Investigation of thermal performance of FRP

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instant low to mid temperature hot water for different applications in small ... Sagade [11-12] used parabolic dish collector for instant water heating application.
Investigation of thermal performance of FRP parabolic trough collector using different receivers Satish Aher1, Atul Sagade2, Narayani Sagade3 1

2

Amrutvahini College of Engineering, Sangamner, Maharashtra, India New Satara College of Engineering and Management (Polytechnic), Korti-Pandharpur - 413304, Maharashtra, India 3 REIRF, Pandharpur -413304, Maharashtra, India

Abstract: Along with low temperature applications, there are several prospective fields of application for solar thermal energy at a medium-high temperature (800C -1500C). From the context of hotter climate countries, an extensive potential is available for low cost solar concentrating technologies for domestic applications as well as industrial process heat. The present work describes the experimental results of the low cost prototype parabolic trough made of fiberglass reinforced plastic. Aperture area of collector is coated with aluminum foil as a selective surface coating with reflectivity 0.86. Proposed parabolic trough is tested with two black epoxy coated and glass covered receivers, i.e. mild steel and aluminum under standard test conditions. From the field experiments, it is seen that, the proposed systems yields an instantaneous efficiency of 49.3% and 53.1% with glass covered mild steel and aluminum receivers respectively. It is demonstrated that, the proposed system can play a vital role in delivering instant low to mid temperature hot water for different applications in small scale industries.

Key Words: Solar parabolic collector; Solar thermal heat for industrial process; Solar collector materials; Selective coatings.

1. Introduction Solar and renewable heating is growing at a large rate all over the world. Especially, the hotter climatic counties planned to utilize full potential solar heating. From the Indian perspective, solar thermal systems can play a vital role in different low temperature (800C -1500C) applications. Upcoming section discusses, a brief literature reported by researchers all over the world which explores the performance of such low temperature solar thermal systems that can be effectively implemented for domestic and industrial applications. GarcΓ­a et al. [1] represents the overview of parabolic-trough collectors that were built and marketed during the past century, as well as the prototypes currently under development. This work also presents a survey of systems which could incorporate this type of concentrating solar system to supply thermal energy up to 400 Β°C, especially steam power cycles for electricity generation, including examples of each application. Tsai and Lin [2] proposed as a solar thermal concentrator system that comprising a cylindrical heat-pipe receiver and a variable-focus-parabolic-trough (VFPT) reflector. The authors analyzed the concentrator system using a ray tracing approach and optimized the geometry of the concentrator system for uniform irradiance distribution on the heat-pipe surface. They showed that, the optimized VFPT concentrator yields a significant improvement in the irradiance uniformity and the heating efficiency compared to conventional cylindrical-trough and parabolic-trough concentrators. Huang et al. [3] proposed a new analytical model for optical performance and a modified integration algorithm are proposed and applied to simulate the performance of a parabolic trough solar collector with vacuum tube receiver. The authors discussed the effects of different optical and design optical error, tracking error, position error from installation of receiver, optical properties of reflector, transmittance and absorptivity of the vacuum tube receiver on the efficiencies of the trough system. Eck and Zerza [4] studied the direct steam generation (DSG) in parabolic trough collectors for solar thermal electricity generation as a part of pre-commercial DSG solar thermal power plant with the aim of minimizing the risk for potential investors. They investigated the DSG with steam cycles using superheated steam and concluded on the advantages, disadvantages, and design considerations of a steam cycle operated with saturated steam. Kalogirou [5] detailed thermal model of a parabolic trough collector and the thermal analysis of the collector -receiver system was done by considering all the modes of heat transfer. He developed a mathematical model and it validated with known performance of existing collectors and performed an analysis of the collector – receiver system described in his work. Tao et al. [6] explored an operation principle and design method of a new trough solar concentrator. They analyzed and optimized the new concentrator for design parameters. They commented on the applicability of their newly designed reflector for better thermal performance and indicated that, the new concentrator can realize reflection, focusing for the sunlight using multiple curved surface compound method. Kalogirou et al. [7] compared the advantages and disadvantages of concentrating collectors against conventional flat-plate collectors. They designed a parabolic-trough solar-collector system to optimize collector-aperture and rim-angle, and the receiver-diameter selection. They showed that, the variation in the collector characteristic curve can contribute to minimize heat losses from the receiver. Sagade et al. [8-10] reported the thermal performance of the compound parabolic trough collector with top glass cover for industrial

heating. They explained the effect of receiver temperature and application of different absorber selective coatings on the thermal performance of parabolic trough suing regression models. They concluded that, the glass covered trough system can able to reduce heat losses from absorber and may increase the life of the collector - receiver assembly. Sagade [11-12] used parabolic dish collector for instant water heating application. He explained the effect of variation of natural and forced convection heat losses and mass flow rates on the thermal performance of prototype parabolic dish water heater using coated and non-coated receivers. He reported an instantaneous efficiency of 63% and 48 % for a parabolic dish- receiver system with coated and non-coated receivers. Although there is a massive similar literature is available on the proposed topic, but cannot be accommodated here because of space limitation. It is important to note that, the proposed system is designed with the aim of standalone low temperature industrial and domestic heating.

