Experimental studies of active solar still integrated ...

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ScienceDirect Solar Energy 130 (2016) 207–223 www.elsevier.com/locate/solener

Experimental studies of active solar still integrated with two hybrid PVT collectors D.B. Singh a,⇑, J.K. Yadav a, V.K. Dwivedi b, S. Kumar c, G.N. Tiwari a, I.M. Al-Helal d a Centre for Energy Studies, Indian Institute of Technology Delhi, HausKhas, New Delhi 110016, India Department of Mechanical Engineering, Galgotias College of Engineering and Technology, Greater Noida, UP, India c Department of Training and Technical Education, Delhi, India d Department of Agricultural Engineering, College of Food & Agricultural Sciences, King Saud Univ., P.O. Box 2460, Riyadh 11451, Saudi Arabia b

Received 7 May 2015; received in revised form 9 February 2016; accepted 11 February 2016

Communicated by: Associate Editor Yanjun Dai

Abstract This paper deals with the experimental studies and performance analysis of partially covered hybrid photovoltaic thermal (PVT) flat plate collector (FPC) solar still. The thermal model of the system has been developed. The data have been collected for the composite climate condition of New Delhi. The experimental results have been compared with theoretical results. It has been observed that there is a fair agreement between the experimental and theoretical values with the correlation coefficients varying between 0.97 and 0.99. The coefficient of determination varies between 0.94 and 0.98. The annual productivity has been found to vary between 120.29% and 883.55%. The thermal, exergy and overall thermal efficiency have been evaluated. It has also been observed that the proposed system is self sustainable and it can meet potable water requirement as well as electricity requirement. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: Photovoltaic thermal (PVT) collector; Active solar distillation; Potable water; Productivity

1. Introduction Potable water and energy are two basic requirements for the survival of life on earth. Water available in rivers, lakes and underground reservoirs has been polluted badly in recent years due to global industrialization, fast growth in agriculture and population. Also, these water resources are not adequate to support life on the earth and may not be able to fulfill the requirement in future. It has been reported that an over 76 million people will die due to water borne diseases by the year 2020 (Pacific Institute of ⇑ Corresponding author.

E-mail address: [email protected] (D.B. Singh). http://dx.doi.org/10.1016/j.solener.2016.02.024 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.

Oakland, California), Dwivedi (2009). Solar still which is simple in technology, eco-friendly, more economical and easily maintainable can provide an attractive alternative solution for getting potable water in remote area for purifying underground brackish water (salinity 10,000 ppm). It needs only sunshine to operate and unskilled labor to maintain. So, it can be most suitable for under developed and developing countries. It can be broadly classified into passive solar still and active solar still. In the case of passive solar still, evaporation occurs in natural mode and no external energy in any form is fed to the basin. It operates at low temperature and the average annual daily yield which is a function of local climatic condition and salinity of water can be around 1.5 kg to 2 kg

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D.B. Singh et al. / Solar Energy 130 (2016) 207–223

Nomenclature Am Ac Ag Ab C F0 FR h PF1 PF2 I(t) Ic(t) Is(t) i K L Lg m_ f Q_ u T Utc,a Utc,p UL,m Va V oc VL I sc IL h1w h1g hbw hba hr hc hewg m_ ew R

area of module, m2 area of collector, m2 area of glass cover, m2 area of basin liner, m2 specific heat capacity, J/kg K flat plate collector efficiency factor, dimensionless heat removal factor, dimensionless heat transfer coefficient, W/m2 K penalty factor first, dimensionless penalty factor second, dimensionless incident solar intensity, W/m2 incident solar intensity on collector, W/m2 incident solar intensity on solar still, W/m2 rate of interest, % thermal conductivity, W/m K latent heat, J/kg thickness of glass, m rate of flow of water mass in collector, kg/s rate of useful energy transfer, kW temperature, °C total heat transfer coefficient from solar cell to ambient through glass cover, W/m2 K total heat transfer coefficient from solar cell to plate through glass cover, W/m2 K an overall heat transfer coefficient from blackened surface to ambient, W/m2 K velocity of air, m/s open circuit voltage, V load voltage, V short circuit current, A load current, A total heat transfer coefficient from water surface to inner glass cover, W/m2 K total hear transfer coefficient from glass cover to ambient, W/m2 K heat transfer coefficient from blackened surface to water mass, W/m2 K heat transfer coefficient from blackened surface to ambient, W/m2 K radiative heat transfer coefficient, W/m2 K convective heat transfer coefficient, W/m2 K evaporative heat transfer coefficient from water to glass, W/m2 K hourly distillate output, kg/h reflectivity, dimensionless

per day. A lot of design of passive solar still have been reported by different authors [Malik et al. (1982), G.N. Tiwari and A.K. Tiwari (2007), Cappelletti (2002), Hayek and Badran (2004), Al-Hinai et al. (2002), multi-basin, Tiwari et al. (1993), inverted trickle, Badran (2001),

r e UI FF Re Rw SP Mw E_ e Ee UAC FCR,i,n FSR,i,n Ps P Pp Cw Ce

correlation coefficient, dimensionless root mean square percent deviation, % standard uncertainty fill factor, dimensionless annual revenue earned from electricity gain, Rs. annual revenue earned from distillate, Rs. selling price, Rs. annual yield, kg hourly electricity gain, kW h annual electricity gain, kW h uniform end-of-year annual cost, Rs. capital recovery factor, dimensionless sinking fund factor, dimensionless net present cost of system, Rs. initial cost of system, Rs. cost of pump, Rs. production cost of water, Rs. cost of electricity gain, Rs.

Subscripts a ambient c solar cell eff effective w water in solar still f water in flat plate collector fi water at Inlet of FPC fo water at outlet of FPC g glass gi inner glass cover go outer glass cover m module N number of collectors p plate b blackened surface r radiative Greek letters a absorptivity ðasÞeff product of effective absorptivity and transmittivity b packing factor bc electrical efficiency g efficiency s transmittivity

multi-effect, Yuichi and Haruki (2003), regenerative, Abu-Arabi and Zurigat (2005), with reflectors, Tanaka and Nakatake (2006), spherical, Ismail (2009), triangular, Rubio-Cerda et al. (2002) and pyramid type solar still, Fath et al. (2003)]. Malik et al. (1982) reviewed the work


on passive solar distillation system till 1982. Further Tiwari and Lawrence (1992) updated, which also included active solar distillation. In the case of active solar still, thermal energy from outside is fed to the basin with an aim to increase the water temperature in basin so that rate of evaporation increases and ultimately higher yield is obtained. Energy can be fed to the basin in many ways. However, integration of flat plate collector to the basin is most recommended method. The integration can be done either directly (Yadav and Yadav, 2004; Badran and Al-Tahaineh, 2004; AbdelRehim and Lasheen, 2007; Tripathi and Tiwari, 2005; Badran et al., 2005; Tiwari et al., 2009; Dwivedi and Tiwari, 2008) or through heat exchanger (Arslan, 2012; Taghvaei et al., 2014; Eltawil and Omara, 2014). Further, Lilian et al. (2014) designed a solar still in which a partially submerged slowly rotating light weight hollow drum was introduced within the still cavity. They predicted an average enhancement in yield by 20–30% as compared to simple solar still. Rajaseenivasan et al. (2014) integrated flat plate collector with solar still and the basin of solar still was divided into smaller compartments. They have also reported an increase in yield by 60% in comparison to the conventional solar still. Hamadou and Abdellatif (2014) found an increase in productivity by heating the solar still fluid at its bottom by a circulating heat transfer fluid. They predicted that the relation between distilled water production and the heat transfer fluid rate is not linear as doubling this rate increases the yield by only 9%. Estahbanati et al. (2015) studied the effect of number of stages on productivity of multi-effect active solar still in continuous and non-continuous mode. They predicted the distillate production with a quadratic function with increased number of stages. They concluded that a 5stage system would produce 25% more freshwater in continuous mode compared to non-continuous mode. Calise et al. (2014) investigated a novel trigeneration system integrating photovoltaic/thermal collectors and seawater desalination. They designed it for small communities in European Mediterranean countries, rich in renewable sources and poor in fossil fuels and water resources. Kumar and Tiwari (2010) fabricated the hybrid (PVTFPC) single slope active solar still to operate in remote locations without grid connection based on work of Huang et al. (2001), Chow (2003), Tiwari and Sodha (2007) and Dubey et al. (2009) on photovoltaic thermal flat plat collector (PVT-FPC). Their experimental setup consists of the integration of two flat plate collectors in series and pump to the basin with insulated pipe. Out of the two flat plate collectors, only one is partially covered by photovoltaic module. Singh et al. (2011) has extended the same work for double slope active solar still. The work of Kumar and Tiwari (2010) has further been extended in this paper. Here, both flat plate collectors are partially covered by photovoltaic module. Tiwari et al. (2015) has reported the exergoeconomic and enviroeconomic analysis of partially covered photovoltaic flat plate collector active

