PRODUCING COOLING FROM SOLAR ENERGY BY USING ADSORPTION CYCLE WITH ACTIVATED CARBON AND METHANOL PAIR Himsar Ambarita Mechanical Engineering Department, Engineering Faculty, University of Sumatera Utara Jl. Almamater, Kampus USU Padang Bulan, Medan, Indonesia Email: himsar@gmail.com Abstract Indonesia is blessed with abundant supply of solar energy which can be harvested in a whole year. According to Ministry of Energy and Mineral Resources Republic of Indonesia, total energy of solar radiation in Indonesia varies between 16-18MJ/m 2 /day. A refrigeration machine with adsorption cycle is an alternative that can be used to collect the solar energy and use it to produce cooling. This paper deals with design, fabrication, and testing a small scale solarpowered adsorption cyle refrigeration machine. It consists of condenser, evaporator, absorber and generator. The absorber has a surface area of 0.25 m 2 and it is loaded with 6 kg local produced activated carbon, as a generator. The evaporator is filled with 2 litres of methanol as refrigerant. The refrigeration machine has been tested by exposing directly to solar radiation in Medan city. Using data acquisition system, operational temperatures and solar radiations are measured and collected. The results show that solar radiation can heat the absorber up to 120 o C. Thus the methanol is desorbed from the activated carbon and condensed by condenser and sent it back to evaporator. During night, the methanol is absorbed from evaporator and produces cooling. The lowest temperature can be reached here is 10 o C. The conclusion can be drawn here is that solar radiation can be used to produce cooling in a very simple way and without any moving part. Keywords: renewable energy, solar, refrigeration, adsorption 1. Introduction As a matter of fact, in some of remote areas of Indonesia, there are villages where electricity is presently unavailable or far from sufficient. These areas, however, need refrigeration machine, in order to preserve foods or vaccines. Most of the refrigeration machines, currently in servis, based on vapour compresion cycle which is powered by electicity. On the other hand, Indonesian archipelagos are located around equator. Such areas receive solar radiation in a relatively constant for entirely year. According to Ministry of Energy and Mineral Resources Republic of Indonesia, total energy of solar radiation can reach of 16-18MJ/m 2 in every day. Therefore, solar-powered refrigeration machine is proposed to harvest the abundant solar thermal energy for preserving foods and vaccines. Many researchers have been reported their works that dealt with producing cooling based on adsorption cycle powered by solar radiation. Pons and Guillminot [1] pioneered the research of adsorption cycle to produce cooling. They designed and tested a solarpowered ice maker based on adsorption cycle. The used solar collector is flat plate type with collector area of 6 m 2. It is loaded with 130 kg activated carbon and 18 kg of methanol as refrigerant. The machine was tested by exposing to solar radiation in Orsay, France with latitude 48 o N. The solar radiation varied from 19 22 MJ/m 2 /day. The machine produced 30-35 kg of ice per day. Li et 26
al. [2] carried out analysis and performace testing of a solar-powered refrigeration machine. The collector consists of two flat plat type with area of 1.5 m 2. The absorber is loaded with activated carbon and methanol as refrigerant. Performance testing was carried out in laboratory by exposing the collector to quartz lamps as solar simulator. By using radiation of 28 MJ 30 MJ, the machine can produce ice of 7-10 kg ice. Khattab [3] studied a small scale of solar powered adsorption cycle. The machine is designed, fabricated and tested in Cairo (30 o N). The local produced activated carbon and methanol are used. In order to enhance the heat transfer rate in the activated carbon grains are mixed with small pieces of blackened steel. Test results show that the bed temperature is above 100 o C was found to be 5 hours with maximum temperture of 120 o C in winter. In summer, the corresponding values 6 hours and 133 o C. The daily ice production is claimed to be 6.9 and 9.4 kg/m 2 and COP is 0.13 and 0.159 for winter and summer climate, respectively. Li et al. [4] developed a solar-powered ice maker with no valve. The used collector is flat plate type with area of 1 m 2 and it contains 19 kg activated carbon produced in China. The machine was tested by exposing directly into solar radiation of 18 22 MJ/m 2 /day and produced ice about 5 kg. Solid adsortion cycle is the most promising cycle for solar-powered refrigeration machine [5]. Thus it has received increased attention since the 1990s. This is due to its simple operation, low cost, can be powered by low grade heat, and utilizes refrigerant with zero ODP and GWP. There are many adsorption pairs have been studied and reported in literatures. It seems that activated carbon and methanol is the most suitable pair for solar-powered refrigeration machine, due to it is less expensive than other pairs, and can be driven by heat with relatively low, near the ambient temperatures. This is a very important condition, since a solar collector with relatively lower temperature can be operated with a higher efficiency. Those paper show that solar-powered refrigeration machine based on adsorption cycle and using activated carbon and methanol is an antractive aplication for harvesting the abundant solar energy. In order to keep the simplicity the flat plat type of solar collector are usually employed. However, to the best knowledge of the