Design optimization of the flat plate collector for a solid absorption solar refrigerator

A study of the effects of various collector design parameters on the performance of a solar powered solid absorption refrigerator is presented. The refrigerator uses specially treated CaCl₂ as absorbent and NH₃ as refrigerant and operates intermittently in a diurnal cycle. The study is undertaken using version 4.0 of a simulation programme, COSSOR, developed from a transient analysis of the system. A large number of simulations was undertaken to test the performance of the refrigerator for various choices of the collector design parameters. The latter include the plate emissivity and material; absorbent pellet diameter, thermal conductivity and packing density; collector tube size, spacing and material; and number of glazing. The refrigerator performance indicators, namely total condensate yield, mass of ice produced, coefficient of performance and effective cooling, are presented for the range of values of the collector parameters of interest. Using a multiple linear regression technique, the performance indicators are correlated with the collector parameters by simple linear polynomial expressions. An objective function, suitable for selecting optimal values of the parameters, is defined, subject to specified constraints. Optimization was then carried out for the objective function. For the collector with steel tubes and steel plate, the refrigerator coefficient of performance obtained with optimal choices of tube size, spacing and plate emissivity is 0.073, representing an improvement of at least 30% with respect to the reference collector. A similar level of improvement was obtained for a collector with aluminium tubes and plate.     

Transient analysis and performance prediction of a solid absorption solar refrigerator

The transient analysis and performance prediction of an intermittent solar powered solid absorption refrigerator is presented. The refrigerator uses CaCl₂ stabilized with 20% CaSO₄ as absorbent and NH₃ as refrigerant. Transient models of heat transfer in the collector plate and heat and mass transfers in the absorbent are developed and linked with heat and mass transfer models of the refrigerant as appropriate for each operating mode. Vapour accumulation in the absorber/generator tubes is neglected. Lumped parameter analysis is utilised for the glazing covers, the collector tubes, the refrigerant and the entrapped air. The moving boundary problem arising from ice formation or melting is solved using the enthalpy method. System pressure prediction models are incorporated in the analysis. The analytical model of the refrigerator is used to build a computer simulation programme, COSSOR, version 4.0, for its performance prediction. Predicted results compared well with experimental data over the major seasons of the year at Nsukka, Nigeria. The model, with its implementation in COSSOR, is therefore a useful tool for the design, performance analysis and optimization studies of the refrigerator, and may find applications in related systems.     

A thermo-electric refrigerator powered by photo-voltaic solar collectors

In developing countries and remote areas where an electric supply is not available, a thermo-electric refrigerator is often needed for food and medical drugs storage. Such a refrigerator, which requires a direct current supply is suitable for matching with a photo-voltaic collector (PV). A normal automobile 12-volt lead-acid battery may be used as the storage system. This refrigerator is rugged and may be operated by unskilled people: a prototype has been built and its components are available commercially.     

Charcoal-methanol adsorption refrigerator powered by a compound parabolic concentrating solar collector

A compound parabolic concentrating solar collector (CPC) of concentration ratio 3.9 and aperture area 2.0 m² was used to power an intermittent solid adsorption refrigerator and ice maker using activated charcoal (carbon) as the adsorbing medium and methanol as the working fluid. The copper tube receiver of the CPC was packed with 2.5 kg of imported adsorbent 207E3, which was only utilised when the performance of activated charcoal (ACJ1, produced from local coconut shells) was found to be inferior to the imported adsorbent. Up to 1 kg of ice at an evaporator temperature of −6°C was produced, with the net solar coefficient of performance (COP) being of the order of 0.02. Maximum receiver/adsorbent temperature recorded was 154°C on a day when the insolation was 26.8 MJ/m⁻². Temperatures in excess of 150°C are undesirable since they favour the conversion of methanol to dimethyl ether, a noncondensable gas which inhibits both condensation and adsorption. The major advantage of this system is its ability to produce ice even on overcast days (insolation 10 MJ/M⁻²).     

