A new solar desalination system with heat recovery for
decentralised drinking water production
K. Schwarzer
a, E. Vieira da Silva
b, B. Hoffschmidt
a, T. Schwarzer
aa
Solar-Institute Ju¨ lich (SIJ) of the Aachen University of Applied Science, Heinrich-Mußmann-Str. 5., Germany
Tel. 0049 2467 54049; Fax 0049 2467 347372; email: [email protected]
b
Laborato´rio de Energia Solar e Ga´s Natural, Bloco 718-CT, Universidade Federal do Ceara, Brazil
Received 31 January 2008; revised accepted 15 May 2008
Abstract
A new solar thermal desalination system has been developed and tested in field conditions. The system has two components: a desalination tower with multiple stages and one or more solar collectors. Sea and ground water can be used to feed the desalination tower, which produces desalinated and decontaminated water in its 5–7 horizontal stages. The solar collectors are used to absorb solar energy and a fluid, the desalinated water itself, transports the heat energy to the tower. Four different systems have been tested in Germany, Spain, India, and Brazil. The field tests show some important results: the system produces about 15–18 L/m2/day, which represents a factor of 5–6 times higher than the production of a still type distiller; it is modular and each unit produces approximately 35 L/day that allows the installation of larger systems (1000–2000 L/day).
Keywords:Solar thermal desalination; Heat recovery
1. Introduction
The shortage of drinkable water in many areas of the World is an old problem. Many regions have a lim-ited supply of conventional energy, although some have a great potential in solar energy. In many coastal areas and arid zones worldwide, drinking water supply is an increasing problem. Overpopulation, growing tourism, industrialization and the use of chemical pro-ducts in agriculture constantly degrade the quality of
drinking water. Many arid areas have only limited underground water resources and as a result of exten-sive pumping, the water quality has deteriorated and become brackish.
Regardless of the need for a more sustainable sup-ply of drinking water, in many regions it has to be pro-duced from brackish or salt water sources. Various large scale desalination techniques have been devel-oped and used. Due to high investment costs, lack of infra-structure, and operation costs, the use of these large scale plants has not been made possible in many coastal regions nor in developing countries, where
Presented at the Water and Sanitation in International Development and Disaster Relief (WSIDDR) International Workshop Edinburgh, Scotland, UK, 28–30 May 2008.
there is a demand for a lower-price, low maintenance, environmentally friendly, and decentralized small-scale desalination systems.
Another problem is the quality of the drinking water. In many parts of the world, the fresh water avail-able for drinking is of limited supply and the popula-tion is forced to drink poor ‘‘quality’’ water. The quality of water acceptable for human consumption has been generally defined according to dissolved solids in parts per million [ppm]. Most desalination facilities seek to reduce the salinity of the water produced to less than the maximum amount of the dissolved solids allowed for potable water (WHO- and EU-Drinking Water Standard [12,13]).
2. Solar thermal desalination system
Solar thermal systems that produce potable water from salty water have been studied for quite some years and the use of solar energy to produce potable water remotes to ancient Egypt. Various solar thermal sys-tems have been presented in the literature [1–3]. The most studied model, the still type distiller, has the advantage of low installation cost, but the disadvan-tages of low efficiency, accumulation of salt and algae at the basin of the unit, and glass breakages from storms and vandalism.
The simple solar still desalination unit consists of a black painted basin that contains sea or brackish water, covered by a transparent glazing. The solar radiation passes through the transparent cover and is absorbed by the water in the black basin area. Thus, water con-tained in the basin is heated up and evaporates inside the still. Vapor rises until it comes in contact with the cooler inner surface of the glass cover. There it con-denses in pure water, run down along the tilted cover surface and is collected in vessel. This solar still type is characterized by its easy construction, using locally available material. However, it has low output of desa-linated water (2–6 L/m2day).
2.1. New solar desalination system with heat recovery
A new solar thermal desalination system with heat recovery is schematically presented in Fig. 1. The two basic system components are flat plate or vacuum tube solar collectors and a desalination tower.
