SOLAR-POWERED ULTRA-VIOLET WATER PURIFICATION SYSTEM ...
SOLAR-POWERED ULTRA-VIOLET WATER PURIFICATION SYSTEM Gong-Ding Hu Department of Electrical and Computer Engineering University of Auckland, Auckland, New Zealand Abstract Lack of clean, drinkable water can often have a serious impact on the health of human beings. Ultra-violet (UV) radiation at the so called “UV-C” wavelength range presents an effective, non-chemical agent in disabling the harmful micro-organisms found in untreated water. The device is designed to provide a household with water purification capability by utilising a Philips UV-C lamp in a compact, low-maintenance and cost effective manner. The device runs at 12 V DC hence the source of power can interchange between a solar cell and a car battery. Full capacity of the device is 3.2 litres, which by running the device at full capacity four times a day should be enough to sustain a household of six people. The UV-C lamp is driven by the electronic circuitry centred on a DC-AC converter, with additional features such as an auto-shutdown timer to minimise unnecessary power consumption and a safety mechanism to prevent UV-C radiation leakage during operation. The overall germ-killing efficiency of the system is calculated to be 90%, but experimental verification on this particular aspect is still needed.
1. Introduction Access to clean, safe drinking water is absolutely crucial for human well being and development, but clean water is often not in great supply in some parts of the world, especially in undeveloped countries. According to the United Nations Department of Public Information, there are more than one billion people worldwide who lacks a stable supply of clean water. And each year more than 2.2 million people, with the majority concentrated in third world countries, die from diseases associated with poor quality of water. [1] These diseases are often caused by bacteria and virus found in water, common ones such as Cryptosporidium, Giardia and Escherichia coli (E. coli) are known to cause serious health problems. Ultra-Violet (UV) radiation provides a non-chemical alternative for water sterilization purpose. Unlike traditional treatments such as chlorine and other chemicals, UV is odourless and produces no by-product. It is very effective in killing disease-causing water-borne miro-organisms, especially at the wavelength of 253.7nm, also known as the “germicidal” wavelength [2].
The objective of this project is to develop a small-scale water purification system for a normal sized family by utilizing a commercially available UV germicidal lamp as the source of UV radiation. Portability (compactness), cost and low maintenance are the three primary concerns when designing the system. Although this application is intended to incorporate solar power, a 12V car battery is still used as the source of power during the prototype development stage. The reason being that solar battery comes in standard package nowadays hence should be able to replace the car battery in the prototype without too much difficulty.
2. System Overview The device is currently at prototype level, the purpose of this prototype is to test if the feasibility of the design approach adopted. 2.1. Physical Structure of the Device Since this is intended to be a small-scale application, only a relatively small amount of water is to be handled by the device. The UV lamp sits in the middle of the treatment area with four 800ml water containers around it. Figure 1 shows the arrangement at the water treatment area of the prototype. Due to the hazardous nature of UV-C radiation, a UV shield has to cover the entire treatment area during operation to prevent any harm to the user.
UV shield
Lamp
Beaker
Figure 1: Schematic diagram of speech production.
The power source (car battery) and the electronic circuitry are placed right next to the treatment area. Figure 2 is a picture of the prototype; note that the UV shield is not shown in the picture.
per household per day is calculated to be 15 liters. Due to concerns for the device’s compactness, the capacity is lowered to 3.2 liters in order to reduce the physical size, meaning by running the system four times a day will be sufficient to sustain a family’s daily drinking water need.
3. Electronic Circuitry and Equipments Selection 3.1. UV-C Lamp
Figure 2: picture of the prototype. 2.2. Water Container The water container has to possess the right kind of optical property to allow UV-C penetration. Typical optical materials used in UV applications are quartz and Vycor, they possess excellent optical characteristic in terms of transmission efficiency at the UV wavelength range, but are not mass produced and often expensive. The alternative water containers used in the prototype are standard laboratory beaker made of the material “Pyrex”. Figure 3 shows the transmission curve of Pyrex. [3] It has the desired optical property and yet cost-effective.
The lamp chosen for the system is the Philips TUV 11W PL-S Germicidal lamp, with a UV radiation emission almost exclusively at 253.7nm (shown in figure 4), which is about 85% of the maximum germicidal effect [6]. It is compact in size and only requires one base fitting (G23 base), unlike the conventional tubular models where two base fittings are needed on both ends. One of the special features of the PL-S series is the specially adopted starter already built into the lamp base providing almost instant starting; this saves the need for an external starter as commonly used in fluorescent lamp applications. Other UV lamps similar in size and specifications but with a different make (e.g. OSRAM HNS 11W OFR germicidal lamp) have also been considered. The decision to go for the Philips one was made based on the availability of stocks in New Zealand, which the OSRAM HNS series cannot be successfully located locally.
