مزایای حرارت در دمای پایین: تاثیر حرارت مخزن بر عملکرد و اقتصاد

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.

Valorisation of low-temperature heat is a broad area. It covers heat recovery by compression heat pumps, sorption heat pumps, and heat transformers, as well as conversion into cold or mechanical energy. The means to do so are abundant as well: conventional conversion systems use thermodynamic mono-fluid cycles (closed steam cycles, open steam cycles, or even gas cycles), or dual-fluid cycles such as sorption cooling processes or sorption power processes. Some options which are state of the art, or technically feasible, or at least in discussion, are listed in Table 1.

From the fundamental relationships which have been presented in this communication it can be concluded that the impact of the temperature and nature of the heat sink on performance and economics in the field of valorisation of low-temperature heat is predominant - except, of course, in the case of simple heat pumps. In all other cases the heat flow into this sink accounts for a large fraction of the energetic turnover. For the sake of efficiency, the driving temperature difference must be small which in turn necessitates large heat transfer areas. Heat pumps seem to be the most cost-efficient devices for valorisation of low-temperature heat. Of course a rising ambient or heat sink temperature in any case reduces efficiency. This effect is especially large for heat driven cooling machines. These devices have been studied in more detail using the approach of the characteristic functions. The impact of the heat sink temperature on heat flows (capacity) and performance (COP) is about twice as large as that of the other temperatures. Finally, a first attempt has been made to include the impact of humidity via the wet bulb temperature into the calculus. This, however has to be refined further by using a realistic model of the cooling tower. Summarising, the heat sink deserves more attention in the design of processes which re-valorise low-grade heat. It can be stated that the research which is dedicated to the field of heat rejection does not match the importance within the area of energy engineering.