209

solar distillation system. They have compared the efficiency of system under study with system proposed by Kumar and Tiwari (2010) and Singh et al. (2011). Further, they have concluded that the thermal efficiency of the system under study is lower. However, exergy efficiency and overall thermal efficiency are better. They did not discuss the modeling, validation and productivity analysis of partially covered photovoltaic flat plate collector active solar distillation system. Hence, to fulfill this knowledge gap, this paper mainly deals with the thermal modeling, validation and productivity analysis of active photovoltaic thermal flat plate collector solar distillation system. The proposed system is unique and feature of this system is that distillate produced is alone able to compensate the cost of water. Some portion of the electric power generated during the sunshine hour can be used for the society particularly in remote area where electricity is not available. Also, this system is self sustainable. 2. Experimental setup The schematic of partially covered photovoltaic flat plate collector active solar distillation system is shown in Fig. 1a. The top view and side view have been shown in Figs. 1b and 1c respectively. It consists of partially covered PVT flat plate collector, solar still and pump driven by DC motor. The detailed specification is presented in Table 1. The experimental set up has two flat plate collectors which are connected in series and it is further connected to the basin of solar still with the help of insulated pipes. Here, both flat plate collectors are partially covered by photovoltaic module. The effective area of each collector is

Fig. 1a. Schematic diagrams for partially covered photovoltaic flat plate collector active solar distillation system.

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D.B. Singh et al. / Solar Energy 130 (2016) 207–223 Solar radiation

Glazing surface

Saline water

Flat plate collector

Riser Tubes

Pump

PV Module

Distillate

Supporting Stand

Fig. 1b. Sectional top view of partially covered photovoltaic flat plate collector active solar distillation system.

Solar radiation Distilled water

Saline water Flat plate collector

Solar radiation

convected from back surface of PV module and solar radiation being transmitted through non packing area of module are utilized for heating water passing through the pipe in the collector. The collector is connected to the single slope solar still which consists of an effective basin area of 1 m 1 m. It was fabricated using fiber reinforced plastic. Coating of unsaturated polymer resin on 3-D mat of glass wool was used for fabricating body. The entrapped air between cavities after drying offers high degree of insulation to heat flow. The top of the solar still is covered by a transparent glass inclined at an angle of 30° with the horizontal with the help of iron clamps and rubber kept in between glass and iron frame and it is sealed with the help of windowputty for preventing vapor leakage to outside. The inside surface of side walls and bottom are blackened for maximum absorption of solar radiation. The solar radiation falling on the outer surface of transparent glass after reflection and absorption is transmitted to water and basin liner where it gets absorbed. The smaller height (front side) of the basin is taken 0.3 m and a trough is fixed to it for collecting distillate which is further allowed to flow down to the external jar with the help of plastic pipe. The rear wall of solar still is fitted with inlet pipe for allowing saline/brackish water into the still and the height of water in the still is measured with the help of scale fixed at the center of basin using adhesive known as m-seal. The opening provided at the bottom facilitates the flushing out the layer of sludge that may develop as the time passes. A thermocouple is also provided to measure the temperature. The entire unit is fixed on iron stand of size 1.5 m 1 m 1 m. The whole unit faces south to get the maximum radiation throughout the year. 3. Instrumentation

DC motor driven Pump PV module Supporting stand

Fig. 1c. Side view of partially covered photovoltaic flat plate collector active solar distillation system.

2 m2 and hence the total area of collector is 4 m2. The entire absorber is placed in an aluminum box having 0.1 m glass wool at base and side for reducing thermal losses. The top of the box is covered by 0.004 m thick glass which is kept there with the help of aluminum frame and screw. A photovoltaic thermal module having area 0.55 m 1.25 m is integrated with each of the collector at lower side (Chow et al., 2006) where low temperature water enters the collector. The electrical energy generated by PV module is supplied to DC motor water pump which compels the water to circulate under forced mode of operation and hence overcomes the pressure drop in collectors and piping arrangement. The radiation directly absorbed by the blackened surface of the collector, the thermal energy

The parameters were measured with the help of measuring instruments. Electronic digital anemometer model of LUTRON AM-4201 was used to measure air flow velocity. Hourly total solar radiation intensity was measured by Solarimeter. It has a least count of 20 W/m2. Temperatures of water and glass cover were measured by Copperconstantan thermocouples with a digital temperature indicator. It has a least count of 0.1 °C. The ambient temperature was measured with the help of a calibrated mercury thermometer. The distillate output was measured by measuring flask. Digital clamp meter was used to measure the short circuit current (ISC), load current (IL), open circuit voltage (VOC) and load voltage (VL). It has the least count of 0.1 A for measuring current and 0.01 V for measuring voltage. 4. Methodology The experiment was conducted for the water depth of 0.05 m in solar still for a year from August 2013 to July 2014 on the roof top of Block V, IIT Delhi. However, data for typical days (February 18, 2014 and November 22,


211

Table 1 Specifications of PVT-FPC active solar still. Components of PVT-FPC active solar still

Specifications

(i) Flat plate collectors Collector type No. of collectors Area of each collector Tube material Plate thickness Riser-outer diameter Riser thickness Spacing between two risers Thickness of insulation Weight of the collector Angle of collectors Thickness of top glass Motor used for water pump Effective area of collector under glass Effective area of collector under PV module

Tube in plate type 2 2.00 m2 Copper tubes 0.002 m 0.0127 m 0.56 x 103 0.112 m 0.1 m 48 kg 450 0.004 m (Toughened) Dc shunt motor (18 V, 40 W and 2800 rpm) 1.34 m2 0.66 m2

(ii) PV Module (under standard test conditions) Area of single solar cell Size of PV module No of solar cells Packing factor (bc ) Efficiency of module Max power rating Pmax

0.007 m2 1.25 m 0.55 m 36 0.44 12% 40 W

(iii) Fiber reinforced plastic (FRP) solar Still Length Width Thickness of glass cover Inclination of glass cover

1m 1m 0.004 m 300

2013) have been presented in Tables 2 and 3. The solar still was filled up by underground brackish water 24 h before the commencement of experiment so that water achieves the steady state condition before starting the experiment. The experiment was started at 8 am (local time) and it was continued until 7 am next day. The radiations were measured on the surface of solar still, collector and PV module with the help of Solarimeter. The following parameters were measured on hourly basis for a period of 24 h. (a) Water temperature (Tw). (b) Glass temperature (Tgi). (c) Total radiation on the solar still cover (Is(t)) and on the collector (Ic(t)). (d) Ambient temperature (Ta). (e) hourly distillate output (m_ ew ). (f) Air velocity (Va). (g) Open circuit voltage (VOC) and load voltage (VL). (h) Short circuit current (ISC) and load current (IL). Hourly observations were recorded and data have been reported in Tables 2 and 3. 5. Thermal modeling The following assumptions are made to write the energy balance equations of solar still (W/m2).

