ES2636800B1 - Solar power generation plant - Google Patents

Abstract

Power generation plant (1) for the use of solar energy comprising: # - a solar field (2) comprising photovoltaic panels (3) and their auxiliary elements of photovoltaic production, # - selective reflection filters (4) of light depending on its wavelength, arranged in some or all of the photovoltaic panels (3), # - at least, a tower (5a) provided with a central thermosolar receiver (5b) towards which the reflected beams are directed ( 101) by the filters (4) of the photovoltaic panels (3), and # - at least, a block of use of the thermal energy captured.

Description

OBJECT OF THE INVENTION

The present invention relates to a power generation plant by harnessing solar energy, which integrates the two main solar technologies: photovoltaic and solar thermal tower. Alternatively, the option of using the hot fluid generated in the tower receiver can be considered for industrial applications that require hot water or superheated fluid.

SECTOR OF THE TECHNIQUE

The main sector in which the project is framed is that of electricity generation through renewable energy.

BACKGROUND OF THE INVENTION

Within the main solar technologies, two large blocks can be distinguished: solar thermal concentration and photovoltaic solar technology.

The principle of operation of both is completely different, each having its advantages and disadvantages.

Solar thermal energy uses optical means, usually mirrors, to generate concentrated light that is used to heat a heat transfer fluid. Said superheated fluid is used as an input in a traditional turbine cycle to heat another fluid, which is the one entering the said cycle.

However, photovoltaic solar energy is characterized by the use of semiconductors, mainly polycrystalline silicon, which generate direct electricity after the solar radiation is affected by photoelectric effect.

The solar thermal energy has the great advantage that, because it uses fluids, these can be stored in tanks and introduced into the turbine cycle at the time of the day of interest, or even at night. This means that, solar thermal has the competitive advantage that it is an energy that can be stored. As a major drawback, said energy is significantly more complex to handle than photovoltaic solar and other conventional sources, being therefore more expensive to produce electricity by this means than by other sources.

Photovoltaic solar energy is, however, much simpler. In addition, certain governments have given tremendous financial momentum for large-scale manufacturing. All this means that the costs are much lower than those of the solar thermal system, and comparable to that of conventional sources. The great disadvantage that it presents is that since it is a direct production of electricity, its storage is not viable unless batteries are used, which is tremendously expensive and would require several replacements for the life of the plants. Therefore, photovoltaics does not allow large-scale storage in commercial plants, which implies that the delivery of energy to the grid is not synchronized with the real demand that an electricity company may have.

Within the area of solar thermal systems, the two technologies that currently dominate the market are parabolic trough and tower. In those of parabolic cylinder, a conduit or tube with the fluid to be heated circulates through the scope of one or more parabolic mirrors that concentrate solar radiation in said conduit. In the tower technology the solar field concentrates the radiation in a single point located in the tower, where the receiver where the heat transfer fluid circulates

Parabolic trough technology is the most mature and has been the dominant throughout the historical development of solar thermal energy. However, recently, the thermosolar towers are being imposed, since they have among others the advantage that the concentration of light is more effective than in the parabolic cylinder, and therefore higher temperatures can be achieved and the efficiency of thermodynamic cycles increased. In addition the circulation of heat transfer fluids is limited to the central area of the plant where the tower is located, while in the parabolic cylinder, being a linear system, the tubes extend absolutely throughout the plant, which greatly increases their complexity.

That is why currently the solar thermal towers have lower generation costs than those of the parabolic cylinder and are, without a doubt, the future bet within this type of technologies.

Relating to photovoltaic technology, the clearly dominant technology is that of mono or polycrystalline silicon. These are simple systems with large economies of scale, therefore very cheap, and that can compete in cost with conventional generation sources.

Some applications for the use of selective filters for solar applications have been reported. Highlight the system reported by scientists of the ~ Australian Center tor Advanced Photovoltaics ~, which have developed a purely folovoltaic tower with a selective filter at the top of the tower, so that it separates the light by redirecting it to two different types of photovoltaic cells located around said filter. However, this system has no thermal receiver and would not store energy with the ease of solar thermal technology. In addition, the photovoltaic part is not based on conventional technology and on concentration cells, much more expensive; meanwhile, the filter only occupies a small area at the top of the tower.