2. Theory of Basic Thermal Performance: The thermal performance of the proposed system can be evaluated sung conventional equations and described in the upcoming section. The useful energy delivered from the concentrator can be calculated using eq. (I) and (II) [13, 14]. 𝑄𝑒 = π‘šπ‘π‘ (𝑇0𝑒𝑑 βˆ’ 𝑇𝑖𝑛 ) (I) 𝐢𝑆

𝑄𝑒 = π‘šπ‘π‘ [

π‘ˆπ‘™

+ π‘‡π‘Žπ‘–π‘Ÿ βˆ’ 𝑇𝑖𝑛 ] [1 βˆ’ exp {βˆ’

𝐹′ πœ‹π·π‘œπ‘ˆπ‘™ 𝐿 π‘šπ‘π‘

}]

(II)

Where, Qu = Useful energy delivered from the concentrator (W) m = Mass flow rate, kg/s, Tout = Outlet fluid temperature (Β°C) Tin = Inlet fluid temperature (Β°C), Cp = Specific heat of water, kJ/kg Β°C C= Concentration ratio, S= Incident solar flux absorbed in the absorber plate (W/m2) Ul = Overall heat loss coefficient, W/m2Β°C, Tair = Ambient temperature, (Β°C) F' = Collector efficiency factor, D0 = Outer diameter of tube (m) L = Length of concentrator (m) The useful energy gain per unit of the collector length can be expressed in terms of the local receiver temperature T r using eq. (III) and (IV) [13, 14]. 𝑄 𝑄𝑒′ = 𝑒 (III) 𝑄𝑒′ =

𝐿 𝑄𝑒 𝐿

= 𝐹′ [𝑆 βˆ’

π‘ˆπ‘™ 𝐢

(π‘‡π‘Ÿ βˆ’ π‘‡π‘Ž)] (π‘Š βˆ’ π·π‘œ)

(IV)

Where, Qu’= useful energy gain per unit of the collector length (W) Tr = Mean receiver surface temperature (Β°C) W = Width of Parabolic reflector (m) F’ is the collector efficiency factor defined by eq. (V). [13, 14] 1

π·π‘œπ‘’π‘‘π‘’π‘Ÿ

π‘ˆπ‘™

𝐷𝑖𝑛 Γ— β„Žπ‘“

𝐹 β€² = 1/π‘ˆπ‘™ [ +

]

𝑄𝑒 = 𝐹𝑅 (π‘Š βˆ’ π·π‘œ)𝐿 [𝑆 βˆ’

(V) π‘ˆπ‘™ 𝐢

(𝑇𝑖𝑛 βˆ’ π‘‡π‘Žπ‘–π‘Ÿ )

(VI)

Where, Din = Inner diameter of tube, (m) hf = Heat transfer coefficient on inside surface of tube, (W/(m2Β°C)) FR = Collector heat removal factor The heat removal factor can be given by eq. (VII) [13]. 𝐹𝑅 =

π‘šπ‘π‘ πœ‹π·π‘œπ‘’π‘‘π‘’π‘ŸΓ—πΏΓ—π‘ˆπ‘™

[1 βˆ’ exp{βˆ’

𝐹′ πœ‹π·π‘œπ‘’π‘‘π‘’π‘ŸΓ—π‘ˆπ‘™ ×𝐿 π‘šπ‘π‘

}]

(VII)

And collector efficiency can be obtained by dividing Qu by Ib WL. The instantaneous collection efficiency can also be calculated by eq. (VIII) [13, 14]. 𝑄𝑒 πœ‚π‘–π‘› = (VIII) πΌπ‘π‘Ÿπ‘π‘ŠπΏ Where, πœ‚π‘–π‘› = instantaneous collection efficiency Ib = Incident beam radiation (W/m2)

