- Email: [email protected]
Assessment of the possibility of unglazed transpired type solar collector to be used for drying purposes: a comparative assessment of efficiency of unglazed transpired type solar collector with glazed type solar collector
Available online at www.sciencedirect.com
ScienceDirect Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 212 (2018) 1295–1302
7th International Conference on Building Resilience; Using scientific knowledge to inform policy and practice in disaster risk reduction, ICBR2017, 27 – 29 November 2017, Bangkok, Thailand
Assessment of the possibility of unglazed transpired type solar collector to be used for drying purposes: a comparative assessment of efficiency of unglazed transpired type solar collector with glazed type solar collector WBMAC Bandaraa*, BK Amarasekaraa, CP Rupasinghea a
Department of Agricultural Engineering, Faculty of Agriculture, University of Ruhuna, Sri Lanka
Abstract Energy, which has become a key issue in global scale, has already posed a threat for the existence and development of mankind. Due to environmental degradation associated with energy consumption in all sectors, it is required to focus on utilization of sustainable energy sources. Solar energy, which is an environment friendly and sustainable energy source, is a promising alternative for fossil fuels. Drying processes in industrial sector accounts for about 12 to 20% of the total energy consumption. In developing countries, solar drying is utilized as a decentralized thermal application for the food preservation. Though natural sun-drying does not involve any cost, there are many disadvantages such as long drying time, contamination and intrusion of insects and rodents, which will result in low quality dried material. Currently, unglazed transpired type solar collectors are commonly used for building heating purposes but rarely for agricultural purposes. To check the possibility of unglazed transpired type solar collectors to be used for drying purposes, this study was conducted to compare the efficiency and performance of unglazed transpired type solar collector with conventional glazed type solar collector in local conditions. Dimensions of both solar collectors were designed as 1m x 1m in size. 5mm thick transparent glass sheet and perforated black painted metal sheet were used for solar radiation transmission in glazed type solar collector and unglazed transpired type solar collector respectively. Inlet and outlet temperature of both types of solar collectors, solar radiation, ambient temperature, wind speed and surrounding relative humidity were monitored. When compare the two types of solar collectors, there were no significant difference of solar collector’s temperature at their outlets with various levels of ambient temperature (28-32 oC), solar radiation (95.21-504.28 W/m2) and wind speed (0.43-1.05 m/s). The efficiency of unglazed transpired type solar collector was observed as 38.41 % under 0.0077 m3/s airflow rate, with average solar radiation of 304.24 W/m2. During lower solar radiation levels, comparatively higher collector’s efficiency was observed in unglazed transpired type solar collector than in glazed type solar collector. In addition, unglazed transpired type solar collector is a low cost
* Corresponding author. Tel.: +94-71-8765876 E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 7th International Conference on Building Resilience.
1877-7058 © 2018 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 7th International Conference on Building Resilience 10.1016/j.proeng.2018.01.167
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and robust heating system. Hence, there is a potential to utilize unglazed transpired type solar collector as a substitute for glazed and heating system. Hence, there a potential to utilize unglazed transpired type solar collector as a substitute for glazed typerobust solar collector for agricultural dryingisprocesses. type solar collector for agricultural drying processes. © 2017 The Authors. Published by Elsevier Ltd. © 2018 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Ltd. committee of the 7th International Conference on Building Resilience. Peer-review under responsibility of Elsevier the scientific Peer-review under responsibility of the scientific committee of the 7th International Conference on Building Resilience. Peer-review under responsibility of the scientific committee of the 7th International Conference on Building Resilience. Keywords: Collector’s efficiency; Drying; Solar radiation; Glazed type solar collector; Transpired solar collector. Keywords: Collector’s efficiency; Drying; Solar radiation; Glazed type solar collector; Transpired solar collector.
1. Introduction 1. Introduction It is estimated that global primary energy consumption will increase by 33.33% by 2040 [1]. Crude-oil consumption is estimated global primary energy will increase by 33.33% [1]. Crude-oil as aItmain energy that source, also expected to beconsumption risen from 90 million barrels per daybyto2040 104 million barrelsconsumption per day [1]. as a2 main energy source, expected be risen million barrels per day to 104ofmillion per day [2]. [1]. emissions related to also energy sector,towith fossilfrom fuel 90 combustion represents 66.66% global barrels CO2 emissions CO CO related to energy of sector, with fossil fuel combustion represents 66.66% of global CO emissions [2]. 2 emissions related to energy sector And2 emissions according to the predictions International Energy Outlook 2016, global CO 2 And accordinguptotothe predictions of International Energy Outlook global COresource to energy sector 2 emissions will increase 43.2 billion metric tons in 2040 [3]. Fossil fuel2016, is a depleting andrelated its burning causes the will increase up 43.2 billion metric tonsininthe 2040 [3]. Fossil fuel has is a now depleting resource andthe its“dangerously burning causes the accumulation of to greenhouse gases (GHG) atmosphere which already exceeded high” accumulation of greenhouse [2]. (GHG) in the atmosphere which has now already exceeded the “dangerously high” threshold of 450 ppm CO2-e gases threshold 450 ppm in COindustrial 2-e [2]. Dryingofprocesses sector accounts for about 12 to 20% of the total world energy consumption. Drying processes industrial sector accounts for aboutis 12 20% of the total energy consumption. Consumption of fossilinfuels for industrial drying purposes, nowtowidely accepted as world an unsustainable solution to Consumption of current fossil fuels for demand. industrialSo, drying purposes, acceptedenergy as an sources unsustainable solution to compensate the energy the solution is is to now find widely out alternative that are not only compensate thealso current energy demand. So, the solution to find out alternative energy sources that are not only renewable but sustainable. Solar energy, which is anisenvironmentally friendly, economically sustainable, and renewable but also sustainable. Solar energy, which is an environmentally friendly, economically sustainable, and promising alternative for fossil fuels. promising forenergy fossil fuels. Drying, alternative which is an intensive process [4], is one of the most important techniques of food processing Drying, which is anfor energy intensive process [4],crop is one theproducts. most important techniques of food industry which is used the preservation of surplus and of food This process consumes high processing amount of industry which used forenergy. the preservation cropwidely and food process consumes high amount of heat energy andiselectric Sun dryingofissurplus the most