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Article

Comprehensive Parametric Study of a Solar Absorption Refrigeration System to Lower Its Cut In/Off Temperature

College of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150000, China
*
Author to whom correspondence should be addressed.
Energies 2017, 10(11), 1746; https://doi.org/10.3390/en10111746
Submission received: 6 September 2017 / Revised: 15 October 2017 / Accepted: 25 October 2017 / Published: 31 October 2017
(This article belongs to the Special Issue Solar-Assisted Heat Pump Systems for Heating and Cooling)

Abstract

:
Solar-driven ammonia-water absorption refrigeration system (AARS) has been considered as an alternative for the conventional refrigeration and air-conditioning systems. However, its high initial cost seems to be the main problem that postpones its wide spread use. In the present study, a single-stage NH3/H2O ARS is analyzed in depth on the basis of energetic and exergetic coefficients of performance (COP and ECOP, respectively) to decrease its cut in/off temperature. This study was carried out to lower the required heat source temperature, so that a less-expensive solar collector could be used. Effects of all parameters that could influence the system’s performance and cut in/off temperature were investigated in detail. Presence of water in the refrigerant and evaporator temperature glide was considered. Results revealed that appropriate selection of system’s working condition can effectively reduce the driving temperature. Besides, the cut in/off temperature can be significantly decreased by inserting an effective solution heat exchanger (SHX). Required driving temperature can be lowered by up to 10 °C using SHX with 0.80 effectiveness. The results also showed that effects of water content in the refrigerant could not be neglected in studying NH3/H2O ARS because it affects both COP and ECOP. Additionally, a large temperature glide in the evaporator can substantially decrease the ECOP.

1. Introduction

According to the International Institute of Refrigeration, about 15% of total world’s electricity generation is consumed for refrigeration and air-conditioning purposes [1]. Approximately 80% of the world’s electricity is generated by using fossil fuels that significantly enhance CO2 emissions, leading to augment the global warming [2]. Furthermore, many developing countries face a shortage of electricity supply [3], particularly during the summer when the need for refrigeration and air-conditioning is obviously increased [4]. On the other hand, many countries enjoy abundant amounts of solar energy, which is freely available throughout the year [5]. To reduce the consumption of fossil fuels and at the same time to promote the use of sustainable energy, exploitation of existing solar energy to operate a refrigeration system is rational. It is quite feasible to harness the solar energy as the demand for cooling and amount of available solar energy are directly proportional to each other [6].
In the field of solar refrigeration, absorption refrigeration seems to be the best option since these systems can be thermally driven by using solar collectors [7]. Advantages of these systems include: use of environment friendly refrigerants, high reliability, extended service life, easy capacity control and efficient and economic use of low-grade energy sources. Absorption refrigeration systems (ARSs) are considered as one of the oldest cooling technologies. These systems essentially work under the same principle as the conventional vapor-compression systems, however, the mechanical compressor is replaced by a thermal compressor [8]. Main components of a typical absorption system are: generator, condenser, evaporator, absorber and solution pump [9]. To improve the system’s performance, usually, two components are added: solution heat exchanger (SHX) and refrigerant heat exchanger (RHX) (Figure 1).
The NH3/H2O system can be used for both refrigeration and air-conditioning purposes [10]. The present study concerns a single-stage solar driven ammonia/water absorption refrigeration system (AARS). Use of NH3/H2O as working fluid in ARSs remains an attractive application. It performed better than fluorocarbon refrigerants. It is free from limitations imposed by the high freezing temperature of the refrigerant, low crystallization temperature of the solution as in H2O/LiBr systems and extreme corrosiveness as in ammonia/sodium thiocyanates systems [11,12]. Besides, NH3/H2O mixture has zero ozone depletion and zero global warming potentials [13]. Other distinguished benefits include: it can be air cooled, though higher driving temperature is required; and it can freeze water and hence can also store the cold. The only disadvantage of this pair is the volatile nature of water, which is usually overcome by incorporating a rectifier [14]. In the current study, the term strong solution means strong in refrigerant and weak solution indicates weak in refrigerant.
An important parameter that has been neglected in all previous studies in field of solar-driven AARSs is the system’s cut in/off temperature. It can be defined as the minimum generator temperature at which the strong solution just starts boiling [15]. This parameter is of paramount importance for solar driven systems. Cut in/off temperature has a major influence on required heating source temperature and hence it determines the operational hours of system during daytime. This temperature, in this way, influences the technical and economic soundness of the system. The cut in/off temperature is mainly affected by the generator pressure and strong solution concentration (Xss). Accordingly, it is also dependent on all sub factors that affect these two parameters such as condenser, evaporator and absorber temperatures, and the pressure losses.
Majority of the simulation models found in the literature for evaluation of AARS are relatively simple. Most of them do not account for the presence of water in refrigerant entering the condenser and evaporator. Due to this water content, evaporation of refrigerant will not be completed without a large temperature glide [16,17]. For example, at evaporator pressure 3.5 bar and refrigerant concentration (Xr) 0.998, the required temperature glide across the evaporator is about 29 °C to achieve complete evaporation. This degree of glide is impractical, so the assumptions of pure refrigerant or saturation vapor at the evaporator outlet can lead to overestimate the system’s performance. Few studies discuss the effects of water content in refrigerant. Bogart reported its harmful effects but did not quantify them [18]. Fernandez-Seara and Sieres analyzed the effect of water content on the system’s coefficient of performance (COP) but its effect on the exergetic performance was not included in their studies [16,19]. Táboas et al. [20] considered a temperature glide of 10 K in evaporator to reduce the amount of liquid refrigerant that was not evaporated and consequently increased cooling effect and COP. However, the effect of this large temperature glide on exergetic coefficient of performance (ECOP) was not reported in that study.
In last decades exergy analysis gained more attention and has been widely applied to show how the system effectively utilizes given energy resources [21]. Then, it could be decided which system’s component need further improvement to reduce irreversibility [22,23]. Knowledge of ECOP can give idea whether the system should be more improved or not. In current comprehensive study, a single-stage solar driven NH3/H2O ARS is put under detailed consideration. The main objective of this study is to decrease driving temperature (cut in/off temperature) of the system. It is analyzed on the basis of energetic and exergetic coefficients of performance (COP and ECOP), circulation ratio (CR), electric power consumed by solution pump (Wpump) and components’ thermal loads. In the present model, effects of water content in refrigerant through the condenser and evaporator are taken into account. All parameters that can affect the system’s cut in/off temperature have been investigated. Effect of preheating the strong solution on reducing the cut in/off temperature is also examined in this study. Besides conventional parameters, special attention has been paid to see the effects of temperature glide in evaporator and SHX effectiveness.

