An enhanced roof-mounted light pipe supplemented with LED lamps

Transcription

1 An enhanced roof-mounted light pipe supplemented with LED lamps Surapong Chirarattananon 1,2,*, Pichet Lertboonkankit 1, Jintana Srimuk 1 1 The Joint Graduate School of Energy and Environment, Center of Excellence on Energy Technology and Environment, King Mongkut s University of Technology Thonburi, Bangkok, Thailand 2 Science and Technology Postgraduate Education and Research Development Office, Ministry of Education, Thailand Abstract: As the first service offered by electric utilities, lighting is a dominant electricity end-use. Globally, lighting is responsible for 19% of electricity consumption and substantial greenhouse gas emissions IEA (2006). In Thailand, lighting is the second most electricity intensive end-use after air-conditioning. In deep interior spaces, certain building configurations allow the use of light pipes to bring daylight into spaces that are beyond the reach of daylight that enter through windows. In some applications, variation of daylight that illuminates the spaces may be tolerated, but in some other cases, such variation may not be desirable. In convenient stores that operate 24 hours a day, it is desirable to use electric lighting to supplement daylight to even out the variation and to provide full lighting outside of daylight time. In such situation, dimmable light emitting diode (LED) lamps that are dimmed by daylight installed in light pipes can be suitably utilized. This paper presents results of experiments conducted on circular light pipes with reflectors and with dimmable LED lamps to enhance the capturing of sunlight when daylight is not available or insufficient. Keyword: Light pipe; Beam sunlight; Diffuse skylight; Daylighting; LED lighting * Corresponding author. Tel.: , Fax: address: surapong.chi@kmutt.ac.th, surapong@jgsee.kmutt.ac.th 1. Introduction As the first service offered by electric utilities, lighting is a dominant electricity end-use.globally, lighting is responsible for 19% of electricity consumption and a massive greenhouse gas emission (IEA, 2006). In Thailand, lighting is the second most electricity intensive end-use, taking up to 20 % in a building. Electric lighting also contributes 20% of air-conditioning load (Chirarattananon et al., 2010). Daylight is plentiful in the tropical region especially near the equator.in deep interior spaces, certain building configurations allow the use of light pipes to bring daylight into spaces that are beyond the reach of daylight that enter through windows. A roof-mounted light pipe comprises an inlet port that collects sunlight and daylight from the whole sky, a light transmission section, and an exit port to distribute transmitted daylight into the space below (CIE, 2006). The amount of luminous flux through a pipe can be increased by using light concentrator such as an anidolic (anidolic is synonymous with non-imaging) reflecting concentrator. Anidolic concentrators intercept light flux from certain directions through a larger area and reflect and transmit it to a smaller area. Anidolic daylighting systems have performed well under different sky conditions; therefore they are considered to be a very interesting option for daylighting in tropical regions as well (Friedrich et al., 2010). 2.Potential and limitation of the simple circular light pipe 2.1 Potential application Circular light pipes can be used in low-rise buildings such as stand-alone convenient stores or hypermarket or detached houses. In some applications, variation of daylight that illuminates the spaces may be tolerated, but in some other cases such as in commercial spaces, such variation may not be desirable.in such cases, daylight can be supplemented with light from dimmable LED lamps. Such device utilizes both daylight, a renewable and free energy source, as well as the efficacious electric lamps. With the use of such dimmable system, the translucent exit aperture of a light pipe appears as a lamp and the light pipe will operate unobtrusively as if it is an electric lamp beyond daylight hours.moreover, the cost of the light exit port of the light pipe is shared by the cost of the fixture of the luminaire. 223

