Performance Evaluation of Series and Parallel Two-Stage Absorption Chillers Driven by Solar Energy: Energetic Viewpoint

1. Introduction

Nowadays, a large number of refrigeration systems use mechanical compression that is energy-intensive. Nevertheless, there is a growing concern about conventional refrigeration system operating fluids which contribute to the destruction of the ozone layer, greenhouse effects, and global warming. Improvement of refrigeration systems more economically and more environmentally-friendly is one of the alternative options to tackle these challenges. Recently, there has been a growing attention in research to improve the absorption refrigeration systems (ARSs) (Kaynakli and Kilic, 2007). This system is a possible choice for using the remaining heat and renewable resources such as geothermal and solar energies (Soltani et al., 2019a, 2019b). The use of renewable and abundant solar radiation is suggested as an appropriate substitute. Recently, solar chillers may represent a suitable alternative for water and gas air conditioners, which consume electricity to a large extent. Moreover, the working fluids of these processes are compatible with the environment (Sun et al., 2012). Although the overall cooling effect per unit of supplied energy performance of the ARS is usually low, the remaining heat such as the one rejected from power plants may be used to increase the global energy utilization (McQuiston et al., 2004).

In the absorption refrigeration cycle, the required heat for the generator is supplied by solar energy. In order to run the working fluid in absorption chillers, thermal energy is used, which may be supplied through steam, hot water resulting from direct combustion of fuel, or through solar energy. Many researchers have worked on the double-effect H₂O-LiBr ARS by performing energy, exergy, and economic analyses (Kaushik and Arora, 2009; Kaynakli et al., 2015; Gomri, 2009; Talukdar and Gogoe, 2016, Morosuk and Tsatsaronis, 2008; Misra et al., 2005; Bereche et al., 2009). One main characteristic of the double-effect ARSs is its capability to work in parallel, series, and reverse parallel flow configurations with respect to the operating solution flow via the heat exchangers and generators (Arun et al., 2000, 2001; Garousi Farshi et al., 2011, 2012). Liu and Wang (2004) studied the performance of a LiBr-H₂O gas/solar double-effect ARS. In their intended system, the high-pressure generator is run by conventional energy, natural gas, and solar energy. Results indicated that this system is economically feasible and efficient. Molero-Villar et al. (2012) carried out a comparison between solar-powered ARSs to select the most appropriate condition of cooling systems containing solar energy. Boudéhenn et al. (2012) investigated the LiBr-H₂O absorption chiller system equipped with a solar energy collector. Sun et al. (2015) studied a gas/solar absorption cooling and heating system in a commercial building. Results indicated that 49.7% saving is achieved in the consumption of gas using this system. Mussati et al. (2018) investigated the configuration optimization of series flow double-effect LiBr-H₂O ARSs by minimizing the costs. Razmi et al. (2018) presented a system that works by hybridization of an ideal vapor compression system with a single-effect absorption system.

Performance analysis and thermodynamic modeling of ARSs are common in the study of vapor absorption cooling research. Performance of double-effect ARSs have been assessed in the literature (Garousi Farshi et al., 2011, 2012; Talpada and Ramana, 2019; Kaushik et al., 2018; Lima et al., 2019; Sioud et al., 2018). A comprehensive review of vapor ARSs and hybrid ARSs is performed for solar cooling applications by Kaushik et al. (Kaushik et al., 2018). The thermodynamic model developed for a prototype absorption chiller using NH₃/LiNO₃ is investigated by Lima et al. (2019). Recently, Sioud et al. (2018) investigated the feasibility and the eventual improvement in the performance of an ejector powered LiBr-H₂O double-effect absorption/recompression refrigeration cycle driven by high-temperature heat sources.

Given the high consumption of absorption chillers and the amount of power and energy required in the generator of the system, evaluating the savings in gas consumption by employing a solar collector, as an alternative solution, can be very useful and needs to be further investigated. The present study will address the effect of employing solar energy on fuel consumption of series and parallel two-stage ARSs. First, two intended systems are designed and then, the COPs of the refrigeration system and also power consumption are calculated by connecting the solar collector to the ARS. In the following, the impact of varying the evaporator temperature on the COP of the system will be studied. A thermodynamic analysis of the system is conducted and the internal variables such as pressure and temperature of different components including the condenser and the evaporator are calculated.