2.1 Overall loss coefficient and heat correlations Overall loss coefficients based on convection and re-radiation losses per unit length were computed by eq. (IX) and (X) [13] 𝑄𝑙 π·π‘œπ‘’π‘‘π‘’π‘Ÿ = β„Žπ‘π‘ (π‘‡π‘Ÿπ‘’π‘π‘’π‘–π‘£π‘’π‘Ÿ βˆ’ 𝑇𝑐)πœ‹π·π‘œπ‘’π‘‘π‘’π‘Ÿ + πœŽπœ‹π·π‘œπ‘’π‘‘π‘’π‘Ÿ(π‘‡π‘Ÿ 4 βˆ’ 𝑇𝑐 4 )/{ 1 } (IX) 1 𝐿

π‘žπ‘™ 𝐿

πœ€π‘

4

= β„Žπ‘€ (𝑇𝑐 βˆ’ π‘‡π‘Ž)πœ‹π·π‘π‘œ + πœŽπœ‹π·π‘π‘œπœ€π‘(𝑇𝑐 βˆ’ π‘‡π‘Ž

4)

+𝐷𝑐𝑖 ( βˆ’1) πœ€π‘

(X)

Where, Tc = Temperature of cover (oC) hw = Wind heat transfer coefficient, W/m2-K Ξ΅p = Emissivity of absorber surface for long wavelength radiation Ξ΅c = Emissivity of cover for long wavelength radiation 2.2 Heat Transfer coefficient between the absorber tube and the cover The heat transfer coefficient hpc for the enclosed annular space between a horizontal absorber tube and a concentric cover is calculated using eqs. XI, XII, XII and XIII [13] 𝐾𝑒𝑓𝑓 = 0.317(π‘…π‘Žβˆ— )0.25 (XI) π‘˜

𝐷

(π‘…π‘Ž βˆ— )0.25 = *

Where Ra = Rayleigh’s Number Thus, β„Žπ‘π‘ =

𝑐𝑖 ) ln(π·π‘œπ‘’π‘‘π‘’π‘Ÿ

1 1 𝑏 0.75 ( 0.6 + 0.6 ) π·π‘œ 𝐷𝑐𝑖

2𝐾𝑒𝑓𝑓 π·π‘œπ‘’π‘‘π‘’π‘Ÿ ×𝑙𝑛(

𝐷𝑐𝑖 ) π·π‘œπ‘’π‘‘π‘’π‘Ÿ

π‘…π‘Ž0.25

(XII)

(XIII)

2.3 Heat transfer coefficient on the inside surface of the absorber tube The convective heat transfer coefficient hf on the inside surface of the absorber tube can be calculated. For Reynolds Number greater than 2000, the flow is turbulent and heat transfer coefficient may be calculated from XIV [13] 𝑁𝑒 = 0.023𝑅𝑒 0.8 Pr 0.4

(XIV)

3. System Description Test Setup shown in fig. 1 consist of solar collector of a storage tank of 100 liter capacity, non- return valves fitted in the pipeline to define the flow direction and control valve is used to regulate the flow rate through the circuit. The necessary instruments were attached to the apparatus and then connected to the data acquisition system. Following measuring instrument were used for the data recording. i) Solar radiation – Pyranometer (Kipp and Zonen CM6b) ii) Wind Velocity- Digital Anemometer (Lutron AM-4201) iii) Mass flow rate- Flowmeter. iv) Temperature measurements- set of J type thermocouple with digital displays. Table 1 indicates the values of system parameters used in the experiments.

Fig.1. Schematic diagram of test setup

Table 1: Parameters used in the Experimentation System Parameters Aperture of the Concentrator (W) Specular reflectivity of concentrator (ρ) Outer diameter of glass tube Outer diameter of absorber tube (Do)

Values 1.10 m 0.86 0.056 m 0.025 m

Length of parabolic trough Intercept factor Inner diameter of glass tube Inner diameter of Absorber tube (Di) Glass cover transitivity for solar radiation(Ο„) Emissivity of Glass(Ξ΅c)

1.21 m 0.95 0.050 0.020 m 0.85 0.82

Emissivity of absorber tube surface (Ξ΅p) Concentration ratio (C) Collector aperture area (Ac) Absorber tube emissivity/emissivity (Ξ±)

