Research and development of the hottest second typ

2022-08-21
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Research and development of the second type of absorption heat pump

Abstract: This paper introduces the research situation of the second type of absorption heat pump in the past 10 years, and explains the development trend of the second type of absorption heat pump

key words: absorption heat pump, working fluid pair, solar energy, heat transfer, mass transfer

Introduction

the second type of absorption heat pump is abbreviated as AHP Ⅱ, more often abbreviated as AHT (absorption heat converter or ITB industrial warming machine). With the rising price of energy and the increasingly tight supply, AHT technology, which recovers industrial waste heat and utilizes solar energy, has become a research hotspot of effective energy utilization abroad. From 1976 to 1985, more than 50 literatures on AHT were published, and practical benefits began to be achieved in production. Especially in Japan, Western Europe and other countries with energy shortage, they pay more attention to the research, development and application of AHT. Although the one-time investment of AHT is high, its investment cost can be recovered within 2 years, so its economic benefit is very obvious

1 research work of AHT in the past 10 years

AHT mainly describes its technical and economic performance by three indicators, (1) heating coefficient, namely performance coefficient cop; (2) Heating capacity Δ T; (3) Volume of equipment. In order to improve COP and Δ T. To reduce the volume of equipment, many experts have conducted extensive research on the thermodynamic cycle, transmission process mechanism and enhancement, working fluid pairs, system simulation and optimization of AHT. This paper makes a brief review of the literature reports on AHT in the past decade

1.1 thermodynamic analysis

aht is a device that takes waste heat as the power and utilizes the absorption analysis cycle of working fluid pairs to realize waste heat temperature increase and recovery. From the viewpoint of thermodynamics, AHT cycle is a combined cycle process composed of heat engine cycle and heat pump cycle. Thermal engine cycle refers to the concentration of solution and condensation of water vapor, which is composed of regenerator and condenser. Waste heat is discharged to the environment through regenerator and condenser while doing work. This part of work shows that the solution is concentrated. Heat pump cycle refers to the evaporation and absorption of working medium water, which is composed of evaporator and absorber. Under the action of releasing work with the dilution of solution, waste heat flows from evaporator to absorber and is raised to useful temperature

perez blance and other studies show that [3, 4] the copmax of AHT is only related to the temperature of absorption, regeneration, evaporation and condensation, and it is concluded that in order to achieve the highest possible actual cop, it is necessary to increase the concentration of working fluid pairs, reduce the flow rate ratio, improve the absorption, evaporation temperature and the efficiency of solution heat exchanger, and reduce the condensation temperature; To get a higher Δ T. It is necessary to increase the concentration of absorbent and the operating pressure of absorber at the cost of reducing cop. Many experts are also interested in cop Δ T. A lot of work has been done on the relationship between the regeneration temperature, condensation temperature and the efficiency of the solution heat exchanger and other operating conditions []. From the perspective of theory and practice, it is concluded that only by improving the thermodynamic cycle process, making it close to the reversible process, and minimizing the irreversible loss of the system (such as reducing the heat transfer temperature difference, increasing the adiabatic process during absorption, etc.), can better operating performance be obtained

1.2 transfer process mechanism and enhancement

the evaporator, absorber and regenerator in the AHT system all adopt the natural decorative stone experimental method of falling film heat and mass transfer outside the vertical tube gb/t9966.1 (⑼ 966.8) (2) 001, but the falling film heat and mass transfer process of water lithium bromide solution is a relatively complex process. Although a lot of work has been done in this regard [], no practical formula for calculating the heat and mass transfer coefficient has been obtained so far. Based on some assumptions and simplified mathematical models, some literatures use computers to solve the continuity equation, momentum equation, energy equation and definite solution conditions directly; Some literatures have carried out experimental research on the basis of theoretical analysis, and summarized semi empirical and semi theoretical formulas applicable to a certain range. These documents have certain reference value for the design of absorber and regenerator in AHT system

the investment cost of AHT, which is mainly composed of evaporator, absorber, regenerator, condenser, solution heat exchanger and other equipment, depends on the transfer area of these equipment, accounting for about 80% of the total investment. Its operation and regulation are closely related to understanding the mechanism of heat and mass transfer in the above equipment and the influence of changes in operating parameters on heat and mass transfer. According to the mechanism of heat and mass transfer process, some scholars seek to take effective strengthening measures. For example, Kim and others use the liquid level fluctuation caused by fluid instability to continuously destroy the diffusion boundary layer, which greatly enhances the mass transfer rate [15]. Rush et al. Summarized the effect of additives on enhancing absorption [16]. Kashiwagi et al. Analyzed the mechanism of Marangoni effect strengthening absorption heat and mass transfer [17, 18]. These work points out an effective way to further reduce the transmission area and reduce the equipment investment cost

1.3 research on working pair (refrigerant absorbent)

the working pair used by AHT determines its operating conditions and performance, the energy consumption and greenhouse gas emissions of the materials used in the equipment in the use stage, as well as the investment scale to a considerable extent

water as refrigerant and lithium bromide as absorbent are the main working pairs at present. Due to the freezing of water and the crystallization and corrosion of lithium bromide, its service temperature is limited (5-150 ℃). In order to improve its performance and expand its application range, other components are now being considered to be added, such as H2O LiBr ZnCl2, H2O LiBr CaCl2, and new working fluid pairs H2O LiCl, H2O NaOH, etc. [19, 20]. In addition, in order to completely solve the corrosion problem, some studies on organic series working fluid pairs have been carried out []. In Table 1, TFE pyrrolidone (trifluoroethanol pyrrolidone) is a promising one, and an AHT test device using TFE pyrrolidone working fluid pairs has been established []. Although the above working pairs are better than H2O LiBr in some aspects, their comprehensive performance is not as good as H2O LiBr, and they cannot be applied in AHT in the near future [28]. Table 1 organic working medium pair

the drawing of "○" in the table indicates that the two can form the improvement of working medium pair

