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Calculation and Analysis of Gas Engine Heat Pump Heating Process
1 Introduction Gas engine-driven heat pump (Gas Engine-Driven Heat Pump, hereinafter referred to as gas engine heat pump) has been widely used in Japan, the United States and Europe and other countries, but in our country, such heat pumps have not yet begun to promote the application. With the smooth progress of the West-East natural gas transmission project and the worsening power peak-valley difference, the application of natural gas as a gas engine heat pump for the refrigeration and air-conditioning equipment has begun to be emphasized. As the gas engine heat pump winter heating introduced natural gas engine cylinder liner and exhaust heat, therefore, in the heating mode gas engine heat pump and ordinary electric drive heat pump are quite different. In this paper, an experiment study is made on the rule of heat generation of a natural gas engine, and the steady-state calculation model of the gas engine heat pump system is established by using the principle of energy conservation. Through the calculation and analysis of the characteristics of gas turbine heat pump heating in winter and operating energy consumption. 2 gas engine heat pump heating process calculation model discussed in this article gas engine heat pump principle and system cycle shown in Figure 1, the gas engine heat pump air-water heat pump unit for heating and cooling cycles. When the heating cycle, the three-way valve 1 and 2 are switched to the location of waste heat recovery, hot water absorption plate heat exchanger after the heat continues to absorb the engine heat, and then the heat delivered to the hot user. As the residual heat of the gas turbine heat pump heat greater impact, this article focuses on gas turbine heat pump heating cycle. Gas engine heat pump cycle process diagram 2.1 engine waste heat calculation model As the engine work process is more complex, it is difficult to deduce the engine's waste heat calculation model with purely mathematical relationship. Some literatures [1, 2] provide the calculation formulas of engine residual heat. However, these formulas are not universal enough and are suitable for only one type of engine. In this paper, the practical method of gas turbine heat pump system to obtain the required engine waste heat data. By measuring the physical quantity, you can indirectly calculate the engine's residual heat. Under steady-state conditions, the working state of the engine is related to the engine speed and torque. When the engine speed and torque is constant, the engine's working condition is determined. Experiment, the engine is stable at a certain working condition, and then measured the engine cooling water flow, cooling water inlet and outlet temperature, exhaust gas temperature, natural gas flow, excess air coefficient and other data. Based on these data can be calculated engine waste heat data. Figure 2 to 5 for the experimental data after conversion results. From the experimental results, it can be seen that the residual heat, the exhaust flow and the exhaust temperature of the cylinder of the engine increase with the increase of the rotation speed and the torque. As the engine cooling water circulation pump driven by the engine, so the flow of cooling water is only related with the speed. The above results by surface interpolation and linear fitting method can be obtained under other non-test state of physical quantities, resulting in engine waste heat calculation required data. On the basis of the thermal calculation data of the engine, a heat balance relationship can be established for the heat recovery system of the engine. If the heat loss is not taken into consideration, then for the liner heat recovery device, Qcj should be the cylinder heat recovery device recovery (KW), cpw is the specific heat of water (kJ / kg · ℃), mw is the flow rate of heating hot water (kg / s), tw2 and tw3 are the temperature of the hot water entering and exiting the liner heat recovery unit ℃), Kcj is the heat transfer coefficient (kW / m2 · ℃) of cylinder heat recovery unit, △ Tm ,, cj is the logarithm average temperature difference (℃) of cylinder heat recovery unit, mcj is the flow rate of engine cooling water kg / s), tcj, in, tcj, out is the engine cooling water temperature (° C) entering and leaving the liner heat recovery unit. Similarly, for exhaust gas heat recovery unit, Qeg is the heat recovered by the exhaust gas heat recovery unit (kW), tw3 and tw4 are the heating hot water temperature (° C) of the exhaust gas heat recovery unit, and Keg is the exhaust heat recovery (KW / m2 · ° C), ΔTm, eg is the logarithmic mean temperature difference (° C) of the exhaust heat recovery unit, meg is the flow rate of the exhaust gas (kg / s), and teg, in, teg, Exhaust gas temperature entering and exiting the exhaust heat exchanger (° C). The relationship between the engine power, the speed and the torque is as follows: P is the engine shaft power (kW), M is the torque (N · m), and n is the speed (kr / min). The primary energy consumption of fuel can be calculated from the fuel consumption and the high calorific value of fuel. Qp is the primary energy consumption (kW), mfuel is the natural gas flow rate (kg / s), r is the natural gas high heat value (kJ / kg). 