Another application of selective light filter in solar applications has been reported in the PV Mirror project. This project is developing the deposition of a multi-layer (up to 60 layers) on a film. Said multilayer lets the useful radiation pass to the photovoltaic cell and reflects the rest to the absorber tube of the parabolic trough. Said film is suitable for use in parabolic cylinder applications, but has drawbacks in central tower systems, such as: -The parabolic trough is based on linear concentration of light while the tower concentrates the light of a field of two-dimensional heliostats in the receiving plane. This means that the concentration in a tower is much higher than that of a parabolic cylinder and allows, even removing effective radiation to the receiver by the inclusion of the filter, to reach temperatures high enough to have thermodynamic cycles

efficient. -The parabolic cylinder is based on mirrors or curved facets, which implies the need to bend the crystalline structure of silicon cells to have a filter-cell optical continuity, and for reliability and performance issues it is not advisable to perform these operations on this type of structures since it can be easily induced to breakage. -Its parabolic-cylinder application cannot be made directly on commercial elements -As the parabolic cylinders are highly curved surfaces, the area of useful radiation uptake is much smaller than the area of mirrors to be installed. Specifically, this relationship would be the diameter of the parabola divided by the perimeter of the semi-parabola. This means that in order to capture the same light, a greater amount of filter surface must be applied, affecting the cost of the installation. -The thermo-solar tower plants are simpler and have lower costs than those of parabolic troughs. This is due in large part to the fact that the entire thermodynamic cycle and fluid circulation is limited to where the tower is located. However, in the case of the parabolic trough, the fluids have to make lakes traveled throughout the extension of the solar field, which increases the complexity of the installation and the cost of it.

EXPLANATION OF THE INVENTION

The solar power generation plant of the invention implies a new type of solar plant, which solves the described drawbacks.

According to the invention, the plant comprises a solar field formed by photovoltaic (PV) modules that, on the one hand, absorb part of the sunlight by injecting it into the network in the same way as a conventional PV photovoltaic plant while, on the other , they reflect infrared and other rays outside the visible spectrum to a central receiver solar thermal power plant (CSP), which allows energy storage by heating fluids. This is achieved through the insertion into the photovoltaic modules of selective light filters, which perform a spectral separation of the solar radiation, which can be done at very low cost using transparent oxidization techniques of high / low refractive index (eg sputtering or dip-coating).

It is, therefore, a new concept of PV-CSP plant, whose conceptual technological core lies in the hybridization of a photovoltaic solar field and a tower solar thermal power plant.

The main elements that make up our system are listed below:

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Solar field: with photovoltaic panels with integrated filters for selective reflection of light depending on their wavelength, and ideally mounted on solar trackers (or heliostats), and provided with their auxiliary elements of photovoltaic production,

•

at least, a tower provided with a central solar thermal receiver.

•

Exploitation block

•

Storage media

Next, a brief description of the entire system will be carried out, including solar field, tower, central receiver and power block. However, the text will focus on the part related to the solar field and selective filter integration because the other components would be standard.

Solar field

The solar field is, without a doubt, the part that most impacts the cost of a solar thermal power plant and the one that will undergo modifications with respect to conventional tower technology in this project.

The receivers used in tower technology require high values of solar radiation concentration, so the use of a multitude of mirrors, arranged in solar trackers or heliostats, which thanks to the action of a servomechanism can follow the movement of the sun and they are used to reflect the direct solar radiation incident in a common focus.

Each heliostat of a central receiver plant has about 140m2 of reflective surface, usually formed by several slightly concave mirrors installed on a common pillar to reduce costs. The great novelty that is introduced within the invention is the replacement of the mirrors by photovoltaic modules with the integrated filter. Said filter will be responsible for transmitting the part of the solar spectrum in which the cell is very efficient and will reflect the rest to the receiver located in the tower.