usedproducts. method This for agricultural drying purposes in the heat electric energy. Sun drying is the most widely used method for agricultural drying purposes in the worldenergy as welland as in Sri Lanka. world as sun welldrying, as in Sri Lanka. Open which is one of oldest methods of food preservation is not hygienic as it is practiced by spreading Open sun drying, which is lying one ofon oldest methods food preservation is not hygienic it is practiced by spreading food products on a thin mat the floor [5].ofThough this is a low-cost method as requiring no skills, there are food products on a thin mataslying on the floor [5]. due Though this exposure is a low-cost method requiring skills, alteration there are several disadvantages such product deterioration to direct to atmosphere, nutrientnocontent several disadvantages such as product deterioration due toand direct exposure to atmosphere, nutrient content due to UV radiation [6], contamination by insects, dusts volatile chemicals, intermittent moistening byalteration rain and due to UV radiation [6], contamination by insects, requirement of long period and higher labor in largedusts scale.and volatile chemicals, intermittent moistening by rain and requirement of long and higher labor in the large scale. Solar dryers haveperiod introduced to overcome consequences of open sun drying. Solar dryers generate a higher Solarof dryers haveconsequential introduced tolower overcome thehumidity consequences of open sun drying. Solarwhich dryersingenerate a highera amount heat and relative compared to open sun drying, return produce amount of heat and consequential lower relative humidityinfection compared open sunthe drying, which incontent return produce good quality product by reducing the spoilage, microbial andtolowering final moisture [6]. Solara good product reducing the microbial infection and lowering the finalsolar moisture content Solar dryingquality systems can bebycategorized as;spoilage, Active solar energy drying systems and Passive energy drying[6]. systems drying systems can be categorized as;solar Active solar energy drying and Passive solar drying systems which are further classified as Direct dryers and Indirect solarsystems dryers. Passive dryers use energy the natural convection which aredrying further classified as Direct and Indirect solarfor dryers. Passive dryers use dryers, the natural convection for their purposes while active solar dryersdryers use forced convection the same. In direct solar internal surface for their drying purposes while dryers use convection formaterials the same.toInbedirect dryers, internal of drying chamber is heated by active the absorption offorced solar radiation, and driedsolar are directly exposuresurface to the of drying chamber is heated byheat the absorption solar radiation, materials to be are directly exposure to the radiation. Indirect solar dryers the air in a of solar collector andand then, the heated airdried is ducted to the drying chamber radiation. Indirect solaris dryers heat the air in aonsolar collectortoand so that solar radiation not directly incident the material be then, dried.the heated air is ducted to the drying chamber so that radiation is notconsist directlywith incident on chamber, the material to collector be dried. and absorbent medium, can be classified into Solarsolar dryers that mainly heating solar Solartypes dryersdepending that mainly with heating solar collector andkind absorbent canemployed be classified into various onconsist their collector type.chamber, Solar collector is a special of heatmedium, exchanger to gain various types depending on incident their collector type. Solar a specialthe kind of heat radiation exchangerand employed useful heat energy from the solar radiation [7].collector Simply, is it absorbs incoming convertstoit gain into useful heattransfers energy from the incident radiation [7].the Simply, it absorbs the incoming radiation energy and converts it into heat, then that heat into fluidsolar flowing through collector. It transforms solar radiation into internal heat, then transfers that medium. heat into fluid flowingfluid through It transforms solar into internal energy of the transform The medium maythe be collector. air, water or oil [8]. There areradiation two typesenergy of solar collectors, energy the transform medium. The medium may be and air, water or oil [8]. Theretracking are two solar types of solar collectors, namely;ofNon – concentrating/stationary solarfluid collectors Concentrating/Sun collectors. Non – namely; Non solar – concentrating/stationary solar fixed collectors and Concentrating/Sun tracking solar collectors collectors.have Nonthe– concentrating collectors are permanently in position and do not track the sun. These concentrating solar collectors permanently fixed in position trackcategorized the sun. These collectors have the same area for intercepting andare absorbing of solar radiation. Theyand cando benot further as; Flat plate collectors, same area for intercepting and absorbing solar radiation. They can be further categorized as;collector Flat plate collectors, Stationary Compound Parabolic Collectorsofand Evacuated Tube Collectors. Concentrating solar focuses solar Stationary Compound Parabolic Collectors and Evacuated Tube Collectors. Concentrating solar collector focuses solar
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beams to a smaller receiving area with its concave reflecting surface that is used to intercept sun rays which eventually increases the radiation flux [8]. Furthermore, solar energy collectors are classified as; Glazed Solar Collectors (GSC) and Covered Unglazed Transpired type Solar Collectors (UTSC). GSC is a collector with glazing appearance, which transfers fluids and enable them to absorb solar radiation more effectively than unglazed transpired type solar collectors. These collectors can be used during the entire year and they can be used in many different climates. The glazing material that is usually used as the top cover of this kind of solar collectors has three functions; minimizing the convective and radiant heat loss from the absorber, transmitting the incident solar radiation to absorber with minor losses and protecting the absorber plate from the outside environment [9]. Therefore, a glass material with qualities such as good strength, durability and non-degradability when exposed to UV light is normally selected for these solar collectors. Though, GSC is commonly used, cost of the glass material and inconvenience of handling have lowered its popularity. UTSC, which is a perforated solar collector used for pre-heating of outside air, is mainly used in European countries for ventilation rather than agricultural practices. This is a development technology that introduced in early 1990s to eliminate energy cost of ventilation and heating. Instead of the matrix of solar absorber, UTSC uses a dark single thin perforated surface made of metal [10]. This system was popularized and installed in 25 different countries on institutional, commercial, multi residential and industrial buildings [11]. According to [12], large scale systems about thousands of square meters of UTSC have been successfully installed and operated in many fields specially in Germany, United States and Canada to acquire energy for different purposes. Heat loss occurred in this device will be small as it directly sucks the air through the perforated plate and operates the temperature of absorber surface relatively low. As there is no glazing cover is utilized, cost of instillation of UTSC is lower than glazed flat plate collectors [13] and other solar heater types [14]. As the usage of UTSC in agriculture is critically low, it is required to determine the possibility of using UTSC effectively in agricultural practices. Therefore, this study was focused on comparing Glazed type Solar Collector and Covered Unglazed Transpired type Solar Collector for their efficiency. Nomenclature GSC UTSC
