Here is an essay on the ‘Types of Solar Thermal Power Plants’ especially written for school and college students.
Essay # 1. Parabolic Trough Solar Power Plant:
It is also called solar farm power plant as a number of solar modules consisting of parabolic trough solar collectors are interconnected. Every module consists of a collector as shown in Fig. 8.9. It is rotated about one axis by a sun tracking mechanism.
Thermo-oil is mostly used as Glass Tube heating fluid as it has very high boiling point. Water/steam working fluid can also be used. The tubes have evacuated glass enclosure to reduce the losses. The concentration ratio is between 40 and 100. The maximum oil temperature is limited to 400°C as oil degrades above this temperature. Alternately steam at 550°C can be directly generated in the absorber tube.
These are commercially under operation. Fig. 8.10 shows a flow diagram of parabolic trough solar power plant. The working fluid is heated in collectors and collected in hot storage tank (2). The hot thermo-oil is used in boiler (5) to raise steam for the steam power plant. The boiler also is provided with a backup unit (6) fired with natural gas. The cooled oil is stored in tank (3) and pumped (4) back to collector (1).
The specification of two such plants is given below:
Performance Analysis:
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The heat output,
Qc = n Ib Aa ƞc [W]
where,
Aa = Aperture area of a parabolic through module [m2]
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Ib = Radiation intensity [W/m2]
n = No. of collectors.
Ƞc = Collectors efficiency
Ƞmod. = ραabs – Uc(Tabs – Ta)/CIb
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where,
ρ = Reflectivity of mirror
αabs = Absorptivity of tubes.
Uc = Overall heat loss coefficient [W/m2 – K]
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Tabs = Temperature of absorber [K]
Ta = Ambient temperature [K]
C = Concentration ratio
Ib = Intensity of beam radiation [W/m2]
Essay # 2. Central Receiver Solar Power Plants:
These are also called solar tower power plants. It can be divided into solar plant and conventional steam power plant. The flow diagram is given in Fig. 8.11.
A heliostat field consists of a large number of flat mirrors of 25 to 150 m2 areas which reflects the beam radiations onto a central receiver mounted on a tower. Each mirror is tracked on two axes. The absorber surface temperature may be 400 to 1000°C. The concentration ratio (total mirror area divided by receiver area) may be 1500. Steam, air or liquid metal may be used as working fluid. Steam is raised for the conventional steam power plant.
The technical data of an experimental solar power plant is given below:
Useful solar energy output,
E = A Ib ƞ [W]
A = Total area [m2]
Ib = Intensity of beam radiations [W/m2]
ƞ = Efficiency of heliostat field ≈ 55 to 80%.
Performance Analysis:
The rate of useful heat output of central receiver.
Qc = E – ∑∆El – ∑∆Ql [W]
Where,
E = output of heliostat field [W]
∑∆El = Electrical losses [W]
(Radiation losses due to reflection from the absorber surface)
∑∆Q1 = Heat losses [W]
(Due to conduction convection and radiation in the receiver walls)
The rate of useful heat output is also equal to
Qc = Aa [Ia – FR Uc (Tin – Ta)] [W]
= ṁ (ho – hi) [W]
The absorbed solar radiation flux,
Ia = (E – ∑∆El)/Aa
= IbCρ τ α [W/m2]
where,
Ib = Beam radiation intensity on heliostat surface [W/m2]
C = Concentration ratio of heliostat receiver system
ρ = Reflectivity of heliostat
γ = Intercept factor
τ = Transmissivity of transparent cover.
α = Absorptivity of absorber surface.
The efficiency, ƞ = Qc/E
where,
E = Useful solar radiation flux of heliostat [W]
Essay # 3. Dish-Stirling Engine System:
This is suitable for autonomous power generation. A hot gas Stirling engine receives solar irradiation from a parabolic trough concentrator (Dish). It has been very high efficiency. Glass mirrors are used for radiation concentrator. The working fluid is hydrogen or helium for Sterling engine. The maximum pressure and temperature of the engine cycle are 20 MPa and 1000K. A schematic diagram of the power plant is shown in Fig. 8.12.
The specifications of such a system are given below:
Essay # 4. Solar Chimney Power Plant:
The air stream is heated by solar radiation absorbed by the ground and covered by a transparent cover. The hot air flows through or chimney which gives the air a certain velocity due to pressure drop caused by the chimney effect. The hot air flows through an air turbine to generate power. The schematic diagram of the plant is shown in Fig. 8.13.
The pressure difference due to chimney effect,
∆p = gH (ρc – ρw) [N/m2]
where,
g = acceleration due to gravity [9.81 m/s2]
H = Chimney height [m]
ρc = Density of cold air [kg/m3]
ρw = Density of warm air [kg/m3]
The pressure energy is converted into kinetic energy of air. The air velocity in the chimney.
The power output of air turbine is a function of air velocity and chimney area. The plant efficiency,
Ƞ = P/IA
where,
P = Power output of air turbine [W]
I = Intensity of global solar radiation [W/m2]
A = Area of ground receiving solar radiator [m2]
The specification of a chimney power plant in operation is:
