Application of High Temperature Heat Pump System in Industry

The application of high temperature heat pump system in the industrial field has the following meanings:

First of all, a large amount of industrial waste heat solves the problem of low temperature heat sources (see Table 2), which allows the heat pump to make full use of the industrial waste heat harder than other methods and much higher efficiency than other heat supply systems Cycle efficiency is generally around 3) to provide industrial heat required. From the ASHRAE survey results of the United States and Canada, the prospect of this idea is optimistic [4]; secondly, the industrial waste heat can be discharged at a lower temperature by the heat pump, reducing the thermal pollution to the environment; Thirdly, the application of heat pump heating also has a positive effect on reducing the greenhouse effect. Compared with coal-fired boilers, carbon dioxide emissions can be reduced by 30-50% by saving fuel [5]. At present, the condensing temperature is below 50 â„ƒ at room temperature heat pump has been more mature, its end cooling device is mainly fan coil; condensing temperature of 50 ~ 65 â„ƒ intermediate temperature heat pump cycle refrigerant is mostly R22, by condensing pressure Limited, its water supply temperature should be below 55 â„ƒ, so it is necessary to use special end cooling type (such as: fan coil or floor radiant heating); Condensation temperature of 80 ~ 100 â„ƒ high temperature heat pump can increase the water supply temperature to 70 â„ƒ Or above 80 â„ƒ, which basically meets the design standard of northern heating. Therefore, the end cooling device of the high-temperature heat pump can be a low-cost cast iron radiator. Therefore, the heat pump can be used as a heating source for ordinary buildings and can also be used in old buildings Transformation (instead of coal-fired boiler), in fact, high-temperature heat pump can not only be used as heating source, but also can be used for a variety of other industries, such as drying.
As the high temperature heat pump has good application prospects, making it a recent international heat pump research a basic direction. High-temperature heat pumps are one of the key research topics in Japan's super heat pump project [6], the IEA heat pump center in the United States and the IIR heat pump development plan [7] and the European large-scale heat pump research project.
In summary, high-temperature heat pump has obvious economic and social benefits, the market has great potential, therefore, to how to improve its efficiency and ensure the normal operation of various research projects for the purpose of great significance.
2 working fluid research progress Talking about the high temperature heat pump system, the first will have to face the selection of circulating fluid in the heat pump system. Only select the appropriate circulating refrigerant, heat pump system to meet the set conditions.
At present, there are more than 5 million kinds of organic matter identified and more than 50,000 kinds of inorganic matter. However, there are not many suitable ones for circulating high temperature heat pump fluids. The main factors to be considered when choosing a working fluid are as follows:
(1) Appropriate condensing pressure should be below 2.4MPa (compressor pressure capacity is generally 2.5MPa);
(2) Appropriate evaporation pressure, should be above 0.1MPa, in order to avoid negative pressure in the system;
(3) the appropriate volume of heat, should be 2.5J / cm3 or more, so as not to over-size compressor, resulting in increased equipment costs;
(4) As other common requirements of the circulating working medium, such as non-toxic, less harmful to the environment, stable chemical properties, excellent thermal properties;
(5) In the premise of meeting the above requirements, the pursuit of higher cycle performance coefficient.
With Japan as the leader, many scholars in the world have done more researches on non-azeotropic mixed refrigerants in high-temperature heat pump systems. For example, Akio Miyara made an experimental study with R22 / R114 as the working medium in l993 (condenser inlet temperature 40 Â° C, outlet temperature 60 Â° C; evaporator inlet water temperature 30 Â° C, outlet 10 Â° C); Kazo Nakatani et al. R22 / 134a, R22 / R152a, R22 / R142b and R22 / R123 working fluid, the condensing temperature of 70 â„ƒ heat pump performance was tested [9]; Japan's super heat pump developed in the highly efficient heat pump, using R123 / R134a In addition, Ieon Liebenberg and others use R22 / R142b as working fluid for hot water heat pumps to provide hot water at 60 Â° C [11]; Vance Payne et al. Tested the heating and cooling performance of heat pumps using R32 / R290, R32 / R152a and R290 / R600a as working fluids [12]; Sukumar Devotta [13] equaled the 1994 proposal for HFCs and HFEs Conducted a theoretical cycle analysis. Calculated condensing temperature range of 80-120 â„ƒ, all materials are specified for the cycle conditions of no subcooling, no overheating, no pressure drop, adiabatic compression, temperature rise 40 â„ƒ. The results show that the COP of R143 and R134 is relatively high. The above results are mainly from the thermodynamic cycle performance of non-azeotropic mixtures.
