Abstract AND FULL PAPER ON FABRICATION AND PERFORMANCE EVALUATION OF THE SOLAR STILL

FABRICATION AND PERFORMANCE EVALUATION OF THE SOLAR STILL

INTRODUCTION

Energy is the basic input required to sustain economic growth and to provide basic amenities of life to the entire population of a country. Energy can be an effective weapon in the battle against abject poverty in the country like India. Like other developing countries, India is also in the process of planning and development as such needs a quantum of energy for its development plans. It is the level and pattern of utilization of energy from different sources in any country, which is an index of industrial development and standard of living. Basically energy is utilized in four key sectors of our economy namely Agriculture, industry, commercial and the households.

               Due to increasing gap between demand and supply of energy                                               there is an urgent need to utilize the different forms of non-conventional energy sources such as solar, wind, biomass etc. Among these energy sources one of the most important sources is solar energy.

SOLAR ENERGY

               The sun is a gaseous globe with a radius of 7.0 x 10⁵     Km, it has a mass of about 2.0 x 10³⁰  Kg this is greater than the earth’s mass by a factor of about 330,000. The total rate of energy output from the sun is

3.8 x 10²³   KW. At a mean distance of 1.49 x 10⁸ Km from the sun, the earth intercepts about 1 part in 2 billion of this energy.

             All countries in the world receive solar energy. This amount varies from a few hundred hours per year as in the northern countries and the lower part of the South America, to four thousands hours per year as in the case of most of the Arabian Peninsula and the Sahara desert. In estimating the amount of solar energy falling on the earth, let us consider the natural deserts of the world which is about 20 x 10⁶ Km² with an average solar insulation of 538.30 w/m²/day. Another 30 x 10⁶ Km²receive about

 291.65 w/m²/day. If we ignore the area of the sea and rest of the land then the amount of solar energy received by this 50 x 10⁶ Km² area of the earth is 162.2 x 10¹² kWh/day, assuming eight hours of sunshine would be approximately 60 x 10¹⁵ kWh/year. Using 5% of this result in 300 x 10¹³ kWh. And comparing this with the worlds energy demand for the year 2000, it can be seen that it is 60 times what the world energy requirement. Solar energy, which is ultimately    source of most forms of energy used now, is clean, safe and exists in viable quantities in many countries. The drawbacks in using solar radiation as energy are that it cannot be stored and it is dilute form of energy.

                      Solar energy is received in the form of light and heat radiation. The radiant energy of the sun can be converted to different forms like thermal, electrical & mechanical or other energies utilizing the modern methods of conversion. Out of these models of converting solar radiant energy, the thermal mode of conversion is very easiest and the most convenient. The thermal energy can be further used for purposes, such as: 1) To heat water or any other fluid 2) To cook food 3) To dry industrial/ agricultural products 4) To generate refrigeration effect 5) To generate power 6) To purify water, and 7) To create appropriate living conditions in cold/hot climatic regions etc.,

NEED FOR SOLAR STILLS

                          Adequate quality and reliability of drinking water supply is a fundamental need of all people. Without potable or fresh water there is no human life. Industries and agriculture also need fresh water without which they cannot function or thrive. Water is therefore key to man’s prosperity, hence it is aptly said that water is every body’s business. Fresh water which is obtained from rivers, lakes, and ponds is becoming scarce because of industrialization and population explosion. Presently more than 2000 million people are not getting potable water which leads to many diseases. Many U.N organizations like UNDP, along with the World Bank are now actively involved throughout the world in promoting projects concerning to supply of fresh water for drinking purposes.

                               There are many rural places in India where normal water supply schemes are impossible and the only available source of water is highly salty and unfit for drinking. With the present steep escalation of energy costs serious efforts are being made to use the freely available solar energy. Solar stills design and fabrication is easy and could be manufactured with the locally available materials and skill. Use of solar stills is going to play a significant role in reducing water borne diseases in the rural areas.

BENEFITS OF THE SOLAR STILL

                        

·       Free to operate using the power of sun.

·       Simple to operate as no filters or chemicals are used

·       Uses any sources of water

·       Produces clinically pure water suitable for domestic, industrial,          

     Agricultural needs.

