Operation of Photovoltaic Electro-Chlorination Process in Salt Water

Transcription

Operation of Photovoltaic Electro-Chlorination Process
Khouzam
Operation of Photovoltaic Electro-Chlorination Process in
Salt Water Pools
K.Y. Khouzam
School of Electrical and Electronic Systems Engineering
Queensland University of Technology
GPO Box 2434 Brisbane 4001
AUSTRALIA
E-mail: k.khouzam@qut.edu.au
Abstract
Chlorine is used in swimming and spa pools to oxidise organics, to kill bacteria and to control the
growth of algae. In this project, direct coupled photovoltaic power was applied to an electrolytic cell to
produce liquid chlorine using brine of sodium chloride. Tests were conducted to investigate the effect
of applied voltage, current, salt concentration, water flow rate, and duration of chlorination. Seven
photovoltaic (PV) chlorinator installations were completed over the past twelve months in public and
private pools. Results showed that proper matching can be achieved by carefully selecting the PV
array parameters with respect to the load parameters. Also, the variation in solar radiation matches
well the need for chlorine production and usage during the day. Using photovoltaics for salt-water
chlorination is an effective way to semi-automate the input of chlorine into the swimming pool. If
adopted, PV-based chlorinators promise a cheaper and cleaner alternative to mains power.
INTRODUCTION
Chlorine is a standard agent for sanitation in swimming and spa pools. Various compounds of liquid
and dry chlorine additives are available in the market. Alternatively, liquid chlorine (sodium
hypochlorite) can be produced using salt-water chlorinators by the electrolysis of salt (sodium
chloride). This involves the passage of direct current through salty water to produce chlorine gas at the
positive electrode. The amount of chlorine produced is proportional to the amount of charge passing in
the electrodes.
Salt-water chlorinators have been in the market for over twenty five years and their technology is well
mature. In Australia, over 80% of swimming pools rely on salt water chlorination. In fact, all industrial
chlorine production uses exactly the same technique used by the salt-water chlorinator. Conventional
chlorinators use a rectifier to convert 240 V to low volt direct current to power the electrolysis plates.
Tests showed that the efficiency of commercially available power converter units is around 50%.
A research grant was funded under the Queensland Sustainable Energy Innovation Fund (QSEIF), to
develop a photovoltaic (PV) source for the production of chlorine. The system must offer the same
level of performance (if not better) as offered by commercial units. The PV chlorinator system must be
simple to operate; safe and cost effective in order to compete with mains powered chlorinators.
The project was able to work with Allchlor Repairs Pty Ltd, an industry partner to trial the application of
photovoltaic to electro-chlorination and to progress the system into commercialisation. Through this
project it is planned to:
•
•
•
•
Design and install a number of PV chlorinator systems for water treatment in public and private
swimming pools.
Monitor and analyse chlorination parameters, along with solar radiation and pool usage. This will
be used to get customer feedback on the performance of PV chlorinators.
Prepare a business plan for marketing and commercialisation of the PV chlorinator system
Develop a commercial turnkey product packages for residential and public swimming pools
suitable for Australian and overseas markets.
Destination Renewables - ANZSES 2003
Reviewed as full paper
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HISTORY OF WATER CHLORINATORS
Australia, with a high standard of living, and with favourable climate, is a large market for the sales
promotion of swimming pools. With the introduction of salt-water chlorinators in early seventies and to
persuade prospective pool buyers, some pool builders offered a salt-water chlorinator as an option
when the pool was to be built.
Nowadays, there are approximately 25,000 in-ground pools added each year in Australia, and many
are fitted with a chlorinator as standard equipment. Many old pools are also upgraded, with the result
that around 75% of residential pools benefit from salt chlorination. Pools using salt chlorination have a
concentration of approximately 0.4% to 0.8%. Salt is periodically added since the salinity level may
drop due to flooding, or when water is lost through overflowing. Evaporation of water will only
concentrate the salt level and will be diluted with water. The simple chemical reaction is given by:
Salt + Water + DC current = Sodium Hypochlorite + Hydrogen
With modern pumps, filters, heaters, PVC plumbing and other equipment, the salt-water chlorinator
has become a reliable and standard part of the Australian swimming pool market. Apart from its
sanitising effect, a salt water chlorination system offer the following advantages:
•
•
•
•
Automatically produces chlorine whenever the system is switched on, thereby eliminating the daily
chore of adding dry or liquid chlorine and eliminating the risk of storing chlorine additives.
