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Comparison of CFL-based and LED-based solar lanterns
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Comparison of CFL-based and LED-based solar lanterns A.K. Mukerjee Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi-110 016, India E-mail: [email protected]
Solar lanterns are designed to replace hurricane lanterns for use in remote places where conventional electricity is not available. They not only provide illumination at night but are also instrumental in avoiding the emission of greenhouse gases normally emitted from burning the wick and kerosene in the latter. Light is produced in a solar lantern by igniting either a compact fluorescent lamp (CFL) or a light emitting diode (LED) while the energy to do so is generated by a string of solar cells, called a solar panel, exposed to sunlight. This energy, generated during sunlight hours, is stored in a battery to be used at night. LEDs are the latest development as a source of light. Therefore the cost and performance of two solar lanterns, one based on a CFL and the other based on an LED, have been studied and compared to determine the better device for low-level lighting in the future. The comparison is based on lumens/watt (lm/W) output of the device. It has been found that the LED- and the CFL-based system cost almost the same if they are used to replace the hurricane lantern which has a light output of 40 to 50 lm. The life of the former is more, typically 10,000 to 50,000 hours with luminous efficacy of 47 lm/W, and its driving and control circuit is less complicated than that of the latter. The luminous efficacy of the latter is slightly higher, 50 to 60 lm/W, and it has a life span of up to 15,000 hours. On the other hand, LEDs are available from powers ranging from a few mW to 3 W, enabling lower-illumination lanterns to be constructed as an almost exact replacement of the hurricane lantern. However, at higher levels of illumination the CFL is still a cheaper source. The electrical efficiency of the circuit used with the CFL is about 82 % whereas that used with the LED is 69 % since it is based on an analog constant current generator. The efficiency of the latter can be improved by using a switch-mode constant current generator. 1. Introduction Solar lanterns were made as a viable alternative to the hurricane lantern for use in the remote areas of India where conventional electric power was not available [Bhargava and Sastry, 1982]. Subsequently a model was designed and fabricated [Mukerjee, 2000] to be marketed as desired in a project financed by the Ministry of NonConventional Energy Sources (MNES) (now renamed Ministry of New and Renewable Energy, MNRE), Government of India, in 1992 with the objective of supplying the basic lighting needs of people living in those areas where they were dependent on the hurricane lantern for illumination [Dutta et al., 1993]. Since sunlight is freely and abundantly available at all places not covered by clouds, it can be easily converted into electricity by solar cells, a string of which makes a solar panel. Details about solar panels are given in the catalogue of Central Electronics Limited (CEL) [CEL, 2007] and other manufacturers. This electricity can then be stored in suitable batteries, normally lead-acid maintenance-free type, to be used at night or in dark places. To produce light the options were limited to either a general lighting service (GLS) incandescent filament lamp, the ordinary household electric bulb, or the compact fluorescent lamp (CFL), 24
since other devices such as the 1.2-m fluorescent tube are either too bulky or consume too much power or both. The objective was simply to produce enough light to equal or slightly exceed that of the kerosene lantern – hence the name solar lantern. Since the cost of a solar panel is about (US)$ 4 per peak watt (Wp), the light output of the lamp per unit power input, defined as its efficacy, had to be as high as possible to keep the cost of the solar lantern within the reach of the buyer. The clear choice was therefore the CFL with an efficacy of 50 to 60 lumens/watt (lm/W) [Osram, 2007] over the GLS lamp with its efficacy of about 10-14 lm/W [Cayless and Marsden, 1983, p. 475]. CFLs are gas discharge tubes and therefore require high voltages, of the order of hundreds of volts, to start glowing. For this reason an oscillator circuit with high output voltage and the required power to operate the tube has to be constructed. This also must have a high electrical efficiency, exceeding 80 %, so that most of the solar electricity actually reaches the tube instead of being wasted in the circuit. The block diagram of the solar lantern is shown in Figure 1. A solar panel generates the electricity, which is given to the oscillator and the control circuit. The control circuit, in addition to operating the oscillator, also contains a charge controller which saves the battery
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Figure 1. Block diagram of a solar lantern
Figure 2. A p-n junction (left) as a light emitting diode (right)
from being damaged by preventing it from getting either over-charged or over-discharged. The ballast, in this case a capacitor, ensures that excessive current does not flow through the lamp and thus prolongs the lamp’s life. The CFL is a miniature version of the normal 1.2 m (fluorescent) tube light. The 1.2 m lamp is a low-pressure gas discharge tube with two filaments at the ends. The finished lamp is filled with an inert gas such as argon, to a pressure of 200-660 Pa (1.5 to 5 mm of mercury). A tiny droplet of mercury is introduced which is used in the discharge to create light. The discharge produces, in addition to visible light, ultraviolet radiation which is then converted to visible light by a coating of phosphor on the inside of the tube [Cayless and Marsden, 1983, p. 185]. This conversion of wavelength is responsible for the increased emission of visible light resulting in a higher efficacy. Light emitting diodes (LEDs) have now come up as an energy-efficient alternative to the CFL at low-level illumination, with a considerably longer life. Being solid state devices, they can be operated with direct current at low voltage levels of 2 to 4 V. Hence it was an interesting exercise to compare lanterns made with these two light sources.
The efficacy of LEDs has steadily increased over the last few years. An interesting comparison among efficacies of different light sources, and the progress the LEDs have made with time, can be seen in [Craford et al., 2001]. It shows a steep rise in the efficacy of LEDs from 1985 to 2000. The white light LED was in its infancy with an efficacy of 20 lm/W in 2000. Now it is 47 lm/W [Cree, 2007] and is a strong rival of the CFL. The basic LED consists of a junction between two different semiconductor materials, one n-type and the other p-type (illustrated in Figure 2), in which an applied voltage produces a current flow, which generates light when charge carriers injected across the junction recombine. This p-n junction in the form of an integrated circuit chip is placed in a concave reflector with two wires attached to carry current to it, with an epoxy-based lens in front [Spring et al., 2003]. The reflector sends the backward-going light to the front whereas the lens focuses it at the desired angle, which can range from 15º to 100º. The frequency, and perceived colour, of emitted photons is characteristic of the semiconductor material, and consequently, different colours are achieved by making changes in the semiconductor composition of the chip.