2.2 Heat Pump System Calculation Model The heat pump system mainly considers the energy balance of the compressor, evaporator and condenser. The throttling process is considered as adiabatic process. If you ignore the system of heat loss, then the compressor exhaust heat, condenser heat transfer and hot water heat absorption should be equal, the same compressor cooling capacity, evaporator heat transfer and outdoor air heat release Should also be equal. For compressors, the relationship between the capacity and heat output of the compressor and the condensing temperature, the evaporating temperature and the speed of the compressor can be obtained by fitting the sample data provided by the manufacturer. The following is a compressor based on an open model fitted to the compressor [3] Computation formula In the formula, Qe, Qc respectively for the cooling capacity and heat output (kW), Te, Tc, respectively, the evaporation temperature and condensation temperature ( ℃), Q0 is the compressor cooling capacity in standard conditions (kW). Thus, the input power of the compressor can be obtained considering the power loss of the engine and the compressor during transmission, where η is the transmission efficiency. For the fin tube heat exchanger (evaporator) and plate heat exchanger (condenser) heat transfer process, to meet the above two types, K refers to the heat transfer coefficient of the heat exchanger (kW / m2 · ℃), F The heat transfer area of ​​the heat exchanger (m2), △ Tm heat exchanger logarithmic mean temperature difference (℃), subscripts e and c, respectively, on behalf of the evaporator and condenser. After outdoor finned tube heat exchanger air should meet the type, cpa is the specific heat of air (kJ / kg · ℃), ma is the air flow rate (kg / s), ta, in and ta, Evaporator air temperature (° C). And after the plate heat exchanger should meet the type of hot water, tw1 into the plate heat exchanger hot water temperature (℃). For the heating mode, the heat pump heat supply consists of three parts, one part from the heat pump condenser (plate heat exchanger) of the heat, the other two parts from the engine block and waste heat, respectively, so the gas The total heat supply of the heat pump is ξ, which is the correction of heat pump heating by defrosting at different outdoor temperatures, which can be corrected by the coefficient indicated in [4]. 2.3 Model Solution In the gas turbine heat pump to determine the structural parameters, fan air flow and pump flow is known, the simultaneous establishment of all the equations can be solved on the model. There are two solutions to this model: (1) Given the speed of the engine, the gas turbine heating capacity at that speed can be obtained; (2) Given the heating capacity required by the system, the required supply Thermal capacity of the gas engine heat pump speed. Then you can get gas turbine heat pump energy consumption, heat and other quantities. As fan pump energy consumption accounted for the proportion of system energy consumption is relatively small, and fixed, so the calculation does not involve this part of the energy consumption. 3 gas engine heat pump heating process calculation and analysis 3.1 heating season heat load calculation The purpose of this paper is mainly on the gas engine heat pump in the heating season work performance calculation and analysis, it must first determine the building's heat load. For the convenience of calculation, assuming that the thermal load of a building is only caused by the temperature difference between indoor and outdoor, the building can be calculated at any outdoor temperature tout given the known indoor design temperature tn and outdoor design temperature tout, d and design load Qd Heat load Qx 3.2 Gas engine heat pump heating process One of the outstanding advantages of the gas engine heat pump is that the gas engine has good speed performance, that is, when the load changes, you can adjust the gas supply to regulate the gas engine heat pump Speed, in order to achieve partial load of gas turbine heat pump heating capacity adjustment. It should be pointed out that the gas engine heat pump speed regulation is limited and can not be arbitrarily adjusted, which is related to the operating characteristics of the engine and compressor. Experimental results of the engine speed - torque curve, that is, the engine's external characteristics curve, and calculate the compressor speed - torque curve. The solid line in the figure shows the maximum torque that the engine can provide at different speeds, while the dotted line shows the torque that the compressor needs to work normally when the load changes. It can be seen from the figure that when the engine speed is lower than 1200r / min, the maximum torque of the engine is not enough to provide the required torque of the compressor, and the gas heat pump can not work normally at this moment. Therefore, in practice, there is a minimum stable engine speed, gas engine heat pump through the speed of energy regulation, the speed can not be less than the minimum stable speed. According to this situation, in order to ensure the stability of the unit, the minimum steady speed in this paper is taken as 1400r / min. When the speed is lower than this speed, the compressor cylinder is used to adjust the energy of the compressor. Different outdoor temperature gas