Patent Application
20130118479
The present invention relates to a thermal absorber for a solar receiver of a solar power plant and to a solar receiver for a solar power plant comprising at least one such absorber, in particular for a Fresnel-type concentrating solar power plant.
Concentrating thermal solar technology consists in using solar radiation to heat a heat transfer fluid used as the heat source in a thermodynamic cycle. Concentration enables relatively high temperatures to be attained, and thus relatively substantial thermodynamic conversion efficiencies to be enjoyed. The developed technologies are distinguished by the means used to concentrate the solar rays, by the means by which heat is transferred, and possibly by the means used to store heat, i.e. the heat transfer fluid used and thermodynamic conversion means which are, for example, steam turbines, gas turbines or Stirling engines.
There are typically four families of Concentrating Solar Power (CSP) systems:
linear-focus parabolic cylinder collectors,
Fresnel linear concentrators,
central receiver tower systems, and
adjustable-focus parabolas.
Each concentrating solar power system comprises a solar receiver the function of which is to transfer to a fluid, such as water, oil or a gas, the heat of the solar radiation. This solar receiver therefore forms a heat exchanger. This exchanger is formed of one or more tubes installed parallel to one another, in which the heat transfer fluid flows.
In the particular case of a Fresnel-type concentrating solar power plant, the solar receiver receives the light rays reflected by mirrors, and transmits them to the heat transfer fluid in the form of heat.
A solar receiver typically comprises:
an absorber which receives the solar flux on its lower face, and in which the heat transfer fluid flows,
possibly, a layer of thermal insulating material enabling the heat losses from the absorber to the exterior to be limited,
possibly, a glazed panel enabling the absorber to be insulated from the external environment, and delimiting a closed cavity between the absorber and the glass.
In such a device, the flux received by the absorber varies greatly across the width of the absorber and along the length of the absorber. This variety is notably due:
to the fact that the concentrated flux is obtained by the superimposition of unit fluxes of each mirror either side of the receiver, where each mirror produces a solar patch on the absorber with focusing and flux which vary according to the position of the sun in the course of the day;
to errors of positioning of the mirrors and of focusing relating to the manufacture and drive precision of the mirrors;
to the passage clouds across the solar field, causing sudden variations of flux.
The surface of the absorber which receives the flux is generally covered with a selective surface coating which absorbs the solar energy whilst having a low emissivity in the infrared spectrum, limiting losses by infrared re-emission. This coating is, for example, a black paint. The lifetime of this surface treatment is an important parameter for the performance of the solar power plant. But this selective surface treatment can be heat damaged; it is therefore important to prevent the appearance of hot points.
An absorber for a solar receiver of the Fresnel type is described, for example, in documents US 2009/0056703 A1 and US2009/0084374 A1. The absorber is formed by multiple tubes positioned next to one another in order to transfer the energy of the concentrated solar flux to the fluid. The tubes are particularly suitable for pressurised fluids such as steam. However, since the concentrated solar flux varies greatly over the width of the absorber and, possibly, along the length of the tubes, the fluid is heated differently in different tubes. Since the fluids exiting the different tubes are not at the same temperature a reblending zone is then required.
In addition, the zones located between the tubes which also receive the concentrated solar flux are not used to heat the fluid, and the efficiency of the receiver is therefore not optimal.
In addition, in the zones of the absorber where the solar flux is potentially very concentrated, for example due to a mirror focusing error, the temperature of the wall of the tube increases suddenly and the selective surface treatment applied to the tube may be damaged rapidly, and there may therefore be a drop in performance of the solar receiver.
It is consequently one aim of the present invention to provide a solar absorber the efficiency of which, even in the case of a varied luminous flux, is increased, and having reduced risks of the appearance of hot points.
The aim set out above is attained by a module for the production of an absorber for a solar receiver of a thermal power plant comprising a wall which is roughly flat, and the outer face of which is intended to receive the luminous flux, a wall opposite said wall, and side walls, where said walls define a single channel for the flow of a pressurised heat transfer fluid, and where the wall the face of which is intended to receive the luminous flux and the wall opposite the latter are connected mechanically by means installed in the flow of the fluid, so as to keep the absorber pressurised. The module according to the invention also comprises means for reblending the fluid in the single channel, homogenising the temperature of the fluid, and preventing the appearance of hot points.
In a particularly advantageous manner, the elements mechanically connecting the two walls also form deflectors for the fluid, so as to reblend the fluid within the module.
The two mechanically connected walls are preferentially connected by a single central element which is roughly aligned with the lengthways axis of the absorber, in which windows are made to cause the fluid to flow from one side to the other of the central element. Even more preferentially, vanes are installed to force the fluid to flow through the windows, and thus to change direction.
The absorber according to the present invention comprises one or more modules according to the present invention placed end-to-end, and collectors upstream and downstream for connection to a liquid supply circuit or to other absorbers.
