unit 2 solar thermal energy

8/12/2019 Unit 2 Solar Thermal Energy

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Solar collectors

A solar collector absorbs the incident solar radiation

and convert it to useful heat which is used for heating a

collector fluid such as water, oil or air.

The surface of a solar collector is designed for high

absorption and low emission.

It is classified into two types: 1) Flat plate collectors 2)

Concentrating (focusing) collectors.

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Flat Plate Collectors

A flat plate solar collector as shown in fig.1 consists of

a transparent cover, an absorber plate, a series oftubes, and a housing with thermal insulation.

The absorber plate with several parallel tubes is

fabricated from copper tube and sheet by soft

soldering.

The plate is blackened in order to absorb maximum

amount of sun light.

It is also kept in a box to insulate it from sides and

bottom so as to prevent heat losses and thereby attain

high temperatures. 3


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Flat Plate Collectors Contt

The complete structure is placed at a proper inclination

to the sun so as to absorb solar radiations.

The heat absorbed by plate is removed by circulating

water, or some other fluid or air in contact with it or

through the tubes.

The flat plate collector gives a temperature of only

about 60 degree above ambient with an efficiency of

30-40%.

They are mainly used for water heating, space heating,

space cooling etc.

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Flat Plate Collectors Contt

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Fig.1 Schematic Diagram of a Flat Plate Solar Collector


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Materials For Flat Plate Collectors

The material for absorber plate should have high thermal

conductivity, adequate tensile strength and good corrosionresistance.

The most common material used for absorber plate is copper

because of its high conductivity and resistance to corrosion.

Other materials for absorber plate are Aluminium, Iron, Brass,

Silver, Tin and Zinc.

The materials for insulation which should have low thermal

conductivity and stability at high temperature are crown white

wool, glass wool, calcium silicate, cellular foam etc.

Tempered glass is commonly used material for cover plate.

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Solar Concentrating Collectors

For applications such as air conditioning, central

power generation, and numerous industrial heat

requirements, flat plate collectors generally cannot

provide carrier fluids at temperatures sufficiently

elevated to be effective.

Alternatively, more complex and expensive

concentrating collectors can be used.

These are devices that optically reflect and focus

incident solar energy onto a small receiving area.

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Concentrating Collectors Contt

As a result of this concentration, the intensity of the

solar energy is magnified, and the temperatures that

can be achieved at the receiver (called the "target")

can approach several hundred or even several

thousand degrees Celsius.

The concentrators must move to track the sun if they

are to perform effectively

Concentrating, or focusing, collectors intercept direct

radiation over a large area and focus it onto a small

absorber area.

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Concentrating Collectors Contt

These collectors can provide high temperatures more

efficiently than flat-plate collectors, since the

absorption surface area is much smaller.

However, diffused sky radiation cannot be focused

onto the absorber.

Most concentrating collectors require mechanical

equipment that constantly orients the collectors toward

the sun and keeps the absorber at the point of focus.

Therefore; there are many types of concentrating

collectors

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Types of Concentrating Collectors

1. Parabolic trough system

2. Parabolic dish

3. Power tower4. Stationary concentrating collectors

There are four basic types of concentrating collectors:

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1. Parabolic Trough SystemParabolic troughs are devices that are shaped like the

letter u. The troughs concentrate sunlight onto areceiver tube that is positioned along the focal line ofthe trough.

Fig.3 Parabolic trough systemFig. 2 Crossection of parabolic trough

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Parabolic troughs often use single-axis or dual-axis

tracking.

Fig. 4. One Axis Tracking Parabolic

Trough with Axis Oriented E-WFig. 5. Two Axis Tracking Concentrator

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Temperatures at the receiver can reach 400 C and

produce steam for generating electricity. Multi-megawatt

power plants have been built using parabolic troughscombined with gas turbines (California).

Fig. 6 Parabolic trough combined with gas turbines 13


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2. Parabolic Dish Systems

A parabolic dish collector is similar in appearance to a

large satellite dish, but has mirror-like reflectors and an

absorber at the focal point. It uses a dual axis sun tracker

Fig. 7. Crossection of parabolic dish 14


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A parabolic dish system uses a computer to track the

sun and concentrate the sun's rays onto a receiver

located at the focal point in front of the dish. Parabolic

dish systems can reach 1000 C at the receiver.