The concept uses multiple condenser stages, arranged vertically one over the other. Each stage recovers heat of condensation from the vapor produced in the stage directly below. The condensate drains on the tilted condenser surface, moves through flow channels to be collected in a tank. The solar heat can be supplied by flat plate collector, parabolic trough, or evacuated tube collectors. Salt water in the lower stage is heated up to 95–100C. The working fluid in the collector is the desalinated water, avoiding the use of an additional heat exchanger and corrosion in the collectors. The flow of salt water is from the top to the bottom, in counter flow with the heat flux. The mechanism that supplies salt water can be electronically or mechanically controlled. The multiple heat recovery of the evaporation enthalpy leads to a much higher production of desali-nated water per m2of solar collector, when compared to the still type distiller. Depending on the number of condenser stages, the production rate can be increased by a factor of 3–4. In other words, for the production of 1 m3of drinking water, only 125–200 kWh are needed as compared to 650 kWh. Depending upon solar radia-tion, a daily output rate of 10–17 L of drinking water per m2of collector area can be reached. Small RO installations need approximately 5–8 kWh/ms3; how-ever, they are not suitable, as a rule, for state alone use in many locations in developing countries.
The desalination unit is modular, allowing various system sizes, large and small, to be installed. It is pos-sible to build systems with capacity up to 5 m3/day. The desalination tower has 5–7 stages for the heat recovery process. Another important characteristic is that there is no need for chemical products neither to operate nor to clean the components, reducing maintenance costs and disposal to the environment. The equipment can be cleaned by simply removing the stages from the desalination unit and washing them.
Considering its characteristics, the system is suita-ble for decentralized operation in rural area zones where brackish water or contaminated ground water is found. Its use can contribute to a sustainable devel-opment and improve the environmental and living con-ditions of the people.
3. Energy analyses
To optimize the performance of the system, two studies were carried out, one determine the field
performance and the other to develop a reference model, used in comparison with field test results.
The field tests include measurements made under real weather conditions to determine the system’s effi-ciency, its daily production, and the temperature pro-files in the solar collectors and in the desalination tower. In the laboratory studies, a single stage desalina-tion unit with controlled evaporadesalina-tion and condensadesalina-tion temperatures was built to generate reference values. These latest measurements were needed because other results found in the literature gave very different pro-duction rate values for evaporation temperature higher than 60C.
3.1. Multiple stage desalination unit
The energy balance equation for the storage volume is given by Eq. (1), where the energy to run the system comes from the solar collectors. This energy is
used to vaporize part of the water, to heat up the water in this volume, and it is partly lost either to the ambient or to the brine that leaves the tower:
AcolEsol_ ðtÞZcol¼mwcp;w
dT0
dt þaA T0ð T1Þ
þEsA T04T14
þlAwallðT0T1Þ
þmcirc_ cpðT0T1Þ þQleak_ ð1Þ
In this equation,arepresents the heat transfer
coef-ficient by evaporation and convection,ethe emissivity from the surface of the water,lthe thermal conductiv-ity of the insulation material,mcirc_ the rate of mass
cir-culated through the stage, andQleak_ the rate of heat loss through vapor leakage. The subscripts ‘‘0’’ and ‘‘1’’ refer to the bottom volume and to the first stage
in the tower, and1 to the ambient air outside the tower.
For the other stages in the desalination tower, the energy balance equation has the same form of Eq. (1), except that the energy enters these stages by con-densation, convection, and radiation from the stage below, not from the solar collector.