In other words, the absorber according to the present invention is a heat exchanger having a single duct, used to collect a concentrated solar energy flux on a single one of its faces, which is particularly suitable for a flow of great variability across the width and along the length of the exposed face. This exchanger transfers this flux to a heat transfer fluid, undertaking an internal blending in order to homogenise the output temperature of the fluid and the wall temperature, which prevents thermal degradation of the fluid and of the surface coating on the face exposed to the flux.
The subject-matter of the present invention is then a module for the production of a thermal absorber for a solar receiver of a solar power plant, with a lengthways axis, comprising a first roughly flat wall having a face intended to be subjected to a luminous flux, a second wall opposite the first wall and side walls connecting said first and second walls, where said module is delimited at its lengthways ends by transverse end planes at which points said module is intended to be connected to upstream and/or downstream modules, and/or to collectors for the supply and/or evacuation of a heat transfer fluid intended to flow under pressure in the module, and where said module comprises means rigidly connecting the first and second walls, where said means are positioned in the flow of the heat transfer fluid, and means to enable the heat transfer fluid to flow in directions which are inclined relative to the lengthways axis.
The heat transfer fluid is a liquid.
The pressure of the heat transfer fluid in the module is preferably between 2 Bar and 6 Bar.
For example, the means to enable flows in directions which are inclined relative to those of the lengthways axis comprise deflectors which are distributed throughout the volume of the module, and which cause the fluid to change direction.
In an advantageous example, the module according to the invention comprises at least one rib extending in a lengthways direction, attached to the first and to the second walls, where said rib comprises windows and deflectors associated with the windows, and where said deflectors cause a portion of the fluid to flow through said windows.
According to an additional characteristic, said at least one rib extends in a lengthways direction along the entire length of said module, delimiting two half-channels which are in fluid communication.
The rib is produced for example from a metal alloy strip from which the deflectors are cut, where said deflectors are folded such that they are inclined relative to the lengthways axis and open up the windows.
Two successive deflectors are preferably located either side of the plane of the rib.
The rib can be welded on one side to the internal face of the first wall, where said rib comprises tabs on its side which is welded to the second wall which are inserted into notches made in said second wall.
The module can be of a roughly rectangular parallelepipedic shape, where the first and second walls have the largest areas.
The face intended to be subjected to a luminous flux advantageously comprises a coating improving the absorption of the luminous flux. The coating preferably has low infrared emissivity properties.
Another subject-matter of the present invention is a thermal absorber for a solar receiver of a solar power plant comprising one or more modules according to the present invention, where the modules are connected in series in sealed fashion, and where said absorber comprises, at a first lengthways end, a heat transfer fluid supply collector and, at a second lengthways end, a heat transfer fluid evacuation collector.
Another subject-matter of the present invention is a solar receiver comprising at least one absorber according to the present invention, and a skirt formed from two inclined panels at some distance from the lengthways axis, positioned either side of the absorber relative to the lengthways axis, where said skirt redirects the luminous flux on to the absorber.
The receiver may comprise thermal insulation means positioned outside the absorber on the second wall and on the sides of the latter.
The receiver is preferably of the Fresnel type.
Another subject-matter of the present invention is a method for manufacturing an absorber according to the present invention, comprising the following steps:
manufacture of one or more modules,
attachment of the mechanical connection means to the first and second walls, and installation of the means to enable flows in directions which are inclined relative to the lengthways axis,
installation of the supply and evacuation collectors at the ends of the assembly formed by the module or modules.
The mechanical connection means and the means to enable flows in directions which are inclined relative to that of the lengthways axis are produced, for example, from a metal alloy strip:
by cutting vanes from the strip, where said vanes are attached on one side to the strip, and
by folding said vanes such that they form an angle with the plane of the strip, where the production of said vanes simultaneously forms windows in the strip.
The mechanical connection means are attached, for example, by welding.
The manufacturing method according to the invention advantageously comprises a step of surface treatment of the outer face of the first wall. Said surface treatment can be accomplished by applying a layer of paint on to said face after the module or modules are manufactured.
Another object of the present invention is a method for manufacturing a solar receiver, where said method comprises:
the step of manufacturing multiple absorbers by the manufacturing method according to the present invention,
the step of sealed connection by welding of said absorbers by their collectors.
The present invention will be better understood using the description which follows and the appended illustrations, in which:
We shall firstly briefly describe a solar receiver in which a solar absorber according to the present invention may be installed. Such a solar receiver is represented in
Solar receiver 2 comprises a skirt 4 formed by two panels 4.1, 4.2 which define, seen as a transverse section, a trapezoid space, in the base of which an absorber 6 is positioned. Skirt 4 advantageously redirects the luminous flux from mirrors (in
The solar receiver forms part of a thermal power plant and transfers the heat from the luminous flux to the liquid.
Absorber 6 delimits a channel 8 in which a heat transfer fluid is intended to flow in a direction roughly perpendicular to the plane of the sheet of paper in the representation of
Thermal insulation means 10 are advantageously installed on the other wall and on the sides of absorber 6 to limit the heat losses from the absorber to the exterior, and more specifically from the heat transfer fluid heated by the luminous flux towards the exterior.
