Research on waste heat recovery from gas engine for auxiliary heating: An emerging operation strategy to gas engine-driven heat pumpRecherche sur la récupération de la chaleur résiduelle des moteurs à gaz pour le chauffage d’appoint : une toute nouvelle stratégie d’exploitation de la pompe à chaleur entraînée par un moteur à gaz
Introduction
With the acceleration of urbanization around the world, building energy consumption has become an important part of the world energy consumption. And the energy consumption ratio related to building heating and air conditioning can reach 60% for residential buildings (Li et al., 2015). It is necessary to develop an efficient and clean way for building heating/cooling. Due to the advantages of efficiency, energy-saving and environmental protection, gas engine-driven heat pump (GEHP) has attracted wide attention in recent years. Compared with the electrical heat pump (EHP), the compressor of a GEHP is driven by engine that uses the nature gas to generate mechanical work. In addition, a substantial amount of waste heat is recovered from engine jacket and exhaust gas to enhance system performance (Xu and Yang, 2009) and other filed (Zhang et al., 2014; Sanaye and Asgari, 2013).
The researches on GEHP are mainly focused on the analysis of operation characteristics (Sanaye and Chahartaghi, 2010), waste heat utilization technology (Liu et al., 2018) and intelligent control of the system (Li et al., 2005). By using the methods of simulation analysis and experimental research, Hepbasli et al. (2009), Zhang et al. (2005), Elgendy and Schmidt (2010), Elgendy et al. (2011a, 2011b), Sanaye and Chahartaghi (2010) and Sanaye et al. (2013) analyzed the effects of environmental temperature, condensation/evaporation temperature, water flow rate, as well as engine speed on the thermodynamic parameters of the GEHP system. The influence degree of variation factors on the performance of the system was analyzed basis on the actual environmental conditions. Results showed that the effect of gas engine speed and evaporation temperature on the system performance was more significant than that of condensation temperature and water flow rate. Liu et al. developed an air-source GEHP (ASGEHP) system with recovering waste heat to produce domestic hot water. The performance of the ASGEHP system under different operation modes such as “heating and domestic hot water” (Liu et al., 2017) and “cooling and domestic hot water” (Liu et al., 2018) were analyzed. The system had obvious energy-saving advantages, and the max primary energy efficiency (PER) of the ASGEHP system was 1.45 in summer. However, due to the heat dissipation of gas engine, the condensation temperature is higher, which is disadvantageous for the system operation. Hence, the advantage of ASGEHP system in single cooling mode is not significant. Therefore, Liu et al. applied evaporative cooling technology in GEHP system, and built a GEHP system with evaporative condenser (Liu et al., 2016). Compared with the traditional system, the PER saving of the GEHP with evaporative condenser was 28%.
Xu and Yang (2009) built a heat-pump air conditioning system driven by gas engine to adjust indoor air temperature and humidity. The results indicated that PER of system was higher in winter than in summer. The advantage of energy saying was remarkable with the decrease of environmental temperature in winter. In addition, to relieve the load fluctuation of buildings, the energy storage technology were combined with GEHP system (Zhang et al., 2017), and four different operation strategies were put forward (Zhang et al., 2018). The results indicated that PERs of four typical buildings (residential, hotel, office and school) with energy storage technology were 21.4%, 35.2%, 23% and 26.6% higher than traditional GEHP system, respectively. Besides the operation strategy, the control theory of the GEHP system was focus areas. Due to the time constants of GEHP system is large and changeable, Li et al. (2005) compared the control character of cascade furry control and cascade PI control strategy. Results displayed that fuzzy control strategy had short reaction time and small overshoot. According to the characteristics of nonlinear dynamic and greatly to be affected by load fluctuation, Wang et al. (2018, 2013) structured different speed controllers. The engine speed setting control consumed less than 40 s to make speed steady, and anti-disturbance control make the error of the engine speed less than ±50 rpm.
One of the core issues of GEHP filed is the utilization of waste heat recovery. The waste heat from engine is mainly used in the production of domestic hot water (Elgendy et al., 2010), auxiliary evaporation (Elgendy and Schmidt, 2014), defrosting (Yang et al., 2013) and so on.
