Theoretical model of circular solar collector for close water open air (CWOA) heated humidification and dehumidification

Transcription

1 International Journal of Renewable Energy and Environmental Engineering ISSN , Vol. 03, No. 01, January 2015 Theoretical model of circular solar collector for close water open air ANIL MOTIR PATEL 1, MANDAR MUKUNDRAV SUMANT 2, V.SHIVA REDDY 3 1 Dept. of Mechanical Engg, M.S.University of Baroda, Vadodara, Gujarat, India 2 Department of Mechanical Engineering, BITs edu campus, Vadodara, Gujarat, India 3 SPERI, V.V.Nagar, Anand, Gujarat, India anilpatel0265@gmail.com,mmmm.four@gmail.com Abstract: The direct contact between the collector and the saline water, result in corrosion and scaling in the still, which is the biggest problem that negatively influence the still performance and thereby reduce the thermal efficiency. In flat plate air heater, cost incurred due to insulation cladding is more so the air heater without insulation is required to develop..in circular solar collector air heater, air is used as a working fluid, which eliminates this problem. The constructions are usually lightweight and inexpensive, and work at atmospheric pressure. A solar air heater is a component, which increases the temperature of the air to a very high temperature using the principle of solar radiation. The use of air as a working medium eliminates the need of a heat exchanger. If the θ is the angle between an incident beam radiation I and normal to the plane surface, then the equivalent flux or radiation intensity falling normal to the surface is given by I Cos θ is the incident angle and Cos θ is the incident angle effect which reduce the radiation intensity. We cannot get constant radiation intensity in case of flat plate air heater. Incident angle effect for airtight circular collector is varying by small change with high value for different months at different hours. Due less incident effect maximum solar radiation can be received by collector surface. Hence, circular geometry collector is consider to minimize the angle of incidence of beam radiation on the collector surface and thus maximize the incident beam radiation Keywords: Circular Water Collector, Radiation Intensity, Incident Angle 1. Introduction: Solar energy is used to heat air in association with solar air heater. Seed vegetable and fruits are drying with the help of solar air heater. Solar energy is clean noiseless and reaction free, low grade energy. (Niles el al., 1979).At north pole and south pole where temperature is very low as well as in winter seasons room heater is a good option among others. Conventional air heater contain absorber plate, glass covers, for preventing heat transfer at sides and back side insulation are used. There are many design available for air passage such a way that air stream remain more time with contact to absorber plate,result in higher heat transfer coefficient achieved. Air heater is a simple device, also it required less maintenance, result in more in practice. To avoid corrosion and leakage problem air heater is a good application against liquid solar heater system. Main drawback of air heater is lower thermal efficiency because of low heat transfer between flow of air steam and absorber plate. To improve heat transfer coefficient between absorber plate and air stream, different suggestion and modification are applied as under (Garg et al., 1991) utilized absorber plate with fin. Choudhury et al., 1988 are worked with corrugated absorber plate. Air heater with solid matrix applied by Sharma et al., 1991.Hollow sphere used by (Swartman and Ogunade, 1966).Above modification increase thermal efficiency effectively, beside pressure drop is more at higher flow rate of air stream. Generally the back and side of air heater are insulated; result in less heat loss but front glass cover which is exposed at the ambient temperature where maximum heat loss occurs. The temperature of glass and absorber plate increase with flow distance travel by air stream, heat losses increase. Collector design developed in such a way that air flowing over a glass cover, result in to preheat of air, now it is passing over the absorber plate.pre heat air absorbed heat from glass cover and increase the heat transfer coefficient. Two glass cover used in air heater, to reduce cost of heater one is plastic cover. Second cover is pours in design, increase heat transfer from absorber to air stream, result in less heat transfer from air to ambient. A.A.Mohamad introduced matrix absorber instead of porous material absorber plate. In proposed circular solar collector reduce incident angle, also circular geometry increase absorber area, simple in construction and easy to assemble. Also connections of this air heater to other equipments are less tedious. 2. Theoretical modeling of proposed solar air heater: The proposed solar air heater is air tight circular collector in which absorber tube is place concentrically of glass tube, between glass tube and absorber tube only air is present and at the both end, neoprene rubber air seals are kept between glass tube and absorber tube which is shown in fig. 3.2 IJREEE Copyright 2015 BASHA RESEARCH CENTRE. All rights reserved

2 ANIL MOTIR PATEL, MANDAR MUKUNDRAV SUMANT, V.SHIVA REDDY Figure 3.2 The equation of time E in minutes, as a function of time of year Figure 3.1 Proposed solar collector tube Q h,g amb Q r,g amb If the θ is the angle between an incident beam radiation I and normal to the plane surface, then the equivalent flux or radiation intensity falling normal to the surface is given by θ is the incident angle and is the incident angle effect which reduce the radiation intensity. We cannot get constant radiation intensity in case of flat plate air heater. Hence, circular geometry collector is considering minimizing the angle of incidence of beam radiation on the collector surface and thus maximizing the incident beam radiation. For the circular geometry collector in which plane rotated about a horizontal north-south axis with continuous adjustment following correlations are consider. Where, is zenith angle, which is given by, Declination angle δ is the angular distance of sun's rays north (or south) of the equator. It is the angle between line extending from center of sun to the center of the earth and projection of this line upon the earth equatorial plane. This is the direct consequence of tilt and it's vary between 23.5 o on June 22 to 23.5 o on December 22, which is given by Where, n is the day of the year, Where, t is the local solar time, given by Negative sign is applicable for the eastern hemisphere. Where, L st is the standard meridian for the local time zone. L loc is the longitude of location. And E is the equation of time in minutes. Stagnant Air Figure 3.3 Q h,t g Q r,t g Heat losses through air heater A theoretical model is used to estimate the U-value of air tight circular collector. For calculating the overall heat loss coefficient from the absorber to ambient, the model assumes in a steady state condition. Energy balance for radiation absorb by absorber Q a, heat loss and useful energy gain is given by A=Collector area, (m 2 ) α a = Absorbivity of absorber Tg =Trasmitivity of glass cover Also Glass Absorber Cp= coefficient of specific heat A measure of collector performance is the collector efficiency, defined as ratio of useful gain over some specific time period to the incident solar energy over same time period. A c =Effective collector area, (m 2 ) Under steady state, conditions heat transfer, Are same as the heat loss through collector having area 1. From absorber tube to inner glass cover ( ) 2. By conduction through glass (Qcond.) and 3. From glass upper surface to ambient ( ) Q cond.

