Lecture Notes

Unit I

A solar radiation sensor measures solar energy from the sun.

Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum.

The units of measure are Watts per square meter.

A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to solar hot water panels, but may refer to installations such as solar parabolic troughs and solar towers; or basic installations such as solar air heaters. Concentrated solar power plants usually use the more complex collectors to generate electricity by heating a fluid to drive a turbine connected to an electrical generator. Simple collectors are typically used in residential and commercial buildings for space heating. The first solar thermal collector designed for building roofs was patented by William H. Goettl and called the "Solar heat collector and radiator for building roof.

Concentrating (or Focusing) Solar Collectors:

Concentrating collector is a device to collect solar energy with high intensity of solar radiation on the absorbing surface by the help of reflector or refractor. In these collectors, the area of collector is kept less than the aperture through which the radiation passes, to concentrate-the solar flux and has high concentration ratio.

Need of Orientation in Concentrating Collectors:

Such collectors generally use optical system in the form of reflectors or refractors. A concentrating collector is a special form of flat-plate collector modified by introducing a reflecting (or refracting) surface (concentrator) between the solar radiations and the absorber. These types of collectors can have radiation increase from low value of 1.52 to high values of the order of 10,000. In these collectors radiation falling on a relatively large area is focused on to a receiver (or absorber) of considerably smaller area. As a result of the energy concentration, fluids can be heated to temperatures of 500°C or more.

Orientation of sun from earth changes from time to time. So to harness maximum solar rays it is necessary to keep our collector facing to sun rays direction. This is the reason why orientation in concentrating collector is necessary. This is achieved by the use of “Tracking device”.

Types of Concentrating Collectors:

Solar collector

The different types of focusing/concentrating type collectors are:

i. Parabolic trough collector.

ii. Mirror strip collector.

iii. Fresnel lens collector.

iv. Flat-plate collector with adjustable mirrors.

v. Compound parabolic concentrator (CPC).

vi. Parabolic dish collector.

i. Parabolic Trough Collector:

Fig 3.4. shows the principle of the parabolic trough collector which is often used in focusing collectors. Solar radiation coming from the particular direction is collected over the area of reflecting surface and is concentrated at the focus of the parabola, if the reflector is in the form of a trough with parabolic cross-section, the solar radiation is focused along a line. Mostly cylindrical parabolic concentrators are used in which absorber is placed along focus axis [Fig. 3.5].

ii. Mirror Strip Collector:

Refer to Fig. 3.6. A mirror strip collector has a number of planes or slightly curved or concave mirror strips which are mounted on a base. These individual mirrors are placed at such angles that the reflected solar radiations fall on the same focal line where the pipe is placed. In this system, collector pipe is rotated so that the reflected rays on the absorber remain focused with respect to changes in sun’s elevation.

iii. Fresnel Lens Collector:

In this collector a Fresnel lens is used in which linear grooves are present on one side and flat surface on the other. The solar radiations which fall normal to the lens are refracted by the lens and are focused on the absorber (tube) as shown in Fig. 3.7. Both glass and plastic can be used as refracting materials for Fresnel lenses.

iv. Flat-Plate Collector with Adjustable Mirrors:

Fig. 3.8. shows a flat-plate collector with adjustable mirrors. It consists of a flat- plate collector facing south, with mirrors attached to its north and south edges. If the mirrors are set at the proper angle, they reflect solar radiation on to the absorber plate.

Thus, the latter receives reflected radiation in addition to that normally falling on it. In order to make the mirrors effective, the angles should be adjusted continuously as the sun’s altitude changes. Since the mirrors can provide only a relatively small increase in the solar radiation falling on the absorber, flat-plate collectors with mirrors are not widely used.

v. Compound Parabolic Concentrator (CPC):

Fig. 3.9 shows the compound parabolic concentrator. It was designed by Winston (and Baranov). It consists of two parabolic segments, oriented such that focus of one is located at the bottom end point of the other and vice versa. The receiver is a flat surface parallel to the aperture joining of two foci of the reflecting surfaces.

For thermal and economic reasons the fin and the tubular type of absorbers are preferable. It is claimed that Winston collectors are capable of competitive performance at high temperatures of about 300°C required for power generation, if they are used with selectively coated, vacuum enclosed receivers.

The maximum concentration ratio available with paraboloidal system is of the order of 10,000.

Advantages of Compound Parabolic Concentrator (CPC):

1. High concentration ratio.

2. No need of tracking.

3. Efficiency for accepting diffuse radiation is much larger than conventional concentrators.

vi. Paraboloidal Dish Collector:

Refer to Fig. 3.10. In this type of collector all the radiations from the sun are focussed at a point. This collector can generate temperature up to 300°C and contraction ratio from 10 to few thousands. Its diameter is of the range between 6 to 7 m and can be commercially manufactured.

Comparison between Flat-Plate and Concentrating Collectors:

The comparison between flat-plate and concentrating collectors is given below:

Advantages of Concentrating Collectors:

1. High concentration ratio.

2. High fluid temperature can be achieved.

3. Less thermal heat losses.

4. System’s efficiency increases at high temperatures.

5. Inexpensive process.

Disadvantages of Concentrating Collectors:

1. Non-uniform flux on absorber.

2. Collect only beam radiation components because diffuse radiation components cannot be reflected, hence these are lost.

3. Need costly tracking device.