Effect of Selective Coatings on Solar Absorber for Parabolic Dish Collector

A solar parabolic dish concentrator is a collector system in which designed to collect thermal energy by concentrating direct sunlight. The solar concentrating system in which the absorber is plays an important role to collect the heat energy. In the working of the solar collectors, the solar absorber gets heated up due to the solar radiation incident and transfers the heat to the Heat Transfer Fluid (HTF). The different types of solar thermal collectors with the optical and thermodynamic performance of the collectors reviewed by1. The modeling and experimental study on solar collectors are carried out by 2–4. Energy conservation measures on solar, and wind energyis discussed by 5–7. The effect of temperature distribution on surface absorption receiver in a parabolic dish solar concentrator with and without PCM was studied by8–10. The improvement in efficiency of solar cells is demonstrated by 11. The study of black coating for solar applications reported by several authors. The objective of present work is to explore and investigate the performance of parabolic solar collector with coated absorber. The different absorber coatings investigated for PDC, and the results reported, testing regular sunny days.


Introduction
A solar parabolic dish concentrator is a collector system in which designed to collect thermal energy by concentrating direct sunlight. The solar concentrating system in which the absorber is plays an important role to collect the heat energy. In the working of the solar collectors, the solar absorber gets heated up due to the solar radiation incident and transfers the heat to the Heat Transfer Fluid (HTF). The different types of solar thermal collectors with the optical and thermodynamic performance of the collectors reviewed by 1 . The modeling and experimental study on solar collectors are carried out by [2][3][4] . Energy conservation measures on solar, and wind energyis discussed by [5][6][7] . The effect of temperature distribution on surface absorption receiver in a parabolic dish solar concentrator with and without PCM was studied by [8][9][10] . The improvement in efficiency of solar cells is demonstrated by 11 . The study of black coating for solar applications reported by several authors. The objective of present work is to explore and investigate the performance of parabolic solar collector with coated absorber. The different absorber coatings investigated for PDC, and the results reported, testing regular sunny days.

Methods and Materials
An experimental set-up is used to analyze the thermal performance of solar PDC with coated absorbers. The experimental set-up consists of 16-square meter 2 PDC, absorber, storage tank, pump, and flow meter and flows control value. The schematic diagram of the experimental set-up is shown in Figure 1. The incident solar radiation on PDC and concentrated on the absorber. The water storage tank is 110 liters which are filled entirely with soft water, and the connected pump used for circulation. The flow controlled by a control valve and measured with a rotameter. The receiver is 400 mm in diameter and 100 mm deep. The metal used is 5 mm thick. MS receiver is a hollow circular vessel with 1-inch pipes coming out of its top and bottom. It has a provision for changing its covering receiver plate, by removing the eight bolts that hold the plate to the hollow receiver. The inlet and outlet pipes coming out of the receiver welded on the receiver. The absorber has an inlet and an outlet port. K-type thermocouples inserted into the entry and exit of the receiver. The entire system insulated with glass wool of 30 mm thickness. All three plates are made from MS plate with a thickness of each plate 5 mm.
The choice of MS is made due to the relative ease of availability of the metal, its cost effectiveness, and most importantly, its physical properties that are essential for its performance as a good absorber plate. MS has a melting point of 1450 o C, density of 7.85 g/cm 3 and emissivity of 0.16. A low emissivity value leads to low emissive heat loss. The plates of the receiver are changed to compare the efficiency of the receiver. The plates with and without coating are tested in the experiment. Ni has high absorptivity and low emissivity values. These parameters are pertinent to high absorptivity. A low emissivity value means that a low amount of the incident radiation would be emitted by the plate and would mostly retain. The properties of MS plate electroplated with Ni are emissivity of 0.08 and the absorptivity of 0.92. The properties of Cr, though not as good as Ni, are still good enough to be tried as an alternative to the MS Plate. The properties of MS plate electroplated with Cr emissivity of 0.11 and absorptivity of 0.88. The plates are placed on the receiver and fixed with bolts. The experiments conducted as per ASHRAE standards 12 .

