Operational Performance Study Of Dual-source Heat Pump Sludge Drying System

With the development of economy and the continuous improvement of urbanization rate,the discharge of sewage and the annual production of sludge in China are increasing.Sludge drying is an energy-intensive industry.Sludge drying by heat pumps can not only achieve rapid reduction of sludge,but also save energy and realize efficient use of low-grade energy such as air and sewage,which can contribute to achieving carbon neutralization in sewage plants.Therefore,it is of great significance to carry out research on heat pump sludge drying systems.In order to adapt to different operating conditions and make full use of various low-grade waste heat,this paper designed a dual-source(air-source and wastewater-source)heat pump sludge drying system,which had four operation modes: air-source heat pump closed operation mode,air-source heat pump open operation mode,wastewater-source heat pump closed operation mode and wastewater-source heat pump open operation mode.Through theoretical calculations,the components of the dual-source heat pump system were reasonably designed and selected.The low-temperature drying characteristics of modified sludge with different ratios,the effect of drying temperature and wind speed on the drying rate of modified sludge were investigated using a low-temperature drying experimental bench.It was found that the drying time of the modified sludge decreased with the increase of the component addition ratio,and the average and maximum drying rates of the modified sludge(with both Ca O and wood residue)increased by 14.23% and 25.71%,respectively,compared with the average and maximum drying rates of the original sludge.The increase of drying temperature and air speed also greatly reduced the drying time of sludge,and the effect of drying temperature on sludge drying time was found to be greater than that of drying air speed by two-factor ANOVA.By fitting and analyzing the experimental data,a model for drying modified sludge at low temperature was constructed to accurately predict the variation of moisture content of modified sludge with time.Based on the assumption of steady-state operation of the heat pump system,a steady-state model of each component of the dual-source heat pump sludge drying system was established.According to the data provided by the manufacturer,the compressor steady-state model was constructed by polynomial fitting;most of the existing studies used the dichotomous method to model the condenser and evaporator,which had problems such as too many assumptions,troublesome operation,and only one true value that satisfies the judgment conditions,etc.This paper innovatively proposed an iterative algorithm to set the iterative interval for the input variables,and found the operating condition range that met the heat pump through multiple nesting of for cycles.The simulation results were more accurate and reasonable.Through the process simulation of the dual-source heat pump sludge drying system,the performance parameters of the system under different operation modes were compared and analyzed to find out the reasonable operation range of the system taking into account the drying rate and drying energy consumption.In the closed operation mode,the SMER value of the air source heat pump increased with the increase of the set drying temperature,and the system COP decreased gradually.Under the same set drying temperature condition,the COP of the wastewater source heat pump increased with the increase of the effluent inlet temperature,and the SMER value decreased with the increase of the effluent inlet temperature;under the same effluent inlet temperature condition,the SMER value of the wastewater source heat pump increased with the increase of the set drying temperature,and the system COP decreased gradually.In the open mode of operation,it was difficult for both the wastewater source heat pump and the air source heat pump to reach the set air temperature of more than55°C,and it required a large evaporator heat exchange area and high fan(or wastewater pump)energy consumption,resulting in a low SMER value of the system,so the system was not selected to operate in the open mode as much as possible.A comparative analysis of the two closed modes of operation showed that the SMER value of the sewage source heat pump at higher temperatures(greater than 60°C)was greater than that of the air source heat pump.Therefore,the system tries to choose the closed mode of operation of the sewage source heat pump when the sewage temperature exceeds 12°C.