Study On Performance Of A Heat Pump Water Heating System With Cascade Utilization Of Waste Heat

With the development of global economy, the demand for energy has increased dramatically in these years,and the environmental pollution also becomes more and more serious. Thus, the international society attaches great importance to energy conservation and emission reduction.Heat pump is a kind ofenergy conservation technology, which has good economic and social benefits. Many places, such as factories, schools, hotels, require large quantities of fresh hot water and also produce plenty of wastewater, and the average wastewater temperature is normally higher than ambient temperature. If using wastewater as heat source of heat pump, not only the efficiency of heat pump could be improved, but also the waste heat could be recovered.Therefore, this thesis carried out the theoreticalanalysis for wastewater heat pump water heater(WHPWH) system.Firstly, a WHPWH system with cascade utilization of waste heat from wastewater was proposed in this thesis. The proposed system includes a preheating heat exchanger and a water source heat pump. The waste heat of wastewater can be cascaded recovered through preheating the fresh water and being the heat source of the heat pump. A thermodynamic model was developed to evaluate and optimize the performance of the proposed WHPWH. The energetic analysis was carried out to investigate the influence of the temperature of the wastewater at the outlet preheating exchanger on the system COP, COP of the heat pump and heating capacity. The comparison analysis was also performed for the performaces of the WHPWH systems with and without preheating. The results indicate that preheating could improve the system COP.Working fluids in heat pump systems are currently mainly hydrofluorocarbons (HFCs), hydrocarbons (HCs) and inorganics (such as CO2, NH3). However, the HFCs, such as HFC-134a, HFC-143a, HFC-125, havehigh GWP and will be restricted. In order to seek working fluids with both excellent thermodynamic performance and low GWP,more and more research work focus on the new mixtures.The vapor liquid equilibrium (VLE) property is the essential property for the cycle thermodynamic analysis,thusthe VLE properity of HFC-134a/HFO-1234yf/HC-600aternary mixture was measuredat temperatures from 283.15 to 323.15 K using a recirculation apparatus. The uncertainties of the experimental data are estimated at 5 mK,0.5 kPa, and 0.003 for the temperature, pressure, liquid and vapor mole fractions, respectively. The experimental data were compared with the predicted data by the Peng Robinson equation of state with the vdW mixing rule. The results show this ternary mixture exhibits zeotropic behavior.Finally, the HFC-134a/HFO-1234yf/HC-600a ternary mixture(0.248:0.363:0.389, GWP=349) was adopted as working fluid of the proposed WHPWH systemwith cascade utilization of waste heat. The thermodynamic analysis was carried out and the results show that the system performance of this ternay mixture is better than that of HFC-134a.