Performance Analysis
Heat losses occur from a receiver due to the temperature difference between the receiver and its surroundings. Total heat losses from the receiver are conductive losses, convective losses, and radiation losses. The geometry of the set-up is as given in Table 1.
The following relation can use as to find shape factor consider the reflector and receiver as two co-axial discs, where, The shape factor between the surfaces obtained by Substituting the values for 1 r , 2 r and L in the equation and also using the summation rule and by the Reciprocity relation. For enclosures with specified surface temperatures, the following relation can be used to obtain the radiosities (J) of the surfaces.
Applying the above equation, For the receiver, reflector and side walls determined by using the expression: Convective heat loss of the receiver to the surroundings can express as, where, U L is the heat loss coefficient determined in W/m 2 K, A r is the effective receiver area in square meter, T av is the average of T in and T out, and T amp is the ambient temperature in K.
The following equation uses the general equation for total radiation heat transfer due to emission, 4 4 ( ) Thermal efficiency obtained by measuring the temperature difference of working fluid through the receiver, together with fluid properties, mass flow rate, and solar direct radiation incident. Thermal energy efficiency of the collector based on heat gained by working fluid expressed as: where, m  is the rate of flow of HTF, C p is the specific heat of HTF, p A is the absorber plate area b I is the incident beam radiation, out in T T − is the temperature difference of water inlet and outlet in the absorber.

Results and discussion
The experimental test is carried out to investigate the performance behavior of the PDC with different coating absorber. The performance of the dish solar concentrator can characterize by an estimation of the stagnation temperature and performance test at constant solar energy input with the same period interval. The direct solar radiation, wind speed, and ambient temperature are observed using pyranometer, anemometer, and K-type thermocouples respectively. The experiments conducted during April 2016 in Chennai climate. The average solar radiation around 760 -820 W/m 2 only considered in the heat loss and performance calculations. The stagnation test is often the preliminary test to compare the characteristics of the different plates. The temperature of a collector system measured in periods of time under no fluid flow conditions as referred the stagnation temperature. It considered for designing of the solar collector, absorber, selection of absorber material, working fluid, hot water tanks and other auxiliary equipment. The useful heat gains by the absorber plate yields evaluated with the following equation: At stagnation condition, Q u = 0, Where S is the incident solar flux and l Q is the total heat loss.  Figure 2. The HTF temperature at inlet and outlet temperature of the absorber plate and atmosphere temperature readings are taken and recorded in every five-minute time interval. The experiment tests restricted to collector within operating temperature of 100 o C. A temperature difference of inlet and outlet heat transfers fluid (HTF) variation for different coated absorber plates obtained and as shown in Figure 3.  Figure 4 indicates that the absorber temperature obtained by the Ni plate is higher than that of the other two plates, thereby providing the higher amount of energy for the heat exchanger to absorb. So the among three plates, by using MS with Ni coated plate provides higher performance of collector.  Figure 5 shows that the efficiency of our collector system is highest when the MS absorber plate is electroplated with Ni. It would mean that the heat available for useful work would be highest with the Niplated absorber, thereby justifying our hypothesis and preliminary tests which suggested that MS Electroplated with Ni is the best choice for our PDC.

Figure 5.
Variation in efficiency for different coating absorber plates.

Conclusion
Experimental investigations of the solar PDC with and without coating on absorber plates were carried out to determine the efficiency of the receiver. MS plate with Ni coating has the highest stagnation temperature about 531ºC among the three plates. The heat available for useful work would be highest with the Ni-plated absorber. The radiation heat loss is more for the MS absorber without coating about 94.24 W. Ni, and Cr exhibited low heat losses under similar radiation conditions. The convective heat loss found as lowest in the Cr plated absorber about 148.6 W. This indicates that the heat loss due to convection mostly controlled by the electroplating of Cr on MS. The Ni coating on MS absorber produces effective heat absorption.