Effect of a Pipe Number on The Heat Transfer Rate For a Granit Stone Absorber Solar Collector

Transcription

1 Effect of a Ppe Number on The Heat Transfer Rate For a Grant Stone Absorber Solar Collector M. Mrmanto*, M. Wrawan, B.H. Saputra Teknk Mesn Fakultas Teknk Unverstas Mataram, Jln. Majapaht No.62 Mataram, NTB *Emal : m.mrmanto@unram.ac.d Abstract As the fossl energy has been gettng depleton, t s mportant to utlze alternatve energes such as solar energy. An eupment that can be used for capturng and convertng solar energy s solar collector. Two dentcal collectors used n ths study were placed facng to the North wth an nclnaton angle of 15. The collector dmenson was 0.8 m x 1 m x 0.05 m. One collector contaned 5 parallel ppes and the other contaned 7 parallel ppes. The water used n the test was flowed naturally due to the gravty force from a hgher tank and was adjusted usng a valve ftted at the end of the collector. The water flow rates employed were 200, 250 and 300 cc/mnutes and measured usng a volume meter and a stop watch. The results showed that the energy comng nto the collector, energy absorbed by the absorber and water, ncreased wth an ncrease n the observaton tme and reached the peak value at about 2.30 pm. After that, they decreased. The effect of the ppe number s sgnfcant and the performance of the collector contanng 7 ppes s better than that contanng 5 ppes. Keywords: solar collector, stone grante absorber, collector performance. INTRODUCTION Energy s somethng that s always needed for human survval. In all condtons human reures or uses energy. However, most human lfe stll depends on fossl energy. Fossl energy such as petroleum and coal s an energy source that cannot be renewed. Therefore, efforts to conserve energy and utlze alternatve energy are reured. One of the alternatve energes s solar energy. Solar energy s one of the renewable energes that s cheap and free and t has hgh potences to be used. The use of solar energy n Indonesa s really sutable because Indonesa s a regon that locates under the euator and between 6 NL and 11 SL. In ths poston, Indonesa has a tropcal weather and sun rays along the day spread unformly. Therefore, Indonesa has solar energy wth a radaton ntensty of about 1000 W/m² or almost 4.8 kwh/m² a day, Rahardjo and Ftranan [1]. To ncrease the effectveness of solar energy use, a devce called solar collector can be utlzed. Ths devce can be used for convertng solar energy to be useful energy. One type of ths devce s flat plate solar collector. In general, from the vew pont of capturng the solar energy, collectors are grouped nto two types: () concentratng collector and () non-concentratng collector. For the nonconcentratng collector, the absorber area s the same as the aperture, Lunde [2]. The collectors used n ths study are nonconcentratng collectors, they are flat plate collectors wth grante stone absorbers. Generally, solar collectors use metallc absorbers to absorb the solar heat. However, n ths paper, the authors wanted to know f the grante stone could be used as an absorber. So the man objectve of ths study s not to compare the performance of the metallc absorber to the grante stone absorber, but just to know the performance of the grante stone as an absorber of a flat plate solar collector. The prevous study conducted by Jatmko [3] reported that grante stone absorber gave better performances than sand absorber. The dfferences between the current study and the study of Jatmko [3] are bgger dmenson of the collector and the number of the ppe, whle n Jatmko [3], they used ppes arranged n a serpentne form. When the sun rays falls down on the absorber of a collector, part of the sun energy s reflected to the ambent and other part absorbed by the absorber and converted nto useful energy. Heat energy absorbed by the absorber s transferred to the flud flowng nsde the ppes mounted n the absorber. Fnally, the heat can be used for other purposes, Permana and Nasbey [4]. The man components of the flat plate collector as also mentoned n Permana dan Nasbey [4] are: 1. Cover. Ths component s functoned to reduce the heat lost from the top of the absorber n convecton ways. 2. Absorber. Ths component s used for absorbng the solar heat from the solar radaton. 3. Ppe. Ths s to transfer the heat from the absorber to the workng flud. 4. Isolator (nsulaton). Ths component s employed to mnmze the heat loss though the walls n conducton manners. 46

