Trigeneration: Cooling demand

The potential for air-conditioning systems in Middle East is huge. Absorption refrigeration in combination with gas engine cogeneration technology can be a good solution.

The use of refrigeration has become commonplace in our industrialised society. It is required for air-conditioning systems in office blocks, shops, hotels, public buildings and conference centres. There are also industrial processes for which refrigeration is an absolute necessity. This is particularly true of the food industry, the chemical industry, pharmaceutical plants, the textile industry and the computer industry.

In the Middle East, refrigeration effect for air conditioning is usually produced by compression-type refrigeration plants. In these plants a ‘refrigerant’ is compressed by means of mechanical energy. This condenses, releasing into the environment the heat previously removed from the object and absorbed at low pressure during the evaporation process. The overall process, however, can be inefficient and damaging to the environment. Almost without exception, the equipment is driven by electricity generated in conventional power stations which only utilise one third of the energy source. Added to this is the fact that the existing refrigerants, which mainly consist of chlorofluorocarbon compounds (CFCs), damage the ozone layer and contribute to the greenhouse effect.

This situation can be improved by using gas fired distributed generation to power the compression refrigeration machines. Using low-pollution natural gas reduces environmental impacts while siting the generation at the point of use avoids transmission losses and therefore increases the overall energy conversion efficiency of the process. The use of water as refrigerant in absorption-type refrigeration machines instead of CFCs, which are regarded as a key factor in the depletion of the ozone layer and contribute to the greenhouse effect, is a considerable contribution to environmental protection.

One reference example for such an installation operates in Mussafah, an industrial city near Abu Dhabi, UAE. The plant operated by Tabreed ” The National Central Cooling Co. ” is powered by three GE Jenbacher natural gas engine gensets JGS 616 GS-N.L which produce 6.0 MW of electrical power for continuous operation of the compression chillers. They provide 4à‚°C chilled water in a 43 km long distribution pipeline network in a residential area incorporating apartments, shopping centres, mosques, a police station and a cinema complex.

Figure 1. Three GE Jenbacher JGS 616 GS-N.L installed in the UAE Mussafah City cooling distribution plant
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Gas engine cogen plants

Gas engine cogeneration plants consist of an engine/generator set with heat exchangers for the utilisation of the thermal energy in the intercooler, jacket water, lubricating oil and exhaust gas. Gas engine cogeneration plants typically attain total efficiency of up to over 90 per cent (approximately 40 per cent electrical and 50 per cent thermal). Either a spark-ignited gas engine, a gas-diesel (dual-fuel) or a diesel engine can be used as the drive. The main disadvantage of the latter two is that emissions are considerably higher

Refrigeration using absorption-type machines is a relatively old, proven technology. The absorption cycle has been known as a refrigeration process since the 18th century. The first ammonia/water absorption plant to go into continuous operation was invented by Francois Carré in 1859. The first lithium bromide/water plant was installed by a company called Carrier in 1945. Since then, refrigeration from absorption machines has been gaining in importance over conventional refrigeration with compression machines.

Absorption-type refrigeration machines work by separating a pair of substances ” water as the refrigerant and lithium bromide as the absorbent ” through the addition of heat and combining them through the removal of heat. The main physical effect that is used is the lower boiling point of water at very low pressures i.e. vacuums. At about 8 ” 10 mbar abs. water boils at 4.5à‚°C. This fact is used in a high vacuum vessel (vaporizer), where water is evaporated by spraying it on a pipe bundle, cooling the chilled water inside down to 5-6à‚°C. The most widespread simple, single-stage absorption machine consists of an ejector, condenser, evaporator, absorber, throttle and solvent or refrigerant pump.

The water vapour is absorbed in the absorber by the LiBr solution. The diluted solvent (LiBr/H2O) is compressed to about 80 mbar abs. by means of a solvent pump and is conveyed to the ejector. To enrich the solvent again the water is evaporated in the ejector through the addition of heat at temperatures between 90à‚°C and 130à‚°C. At that point the heat of the cogeneration plant comes into the process.

What sets absorption-type refrigeration machines apart is the fact that they behave very well under part-load conditions and can be operated in a range from 10 -100 per cent with no appreciable loss of efficiency. Efficiencies of about 70 per cent are typical with single-stage indirectly heated absorption machines of a suitable design. Of considerable importance when designing an absorption refrigeration plant is the temperature of the hot water and the temperature of the heat removed via the cooling circuit. The higher the hot water temperature and the lower the temperature of the cooling circuit the better, the efficiency and performance of the absorption machine.

By its very nature, the demand for refrigeration in air conditioning systems is dependent on the ambient temperature and increases with the outside temperature. This means that low-temperature heat most often occurs when ambient temperatures are high and the only way to remove it economically is to use open or closed cooling towers.

Figure 2. Principle of the compression refrigeration process
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Refrigeration comparisons

Currently, refrigeration effect is usually produced using conventional compression-type refrigeration machines. This is mainly due to their high power density and the lower capital outlay required. However, there are also clear advantages in using absorption-type refrigeration machines for refrigeration:

  • No powerful mechanical compressors are required, which translates into lower maintenance costs and a longer service life. At about one per cent of refrigeration capacity, the electrical energy required is substantially less than is needed with compression-type refrigeration machines (25 per cent ” 40 per cent of refrigeration capacity) and reduces the installed load of the plant as a whole.
  • The principal driving power is not expensive electrical energy but heat, which often occurs as a byproduct, particularly in summer.
  • Low noise emissions and step-less load control while maintaining almost constant efficiency are other important factors in favour of absorption machines.
  • The ability to produce refrigeration effect without CFCs or similarly critical refrigerants.
  • The combination with cogeneration plants allows maximum overall efficiencies to be achieved during prolonged running times and therefore forms the basis of an economic total concept.

The layout of the recooling plant also has to be taken into account at the design stage, as well as the considerable size of the absorption machine. The cooling required is about double that required in compression-type refrigeration machines at the same refrigeration capacity and allowance has to be made for the amount of space required. Any acoustic requirements may necessitate additional investment. Where cooling towers are used, the cost of water consumption and conditioning must be taken into account.

Figure 3. Single-stage absorption-type refrigeration machine (York International)
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The combination of cogeneration plant and absorption chiller has major advantages:

  • Beneficial use of waste heat
  • Satisfactory load on the cogeneration plant
  • Reduction in electricity demand compared with mechanical cold production.

In order to achieve these goals, the absorption cooler and the cogeneration plant must be combined technically, thermodynamically and in terms of energy to create an optimum system.

The potential for the utilisation of the absorption refrigeration in combination with cogeneration technology in the Middle East is huge. Especially in those areas where natural gas is available the combination of distributed generation of electricity and the utilisation of the waste heat for the production of the high demands of refrigeration are the optimal technical and economical solution.

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