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Table 1. Some characteristics of different light sources Flux (lm)
Colour[1]
Luminous efficacy (lm/W)
Specific fuel consumption (kg/klmh)
Fuel consumption rate (kg/h)
Candle
12
Excellent
0.2[2]
0.5[3]
0.006
Kerosene wick
40
Excellent
0.1
0.8
0.032
Kerosene mantle
400
Poor
0.8
0.1
0.040
60 W incand.
730
Excellent
12
0.21[4]
0.016
16 W compact fluorescent
900
Good
56
0.005
0.004
[5]
56.4
Good
47
-
-
Light source
1.2 W LED
Source: [van der Plas, 1988]; original includes additional non-electric sources – carbide lamp, butane lamp, and gasoline mantle lamp; compact fluorescent data calculated here from data presented earlier in the chapter. Notes 1. Colour rendering index (CRI) is difficult to measure for non-electric sources, according to van der Plas [1988]. Thus, colour quality is shown here on a qualitative scale. 2. For non-electric lamps, the power input in W has been calculated using the higher heating value of the fuel. 3. Specific fuel consumption is expressed as kg of fuel use per 1000 lm-h of light (kg/klm-h). 4. For electric sources, we assume a conversion efficiency of 0.30 from kerosene to electricity, and express specific fuel consumption and fuel consumption rate as equivalent amounts of kerosene. 5. The LED data has been taken from [Cree, 2007].
Table 2. Another comparison of electric and non-electric light sources Light source
Flux (lm)
Luminous efficacy Specific fuel consumption (lm/W) (kg/klmh)
Fuel consumption rate (kg/h)
Kerosene Chirag (diya)[1]
21
0.26[2]
0.299[3]
0.006
Hurricane lantern
69
0.38
0.203
0.014
Petromax (mantle)
1303
1.25
0.056
0.073
25 W incandescent
225
9
0.029[4]
0.007
60 W incandescent
660
11
0.023
0.016
Electricity
Source: Selected from [Rajvanshi, 1987] as quoted in [Sinha and Kandpal, 1991]. Notes 1. The chirag or diya is a simple lamp made using a wick and a glass bottle. 2. Same as Note 2 of Table 1 3. Same as Note 3 of Table 1 4. Same as Note 4 of Table 1
[Mills, 2007] gives figures to indicate that there is a large variation in the efficacies of the LEDs available in the market. Therefore, care should be taken to select suitable products while fabricating lanterns. Dependence on fossil fuels like kerosene and the resulting polluting greenhouse gases had also to be reduced for which the solar lantern was the obvious answer. Besides, the cost of transporting the kerosene to such locations is high and the buyers, especially in hilly regions, have to travel long distances over difficult terrain to fetch it. Fuel-based lighting was shown to be significantly costlier than solar-powered CFL and LED alternatives, per unit of lighting service delivered. It was calculated [Jones et al., 2005] that the running cost of a hurricane-style kerosene lamp is approximately $ 0.40 per 1000 lumen 26
hours (klmh), while that of a solar-CFL lantern is $ 0.17/klmh, and a solar-LED device costs $ 0.15/klmh. Furthermore, as LED efficiencies continue to improve, solar-LED products will become even more economical. [Dutt, 1994] gives the data about the output of different sources as shown in Tables 1 and 2. The two tables show difference in the light output from the kerosene wick since the material, the thickness and the burning conditions may not have been the same in the two cases. However, an idea of the light outputs can be had by comparing the two. [Rajvanshi, 2005; 2006] gives additional data on the output from different sources, but the data of interest, the light output from kerosene lanterns, does not differ much from that given in Table 2. The 1 W white LED from Cree Inc., type XR7090WT, gives an output of 47 lm/W
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[Cree, 2007] and can be ranked as the second best source of light next to the 16 W CFL. This device costs $ 3 per unit in strip package and $ 3.50 if reflow-mounted on a 1.9 cm round or star printed circuit board. It also has a built-in lens that can diverge light at an angle of 100º. Two lanterns were, therefore, constructed for comparison. One was made from white LEDs and the other was made from a 5 W CFL with a soft/cold start, the most efficient firing mechanism, to give the maximum electrical circuit efficiency [Mukerjee and Dasgupta, 2007]. 2. Experiments Comparisons were made between the two circuits for their electrical efficiency and cost. Both these circuits were designed and fabricated by the author; one for the MNES, as mentioned earlier, but subsequently modified to compensate for the changes in the battery voltage with temperature, and the other to operate a white LED from a constant current source. The first circuit, for the CFL, has been widely manufactured and sold all over India and is very reliable. The basis for this comparison is to find the most cost-effective solution for the replacement of the hurricane lantern, which should provide light for about three hours with the required protections for the battery [TEC, 1993]. 2.1. The CFL-based circuit The circuit of the whole lantern is split into two parts: (1) the charge-discharge controller as shown in Figure 3 and the (2) oscillator plus the lamp as shown in Figure 4. These are discussed below. 2.1.1. The charge-discharge controller Figure 3 shows the circuit of the charge-discharge controller of the CFL-based lantern, which can operate either a 4-pin 5 W, or a 7 W CFL of any standard make. This circuit is needed to prevent the battery, of the sealed maintenance-free lead-acid type [Linden, 1995], from being over-charged or over-discharged, thus prolonging its life. This also controls the switching of the oscillator, which in turn drives the CFL, as shown in Figure 4. The description of the oscillator follows later. An LM 324 14-pin quad operational amplifier (four opamps in one envelope) is used to operate the controller. The four sections of the opamp are indicated by four triangles with their accompanying components. One section, U3C, with its associated components, makes a stable 5 V regulator used as a reference source. U1A uses this voltage to form a 14.5 V feedback regulator with the P-channel MOSFET MTP 2955E as the series pass element. Diodes D1, D2, and D3 are low-voltage drop Schottky diodes, which protect the circuit against accidental reversal of photovoltaic panel and battery polarities. Diodes D7 to D14 are 1N 4148 silicon diodes in the feedback loop of the regulator, used to provide temperature compensation for the battery during the process of charging. The 12 V lead-acid maintenance-free battery is connected between the B+ and B- terminals to store the charge generated by the PV panel during daytime while the PV panel is connected between the PV+ and PV- terminals. U2B acts as a comparator by detecting the voltage across diode
D3. When the battery is being charged the terminal 5 of U2B is more positive than the terminal 6 and the green light emitting diode, D11, glows to indicate it. When the battery voltage goes lower than 11.5 V the output voltage of operational amplifier U4D goes low and the transistor Q1 stops conducting. This shuts down the CFL-driving oscillator, shown in Figure 4, while the red LED D14 gives an indication of “battery low”. When the battery is charged from a partially discharged condition the series resistance of the PV panel reduces its output voltage to latch on to that of the battery plus some voltage drop across the MOSFET and diodes that come in series. When the battery is fully charged the output of the regulator settles down to a fixed value of 14.5 V so that overcharging does not damage the battery. However, the regulator continues to furnish a small current to compensate the loss of charge, an inherent property of lead-acid cells, which is known as trickle charging. A double-pole, double-throw switch, SW 1, turns the lantern on/off manually. A 10 W solar panel is used to charge the 12 V battery. Although this is the nominal battery voltage its actual value really depends on its state of charge. This may vary from 10.8 V, for deep discharge, to 14.5 V for a fully charged state for both lead-acid and valve-regulated leadacid types [TEC, 1993; Linden, 1995; Berndt, 1993]. Therefore, the open circuit voltage of the panel should be 12/0.8 = 15 V so that it operates at its maximum power point. Since there are some diodes and a MOSFET in the charging path the panel voltage is usually kept between 16 to 17 V to also account for the temperature rise in the tropics. 