The solar still is vapor leakage proof. If the leakage occurs, the steady state condition no longer exits. It will become transient and energy equation will become too complex. So, the vapor leakage which is very small has been neglected because it will not affect the result much and steady state condition exists. The level of water in the basin is constant. The maximum daily yield is 7.74 kg. The corresponding change in level of water is less than 0.01 m during 24 h. The variation in daily yield is 2.85% corresponding to the change in level of water from 0.04 m to 0.05 m in the case of active solar still as reported by Tiwari et al. (2009). Hence, the small change in level of water in the basin can be neglected. No stratification (i.e. no layered configuration) of water occurs in the basin of the solar still. In active solar still, pump sucks water from basin and compels the hot water in flat plate collector to flow to the basin. In this way, the pump mainly circulates the water throughout the system. Due to the circulation of water, there is a stirring of water in the basin which results in continuous mixing. Hence, stratification of water can be neglected. The heat capacity of the glass cover, absorbing and insulation materials (bottom and sides) is negligible. The heat contained by glass can be calculated using the relation ðm C DT Þ, where m, C and DT are the mass, specific heat capacity and temperature difference between inner and outer surfaces of glass cover respectively. The average temperature difference between inner and outer surfaces of

212


Table 2 Hourly variation of various parameters of hybrid (PV/T) active solar still for 0.05 m water depth on 18th February, 2014. Time

Tgi (°C)

Tw (°C)

Ta (°C)

Is(t) (W/ m2)

Ic(t) (W/ m2)

VL (V)

IL (A)

VOC (V)

ISC (A)

Va (m/s)

Yield (kg/h)

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00

10.5 10.4 11.1 13.0 24.1 34.2 41.0 46.0 44.2 53.7 47.0 45.5 37.8 27.3 21.8 18.6 15.6 13.8 12.2 10.7 9.3 8.1 7.1 6.2

10.0 10.2 14.2 18.8 29.6 39.0 49.3 54.9 62.4 61.1 57.3 51.2 42.5 36.2 32.6 29.1 27.3 25.1 23.1 21.2 19.5 17.7 15.9 14.2

9.5 9.5 14.0 17.5 21.0 22.0 23.0 24.5 23.5 23.0 21.0 18.0 16.0 14.6 13.4 11.8 11.8 11.0 10.5 10.5 10.0 10.0 9.0 9.0

60 120 320 420 460 500 480 440 340 200 60 40 0 0 0 0 0 0 0 0 0 0 0 0

80 140 280 440 480 520 500 440 340 200 80 40 0 0 0 0 0 0 0 0 0 0 0 0

0 13.3 14.2 14.6 16.8 16.4 16.5 15.5 13.4 13.2 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 2.1 2.6 2.4 3.2 2.7 2.6 2.5 2.4 2.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 20.8 20.2 19.4 18.9 19.1 18.4 18.2 17.5 16.1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 2.3 4.1 5.8 7.5 7.4 7.1 5.1 4.2 3.4 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1.0 1.0 0.0 0.0 0.0 0.0 0.0 0.2 0.1 0.3 0.4 0.5 0.4 0.5 0.0 0.4 0.5 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.000 0.010 0.010 0.009 0.079 0.195 0.342 0.333 0.327 0.260 0.155 0.138 0.121 0.115 0.085 0.083 0.080 0.078 0.050 0.045 0.038 0.030 0.021 0.008

Table 3 Hourly variation of various parameters of hybrid (PV/T) active solar still for 0.05 m water depth on 22nd November, 2013. Time

Tgi (°C)

Tw (°C)

Ta (°C)

Is(t) (W/ m2)

Ic(t) (W/ m2)

VL (V)

IL (A)

VOC (V)

ISC (A)

Va (m/s)

Yield (kg/h)

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00

21.0 23.1 31.4 35.4 43.3 48.3 50.6 41.9 38.7 34.2 31.9 30.5 27.3 24.9 21.3 19.1 17.5 15.9 14.7 13.8 13.1 11.8 11.6 11.5

24.0 27.5 35.1 54.2 60.1 62.8 60.5 53.0 47.1 40.7 36.4 33.9 31.4 29.6 28.5 26.5 25.1 23.9 22.5 20.6 21.0 19.4 19.0 18.5

19.0 22.0 23.0 24.0 25.0 22.6 23.8 25.5 25.8 26.0 25.0 24.0 21.5 19.5 18.0 17.0 16.0 15.8 14.8 14.8 14.5 14.0 13.6 13.5

220 520 580 600 480 400 260 120 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

240 540 600 620 480 420 260 120 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 13.2 16.9 17.3 16.9 16.8 16.6 16.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 2.2 2.9 3.1 3.1 3.1 2.9 2.8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 20.7 20.4 19.9 19.3 19.0 18.6 18.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 2.4 3.8 5.9 7.0 6.7 4.3 4.2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0.8 0.8 0.5 0.3 1.1 0.0 1.1 1.2 1.3 1.2 0.5 0.3 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 1.5 1.6 1.8

0.008 0.100 0.150 0.250 0.340 0.375 0.250 0.210 0.155 0.130 0.120 0.100 0.092 0.063 0.063 0.052 0.045 0.040 0.038 0.030 0.028 0.021 0.017 0.027

glass cover is 2.84 K in the case of active PVT solar still as reported by Kumar et al. (2010a). Mass of glass in the proposed model is 17.32 kg considering the density of glass as 2500 kg/m3. The heat consumed by glass comes out to be 41.32 kJ taking specific heat capacity of glass as 0.84 kJ/ kg K whereas daily heat output of the system is of the order

of 104 kJ. So, the heat consumed by glass is 0.41% in comparison to the amount of heat output from the solar still. Similarly, heat consumed by absorbing and insulating material can also be calculated which will come out to be very small. Hence, amount of heat consumed by glass, absorbing and insulating material can be neglected as it is


not going to affect the result much. If the same is not neglected, energy balance equation will become complex. The film type condensation occurs through the glass. A small inclination to the glass cover is provided to facilitate the same as the component of gravitational force will act downward along the surface which will help in trickling down the condensate. Also, the inner surface of glass cover is cleaned to prevent the formation of water droplets. Further, if the condensation occurring at the glass surface is not film type then there will be formation of water droplets on the inner surface of glass cover. These water droplets will act as mirror and may reflect solar radiation. Lesser amount of solar intensity will be available to the water surface that will result in lesser increase in temperature. The temperature difference between water temperature and glass temperature will be lower which will further adversely affect the yield. Hence, film type condensation has been ensured during the design phase of experimental setup. However, some droplets may still form which has not been taken into account as it is negligibly small. 5.1. Useful energy gain for N identical PVT hybrid flat plate collector (FPC) Following Dubey and Tiwari (2009), the rate of useful thermal output from N identical PVT hybrid flat plate collectors connected in series can be expressed as, h i Q_ u;N ¼ N ðAm þ Ac Þ ðasÞeff ;N I ðtÞ U L;N T fi T a R ðasÞÞ1 Here, ðasÞeff ;N ¼ ðAF ðAm þAc Þ N 1ð1K K;A Þ ; K K;A ¼ ðAFm_ Rf CUfL Þ1 NK K;A

1ð1K K;A Þ NK K;A

N

; U L;N ¼

ð1Þ ðAF R U L Þ1 ðAm þAc Þ

5.2. Energy balance equations for solar still

where h1g ¼ hr;g þ hc;g or h1g ¼ 5:7 þ 3:8 V a 5.2.3. Water mass in basin Q_ uN þ aw I s ðtÞAb þ hbw ðT b T w ÞAb dT w ð4Þ ¼ h1w T w T gi Ab þ M w C w dt where aw ¼ 1 Rg 1 ag ð1 Rw Þaw which is solar flux absorbed by water mass and Q_ uN is the rate of useful thermal output from N identical hybrid PVT collectors connected in series. 5.2.4. Basin liner ab I s ðtÞAb ¼ hbw ðT b T w ÞAb þ hba ðT b T a ÞAb ð5Þ where ab ¼ 1 Rg 1 ag ð1 Rw Þð1 aw Þab which is solar flux absorbed by basin liner. Using Eqs. (1)–(5), one can get the following differential equation for water temperature in basin. dT w þ aT w ¼ f ðtÞ dt where

f ðtÞ ¼

ðNAc U LN þU s Ab Þ M w Cw

(i) The time interval (Dt ) is small i.e. (0 < t < Dt). (ii) The values of Ta and I(t) can be considered as average value between ‘0’ and ‘t’ i.e. T a and IðtÞ, hence f ðtÞ is constant and its average value will be f ðtÞ. (iii) a is constant for interval Dt. One can get the solution of Eq. (6) with initial condition, T w ¼ T w0 at t = 0, as follows.

5.2.1. Inner surface of glass cover

After evaluating Tw from Eq. (7), one can obtain Eqs. (2) and (3) as follows,

ag I s ðtÞAg þ h1w T w T gi

Kg Ab ¼ T gi T go Ag Lg

ð2Þ

and

a¼ The following assumptions have been made to get an approximate solution of differential Eq. (6).

