Experimental comparative analysis of solar thermal and photovoltaic integrated vapor absorption refrigeration systems for low-GWP sustainable cooling under tropical conditions

Abstract

Refrigeration, an indispensable modern technology with ancient origins, continues to evolve under the pressing demand for sustainability. In India, where the domestic refrigeration market is projected to reach ₹285 billion by 2034. The growing dependence on conventional vapor compression systems poses significant environmental challenges due to their reliance on high-GWP refrigerants such as R134a (GWP = 1430). Vapor Absorption Refrigeration (VAR) systems provide a sustainable alternative because they utilize low-grade thermal energy, operate with minimal mechanical components, and can be driven by renewable energy sources. This study presents a comprehensive experimental and comparative evaluation of solar-driven VAR systems integrating two renewable energy conversion pathways—a solar thermal flat plate collector (FPC) and a solar photovoltaic (PV) system. A 100 LPD solar flat plate collector was thermally coupled with a VAR generator through a copper heat exchanger optimized for insulation and mass flow, while a 100 W polycrystalline solar PV module with a 12 V/8 A charge controller and 12/40 Ah battery was employed to electrically drive the same system. The FPC-integrated VAR system achieved an evaporator temperature of 12 °C, and an average thermal efficiency of 35.3%, corresponding to a coefficient of performance (COP) of 0.14. Conversely, the PV-integrated VAR system attained a minimum evaporator temperature of 9 °C, maintaining a stable operational efficiency of approximately 9% and a COP of 0.12.

Introduction

Refrigeration or cooling can be fundamentally described as the thermodynamic process of removing heat from a selected space, substance, or system and rejecting it to a surrounding environment at a higher temperature. This controlled heat transfer enables the maintenance of temperatures below ambient conditions and forms the basis of all modern cooling technologies. The importance of refrigeration spans a wide range of critical applications, from the preservation of temperature-sensitive pharmaceuticals, vaccines, and medical supplies to food processing, storage, and transportation, where it plays a vital role in ensuring food safety and reducing post-harvest losses^1,2. In recent decades, cooling has also transitioned from being a luxury to a basic necessity, with space cooling for residential and commercial buildings becoming an integral component of modern lifestyles, particularly in regions experiencing high ambient temperatures and rapid urbanization³.

According to an International Energy Agency report, 20% of total electricity in a building is for air conditioners and fan which has also stated that, by 2050, around two third of world’s household could have an air conditioner. Half of that total number will be coming from China, India and Indonesia. This increase in air conditioner would increase in demand for electricity and also increases carbon dioxide emission^4,5. Energy demand due to air conditioner also is expected to triple by that time⁶. In the modern world, air cooling and refrigeration has transformed from luxury to necessity. So, the necessity to look for an alternative option that are available has increased.

Vapor compression cycle has been a popular choice of technology for a long time. But it has its own disadvantages like, has wear and tear problems due to moving parts, high vibrations resulting in high maintenance cost, noise pollution and also it depends only on high grade energy like electricity to drive the system. On top of it vapor compression system has the history of using refrigerants that has huge impact on environment⁷. Even with all these disadvantages it is still dominating the market due to low initial cost and high efficiency. Vapor absorption cycle does not have any of those above-mentioned disadvantages. Since vapor absorption system is driven by low grade energy, it has other options to be considered as an energy source. But still it needs to focus on improving its performance to become a competitor^8,9.

In recent years, Vapor Absorption Refrigeration (VAR) systems have attracted renewed attention as a sustainable alternative to conventional vapor compression cycles, primarily due to their ability to operate on low-grade thermal energy rather than high-grade electrical energy¹⁰. These systems can be powered by renewable or waste heat sources, including solar energy, geothermal energy, industrial waste heat, and internal combustion engine exhaust, thereby reducing both electricity demand and greenhouse gas emissions. Furthermore, the absence of mechanical compressors and the use of environmentally friendly working pairs such as H₂O–LiBr, NH₃–H₂O, NH₃–NaSCN, and NH₃–LiNO₃ significantly lower the system’s overall environmental footprint, making VAR technology compatible with low-GWP and zero-ODP goals set under the Kigali Amendment to the Montreal Protocol¹¹. The transportation sector of European Union risks suitability due to the refrigeration technology being used. But integrating renewable technologies and proper sealing of refrigerator would reduce the annual energy consumption up to 28% and annual CO₂ equivalent emission can be reduced up to 72%¹².

However, despite these advantages, the commercial adoption of VAR systems remains limited, primarily due to their lower Coefficient of Performance (COP) and larger initial setup cost compared to conventional systems. Recent studies have therefore focused on improving thermodynamic efficiency through cycle modifications (single-effect, double-effect, and multi-pressure configurations) and the inclusion of heat recovery components such as Solution Heat Exchangers (SHE), Refrigerant Heat Exchangers (RHE), and Solution-Refrigerant Heat Exchangers (SRHE). These advancements have enhanced heat utilization and demonstrated that, under optimized configurations, VAR systems can achieve COP values approaching those of smaller-scale compression systems while maintaining environmental superiority.

Among renewable energy sources, solar energy represents one of the most promising options for driving absorption refrigeration systems, particularly in tropical regions like India, where solar irradiance exceeds 5 kWh/m² per day for most of the year. Solar-assisted absorption systems can be configured in two principal ways:

• Solar Thermal Integration, where heat from a flat plate or concentrating collector directly drives the generator; and.

• Solar Photovoltaic (PV) Integration, where electrical power from solar panels energizes the heating element of the VAR generator.

Each approach offers unique benefits—thermal systems maximize direct utilization of solar heat, while PV systems offer modularity, ease of installation, and integration with existing DC-powered components. However, a comprehensive comparative evaluation of these two solar-integration methods, under identical environmental and operational conditions, remains limited in literature.

Therefore, this study presents a comprehensive experimental investigation and direct performance comparison of two solar-driven vapor absorption refrigeration (VAR) systems operating under identical tropical climatic conditions. The first configuration integrates a solar flat plate collector (FPC) to directly supply low-grade thermal energy to the VAR generator through a purpose-designed heat exchanger, while the second configuration employs a solar photovoltaic (PV) module coupled with electrical storage to power the generator heater. By maintaining the same absorption refrigeration unit and operating environment for both configurations, the study ensures a fair and meaningful comparison focused solely on the solar energy conversion pathways. The contribution of this study, compared to already existing models are as follows.

• Single platform experimental evaluation and comparison of FPC thermal heating and PV-powered electrical heating, enabling a true performance and feasibility evaluation in the real-world framework.

• Operational stability analysis under real solar variability highlighting PV sensitivity to transient shading versus the thermal inertia advantage of FPC systems.

• Based on the experimental deployment framework, identifying auxiliary storage needs for both solar PV system (battery pack) and Flat plate collection (thermal storage tank) for uninterrupted cooling in the rural tropical environments.

• The comparative findings provide valuable insights for selecting appropriate solar integration approaches for decentralized refrigeration applications, particularly in rural and off-grid regions where grid electricity is unreliable or unavailable.

The performance evaluation encompasses detailed monitoring and analysis of key thermal and electrical parameters, including generator and evaporator temperature profiles, hot water and electrical power inputs, system efficiencies, and the resulting coefficient of performance (COP) for each integration strategy. Through this experimental assessment, the study demonstrates the practical feasibility of utilizing low-grade solar energy to drive absorption-based cooling systems and elucidates the relative advantages and limitations of thermal and photovoltaic integration in terms of efficiency, operational stability, and cooling effectiveness.

By experimentally validating solar-assisted VAR systems as low-GWP, energy-autonomous cooling solutions, this work contributes to the advancement of sustainable and low-emission refrigeration technologies and directly supports the United Nations Sustainable Development Goals, specifically SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action), through practical pathways for decarbonizing the cooling sector via renewable energy integration¹³.

Refrigerant

In vapor absorption system the working fluid is in the form of refrigerant-absorber pair. Working fluid is nothing but a medium which is used to transfer heat from the specific region and release it to the surrounding. A good refrigerant should have the following properties¹⁴.

• It should be non-corrosive, non-explosive, low cost, chemically stable and the newly required environmentally friendly with low Global Warming Potential and Ozone Depleting Potential.

• It should have low viscosity.

• Refrigerant should have high thermal conductivity.

• Achieve improved cooling with low specific heat.

• It should have low freezing point.

• The difference between the boiling of refrigerant and refrigerant/absorbent mixture should be as high as possible.