2.1.2. The oscillator and the lamp circuit The circuit diagram of the oscillator used to drive the CFL is shown in Figure 4. This is a current-fed, class-D sinusoidal circuit in a push-pull configuration, described in [Yarrow, 1959] and [Baxandall, 1959], that has a theoretical efficiency of 100 %, which is the reason for its selection. Diode D1 ensures that a negative polarity bus voltage from a solar panel or a battery does not damage the circuit while the inductor L2 maintains a current-fed topology. The value of L2 should be at least five times the primary inductance of the transformer T1 from its centre tap. The primary voltage is then stepped up to about 125 Vrms (root mean square voltage value) to ignite the CFL, which is either a 5 W or a 7 W device, through a ballast capacitor C2. To help start the discharge in the tube, a shield, a thin strip of copper, is placed between the two bent columns of the CFL and tied to its horizontal upper part with a few turns of thin copper wire. This effectively reduces the length of the tube for creating a glow. A Hewlett-Packard digitizing oscilloscope model 54501A was used to measure the voltage across the lamp at F3 and ground and the signal at point 4 of the secondary of the transformer, that is across R2, which was proportional to the current through the lamp. A built-in multiplier gave the product of these two quantities to display the power waveform. And since it is point-to-point multiplication, the power factor was automatically
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Figure 3. Charge-discharge controller of the CFL-based solar lantern
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Figure 4. Circuit diagram of the current-fed oscillator
taken into account. The power was then averaged over 16 cycles to get a stable power value. A 3n3F capacitor, C3, is placed between the two filaments of the tube so that it provides a current path through them when the inverter is switched on. Although this current of approximately 200 mA is adequate for pre-heating the filaments, it also reduces the voltage across the tube because of the capaci-
tive divider formed by C2 and C3. Initially this low voltage fails to start a discharge but when the hot filaments emit enough electrons the voltage is sufficient to cause a glow. The slow heating of the filaments thus delays the ignition and allows for a gradual increase in the intensity of the emitted light, which is called a “soft start”. Once the tube attains full conduction the voltage across it comes
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down to its sustaining value, which is about 30 Vrms for a 5 W CFL, and therefore the current through C3 at this lower voltage reduces. Another factor that affects the current flow through the filaments is the reduction of the oscillator frequency when the CFL lights up. The ballast capacitor C2, in series with the tube effective resistance of about 170 ohms (Ω), comes in parallel with the secondary winding of the transformer. The shift in frequency is quite drastic as can be seen from Table 3. The two effects combined reduce the current through the capacitor C3 since its reactance increases at low frequency while the voltage across it drops to 30 V from the initial value of 125 Vrms. For cold-starting of the CFL capacitor C3 is removed from the circuit. The circuit was cold-started and switched off 5000 times without any appreciable blackening of the CFL near its filaments, which was earlier observed for a limited number of cold start operations [Cayless and Marsden, 1983, p. 186]. The oscillator is started when the winding of the base terminal of the transistors 2N6292 is raised to 11 V by Q1 of Figure 3, the output transistor of the controller. When the battery voltage falls below a certain value, corresponding to a level of discharge, the voltage at the emitter of Q1 goes to zero, thereby shutting down the oscillator. All resistor values are in Ω and 250 mW, 5% tolerance unless stated. All capacitor values are in microfarad (µF) unless stated. 2.2. The LED-based circuit Figure 5 shows the circuit diagram of the 12 V LEDbased solar lantern complete with the charge-discharge controller. The circuit is similar to that given in Figure 3 except that a constant current source comprising transistors Q1 and Q2 has been added. Resistor R 31 has to be adjusted to give a current of 350 mA through the LEDs D17 to D19, which are placed in series. This circuit has a poor efficiency, as shown in Table 4, compared to the one with the CFL. This is because of the power dissipated in the transistors and the resistance R31. 3. Results The solar lantern with a 5 W CFL gives a total light output of about 250 lm whereas the one with the LEDs gives 150 lm. The light output data are the nominal values supplied by the manufacturers in their data sheets. This is because the lowest-power CFL available is 5 W, which yields 250 lm, whereas the circuit of Figure 3 can support only 3 white LEDs of 1 W each, which together emit 150 lm. The overall electrical efficiency of the CFL-based lantern exceeds 80 % with a 12 V DC input. The corresponding figure for the LED-based lantern is 69.2 % because
0.445 (1.78 × 0.25) W of power is wasted in the constant current generator. This wastage increases to (4.2 × 0.327) = 1.373 W, thereby reducing the efficiency to 63.7 %. A switch-mode design will yield a better efficiency figure. The suggested part is LM 3402/LM 3402HV by National Semiconductor [NS, 2006]. 4. Conclusion A replacement for a hurricane lantern needs a light source that can give an output of 40 to 69 lm, as given in Tables 1 and 2. 5 W CFLs, used earlier since lower-power tubes were not available, gave a lot more light than required. However, with the arrival of high-intensity LEDs in the market only one or two are needed to give the required light. With one LED in circuit the battery and the solar panel size will also reduce and the whole system will cost between $ 20 and $ 25. The circuit given in Figure 5 has three LEDs and costs as much as that given in Figures 3 and 4 because the cost of the oscillator is eliminated. This also requires a solar panel of 8 W as against the 10 W panel used for the CFLbased lantern, which amounts to a reduction of 20 % in the panel cost. However, the life of a white LED is very sensitive to temperature [Narendran and Gu, 2005]. Thus, the following conclusions are drawn. 1. For a direct replacement of the hurricane lantern, with a light output of 47 lm, a single LED-based lantern is cheaper than a CFL-based lantern. The cost of this Table 3. Key performance parameters shown for soft- and cold-start CFLs Type of circuit
4-pin (5 W) soft start
DC input voltage (V)
4-pin (5 W) cold start
12.06
12.06
474
470
29.66
29.66
Output voltage (Vrms) without load
212
212
Output current (Irms) (mA)
156
150
Power output (W)
4.58
4.59
80.12
80.96
Frequency (kHz) on load
25.8
26.5
Frequency (kHz) without load
41.0
80.6
Short circuit current DC (mA) on load
82.0
83.0
Short circuit current DC (mA) without load
34.0
34.0
0.995
0.995
DC input current (mA) Output voltage (Vrms) on load
Efficiency (%)
Power factor
Table 4. Parameters when the LEDs are operated from the circuit of Figure 5
30
Input DC voltage (V)
Input DC (A)
DC voltage across lamp (V)
DC through lamp (A)
DC voltage across lamp & Q1 (V)
Voltage drop across R31 & Q1
15.1
0.34
10.0
0.327
14.2
4.2
63.7
12.2
0.288
9.72
0.25
11.5
1.78
69.2
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Figure 5. Circuit diagram of charge-discharge controller for LED-based lantern
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whole system will be between $ 20 and $ 25. 2. For higher illumination, a lantern based on a 5 W CFL, because it is the minimum power available, is cheaper. For solar-powered street lights that should give higher light output, CFL-based designs are still cheaper than their LED counterparts.
