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ENGeniousdesign to inspire...

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Solar Water Purifier

Introduction

Distillation is one of many processes available for water purification, and sunlight is one ofseveral forms of heat energy that can be used to power that process. Sunlight has theadvantage of zero fuel cost but it requires more space (for its collection) and generally morecostly equipment.

To dispel a common belief, it is not necessary to boil water to distill it. Simply elevating itstemperature, short of boiling, will adequately increase the evaporation rate. In fact, althoughvigorous boiling hastens the distillation process it also can force unwanted residue into thedistillate, defeating purification. Furthermore, to boil water with sunlight requires morecostly apparatus than is needed to distill it a little more slowly without boiling.

Many levels of purification can be achieved with this process, depending upon the intendedapplication. Sterilized water for medical uses requires a different process than that used tomake drinking water. Purification of water heavy in dissolved salts differs from purificationof water that has been dirtied by other chemicals or suspended solids.

For people concerned about the quality of their municipally-supplied drinking water andunhappy with other methods of additional purification available to them, solar distillation oftap water or brackish groundwater can be a pleasant, energy-efficient option.

Solar distillation systems can be small or large. They are designed either to serve the needs ofa single family, producing from to 3 gallons of drinking water a day on the average, or toproduce much greater amounts for an entire neighborhood or village. In some parts of theworld the scarcity of fresh water is partially overcome by covering shallow salt water basinswith glass in greenhouse-like structures. These solar energy distilling plants are relativelyinexpensive lowtechnologysystems especiallyusefulwheretheneedfor small plantsexists

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Prelude

There are four possible ways of purifying water for drinking purpose:

1. Distillation2. Filtration3. Chemical Treatment4. Irradiative Treatment

Considering the areas where the technology is intended to be used we can rule out few of

the above mentioned methods based on the unavailability of materials or costs. Chemicaltreatment is not a stand alone procedure and so is irradiative treatment. Both can act onlyremove some specific impurities and hence can only be implemented in coordination withother technologies.

This analysis leaves us with two methods Distillation and Filtration. By weighting thepositive and negatives of both the methods we decided to go by the first one. The mostimportant considerations were that of complexity, higher maintenance and subsequent costs

coupled with need of other sophisticated supporting equipments.

Finally we decided to go by distillation method owing to the following benefits:

1. It produces water of high quality.2. Maintenance is almost negligible.3. Any type of water can be purified into potable water by means of this process4. The system will not involve any moving parts and will not require electricity to

operate.5. Wastage of water will be minimum unlike reverse osmosis in which almost 30% of

the loaded water flows out in form of unusable water that can only be used for toiletor other cleaning purposes.

Current Designs & their problems

We will use a conical multi stage solar still design. The basic improvement that we suggest isto use low pressure inside the distiller. This will greatly affect the rate of evaporation andhence rate of condensation on the cooler surface. As we have already stated above the rateof evaporation is dependant on

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4. Most of the losses can be attributed to heat transfer losses.5. Thermal losses are mostly in form of conduction and convection and very little by

radiation owing to low temperatures. So we can assume radiative losses to benegligible.

Also the cost of a solar still which produces reasonable amount of purified water is high. Thecost of water produced by the still is high. This fact attributes to almost negligiblepenetration of solar stills in Indian villages.

While perusing and pondering about the ways to reduce costs the first factor that comes to

mind is why not increase the efficiency. But as we all know this is much easier said thandone. After giving it a considerable thought we came up with a design that can greatlyimprove the efficiency of a solar water distillation system by minimizing thermal losses.

The equations governing the heat transfer rates are:a. Conduction

Q = - k A dT / dx

b. ConvectionQ = h A ( Tsurface- Tambient)

Both the losses are greatly dependant on the area and temperature difference between themedium i.e., water and ambient. Hence if we can reduce temperature of the whole system wecan reduce the heat loss and hence improve the efficiency.

But reducing operating temperature will come at the cost of lower rated of evaporation andconsequently lower rated of condensation leading to slower distillation. So now the problemboils down to increasing the rated of evaporation at lower temperature.

(Mass loss rate) / (Unit area) = (Vapor Pressure - Ambient Partial Pressure) * sqrt ((Molecular Weight)/(2*pi*R*T))

(from Zemansky and Dittman, Heat and Thermodynamics, McGraw H ill, copyright dates from 1937 to1981).

The Vapor Pressure of a liquid at a given temperature is a characteristic property of thatliquid. Vapor pressure of a liquid is intimately connected to boiling point.

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R = universal gas law constant = 8.31 J/mol-K = 8.31 X 10-3Kj / mol-K

P1and P2= vapor pressure at T1and T2

T1and T2= Kelvin Temperature at the initial state and final state

At 373K the pressure is 1 atm.

We all know that boiling takes place when the ambient temperature equals that of the vaporpressure of the liquid. This means that we can increase the rate of evaporation by reducing

the pressure of the vessel. This will ensure higher rates of evaporation even at lowtemperatures.

