Performance of cooling capacity adjustment in gas engine-driven heat pump

Abstract: Gas engine driven heat pump (GEHP), which mainly consists of a gas engine, an evaporator, a condenser and an expansion valve, can make full use of the waste heat from cylinder jacket and exhaust gas and achieve a higher primary energy ratio (PER) than other forms of heating/cooling systems, and therefore has been considered as a preferable choice in the air-conditioning scheme. Compared with the electric-driven heat pump (EHP), the GEHP has two distinguished advantages: 1) the ability to recover the gas engine waste heat from cylinder jacket and exhaust gas; 2) easy modulation of gas engine speed to meet the cooling loads. In the present article, a novel GEHP which could independently provide heating, cooling and hot water for the buildings was presented. The capacity adjustment and stable operation of GEHP could be achieved by controlling engine rotary speed. The goals of engine rotary speed control were to match the rotary speed and cooling/heating capacity, and keep robust to disturbance. In order to control engine rotary speed effectively, the engine rotary speed expert proportion-integration-differentiation (PID) controller was designed according to the engine rotary speed control knowledge base and the controlling rules in this study. Meanwhile, the energy analysis of GEHP was presented as well as the GEHP operating parameters (such as ambient air temperature, evaporator water flow and engine rotary speed). The engine rotary expert PID controller was applied to the engine rotary speed control in a GEHP system experimentally under different conditions (modulation on cooling loads and anti-disturbance), and the cooling performance characteristics of GEHP were investigated experimentally over a wide range of engine rotary speed (1 400-2 200 r/min). The performance of GEHP was characterized by cooling capacity, waste heat amount recovered, coefficient of performance (COP) and PER. The relationships between engine rotary speed and cooling capacity, waste heat amount recovered from cylinder jacket and exhaust gas, COP and PER were studied based on theoretical analysis and experimental data. The experimental results indicate that the expert PID controller of engine rotary speed plays a better performance, i.e. small overshoots, flat output and the time before steady state being less than 20 s. The expert PID controller can effectively suppress the disturbance of superheat change, and improve the control quality observably. Furthermore, the cooling capacity, gas engine energy consumption, waste heat amount recovered from cylinder jacket and exhaust gas have increased by 32.8% and 58.4%, 50.1% and 82.3%, 39.1% and 71.1% respectively when the engine rotary speed increased from 1 400 to 1 800 and 2 200 r/min, but the COP and the PER of GEHP has decreased with the increase of gas engine speeds. Finally, the PER of GEHP is 1.23-1.66 and the COP is 2.77-4.25 under experimental condition. Therefore, the GEHP system can significantly improve energy efficiency, environmental quality and energy consumption structure effectively, and should be promoted by governmental incentive policies. The research results will contribute to the study on the engine rotary speed control of GEHP and provide the theoretical basis for the design and optimization of the GEHP.