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Efficiency of Incandescent and Fluorescent Light Bulbs: a Comparative Analysis on Cost and Power Usage. Ng, Joan 2009

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Efficiency of Incandescent and Fluorescent Light Bulbs: a Comparative Analysis on Cost and Power Usage. Joan Ng, Science One 2008-2009 Abstract Light bulbs play an indispensable role in our lives as they provide unhindered ability to see the world even during the night. Efficiency of light bulbs is becoming increasingly important, and through this experiment, light bulb efficiencies are analyzed from measurements of relative light intensity and power input of various bulbs. Furthermore, this report is aimed to increase awareness of the efficiency advantage in fluorescent bulbs and to encourage their usage in the interest of conserving energy.  Introduction Light is an essential part of our daily lives. When natural sunlight is not available, we turn to artificial sources such as electrical light bulbs. Such light bulbs include, but are not limited to, incandescent and fluorescent light bulbs. Incandescent bulbs have been known since 1879 when Sir Joseph Swan and Thomas Edison introduced them to the world [1]. These bulbs operate by running current through a tungsten filament that then emits light as its atoms absorb and release energy [1]. In 1976, Thomas Lo Guidice patented the fluorescent light bulb and claimed that fluorescents consume 1/6 of the energy of incandescents with the same light output [2]. Fluorescent bulbs work by running current through an electrode and exciting mercury, which in turn excites the phosphor coating the fluorescent tube to give off light [1]. In the present day energy crisis, the use of fluorescent bulbs is important as they are much more efficient than incandescents. In this experiment, the relative efficiencies of various wattages of fluorescent and incandescent bulbs are measured and analyzed. This includes mainly a cost efficiency analysis, as well as a power analysis. The goal of the experiment is to justify and relatively quantify the efficiency advantage that fluorescents have over incandescent bulbs.  Methods To acquire data, two sets of four pairs of new light bulbs were used. Each pair had one fluorescent and one incandescent light bulb, both commercially reported to have very similar lumen outputs and are thus comparable. The light bulbs used were: 5W fluorescent and 25W incandescent, 9W fluorescent and 40W incandescent, 13W fluorescent and 60W incandescent, and 23W fluorescent and 100W incandescent (Figure 1). All light bulbs were of Philips brand to ensure similar quality. With eight bulbs per trial, a total of sixteen bulbs were used throughout the whole experiment. Each light bulb was tested three times, leaving ample time between each replicate to allow the light bulb to cool off. The experiment was carried out in the crawlspace of a house, which offered an absolutely dark environment so that the photocell would be isolated and unaffected from external light sources. A photocell is a small resistor of cadmium-sulphide that changes resistance with changing brightness [3]. There were two parts to the measurements done: 1) Measuring the current each light bulb draws: 1  Figure 1. Light bulb pairs are shown left to right from lowest to highest wattages.  Figure 2. Experimental set-up for measuring current. Pictured here is the light source and multimeter.  With the lamp initially turned off, a digital multimeter was connected into circuit between the lamp and the electricity supply (Figure 2). The multimeter dial was then turned to “20m/10A” with alternating current, the lamp was turned on, and the current reading for each light bulb was then recorded. 2) Using the photocell resistor to indirectly measure relative intensity: Using the same lamp set up, a measuring tape was set out on the ground, extending straight away from the lamp. The photocell was connected to the multimeter with the dial set at “200kΩ”, and the photocell was then moved along the measuring tape so that the reading on the multimeter was “14.0 kΩ”. This number was arbitrarily set to make the light bulbs comparable to each other. The distance that the photocell was away from each light bulb was recorded. See Figure 4 for apparatus set-up. To ensure easy and accurate data acquisition, the photocell was attached to a mini art easel (Figure 3) so that the detecting surface of the photocell is vertically in-line with the front edge of the easel’s base. This way, the distance reading on the front of the base was equal to the distance between the photocell and the light bulb.  Results & Discussion  Figure 3. Photocell shown here, taped into position on the mini easel.  Figure 4. Set-up for measuring light intensity. Shown here are the light source, measuring tape, photocell facing the light, and multimeter.  