CHAPTER 4: EXPERIMENT
4.1 SET UP
Table 4.1: List of parabolic solar concentrator materials
SI.No. Component Materials
1 Concentrator or Reflector Aluminum, Silver, Mirror etc
2 Absorber or Receiver Copper, Aluminum, Steel, Stainless steel, Plastic 3 Shield or Cover Glass, Mylar, Fiberglass
4 Insulation Vacuum
5 Coating or paint Flat paint, Glossy paint, Thurmalox, DLC, Cermet etc
4.1.2 Concentrator
It is the most important component of the system. It reflects the sun’s ray and concentrates it into the absorber or receiver. For practical application, reflectors must have high reflectance along solar spectrum and also reflect with high specularity. That’s why surface of the concentrator should be smooth. It shouldn’t be dull or rough because smooth surfaces reflect more light than rough surfaces. Mirror is used as a concentrator because its reflectivity is better than other materials. It gives highest level of reflection. It can almost reflect 95% of the solar radiation. Almost forty pieces of mirror strips has been used in this system. As an operational purpose mirror is very good. After more than five years mirrors can still be cleaned to their as new reflectivity. It doesnot form rust like other materials. Again mirror is better than other reflector because it is easy to clean. Aluminum also shows high reflectivity.
Now-a-days concentrators are also made of Aluminum with reflective coating. Some other concentrators or reflectors are-
1. Silver substrate protected by an aluminum coating
2. PET(Polyethylene Terephthalate) laminated to stainless-steel foil 3. Chrome-plated carbon steel strip
Table 4.2: Characteristics of solar concentrator
Width of the parabolic concentrator 55 inches Length of the parabolic concentrator 36 inches
Focal length 13 inches
Types of reflector Mirror strips
Length of the strips 34 inches
Width of the strips 30 mm
Thickness of the strips 3 mm
4.1.3 Receiver or absorber plate
The function of absorber plate is to absorb the solar radiation as efficiently as possible. It should convert the solar radiation into thermal energy and deliver it to the heat transfer medium (water or thermal fluid) with minimum losses. It should contain some properties like -It should have high thermal conductivity.
-Good corrosion resistance.
-Durability.
-Adequate compressive and tensile strength.
The absorptance of the receiver surface for short wavelength depends on the nature and the color of the coating and glazing. However black color is used but other colors are also used for aesthetic reason. By suitable electrolyte or chemical treatments, surface can be produced with high values of solar radiation absorptance and low long wave emittance.
Fundamentally typically receiver surfaces consist of a thin upper layer, which is highly absorbent to short wave of solar radiation but relatively transparent to long wave thermal radiation, deposited on a surface that has a high reflectance and a low emittance for long wave radiation. Solar absorbers are based on two layers of different optical properties which are referred as tandem absorbers. Now-a-days commercial solar receivers are made by electroplating, anodization, evaporation, sputtering and by applying solar selective paints.
Much of the progress during recent years has been based on the implementation of vacuum techniques.
Generally copper is preferred for its high thermal conductivity and resistance to corrosion.
But it has a disadvantage, it reacts with water. Besides this it can be made of -Aluminum
-Steel
-Thermoplastics like PB, PE, PEX, CPVC, PVC, PP and PVDF
Aluminum and Steel are restricted by corrosion inhibited heat transfer fluid. In this experiment Aluminum is used because of its light weight, low cost and ease of fabrication.
Besides it does not react with water.
Table 4.3: The specification of the receiver
Effective length of the receiver 36 inches Diameter of the receiver 1 inch
Position of the receiver At the Focal Point
Cover or shield Glass
Diameter of the cover 2 inches
Insulation Vacuum
Coating or Paint Black glossy paint
Coating or paint:
Coating or paint is an important part in this experiment. It has impact on the variation of temperature. Coating requires some properties like
-It should be heat resistant.
-It should selectively absorb solar wave length with greatest heat content, visible and infrared wave length.
-It should collect heat as efficiently as possible.
There are several types of paints. To ensure maximum performance film thickness should be carefully controlled. It should be very thin, only enough to just hide the metal surface and as uniform as possible. It should be dried at least for thirty minutes before taken into operation.
There are several types of paints. Like -Flat paint
-Glossy paint -Thurmalox paints
Flat paint comparatively gives good result than glossy paint. But because of the unavailability the glossy paint has been used in this system.
Cover or shield:
The cover should maintain good strength and transmittance under high temperature. A glass tube is used which is transparent to solar radiation over the solar absorber surface. It reduces convection and radiation loss to the atmosphere.
Insulation:
The annulus gap between glass tube and receiver is vacuumed in order to reduce conduction and convection.
4.1.4 Variable Inclined Stand
There should be a system to vary the tilted angle of the concentrator. This cannot be achieved with a fixed inclination because of the direction of the altitude of the sun vary over the year.
In order to overcome this problem a variable inclination stand is made. It gives inclination of the angle starting from 0º to a maximum 90º with the horizontal. This system is made of M.S flat bar.