                               

        REVIEW OF LITERATURE

                   

    The simplest application of a thermal solar thermal energy installation

Is in the distillation of water. The solar distiller purifies water by first evaporating and then condensing it. Distilled water contains no salts, minerals or organic impurities. It is not, however, aseptic, as is sterilized water, of which more later. Distilled water can be used for: drinking purposes, applications in hospitals replenishing batteries, and so on. Such an installation is suited to areas where water is ample but polluted, salty or brackish; naturally, there must also be abundant sun. Finally, glass or UV-resistant transparent foil which is the most important material in the construction must be available and affordable. A reasonably functional solar distiller is able to produce adequate amount of water, so as to make it economically justifiable.

                          The operation of the distiller described with reference to

Fig2.1. the radiation (A) falls through glass or plastic screen (D) on to the absorber. In this case the absorber is a tray or basin filled with brackish water (B) Just as in the flat belt collector, this absorber works best if the basin is coated black. This is especially important if water is clear, turbid water absorbs well enough on its own. The radiation warms the basin and, gradually, the water. To reduce heat loss to a min it is vital to insulate the sides and bottom of the basin; if the basin rests on a dry surface this actually forms a reasonable insulation.

                        The water warms and evaporates, leaving the impurities behind. This vapour condenses on the under side of screen (D) when it has a temperature appreciably lower than that of water and the water vapour. Condensation will be possible if wind is cooling the screen, or the outside temperature. The condensate runs along the sloping screen and into a collecting gutter. To prevent the condensate from falling back into water, the screen must be tilted by at least 10 degrees from the horizontal. The whole distiller must be made air tight to prevent loss of vapour. To achieve the best results the turbid inlet water must be replaced daily. Setting the whole distiller at a slight angle makes this easier.

TYPES OF SOLAR STILLS

1)     Shallow basin type solar still          

2)   Life raft type solar still

3)   Tilted tray solar still

4)   Steeped basin type solar still

5)   Regeneration inclined step solar still

6)   Wick type solar still.

7)   Tilted multiple wick type solar still

8)   Tilted wick type solar still

                   SHALLOW BASIN TYPE SOLAR STILL

   There are several minor variations in the geometric configuration of single basin stills. Some of the geometric configurations of the solar stills are shown in Fig 2.3a-g. The solar still shown in fig 2.3a is double sloped symmetrical still with continuous basin which is made air-tight by using heavy asphalt sheets. Glazings are supported on pre-cast concrete pillars placed on lining. The distillate is collected in stainless steel channel embedded in the in the concrete pillar on the lower side of the glass pane. The average water depth in the basin is about 10cm.asphaltic, mastic and caulking material is used for sealing all glass joints.

                  LIFE RAFT TYPE SOLAR STILL

        Life raft type solar still shown in fig 2.4 are constructed of a black felt pad of 0.2sq.m area saturated in sea water placed inside a transparent inflatable plastic envelope and a distillate collector bottom connected to the bottom of the plastic envelope. While in use the whole assembly floats in the sea along with the raft after infiltration and the solar radiation striking the black felt pad makes the water evaporate which condenses on the inside of plastic envelope and finally dripping in to the bottles at the bottom of the assembly. It is reported that more than 2 lacks such units were produced during the World War II and each device was able to supply about one lit of fresh water on a clear day.

                TILTED-TRAY SOLAR STILL

                     Some of the limitations of a single effect horizontal basin solar still are:

·       The water surface is horizontal and hence it receives less radiation in winter particularly at those places away from the equator.

·       Very shallow water depths are not possible, and

·       Large space between the basin water surface and the condensing surface.

These limitations are overcome by tilted tray type solar still shown in fig 2.5, where both the water tray and glass cover are at an optimum tilt angle receiving more radiation, less water increases water temperature in resulting in higher output. The tray and glass cover are parallel and closer and then reduces the reflection losses and has less thermal inertia. The still is sloped at an angle so that direct radiation is received at near normal incidence which is not possible in the horizontal still. The still consists of a series of steps with narrow widths and shallow depths of water with insulation on the rear side and glass cover on the exposed side parallel to the tilted tray. The saline water is supplied at the top step which flows down the steps and finally drained at the bottom. The water distillate is collected in a trough is attached to the glass at its lower end. Less water depth increases water temperature resulting higher output.

