International Journal of Emerging Technology and Advan - PDF document

International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com ( ISSN 2250 - 2459, ISO 9001:2008 Certified Journal, Volume 2, Issue 12, December 2012) 539 Study on Design of an Evacuated Tube Solar Collector for High Temperature Steam Generation Dr.S.P.Vendan 1 , L.P.A.Shunmuganathan 2 , T.Manojkumar 2 , C.Shiva Thanu 2 1 Associate Professor, Department of Mechanical Engineering, PSG CT. 2 Student, Department of Mechanical Engineering, PSG CT. Abstract — The eco - friendly and non - polluting solar energy can be used for direct conversion into electricity by photo - voltaic conversion and into thermal energy by using Flat Plate Collectors (FPC) or Evacuated Tube Collectors (ETC). Evacuated Tube Collectors are more efficient compared to Flat Plate Collectors as the former eliminates the convective heat losses. This paper discusses design calculat ions involved in the development of an Energy Efficient Solar Thermal System for generating steam at atmospheric pressure. This finds applications such as Steam cooking, laundry etc. It includes calculation of collector area and number of evacuated tubes r equired. It also reports comparison of its performance with the conventionally used steam boilers in terms of fuel usage and carbon emissions. Keywords — Evacuated Tube C ollectors, Flat plate collectors, Steam Generation, Low pressure steam applications, Design of ETC. I. I NTRODUCTION In present scenario, the energy crisis is predominant as conventional sources of power such as fossil fuels are depleting at a faster rate. Also, the usage of these fuels, adversely affect our environment resulti ng in Global warming, Ozone layer depletion, Acid rain etc. Hence, there is a necessity to shift the focus towards other non - conventional and renewable energy sources. Solar energy finds its application over diverse fields. This paper focuses on how solar energy can be used in ironing process of laundry. The steam generated at 175 o C serves this purpose. Steam, generated using boilers or electrical systems can be replaced by cost - effective solar thermal collector. The selection of appropriate solar collecto r, design calculation to estimate the approximate collector area and experimental setup required to determine the collector performance are discussed in this paper. II. C OMPARISION BETWEEN FPC AND ETC An evacuated tube collector is slightly more efficient tha n a flat plate system due to lower heat losses. With less heat loss through the vacuum air layer in the evacuated tubes, these systems are more effic ient at lower ambient air temperatures. Compared to flat plate collectors, evacuated tube collectors are less expensive . Require a smaller roof area than comparable flat plate collectors . Maintaining vacuum is difficult in case of evacuated tube collector . For low temperature applications, flat plate collectors are more efficient compared to evacuated tube collectors. But, f or the purpose of high temperature applications like steam generation, evacuated tube collectors are best suited because of its minimum heat loss as shown in Figure. 1 . Figure 1 Comparison between FPC and ETC III. E VACUATED TUBE COLLECTOR ET C’s are built to reduce conv ective and heat conduction loss (vacuum is a heat insulator). Each evacuated tube consists of two glass tubes. The outer tube is made of extremely strong transparent glass that is able to withstand changing climatic conditions . The inner tube is also made of glass, but coated with a special selective coating (ALN/AIN - SS/CU) which features excellent solar heat absorption and mini mal heat reflection properties. The air is evacuated from the space between the two glass tubes to form a vacuum . International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com ( ISSN 2250 - 2459, ISO 9001:2008 Certified Journal, Volume 2, Issue 12, December 2012) 540 In order to maintain the vacuum between the two glass l ayers, a barium getter is used . During manufacture this getter is exposed to high temperatures which cause the bottom of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO 2 , N 2 , O 2 , H 2 O and H 2 outgases from the tube during storage and operation, thus helping to maintaining the vacuum. IV. D ESIGN PROCEDURES A. Nomenclature τ Transmittance of glass cover α η Absorptance of absorber surface M Mass of water (kg) C Average heat capacity of receiver tube (J/kg K) ΔT Change in mass temperature (T) Δt Change in time period (sec) I Solar radiation (W/m 2 ) A Area of receiver tube exposed to radiation (m 2 ) η Effciency B. Collector Area De sign The collector area A can be determined from the following eqn. 1,[1] ௧ (1) C. Number of tubes The determination of number of tubes depends on the size of each tube available commercially. It is given by eqn.2, ݑ ݎ ݐݑ ݏ ௘௖௧ ௥ ௥௘௔ ௘௤௨ ௥௘ௗ ௥ ௘௖௧௘ௗ ௔௥௘௔ ௙ ௦௧௔ ௗ௔௥ௗ ௧௨௕௘௦ (2) D. Efficiency The efficiency is calculated using emp irical relation [2] as follows: η (3) V. D ESIGN CALCULATIONS The solar irradiation data is collected for Coimbatore region over a period, which is shown in table 1. The lowest value has been recorded in the month of July and it has been used in the calculation of collector area. TABLE I [3] Month Radiation (kWhr/m2/Day) January 5.46 February 5.94 March 6.39 April 5.93 May 5.35 June 3.87 July 3.71 August 4.08 September 4.83 October 4.54 November 4.65 December 4.99 For the design of ETC system, the material of outer tube is considered as Borosilicate Glass and its properties are listed in the table II below: TABLE II I Material Borosilicate Glass Selective coating materials AL/SS/CU Absorption coefficient( α ) 0.93 Emission coefficient 0.065 Transmission coefficient ( τ ) 0.890 A model calculation for producing 5Kg of steam at 175 o C has been worked out and results have been listed in the following table II. TABLE III I S.No Description Values Units 1 Collector Area 0.31 m 2 2 Number of Tubes 4 Nos 3 Theoretical Efficiency 0.251 - International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com ( ISSN 2250 - 2459, ISO 9001:2008 Certified Journal, Volume 2, Issue 12, December 2012) 541 VI. R ESULTS AND DISCUSSIONS This paper re views the use of solar energy in laundry applications. A Flat Plate collector does not operate efficiently beyond a temperature difference of 70 0 C. Hence an evacuated tube collector serves as the best alternative solar thermal technology for high temperature applications upto 200 0 C. Great er efficiencies at high temperatures can be achieved by the use of a solar concentrator technology. Replacing the conventional method of ironing using boilers or electrical heaters , by a solar thermal collector reduces carbon footprint to great extent. VII. F UT URE SCOPE Our work can be extended to design an ETC system with production capacity of 200 kg steam/ day for industrial purposes. In order to minimize the collector area and improve the overall efficiency of the system, a concentrating collector can be used. It is used to focus a beam of solar irradiation towards focal point with high intensity, where absorber tube is placed. Further, this could be optimized for different reflective material of concentrator. An collector setup with sun - tracking system could be used for improved efficiency. [3] R EFERENCES [1] Sukhatme, S.P. and Nayak, J.K. Solar Energy: Principles of Thermal Collection and Storage. [2] Rai, G.D. Solar Energy Utilisation. [3] Website of National Renewable Energy Limited (NREL).