Figure 4: Spectral power distribution with respect to emission wavelength for Philips TUV 11W PLS disinfection lamp The rated useful life of the lamp is about 8000 hours. Figure 3: Optical transmission curve for PYREX 2.2.1.
Treatment Capacity
According to the World Health Organization, the basic hydration requirement for a human being under normal conditions is around 2.5 liters per day [4]. Assuming a normal sized family of 6, the amount of drinking water
3.2. Power Source Although it is originally intended to be a solar-powered application, a 12V DC car battery power will be used as the source of power during the research and development stage. 12 volts is selected as the reference voltage in designing the electronic control circuit. This particular value is chosen because there are 12V solar batteries available in the market, also car battery runs at
12volts, which leaves us with wider options. 3.2. Circuitry Overview 12 V DC Car Battery
Output “A”
Safety Mechanism
Output “B”
Auto-Shutdown Timer Figure 6: CD4011 quad NAND gate IC configured as a square wave oscillator.
DC-AC inverter
Power Amplifier
3.2.2. Driving circuit
18 – 230 center-tap transformer
230V 50 Hz Ballast
Power Amplifier and the Transformer
It would be very hard to drive the lamp and the ballast directly with the outputs of the DC-AC inverter IC discussed in section 3.2.1. As a result, a power amplification stage is put in to increase the current and a transformer is used to step up the voltage. According to the specifications of the ballast and the UV lamp, the goal is to deliver 230V, 190mA to the ballast, which will be enough to drive the lamp. Figure 7 shows how the power amplification stage is implemented.
Philips TUV 11W PL-S disinfection lamp Figure 5: Electronic Circuit Overview 3.2.1.
DC-AC Inverter
Similar to ordinary fluorescent lamps, the UV lamp requires an AC input. The power source of the system is a DC car battery so DC-AC conversion is needed. As shown in figure 6, a simple quad 2-input NAND gate IC (CD4011) is used to generate the AC oscillation waveforms. Two 50 Hz, out of phase square wave outputs are generated using the IC. The reason for generating two out of phase waveforms is to create two opposite directions of current flow in the primary side of the transformer to mimic the AC action; more will be discussed in the next section.
Figure 7: power amplification stage Each set of transistors (BJT) at each output is connected as a “Darlington pair”. This Darlington configuration is typically used to obtaining higher gain factor “beta”. Because the current has to be amplified to a relatively high level, “high gain” type (high “beta”) transistors are needed. The transistors also have to be able to handle high collector current Ic. TIP 140 (NPN) and 2SD880 (NPN), are selected as they have a combined DC current gain (“beta”) of 1000(amplifies the current 1000 times) as well as a maximum Ic rating of 10 A.
Since the two outputs (output A, B) from the IC are out of phase, one set of transistor will be on while the other set is off. The current flow in the circuit can then be illustrated in figure 8 and 9. As shown in figure 8, output “A” is at HIGH while output “B” is at LOW, hence only the Darlington transistors connected to output A will be activated and therefore draw current from the battery, which creates the upward direction of current flow in the primary side of the transformer.
even square waveform at the secondary side. Both output A and output B are switching at 50 Hz, which means the current direction in the primary side of the transformer switches every 1/50 of a second (i.e. 50Hz). The turns ratio, voltage and current of a transformer are governed by equation, Np / Ns = Vp / Vs = Is / Ip
(1)
Where, Np = number of turns at primary Ns = number of turns at secondary Vp = voltage at primary Vs = voltage at secondary Ip = current at primary Is = current at secondary Currently the combined amplification output stage is able to deliver 200V AC, 100mA to the lamp ballast. It is lower than the desired level as mentioned before (230V, 190mA), however, the lamp still responded to this level of power and runs correctly.
Figure 8: illustration of current flowing in the upward direction. As shown in figure 9, output “A” is at LOW while output “B” is at HIGH, hence only the Darlington transistors connected to output B will be activated and therefore draw current from the battery, which creates the downward direction of current flow in the primary side of the transformer.