Vapor absorption cycle

The basic design is single effect absorption system, with components include generator, condenser, evaporator and absorber. When volatile absorbents like water/ammonia are used, an additional component called rectifier is used to purify the refrigerant entering the condenser. Other types include multiple effect system, half effect system and combined ejector system etc., Half effect system have two absorber and two generator which can operate at a relatively lower temperature¹⁵. Two stage absorption cycle would be divided into high pressure stage and low-pressure stage with high pressure generator, high pressure absorber, low pressure generator and low-pressure absorber. High pressure generator will supply the refrigerant to the evaporator and the refrigerant from the evaporator will be collected in the low-pressure absorber. After a detailed discussion author has concluded that double effect system will show improvement in COP and two stage system will make it possible to drive the system by a simple flat plate collector¹⁶.

Absorption cycle can be used for both air conditioning and refrigeration. As the demand for the cooling technology increases, the energy demand would also increase. Vapor Compression cycle has inductive load which would introduce THD in the system. because of this power quality varies between 0.6 and 0.8. Absorption cycle does not have any inductive load so, it maintains a unity power factor with zero THD. In terms of power quality Vapor Absorption refrigeration system has shown better performance¹⁷.

Coefficient of performance and circulation ratio

COP is defined as the ratio of cooling capacity obtained at evaporator to the sum of generator heat input and pump work input. It does not have any unit. Pump work input is negligible compared to the generator heat input and neglected in many cases. In certain cases, where waste heat is utilized generator heat input would be neglected as it is drawn from waste. Circulation Ratio is the ratio of mass flow rate of solution entering the generator to the mass flow rate of refrigerant.

1

Single stage absorption cycle

The schematic of Absorption cycle is shown in Fig. 1. The working fluid of absorption cycle is a mixture of refrigerant absorbent pair. The low-pressure refrigerant from the evaporator would enter he absorber. In absorber, weak solution mixes with the refrigerant to form strong solution. This concentrated solution is pumped to the generator where it is heated. The refrigerant with low boiling point would vaporize leaving behind weaker solution which returns back to absorber to produce strong solution. The refrigerant vapor is condensed in the condenser to form high pressure low temperature refrigerant. It is passed through expansion valve to reduce the pressure. The low pressure low temperature saturated liquid refrigerant would absorb the heat from the evaporator¹⁷. The most commonly used refrigerant for absorption cycle in commercial market are ammonia-water and water- lithium bromide. Both the refrigerant has its own disadvantages like water-lithium bromide pair has crystallization problem and its refrigerant turns into ice at 0 °C. So, it is mainly recommended for air conditioning system. Ammonia - lithium bromide solution also has its disadvantages like water vapor would enter the refrigeration cycle reducing the performance of the system. In order to overcome this, it requires a separator/rectifier making the system more complex. So it is advised to use it for industrial applications¹⁸.

Fig. 1.

Single stage absorption cycle¹⁸.

After design modification of a water ammonia absorption system, just by adding a six-mass transfer stage separator, improvement in COP is noted, as it can reduce the weak solution entering the cycle. In the same system, if an additional throttling valve is added before the separator, then the COP value will be further improved from 0.45 to 0.54 as, it can help separate the refrigerant from the strong solution¹⁹.

In the view of improving the COP, author has made some design changes by introducing SHE, RHE, and SRHE into the system as shown in Fig. 2. Tables 1 and 2 gives the performance of system with heat exchangers. Adding heat exchangers would help increase the energy utilization. The output from the condenser and evaporator are passed through RHE to transfer heat. The heat from the condenser output would be transferred to evaporator output to produce sub-cooled refrigerant. Strong solution from absorber is passed through SRHE and SHE to generator. In SRHE the heat exchange happens between strong solution and refrigerant output from the generator. Output. High temperature refrigerant would transfer heat to preheat the strong solution. This strong solution is passed through SHE to absorb heat from the high temperature weak solution returning from the generator to absorber. The result has clearly shown that SHE has made big improvement in COP with 66% increase. While, RHE and SRHE can provide only 14% and 4% improvement respectively. So, for final study only SHE and RHE is used. This developed system is studied using H₂O-LiBr, NH₃-H₂O, NH₃-LiNO₃ and Acetone- Zinc Bromide as the working fluid. Compared to H₂O-LiBr and NH₃-H₂O, NH₃-LiNO₃ has best performance for low generator and high condenser temperatures. Acetone- Zinc Bromide in this system can provide better performance even for a low generating temperature²⁰.

Fig. 2.

A schematic of single effect VAR system with SHE, RHE and SRHE²⁰.

Table 1.

COP based on design changes²⁰.

Refrigerant	COP without heat exchanger	COP with SHE	COP with RHE	COP with SRHE
NH3-H2O	0.39	0.65	0.45	0.415

Table 2.

Performance variation with design modification²⁰.

Refrigerant	Operating condition				Other parameter	COP	Flow ratio
	Tg(oC)	Tc (oC)	Ta(oC)	Te(oC)
H2O-LiBr	90	35	35	4	εSHE = 0.6 εSHE = 0.6 ηp = 0.90	0.76	7.37
NH3-H2O	90	35	35	4	εSHE = 0.6 εSHE = 0.6 ηp = 0.90	0.54	5.09
NH3-LiNO3	90	35	35	4	εSHE = 0.6 εSHE = 0.6 ηp = 0.90	0.55	5.85
Acetone-Zinc Bromide	53	28	28	12	εSHE = 0.75 εSHE = 0.75 ηp = 0.90	0.7	Less than 5

An alternative refrigerant acetone-zinc bromide is used in simple absorption system with SHE and RHE is taken for study. Mass balance equation for each component is derived and studied for different conditions using simulation model and compared with the theoretical model. Maximum COP of 0.73 is recorded under the following conditions: generator temperature of 57 °C, condenser temperature of 28 °C, absorber temperature of 28 °C, and evaporator temperature of 15 °C for heat exchanger effectiveness of 0.75. This system can be driven by generator temperature as low as 50 °C, under which, evaporator temperature up to 13 °C can be achieved with the COP of 0.40²¹.

Finding a perfect alternative refrigerant will help in making this technology even more competitive. So, author has studied the performance of NH₃-H₂O, NH₃-LiNO₃ and NH₃-NaSCN in a single effect system. COP of NH₃-H₂O, NH₃-LiNO₃ and NH₃-NaSCN at T_g=100 °C, T_c=30 °C, T_a=25 °C to achieve T_a=−5 °C are 0.616, 0.6247, 0.639 respectively. They are almost similar so; circulation ratio has to be checked. The circulation ratio of NH₃-H₂O, NH₃-LiNO₃ and NH₃-NaSCN at T_g=90 °C, T_c=20 °C, T_a=25 °C to achieve T_a=−5 °C are 3.5, 4, and 5.1 respectively. All three have similar COP value so decision can be made using flow ratio value. Both the alternatives considered have shown better performance and NaSCN has shown even better performance for achieving low evaporator temperature but not below − 10 °C. At the same time, NH₃-H₂O has shown better performance than the considered alternatives for higher evaporator temperature²². Author has compared H₂O-LiBr, H₂O-LiCl, H₂O-NaOH, H₂O-LiI, H₂O-LiBr + LiI (salt mole ratio 4:1), H₂O-LiCl + LiNO₃ (2.8:1), H₂O-LiBr + LiNO₃ (4:1), H₂O-LiBr + ZnBr₂ (2:1), H₂O-LiBr + LiSCN (1:1), H₂O-LiBr + LiCl + ZnCl₂ (3:1:4), H₂O-LiBr + ZnCl₂+CaBr₂ (1:1:0.13), H₂O-LiBr + ZnBr₂+LiCl (1:1.8:0.26), H₂O-LiBr + LiI + C₂H₆O₂ (3:1:1), H₂O-NaOH + KOH + CsOH (4.3:3.6:2.4). H₂O-LiNO₃+KNO₃+NaNO₃ (5.3:2.8:1.9), H₂O-LiCl + CaCl₂+Zn (NO₃)₂ (4.2:2.7:1) with the aim of finding low cost, environmentally friendly working fluid using a simulation model. A regression analysis equation is developed to calculate the COP and circulation ratio which are given in the Table 3. It was evident that H₂O-LiBr + LiCl + ZnCl₂ has shown better performance in terms of COP while, H₂O-LiCl performs better in terms of circulation ratio²³. Effect of operating condition on COP and circulation ratio is given in Table 3.

Table 3.

Performance for the operating condition of T_g=90.15, T_c=35.15, T_a= 35.15, T_e=5.15²³.