Glazed Solar Collector Unglazed Transpired type Solar
2. Material and methods 2.1 Location and duration: The experiment was carried out at Department of Agricultural Engineering, Faculty of Agriculture university of Ruhuna at Mapalana, Sri Lanka, from August to November 2016. 2.2 Material selection: Based on the availability and durability, following materials were selected for the fabrication of solar collectors; Plywood board, V- ribbed metal sheets, Transparent glass, Matt enamel black color paint and Styrofoam sheets. 2.3 Fabrication of solar collectors: Plywood is a good insulator and has good strength properties (Thermal conductivity of plywood is 0.13WK-1m-1). Plywood board was used for the fabrication of insulation box (1m x 1m x 15cm) of both types of solar collectors. The boxes were internally insulated with Styrofoam. V- ribbed metal sheets, 1m x 1m x 0.45mm (Coating composition Aluminum 55%, Zinc 43.4% & Silicon 1.6%) were used as the absorption material of solar radiation in both GSC and UTSC [15], because it has a high absorption value of 88% and a high emissivity value of 32% [16]. As color of the collector is a good indicator of the absorbability and converting ability of solar radiation into heat, surface of the absorber was painted with black color paint so that it
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will enhance the solar radiation absorbability of the collector. Moreover, it has low reflectivity when applied on metal surface. The absorber was fixed in the bottom of the plywood box of the GSC and it was covered with 1m x 1m x 5 mm glass sheet which was placed at the top of the plywood box. So, that it permits solar radiation into the system while resisting the heat energy flows out of the system. A perforated V- ribbed metal sheet was used as the absorber of UTSC. The absorber plate was perforated with holes with diameter of 1.5mm, so that the porosity would be about 0.5% [17]. It was fixed on the roof of the plywood box. This perforated metal sheet allows the passage of cool air to the collector and converts the cool air into hot air within the collector. The lower side of GSC was made with an air inlet of 0.005m2, which is equal to the porosity area of total air intake of UTSC. In the upper part of the collector, a single exhaust fan was located to trap hot air from inside of the collector. The outlet of the collector was consisted with an exhaust fan (12v/0.17A) to force the heated air to the drying chamber. UTSC was built with no any air inlet in the lower side. In the upper part of the collector, a single exhaust fan was set up to trap the hot air inside the collector. Outlet of the collector was consisted with an exhaust fan (12v/0.17A) to force the heated air to the drying chamber. Due to forced aeration of the exhaust fan, outside air is entered to the collector through the perforated metal sheet. Each solar collector was placed on a four-legged woody structure. The front two legs were 0.20m in height and back legs were 0.75m in height. The supporting angle was calculated as 160 to the floor (angle of tilt (ß) of solar collectors is ß = 10 + lat Ø, Where lat Ø = latitude of the place that the drier was designed [18] & [19]). Fabricated UTSC and GSC are shown in Fig. 1.
Fig. 1. Unglazed Transpired type Solar Collector (left) and Glazed type solar collector (right)
2.4 Experimental procedure: Experiment was conducted in mid-November, in faculty of Agriculture (latitude 6° 3'37.64"N, longitude 80°34'0.17"E), Mapalana, Sri Lanka. Experiment was started at 9 am to 4 pm and carried out for four days. Data were recorded at regular interval of 10 minutes and data obtained during the experiment were; inlet & outlet air temperature, ambient temperature, solar radiation, relative humidity and wind speed. A TECPEL 322 type hydro-thermo meter was used to measure the inlet, outlet temperature and ambient temperature. Experiment was designed to measure the readings with 10 minutes intervals. Therefore, ambient temperature and temperature of the outlet of both collectors were measured with 10 minutes interval. MES-39461-001 type pyranometer was used for the measurement of instantaneous radiation while Dry and wet bulb thermometer were used to measure the relative humidity of the environment. Using above collected data, efficiency of both solar collectors were analyzed. 2.5 Calculation of solar collector’s efficiency: Solar collector’s efficiency is defined as the ratio of heat received by the drying air to the insulation upon the absorber surface. Collector’s efficiency was calculated using following equation (1) [20].
ŋ" =
%& × ( × ∆* × +, -. × /.
(1)
Bandara et al./ Procedia Engineering 00 (2017) 000–000 WBMAC Bandara et al. / Procedia Engineering 212 (2018) 1295–1302
5 1299
Where; ŋc = Collector’s efficiency, Va = Volumetric flow rate of air (m3/s), ρ = Density of air (kg/m3), ΔT = Temperature elevation (K), Cp = Specific heat capacity of air (J / kg.K), Ac = Area of solar collector (m2), Ic = Solar insolation on collector surface (W/m2) [20]. 3. Results & discussion 3.1 Temperature variation of the outlets of solar collectors; When consider the outlet temperature variation of solar collectors under 0.05 significance level, there was no significant difference between the observed average temperature values of outlets of UTSC and GSC (p=0.43) with ambient temperature (28-32 oC) (Fig. 2), solar radiation (95.21-504.28 W/m2) (Fig. 4) and wind speed (0.43-1.05 m/s) (Fig. 5). The highest temperature values of the outlets of both UTSC and GSC were observed at 32.13 oC of ambient temperature, which was recorded at around 1200 noon and 460.92W/m2 of radiation level. Fig. 2 represents the temperature variation of solar collectors with respect to the time of the day. And the charts of temperature variation of solar collectors with solar radiation and wind speed are shown in Fig. 4 and Fig. 5 respectively.
Fig. 2. Outlet temperature variation of solar collectors with the average ambient temperature (r2=0.8061)
Fig. 3. Outlet temperature variation of solar collectors with respect to the time of the day
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Fig. 4. Outlet temperature variation of solar collectors with solar radiation (r2=0.8366)
Fig. 5. Outlet temperature variation of solar collectors with wind speed (r2=0.0612)
3.2 Variation of the efficiency of solar collectors; At the beginning of the experiment, efficiency of UTSC was recorded as higher than GSC (Fig. 7). But then efficiency of both GSC and UTSC were gradually increased with the increasing ambient temperature. Then after the maximum ambient temperature was reached, efficiency of GSC was tend to be decreased, but the efficiency of UTSC was recorded higher than that of GSC. At the final stage (2.00 - 4.00 pm) with the lowest ambient temperature and the lowest solar radiation level, the efficiency of UTSC was recorded significantly higher than GSC (p=0.0489). It indicates that among GSC and UTSC, the latter can be utilized more effectively under conditions with low ambient temperatures and low solar radiation levels. Fig. 6 represents the variation of average values of solar radiation, ambient temperature and wind speed with time of the day. And variation of the efficiency of solar collectors is given by Fig. 7.
