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Design Considerations For Optimizing Flat Plate Collector Effectiveness Engineering Essay

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Published: Mon, 5 Dec 2016

Abstract- Application of solar energy for domestic and industrial heating purposes has been become very popular. However the effectiveness of presently used fixed flat plate collectors is low due to the moving nature of the energy source. In this paper, an attempt has been made to optimize the performance of flat plate collector by developing a detailed model that includes all of the design features of the collector such as:Geograhic location, absorber plate material and size, selective coating, tube diameters and spacing, number of glazing covers and material, back and edge insulation dimensions, material of water storage tank, collector orientation.,etc. This well-designed collector can produce hot water at temperature up to the boiling point of water.

Keywords-Solar radiation,Collector efficiency,tracking,Tilt angle.

Introduction

Solar energy is a very large, inexhaustible source of energy. Quantitative assessment of solar radiation incident on a tilt plane is very important to engineers for designing solar energy collector. In the solar-energy industry great emphasis has been placed on the development of “passive” solar energy systems, which involve the integration of several subsystems: area that intercepts the sun’s rays. This device absorbs the incoming solar radiation, converting it into heat at the absorbing surface, and transfers this heat to water flowing through the Flat plate collector. Lot of research work is going on to use the available solar energy to maximum extent. One such area is tracking mechanism to obtain maximum energy. Just by keeping the collector fixed; it is not possible to get maximum energy from sun. It is possible to obtain the maximum energy only when it is rotated along the sun direction. In this context, tracking plays an important role. Tracking is desirable for orienting a solar device towards the sun there by collecting maximum solar energy and improving efficiency.

Flat-plate collector

It is a metal box with a glass or plastic cover (glazing) on top and a dark-colored absorber plate on the bottom with insulation at sides and bottom of the collector to minimize heat loss-typically mounted on a roof that heats water using the sun’s energy.

Fig.1 shows the main components of a flat plate solar collector are:

Absorber plate

Tubes or fins

Glazing

Thermal insulation

Cover strip

Container or Casing

Figure 1. Flat Plate Collector Section

Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate. Absorber plates are commonly painted with “selective coatings,” which absorb and retain heat better than ordinary black paint. Absorber plates are usually made of metal-typically copper or aluminum-because the metal is a good heat conductor [1].

Materials for Flat Plate Collectors

This section describes briefly some of the principal requirements for and the Properties of materials employed in solar flat plate collectors used for the transformation of solar energy into thermal energy.

Glazing Materials

The purpose of the glazing is to admit as much solar radiation as possible and to reduce the upward loss of heat to the lowest attainable value. From the standpoint of the utilization of solar energy, the important characteristics are reflection (ρ), absorption (α), and transmission (τ). The first two should be as low as possible and the latter as high as possible for maximum efficiency [2]. Tempered glass (4mm thickness) of low- iron content has been the principal material used to glaze solar collectors [3]. Because it has relatively high transmittance as much as 91% of the incoming short-wave radiation as compared to other glazing materials as shown in table 1

TABLE 1 GLAZING MATERIALS

Material

Transmittance (Ï„)

€± Tempered glass of low- iron content

€² Window glass

€³ Polymethyl methacrylate(acrylic)

€´ Polycarbonate (Lexan, Merlon)

€µ Polyethylene terephthalate (polyester)

ۦ Polyvinyl fluoride (Tedlar)

€· Polyamide (Kapton)

€¸ Fluorinated ethylene propylene (FEP Teflon)