Refrigerant	COP	Circulation ratio
H2O-LiBr	0.85	4.5
H2O-LiCl	0.93	2
H2O-NaOH	0.81	4.5
H2O-LiI	0.81	6.5
H2O-LiBr + LiI (salt mole ratio 4:1)	0.79	6
H2O-LiCl + LiNO3 (2.8:1)	0.84	3.5
H2O-LiBr + LiNO3 (4:1)	0.84	5.5
H2O-LiBr + ZnBr2 (2:1)	0.83	11.5
H2O-LiBr + LiSCN (1:1)	0.79	5
H2O-LiBr + LiCl + ZnCl2 (3:1:4)	0.96	9.5
H2O-LiBr + ZnCl2+CaBr2 (1:1:0.13)	0.85	11
H2O-LiBr + ZnBr2+LiCl (1:1.8:0.26)	0.86	11.5
H2O-LiBr + LiI + C2H6O2 (3:1:1)	0.84	11
H2O-NaOH + KOH + CsOH (4.3:3.6:2.4)	0.94	6
H2O-LiNO3+KNO3+NaNO3 (5.3:2.8:1.9)	0.95	10
H2O-LiCl + CaCl2+Zn(NO3)2 (4.2:2.7:1)	0.85	9

Performance of LiBr +H₂O, LiBr + H₂N(CH₂)₂OH + H₂O, LiBr + HO(CH₂)₃OH + H₂O, and LiBr + (HOCH₂CH₂)₂NH + H₂O were studied for air cooled absorption cycle and results has shown that all the alternatives considered has performed better than LiBr +H₂O in terms of both f and COP under the selected condition. Alternative pairs considered had no problem operating at high absorber and condenser temperature. For high operating conditions of T_c= 50 °C, T_a= 50 °C and T_e=5 °C COP and f can be given as follows²⁴. Table 4 gives the COP and circulation ratio for different refrigerants at different generator temperature.

Table 4.

COP and f for high operating conditions of T_c= 50 °C, T_a= 50 °C and T_e=5 °C²⁴.

Refrigerant	Tg	COP	F
LiBr +H2O	175.2	1.36	22
LiBr + HO(CH2)3OH + H2O	185.1	1.38	10.6
LiBr + (HOCH2CH2)2NH + H2O	185.05	1.5	10.2
LiBr + H2N(CH2)2OH + H2O	200	1.48	10.1

By testing the performance of R134a as refrigerant with DMETEG, MCL and DMEU as absorbent, it had clearly shown that, there is not much difference in the COP for achieving evaporator temperature of −5 °C which can be given by 0.46, 0.47, and 0.49 respectively. So, once again flow ratio does its role selecting the superior refrigerant absorbent pair as DMETEG, MCL and DMEU have flow ratio of 7, 10, and 13 respectively²⁵. The study is done in a 15-ton prototype model. It has an electric circulation fan and thermal load was simulated. Alternative refrigerants considered are R22 with NMP, DMMP, MCL, DDMF, and DMETEG. R22- NMP, R22-DMMP, R22-MCL, R22-DMF and R22-DMTEG has COP of 0.55, 0.51, 0.46, 0.58 and 0.47 respectively so it is suggested to make decision based on the circulation ratio. The developed system has low energy, installation and maintenance cost. Alternatives considered are also not toxic²⁶. During the selection of refrigerant, absorption capacity should also be considered. Due to low solubility LiBr + CH₃CO₂K (potassium acetate) + water (LiBr/CH₃CO₂K) 2 by mass), and LiBr + CH₃CH (OH) CO₂Na (sodium lactate) + water (LiBr/CH₃CH (OH) CO₂Na) 2 by mass) are not taken for further study. Lithium bromide +sodium formate (CHO₂Na) has proven this true by having high COP of 0.94 when compared with LiBr + CHO₂K (potassium formate) + water (LiBr/CHO₂K) 2 by mass) which has COP of 0.91 for the given operating conditions as follows T_g=61.15 °C, T_c=46.25 °C, T_a=12.15 °C to achieve T_a=−5.65 °C²⁷.

A simple vapour absorption refrigeration system with ejector type mixture and a pre-absorber is selected for study. For generator temperature of 100 °C, condenser temperature of 32 °C and absorber temperature of 28 °C for achieving an evaporator temperature of −5 °C, R134a-DMAC, R134a-DMETEG, R124-DMAC and R124-DMETEG has COP of 0.52, 0.46, 0.53 and 0.46 respectively and the flow ratio can be given by 4.5, 6.5, 2.5 and 4 respectively. Thermophysical properties and stability of R134a and R124 in combination with DMAC and DMETEG are also studied. All the considered alternatives are found to stable. Based on the study R124-DMAC stand superior²⁸. A specially designed system, entirely in the form of heat exchanger, with ammonia/lithium nitrate is taken for study. The generator and absorber are in the form of coil heat exchanger while condenser, evaporator and SHE are in the form of plate heat exchanger. It operates at generator temperature as low as 85 °C making it possible to use solar in future. Maximum COP of 0.7 was recorded for hot water temperature of 105°C²⁹.

According to Mehrdad Khanooshi et al., after doing a state of art review, ionic liquids would also be a great alternative as they have less crystallization, less corrosively and also less toxicity³⁰. As it is non-inflammable at operating temperature, low melting point, no crystallization issue, metal compatibility and less toxicity ionic liquids are selected as working fluid for electronic cooling. Heat energy input is neglected for COP calculation, as it is drawn from the waste heat. COP values are listed in Table 5. Water/[emim][BF₄] pair has high COP value at desorber temperature (T_d)of 90 °C and can cope with the chip temperature of 85 °C³¹.

Table 5.

Performance of ionic liquid based on desorber temperature³¹.

Working fluid	Td (oC)	COP
Water/[emeim][BF4]	70	0.91
R32	74	0.42
R134a/[hmim][PF6]	79	0.15
R134a	82	0.22
R134a/[hmim][Tf2N]	80	0.19
R32/[bhmim][BF4]	85	0.41
R134a/[hmim][BF4]	80	0.19

Absorption system with pressure levels

In a single stage system with TPLAC as shown in Fig. 3, study has been done with DMEU as absorbent along with six different refrigerants. TPLAC has three S pressure levels to increase the cooling. The chosen system has SHE, RHE and a jet ejector is introduced between SHE and absorber to improve mixing of strong and weak solution. The heat exchanger helped to transfer low-grade heat to improve the efficiency of the system³² and datasets of temperature and COP values are showcased on Table 6.

Fig. 3.

Schematic of single stage TPLAC³³.

Table 6.

Performance of TPLAC³².

Refrigerant	Tg	Tc	Ta	Te	COP	Circulation ratio
R22-DMEU	100	32	28	−5	0.685	3.74
R32-DMEU	125	32	28	−5	0.658	10.06
R124-DMEU	98	32	28	−5	0.642	3.44
R152a-DMEU	128	32	28	−5	0.598	12.91

R125-DMEU as working fluid is studied for TPLAC system and found that it requires lower heat input to achieve even − 15 °C. A jet ejector with no moving parts is introduced between the absorber and generator. To improve performance, it has additional heat exchanger like SHE and RHE. COP and f for condenser temperature of 32 °C and absorber temperature of 28 °C are given in the Table 7³⁴.

Table 7.

Comparison of single stage absorption cycle and TPLAC³⁴.

Working fluid	Single stage absorption cycle				TPLAC
	Tg (oC)	Te (oC)	COP	Flow ratio	Tg (oC)	Te (oC)	COP	Flow ratio
R125-DMEU	90	0	0.57	3.5	81	0	0.595	3.5
	102	−5	0.53	4	91	−5	0.555	3.9
	115	−10	0.485	4.6	101	−10	0.515	4.4
	132	15	0.445	5.5	113	15	0.480	5.2

To produce cooling less than 0 °C using low grade energy TPL and DPL system are studied. Performance of DPL and TPL system with the following working fluid R125-DMETEG, R125-NMP, R125-MCL, R125-DMAC, R125-DMEU and R125-DMPU are studied. TPL had low f although it had similar COP with DPL. Relative size of heat exchanger is small for TPL cycle. COP and f for the generator temperature of 100 °C, condenser temperature of 32 °C, absorber temperature of 28 °C and evaporator temperature of −5 °C are given in the Table 8³³. It is evident from the Table 8 that DMEU has higher COP still, NMP would be preferred because of its low flow ratio than DMEU.

Table 8.

Comparison of DPL and TPL cycle³³.

Refrigerant	DPL		TPL
	COP	Flow ratio	COP	Flow ratio
R125-DMETEG	0.46	4.2	0.49	3.35
R125-NMP	0.54	3	0.56	2.6
R125-MCL	0.53	4.1	0.55	3.35
R125-DMAC	0.54	3.4	0.56	2.9
R125-DMEU	0.56	4.2	0.59	3.3
R125-DMPU	0.52	3.9	0.54	3.25

Half effect absorption system

Half effect system has condenser and evaporator operating at two pressure levels. The refrigerant from the evaporator first enters low absorber. The strong solution then enters the low generator through a low heat exchanger. The weak solution would return to low absorber. The low generator output would enter the high generator through high absorber and high heat exchanger. The refrigerant output from the high generator is then passed through the condenser and through evaporator to complete the cooling cycle. Using a half effect refrigeration system will reduce the amount of input requirement from 100 °C to 80 °C, even for H₂O-LiBr and NH₃-H₂O. If R134a-DMAC is used as a refrigerant in this system, it will allow even the low-grade energy from the solar to drive the system. Half effect cycle requires lesser generator temperature than the single effect system and hence it can be considered in utilizing waste heat recovery system to meet the cooling requirements³⁵. When a two-stage half effect system with R134a-DMAC as working fluid is used, then the energy required to drive the system gets reduced to a much lower level. This system can achieve up to 8 °C evaporator temperature using a 55 °C heat source. So, even waste heat can be used to run the system³⁶. Table 9 gives the performance of half effect adsorption refrigeration system.