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Fig. 6. Variation of average values of solar radiation, ambient temperature and wind speed with time of the day
Fig. 7. Variation of the efficiency of solar collectors
3.3 Material cost: According to the material cost calculations, UTSC (1 m x 1m x 15cm) was cost $ 30.13 for fabrication while GSC was cost $ 44 for the same. 4. Conclusions Unglazed Transpired type Solar Collector is a new concept for agricultural drying purposes, since it is usually utilized in buildings for indoor ventilation, cooling and drying. Efficiency of UTSC was observed as 38.41% under 0.0077m3/s air flow rate, with average solar radiation of 304.24W/m2. Efficiency of Glazed type solar collector was observed as 38.04% under the same conditions. When consider both types of solar collectors, there was no any significant difference observed in the temperatures at the outlet of each solar collector. Average outlet temperature of unglazed transpired type solar collector was observed as 45.5oC while Glazed type solar collector recorded it as 45.7oC. Cost of UTSC and GSC (1m x 1m x 15cm) for construction were estimated as $ 30.13 and $ 44 respectively. It revealed that unglazed transpired type solar collector is a low cost and robust solar heating system.
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When considering operating and maintenance cost, UTSC is more flexible than GSC. Also, unglazed transpired type solar collector is easier to handle than glazed type solar collector. According to the results of measured parameters, there was no significant difference between the performance of GSC and UTSC. When consider about the cost and handling flexibilities, there is a potential to substitute the commonly used glazed type solar collector with unglazed transpired type solar collector. As industrial drying processes accounts for about 12 to 20% of the total world energy consumption, unglazed transpired type solar collector which utilizes solar power as an alternative energy source, can be successfully utilized for agricultural drying processes rather than using drying and cooling of buildings. References [1] Fatih, Birol. "World energy outlook 2015." Organisation for Economic Co-Operation and Development (OECD): Paris, France (2015). [2] Marchal, V., Dellink, R., Van Vuuren, D., Clapp, C., Chateau, J., Magné, B. and van Vliet, J., 2011. OECD environmental outlook to 2050. Organization for Economic Co-operation and Development. [3] International Energy Outlook 2016, U.S. Energy Information Administration (EIA) [4] Vigants, E., Vigants, G., Veidenbergs, I., Lauka, D., Klavina, K. and Blumberga, D., 2015. Analysis of Energy Consumption for Biomass Drying Process. In Proceedings of the 10th International Scientific and Practical Conference. Volume II (Vol. 317, p. 322). [5] S. Boughali, H. Benmoussa, B. Bouchekima, D. Mennouche, H. Bouguettaia, and D. Bechki, "Crop drying by indirect active hybrid solar– Electrical dryer in the eastern Algerian Septentrional Sahara," Solar Energy, vol. 83, pp. 2223-2232, 2009. [6] W. Senadeera and I. Kalugalage, "Performance evaluation of an affordable solar dryer for crops," 2004. [7] O. Ekechukwu and B. Norton, "Review of solar-energy drying systems II: an overview of solar drying technology," Energy Conversion and Management, vol. 40, pp. 615-655, 1999. [8] S. A. Kalogirou, "Solar thermal collectors and applications," Progress in energy and combustion science, vol. 30, pp. 231-295, 2004. [9] B. Bolaji, "Development and performance evaluation of box-type absorber solar air collector for crop drying," Journal of food technology, vol. 3, pp. 515-600, 2005. [10] C. Dymond and C. Kutscher, "Development of a flow distribution and design model for transpired solar collectors," Solar Energy, vol. 60, pp. 291-300, 1997. [11] V. Delisle and M. R. Collins, "Model of a PV/thermal unglazed transpired solar collector," in Proceedings of 32nd Solar and Sustainable Energy of Canada Inc. conference, Calgary, Alberta, Canada, 2007, pp. 10-13. [12] L. Gunnewiek, E. Brundrett, and K. Hollands, "Flow distribution in unglazed transpired plate solar air heaters of large area," Solar Energy, vol. 58, pp. 227-237, 1996. [13] A. A. Pesaran and K. B. Wipke, "Use of unglazed transpired solar collectors for desiccant cooling," Solar energy, vol. 52, pp. 419-427, 1994. [14] M. R. Collins and H. Abulkhair, "An evaluation of heat transfer and effectiveness for unglazed transpired solar air heaters," Solar Energy, vol. 99, pp. 231-245, 2014. [15] A792/A792M-10, "Standard Specification for Steel Sheet, 55 % Aluminum-Zinc Alloy-Coated by the Hot-Dip Process," ed: ASTM International, West Conshohocken, PA, 2015, 2015. [16] M. Hanif, M. K. Khattak, M. Rahman, M. Khan, M. Amin, and M. Ramzan, "Performance evaluation of a flat plate solar collector as a drier for chillies and tomatoes," J. Sci. Tech. and Dev, vol. 33, pp. 63-67, 2014. [17] G. Van Decker, K. Hollands, and A. Brunger, "Heat-exchange relations for unglazed transpired solar collectors with circular holes on a square or triangular pitch," Solar Energy, vol. 71, pp. 33-45, 2001. [18] Y. Abdullahi, M. Momoh, M. M. Garba, and M. Musa, "Design and construction of an adjustable and collapsible natural convection solar food dryer," Int J Comput Eng Res, vol. 3, pp. 1-8, 2013. [19] K. Sukhatme and S. P. Sukhatme, Solar energy: principles of thermal collection and storage: Tata McGraw-Hill Education, 1996. [20] M. A. Leon, S. Kumar, and S. Bhattacharya, "A comprehensive procedure for performance evaluation of solar food dryers," Renewable and Sustainable Energy Reviews, vol. 6, pp. 367-393, 2002
ScienceDirect Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 212 (2018) 1295–1302
7th International Conference on Building Resilience; Using scientific knowledge to inform policy and practice in disaster risk reduction, ICBR2017, 27 – 29 November 2017, Bangkok, Thailand
Assessment of the possibility of unglazed transpired type solar collector to be used for drying purposes: a comparative assessment of efficiency of unglazed transpired type solar collector with glazed type solar collector WBMAC Bandaraa*, BK Amarasekaraa, CP Rupasinghea a
Department of Agricultural Engineering, Faculty of Agriculture, University of Ruhuna, Sri Lanka