Tw ¼

ð6Þ

I s ðtÞAb þðNAc U LN þU s Ab ÞT a NAc ðasÞeff ;N I c ðtÞþaeff M w Cw

Following Dwivedi and Tiwari (2009) and Kumar et al. (2014), energy balance equation for solar still can be written as follows.

213

f ðtÞ ð1 eat Þ þ T w0 eat a

T gi ¼

a_ g I s ðtÞAg þ h1w T w Ab þ U c;ga T a Ag U c;ga Ag þ h1w Ab

ð7Þ i

from ð8Þ

where ag ¼ 1 Rg ag represents the solar flux absorbed by the glass cover and h1w ¼ hr;wg þ hc;wg þ he;wg represents rate of total heat transfer coefficient from water surface to inner surface of glass cover.

The rate of evaporative heat transfer from water surface is given by ð9Þ q_ ew ¼ hew T w T gi

5.2.2. Outer surface of glass cover

m_ ew ¼

Kg T gi T go Ag ¼ h1g T go T a Ag Lg

where L is the latent heat of evaporation and can be expressed as (Fernandez and Chargoy, 1990; Toyama and Kagakuv, 1972)

ð3Þ

The hourly yield ðm_ ew Þ can be expressed as q_ ew Ab 3600 L

ð10Þ

214


L ¼ 3:1615 106 1 7:6160 104 T w if T w > 70 C ð10aÞ

% Uncertainty ¼

100

and

L ¼ 2:4935 106 1 9:4779 104 T w þ 1:3132 107 T 2w 4:7974 109 T 3w if T w < 70 C

UI Average of total number of observations ð14Þ

ð10bÞ

7. Calculation of efficiency

The daily yield can be calculated by adding the hourly yield for 24 h.

7.1. Thermal efficiency

6. Statistical analysis

Following Tiwari (2002), the hourly thermal efficiency of hybrid (PVT) active solar still can be expressed as

6.1. Correlation coefficient (r) and root mean square percent deviation (e):

ghourly;thermal ¼

The correlation coefficient (r) and root mean square percent deviation (e) have been calculated with an aim to find the relationship between theoretical and experimental values of Tw, Tg and yield. KARL PEARSON’S Coefficient of correlation (r) and mean square percent deviation (e) are respectively given by (Chapra and Canale, 1989) P ðxi xÞðy i y Þ ffi r ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð11Þ P 2P 2 ðy i y Þ ðxi xÞ vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi uP 2 xi y i u t xi 100 ð12Þ and e ¼ No where N o ¼ no: of observations. Coefficient of determination (Cooper and Schindler, 2003; Chapra and Canale, 1989) has also been calculated which gives a measure of how well observed values are replicated by the model. It is given as square of coefficient of correlation (r2).

m_ ew L 100 ½Ac I c ðtÞ þ As I s ðtÞ 3600

and daily thermal efficiency of hybrid (PVT) active solar still can be expressed as P24 _ ew L t¼1 m gdaily;thermal ¼ P24 t¼1 ½AC I C ðtÞ þ AS I sðtÞ 3600 100

7.2. Thermal exergy efficiency Following Jafarkazemi and Ahmadifard (2012), the hourly thermal exergy efficiency of hybrid (PVT) active solar still can be expressed as ðT w þ273Þ hewg T w T gi ðT a þ 273Þ ln T þ273 ð gi Þ ghourly;exergy ¼ 0:933 ðAS I S ðtÞÞ 100 ð17Þ

Fixed and random error contributes to uncertainty in measurement. The internal uncertainty has been estimated for the experiment. The standard uncertainty is given by the following expression (Bell, 1999; Nakara and Chaudhary, 2004).

ð13Þ

where r is the standard deviation and it is given by rP ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi No

r¼

t¼1

ð16Þ

where t represents time. L is the latent heat of vaporization (in J/kg) and it can be calculated using Eqs. (10a) and (10b). Thermal efficiency is based on first law of thermodynamics.

6.2. Uncertainty analysis

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffi u uX N r2t s t UI ¼ t¼1 No 1

ð15Þ

Here, exergy has been calculated on the basis of second law of thermodynamics. The factor 0.933 can be calculated using the expression given by Petela (2003). It has been used to convert sun radiation to exergy. The daily thermal exergy can be calculated by adding hourly thermal exergy for 24 h. Hence, the daily thermal exergy efficiency of hybrid (PVT) active solar still can be expressed as hewg gdaily;exergy ¼

P24 ðT w þ273Þ T T þ 273Þ ln ðT w gi a t¼1 ðT gi þ273Þ 100 P ðA I ðtÞÞ 0:933 24 S S t¼1

ð18Þ

2 ðX t X Þ

No

Here, ðX X Þ is the deviation of observation from the mean. Ns and No represent the number of sets and the total number of observations in the set respectively. The percentage uncertainty can be expressed as

7.3. Electrical exergy efficiency The hourly electrical exergy efficiency (Dubey and Tiwari, 2009) of hybrid (PVT) active solar still can be expressed as


ghourly;electrical

exergy

¼

FF V o I SC V L I L 100 0:933 Am I C ðtÞ

ð19Þ

where FF is fill factor. Its value is 0.88 in the present case. The daily electrical exergy from solar still can be calculated by adding hourly electrical exergy for 24 h. The hourly input exergy can also be calculated by adding hourly input exergy for 24 h. Hence, the daily electrical exergy efficiency of hybrid (PVT) active solar still can be expressed as P24 ½FF V o I SC V L I L gdaily;electrical exergy ¼ t¼1 100 P24 0:933 t¼1 Am I C ðtÞ ð20Þ

215

used to convert electrical energy to equivalent thermal energy as electrical energy is high grade energy. The daily overall thermal energy can be calculated by adding hourly overall thermal energy for 24 h. Hence, the daily overall thermal efficiency of hybrid (PVT) active solar still using Eq. (23) can be expressed as P24 _ t¼1 ðmew LÞ 100 gdaily;overall thermal ¼ P24 ½A I ðtÞ þ As I s ðtÞ 3600 c t¼1 c P FF 24 ðV OC I SC Þ þ 100 ð24Þ Pt¼1 24 0:38 t¼1 ðAm I C ðtÞÞ

Here, summation has been done for 24 h. However, voltage, current and intensity will appear there for sunshine hour only. Their values will be zero for off sunshine hour.

8. Productivity analysis and production cost of water

7.4. Overall exergy efficiency

Productivity refers to the relationship between product and the factors used in obtaining it. It means getting more and more with less and less input of resources. It can also be defined by ILO (1979) as the ratio of effectiveness and efficiency. Following Ashcroft (1950), Cox (1951), Benson (1952) and ILO (1979), productivity can be expressed as

The overall exergy can be evaluated by adding thermal exergy and electrical exergy. Hence, the hourly overall exergy efficiency of hybrid (PVT) active solar still using Eqs. (17) and (19) can be expressed as

hewg

T w T gi ðT a þ 273Þ ln

ghourly;exergy ¼

ðT w þ273Þ ðT gi þ273Þ

hewg gdaily;exergy ¼

100

Following Tiwari and Sodha (2007), the hourly overall thermal efficiency of hybrid (PVT) active solar still can be expressed as ¼

Productivity ¼

Output of the system 100 Input to the system

Pt¼24 P24 ðT w þ273Þ ðT T T þ 273Þ ln þ t¼1 ðFF V O I SC V L I L Þ w gi a t¼1 ðT gi þ273Þ 100 P 0:933 24 t¼1 ½ðAC þ Am Þ I C ðtÞ þ AS I S ðtÞ

7.5. Overall thermal efficiency

thermal

þ ðFF V OC I SC V L I L Þ

0:933 ½ðAC þ Am Þ I C ðtÞ þ AS I S ðtÞ

Using Eqs. (18) and (20), the daily overall exergy efficiency of hybrid (PVT) active solar still can be expressed as

ghourly;overall

8.1. Productivity analysis

m_ ew L ½Ac I c ðtÞ þ As I s ðtÞ 3600 FF V OC I SC 100 þ 100 0:38 Am I C ðtÞ

The factor 0.38 is electric power generation efficiency for a conventional power plant (Huang et al., 2001). It has been