Table 9.

Performance of half effect absorption system.

System type	Working fluid	Tg	Tc	Ta	Te	Other parameters	COP	Reference
Half effect absorption system	R134a- DMAC	70	20	25	5	εCPC = 0.8	0.46	35
	NH3-H2O	70	20	25	5		0.34
Two stage half effect vapour absorption system	R134a- DMAC	75	20	Nil	−7	Tha= 25 Tla= 20	0.37	36

Double effect absorption system

Author has chosen double effect absorption refrigeration system with lithium nitrate and ammonia sodium thiocyanate as working solution. Its performance is studied for series, reverse and parallel configuration. NH₃- NaSCN has recorded a maximum COP of 1.5 for double effect parallel system, while NH₃-LiNO₃ has a maximum COP recorded for a much lower generator temperature and tabulated on Table 10. NH₃- NaSCN has limited operating range due to crystallization problem³⁷.

Table 10.

Performance of double effect absorption system³⁷.

Working fluid	Tc	Te	THGT	COP
LiNO3 series	–	0	165	0.8
LiNO3 Parallel	–	0	192	0.88
LiNO3 Reverse	–	0	192	0.82
LiNO3 Single	–	0	100	0.55
LiNO3 Series	–	−10	190	0.69
LiNO3 Parallel	–	−10	195	0.75
LiNO3 Reverse	–	−10	200	0.72
LiNO3 Single	–	−10	155	0.49
NaSCN Series	–	0	183	0.89
NaSCN Parallel	–	0	195	0.98
NaSCN Revese	–	0	200	0.92
NaSCN Single	–	0	113	0.63
NaSCN Series	–	−10	195	0.72
NaSCN Parallel	–	−10	200	0.81
NaSCN Reverse	–	−10	113	0.63
NaSCN Single	–	−10	110	0.55
LiNO3 series	30	–	145	0.99
LiNO3 Single	30	–	85	0.61
LiNO3 Reverse	35	–	192	0.82
NaSCN Parallel	30	–	200	1.5
NaSCN Series	35	–	180	0.89
NaSCN Single	35	–	110	0.63
LiNO3 Parallel	30	–	145	0.95
LiNO3 Series	35	–	155	0.8
LiNO3 Single	35	–	100	0.55
NaSCN Revese	30	–	200	1.2
NaSCN Parallel	35	–	190	0.99
LiNO3 Reverse	30	–	200	0.9
LiNO3 Parallel	35	–	190	0.88
NaSCN Series	30	–	155	0.98
NaSCN Single	30	–	80	0.65
NaSCN Reverse	35	–	200	0.9

A state of art review shows that water is the highly chosen refrigerant because of very low cost and low environmental effect³⁰³⁸. While deciding the type of system, it is required to make an economic analysis. Especially, when solar is integrated, cost of both the solar and integration unit should be included in the initial cost⁴³⁹. By analyzing vapor compression system with single and double effect absorption refrigeration system the following results are found. Double effect system costs 30% less than the vapor compression system and 45% less than the single effect system. Vapor compression system would cost 11% lower than the single effect system. So, the double effect system is recommended as performs better economically⁴⁰. To improve heat transfer and reduce energy consumption nano-lubricants are also considered⁴¹.

Effects of temperature on the performance of the system

Absorption system components are generator, condenser, expansion valve, evaporator and absorber. Temperature of each of these components has its effect on both COP and circulation ratio. Temperature of each of the components of the system can also affects the overall performance of the system. When the generator temperature is increased, the amount of weak solution in the cycle will be reduced and hence improvement in COP can be noted. When the evaporator temperature is increased then the amount of cooling need to be produced will be reduced and hence the COP of the system gets improved²⁰²⁴. By increasing the desorber temperature circulation ratio will be decreased. If it is increased more than the limit, then this effect will be nullified due to increase in enthalpy. So, instead of providing useful improvement, it will only result in wastage of energy source³¹.

Increased generator temperature would increase the heat transfer. At the same time compressor temperature would also increase⁴². For higher condenser and absorber temperature both COP and circulation ration gets affected³⁶. Increase in absorber and condenser temperature also has crystallization problems²⁴. Change in condenser temperature by ±3 °C will affect the COP by ± 2–4.5.5% and circulation ratio by ± 19–27%³². This effect is more prominent in H₂O-LiBr and NH₃-H₂O, where the COP falls rapidly when the for condenser temperature rises above 45 °C²⁰. In half effect system, temperature of low absorber has more effect on the system performance than the temperature of high absorber³⁵.

Climatic condition also affects the performance of the system⁴³. It is not possible to control the climate. So, it is necessary to select either water cooling or air-cooling option based on the location. In water cooled system, condenser temperature up to 30 °C can be achieved. At the same time, if air cooled system is used, then the condenser temperature will be up to 50 °C²⁴. Using TRNSYS model simulation study has been done to improve the COP. For this, water from the close by well at a temperature of 17 °C is collected in a water tank of 25m³, which will be filled every day to irrigate the sports ground, which in turn can used to remove the heat from the absorber and condenser. From the result it was evident that, geothermal cooling will help improve the mean COP of the system by 42%⁴³.

Renewable energy sources

As stated earlier, vapor absorption cycle can be driven by low grade energy, which can be obtained from variety of ways like solar energy, waste heat recovery etc. One such example is utilizing engine exhaust. IC engine operates with only 35–40% efficiency level, while the remaining will be waste heat⁴⁴.

Solar driven system

A 14,865kWh solar power tower used to drive a vapor absorption and compression cascaded refrigeration system. The exergy and energy efficiency of the system is 39.53% and 28,82% respectively. An evaporator temperature of −20 °C can be reached. This performs better than the supercritical CO₂ and Rankine cycle⁴⁵. The performance of the system is better than the A solar FPC with 48m² collector area was used to drive a simple vapor absorption refrigeration system. COP of the refrigeration unit is 0.686 and the COP of overall system including water heater is 0.5832^46,47. For rural areas, absorption refrigeration system with CPC, to supply the required heat is used to produce 8 kg of ice per day. It is shown in Fig. 4. But it can only provide intermittent refrigeration. Even with intermittent operation, evaporator temperature up to −11 °C can be achieved⁴⁸.

Fig. 4.

Solar thermal integrated absorption refrigeration system⁴⁸.

A PTC integrated vapor absorption system model was studied for intermittent cooling. The output from the thermal collector is enough to produce cooling effect. The COP of the system is 0.209 for producing an evaporator temperature of 4°C⁴⁹. PTC is integrated with Vapor Absorption refrigeration system with a heat exchanger was studied. The study location was Dindigul, Tamil Nadu lies in the tropical region and they have average temperature of 30 °C. The average efficiency of the thermal collector is 60% during the period of operation. After 3 h of operation temperature as low as 11 °C is reached. A TRNSYS simulation model with a storage tank has shown that 24 h cooling at a temperature of 7.9 °C is possible. the COP of the system is 0.46⁹. The solar photovoltaic based micro cold storage integrated in the electric vehicle could solve the last mile cold chain connectivity with the temperature range between 5 and 9 °C effectively⁵⁰⁵¹,. Solar energy is an inherently intermittently due to the diurnal and seasonal variability as well as stochastic atmospheric conditions necessitating the system level design modification for the 24-hour operation⁵⁰⁵²,. To match the gap between cooling demand and available solar energy a storage tank is necessary. With optimum energy storage the COP is around 0.41⁵³. For this author has discussed about adding a storage unit with the system. In refrigeration storage system it can store enough energy, but due to low solution concentration and increased pressure, performance of overall system gets affected. In this same paper author has also studied the possibility of considering solar pond as an alternative for commercial solar collector¹⁶. A 24-hour operating solar driven system has been developed with the help of three types of storage tanks, which are heat storage, cold storage and refrigerant storage. It has two solar collectors, one for day time use and the second is for night time use. Study was done for evaporator target temperature of −9°C. Heat storage system would require higher heat energy so, it requires concentrating collector, which in turn would increase the overall cost of the system. Among them, refrigerant storage has performed better⁵⁴. The hot water storage tank used in solar thermal VAR can be utilized for membrane distillation as a secondary operation⁵⁵. The use of Phase Change Material(PCM) for latent heat thermal energy storage can reduce the auxiliary energy consumption by 6.2%⁵⁶. Latent heat storage system would store 10–15 times more energy than the sensible heat storage system⁵⁷. A seasonal storage absorption storage tank in a solar energy cascaded absorption compression system would reduce the seasonal energy consumption by 31.58% and a 30,431 kg of CO₂ consumption can be reduced⁵⁸.

