An experimental investigation on a novel WWSHP system with the heat recovery through the evaporation of wastewater using circulating air as a medium

Heat recovery from wastewater is of considerable significance to energy conservation and environmental protection. However, all types of wastewater contain suspended foulant. This leads to fouling on heat exchanger surfaces, resulting in low heat transfer efficiency and blocking heat exchanger. Therefore, the use of wastewater source heat pump (WWSHP) is considerably restricted. In order to avoid the fouling on wastewater heat exchanger (WWHEX) to achieve a better performance, a novel WWSHP system was proposed. The detailed structure and experimental performance evaluation of the operating performances of the novel WWSHP system are described in this paper. The core of this novel WWSHP system was a wastewater tower where circulating air extracted heat from wastewater through the evaporation of water, while the foulant in wastewater would stay. An experimental set-up for the novel WWSHP was established and five experimental cases were organized. In the first four cases, the effects of the variations in key system operating parameters on the operating performances of the novel WWSHP system were experimentally examined. In the fifth case, the overall operating performances of the experimental setup when recovering heat from waste bath water at 29 °C at a preset hot water temperature of 45 °C were experimentally examined. Within the wastewater tower, the main heat transfer process was via latent heat through the evaporation of water, accounting for 72.1% of the total heat transfer exchange. The results of cases 1–4 suggested that with an increase in wastewater temperature, the percentage share of latent heat exchange was also increased. However, the increases in both wastewater flowrate and circulating air flowrate had little effect on the percentage of latent heat exchange in the total heat transfer of the wastewater tower. The novel WWSHP system with the wastewater tower worked well at wastewater temperature of 8 °C, with its COP _unit of 2.97 and a COP _sys of 2.0 for water heating. Increasing wastewater flowrate from 1.29 m ³ /h to 1.77 m ³ /h could help improve the heat transfer rate of the wastewater tower by 4.4% and the COP _sys by 2.5%, respectively. On the other hand, the experimental results for case 5 suggested that at wastewater temperature of 29 °C and a preset hot water temperature of 45 °C, the average COP _unit was 4.99 and the average COP _sys was 3.43, both being higher than those of conventional WWSHPs.