Abstract: This paper introduces two typical ways of engine waste heat utilization when gas-driven air-cooled heat pump is heating in winter, and analyzes the characteristics of two waste heat utilization modes from the aspects of structure type, control method, equipment installation and frost formation. Primary energy utilization as a comparative indicator of the effect of waste heat utilization, analysis and calculation of the effect of two kinds of waste heat utilization. Keywords: gas turbine air-cooled heat pump waste heat utilization primary energy utilization 1 Introduction With the "west to east" major project smoothly, the application of natural gas has been more and more widespread attention. In the total social energy consumption, the energy consumption of the construction industry accounts for a large proportion, and refrigeration and air conditioning equipment is the main building energy consumption. Therefore, the rational allocation of natural gas energy to the construction industry is an important means for the rational utilization of natural gas. Natural gas in the refrigeration and air conditioning industry, the application of the following three ways: (1) gas-driven compression heat pump air-conditioning system; (2) natural gas-fired direct-fired absorption heat pump air conditioning units; (3) other types Combined system or composite air conditioning system. Among them, the gas-driven compression type air-cooled heat pump (hereinafter referred to as gas-fired heat pump) has the advantages of high efficiency, energy saving, a primary energy utilization, low emissions, load easily adjusted, flexible configuration, etc., in some developed countries have been A wide range of applications, while in China, this gas engine heat pump is still in the development stage. In this paper, two kinds of foreign common gas engine heat pump waste heat utilization methods are introduced, and qualitative and quantitative comparative analysis, aimed at the domestic gas engine heat pump design to provide some reference. 2 Foreign gas-fired air-cooled heat pump common two ways of using waste heat Under normal circumstances, the gas engine will only convert about 30% of the fuel energy into mechanical work [1], and the remaining 70% of the energy are waste heat Form discharge into the atmosphere. Therefore, gas turbine heat pump, the heat must be effective use of waste heat. For the common small and medium-sized air-to-air gas heat pump [2-7], there are mainly two ways to utilize the waste heat of the engine: (1) Waste heat supply low-pressure side refrigerant, and (2) Waste heat supply indoor air supply. (1) Waste heat supply Low-pressure side refrigerant The connection of the outdoor unit of this gas engine heat pump and the indoor unit is two, so-called two-pipe gas-fired heat pump. Figure 1 shows the main equipment of a typical process. 1. Engine 2. Compressor 3. Four-way valve 4. Indoor fin tube heat exchanger 5. Expansion valve 6. Outdoor fin tube evaporator 7. Finned tube radiator 8. Plate heat exchanger 9. Exhaust heat Recycler 10. Cylinder heat recovery unit Figure 1 two-pipe gas heat pump system processes and major equipment In the heat pump heat, the cooling water through the cylinder heat recovery device 10 indirect recovery of engine block waste heat, and then through the exhaust heat recovery 9 to absorb waste heat in the exhaust gas. The higher temperature cooling water enters the plate heat exchanger 8. And then back to the block heat recovery device 10 to complete the cooling water cycle. Cooling, the cooling water through the fin tube heat exchanger 7 to heat, to ensure the normal operation of the engine. In this waste heat utilization mode, waste heat is released to the low-temperature-side refrigerant through a plate heat exchanger (or a bushing heat exchanger) 8. This gas heat pump system refrigerant logP-h chart compared with the conventional heat pump system shown in Figure 2. Figure 2 Two-pipe gas-fired heat pump heating logP-h In the figure, the loop 1-2-3-4-1 for the gas heat pump cycle, 1'-2'-3-4'-1 'for the conventional electric heat pump Loop process. Since a portion of the heat of the refrigerant is provided by the heat exchanger 8 during evaporation, the heat load of the outdoor side air-cooled fin tube evaporator is much smaller than that of a conventional heat pump system. In the case that the heat exchange area of ​​the outdoor fin tube heat exchanger is the same as that of the conventional heat pump, the evaporation temperature of the heat pump during heating can be increased, and the heat coefficient of the heat pump has been correspondingly increased. (2) waste heat supply air supply Waste heat supply air supply gas heat pump system typical process shown in Figure 3. 