Non Vapour Compression Cycles

Although work is underway to develop alternative refrigerants and improve refrigerant management strategies, the phase-down of HFCs will impose constraints on vaporcompression equipment that will require trade-offs among cost, efficiency, and safety. These constraints could present market opportunities for alternative space-conditioning technologies. Alternative technologies based on their development status (some technologies are in very early stages of development), potential for energy savings, and other factors that may affect their ability to compete with vapor-compression systems.

Figure below groups 22 non vapor compression technologies analyzed in the DOE study, classified by energy source and primary working fluid or material. Although vapor-compression systems are also used in refrigeration, transportation, and process cooling applications, the DOE study focuses solely on building HVAC applications.

Some alternative technologies are impractical for spaceconditioning applications due to low efficiencies and capacities, and some are too early in their development cycle to be fully evaluated (e.g., Bernoulli heat pump, critical-flow cycle, and electrocaloric heat pump).

This sections explains some of Alternatives to Vapor-Compression cycles.

Transcritical carbon dioxide cycle

The low critical temperature for carbon dioxide can be seen in the pressureenthalpy diagram ( Figure below ). A cycle with heat rejection at 31°C would have a much lower refrigerating effect than one condensing at, say 27°C. Above the critical point the gas cannot be condensed, and it is necessary to move into this region if the temperature of heat rejection approaches 30°C. If the gas can be cooled, to say 40°C as shown in Figure, the refrigerating effect is similar to that with heat rejection at 30°C. In the cycle shown, the gas is cooled from 120°C to 40°C at a constant pressure of 100 bar in a heat exchanger described as a gas cooler .

Liquid formation only takes place during expansion to the lower pressure level. It may be possible to operate a system designed for transcritical operation in the subcritical mode, i.e. as a vapour compression cycle, under low ambient conditions in which case the gas cooler becomes a condenser.

Regulation of the high pressure is necessary for the transcritical cycle. The optimum pressure is determined as a function of the gas cooler outlet temperature and is a balance between the highest possible refrigerating effect and the smallest amount of compressor energy.

Total loss refrigerants

Some volatile fluids are used once only and then escape into the atmosphere. Two of these are in general use: carbon dioxide and nitrogen. Both are stored as liquids under a combination of pressure and low temperature and then released when the cooling effect is required.

Carbon dioxide is below its triple point at atmospheric pressure and can only exist as ‘ snow ’ or a gas. The triple point is where solid, liquid and vapour phases co-exist. Below this pressure, a solid sublimes directly to the gaseous state. Since both gases come from the atmosphere there is no pollution hazard. The temperature of carbon dioxide when released will be 78.4°C. Nitrogen will be at 198.8°C. Water ice can also be classifi ed as a total loss refrigerant.

Absorption cycle

Vapour can be withdrawn from an evaporator by absorption into a liquid ( Figure below ). Two combinations are in use, the absorption of ammonia gas into water and the absorption of water vapour into lithium bromide. The latter is non-toxic and so may be used for air conditioning. The use of water as the refrigerant in this combination restricts it to systems above its freezing point. Refrigerant vapour from the evaporator is drawn into the absorber by the liquid absorbant, which is sprayed into the chamber. The resulting solution (or liquor) is then pumped up to condenser pressure and the vapour is driven off in the generator by direct heating.

The high-pressure refrigerant gas given off can then be condensed in the usual way and passed back through the expansion valve into the evaporator. Weak liquor from the generator is passed through another pressure-reducing valve to the absorber. Overall thermal effi ciency is improved by a heat exchanger between the two liquor paths and a suctionto-liquid heat exchanger for the refrigerant. Power to the liquor pump will usually be electric, but the heat energy to the generator may be any form of low-grade energy such as oil, gas, hot water or steam. Solar radiation can also be used. The overall energy used is greater than with the compression cycle, so the COP is lower. Typical figures are as shown in Table.

Air cycle

Air cycle refrigeration works on the reverse Brayton or Joule cycle. Air is compressed and then heat removed; this air is then expanded to a lower temperature than before it was compressed. Heat can then be extracted to provide useful cooling, returning the air to its original state (see Figure below ). Work is taken out of the air during the expansion by an expansion turbine, which removes energy as the blades are driven round by the expanding air. This work can be usefully employed to run other devices, such as generators or fans. Often, it is used to help power the compressor, as shown. Sometimes a separate compressor, called a ‘ bootstrap ’ compressor, is powered by the expander, giving two stages of compression. The increase in pressure on the hot side further elevates the temperature and makes the air cycle system produce more useable heat (at a higher temperature). The cold air after the turbine can be used as a refrigerant either directly in an open system as shown or indirectly by means of a heat exchanger in a closed system. The effi ciency of such systems is limited to a great extent by the effi ciencies of compression and expansion, as well as those of the heat exchangers employed.

Originally, slow-speed reciprocating compressors and expanders were used. The poor effi ciency and reliability of such machinery were major factors in the replacement of such systems with vapour compression equipment. However, the development of rotary compressors and expanders (such as in car turbochargers) greatly improved the isentropic effi ciency and reliability of the air cycle. Advances in turbine technology together with the development of air bearings and ceramic components offer further effi ciency improvements.

The main application for this cycle is the air conditioning and pressurization of aircraft. The turbines used for compression and expansion turn at very high speeds to obtain the necessary pressure ratios and, consequently, are noisy. The COP is lower than with other systems.

Stirling cycle

The Stirling cycle is an ingenious gas cycle which uses heat transferred from the gas falling in temperature to provide that for the gas rising in temperature. The Stirling cycle has been successfully applied in specialist applications requiring low temperatures at very low duties.

Thermoelectric cooling

The passage of an electric current through junctions of dissimilar metals causes a fall in temperature at one junction and a rise at the other, the Peltier effect. Improvements in this method of cooling have been made possible in recent years by the production of suitable semiconductors. Applications are limited in size, owing to the high electric currents required, and practical uses are small cooling systems for military, aerospace and laboratory use.

Magnetic refrigeration

Magnetic refrigeration depends on what is known as the magnetocaloric effect , which is the temperature change observed when certain magnetic mater-ials are exposed to a change in magnetic field. Magnetic refrigeration is a research topic, and historically has been used at ultra-low temperatures. Only recently has it been seen as a possible means of cooling at near room temperatures.

Refrigeration and Air-Conditioning
G. F. Hundy, A. R. Trott, T. C. Welch, and T C Welch