Mills, E., 2007. The Lumina Project, Research Memo #1*, “Assessing the performance of 5mm white LED light sources for developing-country applications”, Lawrence Berkeley National Laboratory, MS 90-4000, Berkeley, CA 94720, USA, May, http://light.lbl.gov/pubs/rm/Lumina-RM1.pdf. Mukerjee, A.K., 2000. “Comparative study of solar lanterns”, Energy Conversion and Management, Vol. 41, pp. 621-624.
References
Mukerjee, A.K., and Dasgupta, N., 2007. “Firing options for CFLs used in solar lanterns”, Proceedings of 3rd International Conference on Solar Radiation and Day Lighting (SOLARIS 2007), February 7-9, New Delhi, India.
Baxandall, P.J., 1959. “Transistor sine wave LC oscillators, some general considerations and new developments”, IEE, Vol. 106, p. 748.
Narendran, N., and Gu, Y., 2005. “Life of LED-based white light sources”, IEEE/OSA Journal of Display Technology, Vol. 1, No. 1, September.
Berndt, D., 1993. Maintenance-free Batteries, Research Studies Press, Taunton, Somerset, England, pp. 165-204.
NS (National Semiconductor), 2006. Data sheet of LM 3402, October, http://www.national.com/JS/searchDocument.do?textfield=lm3402
Bhargava, B., and Sastry, E.V.R., 1992. “Solar lanterns: problems and prospects”, 6th International Photovoltaic Science and Engineering Conference (PVSEC-6), New Delhi, India, February 10-14, pp. 885-889.
Osram, 2007. OSRAM catalogue, http://www.osram.com/osram_com/Professionals/ General_Lighting/Compact_Fluorescent_Lamps/CFL_Energy_Saver.html
Cayless, M.A., and Marsden, A.M., 1983. Lamps and Lighting, (3rd ed.), Edward Arnold Ltd., London. CEL (Central Electronics Limited), 2007. Catalogue of Central Electronics Limited, 4, Industrial Area, Sahibabad 201010, Ghaziabad, UP, India, http://www.celindia.co.in/solar_photoVolt_module.asp Craford, M.G., Holonyak, Jr., N., and Kish, Jr., F.A., 2001. “In pursuit of the ultimate lamp”, Scientific American, February, p. 62. Cree, 2007. Data sheet of Cree XLamp 7090 LEDs, http://www.cree.com/xlamp. Dutt, G.S., 1994. “Illumination and sustainable development, Part I: Technology and economics”, Energy for Sustainable Development, Vol. I, No. 1, May, p. 23. Dutta, V., Saxena, A.K., Mukerjee, A.K., and Joshi, J.C., 1993. Performance Evaluation of Photovoltaic Based Lighting System, Centre for Energy Studies, Indian Institute of Technology, Delhi, May 20, report submitted to the Ministry of Non-Conventional Energy Sources, Government of India. Jones, R., Du, J., Gentry, Z., Gur, I., and Mills, E., 2005. “Alternatives to fuel-based lighting in rural China”, paper presented at Right Light 6, the 6th International Conference on Energy-Efficient Lighting, Shanghai, 9-11 May. Linden, D., (ed.), 1995. Handbook of Batteries, (2nd ed.), McGraw-Hill Inc., New York, pp. 25.1-25.39.
Rajvanshi, A.K., 1987. Design and Development of Improved Lantern for Rural Areas, report prepared for the Advisory Board on Energy, New Delhi. Rajvanshi, A.K., 2005. Rocket Science for Rural Development, http://nariphaltan.virtualave.net/rocketscience.pdf Rajvanshi, A.K., 2006. Development of Improved Lanterns for Rural Areas, Nimbkar Agricultural Research Institute (NARI), Phaltan-415523, Maharashtra, India, February, http://nariphaltan.virtualave.net/lantern.htm Sinha, C.S., and Kandpal, T.C., 1991. “Optimal mix of technologies for rural India: the lighting sector”, International Journal of Energy Research, Vol. 15, pp. 653-665. Spring, K.R., Fellers, T.J., and Davidson, M.W., 2003. Molecular Expressions Microscopy Primer Physics of Light and Color - Introduction to Light Emitting Diodes, October 9, www.micro.magnet.fsu.edu/primer/lightandcolor/ledsintro.html TEC (Telecommunication Engineering Centre), 1993. “Solar photovoltaic power source for single channel VHF and similar systems”, Specification No. P520S93, Power Plant, Issue-II, Telecommunication Engineering Centre, Khurshid Lal Bhawan, Janpath, New Delhi-110001, India, September. Van der Plas, R., 1988, Domestic Lighting, Working Paper WPS 68, Industry and Energy Department, World Bank. Yarrow, C.J., 1959. “Transistor converters for the generation of high-voltage low-current D.C. supplies”, IEE, Vol. 106, p. 1320.
Contributions invited Energy for Sustainable Development is a venture in the field of journals on energy with a special focus on the problems of developing countries. It attempts a balanced treatment of renewable sources of energy, improvements in the efficiency of energy production and consumption, and energy planning, including the hardware and software (policy) required to translate interesting and useful new developments into action. With such a multi-disciplinary approach, Energy for Sustainable Development addresses itself to both specialist workers in energy and related fields, and decision-makers. It endeavours to maintain high academic standards without losing sight of the relevance of its content to the problems of developing countries and to practical programmes of action. It tries to provide a forum for the exchange of information, including practical experience. Articles and short articles published are subject to a formal process of anonymous peer review. Material for publication may be e-mailed to the Editor, Gautam Dutt, at [email protected]. For guidelines to authors on the preparation of the text and other material, see Page 4.