Figure 9: illustration of current flowing in the download direction Note: “HIGH” represents the output voltage level is at 9V while “LOW” represents 0V. The transformer used is a 50VA, 9V-18V to 230V center tap transformer. By using the existing configuration as explained above, the upward and downward current will go through the same number of turns (windings) in the primary side, which creates an
There are many different ways to implement a power amplification stage and several other designs such as the typical class B output stage, or using MOSFET rather than BJT, are also considered. The problem with incorporating class B output stage in this design is because it requires two power sources, one positive and one negative [5], which means two batteries are needed and by doing so, severely compromise the compactness and portability of the system. During the Research and Development stage, a MOSFET driving circuit was also proposed. The MOSFET design strategy proposed has similar switching actions as the one discussed earlier which is proven to work, but one major problem is that MOSFET with high current rating often costs a lot more than BJT with high current rating. The proposed MOSFET design requires two N-Channel and two PChannel MOSFETs (2SJ162 and 2SJ1058) that costs $22 dollars each, whereas the BJTs used in the current design costs only $1.50 each. Since cost is a major factor in this project, the design using N and P channel MOSFET, although looked promising, was not further developed. 3.2.3.
Auto-Shutdown Timer
The timer is designed to automatically switch off the system when the preset sterilization time is reached. This time is currently set at two and half hours, which enable a 90% kill rate of all the micro-organisms present in the water. Sterilization time calculation will be discussed in more detail in section 4.1. The purpose of this timer is to prevent unnecessary draining of the battery during operation.
A common NE555 timer IC is used as the core of the auto-shutdown timer. Figure 10 shows the schematic diagram of the timer. Two LEDs (green and red) are added to indicate the status of the water treatment process. The green LED will be on when the system is in operation, and when the process is complete (i.e. two hours of treatment is reached), the red LED will be on and the system will be turned off.
circuitry to be connected to the power source. If the shield is removed, the spring contact will tilt back to its natural position and hence the internal switch is moved to position 1, leaving an open circuit between the circuit and the battery.
Figure 11: safety mechanism design 3.2.5
Figure 10: Schematic of the auto-shutdown timer The resistor connected to pin 6 controls the timing length. Note the 22-microfarad capacitor connected to the “reset” pin of the timer. The purpose of the capacitor is to reset the timer every time the power is switched on without the need of an external trigger or push-button. This is because the capacitor takes a short period of time to charge, briefly holding the input close to 0V when the circuit is switched on, which creates a similar action as a press of the reset button. The NE555 timer outputs a constant DC voltage when the counting is in operation. The inverter IC takes this signal as its input power source and therefore kicks into operations as well. When the timer’s counting process ends, the output goes to nearly zero volt hence shutting down the inverter IC and consequently the entire system. The inverter IC requires a minimum Vcc of 3V, but the timer can only output around 2.8V when it is in operation. Therefore, a simple non-inverting amplifier is implemented using a LM324 quad operational amplifier IC to bring the voltage up to 9V, which is sufficient to drive the inverter IC. 3.2.4
Safety Mechanism
To prevent any accidental opening of the UV shield during operation, a simple yet reliable spring-contact mechanism is used to turn off the power when the shield is removed. Figure 11 is a diagrammatical illustration of the device. If the shield is properly in placed, the weight of the shield will push down the spring contact and the internal switch will move to position 2, which allows the
System specification
Electrical Input Voltage Input Current Output Power Efficiency
: 12V DC : 2.6 A : 20W : 64%
Physical System Dimension System Weight Treatment capacity Ideal operation time (For 90% sterilization)
: 50cm x 40cm x 35cm : 15 Kg (including battery) : 3.2 L : 2.5 Hours
The overall cost of the system comes to $63.5 dollars, $40 for the lamp and ballast, and $23.5 for the electronic components. Note that car or solar battery is not included.
4. 4.1
System operation guidelines
Operation duration
The untreated water has to be left under UV-C exposure for a certain period of time in order for the germicidal action to effectively take place. The exposure duration approximation needed for a 90% kill rate, suggested by the lamp manufacturer (Philips), is calculated as follows [6]: Nt / N0 = exp. (-k x Eeff x t) Hence ln Nt / N0 = -k x Eeff x t • • • •
(2) (3)
Nt is the number of germs at time t(s) N0 is the number of germs before exposure k is a rate constant depending on the species Eeff is the effective irradiance in W/m2
For Philips TUV 11W PL-S, Eeff is 0.32 W/m2 [6] From Eqn. 3, the equation for 90% kill rate is calculated to be: 2.303 = k x Heff •
(4)
Heff is called “effective dose”, Heff = Eeff x t
Table 1 shows some common water-borne microorganisms and their respective k value, effective dose, and time needed to reduce their numbers by 90%. [6] Table 1: Time needed to reduce the number of microorganisms by 90% Time Effective Bacteria/Virus Dose (W. s / K required m2) (s) Escherichia Coli 30.0 0.077 94 Polio virus 58.0 0.040 180 Giardia Lamblia 11.0 0.209 35 From table 1, it is clear that 180 seconds of UV exposure under TUV 11W PL-S lamp is enough to kill 90% of those three types of micro-organism. According to Table C.1 in Appendix C, Blue Green Algae has the highest effective dose hence requires the longest time to reduce its number by 90% (i.e. 2.5 hours). As a result, by running the system for 2.5 hours during each time of use, the system is able to reduce most common microorganisms present in drinking water by 90%.