A 2-kW absorption system using concentrating solar collector is selected for study. Solar collector will heat the oil which in turn would be used to supply heat to the generator. Completely manual system with concentrating collector for isolated area has been studied. It is completely based on manual operation so; it is not possible to achieve uniform operation. Making the system automatic would make it a more reliable technology⁵⁹. A 10-ton LiBr/H₂O system with heat supplied by 72 m² evacuated tube collector. LPG for auxiliary heating and a storage tank is integrated to make it a continuous process. The entire system is controlled and monitored using a computer system which will help maintaining uniform operation. Making the solar collector in mass production will help bring down the cost⁶⁰.

A simulation model of 11 kW solar powered water LiBr absorption unit with solar collector, storage tank and a boiler are taken for study. In this system, CPC with a reflecting surface to focus on a collector area is selected. This system has a limitation and can be used only for areas with high direct radiation. It has also shown that collector slope angle, storage tank size and collector area have its effect on the overall performance of the system⁶¹. Extra energy from overly sized solar water heater was compensated by utilizing it for, air cooling in Zaragoza (Spain). A TRNSYS system model is developed and used to improve the COP of the system. With the suggested alteration that is mentioned in the Sect. 3 the simulation model has shown up to 42% improvement in COP⁴³.

For the same system experimental study has been done. For the existing system without alteration, there is only 5% variation between the simulation and experimental result during the steady state period of the day. Then it is studied with the design alteration but they are completely different. This is because the storage tank temperature is considered to be 17 °C, same as the well temperature, but in fact, the storage tank temperature is equivalent surface temperature i.e., 25 °C. Due to high thermal conductivity of ground, a buried heat exchanger in the form of flat plate heat exchanger is designed to overcome this mistake. This will help to improve the COP from 0.51 to 0.6⁶². A 25-kW air cooled system with two generator is taken for study. A heat pipe in the form of heat exchanger is used to reduce the input heat required. Solubility of AB150, AB300, clav 68, clav 32 are studied and AB300 oil along with alkylated benzene is chosen as a working fluid⁶³.

For high temperature areas, two stage absorption system is suggested. This system uses a double line heat exchanger. This design change helps to improve the COP by 4%⁶⁴. In cases where flat plate collector is not enough, instead of using concentrating collector which would increase the cost, a double-glazed collector can also be considered⁶⁵.Solar powered absorption cycle with two generators and design modification to adapt a pump free system has been studied. LiBr – H₂O is used as working fluid. A specially designed falling film evaporator tube which extends to the absorber is used. This helps in making the process continuous without a pump⁶⁶.

Author has used a simulation model of a newly developed absorption cycle that can be driven with both solar and electrical energy. This modified system has an additional compressor added between the generator and the condenser. This system has shown a better performance with a steady refrigeration capacity and has recorded a maximum COP of 1.6 in 16 h⁶⁷. Author has designed a system using MATLAB which can produce power and provide refrigeration using solar thermal energy, in this case it is cylindrical parabolic collector. A dual effect vapor absorption refrigeration system was used. One generator was used for cooling and another was used for power generation. With the help of a simple valve arrangement only cooling and only power generation can also be done. Power generation depends on the concentration of aqua ammonia solution. The maximum power output it can generate is 58.751 kW and has a cooling capacity of 91.57kW⁶⁸.

Author has discussed about the various chiller technologies especially absorption and adsorption cooling. Due to size and cost, absorption system has won over the adsorption system in the current scenario. Based on the Lahore, Pakistan climate evacuated tube collector is chosen for supplying energy. A system with collector area of 998.4m² to provide a generator temperature of 80 °C is designed. Due to high solar potential and high electricity bill and a payback period of 4.1 years make will it worth investing⁶⁹.

A 40-liter experimental model driven by 360watts polycrystalline solar panel was studied. It can save 569.4kWh of energy per year. It started cooling after 15 min of starting. It reached the temperature of 10 °C. This system has a COP of 0.14 with a payback period of around 10 years⁷⁰.

Waste heat driven system

For automobile cooling, if vapor compression system is used, it adds additional load on the engine as it will require additional energy to drive it. But using Vapor Absorption system with ammonia water as working fluid will allow it to utilize the waste heat from the engine exhaust. It can be done easily, by just placing the generator of the system closer to exhaust pipe⁴⁴.

A study model with LiBr as a working fluid was driven by waste heat from IC engine shows that it is possible to operate it but the COP of the system would be low⁴². For absorption refrigeration system driven by engine exhaust, performance has been studied for different valve opening and it is found that, at 25% opening it have low cooling capacity but comparatively high COP. When it is opened 100%, it had high cooling capacity but lesser COP⁷¹. A prototype model of single stage system with ammonia water as the working fluid driven by engine exhaust, to cool a 40-ton truck of cooling capacity 5 kW is designed and studied. It is found to be suitable for long distance flat road travel⁷². Due to low viscosity, synthetic polyester is selected for R290/oil pair which is used as a refrigerant for a simulation system model. Both exergy and energy studies are made. It has recorded a COP of 0.4649⁷³. A prototype of engine exhaust was used to drive absorption cycle with NH₃-NaSCN as working fluid. Waste heat is transferred to the generator using a fin-type heat exchanger. The evaporator temperature reached up to −24.56 °C in 2 h and 55 min. The waste heat supplied to the generator was at 233 °C. The overall COP of the system was 0.3507. Even in the different study conditions the refrigerant does not cause any crystallization problems. By controlling the pressure valve the evaporator temperature can be controlled. So, it can be used for both room cooling and creating ice. The author has stated that as far as there is an uninterrupted waste heat is supplied at 150–350 °C, this system can be adapted for long distance application in both road and water transport⁷⁴. An absorption-compression cycle cascaded refrigeration system was supplied with waste heat from IC engines. The exhaust gas heat is supplied to the high-pressure generator and the engine coolant is used for low pressure generator. In this system evaporator temperature reached − 35 °C for condenser temperature of 40°C⁷⁵.

Even the most efficient steam power plant will only have 40% efficiency with remaining 60% wasted as heat energy. Considering this, author has studied the performance of a vapor absorption system with water lithium bromide solution as working fluid, driven by utilizing that 60% waste heat. At a 97 °C temperature from waste heat source for environmental temperature of 30 °C to achieve an evaporator temperature of 17 °C the COP is found to be 0.6569. This system provides an opportunity to reduce energy cost⁷⁶. Author has studied the possibility of utilizing waste heat from the gas turbine in the oil and gas industry for combined heat and power (CHP). Usually, this system has a propane vapor compression system which utilizes electrical energy. As vapor absorption system can be driven by the waste heat from the turbine, this electrical energy can be completely saved and also reduces the emission. This system was designed using engineering equation solver. A simple ammonia water solution has COP of 0.57 and 0.17 in first and second chiller respectively and the overall system COP would be 0.41. A double lift ammonia water cycle with low pressure level absorber and evaporator and intermediate pressure level absorber and evaporator. This system has a rectifier, flash drum to separate steam and pass it through intermediate pressure absorber, and a SHE to increase system efficiency. This new system had an overall COP of 0.47. Single effect system is preferred because it provide required cooling for both the chillers. A 9 MW electricity generating plant provide a 5.2 MW waste heat and this modelled system could save 1.9 MW of electricity⁷⁷.

A single effect lithium bromide water absorption cycle and a sub critical carbon dioxide cycle in cascade for naval application has been designed. This system is driven by exhaust from gas turbine for providing both heating and cooling. The heat rejected in the absorber and condenser would be used for dehumidifying. COP of the absorption cycle is 0.7803 and compression cycle is 2.173 and the overall COP can be given by 0.594. Instead of using only vapor compression system, this cascaded system would help reduce electricity demand up to 31%. This system can meet 85 MW cooling demand at 5 °C provided by vapor absorption system and a 51 MW of cooling demand at −40 °C provided by vapor compression system. As waste heat is the input, COP calculated neglecting this would reach up to 8 for overall cycle⁷⁸. The COP of a solar driven system can be affected by seasonal changes as it varies from 0.07 to 0.33 between seasons⁷⁹. The waste heat from the solid waste incinerator can be utilized to drive the VAR system. The cold chamber temperature has been reduced by 9.1 °C⁸⁰. A review on solar driven absorption cooling system has COP in the range of 0.1 to 0.9 in system implementation and simulation models has shown COP more than unity⁸¹. Based on the type of vapor absorption refrigeration, type of solar collector and solar thermal energy storage the COP of the system can vary from 0.3 to 1.8⁸². Table 11 gives the performance of Vapor Absorption Refrigeration integrated with alternative energy. It shows that renewable energy driven VAR system has COP ranging from 0.055 to 0.787.

