Concentrating Collectors

Concentrating Collectors:

SOLAR ENERGY COLLECTION: Flat plate and concentrating collectors classification of concentrating collectors, orientation and thermal analysis advanced collectors.

Introducation:

Many power plants today use fossil fuels as a heat source to boil water. The steam from the boiling water spins a large turbine, which drives a generator to produce electricity. However, a new generation of power plants with concentrating solar power systems uses the sun as a heat source. The three main types of concentrating solar power systems are: linear concentrator, dish/engine, and power tower systems.

Concentrating collectors

• Concentrating collectors intercept direct radiation over a large area and focus it onto a small absorber area.
• These collectors can provide high temperatures more efficiently than flat-plate collectors, since the absorption surface area is much smaller.
• Concentrating collectors consists of concentrating device like mirrors attached to a flat plate. The mirrors are set to be an angle so that they reflect solar radiation on to the absorber plate.
• Some part of the radiation falls directly on to the absorber plate.
• There are two types of concentrating collectors:

Types of concentrating collectors:

1.Focusingtype and

2.Non-Focusingtype.

1.Focusingtype:

Focusing type collector is a device to collect solar energy with high intensity of solar radiation. Such collectors generally use optical system in the form of reflectors and refractors.

Types:

Concentrating or focusing collectors may be in two general categories:

(A) Line focusing

(B) Point focusing types.

(A)Line focusing :

In practice, the line is a collectors pipe and the point is a small volume through which the heat transport fluid flows.

1. Parabolic through collector
2. Mirror strip reflector
3. Fresnel lens collector
4. Flat plate collector with adjustment mirrors
5. Compound parabolic concentrator (C.P.C)

1. Parabolic through collector:

Parabolic through collector: Parabolic troughs are devices that are shaped like the letter “u”.

Parabolic Trough ReflectorsorPTR, are made by simply bending a sheet of reflective or highly polished material into a parabolic shape called a parabola. Since solar light waves essentially travel parallel to each other, this type of solar collector can be pointed directly into the sun and still achieve a total focal output from all parts of the trough shaped reflector as shown.

2. Mirror strip reflector:

In this focusing collector, a number of plane or slightly curved (concave) mirror strips mounted on a flat base. The angles of the individual mirrors are such that they reflect solar radiation from a specific direction on to the same focal line.

3 Fresnel lens collector:

In this type of concentrating collector uses a Fresnel lens. The Fresnel lens collector

consists of a thin glass sheet whose one surface is plain and the other surface has

vertical groves. The plain surface is exposed to the solar radiation.

Solar Dish Collectortype systems can also be part of another solar technology called a "solar dish-engine" system.

(II) Point focusing collector (paraboloidal dish):

The parabolic solar dish is covered with many small mirror reflectors all around its shape to help concentrate the thermal energy into a single focal point were the heat absorber is located producing more overall thermal energy per square meter of dish. These highly polished mirrors can reflect more than 90% of the sunlight that hits them increasing the efficiency of the dish by more than 20% compared to the parabolic trough collector.

2.Non-Focusingtype:

(I) Compound parabolic concentrating collector: Compound parabolic

concentrating collector consists of reflectors which are parabolic in shape. This type of collector is a non-imaging type. The concentrating ratio of this type od collector varies between 3 and 10.

(II) Moving Absorber and Fixed Concentrator type concentrating collector:

ln this type of concentrating collector, the concentrator is fixed and the absorber or receiver rotates.

Advantages:

1.Reflecting surfaces required less material and are structurally simpler than flat-plate collectors. For a concentrator system the cost per unit area of solar collecting surface is therefore potentially less than that for flat-plate collectors.

2.The absorber area of a concentrator system is smaller than that of a flat-plate system for same solar energy collection and therefore insulation intensity is greater.

3.Because of the area from Which heat is lost to the surroundings unit of the solar energy collecting area is less than that for flat-plate collectors.

The main advantages of concentrator systems over flat-plate type collectors are :

Central Receiver Systems:

Central receivers (or power towers) use thousands of individual sun-tracking mirrors called "heliostats" to reflect solar energy onto a receiver located on top of tall tower.

Solar energy should be given a chance if we want to protect the environment. We own it to our children, our grandchildren and the generations to come.