Air-blown solar still with heat recycling

A laboratory-scale solar still of 1 m² area, designed to recycle the condensation heat of the distillate has been constructed and tested through in-door experiments, using incandescent lamps for irradiation. In this still a considerable fraction of the exothermal enthalpy of condensation and the sensible heat of the warm condensate are successfully recycled and reused for preheating and evaporating the feedstock via a much simpler technique, relative to that used in either multi-stage distillation or multi-flash systems. For heat transfer the most effective processes, forced thin layer evaporation and film condensation, have been applied in counter-flow of the heat exchanging fluids. As a result, about a threefold increase in yield could be achieved, compared with that of a basin-type still of the same area and with the same irradiation. The variation of the yield and the steady-state local temperatures with regard to the variations in the air and feedstock flow rates anre discussed in detail. Optimum conditions, together with the heat and mass balance calculations are also given. The effect of scale-up on the still productivity is estimated. These estimations promise enhanced thermal efficiency with increase in size, and thus a further increase in the productivity per unit area.     

Analysis of heat transfer coefficients and evaporation in a solar still

The dependence of the glass cover temperature, the individual heat transfer coefficients, the overall upward heat flow factor, the fraction of upward heat flow utilized for evaporation, and the rate of water evaporation on the basic parameters has been studied. A semi-empirical equation for estimation of the glass cover temperature has recently been proposed by the authors. An analysis has been made of the capability of the new method to compute accurately the glass cover temperature, the overall upward heat flow factor, the rate of water evaporation, the fraction of upward heat flow utilized for evaporation over an extensive number of combinations of the basic parameters.     

Use of waste hot water in double basin solar still

A periodic analysis of a double basin solar still is presented in this paper. In this still waste hot water is fed into the lower basin at a constant rate such as is available from power stations or other industries. The effect of various parameters on the distillate output of the still is also investigated.     

The steady-state performance of a solar still

The predicted steady-state performance of a single-effect solar still has been in the technical literature and solar energy textbooks for some time, but recent measurements of steady-state solar still efficiencies and temperatures appear in conflict with earlier, apparently similar measurements which partially support the long-standing theory. This paper reviews that controversy and offers new experimental evidence which is used as a basis for altering Dunkle's original model.     

Development of a tube-type solar still equipped with heat accumulation for irrigation

A tube-type solar still is found to be suitable for use in desert irrigation. The effectiveness of a heat accumulator with regard to distillate productivity is experimentally and numerically verified. The heat accumulator consists of tube bundles immersed in wax in order to utilize the latent heat of wax. The dynamic response to stepwise variation of irradiative intensity verified the contribution of wax to an increase of productivity only when the phase change of wax occurred. The effective distillate productivity was found to be 294.3 g/m² during the cyclic stepwise change of irradiative intensity, from 200 to 600 W/m² and back. Velocity vectors driven by natural convection and temperature contours estimated by numerical simulation verified the effectiveness of the heat accumulator especially after peak solar intensity. The latent heat of wax effectively contributed to a 15% increase in total distillate productivity per day. The still can feasibly meet irrigation water supply demands above an irrigative threshold of 17 MJ/m² d.     

Performance of a single-basin solar still

A single-basin solar still having a base area of 1.5 m² (1.5 m × 1.0 m) was designed and fabricated from galvanized steel sheet with an inclined glass cover. The unit was insulated with Staropor. Hourly and daily measurements of the still productivity, temperature of water, glass cover, and ambient air were recorded during both summer and winter seasons. Efficiency of the still was found to be independent of solar radiation, however, an increased diffused radiation lead to slight decrease in its efficiency. An increase in still productivity was observed with the increase in ambient temperature and decrease in wind velocity.     

Optimum arrangement and use of heat pumps in recovering waste heat

Relations are derived for the coefficient of performance of heat pump systems used to transfer heat from a low temperature heat source stream to a high temperature heat sink stream. The manner of use and operation of a number of heat pumps in such a system has been determined for the thermodynamic optimum for reversible and irreversible heat pumps.     