Costs less than liquid or granular chlorine additives and gives water a sparkle and pleasant
appearance.
Eliminates skin irritation and health-related problems with prolonged bathing and helps relieve
tense muscles.
Causes much less strain on the eyes than ordinary pools (study by School of Optometry at QUT).
Salt-water chlorinators require high current low volt (7 to 9 V) direct current to operate the electrolytic
process. This is provided by an inefficient power supply, which converts 240 V alternating current to
low volt uni-directional current. Water must generally be pumped into the electrolysis unit (called in-line
system). The electrode plates are made of titanium with the anode often coated in platinum. Platinum
serves as an effective catalyst, that speeds up chemical reactions but is not itself changed in the
reaction.
Photovoltaic cells produce dc power when exposed to solar radiation. Cells are connected in parallel
and in series to form PV modules to produce the required voltage and current. Experiments showed
that proper matching between the characteristics of the PV panel and the chlorinator load could be
achieved by carefully selecting the comparative parameters. The current required is dependent on the
volume of water, temperature and available free chlorine. Various types of electrode plates were
tested and enhancements were made to design a chosen cell. The cost of PV devices has fallen in
recent years making PV a feasible option for water treatment.
PROJECT OBJECTIVES
The photovoltaic chlorination project received funding through the Office of Sustainable Energy
Industries; Environmental Protection Agency under a state approved “Queensland Sustainable Energy
Innovation Fund”. Additional funding was provided by Queensland University of Technology to achieve
the following goals:
•
•
•
Expand the use of solar power to water pumping and filtration.
Improve overall system performance. This includes integrating the system to the grid and
considering alternative backup systems for chlorination.
Consider alternative electro-chlorination methods such as convection and submersible.
Allchlor Pty Ltd has been collaborating with Queensland University of Technology during the past two
years to research and develop components with the aim of assessing the feasibility of PV chlorinators
for Australian and overseas markets.
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The project offers the following advantages:
•
Contributes to the renewable energy industry as the new PV chlorinator uses direct electricity
produced using PV panels. The inherent matching of the source and the load is accomplished
without the need for costly power conditioning or inverters.
•
Efficiency improvement and reduction in electricity consumption and associated emissions. The
PV system offers greater safety and cost reduction in the maintenance of pools. The resulting
reduction in emission amount to more than 1000 kg of CO2 annually for the average size pool.
•
Contributes to the growth in the swimming pool market and to establishing a commercial PV
based chlorine industry. There is a niche market for the product in Australia and overseas.
•
The demonstration projects will contribute to increasing public awareness of environmental
problems, the need to exploit alternative energy technologies, and to community acceptance of
new energy technologies.
PROJECT DESCRIPTION
The first phase of the project was completed in 2001. This involved testing of various electrolysis
parameters and electrode materials and chemicals. Experiments were conducted to examine the
effect of applied voltage, current, salinity level, water flow rate, and duration of chlorination. Other tests
included the effect of the dimensions of the electrodes and the spacing between them. Results
showed that proper matching can be achieved by carefully selecting the PV parameters with respect to
the electrolytic load.
In the second phase of the project seven PV chlorinator systems were designed and installed in
selected public and residential swimming pool sites in Queensland. Various systems configurations
were considered and some were implemented.
Figure 1 shows a schematic diagram of a PV installation in a public pool at Palmwoods, Maroochy
Shire in the Sunshine Coast. The control unit is supplied with low volt dc from the PV array which in
turn supplies the electrolytic cell. Chlorine is produced in the electrode cell by electrolysis and returns
to the pool. The optional mains chlorinator operates in parallel and its use is limited to few hours on
cloudy days. A data acquisition system is installed to record data for the purpose of monitoring.
Figure 1. Schematic diagram of PV chlorinator at Palmwoods in Sunshine Coast.
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PHOTOVOLTAIC CHLORINATOR INSTALLATIONS
A list of the PV chlorinator sites and their specifications is given in table 1. Specific recommendations
on each site follow in sub-sections.
Table 1. Photovoltaic chlorinators installed in and around Brisbane, Queensland.
Size of
System Specification
Pool
• 4 SX-80 W PV panels wired for 6 V provide total chlorination using
Taringa
in-line system.
