Absorption refrigeration – cooling from heat

The huge variety of today's product palette is reflected in the great differences between and complexity of the necessary production conditions. In addition to power, some processes require steam, hot water and cooling. These different forms of energy can be generated together through the use of combined cooling, heat and power (CCHP) stations and can therefore be supplied in a highly efficient manner.

Even companies that have no significant need for heating can take advantage of CHP by using the excess heat for cooling purposes with the aid of absorption chillers. Thus, absorption refrigeration technology offers a way to make expedient use of the motor heat from a CHP unit, and therefore to supply production processes with cooling, power and, if required, with steam and hot water.

Expedient use of combined heat and power motor heat

In order to remain competitive, many companies in the industrial sector depend on low energy costs. Their competitiveness can be threatened by successive increases in the state mandated pricing components such as the EEG levy added to the price of electricity. Because only a small number of companies are exempt from the EEG levy, the majority of firms are affected by it.

One solution for these companies is to generate their own electricity on site, as this approach benefits from reduced levies. This can lead to an overall reduction in the cost of power, despite a larger capital expenditure and ongoing costs, for example for maintenance and repairs. Motor-driven CHP unit is often the technology of choice due to the comparatively lower capital investment requirement. In addition to power generation, the hot exhaust gasses can be used for the production of steam, which can then be fed into the factory distribution grid. Systems that generate heat and power at the same time are referred to as combined heat and power systems and are characterised by their high fuel utilisation ratios and, therefore, by their environmental-friendliness. Because their special characteristics and efficiency, systems of this type are eligible for state subsidies and therefore contribute to lower energy costs.

In the course of cooling the engine in a CHP unit, the cooling water is heated to around 90 °C. But, to ensure that the engine is sufficiently cooled, the cooling water needs to be cooled down again to around c. 70 °C. It has been shown that most companies in the production sector have an electricity and steam requirement that can at least be partially covered with the aid of a CHP unit. However, many companies have only a very limited or no requirement for hot water. To operate a CHP unit in an efficient, economically sound and environmentally-friendly manner, it is necessary to make expedient use of the excess heat. Absorption refrigeration units are ideally suited to this purpose. In some circumstances, it only becomes feasible to operate a CHP unit or else to install a higher performing CHP unit that can cover a higher percentage of the energy requirement, if it is used in conjunction with absorption refrigeration units.

How an absorption refrigeration system works

Most often, cooling is created with the aid of compression refrigeration systems, which use an electrically powered compressor to increase the pressure of the gaseous coolant.

In place of this, absorption refrigeration systems use a so-called thermal compressor. By contrast with the traditional compressor, the thermal compressor is powered by heat rather than electricity. The thermal compressor works on the basis of a mixed substance comprising coolant and a sorption medium. The sorption medium is able to absorb the coolant, hence the term 'sorption medium'.

At the start of the thermal compressor the coolant-sorption mixture is in the form of a fluid, sometimes referred to as a 'rich mix', because a comparatively large volume of coolant is dissolved in it. It is possible to increase the compression level of the fluid with an extremely small compression force from an electric pump, which means that the absorption refrigeration units do consume a small amount of electricity.

Because they boil at different temperatures, it is possible to separate the coolant from the sorption fluid by means of heating. This is done in the generator, otherwise known as the expulsion unit, through the introduction of external heat, for example the excess heat from the CHP unit. The boiled off coolant vapour is channelled to the condenser (liquefier), where it turns back into a liquid whilst emitting heat to the surrounding environment. The coolant is de-pressurised to the lower pressure level with the aid of a throttle and channelled to the evaporator. Due to the low pressure of the gas, the residual heat in the cooling circuit return flow piping is sufficient to evaporate the coolant. This removes heat from the cooling circuit which can then provide refrigeration. In a subsequent step, in the absorber the gaseous coolant is brought into contact with the low pressure and at this point coolant-depleted solution from the generator, whereby the coolant is absorbed into the solution and can then be pumped back to the generator.