1. Engine 2. Compressor 3. Four-way valve 4. Indoor fin tube heat exchanger 5. Fin tube heat exchanger 6. Expansion valve 7. Outdoor evaporator 8. Finned tube radiator 9. Exhaust heat recovery 10 cylinder heat recovery Figure 3 four-tube gas heat pump system processes and major equipment This gas engine heat pump outdoor unit and the indoor unit is connected to the four tubes, it is called the four-tube gas turbine heat pump. Compared with the first type (1), this method of waste heat utilization has a relatively small impact on the heat pump system itself. Waste heat utilization is also very simple, that is, after the indoor air is heated by the indoor condenser, heat exchanger 5 again through the finned tube to increase the temperature of the air supply to increase the heat pump heating capacity. Relatively speaking, the effect of this waste heat utilization on the heat pump cycle itself is relatively small. In fact, the condensing temperature of the gas heat pump cycle can be reduced when the indoor heat supply is kept constant. In this case, the logP-h diagram of the heat pump cycle is shown in FIG. 4. The gas heat pump cycle is 1-2-3-4-1 and the conventional electric heat pump cycle is 1-2'-3'-4-1. As the heat pump condensation temperature decreases, the heat pump coefficient of performance has also been improved. In addition to the above two more typical ways, there are many other ways: If some gas engine heat pump design will be directly heated exhaust gas compressor outlet Of the refrigerant steam to improve the state of the compressor outlet steam point to increase the system heat; gas heat pump some (1) way the plate heat exchanger and the outdoor side fin tube heat exchanger position For the purpose of making the refrigerant at the outlet of the indoor heat exchanger to achieve a greater degree of subcooling, increase the amount of heat, and then through the plate heat exchanger to re-heat the refrigerant; also some gas heat pump heat only used to heat Outdoor air. The design of these gas heat pump is not very common, energy efficiency is not high, therefore, not here for analysis and discussion. 3 Comparison and Analysis of Two Typical Air-cooled Gas-fired Heat Pumps The above two typical gas-fired heat pump have a wide range of applications in practice. Here, the two typical air-cooled compressed gas heat pump from the structure itself, control, defrost and waste heat utilization results were analyzed and compared. From the structural type speaking, the first (1) gas engine heat pump, that is, two-tube gas engine heat pump, waste heat directly on the heat pump system of circulating refrigerant, outdoor unit structure is relatively complex, and (2) four-tube gas engine Heat pump indoor unit is relatively complex, and four control gas engine heat pump waste heat needs to be sent to the interior, so the waste heat utilization rate is relatively low. One of the most important advantages of a two-pipe gas heat pump is that the indoor unit is exactly the same as a conventional electric heat pump. Therefore, if the original electric heat pump air conditioning system to be modified, the use of gas engine heat pump, indoor unit without modification, saving equipment investment and installation costs. This type of gas heat pump is more advantageous for situations where there is more than one indoor unit (more than one trailer) or the indoor piping is more complicated. From the system control point of view, the disadvantage of two control gas engine heat pump control system is more complicated. As the heat pump heats up, there are two evaporators in the outdoor unit. How are the controls of these two devices controlled so that the heat pump can exert its maximum heating capacity, which requires experimental and theoretical analysis of the unit according to the outdoor climatic conditions . The experiment of [3] pointed out that when the outdoor temperature is lower than a certain value, the heat supply of the system when closing the outdoor fan is higher than that when opening the outdoor fan. The reason for the following qualitative analysis. As shown in Figure 5, waste heat is the only heat source of the heat pump when the outdoor fan is off. Therefore, the evaporation pressure of the system is basically unaffected by the outdoor temperature. The curve of the evaporation pressure with the outdoor temperature is relatively flat on the graph. The evaporation pressure curve must intersect the refrigerant saturation pressure curve at a certain point (A in the figure). When the outdoor temperature is below point A, the system evaporation temperature is higher than the outdoor ambient temperature, at this time, if you turn on the fan, heat will be lost from the outdoor fin tube. Therefore, for two-tube gas engine heat pump, to consider the performance of heat pumps at low temperatures and the corresponding control measures. For the four-tube gas heat pump, compared with the conventional electric heat pump, only one heat exchanger is added near the indoor unit. Therefore, the control method can still use the conventional electric heat pump. Figure 5, the fan start-stop mode of evaporation pressure changes in the system air-cooled heat pump is a prominent feature of the heat pump in the lower temperature environment requires the operation of the outdoor evaporator defrosting. Air-cooled heat pump frost on the system performance is very large, many researchers on the air-cooled heat pump defrosting a lot of research work has taken many ways. In two-tube gas-fired heat pumps, the frost formation of heat pumps has been largely improved due to the fact that the waste heat entails a portion of the heat needed to evaporate the system [3]. Therefore, two control gas engine heat pump is suitable for cold areas.
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