55,000 L
• Electrode is installed below water surface and thus provide
Residential
independent chlorination and pumping (convection effect).
• Chlorination operates during daylight hours.
• An AC pump runs filtration for 4 hours (winter) to 8 hours (summer)
per day.
• No backup system is employed for chlorination.
• A selector switch may be required to prevent excess chlorine.
• System has been running satisfactorily for over two years.
• 6 SX-80 PV panels wired for 6 V provide water chlorination.
Palmwoods
• A backup system is employed in parallel.
32,000 L
• A control switch is installed to switch on 2 panels at a time to control
Public Pool
chlorine level and prevent excess chlorination.
• AC motor is used for water pumping.
• Water is heated using electric heat pump.
• PV chlorinator is oversized and excess electricity may be exported.
• 3 SX-80 PV panels wired for 6 V provide chlorination.
Pallara
• 1 SX-80 PV panel (12 V) provide dc pumping during daylight hours
60,000 L
but bypassing water filter and using a non-return valve.
Residential
• A backup system is employed in parallel.
• AC power is used for pumping for water filtration using night tariff.
• Reverse polarity will be advantageous; otherwise system is running
satisfactorily.
• 8 SX-80 PV panels wired for 12 V and connected to two electrodes
Logan
in series provide water chlorination.
Central
• A backup chlorinator of 50 A and 14 V is employed on another line
97,000 L
but set to low.
Public Pool
• Two AC motors are used for water pumping with two lines for water
chlorination; one on PV and the second on grid power.
• Water is heated using gas.
• Solar chlorinator is performing well.
• 4 SX-80 W PV panels wired for 6 V provide chlorination.
Arana Hills
• Chlorination operates during daylight hours.
100,000 L • Two AC pumps run independently with a combined operation of 6
Learn to
hours (in winter) to 12 hours (in summer) per day.
Swim Club
• A backup chlorinator system is employed for chlorination on one of
the two pump lines.
• 6 SX-80 W PV panels wired in series provide power for dc motor
Burpengary
pumping for water filtration.
65,000 L
• 2 SX-80 W PV panels wired in parallel for 6 V provide for water
Residential
chlorination.
• System runs entirely on solar power but a backup power may be
required for consecutive cloudy days mainly for filtration.
• 4 SX-60 W PV panels are wired in parallel for 6 V to power the cell.
Acacia Ridge
• A 1.5 hp ac motor pump runs the water filtration.
75,000 L
• System operates on timer but during daylight hours.
Fitness and
• A conventional backup chlorinator is set to low.
Squash Club
• System was recently installed and is operating satisfactorily.
Location
and Type
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Taringa
The system at Taringa is privately owned. There are three pumps installed at this pool. The first pump
is used to pump water to a solar heater and back to the pool. A second pump is used to operate the
pool cleaner. The third pump is used to pump water through the filter, into the chlorination cell, and
back to the pool. Figure 2 shows the chlorinator cell at Taringa system.
Because the electrode cell is installed below water level, even when the pumps are not running, there
is still water ni the chlorination cell and PV current flows between the electrodes, producing chlorine.
This causes gas to build up in the cell, which becomes warm. This causes the chlorine to dissipate into
the pool by convection effect. While the filter pump is running the water is stirred up for 4 hours in
winter and up to 8 hours in summer. The cleaner pump is run for one hour a day and the heater pump
is run mostly in summer and whenever the pool is used.
Figure 2. The electrode cell housing is installed below water surface at Taringa system.
The solar chlorinator works very satisfactorily and without a backup source. The owner could not recall
having to manually add any chlorine during its entire two years of operation. The owner had to
disconnect two PV modules in winter to stop over chlorinating. Alternatively, the salt level would be
allowed to drop to below 2,000 ppm to reduce chlorine production.
Palmwoods
There are six SX-80 PV modules at Palmwoods (see
heated public swimming pool owned by the Maroochy
extensively used all year round. As such the filter pump
hours in summer per day. At the time of installation
regarding the installation of a grid interactive PV system.
Figure 3) which chlorinates a 32,000 litres
Shire in the Sunshine Coast. The pool is
runs a minimum of 20 hours in winter to 24
there was discussion with council officials
Data collected showed that by midday the chlorine level reaches 3 ppm especially on clear sunny
days. For fear of over-chlorination, the operator shut the system entirely down manually until next day.
