Experience with fully operational solar-driven 10-ton LiBr/H2O single-effect absorption cooling system in Thailand

A solar-driven 10-ton LiBr/H₂O single-effect absorption cooling system has been designed and installed at the School of Renewable Energy Technology (SERT), Phitsanulok, Thailand. Construction took place in 2005, after which this system became fully operational and has been supplying cooling for our main testing building's air-conditioning. Data on the system's operation were collected during 2006 and analyzed to find the extent to which solar energy replaced conventional energy sources. Here, we present these data and show that the 72 m² evacuated tube solar collector delivered a yearly average solar fraction of 81%, while the remaining 19% of thermal energy required by the chiller was supplied by a LPG-fired backup heating unit. We also show that the economics of this cooling system are dominated by the initial cost of the solar collector array and the absorption chiller, which are significantly higher than that of a similar-size conventional VCC system.     

Constant flow solar pond collector/storage system

This paper presents a periodic analysis of the process of heat extraction by the brine layer circulating at constant flow rate through the bottom convective zone of a solar pond. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted, as a function of time, depths of convective as well as non-convective zones and the flow rate, are derived. Extensive analytical results for the optimum performance of a pond during its year round operation are presented. In a pond with an upper convective zone depth of 0.2 m optimum heat extraction efficiencies of 24 per cent, 29 per cent and 32 per cent corresponding to heat extraction temperatures of 89, 55 and 42°C are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s m², respectively. The load levelling in the extracted heat flux as well as in its temperature improves as the flow rate is lowered and the non-convective zone is over sized. An increase in the total depth of the solar pond improves the load levelling in extraction temperature, but influences the extracted heat flux differently; shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the heat extraction zone and larger depths of the non-convective zone.     

Maintenance strategy for a salt gradient solar pond coupled with an evaporation pond

In a previous study, the authors presented a simple mathematical model for predicting the ratio of the evaporation pond area to that of a salt gradient solar pond area. The evaporation pond idea provides a very attractive method of salt recycling by evaporation, especially in areas of high evaporation and low rates of rain as it is the case for North Africa.In this paper, the model was elaborated upon and applied to two types of surface water flushing (fresh water and seawater) under the prevailing conditions of Tripoli, Libya (latitude=32.86°N). All the results presented were predicted for the first three years of operation. The daily variations of brine concentration in the of Tajoura's Experimental Solar pond and those based on different designs were predicted and discussed under different scenarios. The quantities of brine provided by the evaporation pond and that required by were predicted for both cases of surface water flushing (fresh water and seawater) under the different design conditions. It was predicted that the can provide 20–40% during the first year and 45–95% during the third year depending on the design selected.     

The philosophy of design and construction of the Kuwait 1700 m2 solar pond

This paper discusses the design and construction philosophy of the Kuwait solar-pond/multistage-flash system. The present work came about to study the performance of a solar pond as a main source for energy collection and storage and to use the collected heat in producing fresh water, which is difficult to obtain in remote areas. The pond is built with 1700 m² of surface area and 45° sloping sides. Taking the natural surroundings and the nature of the climate into account, the pond is estimated to perform at 18% efficiency. The collected energy, which is estimated at 1800 kWh/day, will be used to produce 25 m³ of fresh water daily.     

Experimental study of the salt gradient solar pond stability

Many natural systems such as oceans, lakes, etc.…, are influenced by the effect of double-diffusive convection. This phenomenon, which is a combination of heat and mass transfer, can destroy the stability of system-flows.In the case of solar ponds the middle layer, that is linearly stratified, acts as a thermal and mass insulator for the lower layer. This middle layer, called the Non-Convective Zone (NCZ), needs special care to avoid convection and to maintain its stability. In fact, due to an excess of heat stored, a thermal gradient occurs within the NCZ. A convective movement appears at the bottom of the stratified-layers and then grows to a double-diffusive convection movement. This movement transforms the stratified-layers into a well mixed layer, reducing the storage capacity of the pond.Laboratory small-scale pond and middle-scale outdoor solar ponds were designed and built to provide both quantitative data and to study the dynamic processes in solar ponds, including the behavior of the gradient zone.Particle Image Velocimetry (PIV) visualization-experiments carried out in the mechanical and energetic laboratory in the engineering school of Tunisia and experiments in the field showed that the instability of solar ponds could be limited by using porous media placed in the lower layer of the stratification.     

