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.     

A tubular solar still integrated with a heat pipe

The aim of this work is to explore the thermal performance of a tracked tubular solar still (TSS) with a parabolic trough concentrator in Baghdad (33.27° N, 44.37° E) in September 2022. The present tubular still is distinguished by its hexagonal glass cover. The effect of integrating the TSS with a heat pipe, the still tilt angle (10°, 15°), and the depth of saline water inside the still partitions on the productivity of freshwater are investigated. The results showed that using heat pipe enhances the freshwater productivity by 25%–40% and the efficiency by 25%. For the still integrated with heat pipe, as the water depth is increased from 5.5 to 6.5 cm the productivity of freshwater is increased by 16% and 20% for tilt angles 10° and 15°, respectively.     

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.     

Performance analysis of waste brick magnesia as a storage material in a solar still

In this current study, a single basin solar still (SBSS) was explored in the climatic state of Patan (23.84° N, 72.12° E) India over March 2019. The SBSS was examined with the use of waste brick magnesia (WBM) for the same amount of water (10 L). For experimentation, three same-sized SBSS were taken with 3 and 6 kg WBM and without storage materials. It was found that a daytime yield for SBSS of 2.27 L without storage materials was obtained; and this was reduced for SBSS with 3 and 6 kg WBM to 1.96 and 1.75 L, respectively. It has also been noticed that the yield of SBSS during off-sunshine hours due to the energy storage capacity increased with WBM but the combined (daytime + off-sunshine hours) yield was found reduced with an increment in a mass of the WBM. It was found that the maximum instantaneous thermal efficiency of the SBSS without WBM, 3 and 6 kg WBM was 60.24%, 45.65%, and 54.26%, respectively. Overall thermal efficiency was also determined and obtained to be higher for SBSS without WBM as storage.     

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.     

New method to store heat energy in horizontal solar desalination still

A new approach is proposed to store excess heat energy in horizontal solar desalination stills during daytime for the continuation of the process at night. This technique divides the horizontal still into evaporating and heat storing zones and combines the advantages of shallow and deep stills. The performance of heat storing zone was studied over one year and exhaustive data were collected, analyzed and presented. To show the effectiveness of the system, its performance was compared with that of the shallow still. The heat storing capacity of the system during the daytime was found to be an average of 35.7% of the total amount of solar energy entering the system. The efficiency of recovering process, in the form of portable water produced at night, was found to be an average of 47.2% of the total amount of energy stored during the day. Furthermore, this technique does not require any kind of external power for storing and recovering processes.     

The use of a thermal energy recycle unit in conjunction with a basin-type solar still for enhanced productivity

A recently developed thermal energy recycling unit operating under forced air circulation was attached to a conventional, basin-type solar still to enhance overall still productivity. In this unit, a relatively large fraction of the latent heat of condensation of the distillate is utilized to preheat and evaporate the feedstock. The system performance was tested in the laboratory using a solar simulator. The solar still was double glazed and no condensation was observed on the inner glazing when operating in the thermal energy recycling mode. The overall system productivity was about three times that of a conventional (single-effect) basin-type solar still. The advantages of the proposed system design are the following: (i) the solar still productivity can be enhanced significantly and at a reasonable cost; (ii) non-wetting glazings (e.g. certain plastic glazings) can be utilized, since in this mode of operation the glazing does not function as a condensation surface; (iii) as a result, the thermal losses from the outer surface of the glazing to the ambient can be reduced significantly by the use of double glazings; (iv) the system is very adaptable to the utilization of an external waste energy source (e.g. wet steam or hot saturated air) for nocturnal distillation, viz. operation in the absence of solar radiation.     

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.     

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.     

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.     

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.     

Enhancement of the yield of solar still with the use of solar pond: A review

A solar pond (SP) is a remarkable growth of renewable energy technology that has stored solar energy for storage purposes and is used in many solar thermal applications. It is also utilized for many purposes, such as heating, cooling, space heating, air conditioning, and many more. The present paper shows the use of an SP to improve the yield of solar still (SS) by providing hot water through the heat energy stored in it. It also reveals the use of shallow and mini SPs with SS to improve yield. Various future research works on SS using SPs have also been included in this paper. From the current review paper, it was concluded that the SP increases the yield of the SS.     

Transient analysis of double basin solar still with intermittent flow of waste hot water in night

A transient analysis of a double basin solar still has been presented in this paper, incorporating the effect of intermittent flow of waste hot water into the lower basin at a constant rate during off sunshine hours. The waste hot water can be obtained either from a thermal power plant or any other industry. The effect of various parameters, e.g. inlet temperature, flow rate, water mass, etc., on the distillate output of the still has been investigated in detail. It has been found that the yield increases with flow rate if the inlet waste hot water temperature is above its optimum value. If the inlet temperature is below its optimum value, the yield decreases as the flow rate increases. The yield also decreases with an increase of water mass in the lower basin.