Abstract Energy, which has become a key issue in global scale, has already posed a threat for the existence and development of mankind. Due to environmental degradation associated with energy consumption in all sectors, it is required to focus on utilization of sustainable energy sources. Solar energy, which is an environment friendly and sustainable energy source, is a promising alternative for fossil fuels. Drying processes in industrial sector accounts for about 12 to 20% of the total energy consumption. In developing countries, solar drying is utilized as a decentralized thermal application for the food preservation. Though natural sun-drying does not involve any cost, there are many disadvantages such as long drying time, contamination and intrusion of insects and rodents, which will result in low quality dried material. Currently, unglazed transpired type solar collectors are commonly used for building heating purposes but rarely for agricultural purposes. To check the possibility of unglazed transpired type solar collectors to be used for drying purposes, this study was conducted to compare the efficiency and performance of unglazed transpired type solar collector with conventional glazed type solar collector in local conditions. Dimensions of both solar collectors were designed as 1m x 1m in size. 5mm thick transparent glass sheet and perforated black painted metal sheet were used for solar radiation transmission in glazed type solar collector and unglazed transpired type solar collector respectively. Inlet and outlet temperature of both types of solar collectors, solar radiation, ambient temperature, wind speed and surrounding relative humidity were monitored. When compare the two types of solar collectors, there were no significant difference of solar collector’s temperature at their outlets with various levels of ambient temperature (28-32 oC), solar radiation (95.21-504.28 W/m2) and wind speed (0.43-1.05 m/s). The efficiency of unglazed transpired type solar collector was observed as 38.41 % under 0.0077 m3/s airflow rate, with average solar radiation of 304.24 W/m2. During lower solar radiation levels, comparatively higher collector’s efficiency was observed in unglazed transpired type solar collector than in glazed type solar collector. In addition, unglazed transpired type solar collector is a low cost
* Corresponding author. Tel.: +94-71-8765876 E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 7th International Conference on Building Resilience.
1877-7058 © 2018 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 7th International Conference on Building Resilience 10.1016/j.proeng.2018.01.167
21296 2
Bandara al./ Procedia 00 (2017) 000–000 WBMAC et Bandara et al. /Engineering Procedia Engineering 212 (2018) 1295–1302 Bandara et al./ Procedia Engineering 00 (2017) 000–000
and robust heating system. Hence, there is a potential to utilize unglazed transpired type solar collector as a substitute for glazed and heating system. Hence, there a potential to utilize unglazed transpired type solar collector as a substitute for glazed typerobust solar collector for agricultural dryingisprocesses. type solar collector for agricultural drying processes. © 2017 The Authors. Published by Elsevier Ltd. © 2018 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Ltd. committee of the 7th International Conference on Building Resilience. Peer-review under responsibility of Elsevier the scientific Peer-review under responsibility of the scientific committee of the 7th International Conference on Building Resilience. Peer-review under responsibility of the scientific committee of the 7th International Conference on Building Resilience. Keywords: Collector’s efficiency; Drying; Solar radiation; Glazed type solar collector; Transpired solar collector. Keywords: Collector’s efficiency; Drying; Solar radiation; Glazed type solar collector; Transpired solar collector.
1. Introduction 1. Introduction It is estimated that global primary energy consumption will increase by 33.33% by 2040 [1]. Crude-oil consumption is estimated global primary energy will increase by 33.33% [1]. Crude-oil as aItmain energy that source, also expected to beconsumption risen from 90 million barrels per daybyto2040 104 million barrelsconsumption per day [1]. as a2 main energy source, expected be risen million barrels per day to 104ofmillion per day [2]. [1]. emissions related to also energy sector,towith fossilfrom fuel 90 combustion represents 66.66% global barrels CO2 emissions CO CO related to energy of sector, with fossil fuel combustion represents 66.66% of global CO emissions [2]. 2 emissions related to energy sector And2 emissions according to the predictions International Energy Outlook 2016, global CO 2 And accordinguptotothe predictions of International Energy Outlook global COresource to energy sector 2 emissions will increase 43.2 billion metric tons in 2040 [3]. Fossil fuel2016, is a depleting andrelated its burning causes the will increase up 43.2 billion metric tonsininthe 2040 [3]. Fossil fuel has is a now depleting resource andthe its“dangerously burning causes the accumulation of to greenhouse gases (GHG) atmosphere which already exceeded high” accumulation of greenhouse [2]. (GHG) in the atmosphere which has now already exceeded the “dangerously high” threshold of 450 ppm CO2-e gases threshold 450 ppm in COindustrial 2-e [2]. Dryingofprocesses sector accounts for about 12 to 20% of the total world energy consumption. Drying processes industrial sector accounts for aboutis 12 20% of the total energy consumption. Consumption of fossilinfuels for industrial drying purposes, nowtowidely accepted as world an unsustainable solution to Consumption of current fossil fuels for demand. industrialSo, drying purposes, acceptedenergy as an sources unsustainable solution to compensate the energy the solution is is to now find widely out alternative that are not only compensate thealso current energy demand. So, the solution to find out alternative energy sources that are not only renewable but sustainable. Solar energy, which is anisenvironmentally friendly, economically sustainable, and renewable but also sustainable. Solar energy, which is an environmentally friendly, economically sustainable, and promising alternative for fossil fuels. promising forenergy fossil fuels. Drying, alternative which is an intensive process [4], is one of the most important techniques of food processing Drying, which is anfor energy intensive process [4],crop is one theproducts. most important techniques of food industry which is used the preservation of surplus and of food This process consumes high processing amount of industry which used forenergy. the preservation cropwidely and food process consumes high amount of heat energy andiselectric Sun dryingofissurplus the most usedproducts. method This for agricultural drying purposes in the heat electric energy. Sun drying is the most widely used method for agricultural drying purposes in the worldenergy as welland as in Sri Lanka. world as sun welldrying, as in Sri Lanka. Open which is one of oldest methods of food preservation is not hygienic as it is practiced by spreading Open sun drying, which is lying one ofon oldest methods food preservation is not hygienic it is practiced by spreading food products on a thin mat the floor [5].ofThough this is a low-cost method as requiring no skills, there are food products on a thin mataslying on the floor [5]. due Though this exposure is a low-cost method requiring skills, alteration there are several disadvantages such product deterioration to direct to atmosphere, nutrientnocontent several disadvantages such as product deterioration due toand direct exposure to atmosphere, nutrient content due to UV radiation [6], contamination by insects, dusts volatile chemicals, intermittent moistening byalteration rain and due