2. System Description

The system consists of a solar thermal collector linked to a single-stage NH3/H2O absorption chiller as shown in Figure 1. Hot water from solar collector is used to heat the strong solution in generator to produce ammonia vapor at high pressure. During the process, some water vapor is also generated and passes with the ammonia vapor to rectifier (3). Function of the rectifier is to reduce water content in NH3 vapor. High concentrated NH3 vapor enters the condenser (5), where it is condensed and then collected in receiver (6). Afterwards, liquid refrigerant passes through reflux valve which divides the flow into two streams: the first stream (7) represents a small fraction of liquid refrigerant, which returns to rectifier by means of reflux; and the second stream (8) passes through refrigerant heat exchanger (RHX) where it is sub-cooled by refrigerant vapor leaving the evaporator (11). Sub-cooled liquid (9) passes through expansion valve (10) to evaporator, where it is evaporated at low pressure and temperature to induce cooling effect. The evaporated refrigerant enters absorber (12) after passing through RHX. In absorber the vapor is absorbed by weak solution returning from generator through solution heat exchanger SHX. Pressure of weak solution is reduced by an expansion valve before entering the absorber from generator pressure to absorber pressure. The purpose of using SHX is to preheat the strong solution and to precool the weak solution before entering the absorber [24]. An electric-driven pump is used to boost the strong solution pressure coming from absorber and leading to the generator. Water is used to cool both the condenser and the absorber. To achieve this, two additional pumps and fans (termed as auxiliaries) are employed.

3. System Modeling

A simulation model was developed to evaluate the performance of a single-stage NH3/H2O ARS. The developed model is based on energy, mass and species conservation equations. General forms of these equations are specified as [4,7]:
Energy conservation-
( m ˙ h ) i n ( m ˙ h ) o u t + [ Q ˙ i n Q ˙ o u t ] ± W = 0
Mass conservation-
m ˙ i n m ˙ o u t = 0
Species conservation-
( m ˙ x ) i n ( m ˙ x ) o u t = 0
Exergy balance-
E ˙ i n E ˙ o u t + ( 1 T o T j ) Q ˙ j = E ˙ d e s t
By applying these general forms to each individual component in the system, energy and exergy balances can be written as following:
Q ˙ g e n = m ˙ 3 h 3 + m ˙ 4 h 4 m ˙ 2 h 2
E ˙ d e s t ,   g e n = m ˙ 2 E 2 m ˙ 3 E 3 m ˙ 4 E 4 + Q ˙ g e n [ 1 ( T o / T g e n ) ]
m ˙ 2 h 2 + m ˙ 5 h 5 = m ˙ 1 h 1 + m ˙ 3 h 3 + m ˙ 7 h 7
E ˙ d e s t ,   r e c t = m ˙ 1 E 1 + m ˙ 3 E 3 + m ˙ 7 E 7 m ˙ 2 E 2 m ˙ 5 E 5
Q ˙ c o n d = m ˙ 5 h 5 m ˙ 6 h 6
E ˙ d e s t ,   c o n d = m ˙ 5 ( E 5 E 6 ) Q ˙ c o n d [ 1 ( T o / T c o n d ) ]
Q ˙ R H X = m ˙ 8 ( h 8 h 9 ) = m ˙ r ( h 12 h 11 )
E ˙ d e s t ,   R H X = m ˙ 8 ( E 8 E 9 + E 11 E 12 )
Q ˙ e v a = m ˙ 8 ( h 11 h 10 )
E ˙ d e s t ,   e v a = m ˙ 8 ( E 10 E 11 ) + Q ˙ e v a [ 1 ( T o / T e v a ) ]
Q ˙ a b s = m ˙ 12 h 12 + m ˙ 14 h 14 m ˙ 15 h 15
E ˙ d e s t ,   a b s = m ˙ 12 E 12 + m ˙ 14 E 14 m ˙ 15 E 15 Q ˙ a b s [ 1 ( T o / T a b s ) ]
Q ˙ S H X = m ˙ 4 ( h 4 h 13 ) = m ˙ 16 ( h 1 h 16 )
E ˙ d e s t ,   S H X = m ˙ 4 ( E 4 E 13 ) + m ˙ 16 ( E 16 E 1 )
where To is the reference temperature taken as 25 °C.
The reflux ratio is defined as:
R e f l u x = m ˙ 7 m ˙ 6 = m ˙ 7 m ˙ 7 + m ˙ 8
ε R H X = T 8 T 9 T 8 T 11
ε S H X = T 4 T 13 T 4 T 16
where εRHX and εSHX are effectiveness of RHX and SHX, respectively.
Electric power consumed by solution pump can be given by:
W p u m p = m ˙ 15 ν 15 ( P 16 P 15 ) η p u m p
Here, ν is the specific volume of solution (m3/kg), and ηpump is the pump overall efficiency. Equation (22) gives basic power required for pumping the working solution. However, additional power is needed to run the auxiliaries (fans and cooling water pumps). Palomba et al. [25] recently reported that the power consumed by auxiliaries in AARS can be estimated as 23 W/kWcooling. Hence, for a system with 100 kW cooling power, the auxiliaries’ power will be 2.3 kW.
System analysis is carried out based on following assumptions:
  • Vapor at the rectifier outlet is saturated at its pressure and mass concentration.
  • Rectifier has a constant efficiency.
  • Refrigerant, which passes through the condenser, evaporator and RHX, is a mixture of water/ammonia with a specific concentration, so that effects of water presence in refrigerant can be quantified.
  • Condensation through the condenser is complete.
  • Refrigerant at condenser outlet is saturated liquid at condenser temperature.
  • Strong solution at absorber outlet is saturated and it is at absorber temperature.
  • Expansion valves are adiabatic.
  • Weak solution leaves generator at its temperature.
Some parameters used in this study were taken from previous studies at their practical values [20,24]. Therefore, for the system at base condition; εSHX = 0.80, εRHX = 0.80 and pump efficiency is 0.65. Minimum reflux ratio is taken as 0.2 to obtain a refrigerant with a concentration 0.998. Temperature of heating source decreases by 10 °C across the generator. Pressure loss between evaporator and absorber is taken as ΔP/P = 0.075 and ΔP/P = 0.05 between the generator and condenser where P is pressure at the pipe exit.
Performance of ARSs is usually measured by COP, ECOP and CR of the system. The COP of ARS is given by:
COP = Q e v a Q g e n + W p u m p
and the ECOP is given by:
ECOP = Q e v a ( 1 T o / T e v a ) Q g e n ( 1 T o / T g e n ) + W p u m p
The circulation ratio is given as:
CR = m ˙ 1 m ˙ 8