                TILTED WICK TYPE SOLAR STILL 

               Use of black porous fabric dipped in water placed over insulation in an air tight box with glass cover at the top can acts as an efficient solar still. Saline water is supplied from the top side of the still as shown in fig 2.9 to the entire width of the wick with the help of a distributor at slow flow rates such that the entire area of black cloth wick remains well at a time. A water proof liner is placed between the insulation and the wick. Solar energy is absorbed by the water in the wick and gets evaporated and condensed on the underside of the glass and finally collected in the condensate channel fixed on the lower side of the glass.

                   DOUBLE BASIN SOLAR STILL

     There are two glass covers as shown in fig 2.11. Over the second glass cover i.e. inner glass cover a thin layer of saline water is filled. The second basin is usual in case of conventional single basin still. The water vapour evaporated from the lower basin will condense on the lower glass of the second upper basin giving its latent heat to the saline water. The condensate is collected in a channel. The latent heat which is used in heating the water in the second basin and the heat which is directly absorbed by the second basin from the sun is used in evaporating the water from the second basin which gets condensed on the lower side of the to glass cover and collected in the condensate channel. The heated collected by the upper glass is finally lost to the atmosphere. The upper basin reduces the solar radiation reaching the solar basin. The additional distillate obtained in the multiple effect solar still compared to the single basin solar still does not compensate for additional cost, sophistication and maintenance. The cost of distilled water from the single basin solar still is always lower than the cost of multiple effects solar still.

                 WICK TYPE SOLAR STILL

                          The advantage of the inclined wick type solar still is that due to its very low thermal capacity and exactly parallel transparent cover absorb more solar radiation and therefore produces high distilled water. The main problems are in the chocking of pores of wick with salt in due courses of time, deterioration of the wick cloth, decoloring of wick cloth, and maintaining uniform flow of water.

                MULTI-TRAY MULTIPLE EFFECTS SOLAR STILL

                     Here several transparent basins full of water are formed on the glass sheets using glass vertical retaining walls. The still consists of shallow water basin kept on insulation and a clear window glass at the top inclined at 10⁰ to the horizontal. On two other glass sheets water basins are formed by taking 9 vertical retaining walls separating 10 channels which contain saline water required to be distilled. Condensation of water takes place on the under side of three plates.

MEASURING INSTRUMENTS

                       While working on various kinds of solar energy systems, it is extremely necessary to understand about the radiation emitted from the sun, its measuring instrument and recording devices. The radiation reaching from the sun to the earth’s surface consists of two components such as direct radiation and diffused radiation. Different kinds of instruments, e.g., pyrheliometer, shading ring pyranometer are required to measure these radiations which give output in mV and have to be converted into proper units by multiplying with their calibration factors. These outputs are then recorded by various devices available.

                 1.  PYRANOMETER

              Pyranometer is highly sensitive instrument which measures the intensity of total radiation received at earth’s surface over a hemispherical field view.

DESCRIPTION:

              Fig2.12 shows the photograph of a typical pyranometer and fig shows its schematic dia. basically it consists of a thin blackened surface which is enclosed in two concentric hemispherical glass domes so as to protect it from wind, rain and dust, this surface is connected to a multi junction thermopile and is supported inside a relatively massive well polished case. The active or hot junctions of this thermopile lie along ring on the upper surface of the sensor. While passive are cold junctions are located in such away, that they do not receive ant radiations.

                INSTALLATION:

             It is essential for the pyranometre to be mounted in the open, in such a position that there is no obstacle to obstruct sun rays in all seasons between sunrise and sun set. A suitable platform or pillar on the flat roof of the building with no tall tree or building obstructing the radiation from any part of the sky would be the best.