Domestically, there are mainly four research institutes (Tsinghua University, Tianjin University, Shanghai Jiaotong University and Guangzhou Institute of Energy, Chinese Academy of Sciences) on the high temperature heat pump. Tsinghua University [14] has declared the patent for R124 / R142b mixed working medium. The condensing temperature of a given high-temperature water source heat pump is 90 Â° C. Shanghai Jiaotong University, the use of mixed refrigerant R22 / R141b condensate from 70 â„ƒ to 80 â„ƒ, and the relationship between the compressor frequency and COP conducted a preliminary study [15]. Tianjin University since the late 80s of last century for a series of high-temperature heat pump to carry out a series of research work, including a fixed temperature rise of 40 â„ƒ, condensation temperature of 60-80 â„ƒ, superheat of about 10 â„ƒ, undercooling of 5 R142b, R227ea / R600, R22 / R142b and R124 / R141b were studied theoretically and experimentally on the heat pump performance [16]. By the combination of R22 / R142b / R21 and R290 / R600a / R123, The experimental results of the heat pump outlet temperature of 85 Â° C and the COP of more than 3 were obtained [17]. Recently, the condensing temperature was raised to 95 Â° C to provide 90 Â° C condensate [18];
The above research has made a positive exploration in the working conditions of high temperature heat pump. Generally speaking, however, there are not many related literatures, and most of them are in simple thermodynamic cycle analysis and preliminary experimental verification of non-azeotropic mixed refrigerants, which is not enough to provide sufficient theoretical guarantee for the actual operation of high-temperature heat pump systems.
3 system research advances In order to promote the application of high temperature heat pump, the researchers are optimizing the cycle itself. The advantages and disadvantages of the system cycle are mainly attributed to three aspects: (1) the thermal properties of the working fluid on the circulatory system; (2) the system's own matching performance; (3) the system's control strategy.
In heat pump systems, the issue of heat transfer enhancement in evaporators is as important as the problem of intensification in condensers. The difference is that in the evaporator working fluid boiling heat exchanger. The commonly used method is based on reductionism. Mathematical methods are linear. Boiling systems are non-equilibrium, non-linear, random, complex and non-reductive. They must be based on holistic system theory Linear mathematical tools to study. For this reason, the Department of Mechanics of Tsinghua University introduced the theory of bifurcation and catastrophe in chaotic mathematics into the boiling system in order to make a breakthrough in the boiling mechanism [19]. In view of the phase change heat transfer of working fluid in the evaporator, Kedzierski and Bryant carefully studied the influence of the heat transfer surface angle on the heat transfer in the evaporator [20]. Bivens studied the evaporation process of the mixed refrigerant R32 / R125 / R134a in the evaporator [21]. Taking G. Venkatarathnam [22,23,24] as the main research object, the phase change heat transfer characteristics of non-azeotropic mixtures were studied in detail. The conditions for the existence of heat exchange narrow spots in evaporator and condenser and The possibility and how to avoid the occurrence of heat exchange narrow spots, which laid the necessary theoretical foundation for the application of non-azeotropic mixed working fluids in practical engineering.
In addition, the Institute of Engineering Thermophysics of the Chinese Academy of Sciences has carried out theoretical and experimental studies on the evaporation heat transfer in capillary tubes in refrigeration systems [25]. Shanghai Jiaotong University has also made corresponding theoretical studies on the universal integral model of capillary tubes [26] ]. Tianjin University also made a number of interesting attempts at the frequency conversion characteristics of high temperature heat pump compressors [27], the effect of working fluid leakage on system circulation [28], and the intelligent regulation of heat pump system load [29].