3.1. Simulation Tool

The simulation software Cycle-Tempo was used to compute energy and mass balances in all components of the system. The components have been assembled graphically in the software scheme window as depicted by Figure 2. Necessary data were entered, followed by the simultaneous solution of energy and mass balances equations to obtain energy, exergy and mass flows through all components and pipes. Values for temperature, pressure, enthalpy, entropy, vapor quality and mass fraction at all points (points 1–24) have been specified. Thermodynamic properties of working fluid were obtained from equations of Ziegler and Trepp, implemented in the software [26].

3.2. Model Validation

The simulation model was validated by using two sets of experimental data. First set is experimental results for a commercial solar driven unit (chilli® PSC 10 from SolarNext Company) published in [27]. Second one results from an experimental study conducted by Mendes and Pereira [28] and reported in [29]. Comparison between several key values computed by the model and those published in the cited references is presented in Table 1. As shown, results of the model are close to that in mentioned references.

4. Results and Analysis

By running the simulation model, effects of all operating parameters on system’s performance were obtained. For different working conditions, coefficient of performance (COP), exergetic coefficient of performance (ECOP), cut in/off temperature, circulation ratio (CR), power consumed for solution pumping (Wpump) and the different components’ thermal loads were determined. Results are presented graphically by varying parameter under consideration while keeping remaining parameters at their base condition values. Consequently, different possible design points have been compared. In this study, the system base condition is considered at Tgen = 90 °C, Tcon = Tabs = 30 °C, Teva = −4 °C, the reflux ratio = 0.2 and the strong solution concentration (Xss) by mass = 0.4824. The system cooling capacity ( Q ˙ e v a ) is always 100 kW. Due to water presence in refrigerant, some degree of temperature glide through evaporator is required. Therefore, in all coming subsections, temperature glide between evaporator entrance and exit is equal to 2 °C. Consequently, more refrigerant can be evaporated. This degree of glide was taken in accordance with the results presented in Section 4.7 and experimental outcomes reported in [27]. Table 2 displays results of thermodynamic analysis of the system under base condition. Numbers here refer to the points in Figure 1. Predicted performance results are also presented in same table. Values of working parameters and their simulation ranges are summarized in Table 3.

4.1. Effect of Strong Solution Concentration (Xss)

Xss is an important parameter in AARSs. Performance of the system is highly improved as ammonia concentration increases in the solution entering generator. This improvement occurs because more refrigerant can be released in generator, which leads to lower CR and thermal loads. As appeared in Figure 3, both COP and ECOP increase as the percentage of ammonia in strong solution increases. Initially, both COP and ECOP rise sharply with increase in Xss. It proceeds as such until the percentage of ammonia reaches at about 48%. Afterwards, the increase in COP and ECOP is at a lower rate. Xss also affects the CR noticeably, as illustrated by Figure 4. The CR decreases abruptly as ammonia percentage increases in start, afterwards the decrease of CR continues but at a lower ratio.
Another important parameter affected by the Xss is the cut in/off temperature. Figure 4 expresses the relation between cut in/off generator temperature and Xss. As shown, at constant generator pressure, cut in/off temperature is inversely influenced by ammonia concentration in the solution entering generator. Amount of electric power consumed by solution pump (Wpump) is directly proportional to the value of circulation ratio. Therefore, increase of strong solution concentration has a positive impact on reducing the required pumping power as shown by Figure 5. Figure 6 implies the variations of components’ thermal loads at different Xss. As illustrated, for constant Qeva, the Qgen, Qabs and QSHX fall rapidly (due to decrease in the CR) as the ammonia concentration increases. Later, the decrease of thermal loads is noticed at a low rate, while Qcon and QRHX remain unchanged. In addition, it is clear that QSHX is very sensitive to the change in Xss. To lower the system’s initial cost, size of components (depend on thermal load) should be minimized. Hence, Xss has an important impact on the system’s cost. However, the use of strong solution with higher concentration requires lower absorber temperature which is limited by temperature of available cooling water (or air).

