Thermodynamic solar energy

THERMODYNAMIC SOLAR ENERGY

Themis solar tower

A thermodynamic solar concentration system takes advantage of the sun radiation by directing the sun’s rays with mirrors. This concentrated thermal system achieves temperature levels well above those of conventional thermal systems not concentrated.
While domestic water heaters heat water to about 50°C, it is possible, by concentration, to heat fluids to temperatures between 250 and 1000°C
It then becomes possible to use these systems in industrial processes such as power generation; this is called "thermodynamic solar power".
Other direct or indirect uses hot fluids are possible such as the desalination of sea water, cooling or hydrogen generation.

Concentration principle and functioning of thermodynamic solar plants:
The concept of focusing sun’s rays has been known since ancient times. Starting a fire of dead leaves with a magnifying glass uses this principle.
Most often through reflecting mirrors or magnifiers, a concentration system redirects the solar radiation collected by a given surface on a smaller dimension target.
The thermodynamic solar power plants use a lot of mirrors to orient the solar rays and converge them toward a heat-carrier fluid in order to heat it to high temperature.
To do this, the reflective mirrors have to follow the sun's movement to capture and concentrate the radiation throughout the day. The fluid produces electricity via steam or gas turbines.

There are four main types of solar thermal power plants:

The 4 thermal plants types

The punctual concentration systems:        

• Tower power plants,                              
• Parabolic concentrators Dish-Stirling.     

The linear concentration systems:  

• Thermal power plants with parabolic trough,
• Thermal power plants with Fresnel mirrors.

Punctual concentration systems:  

Solar radiation is concentrated about 1,000 times and oriented towards a single target of small size. The temperature can reach 500 to 1000°C.

 

Solar tower

•Tower power plants:

Hundreds of mirrors follow the sun's path (the "heliostats"), reflect and concentrate sunlight onto a central receiver (target) atop a tower, in which heat carrier fluid flows.

The temperature is so high, at the focal point of the target, that the heat carrier fluid is composed of molten salt.

Compared to a parabolic trough system, the solar tower has the advantage of not having to circulate the fluid in the entire field of mirrors: heat losses are significantly reduced. Furthermore, the level of solar concentration may be much higher, and the efficiency of the thermodynamic cycle is increased. However, these technical gains should also result in an economic gain, which is limited by the cost of building the tower. 

•Parabolic concentrators Dish-Stirling:

Dish-Stirling

This system looks like a satellite dish (needed to receive satellite TV). The receiving antenna is just replaced by a Stirling engine and a receiver.

The base of the parabolic dish is mobile in order to follow the sun's displacement. The parabola focuses the solar radiation on the small target (receiver) in its focal point in order to heat to a high temperature gas (about 200 to 750°C).
This fluid is then used to generate electricity through a Stirling engine.
This technology is not suitable for industrial mass production due to its high cost, hence the delay in its development. However, it is the only thermodynamic technology that can be implemented in small isolated sites.

Dish-Stirling system

How works a Stirling engine?  
The Stirling engine is a closed-cycle system, using the heat of the sun to expand hydrogen gas, pushing a piston (1), which rotates a crankshaft (2) to power an electrical generator. The hydrogen is cooled and condensed by a radiator, then sent back to the expansion cylinder.

 (See "Stirling engine solar")

       (See "Infinia Stirling solar generator")

Linear concentration systems:

The solar radiation is concentrated on one or more absorber(s) tube(s) installed along the focal line of the mirror. The tube contains a heat-carrier fluid which is heated to a temperature of about 250 to 500°C.

•Thermal power plant with parabolic trough:

Parabolic trough

This technology is the most prevalent today. The focus of a parabola is a point that of the parabolic trough mirror is an axis, on which is placed an absorber tube (receiver) which is black color in order to capture the most radiation.
In this tube, the heat-carrier fluid, which can reach a temperature of about 500°C, is then sent to the electricity production block.
The set, parabolic trough reflector and receiver follow the sun’s movement.

Heat exchanger for electricity generation

In the parabolic trough system, the high temperature of the heat carrier fluid (see oil loop) is transferred to a steam loop to generate electricity (like in a conventional thermal power plant).


•Thermal power plant with Fresnel mirrors:
Rather than curving the mirrors (expensive industrial processes), the Fresnel reflectors "imitate" the parabolic trough shape with slightly curved mirrors placed at the same horizontal level.

Fresnel mirrors

The focusing (concentration factor), however, is less efficient in this system because the parabola is not perfect (temperature about 270°C):
Only the mirrors are moving, the structure and the absorber tube are both stationary.
The costs of thermal power plants with Fresnel mirrors are lower than those of power plants with parabolic trough (both for Installation and maintenance which are easier).
The systems with Fresnel reflectors are still relatively rare.

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