author, no study on the solar-powered adsorption cycle refrigeration machine carried out in Indonesia has been reported in literature. This is the main back ground of this work. This paper deals with design, fabrication, and testing a small scale solar-powered adsorption cyle refrigeration machine. It consists of condenser, evaporator, absorber and generator. The absorber has a surface area of 0.25 m 2 and it is loaded with 6 kg local produced activated carbon, as a generator. The used activated carbon is made of local palm oil kernel. The evaporator is filled with 2 litres of methanol as refrigerant. The refrigeration machine will be tested by exposing directly to solar radiation in Medan city. 2. Working Principle and designed refrigeration machine The working principle of the solarpowered refrigeration machine is depicted in Fig. 1. A simple solarpowered adsorption cycle consits of a condenser, an evaporator, and a generator contains adsorbents and absorbate which also serves as a solar collector. 27
Condenser Insulation Natural convection Absorber Generator Evaporator Methanol Closed Glas cover flows into evaporator tank. This desorption process will finish untill the temperature of the adsorbent reaches the maximum desorption temperature. In the night period, when the temperature of the adsorbent bed reduces, the refrigerant evaporates from the evaporator and flows back into the adsorbent bed. The evaporation process results a colling effect. The colling effect will be utilized for refrigeration uses. This suggests that the described adsorption cycle is an intermittent cycle. Condenser Water a) Charging (Desorption) Closed Generator Evaporator Methanol Ice Natural convection b) Discharging (Adsorption) Fig. 1 Main components and working principle of solar-powered adsorption cycle The operation cycle can be divided into two periods, day and night periods. In the day period, the adsorbent is heated by solar energy. This raises the temperature of the adsorbent and presure of the refrigerant in the adsorbent bed as well. When the temperature adsorbent reaches the desorption temperature, the refrigerant begins to evaporate and desorbs from the bed. The desorbed refrigerant vapour will be condensed into liquid in the condenser. The refrigerant liquid In the present work, the simplest structure and component of the adsorption cycle are proposed. The pair of adsorbent and absorbate are the local domestic type charcoal and methanol, respectively. The proposed design has been fabricated and it is depicted in Figure 2. The components to be designed are generator, condenser, and evaporator. Since the collector is also a generator, it has unique properties in comparison with a conventional solar collector. In one hand, to provide a better desorption process, in the day period, the collector must absorb solar energy as much as possible but releases the absorbed heat as small as possible to the environment. On the other hand, in the night period, the collector must release the heat as much as possible to provide a better adsorption process. In other words, in the day time, the collector should be a good insulation, but in the night time it should provide a good heat transfer release rate. In order to meet these duties, a double cover glasses will be installed and released manually. The designed collector area of 0.25 m 2 and it contains about 6 kg activated carbon. To enhance the heat transfer process from the collector to the activated carbon, some metal fins will be installed. The objective of the condenser here is to convert the vapour of the refrigerant into fluid. The natural convection heat transfer will be utilized. Tube finned condenser will be used. In order to provide a better performance, the condenser position should be located 28
Jurnal Dinamis,Volume I, No.9, Juni 2011 under the collector to avoid it from directly solar radiation. As a note, the present machine is free of moving part and electricity. The refrigerant (in fluid phase) will be collected in an Evaporator tank in the day period. In the night period, the refrigerant evaporates and absorbs some heat from the cooled medium. In this work water will be used as a medium cooled. ISSN 0216-7492 Fig. 3 Clapeyron diagram (ln P vs. -1/T) of an ideal adsorption cycle The total energy gained by the system during the heating period is the sum of energy from A to B or Q AB and energy from B to C or Q BC. As a note Q AB is energy to raise the temperature of the activated carbon and methanol and Q BC is energy for progressive heating of the activated carbon and for desorption process. A G bs en or er be at r or Fig 4 Experimental setup Fig 2 The designed solar-powered refrigeration machine The analysis of the performance of a solar-powered adsorption cycle can be explained by using Clapeyron diagram as shown in Fig. 3. Q AB (m ac cp ac m m A cp m ) TB T A (1) Q BD m AC cp AC cp m (m m A mm B ) 2 TC TB mma mmc H (2) Where subscribe AC and m stand for activated carbon and methanol, respectively. And H [kj/kg] is heat desorption. During night or cooling period, the refrigeration capacity is calculated by: Q e m ma m mc L (3) Where L [kj/kg] is the latent heat of evaporation of methanol. And the total heat collected from the solar radiation is calculated by equation: tss Q rad tsr I dt (4) 29