4.2. Effect of Generator Temperature (Tgen)

Generator temperature (Tgen) is one of the most important parameters affecting the performance of absorption systems. The COP and ECOP of system under consideration were plotted as functions of Tgen in Figure 7. The Tgen varied from cut in/off limit to 120 °C, keeping all other parameters constant at their base values except heating water temperature. As Tgen rises from cut in/off value, the COP rapidly increases until about 90 °C; after that, gradient of COP curve becomes nearly flat. Meanwhile, ECOP passes through maxima at 86 °C and drops gradually after that. This suggests that exergetic performance of the system is affected more by increasing Tgen than energetic performance. This behavior can be attributed to the increase of solution temperature in generator and absorber which leads to more exergy losses in two components. These results are consistent with those reported by other authors [4,20].
The generator at a higher temperature can produce more ammonia vapor, so CR declines rapidly as Tgen increases until 80 °C. Later, the gradient of CR curve becomes nearly flat, as depicted in Figure 8. Identical behavior is noted for the required pumping power, as portrayed in Figure 9. For the system designed to work at Tgen = 80 °C, the pump power is 2.75 kW. This value is reduced to 1.34 kW for Tgen = 90 °C. On the other hand, as Tgen arises, the refrigerant concentration in vapor leaving generator and entering rectifier descends. Resultantly, a rectifier with higher efficiency is required which means adding more costs. The relation between Tgen and NH3 concentration in vapor from generator is shown in Figure 8. At Tgen = 80 °C, ammonia concentration in vapor is 0.9792, which decreases to only 0.8593 at Tgen = 120 °C due to evaporation of more water at higher Tgen. Furthermore, Tgen affects Qgen, Qabs and QSHX, as described in Figure 10. When Tgen ascends, the thermal loads of the three components decrease. Moreover, it can be seen that QSHX is most sensitive to the change of Tgen.

4.3. Effect of Condenser Temperature (Tcon)

The relationships among COP, ECOP and condenser temperature (Tcon) are implied in Figure 11. Tcon varies from 20 to 42 °C which corresponds to the change in condensation pressure from 8.555 to 16.4 bar. The decrease in COP is from 0.6208 to 0.10 and in ECOP is from 0.3548 to 0.0467 which is in accordance with the [4,24]. The results can be explained as follows: for constant Qeva, increasing Tcon means a higher pressure in generator, so less ammonia vapor is released. This situation increases the CR through the generator, absorber and SHX. Consequently, it intensifies the thermal loads in these three components. Results are illustrated in Figure 12 and Figure 13. Following the increase of CR at high Tcon, required pumping power increases from 1.34 kW at Tcon of 30 °C to 7.23 kW at Tcon = 40 °C, as revealed by Figure 14.
Another imperative parameter that is highly affected by changing Tcon is the generator cut in/off temperature. As Tcon increases from 20 to 40 °C, the generator pressure increases from 8.555 to 15.52 bar. It leads to increase cut in/off temperature from 62 to 87 °C, as shown in Figure 12.

4.4. Effect of Reflux Ratio (Reflux)

The reflux ratio (Reflux) strongly affects both COP and ECOP as indicated by Figure 15. Increase in Reflux witness an abrupt decrease in COP and ECOP. The effect of Reflux can be explained as follows: for constant Qeva, by increasing Reflux, the vapor leaving rectifier and liquid returning from condenser increases. Hence, more liquid returns to the generator and more vapor must be produced. It implicates that the Qgen must increase and, consequently, the system COP and ECOP decrease, which is in good agreement with the results in [30].
Since the mass flow rates through generator and condenser increase as the Reflux increases, so the thermal loads of these two components increase obviously. The Qabs and QSHX increase at a lower rate. Figure 16 shows the system components’ thermal loads at different reflux ratios. Practically, the Reflux is dependent on rectifier efficiency and ammonia concentration in vapor produced by the generator.

4.5. Effect of NH3 Concentration in Refrigerant (Xr)

In AARSs, the refrigerant entering the evaporator always contains a small fraction of water. This water content in the evaporator, though very less, significantly impacts the system’s performance and thermal loads. It occurs due to the reason that complete vaporization of refrigerant cannot be arrived without relatively larger temperature glide in the evaporator. For example, with 2 °C temperature glide, the vapor quality at evaporator exit is 0.9886 for Xr = 0.999 (remaining refrigerant not evaporated). This quality reduces to only 0.8375 for Xr = 0.986 articulating that 16.25% of the refrigerant passes through evaporator without inducing cooling effect. The relation between Xr and vapor quality at evaporator exit is presented in Figure 17. As per the figure, variation of Xr from 0.986 to 0.999, COP increases from 0.4733 to 0.5997 and ECOP increases from 0.2647 to 0.3352. Reason being the refrigerant with a higher concentration can evaporate in evaporator to reach a higher vapor quality (most of the refrigerant evaporated) than the refrigerant with a lower concentration. Therefore, the required refrigerant amount is decreased for constant Qeva. Subsequently, Qgen, Qcon, QRHX, Qabs and QSHX are effectively lowered with an increase in COP and ECOP, as shown in Figure 18.
Another explanation for the improvement in system’s performance with increase of Xr is as follows: for same evaporator temperature, higher concentration refrigerant evaporates at slightly higher pressure than the refrigerant with lower concentration. This increases the absorber pressure which allows strong solution to reach a higher concentration, hence the system’s performance increases. For example, at evaporator temperature (Teva) −4 °C and Xr = 0.97, the evaporator pressure is 3.58 bar and strong solution at absorber exit is 0.4767. While at same temperature and Xr = 0.999, the evaporator pressure is 3.685 bar and related Xss is 0.4828.