Table 11.

Alternative energy driven absorption cycle.

Source	System specification		COP	Reference
Solar power tower	Cascaded VAR and VCR system		0.5391	45
Solar flat plate collector	Single stage cooling system		0.72	43
Solar water heater	With second generator		0.787	66
	Without second generator		0.53
Cylindrical parabolic collector	Lithium nitrate is the working fluid		0.08	48
IC engine exhaust gas	Single stage absorption system with ammonia water as the working fluid.		0.049	71
Solar water heater	Tg=70 °C; Refrigerant storage for ambient temperature of 20 °C		0.65	54
	Tg=120 °C; Evacuated tube collector for cold storage		0.372
	Tg=120 °C; Evacuated tube collector for refrigerant storage		0.427
	Concentrated collector heat storage system		0.4343
Flat plate solar collector	Uses propane/alkylated benzene (AB300 oil) as he refrigerant. Te= 3 °C, Ta= 36 °C, Tc= 25 °C, Tg=61 °C		0.6	63
Automobile exhaust	Te= −20 °C, Tatm= 20 °C		0.28	72
	Te= −20 °C, Tatm= 30 °C		0.27
	Te= 0 °C, Tatm= 20 °C		0.30
	Te= 0 °C, Tatm= 30 °C		0.28
Resistance to heat oil tank	CI COP calculated without electrical energy input		0.49	83
	COP calculated with electrical energy input		0.37
Flat plate collector	5 ≤ Te≤15 °C, 30 ≤ Tc≤70 °C, 20≤(Tg)≤60 °C		0.43–0.51	64
Solar concentrating collector	Water ammonia working fluid Te=10 °C		0.05	59
Parabolic Trough collector	Diffusion absorption refrigeration system		0.38	84
Flat Plate Collector	Single stage system		0.02–0.12	85
Compound Parabolic Concentrator - photovoltaic thermal –thermoelectric generator	Single stage system, tedlar based PV back cover		0.28	86
Compound Parabolic Concentrator	Intermittent system	ammonia/lithium nitrate working fluid	0.066	87
		ammonia/lithium nitrate/water working fluid	0.093
Solar PV	Diffusion absorption system with evaporator temperature of − 9.5 °C and 3.5 °C		0.055	88

Solar energy integrated VAR system–case study

Sun is the primary energy source for the earth to operate. The energy source for living organism is also from sun that flows from producers to other organism through consumption. Solar energy possesses light and heat energy which can be harvested by solar PV and solar water heater respectively. A tropical country like India has abundant solar energy for 360 days a year. The study location is in the state of Tamil Nadu. According to NREL GHI map, Tamil Nadu lies in the high radiation region. In this region summer occurs during the month of March through June. During these months the average temperature would be 40 °C. During winter the average temperature lies in 21 °C.

The radiation entering the earth’s surface has two types of radiation. They are beam radiation and diffuse radiation. Beam radiation and diffuse radiation are also called Direct Normal Irradiance (DNI) and Direct Horizontal Irradiance (DHI), respectively. The total radiation reaching the earth’s surface is called Global Horizontal Irradiance (GHI). Type of radiation is important for understanding the solar thermal application as different thermal application depends on different radiation. Solar thermal applications include water heater, solar pond, solar furnace, space heating and cooling, water distillation, cooker et., Most of the applications can operate using GHI. But when it comes to concentrating type application concentrating collector, it depends on the DNI. So, in this case, it is necessary to continuously align the collector towards the Sun to receive direct radiation without any barriers. As there are more technologies and lower costs, it became popular even before the solar PV system. As these applications depend on thermal energy, they must absorb most of the energy falling on them. In order to achieve more thermal energy absorption, all solar thermal applications are coated with black material or coated with a selective coating. They can automatically help improve the overall efficiency of the device itself.

Solar water heating is a popular solar technology because of its economic viability. It was also explained that this solar water heating would be more feasible for large-scale plants than the small unit for household usage. Even to improve both the performance and economics of solar PV, it could be used by integrating with solar water heaters⁸⁹. So, solar thermal and solar PV is a commonly available technology in India.

Performance of the solar integrated system

Karamangil et al.²⁰, has presented the COP equation single-stage VAR system. It is given in the formula 2.

2

3

4

5

Q_E = heat capacity of the evaporator (kj/kg).

Q_G = heat capacity of the generator (kj/kg).

W_p= work done by pump.

h_le = enthalpy of solution leaving the evaporator (kj/kg).

h_ee = enthalpy of solution entering the evaporator (kj/kg).

h_lg = enthalpy of solution leaving the generator (kj/kg).

h_e.g. = enthalpy of solution entering the generator (kj/kg).

h_w = enthalpy of weak solution returning to the absorber (kj/kg).

FR= flow ratio.

m_ss = mass flow rate of strong solution (kg/s).

m_wf= mass flow rate of working fluid (kg/s)

6

The thermal efficiency of the solar thermal collector is,

7

Not all the heat absorbed by the receiver is utilized for water heating. A certain amount of heat gets lost due to various parameters like wind velocity, ambient temperature, and emissivity of the receiver.

8

9

Once the water is heated will leave the receiver and enter the VAR generator unit. There will be a fall in temperature during this transmission through the pipe.

10

Where,

A - collector area, (m²)

C_p - specific heat capacity, (kJ/kg/°C)

m - mass flow rate of the heat-transfer medium, (kg/s)

S - wind velocity, (m/s)

T_a - Ambient temperature, (°C)

I_b - incident beam radiation, (W/m²)

T_i - tube inlet temperature, (°C)

T_iVAR - vapor absorption refrigerator inlet temperature, (°C)

T_o - tube outlet temperature, (°C)

T_r - Receiver temperature, (°C)

U_L - temperature coefficient

11

12

U_ins - Uncertainty in instrument.

U_RJC – Uncertainty of Reference Junction Compensation.

U_fluc – Uncertainty of fluctuation.

The overall efficiency for solar integrated system,

13

Solar flat plate collector integrated VAR

Solar Flat Plate Collector (FPC) is a non-concentrating type device. So, they can make use of both direct and diffuse radiation. The flat plate collector shows a maximum rise in temperature during the high radiation hours. The maximum heat absorbed by the absorber means high efficiency, and this happens during the mid-day. Wind velocity could affect the flat plate collector performance. With the increase in wind velocity, its efficiency decreases. Even with all the supportive conditions (high radiations and low wind velocity), the wrong tilt angle would affect the efficiency of the flat plate collector. To enhance maximum heat transfer, a lower mass flow rate would be profitable⁹⁰. Optimum water flow rate is very important to obtain the best possible performance from the system⁹¹. A flat plate collector is less costly than the other solar water heating systems. It also requires low maintenance. The TRNSYS model designed for the Indian climate with one-year data has proven the best performance of the system⁹². Bharath Subramaniam et al.⁹³, has studied the flat plate collector in combination with phase change material and thermal storage system. This has helped in increasing the overall efficiency of the system. Successful implantation of fiber-reinforced plastic flat plate collector was presented by Srithar & Mani⁹⁴ for the treatment of tannery effluent treatment. The insulation of the system ensures the enhanced system performances⁹⁵. This proves that flat plate collectors can be used in much more applications along with water heating. Flat plate collector does not require any tracking and hence operation is easy. It is necessary to fix the collector such that it is facing the Sun for the longest duration.

Thermal integration: structural configuration

The main function of the integration structure is to receive the hot water from the source and transfer the heat to the working fluid of the vapor absorption refrigeration system. Figure 5 illustrates the structural integration of the heat exchanger.

Fig. 5.

Solar thermal integration heat exchanger.

The solar thermal integration heat exchanger structure is made up of copper, as copper is a good conductor of heat. As it is a heat exchanger setup, there is no chance for water to come in contact with ammonia. So, the actual refrigeration cycle would not be affected by this solar thermal integration heat exchanger. It is provided with an inlet and outlet opening. It is fixed across the generator, replacing the electric heater. Hot water from the source is passed through the pipe and tube arrangement to the heat exchanger. The rate of flow of hot water can be controlled by the control valve present in the inlet tube. Another pipe tube setup is provided to remove the water after transferring the heat to the working fluid from the integration heat exchanger. To avoid any thermal energy wastage, the entire heat exchanger setup is insulated by an asbestos rope. Figure 6 shows the solar thermal heat exchanger after adding the insulation layer.

Fig. 6.

Solar thermal heat exchanger with insulation.

