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solar powered hybrid system for household wastewater treatment
Sustainable Energy Technologies and Assessments xxx (2017) xxx–xxx
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Original article
Adaptable wind/solar powered hybrid system for household wastewater treatment Akhilesh Soni a,1, Jacqueline A. Stagner b,⇑, David S.-K. Ting b a b
Mechanical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, India Turbulence & Energy Laboratory, University of Windsor, Windsor, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 20 September 2016 Revised 15 February 2017 Accepted 16 February 2017 Available online xxxx Keywords: Wastewater management Solar energy Vacuum evaporation Solar still Economic analysis
a b s t r a c t Sustainable and cost-effective water treatment systems are critical elements to developing nations. In India, the human population is escalating while the water availability is lagging behind. An adaptable, affordable, and sustainable wastewater treatment system powered by wind/solar energy is proposed based on proven theory and technology. A household in India is singled out to illustrate the workings of the proposed system, where the wastewater is recirculated through a hybrid of water purifiers powered by solar/wind energy. The system demonstrated here is specifically designed for small-scale applications, i.e., for a single household. The solar still has been divided into four stages. Partial vacuum is created inside the still so as to obtain boiling point temperatures of 70 °C, 67 °C, 62 °C and 50 °C in the four stages. Dhanbad, India 23.79°N, 86.43°E, with an average solar intensity of 850 W/m2 for 6 h a day, has been used for this study. A lumped parameter mathematical model was developed for this study. With an aperture area of 2.5 m2, the total amount of water distilled is found to be 43.3 kg/day. The system proposed is more efficient than existing systems as it is able to achieve efficiencies as high as 53%. The effect of wind speed on distillate output yield has also been discussed. Ó 2017 Elsevier Ltd. All rights reserved.
Introduction With modernization, urbanization and industrialization, the human race is advancing at one front and sacrificing at the other. The population is rising and the potable sources of water are diminishing. Global climate change and speedy growth in population have many cities under water threat [1,2]. With a few exceptions, water has always been a natural resource that mankind has taken for granted. The worldwide supply is not distributed evenly around the planet, nor is water equally available at all times throughout the year. Strategic reuse has gained importance over the last three decades as demand for water increased dramatically [3]. Reuse of wastewater for domestic and agricultural purposes has been executed to boost water usage efficiency since historical times. Initially, the emphasis was on reuse for agricultural and non-potable purposes but, with innovative advancements in technology, the domain for reused water has widened [4]. Conventional wastewater treatment processes are acceptable for non-potable water reuse applications (e.g. turfgrass, landscape, and agricultural ⇑ Corresponding author. E-mail addresses: [email protected] (A. Soni), stagner@uwindsor. ca (J.A. Stagner). 1 Globalink Mitacs Undergraduate Intern at Turbulence & Energy Laboratory.
irrigation) that do not require the wastewater to be treated to drinkable standards. Treatment of raw water from lakes, rivers, or wells is required in order to meet drinkability criteria. To do so, it requires work, and thus energy, which is yet another issue in rural areas. Access to modern energy is a social and economic priority to the rural population because of its direct socioeconomic and environmental benefits. People living in these communities also face financial problems. With this in mind, this work aims to design a self-sustainable stand-alone water treatment system. There is already an immense amount of research ongoing in the field of applications of renewable sources of energy in an offgrid community [5]. Kulworawanichpong et al. [6] suggested a design for a stand-alone solar photovoltaic system for a rural Tanzanian household. The application of the system presented here, which is driven by renewable sources of energy, is primarily for an off-grid community. The design philosophy is to use solar energy as a primary source, and wind energy as a secondary source, to power the water purification system. Most filtration techniques like reverse osmosis (RO) require the water to be pumped at high pressures, thus requiring a pump. Supplying power to the pump is another challenge in rural areas. Moreover, filtration techniques have a limitation of the minimum molecular size particles they can remove from water. With distillation, almost 99% of the particles can be removed, leaving only the
http://dx.doi.org/10.1016/j.seta.2017.02.015 2213-1388/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Soni A et al. Adaptable wind/solar powered hybrid system for household wastewater treatment. Sustainable Energy Technologies and Assessments (2017), http://dx.doi.org/10.1016/j.seta.2017.02.015
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Nomenclature A AC AMC C Cp CT Cum d eT f fl F FR GRP hci hewi hfg Hi H0 I ki K Lc LH LT mc;i mei msi M MY
aperture area (m2) annual cost (Rs) annual maintenance cost (Rs) cost (Rs) specific heat (J/kg K) collector thickness (m) cost of unit mass of water (Rs/kg) diameter of pipe (m) edge insulation thickness (m) capital recovery factor frictional loss coefficient sinking fund factor heat Removal Factor glass reinforced polymer convective heat transfer coefficient (W/m2K) evaporative heat transfer coefficient (W/m2K) latent heat (J/kg) inlet head of cold water in solar collector outlet head of water from solar collector solar intensity (W/m2) thermal conductivity of insulation (W/m K) minor loss coefficient in Heat Exchanger length of piping of solar collector length of piping of Heat Exchanger insulation thickness of flat plate collector (m) mass of water condensed in the ith stage (kg/s) mass of water condensed in the ith stage (kg/s) mass of water in ith stage (kg) maintenance cost (percentage of annual cost) annual distillate yield (kg)
organic particles which have a boiling point less than water. To accommodate this, a carbon filter can be used in conjunction with the distillation system. The proposed system uses distillation to purify wastewater. Advancements in solar thermal energy technologies are continuing to drive down investment costs, increase ease of arrangement, and optimize direct coupling with other process systems. The system presented here is simple, reliable, affordable, sustainable, and effective. Status of water consumption in an Indian household People in some parts of India have adjusted their habits based on the supply such that they do not feel that more water is needed. This, in turn, creates hygiene and sanitation problems, resulting in several health consequences. The effects of consuming water of substandard quality cause a myriad of health issues. In many Indian cities, the supply is very erratic, in addition to the inferior quality of the water. Thus, there is dire need of a system that meets the daily requirements of water adequately. Fig. 1 shows the average consumption of water in liters per capita per day (LPCD) in major Indian cities [7]. Five major Indian cities have been chosen to conduct this study. In this paper, we design our system as per the average water consumption in these cities. The Indian government [8] has reported that approximately 60% of households in major Indian cities are water-deficient. The same report shows that 72% of those people are from lower income families. The average water consumption is approximately 91 LPCD. Fig. 2 depicts the distribution of water in a typical Indian household. Approximately 50% of the total water consumption is for bathing and toilet activities. 25% consumption is for house cleaning and washing clothes. The remaining 25% needs to meet the basic requirements for human consumption.
N p P Δp Qh QL Qu r Rs S UB UE UT Vw T ci T in T si DT K
g q gc go
number of years of operation partial vapor pressure (Pa) collector perimeter (m) pressure difference heat absorbed from solar water heater (W) heat leaked (W) useful Energy Gain (W) inflation rate Indian currency savage value (Rs) heat transfer coefficient for bottom losses heat transfer coefficient for edge losses heat transfer coefficient for top losses wind speed (m/s) condensing surface temperature (°C) Inlet temperature of water (°C) water bed temperature in ith stage (°C) temperature difference (°C) minor loss coefficient in solar collector collector efficiency of top stage (%) density of water (kg/m3) flat plate collector efficiency overall distill efficiency
Subscripts c solar collector h heat exchanger i inlet s still TC total cost (Rs)
History of water purification techniques Purifying water for reuse has been practiced for years [9,10]. Some of the traditional treatment methods are:
Filtration through winnowing sieve Filtration through cloth Filtration through clay vessels Clarification and filtration through plant material Jampeng stone filtration method
However, these methods are not reliable as each of them has its own limitations. The sieve cannot filter the fine suspended particles in the water. Cloth filters water only to a small extent. Pores
Kanpur
Hyderabad
Kolkata
Mumbai
Delhi 0
50
100
150
Fig. 1. Water consumption per capita per day (in liters) in distinctive Indian cities.
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Washing Utensils 14%
3
that incorporates solar still technology with wind energy has been designed for purifying water for a household.
Other 5% Bathing 28%
Working principle Cleaning House 8% Drinking & Cooking 7% Washing Clothes 18%
Toilets 20%
Fig. 2. Activity-wise water consumption distribution.
of clay vessels get blocked very often and cannot be used long term. As well, particles finer than pore size cannot be filtered through clay vessels. The last two methods mentioned above do remove certain types of particles but do not make the water suitably fit for human consumption. Biological slow sand filters have also been used but moving water through them requires energy. As well, reverse osmosis (RO) has been used widely but one of the largest challenges facing RO is the reliability of membranes that treat water having high concentrations of sparingly soluble salts, particulates, and organic matter [11]. Due to this, the application of RO is restricted as the chemical and energy costs are high. Multi-effect distillation (MED) integrated with concentrated solar thermal power (CST), called a concentrated solar still (CSS), has been demonstrated to treat agricultural drainage water at the Panoche Drainage District in the San Joaquin Valley [12]. Ahmed et al. [13] studied the characteristics of a multi stage solar still. Thus, water purification by distillation is not a totally new concept, but it has not been employed for domestic water treatment until now. Moreover, owing to their high costs and poor efficiencies, the existing distillation systems need to be improved for extensive effective applications. To the authors’ best knowledge, no system
The wind-solar hybrid system presented here is a selfsustaining system for pumping domestic wastewater from ground level using wind energy and making it reusable with the aid of solar energy. The system is designed in such a way so as to be installed in households without disturbing the existing plumbing systems. In most Indian households, the household water, excluding the sewage, collects at a common point so as to be disposed of to drains. The system presented here does not incorporate sewage waste, due to sanitary and environmental issues. This wastewater, excluding sewage, needs to be pumped from ground level to roof level where it is to be distilled in order to obtain pure water. Thus, there are two major tasks to be performed by the system, first, lifting water from ground level and, second, making it consumable by purifying it. Water is lifted from ground level with the help of a wind pump. If the wind does not blow for a couple of days, the storage tank may get empty. To counter this, a crank is attached to the wind driven rotor. When the wind does not blow, a handdriven wheel achieves the rotary motion of the rotor. The distillation system presented here is a combination of flat plate collector, tubular heat exchanger, and an evaporative condenser unit. The distiller consists of multiple reservoirs created by a stacked array of distillation trays that act as condensers for the tray below, as shown in Fig. 3. Each stage consists of an extruded cylindrical opening which is used to create the required partial vacuum. A valve is provided in each stage in order to regulate the pressure. There are multiple stages inside the distiller. Perfect sealing is maintained between the stages to prevent any vapor loss through the contact surfaces between the stages. Fig. 4 illustrates how sunlight captured by the glass cover is concentrated on a black surface, heating the water in the topmost chamber and, thus, evaporating it. This vapor is then condensed to form droplets. Fig. 5 provides a flow chart depicting the flow of water through the system Wastewater is fed into each stage from the tank. Each stage consists of an evaporator and a condenser surface.
