Solar Desalination Plant for South African industry
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Development of a Solar Desalination Plant
Article in South African Journal of Geology · March 2016
DOI: 10.2113/gssajg.119.1.39
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3. The Sustainable Development Goals (SDGs), as part of a new
agenda to complete the aims of the MDGs, aims to ensure
availability and sustainable management of water and
sanitation for all. Lack of water security is not only due to
inadequate access to water, but is also worsened by poor water
quality (Lougheed, 2013).
Water security is defined as “the reliable availability of an
acceptable quantity and quality of water for health, livelihoods
and production, coupled with an acceptable level of water-
related risks” (Grey and Sadoff, 2007). Water scarcity occurs
where a discrepancy exists between the availability of water
and the demand for adequate quantities of water for human
and environmental uses (Muller et al., 2009). Water scarcity
would be a major constraint on food production, human health
and environmental quality (Seckler et al., 1998). The study by
Seckler et al. (1998) places South Africa in a group of countries
expected to experience water scarcity by 2025. The official
water shortage estimate of between 2 and 13% by 2025,
however, do not consider the impact of climate change and
water quality deterioration (de Villiers and de Wit, 2010).
De Villiers and de Wit (2010) showed that when these factors
are taken into consideration a water shortage of between 19 to
33% can be expected by 2025. In line with the Millennium
Development Goals, South Africa was expected to eradicate
the current backlog of communities without sustainable access
to safe drinking water supplies. Despite the investment in
water and sanitation infrastructure by the South African
government, 8% of South Africans still do not have access to
safe piped in-house water (Census, 2011). A large percentage
of those without access to safe piped in-house water live in the
rural areas where mainly untreated water is still collected from
up to 650 metres from their dwellings (Goldie et al., 2004;
Geere et al., 2010).
In addition, many rural areas utilise water that has a high
salt concentration with implicit health implications. One
solution is for such water to be desalinated. Thus, whilst water
needs to be treated to comply with national drinking water
standards, brackish water or sea water is increasingly being
used in areas devoid of potable water.
Desalination processes remove salts from water and are
generally used in industrial processes as well as providing
potable water for human consumption. There are four main
desalination technologies currently being used globally, based
on the following process principles (Department of Agriculture,
Fisheries and Forestry- Australia (DAFFA), 2002):
• Processes based on chemical bonds (Ion exchange)
• Processes based on membrane technology
– Pressure-driven membrane filtering: reverse osmosis
(RO), nanofiltration (NF), ultrafiltration, and microfiltration
– Electrically-driven membrane dialysis: electrodialysis
(ED) and electrodialysis reversal (EDR)
• Processes based on precipitation techniques
• Processes based on thermal distillation or freezing.
Membrane processes are used in drinking water treatment to
separate dissolved and colloidal particles (salts, pathogens such
as viruses and bacteria) from water by using pressure, electrical
potential, or a concentration gradient mechanism (DAFFA,
2002). Thermal distillation technologies on the other hand
mimic the natural water cycle. Water is heated, vaporised,
and condensed. Pure (de-mineralised) water is precipitated and
collected in a separate container, allowing salts to remain
behind as a brine stream (DAFFA, 2002). The main thermal
desalination technologies include the following processes:
• Multiple-stage flash distillation (MSF)
• Multiple effect distillation (MED)
• Vapour compression (VC)
– Mechanical vapor compression (MVC) and
– Thermal vapor compression (TVC).
Future water demand suggests that desalination must be
considered as a viable approach to the water provision sector
DEVELOPMENT OF A SOLAR DESALINATION PLANT
SOUTH AFRICAN JOURNAL OF GEOLOGY
Figure 1. Kerksplaas solar still plant near Ladysmith. Note the deterioration particularly of the basins and glass covers.
SOUTH AFRICAN JOURNAL OF GEOLOGY
40
4. D.M. VAN TONDER, C.J.S. FOURIE AND J.M. MAREE
in South Africa (DWA, 2013). Local municipalities have already
invested in the development of RO seawater desalination
systems linked to the electricity grid. These desalination plants
include: Robben Island, Eastern Cape towns of Kenton-on-Sea,
Bushman’s River Mouth, Sedgefield, with Mossel Bay, George,
and Bitou all at various stages of acquiring RO plants. The City
of Cape Town investigated the desalination potential at the
Koeberg Nuclear Power station (McGrath, 2010). Although
current desalination technologies can create new sources of
fresh water from highly saline waters such as seawater or
brackish water, the financial and energy costs currently keep
these technologies out of the reach of small communities.
