Heating calculation and analysis of natural gas engine heat pump unit

1 Introduction
Natural gas engine-driven heat pump units (GasEngine-DrivenHeatPump, hereinafter referred to as gas engine heat pumps) have been widely used in countries such as Japan, the United States, and Europe. However, in my country, this type of heat pump has not yet begun to be widely used. With the smooth progress of the West-East Gas Transmission Project and the increasingly serious power peak-valley difference, the application of gas engine heat pumps that use natural gas as the energy source for refrigeration and air-conditioning equipment has begun to receive attention. Since the gas engine heat pump introduces the waste heat of the cylinder liner and exhaust gas of the natural gas engine during winter heating, there is a big difference between the gas engine heat pump and the ordinary electric-driven heat pump in the heating mode. This paper conducts an experimental study on the waste heat generation law of a natural gas engine, and uses the principle of energy conservation to establish a steady-state calculation model for the gas engine heat pump system. Through calculation, the characteristics of the winter heating process and operating energy consumption of the gas engine heat pump are analyzed.
2 Calculation model of gas engine heat pump heating process
The gas engine heat pump principle and system cycle process discussed in this article are shown in Figure 1. The gas engine heat pump is an air-water heat pump unit and can perform heating and refrigeration cycles. During the heating cycle, both three-way valves 1 and 2 are switched to the waste heat recovery position. After the hot water absorbs the heat from the plate heat exchanger, it continues to absorb the engine waste heat, and then delivers the heat to the heat user. Since the waste heat of the gas engine has a greater impact on the heat supply of the heat pump, this article mainly discusses the heating cycle of the gas engine heat pump.
2.1 Engine waste heat calculation model
Due to the complexity of the engine’s working process, it is difficult to derive the engine’s waste heat calculation model using purely mathematical relationships. Some literature provides calculation formulas for engine waste heat, but these calculation formulas are less versatile and are only suitable for a certain type of engine. This paper uses experimental methods to obtain the engine waste heat data required in the gas engine heat pump system model for practical application. By measuring relevant physical quantities, the waste heat of the engine can be calculated indirectly.
Under steady-state operating conditions, the working state of the engine is related to the engine speed and torque. When the engine speed and torque are constant, the engine’s working state is determined. During the experiment, the engine was stabilized in a certain working state, and then the engine cooling water flow rate, cooling water inlet and outlet temperature, exhaust gas temperature, natural gas flow rate, excess air coefficient and other data were measured. Based on these data, the engine waste heat data can be calculated.
From the experimental results, it can be seen that the waste heat, exhaust gas flow and exhaust temperature of the engine cylinder increase with the increase of speed and torque. Since the engine cooling water circulation pump is driven by the engine, the flow rate of the cooling water is only related to the rotation speed. For the above results, the physical quantities at other non-test state points can be obtained through surface interpolation and linear fitting, thereby obtaining the data required for engine waste heat calculation.
2.2 Heat pump system calculation model
The thermal model of the heat pump system mainly considers the energy balance relationship between the three major components of the compressor, evaporator, and condenser. The throttling process is considered an adiabatic process. If the heat loss of the system is ignored, then the heat rejection of the compressor, the heat exchange of the condenser and the heat absorption of hot water should be equal. Similarly, the cooling capacity of the compressor, the heat exchange of the evaporator and the heat release of the outdoor air should also be equal.
For compressors, the relationship between the compressor’s cooling capacity and heat rejection, condensation temperature, evaporation temperature and compressor speed can be obtained by fitting the sample data provided by the manufacturer. 2.3 Solution of the model
When the structural parameters of the gas engine heat pump are determined and the fan air volume and water pump flow rate are known, the model can be solved by combining all the above equations. There are two ways to solve this model: (1) If the engine speed is given, then the gas engine heat supply capacity at that speed can be obtained; (2) If the heat supply capacity required by the system is given, then the required heat supply capacity can be calculated. Gas engine heat pump speed under thermal capacity. Then correspondingly, other quantities such as gas engine heat pump energy consumption, waste heat, etc. can be obtained. Since the energy consumption of the fan and water pump accounts for a relatively small proportion of the system energy consumption and is fixed, this part of the energy consumption is not involved in the calculation.
3 Calculation and analysis of gas engine heat pump heating process
3.1 Calculation of heat load during heating season
The purpose of this article is mainly to calculate and analyze the performance of the gas engine heat pump throughout the heating season, so the building’s heat load must first be determined. For the convenience of calculation, assuming that the heat load of a building is only caused by the indoor and outdoor temperature difference, then when the indoor design temperature tn and outdoor design temperature tout, d and the design load Qd are known, the building can be calculated at any outdoor temperature tout The heat load Qx
3.2 Characteristics of gas engine heat pump heating process
A prominent advantage of the gas engine heat pump is that the gas engine has good speed regulation performance. That is, when the load changes, the speed of the gas engine heat pump can be adjusted by adjusting the air supply volume of the gas engine to achieve partial load. Regulation of thermal capacity. It should be pointed out that the speed adjustment of the gas engine heat pump is limited and cannot be adjusted arbitrarily, which is related to the working characteristics of the engine and compressor.