Two important parameters are used to quantify the performance of the system: the Coefficient of Perfor-mance (COP) and the Gain Output Ratio (GOR) values. The COP is defined as,
COP¼
Pn
i¼1 mdest;i
mdest;1
ð2Þ
wherePni¼1mdest;irepresents the total amount of
desalinated water produces by all stages, andmdest;1
the amount of desalinated water produced by the first stage. The COP is used to analyze to heat recovery processes and to optimize the efficiency of the stages. The GOR value is defined as,
GOR¼
Pn
i¼1
mdest;iDh
Qin ð3Þ
and the energy input to the desalination tower from the solar collector,Qin, as
Qin¼Acol Z
ZcolE_ðtÞdt ð4Þ
whereDh represents the evaporation enthalpy of water andEsol_ ðtÞthe solar radiation on collector. The GOR value is used to analyze the performance of the system as a whole, equivalently to the thermal effi-ciency. The heat recovery mechanism can be studied using the definition of the stage efficiency as,
Zi;s¼
_
mdest;iDh _
Qr;jþ _
Qc;jþ _
mdest;jDh
þQcirc_ þQl_;j ð5Þ
whereZi;s is the efficiency of stage ‘‘i’’, _
Qr;jthe
radiation transfer from stage ‘‘j’’ (below) to stage ‘‘i’’, Qc_ ;j the convection transfer rate,
_
Ql;j the sum
of all energy rates lost through the walls, back drops,
vapor leakage from stage,Q_circthe sensible energy of
the feeding water, and mdest_ ;jDh
the rate of energy used to evaporate the mass rate (vapor pro-duction) from stage ‘‘i’’. The overall stage efficiency for one day, Zi, related to the production of the
Zi
;sstage, is
Zi¼
P 1day
mdest;iDh
Qin ð6Þ
and the GOR value can also be determined as,
GOR¼X
n
i¼1
Zi ð7Þ
3.2. Single stage unit
To develop controlled laboratory results, a one-stage desalination unit was built. The evaporation and condensation temperatures were controlled using external electrical heat sources. The upper plate was tilted at different angles with the horizontal plane vary-ing from 5 to 16. The number of measured data points was 379. The horizontal plate dimensions were 40 cm40 cm and the average distance from the water level to the condensation surface was 14 cm. The walls were isolated with 5 cm PU-foam.
The temperatures were measured with type-K (NiCrNi) thermocouples. The amplifier had integrated cold-junction compensation. There were four sensors on each surface, and the measures values were used to calculate a mean value to represent the temperature of the surface. The distillate mass was measured with a digital laboratory scale with 0.1 g resolution. The eva-poration and condensation surfaces were heated by two 700 W cartridge heaters. To achieve a uniform tempera-ture distribution on the condensation surface, the heater was placed in a water cushion. The evaporation basin contained a metallic grid for better heat distribution.
4. Results
This section presents the laboratory and field results of the work done in Germany, India, Spain, Bra-zil, and India [4–11].
Using the laboratory measurements (controlled evaporation and condensation temperatures, condensa-tion at 16tilted plane), it was possible to study the pro-duced desalinated water rate as a function of the evaporation temperature and the temperature differ-ence between the evaporation and the condensation surfaces (Fig. 2).
Fig. 2 is used as a reference or maximum measured value that the desalination stages could produce in field tests. Other studies were carried out to find the most appropriate geometry (tilt angle, number of stages per tower, area of each stage), condensate flow in the stages, and the amount of water in the stages and in the storage tank. As optimized values, the number of stages per tower should vary from 5 to 7 and the GOR value from 2.8 to 3.5.
Two prototypes with a daily drinking water output of 30–65 L were developed. In the frame of the research project AQUASOL, funded by the German Federal Ministry of Research and Education (BMBF), Thames Water and IBEU the systems have been tested and optimized under real conditions in the Canary Islands. Fig. 3 shows four prototypes of the solar stills with two types of solar collectors. One system uses flat plate collectors and the other uses vacuum tube collec-tors. The tests started in July 2005 and lasted for more than one year.
Both systems run without additional energy or elec-tric pumps, and no complex control units are required,
as in other systems. This is a very important advantage in remote areas, where electricity is not available or too expensive.
