PERFORMANCE ANALYSIS

CHAPTER 5: ANALYTICAL STUDY

5.1 PERFORMANCE ANALYSIS

Reflectance loss or specular reflectance is defined as the portion of the incident beam which is reflected such that at the angle of reflection equals the angle of incidence. It is function of

 Nature of the surface

 Surface smoothness To improve the reflectance loss

 Metal deposits or coating on the surfaces on front face or on the black glass can be used.

 Anodized aluminum sheets may be used as reflectors.

 Vacuum metalized plastic films can be used.

Reflectance losses can be reduced by etching from 4 percent surface to about 1 percent at normal incidence. A double etching process can yield even better results, although at higher costs.

The proper value of the transmittance and absorptance product must be arrived by integration of the radiation passing through the cover and incident on the receiver from all portions of the concentrator. By proper design, it should be possible to keep all angles of incidence less than 60° by shaping the receiver.

Intercept factor represents the function of the specularly reflected radiation that is intercepted by the energy absorbing surface. It has considerable influenced on the determination of energy collected and represents a significant factor in the energy balance. The intercept factor in a property of the concentrator and its orientation in producing in an image and of the receiver and its positioning relative to the concentrator in intercepting part of that image.

The loss coefficient UL is found from

U

L

= (

_h¹

wind

+

_h¹

r

)

^-1

(4.2)

Where

, h

wind= film coefficient due to wind

Or,

h

wind= 5.7 + 3.8V (4.3)

V is the velocity of wind found from the following table.

Table 5.1: Average wind speed

(Source: http://www.weatherbase.com/weather/weather.php3?s=032914) Km

/h

Annua l

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

6 4 4 8 9 8 6 6 8 6 4 4 4

hr = radiation coefficient and can be determined from the following equation h_r(T_r−T_a)

= ε σ (

^Tr4

−T_a⁴¿

Or,

^hr

= 4 ε σ

(T_r+T_a 2 ) ³ (4.4)

ε

= emissivity of coating of the receiver

σ

= Stephen Boltzman Planck’s constant = 5.67 X 10^-8 Jm^-2s^-1k^-4

Focusing collector differ in their thermal behavior from the flat type, because for receivers the shapes are widely variable, the temperatures are higher, the edge effects are more significant, conduction terms are quite high, and radiation flux on the receivers is not uniform. The effects of variation in direct solar radiation, selective and non-selective coatings, absorber tube material and configuration, insulation and orientation of concentrator etc., on collector performance can be analyzed experimentally.

The collector performance is described by an energy balance with the incidence energy distributed into optical losses, thermal losses from the receiver and the useful energy gain.

For determining the collector efficiency the whole absorber length is divided into three portions.

Heat loss due to conduction is considered to be negligible in the analysis. Collector efficiency is calculated as follows:

η_{th ;exp}=mC´ _pw(T_o−T_i)

I_bA_a (4.5)

Where,

Ti = initial temperature, °C

To = final temperature, °C

Cpw = specific heat of water, J/kgk m´ = mass flow rate of water, kg/s

m´

=

^ρwVw

t (4.6)

ρ_w = density of water, kg/m³ Vw = volume of water, m³

t = time taken to raise the temperature, s

η_{th ;theo}=ταργ A_aI_b−U_LA_r(T_r−T_a)

I_bA_a (4.7)

The transmissivity, absorptivity and emissivity of the coating is found from the table below.

Table 5.2: Selective Surfaces (Source: Solar Energy Utilization, G.D. Rai, Khanna Publisher, 6^th Edition, 9^th re-print)

Type of coating

α ε

Temperature

°C Air/Vacuum

Life (year) Air/Vacuu

m

Interference 0.94 0.10 175/200 5/30

Black Chrome 0.96 0.12 175/200 15/30

Black Nickel 0.90 0.07 - -

CuO 0.88 0.15 100 -

Enamels 0.95 0.8-0.9 250/250 ∞

PbO2 0.98 0.30 100/150 20

Stainless steel 0.90 0.40 75 30/30

Anodic

aluminum 0.95 0.80 250 50

Aluminum

conversation 0.93 0.85 100/100 10-20

Paints 0.97 0.91 70/80 5-20

Table 5.3: Thermal and relative properties of Collector Cover Materials (Source: Solar Energy Utilization, G.D. Rai, Khanna Publisher, 6^th Edition, 9^th re-print)

Material name Index of refraction(n)

Transmissivity ( τ ) (for λ =.2-4 μ

m)

Temperature limits °C Lexan

(Polycarbonate) 1.586 72.6 125-135

Plexiglass (acrilyc) 1.49 79.6 90-100

Teflon FEP

(flurocarbon) 1.343 89.8 200

Tedlar R.V.F.

(flurocarbon) 1.46 88.3 110 (continuous use)

170 (short time use)

Sunlight (fiber glass) 1.54 75.4 100 (continuous use)

Cause 5% loss in τ

Float glass 1.518 78.6 650

Temper glass (glass) 1.518 87.5

200-250

(continuous use) 250-275

(short term use) Clear lime sheet glass

(low iron oxide glass) 1.51 87.5 200 for continuous

operation Clear lime temper

glass (low iron oxide) 1.51 87.5 200 for continuous

operation

Sunadex white

crystal glass(.01%

iron oxide glass)

1.50 91.5 200 for continuous

operation

In mass production, etching of the cover glazing for the receiver will always be cost effective because of the relatively small area and because this glazing is sufficiently well protected from the environment not to degrade by accumulation of dirt. If the glazing is expected to be exposed to temperature, borosilicate (Pyrex) glass is recommended; it is more expensive than soda lime glass. Acrylic is suitable for cover material for a whole collector, and in particular it is excellent material for Fresnel lenses.

When a high α or low ε are important will depend on collector type and on application. In non-evacuate collectors conduction and convection will begin to dominate radiation losses when ε is reduced below about 0.2. in evacuated collectors, on the other hand there is

incentive to achieve the lowest possible values of ε provided that α is at least 0.85. Better coatings are likely to be produced as a result of intensive research.

When interrupting coated values of α and ε , following points should be keep in mind

 α depends on the solar spectrum that has been assumed.

 α will in general decrease with angle of decrease, but usually only normal incidence values are coated. In concentrating collectors the angle of incidence of some rays at the absorber will deviate strongly from the normal direction. Black chrome maintains a high absorptance up to 60°, but multilayer interference films tend to drop off significantly around 45°.

 ε increases with temperature, because of spectral shift of black body radiation and because of changes in the surface properties.

 A single number such as α/ε does not adequately characterize a selective surface.

Reflector is divided into two parts:

i. Reflective lining and

ii. The shell, supporting and orienting structure.

Lining is desirable to use a reflective material with a maximum specular reflectance over the period of use consistent with cost. Also there is possibility of renewing a lining by putting on a new layer of reflective plastic tape.

Shell and supporting structure influences the intercept factor and the collector operation is dependent upon the ability of structure to maintain the shape and orientation of the reflector.

Since orientation and shape are critical factors, the following points must be keep in mind while designing:

i. Shell and structure must be supported in various positions so as to have minimum distortion due to its weight.

ii. Wind effects.