A new solar powered adsorption refrigerator with high performance

An adsorptive solar refrigerator was built in September 2000 in Yverdon-les-Bains, Switzerland. The adsorption pair is silicagel + water. The machine does not contain any moving parts, does not consume any mechanical energy except for experimental purposes and is relatively easy to manufacture. Cylindrical tubes function as both the adsorber system and the solar collector (flat-plate, 2 m² double glazed); the condenser is air-cooled (natural convection) and the evaporator contains 40 l of water that can freeze. This ice functions as a cold storage for the cabinet (320 l).The first tests (September 2000) showed a very promising performance, with a gross solar cooling COP_SR of 0.19. After minor modifications, a second test series was carried out during summer 2001. This test series shows how the external parameters influence the machine with respect to the COP_SR (irradiation and external temperature). The latter varies between 0.10 and 0.25 with a mean value of 0.16. These values are higher than those obtained by earlier solar powered refrigerators (0.10–0.12).This paper describes the principle of the cycle, the different components of the machine, and the test procedure. The test procedure includes a constant daily cooling requirement. The experimental results presented were taken over a period of two months.     

Study of a new solar adsorption refrigerator powered by a parabolic trough collector

This paper presents the study of solar adsorption cooling machine, where the reactor is heated by a parabolic trough collector (PTC) and is coupled with a heat pipe (HP). This reactor contains a porous medium constituted of activated carbon, reacting by adsorption with ammonia.We have developed a model, based on the equilibrium equations of the refrigerant, adsorption isotherms, heat and mass transfer within the adsorbent bed and energy balance in the hybrid system components. From real climatic data, the model computes the performances of the machine. In comparison with other systems powered by flat plate or evacuated tube collectors, the predicted results, have illustrated the ability of the proposed system to achieve a high performance due to high efficiency of PTC, and high flux density of heat pipe.     

A study of the effects of collector and environment parameters on the performance of a solar powered solid adsorption refrigerator

Based on the heat and mass transfer model validated by experiment, the performance of the plate solar ice-maker is analyzed systemically with the opinion of two-type characteristic parameters, which includes parametric effects of adsorbent bed of solar ice-maker and outer parameters referring to circumstance. A large number of simulations were undertaken to test the performance of the refrigerator for various collector design parameters and environmental parameters. These works are beneficial to further study the optimization design of a solar cooling system.     

Design improvements for a collector/generator/adsorber of a solid adsorption solar refrigerator

A study of the effects of different collector design parameters on the performances of a solar powered solid adsorption refrigerator is presented. The refrigerator uses activated carbon/methanol as the adsorbent/refrigerant pair. The study was undertaken using a computer simulation program developed from a transient analysis of the system. The parameters tested are the collector plate emissivity/absorptivity combination, adsorbent packing density, tube spacing, outer tube outside diameter, adsorbent thermal conductivity, heat transfer coefficient at adsorbent/tube interface, and adsorbent tube/collector plate materials combination. Two performance indicators namely, condensate yield and coefficient of performance (COP) were used in the study as figures of merit. A multiple regression technique was used to correlate the performance indicators with the collector parameters through a quadratic relation. Consequently an objective function, suitable for selecting optimal values of the parameters is defined, subject to specified constraints. Selecting the COP as the preferred indicator parameter, optimization was then carried out. Improvements in the ranges of 29–38% for COP and 26–35% for condensate yield were obtained with optimal choices of tube spacing, adsorbent packing density and collector plate/adsorbent tube material combinations.     

Transient analysis and performance prediction of a solid adsorption solar refrigerator

The transient analysis and performance prediction of a solid adsorption solar refrigerator, using activated carbon/methanol adsorbent/adsorbate pair are presented. The mathematical model is based on the thermodynamics of the adsorption process, heat transfer in the collector plate/tube arrangement, and heat and mass transfers within the adsorbent/adsorbate pair. Its numerical model developed from finite element transformation of the resulting equations computes the collector plate and tube temperatures to within 5 °C. The condensate yield and coefficient of performance, COP, were predicted to within 5% and 9%, respectively. The resulting evaporator water temperature was also predicted to within 5 °C. Thus the model is considered a useful design tool for the refrigerator to avoid costly experimentation.     

An irreversible thermodynamic model for solar absorption refrigerator

A solar refrigerator is made of a solar collector and a refrigeration system. Real solar refrigerators usually operate between two limits, maximum coefficient of performance (COP) and maximum cooling load. A new model is presented to describe an irreversible absorption refrigerator, in which not only the irreversibilities of heat conduction but also those resulting from friction, eddy and other irreversible effects inside the working fluid are considered. The influence of these irreversible effects on the performance of an absorption refrigerator with continuous flow is investigated. The analytical expressions of the optimal refrigeration coefficient and the cooling rate of the refrigerator are derived. The predictions of the model are compared with semi-empirical cycle model of single-stage absorption refrigeration machines. The results obtained here can describe the optimal performance of a four temperature level absorption refrigeration affected simultaneously by the internal and external irreversibilities and provide the theoretical bases for the optimal design and operation of real absorption refrigerators operating between four temperature levels.     