€¹ Fiberglass-reinforced polyester

0.91

0.85

0.89

0.84

0.84

0.93

0.80

0.96

0.87

Number of Cover

A flat-plate solar collector consists of none, one or more transparent covers as per requirement of hot water temperature. Two layers of glass are sometimes used in order to reduce the heat losses from the absorber.This however will reduce the amount of solar energy entering the collector.A double layer of 4 mm glass will transmit only 71% of the radiation it receives.Double glazing is beneficial only in the circumstances where there are very high heat losses and water is to be heated more than about 35°C above the temperature of the outside air.For double glazing,spetial attention must be paid to the fixing of inner layer. Fig. 2 shows the effect of the number of covers on the collector instantaneous efficiency. In the case of two covers, they are assumed to be optically identical and the cover-to-cover air spacing is set to be 0.5 cm. In this figure, the value of intercept FR(´ ¡ ) decreases with the increase of the number of covers since the value of transmittance of the cover system decreases. The slope of curves, FRUL, decreases as the number of covers increases since the overall heat loss coefficient is a function of the

mean plate temperature and ambient conditions and its dependence on the plate temperature decreases with the increasing number of covers.

Figure 2. Effect of the number of covers on the collector efficiency

Absorber Plates

The primary function of the absorber plate is to absorb as much as possible of the radiation reaching through the glazing, to loose as little heat as possible upward to the atmosphere and downward through the back of the container, and to transfer the retained heat to the circulating fluid. Factors that determine the choice of absorber material are its thermal conductivity, corrosion resistance, withstands the stagnation temperature its durability and ease of handling, its availability and cost, and the energy required to produce it. Of the commonly available materials copper is the best because of its high thermal conductivity as given in table 2.

TABLE 2 THERMAL CONDUCTIVITY OF ABSORBER MATERIALS

Material

Thermal conductivity in W/mk

Copper

376

Aluminium

205

Mild steel

50

Stainless steel

24

The ability to conduct the absorbed heat to the tubes can be improved by bringing the tubes closer together and by using thicker sheets. The thicker the sheet, the less the resistance to heat flow across it. Spacing and thickness for the different metals is given in table 3

TABLE 3 ABSORBER MATERIAL SPACING AND THICKNESS

Material

Thickness in mm

Spacing in mm

Copper

0.25

138

Aluminium

0.50

138

Mild steel

1.0

100

D. Selective Absorber

A surface that has a high absorptance and is a good absorber of solar radiation usually has a high infrared emittance as well and is a good radiator of heat. For such a surface, α = 1 and ε = 0. Selective absorbers can be manufactured that approach this ideal, and several are available commercially in Table 4.

To date, the most successful and stable selective absorber is made by electroplating a layer of nickel onto the absorber plate and then electrodepositing an extremely thin layer of chromium oxide onto the nickel. This combination is more resistant to water damage than the commonly used nickel- oxide coating.

TABLE 4 : PROPERTIES OF SELECTIVE COATINGS

Selective Coatings

α

ε

Black Chrome

0.93

0.10

Black Nickel on polished nickel

0.92

0.11

Black Nickel on galvanized iron

0.89

0.12

CuO on nickel

0.81

0.17

Co3O4 on silver

0.90

0.27

CuO on aluminum

0.93

0.11

CuO on anodized aluminum

0.85

0.11

Riser and Header Tubes

Riser tubes or fins for conducting or directing the heat transfer fluid from the inlet header to the outlet. While designing, the factors such as choice of materials, inner and outer diameter, volumetric flow rate (m), inlet pressure, number of tubes, spacings of tubes and the thermal conductivity of bond between tube and absorber sheet must be exercised carefully.

A good thermal bonds such as a braze, weld or high temperature solder are required for tube and plate designs, in order to ensure good heat transfer from the absorbing surface into the fluid as shown in figure 3.

Figure 3. Absorber plate and tube thermal bond

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Parallel Tube Configuration

This is the most common pipe work arrangements as shown in figure 4.

Two manifolds (headers), normally 25.4mm copper pipe are connected within the panel by a series of copper tubes (12mm or 12.5mm are the most common sizes). The colder water is fed into the bottom and the hot is drawn off the top.

Figure 4. Parallel Tube Configuration

For optimization, absorber plate and tube sections for better surface contact as shown in figure 5.