Fig. 3. Schematic representation of distillation and evaporation/condenser unit.
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Fig. 4. Schematic of evaporation and condensation of water.
Wind Pump
Waste water stored in tank (Ground Level)
Waste water in storage tank at roofl evel
Water gets contaminated
Water used for domesc needs
Water moves to chambers at different stages
Pure water collects in a storage tank
Solar Radiaon
Water evaporates ,and condenses
Fig. 5. Flow chart depicting the flow of water in the system.
Table 1 Vapor pressure and associated boiling point of water in each stage. Stage
Vapor pressure (kPa)
Boiling point (°C)
First Second Third Fourth
31 27 20 18
70 67 62 50
There is a pressure gradient inside the chamber. The pressure decreases in each stage in order to obtain a lower boiling point in upper chambers. An economic and productivity analysis of the number of stages is shown in a later section where the number of stages are optimized to be four. The pressure in each stage is shown in Table 1. These temperatures have been used as the solar still works best in this range [14,15]. A pump is used to create a partial vacuum inside the distillation chamber. To achieve high temperatures, heat is supplied to the wastewater in the lowest reservoir via a heat exchanger through which water is circulated from the solar collector, passed through the heat exchanger tubes and then returned back to the solar collector again. Cold water gets heated and moves to the surface so that it can evaporate faster. Vapour generated in the lower stage condenses on the bottom surface of the intermediate stage, thus giving its heat to the water in the intermediate
stage. Vapour from the intermediate stage condenses on the upper stage, transferring its latent heat of condensation to the water in the upper stage. Water in the top-most reservoir, which is painted black to maximize radiation capture, is also heated directly by solar radiation. In the intermediate stages, heat transfer apart from radiation and convection, occurs by evaporation and condensation, thus utilizing the latent heat of condensation and improving the system’s efficiency. Radiation and convection constitute minor energy transfer between the stages and, hence, have been ignored. The number of stages has been decided by taking the cost factor into consideration. It has been observed that an increase in distillate yield corresponds to an increase in the number of stages; however, fractional increases in the distillate decrease with the addition of every stage [16]. An economic and productivity analysis of the number of stages is shown in a later section. Also, Reddy et al. [17] showed that the gap between stages should be 10 cm. Evacuated solar stills have already been demonstrated in the past but no one has used a pressure gradient along the stages to get a variable evaporating point of water in the stages. The suggested concept can improve the efficiency of the existing systems to a great extent. The only problem associated with the working of the still is that the plates need to be cleaned daily. To prevent algae and scaling on inner black surfaces, a still would be required to be dried completely once a week. Bleaching or chlorination can also be used to prevent algae formation. Ahmed et al. [18] experimentally investigated the multi stage evacuated solar still. The productivity of this new system was found to be about threefold greater than the maximum productivity of the basin type solar still. While designing the above system, some keys points have been taken into consideration: 1. Simple, appropriate technology has been adopted, as an overly complex system would be challenging due to its maintenance problems. 2. The system is flexible as per the demand of the people in a particular area i.e. its cost will vary as per the quantity of water required. 3. The system operates in a sustainable manner. This means being funded, owned, and operated by the individuals using the water supply. 4. The system is independent of any external power source. The basic principle used is reducing the air pressure in order to reduce the boiling point of water.
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Vacuum pump Design methodology The design parameters are the most important factors that affect the productivity of the proposed system and can be easily controlled, developed and designed to improve productivity. These parameters include evaporation area, water depth, cover condensing angle, and insulation. The individual components of the proposed system include:
Wind pump A conventional method of pumping water from ground level to roof level where the solar distillation system is located has been employed. A wind-energy driven piston pump is to be used. The wheel, having wide blades, is rotated by the wind, and is attached to a shaft by long arms. The shaft has small pinion gears at the other end, inside a small gearbox. The pinion gears drive the bull gears, which move pitman arms. The pitman arms push a sliding yoke up and down, above the bull gears. The yoke lifts and drops the pump rod and, hence, the necessary motion required for the working of the piston pump is achieved.
Tempered glass plate Tempered glass lets only high-energy radiation to pass and this high-energy radiation is captured. The tempered glass surface also serves as a condensing surface for the top reservoir. It is always at a temperature less than the water in the reservoir. Due to the slant, droplets slide to one side and deposit all condensate into the collector. A thermally treated low iron glass has been used to enhance its toughness. The inclination angle of the glass plate is fixed as per the location. For this case, the location used is Dhanbad, 23.79°N, 86.43°E. The optimum inclination angle for the month of April is found to be 26° [19].
The vacuum pump used here is simply a modified cycle pump as shown in Fig. 6. The vacuum pump is to be manually operated by a person before the still can be used. Three one-way valves, one each on the inlet, piston and exit are installed. Its working is similar to that of a 2-stroke engine. The inlet joins to the distillation chamber. The downward stroke of the piston draws air from the distillation chamber into the upper part and compresses air in the lower. Both piston and exit valves are closed during this period. As the air pressure in the lower part exceeds atmospheric pressure, the exit valve opens to discharge this air into ambient. During the upward stroke, the piston valve is kept open while the inlet and exit valves close. Thus, air is transferred into the exit chamber. This process continues until the required pressure is achieved.
Heat exchanger A small solar thermal collector is used to heat the water in the bottom-most chamber. The thermosiphon process, represented in Fig. 4 achieves this. The bottom most chamber is kept at a level higher than the thermal solar collector. Use of an evacuated thermal collector (ETC) is a way in which heat loss to the environment, inherent in flat plates, can be reduced. Since heat loss due to convection cannot cross a vacuum, it forms an efficient isolation mechanism to keep heat inside the collector pipes [20]. In this case, the vacuum is created between two concentric tubes. The water piping in an ETC is surrounded by two concentric tubes of glass with a vacuum in between that admits heat from the sun (to heat the pipe) but which limits heat loss back to the environment. The inner tube is coated with a thermal absorbent [21]. The tubes used are made from borosilicate [22]. The water in the tubes lying on the surface of the collector gets heated, and moves up to the bottommost reservoir, and cold water from there comes down in the tubes. The borosilicate tubes are connected to copper tubes inside the bottom stage of the still so as to lose heat to the water present
Fig. 6. Mechanism of vacuum pump.
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in the lowermost stage. Due to the thermosiphon process, the water in the bottom reservoir gets heated and increases the rate of evaporation. Condensing cover/surface angle The surface must be at such an angle so as to ensure that sufficient condensing water runs smoothly to the collector plate without forming large droplets, which might fall back into the distillation chamber instead of the condensate channel. The angle should be such that it provides a cool surface for water vapor condensation, maximizes the absorption of solar radiation, and reduces heat losses to the surroundings. Various studies [23–24] suggest that the optimum value for this lies in the range of 30–35°. Hence, an angle of 32° is used in the present work. Insulation Proper insulation is necessary to prevent the release of useable thermal energy to the environment. The side and bottom walls are insulated to prevent wastage of heat. Numerous investigations [25,26] propose the usage of glass wool or wooden boxes insulated with sawdust. In the model presented in this paper, glass wool has been used. The heat conductivity coefficient for it is 0.04 W/m K.
4. All heat from the collector is transferred to the water in stage 1. Fig. 7 shows the hourly solar intensity variation as recorded on 15th April 2016 using a digital solar pyranometer. Extreme summers in India are during April-June. Hence, the solar intensity data has been collected during the same period. The average solar intensity is found to be 850 W/m2 during the six-hour period. The data have been recorded in Dhanbad, 23.79°N, 86.43°E, a city located in the state of Jharkhand, India. Analysis of solar still The basic model of the solar still is quite similar to the one presented by Adhikari et al. [27]. The energy flow rate through the system is presented in Fig. 8. A static water feed system has been adopted. Thus, while feeding the water to the still, the entire mass of water is supplied before the still starts functioning.
m_ e ¼ m_ c
ð1aÞ
dmsi ¼ m_ei dt
ð1bÞ
Energy balance equation for first or bottom stage:
ms1 C p Nozzle The recirculating loop and still are at a pressure less than atmospheric pressure. Household drains work at atmospheric pressure. Hence, to connect the two, a constricting nozzle is used. In the common line between feeder tank and the solar still, a constricting nozzle is used. The ratio of diameters at the two ends in the first and subsequent stages is (1:7.5), (1:7.7), (1:8) and (1:8.4), respectively. The inlet water head is considered to be 2 m for all stages. Mathematical model The lumped parameter mathematical model has been developed to conduct this study. Fig. 3 shows the fluid from the heat exchanger transfers its workable heat to the impure water in the first stage. The temperature in the first stage increases and it loses its heat to the second stage through evaporation. To develop the mathematical model for the system, the following assumptions have been made 1. Due to small temperature differences between adjacent stages and the absence of non-condensable gases, convection and radiative heat transfer are ignored [18]. 2. The amount of water evaporated and distilled in the nth stage is equal i.e. mc;i = me;i 3. The distillate leaves the chamber at the condensing surface temperature.
dT s1 ¼ Q_ h m_c1 C p T c1 m_e1 hfg1 Q_L1 dt
Q_ h ¼ IAg
ð3Þ 2
Area A of the thermal solar collector plate is set to 2.5 m so as to meet the required demand of water, as an area less than this was not meeting the required demand. Energy balance equation for consecutive stages,
ms2 C p
dT s2 ¼ m_e1 hfg1 mc2 C p T c2 m_e2 hfg2 Q_L2 dt
ð4Þ
ms3 C p
dT s3 ¼ m_e2 hfg2 m_c3 C p T o3 m_e3 hfg3 Q_L3 dt
ð5Þ
ms4 C p
dT s4 ¼ IAg þ m_e3 hfg3 m_c4 C p T o4 m_e4 hfg4 Q_L4 dt
ð6Þ
In the fourth stage, term IAgð3600Þð6Þ indicates the solar energy due to direct heating. This solar energy is captured using the tempered glass plate. Collector efficiency can be as high as in the range of 0.5–0.6 for zig-zag type solar collectors [28]. However, the collector efficiency is a strong function of solar intensity and wind speed. The following set of equations is used to study the effect of wind speed on collector efficiency [29].