The solution saturation difference between the coolant rich and coolant depleted solution is known as the degassing spectrum, and can be regarded as an efficiency measure for the thermal compressor, whereby the objective is to maximise the degassing spectrum. The back-cooling temperature is determined by the ambient temperature of the surrounding environment, and the target refrigeration temperature is dependent on the process conditions. The result of this is that the temperature of the heat source which is used to expel the coolant in the generator needs to be raised in order to increase the degassing spectrum. The greater the degassing spectrum, the greater the extent to which the heating medium can be refrigerated, thus enable greater transfer capacities.

If one only takes the capacities into account, one can use the thermal ratio, which describes the refrigeration capacity in relation to the thermal capacity used at the applicable system temperatures.

Additional internal heat exchangers are used to improve the processes inside the absorption refrigeration unit, by means of which it is possible to further increase the efficiency of the system.

Two systems have become established in the market, each of which uses a different media mixture: water with lithium bromide, and ammonia with water. The first is used when cooling temperatures above 0 °C are required. If deep freezing is required, then ammonia has to be used as an alternative coolant. Units of this type are considerably more complex and require rectification systems in order to produce a highly concentrated coolant vapour.

Combined cooling, heat and power for chilled air generation

Lithium-bromide-based absorption refrigeration systems designed as air conditioning systems are now available on the market in the form of compact units with a range of different performance capacities. In combination with the low-temperature excess heat from a CHP system, these can be used for the efficient generation of process cooling, air conditioning in office buildings, or chilled air for refrigerated storage areas.

Combined cooling, heat and power for the generation of deep-freeze refrigeration

The use of the low-temperature excess heat from a CHP system for the generation of deep-freeze refrigeration in an absorption refrigeration system is sharply constrained by the heating temperature of around 90 °C. On the one hand, single-stage units are only capable of producing cooling in the single-digit minus range, and on the other hand, the thermal ratio of this type of plant is rather low. Moreover, under certain circumstances, the refrigeration of the heating medium will not be sufficient to ensure the cooling of the CHP motor circuit within safe limits, which is why engineering solutions are necessary for the utilisation of the residual thermal volume. The use of multi-stage units may enable the achievement of lower temperatures whilst ensuring sufficient cooling for the CHP plant. Yet, due to the significant amount of additional equipment involved, the use of this type of system is currently restricted to exceptional cases and research environments.

Nevertheless, in comparison with a baseline reference system comprising third-party power supply, compression refrigeration and the use of a natural gas boiler to cover the total steam requirement, it is possible to achieve both economic and ecological benefits through the use of a combination of CHP unit, a waste heat recovery boiler and an absorption refrigeration system.

Because of the high capital investment requirement for the cooling plant and various treatment stages, the economic benefits accrue less from the fact that the absorption refrigeration system obviates the need for cooling from a compression refrigeration unit, and more from the fact that expensive third-party power supply costs are replaced by on-site electricity generation, and that steam can be produced in a waste heat recovery boiler in an extremely cost-effective manner.

The ecological benefits of this type of system are the result of effective fuel exploitation and the fact that the primary energy factor for natural gas is significantly lower than that of electrical power. Moreover, because third-party power consumption is reduced, the company's expenditure on primary energy is also reduced.

In the case of a CHP system with an electricity generation capacity of 2 MW, which also contributes to the provision of saturated steam at 10 bar(g) by way of a waste heat recovery boiler, and provides refrigeration at around -7 °C by way of an absorption refrigeration system, a primary energy cost saving in excess of 15 % is more than feasible! Should the saturated steam be required at a lower pressure or the cooling at a higher temperature, then the system efficiency level of the CCHP plant will increase, which will, in turn, result in a greater primary energy cost saving.

Benefits of absorption refrigeration

Refrigeration through absorption refrigeration units is climate friendly because

  • it utilises waste heat and
  • less electricity

The generation of cooling within a combined system comprising a CHP unit (and waste heat recovery boiler) is more cost-effective than separate energy supplies for the following reasons:

  • Economic benefits through CHP and EEG compensation
  • Exemption from energy / electricity tax
  • Subsidy potential from absorption chillers and cooling grids

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