Abstract
Waste heat recovered from a refrigeration machine is associated with the double benefit of generating cold and heat with just one unit. Additional energy is required in most cases to achieve these benefits. To evaluate the efficiency of waste heat recovery, two novel efficiency indicators are described. The overhead coefficient of performance (OCOP) describes additional electrical power required to raise the temperature to make waste heat usable. The coefficient of savings describes power reduction when condenser heat is fed into a cold district heating network instead of exhausting it to high-temperature outside air. Results are reported from a case study in a food logistic center with high cooling demand in Isny, Germany. Waste heat at this facility was previously released unused to outside air. We describe how this waste heat can be used to supply sustainable heat supply to a new residential area. During the design phase, it is difficult to choose the best operating temperature for district heating networks (DHNs). The novel indicators are used to value the effort to make waste heat usable. Whereas a supply temperature of 20 °C has no disadvantages for the operator, a supply temperature of 40 °C is associated with an increase in electricity consumption. The resulting OCOPs are above 5.0 even under unfavorable conditions and exceed the theoretically calculated (Holm and Pehnt, 2023, Wärmeschutz und Wärmepumpe-Warum Beides Zusammengehört, Forschungsinstitut für Wärmeschutz e.V., Institut für Energie und Umweltforschung, München/Berlin/Heidelberg, p, 13, 14; Agora Energiewende, Fraunhofer IEG, 2023, Roll-out von Großwärmepumpen in Deutschland, Strategien für den Markthochlauf in Wärmenetzen und Industrie, Berlin) and measured (Fraunhofer Institut für Solare Energiesysteme, 2020, “Wärmepumpen in Bestandsgebäuden, Ergebnisse aus dem Forschungsprojekt, WP-Smart im Bestand.”) coefficients of performance (COPs) for air-sourced heat pumps. Although using waste heat is not free, it is beneficial when overall efficiency is considered.