Figure 5. Absorber plate and tubes sections

Number of Tubes

As the number of tubes increases, the tube-to-tube spacing W decreases and the collector heat removal factor FR increases while the value of (´ ¡) remains constant. Thus, the instantaneous efficiency improves with the increase of the number of tubes [4]. The effect of the number of tubes on the instantaneous efficiency of the collector is illustrated in Figure 6. The degree of improvement decreases and there exists the optimum number of tubes for the collector considering the manufacturing cost. As shown in this figure the instantaneous efficiency for 11 tubes is almost identical to that for 10 tubes.

Figure 6. Effect of the number of tubes on the collector efficiency

Collector Thermal Insulation

Flat-plate collectors must be insulated to reduce conduction and convection losses through the back and sides of the collector box. As the collector may have to operate at temperature as high as 200°C the insulation must not deteriorate, outgas, expand or contract at temperatures between 30°C and 200°C. It must also have structural stability and must not become compact or settle with time.It should not attract moisture and should be fire resistant.It should be light in weight. The section of collector box shows back and side insulation of rock wool in figure 7.

Figure 7. Rock wool thermal insulation

We know, the increase in back and edge thermal conductivity causes an increase in the heat loss from the collector and is equivalent to the increase of thermal leakage from the collector. The efficiency of the collector decreases with increase in thermal conductivity of insulation material. Usually, rock wool or glass-wool is recommended for back and sides insulation of the collector because of its less thermal conductivity.

Collector Housing

The collector housing is provided to protect the

insulation and absorber plate from the environment and also to minimize heat loss.Materials like aluminium, galvanised steel, fibre glass, etc.are used for making the collector housing.It is advisable to use the same material for collector support structure and housing to avoid contact corrosion. Such problems however may not arise with fibre glass or plastic housing.

Storage Tank

In the simplest arrangement of a solar water heating system, the hot water storage tank is placed at a higher level than the collector so that the heated water will run from the collector to the tank and induce natural circulation by convection (thermosyphon). For better result, Stainless steel -304 L grade- thickness 2 mm for tank material I.S.I. marked G.I pipe fittings for hot water insulation pipe line are preferred.Rock wool or glass wool insulation (100 mm) for hot pipe line for minimum heat loss.The capacity of storage tank as per demand of daily hot water The shape of tank is cylindrical horizontal.Provision of man hole of 500 mm and provision of flanged outlet to connect to another tank for increasing system capacity in future may be made. At night it is possible for the collector to lose heat by radiation and the circulation will be in the opposite direction, so the water will cool. This can be overcome by use of a suitable non-return valve.

Collector Support

It is advisable to use the same material for collector support structure and housing to avoid contact corrosion. M.S channel or square pipe stand is suitable to withstand the weight of collector and water storage tank. Collector support would be grounded in cement concrete block or faster bolts used of appropriate quality.

Collector Orientation (Tilt Angle)

The most common form of solar energy utilization in the world is for water heating applications, mainly during the winter months. Generally conventional liquid flat-plate collectors are used in domestic hot water applications and the roofs of many houses and flats in cities are fitted with such collectors. These systems are manufactured and installed by ordinary ironmasters with little or no knowledge of solar or heat theory. These collectors are mounted with their surfaces facing towards the equator and the tilt angle is set approxi- mately equal to latitude.

Tracking Systems

Optimum tilt can be achieved by use of tracking systems. There are two types of tracking systems, Manual and Automatic. Following methods have been adopted in order to achieve optimum tilt. (1) Monthly based optimization: Both manual and automatic tracking system can be used for monthly based optimization. (2) Season based optimization: In this case also manual and automatic both tracking systems can be used. Automatic tracking system will be expensive as compared to manual one. (3) Annual based: In this case no need of automatic tracking system and hence only manual tracking system is used.