A heliostat is mainly composed of a central post cemented to the ground, two helical tubes or arms (one on each side) that provides rigidity to the system and torsional strength; and a series of trusses that serve as an anchor for mirrors in conventional systems or for photovoltaic panels in this case. Likewise, a mechanism, normally hydraulic, is installed at the junction of the two arms with the pedestal to provide the entire system with the ability to follow the sun accurately, in azimuth as well as normally in inclination (two tracking axes).

The photovoltaic module or board would be a commercial product of the many manufacturers that currently exist, so that selective sunlight filters would be laminated on them.

The main configurations of the heliostat field are reduced to two possibilities: north / south field and circular field. Depending on the latitude of the site and the size of the plant, one or the other configuration will be chosen. In general, the farther the central of Ecuador is, the greater the efficiency of a north / south field compared to a circular one. However, the north / south field requires higher towers - which implies higher costs - than the circular field for the same thermal power in the receiver. Thus, a circular field to the detriment of a north / south field will be convenient for large-scale plants.

Dichroic filter

To achieve a differentiated treatment of sunlight according to the wavelength, so that a fraction of the spectrum is selectively reflected, while the other is transmitted through the device, different types of optical devices can be applied. Surely the option with more technical feasibility and better possibilities of industrial scaling is the one constituted by a dichroic filter: an optical device used to reflect or transmit light selectively according to its wavelength. The cut-off wavelength is chosen at will according to the characteristic needs of the devices (photovoltaic, thermal or other solar collectors) to which the light is redirected.

In general, a dichroic mirror consists of a stack of layers of two transparent materials of different refractive index. The low index layer / high index layer set may have a periodic sequence or not, depending on the characteristics of the desired reflection and transmission spectra.

The control of transmitted / reflected light depends on several factors:

•

refractive index of the individual components.

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Thickness of the layers and number of stacked layers.

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Angula of incidence of light.

Dichroic mirrors are usually manufactured by sputtering techniques, although in our case they can be prepared using a technique based on precursors obtained using the sol-gel method, which is specifically dip dip coating, with a clear industrial application. This technique allows the manufacture of mirrors on large substrates, so they are considered suitable to replace the techniques based on "sputtering" or other physical vapor depositions used to obtain small dichroic filters, the cost of which is also an order of magnitude greater.

Tower and central receiver

The receiver is the unit where the solar energy from the heliostats is concentrated to transform it into thermal energy in the working fluid. To ensure that the energy that reaches the receiver is as high as possible, it must be placed in height, reducing as far as possible the effects of shadows and blockages that may occur in the field of heliostats. In this sense, the main mission of the tower is to provide a height support to the receiver. The towers built to date consist of metal or concrete structures and can easily reach 150 meters high.

There are currently several types of receivers (cavity or external) whose choice depends on technical-economic factors since it is not proven that one technology or another is better than the others. The basic design of the receiver and the tower are usually done taking into account the following factors:

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Optimal size to minimize thermal losses

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High incident radiation flow

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Optimized design to work at the maximum temperature limits of metal components

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Maximum thermal efficiency

The heat is transported, from the receiver to the point of demand, by means of a heat transfer fluid and stored in thermal tanks in order to adapt, as far as possible, production to demand. The main fluids used are steam and molten salts.

Storage media

Solar radiation cannot be stored. However, it is possible to do it with the thermal energy that carries the heat transfer fluid, which allows the plant to operate in periods of absence or high variability of solar radiation. At present, the most viable solution to conserve this thermal energy is the storage in two tanks of molten salts. The system consists of two large thermal deposits, one hot and one cold. The hot thermal fluid, which leaves the receiver, is directed to the hot tank, which is maintained at the thermal level required by the power cycle. In the cold reservoir, the cooled thermal fluid accumulates, which has already exhausted its ability to yield heat and returns to the top of the tower, at the lowest possible temperature. Due to the existence of these two deposits, the hot tank can accumulate heat when solar production exceeds demand.