5.
Discussions, Recommendations and Possible Future Work
Unfortunately, the experimental verification on the germ-killing efficiency of the system could not be performed in time due to time constraints. The expected 90 % germ-killing efficiency system was calculated based on theoretical data only, actual testing must be carried out before any conclusion regarding to this aspect is made. Possible future work on this project includes the incorporation of solar energy as the source of power as originally intended. Some aspects include replacing the existing car battery with a solar battery, or to use solar battery as the charging device. The system is ultimately hoped to run as a dual supply application, with solar energy as primary power and car battery as back up. The electrical efficiency of the system is only at 64%, which is quite poor in today’s standard. Some of the
reasons contribute to this low efficiency include the use of the ballast and the square signal waveform. The lamp’s ballast is designed for the lamp to work off the AC-main. As the result, the ballast is like a piece of inductor, which steps the power down to the desired level for the lamp during operation. Basically the electronics currently in the prototype was design to create an AC-main like power signal from a 12V car DC battery. In a way this is quite inefficient because voltage is to be stepped up in the transformer and later on to be stepped down again by the ballast. One possible solution to this problem is to exclude the ballast and drive the lamp directly from the DC-AC converter, but the converter will have to be designed as a “fly-back” type converter that provides a large spike of voltage at start to start up the lamp and then settles back to the lamp’s ideal operation condition. This approach should be able to improve the efficiency of the system. Another factor contributing to the loss of efficiency is the waveform the power is transmitted at; in this case it is a square wave. Ideally, AC signals are best to be transmitted in a sinusoidal waveform, but since this design is only a low-powered application, square wave is acceptable although less efficient. The reason being that transformers don’t respond that well to square waves (high frequency components), it causes a 20% rise in temperature co-efficient meaning higher power loss in the transformer due to heat. Maximum treatment capacity of the system is yet to be determined due to the lack of experimental testing results.
6. Conclusions The overall system is able to drive the UV lamp with a 12V DC car battery. An auto shutdown device is added to control the sterilization time. Due to the hazardous nature of the UV radiation, a safety mechanism is included in the system to prevent any UV exposure to the user. Electronics used to build the prototype are relatively cheap and easy to obtain. The overall cost of the system is $63.5 dollars. The full treatment capacity of the system is 3.2 liters, which by running the system at full capacity four times a day should be enough to sustain a average sized family. Total sterilization efficiency of the system is expected to be at 90% but it is yet to be experimentally verified. The system is currently at prototype level and there is still room for improvements such as incorporation of solar power and increase the overall efficiency of the electronic circuitry.
Acknowledgements
Firstly, I would like to specially thank our project supervisor, Mr. Chris Smaill, for his valuable knowledge inputs and the guidance he provided us with throughout the course of the project. Other academic staffs include Associate professor Dr. Gillian Lewis from the Department of Biological Science and Dr. P.T. Elefsiniotis from the Department of Civil & Environmental Engineering, for their help on the bacterial-content testing aspect of the project. My project partner, Yi-Che Wu, for his contribution to the project. I would also like to thank Mr. Ian Farquhar and Mr. Jason Hitchen from Lamp Specialists Ltd. (Penrose branch), for kindly sponsoring the Philips ultraviolet germicidal lamp and its ballast, as well as Mrs. Katrina Graaf from the Chemistry department for lending the lab beakers which enabled us to build the prototype.