Most desalination processes require a pre-treatment step. Pre-
treatment can either be coagulation, flocculation, disinfection and
oxidation, physical screening, pre-sedimentation clarification or
filtration. The type of pre-treatment is highly dependent upon the
composition of the source water. In membrane processes the pre-
treatment step is used to remove large particles to prevent
equipment damage such as scaling and fouling of membranes,
suspended solids plugging and biological fouling or attack
(du Plessis et al., 2006).
Producing fresh water through desalination technologies
driven by renewable energy is considered to be a viable
solution especially in remote areas characterised by poor water
quality and lack of grid electricity connections (Tzen and
Morris, 2003).
Solar desalination systems
Solar energy may be used to supply the required energy for a
desalination process either in the form of thermal energy or
electricity. Solar desalination is divided into direct and indirect
systems. Indirect systems convert solar energy into heat or
electricity to be utilised as an energy source for the desalination
process (such as RO, ED, MSF and VC). These systems are
generally large-scale (100 to 300 Ml/d) operations that have
difficulties in operating optimally at low (<1Ml/d) volume and
may not be economic for small communities. On the other
hand a direct system transforms solar energy into thermal
energy that is used to drive the distillation process
(Mathioulakis et al., 2007). The most common direct small scale
thermal solar technology is a solar still. A number of derivatives
of the basic solar still were developed over the years with
the aim of increasing the efficiency. Modifications to improve
the performance of solar stills include linking the desalination
process to a solar energy collector (Badran and Al-Tahaineh,
2005). It incorporates a number of effects to recover the latent
heat of condensation by improving the configurations and flow
patterns. This increases the heat transfer rates (Sampathkumar
et al., 2010), while using low-cost construction materials to
reduce the initial cost.
Although South Africa’s grid electricity generation is mainly
coal-based, conventional electricity resources are not available
in many rural areas. It is therefore important to explore how
renewable energy sources can be linked to desalination
systems for sustainable freshwater production in rural
South Africa. South Africa experiences average solar radiation
levels between 4.5 and 6.5kWh/m2
per day (Eberhard, 1990).
Many rural communities in South Africa without reliable access
to clean drinking water are situated in semi-arid to arid regions
with a high potential for solar energy generation. It is therefore
plausible that solar energy be implemented for desalination of
water with significant salt loading in arid to semi-arid regions
of the country.
Solar desalination installations in South Africa were
explored in only a few studies (Goldie et al., 2003; Goldie
et al., 2004; Goldie, 2003; Hartwig, 2013). The concept of solar-
assisted distillation is still mainly conceptual with a limited
number of prototype developments. The work by Goldie et al.
(2003 and 2004) focused on solar thermal desalination by using
a basic solar still. Their studies resulted in the installation of
solar stills in two rural communities in the Western Cape at
Kerkplaas and Algerynskraal, near Ladysmith. The efficiency of
these stills was between 20% and 35% and served a small
community and a rural school. Both plants deteriorated and are
no longer in use. The Kerkplaas plant fell into disrepair due to
the impact of environmental conditions as well as lack of
maintenance (Figure 1). Experience has shown that attempts at
delivering solar desalination solutions to remote rural
communities in South Africa is generally unsuccessful due to
the need for maintenance, reliable technical support, education
and the use of material sensitive to harsh environmental
conditions. In a more recent study (Hartwig, 2013) grey water
and seawater desalination by a two-stage indirect solar energy
multiple effect humidification dehumidification (MEHDH)
system operating without a vacuum was investigated at the
Lynedoch Eco-village, Stellenbosch.
New solar desalination system prototype
It was important to consider past experiences with the harsh
local environmental conditions and socio-economic aspects
when selecting components and material for the prototype.
Vital prerequisites include a design that is robust, sustainable,
and require minimal supervision and maintenance, and
mobility for easy transport to remote rural areas. The aim is to
keep the construction cost as low as possible while ensuring
quality. Since the system is intended for remote rural areas
the aim is to develop a system that is completely driven by
solar energy.