4.6. Effect of Evaporator Temperature (Teva)

Evaporator temperature affects the pressure of evaporator and absorber. The effects of evaporator temperature (Teva) on system’s energetic and exergetic performances are illustrated in Figure 19. The Teva varies from −12 to 12 °C which increases the COP from 0.3816 to 0.6613. The higher Teva increases absorber pressure which let the concentration of strong solution (Xss) increase. It consequently decreases the Qgen and leads the COP to increase. For the cause of ECOP, the situation is different: as Teva increases from −12 °C, the ECOP rises to touch its maximum at Teva of −8 °C. This occurs due to decrease in exergy losses in generator, absorber and SHX as their thermal loads are effectively reduced by increase of Teva from −12 to −8 °C (Figure 21). However, with more increase of Teva the ECOP decreases distinctly from 0.3375 to 0.1527. It happens as the evaporator has a lower potential for cooling at a higher temperature. Thus, high increase Teva has a negative effect on ECOP, which corresponds to the results reported in [4,15,31].
On the other hand, since increase in Teva causes an increase in Xss, CR decreases as well as the Qgen, Qabs, QSHX and QRHX. However, the Qcon slightly increases with the increase in Teva. It takes place because the refrigerant at higher evaporator pressure has a marginally lower vapor quality than that at lower evaporator pressure. This means at a higher Teva, the refrigerant requires more temperature glide to be evaporated completely. Effects of Teva on CR and thermal loads are presented in Figure 20 and Figure 21, respectively. Due to the decrease in CR with increase of Teva, the required pumping power declines from 4.5 kW at Teva = −12 °C to 1 kW at Teva of −2 °C as depicted by Figure 22.
As mentioned previously, generator cut in/off temperature depends mainly on Xss and generator pressure. Hence, an increase in evaporator pressure, which lifts the Xss in absorber, has a significant impact on decreasing the cut in/off temperature as presented in Figure 20. Increase in Teva from −12 to 12 °C abruptly decrease the generator cut in/off temperature from 86 to 54 °C.
From these results, it can be comprehended that the increase in Teva has positive effects. The fact is endorsed by the values which increase for COP and decrease for CR, cut in/off temperature, Wpump and thermal loads. Only exception is the condenser, which undergoes a small increase in thermal load. At the same time, higher increase in Teva has a negative effect as well since it evidently decreases the ECOP. It is worth mentioning that evaporator temperature is determined generally in accordance to the application requirement, either refrigeration or air-conditioning. Thus, the choice of Teva usually is within a limited range.

4.7. Effect of Evaporator Temperature Glide

In AARSs, required temperature glide to achieve complete vaporization is dependent on water content in the refrigerant and evaporator pressure. For the present cycle, which has a value of 0.998 for Xr, the temperature glide must be kept at about 29 °C to accomplish complete evaporation. Figure 23 shows the effect of temperature glide in evaporator on COP and ECOP.
As interpreted from Figure 23, a small evaporator temperature glide increases both COP and ECOP. At about 2 °C glide, the ECOP reaches its maximum value. Afterwards as the glide increases, the COP continues to increase while the ECOP decreases significantly. This figure also illustrates that complete evaporation could not be attained in evaporator up to 14 °C of glide even though water in refrigerant is 0.2% only. For the present system, until this glide degree, refrigerant can be 99.5% evaporated. Complete evaporation at evaporator exit was achieved by 29 °C of glide. In the same manner, for constant Qeva, evaporator temperature glide by 2 °C is enough to effectively reduce the Qgen, Qcon, Qabs, QRHX and QSHX. It should be noticed that further increase in evaporator temperature glide does not produce a remarkable reduction in the thermal loads, as shown by Figure 24. Thus, a small glide of evaporator temperature will improve the system’s performance and reduce thermal loads despite the existence of a small amount of liquid refrigerant at the evaporator exit. However, it seems to be compensated since the remaining liquid will be dragged to absorber by vapor flow through RHX. This liquid will be evaporated completely in RHX and its refrigeration effect will be used to sub cool the liquid refrigerant from condenser. This may add more importance for utilizing RHX in situations of low temperature glide, especially when refrigerant concentration is not too high.

4.8. Effect of Absorber Temperature (Tabs)

Increasing Tabs directly affects the Xss, which decreases as Tabs increases. At high Tabs, the Xss approaches that of weak solution and leads CR to increase dramatically. The values of Qgen, Qabs and QSHX then follow considerable increase with a consequent decrease in COP. In addition, increase of Tabs increases the absorber exergy loss as well as the SHX. Resultantly ECOP also decreases. In Figure 25, the Tabs varies from 15 to 42 °C, the COP is decreased from 0.6474 to 0.2851 and ECOP is reduced from 0.3682 to 0.1487 only, which are in a good agreement with results published by Aman et al. [4].
For Tabs above 40 °C, difference in the concentrations between strong and weak solutions becomes so small, the CR and thermal loads extremely increase and both COP and ECOP rapidly decrease. The effect of Tabs on CR and thermal loads are shown in Figure 26 and Figure 27, respectively. As Tabs inversely affects the Xss substantially, it will strongly influence generator cut in/off temperature, as illustrated by Figure 26. Increase in Tabs from 15 to 42 °C will conspicuously rise the cut in/off temperature from 57 to 88.5 °C. Other negative effect of increasing Tabs is the noticeable increase of Wpump, as revealed by Figure 28. By increase of Tabs from 30 to 42 °C, consumed power by pump sharply increases from 1.34 kW to reach 8.4 kW.
Usually, Tabs and Tcon are determined by temperature of available cooling medium (water or air). In current study, the condenser and absorber are water cooled. To keep heat transfer rates between working solution and cooling water at required values, the minimum temperature difference must not be less than 6 °C in these components. In other words, for Tcon and Tabs of 30 °C the cooling water temperature must be 24 °C or lower. Otherwise, the condenser and absorber will work at higher temperatures which decrease COP, ECOP and increase the cut in/off temperature, CR and Wpump.