Experimental setup of solar thermal integrated VAR

A 60 W VAR system with ammonia water as a working fluid was taken for study. Ammonia- water, water-LiBr are the commercially used refrigerants. Ammonia-water is selected because, it is preferred for refrigeration operation while water-LiBr is used for air conditioning operation. The solar thermal integrated VAR block diagram has been illustrated in Fig. 7. The dimension of the water heater is 1m 2 m (lengthbreadth). Flat plate collector should be placed facing the sun for longer duration of time. As the selected location is slightly above the equator, a flat plate collector is placed facing south. Due to the earth’s tilt Sun is positioned towards the south for the most part of the year for India. The flat plate collector is placed at a tilt angle of 25^o with the ground surface. Natural circulation would require some time to start the operation. So, for this experiment forced circulation method is used. The water is circulated through the flat plate collector with the help of a pump.

Fig. 7.

Block diagram of FPC integrated VAR system.

The overall experimental setup has been illustrated in the Fig. 8. The systematic thermal integration of the system made thermodynamics as closed loop system condition. With the help of the pump water is continuously fed into the water heater. The black-coated absorber pate would readily absorb thermal energy from the solar radiation and transfer it to the water passing through it. Water fed through the inlet header passes through the riser to the outlet header. To increase the area of contact the fishbone-like stricture called fine is attached to the raiser tubes. This will increase the thermal energy transmission to the heating medium.

Fig. 8.

Experimental setup of Flat plate collector integrated VAR system.

Experimental setup for solar PV integrated VAR

A 100 W poly-crystalline solar panel was used to supply the power to VAR system. A 100 W peak solar PV module was selected for 60 W resistive generator type heater to ensure the real-world derating and to maintain the stability of the generator performance in desorption process. In order to maintain the generator temperature above 70–95 °C, an additional capacity of overhead solar PV capacity was selected. Here, when temperature reduces less than the lower operating point of 55 W, vapor generation could also be got reduced. Therefore, providing 40% PV oversizing stabilizes generator temperature, sustains continuous refrigerant flow, improves cooling consistency and offsets long-term degradation, thereby ensuring robust and reliable VAR system performance under fluctuating solar conditions. Solar panel power output is dependent on the input radiation which changes time to time due to factors like cloud movement. So, a 12 V, 8 Amps charge controller is used to regulate the VAR input and supplied through a 12/40Ah battery. Since the heater used in the generator is DC it does not require any inverter setup. Solar PV should also be placed facing the Sun for longer duration of time. Similar to the flat plate collector, solar PV is also placed at a tilt angle of 25^o with the ground surface to receive maximum radiation throughout the year. Figures 9 and 10 gives the illustrations of schematic and experimental setup of VAR system respectively.

Fig. 9.

Block diagram of PV integrated VAR.

Fig. 10.

Experimental setup of PV integrated VAR.

.

Uncertainty analysis

To measure the temperatures three k-type thermocouples T₁, T₂ and T₃ are used. The internal temperature of the chino data logger is tested and found that all the terminals have same internal temperature. The temperature measurement of each thermocouple is measured for three times each. The average U_RJC of T₁, T₂ and T₃ are ± 0.32 °C, ± 0.30 °C and ± 0.31 °C respectively. Uncertainty in measurement for all three thermocouples is equal to ± 0.21 °C. The uncertainty of fluctuation of T₁, T₂ and T₃ are ± 0.0123 °C, ± 0.0136 °C and ± 0.0120 °C respectively. Uncertainty of each thermocouple is measured using the Eq. 12. The temperature uncertainty of T₁, T₂ and T₃ are ± 0.39 °C, ± 0.36 °C and ± 0.37 °C respectively. Considering chino data logger KR2000 series, the power uncertainty is ± 0.2 watts. For the measurement range and operation temperature the pyranometer has < 5 W/m². The efficiency of flat plate collector has ± 3.2% error and the solar PV efficiency has 3.6% error.

Experimental procedure for solar thermal integrated VAR

The experimental procedure involved leveraging a flat plate collector (FPC) to generate hot water, which was then directed to a heat exchanger integrated with a VAR system. This hot water, after transferring its thermal energy to the working fluid within the generator, exited through an outlet pipe. To accurately assess the solar energy input, a pyranometer was systematically positioned on the FPC surface, allowing for precise measurement of incident solar radiation for effective efficiency quantification. Throughout the experimentation, key parameters including generator temperature, evaporator temperature, hot water temperature, and solar radiation were continuously monitored with one minute once integration data logging simultaneously recorded using K-type thermocouples. T1 is used to measure the FPC output hot water temperature while T2 and T3 are used to measure the generator and evaporator temperature. The close proximity of the FPC and VAR system minimized pipe losses, ensuring efficient heat transfer within the integrated setup.

Experimental procedure for solar PV integrated VAR

For the experimentation, a solar photovoltaic (PV) module is strategically positioned in a no-shadow region to ensure optimal energy harvesting with optimal tilt angle. The power generated by the solar PV, regulated by a charge controller, directly supplies a heater within the generator of the refrigeration system. The cooling process is initiated upon the generator reaching its predetermined cut-off temperature. Throughout the experiment, incident solar radiation is precisely measured using a pyranometer, while the electrical output from the solar panel is monitored with a voltmeter and ammeter. Concurrently, the temperature within the cold chamber and generator temperature is accurately recorded using a K-type thermocouple T2 and T3 respectively. In addition, an IV characterization circuit also been employed to obtain the maximum power point operation and performance of the SPV system.

Results and discussions

Figures 11 and 12 give the VAR inputs i.e., the output hot water temperature of the flat plate collector and the power output from the solar panel. The FPC temperature reached up to 88 °C during the operation period. With the gradual increase in temperature, the peak temperature is recorded after the Sun reaches its peak. The fall in temperature can be noticed in the afternoon when the Sun starts going down. During peak hour power lies in the range of 63–60watts, with average output of 62 watts in the study duration. But after 4:30pm an abrupt fall in power output can be noticed which reaches up to 38watts. This is because of low radiation after 4:00pm. Unlike FPC output, PV output is not maintained at the same level. This is because, solar PV panel is more sensitive to shadows cast by clouds and birds flying over the PV panel than the FPC system.

Fig. 11.

Flat plate collector output temperature.

Fig. 12.

Solar PV output of the system under investigation.

Figure 13 gives generator temperature of the VAR heated by the hot water from the FPC and generator temperature of PV integrated VAR system. The cooling operation of the VAR system starts at the generator temperature of 60 °C. So, the hot water temperature is enough to drive the VAR system. Comparing the generator temperatures of the both the system, it is noticed that the FPC hot water temperature is proportional to the generator temperature. Whenever the fall in hot water temperature prevails for a longer duration, generator temperature also experiences a considerable fall in temperature. The solar PV–integrated VAR system attained a generator temperature of 89 °C and lies in 80 to 89 °C range throughout peak hours. After 4:00pm, when the power supplied is not enough to operate the heater of VAR system, then the cooling process would not operate in the optimum level. VAR system performance cannot be affected by load but the performance of Solar PV is affected by cloud as output could drop from 10% to 80% based on the cloud condition. Solar-PV integrated system more sensitive to shade since, the operation is based on photons from the solar radiation.

Fig. 13.

VAR generator temperature.

The efficiency of the FPC is collected using the formula 7. Figure 14 gives the efficiency of the FPC during operation hours. The system efficiency depends on solar radiation; therefore, as solar irradiation reduces in the evening, the efficiency of the system is also reduced. The average efficiency of the FPC is 35.3%. All through the peak radiation, up to 40% efficiency can be reached. But during low radiation duration, efficiency falls up to 31%. The efficiency and cold chamber temperature get affected at the same after the peak hour. The solar PV system efficiency was sustained significantly around 8–10% ranges between 12:00 to 14:50 h and highlighted in Fig. 14. In addition, quasi-steady range of 8.5–9.2% around 13:00 to 15:30, which ensures the reliability of the proposed system. Also, the measurement uncertainty ranges within ± 0.3–0.5% to validate the results. A significant drop in efficiency was observed after 15:40, with drop in efficiency up to ~ 6% at 16:50 due to lower solar radiation. Therefore, the overall system performance ensures a stable operational efficiency of ~ 9% for over 3.5 h, establishing its robustness for solar-driven energy applications for rural stand-alone applications.

Fig. 14.

Efficiency of the VAR input.