RQ
gc ¼
dt RI Ac dt
ð7Þ
u
Q u ¼ F R ½I U L ðT i T a Þ
Intensity (W/m2)
gc ¼ F R fa U L
1200 1000 800 600 400 200 0
ð2Þ
FR ¼
Ti Ta I
ð8Þ ð9Þ
mcp Ac U L 1 exp Ac U L mcp
ð10Þ
U L¼ U T þ U B þ U E 8
10
12
14
Time (Hours)
16
0 UT ¼ @
11 1A h iþ þ T pm T a hw N
C T pm
Nþf
ð11Þ
rðT pm þ T a Þ T 2pm þ T 2a 1
ep þ0:00591hw
þ
1þf þ0:133ep
eg
1
ð12Þ
Fig. 7. Hourly variation of solar intensity.
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Fig. 8. Flow chart showing energy flow through the system.
hw ¼ 5:7 þ 3:8V w
UE ¼ UB ¼
PC t
ð13Þ
et ki
ð14Þ
Ac ki Lt
ð15Þ
Condensing surfaces for the nth stage and waterbed for (n + 1) th are considered to be at identical temperatures, Mass flow rate of the vapor evaporated is given as
m_ei ¼
ðT si T ci Þ hewi A hfgi
ð16Þ
psi pci T si T ci
The driving force needed to circulate the water in the solar collector is provided by the buoyancy driven natural convection thermosiphon, thus, eliminating the need of an electric pump. The only significant pressure losses occur in the solar collector and heat exchanger in the system. These pressure losses are to be overcome by the thermosiphon process. The net hydrostatic pressure across the heat exchanger, i.e. driving force of the thermosiphon is given by [33]:
Dpnet ¼ gðqHo qHi Þ þ Dploss
ð17Þ
Dploss ¼ Dph þ
Convective transfer coefficient hci is given as
" hci ¼ 0:884 ðT si T ci Þ þ
ðpsi pci ÞðT si þ 273Þ
#13 ð18Þ
268:9 103 psi
The latent heat and specific heat of water are also a function of temperature [31,32].
hfg ðTÞ ¼ 1000 ½3161:5 2:4075ðT þ 273Þ 5
ð19Þ 7
C p ðTÞ ¼ 1000 ½4:2101 0:0022T þ 5 10 T 3 10 T
ð20Þ
Overall thermal efficiency for the multi-stage evacuated solar desalination system (%) is defined as the ratio of total heat content output from all the stages by the cumulative daily distillate yield of all the stages to that of the total heat content supplied to system throughout the day. The total heat content input is through the flat plate collector and direct heating of water in the top chamber. The overall efficiency of the yield is calculated using the following formula.
P4
m h
go ¼ _ 1 i fg ½Q h þ IA 3600 6
ð22Þ
The inclination angle of solar collector is 26° as already discussed in Section 3.2. Hence, Hi = 0 and Ho = 2Sin (26°),
Evaporative heat transfer coefficient hewi is given as [30]
hewi ¼ 16:273 103 hci
Hydraulic analysis of solar collector
ð21Þ
Dph ¼
qv 2 2
fl
qv 2 2
s þ fl
LH þK D
Lc d
ð23Þ
ð24Þ
The friction factor for laminar flow is defined as [31]
fl ¼
64 Re
ð25Þ
It is not possible to predict the exact pressure losses theoretically. Moreover, an in-depth analysis of the pressure drop is beyond this study. However, an approximation can be done for the pressure losses occurring. The friction loss coefficient is approximated from the Moody chart, assuming the flow to be laminar. Cramp and Harrison [33] showed that the thermosiphon drives the fluid with a flow rate in the range of 0.2–1 LPM. The minor loss coefficient for the return bend in the heat exchangers can be assumed to be 0.2. Thus, based on the assumed value, the total pressure head loss is found in the range of 0.003–0.005 m. The low value is attributed to the low driving force of the thermosiphon which leads to low velocities of fluid flow in the pipes.
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summarizes the various cost parameters with cost given in Rs (1 Rs = 0.015USD).
Economic analysis of proposed system The distillation system consists of various metallic plates stacked in a way so as to form different stages inside the still. The utilization of the proposed system as a source of distilled water for commercial and household purposes should be determined by its economics. An economic model also helps to optimize the number of stages. The better economic return on the investment depends on the production cost of the distilled water and its applicability. To take into consideration time value of money, capital recovery factor (f), fund sinking factor (F), and the inflation rate (r) have been employed in the analysis. The annual cost of the evacuated distillation system coupled with a solar flat plate collector can be written as follows: Total Cost = Cost of still + Cost of solar collector + cost of vacuum pump + Cost of heat exchanger
C TC ¼ ðC s f s þ M s C s Ss F s Þ þ ðC c f c þ M c C c Sc F c Þ þ ðC p f p þ Mp C p Sp F p Þ þ ðC h f h þ M h C h Sh F h Þ f ¼
F¼
rð1 þ rÞN
ð27Þ
ð1 þ rÞN 1 r
ð28Þ
ð1 þ rÞN 1 Annual Cost of system Annual distillate yield
Cost of unit mass of water ¼
C um ¼
ð26Þ
C TC MY
ð29Þ
The life cost analysis has been carried out by considering the life of the proposed system as 20 years. The quantity of material used in the fabrication and the cost breakdown of the proposed system are given in Table 2. The scrap value in developed countries is almost negligible, whereas the scrap value, inflation rates etc. in developing countries like India are higher than the developed countries. The scrap value of iron, copper and aluminium will also increase with time due to inflation. The expected rates of these materials have been considered at an inflation rate of r = 3.6% in the Indian market. The AMC (annual maintenance operational cost) of the solar still is required for regular filling of water, collecting the distilled water, cleaning of the glass cover, removal of impurities deposited and maintenance of the pump. As the system life increases, the maintenance on it also increases. Therefore, 10% of net present cost [34] has been considered as maintenance cost. Table 2
Results Distill thermal analysis A mathematical model has been developed for the abovepresented system and operated for 6 h a day under a constant solar flux of 850 W/m2. Inlet mass flow rate will have a significant effect on the productivity of the distillation system, as shown in Fig 9. Results in Fig 9 are shown for a distillation system consisting of four stages as it can be seen from the plot of productivity vs number of stages (Fig 10) that increasing the number of stages beyond four does not have a significant increase in still productivity. It can be observed from Fig 9 that the productivity increases initially with increase in mass flow rate, attains a maximum at a mass flow rate of approximately 50 kg/m2/day, and then decreases with further increase in mass flow rate. This is due to the storage effect which results in a lesser amount of latent heat being available for reuse. Hence, it is recommended to use mass flow rate of 50 kg/m2/day (or a water depth of 5 cm) Variation of the waterbed temperature of different stages with time is represented in Fig. 11(a). Linear variation is attributed to the constant solar flux. At wind speed of 3 m/s, the difference in the water bed and condensing surface temperatures in the four stages are found to be 3 °C, 5.1 °C, 4.7 °C and 14.2 °C, respectively. The variation of waterbed temperatures at wind speed of 6 m/s is shown in Fig. 11(b). It can be seen that the temperature difference between the consecutive stages is reduced drastically. This leads to poor yield. Fig. 12(a) indicates the distillate yield for the given span of time of 6 h at different wind speeds. Distillate product depends on the waterbed temperatures, the temperature difference between the condensing and evaporating surfaces and on the wind speed. As the wind speed increases, thermal losses severely increase leading to poor yield. Similar behaviour is observed with still efficiency, as shown in Fig. 12(b), which monotonically decreases with increase in wind speed. It is also observed that distillate product increases on going from the bottom to the top stage except for the third stage as shown in Fig. 13. This is because of the low temperature difference between the condensing and evaporating surfaces in the third stage. Water in the uppermost chamber, apart from the latent heat of condensation, is heated directly from solar radiation. Hence, the waterbed temperature for the fourth stage and condensing temperature for the third stage is comparatively higher than expected.
Table 2 Cost breakdown of components of the proposed system. Cost (Rs)
Salvage value (Rs) at inflation rate 3.6%
GRP Body (5 mm thick) Iron Stand Metal Sheet (Aluminium 1.5 mm thick) Glass Cover (for still) Vacuum Pump Solar collector Heat Exchanger (Copper Tubes of ID = 5/800 ) Glass wool (50 mm thick) Nozzles Pressure Gauge
5 m2 15 30.75 kg
500/m2 70/kg 170/kg
400 500 4000
2.5 m 1 1 9.4 kg
1000/m 1000 20,000 500/kg
4.8 m2 4 4
220/m2 100/piece 600/piece
– – 15,000 4110
Sll Yield (kg/day)
Quantity
2
Wind Speed=3m/s
Wind Speed=5m/s
Wind Speed=7m/s
50
Component
2
Wind Speed=1m/s
40 30 20 10 0 0
– – –
50
100
150
Mass flow rate (kg/m²/day) Fig. 9. Effect of mass flow rate on output yield for 4 stages.
