A SOLAR EJECTOR COOLING SYSTEM USING REFRIGERANT R141b

A high-performance solar ejector cooling system using R141b as the working fluid was developed. We obtain experimentally a COP of 0.5 for a single-stage ejector cooling system at a generating temperature of 90°C, condensing temperature of 28°C, and an evaporating temperature 8°C. For solar cooling application, an optimum overall COP can be obtained around 0.22 at a generating temperature of 95°C, evaporating temperature of 8°C and solar radiation at 700 W m⁻².     

Experiments on vapour jet refrigeration system suitable for solar energy applications

Experiments are carried out on a R11 vapour jet refrigeration system (VJRS) to study the influence of ejector configuration and operating conditions on the performance. Eight ejector configurations, formed out of five nozzles and seven diffusers are investigated. The influence of boiler temperature, which represents the solar energy collection temperature, and that of evaporating temperature, which denotes the cooling load temperature, are studied. Overall COPs in the range of 0.08-0.33 and evaporating temperatures in the range of −3–18°C are obtained for boiler temperatures from 75 to 85°C. Ejector configuration has significant influence in deciding the operating range.     

Estimation of ejector's main cross sections in steam-ejector refrigeration system

This paper describes the calculation of main cross sections of a steam-ejector, operating in a refrigeration system. For the detailed calculation of ejector a method has been developed which employs functions describing the thermodynamic properties of steam. The influence of three major parameters: generator pressure, condenser and evaporator temperature on cross sections of ejector is discussed. The design conditions were: generator pressure (6–8 bar), condenser temperature (40–50 °C) and evaporator temperature (4–10 °C). It is confirmed that the dimensions of ejector depend on the operation conditions.     

An exergy analysis of a solar-driven ejector refrigeration system

Exergy analysis is used as a tool to analyse the performance of an ejector refrigeration cycle driven by solar energy. The analysis is based on the following conditions: a solar radiation of 700 W/m², an evaporator temperature of 10 °C, a cooling capacity of 5 kW, butane as the refrigerant in the refrigeration cycle and ambient temperature of 30 °C as the reference temperature. Irreversibilities occur among components and depend on the operating temperatures. The most significant losses in the system are in the solar collector and the ejector. The latter decreases inversely proportional to the evaporation temperature and dominates the total losses within the system. The optimum generating temperature for a specific evaporation temperature is obtained when the total losses in the system are minimized. For the above operating conditions, the optimum generating temperature is about 80 °C.     

Study on a dual-mode, multi-stage, multi-bed regenerative adsorption chiller

In this paper, a detailed parametric study on a dual-mode silica gel–water adsorption chiller is performed. This advanced adsorption chiller utilizes effectively low-temperature solar or waste heat sources of temperature between 40 and 95 °C. Two operation modes are possible for the advanced chiller. The first operation mode will be to work as a highly efficient conventional chiller where the driving source temperature is between 60 and 95 °C. The second operation mode will be to work as an advanced three-stage adsorption chiller where the available driving source temperature is very low (between 40 and 60 °C). With this very low driving source temperature in combination with a coolant at 30 °C, no other cycle except an advanced adsorption cycle with staged regeneration will be operational. In this paper, the effect of chilled-water inlet temperature, heat transfer fluid flow rates and adsorption–desorption cycle time effect on cooling capacity and COP of the dual-mode chiller is performed. Simulation results show that both cooling capacity and COP values increase with the increase of chilled water inlet temperature with driving source temperature at 50 and 80 °C in three-stage mode, and single-stage multi-bed mode, respectively. However, the delivered chilled-water temperature increases with chilled-water inlet temperature in both modes.     

Crystallization as a limit to develop solar air-cooled LiBr–H2O absorption systems using low-grade heat

In the present work the use of low-temperature solar heat is studied to produce cooling at 5°C, using a double-stage LiBr–H₂O air-cooled absorption cycle. A solar plant, consisting of flat plate collectors feeding the generators of the absorption machine, has been modeled. Operating conditions of the double-stage absorption machine, integrated in the solar plant without crystallization problems for condensation temperatures up to 53°C, are obtained. Results show that about 80°C of generation temperature are required in the absorption machine when condensation temperature reach 50°C, obtaining a COP equal to 0.38 in the theoretical cycle.A comparative study respect to single-stage absorption cycles is performed. Efficiency gain of the double-stage solar absorption system, over the single-stage one, will increase with higher condensation temperatures and lower solar radiation values. Single-stage cycles cannot operate for condensation temperatures higher than 40°C using heat from flat plate collectors. For higher condensation temperatures (45°C) the generation temperatures required (105°C) are very high and crystallization occurs. Condensation temperatures able to use in double-stage cycles may be increased until 53°C using heat from flat plate collectors without reaching crystallization.     