Solar radiation data is usually measured in the form of global and diffuse radiation on a horizontal surface at the latitude of interest. Flat-plate solar collectors are tilted so that they capture the maximum radiation and the problem of calculating solar radiation on a tilted surface is in determin-ing the relative amount of beam and diffuse radiation con-tained in the measured horizontal global radiation. Since the fiat plate solar collectors are positioned at an angle to the horizontal, it is necessary to calculate the optimum tilt angle which maximizes the amount of collected energy. The best way to collect the maximum solar energy is by using solar tracking systems to follow the sun as it moves each day, and thus to maximize the collected beam radiation. It is possible to collect 40% more solar energy by using a two-axis track-ing system and it is estimated that in sunny climates, a flat-plate collector moved to face the sun twice a day can intercept nearly 95% of the energy collected using a fully automatic solar tracking system [5]. Tracking systems are expensive, need energy (usually solar energy is used) for their operation.

It is generally known that in the northern hemisphere, the optimum collector orientation is south facing and the optimum tilt depends upon the latitude and the day of the year. In winter months, the optimum tilt is greater (usually latitude +15°), while in summer months, the optimum tilt is less (usually latitude -15°).

Experimental Data

For Indian stations, long term monthly-mean hourly global radiation data for a measuring site are obtained [6].

Figure 8 shows the monthly solar energy collected when the angle of tilt is optimum, when the seasonal average angles are used, and when the yearly average angle is used throughout the year. The seasonal average was calculated by finding the average value of the tilt angle for each season and the implementation of this requires the collector tilt to be changed four times a year. The collected energy is tabulated in detail in Table 5. When the monthly optimum tilt angle was used, the yearly collected solar energy was 9103 MJ/m2. With the seasonally adjusted tilt angles, the yearly collected solar energy was 9015 M J/m2. Finally, with the yearly average tilt angle, the yearly collected solar energy was 7879 MJ/m2.

Figure 8. Total monthly solar radiation for optimum, seasonally adjusted and yearly fixed tilt angles

TABLE 5 Optimum Tilt, Seasonally Adjusted Tilt and Yearly Average Tilt and Monthly Solar Radiation on a Tilted Plane in India as follows

Tilt

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Yearly

βopt

870.29

721.67

783.86

780.82

803.30

795.84

760.89

705.47

721.92

704.88

694.26

758.86

9102.1

(56)

(45)

(32)

(14)

(0)

(0)

(0)

(6)

(25.5)

(40)

(53)

(58)

Seasonally

870.29

701.43

783.52

760.08

803.30

795.84

760.89

684.04

721.75

681.91

693.58

758.38

9015.0

adjusted β

(56)

(30)

(30)

(30)

(0)

(0)

(0)

(24)

(24)

(24)

(56)

(56)

Yearly

795.34

701.43

783.52

760.08

732.22

691.05

678.16

667.52

720.33

695.73

649.20

682.22

7878.6

average β

(30)

(30)

(30)

(30)

(30)

(30)

(30)

(30)

(30)

(30)

(30)

(30)

It is clear from Table 5 that the loss in the amount of collected energy is less than 1% (0.97) if the angle of tilt is adjusted seasonally instead of using βopt for each month of the year. The loss of energy when using the yearly average fixed angle is around 21% compared with the optimum tilt. It can be concluded that a yearly average fixed tilt can be used in many general applications (e.g. domestic water

heating) in order to keep the manufacturing and installation costs of collectors low. For higher efficiency, the collector should be designed such that the angle of tilt can easily be changed at least on a seasonal basis, if not monthly.

Acknowledgment

From the above design considerations, we can conclude that the efficiency of the system considerably improve at reasonable cost. We further improve the performance of the collector by providing the manual seasonal tracking system to the collector with respect to solar beam than the fixed plate collector by 21 %. Hence, flat plate collector with tracking method utilizes maximum beam radiation and gives high efficiency when compared to fixed flat plate collector.

Greek Symbols:

¨ Collector efficiency

transmission coefficient of glazing

absorption coefficient of plate

β Surface slope from the horizontal degrees

Nomenclature:

A

collector area, m2

FR

collector heat removal factor

I

intensity of solar radiation, W/m2

Tc

collector average temperature, °C

Ti

inlet fluid temperature, °C

Ta

ambient temperature, °C

U L

2

collector overall heat loss coefficient, W/m

Qi

collector heat input, W

Qu

useful energy gain, W

Qo

heat loss, W


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