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A. Soni et al. / Sustainable Energy Technologies and Assessments xxx (2017) xxx–xxx
Wind Speed=1m/s Wind Speed=1m/s
Wind Speed=7m/s
50 40
50 40
Sll Yield (kg/day)
SllYield Yield(kg/day) (kg/day) Sll
WindSpeed=7m/s
Wind Speed=5m/s
6050
a
Wind Speed=3m/s Wind Speed=3m/s
Wind Speed=5m/s
40 30 30 20 20 1010 00 10
3 50
5
100
7
Number of stages
150
30 20 10
Fig. 10. Effect of number of stages on output yield.
0
b Stage 4
Stage 3
Stage 2
Stage 1
3
5
7
Wind Speed (m/s) 60 50
Sll Efficiency (%)
a
70
Temperature (°C)
1
60 50 40
40 30 20 10
30
0 20
1 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Operang Time (hours)
b
Stage 4
Stage 3
Stage 2
Stage 1
70
Temperature (°C)
3
5
7
6
60 50 40 30 20
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
Operang Time (hours) Fig. 11. (a) Waterbed temperature variation at Vw = 3 m/s. (b) Waterbed temperature variation at Vw = 6 m/s.
Wind Speed (m/s) Fig. 12. (a) Variation of output yield with wind speed. (b) Variation of distill efficiency with wind speed.
Badran et al. [35] investigated a basin still with a 1 m2 area coupled with a 1.34 m2 flat plate collector and a maximum production capacity of 4.6 kg/day. Fernandez and Chargoy [36] studied experimentally and theoretically, a multi-stage desalinisation system with a W-shaped condensing surface. During the experiment, a maximum of 2.5 kg of water per collector was produced when operated at atmospheric pressure. Thus, the solar still described in this paper is quite efficient compared to the existing ones, as it uses only a single collector and produces 43.41 kg of water for an operation of 6 h a day, considering wind speed to be 1 m/s. The maximum still efficiency, 53.2%, is observed at a wind speed of 1 m/s. This is greater than the maximum still efficiency for fresh water in the work conducted by Kumar et al. [37] where they obtained a maximum efficiency of 50.5% for fresh water. It should be noted that Kumar et al. have considered the pressure to be as low as 0.03 bar which is quite low compared to our system where minimum pressure is 0.18 bar. The work done by the vacuum pump, and hence, the power input to the system, is less in the system presented in this paper. Study of the year-round transient analysis of a multi-stage evacuated solar desalination system was done by Reddy et al. [38] and the results show that the system
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Vw=1m/s
Vw=3m/s
Vw=6m/s
20 18
Output Yield (kg/day)
16 14 12 10 8 6 4 2 0 1
Stage
2
3
4
Fig. 13. Variation of output yield in different stages with wind speed.
Table 3 Average solar radiance in various Indian cities. City Solar radiance (MJ/m2/day)
Kanpur 17.68
Hyderabad 20.34
produces a maximum distillate yield of 16.4 kg/m2/day at an average efficiency of 45%. Thus, the system presented here is more efficient than the existing ones. The solar energy utilised by the presented system is 850 W/m2 for 6 h a day i.e. 18.36 MJ/m2/day. As per the data of Solar Energy Centre, MNRE Indian Metrological Department [39], the average solar radiance in various cities is found in Table 3. Hence the system presented is suitable for being used across the country.
Table 4 Dimensions and capital cost of proposed system. Length Width Height (lower side) Height (upper side) N (number of years) Initial Investment Salvage Value (after 20 years) CRF, f SFF, F AMC AC
1.6 m 1.4 m 0.5 m 1.2 m 20 Rs 40,834 Rs 24,010 0.071 0.035 Rs 4100 Rs 6143
Wind Speed=1m/s
Wind Speed=3m/s
Wind Speed=5m/s
Wind Speed=7m/s
Cum (Rs/kg)
2.4 1.9 1.4 0.9 0.4 2
4
6
Number of stages Fig. 14. Effect of number of stages on per unit cost of water.
Mumbai 18.25
Delhi 18.25
Economic analysis Economic analysis has been done for the proposed system based on the model developed in Section 4.2. Dimensions and capital cost of the system are shown in Table 4. Annual yield is calculated assuming the number of sunny days to be 300 in a year [40]. The annual cost of the proposed system is approximately Rs 7384. This provides a unit cost of water in the range of 0.5–1.2 Rs/kg for the wind speed range of 1–5 m/s. It is more economical in comparison to the bottled water available in India which costs approximately 10–12 Rs/kg for consumers. The combined analysis of productivity and economics of the system is studied by seeing the variation of cost per unit mass of water produced as shown in Fig 14. It can be seen that Cum (cost per unit mass of water produced) decreases with increasing number of stages until it attains a minimum value when the number of stages is equal to 4. Hence, any further increase in the number of stages after 4 is not justifiable from an economic point of view. The increase in productivity is not at par with the increase in cost of the system. Variation of the unit cost with wind speed is also shown in Fig 14. Conclusions
2.9
0
Kolkata 16.17
8
The water purification system presented above utilizes two modes of renewable energy, solar and wind. Wind energy is used to drive a vacuum pump which reduces the air pressure inside the system. The number of stages has been optimized to be four, as any further increase in the number of stages is not justifiable from an economic point of view. The vapor pressure maintained in the successive stages of the still are 31 kPa, 27 kPa, 20 kPa and 18 kPa. Constricting nozzles are used to connect the household drain with the recirculating loop and still. Solar energy is used to heat the water lying in the chambers of the still. Solar energy is transferred to the system from the bottom chamber via a heat exchanger, and from the top by direct heating of the water. The fresh water production capacity of the investigated solar-collector four-stage solar still, when operated for 6 h a day at a constant flux of 850 W/m2 , is found to be 17.4 kg/m2 /day at Vw = 1 m/s, which is greater than conventional multi-stage solar stills [17,32,35–37]. The annual cost of the system is approximately Rs7450, with the per unit water cost in the range of
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0.5–1.2 Rs/kg for the wind speed range of 1–5 m/s. Water evaporates in four different stages, each separated by a distance of 10 cm. In the absence of wind, a hand-driven wheel can be used to drive the reciprocating pump to propel the water from ground to roof level. The suggested multistage solar desalination system can meet the fresh water needs of rural and urban communities by distilling 25–45 kg/day, considering wind speed is in the range of 1-5 m/s. Acknowledgement Authors would like to express their sincere gratitude to Globalink Mitacs for its continuous support during this study. References [1] Chen J, Shi H, Kumar BS, Peart MR. Population, water, food, energy and dams. Renewable Sustainable Energy Rev 2016;56:18–28. [2] Dale AT, Bilec MM. The regional energy & water supply scenarios (REWSS) model, part II: case studies in Pennsylvania and Arizona. Sustainable Energy Technol Assess 2014;7:237–46. [3] Shaban A, Sharma RN. Water consumption patterns in domestic households in major cities. Econ Political Weekly 2007;23:2190–7. [4] Taylor M, Clarke WP, Greenfield PF. The treatment of domestic wastewater using small-scale vermicompost filter beds. Ecol Eng 2003;21:197–203. [5] Byrne J, Zhou A, Shen B, Hughes K. Evaluating the potential of small-scale renewable energy options to meet rural livelihoods needs a GIS- and lifecycle cost-based assessment of Western China’s options. Energy Policy 2007;35:4391–401. [6] Kulwoawanichpong T, Mwambeleko Joachim J. Design and costing of a standalone solar photovoltaic system for a Tanzanian rural household. Sustainable Energy Technol Assess 2015;12:53–9. [7] Soni V. Water and carrying capacity of a City – Delhi. Econ Political Weekly 2003;38:4745–9. [8] Government of India, Tenth Five-Year Plan 2002-2007, Planning Commission, New Delhi; 2002. [9] Holloway RW, Miller-Robbie L, Patel M, Stokes J, Munakata J, Dadakis J, Cath T. Life-cycle assessment of two potable water reuse technologies: MF/RO/UV– AOP treatment and hybrid osmotic membrane bioreactors. J Membr Sci 2016;507:165–78. [10] Woltersdorf L, Scheidegger R, Liehr S, Doll P. Municipal water reuse for urban agriculture in Namibia: modeling nutrient and salt flows as impacted by sanitation user behavior. J Environ Manage 2016;169:272–84. [11] Cohen Y, McCool B, Rahardianto A, Kim M, Faria J, Chang AC, Silva DB. Membrane desalination of agricultural drainage water. Dordrecht: Springer Science Business Media; 2014. p. 303. [12] Stuber MD, Sullivanb C, Kirk SA, Farrand JA, Schillaci PV, Fojtasek BD, Mandell AH. Pilot demonstration of concentrated solar-powered desalination of subsurface agricultural drainage water and other brackish groundwater sources. Desalination 2015;355:186–96. [13] Ahmed MI, Hrairi M, Ismail AF. On the characteristics of multistage evacuated solar distillation. Renewable Energy 2009;34:1471–8. [14] Clark JA. The steady state performance of a solar still. Solar Energy 1990;44:43–9. [15] Adhikari RS, Kumar A. Estimation of mass transfer rates in solar stills. Int J Energy Res 1990;14:737–44.