Here we will present our design.

Design

As stated already, we need to reduce the working pressure inside the distiller to increase therate of evaporation at lower temperatures and hence increase efficiency. One moreadditional feature in the distiller that we are proposing is that it would use the latent heatwhich is released during condensation to heat up the water at lower temperature. This isachieved by using an innovative staged still design.

The basic arrangement of the system can be described as follows

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Before proceeding further we would like to mention a few assumptions that we made for thedesign:

1. The system will serve a family of 5. The number is assumed to be the average size ofa rural household. Data has also been confirmed with the census data.

2. Average requirement of water per person in a house is assumed to be around 1.5liters/ day. This gives the total water consumption to be around 7.5 liters/ day. Alsoconsidering the requirement for cooking we roughly evaluate net water consumptionper household is around 30 liters/day.

3. The solar constant equals 1.3 kW/m2but owing to losses incurred while passingthrough atmosphere we can consider the solar irradiation to be 1kW/m

2

.

Some other important data required for design is given below.

Specific heat of water = 4.2 kJ/ kgLatent heat of vaporization = Latent heat of condensation = 2260 kJ/ kg

In the design we have incorporated a pump which is a simple manually operated vacuum

pump to reduce air pressure inside the distillation chamber. We are looking at operatingconditions of about 60 oC to ensure low heat transfer losses.At this temperature thevapor pressure of water is 20 kPa. So we need to operate the pump to reduce the pressure tothis value and then leave it in the sun for distillation. This will ensure boiling of water insidethe distiller as soon as the temperature reaches 60 oC, which is pretty low and easilyachievable by using simple designs. Actually we plan to use a slight modification of theregular cycle pump that is available everywhere.

For the purpose of design we will assume a very low conversion efficiency of around 20%.This will ensure that water is available in excess and also when there is not ample sunlight.Given the highly erratic supply of sunlight which depends greatly on weather conditions wehave to over design it for high factor of safety in this case 2. In real life we expect theefficiency to be higher than 40%.

The first step in design is to calculate the aperture area.

Aperture Area= Energy required for distillation of 30 liters of water / Solar energy

available per m2* conversion efficiency

= (30 kg/day * 4.2kJ/ kg oC * (60-30) oC)/ (1 kW/ m2 * 3600 s/hour *6hours/day)*(0.2)

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Specifications of Distiller Design

Fig.2. Isometric V iew of the Solar Water Disti ller

Design Specifications

The Distiller is made of the following parts:

1. Tempered Glass PlateGlass has the property of selectively allowing only the higher energy radiation topass through and blocking the longer wavelengths. This particular property aidsin the distiller as it captures most of the incoming higher energy radiation butdoes not allow it to radiate back. This also serves as a condensing surface being

t t h it ill l b t l t t th th t

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before use to prevent any sort of oil or volatile contents. The side reservoir wallsare insulated and the bottom is conducting.

Fig.3. A cutaway view of the Distiller

3. Staged Water ReservoirsBelow the topmost reservoir lie two more stages of water reservoirs and side

walls insulated and the bottom wall conducting. The shape is such that allcondensate moves down to a point and drops into the collecting tubes which runall the way down and out of the distiller into storage.

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Fig. 4. Structure of the multi-stage reservoirs also showing the lowermost reservoir and

the collector tubes.

4. Lowermost ReservoirThe lowermost reservoir is designed differently than the above 3. We need tomake it different so that evaporation takes place from this and then condenseson the first stage and delivers heat to it. Then it repeats with the second stageand so on. For achieving higher temperatures we have added tubes coated withblack joining the deeper water level to the surface water. This will ensure that the

cooler water at bottom is heated up by absorbing solar radiation and added tothe surface where it can evaporate faster. There is no need of a pump formaintaining the circulation natural convection induced because of densitydifference will take care of that.

5. PumpTh d i di l ith f i difi ti W

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Fig. 5. Design of the pump, modified from an ordinary cycle pump.

Design of the pump

The working is fairly simple The inlet joins to the distiller and when the pumpis pushed down the air is drawn from inside the distiller (a) is open and other twoare closed. When pressure inside lower part increased beyond atmospheric theexit valve will open to discharge air into ambient. While pulling back the pistonvalve will open and other two will close thus transferring the air into the exitchamber from inlet chamber. The process will continue till the required pressure

is achieved.

Assembling and manufacture

Fabrication of the whole unit is pretty straight forward and involves metal cutting, welding,glass cutting, sealing, painting and drilling. All these processes can be done at any localworkshop using simple machines lathe, drill, welding, milling etc.

The steps in the process of assembling are outlined as follows:

1. The outer box will be fabricated first. It will be made of double wall and will be filledwith glass wool to provide insulation.

2. The stages will be fabricated second the collector holes will be made at the time off b i i i ll h ill b bl di id h i

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6. Finally after the whole procedure is over the glass cover is installed on top. This isthe last step as glass is the most fragile material we are using for the design.

