沙漠研究 22-1, 207 -210 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－Simple Solar Still Using Solar Energy and Compost Heat for Family UseShotaro KIKUCHI*1), Hiromichi TOYODA2), Akane TAKAMI1),
Sawahiko SHIMADA2), Makoto OOBA3) and Tetsuo SEKIYAMA4),
Abstract: In this study, we conducted vapor distillation experiments at National Crops Resources Research Institute and Tokyo
University of Agriculture (TUA). The objectives of this study were to boost the performance of distillation by using compost heat,and to activate natural convection in a solar still. The compost was made from cow dung and rice straw in a box. As a result of themeasurement of the compost temperature distribution the effective depth of the compost, as a source of heat underneath the still, was
20 cm from the surface. The surface temperature of the solar still cover, made of polyethylene sheets, and the amount of distillation atexperimental field in Uganda and an environmentally controlled room at TUA. According to the field experiment in Uganda,
distilled water was obtained mostly from the western side in the morning, and the eastern side in the afternoon. Experiment in theenvironmentally controlled room revealed that increases in the temperature variance of the solar still cover tended to increase theamount of distillation and the Ra number. We assumed that there would be a close relationship between the temperature variance ofthe solar still cover, natural convention and distillation.
Key Words: Compost heat, Distilled water, Natural convection, Solar energy, Solar still

1. Introduction
According to UNICEF (2010), 884 million people in theWorld still don’t have access to safe drinking water. Most ofthese people are living in Sub-Saharan Africa. Moreover, thisnumber is expected to increase in event of natural disasters.
In this study, we developed a simple solar still which can makeclear water and be managed by a household. We attempted toestimate the performance of the solar still and investigateddistillation by compost heat and natural convection in the stillin order to increase the amount of distilled water.

Fig. 1. Developed solar still.

2. System Description

Resources Research Institute from Aug. 2010 to Sep. 2010 andinside the environmentally controlled room at Tokyo
University of Agriculture (TUA) in Apr. 2010.

2.1. Construction
The solar still basin had a basal area 1 × 1 m, and it wasmade of plywood (Fig. 1). A black mulch plastic film waslaid on the bottom, and the cover was made of PVC transparent
film of 0.05 mm thickness. It was tilted at an angle of 60
degrees to the horizontal surface. The compost was set underthe basin. The well water in the basin was heated andevaporated by solar energy and compost heat, and theevaporated water was condensed near the cover. Finally, it
was collected and analyzed. We performed the distillation
experiments and distilled water analysis at National Crops

2.2. Practicability
The still material was purchased locally in Ethiopia in 2009
and cost approximately $4.00 U.S., which is 1% of the cost of acommercial solar still.
Distilled water analysis was performed by electric
conduction (EC), pH and bacteria counting, and by comparing
distilled water with well water. As a result, EC decreased
from 0.175 ms cm-1 to 0.013 ms cm-1. The bacteria counting
was conducted using an agar medium. A 0.05 ml sample ofwhich well water and distilled water diluted 200 times, wasthen dropped in to an agar medium. The colony number ofCorresponding Author: 65100001@nodai.co.jp
1-1-1, Sakuragaoka, Setagaya, Tokyo, 156-8502 Japan
1) Graduate School of Agriculture, Tokyo University of Agriculture3) Asian Environment Research Group, National Institute for Environmental Studies, Japan

2) Faculty of Regional Science, Tokyo University of Agriculture4) Ibaraki Agricultural Academy

bacteria and E. coli decreased from 33 to 3 and 34 to 0,
respectively. However, pH did not show any significant
difference.
2.3. Mathematical model
Distillation efficiency is adopted as an index of energy
efficiency of solar still in this study. Distillation
efficiency (EDis) is given by
EDis = le / Aw (rn+qe -hw)
(1)
where l is latent heat of vaporization (J g-1), e is evaporation
rate (g s-1), Aw is area of basin (m2), rn is solar radiation ofpassed the film(W m-2), qe is compost heat input per basin unit
area (W m-2), hw is sensible heat per unit area from basin (W m-2).
In this study, the Rayleigh number (Ra) was used foranalyzing temperature distribution and natural convection in
the still. The Ra number is dimensionless number associatedwith buoyancy driven flow. The Ra number is defined as:gβ (Tw − Tg ) 3L
Ra =(2)v・αwhere g is gravitational acceleration (m sec-2), β is coefficientof volume expansion (K-1), Tw is temperature of water surface(K), Tg is air temperature near the cover (K), ν is kinematic
viscosity coefficient (m2 sec-2), α is thermal diffusivity (m2sec-2), L is distance between water surface and cover (m).
Omari (2005) explains the relationship between the amount ofdistilled water and the Ra number from numerical simulation.
He also states the Ra number increases with the accumulated
amount of distilled water. Therefore, the temperature
difference between the water surface at the basin and cover atthe upper sides may play an important role in producing distilledwater in terms of activating convection inside of the still.

