Looking at Figure 1, the absorption cycle is very similar to the vapour compression cycle, in that it uses a circulating refrigerant, an evaporator, a condenser and an expansion device. The difference is that the compressor of the vapour compression cycle is replaced by a chemical absorption process and generator, with a pump to provide the circulation and pressure change.
Figure 1 : Basic absorption cycle
The vapour compression cycle is described as a work-operated cycle because it uses a compressor that requires work in the form of electrical energy to operate. The absorption cycle is referred to as a heat-operated cycle because most of the energy required to operate the cycle is heat energy. This immediately opens up options for its application and use, over an electrically driven cooling cycle. Absorption cooling is worth considering if any of the following factors apply:
- For a CHP unit where there is spare heat available, or when a new CHP plant is being considered;
- Waste heat is available (eg, exhaust steam);
- A low cost source of heat is available (eg, landfill gas, geothermal);
- An existing site has an electrical load limit that would be expensive to upgrade;
- A site is particularly sensitive to noise and/or vibration;
- Solar energy can be harnessed.
Absorption cycle theory
As well as a refrigerant, an absorption system needs an absorbent solution and various pairs of fluids are available. For air conditioning applications operating with evaporating temperatures above 0ºC, lithium bromide solution is the absorbent, while water is the refrigerant. Below 0ºC, the most common pairing is water as the absorbent and ammonia as the refrigerant. In this article, only the air conditioning case is considered.
Figure 2 : Temperature/pressure/concentration data for lithium bromide solution
Lithium bromide is a solid salt crystal that readily absorbs water vapour (it is used to keep electronic equipment like cameras free of moisture), eventually becoming a liquid solution of lithium bromide and water. This solution exerts a water vapour pressure that is a function of the solution temperature and concentration. These temperature/pressure/ concentration properties are shown in Figure 2 for lithium bromide/water solutions.
For example, a lithium bromide solution at 50% concentration and 25ºC would have a vapour pressure of 0.87kPa. This is a typical condition for the solution in the absorber vessel in Figure 1. Connected to the absorber is the evaporator, containing water as refrigerant, which we would like to be at a saturation temperature of 5ºC for producing chilled water at say 7ºC. If the evaporator temperature starts at 7ºC, its vapour pressure as pure water is 1.0kPa and for equilibrium between the evaporator and absorber to be achieved, water would have to evaporate in the evaporator and condense into the solution in the absorber. Provided that a fresh supply of lithium bromide solution at 25ºC is continuously available, the process could go on indefinitely and the water in the evaporator would evaporate until its vapour pressure drops to about 0.87kPa, and a temperature of 5ºC, cooling the chilled water. The lithium bromide solution acts like a compressor in drawing off “refrigerant vapour”, in this case water, from the evaporator, causing the pressure and saturation temperature to reduce to the required cooling temperature. This process is the basic principle by which the absorption cycle operates.
To complete the cycle shown in Figure 1, the ‘weak’ solution in the absorber is pumped to a generator, where external heat is applied to boil off or vaporise the water from the solution. This results in the water (refrigerant) vapour leaving the generator and being condensed in a water or air cooled condenser, back to a liquid. Its pressure is then reduced before feeding back into the evaporator to continue the cooling process. Meanwhile, the now ‘strong’ solution in the generator is fed back to the absorber, also reducing in pressure as it goes and continuing the absorption process.
The energy flows in Figure 1 indicate:
- The cooling duty heat input is to the evaporator; and
- Heat is generated by the absorption process and this heat has to be removed;
- The heat input at the generator will be the heat source selected; and
- The heat rejected from the condenser produces the condensation of the refrigerant (water).
The only electrical input is for circulating pumps (see figure 3) and control valves. Note that the removal of heat from the absorber and condenser can be by ambient air in small absorption units and are available as air cooled, air cooling units up to about 80kW cooling duty. Perhaps the most common absorption application since its conception has been in domestic refrigeration, where a system has been developed that has no electrical requirement and the system is driven by gas. In building services applications it is more common to find large capacity absorption plant, chilling water and rejecting heat through water cooled condensers and absorbers by cooling tower/ dry cooler water, passing through the absorber first, then the condenser.