Exploitation block

The utilization block incorporates the elements capable of harnessing the energy collected in the form of heat for any use

A possible first set of elements would be a power cycle, type Rankine cycle. The mission of this water-steam cycle is to transport water vapor from the steam generator to the steam turbine and, once expanded and subsequently condensed, pump the water to the generator, starting the cycle again.

Depending on the type of control panel you have, different cycle settings will be used. Generally, larger plants will require reheating and / or regeneration stages, which increase the yield of the cycle but in turn also make it more expensive.

It is also necessary to take into account what kind of thermal fluid the plant will use because, depending on whether it is water or molten salts, the steam generation stage will be different for each case.

Another possible second set of elements, alternative or complementary to the previous one, would be a set of exchangers to transfer heat from the heat transfer fluid to other fluids, for example to heat water.

BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, a set of drawings is attached as an integral part of said description, where illustrative and non-limiting nature has been represented. next:

Figure 1.- Shows a diagram of an embodiment of the plant of the invention. In it, the main elements of the photovoltaic generation8 appear in detail.

Figure 2.- It shows a section of a top glass of a photovoltaic plate with the built-in filter, and the scheme of optical operation of the filter.

PREFERRED EMBODIMENT OF THE INVENTION

The power generation plant (1) for use of solar energy of the invention comprises (see fig. 1): a solar field (2) comprising photovoltaic panels (3) (PV), configuring the entire solar field (2 ) said photovoltaic panels (3), or being able to coexist with mirrors, not shown, with the same ones - filters (4) (see fig 2) of selective reflection of light depending on their length of

wave, arranged in some or all of the folovoltaic plates (3), -at least, a tower (5a) provided with a central solar thermal receiver (5b) (see fig 1) AND towards whose solar thermal receiver (5b) the reflected beams are directed (101) by the filters (4) of the photovoltaic panels (3), and of the mirrors where appropriate, and -at least, a block of use of the thermal energy captured.

Said utilization block may comprise for example a power cycle (Se) for generation by turbines and connection to the network (80) -8 through a transformation center (81) -and / or heat exchangers, not shown, for heating fluids (for example water for other uses)

The optional arrangement of thermal energy storage means (5d) (molten salt tanks for storage of this heat transfer fluid for example) for storage of thermal production surpluses in consumption / generation valley periods has also been provided.

In this way, the integration, within a solar thermal system (5) of a central tower receiver, of a solar field where some or all the elements reflecting solar radiation towards the central receiver (5b) include photovoltaic panels is proposed (3) (PV) on which a selective light filter (4) is deposited. Said filter (4) will allow a certain strip of the visible spectrum useful for the photovoltaic cells of the same to the corresponding photovoltaic plate (3), and will reflect the rest of the wavelengths towards the central receiver (5b) located at the top of the tower (5th). Therefore the photovoltaic panels (3) also cooperate in the generation of energy by photoelectric effect increasing the performance of the plant (1), and will also be connected to the network (80) through the corresponding inverter (30), and the center of transformation (81), also being able to comprise batteries (82) for storing energy generated in this way.

Preferably, the filters (4) are configured to let the solar radiation of visible wavelengths pass towards the corresponding photovoltaic plate (3), and reflect the solar radiation of shorter and longer wavelengths relative to the visible radiation towards the central receiver (5b).

A preferred spectral plot of the reflected wavelengths would be the

next, where the thick line reflects these reflected wavelengths:

0.8

•

g 0.6

~

~ 0.4

or

5 Most preferably (see fig. 2), the filters (4) comprise dichroic filters, ideally Bragg reflectors comprising layers of transparent conductive oxides (4a, 4b) of high / low refractive index laminated on the photovoltaic plates (3 ). Said layers of metal oxides (4a, 4b) are ideally laminated on the photovoltaic panels (3) by immersion coating,

10 although they can also be arranged by sputtering or any other means, and said oxides will preferably be silicon oxide as a low refractive index element and titanium oxide as a high refractive index element.