As a consequence, the operator adds liquid chlorine to the pool early the following day at 6 am.
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Figure 3. PV chlorinator in public swimming pool at Palmwoods in the Sunshine Coast.
The system at Palmwoods requires a controller to better match chlorine requirement and make use of
excess energy. According to Australian Guidelines (1989) and Queensland Health (2000) a high level
of chlorine may be permitted and would therefore reduce the frequency to super-chlorinate the pool in
times of heavy demand. Thus, although the system is considered oversized, the operator could leave
it on and allow the chlorine level to rise to up to 8 ppm, which will avoid adding liquid chlorine.
Pallara
The system (Figure 4) at Pallara is privately owned. It comprises four SX-80 W PV modules. Three
modules are connected in parallel (6V) and a backup power pack is used. To reduce electricity bills for
pumping an 80 W 12 V PV module is used to circulate the water through the electrode cell using a dc
motor pump and a non-return valve. Because of its low power rating the plumbing line is designed to
bypass a large sand filter. A second motor runs the pump for water filtration on night tariff.
Figure 4. PV water pumping and chlorination system at Pallara, south of Brisbane.
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Although a solar powered motor was used to pump the water through the electrode, it was found that
the cell plates require very frequent cleaning. This is probably because the water was not filtered prior
to chlorination. It is considered to use a current booster to the motor for pumping the water through an
auxiliary low-pressure filter. A control circuit for switching the plates polarity for self-cleaning has been
recommended.
Logan Central
This system comprises eight SX-80 PV modules wired for 12 V, connected to two electrodes in series
via a selector switch and provide a peak current of 42 A. Due to some building restrictions with the
design of the support structure, the system suffers some shading loss of 10% until around 10 am. A
photograph of the system is shown in Figure 5. Two other electrodes are connected in series to a
mains-power pack rated 50 A at 14 V and is almost always set to minimum. This heated pool capacity
is 97,000 litres and is used all year round.
Figure 5. PV chlorination system at Logan City Gardens Centre public swimming pool.
The chlorine level of the pool during the test period (12 months) was within recommended values and
the PV system was able to meet the chlorine demand. The operator of the pool is experienced in pool
maintenance and has the system well tuned. If there is no high load, the operator will usually turn off
the power pack and just keep the solar system. The system demonstrated that the backup unit was
only required on severely consecutive overcast days. Thus, there is no need to expand the size of the
PV system, and alternatively liquid chlorine could be used if necessary.
Arana Hills
The pool at Arana Hills is part of a health and fitness club. The chlorinator system comprises four SX80 W PV modules, in-line electrode, and an ac motor-pump for filtration. For greater reliability, there is
a similar plumbing system running in parallel but employing a conventional power pack. Thus water
chlorination uses nearly 50% of the energy requirement from PV while mains supplies the remaining.
The proper place for the installation of the PV array would have been on top of the equipment room,
which is also facing north. However, for reasons of security and vandalism, the PV modules were
installed away from the entrance of the pool, as shown in Figure 6.
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Figure 6. Photovoltaic chlorinator in swimming pool at a sports club in Arana Hills.
The club manager expressed interest in the installation of a larger PV system with the view to reduce
the club’s electricity bill. A grid interactive PV system may be integrated with the pool chlorinator.
Burpengary
The installation at Burpengary comprises eight SX-80 PV modules, a dc motor driving a centrifugal
pump, a chlorinator cell and a control and instrumentation circuit. Six modules are used for pumping
for water filtration and two modules run the chlorinator cell in an in-line system. The system normally
works from sunrise to sunset unless disconnected manually by the owner. This is the first system that
runs entirely on solar power and no backup power was put in place.
Figure 7. Eight SX-80 W PV modules are used to run the swimming pool at Burpengary.
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The system was installed in September 2002; but a trial dc motor driving the pump was replaced in
May 2003. The chlorinator has been performing well with only two PV modules during low pool usage.
On clear sunny days with solar radiation intensity of around 5 kWh/sq metre, the chlorine level remains
adequate until around 5 pm. However, on consecutive cloudy days, insufficient chlorine level was
noticed. This is mainly due to the reduced filtration time on cloudy days. A larger electrode plate rated
50 A was later installed and improved the chlorine level substantially including on cloudy days.
Implementing a backup system may improve water quality. An option would be to construct a
convection electrode cell in the pool walls and use liquid chlorine for superchlorination as required.