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[16] Adhikari RS, Kumar A. Transient simulation studies on a multi-stage stacked tray solar still. Desalinisation 1993;91:1–20. [17] Reddy KS, Ravi Kumar K, O’Donovan TS, Mallick TK. Performance analysis of an evacuated multi-stage solar water desalination system. Desalination 2012;288:80–92. [18] Ahmed ML, Hrairi M, lsmail AF. On the characteristics of multi-stage evacuated solar distillation. Renewable Energy 2009;34(6):1471–8. [19] Michael Boxwell, Solar electricity handbook: Green Stream Publishing; 2016. [20] Kim Y, Seo T. Thermal performances comparisons of the glass evacuated tube solar collectors with shapes of absorber tube. Renewable Energy 2007;32:772. [21] Yueyan S, Xiaoji Y. Selective absorbing surface for evacuated solar collector tubes. Renewable Energy 1999;16:632. [22] Hunasbikatti PT, Suresh KR, Pratbima B, Sachdeva G. Development of desalination unit using solar still coupled with evacuated tubes for domestic use in rural areas. Curr Sci 2014;107:1683–93. [23] Khalifa AJN, Hamood AM. On the verification of the effect of water depth on the performance of basin type solar stills. Sol Energy 2009;83:1312–21. [24] Ali Samee M, Mirza UK, Majeed T, Ahmad N. Design and performance of a simple single basin solar still. Renewable Sustainable Energy Rev 2007;11:543–9. [25] Hashim AY, Al-asadi JM, Taha WA. Experimental investigation of symmetrical double slope single basin solar stills productivity with different insulation. J Kufa-Phys 2009;1:26–32. [26] Boukar M, Harmim A. Effect of climatic conditions on the performance of a simple basin solar still: a comparative study. Desalination 2001;137:15–22. [27] Adhikari RS, Kumar A, Sootha GD. Simulation study on a multistage stacked tray solar still. Solar Energy 1995;54:317–25. [28] Sivakumar P, Christraj W, Sridharan M, Jayamalathi N. Performance improvement study of solar water heating system. ARPN J Eng Appl Sci 2012;7(1):45–9. [29] Duffie J, Beckman W. Solar engineering of thermal processes. John Wiley & Sons; 2006. [30] Cooper P. The absorption of radiation in solar stills. Solar Energy 1969;12:333–46. [31] Incropera FP, DeWitt DP. Fundamentals of heat and mass transfer. New York: John Wiley & Sons; 1996. p. 912. ISBN-13:978–0471163541. [32] Eames I, Maidment G, Lalzad AK. A theoretical and experimental investigation of a small-scale solar-powered barometric desalination system. Appl Therm Eng 2007;27:1951–9. [33] Cramp N, Harrison SJ. A simplified model for sizing and characterizing a natural convection heat exchanger system. In: Proceeding of international building simulation performance association e Sim, May 3–6, 2016. [34] Govind J, Tiwari GN. Economic analysis of some solar energy systems. Energy Convers Manage 1984;24:131–5. [35] Badran AA, Al-Hallaq IA, Eval Salman IA, Odat MZ. A solar still augmented with a flat plate collector. Desalinisation 2005;172(3):227–34. [36] Fernandez J, Chargoy N. Multi-stage indirectly heated solar still. Solar Energy 1990;44:215–23. [37] Kumar PV, Kaviti AK, Prakash O, Reddy KS. Optimization of design and operating parameters on the year-round performance of a multi-stage evacuated solar desalination system using transient mathematical analysis. Int J Energy Environ 2012;3(3):409–34. [38] Reddy KS, Kumar KR, Vishwanath PK, Mallick TK, O’Donovan TS. Transient analysis of multi-stage solar desalination system. Int J Green Energy Energy Environ 2010;1(1):27–35. [39] Solar Energy Centre, MNRE Indian Metrological Department, Typical Climatic Data for Selected Radiation Stations, Solar Radiation Hand Book; 2008. [40] Average sunshine in a year in India, https://www.currentresults.com/ Weather/India/annual-sunshine.php [accessed 10.01.17].
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Original article
Adaptable wind/solar powered hybrid system for household wastewater treatment Akhilesh Soni a,1, Jacqueline A. Stagner b,⇑, David S.-K. Ting b a b
Mechanical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, India Turbulence & Energy Laboratory, University of Windsor, Windsor, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 20 September 2016 Revised 15 February 2017 Accepted 16 February 2017 Available online xxxx Keywords: Wastewater management Solar energy Vacuum evaporation Solar still Economic analysis
a b s t r a c t Sustainable and cost-effective water treatment systems are critical elements to developing nations. In India, the human population is escalating while the water availability is lagging behind. An adaptable, affordable, and sustainable wastewater treatment system powered by wind/solar energy is proposed based on proven theory and technology. A household in India is singled out to illustrate the workings of the proposed system, where the wastewater is recirculated through a hybrid of water purifiers powered by solar/wind energy. The system demonstrated here is specifically designed for small-scale applications, i.e., for a single household. The solar still has been divided into four stages. Partial vacuum is created inside the still so as to obtain boiling point temperatures of 70 °C, 67 °C, 62 °C and 50 °C in the four stages. Dhanbad, India 23.79°N, 86.43°E, with an average solar intensity of 850 W/m2 for 6 h a day, has been used for this study. A lumped parameter mathematical model was developed for this study. With an aperture area of 2.5 m2, the total amount of water distilled is found to be 43.3 kg/day. The system proposed is more efficient than existing systems as it is able to achieve efficiencies as high as 53%. The effect of wind speed on distillate output yield has also been discussed. Ó 2017 Elsevier Ltd. All rights reserved.
Introduction With modernization, urbanization and industrialization, the human race is advancing at one front and sacrificing at the other. The population is rising and the potable sources of water are diminishing. Global climate change and speedy growth in population have many cities under water threat [1,2]. With a few exceptions, water has always been a natural resource that mankind has taken for granted. The worldwide supply is not distributed evenly around the planet, nor is water equally available at all times throughout the year. Strategic reuse has gained importance over the last three decades as demand for water increased dramatically [3]. Reuse of wastewater for domestic and agricultural purposes has been executed to boost water usage efficiency since historical times. Initially, the emphasis was on reuse for agricultural and non-potable purposes but, with innovative advancements in technology, the domain for reused water has widened [4]. Conventional wastewater treatment processes are acceptable for non-potable water reuse applications (e.g. turfgrass, landscape, and agricultural ⇑ Corresponding author. E-mail addresses: [email protected] (A. Soni), stagner@uwindsor. ca (J.A. Stagner). 1 Globalink Mitacs Undergraduate Intern at Turbulence & Energy Laboratory.
irrigation) that do not require the wastewater to be treated to drinkable standards. Treatment of raw water from lakes, rivers, or wells is required in order to meet drinkability criteria. To do so, it requires work, and thus energy, which is yet another issue in rural areas. Access to modern energy is a social and economic priority to the rural population because of its direct socioeconomic and environmental benefits. People living in these communities also face financial problems. With this in mind, this work aims to design a self-sustainable stand-alone water treatment system. There is already an immense amount of research ongoing in the field of applications of renewable sources of energy in an offgrid community [5]. Kulworawanichpong et al. [6] suggested a design for a stand-alone solar photovoltaic system for a rural Tanzanian household. The application of the system presented here, which is driven by renewable sources of energy, is primarily for an off-grid community. The design philosophy is to use solar energy as a primary source, and wind energy as a secondary source, to power the water purification system. Most filtration techniques like reverse osmosis (RO) require the water to be pumped at high pressures, thus requiring a pump. Supplying power to the pump is another challenge in rural areas. Moreover, filtration techniques have a limitation of the minimum molecular size particles they can remove from water. With distillation, almost 99% of the particles can be removed, leaving only the
http://dx.doi.org/10.1016/j.seta.2017.02.015 2213-1388/Ó 2017 Elsevier Ltd. All rights reserved.
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A. Soni et al. / Sustainable Energy Technologies and Assessments xxx (2017) xxx–xxx
Nomenclature A AC AMC C Cp CT Cum d eT f fl F FR GRP hci hewi hfg Hi H0 I ki K Lc LH LT mc;i mei msi M MY
aperture area (m2) annual cost (Rs) annual maintenance cost (Rs) cost (Rs) specific heat (J/kg K) collector thickness (m) cost of unit mass of water (Rs/kg) diameter of pipe (m) edge insulation thickness (m) capital recovery factor frictional loss coefficient sinking fund factor heat Removal Factor glass reinforced polymer convective heat transfer coefficient (W/m2K) evaporative heat transfer coefficient (W/m2K) latent heat (J/kg) inlet head of cold water in solar collector outlet head of water from solar collector solar intensity (W/m2) thermal conductivity of insulation (W/m K) minor loss coefficient in Heat Exchanger length of piping of solar collector length of piping of Heat Exchanger insulation thickness of flat plate collector (m) mass of water condensed in the ith stage (kg/s) mass of water condensed in the ith stage (kg/s) mass of water in ith stage (kg) maintenance cost (percentage of annual cost) annual distillate yield (kg)
organic particles which have a boiling point less than water. To accommodate this, a carbon filter can be used in conjunction with the distillation system. The proposed system uses distillation to purify wastewater. Advancements in solar thermal energy technologies are continuing to drive down investment costs, increase ease of arrangement, and optimize direct coupling with other process systems. The system presented here is simple, reliable, affordable, sustainable, and effective. Status of water consumption in an Indian household People in some parts of India have adjusted their habits based on the supply such that they do not feel that more water is needed. This, in turn, creates hygiene and sanitation problems, resulting in several health consequences. The effects of consuming water of substandard quality cause a myriad of health issues. In many Indian cities, the supply is very erratic, in addition to the inferior quality of the water. Thus, there is dire need of a system that meets the daily requirements of water adequately. Fig. 1 shows the average consumption of water in liters per capita per day (LPCD) in major Indian cities [7]. Five major Indian cities have been chosen to conduct this study. In this paper, we design our system as per the average water consumption in these cities. The Indian government [8] has reported that approximately 60% of households in major Indian cities are water-deficient. The same report shows that 72% of those people are from lower income families. The average water consumption is approximately 91 LPCD. Fig. 2 depicts the distribution of water in a typical Indian household. Approximately 50% of the total water consumption is for bathing and toilet activities. 25% consumption is for house cleaning and washing clothes. The remaining 25% needs to meet the basic requirements for human consumption.
