Thermodynamic analysis of monomethylamine–water solutions in a single-stage solar absorption refrigeration cycle at low generator temperatures

A theoretical analysis of the coefficient of performance COP was undertaken to examine the efficiency characteristics of the monomethylamine–water solutions for a single-stage absorption refrigeration machine, using low generator temperatures (60–80°C), which allows the use of flat plate solar collectors. The thermodynamic analysis considers both, basic and refined cycles. The refined absorption cycle included a sensible heat recover exchanger (that is a solution heat exchanger). The thermal coefficients of performance COP_h for the basis cycle and COP_SHE for the refined cycle were calculated using the enthalpies at various combinations, at the operating temperatures and concentrations. The flow ratio F_R has been calculated as additional optimization parameter. Due to the relative low pressure and the high coefficients of performance, the monomethylamine–water solutions present interesting properties for their application in solar absorption cycles at moderate condenser and absorber temperatures (25–35°C), with temperatures in the evaporator from −10°C to 10°C which are highly usable for food product preservation and for air conditioning in rural areas.     

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.     

Modeling of an intermittent solar absorption refrigeration system operating with ammonia–lithium nitrate mixture

The theoretical performance of an intermittent absorption refrigeration system operating with ammonia–lithium nitrate mixture is presented. The analysis was done for representative days of each season of 2001. Meteorological data were taken from a local meteorological station installed in the Energy Research Centre of the National University of Mexico in Temixco, Morelos, Mexico. The system consists of a generator-absorber, a condenser, a valve and an evaporator. A compound parabolic concentrator (CPC) with a glass cover, operates as the generator-absorber of the cooling system. Since lithium nitrate does not evaporate during the generation, it is not necessary to use a rectifier. The theoretical efficiencies of the CPC varied from 0.78 to 0.33 depending on the time of the day and the season. Also, the results showed that with the proposed system it is possible to produce up to 11.8 kg of ice at generation temperatures around 120°C and condensation temperatures between 40°C and 44°C. These temperatures allow the system to be chilled with air or water. The overall efficiencies of the systems were between 0.15 and 0.4 depending on the generation and condenser temperatures. The efficiencies are satisfactory considering the simplicity of the system.     

An energy efficient hybrid system of solar powered water heater and adsorption ice maker

A new hybrid system of solar powered water heater and adsorption ice maker has been proposed. The working principle of the combined cycles of solar refrigeration and heating is described, theoretical simulation to the thermodynamic processes has been made. Experiments have been performed in a developed prototype hybrid system; it is verified that the hybrid system is capable of heating 60 kg water to about 90°C as well as producing ice at 10 kg per day with a 2-m² solar collector.     

Solar assisted heat pump

The paper describes a series solar heat pump, using Freon 11 as the working fluid. The heat pump is specifically designed for use in a tropical climate where the normal daytime ambient of above 25°C permits the evaporator to be operated at a high temperature (15–50°C depending on solar input). The use of Freon 11 permits conventional reciprocating refrigeration compressors to be used at elevated temperatures without exceeding design pressure limits. A single unit acts as the evaporator and solar collector. When solar insolation is low the evaporator pressure automatically drops so that energy is received from the atomsphere. However the C.O.P. and output are so low in this mode that the system cannot correctly be termed dual source. The water cooled condenser operates in the temperature range of 35–90°C, the heated water representing the useful output of the system. Operation in the air conditioning mode is not possible due to the large specific volume of Freon 11 at low temperatures. A theoretical analysis is presented to describe the system operation, and the experimental results are shown to agree well with the computer simulation. Average values of C.O.P. of between 2.5 and 3.5 were obtained for the small prototype developed with high side storage temperatures of up to 80°C.     

Thermal analysis of a natural circulation solar air heater with phase change material energy storage

The transient thermal analysis of a natural convection solar air heater is presented. The heater consists of a single-glazed flat plate solar collector integrated with a paraffin type phase change material (PCM) energy storage subsystem and a rectangular enclosure which serves as the working chamber. The PCM is prepared in modules, with the modules equispaced across the absorber plate. The underside of the absorber plate, together with the vertical sides of the PCM module container, serve as air heating vanes. Air flow through the system is by natural convection. Energy balance equations are developed for each major component of the heater and linked with heat and mass balance equations for the heated air flowing through the system. The airflow rate is determined by balancing the buoyancy head resulting from thermally induced density differences and the friction head due to various flow resistances. The predicted performance of the system is compared with experimental data under daytime no-load conditions over the ambient temperature range of 19–41 °C and daily global irradiation of 4.9–19.9 MJ m^–2. Predicted temperatures at specific locations on the absorber plate, heat exchanger plate, glazing, and heated air agree closely with experimental data to within 10, 6, 8, and 10 °C, respectively. Maximum predicted cumulative useful and overall efficiencies of the system are within the ranges 2.5–13 and 7.5–18%, respectively. Correlations of the predicted efficiencies are presented.     