to UV radiation [6], contamination by insects, requirement of long period and higher labor in largedusts scale.and volatile chemicals, intermittent moistening by rain and requirement of long and higher labor in the large scale. Solar dryers haveperiod introduced to overcome consequences of open sun drying. Solar dryers generate a higher Solarof dryers haveconsequential introduced tolower overcome thehumidity consequences of open sun drying. Solarwhich dryersingenerate a highera amount heat and relative compared to open sun drying, return produce amount of heat and consequential lower relative humidityinfection compared open sunthe drying, which incontent return produce good quality product by reducing the spoilage, microbial andtolowering final moisture [6]. Solara good product reducing the microbial infection and lowering the finalsolar moisture content Solar dryingquality systems can bebycategorized as;spoilage, Active solar energy drying systems and Passive energy drying[6]. systems drying systems can be categorized as;solar Active solar energy drying and Passive solar drying systems which are further classified as Direct dryers and Indirect solarsystems dryers. Passive dryers use energy the natural convection which aredrying further classified as Direct and Indirect solarfor dryers. Passive dryers use dryers, the natural convection for their purposes while active solar dryersdryers use forced convection the same. In direct solar internal surface for their drying purposes while dryers use convection formaterials the same.toInbedirect dryers, internal of drying chamber is heated by active the absorption offorced solar radiation, and driedsolar are directly exposuresurface to the of drying chamber is heated byheat the absorption solar radiation, materials to be are directly exposure to the radiation. Indirect solar dryers the air in a of solar collector andand then, the heated airdried is ducted to the drying chamber radiation. Indirect solaris dryers heat the air in aonsolar collectortoand so that solar radiation not directly incident the material be then, dried.the heated air is ducted to the drying chamber so that radiation is notconsist directlywith incident on chamber, the material to collector be dried. and absorbent medium, can be classified into Solarsolar dryers that mainly heating solar Solartypes dryersdepending that mainly with heating solar collector andkind absorbent canemployed be classified into various onconsist their collector type.chamber, Solar collector is a special of heatmedium, exchanger to gain various types depending on incident their collector type. Solar a specialthe kind of heat radiation exchangerand employed useful heat energy from the solar radiation [7].collector Simply, is it absorbs incoming convertstoit gain into useful heattransfers energy from the incident radiation [7].the Simply, it absorbs the incoming radiation energy and converts it into heat, then that heat into fluidsolar flowing through collector. It transforms solar radiation into internal heat, then transfers that medium. heat into fluid flowingfluid through It transforms solar into internal energy of the transform The medium maythe be collector. air, water or oil [8]. There areradiation two typesenergy of solar collectors, energy the transform medium. The medium may be and air, water or oil [8]. Theretracking are two solar types of solar collectors, namely;ofNon – concentrating/stationary solarfluid collectors Concentrating/Sun collectors. Non – namely; Non solar – concentrating/stationary solar fixed collectors and Concentrating/Sun tracking solar collectors collectors.have Nonthe– concentrating collectors are permanently in position and do not track the sun. These concentrating solar collectors permanently fixed in position trackcategorized the sun. These collectors have the same area for intercepting andare absorbing of solar radiation. Theyand cando benot further as; Flat plate collectors, same area for intercepting and absorbing solar radiation. They can be further categorized as;collector Flat plate collectors, Stationary Compound Parabolic Collectorsofand Evacuated Tube Collectors. Concentrating solar focuses solar Stationary Compound Parabolic Collectors and Evacuated Tube Collectors. Concentrating solar collector focuses solar
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beams to a smaller receiving area with its concave reflecting surface that is used to intercept sun rays which eventually increases the radiation flux [8]. Furthermore, solar energy collectors are classified as; Glazed Solar Collectors (GSC) and Covered Unglazed Transpired type Solar Collectors (UTSC). GSC is a collector with glazing appearance, which transfers fluids and enable them to absorb solar radiation more effectively than unglazed transpired type solar collectors. These collectors can be used during the entire year and they can be used in many different climates. The glazing material that is usually used as the top cover of this kind of solar collectors has three functions; minimizing the convective and radiant heat loss from the absorber, transmitting the incident solar radiation to absorber with minor losses and protecting the absorber plate from the outside environment [9]. Therefore, a glass material with qualities such as good strength, durability and non-degradability when exposed to UV light is normally selected for these solar collectors. Though, GSC is commonly used, cost of the glass material and inconvenience of handling have lowered its popularity. UTSC, which is a perforated solar collector used for pre-heating of outside air, is mainly used in European countries for ventilation rather than agricultural practices. This is a development technology that introduced in early 1990s to eliminate energy cost of ventilation and heating. Instead of the matrix of solar absorber, UTSC uses a dark single thin perforated surface made of metal [10]. This system was popularized and installed in 25 different countries on institutional, commercial, multi residential and industrial buildings [11]. According to [12], large scale systems about thousands of square meters of UTSC have been successfully installed and operated in many fields specially in Germany, United States and Canada to acquire energy for different purposes. Heat loss occurred in this device will be small as it directly sucks the air through the perforated plate and operates the temperature of absorber surface relatively low. As there is no glazing cover is utilized, cost of instillation of UTSC is lower than glazed flat plate collectors [13] and other solar heater types [14]. As the usage of UTSC in agriculture is critically low, it is required to determine the possibility of using UTSC effectively in agricultural practices. Therefore, this study was focused on comparing Glazed type Solar Collector and Covered Unglazed Transpired type Solar Collector for their efficiency. Nomenclature GSC UTSC
Glazed Solar Collector Unglazed Transpired type Solar