A high-efficiency, compound NH3/H2O-H2O/LiBr absorption-refrigeration system

A high-efficiency, compound absorption-refrigeration system is considered, which is composed of two cooperating absorption units using NH₃/H₂O and H₂O/LiBr solutions, respectively. The heat output from the NH₃/H₂O unit is employed to drive the H₂O/LiBr unit. The thermodynamics of the new system are simulated by using a procedure which showed that very high theoretical coefficients of performance may be obtained (up to 230%) compared to the corresponding theoretical values for the usual single absorption units, which do not exceed 100%.     

Development of electrolyte solution concentration measurement system and application in solar pond

In order to measure the concentration of an electrolyte solution, especially the salt gradient in a solar pond in which the salinity is changed continually, the C.E.T. sensor and its special circuit were developed on the basis of the electroconductivity-temperature measurement method, by which the salt gradient could be measured quickly at any experimental site. Based on a thorough analysis of the conventional sensor with needle electrodes, the C.E.T. sensor effectively overcame the fault (wall-approach-effect) of the conventional sensor with a special structure formed by several cylindrical electrodes and a shield plate on the top. Here, the circuit was designed to obtain high S/N signals and a relation function with a high multiple correlation coefficient of 0.998 was obtained to adapt the wide ranges of concentration 020 wt% and temperature 1080°C on measuring electrolyte-solution concentration. The applicability of the C.E.T. sensor was verified and a new design and measurement method was described in this research.     

Effect of thermal trap on the performance of an underground shallow solar pond water heater

A simple analysis of an underground shallow solar pond water heater has been presented. The effect of a thermal trap at the top of the system has also been incorporated in the analysis. Using the model, the effect of various system parameters, viz. thermal trap thickness, heat capacity of water mass, flow rate and duration of flow rate have been studied in detail. Numerical calculations have been made for a typical winter day at New Delhi (India). It is concluded that the system with thermal trap gives better performance in comparison with a system with a movable insulation system.     

Spectral calculation of the thermal performance of a solar pond and comparison of the results with experiments

This paper deals with a method and the result of the spectroscopic calculation on heat balance of a salt-gradient solar pond under the conditions of spectral solar radiation. Furthermore, reflection of the ray incident upon the surface of the pond water, refraction of the rays within the salt-water layer and diffusion of salt in the pond water are considered. On the other hand, in order to make a clear mechanism of the heat collection and heat storage of the solar pond, we conducted an indoor experiment and a numerical analysis on a small scale model of the salt-gradient solar pond with 2 m² surface area and 1.6 m depth, under the incident rays from a Xe-lamp solar simulator. According to the above experimental analysis, we made a simulation model of thermal performance for a solar pond and carried out the calculation from the heat balance. We found that the simulation calculations correspond well to the experimental results, so that our thermal simulation model and method might be correct. We also did the thermal calculation by changing the incident rays from a Xe-lamp into natural ray (Moon’s spectrum) and Halogen lamp. As a result, it was found that the temperature distributions in the solar pond were notably different due to spectral characteristics of the incident ray. Therefore, the spectroscopic consideration for thermal performance of any solar pond is necessary to obtain a correct solution under the spectral incidence with special wavelength distributions.     

A one-dimensional numerical study of the salt diffusion in a salinity-gradient solar pond

A one-dimensional transient mathematical model is used for the study of the salt diffusion and stability of the density gradient in a solar pond. A finite difference method with a diffusion coefficient dependent on both temperature and salt concentration is used to solve the salt diffusion equation. On the basis of simple considerations we analyze the influence of the salinity-gradient thickness on the useful energy which can be withdrawn from the bottom layer of the solar pond. Finally some considerations on the effect of the velocity of injected brine in rising solar ponds are presented, making use of the Rayleigh analysis of the small perturbations in order to study the stability of the system.     

Experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator

This paper is concerned with experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator. By using the second generator together with a lunate thermosiphon elevation tube, the required minimum driving temperature of the heat source is only 68 °C compared to above 100 °C in traditional absorption refrigeration systems. Based on the horizontal-tube falling-film method, the performance of the absorber can be enhanced by the second generator due to an increase in the differential concentration of the solution between the inlet and the outlet of the absorber and an increase in the temperature difference between the inlet and the outlet of the cooling water in the absorber. The yield of condensate with the second generator open is increased by 68% compared to that with the second generator closed. The performance of the evaporator is significantly improved due to the increase in temperature drop of the chilled water and the decrease in the outlet temperature of the chilled water. This leads to an improvement of the performance of the overall refrigeration system. The maximum coefficient of performance (COP) approaches 0.787.     

Examining potential benefits of combining a chimney with a salinity gradient solar pond for production of power in salt affected areas

The concept of combining a salinity gradient solar pond with a chimney to produce power in salt affected areas is examined. Firstly the causes of salinity in salt affected areas of northern Victoria, Australia are discussed. Existing salinity mitigation schemes are introduced and the integration of solar ponds with those schemes is discussed. Later it is shown how a solar pond can be combined with a chimney incorporating an air turbine for the production of power. Following the introduction of this concept the preliminary design is presented for a demonstration power plant incorporating a solar pond of area 6 hectares and depth 3 m with a 200 m tall chimney of 10 m diameter. The performance, including output power and efficiency of the proposed plant operating in northern Victoria is analysed and the results are discussed. The paper also discusses the overall advantages of using a solar pond with a chimney for production of power including the use of the large thermal mass of a solar pond as a practical and efficient method of storing collected solar energy.     

A solar and wind driven energy system for hydrogen and urea production with CO2 capturing

A novel solar PV and wind energy based system is proposed in this study for capturing carbon dioxide as well as producing hydrogen, urea and power. Both Aspen Plus and EES software packages are employed for analyses and simulations. The present system is designed in a way that PEM electrolyzer is powered by the wind turbines for hydrogen production, which is further converted into ammonia and then synthesizes urea by capturing CO₂ and additional power is supplied to the community. The solar PV is employed to power the cryogenic air separation unit and the additional power is used for the industrial purpose. In the proposed system, ammonia does not only capture CO₂ but also synthesizes urea for fertilizer industry. The amount of electrical power produced by the system is 2.14 MW. The designed system produces 518.4 kmol/d of hydrogen and synthesizes 86.4 kmol/d of urea. Furthermore, several parametric studies are employed to investigate the system performance.     

Validation of a model for the absorption process of H2O(vap) by a LiBr(aq) in a horizontal tube bundle, using a multi-factorial analysis

A multi-factorial experimental test of H₂O vapour absorption by a falling film of LiBr(aq) over an horizontal smooth tube absorber is presented. The response parameters of the study are the average convective heat transfer and mass transfer coefficients of the falling film. The response is expressed as a function of the factors used in the study. Also a mathematical model for the prediction of the performance of the absorber is exposed which takes into account the wetting effects. Finally the predictions from the model and the experimental multi-factorial runs are compared and the conclusions discussed.     

Performance investigation of a salt gradient solar pond coupled with desalination facility near the Dead Sea

Solar ponds provide the most convenient and least expensive option for heat storage for daily and seasonal cycles. This is particularly important for a desalination facility, if steady and constant water production is required. If, in addition to high storage capacity, other favorable conditions exist, the salt gradient solar ponds (SGSPs) are expected to be able to carry the entire load of a large-scale flash desalination plants without dependence upon supplementary sources. This paper presents a performance investigation of a SGSP coupled with desalination plant under Jordanian climatic conditions. This is particularly convenient in the Dead Sea region characterized by high solar radiation intensities, high ambient temperature most of the year, and by the availability of high concentration brine. It was found that a 3000 m² solar pond installed near the Dead Sea is able to provide an annual average production rate of 4.3 L min⁻¹ distilled water compared with 3.3 L min⁻¹ that would be produced by El Paso solar pond, which has the same surface area. Based on this study, solar ponds appear to be a feasible and an appropriate technology for water desalination near the Dead Sea in Jordan.     