Thermodynamic design procedure for solid adsorption solar refrigerator

The thermodynamic design procedure for solid adsorption solar refrigeration is presented and applied to systems using activated carbon/methanol, activated carbon/ammonia and zeolite/water adsorbent/adsorbate pairs. The results obtained showed that zeolite/water is the best pair for air conditioning application while activated carbon/ammonia is preferred for ice making, deep freezing and food preservation. In all cases, the system depends strongly on adsorption and condensation temperatures and weakly on the evaporator temperature. The maximum possible net solar COP was found to be 0.3, 0.19 and 0.16 for zeolite/water, activated carbon/ammonia and activated carbon/methanol, respectively, when a conventional flat plate solar collector is used.     

Theoretical study of a solar powered absorption/MED combined system

In this article, a theoretical study is presented for a solar powered combined system comprising a LiBr---H₂O absorption cooling machine and a multiple-effect distillator (MED). The MED has 8 VTE of the falling film type, and it replaces the condenser in conventional absorption machines. Steam released at the generator pressure is supplied to the effect which matches its conditions, and the condensate follows its usual route towards the evaporator of an A/C unit. Thus, the MED is powered by the waste heat of the absorption machine which improves the overall gain and the thermodynamic characteristics significantly.

Governing equations for the combined system are given and are numerically solved. Medium parabolic concentrators are used to power the system, and a transient simulation for the combined arrangement is presented.

Results are given for a typical design summer day in Jeddah, Saudi Arabia, for a range of firing temperatures 150–190°C with a storage temperature amplitude of 10–20°C over a daily working period of 12 h. For a given cooling load of 100 ton refrigeration, the system can produce up to 40m³ of fresh water at a specific collector area of 12.41. H₂O plus 0.03 TR/m². The overall COP_o reaches 1.44, which is more than twice that of a conventional absorption machine at the same temperature levels.     

Design and performance study of a solar energy powered vaccine cabinet

An insulated steel sheet cabinet of 0.6 × 0.3 m face area and 0.5 m depth was designed. The cabinet was intended to store vaccine in remote desert areas, away from the electrical national grid. World Health Organization regulations limit the inside temperature of such vaccine storage cabinets to the range of 0–8°C. A solar energy powered absorption refrigeration cycle using Aqua-Ammonia solution was designed to keep this cabinet temperature in the range of required temperatures, away from the outside temperature, which reaches about 45°C in August. A computer simulation procedure was developed to study the performance and characteristics of the cooling cycle. The simulation included MATLAB computer programs for calculating the absorption cycle, thermodynamic properties and quantities as subroutines for the main program, and a detailed mathematical model using EXCEL for the solar side of the unit. A year round work was predicted. Refrigeration cycle coefficient of performance ranged between 0.5 and 0.65; cylindrical solar concentrator extended the daily operating time to about 7 h and increased the output temperature up to >200°C, while the temperature which gives optimum condition (of COP = 0.65) was 120°C.     

Design and development of a water piston solar powered steam pump

Most solar pumping systems are based on photovoltaic receivers driving electric pumps. An alternative system is to use a boiling water solar receiver to operate a direct acting steam pump. The advantages lie in the relative simplicity of the pump giving ease of manufacture, maintenance, reliability and low cost. At De Montfort University a simple direct acting steam pump has been developed over many years, so it has been a natural development to link this with a boiling water solar receiver. This paper is primarily concerned with the operation of the pump and overall system employed.     

Design and experimental investigation of portable solar thermoelectric refrigerator

The main objective of this study is to design and build an affordable solar thermoelectric refrigerator for the Bedouin people (e.g. deserts) living in remote parts of Oman where electricity is still not available. The refrigerator could be used to store perishable items and facilitate the transportation of medications as well as biological material that must be stored at low temperatures to maintain effectiveness. The design of the solar-powered refrigerator is based on the principles of a thermoelectric module (i.e., Peltier effect) to create a hot side and a cold side. The cold side of the thermoelectric module is utilized for refrigeration purposes; provide cooling to the refrigerator space. On the other hand, the heat from the hot side of the module is rejected to ambient surroundings by using heat sinks and fans. The designed solar thermoelectric refrigerator was experimentally tested for the cooling purpose. The results indicated that the temperature of the refrigeration was reduced from 27 °C to 5 °C in approximately 44 min. The coefficient of performance of the refrigerator (COP_R) was calculated and found to be about 0.16.     