3. Materials and Methods
3.1. Natural convention
Figure 2 shows a schematic diagram of the solar still.
Thermocouples recorded temperatures at various points in thestill (four directions on the covers and air temperature inside of
1 cm, and 25 and 75 cm above water surface at the east side).
An experiment in the environmentally controlled room wasconducted to monitor the relative humidity at the center of thestill, where the air was affected by heat aggregation. An
electric heat cable (100 V, 125 W) and two lamps (500 W at anangle of 45 degrees) were used to illuminate and heat the basinwater instead of solar radiation.
3.2. Compost heat
Compost produced from cow dung (500 kg) and rice straw(100 kg) was set inside wooden box (1×1×0.7 m). The
compost temperature was measured at intervals of 10Fig. 2. Schematic diagram of the experiment.

(a) 11:00(b) 14:00

Fig. 3. Temperature distribution of solar still in 5th of September.

cm from the top surface to the bottom by the thermocouples(Fig. 2).

4. Results and Discussion
4.1. Natural convection
4.1.1. Temperature distribution of solar still in Uganda
All data was recorded between 26 August to 17 September,2010. Temperature distribution in the still at 11:00 and 14:00
in 5 Sep. are shown in Figure 3. This diagram was derived
by interpolating 10 measuring points (Fig. 2) using FEM
software (XFEM). The temperature at the center of the still at
11:00 was the highest temperature of the ten measuring points.
On the other hand, temperature at every measuring point in thestill at 14:00 recorded maximum and the surface of the basinwater was the highest point.
In the environmentally controlled room, the humidity inside
the still remained of 100% when the still was heated by theheating cable, whereas it decreased to 80% when heated byheating lamp. Therefore solar radiation is assumed to have
two heating effects, namely; basin water heating and air heating
inside the still. Also temperature differences have been
observed around the cover’s four sides. These temperature
differences are assumed to reflect the ability to produce distilledwater from each cover side (Table 1). The relatively cool
cover on the opposite side of the solar azimuth was found to be
the most appropriate zone to produce distilled water. The
amounts of distilled water from the four direction sides from8:00 to 13:00 were nearly equal. However, the cover iseastern side from 13:00 to 18:00 produced more than twice theTable 1. Amount of distilled water of north south east and west side.
Distilled water of each four direction film
surface (ml)3/9/2010

Watertemperature (℃)

Solar intensity(W/m2)

EastWestSouth

NorthSUM

8:00 to 13:00

39.3

677

166

195

147120628

13:00 to 18:00

40.3

281

228

103

52

73456

(a) Ra number(a)Variation of Ra between 8:00 to 11:00(b) Distillation efficiency
Fig. 5. Ra number and distillation efficiency within temperature
difference between the highest zone side and the opposite side.

(b) Variation of Ra between 11:00 to 13:00

Fig. 6. Temperature of sealing compost.
(c) Variation of Ra between 14:00 to 16:00
Fig. 4. Relationships between Ra number and solar intensity.

amount of distilled water compared with the other sides during
each experiment.
4.1.2. Ra number
A relationship between the Rayleigh (Ra) number and solar
intensity is shown in Figure 4. The measurements weretaken three times a day, morning, noon and afternoon (8:00 to11:00, 11:00 to 13:00 and 14:00 to 17:00). The Ra number
increased as solar intensity rose in the morning due to the risein basin water temperature. However, the Ra number
decreased from noon because the temperatures at all sides of
the cover were equally high due to the solar radiation from thezenith. The Ra number tended to increase in the afternoon,
but this was not as remarkable during the morning period. A
strong correlation was found between the Ra number and solar
radiation especially during the morning.
4.1.3. Surface temperature difference between east andwest sides
This experiment was undertaken to investigate for therelationship between the temperature at the cover’s highest
zone side and its opposite side (the lowest side). Figure 5shows the Ra number and distillation efficiency visaris thetemperature difference. The electric heat cable was used in

the environmentally controlled room for heating the water inthe basin. As a result, both the Ra number and distillation
tended to increase from around 0.75oC. The temperature
difference is explained by the close relationship between
natural convention and distillation.
4.2. Compost heat
4.2.1. Temperature distribution of compost
The temperature distribution of sealed compost from 1 cm
to 60 cm depth is shown in Figure 6. The temperature wasmonitored from 18 to 22 August 2010. A box was filled withcompost and covered with a plastic sheet. As a result, thetemperature at 1 cm depth varied due to air temperature
variations, and deeper part of the compost temperature (1- 60cm) tended to decrease with the depth increased inside thecompost. From the compost depth between 10 cm to 30 cm,