ð25Þ

ð22Þ

Here, ouput includes annual yield of water (kg) and annual electricity gained (kW h). Annual yield of water (kg) can be expressed in terms of rupees by multiplying with selling price of water (Rs./kg). Electricity gain (kW h) can also be expressed in terms of rupees by multiplying with selling price (Rs./kW h). Hence output can be expressed as Output ¼ Rw þ Re

ð23Þ

ð21Þ

ð26Þ

where, Rw ¼ ðM w ðSP Þw Þ and Re ¼ Ee ðSP Þe Here, Rw, Re, Mw, Ee, (SP)w and (SP)e are annual revenue (Rs.) earned from water, annual revenue (Rs.) earned from electricity gain, annual yield of water (kg), annual electricity gain (kW h), selling price of water (Rs./kg) and selling price of electricity (Rs./kW h) respectively. The

216


daily yield can be obtained by adding hourly yield from Eq. (10) for the period of 24 h. The monthly yield can be calculated by multiplying daily yield with number of clear days observed in a month. Hence, daily yield and monthly yield can be expressed as Daily yield ¼

24 X

m_ ew

ð27Þ

t¼1

and Monthly yield ¼

12 X

ðDaily yieldÞi

ð27aÞ

UAC ¼ P s F CR;i;n þ P s F CR;i;n M s S s F SR;i;n

ð29Þ

where, Ps, Ss and Ms are the net present cost, salvage value and maintenance cost (fraction) of the solar still respectively. The value of Ms has been taken as 0.1. The first term in Eq. (29) represents the part of UAC that comes from net present cost, the second term represents the part of UAC that comes from present value of maintenance cost and third term is the part UAC corresponding to salvage value. FCR,i,n and FSR,i,n are capital recovery factor and sinking fund factor respectively. They are given by

i¼1

Further, Annual yield can be obtained by adding monthly yield for 12 month. Hence, annual yield (Mw) can be expressed as Mw ¼

12 X

ðMonthly yieldÞi

ð27bÞ

i¼1

The hourly electricity gain (E_ e ) can be expressed as E_ e ¼ FF V oc I sc V L I L

ð28Þ

The daily electricity gain can be calculated by adding hourly electricity gain for 24 h. The monthly electricity gain can be obtained by multiplying daily electricity gain with number of clear days obtained in a particular month. Hence, daily and monthly electricity gain can be expressed as 24 X Daily electricity gain ¼ E_ e ð28aÞ t¼1

F CR;i;n ¼

i ð1 þ iÞn ð1 þ iÞn 1

and

F SR;i;n ¼

i : ð1 þ iÞn 1

where i and n are the rate of interest and life of the system. The capital recovery factor (Tiwari, 2013) converts the present cost into uniform end-of-year annual cost. The sinking fund factor (Tiwari, 2013) converts the future cost into uniform end-of-year annual cost. The replacement time of DC water pump is estimated as 10 years. It has been used during the analysis to calculate the net present cost of the system. It has been assumed that the cost of the pump remains same on each purchase after adjusting its salvage value. Therefore, net present cost (Ps) for the 30 years life span of active solar still can be expressed as (Tiwari, 2002) Ps ¼ P þ Pp þ

Pp ð1 þ iÞ

10

þ

Pp

ð30Þ

ð1 þ iÞ20

and Monthly electricity gain ¼

24 X

Net present cost (Ps) for the 50 years life span of active solar still can be expressed as (Tiwari, 2002)

ðdaily electrcity gainÞi

i¼1

ð28bÞ

Ps ¼ P þ Pp þ

Pp ð1 þ iÞ

10

þ

Further, the annual electricity gain (Ee) can be evaluated as Ee ¼

12 X ðmonthly electricity gainÞi

ð28cÞ

i¼1

Input means uniform end-of-year annual cost (UAC) of the system. The calculation of UAC is based on the present value method. Table 5 represents the cost of different components. The salvage value is based on the current price of different materials in Indian local market. The uniform end-of-year annual cost (Kumar and Tiwari, 2009; Tiwari and Ghosal, 2005) for a given initial investment of solar distillation systems can be expressed as

Hourly productivity ¼ Daily productivity ¼

Pp ð1 þ iÞ

20

þ

Pp ð1 þ iÞ

30

þ

Pp 40

ð1 þ iÞ ð30aÞ

where, P and Pp are the initial cost of the system and the cost of the pump respectively. The initial cost of the system can be calculated as P = Cost of solar still + Cost of PV + Cost of FPC + Fabrication cost (Cost of piping and labor). The uniform end-of-year annual cost (UAC) has been calculated using Eq. (29) and presented in Table 7. Life span of system has been taken as 30 years and 50 years. Following Ashcroft (1950), Cox (1951), Benson (1952) and ILO (1979), the hourly productivity and daily productivity respectively can be expressed as

ðHourly yield ðSP ÞwÞ þ ðHourly electricity gain ðSP Þe Þ 100 Hourly cost

ðDaily yield ðSP ÞwÞ þ ðDaily electricity gain ðSP Þe Þ 100 Daily cost

ð31Þ ð32Þ

8.2. Production cost of water and cost of electricity gain Following Kumar and Tiwari (2009), the cost (Cw) of water production in Rs./kg and cost (Ce) of electricity gain in Rs./kW h respectively can be expressed as Cw ¼

UAC Re Mw

ð33Þ

Ce ¼

UAC Rw Ee

ð34Þ

217

1.5

105

85

Tw-th. Tw-ex. Tg-th Tg-ex hourly yield-th. hourly yield-ex.

r for Tw = 0.964 r for Tg = 0.98 r for hourly yield = 0.988 e for Tw = 16.59 % e for Tg = 26.52 % e for hourly yield = 29 %

0.75 45 0.5 25

0.25

0

5 8 am 10:00 12:00 14:00 16:00 18:00 20:00 22:00 24:00 2:00

4:00

6:00

40 th ex

65

r for Tw = 0.979 r for Tg = 0.977 r for hourly yield = 0.984 e for Tw = 13.75 % e for Tg = 21.26 % e for hourly yield = 21.97

1.25

45

Hourly Thermal Eff. in %

r = 0.985 e = 16.53 %

25 20 15 10

0 8 am

10:00

12:00

14:00

16:00

Time

Fig. 4. Variation of hourly thermal efficiency for 22–23 November, 2013. 100 th ex

r = 0.99

90

e=33.37

80 70 60 50 40 30 20

1

0.75

30

5

Hourly Thermal Eff. in %

85

1.5 Tw-th. Tw-ex. Tg-th Tg-ex yield-th. yield-ex.

Yield in kg

Water temperature (Tw) and glass temperature (Tg) in degree C

105

8:00

Fig. 3. Variation of Tw, Tg and yield for 18–19 February, 2014.

35

The data presented in Tables 2 and 3 were fed to MATLAB program and the output obtained is presented in Figs. 2–13. MATLAB program incorporates all relevant equations. Figs. 2 and 3 present the theoretical and experimental variation of water temperature (Tw), glass cover temperature (Tg) and hourly yield (m_ ew ). Statistical parameters r and e were also calculated and fair agreements were found between theoretical and experimental values. Here, it should be noted that the yield depends on temperature difference between water temperature and glass cover temperature. It also depends on evaporative heat transfer coefficient. A lower value of yield may be obtained even at higher value of Tw if ambient temperature is higher because Tg will also be higher resulting in lower temperature difference. Also, plots for temperature and yield are on different scale. The difference in temperature is in whole number whereas difference in yield is in fraction. So,

1

65

Time

If (UAC-Rw) comes out to be negative, then it can be taken as zero because negative cost does not make any sense. It will make value of Ce zero which will mean that we are getting electricity at no cost. 9. Result and discussion

1.25

Yield in kg

The daily cost can be calculated by dividing uniform endof-year annual cost (UAC) with the number of clear days obtained in a year. The hourly cost can be calculated by dividing daily cost with 24 h.