The new design consists of a single stage active solar
distillation system operating without a vacuum. It utilizes a flat
plate solar collector and a desalination module (Figure 2).
Brackish (1500 mS/m to 1650 mS/m) water from a feed water
tank is fed into a 50 litre evaporation chamber (EVC) at
ambient temperature. The heat exchange fluid in the solar
collector module is circulated in a closed loop system by two
solar powered 12V DC pumps. The liquid circulates through a
coiled copper pipe heat exchanger (HE) situated along the
inner edge of the EVC tank. Heat energy generated in the solar
panels is transferred to the feed water in the EVC, heating it to
between 60°C and 100°C. Evaporate is generated by the heated
feed water and is extracted by a fan, located in the roof of the
EVC tank. The evaporate is allowed to cool and condense
through heat exchange between cooling water inside the
50 litre CC tank and another set of copper coils inside the tank.
The cooling water is continuously cooled by the circulation
thereof between the CC tank and an interconnected car
SOUTH AFRICAN JOURNAL OF GEOLOGY 41
5. DEVELOPMENT OF A SOLAR DESALINATION PLANT
SOUTH AFRICAN JOURNAL OF GEOLOGY
radiator. The basic energy balance of the system can be
expressed as:
(1)
The condensed (product) water is fed from the CC to the
storage tank from where it can be collected and used.
All connections are via pvc pipes. Flat plate collectors,
although known for heat losses, typically cost less than
evacuated tubes, and were selected based on simplicity and
cost-effectiveness. Choosing components that will be in
contact with the feed and product water represented a
challenge. Plastics, due to their flexibility, toughness, excellent
barrier and physical properties, and ease of fabrication are now
rivaling metals and were therefore selected for all containers.
Analysis of efficiency
Evaluation of the pilot plant was conducted at the Tshwane
University of Technology (TUT) Arcadia campus outdoor
rooftop laboratory. The potential impact of local weather
conditions on the distillate production rate of the prototype
was evaluated by recording the weather conditions by an
onsite weather station at 5 minute intervals. Ambient weather
conditions included ambient temperature (shade and sun),
wind speed, wind direction and wind chill temperature, as well
as rainfall and humidity. Radiation measurements
(Direct Normal Irradiance (DNI), Diffuse Horizontal Irradiance
(DHI), Global Horizontal Irradiance (GHI)) at one hour
intervals, were obtained from the Southern African Universities
Radiometric Network (SAURAN, 2014) from the UPR – GIZ
station at the University of Pretoria, some 5 km from the TUT
site. It was assumed that there is no difference between the
radiation levels occurring at the two sites.
The differential and cumulative yields of product water,
along with water quality parameters (pH, electrical
conductivity (EC) and temperature), were recorded hourly
throughout a 10 hour day. Experiments relating to the
efficiency of the prototype were performed using ordinary tap
water. It was assumed that the use of tap water would result in
the same distillate flow rate as using brackish water.
Experiments relating to water quality were performed using a
concentrated NaCl solution as proxy for saline water.
The experimental setup included continuous measurements
and recording of the temperatures at various critical points
within the plant. Thermocouples (PT100 and T-type) were
used for measuring plant temperatures at the following
localities: evaporation chamber, condensation chamber, solar
panel, sun and shade. The data were recorded at 1 minute
intervals with a Squirrel continuous data logger.
Results
Plant temperature profile
Thermocouple temperature measurements for each plant
component were plotted throughout a typical winter’s and
summer’s day respectively (Figures 3 and 4). The daily
temperature profiles for the plant show three main sections; an
initial warming-up phase followed by a constant temperature
phase, and then a cooling down phase. A start-up time of
60 to 120 minutes was needed for heating of the heat exchange
fluid in the solar panels before the circulation pumps were
switched on. Temperature increases of the heat exchange fluid
at the solar panel and the EVC (reactor) are more dynamic,
whereas the other components exhibit a gradual initial
temperature increase (Figures 3 and 4). In both winter and
summer profiles the EVC and heat exchange fluid/solar panel
curve follows much the same pattern which is attributed to the
circulation of the heated exchange fluid which is dependent on
SOUTH AFRICAN JOURNAL OF GEOLOGY
42
Figure 2. Solar desalination plant set-up on the rooftop of Tshwane University of Technology showing the components of the solar and
desalination modules.