7. References [1] United Nations, Fact Sheet: Water: A Matter of Life and Death, Retrieved 6.May.2005, from http://www.un.org/events/water [2] Masschelein, W.J and Rice, R.G. UV Light in Water and
Wastewater Sanitation, Lewis Publishers, Boca Raton, FL, 2002, pp. 60-64. [3] Prazisions Glas & Optik, PYREX, Retrieved 13.Apr.2005, http://www.pgo-online.com/intlframes/pyrexset.html [4] World Health Organisation, Domestic water quantity, service level and health, Retrieved 13.Mar.2005, from http://www.who.int/water_sanitation_health/diseases/wsh 0302/en
[5] Sedra, A.S. and Smith, K.C., Microelectronic Circuits, Oxford University Press, New York, 2004, pp. 1229-1271. [6] Phillips, Appl. Note: UV Disinfection 3222 635 43401, pp. 8-11.
1. Introduction Access to clean, safe drinking water is absolutely crucial for human well being and development, but clean water is often not in great supply in some parts of the world, especially in undeveloped countries. According to the United Nations Department of Public Information, there are more than one billion people worldwide who lacks a stable supply of clean water. And each year more than 2.2 million people, with the majority concentrated in third world countries, die from diseases associated with poor quality of water. [1] These diseases are often caused by bacteria and virus found in water, common ones such as Cryptosporidium, Giardia and Escherichia coli (E. coli) are known to cause serious health problems. Ultra-Violet (UV) radiation provides a non-chemical alternative for water sterilization purpose. Unlike traditional treatments such as chlorine and other chemicals, UV is odourless and produces no by-product. It is very effective in killing disease-causing water-borne miro-organisms, especially at the wavelength of 253.7nm, also known as the “germicidal” wavelength [2].
The objective of this project is to develop a small-scale water purification system for a normal sized family by utilizing a commercially available UV germicidal lamp as the source of UV radiation. Portability (compactness), cost and low maintenance are the three primary concerns when designing the system. Although this application is intended to incorporate solar power, a 12V car battery is still used as the source of power during the prototype development stage. The reason being that solar battery comes in standard package nowadays hence should be able to replace the car battery in the prototype without too much difficulty.
2. System Overview The device is currently at prototype level, the purpose of this prototype is to test if the feasibility of the design approach adopted. 2.1. Physical Structure of the Device Since this is intended to be a small-scale application, only a relatively small amount of water is to be handled by the device. The UV lamp sits in the middle of the treatment area with four 800ml water containers around it. Figure 1 shows the arrangement at the water treatment area of the prototype. Due to the hazardous nature of UV-C radiation, a UV shield has to cover the entire treatment area during operation to prevent any harm to the user.
UV shield
Lamp
Beaker
Figure 1: Schematic diagram of speech production.
The power source (car battery) and the electronic circuitry are placed right next to the treatment area. Figure 2 is a picture of the prototype; note that the UV shield is not shown in the picture.
per household per day is calculated to be 15 liters. Due to concerns for the device’s compactness, the capacity is lowered to 3.2 liters in order to reduce the physical size, meaning by running the system four times a day will be sufficient to sustain a family’s daily drinking water need.
3. Electronic Circuitry and Equipments Selection 3.1. UV-C Lamp
Figure 2: picture of the prototype. 2.2. Water Container The water container has to possess the right kind of optical property to allow UV-C penetration. Typical optical materials used in UV applications are quartz and Vycor, they possess excellent optical characteristic in terms of transmission efficiency at the UV wavelength range, but are not mass produced and often expensive. The alternative water containers used in the prototype are standard laboratory beaker made of the material “Pyrex”. Figure 3 shows the transmission curve of Pyrex. [3] It has the desired optical property and yet cost-effective.
The lamp chosen for the system is the Philips TUV 11W PL-S Germicidal lamp, with a UV radiation emission almost exclusively at 253.7nm (shown in figure 4), which is about 85% of the maximum germicidal effect [6]. It is compact in size and only requires one base fitting (G23 base), unlike the conventional tubular models where two base fittings are needed on both ends. One of the special features of the PL-S series is the specially adopted starter already built into the lamp base providing almost instant starting; this saves the need for an external starter as commonly used in fluorescent lamp applications. Other UV lamps similar in size and specifications but with a different make (e.g. OSRAM HNS 11W OFR germicidal lamp) have also been considered. The decision to go for the Philips one was made based on the availability of stocks in New Zealand, which the OSRAM HNS series cannot be successfully located locally.
Figure 4: Spectral power distribution with respect to emission wavelength for Philips TUV 11W PLS disinfection lamp The rated useful life of the lamp is about 8000 hours. Figure 3: Optical transmission curve for PYREX 2.2.1.