For maximum heat exchange contact in the evaporator, the refrigerant pump sprays refrigerant water over the chilled water tubing – similarly in the absorber, where solution is sprayed over the heat rejection tubing. The heat exchanger improves the efficiency between absorption and generation.
It should be noted that heat rejection from absorption systems will be greater than that for an equivalent vapour compression system, because of the cooling required in the absorber – about 2.5 times the cooling capacity, for air conditioning applications, which means larger heat rejection equipment.
Operation and performance
In a typical absorption system producing chilled water, the evaporating temperature might be 5ºC, chilling water to about 7ºC, and a condensing temperature of 40ºC. The absorber will be at a temperature of about 25ºC. The solution is pumped to the generator, where heat is supplied at a temperature ranging from 80ºC to 140ºC, say 100ºC, driving the refrigerant from the solution to the condenser. The interesting feature of using water as the refrigerant is the low pressures within the system. In the evaporator and absorber the pressure will be 0.87kPa for an evaporating temperature of 5ºC. In the condenser and generator the pressure will be 7.38kPa corresponding to a condensing temperature of 40C. In other words the whole system operates well below atmospheric pressure. This means that any point of leakage in the components will result in air being drawn into the absorption system, which will reduce cooling capacity or at worst stop the process altogether.
The Coefficient of Performance (COP) for an absorption system is defined as:-
COPc = Cooling Duty(kW)/Generator Heating Duty(kW)
The ideal, theoretical Carnot COPc is:
Tr(Ts – Ta)/Ts(Ta – Tr),
- Tr is the evaporator refrigerant temperature;
- Ts is the generator temperature;
- Ta is the absorber temperature.
From the above example the ideal COP is 2.8.
Compare this with the Ideal Vapour Compression COP, operating at the same temperature difference, of 7.9.
In practice a typical COP for an absorption cycle in air conditioning would be about 0.7, compared to about 3.5 for a vapour compression system. It appears that absorption systems require about five times more energy than vapour compression, but of course, the energy for absorption is heat energy, not work (electrical) energy. Heat energy is cheaper than electrical energy and in some applications this heat energy is free, or is waste heat from another use, such as waste steam, hot water, gas, solar energy etc., which makes it advantageous to use absorption.
Note the performance characteristics of absorption systems:
- The higher the heat supply temperature to the generator, the greater the COP.
- The higher the refrigerant evaporating temperature, the greater the COP.
- The lower the ambient temperature (air or water) for heat rejection, the greater the COP.
- Enhanced absorption systems that use double and triple effect generators have improved COPs of 1.2 and 1.7.
Figure 3 : Components of a two shell lithium bromide water chiller
With regard to capital and running cost comparison between absorption and vapour compression, as a very general estimate, figures for a 800 kW cooling plant showed that absorption plant capital cost was 30% higher than, but annual running costs were 10-15% lower than vapour compression plant.
A potential selling point for absorption chillers is that they do not use global warming fluids such as HCFC, or HFC refrigerant fluids found in vapour compression systems. This is an important advantage of absorption units, but it is clear that the environmental effects of refrigerant leakage on ozone depletion and global warming is minimal compared to the effect on global warming of CO2 generation from the energy production required to operate the system. Absorption chillers are also marketed as environmentally friendly because their power input is not primarily electricity, but a heat source. This would appear to produce lower CO2 emissions than vapour compression systems, but this will depend on the energy source for generating the electricity used in vapour compression systems. If the electricity generation for vapour compression is from fossil fuel, then overall CO2 emissions may be lower from a gas powered absorption system. However, if greener electricity is produced, say from hydropower plants, then vapour compression systems will have lower CO2 emissions than gas fired absorption. The situation is fairly complex and each application would need to be considered with all the relevant data.
From an environmental position, considering primary energy requirements only, today’s absorption systems can be effectively applied for use with integrated energy systems such as waste heat or Combined Heat and Power(CHP).
© Terry Welch