2 2.2 Limitation Fig. 1(a) shows that during the early morning or late afternoon periods, the projected area of the entry aperture for daylight from the sun is reduced to Acos, where A is the aperture area and is the angle between the direction to the sun and the normal of the entry aperture. Daylight from the sun in the morning or late afternoon is relatively low as sunlight during the two periods travels over larger air masses. The problem of weak morning or afternoon sunlight is exacerbated by the reduced entry area. Such limitation does not strictly apply to the case of daylight from the sky where the view factor to the sky remains identical for all the time of a day. One way of overcoming the problem of reduction in effective area is to use a reflector. Parabolic Clear hemispheric dome W wedge reflector E ) a( A light pipe with reduced effective aperture area Fig. 1 Configuration of reduced effective aperture area and reflection of sun rays from a reflector attached to the top of a light pipe 3.Design and fabrication of a deconcentrating reflector The limitation mentioned can be partially resolved by increasing the area of reception of sunlight in the morning by using a reflecting surface above the pipe as in Fig. 1(b) that has an inverse parabolic shape. The reflector has a so-called deconcentrating surface as the incoming rays are in opposite direction to those that would be reflected to the focus of the parabolic reflector. The figure also shows the use of a wedge reflector placed beneath the center between two east-west reflectors to reflect sunlight passing through the middle of the two reflectors from directly reaching the exit port of the pipe. Fig. 2(a) shows the geometrical details of the side view of a deconcentrating reflector under a clear dome and over the top of a tubular light pipe. Fig. 2(b) shows a photograph of the tops of two pipes each with a reflector. It is designed to reflect sun rays of altitude angle from 0 to. Two coordinates are erected in the figure. Coordinate (x n,z n ) coincides with the aperture and the pipe. Coordinat (x b,y b ) si the coordinate of the parabolic reflector where the center of the coordinate is where the focus of the parabola is located and where the vertex of the parabola is at a distance a from the focus. In the figure, the lower edge of the reflector is coincident with the entry aperture of the pipe at P. The reflectors are apart by w and each is 2 w away from the center of the pipe C. The radius of the * * pipe is R. The location ( xb, y b) of point P at the point of intersection of the reflector with the coordinate (x b,y b ) is at distance d from the coordinate y b. The line drawn from C, the center of the pipe, to the vertex of the parabola at Q makes angle with the coordinate x n. The angle is independent from and is larger than. Consideration of geometrical relationships and application * of the parabolic equation leads to the following equation for x : ) 1( * 2 * ( xb) 4aRsin( ) 4a tan xb 4aR cos( ) tan 0 b (b) Side view 224

3 z n Q a R C w R x (a) Detailed geometry of the deconcentrator P d (b) A photograph of two pipes each with reflectors Fig. 2 Geometrical configuration of a deconcentrator and a photograph of fabricated deconcentrators each above a light pipe By choosing the radius R, the value for a, the angle, and, then the value of * x b and w can be calculated by first solving the second-order polynomial Eq. 1 F x n 4.Experimental results and discussion Two reflectors, one with angle =60 o and another with =45 o were fabricated and used with light pipes each of diameter 0.40m, both shown in Fig. 2(b) in experiments conducted mainly over a rainy season. The interior pipe surfaces and the diffusers were covered with films of specular reflectance of 95%. The wedge diffusers in both pipes were also covered with the specular film. Three sets of experiments were conducted to compare results from the outlets of light pipes of different configurations. The experimental site is the flat roof of the building of the School of Bioresource Technology in the seaside Bangkhuntian campus of KMUTT where a daylight measurement station is also located.three equally spaced illuminance sensors (Li210SA photometric sensors of Licor Inc.) connected to a data logger were placed under each light pipe in each experiment and the results are reported in the followings. 4.1 Light pipes without diffusers This is a set of experiments to compare the performance between the light pipe with the 60 o reflector, the pipe with the 45 o reflector, and the pipe without reflector. A pair of pipes are set sideby-side outdoor, each with three illuminance sensors to take measurements for direct comparison. Each pipe was covered by a clear dome, and no diffuser was used at the outlet port. Fig. 3 shows illuminance values. In the figure, global and beam illuminance values are taken from the station and 60 Degree and 45 Degree are each the average measured illuminance value from 3 sensors placed at the outlet port of each pipe. Both days are cloudy, beam illuminance appears sporadically. In Fig. 3(a), the outlet illuminance of the pipe with the 60 o reflector is clearly higher, thus more effective, than that of the 45 o pipe for all time. Furthermore, the pattern of outlet illuminance seems to follow that of global illuminance. Beam illuminance seems to have less influence. During 9.00 to about 9.30 AM, the beam illuminance seems to have small influence on outlet illuminances. The reflectors seem to raise outlet illuminances during early morning and late afternoon periods, and subdue them near noon time, thus both perform as they are designed for. In Fig. 3(b), the solid orange line of the outlet illuminance from the pipe without reflector is clearly higher than that of the pipe with the 60 o reflector, especially near noon time and less so during the y 225