Treatment Capacity
According to the World Health Organization, the basic hydration requirement for a human being under normal conditions is around 2.5 liters per day [4]. Assuming a normal sized family of 6, the amount of drinking water
3.2. Power Source Although it is originally intended to be a solar-powered application, a 12V DC car battery power will be used as the source of power during the research and development stage. 12 volts is selected as the reference voltage in designing the electronic control circuit. This particular value is chosen because there are 12V solar batteries available in the market, also car battery runs at
12volts, which leaves us with wider options. 3.2. Circuitry Overview 12 V DC Car Battery
Output “A”
Safety Mechanism
Output “B”
Auto-Shutdown Timer Figure 6: CD4011 quad NAND gate IC configured as a square wave oscillator.
DC-AC inverter
Power Amplifier
3.2.2. Driving circuit
18 – 230 center-tap transformer
230V 50 Hz Ballast
Power Amplifier and the Transformer
It would be very hard to drive the lamp and the ballast directly with the outputs of the DC-AC inverter IC discussed in section 3.2.1. As a result, a power amplification stage is put in to increase the current and a transformer is used to step up the voltage. According to the specifications of the ballast and the UV lamp, the goal is to deliver 230V, 190mA to the ballast, which will be enough to drive the lamp. Figure 7 shows how the power amplification stage is implemented.
Philips TUV 11W PL-S disinfection lamp Figure 5: Electronic Circuit Overview 3.2.1.
DC-AC Inverter
Similar to ordinary fluorescent lamps, the UV lamp requires an AC input. The power source of the system is a DC car battery so DC-AC conversion is needed. As shown in figure 6, a simple quad 2-input NAND gate IC (CD4011) is used to generate the AC oscillation waveforms. Two 50 Hz, out of phase square wave outputs are generated using the IC. The reason for generating two out of phase waveforms is to create two opposite directions of current flow in the primary side of the transformer to mimic the AC action; more will be discussed in the next section.
Figure 7: power amplification stage Each set of transistors (BJT) at each output is connected as a “Darlington pair”. This Darlington configuration is typically used to obtaining higher gain factor “beta”. Because the current has to be amplified to a relatively high level, “high gain” type (high “beta”) transistors are needed. The transistors also have to be able to handle high collector current Ic. TIP 140 (NPN) and 2SD880 (NPN), are selected as they have a combined DC current gain (“beta”) of 1000(amplifies the current 1000 times) as well as a maximum Ic rating of 10 A.
Since the two outputs (output A, B) from the IC are out of phase, one set of transistor will be on while the other set is off. The current flow in the circuit can then be illustrated in figure 8 and 9. As shown in figure 8, output “A” is at HIGH while output “B” is at LOW, hence only the Darlington transistors connected to output A will be activated and therefore draw current from the battery, which creates the upward direction of current flow in the primary side of the transformer.
even square waveform at the secondary side. Both output A and output B are switching at 50 Hz, which means the current direction in the primary side of the transformer switches every 1/50 of a second (i.e. 50Hz). The turns ratio, voltage and current of a transformer are governed by equation, Np / Ns = Vp / Vs = Is / Ip
(1)
Where, Np = number of turns at primary Ns = number of turns at secondary Vp = voltage at primary Vs = voltage at secondary Ip = current at primary Is = current at secondary Currently the combined amplification output stage is able to deliver 200V AC, 100mA to the lamp ballast. It is lower than the desired level as mentioned before (230V, 190mA), however, the lamp still responded to this level of power and runs correctly.
Figure 8: illustration of current flowing in the upward direction. As shown in figure 9, output “A” is at LOW while output “B” is at HIGH, hence only the Darlington transistors connected to output B will be activated and therefore draw current from the battery, which creates the downward direction of current flow in the primary side of the transformer.
Figure 9: illustration of current flowing in the download direction Note: “HIGH” represents the output voltage level is at 9V while “LOW” represents 0V. The transformer used is a 50VA, 9V-18V to 230V center tap transformer. By using the existing configuration as explained above, the upward and downward current will go through the same number of turns (windings) in the primary side, which creates an
There are many different ways to implement a power amplification stage and several other designs such as the typical class B output stage, or using MOSFET rather than BJT, are also considered. The problem with incorporating class B output stage in this design is because it requires two power sources, one positive and one negative [5], which means two batteries are needed and by doing so, severely compromise the compactness and portability of the system. During the Research and Development stage, a MOSFET driving circuit was also proposed. The MOSFET design strategy proposed has similar switching actions as the one discussed earlier which is proven to work, but one major problem is that MOSFET with high current rating often costs a lot more than BJT with high current rating. The proposed MOSFET design requires two N-Channel and two PChannel MOSFETs (2SJ162 and 2SJ1058) that costs $22 dollars each, whereas the BJTs used in the current design costs only $1.50 each. Since cost is a major factor in this project, the design using N and P channel MOSFET, although looked promising, was not further developed. 3.2.3.