6. D.M. VAN TONDER, C.J.S. FOURIE AND J.M. MAREE
the circulation pumps being switched on. The winter’s day
temperature profile for the EVC and heat exchange fluid/solar
panel is generally a smooth curve compared to the profile for
a typical summer’s day (Figures 3 and 4). Although the smooth
EVC temperature profile curve was observed for some
observation days during the summer test period, the influence
of external conditions such as cloud cover and higher wind
speeds, resulted in the irregular pattern observed in Figure 4.
The maximum temperature in the EVC for a typical winter’s day
was below 80°C compared to 80°C to 90°C on a typical summer’s
day. Once the circulation pumps were switched off, when the
production rate fell below 10ml/h in the afternoon, the temperature
profile showed a sharp decline in the EVC temperature.
The CC temperature is highly variable as shown in the
ragged-tooth temperature profile for both summer and winter
test periods (Figures 3 and 4). This temperature pattern is
largely attributed to the fact that regular visual inspection of the
CC was done throughout the 10 hour day with subsequent heat
loss. Small changes in the CC temperature profiles, as observed
around 14:00, can be attributed to external influences such as
changes in wind speed or passing cloud cover (Figure 3).
Production rates and the efficiency
The early morning production rate was low, and it was
subsequently decided to allow an initial heating-up period
before measurements would start. The system required
120 minutes to heat-up the 50 litre saline water in the
evaporation tank to 60°C during the winter period and
60 minutes during the summer period, before a steady state
was reached and significant evaporation occurred.
The production rate was highest between 11h00 and
13h00 for a typical winter’s day (Figure 3). The high initial
production volume was related to a small leakage which
occurred in the CC tank which was subsequently repaired.
During the summer period the maximum production volume
was reached between 11h00 and 12h00, an hour earlier than
during a typical winter’s day (Figures 3 and 4).
Distillation rate relative to ambient temperature for a typical
winter’s and summer’s day showed the distillate production
peaked at least 2 hours before the peak ambient temperature
for the day was reached (Figures 3 and 4). For both cases 65%
of the daily distillate production occurred between 11h00 and
14h00 with around 50% of the summer’s day production
occurring between 11h00 and 13h00 and 45% of the winter’s
day production between 12h00 and 14h00.
Although the maximum hourly production volume for the
selected summer’s day (450 ml) was less than that observed for
the selected winter’s day (550 ml), the cumulative production
volume during the summer test period exceeded that for
winter’s days in general. The average production volume
during a summer’s day (1685 ml/d) was on average 33% larger
than the average winter’s day (1180 ml/d) (Table 1).
The average flow rate of distillate produced per minute during
the summer period is 4.52 ml/min and for the winter period
2.92 ml/min. When the warm-up period is excluded, the
average flow rate of distillate is 5.35 ml/min and 3.95 ml/min
during the summer and winter period respectively.
Radiation intensity and distillate production
Interpolated daily GHI, DHI and DNI radiation are plotted
along with the distillate production rate at different time
SOUTH AFRICAN JOURNAL OF GEOLOGY 43
Figure 3. Plant component temperatures and distillate production rates for a typical winter’s day.
Table 1. Production volume of the prototype.
Summer’s day Winter’s day
Average production volume (ml/day) 1685 1180
Average flow rate (ml/min) 4.52 2.92
Average flow rate excluding 5.35 3.95
warm-up period (ml/min)
7. intervals for different weather conditions (Figures 5 and 6).
The maximum radiation (GHI and DNI) was between
12h00 and 16h00 during winter months, and between 11h00
and 15h00 during summer months. The maximum GHI and
DNI radiation levels coincide with the maximum distillate
production rates.
During the winter months, the lack of cloud cover was
responsible for a smooth solar radiation curve (Figure 5).
This pattern was mirrored by the plant temperatures, although
there was a slight off-set. Occasional cloud cover was
responsible for plant temperature changes, mimicking the
changes in solar radiation for a typical summer’s day (Figure 6).
DHI solar radiation was less affected by cloud cover change
and was substantially higher in summer compared to the
winter test period.