1. fabricate the outer casing

2. fabricate the inner stages

3. fabricate the collector tubes

4. Provide holes for transport of distilled waterand saline water

5. Provide sealing

6. Finally put the glass cover

Fig. 6. A ssembling procedure outline

Working

The working is based on phenomena of evaporation on absorption of heat from sunlightand subsequent condensation of vapor on contact with the cooler walls.

1. Pump is used to reduce the pressure inside the chamber to around 20 kPa to ensureboiling takes place at 60oC.

2. The sunlight is captured by the glass cover and absorbed by the black floor of thetop reservoir.

3. Water gets heated up and evaporates thus making the air saturated with vapor.4. This water also supplies to the two stage troughs by means of tubes running down

and flow is controlled by the level control valves.5. When this saturated air comes in contact with cooler glass cover it condenses and

forms droplets. Due to combined effect of inclination and gravity the droplets moveand drop into the collector. This water is conveyed to the storage from here.

6. In the lowermost reservoir the water is heated up by means of the collector tubes this serves dual purpose, first it introduces turbulence in water and hence enhancesevaporationalso it takeswater fromlower cooler layersandconstantly heats it up to

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8. The same process as described in 7 repeats in the next two stages. This ensuresmaximum utilization of the captured heat and finally all the water falls into a

common collector system which is so designed to minimize any sort of losses.

Materials

1. The side and bottom walls need to be insulated. This can be achieved by using multi-layered insulator. Glass wool will be sand-witched between two metallic plates. Thiswill ensure negligible heat loss to the surroundings.

2. The main frame is composed of steel owing to its corrosion resistance, low weight,long life and easy cleanability.3. The outside of the complete distiller is coated with carbon black to increase

absorption of radiation.4. The cover on the top is made of tempered glass so that the birds cant see their

reflection and hence avoid nuisence.

Usage

The usage will involve following steps:-

1. The user will fill the reservoir tank with water that needs to be purified.2. Attach the vacuum pump to the apparatus and operate it till the pressure inside

drops to around 20kPa. Then remove the pump.3. Attach the pipe for collecting the purified water.4. Then he will lift the whole device up a few meters above the ground (around 2.5 m).This will ensure no shadows fall on the apparatus during any part of the day.5. Leave it there till evening.6. Remove the purified water for use.7. Remove the plates for cleaning and dispose of the remaining water.8. Ready for use on the next day.

Maintenance

1. The only maintenance that the device requires is replacement of the glass in case ofaccidental breakage.

2. Daily cleaning of the plates is required.3. Sometimes the pumpwill need tobeoiledandserviced.This cost might occur once

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Cost of crushed hay and sawdust = Almost free

Cost of carbon black paint = Rs 20

Cost of tempered glass = Rs 200

Cost of insulation and sealing = Rs 80

Cost of the pump = Rs 100

Cost of the hoisting mechanism and other auxiliaries = Rs 50

Cost of labor and machining = Rs 130

Net cost of the device = Rs 980

The per-liter cost of solar-distilled water can be calculated as follows:

(a) estimate the usable lifetime of the still;

(b) add up all the costs of construction, repair and maintenance (including labor)over its lifetime; and(c) divide that figure by the still's total expected lifetime output in liters.

Such a cost estimate is only approximate since there are large uncertainties in both the

lifetime and the yield estimates. Costs are usually considerably higher than current water

priceswhich explains why solar backyard stills are not yet marketed widely in India.

Market Research

The market for the product includes whole of rural Indian population and some of the waterscarce urban areas like Chennai. This comprises around 70% of Indias population. Theexact number being 70cores. Assuming around 10 % of them will actually be interested inbuying this product the demand for the product is 70*.1/5 = 1.4 crore units.

This makes the market size to come up to around Rs 140 crore.

Impact

The impact of thiskind of a product to the rural Indian will be tremendous Oneof the

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Second Design

We would also like to present a second design that does not require vacuum pressures tooperate. This will make the system much simpler and cheaper. For achieving betterevaporation at lower temperatures in this case we make use of the phenomena calledatomization. This is achieved as follows.

The water is stored at a higher location under high pressure. The water is heated at thislocation by using the solar radiation entrapment using glass plate for green house effect.When water comes down to the distillation chamber both pressures and temperatures are

high because of pgh term or pressure head and solar heating. This water is made to passthrough a fine nozzle. This will ensure fine droplet formation and consequently very highrates of evaporation.

The walls of the chamber are cooled by means of evaporative cooling of water on the layerof clay, as demonstrated in the diagram. When this moisture laden air comes in contact withthe cool walls of the chamber condensation takes place and the water moves down to thecollector drain.

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Solar w ater heater

Pipe for raising pressure

S tor a e for water for kee in

the coating cool

Distillation ColumnWa ter will be sprayedfrom nozzle for better evaporation

The outside of thecolumn is coated

with clay to keepit cool by meansof ev aporativecooling

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