Where tsr and tss are time at sunrise and time at sunset, respectively. The COP of the system will be: Qe COP (5) Q rad 3. Experimental Setup The machine and data acquisition system of the experimental setup are shown in Fig. 4. A flat-plate type solar collector with dimension 0.5 m 0.5 m and the slope angle is 30 o. The generator beneath the solar collector and it is loaded with 6 kg of activated carbon. In order to reduce heat loses to atmosphere, the solar collector is covered by double glas cover. Temperatures are measured by using J- type thermocouples with uncertainly equal to 0.1 o C. The number of thermocouples are 20, which are six thermocouples placed in collector, six in evaporator, and the rests in the condenser. Agilent 34972, multi channel data logger is used to record temperatures with interval of 1 minute. Two pressure gauges are used to measure pressure in evaporator and in generator bed. HOBO Microstation data logger is used to measure the ambient. Solar radiation is measured using HOBO pyranometer smart sensor, temperatur and RH are measured using HOBO temperature RH smart sensor with accuarcy 0.2 o C and 2.5%RH, and wind speed with HOBO wind speed smart sensor with accuracy 1.1 m/s. In order to make sure the activated carbon is free of water vapor and other gases, preparation is carried out. It is heated up to 90 o C. While the temperature is kept constant, it is evacuated by using vacum pump for 20 minutes. Then the evaporator is filled with methanol and evacuation is continue for a while until methanol boiling is observed. The machine is now ready for experiment. Fig. 5 Solar radiation and ambient temperature in the first testing day 4. Results and Discussions The solar-powered refrigeration machine has been tested for three days by exposing it to the solar radiation in our Laboratory, Sustainable Energy Research Centre in Medan city. It has latitude of 3 o 35 North and longitude 30
98 o 39 East, 35 above mean sea level. All sensors and machine are placed at elevation of 12 m above ground. In the present report only measurement results of the first day are reported. The first day of test starts from June 3 rd at 8.00 AM and finish on June 4 th at 6.00 AM. The ambient temperature and solar radiation at the testing day are presented in Fig. 5. The maximum solar radiation is 874 W/m 2, recorded at 13.04 WIB. The maximum ambient temperature is 34 o C, recorded at 13.40 WIB. Wind speed is categorized low with a maximum speed of 3.8 m/s. Total solar radiation in this recorded day is 16.2 MJ/m 2 which is in the range of data presented by Ministry of Energy and Mineral Resources Republic of Indonesia. measurement and experiment. Thus, in the figure temperature of the absorber start from 60 o C. It was shown that temperature of the absorber increase as time increasing. This suggests that, solar energy is used to heat up the absorber and the generator. Since the absorber is insulated, temperature of the absorber is far above the ambient. Temperatures of the absorber can be over 100 o C. According to the graph, time duration during which the bed temperature is above 100 o C was shown to be 1.5 hours. It was from 13.00 to 14.30 WIB and the maximum temperature of 120 o C. Based on these facts, it is clear that desorption process takes places in the generator and powered by solar energy. Temperatures of the absorber are presented in Fig. 6. As a note, when the experiment is starting, the thermocouples are in preparation. In other words, there is a delay between Fig. 6 Measured temperatures at absorber in the first testing day 31
Fig. 7 Measured temperatures at evaporator in the first day The measured temperatures at evaporator in the first testing day are shown in Fig. 7. As shown in the figure, temperatures inside the evaporator are strongly affected by ambient. The maximum temperature in the evaporator is 30 o C at 15.00. At 17.00, desorption process in generator is stop and its temperature decreases as time increasing. At this time absorption process is starting in the evaporator. The absorption process makes methanol evaporating. The evaporation will draw heat from surrounding. Measured temperatures decrease with time. The duration of the adsorption process is about 7 hours. It starts from 17.00 and finishes around 24.00. The lowest temperature in the evaporator is about 10 o C. The time duration during which the evaporator reaches the lowest temperature was about 6 hours. This suggests that the present machine can provide low temperature environment for almost 11 hours. It starts from 20.00 until 6.00. However, the lowest temperature can be reached by this machine is 10 o C. 5. Conclusions A small scale soar-powered refrigeration machine has been designed and fabricated. It is also tested by exposing to solar radiation in Medan city for three testing days. The measurements show that the refrigeration machine can produce cooling with temperature of 10 o C for 10 hours. The main conclusion can be drawn here is adsorption cycle can be employed to produce cooling by using typical radiation in Medan city. References [1] M. Pons and J.J. Guilleminot, Design of an experimental solar-powered, solidadsorption ice maker, Transactions of the ASME, Journal of Solar Energy Engineering 108, 1986, p.332-337. [2] M. Li, R.Z. Wang, Y.X. Xu, J.Y.Wu, and A.O. Dieng, Experimental study on dynamic performance analysis of a flatplate solar solid-adsorption refrigeration for ice maker, Renewable Energy 27, 2002, p.211-221. [3] N.M. Khattab, A novel solar-powered adsorption refrigeration module, Applied Thermal Engineering 24, 2004, p.2747-2760. [4] M. Li, C.J. Sun, R.Z. Wang, and W.D. Cai, Development of no valve solar ice maker, Applied Thermal Engineering 24, 2004, p.865-872. [5] R.Z. Wang and R.G. Olivera, Adsorption refrigeration-an efficient way to make good use of waste heat and solar energy, Progress in Energy and Combustion Science 32, 2006, p.424-458. 32