0.08 13.69 1.33 m2 0.15

4. Experimental Procedure The field experiments for the current work were conducted at Shivaji University, Kolhapur at latitude of 16.67o N and longitude of 74.25o E. At the start of every experiment, system is flushed out to eliminate the air and other impurities in the absorber if any. The flow of water in the absorber is started and it is filled with water and the flow rate is adjusted to the required value. The whole system is exposed to sun for over 20 min. to achieve quasisteady state conditions before starting the experiments and checking is done for the proper working of all measuring instruments. The cold water storage tank is located above the level of the collector, to ensure that the cold water from the storage tank enters the absorber of the parabolic trough collector by gravity only and get heated up as shown in the fig.1. Heated water flows by thermo siphon into the top of the water tank and hot water is replaced by cold water from the bottom of the tank. The measurements of ambient temperature, fluid temperatures, receiver

surface temperature, storage tank temperatures, solar radiation and wind speed were recorded at every 30 min. The experiments were performed 7 hours after day from 9.00 am to 5.00 pm and each experiment is repeated for four times to check the reproducibility of the results. During the experiment, the orientation of a cylindrical parabolic collector was kept such that its focal axis pointed in the east-west (E-W) direction and the focal axis is horizontal. Manual tracking was provided to the parabolic trough collector on experimental days. The trough was rotated about a horizontal E-W axis and adjusted manually so that the solar beam makes the minimum angle of incidence with the aperture plane at all times.

5. Results and Discussions Fig. 2 shows the variation of the inlet and outlet temperatures of water obtained with the two receivers. The trend explains that, as solar radiation on receiver enhances, outlet water temperature increases. It is seen that, glass covered receivers both aluminum receiver and mild steel performs better as compared to uncovered receiver and it is possible to achieve higher outlet water temperature with a glass covered receivers. Because the glass cover over the receiver acts as an insulator and outlet water temperature and temperature gradient between inlet and outlet water rises rapidly. It is observed that, outlet water temperature and temperature gradient (i.e. The difference between the inlet and outlet water temperatures at the instant) increased by 68% and 29%, respectively, using the glass covered aluminum and mild steel receivers respectively. It is also observed that, when uncovered receiver is exposed to the atmosphere, there are heavier convective heat losses from receiver due to natural wind as compared to a glass covered receiver. But it is to be noted here that, the results of the system with uncovered receivers are not presented here as it is not the aim of the study discussed in this paper. 0

Outlet water temperature with Aluminium Receiver ( C) 0 Outlet water temperature with M.S.Receiver ( C) 0 Temperature gradient with Aluminium Receiver ( C) 0 Temperature gradient with M.S.Receiver ( C) 70 65 60

0

Temperature ( C)

55 50 45 40 35 30 25 20 15 10 9

10

11

12

13

14

15

16

Time

Fig.2. Variation of outlet water temperatures and water temperature gradient throughout the day Fig.3 shows the effect of variation of the receiver temperature and heat loss coefficient on the collector efficiency. It is seen that, the as receiver surface temperature increases, heat loss coefficient and heat loss from the receiver increases affecting the collector efficiency adversely. Collector efficiency of 49.3% and 53.1% are obtained with the glass covered mild steel and aluminum receivers respectively. It is seen that, higher the wind speed, the higher the convective heat losses from the receiver and lower instantaneous collector efficiency. It is also seen that, convective heat losses from receiver enhances with the increased temperature gradient between receiver and surrounding and instantaneous wind velocity. It is also observed that the average decrease in heat loss coefficient is 70 % when receivers are covered with glass and at the same time the instantaneous efficiency of the collector is increased by 13 % with glass covered receivers.

0

Receiver Temperature with glass covered & coated mild steel receiver( C) 20 Overall heat loss coefficient with glass covered & coated mild steel receiver(W/m C) Collector Efficiency with glass covered & coated mild steel receiver(%) 0 Receiver Temperature with glass covered & coated aluminium receiver ( C) 20 Overall heat loss coefficient with glass covered & coated aluminium receiver (W/m C) Collector Efficiency with glass covered & coated aluminium receiver(%)

Collector Efficiency (%) 0 Receiver Temperature ( C) 20 Overall heat loss coefficient (W/m C)

90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 9

10

11

12

13

14

15

16

Time

Fig.3. Variation of collector efficiency, overall heat loss coefficient and receiver surface temperature throughout the day

Fig.4 shows useful heat gained by the water flowing through the receiver throughout the day. It is to be noted here that, the receiver temperature is the average of three values over the length of the receiver and not over the time. Useful heat gained by the water is affected by various parameters such as wind speed, receiver temperature, and solar radiation. The variation plotted shows that, as the receiver surface temperature increases, the thermal conductivity of air in the near vicinity of the receiver also enhances and therefore, the heat losses through the receiver also get advanced. But this is not only the reason of heat loss from the receiver. It is also observed that, as the beam solar radiation incident on the collector increases, receiver captures more heat. It is seen that, when solar radiation and wind velocity reduces by 4 % and 7 %, respectively, average receiver temperature and useful heat gained by the glass covered aluminum receiver increases by 22 % and 23% respectively as compared to glass covered mild steel receiver.