Auto-Shutdown Timer
The timer is designed to automatically switch off the system when the preset sterilization time is reached. This time is currently set at two and half hours, which enable a 90% kill rate of all the micro-organisms present in the water. Sterilization time calculation will be discussed in more detail in section 4.1. The purpose of this timer is to prevent unnecessary draining of the battery during operation.
A common NE555 timer IC is used as the core of the auto-shutdown timer. Figure 10 shows the schematic diagram of the timer. Two LEDs (green and red) are added to indicate the status of the water treatment process. The green LED will be on when the system is in operation, and when the process is complete (i.e. two hours of treatment is reached), the red LED will be on and the system will be turned off.
circuitry to be connected to the power source. If the shield is removed, the spring contact will tilt back to its natural position and hence the internal switch is moved to position 1, leaving an open circuit between the circuit and the battery.
Figure 11: safety mechanism design 3.2.5
Figure 10: Schematic of the auto-shutdown timer The resistor connected to pin 6 controls the timing length. Note the 22-microfarad capacitor connected to the “reset” pin of the timer. The purpose of the capacitor is to reset the timer every time the power is switched on without the need of an external trigger or push-button. This is because the capacitor takes a short period of time to charge, briefly holding the input close to 0V when the circuit is switched on, which creates a similar action as a press of the reset button. The NE555 timer outputs a constant DC voltage when the counting is in operation. The inverter IC takes this signal as its input power source and therefore kicks into operations as well. When the timer’s counting process ends, the output goes to nearly zero volt hence shutting down the inverter IC and consequently the entire system. The inverter IC requires a minimum Vcc of 3V, but the timer can only output around 2.8V when it is in operation. Therefore, a simple non-inverting amplifier is implemented using a LM324 quad operational amplifier IC to bring the voltage up to 9V, which is sufficient to drive the inverter IC. 3.2.4
Safety Mechanism
To prevent any accidental opening of the UV shield during operation, a simple yet reliable spring-contact mechanism is used to turn off the power when the shield is removed. Figure 11 is a diagrammatical illustration of the device. If the shield is properly in placed, the weight of the shield will push down the spring contact and the internal switch will move to position 2, which allows the
System specification
Electrical Input Voltage Input Current Output Power Efficiency
: 12V DC : 2.6 A : 20W : 64%
Physical System Dimension System Weight Treatment capacity Ideal operation time (For 90% sterilization)
: 50cm x 40cm x 35cm : 15 Kg (including battery) : 3.2 L : 2.5 Hours
The overall cost of the system comes to $63.5 dollars, $40 for the lamp and ballast, and $23.5 for the electronic components. Note that car or solar battery is not included.
4. 4.1
System operation guidelines
Operation duration
The untreated water has to be left under UV-C exposure for a certain period of time in order for the germicidal action to effectively take place. The exposure duration approximation needed for a 90% kill rate, suggested by the lamp manufacturer (Philips), is calculated as follows [6]: Nt / N0 = exp. (-k x Eeff x t) Hence ln Nt / N0 = -k x Eeff x t • • • •
(2) (3)
Nt is the number of germs at time t(s) N0 is the number of germs before exposure k is a rate constant depending on the species Eeff is the effective irradiance in W/m2
For Philips TUV 11W PL-S, Eeff is 0.32 W/m2 [6] From Eqn. 3, the equation for 90% kill rate is calculated to be: 2.303 = k x Heff •
(4)
Heff is called “effective dose”, Heff = Eeff x t
Table 1 shows some common water-borne microorganisms and their respective k value, effective dose, and time needed to reduce their numbers by 90%. [6] Table 1: Time needed to reduce the number of microorganisms by 90% Time Effective Bacteria/Virus Dose (W. s / K required m2) (s) Escherichia Coli 30.0 0.077 94 Polio virus 58.0 0.040 180 Giardia Lamblia 11.0 0.209 35 From table 1, it is clear that 180 seconds of UV exposure under TUV 11W PL-S lamp is enough to kill 90% of those three types of micro-organism. According to Table C.1 in Appendix C, Blue Green Algae has the highest effective dose hence requires the longest time to reduce its number by 90% (i.e. 2.5 hours). As a result, by running the system for 2.5 hours during each time of use, the system is able to reduce most common microorganisms present in drinking water by 90%.