Figure 15 gives the evaporator temperature of FPC and solar PV integrated VAR. Even though the generator does not operate at its optimum temperature, the chilling is started in the cold chamber as the minimum temperature need is reached. It took 25 min to start the refrigeration operation. It can reduce the cold chamber temperature to as low as 12 °C after 4 h and 30 min of operation. Cooling produced in the cold chamber may not be minimal, but it is enough to store the tropical fruits and vegetables. Thus, this FPC integrated VAR system could satisfy the necessity for which it is developed. During the evening time, when the solar radiation is lower, then the cold chamber temperature starts rising as there is not enough thermal energy available. Adding a hot water storage tank would help achieve uninterrupted 24-hour operation. It can act as a heat buffer to supply the required heat. The study starts at 12:00pm to 4:50pm. It took 39 min to start cooling. The lowest temperature of 9 °C was reached after 3 h and 10 min. But a rise in temperature after peak hour can be noticed. This can be solved by adding a battery. Adding a battery bank would help store the solar energy during peak hours and utilize during off-peak hours. Adding hot water storage tank or battery bank based on the type of supply, VAR system would operate 24-hours independent of any power supply. The COP is calculated using Eq. 2. The COP of FPC and Solar PV integrated Var are 0.14 and 0.12 respectively. By selecting double effect or triple effect system with alternative refrigerant it is possible to improve the COP. Even though, COP of VAR is lower than the commercial VCR system, VAR performs better in terms of environmental sustainability. For large scale operation like cold storage, the carbon footprint can be considerably reduced. The overall efficiency of the solar driven VAR is calculated using formula 13. The overall efficiencies of PV integrated VAR and FPC integrated VAR are 1.08 and 4.9% respectively.

Fig. 15.

VAR evaporator temperature.

Table 12 has given the collective results of both the systems. Since, the COP of both the systems are almost similar. But in terms of efficiency, cost and maintenance FPC has shown better results. Since it is a cold storage system, priority should be given to cold storage with lowest possible cold chamber temperature. In that case, solar-PV integrated system has sown better performance.

Table 12.

Comparison of FPC integrated and PV-integrated VAR.

S.No	Parameter	FPC integrated VAR	Solar PV-integrated VAR
1.	Type of input	Solar thermal energy is converted into thermal energy and supplied as a thermal energy to the working fluid	Solar photo energy is converted into electrical energy and supplied to the heater present in the generator to heat the working fluid. so, energy conversion takes place.
2.	Efficiency of input source	35.3%	9%
3.	Complexity	Comparatively integration of FPC is easier.	It is more complex with many electrical devices. So, it requires professional hands for integration.
4.	Auxiliary component for uninterrupted operation	Hot water storage tank	Battery bank
5.	Cold chamber temperature	12 °C	9 °C
6.	Environmental condition	Long duration of shadow can affect the FPC output	Even a passing of cloud can affect the power out of solar PV
7.	Maintenance	Easy	Comparatively, complex
8.	Cost	₹17,000	₹20,500
9.	COP	0.14	0.12

The proposed solar powered VAR system is technically highly scalable for rural cold chain (farmland or household (40–100 L) to community level (1–5-ton capacity) because the system can be scaled enhancing capacities of the generators and heat exchangers. Also, economically, the FPC driven VAR unit costs around ₹ 17,000.00, while PV driven VAR approximately ₹ 20,500.00. These systems can be scaled up to 3.5 kW (1 ton capacity), costing around 2.5–3.5 lakhs (FPC driven VAR) and ₹ 3.5–4.5 lakhs (PV driven VAR)⁹⁶⁹⁷,. These makes them highly economical compared with the diesel powered or conventional electrical units. The experimental investigation clearly highlights the storage unit temperature ranges between 9 and 12 °C is highly suitable for perishables and other agro produces. Here, scaling can be done through the multiple cascaded VAR units, proper insulation, thermal storage tanks for FPC units, battery energy storage system for PV enabled units to ensure the uninterrupted cold storage operation even under stand-alone mode of operation in tropic rural areas.

Conclusion and future research directions

The reason for vapor compression refrigeration to be commercially useful for domestic application is because of better performance in terms of COP. But the environmental issues caused by Vapor compression cycle is undeniable. The refrigerants being used in vapor compression cycle are proven to cause global warming and ozone depletion. Vapor compression refrigeration is listed among one of the leading contributors of CO₂. The refrigerants in the compression cycle have progressed from CFC to HFC as the former has chlorine. Chlorine released to atmosphere would react chemically with the ozone to form free oxygen. As CFC is stable one chlorine atom can destroy multiple ozone atoms. Later, HFC is also found to be the contributor of global warming. After Montreal protocol, it was suggested to phase out HFC usage. R134a the commonly used refrigerant replaces it gradually which is also a contributor of global warming.

In this paper several research papers are reviewed with main focus on refrigerant, COP, and alternative energy sources. Vapor absorption refrigeration system does not have any environmentally damaging refrigerant. Section 3 has discussed a number of refrigerant alternatives available to improve the COP of the system. There are design changes made on absorption cycle to improve the performance in terms of COP and cooling effect. As there are several absorption cycles designs available, location, climatic condition and the type of operation for which it is designed must be considered, while designing the cooling system. By choosing proper absorption cycle in combination with refrigerant COP value can be improved even more than 1. Absorption cycle also performs better than the vapor compression refrigeration in terms of power quality. For small scale application, particularly for domestic application, absorption refrigeration has low cost and maintenance advantage than the vapor compression cycle. The study has shown that absorption refrigeration has the potential to replace vapor compression refrigeration.

Utilizing renewable energy would make refrigeration a sustainable technology. In compression refrigeration, the compressor can be driven by high grade energy, electricity. But for absorption refrigeration, generator requires low grade energy like heat. Since absorption refrigeration can be driven by heat energy, it has various energy option. So, solar thermal energy can be utilized directly without any need for converting into electrical energy.

Absorption refrigeration with SHE and RHE and acetone-zinc bromide as working fluid can operate at 50 °C. When the input requirement is reduced to this level output from flat plat collector and engine exhaust can be utilized. A CPC integrated system can reach sub-cooling temperature and can produce ice during peak hours. Automatic tracking is mandatory for concentrated collectors as continuous adjustment is needed for every 15 min. Absorption cycle driven by exhaust from gas turbine can reach up to 5 °C. Utilizing the waste heat would, reduce the dependence on energy source and a huge opportunity to reduce energy cost. In this case, energy from waste is utilized, COP can be improved up to 8.

The case study has shown that a simple heat exchanger is enough to integrate Vapor absorption cycle with thermal energy source. As the integration is not complex it can easily be adapted. It has also shown that at the study location a flat-plate collector with basic absorption cycle is enough to produce 12 °C. compared to flat plate collector, solar PV integrated VAR has shown better performance in-terms of evaporator temperature by reaching 9 °C during peak hours. The difference between the COP of both the systems is only 0.02. Adding thermal storage tank can make 24-hour uninterrupted operation throughout the year in tropical areas. For regions with less solar availability, auxiliary heating can be provided. Even solar PV output can be used directly as the load in absorption cycle is resistive which can be driven by DC current.

In the futuristic perspectives, a paradigm shift towards vapor absorption refrigeration is rapidly gaining momentum, due to its viability for the lower domestic GWP requirements. Currently experimental studies proven that solar thermal (FPC) and solar PV integrated system could produce 0.14 and 0.12 COP respectively, which ensures the lower domestic GWP requirements. Even though the COP is lower than the commercial system, the environmental effect caused by the commercial refrigerant is high. Advanced alternative refrigerants like acetone-zinc bromide allow absorption refrigeration systems to operate effectively at lower temperature less than 50 °C, which facilitates the effective waste heat recovery and energy recycling from the industrial process. This type of advanced VAR could improve the COP by optimizing the design and refrigerant composition for achieving the global and rural sustainable development goal through sustainable refrigeration practices.

Abbreviations

COP

Coefficient of performance

f

Flow ratio

SHE

Solution heat exchanger

RHE

Refrigerant heat exchanger

SRHE

Solution refrigerant heat exchanger

ε

Effectiveness

η

Efficiency

T

Temperature

CPC

Cylindrical parabolic collector

DMETEG

Dimethyl ether tetraethylene glycol

MCL

N-Methyl ε–carprolactum

DMEU

Dimethyl-ethyleneurea

DMMP

Dimethyl methyl phorponate

NMP

N methyl − 2 pyrrolidone

DMAC

N N’ dimethylacetamide

DMPU

N N’-dimethylacetamide

TPLAC

Triple pressure level absorption cycle

DPL

Double pressure level

TPL

Triple pressure level

ODP

Ozone depleting potential

GWP

Global warming potential

Subscripts

SHE

Solution heat exchanger

RHE

Refrigerant heat exchanger

SRHE

Solution refrigerant heat exchanger

p

Pump

g

Generator

c

Condenser

a

Absorber

e

Evaporator

d

Desorber

ha

Temperature of high absorber

la

Temperature of low absorber

HGT

Temperature of high generator

CPC

Condensate pre-cooler

atm

Atmosphere

Author contributions

Divya Arputham Selvaraj, Lalith Pankaj Raj Nadimuthu: Conceptualization, Methodology, Software, Visualization, Investigation, Writing- Original draft preparation. Kirubakaran Victor: Data curation, Validation, Supervision, Resources, Writing - Review & Editing. Seif Al Bustanji, Mykhailo Panchyk: Project administration, Supervision, Resources, Writing - Review & Editing.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.