Water temperature (Tw) & glass temperature (Tg) in degree C


0.5

10 0

8:00 am

10:00

12:00

14:00

16:00

18:00

20:00

Time

Fig. 5. Variation of hourly thermal efficiency for 18–19 February, 2014. 25

0.25

0

5 8 am 10:00 12:00 14:00 16:00 18:00 20:00 22:00 24:00 2:00

4:00

6:00

8:00

Fig. 2. Variation of Tw, Tg and yield for 22–23 November, 2013.

difference in yield is not visible particulary after 14:00 h. However, it exists there. Moreover, yield depends on difference between water temperature and condensing cover temperature. The gap of difference between theoretical

218

D.B. Singh et al. / Solar Energy 130 (2016) 207–223 80

Variation of hourly electrical exergy efficiency

16 th ex

Nov. Feb.

14

Hourly electrical exergy eff. (%)

Instantaneous efficiency

70 60 50 40 30 20

12 10 8 6 4

10 2 0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45 0

(Tw-Ta)/I

8:00 am

10:00

12:00

14:00

16:00

10

Time

Fig. 6. Characteristic curve for 18–19 February, 2014.

Fig. 9. Variation of hourly electrical exergy efficiency. 50 45

Variation of hourly overall exergy efficiency

th ex

25 Nov. Feb.

35

Hourly overall exergy eff. (%)

Instantaneous efficiency

40

30 25 20 15 10 5

20

15

10

5

0 0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

(Tw-Ta)/I

0 8:00 am

10:00

12:00

14:00

16:00

18:00

20:00

Time

Fig. 7. Characteristic curve for 22–23 November, 2013.

Fig. 10. Variation of hourly overall exergy efficiency. Variation of hourly thermal exergy efficiency

25

Nov. Feb.

90

20

15

10

5

0

Variation of hourly overall thermal efficiency

100

Hourly overall thermal eff. (%)

Hourly thermal exergy eff. (%)

Nov. Feb.

80 70 60 50 40 30 20 10

8:00 am

10:00

12:00

14:00

16:00

18:00

Time

Fig. 8. Variation of hourly thermal exergy efficiency.

0 8:00 am

10:00

12:00

14:00

16:00

18:00

Time

Fig. 11. Variation of hourly overall thermal efficiency.

values of water temperature & condensing cover temperature and difference between experimental values of water temperature & condensing cover temperature is not big.

Also, the gap between theoretical and experimental values of temperatures may be due to time gap in taking the read-

D.B. Singh et al. / Solar Energy 130 (2016) 207–223 2500 n=30, I=2 n=50, I=2 n=30, I=5

Hourly productivity (%)

2000

n=50, I=5 n=30, I=10 n=50, I=10

1500

1000

500

0

8:00 am 10:00 12:00 14:00 16:00 18:00 20:00 22:00 24:00 2:00

4:00

6:00

8:00

Time

Fig. 12. Variation of hourly productivity for 22–23 November, 2013.

2000

n=30, I=2 n=50, I=2 n=30, I=5 n=50, I=5 n=30, I=10 n=50, I=10

1800

Hourly productivity (%)

1600 1400 1200 1000 800 600 400 200 0

8:00 am10:00 12:00 14:00 16:00 18:00 20:00 22:00 24:00 2:00

4:00

6:00

8:00

Time

Fig. 13. Variation of hourly productivity for 18–19 February, 2014.

ing and actual occurrence as readings were noted down manually. Figs. 4 and 5 represent the variation of hourly thermal efficiency for the month of November and February respectively. The thermal efficiency has been found to vary between 2% to 75%. The thermal efficiency shoots up at 17:00 h because the value of solar intensity is very low. The daily thermal efficiency has been estimated as 11.6% and 11.35% for the month of November and February respectively. The daily thermal efficiency is low because some portion of the flat plate collectors are covered by photovoltaic module which results in lower heat gain by flat plate collector and hence lower heat is supplied to water mass in basin. Due to lower heat, rise in temperature of water mass is lesser and hence lesser evaporation occurs. Figs. 6 and 7 represent the characteristic curve of solar still which also gives a fair agreement between theoretical and experimental values. The percentage uncertainty has been calculated using Eq. (14) and it has been found to be 8.8%, 9.8% and 0.73% for water temperature in basin, glass cover temperature and yield respectively. It suggest that the errors are within limits.

219

Fig. 8 represents the variation of hourly thermal exergy efficiency for the month of November and February. They have been found to vary between 0.06% to 20.74%. The daily thermal exergy efficiency has been evaluated as 3.92% and 3.93% for the month of November and February respectively. Fig. 9 represents the variation of hourly electrical exergy efficiency for the month of November and February. They have been found to vary between 2.21% to 28.53%. The daily electrical exergy efficiency has been evaluated as 7.24% and 15.36% for the month of November and February respectively. Fig. 10 represents the variation of hourly overall exergy efficiency. It has been found to vary between 0.03% to 25%. The daily overall exergy efficiency has been evaluated as 5.71% and 10.09% for the month of November and February respectively. Fig. 11 represents the variation of hourly overall thermal efficiency. It has been found to vary between 1.02% to 69.06%. The daily overall thermal efficiency has been evaluated as 51.28% and 68.13% for the month of November and February respectively. A higher value of overall thermal efficiency is obtained because electrical energy has been converted to equivalent thermal energy by dividing with factor 0.38 which is the efficiency of conventional thermal power plant. Also, photovoltaic module provides electrical as well as thermal energy. The variation trend in Figs. 8–11 is irregular. The reason being that the experimental observation has been taken manually. So, there is a time gap in noting down the different observations such as solar intensity, short circuit current, open circuit voltage, load current and load voltage which affects the electrical energy from the system. This time gap may be one of the reason for irregular trend in electrical parameter. In Fig. 8, hourly thermal exergy efficiency of November is higher in comparison to their corresponding values in February. This happens because solar intensity in November is higher in comparison to the solar intensity in February. Further, photovoltaic module performs better at lower temperature. If solar intensity increases, temperature of module also increases. So, electrical exergy obtained from photovoltaic module is lower at higher solar intensity. Figs. 9–11 shows the effect of electrical exergy. Some variation in weather condition and hence variation in solar intensity might have been there during one hour. The measured value might not reflect the actual value. This may be the another reason. Also, Figs. 8–11 represents the experimental values only. The daily, monthly and annual yield have been calculated using Eqs. (27), (27a) and (27b) respectively. They have been presented in Table 4. The maximum daily yield is 7.74 kg which occurs in the month of May. The minimum daily yield is 1.67 kg which occurs in month of December. They occur due to variation in solar intensity. The maximum value of monthly yield is 160.44 kg which occurs in the month of June. The minimum value of monthly yield is 48.43 kg which occurs in the month of December. The variation in monthly yield may be due to variation in intensity as well as number of clear days

220


Table 4 Daily, monthly and annual yield. Month

No. of clear days

January, 14 February, 14 March, 14 April, 14 May, 14 June, 14 July, 14 August, 13 September, 13 October, 13 November, 13 December, 13

Daily yield (kg)

29 24 22 24 20 21 22 14 25 28 29 29

No. of clear days in a year

Monthly yield (kg)

1.87 2.61 4.37 3.83 7.74 7.64 4.06 5.42 4.24 4.94 2.71 1.67

287

54.23 62.69 96.14 91.92 154.80 160.44 96.80 75.88 106.00 138.32 78.42 48.43

Annual yield

1164.07

Table 5 Daily, monthly and annual electricity gain. Month

No. of clear days

January, 14 February, 14 March, 14 April, 14 May, 14 June, 14 July, 14 August, 13 September, 13 October, 13 November, 13 December, 13

29 24 22 24 20 21 22 14 25 28 29 29

No. of clear days in a year

287

Daily electricity gain (kW h) 0.2981 0.6341 0.6836 0.5508 0.5508 0.5381 0.5050 0.2943 0.2936 0.3010 0.2924 0.2981 Ee

Monthly electricity gain (kW h) 8.65 15.22 15.04 13.22 11.02 11.30 11.11 4.12 7.34 8.43 8.48 8.64 122.56

Table 6 Capital investment. S.N.

Parameter

Cost (Rs.)