Performance testing and evaluation of solid absorption solar cooling unit

Based on solid-vapour intermittent absorption system, DORNIER a German Firm designed and fabricated a solar cooling unit, which utilizes thermal energy supplied by heat pipe vacuum tube solar collectors through thermosyphonic flow of water. The unit of 1.5 kWh/day cooling capacity uses ammonia as a refrigerant and IMPEX material as absorbent and does not have any moving part requiring no auxiliary energy. The IMPEX material (80% SrCl₂ and 20% Graphite) has high heat and mass transfer coefficient as well as high absorption capacity. Detailed experiments were performed on a unit in Delhi under real field conditions followed by theoretical analysis. Theoretical maximum overall COP of the unit is 0.143, and it depends upon the climatic conditions. Under field conditions, it was found that if the maximum daytime ambient temperature was 30°C and night time temperature 20°C, it took three sunny days to freeze water in the cooling box. After the second day, the temperature inside the cooling box remained 1°C. The overall COP was found to be 0.081 only. The automatic control valve based on mechanical/thermal principles however has defects and the problem of corrosion of the sealings needs to be solved. In climates where day time temperatures are high (Delhi summer 43°C–47°C during the day, 30°C–35°C during the night) and solar radiation relatively low (4–5 kWh/m²d) because of pollution and sand in the atmosphere, it is most unlikely that pressure in the ammonia circuit can reach values at which ammonia vapours start to condense. The unit, needs to be redesigned for such conditions.     

Novel composite sorbent for resorption systems and for chemisorption air conditioners driven by low generation temperature

The utilization of a composite sorbent (NaBr and expanded graphite) in chemisorption air conditioning systems driven by low-grade heat source, and in resorption systems with simultaneous heating and cooling effects was experimentally investigated using bench-scale prototypes. The mass of ammonia desorbed and adsorbed was measured, and used to calculate the specific cooling capacity. The sorbent produced 219 kJ kg⁻¹ of cooling at 5 °C and 510 kJ kg⁻¹ at 15 °C, when the heat source temperature was 65 °C and the heat sink temperature was 30 °C. The air conditioning system mean specific cooling power (SCP), and mean coefficient of performance (COP) were calculated based on the desorbed and adsorbed masses, and on the variation of temperature in the reactors. For the same heat source and heat sink temperatures mentioned above, the air conditioning system had a SCP of 129 ± 7 W kg⁻¹ and a COP of 0.46 ± 0.01, when cooling occurred at 15 °C. Regarding the utilization of the composite sorbent in resorption machines, the prototype was tested for production of cooling/heating at −5/50 °C, and at 10/70 °C. In the former condition, the COP was only 0.02, but in the latter condition, there was a tenfold increase in the COP, and the combined coefficient of performance and amplification reached 1.11, which indicates the energy saving potential of resorption systems using the studied sorbent.     

Experimental investigation and analysis on a thermoelectric refrigerator driven by solar cells

Experimental investigation and performance analysis on a solar cell driven, thermoelectric refrigerator has been conducted. Research interest focused on testing the system performance under sunshine. Experiment results demonstrated that the unit could maintain the temperature in the refrigerator at 5–10°C, and have a COP about 0.3. Further analysis indicated that the performance of the system is strongly dependent on intensity of solar insolation and temperature difference of hot and cold sides for the thermoelectric module, etc. There exist optimum solar insolation rates, which let the cooling production and COP achieve maximum value, respectively. It was expected that the refrigerator would be potential for cold storage of vaccine, food and drink in remote area, or outdoor conditions where electric power supply is absent.     

Dynamic study of the thermal behaviour of solar thermochemical refrigerator: barium chloride-ammonia for ice production

The results of the theoretical thermodynamic analysis and the dynamic behaviour of the solar heating system of a thermochemical refrigerator, which operates on a heterogeneous solid–gas reaction between barium chloride and ammonia, are presented in this work. The thermodynamic analysis of the barium chloride–ammonia system shows that after energy and mass balance, the global efficiency coefficient (COP) varies very little. The theoretical relative low temperatures of dissociation in this system which are between 50°C and 60°C need simple heating systems such as flat plate collectors are needed, with an advantage over traditional liquid/vapour absorption systems. A simulation of the annual dynamic behaviour of the solar heating system for the operation of a solid-gas reactor is also presented. For an ice production specific cooling load, calculations are made of the different solar fractions of different areas of solar caption as well as the monthly variations of the efficiencies of the refrigeration systems.     