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Comparison of CFL-based and LED-based solar lanterns A.K. Mukerjee Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi-110 016, India E-mail: [email protected]
Solar lanterns are designed to replace hurricane lanterns for use in remote places where conventional electricity is not available. They not only provide illumination at night but are also instrumental in avoiding the emission of greenhouse gases normally emitted from burning the wick and kerosene in the latter. Light is produced in a solar lantern by igniting either a compact fluorescent lamp (CFL) or a light emitting diode (LED) while the energy to do so is generated by a string of solar cells, called a solar panel, exposed to sunlight. This energy, generated during sunlight hours, is stored in a battery to be used at night. LEDs are the latest development as a source of light. Therefore the cost and performance of two solar lanterns, one based on a CFL and the other based on an LED, have been studied and compared to determine the better device for low-level lighting in the future. The comparison is based on lumens/watt (lm/W) output of the device. It has been found that the LED- and the CFL-based system cost almost the same if they are used to replace the hurricane lantern which has a light output of 40 to 50 lm. The life of the former is more, typically 10,000 to 50,000 hours with luminous efficacy of 47 lm/W, and its driving and control circuit is less complicated than that of the latter. The luminous efficacy of the latter is slightly higher, 50 to 60 lm/W, and it has a life span of up to 15,000 hours. On the other hand, LEDs are available from powers ranging from a few mW to 3 W, enabling lower-illumination lanterns to be constructed as an almost exact replacement of the hurricane lantern. However, at higher levels of illumination the CFL is still a cheaper source. The electrical efficiency of the circuit used with the CFL is about 82 % whereas that used with the LED is 69 % since it is based on an analog constant current generator. The efficiency of the latter can be improved by using a switch-mode constant current generator. 1. Introduction Solar lanterns were made as a viable alternative to the hurricane lantern for use in the remote areas of India where conventional electric power was not available [Bhargava and Sastry, 1982]. Subsequently a model was designed and fabricated [Mukerjee, 2000] to be marketed as desired in a project financed by the Ministry of NonConventional Energy Sources (MNES) (now renamed Ministry of New and Renewable Energy, MNRE), Government of India, in 1992 with the objective of supplying the basic lighting needs of people living in those areas where they were dependent on the hurricane lantern for illumination [Dutta et al., 1993]. Since sunlight is freely and abundantly available at all places not covered by clouds, it can be easily converted into electricity by solar cells, a string of which makes a solar panel. Details about solar panels are given in the catalogue of Central Electronics Limited (CEL) [CEL, 2007] and other manufacturers. This electricity can then be stored in suitable batteries, normally lead-acid maintenance-free type, to be used at night or in dark places. To produce light the options were limited to either a general lighting service (GLS) incandescent filament lamp, the ordinary household electric bulb, or the compact fluorescent lamp (CFL), 24
since other devices such as the 1.2-m fluorescent tube are either too bulky or consume too much power or both. The objective was simply to produce enough light to equal or slightly exceed that of the kerosene lantern – hence the name solar lantern. Since the cost of a solar panel is about (US)$ 4 per peak watt (Wp), the light output of the lamp per unit power input, defined as its efficacy, had to be as high as possible to keep the cost of the solar lantern within the reach of the buyer. The clear choice was therefore the CFL with an efficacy of 50 to 60 lumens/watt (lm/W) [Osram, 2007] over the GLS lamp with its efficacy of about 10-14 lm/W [Cayless and Marsden, 1983, p. 475]. CFLs are gas discharge tubes and therefore require high voltages, of the order of hundreds of volts, to start glowing. For this reason an oscillator circuit with high output voltage and the required power to operate the tube has to be constructed. This also must have a high electrical efficiency, exceeding 80 %, so that most of the solar electricity actually reaches the tube instead of being wasted in the circuit. The block diagram of the solar lantern is shown in Figure 1. A solar panel generates the electricity, which is given to the oscillator and the control circuit. The control circuit, in addition to operating the oscillator, also contains a charge controller which saves the battery
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Figure 1. Block diagram of a solar lantern
Figure 2. A p-n junction (left) as a light emitting diode (right)
from being damaged by preventing it from getting either over-charged or over-discharged. The ballast, in this case a capacitor, ensures that excessive current does not flow through the lamp and thus prolongs the lamp’s life. The CFL is a miniature version of the normal 1.2 m (fluorescent) tube light. The 1.2 m lamp is a low-pressure gas discharge tube with two filaments at the ends. The finished lamp is filled with an inert gas such as argon, to a pressure of 200-660 Pa (1.5 to 5 mm of mercury). A tiny droplet of mercury is introduced which is used in the discharge to create light. The discharge produces, in addition to visible light, ultraviolet radiation which is then converted to visible light by a coating of phosphor on the inside of the tube [Cayless and Marsden, 1983, p. 185]. This conversion of wavelength is responsible for the increased emission of visible light resulting in a higher efficacy. Light emitting diodes (LEDs) have now come up as an energy-efficient alternative to the CFL at low-level illumination, with a considerably longer life. Being solid state devices, they can be operated with direct current at low voltage levels of 2 to 4 V. Hence it was an interesting exercise to compare lanterns made with these two light sources.
The efficacy of LEDs has steadily increased over the last few years. An interesting comparison among efficacies of different light sources, and the progress the LEDs have made with time, can be seen in [Craford et al., 2001]. It shows a steep rise in the efficacy of LEDs from 1985 to 2000. The white light LED was in its infancy with an efficacy of 20 lm/W in 2000. Now it is 47 lm/W [Cree, 2007] and is a strong rival of the CFL. The basic LED consists of a junction between two different semiconductor materials, one n-type and the other p-type (illustrated in Figure 2), in which an applied voltage produces a current flow, which generates light when charge carriers injected across the junction recombine. This p-n junction in the form of an integrated circuit chip is placed in a concave reflector with two wires attached to carry current to it, with an epoxy-based lens in front [Spring et al., 2003]. The reflector sends the backward-going light to the front whereas the lens focuses it at the desired angle, which can range from 15º to 100º. The frequency, and perceived colour, of emitted photons is characteristic of the semiconductor material, and consequently, different colours are achieved by making changes in the semiconductor composition of the chip.
