Use of the low-temperature waste heat from a CHP unit for low-temperature refrigeration in an absorption refrigeration unit is limited by the heating temperature of approximately 90°C. For one thing, single-stage units can only produce cooling in the single-digit minus range and, in addition, the thermal ratio of this type of unit is rather low. Not only that, but under certain circumstances, refrigeration of the heating medium will not be sufficient to ensure the cooling of the CHP motor circuit within safe limits, making engineering solutions necessary for the utilization of the residual thermal volume. It might also be possible to achieve lower temperatures and ensure sufficient cooling of the CHP unit by using multi-stage systems. Systems such as these are currently only used in special cases or for research purposes due to the extensive equipment required.
Nevertheless, the combination of CHP, waste heat boiler and absorption refrigeration unit provides a financial as well as an ecological advantage compared to the reference case with external electricity supply, compression refrigeration and complete steam generation from a natural gas boiler.
Due to the considerable investment costs in the refrigeration unit and treatment stages, the economic advantage does not so much result from the fact that the absorption refrigeration unit replaces refrigeration from a compression refrigeration unit, but rather from the fact that expensive external electricity can be replaced through the generation of the unit's own electricity and steam can be provided extremely cost-effectively in a waste heat boiler.
The ecological advantage of such a system arises from the efficient use of fuel and the fact that the primary energy factor for natural gas is significantly lower than the primary energy factor for electricity. The fact that the amount of electricity purchased from outside sources can be reduced also reduces the amount of primary energy required to supply the plant with energy.
For a CHP unit with an electrical output of 2 MW, which continues to contribute to the supply of saturated steam at 10 bar(g) using a waste heat boiler and provides cryogenic cooling of about -7°C from an absorption refrigeration unit, a primary energy saving of more than 15% is possible! If the saturated steam is required at a lower pressure or the refrigeration at a higher temperature level, the efficiency of the CCHP plant improves, resulting in even greater primary energy savings.