SOLAR THERMAL ENERGY BY CONCENTRATION
The thermal solar energy by concentration is a type of technology that uses solar energy to produce electricity.
WHAT IS THERMAL SOLAR ENERGY BY CONCENTRATION?
The Concentrated Solar Power (CSP) is to concentrate heat (from the sun) at one point. In this way, steam is generated to drive the turbine.
In addition, this type of plants can operate constantly, since part of the heat is stored, which allows to continue producing steam and supply energy in the absence of solar radiation.
At Torresol Energy, we are committed to technologies aimed at the design, construction, operation and maintenance of high-power power plants. These technologies allow to pose:
- Central tower stations . A set of heliostats (flat mirrors) concentrate the heat of the sun on a central receiver mounted on the top of a tower over 100 meters high. The fluid that circulates through the receiver (in our case, molten salts) absorbs highly concentrated solar radiation and converts it into thermal energy, to be used in steam generation, which starts the turbine, and produces, in this way , electricity
- Central parabolic trough collectors . Reflectors in the form of a parabolic section channel, arranged in horizontal rows and in whose foci are located collector tubes through which the heat transfer fluid circulates (synthetic thermal oil), concentrate the sunlight on said tubes, which elevates the oil temperature. The hot oil is used to vaporize water that, driven to a steam turbine, drives a generator that injects electrical energy into the network.
Both technologies have a thermal storage system that allows:
- Avoid fluctuations in the supply.
- Continue the production in hours of absence of solar radiation, in which direct generation is not possible.
- Transfer the production peaks according to the needs of the demand.
During the last years, SENER has made important developments in solar technology:
- The heliostat of 120 m 2 of reflective surface, with excellent optical characteristics and high precision of aim, used in Gemasolar.
- The H-54 heliostat, of 180 m 2 of surface, that, after being tested and tested in Gemasolar, has been incorporated into the solar field of the Noor III power plant in Morocco.
- The receiver of molten salts with capacity to operate with high flows of incident solar radiation and provide a thermal efficiency superior to that of its predecessors.
- In addition, SENER has developed and validated the design criteria of this pioneering thermal storage system, with working temperatures above 500º C.
Solar energy by concentration is clean and reliable, can be produced during high peaks of demand and has the potential to satisfy, in the future, the growing needs of a country.
The principle of concentration.
The principle of the concentration of solar radiation has been known since antiquity, as illustrated by the myth of the “burning mirrors” of Archimedes.
Most of the time, with reflecting mirrors or magnifying glasses, a concentration system reorients the solar radiation captured by a given surface into a smaller objective: starting a dead leaf fire with a magnifying glass uses this principle.
Where to use CSP technology?
Concentrated thermodynamic solar energy (CSP) is the set of technologies and systems designed to convert solar radiation into electrical energy, through optical concentration and thermodynamic cycles.
This technology uses only direct radiation and is not suitable, in particular cases or eminently thermal applications, for the construction of small plants.
However, for large systems around or more than the MW and in areas with strong direct radiation, it allows lower electricity production costs than photovoltaic technology.
If we look at the Euro-Mediterranean area, we can foresee a kind of integration between photovoltaic technology in areas, mainly European, less equipped with direct radiation
The operation of solar thermodynamic power plants.
Solar thermodynamic plants use a large number of mirrors that converge the sun’s rays in a heat transfer fluid heated at high temperature.
To do this, the reflecting mirrors must follow the movement of the sun to capture and focus the radiation throughout the daily solar cycle. The fluid produces electricity through steam or gas turbines.
There are four main types of thermodynamic solar power plants: parabolic cylindrical mirrors and their variants Fresnel mirrors, tower plants and, finally, Stirling parabolic troughs.
There are several types of CSP technology plants in the world:
- Linear parabolic collectors: more than 13 TWh (billions of kWh) have been produced and placed on the network at present;
- Central tower systems: the Solar Two plant was the first to use a mixture of “molten salts” consisting of 60% sodium nitrate (NaNO3) and 40% potassium nitrate (KNO3) as a heat transfer fluid. ;
- Plants with parabolic disk collectors.
CSP plants currently find a concrete application only in the production of electricity. According to Anest, the introduction of a reward incentive system for electricity / heat-cold cogeneration plants could also open this front.
With respect to the production of electricity, there are two main aspects that limit the current CSP technology:
- the achievable temperature
- Continuity of the service.
Regarding the first point, most of the current commercial plants use linear parabolic collectors and use a synthetic oil as a heat transfer fluid.
This type of fluid allows operating temperatures that do not exceed 400 ° C; This limits the thermodynamic efficiency. Regarding the continuity of the operation, in order to make the production of electricity less dependent on the intrinsic variability of the solar source, the current technology contemplates the creation of “hybrid” systems, in which the solar field is flanked by a gas burner that supplies energy when the solar radiation is insufficient.
Alternatively, solar heat can be used to produce additional steam for use in a conventional thermoelectric plant; in this case, however, for technical reasons, the proportion of solar production in relation to the total is
The different types of plants
Linear concentration systems
Solar radiation is concentrated in one or more absorbent tubes installed along the focal line of the mirrors. This tube contains a refrigerant heated to a temperature of the order of 250 to 500 ° C.
