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A solar turbine power plant works on the energy received from solar radiation through solar collectors. Solar power is the renewable source of energy. Amount of energy received on earth surface through solar power is around 1.783*10¹⁴ KJ. The energy received per square meter is 1.353KJ/s. Solar power plant operates mainly on closed power cycle which are Rankine cycle (for lower temperature range) and Brayton cycle ( for high temperature range).We have solar plants which operates within the range of few kilowatts to few megawatts. Various type of collectors used for collecting solar energy are of three types namely Low temp (Tmax =100°C) ,Medium temp (Temp=300-400°C) and high temperature(Temp=400-700°C) collectors. The working fluids that can be used are Steam, Freon or Helium. Block diagram indicating the Energy and Fluid flow in a solar turbine power plant is shown in the figure 1.The Constraints associated with solar plants are size, space high capital cost and the variation of solar energy per day.

Figure 1.Energy and fluid flow in a solar turbine power plant

Concentration Ratio

Concentration ratio is the area of concentrator to area of receiver surface. It is an important parameter in determining the temperature of receiver. The amount of solar energy incident on concentrator is directed towards receiver so it would be a measure of energy concentrated towards the receiver. Higher values of CR can be attained by use of large apertures and small receiver. Receiver temperature increases with increase in concentration ratio as shown in Figure 2. Concentration ratio varies from 1.5 to 3000 depending on type of collector i.e. whether it is a medium temperature collector or it is a large temperature collector. It is an important parameter in determining the efficiency of a plant.

${CR}={{aperture\,area\,of\,concentrator} \over {receiver\,surface\,area}}={Ac \over Ar}$ (A)

Optical Efficiency

Optical efficiency of the solar collector indicates the percentage of the solar rays penetrating the transparent cover of the collector (transmission) and the percentage being absorbed.

${\eta _{0}}={{heat\,energy\,received\,by\,the\,receiver} \over {incident\,radiation\,on\,the\,collector}}={Qr \over Qc}$

Where ${Q_{c}}={I_{c}}.{A_{c}}$

I_c = incident solar radiation

Therefore ${Q_{r}}={\eta _{o}}..{I_{c}}.{A_{c}}$

Collector Efficiency

${\eta _{c}}={{useful\,heat\,received\,by\,the\,coolant} \over {incident\,radiation\,on\,the\,collector}}={Qu \over Qc}$

Where, ${Q_{u}}={Q_{r}}-{L}={Q_{r}}-{Losses}$

The losses can be expressed by overall co efficient U based on receiver area

${L}={U}.{Ar}.({Tr}-{Ta})$

${Q_{u}}={\eta _{0}}.{Ic}.{Ac}-{U}.{Ar}.({Tr}-{Ta})$

${\eta _{c}}={\eta _{0}}-({1 \over {cr}}).{{U\,Ta\,} \over {Ic}}.({{Tr} \over {Ta}}-{1})$

ɳ_c= f(CR,TR) ,

where TR is the Receiver Temperature Ratio.

TR increases with the concentration ratio as shown in the figure 2

Figure 2.Variation of Receiver temperature to Concentration Ratio.jpg

However collector efficiency decreases with temperature ratio as shown in figure 3

Figure 3.Variation of Collector efficiency (%) with Temperature ratio.jpg

Solar Receiver

The receiver absorbs heat transmitted by the collector. Sometimes the receiver is an integral part of the system for example in solar ponds and flat plate collectors. Receivers may be stationary or movable.

There are three types of receivers

I) External

II) Tubular

III) Cavity

External receivers

Working fluid is provided on the external surface of vertical body (figure 4).

Figure 4.A Heliostat and External receiver

Major losses are due to:

a) non- focusing,

b)losses due to conduction , convection and radiation.

c)Reflection. CR for this type of reflector is around 1000. Temperature is around 500°C.