For example, a possible section of a configured filter (4) is shown in Figure 2

15 as a Bragg reflector or 1 D photonic glass under the transparent cover of a photovoltaic plate (3). In such a structure, the incident light beam (100) undergoes reflection and refraction processes in all the intercars (40) that exist between the different layers (4a, 4b) and between the last layer (4a) and the air interior and the first layer (4a) and the base or substrate (49) that configures the transparent cover, so

20 that the reflected parts (102) in the different interleaves (40) leave the filter (4) forming a reflected beam (101) in which, since each reflected part (102) travels different optical paths, has generated interference processes optics that cancel out certain wavelength ranges in the resulting reflected beam (101). Precisely that non-reflected range (104) will be the one transmitted to the plate

25 photovoltaic (3).

Preferably the number of layers (4a, 4b) would be from 1 to 200. More preferably it would be from 4 to 100 and even more preferably from 5 to 20.

The design of the filter (4) will be preferably defined by the following expression:

substrate / (a, Ti02) / (b, Si02) / (a, Ti02) / (b, Si02) / ...... ./ (a "Ti02) / (b" Si02) /

Being a1, a2 .... an the thickness of the different layers of the titanium oxide and b1, b2, .. bn the thickness of the layers of silicon oxides, and where the first layers (4a) could correspond to the layers of Ti02 and the second layers (4b) could correspond to the layers of Si02 or vice versa.

Preferably a1, a2, ... an, will be thicknesses with different values between them and in turn different from the values of b1, b2 ..., bn, which will also be different from each other, in order to cause reflections of lengths of Different wave in each layer.

Preferably the thicknesses of both silicon oxide and titanium oxide will be between 50 and 1000 nm.

The layer structure of the filters will preferably be deposited on the inner side of the glass that makes up the commercial photovoltaic panels (3), which will therefore form the substrate (49), and whose photovoltaic plates (3) in turn will ideally be installed in heliostats (6) with two-axis tracking, and where preferably the assemblies of photovoltaic panels (3) in heliostats (6) comprise surfaces between 50-200 m2, and will have a motor or tracking mechanism, not shown, preferably hydraulic or electromechanical. In this way, the filter (4) is integrated in the photovoltaic plate (3) so it is a constituent part of any solar field and, covering much more area, much cheaper manufacturing techniques are needed.

The arrangement of the heliostats (6) will be preferably north if it is in the northern hemisphere, circular if it is installed in equatorial areas and preferably of southern distribution if it is located in said hemisphere.

The solar field (2) may consist entirely of heliostats (6) + photovoltaic panels (3) + filters (4) or there may be a proportion of heliostats (6) + photovoltaic panels (3) + filters (4) I heliostat (6) + traditional mirror. In the latter case, the proportion of heliostats (6) + photovoltaic panels (3) + filters (4) will preferably be from 1% to 80%.

The other parameters of the plant will be conventional with respect to existing commercial solutions. By way of illustration and without serving the following numbers as parameters of defined and closed designs, some of the most representative ranges of these plants are listed. The installed peak power will preferably be between 20-300MW, the heat transfer fluid will preferably be superheated steam or molten salts, the storage system will preferably be molten salts with a delivery capacity at peak power between 1-20 hours. The power cycle will preferably be a Rankine cycle and the tower will preferably be built with a concrete base and will have a height between 50-200 m.

Claims (21)