Experimentation of a solar adsorption refrigerator in Morocco

For countries with a high potential of solar energy, producing cold using solar energy is a promising way to sustainable development since the energy used is free and not harmful for the environment.This work proposes a solar adsorption refrigerator using the pair activated carbon–methanol, which has been totally built and is under experimental tests in the solar laboratory of the Faculty of Sciences of Rabat, the capital of Morocco with Mediterranean climate.The solar adsorption refrigerator is mainly composed of a collector containing the adsorbent, an evaporator and a condenser. The results show that the refrigerator gives good performance in Rabat. The unit produces cold even in rainy and cloudy days and the temperatures achieved by the unit can be less than −11 °C for days with a very high irradiation. The solar coefficient of performance (COP) (cooling energy/solar energy) ranges between 5% and 8% for an irradiation between 12,000 and 28,000 kJ m⁻² and a daily mean ambient temperature around 20 °C.     

Performance of solar driven methanol–water combined ejector–absorption cycle in the Athens area

The performance of a solar driven CH₄O-H₂O combined ejector– absorption unit, operating in conjunction with intermediate temperature solar collectors in Athens, is predicted along the five months (May–September) in case of the unit working as heat pump in an industrial area. The operation of the unit and the related thermodynamics are simulated by suitable computer codes and the required local climatological data are determined by statistical processings over a considerable number of years. It is found that the heat gain factor varies in the range from 2.1330 to 2.4481 for the above period of time. The maximum HGF of about 2.4481 is obtained in July at 14.25 hrs with corresponding specific heat gain power 915 W/m². The maximum Q_gain of about 1086 W/m² is obtained in June at 12.54 hrs with corresponding HGF 2.3572. Also the maximum value of HGF was estimated by correlation of three temperatures: generator temperature (85.0°C–97.2°C), condenser temperature (43.3°C–47.6°C) and evaporator temperature (12.6°C–25.4°C).     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

Development of a metal hydride refrigeration system as an exhaust gas-driven automobile air conditioner

Aiming at developing exhaust gas-driven automobile air conditioners, two types of systems varying in heat carriers were preliminarily designed. A new hydride pair LaNi_4.61Mn_0.26Al_0.13/La_0.6Y_0.4Ni_4.8Mn_0.2 was developed working at 120–200 °C/20–50 °C/−10–0 °C. P-C isotherms and reaction kinetics were tested. Reaction enthalpy, entropy and theoretical cycling coefficient of performance (COP) were deducted from Van’t-Hoff diagram. Test results showed that the hydride pair has flat plateau slopes, fast reaction dynamics and small hystereses; the reaction enthalpy of the refrigeration hydride is −27.1 kJ/mol H₂ and system theoretical COP is 0.711. Mean particle sizes during cycles were verified to be an intrinsic property affected by constitution, heat treatment and cycle numbers rather than initial grain sizes. Based on this work pair, cylindrical reactors were designed and a function proving metal hydride intermittent refrigeration system was constructed with heat conducting oil as heat source and water as heat sink. The reactor equivalent thermal conductivity is merely 1.3 W/(m K), which still has not meet practical requirement. Intermittent refrigeration cycles were achieved and the average cooling power is 84.6 W at 150 °C/30 °C/0 °C with COP being 0.26. The regulations of cycling performance and minimum refrigeration temperature (MRT) were determined by altering heat source temperature. Results showed that cooling power and system COP increase while MRT decreases with the growth of heat source temperature. This study develops a new hydride pair and confirms its application in automobile refrigeration systems, while their heat transfer properties still need to be improved for better performance.     

Performance of a non-metallic unglazed solar water heater with integrated storage system

The performance of a new design of non-metallic unglazed solar water heater integrated with a storage system has been studied. In this system, the collector and storage were installed in one unit. All parts of the system have been fabricated from fiberglass reinforced polyester (GFRP) using a special resin composition that provides good thermal conductivity and absorptivity. The storage tank has a capacity of 329 l. The design of the storage system was sandwich construction, with the core material made out of polyurethane foam, which combines stiffness and lightness of structure with very good thermal insulation. The width and length of the absorber plat were 1.4 and 1.8 m, respectively. The performance of the system has been investigated by two methods. In the first method, the storage tank was filled up with water the night before the test. The tank was then drained during the night, refilled and made ready for the next day’s test. The tests were repeated under varied environmental conditions for several days. The maximum water temperature in the storage tank of 63 °C has been achieved for a clear day operation at an average solar radiation level of 700 W m⁻² and ambient temperature of 30 °C. The decrease of water temperature with and without the thermal diode is 10 and 20 °C, respectively. In the second method, the testing was of the same way, but in this case without draw-off or draining of the hot water from the storage tank. All data readings were recorded from sunrise to sunset over the same period. The temperature was recorded for several days and ranges of 60–63 °C were obtained in the storage tank. A system efficiency of 45% was achieved at an average solar radiation level of 635 W m⁻² and ambient temperature of 31 °C.     