Fig. 7. Compost temperature after the solar still set up.

still without compost showed higher distillation efficiency than
the still with compost when solar intensity was over 12 MJ
day-1. One reason for this may be the basin water becamehotter than the compost temperature at noon. However, thestill’s distillation efficiency with compost was stable at over
12% and was able to produce more distilled water (over 200ml) than the still without compost when solar intensity was lessthan 12 MJ day-1.

(a)Distilled water

5. Conclusion(b) Distillation efficiency
Fig. 8. Distillation efficiency and distilled water for solar still with andwithout compost.
Table 2. Distilled water and distillation efficiency.
Compost

Date

Solar intensity

Distilled water

Efficiency

PlaceNonuse

23/10/2009

5.6MJ/day

34ml

1.54%TokyoUse

26/8/2010

2.9MJ/day

296ml

14.5%

UgandaNonuse3/9/2010

18.0MJ/day

1259ml

17.4%

UgandaUse

17/9/2010

18.0MJ/day

1243ml15.0%

Ugandatemperature increased while from 40 cm to 60 cm it didn’tchange during the 5 day measurement period. The compost
near the bottom had a low temperature, because of the weakfermentation caused by a lack of oxygen.
Figure 7 shows the temperature of the compost, fermented
for 5 days, both one day before and one day after it wasinstalled underneath the still. As a result, from 1 to 20 cm,
temperature decreased greatly, while from 20 cm to 60 cm thedecrease was smaller. Therefore, the valid range of the depth
as a source of heat is from 1 to 20 cm.
The heat transfer value from the compost to the basin wascalculated using the following equation.
Q = c m (T1 - T2)(3)
Where Q is heat value (J), c is specific heat (J kg-1 K-1), m iscompost mass (kg), T1 - T2 is temperature difference. c wasreferred from Ahn (2009). As a result, the heat transfer value
was calculated at 2.85 MJ, and 296 m/s of distilled water wasproduced over 18 hour. The amount of solar radiation was
2.9 MJ during the experiment.
4.2.2. Quantity of compost and effect of compost heat
Heat transfer value from the compost was measured inorder to investigate the heat value difference by compostthickness. This compost was put in the box of 1×1×0.1 m.
The average heat transfer value of 10 cm thick and 60 cm thickwere calculated for 2 days. As a result, they were calculated
to be 2.7 MJ day-1 and 2.9 MJ day-1 for 10 cm and 60 cm,respectively. A comparison of the solar still’s distillation
efficiency (with and without compost) is shown in Figure 8
with measuring date shown in Table 2. The experimental
data from Tokyo was quoted from Hasegawa (2010). The

Based on the results obtained from the experiments, thefollowings can be concluded.
1. Solar radiation has two effects it heats basin water and theair inside solar stills. The still’s humidity (heated by alamp) was about 80% in the environmentally controlledroom.
2. Distilled water can be produced when the cold covercondition was induced at the opposite side of the solar
azimuth within the solar still. The eastern cover in theafternoon produced twice the amount of distilled watercompared with of other directions in Uganda.
3. The Ra number increases according to solar intensity risesin the morning and afternoon. However, the Ra numbertends to decrease at noon.
4. The Ra number and distillation efficiency tend to increase
with temperature differences of the two sides of the solar
still cover.
5. The effective depth of the compost as a source of heat
underneath the still was 20 cm from the surface.
6. The distillation efficiency of the solar still with compost
was stable at over 12%. Moreover, the performance ofthe solar still with compost was found out to be more
efficient in producing 200 ml more distilled water thanthe one without compost.

Acknowledgments
We appreciate Dr. Ryuzo Nishimaki, Dr. Satoru Takahashi,
Dr Masaru Hosaka and Dr. Shinji Suzuki for their professional
help and input.
References
Ahn H.K., Sauer T.J., Richard T.L., Glanville T.D. (2009):
Determination of thermal properties of composting bulking
materials. Bioresource technology, 100:3974-3981
Hasegawa Y. (2010): Development of non-ventilated simple solarstill. Thesis, TUA.
Omari A., Orfi J., S Ben Nasrallah(2005): Natural convection
effects in solar stills. Desalination, 183: 173-178
UNICEF/WHO (2010): Progress on sanitation and drinking
water. UNICEF/WHO