The engine speed-torque characteristic curve obtained experimentally, that is, the engine’s external characteristic curve, and the calculated speed-torque characteristic curve of the compressor. The solid line in the figure represents the maximum torque that the engine can provide at different speeds, while the dotted line represents the torque required for normal operation of the compressor 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 no longer sufficient to provide the torque required by the compressor. At this time, the gas engine heat pump cannot work normally. Therefore, in actual use, the engine has a minimum stable speed. When the gas engine heat pump adjusts energy through speed, the speed cannot be lower than this minimum stable speed. According to this situation, in order to ensure the stability of the unit, the minimum stable speed in this calculation is taken as 1400r/min. When the speed is lower than this speed, the compressor unloading method is used to adjust the compressor energy.
The relationship between the gas engine heat pump heat load, heat supply, engine waste heat, and system primary energy consumption under different outdoor temperatures. Figure 8 shows the changing pattern of primary energy utilization. The primary energy utilization rate PER is defined as the ratio of the actual heat obtained by the system to the primary energy consumed by the system.
(1) In the heat supply of gas engine heat pump, the waste heat of the engine accounts for a considerable share (about 1/3 of the total heat supply). Therefore, even when the temperature is very low, the gas engine heat pump can still meet the heating requirements and there will be no shortage of heat supply. The amount of waste heat is closely related to the load and working status of the engine.
(2) Due to the minimum stable speed problem of the engine, the gas engine heat pump cannot match the heat supply and heat load when the outdoor temperature is higher than 2°C. At this time, the engine can no longer adjust the heat supply of the system through rotation speed. Calculations show that when the heat load rate is high, the gas engine heat pump has better load regulation characteristics, the heat supply and heat load can be better matched, and the system also has a higher primary energy utilization rate. However, as the load factor decreases, the engine speed is limited to run at the minimum speed, part of the system’s heat supply is wasted, and the primary energy utilization rate decreases.
(3) When the load rate is low, the energy adjustment method of compressor unloading does not significantly improve the energy consumption of the gas engine heat pump. Judging from the calculation results of this article, the energy adjustment method of compressor unloading can overcome the rising trend of energy consumption to a certain extent. Further analysis shows that the reason why energy consumption cannot be reduced is still because the speed is stabilized at the lowest speed. Therefore, when the partial load is low, the primary energy utilization rate of the gas engine heat pump is low. In order to save energy at low loads, it is still necessary to use start/stop control for energy regulation.
3.3 Analysis and comparison of seasonal energy consumption and operating costs of gas engine heat pump heating
If the cumulative time of air conditioning operation at each temperature is considered, the energy consumption of the gas engine heat pump during winter operation can be calculated. The operating energy consumption of the gas engine heat pump at various temperatures in winter is calculated based on the outdoor temperature frequency table [5] during air conditioning operation in Shanghai. It can be seen that the points with higher energy consumption are concentrated in the area where the outdoor temperature is 5 to 10°C. This is because the air conditioner operates for many hours in this temperature area. By adding up the energy consumption values at each temperature, the energy consumption of the gas engine heat pump during the entire heating season can be obtained.
Another purpose of gas engine heat pump energy consumption calculation is to examine the economical efficiency of gas engine heat pump operation. The factors that affect the operating costs of gas engine heat pumps are not only the climatic conditions in the region, but more importantly, the local energy price factors.
Comparative analysis of the operating costs of gas engine heat pumps and electric drive heat pumps with variable frequency regulation under the current energy price conditions in Shanghai. It can be seen from the comparison results that the operating cost of the gas engine heat pump is significantly lower than that of the electric heat pump at low temperatures. When the outdoor temperature is high, the gas engine heat pump is slightly worse than the electric heat pump. This is mainly because the gas engine used in this article is This is caused by poor regulation performance when the partial load is low. For the entire heating season, the operating costs of a gas engine heat pump are 16.8% lower than that of an electric heat pump.
4 Conclusion
(1) Through the natural gas engine waste heat experiment and the calculation analysis of the entire system, it can be seen that the engine waste heat accounts for a considerable share of the heat supply of the gas engine heat pump system. The waste heat of the engine allows the gas engine heat pump to have good heating capabilities at low temperatures, and the waste heat has a great impact on the heating performance of the system.
(2) Although the gas engine has a speed regulating mechanism, to maintain the stability of the engine, the engine speed cannot be too low. This feature limits the part-load performance of the gas engine heat pump. To obtain good part-load characteristics, the engine needs to have a lower minimum stable speed. For gas engine heat pumps with higher minimum speeds, the compressor start/stop control method is still very necessary in order to save energy.
(3) Calculations show that the operating economy of gas engine heat pumps varies under different outdoor conditions. Generally speaking, under the climate and energy price conditions in Shanghai, gas engine heat pumps still have better performance than electric heat pumps. economical