2 FACILITY AND EXPERIMENTAL SETUP Ths study uses two prototypes of the flat plate collector wth absorbers made of grante stone. All varables were recorded drectly unless some calculated varables. Tools and Materals Tools or nstruments used were: 1. Thermocouple. Thermocouples used were K type wth measurng ranges rangng from -200 to C. These thermocouples were used to know the temperatures of the glass cover, absorber, collector walls, ambent, nlet and outlet of the water. 2. Glass volume meter and stopwatch. These tools were utlzed for measurng the volumetrc rate of the water. 3. Compass. A compass was used to determne the North-South drecton. Ths was to know the poston of the sun so that the collectors could be placed correctly. 4. Pump. The pump was used for rasng the water from the well to the top reservor. and 300 cc/mnute measured usng the glass volume meter and stopwatch. Data Reducton Euatons used to process the data consst of conducton, convecton, radaton euatons and Newton Law of coolng whch can be obtaned n Holman [5]: use p o mc T T (1) Where use s the useful energy, c p s the specfc heat whch can be attaned n the flud property table based on the bulk temperature / 2 T, T o m represents the mass flow rate of the workng flud whch can be predcted usng euaton (2). m AV (2) Materals employed n ths study were grante stone, water, sterofoam, collector frame, plastc buckets, copper ppes, flexble plastc ppes. The schematc dagram of the research s shown n fgure 1 comprsng a tower, plastc buckets, a flexble plastc ppe, flat plate collectors, a glass volume meter, valves, a pump, an over flow ppe and thermocouples. The water flowed from the upper plastc bucket (2) through the flexble plastc ppe (7). At the collector entrance and ext, the temperature of water was measured. The water receved the heat from the absorber/ppe. Measurng the volumetrc rate of the water was performed at the outlet of the collector usng the glass volume meter and the stopwatch. Meanwhle, the test secton constructon s presented n fgure 2. All temperatures were measured usng calbrated type K thermocouples wth an uncertanty of ± 0,5 C. The volumetrc rates used were 200, 250 Where ρ s the flud densty, A s the cross sectonal area, V s the mean flud velocty nsde the duct. Heat receved by the collector,, can be estmated as: I A (3) T c Where I T s the energy ntensty of the solar radaton whch can be detected usng a pyranometer and A c s the area of the collector absorber or aperture, see Holman [5]. The heat loss to the ambent, loss, s determned as: loss (4) use One ndcator representng the performance of the collector s a collector effcency whch s defned as a comparson between the useful energy and the heat comng and gven by: 47

3 use (5) n lnes ndcatng the useful energy are touchng each other for both collectors (5 ppes and 7 ppes). Ths phenomenon occurs at all the tested volumetrc rates. If they are calculated usng a percentage of the useful energy devaton, ther dfference s stll very small. For example, at the volumetrc flow rate of 200 cc/mnute, ther devaton s only 5,87%, at the RESULTS AND DISCUSSIONS Usng euaton (1) results n calculated useful energy data, use, whch are presented n fgure 3. The data were obtaned from the experments at the volumetrc rates of 200, 250 and 300 cc/mnute. In fgure 3, t s clear that effect of the ppe number on 250 cc/mnute, t s 4,1% and at the 300 cc/mnute the devaton s about 2,5%, see fgure 4. Based on the acceptable error theory wrtten by Adebayo and Osmond [6], Sommer and Kochsek [7], the devatons here are less than 10%, therefore, the devatons can be neglgble. In another word, the the heat transfer rate s nsgnfcant because the 48

4 devatons do not represent the effect of the ppe number on the heat transfer rate. One of the causes of ths phenomenon s due to the same collector area, so that the energy receved by the absorber s the same, conseuently energy absorbed the water s also the same. Of course, there s a dfference of the ppe heat transfer area for 5 ppe and 7 ppe collectors, however, the ppe heat transfer s to small compared to the heat transfer area of the collector, hence, the dfference of the ppe heat transfer area s not enough to make sgnfcantly devatons. Therefore, the effect of the ppe number on the heat transfer rate s nsgnfcant. Smlarly, from the vew pont of the useful energy, see fgure 4, the number of the ppe does not show dfferent useful energy. Nevertheless, n fgure 3, It seems that the useful energy for both 5 ppe and 7 ppe collectors s hgher at the volumetrc rate of 250 cc/mnute. Ths was due to the dfferent solar radaton energy receved by the collectors. The three dfferent volumetrc rates were tested on the dfferent days, when the 250 cc/mnute was beng tested the solar radaton energy was hgher, conseuently at the 250 cc/mnute the useful energy s hgher than those at the other two volumetrc rates. Another result relatng to the useful energy s the water temperature ncrease. The ncrease of the water temperature s presented n fgure 4. From the fgure, t can be seen that the ncrease of the water temperature for the two solar collectors at the same volumetrc rate s not sgnfcant. Ths s one of the factors that causes no dfferent useful energy for both collector. The useful energy s only a functon of the water temperature f the specfc heat and the mass flow rate s the same. Therefore, the graph of the useful energy trend (fgure 3) s almost the same as the graph of the water temperature ncrease (fgure 4). Absorber temperatures are mportant thng n the solar water heater. They relate the heat loss to the ambent. The hgher absorber temperature results n the hgher heat loss and the lower useful energy. The absorber temperatures obtaned from the experments are presented n fgure 5. 49