                CALIBRATION:

      Pyranometer are usually calibrated against standard pyrheliometers. A standard method has been forth in the annals of the international geophysical year 1953, which requires that readings to be taken at the time of clear skies, with the pyranometer shaded and unshaded at the same time when readings are taken with pyrehilometer. Shading is recommended to be accomplished by means of disc held at 1m from the pyranometer, with the disc just large enough to shade the glass envelope. The calibration constant is then the ratio of the difference in the output of the shaded and unshaded pyranometer to the output of the pyrheliometer.and cosθ_z, the angle of incidence of beam radiation on the horizontal pyranometer. Care and precision are required in these calibrations. It is possible to calibrate the pyranometer against a secondary standard pyranometer. Direct comparison of the secondary pyranometer and field instrument can be made to determine the calibration constant of the field instrument.

                2. SUNSHINE RECORDER

                The hours of bright sunshine, i.e. through which the solar disc is visible, is of use in estimating long term average of solar radiation. The instrument used to collect such data is called sunshine recorder.

DESCRIPTION:

It consists of glass sphere mounted in a brass bowl with groove for holding the recorder cards. The glass sphere when exposed to the sun produces its image on the opposite side which burns a trace on the card, mounted concentrically with the sphere. As the sun moves across the sky, so does the position of the spot across the card. When the sun is obscured due to clouds etc., the trace is interrupted. In the end of the day the total length of the traceless gaps give the duration of bright sunshine. The instrument I generally mounted on a marble base, the bowl being supported in a semi circular brass bar. The sphere is held at two ends by brass screws which fit into cups fixed on the sphere. The bowl has three sets of grooves for taking three sets of cards, long curved for summer, short curved for winter and straight cards at equinoxes.

INSTALLATION:

            As in case of pyranometer the sunshine recorder should also be installed at a place where there is no obstruction to the sun.

OPERATION:

               Once the instrument has been setup and properly adjusted, it requires little attention beyond changing the cards everyday. While inserting the cards care must be taken to ensure that noon line on the card coincides exactly with the noon mark on the bowl.

                3. ANEMOMETER

                 Anemometer is a van-type digital instrument used to measure the air velocity in meter per second. Its range is 0-15 m/sec. t6he operation of vane-type anemometer is as follows.

                 DIGITAL TEMPERATURE INDICATOR:

               The most common electrical method of temperature measurement uses the thermo-electric sensor, also known as the thermocouple. The construction of the thermocouple consists of two wires of different metals twisted and brazed or welded together with each wire covered with insulation. The range is of 0-1600⁰C.

           

DESIGN ASPECTS OF SOLAR STILL

                          The performance of the solar still is governed by different factors such as solar insulation, heat transfer characteristics as well as the environmental aspects. The usefulness of the solar still depends upon the economic returns obtained from the solar still. The section summarizes the different relations used for performance evaluation and economic analysis of solar stills.

ESTIMATION OF SOLAR INSOLATION:

              The Global Radiation I_g=I_bn cosθ_z + I_d

              As per the ASHRAE model

                 I_bn=A exp [b/cosθ_z]

              And I_d=C I_bn

            Where A, B, C are constants for determining solar insulation on clear days.

           I_g=I_b + I_d

                ESTIMATION OF SOLAR RADIATION ON TILTED SURFACES

       I_t=I_br_b + I_dr_d + (I_b + I_d) r_r

Where it is the flux falling on the tilted surface at any instant and r_b,r_d,r_r are tilt factor related to beam, diffuse and reflected radiations.

They are obtained using the following relations.

  R_b= cosθ/cosθ_z= [sinδ sin (Φ-Ѕ) + cosδcosWcos (Φ-Ѕ)]/ [sinΦsinδ + cosΦcosδcosW]

R_d = (1+cosЅ)/2

R_r = (1-cosЅ)/2

                COMPUTATION OF THE HEAT TRANSFER COEFFICIENT

         The heat transfer coefficient for the various heat exchanges taking place in the solar still can be grouped as external and internal modes. The external heat transfer modes are mainly governed by conduction, convection and radiation processes taking place outside the still are independent of each other.

Heat transfer with in the solar still is referred to as internal heat transfer, which involves radiation, convection and Evaporation. Usually the convection and evaporation heat transfer takes place in a combined mode while the radiation heat transfer is independent of the two.