The initial data and calculated power input of each light bulb tested are displayed in Table 1 below. The distance and current values are the averages of both trials of the entire experiment. The data is listed with increasing light bulb wattage, and the power uncertainties were propagated from the uncertainty in the measurements of current. There was an expected increasing trend in the distance of the photocell from the testing light bulb. The resistance of the photocell decreases with greater light intensity; to attain the same 14.0 kiloOhms for each light bulb, the dimmer, lower wattage bulbs required a shorter distance away from the photocell, while brighter, higher wattage bulbs required greater distances. The current measured using the multimeter in circuit also yielded an expected increasing trend as the light bulb wattages increased. Since the household voltage in Canada is a constant 120 volts, and power equals the product of voltage and current, the current drawn from higher wattage bulbs must also be higher. Table 1. Initial Experimental Data including photocell resistance [kΩ], distance of photocell from bulb [m], current drawn by the bulb [A], and power [W] calculated using P = VI and V = 120V. Distance Current Reported Photocell photocell is away drawn by Calculated Pair Bulb lumen resistance [kΩ] from bulb [m] bulb [A] Power [W] 1 5W Fluor. * 250 14.0 ± 0.05 0.734 ± 0.002 0.03 ± 0.005 3.6 ± 0.6 25W Inc. 235 14.0 ± 0.05 1.020 ± 0.002 0.19 ± 0.005 22.2 ± 0.6 2 9W Fluor. 500 14.0 ± 0.05 1.243 ± 0.002 0.06 ± 0.005 7.3 ± 0.6 40W Inc. 475 14.0 ± 0.05 1.466 ± 0.002 0.31 ± 0.005 37.4 ± 0.6 3 13W Fluor. 900 14.0 ± 0.05 1.602 ± 0.002 0.10 ± 0.005 11.6 ± 0.6 60W Inc. 830 14.0 ± 0.05 1.988 ± 0.002 0.49 ± 0.005 59.3 ± 0.6 4 23W Fluor. 1600 14.0 ± 0.05 2.227 ± 0.002 0.18 ± 0.005 21.6 ± 0.6 100W Inc. 1440 14.0 ± 0.05 2.781 ± 0.002 0.82 ± 0.005 98.4 ± 0.6 *Henceforth, fluorescent light bulbs will be referred to as “Fluor.” and incandescents as “Inc.” in tables/graphs.  2  3  Distance of photocell away from bulb (m) (Relative Light Intensity ---->)  Figure 5 on the left shows a graphical representation of 2.5 the data from Table 1. In 23W Fluor. Figure 5, it is clear that the 2 60W Inc. fluorescent bulbs require 13W Fluor. much less power than their 1.5 40W Inc. equivalent incandescent to 9W Fluor. produce similar light output. 25W Inc. 1 The distance of the photocell otocell away from the light Fluorescents 5W Fluor. bulbb is analogous to light 0.5 Incandescents intensity of the bulb. Since the photocell was not calibrated 0 and all comparisons from this 0 20 40 60 80 100 120 experiment are relative, these Calculated Power Input( W) Figure 5. This graph shows the relationship between distance the distances can be substitutes photocell is away from the bulb (m) and calculated power of each bulb for light intensities. 100W Inc.  (W). Pairs of light bulbs are indicated by dotted lines.(Note: uncertainties are too small to be seen.  Cost Efficiency To address the efficiency of the light bulbs, cost effectiveness must first be analyzed. Assuming that each light bulb is used 10 hours per day, and there are 30 days in one month, each light bulb is therefore used 300 hours per month. The cost of electric electricity ity per kilowatt-hour, kilowatt according to BC Hydro, is $0.05980. Utilizing this information, the cost per month to run each light bulb was calculated. These costs are displayed in Figure 6 below. 2.50 5W Fluor. → $0.07 9W Fluor. → $0.13 13W Fluor. → $0.21 23W Fluor. → $0.39  $/month  2.00 1.50  100W Inc. → $1.77 60W Inc. → $1.06  1.00 40W Inc. → $0.67  0.50  Fluorescents Incandescents  25W Inc. → $0.40  0.00 0  20  40  60  80  100  120  Calculated Power Input (W) Figure 6. This graph displays the cost to run each light bulb/month (assuming 300 hours of use per month and BC Hydro charges $0.05980/kW.h), calculated power input [W], and the relative surface area [m2] each light bulb illuminates as depicted by the sizes of sphe spheres (with photocell at 14.0 ± 0.05 kΩ) . The uncertainty in cost is ± $0.01. -0.50  In addition to the cost required to run each light bulb, Figure 6 also shows each bulb’s power input as well as its relative illuminated surface area. The size of each sphere indicates the relative surface area of illumination. With fluorescents as light blue spheres, and incandescents as orange spheres, it is clear that the fluorescent fluorescents are much more efficient than the incandescents. incandescent The 3  power and cost gaps between the fluorescents are relatively small compared to those of the incandescents. Most noteworthy are the 23W fluorescent and 100W incandescent. Both produce relatively similar amounts of light, depicted in Figure 6 as similarly sized spheres, but the 23W uses much less power and therefore costs a lot less. $0.04  $/m2  $0.03 $0.02 $0.01 $5W Fluor.  25W Inc.  9W Fluor.  40W Inc.  13W Fluor.  60W Inc.  23W Fluor. 100W Inc.  Light Bulbs Figure 7. The cost/month per surface area illuminated of each light bulb is displayed. The unit for surface area is meters squared. Pairs of light bulbs are distinguished using different colours. Uncertainties were calculated in quadrature from those of electricity cost/ month and surface area illuminated.  