1 2 3 4 5 6

Cost of Solar Still Cost of PV module @ 8000 each Cost of Flat Plate collector Cost of Motor and pump Fabrication Cost Salvage value of the system after 30 years, if inflation remains @ 4% in India, [using present value of scrap material sold in Indian market] Salvage value of the system after 50 years, if inflation remains @ 4% in India, [using present value of scrap material sold in Indian market]

5000 16000 25000 1000 4000

7

obtained in a particular month. The daily, monthly and annual electricity gain have been evaluated using Eqs. (28a), (28b) and (28c) respectively. They have been presented in Table 5. Table 6 presents the cost of different components of the system. The uniform end-of-year annual cost has been calculated using Eq. (29) which is based on present value method and it is presented in Table 7. The annual productivity has been calculated using Eq. (27) and it is presented in Table 8. It has been found to vary between 120.29% to 883.55%. It means that the system is feasible and it will sur-

36767 80561

vive as the value is more than 100% which is expected while performing productivity analysis. The hourly productivity has been calculated using Eq. (31) for the month of November and February. Figs. 12 and 13 represent the variation of hourly productivity for the month of November and February respectively. Its maximum value has been found to be 2100% for the month of November which occurs at 13 h. However, maximum intensity occurs at 12 noon. It is so because increase in temperature of water kept in basin takes some time. Similarly, maximum value of hourly productivity for the month of February is 1900%


221

Table 7 Uniform end-of-year annual cost (UAC) of capital investment. n

i (%)

PS (Rs.)

M @10%

SS (Rs.)

FCR,i,n

FSR,i,n

UAC (Rs.)

30 50 30 50 30 50

2 2 5 5 10 10

52493.30 52364.23 51990.80 52364.23 51534.00 51613.59

5249.33 5236.42 5199.08 5236.42 5153.40 5161.36

36767.00 80561.00 36767.00 80561.00 36767.00 80561.00

0.045 0.032 0.065 0.055 0.106 0.101

0.0250 0.0118 0.0150 0.0048 0.0060 0.0009

1424.65 714.23 3168.78 2483.81 5246.09 5136.42

Table 8 Productivity of active PVT flat plate collector solar distillation system. S.N.

Input (Rs.)

Mw (kg)

(SP)w (Rs./kg)

Rw (Rs.)

Ee (kW h)

(SP)e (Rs./kW h)

Re (Rs.)

Output (Rs.)

Productivity (%)

1 2 3 4 5 6

1424.65 714.23 3168.78 2483.81 5246.09 5136.42

1164.07 1164.07 1164.07 1164.07 1164.07 1164.07

5.0 5.0 5.0 5.0 5.0 5.0

5820.35 5820.35 5820.35 5820.35 5820.35 5820.35

122.56 122.56 122.56 122.56 122.56 122.56

4.0 4.0 4.0 4.0 4.0 4.0

490.24 490.24 490.24 490.24 490.24 490.24

6310.59 6310.59 6310.59 6310.59 6310.59 6310.59

442.95 883.55 199.14 254.06 120.29 122.85

Table 9 Production cost (Cw) of water and cost (Ce) of electricity gain. S.N.

UAC (Rs.)

Mw (kg)

Rw (Rs.)

Ee (kW h)

Re (Rs.)

(UAC-Re) (Rs.)

(UAC-Rw) (Rs.)

Cw (Rs./kg)

Ce (Rs./kW h)

1 2 3 4 5 6

1424.65 714.23 3168.78 2483.81 5246.09 5136.42

1164.07 1164.07 1164.07 1164.07 1164.07 1164.07

5820.35 5820.35 5820.35 5820.35 5820.35 5820.35

122.56 122.56 122.56 122.56 122.56 122.56

490.24 490.24 490.24 490.24 490.24 490.24

934.45 224.03 2678.58 1993.61 4755.89 4646.22

0.00 0.00 0.00 0.00 0.00 0.00

0.80 0.19 2.30 1.71 4.08 3.99

0.00 0.00 0.00 0.00 0.00 0.00

which occurs at 14 h. But, the maximum intensity occurs at 13:00 h. Production cost of water and cost of electricity gain have been calculated using Eqs. (33) and (34) and they have been presented in Table 9. The production cost of water has been found to vary between Rs. 0.19 per kg to Rs. 4.08 per kg. The minimum value occurs for life span of 50 yr. and 2% rate of interest. The maximum value occurs for life span of 30 yr. and 10% rate of interest. The cost of electricity gain has been found to be Rs. 0 per kW h. It means that water produced is sufficient to overcome the cost of system and we do not have to pay for electricity. 10. Conclusions The partially covered photovoltaic flat plate collector active solar distillation system has been designed fabricated and tested and the performance of the solar still has been presented. The theoretical and experimental values of performance have been compared and a fair agreement has been found. The correlation coefficients have been calculated and they have been found to vary between 0.97 and 0.99. The root mean square percent deviation has also been calculated. The characteristic curve for solar still has also

been drawn and it shows a fair agreement between theoretical and experimental values. Coefficient of determination has been found to vary between 0.94 and 0.98. The percentage uncertainties for the experiment and performance analysis have been carried out. Maximum value of thermal, thermal exergy, electrical exergy, overall exergy and overall thermal efficiencies have been found to be 75%, 20.74%, 28.53%, 25% and 69.06% respectively. Productivity analysis has been performed. Production cost of in Rs./kg and cost of electricity gain in Rs./kW h have been evaluated.

Appendix A. In modeling equations, we used the following parameters. Ac F Rc U L;c þ Ac F Rc ðasÞc;eff ðAF R ðasÞÞ1 ¼ Am F Rm PF 2 ðasÞm;eff 1 m_ f C f

Ac F Rc U L;c ðAF R U L Þ1 ¼ Am F Rm U L;m 1 þ Ac F Rc U L;c m_ f C f Am F Rm

F 0 Am U L;m m_ f C f ¼ 1 exp U L;m m_ f C f

222


Ac F Rc ¼

F 0 Ac U Lc m_ f C f 1 exp U Lc m_ f C f

U Lm ¼

U L1 hp;f U L1 þ F 0 hp;f

U L1 ¼

U tc;a U tc;p U tc;a þ U tc;p

ðasÞm;eff ¼ PF 1 ðasÞ1;eff þ ðasÞ2;eff ðasÞ1;eff ¼ sg ðac gc Þbc ðasÞ2;eff ¼ ap ð1 bc Þs2g PF 1 ¼

U tc;p U tc;a þ U tc;p

U L1 ¼

U tc;a U tc;p U tc;a þ U tc;p

PF 2 ¼

hp;f U L1 þ F 0 hp;f

hbw hbw þ hba hbw hba Ub ¼ hbw þ hba h1 ¼

Kg Lg

U c;ga ¼ K g Lg

h1g

þ h1g

h01 ¼

h1w Ag U c;ga Ag þ h1w Ab

Ut ¼

h1w U c;ga Ag U c;ga Ag þ h1w Ab

aeff ¼ aw þ h1 ab þ h01 ag and U S ¼ ðU b þ U t Þ References Abdel-Rehim, Z.S., Lasheen, A., 2007. Experimental and theoretical study of a solar desalination system located in Cairo, Egypt. Desalination 217, 52–64. Abu-Arabi, M., Zurigat, Y., 2005. Year-round comparative study of three types of solar distillation units. Desalination 172, 137–143. Al-Hinai, H., Al-Nassr, M.S., Jubran, B.A., 2002. Effect of climatic, design and operational parameters on the yield of a simple solar still. Energy Convers. Manage. 43, 1639–1650. Arslan, M., 2012. Experimental investigation of still performance for different active solar still designs under closed cycle mode. Desalination 307, 9–19. Ashcroft, H., 1950. The productivity of several machines under the care of one operator. J. Roy. Stat. Soc. (Lond.) Ser. B XII, 145–151. Badran, A.A., 2001. Inverted trickle solar still: effect of heat recovery. Desalination 133, 167–173. Badran, A.A., Al-Hallaq, A.A., Eyal Salman, I.A., Odat, M.Z., 2005. A solar still augmented with a flat-plate collector. Desalination 172, 227– 234. Badran, O.O., Al-Tahaineh, H.A., 2004. The effect of coupling flat plate collector on the solar still productivity. Desalination 183, 137–142. Bell, Stephanie, 1999. A Beginner’s Guide to Uncertainty of Measurement, N0. 11, Issue 2, ISSN 1368-6550, Centre for Basic, Thermal and Length Metrology, National Physical Laboratory, Teddington, Middlesex, United Kingdom, p. TW11 0LW.