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Table 1. Some characteristics of different light sources Flux (lm)
Colour[1]
Luminous efficacy (lm/W)
Specific fuel consumption (kg/klmh)
Fuel consumption rate (kg/h)
Candle
12
Excellent
0.2[2]
0.5[3]
0.006
Kerosene wick
40
Excellent
0.1
0.8
0.032
Kerosene mantle
400
Poor
0.8
0.1
0.040
60 W incand.
730
Excellent
12
0.21[4]
0.016
16 W compact fluorescent
900
Good
56
0.005
0.004
[5]
56.4
Good
47
-
-
Light source
1.2 W LED
Source: [van der Plas, 1988]; original includes additional non-electric sources – carbide lamp, butane lamp, and gasoline mantle lamp; compact fluorescent data calculated here from data presented earlier in the chapter. Notes 1. Colour rendering index (CRI) is difficult to measure for non-electric sources, according to van der Plas [1988]. Thus, colour quality is shown here on a qualitative scale. 2. For non-electric lamps, the power input in W has been calculated using the higher heating value of the fuel. 3. Specific fuel consumption is expressed as kg of fuel use per 1000 lm-h of light (kg/klm-h). 4. For electric sources, we assume a conversion efficiency of 0.30 from kerosene to electricity, and express specific fuel consumption and fuel consumption rate as equivalent amounts of kerosene. 5. The LED data has been taken from [Cree, 2007].
Table 2. Another comparison of electric and non-electric light sources Light source
Flux (lm)
Luminous efficacy Specific fuel consumption (lm/W) (kg/klmh)
Fuel consumption rate (kg/h)
Kerosene Chirag (diya)[1]
21
0.26[2]
0.299[3]
0.006
Hurricane lantern
69
0.38
0.203
0.014
Petromax (mantle)
1303
1.25
0.056
0.073
25 W incandescent
225
9
0.029[4]
0.007
60 W incandescent
660
11
0.023
0.016
Electricity
Source: Selected from [Rajvanshi, 1987] as quoted in [Sinha and Kandpal, 1991]. Notes 1. The chirag or diya is a simple lamp made using a wick and a glass bottle. 2. Same as Note 2 of Table 1 3. Same as Note 3 of Table 1 4. Same as Note 4 of Table 1
[Mills, 2007] gives figures to indicate that there is a large variation in the efficacies of the LEDs available in the market. Therefore, care should be taken to select suitable products while fabricating lanterns. Dependence on fossil fuels like kerosene and the resulting polluting greenhouse gases had also to be reduced for which the solar lantern was the obvious answer. Besides, the cost of transporting the kerosene to such locations is high and the buyers, especially in hilly regions, have to travel long distances over difficult terrain to fetch it. Fuel-based lighting was shown to be significantly costlier than solar-powered CFL and LED alternatives, per unit of lighting service delivered. It was calculated [Jones et al., 2005] that the running cost of a hurricane-style kerosene lamp is approximately $ 0.40 per 1000 lumen 26
hours (klmh), while that of a solar-CFL lantern is $ 0.17/klmh, and a solar-LED device costs $ 0.15/klmh. Furthermore, as LED efficiencies continue to improve, solar-LED products will become even more economical. [Dutt, 1994] gives the data about the output of different sources as shown in Tables 1 and 2. The two tables show difference in the light output from the kerosene wick since the material, the thickness and the burning conditions may not have been the same in the two cases. However, an idea of the light outputs can be had by comparing the two. [Rajvanshi, 2005; 2006] gives additional data on the output from different sources, but the data of interest, the light output from kerosene lanterns, does not differ much from that given in Table 2. The 1 W white LED from Cree Inc., type XR7090WT, gives an output of 47 lm/W
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[Cree, 2007] and can be ranked as the second best source of light next to the 16 W CFL. This device costs $ 3 per unit in strip package and $ 3.50 if reflow-mounted on a 1.9 cm round or star printed circuit board. It also has a built-in lens that can diverge light at an angle of 100º. Two lanterns were, therefore, constructed for comparison. One was made from white LEDs and the other was made from a 5 W CFL with a soft/cold start, the most efficient firing mechanism, to give the maximum electrical circuit efficiency [Mukerjee and Dasgupta, 2007]. 2. Experiments Comparisons were made between the two circuits for their electrical efficiency and cost. Both these circuits were designed and fabricated by the author; one for the MNES, as mentioned earlier, but subsequently modified to compensate for the changes in the battery voltage with temperature, and the other to operate a white LED from a constant current source. The first circuit, for the CFL, has been widely manufactured and sold all over India and is very reliable. The basis for this comparison is to find the most cost-effective solution for the replacement of the hurricane lantern, which should provide light for about three hours with the required protections for the battery [TEC, 1993]. 2.1. The CFL-based circuit The circuit of the whole lantern is split into two parts: (1) the charge-discharge controller as shown in Figure 3 and the (2) oscillator plus the lamp as shown in Figure 4. These are discussed below. 2.1.1. The charge-discharge controller Figure 3 shows the circuit of the charge-discharge controller of the CFL-based lantern, which can operate either a 4-pin 5 W, or a 7 W CFL of any standard make. This circuit is needed to prevent the battery, of the sealed maintenance-free lead-acid type [Linden, 1995], from being over-charged or over-discharged, thus prolonging its life. This also controls the switching of the oscillator, which in turn drives the CFL, as shown in Figure 4. The description of the oscillator follows later. An LM 324 14-pin quad operational amplifier (four opamps in one envelope) is used to operate the controller. The four sections of the opamp are indicated by four triangles with their accompanying components. One section, U3C, with its associated components, makes a stable 5 V regulator used as a reference source. U1A uses this voltage to form a 14.5 V feedback regulator with the P-channel MOSFET MTP 2955E as the series pass element. Diodes D1, D2, and D3 are low-voltage drop Schottky diodes, which protect the circuit against accidental reversal of photovoltaic panel and battery polarities. Diodes D7 to D14 are 1N 4148 silicon diodes in the feedback loop of the regulator, used to provide temperature compensation for the battery during the process of charging. The 12 V lead-acid maintenance-free battery is connected between the B+ and B- terminals to store the charge generated by the PV panel during daytime while the PV panel is connected between the PV+ and PV- terminals. U2B acts as a comparator by detecting the voltage across diode
D3. When the battery is being charged the terminal 5 of U2B is more positive than the terminal 6 and the green light emitting diode, D11, glows to indicate it. When the battery voltage goes lower than 11.5 V the output voltage of operational amplifier U4D goes low and the transistor Q1 stops conducting. This shuts down the CFL-driving oscillator, shown in Figure 4, while the red LED D14 gives an indication of “battery low”. When the battery is charged from a partially discharged condition the series resistance of the PV panel reduces its output voltage to latch on to that of the battery plus some voltage drop across the MOSFET and diodes that come in series. When the battery is fully charged the output of the regulator settles down to a fixed value of 14.5 V so that overcharging does not damage the battery. However, the regulator continues to furnish a small current to compensate the loss of charge, an inherent property of lead-acid cells, which is known as trickle charging. A double-pole, double-throw switch, SW 1, turns the lantern on/off manually. A 10 W solar panel is used to charge the 12 V battery. Although this is the nominal battery voltage its actual value really depends on its state of charge. This may vary from 10.8 V, for deep discharge, to 14.5 V for a fully charged state for both lead-acid and valve-regulated leadacid types [TEC, 1993; Linden, 1995; Berndt, 1993]. Therefore, the open circuit voltage of the panel should be 12/0.8 = 15 V so that it operates at its maximum power point. Since there are some diodes and a MOSFET in the charging path the panel voltage is usually kept between 16 to 17 V to also account for the temperature rise in the tropics. 