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Keywords: gas turbine air-cooled heat pump waste heat utilization primary energy utilization 1 Introduction
With the smooth progress of the major project "West-East Gas Transmission Project", the application of natural gas has drawn more and more 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; 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 to use waste heat
In general, gas engines convert only about 30% of the fuel's energy into mechanical work [1], while the remaining 70% of the energy is released into the atmosphere as waste heat. 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
This gas engine heat pump outdoor unit and the indoor unit for the two connecting pipe, the so-called two-tube gas turbine 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 gas heat recovery 10. Cylinder heat recovery
Figure 1 two-pipe gas-fired heat pump system processes and major equipment
When the heat pump heats up, the cooling water indirectly recovers waste heat from the engine block through the cylinder heat recovery unit 10 and then passes through the exhaust heat recovery unit 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 heat pump heating logP-h chart
In the figure, the loop 1-2-3-4-1 is the circulation process of the gas heat pump, and the loop 1'-2'-3-4'-1 'is the circulation of the conventional electric heat pump. 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-and-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
Figure 3 shows the typical flow of the waste heat heat supply system.
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 gas heat recovery unit 10. Cylinder heat recovery unit
Figure 3 four-pipe gas-fired heat pump system processes and major equipment
This gas engine heat pump outdoor unit and the indoor unit is connected to four tubes, so called four-tube gas engine 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.
Figure 4 four-pipe gas heat pump heating logP-h chart
Exhaust heat utilization of gas engine heat pump In addition to the above two more typical ways, there are a variety of other ways: If some gas engine heat pump design will exhaust heat directly to the compressor outlet refrigerant vapor to improve the state of the compressor outlet steam Point to increase the amount of heat supplied by the system. In some cases, the gas heat pump exchanges the plate heat exchanger in the mode (1) with the position of the outdoor side fin tube heat exchanger so as to make the refrigerant exchange heat in the room Outlet to achieve a greater degree of sub-cooling, increase heat supply, and then through the plate heat exchanger to re-heat the refrigerant; 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.
Analysis and Comparison of Two Typical Air-cooled Gas-fired Heat Pumps
These two typical gas engine heat pump in practice have a wide range of applications. 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-tube gas turbine heat pump needs to be sent to the indoor 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 where 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 fan start-stop mode of evaporation pressure changes
One prominent feature of air-cooled heat pumps is the need to defrost the outdoor evaporator when the heat pump is operating in cooler conditions. 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. For four-tube gas engine heat pump, the system defrost can rely on conventional methods to solve, such as hot gas bypass, electric heating and other methods.
Compared with the conventional electric heat pump, the advantages of the gas engine heat pump are mainly reflected in the primary energy utilization of the system. Therefore, the utilization efficiency of the waste heat of the gas engine heat pump also takes the primary energy utilization rate of the whole system as an index for analysis and comparison. In addition to the important parameter of primary energy utilization, in order to facilitate the calculation and analysis, define the parameters of several other gas heat pumps:
(1) The mechanical efficiency of the engine ηe refers to the ratio of the engine primary energy into mechanical work, then the total waste heat energy in primary energy is;
(2) Waste Heat Recovery ηr Although 70% of the primary energy is waste heat, not all of these waste heat can be utilized. Waste heat that can be recovered due to flue gas emissions and cylinder cooling to the environment is only part of it , Ηr refers to the recovery of waste heat and primary energy in the total waste heat ratio
(3) Waste heat utilization rate ηu In the actual utilization of waste heat, part of the heat will be lost due to heat dissipation in the pipeline and heat loss during heat exchange. Ηu refers to the ratio of the actual utilized waste heat to the recovered waste heat
Among them, W - mechanical work (kW);
Qp - energy of primary energy (kW);
Qw - total waste heat (kW);
Qr - Recycled waste heat (kW);
Qu - Actual utilization of waste heat (kW)
The following test data provided by a certain open-type compressor in the standard operating conditions [6], were calculated two kinds of gas turbine heat pump waste heat utilization. For the defrost condition, the working status of the heat pump is not stable and it is difficult to calculate the heat. In this case, the utilization of the waste heat in the standard condition (outdoor temperature is 7 ° C) is calculated.
A air-cooled gas heat pump, first according to the summer conditions outside the design of finned tube evaporator (summer for the condenser), the structure parameters are shown in Table 2:
Table 2 some outdoor side fin tube heat exchanger structural parameters evaporator heat transfer coefficient K
30W / m2 · ℃
Evaporator area Ae
63.07m2
The maximum air flow G
2.72kg / s
In the heat pump heating, if the use of two control type, the main purpose of its engine waste heat utilization is to increase the heat pump evaporation temperature, in order to improve the heat pump coefficient of heating performance. According to the structure of the outdoor finned tube heat exchanger in Table 2, the evaporation temperature of the conventional thermal electric pump system can be calculated by the following heat balance equation of the fin tube evaporator
Air-heat balance:
(1)
Heat balance of fin tube heat exchanger:
(2)
According to the compressor test data available cooling capacity performance curve:
(3)
Of which: Qe - heat pump system heat (kW);
cpa - Specific heat of air (kJ / kg · ℃);
t1 - air inlet temperature of fin tube heat exchanger
Degree (℃), refers to the outdoor air temperature;
t2 - outlet temperature on the air side of the fin tube heat exchanger
Degree (℃);
t0 - System evaporation temperature (℃);
tc - system condensation temperature (℃).