                EXTERNAL HEAT TRANSFER COEFFICIENT

1)     TOP LOSS COEFFICIENT: - involves the radiation/convection loss from glass cover to the outside atmosphere.

  H_rg= e_gσ [(T_g+273)⁴ – (T_sky +273)⁴]/ (T_g-T_a)

T_sky = T_a-6 and T_g is the glass temperature.

H_cg = j* Re*Cp * V*(Pr)^-2/3

2)     BOTTOM LOSS COEFFICIENT: Is computed using the relation

H_b= [1/ (K_i/l_i) + 1/ (h_cb+h_rb)]

3)     SIDE LOSS COEFFICIENT:

U_s= (L₁+L₂) L₃K_i/ (L₁L₂δ_s)

4)     INTERNAL LOSS COEFFICIENT

Radiative heat transfer coefficient from water surface to glass cover.

H_rw=ε_eff σ [(T_w-T_g+ {(P_w-P_g) (T_w+273)⁴-(T_g+273)⁴]

Ε_eff= [1/ε_g + 1/ε_w-1]-1

5)     CONVECTIVE LOSS COEFFICIENT(Hcw)

H_cw= 0.884[T_w-T_g+ {(P_w-P_g) (T_w+273)/ (768.9*10.3-P_w)]1/3

  P_w and P_g represent saturation pressures of basin water and glass cover at initial temperature.

 Saturation pressure is computed using

   P (t) = exp [25.317-(5144)/ (T+273)]

6)     EVAPORATIVE HEAT TRANSFER

H_ew=16.273*10-3H_cw {(P_w-P_g)/ (T_w-T_g)}

   The total internal heat transfer coefficient

H_iw=H_rw+H_cw+h

EVALUATION OF THE DISTILLATE OUTPUT

    T_w2=T_we^-A1t + A₂/A₁ (1- e^-A1t)

Where A₁=1/M_w*4186.0[(H₃H_b/ (H₃+H_b) +H₁H₂/ (H₁+H₂)}

             A₂= 1/M_w*4186.0[{α₁+H₃/ (H₃+H_b) α₂} It +A₁T_a]

Where H₁=heat transfer coefficient from water to glass

            H₂=heat transfer coefficient from glass to ambient

            H₃=heat transfer coefficient from basin to water

            H_b=overall bottom heat transfer coefficient from basin to liner

GLASS TEMPERATURE

           T_g2= (h₁T_w+h₂ T_a)/ (h₁ + h₂)

    Mass of distillate collected

   M_ew=h_ew [T_w2-T_g2]*t/L

L=latent heat of vaporization 2.56*106j/kg

EFFICIENCY

      Η= Q_ex/I_t

Where q_ex= M_ew C_p (T_w2-T_w)-T_aLn (T_w2/T_w)

It is insulation on tilted surface

LIMITATIONS OF THE STUDY

·       The predicted values/results may be different from the observed results due to variations in the climatic conditions.

·                   The model is based upon the estimation of solar insulation for clear days and hence the actual insulation may vary and so also the distillate collected.

·       The predictions are based upon the empirical relations, which are valid to localized regions/areas and climatic conditions, so the code may show large variations from practical results.

CONCLUSIONS

                    The significant conclusions of the study are:

·       The computer code could be used as a versatile tool for determining the available solar insulation on daily/monthly/annual basis and hence compute the distillate output.

·                    Comparison between basin type solar still and stepped basin type solar still shows that for identical water masses and collector areas the latter type is more efficient and gives higher distillate output.

·       Solar stills can be employed for different climatic conditions and supply the drinking water in a cost effective manner employing the renewable energy source.

·         The approach of design analysis based upon the mathematical modeling is cheap & efficient compared to the analysis based on physical models, which in some cases may become tedious.

·       The iterative design process can be employed to obtain an optimum configuration for the solar still.

·       The deviation from the actual results can be corrected by proper selection of material property values.

SUGGESTIONS FOR FUTURE WORKS

           The present work can be extended for simulation of solar still of other types such as porous type solar still, wicked type solar still etc.

               Different factors like water depth, spacing between covers can be taken up as study parameters to know there influence on the performance of solar still.

     Validation of computed results with experimental results, need to be carried out.

         

           

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