Useful heat gain(W) 20 Overall heat loss coefficient (W/m C) Collector Efficiency (%)

Useful heat gain with gass covered & coated mild steel receiver (W) 20 Overall heat loss coefficient gass covered & coated mild steel receiver(W/m C) Collector Efficiency gass covered & coated mild steel receiver (%) Useful heat gain with gass covered & coated aluminium receiver (W) 20 Overall heat loss coefficient gass covered & coated aluminium receiver(W/m C) Collector Efficiency gass covered & coated aluminium receiver (%) 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0 9

10

11

12

13

14

15

16

Time

Fig.4. Variation of useful heat gain by the receiver, overall heat loss coefficient and collector efficiency throughout day

6. Conclusions Through the field experiments, it is seen that, the proposed parabolic trough–receiver system shows better performance when a black epoxy coated aluminum and mild steel receivers are covered with glass cover. Average efficiencies of 53.1 % and 49.3 % are achieved with the proposed system using glass covered aluminum and mild steel receivers respectively. It is seen that, throughout the day, in comparison with glass covered mild steel receiver, average receiver temperature and useful heat gained by glass covered aluminum receiver increases by 22 % and 23% respectively. It is concluded that, from hotter climate perspective, proposed low cost FRP parabolic trough system can able to play vital role in low temperature industrial heating and domestic heating applications. Particularly, the proposed system can be implemented to fulfill the needs of hot water and low temperature steam in restaurants and hotels, laundries. It is recommended to test the proposed system using different receiver and selective coating materials for possible applicability in high temperature steam generation and similar systems.

7. Acknowledgements Authors want to acknowledge the lab and instrument support provided by Dept. of Energy Technology, Shivaji University, Kolhapur, to accomplish the current work.

8. References [1] FernΓ‘ndez-GarcΓ­a, A., Zarza, E., Valenzuela, L., PΓ©rez, M.: Parabolic-trough solar collectors and their applications. Renewable and Sustainable Energy Reviews. 14, 1695–1721 (2010) [2] Tsai, C.Y., Lin, P.D.: Optimized variable-focus-parabolic-trough reflector for solar thermal concentrator system. Solar Energy. 86(5), 1164–1172 (2012) [3] Huang, W., Peng, H., Chen, Z.: Performance simulation of a parabolic trough solar collector. Solar Energy. 86(2), 746–755 (2012) [4] Eck, M., Zarza, E.: Saturated steam process with direct steam generating parabolic troughs. Solar Energy. 80, 1424– 1433 (2006) [5] Kalogirou, S.A.: A detailed thermal model of a parabolic trough collector receiver. Energy. 48, 298–306 (2012) [6] Tao, T., Hongfei, Z., Kaiyan, H., Mayere, A.: A new trough solar concentrator and its performance analysis. Solar Energy. 85, 198–207 (2011) [7] Kalogirou, S.A., Lloyd, S., Ward, J., Eleftheriou, P.: Design and performance characteristics of a parabolic-trough solar collector system. Applied Energy. 47, 341–354 (1994) [8] Sagade, A.A., Shinde, N.N., Patil, P.S.: Effect of receiver temperature on performance evaluation of silver coated selective surface compound parabolic reflector with top glass cover. Energy Procedia. 48, 212–222 (2014) [9] Shinde, N.N., Sagade, A.A.: Experimental investigation into different selectively coated receivers and silver-coated selective surface compound parabolic reflector using regression modelling for industrial heating, International Journal of Sustainable Engineering. 9(3), 189-196 (2016) [10] Sagade, A.A., Shinde, N.N., Patil, S.: Experimental Investigations on Mild Steel Compound Parabolic Reflector with Aluminum Foil as Selective Surface and Top Cover. Energy Procedia. 57, 3058 – 3070 ( 2014) [11] Sagade, A.A.: Comparative experimental analysis of the effect of convective heat losses on the performance of parabolic dish water heater, International Journal of Sustainable Engineering, 6(3), 258-266 (2013) [12] Sagade, A.A.: Experimental investigation of effect of variation of mass flow rate on performance of parabolic dish water heater with non-coated receiver. International Journal of Sustainable Energy. 34(10), 645-656 (2015) [13] Duffie, J., Beckman, W.: Solar Engineering of Thermal Processes. John Wiley and Sons, New York USA. (2006) [14] Yogi Goswami, D., Kreith, F., Kreider, J.F.: Principals of solar engineering. Taylor and Francis, Philadelphia, USA. (2003)