The first tests in Gran Canaria were carried out with potable water, followed by tests with salty water. The collector areas were 5 m2and 2.2 m2for the flat plate and evacuated tube collectors, respectively. The daily production of drinkable water, for a solar radiation intensity of 6–8 kWh/m2, was 32–60 L. When sea water was used in the systems, a reduction of about 20% in the daily production was observed. Fig. 4 shows the temperature and solar radiation intensity for a system during 1 day. A maximum temperature of 100C was reached in the storage, approximately 2 h
Fig. 2. Desalinated water production as a function of the evaporation temperature and of the temperature difference between the evaporation and the condensation surfaces.
after solar noon. After sunset, the storage temperature was about 80C, and the temperature in the next
morn-ing was near 45C.
Table 1 shows the typical data of one system with vacuum tube collectors (2.2 m2) and seven stages. The daily drinking water production was 37.6 L, with a flux rate of 17.08 L/m2of collector area. The night produc-tion is high due to thermal energy storage in the stages in the tower. This results shows a factor of 4–5 times higher that the water production of a one stage solar still with the same collector area.
Analysis of the produced water shows that the requirements for drinkable water are met. In Gran Canaria, the conductivity of the desalinated water was approximately 72mS/cm for sea water with a conduc-tivity of 51,000mS/cm. These results show that about 99.8%of the solid materials were removed.
Similar systems have been tested in Brazil and Table 2 shown the microbiological and chemical test results when highly polluted sea water was used as sup-ply water. These results show the very high quality of the drinking water output from the desalination tower (thermal process), all results were way below the max-imum allowed values suggested by the World Health Organization [12]. The results of the microbiological contamination analysis show the absence (considering the lowest level of measurement of the instrument) of
Escherichia coli, coli-form and fecal Streptococcus
confirm the desired quality of the produced water.
Pseudomonaswere present in sea water, but not on the distilled water. The high temperature of operation kills the bacteria in the water, but care has to be given to the bacteria that resist temperature above 100C.
In Bangalore, India, the Solar Institute has installed two desalination systems with vacuum tube collectors. A basic problem is the drinking water supply in some states, especially in the state of Kanataka, where the drinking water is contaminated by halogen fluorine.
Table 1
Daily output results of a system with evacuated tube solar collector (2.2 m2)
Daily production rate: day and night 37.6 L/day
Day production 20.35 Litre
Night production 17.25 Litre
Global radiation 6.0 kWh/m2d
Solar energy of the collector area 13.17 kW/d Collector output energy (h¼0.5) 6.58 kWh/d Specific energy consumption per liter 0.175 kWh/L Distillate production per liter 5.71 L/kWh Distillate/collector area 17.08 L/m2d GOR value (desalination tower) 3.65
COP value 4.27
0 200 400 600 800 1.000 1.200
5:00 9:00 13:00 17:00 21:00 1:00 5:00
Time (24 h )
Sol
a
r r
a
di
at
ion (
W
/m
²)
0 20 40 60 80 100 120
Tem
p
er
at
ur
e (
°C
)
Qpsolar(W/m²)
T_ambient(degC)
T_evap_sys2(degC)
T_stage_1_sys2(degC)
T_stage_4_sys2(degC)
T_stage_7_sys2(degC)
Fig. 4. Solar energy radiation and temperature in a system during one day.
According to a study of the World Health Organization (WHO) ‘‘Fluorine in Drinking Water’’, more then 20 million people are affected in India. The ground water concentration of fluorine is, in some areas, 30–40 mg/L. The WHO suggests a value lower than 1.5 mg/L. In the district of Koppal, approximate 70% of all habitants
suffer from the contamination of fluorides in their ground water sources. Koppal district is one of the highest affected areas in South India. The drinking water sources are 100% based on groundwater. If one tube cracks or breaks, it can easily be replaced from outside or the opening closed with some stuffing, and the unit can continue to work. A high consumption of fluorine ground water leads to the well known clinical picture in the tooth area, ‘‘tooth fluorosis’’, and ‘‘articular-fluorosis’’.
5. Conclusion
The present solar still was designed to use the latent heat from the condensation process and to re-use this condensation’s enthalpy in the evaporation process in the next stage, recovering heat several times.
The solar thermal desalination system was designed for stand-alone use in rural areas with low infra-structure. The results from the field tests show the high efficiency of the system, easy handling, and the possibility of bring sea, ground, and contaminated waters into the system.