4.9. Effect of Solution Heat Exchanger Effectiveness (εSHX)

Solution heat exchanger is regarded as a critical component of the AARS because it affects the system in many aspects. The εSHX has a major effect on system’s performance as displayed in Figure 29 where εSHX varies from 0.00% (represent no SHX) to 100%. Keeping all other parameters at their base values, the COP increases sharply from 0.1640 to 0.6906 and ECOP increases from 0.0926 to 0.3857.
Rate of heat transfer from the weak solution to strong solution increases with the increase in εSHX, so the temperature of the strong solution (T1 in Figure 1) increases while that of the weak solution (T13) decreases. As a result, strong solution enters the generator with a higher temperature and weak solution enters the absorber with a lower temperature. Consequently, both Qgen and Qabs decrease. Figure 30 indicates thermal loads as a function of εSHX and energetic and exergetic performance increase ratios (PIR) are shown in Figure 31 and Figure 32, respectively. With increase in εSHX, thermal loads decrease and energetic and exergetic performance improve. As εSHX value reaches 0.80, the Qgen is reduced by 72%, Qabs is reduced by 75%, COP is increased by 256% and the value of ECOP is increased by 253%. The effectiveness of SHX also has a positive impact on CR, as illustrated by Figure 33, while increase in εSHX decreases CR. This directly reduces Wpump, as uncovered by Figure 34.
The increase of εSHX also decreases the generator cut in/off temperature by a reasonable value. For the author’s knowledge, relation between εSHX and generator cut in/off temperature has not been reported before the present study. Figure 33 shows that cut in/off generator temperature can be reduced by more than 10 °C provided a SHX with εSHX = 0.80 is used. This result can be achieved since SHX with a high effectiveness can preheat the strong solution to approach its saturation temperature before entering the generator. Later, the entire heat supplied to the generator can be utilized to vaporize ammonia.
In Figure 35 and Figure 36, performance of the system is compared with different εSHX at different Tgen, as shown graphically. By using a SHX with higher effectiveness, maximum COP and ECOP can be achieved at a lower generator temperature. In addition, the differences in values of cut in/off generator temperature show the importance of using a SHX with high effectiveness.

5. Conclusions and Discussion

Main goals of this study are to analyze an ammonia/water ARS for lowering its required driving temperature (cut in/off temperature), and maximizing its performance, keeping in view the first and second laws of thermodynamics. In this regard, energy and exergy analysis of a 100 kW AARS has been performed and effects of all parameters have been investigated. Influences of water content in refrigerant, pressure losses and temperature glide in evaporator have also been considered. The COP, ECOP, CR, required pumping power and different components’ thermal loads have been obtained. Following results have been concluded:
  • The cut in/off generator temperature is an important parameter in studying AARSs and must be optimized along with other parameters to reduce required driving temperature.
  • Results revealed that cut in/off temperature is dependent on the Xss, generator pressure and the εSHX. It decreases as Xss and εSHX increase and as the generator pressure decreases. Consequently, all variables (e.g., Tcon, Teva, Tabs, and pressure losses) affecting those three parameters also affect the cut in/off generator temperature.
  • In optimization of AARSs, selection of the generator working temperature requires special attention because higher Tgen maximize the COP and reduce both CR and electric power required for solution pumping. However, for the systems designed to work at higher Tgen, maximum achievable ECOP is a little low.
  • Appropriate choice of Tgen (without overheating) can increase the ECOP as well as ensure a high ammonia concentration in vapor leaving the generator, and hence reduce the work required by rectifier.
  • The temperature of cooling water used to cool both of absorber and condenser has a great impact because both Tabs and Tcon significantly affect the generator cut in/off temperature, COP, ECOP, CR and Wpump.
  • In evaporator, due to the presence of water in refrigerant, complete evaporation cannot be accomplished without a relatively higher temperature glide. For this reason, while studying AARSs, the assumption of saturated vapor at evaporator exit can lead to misleading results.
  • A small temperature glide in evaporator (within 5 °C) will improve both COP and ECOP and reduce the components’ thermal loads. However the glide beyond 5 °C will decrease ECOP. On the other hand, increasing the Teva will rise COP and lowers the consumed power by solution pump, but at higher Teva the ECOP decreases sharply.
  • In AARSs, the SHX has a prime importance, especially for the systems driven by low temperature heat sources. Employing a SHX with εSHX = 0.8, the Qgen and Qabs can be reduced by 47% and 53% while the COP and ECOP can be increased by 88% and 87%, respectively. In addition, the CR can be reduced by 41%, Wpump dropped by 61% and the cut in/off temperature can be lowered by about 10 °C.
  • For better and reliable analysis, information about electric power consumption by absorption cycle and auxiliaries is pivotal. For standalone solar-driven absorption cooling systems, it is even more crucial. By comparing the results here, it should be noted that for system at base condition, the electric power consumed by auxiliaries (2.3 kW) is higher than that required for solution pumping (1.34 kW). Resultantly, due consideration of the auxiliaries consumption will be more credible in evaluating solar ARSs.
  • From the indicated results, it can be judged that an appropriate selection of system working condition values in AARSs, i.e., Teva = −4 °C, and COP and ECOP higher than 0.53 and 0.32, respectively, can be achieved at 90 °C generator temperature. This temperature can be supplied by a low-temperature heat source.
Finally, the comprehensive analysis of AARS presented in this study can be considered as a useful source to observe all parameters that affect cut in/off generator temperature, COP, ECOP, CR, Wpump and system thermal loads. These outcomes can facilitate the coming research work in this sector. However, a further study undertaking the effect of water content in refrigerant on exergy loss in each component is needed. Besides, a complete analysis including solar collector, hot water tank and absorption system should also be conducted. These points can be kept in front for future work.