Reflective mirrors follow the movement of the sun throughout the day.
- Central parabolic cylindrical mirrors : this is the most widespread technology at present. The focal point of a parabola is a point, that of a cylindrical-parabolic mirror is an axis, on which an absorbent tube (the receiver) of black color is placed, to capture a maximum of radiation. In this tube circulates the refrigerant, which is heated to a temperature of about 500 ° C and then centralized and transported to the power generation block. The cylindrical-parabolic mirror / receiver assembly follows the movement of the sun.
Examples: Andasol, one of the most powerful thermodynamic power plants in Europe located in Spain (150 MW); Nevada Solar One in the United States (64 MW).
Parabolic mirrors from Andasol, Spain
- Solar energy plants with Fresnel mirror : Fresnel mirrors, instead of flexing mirrors (expensive industrial process), “imitate” the parabolic cylindrical shape with the slightly curved mirrors and are placed at the same horizontal level. Only the mirrors move, the structure and the absorbent tube are both stationary. Therefore, the costs of Fresnel solar mirrors are lower than those of cylindrical parabolic mirrors in installation and maintenance. However, the approach is degraded in this system (since the parable is not perfect): the challenge is that cost reduction “compensates” the degradation of efficiency from an economic point of view. This type of system is still relatively rare.
Examples: Puerto Errado in Spain (31.4 MW), Kimberlina in California (5 MW).
Solar thermodynamic plant with Fresnel mirrors (
Concentration systems per household.
Solar radiation is concentrated approximately 1,000 times in a single house of small size. The temperature can reach 500 to 1000 ° C.
- Tower power stations Hundreds of mirrors that follow the course of the sun (“heliostats”) reflect and concentrate the solar radiation in a central receiver located at the top of a tower, in which the refrigerant circulates. As in parabolic cylinder systems, heat from the fluid is transferred to a conventional steam cycle to generate electricity. In comparison with a cylindrical-parabolic system, the solar tower offers the advantage of not having to circulate the fluid in the whole field of the mirrors: the thermal losses are significantly reduced. In addition, the level of concentration of the irradiation can be much higher and the efficiency of the thermodynamic cycle increases. However, these technical gains must also result in a technical and economic gain,
Examples: Crescent Dunes in Nevada (110 MW), Solar Tres in Spain (19.9 MW), PEGASE project in the French center of Thémis (Pyrénées-Orientales).
- Parabolic mirror plants from Dish Stirling : a parabola concentrates the radiation in a focus at its focal point to power an engine called “Dish-Stirling”. Once heated in a closed circuit, the gas it contains drives a piston that recovers the mechanical energy produced. This technology is not suitable for mass industrial production due to its high cost, hence the delay of its development. However, it is the only thermodynamic technology that can be implemented in isolated small sites.
Example: site of Font-Romeu Oreillo, headquarters of the CNRS research on the subject.
Dish Stirling solar parabolas tested in Albuquerque, New Mexico
Challenges with energy
The electricity produced by thermodynamic solar energy must be economically competitive with fossil fuels within a period of 10 to 15 years (at present, Moroccan experts mention, for example, a price close to 150 euros per MWh for the Noor 1 plant ).
The useful life of a facility is estimated between 25 and 40 years. Some concentration technologies already benefit from important comments. In addition, cogeneration, that is, the use of residual heat after electrical generation to produce, for example, desalinated or cold water, significantly increases the competitiveness of solar thermodynamic plants.
Unlike photovoltaic solar systems whose product is directly electricity, thermodynamic solar technologies generate heat in their process.
The hot fluid that has a certain thermal inertia (capacity to store heat), the thermodynamic electricity production is, therefore, less “unequal” than the production of photovoltaic electricity.
In addition, the systems of dynamic storage of heat can be integrated into the installations, prolonging the generation of electricity until several hours after the disappearance of solar radiation.
Thermodynamic solar energy does not directly produce waste or greenhouse gases.
The first installations were created in the 1980s, but subsequent oil shocks left this technology in the dark until the 2000s.
It is necessary to wait for the adoption of preferential food tariffs during this decade so that the sector can restart industrial activities. In 2015, there were around 4.7 GW of thermodynamic solar capacity installed worldwide, including 2.3 GW in Spain and 1.7 GW in the United States (2) .
The announcements are multiplying: mention among others the project of a plant of more than 1 GW in the Sultanate of Oman (Petroleum Development Oman), the 3 projects of CSP under the solar plan Noor in Morocco (the Saudi company Acwa Power) or the income of the Moroccan solar energy agency (Masen) to the capital of the French company Alsolen for 30 million euros in the summer of 2015.
The United States, Spain, North Africa, China and Australia are the next likely areas of growth. The IEA predicts that this energy source could provide 11% of global electricity by 2050 (3) . This would correspond to an annual production of approximately 4,770 TWh, equivalent to the energy consumption of the USA. UU (With an installed capacity of more than 1,000 GW in this scenario).