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2. Parabolic Dish Systems Contt


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3. Power Tower System

A heliostat uses a field of dual axis sun trackers that direct

solar energy to a large absorber located on a tower. To date

the only application for the heliostat collector is power

generation in a system called the power tower (solar tower).

Fig. 9 HeliostatsFig. 8 Power tower system 16


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A heliostat (from helios, the Greek word for sun, and stat,

as in stationary) is a device incorporating a mirror whichmoves so as to keep reflecting sunlight toward a

predetermined target or receiver, despite the sun's apparent

motions in the sky.

The target is stationary relative to the heliostat, so the lightis reflected in a fixed direction.

Most modern heliostats are controlled by computers. Thecomputer is given the heliostat's position on the earth

(latitude and longitude) and the time and date, and uses

them to calculate the direction of the sun as seen from the

mirror. 17

3. Power Tower System Contt


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A power tower has a field of large mirrors that follow thesun's path across the sky.

The mirrors concentrate sunlight onto a receiver on top of ahigh tower.

A computer keeps the mirrors aligned so the reflected raysof the sun are always aimed at the receiver, where

temperatures well above 1000C can be reached.

High-pressure steam is generated to produce electricity.

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Fig. 10 Power tower system with heliostats 19



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Fig. 11 Power tower system with heliostats 20



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4. Stationary Concentrating Solar Collectors

Stationary concentrating collectors use compoundparabolic reflectors and flat reflectors for directing

solar energy to an accompanying absorber or aperture

through a wide acceptance angle.

The wide acceptance angle for these reflectorseliminates the need for a sun tracker.

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Working principles of concentrating collectors

Unlike solar (photovoltaic) cells, which use light to produce

electricity, concentrating solar power systems generateelectricity with heat.

Concentrating solar collectors use mirrors and lenses toconcentrate and focus sunlight onto a thermal receiver,

similar to a boiler tube.

The receiver absorbs and converts sunlight into heat. Theheat is then transported to a steam generator or engine

where it is converted into electricity.

A concentrating solar power system that produces 350 MWof electricity displaces the energy equivalent of 2.3 million

barrels of oil . 22


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Application of Flat Plate Collectors

1. Water heating

2. Space heating

3. Power generation

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1. Water Heating

24Fig.12 Water heating system


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2. Space Heating

25Fig.13 Space heating system


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3. Power Generation

26Fig.14 Low temperature power generation


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Application of Concentrating Collectors

1. Solar plants for power generation

2. Industrial process heat supply

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Solar Thermal Power Plants

There are two types of solar thermal power plants:

1. Solar distributed collector power plants

2. Solar central receiver power plants

In the first type, collectors may be of two types: 1)Parabolic trough units with line focus, and 2)

Paraboloidal dishes with center focus.

While in second type, there are again two type: 1)Solar furnace systems, and 2) Solar farm systems.

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29Fig. 15 Solar power plant


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Fig. 16 Schematic of a concentrated solar thermal trough

power plant with thermal storage


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2. Solar central receiver power plants

31Fig.17 Solar Furnace (Tower Concept)


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2. Solar central receiver power plants contt

32Fig. 18 Solar Furnace (Tower Concept)


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Limitations of Solar Thermal Energy

A vast amount of solar energy is though freely available from

sun but it has following limitations:

1. Low energy density 0.1 to 1 kW/m2.

2. Large area is required to collect solar thermal energy.

3. Direction of rays changes continuously with time.

4. Solar energy is not available during night and during

clouds.

5. Energy storage is essential.

6. High cost.

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Limitations of Solar Thermal Energy contt

7. It requires hybrid plant with storage facility for supplying

energy during night.

8. Solar central power plants in MW range are not

economical.

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Thermal Energy Storage

Thermal energy storage is essential for both domestic water

and space heating applications.

Thermal energy can be stored in well insulated fluids orsolids

It can be stored in the following two ways:1. Sensible heat storage

2. Latent heat storage

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unit 2 solar thermal energy

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