N p P Δp Qh QL Qu r Rs S UB UE UT Vw T ci T in T si DT K
g q gc go
number of years of operation partial vapor pressure (Pa) collector perimeter (m) pressure difference heat absorbed from solar water heater (W) heat leaked (W) useful Energy Gain (W) inflation rate Indian currency savage value (Rs) heat transfer coefficient for bottom losses heat transfer coefficient for edge losses heat transfer coefficient for top losses wind speed (m/s) condensing surface temperature (°C) Inlet temperature of water (°C) water bed temperature in ith stage (°C) temperature difference (°C) minor loss coefficient in solar collector collector efficiency of top stage (%) density of water (kg/m3) flat plate collector efficiency overall distill efficiency
Subscripts c solar collector h heat exchanger i inlet s still TC total cost (Rs)
History of water purification techniques Purifying water for reuse has been practiced for years [9,10]. Some of the traditional treatment methods are:
Filtration through winnowing sieve Filtration through cloth Filtration through clay vessels Clarification and filtration through plant material Jampeng stone filtration method
However, these methods are not reliable as each of them has its own limitations. The sieve cannot filter the fine suspended particles in the water. Cloth filters water only to a small extent. Pores
Kanpur
Hyderabad
Kolkata
Mumbai
Delhi 0
50
100
150
Fig. 1. Water consumption per capita per day (in liters) in distinctive Indian cities.
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Washing Utensils 14%
3
that incorporates solar still technology with wind energy has been designed for purifying water for a household.
Other 5% Bathing 28%
Working principle Cleaning House 8% Drinking & Cooking 7% Washing Clothes 18%
Toilets 20%
Fig. 2. Activity-wise water consumption distribution.
of clay vessels get blocked very often and cannot be used long term. As well, particles finer than pore size cannot be filtered through clay vessels. The last two methods mentioned above do remove certain types of particles but do not make the water suitably fit for human consumption. Biological slow sand filters have also been used but moving water through them requires energy. As well, reverse osmosis (RO) has been used widely but one of the largest challenges facing RO is the reliability of membranes that treat water having high concentrations of sparingly soluble salts, particulates, and organic matter [11]. Due to this, the application of RO is restricted as the chemical and energy costs are high. Multi-effect distillation (MED) integrated with concentrated solar thermal power (CST), called a concentrated solar still (CSS), has been demonstrated to treat agricultural drainage water at the Panoche Drainage District in the San Joaquin Valley [12]. Ahmed et al. [13] studied the characteristics of a multi stage solar still. Thus, water purification by distillation is not a totally new concept, but it has not been employed for domestic water treatment until now. Moreover, owing to their high costs and poor efficiencies, the existing distillation systems need to be improved for extensive effective applications. To the authors’ best knowledge, no system
The wind-solar hybrid system presented here is a selfsustaining system for pumping domestic wastewater from ground level using wind energy and making it reusable with the aid of solar energy. The system is designed in such a way so as to be installed in households without disturbing the existing plumbing systems. In most Indian households, the household water, excluding the sewage, collects at a common point so as to be disposed of to drains. The system presented here does not incorporate sewage waste, due to sanitary and environmental issues. This wastewater, excluding sewage, needs to be pumped from ground level to roof level where it is to be distilled in order to obtain pure water. Thus, there are two major tasks to be performed by the system, first, lifting water from ground level and, second, making it consumable by purifying it. Water is lifted from ground level with the help of a wind pump. If the wind does not blow for a couple of days, the storage tank may get empty. To counter this, a crank is attached to the wind driven rotor. When the wind does not blow, a handdriven wheel achieves the rotary motion of the rotor. The distillation system presented here is a combination of flat plate collector, tubular heat exchanger, and an evaporative condenser unit. The distiller consists of multiple reservoirs created by a stacked array of distillation trays that act as condensers for the tray below, as shown in Fig. 3. Each stage consists of an extruded cylindrical opening which is used to create the required partial vacuum. A valve is provided in each stage in order to regulate the pressure. There are multiple stages inside the distiller. Perfect sealing is maintained between the stages to prevent any vapor loss through the contact surfaces between the stages. Fig. 4 illustrates how sunlight captured by the glass cover is concentrated on a black surface, heating the water in the topmost chamber and, thus, evaporating it. This vapor is then condensed to form droplets. Fig. 5 provides a flow chart depicting the flow of water through the system Wastewater is fed into each stage from the tank. Each stage consists of an evaporator and a condenser surface.
Fig. 3. Schematic representation of distillation and evaporation/condenser unit.
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Fig. 4. Schematic of evaporation and condensation of water.
Wind Pump
Waste water stored in tank (Ground Level)
Waste water in storage tank at roofl evel
Water gets contaminated
Water used for domesc needs
Water moves to chambers at different stages
Pure water collects in a storage tank
Solar Radiaon
Water evaporates ,and condenses
Fig. 5. Flow chart depicting the flow of water in the system.
Table 1 Vapor pressure and associated boiling point of water in each stage. Stage
Vapor pressure (kPa)
Boiling point (°C)
First Second Third Fourth
31 27 20 18
70 67 62 50
There is a pressure gradient inside the chamber. The pressure decreases in each stage in order to obtain a lower boiling point in upper chambers. An economic and productivity analysis of the number of stages is shown in a later section where the number of stages are optimized to be four. The pressure in each stage is shown in Table 1. These temperatures have been used as the solar still works best in this range [14,15]. A pump is used to create a partial vacuum inside the distillation chamber. To achieve high temperatures, heat is supplied to the wastewater in the lowest reservoir via a heat exchanger through which water is circulated from the solar collector, passed through the heat exchanger tubes and then returned back to the solar collector again. Cold water gets heated and moves to the surface so that it can evaporate faster. Vapour generated in the lower stage condenses on the bottom surface of the intermediate stage, thus giving its heat to the water in the intermediate
stage. Vapour from the intermediate stage condenses on the upper stage, transferring its latent heat of condensation to the water in the upper stage. Water in the top-most reservoir, which is painted black to maximize radiation capture, is also heated directly by solar radiation. In the intermediate stages, heat transfer apart from radiation and convection, occurs by evaporation and condensation, thus utilizing the latent heat of condensation and improving the system’s efficiency. Radiation and convection constitute minor energy transfer between the stages and, hence, have been ignored. The number of stages has been decided by taking the cost factor into consideration. It has been observed that an increase in distillate yield corresponds to an increase in the number of stages; however, fractional increases in the distillate decrease with the addition of every stage [16]. An economic and productivity analysis of the number of stages is shown in a later section. Also, Reddy et al. [17] showed that the gap between stages should be 10 cm. Evacuated solar stills have already been demonstrated in the past but no one has used a pressure gradient along the stages to get a variable evaporating point of water in the stages. The suggested concept can improve the efficiency of the existing systems to a great extent. The only problem associated with the working of the still is that the plates need to be cleaned daily. To prevent algae and scaling on inner black surfaces, a still would be required to be dried completely once a week. Bleaching or chlorination can also be used to prevent algae formation. Ahmed et al. [18] experimentally investigated the multi stage evacuated solar still. The productivity of this new system was found to be about threefold greater than the maximum productivity of the basin type solar still. While designing the above system, some keys points have been taken into consideration: 1. Simple, appropriate technology has been adopted, as an overly complex system would be challenging due to its maintenance problems. 2. The system is flexible as per the demand of the people in a particular area i.e. its cost will vary as per the quantity of water required. 3. The system operates in a sustainable manner. This means being funded, owned, and operated by the individuals using the water supply. 4. The system is independent of any external power source. The basic principle used is reducing the air pressure in order to reduce the boiling point of water.
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Vacuum pump Design methodology The design parameters are the most important factors that affect the productivity of the proposed system and can be easily controlled, developed and designed to improve productivity. These parameters include evaporation area, water depth, cover condensing angle, and insulation. The individual components of the proposed system include:
Wind pump A conventional method of pumping water from ground level to roof level where the solar distillation system is located has been employed. A wind-energy driven piston pump is to be used. The wheel, having wide blades, is rotated by the wind, and is attached to a shaft by long arms. The shaft has small pinion gears at the other end, inside a small gearbox. The pinion gears drive the bull gears, which move pitman arms. The pitman arms push a sliding yoke up and down, above the bull gears. The yoke lifts and drops the pump rod and, hence, the necessary motion required for the working of the piston pump is achieved.
Tempered glass plate Tempered glass lets only high-energy radiation to pass and this high-energy radiation is captured. The tempered glass surface also serves as a condensing surface for the top reservoir. It is always at a temperature less than the water in the reservoir. Due to the slant, droplets slide to one side and deposit all condensate into the collector. A thermally treated low iron glass has been used to enhance its toughness. The inclination angle of the glass plate is fixed as per the location. For this case, the location used is Dhanbad, 23.79°N, 86.43°E. The optimum inclination angle for the month of April is found to be 26° [19].
The vacuum pump used here is simply a modified cycle pump as shown in Fig. 6. The vacuum pump is to be manually operated by a person before the still can be used. Three one-way valves, one each on the inlet, piston and exit are installed. Its working is similar to that of a 2-stroke engine. The inlet joins to the distillation chamber. The downward stroke of the piston draws air from the distillation chamber into the upper part and compresses air in the lower. Both piston and exit valves are closed during this period. As the air pressure in the lower part exceeds atmospheric pressure, the exit valve opens to discharge this air into ambient. During the upward stroke, the piston valve is kept open while the inlet and exit valves close. Thus, air is transferred into the exit chamber. This process continues until the required pressure is achieved.