3 Theoretical model of circular solar collector for close water open air t a L=Length of tube (mm) The recommended relation for effective thermal conductivity 1 hh,g amb 1 hr,g amb t go t gi (Correlation due to Raith by and Hollands) Rayleigh number: 1 hh,t g 1 hr,t g t to Absorber tube Glass Pr= Prandtl number Modified Rayleigh number: Tmean Figure Thermal resistances to heat losses 1. Radiation heat transfer : The radiation heat transfer from the absorber tube to the bottom portion of glass cover is in good approximation according to Duffie and Beckman. Hence, Figure Thermal resistances to heat losses Heat transfer surface. Heat transfer from absorber tube to inner glass from absorber tube to inner glass surface is due to combine natural convection heat transfer and radiation heat transfer, which is given by Natural convection heat transfer : For concentric cylindrical enclosure maintain at uniform but different temperature, is given by[8] T g1 =Glass tube inside surface temperature, ( 0 C) T g0 =Glass tube outside surface temperature, ( 0 C) σ =Stefan Boltzmann constant, (W/m 2 k 4 ) =Emissivity of glass tube =Emissivity of absorber A abs =Area of absorber tube (outside) in mm 2 A gi = Area of glass cover (in side) in mm 2 A go = Area of glass cover (outside) in mm 2 2. Heat transfer by conduction through glass tube: As the glass having significant thickness, heat transfer by conduction through glass is consider, which is given by Dci =Glass tube inside diameter, (mm) Dco=Glass tube outside diameter, (mm) Do=Absorber tube outside diameter, (mm) Ta=Ambient temperature, ( 0 C) Tmean=Absorber surface temperature, ( 0 C) Hence,

4 ANIL MOTIR PATEL, MANDAR MUKUNDRAV SUMANT, V.SHIVA REDDY =Thermal conductivity of glass tube, (W/mk) 3. Heat transfer from outer glass surface to ambient: Heat transfer from outer glass surface to ambient are due to the combined natural convection and radiation heat transfer, which is given by 82 o 30 '. Incident angle effect is calculated and tabulates as follows. Where is combined convection radiation heat transfer coefficient from glass to ambient. Hence, θ z = Zenith angle =Convective heat transfer coefficient between glass and ambient, (ww/m 2 k) =Convective heat transfer coefficient between absorber and glass tube, (W/m 2 k) Correlation for natural convection heat transfer coefficient for cylinder given as follows [8]. Co-relation based on the data of R. Hilpert Overall heat loss coefficient (U): Overall heat loss coefficient of collector i.e. overall heat loss coefficient based on outer glass surface area ( ) is given by, Figure 3.5 Incident angle effect for solar air heater (Airtight circular collector) =Thermal resistance due to glass conductivity, ( 0 C/W) =Thermal resistance between glass and ambient, ( 0 C/W) R t-g = Thermal resistance between absorber and glass tube, ( 0 C/W) 3. Sample calculation for incident angle effect, Cos ɵ: Considering the Vadodara latitude φ = o N, slope β = 35 o, longitude of Vadodara = 72 o 95 ' N and L st the standard meridian for the local time zone =

5 Theoretical model of circular solar collector for close water open air Local time 9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:00 Table-1: Incident angle effect Incident angle effect Jan Feb Mar Apr May for solar air heated on day of every month Jun Jul Aug Sep Oct Nov Dec Conclusion: It is observe that incident angle effect for airtight circular collector is varying from to for different months at different hours. Due less incident effect maximum solar radiation can be receive by collector surface. References: [1] C.Yamal1, I.Solmus, (2008) A solar desalination system using Humidification-dehumidification process: experimental study and comparison with the theoretical results, Desalination, Vol.220, pp [2] Y.J.Dai, H.F.Zhang, (2000) Experimental investigation of a solar desalination unit with humidification and dehumidification, Desalination, Vol.130, pp [3] S.A.El -Agouz, M.Abugderah, (2008) Experimental analysis of humidification process by air passing through seawater, Energy Conversion and Management, Vol.49, pp [4] Chafik E, (2002) A new seawater desalination process using solar energy, Desalination, pp. 153: [5] Chafik E, (2003) A new type of seawater desalination plants using solar energy, Desalination, pp. 156: [6] K.Sopian, M.A.Alghoul, (2009) Evaluation of thermal efficiency of double pass solar collector with porous-nonporous media, Renewable Energy, Vol.34, pp [7] J.Orfi, M.Laplante, et.al, (2004) Experimental and theoretical study of a humidificationdehumidification water desalination system using solar energy, Desalination, Vol.168, pp Proceedings of the International Multi Conference of Engineers and Computer Scientists 2009 Vol. II IMECS 2009, March 18-20, 2009, Hong Kong. [8] S.P.Sukhatme, Solar Energy, Principles of thermal collection and storage, IInd edition,1997