2.1.2. The oscillator and the lamp circuit The circuit diagram of the oscillator used to drive the CFL is shown in Figure 4. This is a current-fed, class-D sinusoidal circuit in a push-pull configuration, described in [Yarrow, 1959] and [Baxandall, 1959], that has a theoretical efficiency of 100 %, which is the reason for its selection. Diode D1 ensures that a negative polarity bus voltage from a solar panel or a battery does not damage the circuit while the inductor L2 maintains a current-fed topology. The value of L2 should be at least five times the primary inductance of the transformer T1 from its centre tap. The primary voltage is then stepped up to about 125 Vrms (root mean square voltage value) to ignite the CFL, which is either a 5 W or a 7 W device, through a ballast capacitor C2. To help start the discharge in the tube, a shield, a thin strip of copper, is placed between the two bent columns of the CFL and tied to its horizontal upper part with a few turns of thin copper wire. This effectively reduces the length of the tube for creating a glow. A Hewlett-Packard digitizing oscilloscope model 54501A was used to measure the voltage across the lamp at F3 and ground and the signal at point 4 of the secondary of the transformer, that is across R2, which was proportional to the current through the lamp. A built-in multiplier gave the product of these two quantities to display the power waveform. And since it is point-to-point multiplication, the power factor was automatically
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Figure 3. Charge-discharge controller of the CFL-based solar lantern
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Figure 4. Circuit diagram of the current-fed oscillator
taken into account. The power was then averaged over 16 cycles to get a stable power value. A 3n3F capacitor, C3, is placed between the two filaments of the tube so that it provides a current path through them when the inverter is switched on. Although this current of approximately 200 mA is adequate for pre-heating the filaments, it also reduces the voltage across the tube because of the capaci-
tive divider formed by C2 and C3. Initially this low voltage fails to start a discharge but when the hot filaments emit enough electrons the voltage is sufficient to cause a glow. The slow heating of the filaments thus delays the ignition and allows for a gradual increase in the intensity of the emitted light, which is called a “soft start”. Once the tube attains full conduction the voltage across it comes
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down to its sustaining value, which is about 30 Vrms for a 5 W CFL, and therefore the current through C3 at this lower voltage reduces. Another factor that affects the current flow through the filaments is the reduction of the oscillator frequency when the CFL lights up. The ballast capacitor C2, in series with the tube effective resistance of about 170 ohms (Ω), comes in parallel with the secondary winding of the transformer. The shift in frequency is quite drastic as can be seen from Table 3. The two effects combined reduce the current through the capacitor C3 since its reactance increases at low frequency while the voltage across it drops to 30 V from the initial value of 125 Vrms. For cold-starting of the CFL capacitor C3 is removed from the circuit. The circuit was cold-started and switched off 5000 times without any appreciable blackening of the CFL near its filaments, which was earlier observed for a limited number of cold start operations [Cayless and Marsden, 1983, p. 186]. The oscillator is started when the winding of the base terminal of the transistors 2N6292 is raised to 11 V by Q1 of Figure 3, the output transistor of the controller. When the battery voltage falls below a certain value, corresponding to a level of discharge, the voltage at the emitter of Q1 goes to zero, thereby shutting down the oscillator. All resistor values are in Ω and 250 mW, 5% tolerance unless stated. All capacitor values are in microfarad (µF) unless stated. 2.2. The LED-based circuit Figure 5 shows the circuit diagram of the 12 V LEDbased solar lantern complete with the charge-discharge controller. The circuit is similar to that given in Figure 3 except that a constant current source comprising transistors Q1 and Q2 has been added. Resistor R 31 has to be adjusted to give a current of 350 mA through the LEDs D17 to D19, which are placed in series. This circuit has a poor efficiency, as shown in Table 4, compared to the one with the CFL. This is because of the power dissipated in the transistors and the resistance R31. 3. Results The solar lantern with a 5 W CFL gives a total light output of about 250 lm whereas the one with the LEDs gives 150 lm. The light output data are the nominal values supplied by the manufacturers in their data sheets. This is because the lowest-power CFL available is 5 W, which yields 250 lm, whereas the circuit of Figure 3 can support only 3 white LEDs of 1 W each, which together emit 150 lm. The overall electrical efficiency of the CFL-based lantern exceeds 80 % with a 12 V DC input. The corresponding figure for the LED-based lantern is 69.2 % because
0.445 (1.78 × 0.25) W of power is wasted in the constant current generator. This wastage increases to (4.2 × 0.327) = 1.373 W, thereby reducing the efficiency to 63.7 %. A switch-mode design will yield a better efficiency figure. The suggested part is LM 3402/LM 3402HV by National Semiconductor [NS, 2006]. 4. Conclusion A replacement for a hurricane lantern needs a light source that can give an output of 40 to 69 lm, as given in Tables 1 and 2. 5 W CFLs, used earlier since lower-power tubes were not available, gave a lot more light than required. However, with the arrival of high-intensity LEDs in the market only one or two are needed to give the required light. With one LED in circuit the battery and the solar panel size will also reduce and the whole system will cost between $ 20 and $ 25. The circuit given in Figure 5 has three LEDs and costs as much as that given in Figures 3 and 4 because the cost of the oscillator is eliminated. This also requires a solar panel of 8 W as against the 10 W panel used for the CFLbased lantern, which amounts to a reduction of 20 % in the panel cost. However, the life of a white LED is very sensitive to temperature [Narendran and Gu, 2005]. Thus, the following conclusions are drawn. 1. For a direct replacement of the hurricane lantern, with a light output of 47 lm, a single LED-based lantern is cheaper than a CFL-based lantern. The cost of this Table 3. Key performance parameters shown for soft- and cold-start CFLs Type of circuit
4-pin (5 W) soft start
DC input voltage (V)
4-pin (5 W) cold start
12.06
12.06
474
470
29.66
29.66
Output voltage (Vrms) without load
212
212
Output current (Irms) (mA)
156
150
Power output (W)
4.58
4.59
80.12
80.96
Frequency (kHz) on load
25.8
26.5
Frequency (kHz) without load
41.0
80.6
Short circuit current DC (mA) on load
82.0
83.0
Short circuit current DC (mA) without load
34.0
34.0
0.995
0.995
DC input current (mA) Output voltage (Vrms) on load
Efficiency (%)
Power factor
Table 4. Parameters when the LEDs are operated from the circuit of Figure 5
30
Input DC voltage (V)
Input DC (A)
DC voltage across lamp (V)
DC through lamp (A)
DC voltage across lamp & Q1 (V)
Voltage drop across R31 & Q1
15.1
0.34
10.0
0.327
14.2
4.2
63.7
12.2
0.288
9.72
0.25
11.5
1.78
69.2
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Figure 5. Circuit diagram of charge-discharge controller for LED-based lantern
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whole system will be between $ 20 and $ 25. 2. For higher illumination, a lantern based on a 5 W CFL, because it is the minimum power available, is cheaper. For solar-powered street lights that should give higher light output, CFL-based designs are still cheaper than their LED counterparts.