Heat exchanger A small solar thermal collector is used to heat the water in the bottom-most chamber. The thermosiphon process, represented in Fig. 4 achieves this. The bottom most chamber is kept at a level higher than the thermal solar collector. Use of an evacuated thermal collector (ETC) is a way in which heat loss to the environment, inherent in flat plates, can be reduced. Since heat loss due to convection cannot cross a vacuum, it forms an efficient isolation mechanism to keep heat inside the collector pipes [20]. In this case, the vacuum is created between two concentric tubes. The water piping in an ETC is surrounded by two concentric tubes of glass with a vacuum in between that admits heat from the sun (to heat the pipe) but which limits heat loss back to the environment. The inner tube is coated with a thermal absorbent [21]. The tubes used are made from borosilicate [22]. The water in the tubes lying on the surface of the collector gets heated, and moves up to the bottommost reservoir, and cold water from there comes down in the tubes. The borosilicate tubes are connected to copper tubes inside the bottom stage of the still so as to lose heat to the water present
Fig. 6. Mechanism of vacuum pump.
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in the lowermost stage. Due to the thermosiphon process, the water in the bottom reservoir gets heated and increases the rate of evaporation. Condensing cover/surface angle The surface must be at such an angle so as to ensure that sufficient condensing water runs smoothly to the collector plate without forming large droplets, which might fall back into the distillation chamber instead of the condensate channel. The angle should be such that it provides a cool surface for water vapor condensation, maximizes the absorption of solar radiation, and reduces heat losses to the surroundings. Various studies [23–24] suggest that the optimum value for this lies in the range of 30–35°. Hence, an angle of 32° is used in the present work. Insulation Proper insulation is necessary to prevent the release of useable thermal energy to the environment. The side and bottom walls are insulated to prevent wastage of heat. Numerous investigations [25,26] propose the usage of glass wool or wooden boxes insulated with sawdust. In the model presented in this paper, glass wool has been used. The heat conductivity coefficient for it is 0.04 W/m K.
4. All heat from the collector is transferred to the water in stage 1. Fig. 7 shows the hourly solar intensity variation as recorded on 15th April 2016 using a digital solar pyranometer. Extreme summers in India are during April-June. Hence, the solar intensity data has been collected during the same period. The average solar intensity is found to be 850 W/m2 during the six-hour period. The data have been recorded in Dhanbad, 23.79°N, 86.43°E, a city located in the state of Jharkhand, India. Analysis of solar still The basic model of the solar still is quite similar to the one presented by Adhikari et al. [27]. The energy flow rate through the system is presented in Fig. 8. A static water feed system has been adopted. Thus, while feeding the water to the still, the entire mass of water is supplied before the still starts functioning.
m_ e ¼ m_ c
ð1aÞ
dmsi ¼ m_ei dt
ð1bÞ
Energy balance equation for first or bottom stage:
ms1 C p Nozzle The recirculating loop and still are at a pressure less than atmospheric pressure. Household drains work at atmospheric pressure. Hence, to connect the two, a constricting nozzle is used. In the common line between feeder tank and the solar still, a constricting nozzle is used. The ratio of diameters at the two ends in the first and subsequent stages is (1:7.5), (1:7.7), (1:8) and (1:8.4), respectively. The inlet water head is considered to be 2 m for all stages. Mathematical model The lumped parameter mathematical model has been developed to conduct this study. Fig. 3 shows the fluid from the heat exchanger transfers its workable heat to the impure water in the first stage. The temperature in the first stage increases and it loses its heat to the second stage through evaporation. To develop the mathematical model for the system, the following assumptions have been made 1. Due to small temperature differences between adjacent stages and the absence of non-condensable gases, convection and radiative heat transfer are ignored [18]. 2. The amount of water evaporated and distilled in the nth stage is equal i.e. mc;i = me;i 3. The distillate leaves the chamber at the condensing surface temperature.
dT s1 ¼ Q_ h m_c1 C p T c1 m_e1 hfg1 Q_L1 dt
Q_ h ¼ IAg
ð3Þ 2
Area A of the thermal solar collector plate is set to 2.5 m so as to meet the required demand of water, as an area less than this was not meeting the required demand. Energy balance equation for consecutive stages,
ms2 C p
dT s2 ¼ m_e1 hfg1 mc2 C p T c2 m_e2 hfg2 Q_L2 dt
ð4Þ
ms3 C p
dT s3 ¼ m_e2 hfg2 m_c3 C p T o3 m_e3 hfg3 Q_L3 dt
ð5Þ
ms4 C p
dT s4 ¼ IAg þ m_e3 hfg3 m_c4 C p T o4 m_e4 hfg4 Q_L4 dt
ð6Þ
In the fourth stage, term IAgð3600Þð6Þ indicates the solar energy due to direct heating. This solar energy is captured using the tempered glass plate. Collector efficiency can be as high as in the range of 0.5–0.6 for zig-zag type solar collectors [28]. However, the collector efficiency is a strong function of solar intensity and wind speed. The following set of equations is used to study the effect of wind speed on collector efficiency [29].
RQ
gc ¼
dt RI Ac dt
ð7Þ
u
Q u ¼ F R ½I U L ðT i T a Þ
Intensity (W/m2)
gc ¼ F R fa U L
1200 1000 800 600 400 200 0
ð2Þ
FR ¼
Ti Ta I
ð8Þ ð9Þ
mcp Ac U L 1 exp Ac U L mcp
ð10Þ
U L¼ U T þ U B þ U E 8
10
12
14
Time (Hours)
16
0 UT ¼ @
11 1A h iþ þ T pm T a hw N
C T pm
Nþf
ð11Þ
rðT pm þ T a Þ T 2pm þ T 2a 1
ep þ0:00591hw
þ
1þf þ0:133ep
eg
1
ð12Þ
Fig. 7. Hourly variation of solar intensity.
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Fig. 8. Flow chart showing energy flow through the system.
hw ¼ 5:7 þ 3:8V w
UE ¼ UB ¼
PC t
ð13Þ
et ki
ð14Þ
Ac ki Lt
ð15Þ
Condensing surfaces for the nth stage and waterbed for (n + 1) th are considered to be at identical temperatures, Mass flow rate of the vapor evaporated is given as
m_ei ¼
ðT si T ci Þ hewi A hfgi
ð16Þ
psi pci T si T ci
The driving force needed to circulate the water in the solar collector is provided by the buoyancy driven natural convection thermosiphon, thus, eliminating the need of an electric pump. The only significant pressure losses occur in the solar collector and heat exchanger in the system. These pressure losses are to be overcome by the thermosiphon process. The net hydrostatic pressure across the heat exchanger, i.e. driving force of the thermosiphon is given by [33]:
Dpnet ¼ gðqHo qHi Þ þ Dploss
ð17Þ
Dploss ¼ Dph þ
Convective transfer coefficient hci is given as
" hci ¼ 0:884 ðT si T ci Þ þ
ðpsi pci ÞðT si þ 273Þ
#13 ð18Þ
268:9 103 psi
The latent heat and specific heat of water are also a function of temperature [31,32].
hfg ðTÞ ¼ 1000 ½3161:5 2:4075ðT þ 273Þ 5
ð19Þ 7
C p ðTÞ ¼ 1000 ½4:2101 0:0022T þ 5 10 T 3 10 T
ð20Þ
Overall thermal efficiency for the multi-stage evacuated solar desalination system (%) is defined as the ratio of total heat content output from all the stages by the cumulative daily distillate yield of all the stages to that of the total heat content supplied to system throughout the day. The total heat content input is through the flat plate collector and direct heating of water in the top chamber. The overall efficiency of the yield is calculated using the following formula.
P4
m h
go ¼ _ 1 i fg ½Q h þ IA 3600 6
ð22Þ
The inclination angle of solar collector is 26° as already discussed in Section 3.2. Hence, Hi = 0 and Ho = 2Sin (26°),
Evaporative heat transfer coefficient hewi is given as [30]
hewi ¼ 16:273 103 hci
Hydraulic analysis of solar collector
ð21Þ
Dph ¼
qv 2 2
fl
qv 2 2
s þ fl
LH þK D
Lc d
ð23Þ
ð24Þ
The friction factor for laminar flow is defined as [31]
fl ¼
64 Re
ð25Þ
It is not possible to predict the exact pressure losses theoretically. Moreover, an in-depth analysis of the pressure drop is beyond this study. However, an approximation can be done for the pressure losses occurring. The friction loss coefficient is approximated from the Moody chart, assuming the flow to be laminar. Cramp and Harrison [33] showed that the thermosiphon drives the fluid with a flow rate in the range of 0.2–1 LPM. The minor loss coefficient for the return bend in the heat exchangers can be assumed to be 0.2. Thus, based on the assumed value, the total pressure head loss is found in the range of 0.003–0.005 m. The low value is attributed to the low driving force of the thermosiphon which leads to low velocities of fluid flow in the pipes.
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summarizes the various cost parameters with cost given in Rs (1 Rs = 0.015USD).