The performance characteristics of the GEHP system under the two modes (producing domestic hot water or auxiliary evaporation) were compared (Elgendy and Schmidt, 2014). When the waste heat recovery was used in producing hot water, the maximum PER could reach 1.83. When it was used to evaporate refrigerant, the maximum PER could reach 1.25. The model of GEHP working as water heater was built by Yang et al. (2013), and the defrost time of three methods was performed. Compared with reverse-cycle defrosting and combine waste heat and reverse-cycle defrosting, removing frost only by waste heat recovery could keep the hot water temperature increasing, but the defrosting time were 1.8 and 4.85 times than the others, respectively. To achieve cascade utilization of the waste heat, a compound system combining with compression-type heat pump and an absorption-type heat pump (GECAHP) was analyzed. The results showed that GECAHP system could produce 6% more heating capacity and saved around 5% of primary energy than GEHP. In addition, the existing research also include economic analysis (Sanaye et al., 2010), select of refrigerant (Liu et al., 2005; Wu et al., 2014), and parameter optimization (Sanaye and Chahartaghi, 2010) of GEHP system.
There are many researches on GEHP system, few of them on waste heat recovery to auxiliary heating and improve the heating temperature. Under the same heating temperature, the condensation pressure and temperature can be reduced, and the operating condition of the system can be improved significantly. In this paper, taking the water-source gas engine-driven heat pump (WSGHP) as the research object. A waste heat auxiliary heating operation strategy is proposed, which recovers the waste heat from the gas engine to heat the heating water produced by the condenser. The operation characteristics of WSGHP system with the auxiliary heating mode in winter are quantitatively analyzed, and focuses on the changing trend of heating capacity, gas consumption, waste heat recovery, refrigerant mass flow, condensation pressure and energy efficiency of heat pump system. In addition, the operation parameters of the WSGHP under the two modes is compared, and the CO2 emission of environment are researched.
Section snippets
Experimental procedure and measurement
The WSGHP system is mainly composed of four circulations: prime circulation, heating water circulation, low-temperature heat source circulation and waste heat recovery circulation. The schematic diagram and experimental prototype of the WSGHP system are exhibited in Figs. 1 and 2, respectively. The shaft work of the compressor (6NFCy) is 11.5 kW under design condition (Teva=4 °C, Tcon=38 °C, Ncom=1450 rpm). And the log (p)-h diagram of the actual cycle of heat pump is displayed in Fig. 3. The
Date analysis
All the measured data are analyzed using REFPROP 9.0 and Matlab program (Wang et al., 2018) to estimate the system performance. The enthalpy of each point (point 1–7) during the operation of the system is calculated by the thermodynamic parameters measured by the sensor.
The heating capacity from condenser (Qheating,con) and shaft work of compressor (W) are calculated from the mass flow rate of refrigerants (Mref) and the enthalpy difference. The specific formulas are as follows:
Analysis and results
The WSGHP system recovers waste heat from gas engine to increase the temperature of heating water. When at the same heating temperature or heating capacity, the condensation pressure and temperature can be reduced and the operating efficiency of the system can be improved significantly. From the view of economy and energy saving of the system, it is a favorable choice. The impact of the source water inlet temperature (Tsou,in,water: 13.4–19.2 C), the condenser water inlet temperature (T
Error analysis
The overall error eq of a quantity q indirectly evaluated from the measured values of the variables can be calculated by:
Eq. (12) agrees with Gaussian law of error propagation in the case of linear dependence of the quantity q on.
According to Eq. (12), the overall error of Qheating,con, W, Qhr, Qheating, QGC, R, COP, COPA and PER are
Conclusions
This paper is to explore the operation characteristics of the WSGHP system using the engine waste heat in auxiliary heating mode, and to provide guidance for the design, operation and application of WSGHP system. The influence of Neng, Tsou,in,water and Tcon,in,water on the operation parameters of the system are analyzed and two different operation modes of GEHP system are compared. The specific conclusions are as follows:
- (1)
When the system is actually running, it is beneficial for users to save
Declaration of Competing Interest
None.
Acknowledgment
Funded by Science and Technology Plans of Ministry of Housing and Urban-Rural Development of the People's Republic of China (2018-K1-008) and Training Program for innovative talents of Young and Middle-aged people in Tianjin.
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Both authors contributed equally to this work.