1.4 absorption cycle

in order to further improve the thermodynamic performance and heating capacity of the cycle, the improved cycle with important application prospects is briefly introduced below

1.4.1 AHT of secondary temperature rise

AHT of secondary temperature rise is referred to as dlht [29], and its working state is shown in Figure 1. It is composed of two-stage AHT. The heat released by the primary circulating absorber A1 is supplied to the secondary circulation, which effectively reduces the maximum stress and maximum deformation of the structure; Then there is the evaporator V2, which was officially certified by Boeing in 2017. The heat between the two can be transferred by heat carrier, or V2 can be placed in the tube of A1. The pilot scale dlht device has been established, and the heating capacity in the test Δ T is nearly 70 ℃, but the corresponding cop drops to about 0.32. The results show that dlht is promising

Figure 1 AHT cycle with secondary temperature rise

1.4.2 self regenerative absorption temperature rise cycle

self regenerative absorption temperature rise cycle is referred to as srata [30], which is referred to as regenerative absorption heat exchange cycle in some literatures and is referred to as Gax [31]. The system operates under the single cycle operating pressure, transferring the heat of the dilute solution discharged from the absorber directly to the regenerator for solution regeneration, thereby increasing the depth of regeneration. As a result, the heating capacity is improved Δ T. But the corresponding cop is also reduced. Figure 2 is the result of computer simulation calculation using TFE-E181 as the working fluid pair, which illustrates the relationship between the output temperature and the waste heat temperature of the conventional AHT and srata cycles. It can be seen from Figure 2 that under the same waste heat temperature, the temperature of srata output heat is significantly higher than that of conventional AHT cycle, so it is more suitable for some industrial needs. Figure 3 shows the implementation scheme of srata. Its design idea is to make the device compact, but a lot of work needs to be done for real application

Figure 2 two areas of cyclic application

condensation temperature: --tc = 25.5 ℃, ---tc = 15.0 ℃

Figure 3 srata implementation scheme

1.4.3 absorption cycle with supercharger

Figure 4 shows the absorption cycle with supercharger [28], which uses supercharger to replace the absorber A2 and evaporator V2 of two-stage absorption cycle (see Figure 1). Its advantage is that it can use the waste heat with lower temperature and improve the heating capacity. Similarly, the absorption compression composite cycle is also under research and development

Figure 4 absorption cycle with supercharging device

1.4.4 double effect AHT

double effect AHT is abbreviated as DEHT [32]. Its COP can reach 0.65, but the corresponding heating capacity is poor. Its working state is shown in Figure 5. Its main feature is to heat regenerator G1 with the heat released by condenser K2, that is, the refrigerant from G2 condenses in G1 pipe. Compared with single effect AHT, it only adds one regenerator and one solution heat exchanger, so the cost of double effect equipment is not much more expensive than single effect

Figure 5 dual effect AHT cycle

1.4.5 cycle combined with two types of absorption heat pumps

cycle combined with two types of absorption heat pumps is the combination of the first and second types of absorption heat pumps, referred to as HPT. Its working state is shown in Figure 6, which is to improve COP and Δ T device. Bokelmann [33] and Scharfe [34] carried out the test of HPT device. It is composed of two-stage AHP and two-stage AHT, and its COP can be as high as 4. In conventional AHT, only one heating unit can be operated, while in HPT, four heating units can be operated. For example, if HPT is installed in the evaporation device of four effect evaporator, the total cop can reach 16. This device is mainly used in the evaporation process of water treatment, food and beverage industry. Compared with other energy-saving evaporation devices such as steam compression and steam jet compression, HPT has the advantage of saving electricity, without considering dust removal and foam catching, and without other technical restrictions

Figure 6 HPT cycle

1.4.6 thermosyphon self circulation aht

reasonably arrange the height of the absorber and regenerator, use the liquid level to establish the pressure difference required for the solution transportation between the absorber and regenerator, and rely on the density difference between the low-temperature high-concentration solution after regeneration and the high-temperature low concentration solution after absorption, so that the solution can circulate automatically between the regenerator and the absorber. In this way, the whole system can not use the circulating pump, which not only saves investment and power consumption, reduces air leakage, but also avoids some troubles caused by pump maintenance. The experimental device of thermosyphon self circulation AHT has been established and has achieved good results [35, 36], which needs to be further popularized and applied

1.4.7 open AHT

the above is mainly the closed AHT. The so-called open AHT is to directly discharge the steam evaporated from the regenerator into the environment, eliminating the condenser in the closed AHT. In addition, the evaporator in the closed AHT is replaced by the flash chamber, and the waste heat directly enters the flash chamber, and the flash steam is absorbed by the absorber to generate high-temperature potential heat. Without condensers and evaporators in the system, the heat transfer temperature difference between the two equipment can be eliminated, and its COP can be significantly improved. Open AHT is very active in the research of solar energy recovery [37]. However, in other applications, due to the fact that the working medium is in contact with the air, it is easy to mix impurities, which affects the operation and increases the corrosion of the equipment (especially for the H2O LiBr system). In addition, it also needs to add air purification equipment, which increases the equipment investment and power consumption, and the application is greatly limited [38, 39]

1.5 computer simulation and optimization design

aht is composed of multiple unit devices. Under given operating conditions, there will be a reasonable matching problem between them, so the optimization of the system should be fully considered in the design. The objective of optimization is to

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