Benson, F., 1952. Further notes on the productivity of machines requiring attention at random intervals. J. Roy. Stat. Soc. Ser. B XIV, 200–210. Calise, F., d’Accadia, M.D., Piacentino, A., 2014. A novel solar trigeneration system integrating PVT (photovoltaic/thermal collectors) and SW (seawater) desalination: dynamic simulation and economic assessment. Energy 67, 129–148. Cappelletti, G.M., 2002. An experiment with a plastic solar still. Desalination 142, 221–227. Chapra, S.C., Canale, R.P., 1989. Numerical methods for engineers. McGraw Hill. Chow, T.T., He, W., Ji, J., 2006. Hybrid photovoltaic-thermosyphon water heating system for residential application. Sol. Energy 80, 298– 306. Chow, T.T., 2003. Performance analysis of photovoltaic-thermal collector by explicit dynamic model. Sol. Energy 75, 143–152. Cooper, D.R., Schindler, P.S., 2003. Business Research Methods. Tata McGraw-Hill Publishing Company Limited, New Delhi 110008. Cox, D.R., 1951. The productivity of machines requiring attention at random intervals. J. Roy. Stat. Soc. Ser. B XIII, 65–82. Dubey, S., Tiwari, G.N., 2009. Analysis of PVT flat plate water collectors connected in series. Sol. Energy 83, 1485–1498. Dubey, S., Sandhu, G.S., Tiwari, G.N., 2009. Analytical expression for electrical efficiency of PVT hybrid air collector. Appl. Energy 86 (5), 697–705. Dwivedi, V.K., 2009. Performance Study of Various Designs of Solar Stills (Ph. D thesis). IIT New Delhi, India. Dwivedi, V.K., Tiwari, G.N., 2008. Annual energy and exergy analysis of single and double slope solar stills. Trends Appl. Sci. Res. 3 (3), 225– 241. Dwivedi, V.K., Tiwari, G.N., 2009. Experimental validation of thermal model of a double slope active solar still under natural circulation mode. Desalination 250 (2010), 49–55. Estahbanati, M.R.K., Feilizadeh, M., Jafarpur, K., Feilizadeh, M., Rahimpour, M.R., 2015. Experimental investigation of a multi-effect active solar still: the effect of the number of stages. Appl. Energy 137, 46–55. Fath, H.E.S., El-Samanoudy, M., Fahmy, K., Hassabou, A., 2003. Energy Res. 31 (14), 1358–1382. Fernandez, J., Chargoy, N., 1990. Multistage, indirectly heated solar still. Sol. Energy 44 (4), 215. Hamadou, O.A., Abdellatif, K., 2014. Modeling an active solar still for sea water desalination process optimization. Desalination 354, 1–8. Hayek, I.A., Badran, O.O., 2004. The effect of using different designs of solar stills on water distillation. Desalination 169, 121–127. Huang, B.J., Lin, T.H., Hung, W.C., Sun, F.S., 2001. Performance evaluation of solar photovoltaic/thermal systems. Sol. Energy 70 (5), 443–448. Ismail, B.I., 2009. Design and performance of a transportable hemispherical solar still. Renew. Energy 34, 145–150. International Labor Office, 1979. Introduction to Work Study. International Labor Organization, Geneva, ISBN 81-204-0602-8. Jafarkazemi, F., Ahmadifard, E., 2012. Energetic and exergetic evaluation of flat plate solar collectors. Renew. Energy 56, 55–63. Kumar, S., Tiwari, G.N., 2009. Life cycle cost analysis of single slope hybrid (PVT) active solar still. Appl. Energy 86, 1995–2004. Kumar, S., Tiwari, A., 2010. Design, fabrication and performance of a hybrid photovoltaic/thermal (PVT) active solar still. Energy Convers. Manage. 51, 1219–1229. Kumar, S., Tiwari, G.N., Gaur, M.K., 2010a. Development of empirical relation to evaluate the heat transfer coefficients and fractional energy in basin type hybrid (PVT) active solar still. Desalination 250, 214–221. Kumar, S., Dubey, A., Tiwari, G.N., 2014. A solar still augmented with an evacuated tube collector in forced mode. Desalination 347, 15–24. Lilian, M., George, M.A., Al-Hindi, M., 2014. The effect of cover geometry on the productivity of a modified solar still desalination unit’. Energy Procedia 50, 406–413.

D.B. Singh et al. / Solar Energy 130 (2016) 207–223 Malik, M.A.S., Tiwari, G.N., Kumar, A., Sodha, M.S., 1982. Solar Distillation: A Practical Study of a Wide Range of Stills and Optimum Design, Construction and Performance. Pergamon Press, New York. Eltawil, M.A., Omara, Z.M., 2014. Enhancing the solar still performance using solar photovoltaic, flat plate collector and hot air. Desalination 349, 1–9. Nakara, B.C., Chaudhary, K.K., 2004. Instrumentation, Measurement and Analysis. Tata McGraw-Hill. Petela, R., 2003. Exergy of undiluted thermal radiation. Sol. Energy 86, 241–247. Rajaseenivasan, T., Nelson, P., Srithar, K., 2014. An experimental investigation on a solar still with an integrated flat plate collector. Desalination 347, 131–137. Rubio-Cerda, E., Porta-Ga’ndara, M.A., Ferna’ndez-Zayas, J.L., 2002. Thermal performance of the condensing covers in a triangular solar still. Renew Energy 27, 301–318. Singh, G., Kumar, S., Tiwari, G.N., 2011. Design, fabrication and performance of a hybrid photovoltaic/thermal (PVT) double slope active solar still. Desalination 277, 399–406. Taghvaei, Hossein, Taghvaei, Hamed, Khosrow, Jafarpur, Karimi Estahbanati, M.R., MansoorFeilizadeh, M., Seddigh Ardekani, A., 2014. A thorough investigation of the effects of water depth on the performance of active solar stills. Desalination 347, 77–85. Tanaka, H., Nakatake, Y., 2006. Theoretical analysis of a basin type solar still with internal and external reflectors. Desalination 197, 205–216. Tiwari, G.N., 2013. Solar Energy, Fundamentals, Design, Modeling and Application. Narosa Publishing House, New Delhi. Tiwari, G.N., Ghosal, M.K., 2005. Renewable Energy Resources: Basic Principles and Applications. Narosa Publishing House, New Delhi.

223

Tiwari, A., Sodha, M.S., 2007. Parametric study of various configurations of hybrid PVT air collector: experimental validation of theoretical model. Sol. Energy Mater. Sol. Cells 91 (1), 17–28. Tiwari, G.N., Tiwari, A.K., 2007. Solar Distillation Practice in Water Desalination Systems. Anamaya Pub. Ltd., New Delhi, India. Tiwari, G.N., Lawrence, S.A., 1992. heat and mass transfer relations for a solar still. Energy Convers. Manage. 31, 201–203. Tiwari, G.N., Singh, S.K., Bhatnagar, V.P., 1993. Analytical thermal modeling of multi-basin solar still. Energy Convers. Manage. (34/12), 1261–1266 Tiwari, G.N., 2002. Solar Energy: Fundamentals, Design, Modelling and Applications. CRC Publication/Narosa Publishing House, New Delhi/ New York. Tiwari, G.N., Yadav, J.K., Singh, D.B., Al-Helal, I.M., Abdel-Ghany, A. M., 2015. Exergoeconomic and enviroeconomic analyses of partially covered photovoltaic flat plate collector active solar distillation system. Desalination 367, 186–196. Tiwari, G.N., Dimri, V., Chel, A., 2009. Parametric study of an active and passive solar distillation system: energy and exergy analysis. Desalination 242, 1–18. Toyama, S., Kagakuv K., 1972. Gijitsu, 24, 159, Maruzen, Tokyo. Tripathi, R., Tiwari, G.N., 2005. Effect of water depth on internal heat and mass transfer for active solar distillation. Desalination 173, 187– 200. Yadav, Y.P., Yadav, S.K., 2004. Parametric studies on the transient performance of a high temperature solar distillation system. Desalination 170, 251–262. Yuichi, Y., Haruki, S., 2003. Development of small-scale multi-effect solar still. In: International Solar Energy Conference, pp. 167–173.