Thermodynamic optimization for central heating system by using industrial waste heat through absorption heat transformer

Urban heating energy consumption in northern China accounts for 40% of total building energy consumption. Meanwhile there are large quantities of low grade waste heat discharged into the environment during industrial processes in northern China. Utilizing these industrial waste heat for central heating in winter is of great importance in energy saving and has raised more concerns during recent years. However, in traditional systems, the low grade waste heat is often used to produce relatively low temperature supply water through heat exchangers, often leading to large mass flow rate in the primary network, which results in huge pump energy consumption after long distance delivery, since the heating users are often far away from the industrial heat sources. To break through such application bottleneck, the key is to increase the temperature difference between the supply and the return water, and decreasing the mass flow rate in the primary network. In this paper, based on the concept of heat adaptor, two kinds of heat adaptors, absorption heat transformer and absorption heat pump, are used at heat source and substations respectively to take place of heat exchangers in central heating system, to decrease the mass flow rate in the primary network. Aiming to minimize the pump energy consumption, the thermodynamic model of the new system is built and the optimal supply and return water temperatures can be obtained through extreme principle. The illustrative example indicates that the optimal supply water temperature in primary network is much higher than the waste heat temperature when the absorption heat transformer is utilized. The results also show that the delivery pump energy consumption can be decreased by almost 96% for the ideal heat adaptor system after optimization, compared to absorption heat exchange system. The theoretically ideal system deduced out in this paper is significant for guiding the optimization design of practical central heating systems using industrial waste heat.