A solar ejector cooling system using refrigerant R134a in the Athens area

This paper describes the performance of an ejector cooling system driven by solar energy and R134a as working fluid. The system operating in conjunction with intermediate temperature solar collector in Athens, is predicted along the 5 months (May–September). The operation of the system and the related thermodynamics are simulated by suitable computer codes and the required local climatologically data are determined by statistical processing over a considerable number of years. It was fount that the COP of ejector cooling system varied from 0.035 to 0.199 when the operation conditions were: generator temperature (82–92 °C), condenser temperature (32–40 °C) and evaporator temperature (−10–0 °C). For solar cooling application the COP of overall system varied from 0.014 to 0.101 with the same operation conditions and total solar radiation (536–838 W/m²) in July.     

The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis

The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO₂·xH₂O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO₂ structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.     

An autonomous solar ammonia-water refrigeration system

Solar refrigeration is especially attractive in isolated regions. The Danube Delta needs ice for fish preservation. This paper describes an intermittent single-stage H₂O---NH₃ solar absorption system of 46 MJ/cycle. Solar collectors heat the generator. Installation details and experimental results are presented. The system coefficient of performance (COP)_system varies between 0.152 and 0.09 in the period of May–September. Solar radiation availability and the theoretical (COP), also applicable to the Trombe-Foex system, are assessed. Reference is made to evacuated solar collectors with selective surfaces. Actual (COP)_system values of 0.25–0.30 can be achieved at generation and condensation temperatures of 80°C and 24.3°C respectively. For bigger capacities of 450–675 MJ/day, the pay-off period is estimated to be 6 and 4 years respectively and the life-time to 15–18 years.     

An organic PCM storage system with adjustable melting temperature

A proper storage temperature is an important criterion for selecting a phase change material (PCM) for a passive solar heating application. Here we describe a novel procedure to produce a mixture of carboxylic acids with a melting temperature adjustable to the climate specific requirements. The approach is based on the ideal solution model and differential scanning calorimetry (DSC). The applicability of the method is demonstrated and it is also applied to a PCM wall design. The accuracy of the theoretical model is ±2°C in the temperature range of 20°–30°C and even a ±0.5°C accuracy can be obtained by the experimental procedure.     

Thermal and economical analysis of a central solar heating system with underground seasonal storage in Turkey

Thermal performance and economic feasibility of two types of central solar heating system with seasonal storage under four climatically different Turkey locations are investigated. The effects of storage volume and collector area on the thermal performance and cost are studied for three load sizes. The simulation model of the system consisting of flat plate solar collectors, a heat pump, under ground storage tank and heating load based on a finite element analysis and finite element code ANSYS™ is chosen as a convenient tool. In this study, the lowest solar fraction value for Trabzon (41°N) and the highest solar fraction value for Adana (37°N) are obtained. Based on the economic analysis, the payback period of system is found to be about 25–35 years for Turkey.     

Is there a need for a rock bed store? Simulation and optimization of solar air heating systems for offices with large thermal capacity walls

A solar air heating system is designed for a floor of 120 m² offices, with large thermal capacity walls, in Israel. A constant air volume system is chosen for its operational simplicity. Representative winter hourly weather data are used to calculate the heating load. The building behavior is modeled in detail with dynamic wall and room temperatures which are linked to the heat input. The heat losses are found to be primarily (70–75%) due to storage in the walls for two different values of wall heat capacity and for two design temperatures (19° and 20°C). The paper deals with the operational details, seasonal performance and economics of the system. Multivariate optimization is carried out using the Simplex method. Optimum collector area, store volume and air flow rate of 30 m², 2–3 m³ and 0.5 kg s⁻¹, respectively, are not affected by economic predictions. A comparison of this system with one which omits the rock bed store and uses only the building material as storage is also made. Results show that for the higher design temperaturre of 20°C, the rock bed store improves system performance, but the same solar fraction can be achieved by increasing the collector area from 30 to 50 m² in the system without active storage. For the lower design temperature of 19°C the improvement in performance made by the addition of the rock bed store is small, and can be obtained by increasing the collector area from 30 to 40 m², obviating the need for the store system. In buildings with a high heat capacity, operated during daytime only, the no-active-store system is recommended for its ease of operation and suitability for retrofitting.     

Design and performance prediction of a novel double heat pipes type adsorption chiller for fishing boats

A novel double heat pipe type adsorber, which uses compound adsorbent of CaCl₂ and expanded graphite to improve the adsorption performance, is designed. The double heat pipes are integrated into the adsorbers in order to solve the problem of the corrosion between seawater and the steel adsorber in ammonia system and improve the heat transfer performance of the adsorber. There are two kinds of heat pipes integrated with the adsorber. One is the split type heat pipe for heating the adsorber in desorption phase, the other one is the two-phase closed thermosyphon heat pipe for cooling the adsorber in adsorption phase. The performance of two-adsorber adsorption chiller integrated with double heat pipes is predicted. The heat transfer performance of the heat pipes can meet the heat demands for adsorption/desorption of the adsorbent when the heating/cooling time is 720 s and mass recovery time is 60 s. When the exhaust gas temperature is 550 °C, the cooling water temperature is 25 °C, the inlet and outlet chilled water is −10 and −15.6 °C, respectively; the simulation results show that the cooling power and COP of this adsorption system are 5.1 kW and 0.38, respectively.     