Adsorption refrigeration—An efficient way to make good use of waste heat and solar energy

This paper presents the achievements gained in solid sorption refrigeration prototypes since the end of the l970s, when interest in sorption systems was renewed. The applications included are ice making and air conditioning. The latter includes not only cooling and heating, but also dehumidification by desiccant systems. The prototypes presented were designed to use waste heat or solar energy as the main heat source. The waste heat could be from diesel engines or from power plants, in combined cooling, heating and power systems (CCHP). The current technology of adsorption solar-powered icemakers allows a daily ice production of between 4 and 7 kg m⁻² of solar collector, with a solar coefficient of performance (COP) between 0.10 and 0.16. The silica gel–water chillers studied can be powered by hot water warmer than 55 °C. The COP is usually around 0.2–0.6, and in some commercially produced machines, it can be up to 0.7. The utilization of such chillers in CCHP systems, hospitals, buildings and grain depots are discussed. Despite their advantages, solid sorption systems still present some drawbacks such as low specific cooling power (SCP) and COP. Thus, some techniques to overcome these problems are also contemplated, together with the perspectives for their broad commercialisation. Among these techniques, a special attention was devoted to innovative adsorbent materials, to advanced cycles and to heat pipes, which are suitable devices not only to improve the heat transfer but also can help to avoid corrosion in the adsorbers. Recent experiments performed by the research group of the authors with machines that employ composite adsorbent material and heat pipes showed that it is possible to achieve a SCP of 770 W kg⁻¹ of salt and COP of 0.39 at evaporation temperatures of −20 °C and generation temperature of 115 °C.     

Model for computation of solar fraction in a single-slope solar still

A new model that calculates the distribution of solar radiation inside a single-slope solar still has been proposed. In this model, the solar fraction on a vertical surface is divided into beam and diffuse parts and the optical view factors of surfaces inside the still are taken into account. To validate the model, outdoor tests of a conventional solar still were conducted under different weather conditions at the University of Strathclyde. The proposed model is compared with the previous one. It is found that the beam solar fraction is affected by both the geometry of the solar still and position of the sun in the sky. In contrast, the diffuse solar fraction is only dependent on the geometry of the solar distiller. The present model exhibited a lower root mean square error than that of the previous model. It appears that splitting the solar fraction into beam and diffuse parts improves the accuracy of modelling the performance of a single-slope solar still.     

Exergy analysis of a passive solar still

This paper presents a steady-state and transient theoretical exergy analysis of a solar still, focused on the exergy destruction in the components of the still: collector plate, brine and glass cover. The analytical approach states an energy balance for each component resulting in three coupled equations where three parameters—solar irradiance, ambient temperature and insulation thickness—are studied. The energy balances are solved to find temperatures of each component; these temperatures are used to compute energy and exergy flows. Results in the steady-state regime show that the irreversibilities produced in the collector account for the largest exergy destruction, up to 615 W/m² for a 935 W/m² solar exergy input, whereas irreversibility rates in the brine and in the glass cover can be neglected. For the same exergy input a collector, brine and solar still exergy efficiency of 12.9%, 6% and 5% are obtained, respectively. The most influential parameter is solar irradiance. During the transient regime, irreversibility rates and still temperatures find a maximum 6 h after dawn when solar irradiance has a maximum value. However, maximum exergy brine efficiency, close to 93%, is found once T_col<T_w (dusk) and the heat capacity of the brine plays an important role in the modeling of collector–brine interaction. Nocturnal distillation is characterized by very low irreversibility rates due to reduced temperature difference between collector and an increase in exergy efficiency towards dawn due to ambient temperature decrease.     

Evaluation of a concrete cascade solar still

The quality of the distilled water produced by the Concrete Cascade Solar Still wasevaluated by the comparison with the conventional electrically powered still used for the samepurpose and the quality of the tap water intake prior to these investigations. The samples weredrawn from the following areas: the cascade solar still (over a continuous five days period) ; theelectrically powered still; and a five day storage period (in the dark) of the previouslymentioned areas. The Water Quality Indices (WQI) were 4.50, 3.76 and 29.35 for the watersamples from the cascade solar still, the electrically powered still and tap water respectively. Theheavy metals and sodium and calcium profiles also showed that the tap water had higherconcentrations of these metals. However, the microbiological assays showed that the cascadesolar still had the highest colonies forming units (1400 CFU/ml) , with respect to the electricallypowered still (22 CFU/ml) and tap water (12 CFU/ml) . The C.V. for the selected parameterswere: 0.875, 0.214, 2.060, 1.443, 2.109, 0.218, 1.888, 0.436, 1.082, 1.081 and 0.939 for theconcentrations of iron, lead, calcium, zinc and sodium, and the values of pH, conductivity,turbidity, total suspended solids, chloride ions and microbiological assays respectively.     

Techno-economic model of a solar still coupled with a solar flat-plate collector

In this paper, a techno-economic model has been developed for a solar still coupled with a flat-plate collector through a heat-exchanger. The model is based on the establishment of periodic steady-state conditions. The economic criterion used in the model is based on the least cost of a unit mass of distilled water evaluated from the life-cycle costing of the system. To evaluate the model, numerical calculations have been made corresponding to the climate of Delhi (India). It is concluded that the addition of a solar collector enhances the distillate yield; however, this is not always economical. This system therefore needs careful and economic system design.     