Pure refrigerant multistage compression heat pump systems and installations are mainly two-stage or three-stage compressors, which use multistage centrifugal or screw refrigeration compressors [30] for district heating or industrial process heating. The working process of this kind of heat pump is similar to that of the two-stage compression refrigeration system, but generally uses multiple condensers with the same number of stages of compression to stepwise heat-up the heat-transfer medium, and its working temperature range and working fluid used and refrigeration Different systems, technology is mature, has been in practical application of heating engineering. Absorption-compression combined heat pump systems and devices combine absorption and compression systems that employ fluids that differ greatly in boiling point as working fluid pairs and utilize their solution's absorption of lower-boiling fluids to improve circulation of certain However, studies have shown that such cycles can greatly improve the performance of absorption or vapor compression heat pumps, with great potential for development [31].
Intelligent control method applied to refrigeration, heat pump unit is only a matter of recent years, Japan is stepping up the development of small variable capacity steam compression heat pump air conditioning system control technology, and the composition of the key components of air conditioning system: inverter compressor, electronic expansion valve, Heat exchanger fan and related technologies were analyzed and commented [32].
Many domestic scholars have carried out more intelligent control research on air conditioners, Xi'an Jiaotong University conducted research on fuzzy control theory for small air conditioners [33]; Southeast University used computer simulation technology to conduct heat transfer characteristics of air conditioning heat exchangers The study [34], Shanghai Jiao Tong University also conducted a large number of basic research on intelligent control technology [35], mainly focused on artificial neural network system components of refrigeration equipment identification, has been very good conclusions, but the practical application of more less.
It can be predicted from the above analysis that the intelligent control method applied to heat pump units is a trend. Its application will make the system run more smoothly, reduce energy consumption and improve human comfort. Reason to believe that the use of intelligent control technology, high-temperature heat pump units, will be more market competitiveness.
4 Domestic Application Introduction After more than ten years of research and development, several high-temperature heat pump systems have been put into practical use in Hebei, Shandong and Tianjin. The following will briefly introduce the characteristics of each system and operating parameters.
(1) A company in Hebei newly built an office building (1800m2) and a production workshop (800m2), and a bungalow building such as a guard and heating area of â€‹â€‹about 2,700m2. Different requirements for office buildings and workshops, the supply of water temperature is also different.
The original conditions and requirements:
a. There are two shallow wells (1 eye, 1 eye reperfusion), water temperature 15 â„ƒ, as a low temperature heat source (cold source);
b. Office system terminal fan coil, winter hot air heating, cooling air conditioning in summer;
c. Production workshop terminal cast iron radiator, winter hot water heating, summer does not require cooling.
Figure 1 high-temperature heat pump system

Design:
Two separate systems are used, one for heating air at room temperature, one for air conditioning in summer, and one for high temperature heat pump systems for hot water heating in the workshop. Among them, the high-temperature heat pump system compressor electric power of 30 horses, 15 â„ƒ water for low temperature heat source, the production of 70 â„ƒ hot water for workshop heating. The system uses mixed working fluid.
The project was completed in early 2004, high temperature heat pump system, the average COP of 3.17, the drainage temperature of about 11 â„ƒ, hot water 70 â„ƒ, to meet the design requirements.
(2) The unit of a unit in Shandong Province was originally boiler heating, in order to save fuel, instead of heating by one geothermal well, the indoor radiator does not move, requiring heating inlet water temperature not lower than 70 â„ƒ, but geothermal well water temperature is only 51 â„ƒ , So install a heat pump temperature.
The original conditions and requirements:
a.10000m2 residential building original indoor radiator unchanged;
b. heating system inlet temperature not lower than 70 â„ƒ;
c. Geothermal well water temperature 51 â„ƒ, 50T / h, depth of 1500m.
Design:
As only the winter heating is required, the summer is not cooling, the system is simple, as long as the heating load can be satisfied. Decided to use six electric power 50 reciprocating presses running in parallel, the use of mixed working fluid. System COP average of 3.3, water temperature 70 â„ƒ, drainage temperature 40 ~ 44 â„ƒ.
The project was completed in early 2002, after two heating season operation, high temperature heat pump system COP average of 3.21, the drainage temperature is about 43 â„ƒ, heating water temperature reached 74 â„ƒ.