2. Mathematical Modeling

2.1. System Description

The main components of two-stage absorption chillers include evaporator, absorber, high-temperature generator (HTG), low-temperature generator (LTG), condenser, steam and liquid separator, high-temperature heat exchanger (HTHX), low-temperature heat exchanger (LTHX), low-temperature soluble pump, soluble pump, and refrigerant pump. In absorption cycles, the efficiency of refrigeration equipment is assessed using the COP. Figs. 1 and 2 respectively illustrate a schematic of the parallel and series double-effect absorption chillers with the absorber soluble of LiBr. These cycles have an evaporator, an HTG absorber, an LTG, two heat exchangers, and two condensers. In the series cycle, an outlet dilute soluble from the absorber is pumped into an LTHX, and is then let into an HTHX and afterward directly into a HTG. However, in the parallel cycle, this dilute soluble is injected into the LTG and the HTG simultaneously, the evaporated water enters the condensers and the concentrated soluble returns the absorber as in the series cycle. In both series and parallel cycles, the evaporated water taken from the LiBr-H₂O solution enters a distinct condenser in each generator.

Figure 1. LiBr-H₂O double-effect cycle of the parallel type

Figure 2. LiBr-H₂O double-effect cycle of the series type

The schematics of the modeled system are shown in Figs.1 and 2. The system is composed of two generators, one absorber, two condensers, one evaporator, two pumps, two expansion valves, and two heat exchangers. As demonstrated by Fig. 1, the system has five temperature values (for LTG, HTG, high-temperature condenser (HTC), condenser, and absorber), and three pressure levels (low pressure in the evaporator and the absorber, medium pressure in the condenser and LTG, and high pressure in HTG and HTC).

The term “double-effect” refers to the fact that the inlet heat is used twice in the cycle of vapor production at high temperatures. The generated vapor is used as the inlet heat in the HTG. This vapor, then, enters the high-temperature, where phase-change or condensation occurs due to loss of heat. This heat is high enough to move vapor out of the heat exchanger in the LTG. Therefore, the heat is utilized twice and demonstrates double effects.

One of the main options of designing in double-effect technology for increasing the COP is the selection method of the flow connections. The major options for this case are the series and the parallel types of flows. The cycle demonstrated in Fig. 1 is drawn with the assumptions of parallel flow and also the same change in mass fraction of each generator. Fig. 2 illustrates a schematic of the series flow, where the flow in this direction first enters the HTG and then the LTG. In both cases, the process of internal heat exchange between the HTC and the LTG restricts the temperature. The HTC must have a high enough temperature for heat transfer to LTG.

2.2. System Modeling

The cycle has been modeled using the equations of energy and mass balance, for each component. The outlet mass flow rate of absorber is denoted by .In general, it is expected that the flow rate at the upper circuit (pump 2) be specified. Yet, the energy balance equation between the HTC and the low-temperature one is utilized to calculate the higher flow rate. The heat exchanger’s mass flow rate is specified in four foreign heat transfer loops. As the initial temperature of each loop is specified, the value of  is assumed the input data for each of the external heat exchangers. This value is also considered for the condenser ( ). The assumptions and the considered inputs are similar to the parallel flow model.

2.2.1. Assumptions of the Problem

To model the double-effect of the ARS, several assumptions are considered as the following:

1- The system and the heat exchanger are under steady-state conditions.

2- At the condenser outlet, water is a saturated liquid.

3- At the evaporator outlet, water is a saturated vapor.

4- LiBr’s temperature at the absorber outlet is the absorber’s temperature.

5- The outlet heat temperature of the absorber and the generator depends on the balance conditions of mixing and separation.

6- Heat loss and pressure drop in the tubes and heat exchangers have been ignored.

7- Heat exchange between the system and the environment does not occur except in the HPG, the evaporator, the condenser, and the absorber.

8- In default conditions of system, water is operated with a temperature of 25 ℃ and pressure of 1 atm.

2.3. Thermodynamic Analysis

The equations and the first and second laws of thermodynamics are taken into consideration in this section. The first law of thermodynamics defines energy transfer, while the second law describes the quality of energy and material. In order to conduct thermodynamic analysis of the ARS, the mass equation, and the first and second laws of thermodynamics have been applied to each component of the system equipment.

Mass equations including the mass balance governing the whole cycle and conservation of concentration equation under steady-state flow and steady-state system conditions are as follows:

where  and X denote mass flow rate and mass concentration of LiBr, respectively. These equations are written for chiller components of the parallel and series cycles.

The first law of thermodynamics demonstrates the energy balance equation for each component, and it is stated as follows:

where Q denotes the value of the heat transfer between the control volume of system and its surrounding environment, and W is the positive work amount of system. Energy balance for components of these systems in parallel double-effect conditions is presented as Table 1. It is noteworthy that the equations of the double-effect series system components are also similar to these equations.