Distillate quality
The product water quality of a typical winter’s and summer’s
day did not vary significantly. The experiments were
conducted with an initial water quality which falls within the
brackish water range of between 1500 mS/m and 1650 mS/m
and a pH of 8.6 to 8.8. The average product water quality
ranged between 3.00 mS/m and 25.00 mS/m throughout the
day with an average daily electrical conductivity of 7.00 mS/m
and a pH of 8.8 to 9.0.
DEVELOPMENT OF A SOLAR DESALINATION PLANT
SOUTH AFRICAN JOURNAL OF GEOLOGY
44
Figure 4. Plant component temperatures and distillate production rates for a typical summer’s day.
Figure 5. Distillation rate relative to solar radiation for a typical winter’s day.
8. D.M. VAN TONDER, C.J.S. FOURIE AND J.M. MAREE
Discussion
The operation of the pilot plant showed that the system is
elementary and would be within the capability of a local
community member to operate over the long-term. Although
small leakages were encountered it was possible to rectify the
problem with the use of low cost material which would be
available in most rural areas.
It was expected that some inertia, due to the initial heating-
up of the system, some inertia would be reflected in the
experimental data. This was demonstrated by both the plant
temperatures and the production rates. The inertia was linked
to the fact that the water in the system will at first absorb the
heat energy received and only then will energy drive
evaporation. Heat of evaporation is removed from the brine
and the brine temperature will therefore decrease. However,
since the PV circuit continuously replaces the heat the process
of evaporation can continue.
The distillation rate relative to plant temperature for a
typical winter’s and summer’s day showed that 65% of the
distillate is produced prior to 14h00, when feed water
temperatures and solar radiation is at its highest. Distillate
production decreased drastically after radiation peaks were
reached. Distillate is produced prior to the feed water reaching
60°C temperatures, suggesting that evaporation occurred
before a steady state was reached. The maximum productivity
occurred during summer when radiation levels were higher
and subsequent heat transfer to the feed water was more
effective. The average flow rate of distillate produced when the
warm-up period is excluded, 5.35 ml/min and 3.95 ml/min
during the summer and winter period respectively, compares
well and outperformed the 3.07 ml/min recorded by the
MEHDH system developed by Hartwig (2013).
The results of the experimental investigation reflect
the impact various environmental conditions have on the
production rate. The temperature profiles for a typical
summer’s day is not a smooth curve, and it can therefore be
assumed that external environmental conditions (e.g. cloud
cover and wind speed), other than radiation, have an influence
on the production rate.
The product water quality of 7.0 to 9.0 mS/m and pH
ranging from 8.6 to 9.1 falls within the national standard limits
for ideal drinking water quality (EC 0 to 70 mS/m; pH 6.0 to
9.0) (DWAF, 1996). By blending the distilled product water with
feed water may provide an option to increase the volume of
acceptable drinking water. However, this option is only viable
where the final water mix has an electrical conductivity below
300 mS/m, and provided the feed water falls within the
microbiological limits for drinking water.
Conclusion
Desalination by means of solar energy is a suitable alternative
to conventional methods of providing fresh water, especially
for rural areas in South Africa where small volumes of water are
required for human consumption.
The new solar thermal desalination technique holds
important advantages with regard to small scale water
treatment systems:
• The operating temperature is between 60°C and 100°C,
which is the temperature range at which thermal solar
collectors perform well, distillation occurs and bacteria are
destroyed through pasteurisation.
• No chemical pre-treatment step is necessary for the feed
water.
• No membrane fouling and scaling occur which would
require regular maintenance and cause interruption of plant
operation.
SOUTH AFRICAN JOURNAL OF GEOLOGY 45
Figure 6. Distillation rate relative to solar radiation for a typical summer’s day.
9. • System efficiency and product water quality are almost
independent from the salinity of the feed water.
• The system is easily scaled-up to meet the requirements of
a small community.
Acknowledgements
Dr. P. Wade and Mr. D. Johnson are thanked for reviewing the
document and providing valuable inputs. The authors would
also like to thank InkabaYeAfrica and THRIP-NRF for funding.
This is Inkaba yeAfrica contribution number 141.
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V i e w p u b l i c a t i o n s t a t s
V i e w p u b l i c a t i o n s t a t s