5 In fgure 5 t seems that the absorber temperature for the 5 ppe collector s always hgher than the absorber temperature for the 7 ppe collector. Ths was due to the lower energy absorbed by the water n the 5 ppe collector. Even so, the dfference of the useful energy for both collectors s Fgure 7 ndcates the heat loss to the ambent for the 5 ppe collector s hgher than that for the 7 ppe collector. Ths can be seen that the blue lne s almost always above the red lne. Fgure 7 confrms the explanaton gven by the fgure 5. Fgure 7(a) shows a dfferent phenomenon. That was due to the stll below 10% or nsgnfcant due to [6,7]. However, the average daly useful energy for the 7 ppe collector s hgher than that for the 5 ppe collector, as seen n fgure 6. Energy receved by the collector s surely the same because the collector area s the same, nevertheless, the heat loss for the 5 ppe collector s hgher than that of the 7 ppe collector. To know the heat loss to the ambent, fgure 7 s presented. condton of the sky. When the sky s cloudy, the sun energy comng nto the collector s small, meanwhle the useful energy s stll bg because of the saved energy n the absorber than can be removed by the water. For that reason, applyng euaton (4) results n the lowest heat loss. One mportant parameter n testng the solar collector s the collector effcency. Theoretcally, the effcency of the collector can be predcted usng 50

6 euaton (5) and the results are shown n fgure 8. The collector effcency s affected by the useful energy and ncomng energy, therefore, the effcency of the collector when the sky s cloudy, s very hgh, see fgure 8(a). Ths s due to the very low ncomng energy when the sky s not clear, whle the useful energy remans hgh, conseuently, the effcency s very hgh. CONCLUSIONS Base on the expermental results and dscusson, several conclusons can be drawn as follows: 1. The effect of the ppe number on the heat transfer rate s nsgnfcant. 2. The heat transfer rate s affected by the observaton tme. 3. The heat loss to the ambent s nfluenced by the absorber temperature. 4. The effect of the volumetrc rate on the heat transfer rate s nconclusve. [4] H. Permana, H. Nasbey, Preparas pengukuran suhu kolektor surya dan fluda kerja dengan datapa easytrack 2 system, Jurnal Fska dan Aplkas (2011). [5] J.P. Holman, Heat Transfer, 8th edton, McGraw-Hll Companes, Inc., USA, [6] O.O. Adebayo, I.C. Osmond, A search for an acceptable margn of valuaton error: A case study of valuers and ther clents n Ngera, Sr Lankan Journal of Estate, Department of Estate Management Valuaton Unversty of Sr Jayewardenepura, Sr Lanka, [7] K.D. Sommer, M. Kochsek, Role of measurement uncertanty n decdng conformance n legal metrology, n Proc. of The 10th Int. Metrology Congress, Sant Lous, France, October ACKNOWLEDGEMENT The authors would lke to acknowledge the Mataram Unversty for the fundng (BOPTN research grand) and the faclty. NOMENCLATURE A : cross-sectonal area (m²) A c : collector area (m²) c p : specfc heat (kj/kg K) I T : solar radaton heat flux (W/m²) m : mass flow rate (kg/s) : heat nput (W) loss : heat loss to the ambent (W) use : useful energy (W) T : water nlet temperature ( C) T o : water outlet temperature ( C) V : average velocty (m/s) ρ : densty (kg/m³) η : effcency REFERENCES [1] I. Rahardjo, I. Ftrana, Analss potens pembangkt lstrk tenaga surya d Indonesa, strateg penyedaan lstrk nasonal dalam rangka mengantspas pemanfaatan PLTU batubara skala cecl, PLTN dan Energ Terbarukan, [2] J.P. Lunde, Solar Thermal Engneerng, John Wley & Sons, New York, [3] H. Jatmko, Kajan ekspermental penggunaan absorber pasr dan absorber batu grant terhadap laju perpndahan kalor yang dterma ar pada kolektor surya tpe plat datar, Tugas Akhr, Teknk Mesn, Fakultas Teknk, Unverstas Mataram,