4 early morning and late afternnon periods. However, it also fluctuates much more. When beam illuminance increases, outlet illuminace from the pipe without reflector increases even more and at times becomes higher than global illuminance. The results show that the reflector perfoms as intended. The illuminance records of the three individual sensors from the pipe without reflector show that they are highly uneven, in contrast to those of the 60 o pipe. The reflector also disperses transmitted beam illuminance more uniformly at the outlet port. (a) 60 o VS 45 o reflector (b) 60 o VS no reflector Fig. 3 Illuminance at outlet ports, Evg:global, Evb:beam, Evb*sin :beam horizontal 4.2 Light pipes with difusers This is a set of experiments to further compare performances of light pipe with reflector with that without reflector, but both pipes now are attached each with a diffuser of tranmittance of 80 %at the outlet port.fig. 4 show illuminance values. The day is cloudy, but rather bright. Even when no significant beam illuminance appears in the afternoon, illuminance from the pipe without reflector fluctuates significantly with fluctuation in global illuminance while that from the pipe with 60 o reflector show less fluctuation.comparing outlet illuminances from the two pipes, that from the pipe without reflector is higher especially for that near noon time, but less so near sunrise and sunset times.this again illustrates that the reflector performs as intended. Fig. 4 Illuminance at outlet ports, Evg:global, Evb:beam, Evb*sin :beam horizontal, Evd:diffuse 4.3 Light pipes with dimmable LED lamps This is a set of experiments to compare the performance of a light pipe with dimmable LED lamp and that without. A daylight sensor is used to sense daylight that passes through the domed pipe 226

5 with reflector and diffuser. If the illuminance of the resulting transmitted daylight through the pipe is below a set-point value, output from the sensor will drive the dimmer-driver to supply more gated currents to two dimmable LED lamps. The lamp is each rated 20W and produces 1,400 lumens. The sensor circuit drives the two dimmable LED lamps to compensate for the daylight output from the pipe. If the daylight output exceeds the set value, the LED lamps will dim off. If the daylight output drops to negligible illuminance such as that during night time, the sensor circuit will turn the LED lamps full on. Fig. 5 show illuminance values of the two cases. For the line graph in the figure, the red line is the illuminance from the pipe with dimmable LED lamps and the blue line is the illuminance from the pipe without LED lamp. The bands and the callouts in the figure indicate that there is a period when the lamps are totally dimmed off, a period when the lamps are partly dimmed off, and two periods that the lamps are fully on. The dimmable LED lamps keep illuminance from the output to 19 klux. Dimmed Partly Fully Fully Fig. 5 Illuminance at outlet ports, Evg:global, Evb:beam, Evb*sin :beam horizontal, Evd:diffuse 5. Conclusion The experimental results illustrate that the light pipes fitted with appropriate deconcentration reflectors perform well to increase the effectiveness of capturing daylight during the morning and afternoon periods and to enhance uniformity of transmitted daylight. When such pipe configuration is fitted with dimmable LED lamps with appropriate compensation circuit, the pipe set is able to fully utilize the free and renewable daylight for quality (uniform) lighting during daytime. The set is also able to utilize the efficacious light from LED lamps for compensation during the times when daylight is partly or totally unavailable. References Chirarattananon, S., Chaiwiwataorakul, P., Hien, V.D., Rakwamsuk, P. and Kubaha, K Assessment of energy savings from the revised Building Energy Code of Thailand. Energy, 35, Commission Internationale de L'Eclairage (CIE) Tubular Daylight Guidance System. International Commission on Illumination, Technical Committee. Friedrich, L., Stephen, K.W. and Scartezzini J.L.2010.Performance of anidolic daylighting system in tropical climates-parametric studies for identification of main influencing factors. Solar Energy, 84(7), International Energy Agency (IEA) Energy Efficiency and Environment Division, Light s Labour s Lost :Policies for Energy Efficient Lighting, Paris. 227