Mills, E., 2007. The Lumina Project, Research Memo #1*, “Assessing the performance of 5mm white LED light sources for developing-country applications”, Lawrence Berkeley National Laboratory, MS 90-4000, Berkeley, CA 94720, USA, May, http://light.lbl.gov/pubs/rm/Lumina-RM1.pdf. Mukerjee, A.K., 2000. “Comparative study of solar lanterns”, Energy Conversion and Management, Vol. 41, pp. 621-624.
References
Mukerjee, A.K., and Dasgupta, N., 2007. “Firing options for CFLs used in solar lanterns”, Proceedings of 3rd International Conference on Solar Radiation and Day Lighting (SOLARIS 2007), February 7-9, New Delhi, India.
Baxandall, P.J., 1959. “Transistor sine wave LC oscillators, some general considerations and new developments”, IEE, Vol. 106, p. 748.
Narendran, N., and Gu, Y., 2005. “Life of LED-based white light sources”, IEEE/OSA Journal of Display Technology, Vol. 1, No. 1, September.
Berndt, D., 1993. Maintenance-free Batteries, Research Studies Press, Taunton, Somerset, England, pp. 165-204.
NS (National Semiconductor), 2006. Data sheet of LM 3402, October, http://www.national.com/JS/searchDocument.do?textfield=lm3402
Bhargava, B., and Sastry, E.V.R., 1992. “Solar lanterns: problems and prospects”, 6th International Photovoltaic Science and Engineering Conference (PVSEC-6), New Delhi, India, February 10-14, pp. 885-889.
Osram, 2007. OSRAM catalogue, http://www.osram.com/osram_com/Professionals/ General_Lighting/Compact_Fluorescent_Lamps/CFL_Energy_Saver.html
Cayless, M.A., and Marsden, A.M., 1983. Lamps and Lighting, (3rd ed.), Edward Arnold Ltd., London. CEL (Central Electronics Limited), 2007. Catalogue of Central Electronics Limited, 4, Industrial Area, Sahibabad 201010, Ghaziabad, UP, India, http://www.celindia.co.in/solar_photoVolt_module.asp Craford, M.G., Holonyak, Jr., N., and Kish, Jr., F.A., 2001. “In pursuit of the ultimate lamp”, Scientific American, February, p. 62. Cree, 2007. Data sheet of Cree XLamp 7090 LEDs, http://www.cree.com/xlamp. Dutt, G.S., 1994. “Illumination and sustainable development, Part I: Technology and economics”, Energy for Sustainable Development, Vol. I, No. 1, May, p. 23. Dutta, V., Saxena, A.K., Mukerjee, A.K., and Joshi, J.C., 1993. Performance Evaluation of Photovoltaic Based Lighting System, Centre for Energy Studies, Indian Institute of Technology, Delhi, May 20, report submitted to the Ministry of Non-Conventional Energy Sources, Government of India. Jones, R., Du, J., Gentry, Z., Gur, I., and Mills, E., 2005. “Alternatives to fuel-based lighting in rural China”, paper presented at Right Light 6, the 6th International Conference on Energy-Efficient Lighting, Shanghai, 9-11 May. Linden, D., (ed.), 1995. Handbook of Batteries, (2nd ed.), McGraw-Hill Inc., New York, pp. 25.1-25.39.
Rajvanshi, A.K., 1987. Design and Development of Improved Lantern for Rural Areas, report prepared for the Advisory Board on Energy, New Delhi. Rajvanshi, A.K., 2005. Rocket Science for Rural Development, http://nariphaltan.virtualave.net/rocketscience.pdf Rajvanshi, A.K., 2006. Development of Improved Lanterns for Rural Areas, Nimbkar Agricultural Research Institute (NARI), Phaltan-415523, Maharashtra, India, February, http://nariphaltan.virtualave.net/lantern.htm Sinha, C.S., and Kandpal, T.C., 1991. “Optimal mix of technologies for rural India: the lighting sector”, International Journal of Energy Research, Vol. 15, pp. 653-665. Spring, K.R., Fellers, T.J., and Davidson, M.W., 2003. Molecular Expressions Microscopy Primer Physics of Light and Color - Introduction to Light Emitting Diodes, October 9, www.micro.magnet.fsu.edu/primer/lightandcolor/ledsintro.html TEC (Telecommunication Engineering Centre), 1993. “Solar photovoltaic power source for single channel VHF and similar systems”, Specification No. P520S93, Power Plant, Issue-II, Telecommunication Engineering Centre, Khurshid Lal Bhawan, Janpath, New Delhi-110001, India, September. Van der Plas, R., 1988, Domestic Lighting, Working Paper WPS 68, Industry and Energy Department, World Bank. Yarrow, C.J., 1959. “Transistor converters for the generation of high-voltage low-current D.C. supplies”, IEE, Vol. 106, p. 1320.
Contributions invited Energy for Sustainable Development is a venture in the field of journals on energy with a special focus on the problems of developing countries. It attempts a balanced treatment of renewable sources of energy, improvements in the efficiency of energy production and consumption, and energy planning, including the hardware and software (policy) required to translate interesting and useful new developments into action. With such a multi-disciplinary approach, Energy for Sustainable Development addresses itself to both specialist workers in energy and related fields, and decision-makers. It endeavours to maintain high academic standards without losing sight of the relevance of its content to the problems of developing countries and to practical programmes of action. It tries to provide a forum for the exchange of information, including practical experience. Articles and short articles published are subject to a formal process of anonymous peer review. Material for publication may be e-mailed to the Editor, Gautam Dutt, at [email protected]. For guidelines to authors on the preparation of the text and other material, see Page 4.
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