Figure 2 heat pump heat system diagram
5 Conclusion and future research hot spots Through the above summary analysis, we can find that the current research work focused on high temperature heat pump to explore the appropriate working fluid, in this regard has made some achievements, and engineering applications. The author believes that the current research on the circulating refrigerant has relatively advanced in the study of high temperature heat pump system, and the latter will directly related to the study of high temperature heat pump system can work long-term stability, high temperature heat pump refrigerant can give full play to their own Cycle characteristics and other important issues, so in the next few years, based on a number of previous achievements made by the predecessors need to carry out the following studies, namely for some non-azeotropic mixed refrigerant system for heat transfer matching and variable conditions Regulation and control strategy, which one of the high temperature heat pump in non-azeotropic mixed refrigerant phase change heat transfer there are two important basic issues to be solved.
One is the research on the heat transfer of non-azeotropic mixed refrigerants in the evaporator and condenser of high-temperature heat pump system. This problem was proposed by Indian scholar G. Venkatarathnam and conducted a more detailed study on the working conditions of the heat pump at room temperature. According to the published literature, no one has studied the issue of heat transfer narrow spots for high temperature heat pump working fluid. The narrow spot problem is the unique problem of the non-azeotropic working fluid during the phase transformation. It is due to the non-linearization of the enthalpy change of the working fluid with the temperature. Different non-azeotropic working fluids have different Narrow spot distribution, reasonable selection of narrow spot distribution and weakening of narrow spot effect are very necessary for optimizing the phase transition heat transfer of working fluid. Meanwhile, the distribution of narrow spot can also be used as a kind of evaluation index of preferred working fluid. In fact, when the system operating conditions are basically established, the non-azeotropic refrigerant heat transfer narrow point problem is mainly related to the physical parameters of the working fluid and the heat transfer medium, which is a relatively static problem in the high-temperature heat pump system.
In the presently proposed high temperature heat pump cycle of working medium, most of the non-azeotropic mixed working medium, and most of the working medium in high temperature conditions when the temperature slip is very obvious. Since the enthalpy of the heat transfer fluid (usually water or air) is basically linear with temperature, the change of enthalpy with the temperature of the non-azeotropic phase tends to be obviously nonlinear, which makes it possible Lead to the occurrence of heat transfer narrow spots;
The second is the non-azeotropic mixture of high-temperature heat pump system evaporator and condenser in the incomplete phase change of the lack of theoretical criteria and the corresponding experimental verification, the other non-complete phase transition and high temperature heat pump system, the relationship between the cycle parameters It is not clear (incomplete transformation refers to the process of transition from normal complete phase transition to non-complete phase transition due to the change of flow parameters of heat transfer medium under certain conditions. The appearance of phase transitions leads to a drastic deterioration of the system cycle.36 The non-azeotropic working fluid phase transformation is not only related to the physical properties of the working fluid and the heat transfer medium, but also to the regulation of the system operation, External system of some kind of excitation parameters reflect, so it is relatively high-temperature heat pump system in the dynamic issues.
Under the premise of the high temperature heat pump system working normally (at this time, the non-azeotropic working fluid is in complete phase change state), the temperature of the heat exchange medium at the inlet of the evaporator or the condenser is maintained to increase the heat exchange medium flow rate; or the evaporator or the condenser The inlet heat exchanger temperature and flow, and the working fluid vapor pressure, reduce the compressor frequency. The above two common control methods are likely to lead to non-azeotropic refrigerant in the heat exchanger in the incomplete phase transition, resulting in deterioration of the cycle (COP significantly reduced). To study the mechanism of non-azeotropic refrigerant phase transformation can clarify the preconditions of this phenomenon, delineate the range of working conditions so as to avoid entering the non-complete phase transition zone in the actual operation of the system and make the system safe, stable and efficient Run
Because the operation regulation strategy of high-temperature heat pump system can be developed through the experimental results of non-azeotropic working fluid with incomplete phase transition, the fluid flow rate, temperature, Continuous measurement of parameters such as temperature slip of working fluid, phase transition pressure and pressure drop in the heat exchanger can obtain the theoretical discriminant of incomplete phase transition, so that the mathematical model can be used to express the high temperature heat pump system to avoid entering incomplete Control strategy of phase change zone.