Design and experimental testing of the performance of an outdoor LiBr/H2O solar thermal absorption cooling system with a cold store

A domestic-scale prototype experimental solar cooling system has been developed based on a LiBr/H₂O absorption system and tested during the 2007 summer and autumn months in Cardiff University, UK. The system consisted of a 12 m² vacuum tube solar collector, a 4.5 kW LiBr/H₂O absorption chiller, a 1000 l cold storage tank and a 6 kW fan coil. The system performance, as well as the performances of the individual components in the system, were evaluated based on the physical measurements of the daily solar radiation, ambient temperature, inlet and outlet fluid temperatures, mass flow rates and electrical consumption by component. The average coefficient of thermal performance (COP) of the system was 0.58, based on the thermal cooling power output per unit of available thermal solar energy from the 12 m² Thermomax DF100 vacuum tube collector on a hot sunny day with average peak insolation of 800 W/m² (between 11 and 13.30 h) and ambient temperature of 24 °C. The system produced an electrical COP of 3.6. Experimental results prove the feasibility of the new concept of cold store at this scale, with chilled water temperatures as low as 7.4 °C, demonstrating its potential use in cooling domestic scale buildings.     

Modeling of the optimum tilt of a solar chimney for maximum air flow

The aim of this work is to develop a mathematical model to determine the tilt that maximizes natural air flow inside a solar chimney using daily solar irradiance data on a horizontal plane at a site. The model starts by calculating the hourly solar irradiation components (direct, diffuse, ground-reflected) absorbed by the solar chimney of varying tilt and height for a given time (day of the year, hour) and place (latitude). In doing so it computes the transmittance and absorbance of the glazing for the various solar irradiation components and for various tilts. The model predicts the temperature and velocity of the air inside the chimney as well as the temperatures of the glazing and the black painted absorber. Comparisons of the model predictions with CFD calculations delineate the usefulness of the model. In addition, there is a good agreement between theoretical predictions and experiments performed with a 1 m long solar chimney at different tilt positions.     

One-pot sulfurized synthesis of ZnIn2S4/S,N-codoped carbon composites for solar light driven water splitting

To enhance the intrinsic active sites and to suppress the recombination of charge carriers, ZnIn₂S₄ modified with S,N-codoped carbon (ZIS/SN-C) composites were prepared for solar light driven water splitting via the one-pot sulfurized route. Compared with g-C₃N₄ and S-doped g-C₃N₄, the combined effect between S,N-codoped carbon and ZnIn₂S₄ can greatly enhance the photocatalytic activity of ZIS/SN-C. The optimal 4-ZIS/SN-C with the Zn(II) content of 13.78% and the calculated In/Zn molar ratio of 2.03:1 presents the H₂ evolution rate of 2937.1 μmol g^?1 h^?1, which is 2.98 and 23.42 times higher than that of one-pot sulfurized ZnIn₂S₄ and S-doped g-C₃N₄, respectively. However, long-term photo-corrosion induces to the declined durability of 4-ZIS/SN/C for water splitting after three cycles. S vacancies of ZnIn₂S₄ serve as the efficient active sites of H₂ evolution reaction, and S, N-codoped carbon acts as the photo-induced electrons trapper. The one-pot sulfurized approach is thus a potential strategy to fabricate metal sulfide-based photocatalysts.     

ERDA of Ni-Al2O3/SiO2 solar thermal selective absorbers

Thin film materials for the use in solar thermal absorbers have been investigated using time-of-flight energy elastic recoil detection analysis (ERDA). The ERDA measurements proved to be very efficient in detecting the elemental depth composition of a selective solar absorber. The three-layer absorber is composed of an 80% nickel-20% alumina film at the base, a 40% nickel-60% alumina layer in the middle and finally an AR film of silica or hybrid-silica film at the top. The difference between solution volume percent and actual volume percent could be investigated when studying individual nickel-alumina films with varying ratios coated on glass substrates. The result showed that there was a maximum difference of 3% between the calculated solution volume percent and the actual volume percentages in the solid films. The ERDA measurements also indicate that about 15% of the nickel found in the nickel-alumina composite films is bound in the form of NiO.