Cavity Receivers

Heat flux enters through the apertures as shown in the figure 5, concentrators transmit the heat flux to the surface of coolant tubes through the apertures. Heat energy is transferred to other parts (where the direct beam is unable to reach) through internal reflection. Overall size is large due to number of coolants tubes.

Figure 5.A Heliostat and External receiver

Tubular receivers

It consists a row of coaxial tubes. Outer tube receive the radiation where as the working fluid enters through the inner tube and leave through the annular space between the two tubes (figure 6). Concentration ratio is around 1.5. Fluid temperature that could be attained is around 200°C.

Figure 6.A tubular collector receiver.jpg

Receiver System

Distributed receiver System

In this system the three collectors (as shown in the figure 7)collect the heat flux and transfer it to receiver from where the coolant takes this energy to the heat exchanger(Path A). The coolant at times serves the purpose of working fluid as depicted by Path B.

Figure 7.Distributed Receiver System.jpg

Central Receiver System

In this system the solar collectors transmit the heat flux to receiver which is large in size. External and cavity types of receiver can be employed for this purpose. Example: Heliostats

Net Efficiency

The collector efficiency (ɳ_c decreases with increase in temperature of receiver. The thermal efficiency (ɳ_th) increases with increase in inlet temperature of working fluid. Therefore overall efficiency (ɳ_n) of plant varies as shown in the figure 8.The curve is flat at maximum efficiency.

Figure 8.Variation of Collector efficiencies with Receiver temperature.jpg

Solar energy storage

Since the solar radiation is not available throughout it becomes necessary to store the energy of the sun in some form. The solar thermal energy storage can be done in

1. Solids: Some of the rocks may absorb the heat. The amount of energy stored depends on the mass of the solid material, its specific heat and the allowable temperature rise.

2. Liquids: If the heat is stored below the boiling point of fluids at ambient pressure then some fluids can be used as heat storage medium. Some of the liquid which can be used for this purpose are Sodium, Hitec, Therminol, oils.

3. Latent heat of fusion : In this type of system suppose we heat a solid then it melts. Thus heat is stored in the body at constant temperature in the form of latent heat. Examples LiF(latent heat = 1050 KJ/Kg melting point =848°C) and LiOH(latent heat = 1080 KJ/Kg melting point =471°C)

4. The combination of any of the above stated phenomenon can also be used to store solar energy.

Solar turbines

The coolants and working fluid along with steam turbines or gas turbines together determine the efficiency of the plant.

Coolants and Working Fluids

A coolant absorbs energy in the receiver and transfers the energy to the working fluid in the heat exchanger. Example water/steam, liquid metals, molten salts, gases and oils.

Water can be used as coolants in low and medium temperature solar power plants.

The maximum temperature deployed in oil type of coolant is 471°C. Oil can be dangerous because it is inflammable. Also these are costly.

Gases that can be used as coolant are air, helium, argon, and carbon dioxide. It can be used for high temperature range (T_max=800°C).

Molten salts are also used for high temperature region. They have high specific heat.

Molten metals (sodium or aluminium) can also be used as coolants. since their density is high they require a smaller receiver.

Steam turbines

Steam turbines operate on Rankine cycle. Values of pressure and temperature in solar plants are 50-100 bar and 400-500°C. 0c respectively. Both impulse and reaction stages can be used. For small values of power impulse stages are preferable.

Gas turbine

Gas turbine operate onBrayton cycle (that is the inlet temperature around 500-800°C. Gas turbine use fewer number of stages, absence of feed water heaters ,condenser and has a small cooling requirement.

Advantages and Disadvantage

Advantages

1. Being a renewable form of energy, the fuel is free and surplus. 2. No fuel storage, processing or handling equipment is required.

3. Being an alternative form of energy saves a lot of Oil/Petrol/Diesel.

4. Less environmental pollution.

5. Can easily be operated in remote places or the places which are unfit for habitation.

Disadvantages

1. Dependant power generation(depends on weather).

2. Large amount of area required for its establishment.

3. High capital cost.

4. Overall efficiency is low.