Acknowledgments

The authors would like to express their gratitude to Northeast Forestry University for financial support.

Author Contributions

All authors contributed to this work, Osman Wageiallah developed the used model and wrote the paper. The whole project was supervised by Guo Yanling.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

ARSAbsorption refrigeration system
AARSAmmonia-water absorption refrigeration system
COPCoefficient of performance
ECOPExergetic coefficient of performance
SHXSolution heat exchanger
RHXRefrigerant heat exchanger
XMass fraction of ammonia
CRCirculation ratio
m ˙ Mass flow rate (kg/s)
Q ˙ Heat transfer rate (kW)
WWork rate (kW)
E ˙ Exergy rate (kW)
hSpecific enthalpy (kJ/kg)
TTemperature (°C)
ESpecific exergy (kJ/kg)
RefluxReflux ratio
PPressure (kPa)
Subscripts
ssStrong solution (strong in refrigerant)
rRefrigerant
inGoing in
outGoing out
oReference state
destDestroyed
genGenerator
rectRectifier
conCondenser
evaEvaporator
absAbsorber
pumpSolution pump
Greek symbols
εEffectiveness
ΔDifference in any quantity
νSpecific volume of solution (m3/kg)
ηEfficiency

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Figure 1. Single-stage NH3/H2O solar absorption refrigeration system.
Figure 1. Single-stage NH3/H2O solar absorption refrigeration system.
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Figure 2. Single stage NH3/H2O absorption system in Cycle-Tempo scheme window.
Figure 2. Single stage NH3/H2O absorption system in Cycle-Tempo scheme window.
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Figure 3. Effect of Xss on energetic and exergetic coefficients of performance (COP and ECOP).
Figure 3. Effect of Xss on energetic and exergetic coefficients of performance (COP and ECOP).
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Figure 4. Effect of Xss on circulation ratio (CR) and cut in/off temperature.
Figure 4. Effect of Xss on circulation ratio (CR) and cut in/off temperature.
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Figure 5. Effect of Xss on pumping power.
Figure 5. Effect of Xss on pumping power.
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Figure 6. Variations of thermal loads with Xss.
Figure 6. Variations of thermal loads with Xss.
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Figure 7. Effect of Tgen on COP and ECOP.
Figure 7. Effect of Tgen on COP and ECOP.
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Figure 8. Variation of NH3 vapor concentration and CR with Tgen.
Figure 8. Variation of NH3 vapor concentration and CR with Tgen.
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Figure 9. Effect of Tgen on pumping power.
Figure 9. Effect of Tgen on pumping power.
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Figure 10. Effects of Tgen on Components’ thermal loads.
Figure 10. Effects of Tgen on Components’ thermal loads.
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Figure 11. Effect of Tcon on COP and ECOP.
Figure 11. Effect of Tcon on COP and ECOP.
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Figure 12. Variation of cut in/off temperature and CR with Tcon.
Figure 12. Variation of cut in/off temperature and CR with Tcon.
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Figure 13. Variations of Components’ thermal loads with Tcon.
Figure 13. Variations of Components’ thermal loads with Tcon.
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Figure 14. Effect of Tcon on pumping power.
Figure 14. Effect of Tcon on pumping power.
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Figure 15. Effect of reflux ratio on COP and ECOP.
Figure 15. Effect of reflux ratio on COP and ECOP.
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Figure 16. Effect of reflux ratio on components’ thermal loads.
Figure 16. Effect of reflux ratio on components’ thermal loads.
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Figure 17. Effect of Xr on COP, ECOP and vapor quality at evaporator exit.
Figure 17. Effect of Xr on COP, ECOP and vapor quality at evaporator exit.
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Figure 18. Variations of thermal loads with Xr.
Figure 18. Variations of thermal loads with Xr.
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Figure 19. Effects of Teva on COP and ECOP.
Figure 19. Effects of Teva on COP and ECOP.
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Figure 20. Effect of Teva on the CR and cut in/off temperature.
Figure 20. Effect of Teva on the CR and cut in/off temperature.
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Figure 21. Effect of Teva on thermal loads.
Figure 21. Effect of Teva on thermal loads.
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Figure 22. Effect of Teva on pumping power.
Figure 22. Effect of Teva on pumping power.
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Figure 23. Effect of temperature glide on COP and ECOP.
Figure 23. Effect of temperature glide on COP and ECOP.
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Figure 24. Effect of temperature glide on thermal loads.
Figure 24. Effect of temperature glide on thermal loads.
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Figure 25. Effect of absorber temperature on COP and ECOP.
Figure 25. Effect of absorber temperature on COP and ECOP.
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Figure 26. Effect of Tabs on CR and cut in/off Temperature.
Figure 26. Effect of Tabs on CR and cut in/off Temperature.
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Figure 27. Effect of Tabs on the thermal loads.
Figure 27. Effect of Tabs on the thermal loads.