Performance of an aqua-ammonia absorption solar refrigerator at sub-freezing evaporator conditions

Ice making by means of a solar-assisted aqua-ammonia absorption refrigeration unit at subfreezing evaporator conditions is considered in terms of its technical and economic feasibility. A computer-aided thermodynamic analysis is performed for various ranges of operation parameters, three climatic locations varying from 15°N to 43°N latitude and four solar collector types, i.e. flat plate, compound parabolic collector, and east-west and north-south axis tracking concentrators. In order to use an air-cooled condenser, the simultation is predicted on an absorber temperature of 35°C and a condenser temperature of 38°C. The results indicate that for generator pressures of 1.02 to 2.07 MPa, generator temperatures greater than 120°C are required. At these conditions, the COP is on the order of 0.5 and a conventional flat plate collector is not satisfactory. For the three climates, the compound parabolic collector (CPC) has a higher output than an east-west axis tracking concentrator but less than the north-south tracker. The shape of the insolation curves at the lower latitudes causes difficulties in obtaining optimal collector sizes. The cost comparison between the CPC and north-south tracker indicates that overall system costs will range between $85 and $155,000 (1981) for a one ton of ice per day system. The projected costs per ton for a 25-year life are in the range of $10 to $20, which are favorable compared to the cost of domestic ice.     

Simulation, construction and testing of a two-cylinder solar Stirling engine powered by a flat-plate solar collector without regenerator

In this research, a gamma-type, low-temperature differential (LTD) solar Stirling engine with two cylinders was modeled, constructed and primarily tested. A flat-plate solar collector was employed as an in-built heat source, thus the system design was based on a temperature difference of 80 °C. The principles of thermodynamics as well as Schmidt theory were adapted to use for modeling the engine. To simulate the system some computer programs were written to analyze the models and the optimized parameters of the engine design were determined. The optimized compression ratio was computed to be 12.5 for solar application according to the mean collector temperature of 100 °C and sink temperature of 20 °C. The corresponding theoretical efficiency of the engine for the mentioned designed parameters was calculated to be 0.012 for zero regenerator efficiency. Proposed engine dimensions are as follows: power piston stroke 0.044 m, power piston diameter 0.13 m, displacer stroke 0.055 m and the displacer diameter 0.41 m. Finally, the engine was tested. The results indicated that at mean collector temperature of 110 °C and sink temperature of 25 °C, the engine produced a maximum brake power of 0.27 W at 14 rpm. The mean engine speed was about 30 rpm at solar radiation intensity of 900 W/m² and without load. The indicated power was computed to be 1.2 W at 30 rpm.     

Investigation of a diffusion absorption refrigerator

A diffusion absorption refrigerator is a heat powered refrigeration system. It can be operated without any use of electrical or mechanical energy. It uses three component working fluids: ammonia–water–helium. This system consists of no moving parts. Circulation of the working fluid is achieved using a bubble-pump. In this paper, a simple mathematical model is developed. The bubble-pump performance was obtained from a simple experiment using air and water. Calculated results were compared with experimental data. The results show that the system performance is strongly dependent upon the bubble-pump characteristics and the evaporator and absorber mass transfer performance.     

Two-phase flow modelling of a solar concentrator applied as ammonia vapor generator in an absorption refrigerator

A detailed one-dimensional numerical model describing the heat and fluid-dynamic behavior inside a compound parabolic concentrator (CPC) used as an ammonia vapor generator has been developed. The governing equations (continuity, momentum, and energy) inside the CPC absorber tube, together with the energy equation in the tube wall and the thermal analysis in the solar concentrator were solved.The computational method developed is useful for the solar vapor generator design applied to absorption cooling systems. The effect on the outlet temperature and vapor quality of a range of CPC design parameters was analyzed. These parameters were the acceptance half-angle and CPC length, the diameter and coating of the absorber tube, and the manufacture materials of the cover, the reflector, and the absorber tube. It was found that the most important design parameters in order to obtain a higher ammonia–water vapor production are, in order of priority: the reflector material, the absorber tube diameter, the selective surface, and the acceptance half-angle.The direct ammonia–water vapor generation resulting from a 35 m long CPC was coupled to an absorption refrigeration system model in order to determine the solar fraction, cooling capacity, coefficient of performance, and overall efficiency during a typical day of operation. The results show that approximately 3.8 kW of cooling at −10 °C could be produced with solar and overall efficiencies up to 46.3% and 21.2%, respectively.