A hybrid solar-assisted adsorption cooling unit for vaccine storage

A concept of a hybrid adsorption cooling unit for vaccine storage utilizing solar energy as a main power supply and a gas burner as an alternative power supply has been developed. The components of the cooling unit have been designed to work under the weathering conditions of Burkina Faso, West coast of Africa according to the requirements of the World Health Organization. For the first adsorber, which is driven by a gas burner, zeolite-13X has been selected. For the second adsorber to be driven by solar energy selective water sorbent SWS-2L has been applied. Water is selected as a refrigerant for both adsorbents. Theoretical investigations of the expected performance of the designed cooling unit have shown a coefficient of performance (COP) of 0.28 for the solar-operated system based on the heat input to the adsorption unit, at the design conditions of T_evap=−5 °C, T_con=55 °C, T_ads=38 °C, T_des(max)=122 °C. For the gas-heated system, also a COP of 0.28 has been estimated at the design conditions of T_evap=−5 °C, T_con=55 °C, T_ads=38 °C, T_des(max)=280 °C. The variations of COP, cooling capacity and the heating power required to operate both systems have been estimated for a broad range of desorption temperatures. It turns out that the SWS-2L/water system is much more sensitive to the operating conditions than the zeolite-13X/water system. The obtained results should serve in designing both control and heating components of the cooling unit.     

Performance improvement of absorption heat transformer

In this study, a mathematical model of absorption heat transformer (AHT) operating with the aqua/ammonia was developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds and used a special ejector located at the absorber inlet. By the use of the ejector, the obtained absorber pressure becomes higher than the evaporator pressure and thus the system works with triple-pressure-level. The ejector has two functions: (i) aids pressure recovery from the evaporator and (ii) upgrades the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. The other advantage of the system with ejector is increased absorber temperature. Therefore, pressure recovery and pre-absorption in the ejector improves the efficiency of the AHT. Under the same circumstances, when compared to an AHT with and without an ejector, the system's COP and exergetic coefficient of performance (ECOP) were improved by 14% and 30%, respectively and the circulation ratio (f) was reduced by 57% at the maximum efficiency condition. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The maximum upgrading of the solar pond's temperature by the AHT was obtained at 57.5 °C and gross temperature lift at 97.5 °C with coefficients of performance of about 0.5. The maximum temperature of the useful heat produced by the AHT was 150 °C. In addition, exergy losses for each component in the system were calculated at different working temperatures and the results of both systems with and without an ejector were compared. Exergy analysis emphasised that both the losses and irreversibilities have an impact on the system performance and exergy analysis can be used to identify the less efficient components of the system. Exergy analyses also showed that the exergy loss of the absorber of AHT with ejector was higher than those of other components.     

A solar ejector air‐conditioning system using environment‐friendly working fluids

In this paper, the performance of the solar‐driven ejector air conditioning with several environment‐friendly working fluids is studied. The effect of the fluid nature and operating conditions on the ejector performance is examined. This performance is calculated using an empirical correlation. Thermodynamic properties of functioning fluids are obtained with a package REFPROP7. It appears that the refrigerant R717 offers the highest coefficient of performance (COP). For generator temperature T_B = 90°C, condenser temperature T_C = 35°C and evaporator temperature T_E = 15°C and with R717, the COP of ejector air‐conditioning system is 0.408. Using a meteorological data for the city of Tunis, the system performance is computed for three collector types. The air‐conditioning season and period were taken for six months from April to September. The daily period is between 8 and 17 h. For the solar air‐conditioning application, the COP of the overall system varied from 0.21 to 0.28 and the exergy efficiency varied from 0.14 to 0.19 with the same working conditions and total solar radiation (351–875 Wm^?2) in July. Copyright © 2008 John Wiley & Sons, Ltd.     

Photovoltaic solar cells performance at elevated temperatures

It is well known that efficiency of photovoltaic solar cells decreases with an increase of temperature, and cooling is necessary at high illumination conditions such as concentrated sunlight, or cosmic or tropical conditions. The purpose of present study was to investigate the opposite option: to make a cell work at relatively high temperature (around 100–200 °C) and use the excessive heat in a hybrid system of some kind to increase the total efficiency of solar energy utilization. We studied the temperature dependence of the solar cell parameters both theoretically and experimentally, for the basic cells with p–n junction and the Schottky barrier, taking account of the different carrier transport mechanisms and recombination parameters of the cell material. The possibility of usage of the concentrated sunlight was also taken into account. The experiments conducted in the temperature interval of 25–170 °C and the calculated data show a real possibility of construction of a two-stage solar-to-electric energy converter with high-temperature second stage, having the overall conversion efficiency of 30–40%.