Valorisation of low-temperature heat: Impact of the heat sink on performance and economics

Low-grade heat is available everywhere; consequently, the valorisation of this heat seems to be attractive in terms of economics. However, irrespective of the form of energy which is produced, any valorisation comes along with the production of another stream of waste heat with even lower value. The dumping of this reject heat often turns out to be the issue which determines cost.This presentation will elaborate on the influence of the heat sink temperature both on conversion efficiency and cost. It first will give a frame on a very generic level. It is easy to reproduce the well-known fact that the change in COP of a compression heat pump with heat sink or source temperatures is in the order of some %/K. The same order of magnitude holds for all generic cycles with one important exception: the influence of the heat sink temperature on the COP of a thermally driven cooling machine is about twice the impact of the other temperatures. In addition, simple equations to account for the cost of heat exchange are presented. They show that heat pumps, be it work driven or heat driven, exhibit the best efficiency-to-cost ratio.In order to leave the generic level, a more detailed analysis is given for an absorption cooling system. It is confirmed that the impact of the heat sink temperature on capacity and COP is significantly larger than that of the other temperatures; in the nominal point a rise in heat sink temperature reduces the cooling capacity by over 10%/K.Finally, the influence of the humidity of the ambient air on performance is presented in a first order approach, also.