2. Material and methods 2.1 Location and duration: The experiment was carried out at Department of Agricultural Engineering, Faculty of Agriculture university of Ruhuna at Mapalana, Sri Lanka, from August to November 2016. 2.2 Material selection: Based on the availability and durability, following materials were selected for the fabrication of solar collectors; Plywood board, V- ribbed metal sheets, Transparent glass, Matt enamel black color paint and Styrofoam sheets. 2.3 Fabrication of solar collectors: Plywood is a good insulator and has good strength properties (Thermal conductivity of plywood is 0.13WK-1m-1). Plywood board was used for the fabrication of insulation box (1m x 1m x 15cm) of both types of solar collectors. The boxes were internally insulated with Styrofoam. V- ribbed metal sheets, 1m x 1m x 0.45mm (Coating composition Aluminum 55%, Zinc 43.4% & Silicon 1.6%) were used as the absorption material of solar radiation in both GSC and UTSC [15], because it has a high absorption value of 88% and a high emissivity value of 32% [16]. As color of the collector is a good indicator of the absorbability and converting ability of solar radiation into heat, surface of the absorber was painted with black color paint so that it
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will enhance the solar radiation absorbability of the collector. Moreover, it has low reflectivity when applied on metal surface. The absorber was fixed in the bottom of the plywood box of the GSC and it was covered with 1m x 1m x 5 mm glass sheet which was placed at the top of the plywood box. So, that it permits solar radiation into the system while resisting the heat energy flows out of the system. A perforated V- ribbed metal sheet was used as the absorber of UTSC. The absorber plate was perforated with holes with diameter of 1.5mm, so that the porosity would be about 0.5% [17]. It was fixed on the roof of the plywood box. This perforated metal sheet allows the passage of cool air to the collector and converts the cool air into hot air within the collector. The lower side of GSC was made with an air inlet of 0.005m2, which is equal to the porosity area of total air intake of UTSC. In the upper part of the collector, a single exhaust fan was located to trap hot air from inside of the collector. The outlet of the collector was consisted with an exhaust fan (12v/0.17A) to force the heated air to the drying chamber. UTSC was built with no any air inlet in the lower side. In the upper part of the collector, a single exhaust fan was set up to trap the hot air inside the collector. Outlet of the collector was consisted with an exhaust fan (12v/0.17A) to force the heated air to the drying chamber. Due to forced aeration of the exhaust fan, outside air is entered to the collector through the perforated metal sheet. Each solar collector was placed on a four-legged woody structure. The front two legs were 0.20m in height and back legs were 0.75m in height. The supporting angle was calculated as 160 to the floor (angle of tilt (ß) of solar collectors is ß = 10 + lat Ø, Where lat Ø = latitude of the place that the drier was designed [18] & [19]). Fabricated UTSC and GSC are shown in Fig. 1.
Fig. 1. Unglazed Transpired type Solar Collector (left) and Glazed type solar collector (right)
2.4 Experimental procedure: Experiment was conducted in mid-November, in faculty of Agriculture (latitude 6° 3'37.64"N, longitude 80°34'0.17"E), Mapalana, Sri Lanka. Experiment was started at 9 am to 4 pm and carried out for four days. Data were recorded at regular interval of 10 minutes and data obtained during the experiment were; inlet & outlet air temperature, ambient temperature, solar radiation, relative humidity and wind speed. A TECPEL 322 type hydro-thermo meter was used to measure the inlet, outlet temperature and ambient temperature. Experiment was designed to measure the readings with 10 minutes intervals. Therefore, ambient temperature and temperature of the outlet of both collectors were measured with 10 minutes interval. MES-39461-001 type pyranometer was used for the measurement of instantaneous radiation while Dry and wet bulb thermometer were used to measure the relative humidity of the environment. Using above collected data, efficiency of both solar collectors were analyzed. 2.5 Calculation of solar collector’s efficiency: Solar collector’s efficiency is defined as the ratio of heat received by the drying air to the insulation upon the absorber surface. Collector’s efficiency was calculated using following equation (1) [20].
ŋ" =
%& × ( × ∆* × +, -. × /.
(1)
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Where; ŋc = Collector’s efficiency, Va = Volumetric flow rate of air (m3/s), ρ = Density of air (kg/m3), ΔT = Temperature elevation (K), Cp = Specific heat capacity of air (J / kg.K), Ac = Area of solar collector (m2), Ic = Solar insolation on collector surface (W/m2) [20]. 3. Results & discussion 3.1 Temperature variation of the outlets of solar collectors; When consider the outlet temperature variation of solar collectors under 0.05 significance level, there was no significant difference between the observed average temperature values of outlets of UTSC and GSC (p=0.43) with ambient temperature (28-32 oC) (Fig. 2), solar radiation (95.21-504.28 W/m2) (Fig. 4) and wind speed (0.43-1.05 m/s) (Fig. 5). The highest temperature values of the outlets of both UTSC and GSC were observed at 32.13 oC of ambient temperature, which was recorded at around 1200 noon and 460.92W/m2 of radiation level. Fig. 2 represents the temperature variation of solar collectors with respect to the time of the day. And the charts of temperature variation of solar collectors with solar radiation and wind speed are shown in Fig. 4 and Fig. 5 respectively.
Fig. 2. Outlet temperature variation of solar collectors with the average ambient temperature (r2=0.8061)
Fig. 3. Outlet temperature variation of solar collectors with respect to the time of the day
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Fig. 4. Outlet temperature variation of solar collectors with solar radiation (r2=0.8366)
Fig. 5. Outlet temperature variation of solar collectors with wind speed (r2=0.0612)
3.2 Variation of the efficiency of solar collectors; At the beginning of the experiment, efficiency of UTSC was recorded as higher than GSC (Fig. 7). But then efficiency of both GSC and UTSC were gradually increased with the increasing ambient temperature. Then after the maximum ambient temperature was reached, efficiency of GSC was tend to be decreased, but the efficiency of UTSC was recorded higher than that of GSC. At the final stage (2.00 - 4.00 pm) with the lowest ambient temperature and the lowest solar radiation level, the efficiency of UTSC was recorded significantly higher than GSC (p=0.0489). It indicates that among GSC and UTSC, the latter can be utilized more effectively under conditions with low ambient temperatures and low solar radiation levels. Fig. 6 represents the variation of average values of solar radiation, ambient temperature and wind speed with time of the day. And variation of the efficiency of solar collectors is given by Fig. 7.