Effect of irreversibilities on performance of an absorption heat transformer used to increase solar pond’s temperature

Absorption thermal systems are attractive for using waste heat energy from industrial processes and renewable energy such as geothermal energy, solar energy, etc. The Absorption Heat Transformer (AHT) is a promising system for recovering low-level waste heat. The thermal processes in the absorption system release a large amount of heat to the environment. This heat is evolved considerably at temperature, the ambient temperature results in a major irreversible loss in the absorption system components. Exergy analysis emphasises that both losses and irreversibility have an impact on system performance. Therefore, evaluating of the AHT in exergy basis is a much more suitable approach. In this study, a mathematical model of AHTs 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. A heat source at temperatures not higher than 100 °C was used to simulate the heat input to an AHT from a solar pond. In this paper, exergy analysis of the AHT were performed and effects of exergy losses of the system components on performance of the AHT used to increase solar pond’s temperature were investigated. The maximum upgrading of solar pond’s temperature by the AHT, is obtained at 51.5 °C and gross temperature lift at 93.5 °C with coefficients of performance of about 0.4. The maximum temperature of the useful heat produced by the AHT was ˜150 °C. As a result, determining of exergy losses for the system components show that the absorber and the generator need to be improved thermally. If the exergy losses are reduced, use of the AHT to increase the temperature of the heat used from solar ponds will be more feasable.     

Experimental studies on bubble pump operated diffusion absorption machine based on light hydrocarbons for solar cooling

An experimental investigation of an air-cooled diffusion absorption machine operating with a binary light hydrocarbon mixture (C₄H₁₀/C₉H₂₀) as working fluids and helium as pressure equalizing inert gas is presented in this paper. The machine, made of copper an available and very good heat conducting metal, is intended to be solar powered heat from flat plate or common evacuated tube collectors. The cooling capacity is 40–47 W respectively for 9 and 11°C chilled water temperature. Cold is produced at temperatures between −10 and +10 °C for a driving temperature in the range of 120–150 °C.     

The effect of temperature on the power drop in crystalline silicon solar cells

The influence of temperature and wavelength on electrical parameters of crystalline silicon solar cell and a solar module are presented. At the experimental stand a thick copper plate protected the solar cell from overheating, the plate working as a radiation heat sink, or also as the cell temperature stabilizer during heating it up to 80°C. A decrease of the output power (−0.65%/K), of the fill-factor (−0.2%/K) and of the conversion efficiency (−0.08%/K) of the PV module with the temperature increase has been observed. The spectral characteristic of the open-circuit voltage of the single-crystalline silicon solar cell is also presented. It is shown that the radiation-rate coefficient of the short-circuit current-limit of the solar cell at 28°C is 1.2%/(mW/cm²).     

Smart solar tanks for small solar domestic hot water systems

Investigation of small SDHW systems based on smart solar tanks are presented. The domestic water in a smart solar tank can be heated both by solar collectors and by means of an auxiliary energy supply system. The auxiliary energy supply system––in this study electric heating elements––heats up the hot-water tank from the top and the water volume heated by the auxiliary energy supply system is fitted to the hot-water consumption and consumption pattern. In periods with a large hot-water demand, the volume is large; in periods with a small hot-water demand, the volume is small.Two small SDHW systems, based on differently designed smart solar tanks and a traditional SDHW system were investigated by means of laboratory experiments and theoretical calculations. The investigations showed that the yearly thermal performance of SDHW systems with smart solar tanks is 5–35% higher than the thermal performance of traditional SDHW systems. Estimates indicate that the performance/cost ratio can be improved by up to 25% by using a smart solar tank instead of a traditional tank when the backup energy system is electric heating elements. Further, smart solar tanks are suitable for unknown, variable, large or small hot-water consumption and the risk of oversized solar heating systems and oversized tank volumes is reduced by using smart solar tanks. Based on the investigations it is recommended to start development of smart solar tank units with an oil-fired boiler or a natural gas burner as auxiliary energy supply system.     

Transient heat conduction in a glass with chemically deposited SnS---CuxS solar control coating

The transient thermal performance of a chemically deposited thin film on a glass substrate is presented. Differential energy balances for the glass are set up assuming a one-dimensional transient state for normal incidence of the air mass to solar radiation. Using the reported properties of a SnS-Cu_xS thin film as a specific example, the net energy flow through a glass and the specific film were calculated for a time period. The effect of absorption in the film is specifically considered. The theoretical time histories of the surface temperatures of the glass plus solar control coating and the clear glass are plotted for an exterior temperature range of 0–50°C. The redistribution to the interior and exterior of the absorbed component of the solar radiation as well as the shading coefficient in time are evaluated for a constant convective heat transfer coefficient for ambient temperatures of 30 and 20°C. Also, the time constant of the system is given.     

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.