Acacia Ridge
This system runs a pool chlorinator at a sports and squas h club in Acacia Ridge. It comprises four SX60 PV modules, a backup mains unit, a salt cell and an instrumentation circuit, as shown in Figure 8.
According to the owner, the system was set to run during daylight hours and has been running without
a problem since it was installed in May 2003. Because the system was recently commissioned
chlorination data are not yet available.
Figure 8. Installation and wiring of the salt cell at swimming pool in Acacia Ridge.
COMMERCIAL POTENTIAL
A business plan for commercialisation was prepared. This included the system’s main strengths,
weaknesses, opportunities and threats (SWOT) against conventional chlorinators. The PV electrochlorination system relies on two existing technologies/products: photovoltaics and electrolysis plates.
Thus, it is envisaged that the set up cost of the production facility can be kept low. It is believed that
the PV chlorinator can be marketed for the following reasons:
•
•
•
The technique of electrolysis of sodium chloride is mature and has been in use for many years.
There is no major change with the introduction of PV.
A 300 W power supply unit for chlorination costing $500 can be replaced with 240 W PV array at
a cost of $1500 and with no running cost saving over $200 a year, making a payback period of
five years.
The PV chlorinator can either be used in new pools or retrofitted to existing ones. Furthermore,
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Khouzam
with a total solar power system for chlorination and filtration, the need for ac cable extensions and
trenching will be eliminated.
In spite the controversy of using chlorine for treatment, many people in Australia and around the
world would regard this product as ‘environmentally friendly’.
The PV array can be installed above the pool thus providing shade and reducing loss of chlorine
due to evaporation.
The solar option could contribute to the growth in the swimming pool market amongst Australians
who regard the pool an important recreation activity.
CONCLUSIONS
The results demonstrated that the use of direct-coupled PV in the water electro-chlorination process
offers the following salient features:
•
•
•
•
The electrolysis makes use of the non-linear characteristic of PV array; in its intrinsic matching
quality, and without a need for expensive power conditioning.
The PV system offers over-current protection caused when salt concentration is exceeded or due
to contaminants in the water (an inherent problem with commercial units).
PV produces high quality dc (without ripple), which in turn offer more effective and sustained ion
separation during electro-chlorination.
There is some inherent matching of solar radiation, chlorine requirement and production and pool
usage. This is true for the daily as well as the seasonal variation in radiation and in temperature.
Seven PV chlorinators are being evaluated and showed that the use of PV for water treatment is a
practical, technically feasible and generally cheaper alternative to mains power. In spite some
challenges with installations, all systems are performing adequately well.
The PV-based chlorinator application will contribute to the renewable energy industry by creating
demand for PV devices. PV offers unpolluted production of electricity, which would result in an annual
saving of one ton of CO2 and other pollutants for an average size pool. The use of PV electrolysis
promises to offer environmental benefits, energy and cost savings if used in electrochemicals and
industrial processes.
ACKNOWLEDGMENTS
This project is funded through the Queensland Sustainable Energy Innovation Fund, Environm ental
Protection Agency. Further contribution was provided by Queensland University of Technology and by
Allchlor Repairs Pty Ltd. The author wishes to thank Dr. Martin Gellender (EPA) and Mr. Jeff
Braithwaite (Allchlor) for their continued support.
REFERENCES
Appelbaum J. (1987), The Quality of Load Matching in a Direct-Coupling PV System, IEEE Transactions
on Energy Conversion, EC-2, No. 5, December 1987, pp.534-541.
Khouzam K.Y. (2000), Demonstration of a Solar Electrochemical Plant in the Form of Salt-Water
Chlorinator”, Final Report prepared for Department of Mines and Energy, Office of Sustainable Energy,
QSEIF, October 2000.
Khouzam K.Y. (1991), Optimum Load Matching In Direct-Coupled PV Systems - Application to Resistive
Loads, IEEE Transactions on Energy Conversion, EC-5, No. 2, June 1990, pp.265-271.
National Health and Medical Research Council (1989), Australian Guidelines for Disinfecting Private
Swimming Pools, Australian Government Publishing Service, ISBN 064410291 8, 1989.
Queensland Health (2000), Swimming and Spa Pool Water Quality and Operational Guidelines,
February 2000.
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Operation of Photovoltaic Electro-Chlorination Process in Salt Water

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