5.
Discussions, Recommendations and Possible Future Work
Unfortunately, the experimental verification on the germ-killing efficiency of the system could not be performed in time due to time constraints. The expected 90 % germ-killing efficiency system was calculated based on theoretical data only, actual testing must be carried out before any conclusion regarding to this aspect is made. Possible future work on this project includes the incorporation of solar energy as the source of power as originally intended. Some aspects include replacing the existing car battery with a solar battery, or to use solar battery as the charging device. The system is ultimately hoped to run as a dual supply application, with solar energy as primary power and car battery as back up. The electrical efficiency of the system is only at 64%, which is quite poor in today’s standard. Some of the
reasons contribute to this low efficiency include the use of the ballast and the square signal waveform. The lamp’s ballast is designed for the lamp to work off the AC-main. As the result, the ballast is like a piece of inductor, which steps the power down to the desired level for the lamp during operation. Basically the electronics currently in the prototype was design to create an AC-main like power signal from a 12V car DC battery. In a way this is quite inefficient because voltage is to be stepped up in the transformer and later on to be stepped down again by the ballast. One possible solution to this problem is to exclude the ballast and drive the lamp directly from the DC-AC converter, but the converter will have to be designed as a “fly-back” type converter that provides a large spike of voltage at start to start up the lamp and then settles back to the lamp’s ideal operation condition. This approach should be able to improve the efficiency of the system. Another factor contributing to the loss of efficiency is the waveform the power is transmitted at; in this case it is a square wave. Ideally, AC signals are best to be transmitted in a sinusoidal waveform, but since this design is only a low-powered application, square wave is acceptable although less efficient. The reason being that transformers don’t respond that well to square waves (high frequency components), it causes a 20% rise in temperature co-efficient meaning higher power loss in the transformer due to heat. Maximum treatment capacity of the system is yet to be determined due to the lack of experimental testing results.
6. Conclusions The overall system is able to drive the UV lamp with a 12V DC car battery. An auto shutdown device is added to control the sterilization time. Due to the hazardous nature of the UV radiation, a safety mechanism is included in the system to prevent any UV exposure to the user. Electronics used to build the prototype are relatively cheap and easy to obtain. The overall cost of the system is $63.5 dollars. The full treatment capacity of the system is 3.2 liters, which by running the system at full capacity four times a day should be enough to sustain a average sized family. Total sterilization efficiency of the system is expected to be at 90% but it is yet to be experimentally verified. The system is currently at prototype level and there is still room for improvements such as incorporation of solar power and increase the overall efficiency of the electronic circuitry.
Acknowledgements
Firstly, I would like to specially thank our project supervisor, Mr. Chris Smaill, for his valuable knowledge inputs and the guidance he provided us with throughout the course of the project. Other academic staffs include Associate professor Dr. Gillian Lewis from the Department of Biological Science and Dr. P.T. Elefsiniotis from the Department of Civil & Environmental Engineering, for their help on the bacterial-content testing aspect of the project. My project partner, Yi-Che Wu, for his contribution to the project. I would also like to thank Mr. Ian Farquhar and Mr. Jason Hitchen from Lamp Specialists Ltd. (Penrose branch), for kindly sponsoring the Philips ultraviolet germicidal lamp and its ballast, as well as Mrs. Katrina Graaf from the Chemistry department for lending the lab beakers which enabled us to build the prototype.
7. References [1] United Nations, Fact Sheet: Water: A Matter of Life and Death, Retrieved 6.May.2005, from http://www.un.org/events/water [2] Masschelein, W.J and Rice, R.G. UV Light in Water and
Wastewater Sanitation, Lewis Publishers, Boca Raton, FL, 2002, pp. 60-64. [3] Prazisions Glas & Optik, PYREX, Retrieved 13.Apr.2005, http://www.pgo-online.com/intlframes/pyrexset.html [4] World Health Organisation, Domestic water quantity, service level and health, Retrieved 13.Mar.2005, from http://www.who.int/water_sanitation_health/diseases/wsh 0302/en
[5] Sedra, A.S. and Smith, K.C., Microelectronic Circuits, Oxford University Press, New York, 2004, pp. 1229-1271. [6] Phillips, Appl. Note: UV Disinfection 3222 635 43401, pp. 8-11.