When the finned tube heat exchanger performance parameters, heat exchanger inlet air temperature and flow rate, and the compressor performance curve is known, the simultaneous establishment of the above three equations, we can solve the unknowns t0, t2 and Qe. Assuming finned tube heat exchanger in the system cooling and heating heat transfer coefficient K are 30W / m2 · ℃, under standard conditions heat pump inlet air temperature is 7 ℃, the condensing temperature is 50 ℃, the fan to run at maximum air volume, The electric heat pump evaporation temperature is -2.1 ℃, the air temperature is 2.5 ℃, the system of heat of evaporation 12.4kW. In the gas engine heat pump, the new evaporation temperature is higher than the conventional electric heat pump evaporation temperature, and the new evaporation temperature t0 'size must meet the new evaporation temperature finned tube evaporator and plate heat exchanger provided The total heat is equal to the required evaporation heat of the system at the new evaporation temperature.
Two control gas engine heat pump finned tube heat exchanger absorbs heat
(4)
The two control gas engine heat pump plate heat exchanger to absorb heat
(5)
Therefore, the gas engine heat pump finned tube evaporator and plate heat exchanger can provide the total heat
(6)
Figure 6 Two-tube gas engine heat pump at various evaporation temperatures
Heat exchanger can provide heat and system requirements of the evaporation heat
Figure 6 shows the variation of the sum of the heat required by the system for the new evaporation temperature and the sum of the heat provided by the two heat exchangers. As can be seen from the figure, as the evaporating temperature increases, the amount of heat absorbed by the heat pump system from air and waste heat decreases, while the evaporative heat required by the system increases with increasing evaporating temperature. Therefore, in actual operation, the two must reach equilibrium at a certain evaporation temperature, that is, the new evaporation temperature t0 '. Assuming that the mechanical efficiency ηe of the engine is 30%, the waste heat recovery efficiency ηr is 60%, and the waste heat utilization efficiency ηu is 90%, it can be calculated that the equilibrium point of the new evaporating temperature of the gas engine heat pump is 1.2 ° C, which is 3.3 higher than the original evaporating temperature ° C, while above 1.2 ° C, the heat absorbed by the system from air and waste heat through the heat exchanger will not meet the system's evaporative heat requirement at that evaporation temperature. If you continue to increase the evaporation temperature, for the normal operation of the system must be achieved by increasing the evaporation area or increase the fan air volume. Knowing the evaporation temperature of the two-pipe gas engine heat pump and the original condensing temperature, the compressor cooling capacity Q0 and the consumed power P can be detected from the compressor sample data. In this way, the heating cycle of the two control gas engine heat pump primary energy utilization is
(7)
If the four-tube gas turbine heat pump, a primary energy efficiency calculation is relatively simple. Compared with the conventional electric heat pump, the use of waste heat to increase the heat pump heat, the primary utilization rate of
(8)
In the above formula, Qh1 is the heat pump itself. Based on the evaporation temperature of 0 ° C and the condensation temperature of 50 ° C, the refrigeration capacity and power consumption of the compressor are found by the sample data and added together. Qh2 for the use of gas turbine heat of the heat, the available amount of waste heat
(9)
Therefore, the four-control gas engine heat pump primary energy utilization rate
(10)
Analysis of two types of gas turbine heat pump structure type and formula (4 ~ 10) shows that the difference between the two systems mainly in the waste heat utilization, and the size of the primary energy efficiency is also closely related with the thermal efficiency ηu. Calculations show that when all other conditions are the same, the PER values ​​of the two types of gas engine heat pumps are related to their respective ηu.
Figure 7 Both systems are under different heat recovery efficiency areas
Primary energy utilization comparison
Figure 7 shows the comparison of the primary energy utilization rates of the two systems. The white and gray areas in the graph represent PERfour conditions above PERtwo and PERtwo above PERFOUR, respectively. The calculation results in the figure show that PERtwo is higher than PERFOUR only in a small area (gray) (ie ηu, four low, and ηu, two higher). This shows that as long as the four-tube gas heat pump waste heat utilization in the process of heat loss is not large, four-tube gas heat pump primary energy utilization is usually higher than the two-tube gas engine heat pump primary energy efficiency.
4 Conclusion
1. In terms of the unit itself, the two-control gas-fired heat pump is slightly more complicated than the four-control system in terms of structure type, control means and system design. However, the two-control gas heat pump is superior in terms of equipment installation, air conditioning system reforming and system frost performance In four control gas heat pump.
2. The calculation of the effect of waste heat utilization shows that if the utilization efficiency of the waste heat of the four-tube gas heat pump is not very low and the heat pump is used for the frost not very severe climatic conditions, the waste heat utilization efficiency of the four-tube gas heat pump is usually better than that of the two-tube gas heat pump .
Analysis and Comparison of Gas Engine Heat Pump Waste Heat Utilization
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.