Introduction

Absorption refrigeration technology utilizes two substances with different boiling points as working fluids. The commonly used working fluids are LiBr/H₂O and NH₃/H₂O. The refrigeration effect is achieved through the absorption and release of heat by LiBr for H₂O or by H₂O for NH₃. Absorption refrigeration technology is already very mature and efficient, and is currently the most widely used solar driven refrigeration method. The lithium bromide absorption refrigeration system driven by solar energy is mainly composed of single effect units and double effect units (Figure 1). The coefficient of performance (COP) of a single effect or double effect bromine refrigeration system varies and has its own suitable operating temperature range. Generally speaking, the COP of a single effect lithium bromide absorption refrigeration system is between 0.7-0.8, and the driving temperature of the generator heat source is between 80 ℃ and 100 ℃; The COP of the dual effect lithium bromide absorption refrigeration system can reach 1.3, and the driving temperature of the generator heat source is around 140 ℃ [1-2].

Research on Single Effect Lithium Bromide Absorption Refrigeration System

The single effect lithium bromide absorption refrigeration system is currently the most mature and common absorption refrigeration method. This system has low temperature requirements for driving the heat source and can effectively utilize low-grade heat sources such as solar energy. It has low cost and is widely used in areas with abundant solar energy. Numerous scholars have conducted corresponding experimental and simulation studies on solar single effect bromine chillers.

One key technical barrier to using LiBr-H₂O as the working fluid in absorption chillers is the risk of crystallization when the absorber temperature increases at fixed evaporating pressure [3]. This phenomenon could block the solution pipes, interrupt the machine operation, and even damage the absorption chiller. Liao et al. [4] and Florides et al. [5] indicated the instances that might increase the probability of crystallization. Based on steady-state models, many methods have been investigated to reduce or avoid crystallization problems [3], such as adding chemical inhibitor additives [6], enhancing heat and mass transfer [7] and adopting appropriate system-control strategies [4]. However, when the disturbance occurred, the absorption chiller initially oscillated, and then reached the steady state progressively. In this dynamic process, crystallization might occur at certain oscillation points, although the steady-state point was far from the crystallization line. Dynamic simulation, which could show whether crystallization happens when the disturbance occurs, could provide unique insight into avoiding crystallization.

In Iranmanesh's study[8], a dynamic analysis of a single-effect LiBr-H₂O absorption chiller regarding the effects of all thermal masses on various parameters of the absorption chiller is represented. Six different cases are considered to investigate the thermal mass effects individually and simultaneously. The ordinary differential equations deduced from continuity, momentum, and energy balances are solved using Runge–Kutta method. The results show that the heat transfer rate of high-pressure components (generator and condenser) are highly dependent of thermal mass of the condenser whereas the heat transfer rate of low-pressure components (evaporator and absorber) are hardly affected by thermal masses. Furthermore, the major components except condenser show approximately the same behavior when thermal masses are ignored.

In Xu's study[9], a dynamic model of a single-effect LiBr–H₂O absorption chiller with improved accuracy is built to explore the dynamic performance and the control strategy. Two control strategies are implemented in the model of the absorption chiller: one is setting the chilled water outlet temperature as the manipulated variable, and the other is setting the generator solution temperature as the manipulated variable. The control performances of the two strategies are compared in detail. Results show that either increasing the heat source temperature or decreasing the cooling water inlet temperature increases the risk of crystallization in the dynamic process. For the single-effect absorption chiller, the heat source temperature must be less than a certain value under different cooling water inlet temperatures to avoid crystallization. When the cooling water inlet temperature is 19 °C, the heat source temperature should be less than 86 °C. If the cooling water inlet temperature is higher than 27 °C, then crystallization would not occur. For this single-effect absorption chiller, the thermal mass of the generator has a major effect on the time to reach the steady state of the chilled water outlet temperature. When the solution mass of generator decreases from 5 kg to 2 kg, the response time of the absorption chiller decreases from 1500 s to 600 s. Therefore, reducing the thermal mass of the generator is necessary to reduce the response time. For the PID controller used in this absorption chiller, the increase of K and  τ_D could obviously decrease the settling time. Moreover, the overshoot significantly decreases as K increases. The variation of τ_I has slight effect on the settling time.The time to reach steady state is highly dependent on the thermal mass of the generator, rather than the thermal masses of the condenser, evaporator, and absorber. The single-effect chiller has better control performance in off-design condition when the generator solution temperature rather than the chilled water outlet temperature is selected as the manipulated variable.

Dual effect lithium bromide absorption refrigeration system 

Although single effect lithium bromide absorption refrigeration machines have the advantages of simple structure and easy operation, their system COP level is relatively low. Therefore, in order to ensure that the temperature of the solution in the generator is within an acceptable range and avoid crystallization, the temperature of the driving heat source of the unit must be controlled properly. By introducing a dual effect lithium bromide absorption refrigeration system, not only can high-grade thermal energy be effectively utilized, but the COP of the system can also be significantly improved. State-of-the-art technology employs direct-fired and steam-fired double-effect machines that use an aqueous solution of lithium bromide as the working fluid. These systems have typically a refrigeration capacity of 100-2000 refrigeration tons (350-7000 kW). They are very reliable and widely used in Japan as highly efficient chillers and district heating and cooling systems. More importantly, water-lithium bromide is environmentally benign and thus not affected by the ozone depletion concerns caused by some refrigerants used for vapour compression systems. The COP of a single-effect water-lithium bromide machine can realistically reach 0.7. Double effect machines reach COPs in the range of 0.9-1.25. An alternative working pair is ammonia-water. Ammonia is environmentally benign and because of its odour, self-alarming in the event of a leak. However, ammonia is toxic at concentrations above 50 ppm, and flammable in a concentration range of 15-25vol % in air. Residential air-conditioners using this fluid traditionally have a COP of 0.45. Large ammoni a - water systems are used in predominantly industrial installations for refrigeration purposes. There exist a small number of units in special applications such as waste heat recovery, geothermal energy usage, and solar energy utilization[10].

The energy and exergy analysis of single effect and series ?ow double effect water–lithium bromide absorption systems is studyed by Kaushik and his colleagues[11].It is shown that an increase in the generator temperature increases the COP and exergetic ef?ciency in both single and series ?ow double effect systems up to an optimum generator temperature. The COP of the double effect system is nearly 60–70% greater than that of the single effect system. The optimum value of the COP is achieved at 91℃ for the single effect system and 150 C for the double effect system for Ta=Tc=37.8℃. The exergetic ef?ciency increases with increase in the generator temperature, initially up to 80℃ for the single effect system and up to 130℃ for the double effect system, for Ta=Tc=37.8℃. Further increase in the generator temperature brings down the exergetic ef?ciency. For both systems, maximum value of the exergetic efficiency achieved is nearly the same. Lowering the value of the absorber temperature brings down the optimum generator temperature and increases the COP and exergetic efficiency, as well astheir maximum values. The increase in evaporator temperature increases the COP but reduces exergetic efficiency. It is also shown that increasing the absorber temperature reduces the system performance influentially as compared to increasing the condenser temperature. Moreover, its effect on the performance of the double effect system is more severe. The pressure drop between evaporator and absorber is a very significant factor and an increase in its value is responsible for a reduction in both COP and exergetic efficiency. It is also established that the temperature difference between cold room and evaporator is of greater importance in comparison to temperature difference between heat source and generator. The computation of efficiency defects proves that the absorber is the main component in which the efficiency defect is highest in comparison to other components in both of the systems.

References

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