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Figure 28. Effect of Tabs on pumping power.
Figure 28. Effect of Tabs on pumping power.
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Figure 29. Effect of εSHX on COP and ECOP.
Figure 29. Effect of εSHX on COP and ECOP.
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Figure 30. Effect of εSHX on components’ thermal loads.
Figure 30. Effect of εSHX on components’ thermal loads.
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Figure 31. COP increase ratio as a function of εSHX.
Figure 31. COP increase ratio as a function of εSHX.
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Figure 32. ECOP increase ratio as a function of εSHX.
Figure 32. ECOP increase ratio as a function of εSHX.
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Figure 33. Effect of εSHX on the CR and cut in/off temperature.
Figure 33. Effect of εSHX on the CR and cut in/off temperature.
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Figure 34. Effect of εSHX on pumping power.
Figure 34. Effect of εSHX on pumping power.
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Figure 35. Variations of COPs with different εSHX at different Tgen.
Figure 35. Variations of COPs with different εSHX at different Tgen.
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Figure 36. Variations of ECOPs with different εSHX at different Tgen.
Figure 36. Variations of ECOPs with different εSHX at different Tgen.
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Table 1. (a) Results of model validation using data from Ref. [27]; and (b) results of model validation using data from Ref. [29].
Table 1. (a) Results of model validation using data from Ref. [27]; and (b) results of model validation using data from Ref. [29].
(a)
Stream No.Temperature (°C)Energy Flow (kW)
Ref. [27]ModelDifference (%)QuantityRef. [27]ModelDifference (%)
162620.00 Q ˙ g e n 17.116.6−2.9
26972.65.2 Q ˙ c o n 9.69.82.1
379790.00 Q ˙ e v a 10.510.14−3.4
470700.00 Q ˙ a b s 15.914.6−8.2
564663.1COP0.600.600.00
630300.00
831310.00External Flows (kg/s)
10880.00QuantityRef. [27]ModelDifference (%)
1111110.00 m ˙ 17 , 18 0.520.520.00
121111.21.8 m ˙ 19 , 20 0.560.560.00
133030.62 m ˙ 21 , 22 0.330.330.00
143030.72.3 m ˙ 23 , 24 0.820.7725.9
1525250.00
162525.20.8
1786860.00
187978.40.8
1922220.00
203129.8−0.6
2122220.00
223130.90.3
2315150.00
2419190.00
(b)
Stream No.Temperature (°C)Energy Flow (kW)
Ref. [29]ModelDifference (%)QuantityRef. [29]ModelDifference (%)
191.9931.2 Q ˙ e v a 5.05.00.00
4104104.30.3 Q ˙ g e n 8.968.66−3.3
59493.7−0.3 Q ˙ c o n 5.945.68−4.4
646.746.80.2 Q ˙ a b s 8.117.97−1.7
914.314.82.1 Q ˙ S H X 11.4211.712.5
107.37.30.00 Q ˙ R H X 0.740.70−5.4
1112.312.30.00COP0.550.561.8
1231.333.88
1354.654.5−0.2
1454.854.6−0.4
1546.946.90.00
1647.147.30.4
Table 2. Results of system analysis at base condition.
Table 2. Results of system analysis at base condition.
Point No.Temperature (°C)Pressure (bar)Flow (kg/s)NH3 Conc. (By Mass)Enthalpy (kJ/kg)Entropy (kJ/kg·K)
166.210.840.4200.50056.740.7747
264.610.320.4270.50049.070.7517
390.010.320.0990.9571501.934.9729
490.010.320.3280.362180.381.1077
546.210.320.1150.9981344.164.5167
626.010.320.1150.998120.870.4384
726.010.320.0230.998120.870.4384
826.010.320.0920.998120.870.4384
94.310.320.0920.99818.660.0838
10−5.33.500.0920.998−25.69−0.0750
11−5.03.500.0920.9981061.733.9837
12−4.73.500.0920.9981163.944.3649
1339.410.320.3280.362−51.940.4187
1439.53.500.3280.362−51.940.4212
1526.03.260.4200.500−126.180.2040
1626.710.840.4200.500−124.740.2057
Q ˙ g e n = 187 kW       Q ˙ c o n = 140.6 kW       Q ˙ e v a = 100 kW       Q ˙ R H X = 9.4 kW
Q ˙ a b s = 143 kW       Q ˙ S H X = 76.3 kW       W ˙ P = 1.01 kW      CR = 3.65
COP = 0.5316         ECOP = 0.3243
Table 3. Base values for system’s parameters and their simulation ranges.
Table 3. Base values for system’s parameters and their simulation ranges.
ParameterBase ValueSimulation Range
Strong solution concentration (Xss) by mass48.24%41–58 (%)
Refrigerant concentration99.8%98.6–99.9 (%)
Generator temperature (Tgen)90 °C74–120 (°C)
Condenser temperature (Tcon)30 °C20–42 (°C)
Evaporator temperature (Teva)−4 °C−12–12 (°C)
Absorber temperature (Tabs)30 °C15–42 (°C)
Evaporator temperature glide2 °C0.0–14 (°C)
Reflux ratio0.20.0–0.80
Solution heat exchanger effectiveness (εSHX)0.800.0–1
Refrigerant heat exchanger effectiveness (εRHX)0.80not changed
Solution pump efficiency (ηpump)0.65not changed
Cooling power (Qeva)100 kWnot changed
Minimum temperature difference in the generator10 °C-
Minimum temperature difference in the condenser6 °C-
Minimum temperature difference in the absorber6 °C-
Minimum temperature difference in the evaporator10 °C-
Minimum temperature difference in the SHX12 °C-
Minimum temperature difference in the RHX6 °C-
Cooling water temperature24 °C-
Auxiliaries’ power2.3 kW-

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Wageiallah Mohammed, O.; Yanling, G. Comprehensive Parametric Study of a Solar Absorption Refrigeration System to Lower Its Cut In/Off Temperature. Energies 2017, 10, 1746. https://doi.org/10.3390/en10111746

AMA Style

Wageiallah Mohammed O, Yanling G. Comprehensive Parametric Study of a Solar Absorption Refrigeration System to Lower Its Cut In/Off Temperature. Energies. 2017; 10(11):1746. https://doi.org/10.3390/en10111746

Chicago/Turabian Style

Wageiallah Mohammed, Osman, and Guo Yanling. 2017. "Comprehensive Parametric Study of a Solar Absorption Refrigeration System to Lower Its Cut In/Off Temperature" Energies 10, no. 11: 1746. https://doi.org/10.3390/en10111746

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