engine heat pump heat load, heat supply, engine exhaust heat, the system of a change in the relationship between energy consumption. Figure 8 shows the variation law of primary energy utilization rate, where the primary energy utilization rate PER is defined as the ratio of the actual system heat gain to the system primary energy consumption (19) where min (Qx, Qtotal) When the heat is greater than the heat load, the heat load is taken as the actually obtained heat, and when the heat supply is less than the heat load, the system heat supply is taken as the actual heat gain. It can be seen from Fig. 7 and Fig. 8 that (1) the heat of gas engine heat pump accounts for a considerable share of the heat (about 1/3 of the total heat supply). Therefore, even in the case of very low temperatures, the gas engine heat pump can still meet the heating requirements, there will be no lack of heat. The amount of waste heat with the engine load and working conditions are closely related. (2) Due to the minimum steady speed of the engine, gas heat pump can not match the heat supply and heat load when the outdoor temperature is higher than 2 ℃. At this time, the engine has been unable to adjust the system through the speed of heat. The calculation shows that when the heat load rate is high, the gas engine heat pump has good load regulation characteristics, the heat supply and heat load can be well matched, and the system has higher primary utilization rate. However, with the decrease of the load rate, the engine speed is limited to the minimum speed operation, a part of the system heat is wasted, and once utilization rate is reduced. (3) In the low load rate, the use of compressor discharge cylinder approach to energy regulation on the gas engine heat pump energy consumption improvement is not obvious. From the calculation results of this paper, after adopting the energy regulation mode of the compressor unloading cylinder, to a certain extent, the trend of energy consumption rise is overcome. Further analysis shows that the reason why the energy consumption can not be reduced is still that the rotation speed is stabilized at the lowest rotation speed. Therefore, the partial load of the lower case, the gas engine heat pump primary energy utilization is low. In order to make the unit save energy under low load, it is still very necessary to adopt the start / stop control mode for energy regulation. 3.3 Gas engine heat pump heating seasonal energy consumption and operating cost analysis and comparison If you consider the accumulation of air temperature at each temperature of the cumulative time, you can calculate the gas heat pump heat pump operation in winter. According to Shanghai outdoor air temperature during the operation of the frequency table [5] calculated gas turbine heat pump at various temperatures in winter operating energy consumption. It can be seen that the higher energy consumption points are concentrated in the outdoor temperature of 5 ~ 10 ℃ area, which is because the operating temperature of the air conditioning a lot of hours. The energy consumption at each temperature value together, get the entire heating season gas engine heat pump energy consumption. Another purpose of gas turbine heat pump energy consumption calculation is to investigate the economy of gas turbine heat pump operation. Factors affecting gas turbine heat pump operating costs are not only the region's climatic conditions, more importantly, the local energy price factors. At present, under the condition of energy price in Shanghai, the running cost of the electric-driven heat pump with gas-turbine heat pump and frequency conversion adjustment mode is comparatively analyzed. It can be seen from the comparison results that the operation cost of the gas engine heat pump is obviously lower than that of the electric heat pump, and the gas engine heat pump is slightly inferior to the electric heat pump in the outdoor temperature, mainly because the gas engine used in this article Part of the lower load adjustment performance due to poor. For the entire heating season, gas-fired heat pump operating costs than the electric heat pump operating costs savings of 16.8%. 4 Conclusions (1) Through the natural gas engine waste heat experiment and the calculation and analysis of the whole system, it can be seen that the engine waste heat accounts for a considerable share of the heat supplied by the gas engine heat pump system. Engine waste heat makes the gas engine heat pump at low temperatures have a good heating capacity, and waste heat on the system heating performance have a greater impact. (2) Although the gas engine has a governor mechanism, to maintain the stability of the engine, the engine speed can not be too low. This feature limits the partial load performance of the gas engine heat pump to play, to obtain good part load characteristics, the engine needs to have a lower minimum steady speed. For the lowest speed gas turbine heat pump, to make the unit energy saving, compressor start / stop control mode is still very necessary. (3) The calculation shows that the operating economy of the gas engine heat pump varies under different outdoor conditions. Generally speaking, the gas engine heat pump still has better performance than the electric heat pump under the climate and energy price conditions in Shanghai The economy.