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Fig. 6. Variation of average values of solar radiation, ambient temperature and wind speed with time of the day
Fig. 7. Variation of the efficiency of solar collectors
3.3 Material cost: According to the material cost calculations, UTSC (1 m x 1m x 15cm) was cost $ 30.13 for fabrication while GSC was cost $ 44 for the same. 4. Conclusions Unglazed Transpired type Solar Collector is a new concept for agricultural drying purposes, since it is usually utilized in buildings for indoor ventilation, cooling and drying. Efficiency of UTSC was observed as 38.41% under 0.0077m3/s air flow rate, with average solar radiation of 304.24W/m2. Efficiency of Glazed type solar collector was observed as 38.04% under the same conditions. When consider both types of solar collectors, there was no any significant difference observed in the temperatures at the outlet of each solar collector. Average outlet temperature of unglazed transpired type solar collector was observed as 45.5oC while Glazed type solar collector recorded it as 45.7oC. Cost of UTSC and GSC (1m x 1m x 15cm) for construction were estimated as $ 30.13 and $ 44 respectively. It revealed that unglazed transpired type solar collector is a low cost and robust solar heating system.
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When considering operating and maintenance cost, UTSC is more flexible than GSC. Also, unglazed transpired type solar collector is easier to handle than glazed type solar collector. According to the results of measured parameters, there was no significant difference between the performance of GSC and UTSC. When consider about the cost and handling flexibilities, there is a potential to substitute the commonly used glazed type solar collector with unglazed transpired type solar collector. As industrial drying processes accounts for about 12 to 20% of the total world energy consumption, unglazed transpired type solar collector which utilizes solar power as an alternative energy source, can be successfully utilized for agricultural drying processes rather than using drying and cooling of buildings. References [1] Fatih, Birol. "World energy outlook 2015." Organisation for Economic Co-Operation and Development (OECD): Paris, France (2015). [2] Marchal, V., Dellink, R., Van Vuuren, D., Clapp, C., Chateau, J., Magné, B. and van Vliet, J., 2011. OECD environmental outlook to 2050. Organization for Economic Co-operation and Development. [3] International Energy Outlook 2016, U.S. Energy Information Administration (EIA) [4] Vigants, E., Vigants, G., Veidenbergs, I., Lauka, D., Klavina, K. and Blumberga, D., 2015. Analysis of Energy Consumption for Biomass Drying Process. In Proceedings of the 10th International Scientific and Practical Conference. Volume II (Vol. 317, p. 322). [5] S. Boughali, H. Benmoussa, B. Bouchekima, D. Mennouche, H. Bouguettaia, and D. Bechki, "Crop drying by indirect active hybrid solar– Electrical dryer in the eastern Algerian Septentrional Sahara," Solar Energy, vol. 83, pp. 2223-2232, 2009. [6] W. Senadeera and I. Kalugalage, "Performance evaluation of an affordable solar dryer for crops," 2004. [7] O. Ekechukwu and B. Norton, "Review of solar-energy drying systems II: an overview of solar drying technology," Energy Conversion and Management, vol. 40, pp. 615-655, 1999. [8] S. A. Kalogirou, "Solar thermal collectors and applications," Progress in energy and combustion science, vol. 30, pp. 231-295, 2004. [9] B. Bolaji, "Development and performance evaluation of box-type absorber solar air collector for crop drying," Journal of food technology, vol. 3, pp. 515-600, 2005. [10] C. Dymond and C. Kutscher, "Development of a flow distribution and design model for transpired solar collectors," Solar Energy, vol. 60, pp. 291-300, 1997. [11] V. Delisle and M. R. Collins, "Model of a PV/thermal unglazed transpired solar collector," in Proceedings of 32nd Solar and Sustainable Energy of Canada Inc. conference, Calgary, Alberta, Canada, 2007, pp. 10-13. [12] L. Gunnewiek, E. Brundrett, and K. Hollands, "Flow distribution in unglazed transpired plate solar air heaters of large area," Solar Energy, vol. 58, pp. 227-237, 1996. [13] A. A. Pesaran and K. B. Wipke, "Use of unglazed transpired solar collectors for desiccant cooling," Solar energy, vol. 52, pp. 419-427, 1994. [14] M. R. Collins and H. Abulkhair, "An evaluation of heat transfer and effectiveness for unglazed transpired solar air heaters," Solar Energy, vol. 99, pp. 231-245, 2014. [15] A792/A792M-10, "Standard Specification for Steel Sheet, 55 % Aluminum-Zinc Alloy-Coated by the Hot-Dip Process," ed: ASTM International, West Conshohocken, PA, 2015, 2015. [16] M. Hanif, M. K. Khattak, M. Rahman, M. Khan, M. Amin, and M. Ramzan, "Performance evaluation of a flat plate solar collector as a drier for chillies and tomatoes," J. Sci. Tech. and Dev, vol. 33, pp. 63-67, 2014. [17] G. Van Decker, K. Hollands, and A. Brunger, "Heat-exchange relations for unglazed transpired solar collectors with circular holes on a square or triangular pitch," Solar Energy, vol. 71, pp. 33-45, 2001. [18] Y. Abdullahi, M. Momoh, M. M. Garba, and M. Musa, "Design and construction of an adjustable and collapsible natural convection solar food dryer," Int J Comput Eng Res, vol. 3, pp. 1-8, 2013. [19] K. Sukhatme and S. P. Sukhatme, Solar energy: principles of thermal collection and storage: Tata McGraw-Hill Education, 1996. [20] M. A. Leon, S. Kumar, and S. Bhattacharya, "A comprehensive procedure for performance evaluation of solar food dryers," Renewable and Sustainable Energy Reviews, vol. 6, pp. 367-393, 2002