Ethyl alcohol distillation in a basin solar still

Prediction of alcohol distillation in a solar still with a horizontal evaporating surface and an inclined condensing surface was carried out, based on the model modified by Spalding [Convective Mass Transfer. Edward Arnold, 1963] and Kiatsiriroat et al. [Energy, 1986, 11, 881–886]. With the temperatures measured at the evaporating and condensing surfaces, including the concentration of alcohol in the liquid at the evaporating surface, the mass fluxes of concentrated ethanol, pure alcohol and water distillate yielded from the unit could be estimated. The predicted results agreed well with those of the experiments.     

Steam jet ejector cooling powered by waste or solar heat

A small scale steam jet ejector experimental setup was designed and manufactured. This ejector setup consists of an open loop configuration and the boiler operate in the temperature range of T_b = 85–140 °C. The typical evaporator liquid temperatures range from T_e = 5 °C to 10 °C while the typical water-cooled condenser pressure ranges from P_c = 1.70 kPa to 5.63 kPa (T_c = 15–35 °C). The boiler is powered by two 4 kW electric elements while a 3 kW electric element simulates the cooling load in the evaporator. The electric elements are controlled by means of variacs.Primary nozzles with throat diameters of 2.5 mm, 3.0 mm and 3.5 mm are tested while the secondary ejector throat diameter remains unchanged at 18 mm. These primary nozzles allow the boiler to operate in the temperature range of T_b = 85–110 °C. When the nozzle throat diameter is increased, the minimum boiler temperature decreases. A primary nozzle with a 3.5 mm throat diameter was tested at a boiler temperature of T_b = 95 °C, an evaporator temperature of T_e = 10 °C and a critical condenser pressure of P_crit = 2.67 kPa (22.6 °C). The system's COP is 0.253.In a case study the experimental data of a solar powered steam jet ejector air conditioner is investigated. Solar powered steam ejector air conditioning systems are technical and economical viable when compared to conventional vapour compression air conditioners. Such a system can either utilise flat plate or evacuated tube solar thermal collectors depending on the type of solar energy available.     

Double basin solar still

This paper presents a periodic analysis and observed performance of a double basin solar still, mounted on a stand. Observations on this type of still are in fair agreement with the results of analysis. The daily distillate production of such a still is on the average 36% higher than that of a single basin still. Some aspects of the operation, design and performance have also been discussed.     

Performance of a multiple-wick solar still with condenser

In a conventional still, because the water condenses underneath a glass cover, its temperature becomes quite high. During the period of maximum sunshine, the glass temperature is higher than the dew point of the air-vapour mixture inside the still. Thus the yields of these kinds of stills are low. In this communication we have investigated experimentally the performance of a still with a condenser. Most of the condensation now takes place in the condenser, consequently the glass temperature remains low causing less heat loss to the ambient environment. The yield of the condenser-type still is higher than that of the non-condenser-type still.     

Demonstration plant of a multi wick solar still

A solar distillation plant of capacity 70 litres/day (annual average) has been installed at the Centre of Energy Studies, Indian Institute of Technology, New Delhi, India. The system has been working for the last 1 year, and its performance is found to be satisfactory. It meets the requirements of distilled water in the Institute.     

Dewatering of wastewater sludge through a solar still

Wastewater sludge was passed through a laboratory solar still in order to determine the dewatering process. A number of parameters describing the distillate and the sludge were measured at the end of each day in order to determine the process behavior under conditions of relatively high solar radiation and temperature levels. It was realized that dewatering is accelerated and further analysis is needed in order to determine optimum conditions and design parameters for a demonstration plant that will operate towards reducing the sludge volume.     

Estimation of internal heat transfer coefficients of a hybrid (PV/T) active solar still

In this paper, an attempt is made to estimate the internal heat transfer coefficients of a deep basin hybrid (PV/T) active solar still. The estimation is based on outdoor experimental observation of hybrid (PV/T) solar still for composite climate of New Delhi (latitude 28°35′N and longitude 77°12′E). The internal heat transfer coefficients are evaluated by using thermal models proposed by various researchers. The comparison of hourly yield predicted using various thermal models to the experimental has also been carried out by evaluating the correlation coefficient and percentage deviation. It is observed that, Kumar and Tiwari model (KTM) better validate the results than the others model. The average annual values of convective heat transfer coefficient for the passive and hybrid (PV/T) active solar still are observed as 0.78 and 2.41 W m⁻² K⁻¹, respectively at 0.05 m water depth.