Figure 7 above effectively contrasts the cost efficiencies of the light bulbs. The cost/month per surface area illuminated was calculated for each bulb. The 23W fluorescent was found to be the most cost efficient per unit meters squared, only costing $ 0.0062 ± 0.0002 /m2. So, the 23W gives the most light for the electricity bill paid (13W very closely following). As a general trend, the fluorescent bulbs were all significantly more cost efficient than the incandescents. $23.43  $25.00  $/year  $20.00 $14.55  $15.00  $10.29  $10.00  $6.56  $5.56  1  2  1  5W Fluor.  25W Inc.  9W Fluor.  $4.76  $9.64 $6.49  $5.00 $0.00  3  1  3  1  3  40W Inc. 13W Fluor. 60W Inc. 23W Fluor. 100W Inc.  Light Bulbs Figure 8. This graph displays the total investment of each type of bulb per year (3600 hours). Factors incorporated: retail price, electricity cost, and projected life span with replacement cost. The white digit within each bar indicates the number of bulbs required to provide light for one year.  Figure 8 shows the total amount of money invested in each type of light bulb to run for 3600 hours, or one year. This takes into account the retail price of each light bulb, the cost of electricity, and the life span of each bulb. None of the incandescent have a predicted life of 3600 hours, so more than one bulb would be needed. Conversely, all fluorescents have predicted lives of 8000 to 10000 hours, and one bulb each is sufficient. In both types of light bulbs, the total investment is greatest in the bulbs of highest wattage. More importantly, even though the retail prices per fluorescent bulb are 4 to 7 times greater than incandescent, the total cost to run the fluorescents can be up to 60% cheaper than incandescents (between 23W Fluor. and 100W Inc.). This information from Figure 8, in combination with Figure 7, shows that it is important to know the light bulb’s use before purchasing it. First, incandescent light bulbs cannot compare to the 4  Powerinc. /Powerfluor.  fluorescents in cost efficiency. Second, although the 23W fluorescent is the most cost efficient per surface area illuminated, it still costs more overall to run than the lower wattages. Therefore, the consumer will save money by buying the lowest possible wattage fluorescent that supplies sufficient light. Power Efficiency 7.5 Figure 9 above shows a plot of 7 the calculated Powerinc./Powerfluor. 6.5 for each light bulb pair. If the 6.17 6 uncertainties were ignored, it 5.5 would seem that there is a 5.12 5.11 decreasing trend on this power 5 ratio as the wattage of the bulb 4.56 4.5 pairs increase. This would suggest 4 that at lower wattages, the 25WI/5WF 40WI/9WF 60WI/13WF 100WI/23WF Light Bulb Pairs (listed in increasing wattage) fluorescent bulb has greater Figure 9. Calculated power comparisons between light bulb pairs. efficiency over its incandescent Uncertainties were propagated in quadrature from power uncertainties. pairing than at higher wattages. However, once uncertainties are taken into account, it is noted that there is no significant difference between the power ratios. This suggests that the fluorescent bulb in all four pairs are similarly power efficient over their incandescent pairings. More importantly, these ratios reflect the claim that Thomas Lo Guidice made in 1976 that fluorescents use 1/6 of the energy that incandescents do [2]; in the above analysis, fluorescents are indeed approximately 6 times more power efficient than incandescents in giving the similar light output. Since this experiment was composed of relative light intensity measurements, an extension to attain more data would be to find an easy and accurate way to calibrate the photoresistor using a light meter that detects light in lumens, which could then be easily converted to power in watts. This would allow calculations of absolute power efficiency. In addition, the Light Emitting Diode could also be analyzed to gain a more comprehensive knowledge of efficiency ranges.  Conclusion Through measurement and analysis of the relative light intensities and power inputs of 8 light bulbs, this experiment gives a very clear picture on the efficiency advantage that fluorescents have over incandescent light bulbs. In terms of power, this experiment confirms Guidice’s claim that fluorescents are 6 times more efficient than incandescents [2]. Cost-wise, fluorescents of similar light output can be up to 60% less expensive to operate than incandescents for one year. This experiment supports that fluorescents are in fact more efficient in saving money as well as valuable energy. Hopefully, as more individuals acknowledge this advantage and switch to the use of fluorescent bulbs, we will advance a small step towards the ultimate goal of energy conservation.  References [1] Harris, T. (2002). How Light Bulbs Work. Retrieved March 9, 2009 from http://home.howstuffworks.com/light-bulb.htm. [2] Guidice, T.L.(1976).U.S.Patent No.3,953,761.Washington, D.C.:U.S. Patent and Trademark Office. [3] Cook, D. (2004). Intermediate Robot Building. New York : Springer-Verlag. 5  

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