The overall performance of energy in refrigeration cycles is demonstrated using the COP, which is defined as follows:

In this study, EES software version 9.4 was used for data analysis and sensitivity analysis. Conducting the calculations related to the first and second laws of thermodynamics, thermal efficiency, component equations, and other thermodynamic and mathematical calculations are achieved by programming in EES.

Table 1. Energy balance equations for different components in double-effect series and parallel systems

3. Results and Discussion

This section analyzes and compares the results obtained from the simulation. After presenting the obtained results and sensitivity analysis of different parameters, the best-operating conditions are selected for the system. First, to examine the accuracy of the calculations, the results of the present simulation for the COP have been compared with the work of Gomri (2009) in Fig. 3. The results of this comparison have indicated a completely desirable match between the two studies. Results of the simulation including the system’s thermodynamic properties are shown for the series and parallel cycles in Table 2. Other specifications including the system’s COP, the pump’s power consumption, the heat transfer rate of each component, and the heat exchanger’s efficiency are presented in Table 3.

Figure 3. Verification of the results of the current study with the study of Gomri (2009) for the conditions of T_con=360K & T_eva=277K

Table 2. Thermodynamic specifications of the parallel and series flow cycles

The assumption in this system is that the maximum mass ratio must be smaller than the point where crystallization occurs because this occurrence is not desired for the system. In other words, this requirement is an assumption made to simplify the modeling.

In the following, by changing the absorber’s outlet flow rate, variations in the value of the system’s COP may be observed for both cycles. Fig. 4 presents the variations in COP versus the absorber outlet flow rate. The COP of the parallel flow is higher than the series flow; however, the reduction of the system’s COP by increasing the absorber’s outlet mass flow rate is similar in both of the cycles. On the other hand, although the COP of the series flow cycle is smaller than that of the parallel flow, it is of higher capacity.

Table 3. Specifications of different components of the cycle in both the parallel and the series flows

The overall heat transfer value has been investigated in Fig.5. This figure studies variations of the overall heat transfer with respect to the absorber’s outlet flow rate for both of the series and the parallel flows. As the results demonstrate, it may be observed that an increase in the overall exchanged heat in terms of the absorber’s outlet flow rate is not linear, and the increasing slope is decreased at higher flow rates.

The results of Fig.6 indicate that the COP is better in the parallel cycle than in series one. As is observed, with an assumption of 280 K temperature for the evaporator, the COP of the parallel cycle is always higher at all temperature levels of the high-pressure generator than that of the series cycle. Furthermore, it was observed at other temperature levels considered for the evaporator that increasing the temperature in the high-pressure generator results in increasing COP. However, the rate of increase in COP at initial temperatures of the high-pressure generator (range of 390-420 K) was higher, and as the temperature increases (range of 420-470 K), the rate of increase in COP reduces. The parallel cycle has consistently shown a better performance than the series cycle.

Figure 4. Variations in COP in terms of the outlet flow rate of the absorber

Figure 5. The amount of total exchanged heat with respect to the absorber’s outlet flow rate

Figure 6. The effect of the evaporator’s and the generator’s temperatures on COP

Figure 7. The impact of the condenser’s and the generator’s temperatures on COP

Fig. 7 shows variations of the cycle’s COP with respect to the variations in the temperature of the condenser as well as the generator’s temperature on the cycle. Results demonstrated the superiority of the parallel cycle’s COP to the series one.

In Fig.8, other parameters including the evaporator and the generator’s temperature variations to the pressure have been investigated. It may be observed that pressure variations in the series cycle are intensively increasing, while such variations in the parallel cycle are insignificant compared to the series cycle. This shows that the parallel cycle is more reliable than series one.

Figure 8. The impact of the evaporator’s and the generator’s temperatures on the pressure of the high-pressure generator

Fig.9 provides investigation of variations in the generator’s pressure by changing the temperature of the condenser and the generator. As is observed, with an increase in the condenser’s temperature in the parallel cycle, variations of pressure at various temperatures are different but insignificant. However, in the series cycle, such variations are almost equal at different temperatures, but pressure variations are intensively increased.

It may be observed in Fig.10 that increasing the efficiency of high-and LTHXs would change COP in the series and the parallel cycles. With an increase in the efficiency of HTHX, the parallel cycle COP is higher than that of the series. Also, in LTHX, with an increase in efficiency, the parallel cycle COP is higher than that of the series. This fact, by itself, indicates the superiority of the parallel cycle to the series one with respect to efficiency in HTHX and LTHXs.

Figure 9. The impact of the condenser’s and the generator’s temperatures on the pressure of HPG

Figure 10. The effect of HTHE and LTHXs on COP