1.-Central (1) of power generation by solar energy use, characterized in that it comprises: -a solar field (2) comprising photovoltaic panels (3) and their auxiliary elements of photovoltaic production, -a few filters (4) for selective reflection of light depending on its wavelength, arranged in some or all photovoltaic panels (3), -at least, a tower (5a) provided with a central solar thermal receiver (5b) to which the reflected beams (101) are directed by the filters (4) of the plates photovoltaic (3), and -at least, a block of use of the thermal energy captured. 2.-Power generation plant (1) for solar energy use according to claim 1 characterized in that the use block comprises a power cycle (5c) for generation and connection to the network (80) and / or heat exchangers for fluid heating. 3.-Power plant (1) for generating energy by using solar energy according to any of the preceding claims, characterized in that it comprises storage means (5d) for thermal energy. 4. Power plant (1) for generating energy by harnessing solar energy according to claim 3, characterized in that the storage means (5d) for thermal energy comprise molten salt tanks. 5. Power plant (1) for generating energy by using solar energy according to any of the preceding claims, characterized in that the filters (4) are configured to allow solar radiation of visible wavelengths to pass to the photovoltaic plate (3) corresponding, and reflect solar radiation of shorter and longer wavelengths relative to the radiation visible to the receiver central (5b). 6. Power plant (1) for generating energy by using solar energy according to any of the preceding claims, characterized in that the filters (4) comprise dichroic filters. 7.-Power generation plant (1) for solar energy use according to claim 6 characterized in that the dichroic filters (4) comprise 8ragg reflectors, comprising layers of transparent conductive oxides (4a, 4b) of high / low index refraction laminated on photovoltaic panels (3). B.-Power plant (1) for generating energy by using solar energy according to claim 7, characterized in that the layers of transparent conductive oxides (4a, 4b) are laminated on the photovoltaic panels (3) by immersion coating. 9. Power plant (1) for generating energy by using solar energy according to claim 7, characterized in that the layers of transparent conductive oxides (4a, 4b) are laminated on the photovoltaic panels (3) by sputtering. 10. Power plant (1) for generating energy by using solar energy according to any of claims 7 to 9, characterized in that each filter (4) comprises a number of layers of transparent conductive oxides (4a, 4b) comprised between 1 and 200 11. - Power generation plant (1) for solar energy use according to claim 10 characterized in that each filter (4) comprises a number of layers of transparent conductive oxides (4a, 4b, 4c) comprised 4 and 100. 12. Power plant (1) for generating energy by using solar energy according to claim 11, characterized in that each filter (4) comprises a number of layers of transparent conductive oxides (4a, 4b, 4c) between 5 and 20. 13.-Power generation plant (1) for solar energy use according to any of claims 7 to 12 characterized in that the layers of transparent conductive oxides (4a, 4b) comprise layers of silicon oxide as a low refractive index element and of titanium oxide as a high refractive index element. 14.-Power plant (1) for generating energy by using solar energy according to claim 13, characterized in that the design of the filter (4) is defined by the expression: substrate / (a, Ti02) / (b, Si02) / (a, Ti02) / (b, Si02) / ...... ./ (a "Ti02) / (b" Si02) / Being a1, a2 .... 8n the thickness of the different layers of the titanium oxide and b1, b2, .. bn the thickness of the layers of silicon oxides. 15.-Power plant (1) for generating energy by using solar energy according to claim 14, characterized in that a1> a2, ... an, have different values between them and in turn different from the values of b1, b2 ..., bn, which are also different from each other. 16. Power plant (1) for generating energy by using solar energy according to any of claims 12 to 15, characterized in that the layers of silicon oxide and titanium oxide have thicknesses between 50 and 1000 nm. 17. Power plant (1) for generating energy by using solar energy according to any of claims 12 to 16, characterized in that the layers of silicon oxide and titanium oxide are laminated on the inner face of the outer glass (3a) of the corresponding photovoltaic plate (3). 18.-Power plant (1) for generating energy by using solar energy according to any of the preceding claims, characterized in that the photovoltaic panels (3) are arranged on heliostats (6). 19.-Power plant (1) for generating energy by using solar energy according to claim 18, characterized in that the heliostats (6) comprise two tracking axes. 20.-Power plant (1) for generating energy by using solar energy according to any of claims 18 or 19, characterized in that the assemblies of photovoltaic panels (3) in heliostats (6) comprise surfaces between 50-200 m2. 21. Power plant (1) for generating energy by using solar energy according to any of the preceding claims, characterized in that the solar field (2) contains a proportion between 1% and 80% of heliostals (6) + photovoltaic panels (3) + filters (4), and the rest of the mirrors. 22.-Power generation plant (1) for solar energy use according to any of the preceding claims characterized in that the solar field (2) has a north configuration for locations in the northern hemisphere, southern configuration for locations in the northern hemisphere and configuration circular for locations in equatorial areas.