System Simulation of an Untreated Sewage Source Heat Pump

The paper studies the characteristics of heat pumps used in untreated-sewage source where the matter enters the heat pump directly in winter. The system model is established first. Then the changes of the variable parameters, including the sewage and water temperatures at the inlets of the heat pump and the compressor frequency, are simulated to assess their influence on the system performance. Then, the system is compared with those used in indirect mode to observe the different heating capacities. The results show that most parameters, except the condensation heat transfer coefficient, change as a sine wave, the same as the sewage and water temperatures at the inlets of the heat pump. COP is about 3.75 in the range of the temperature fluctuation of 1°C. The heating capacity and consumed power are 12.9 and 3.45kW when the sewage temperature is 13°C at the inlet. When the compressor frequency increases from 25Hz, and the sewage and water temperatures at the inlet take on impulse signals, the system parameters produce rapid changes at the impulse point, and become unstable in a short time. By comparison with the indirect-mode system, it is shown that the heating capacity of direct-mode system increases by 7~8%, showing greater advantage of energy conservation.