Solar Turbine Plants

A solar turbine power plant works on the energy received from solar radiation through solar collectors. Solar power is a 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 plants, such as the Blythe Solar Power Project, operate mainly on closed power cycles: Rankine cycles (for low temperature ranges) and Brayton cycles (for high temperature ranges). Solar plants operate within the range of few kilowatts to few megawatts. . The constraints associated with solar plants are size, space, high capital cost, and the variation of solar energy per day.

Efficiency

Concentration ratio

The concentration ratio is the area of concentrator to the area of receiver surface. The amount of solar energy incident on the concentrator is directed towards the receiver, so it measures the energy concentrated towards the receiver.^[1]^: 210

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 the type of collector, i.e., whether it is a medium or large temperature collector. It is an important parameter in determining the efficiency of a plant.

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

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

There are three types of solar collectors: low (100° C),^[2]^: 699 medium (300-400° C)^[2]^: 701, and high (400-700° C)^[2]^: 706 temperature collectors. Each type has its own efficiency.

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

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

The losses can be expressed by overall co-efficient ${U}$ based on receiver area

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

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

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

${\eta _{c}}=$ f(CR,TR)

where TR is the Receiver Temperature Ratio. TR increases with the concentration ratio as shown in Figure 2. Collector efficiency decreases with temperature ratio as shown in Figure 3.

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: external, tubular, and cavity.

External receivers

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

Major losses are due to:

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 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 coolant tubes.

Tubular receivers

This consists a row of coaxial tubes. The outer tube receives the radiation whereas the working fluid enters through the inner tube and leaves through the annular space between the tubes (Figure 6). Concentration ratio is around 1.5. Fluid temperature that could be attained is around 200° C.

Receiver system

Distributed receiver system

In this system the three collectors (as shown in 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.

Central receiver system

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

Net efficiency

The collector efficiency (ɳ_c) decreases with increases in temperature of receiver. The thermal efficiency (ɳ_th) increases with increases in inlet temperature of working fluid. Therefore, overall efficiency (ɳ_n) of the plant varies as shown in Figure 8. The value of net efficiency is between 15-20%^[3]^: 725. The curve is flat at maximum efficiency.

Solar energy storage

Since solar radiation is not always available it becomes necessary to store the energy in some form. Solar thermal energy storage can be done in:

Solids: Some rocks absorb heat. The amount of energy stored depends on the mass of the solid material, its specific heat, and the allowable temperature rise.
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 liquids which can be used for this purpose are Sodium, Hitec, Therminol, and oils.
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).^[2]^: 717
The combination of any of the above 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 an oil type of coolant is 250° C.^[2]^: 717 Oil can be dangerous because it is inflammable. Also, these are costly.

Gases that can be used as coolants are air, helium, argon, and carbon dioxide. It can be used for high temperature range (T_max=800° C).^[2]^: 717

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, freon, or helium are some of gases used as working fluid

Steam turbines

Steam turbines operate on a Rankine cycle. Steam can be generated by receiver directly from solar heat flux which would eliminate the need for a heat exchanger. However, some plants deploy molten salts for attainment of higher temperature this eliminates the need for steam boilers. Values of pressure and temperature in solar plants are 50-100 bar and 400-500° C respectively. Both impulse and reaction stages can be used. For small values of power impulse stages are preferable.

Gas turbine

Gas turbines operate on a Brayton cycle, that is, the inlet temperature around 500-800° C. A conventional gas turbine power plant uses a combustion chamber, but here the combustion chamber is receiver/heat exchanger. However, using the gas solar turbine power plants through stored thermal energy is quite difficult because the power plant operates at a high temperature range and it is quite difficult to store heat energy at this high temperature. Gas turbines use fewer stages, absence of feed water heaters, condenser and has a small cooling requirement.

Advantages and disadvantages

Advantages

Being a renewable form of energy, the fuel is free and surplus
No fuel storage, processing or handling equipment is required
Being an alternative form of energy saves of oil/petrol/diesel
Less environmental pollution
Can easily be operated in remote places or the places which are unfit for habitation

Disadvantages

Dependent power generation (depends on weather)
Large amount of area required for its establishment
High capital cost
Overall efficiency is low

References

^ Sukhatme, Suhas P.; Nayak, J. K. (2008). Solar Energy: Principles of Thermal Collection and Storage (3 ed.). McGraw-Hill. ISBN 978-0070142961.
^ ^a ^b ^c ^d ^e ^f Yahya, S. M. (2005). Turbines, Compressors and Fans (3 ed.). Tata McGraw-Hill. ISBN 978-0070597709.
^ Cite error: The named reference yanhya was invoked but never defined (see the help page).