatically subjected to a magnetic field of about 8 Tesla it will increase its temperature by about 12-14 K. A refrigeration cycle may be enacted by passing a heat transfer fluid between hot and cold heat exchangers in a periodic flow as the magnetic material is alternatively adiabatically magnetized and demagnetized.
One significant problem associated with active regenerative systems employing the magnetocaloric effect is the cost of developing adequate adiabatic temperature changes especially for near room temperature use. Magnetic systems require powerful superconducting magnets to achieve magnetic fields large enough to cause modest temperature ratios. Such superconducting magnets are very expensive and not practical for many applications and the energy required to keep the superconducting magnets cold makes the entire cycle inefficient with the exception of very large systems.
Elastomeric materials may also be used as an active heat transfer element in a regenerative system. U.S. Pat. No. 5,339,653 dated Aug. 23, 1994, DeGregoria, describes refrigeration cycles based on the thermoelastic effect in which certain elastomers, such as rubber, warm upon stretching and cool upon contracting. In particular, a regenerative bed may be formed comprising a porous matrix of elastomeric sheets arranged in layers with space
Barclay John A.
Corless Adrian J.
Kratschmar Kenneth W.
Reid Christopher E. J.
586925 B.C. Inc.
Doerrler William C.
Oyen Wiggs Green & Mutala
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