Economic analysis of proposed system The distillation system consists of various metallic plates stacked in a way so as to form different stages inside the still. The utilization of the proposed system as a source of distilled water for commercial and household purposes should be determined by its economics. An economic model also helps to optimize the number of stages. The better economic return on the investment depends on the production cost of the distilled water and its applicability. To take into consideration time value of money, capital recovery factor (f), fund sinking factor (F), and the inflation rate (r) have been employed in the analysis. The annual cost of the evacuated distillation system coupled with a solar flat plate collector can be written as follows: Total Cost = Cost of still + Cost of solar collector + cost of vacuum pump + Cost of heat exchanger
C TC ¼ ðC s f s þ M s C s Ss F s Þ þ ðC c f c þ M c C c Sc F c Þ þ ðC p f p þ Mp C p Sp F p Þ þ ðC h f h þ M h C h Sh F h Þ f ¼
F¼
rð1 þ rÞN
ð27Þ
ð1 þ rÞN 1 r
ð28Þ
ð1 þ rÞN 1 Annual Cost of system Annual distillate yield
Cost of unit mass of water ¼
C um ¼
ð26Þ
C TC MY
ð29Þ
The life cost analysis has been carried out by considering the life of the proposed system as 20 years. The quantity of material used in the fabrication and the cost breakdown of the proposed system are given in Table 2. The scrap value in developed countries is almost negligible, whereas the scrap value, inflation rates etc. in developing countries like India are higher than the developed countries. The scrap value of iron, copper and aluminium will also increase with time due to inflation. The expected rates of these materials have been considered at an inflation rate of r = 3.6% in the Indian market. The AMC (annual maintenance operational cost) of the solar still is required for regular filling of water, collecting the distilled water, cleaning of the glass cover, removal of impurities deposited and maintenance of the pump. As the system life increases, the maintenance on it also increases. Therefore, 10% of net present cost [34] has been considered as maintenance cost. Table 2
Results Distill thermal analysis A mathematical model has been developed for the abovepresented system and operated for 6 h a day under a constant solar flux of 850 W/m2. Inlet mass flow rate will have a significant effect on the productivity of the distillation system, as shown in Fig 9. Results in Fig 9 are shown for a distillation system consisting of four stages as it can be seen from the plot of productivity vs number of stages (Fig 10) that increasing the number of stages beyond four does not have a significant increase in still productivity. It can be observed from Fig 9 that the productivity increases initially with increase in mass flow rate, attains a maximum at a mass flow rate of approximately 50 kg/m2/day, and then decreases with further increase in mass flow rate. This is due to the storage effect which results in a lesser amount of latent heat being available for reuse. Hence, it is recommended to use mass flow rate of 50 kg/m2/day (or a water depth of 5 cm) Variation of the waterbed temperature of different stages with time is represented in Fig. 11(a). Linear variation is attributed to the constant solar flux. At wind speed of 3 m/s, the difference in the water bed and condensing surface temperatures in the four stages are found to be 3 °C, 5.1 °C, 4.7 °C and 14.2 °C, respectively. The variation of waterbed temperatures at wind speed of 6 m/s is shown in Fig. 11(b). It can be seen that the temperature difference between the consecutive stages is reduced drastically. This leads to poor yield. Fig. 12(a) indicates the distillate yield for the given span of time of 6 h at different wind speeds. Distillate product depends on the waterbed temperatures, the temperature difference between the condensing and evaporating surfaces and on the wind speed. As the wind speed increases, thermal losses severely increase leading to poor yield. Similar behaviour is observed with still efficiency, as shown in Fig. 12(b), which monotonically decreases with increase in wind speed. It is also observed that distillate product increases on going from the bottom to the top stage except for the third stage as shown in Fig. 13. This is because of the low temperature difference between the condensing and evaporating surfaces in the third stage. Water in the uppermost chamber, apart from the latent heat of condensation, is heated directly from solar radiation. Hence, the waterbed temperature for the fourth stage and condensing temperature for the third stage is comparatively higher than expected.
Table 2 Cost breakdown of components of the proposed system. Cost (Rs)
Salvage value (Rs) at inflation rate 3.6%
GRP Body (5 mm thick) Iron Stand Metal Sheet (Aluminium 1.5 mm thick) Glass Cover (for still) Vacuum Pump Solar collector Heat Exchanger (Copper Tubes of ID = 5/800 ) Glass wool (50 mm thick) Nozzles Pressure Gauge
5 m2 15 30.75 kg
500/m2 70/kg 170/kg
400 500 4000
2.5 m 1 1 9.4 kg
1000/m 1000 20,000 500/kg
4.8 m2 4 4
220/m2 100/piece 600/piece
– – 15,000 4110
Sll Yield (kg/day)
Quantity
2
Wind Speed=3m/s
Wind Speed=5m/s
Wind Speed=7m/s
50
Component
2
Wind Speed=1m/s
40 30 20 10 0 0
– – –
50
100
150
Mass flow rate (kg/m²/day) Fig. 9. Effect of mass flow rate on output yield for 4 stages.
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Wind Speed=1m/s Wind Speed=1m/s
Wind Speed=7m/s
50 40
50 40
Sll Yield (kg/day)
SllYield Yield(kg/day) (kg/day) Sll
WindSpeed=7m/s
Wind Speed=5m/s
6050
a
Wind Speed=3m/s Wind Speed=3m/s
Wind Speed=5m/s
40 30 30 20 20 1010 00 10
3 50
5
100
7
Number of stages
150
30 20 10
Fig. 10. Effect of number of stages on output yield.
0
b Stage 4
Stage 3
Stage 2
Stage 1
3
5
7
Wind Speed (m/s) 60 50
Sll Efficiency (%)
a
70
Temperature (°C)
1
60 50 40
40 30 20 10
30
0 20
1 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Operang Time (hours)
b
Stage 4
Stage 3
Stage 2
Stage 1
70
Temperature (°C)
3
5
7
6
60 50 40 30 20
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
Operang Time (hours) Fig. 11. (a) Waterbed temperature variation at Vw = 3 m/s. (b) Waterbed temperature variation at Vw = 6 m/s.
Wind Speed (m/s) Fig. 12. (a) Variation of output yield with wind speed. (b) Variation of distill efficiency with wind speed.
Badran et al. [35] investigated a basin still with a 1 m2 area coupled with a 1.34 m2 flat plate collector and a maximum production capacity of 4.6 kg/day. Fernandez and Chargoy [36] studied experimentally and theoretically, a multi-stage desalinisation system with a W-shaped condensing surface. During the experiment, a maximum of 2.5 kg of water per collector was produced when operated at atmospheric pressure. Thus, the solar still described in this paper is quite efficient compared to the existing ones, as it uses only a single collector and produces 43.41 kg of water for an operation of 6 h a day, considering wind speed to be 1 m/s. The maximum still efficiency, 53.2%, is observed at a wind speed of 1 m/s. This is greater than the maximum still efficiency for fresh water in the work conducted by Kumar et al. [37] where they obtained a maximum efficiency of 50.5% for fresh water. It should be noted that Kumar et al. have considered the pressure to be as low as 0.03 bar which is quite low compared to our system where minimum pressure is 0.18 bar. The work done by the vacuum pump, and hence, the power input to the system, is less in the system presented in this paper. Study of the year-round transient analysis of a multi-stage evacuated solar desalination system was done by Reddy et al. [38] and the results show that the system
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Vw=1m/s
Vw=3m/s
Vw=6m/s
20 18
Output Yield (kg/day)
16 14 12 10 8 6 4 2 0 1
Stage
2
3
4
Fig. 13. Variation of output yield in different stages with wind speed.
Table 3 Average solar radiance in various Indian cities. City Solar radiance (MJ/m2/day)
Kanpur 17.68
Hyderabad 20.34
produces a maximum distillate yield of 16.4 kg/m2/day at an average efficiency of 45%. Thus, the system presented here is more efficient than the existing ones. The solar energy utilised by the presented system is 850 W/m2 for 6 h a day i.e. 18.36 MJ/m2/day. As per the data of Solar Energy Centre, MNRE Indian Metrological Department [39], the average solar radiance in various cities is found in Table 3. Hence the system presented is suitable for being used across the country.
Table 4 Dimensions and capital cost of proposed system. Length Width Height (lower side) Height (upper side) N (number of years) Initial Investment Salvage Value (after 20 years) CRF, f SFF, F AMC AC
1.6 m 1.4 m 0.5 m 1.2 m 20 Rs 40,834 Rs 24,010 0.071 0.035 Rs 4100 Rs 6143
Wind Speed=1m/s
Wind Speed=3m/s
Wind Speed=5m/s
Wind Speed=7m/s
Cum (Rs/kg)
2.4 1.9 1.4 0.9 0.4 2
4
6
Number of stages Fig. 14. Effect of number of stages on per unit cost of water.
Mumbai 18.25
Delhi 18.25
Economic analysis Economic analysis has been done for the proposed system based on the model developed in Section 4.2. Dimensions and capital cost of the system are shown in Table 4. Annual yield is calculated assuming the number of sunny days to be 300 in a year [40]. The annual cost of the proposed system is approximately Rs 7384. This provides a unit cost of water in the range of 0.5–1.2 Rs/kg for the wind speed range of 1–5 m/s. It is more economical in comparison to the bottled water available in India which costs approximately 10–12 Rs/kg for consumers. The combined analysis of productivity and economics of the system is studied by seeing the variation of cost per unit mass of water produced as shown in Fig 14. It can be seen that Cum (cost per unit mass of water produced) decreases with increasing number of stages until it attains a minimum value when the number of stages is equal to 4. Hence, any further increase in the number of stages after 4 is not justifiable from an economic point of view. The increase in productivity is not at par with the increase in cost of the system. Variation of the unit cost with wind speed is also shown in Fig 14. Conclusions
2.9
0
Kolkata 16.17
8
The water purification system presented above utilizes two modes of renewable energy, solar and wind. Wind energy is used to drive a vacuum pump which reduces the air pressure inside the system. The number of stages has been optimized to be four, as any further increase in the number of stages is not justifiable from an economic point of view. The vapor pressure maintained in the successive stages of the still are 31 kPa, 27 kPa, 20 kPa and 18 kPa. Constricting nozzles are used to connect the household drain with the recirculating loop and still. Solar energy is used to heat the water lying in the chambers of the still. Solar energy is transferred to the system from the bottom chamber via a heat exchanger, and from the top by direct heating of the water. The fresh water production capacity of the investigated solar-collector four-stage solar still, when operated for 6 h a day at a constant flux of 850 W/m2 , is found to be 17.4 kg/m2 /day at Vw = 1 m/s, which is greater than conventional multi-stage solar stills [17,32,35–37]. The annual cost of the system is approximately Rs7450, with the per unit water cost in the range of
Please cite this article in press as: Soni A et al. Adaptable wind/solar powered hybrid system for household wastewater treatment. Sustainable Energy Technologies and Assessments (2017), http://dx.doi.org/10.1016/j.seta.2017.02.015
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Please cite this article in press as: Soni A et al. Adaptable wind/solar powered hybrid system for household wastewater treatment. Sustainable Energy Technologies and Assessments (2017), http://dx.doi.org/10.1016/j.seta.2017.02.015