The important advantages, when compared to other solar thermal desalination systems, are: excellent drinking water quality; the stand-alone capability, as the system can operate with solar energy only; self-regulating mechanism and self-start; modular construction (bigger system can be easily implemen-ted: 35–2000 L/day); no moving parts; higher drinking water production rate (approximately 15–18 L/day/m2), when compared to the single stage solar still (3–6 L/day/m2); easy cleaning; low maintenance; possibility of operation with other available energy sources (waste heat, electrical energy from wind mills, etc.) and 24-h/day operation, increasing the production ratio by a factor of 3.
References
[1] A. Palacio and J.L. Fernandes, Numerical analysis of greenhouse-type solar stills with high inclination, Solar Energy, 50(6) (1993) 469–476.
[2] M.A.S. Malik, G.N. Tiwari, A. Kumar and M.S. Sodha, Solar Distillation, Pergamon Press, 1982.
[3] K. Schwarzer, J. Hohmann and C. Faber, Solarthermische Meerwasserentsalzung und Wasseraufbereitung, Abschlußbericht des Solar-Instituts Ju¨lich im Projekt 252 001 91 der AG Solar NRW, Teilprojekt: Solarthermische Anwendungen, Ju¨lich, 1995.
[4] K. Schwarzer, E. Vieira da Silva, C. Faber and C. Muel-ler Solar thermal desalination system with heat recov-ery, Desalination Strategies in South Mediterranean Countries, September 11–13, 2000.
[5] C. Mueller, K. Schwarzer, E. Vieira da Silva and C. Mertes, Modular solar thermal desalination system with multi-stage heat recovery, in: World Renewable Energy Congress, WREC-2004, Denver, CO, 2004.
[6] K. Schwarzer, M.E.V. Silva and C. Faber, Solar thermal desalination system with heat recovery, Desalination, 137 (2001) 23–29.
[7] K. Schwarzer, T. Schwarzer and E. Vieira da Silva, Solare meerwasser-entsalzungsanlage mit mehrstufiger wa¨rmeru¨ckgewinnung, in: 16. Symposium Thermische Table 2
Microbiological and chemical test results before and after desalination
Sea water Ground water Item measured Before After Before After Total coli form
group bacteria (NMP/100 mL)
>2419.20 3.1
Fecal coli form group bacteria
(NMP/100 mL)
2419.17 2.05
pH (25C) 7.95 4.72 5.74 5.71
Alkalinity (mg CaCO3/L)
114 4 4
Total hardness (mg CaCO3/L)
4400 4 170 6
Chloride content (mg Cl/L)
16,500 5 235 9
Conductivity (mS/cm)
53,400 56.3 1072 36
Solarenergie, Kloster Banz. OTTI – 16. Symposium Thermische Solarenergie, 2006.
[8] K. Schwarzer, E. Vieira da Silva, C. Mueller, H. Leh-mann, L.D.E. Coutinho, Modular solar thermal dessali-nation system with flat plate collector, in: Rio3 World Climate and Energy Event, Rio de Janeiro, 2003. [9] E. Vieira da Silva, L.D.E. Coutinho, K.J. Schwarzer and
M.F. Campos, Simulac¸a˜o do Desempenho e Estudo Econoˆmico de um Dessalinizador Hı´brido (Solar e Ga´s Natural), in: Rio Oil&Gas 2004 – Expo and Confer-ence, 2004, Technical Papers – CD – IBP579_04, 2004.
[10] E. Vieira da Silva, K. Schwarzer, C. Faber and C. Muel-ler, Mass transfer correlation coefficients for an evaporation–condensation unit, in: Congreso Latino Americano de Transferencia de Calor y Materia, 2001, Me´xico, Anais do LATCYM2001, 2001. [11] K. Schwarzer and D. Majumder, Solar still generates
international interest, World Water Environ. Eng., 29(3) (2006).
[12] WHO – Drinking-Water Standards, www.lenn.tech. com.