A transparent plate 11 may be installed, for example made of glass, positioned upstream from the absorber in the direction of the luminous flux, to create a sealed cavity and to limit losses by convection. We shall now describe in detail an example embodiment of an absorber 6 according to the present invention represented in
Absorber 6 comprises a casing 12 of roughly parallelepipedic shape extending in a lengthways axis X. The casing comprises two lengthways walls 14.1, 14.2 of greater area, where two side lengthways walls 16.1, 16.2 connect both lengthways walls 14.1, 14.2 of greater area. The casing is delimited at its lengthways ends by two transverse end planes 18.1, 18.2.
The casing can comprise one or more modules; in the represented example it contains two such modules. The absorbers are generally very long, for example several hundred metres; they therefore usually comprise several modules. This type of production in the form of modules enables manufacture to be simplified.
One of the lengthways walls of greater area 14.1 is intended to be illuminated by luminous flux F. This wall is located in the lower portion in order to receive the luminous flux reflected by the mirrors; this wall will be designated below by the description “lower wall”; the opposite wall, for its part, will be designated the “upper wall”.
The absorber also comprises a collector 20 for the supply of “cold” heat transfer fluid connected to the casing at one of the transverse end planes, and a collector 22 for evacuating the heated heat transfer fluid, after it has traversed the entire absorber connected to the casing at the opposite transverse plane. Collectors 20, 22 are connected to a fluid network of the solar power plant.
The heat transfer fluid flows along lengthways axis X, from supply collector 20 to evacuation collector 22, in the direction represented symbolically by arrow 24. The heat transfer fluid is a liquid.
In the example represented in
In the represented example these mechanical connection means 28 are formed by a rib 30 positioned roughly along lengthways axis X and attached to the side walls 14.114.2 of greater area. The rib is more specifically visible in
Rib 30 is perforated to allow fluid to pass from one side to the other of the rib, and to allow the fluid to be reblended.
The absorber advantageously comprises means improving the blend by causing the fluid to change its direction of flow relative to the lengthways direction, such that the temperature of the fluid within the channel is roughly uniform, and such that the appearance of hot points is prevented. A hot point is understood to mean, in particular in the context of this invention, a zone subjected to a greater solar flux over an area extending lengthways along the absorber.
In the represented example, and advantageously, the rib is pierced with windows 32, and has fluid deflectors 34 such that they cause the fluid flow to change from one side to the other of rib 30.
In a particularly advantageous manner, the deflectors are formed by vanes cut from the rib simultaneously producing windows 32.
In addition, in the represented example, the deflectors' direction of inclination is such that it deflects the stream towards the opposite face of the rib. The change of direction of the stream is represented symbolically by arrows 36 in
As can be seen in
The effect of the presence of the rigidification rib is not therefore to divide channel 8 into two independent channels, but is to allow the fluid to flow between the two subchannels, and therefore allows homogenisation of the temperature. If one side of plate 14.1 opposite one of the subchannels is more illuminated than the other, therefore potentially implying a greater heating of the fluid flowing in this subchannel, the blending thus prevents the appearance of this temperature differential. The temperature of the walls is also homogenised, reducing the deformations of the casing by expansion.
The absorber's casing is preferably made of metal plate, as is the rib, which is then welded on to both lengthways walls of greater area.
In the represented example, rib 30 has protruding tabs 37 on its upper edge on the side of wall 14.2 of the casing; these tabs are inserted into notches made in upper wall 14.2 to anchor the rib satisfactorily.
Rib 30 is then welded on to the internal face of lower wall 14.1, which does not damage its outer face 6.1 subjected to the luminous flux, and tabs 37 traverse upper wall 14.2 and are welded to this wall in sealed fashion. It should be noted that the surface condition of outer face 6.1 of upper wall 14.2 is not a factor for the operation of the absorber.
The interval of windows 32 and angle of inclination α of vanes 34 are chosen in accordance with the flow conditions of the fluid in absorber 6.
In the represented example there is a single rib 30 in channel 8. It may be envisaged to have n parallel ribs, where n is a positive integer, extending along the entire length of the channel, and defining n+1 subchannels in communication. The pressure resistance of the absorber is thus increased further.
In addition, in the represented example the rib extends along the entire length of the lengthways axis. Conversely, it is possible to have several ribs of lesser length positioned parallel to the lengthways axis, and distributed throughout the volume of the duct so as to form rigidification and blending elements throughout the entire volume. These elements are, for example, distributed uniformly. Multiple single deflectors 134 can also be distributed throughout the entire volume of duct 8, as is represented diagrammatically in
It can also be envisaged to separate the mechanical connection means and the deflection means. To this end rods which mechanically connect both lengthways walls of greater area may be used, since the latter have little effect on the flow of the liquid. The deflector means would be formed, for example, by plates inclined relative to the lengthways axis. The plates could then be attached only to one or other of the lengthways walls of greater area.
A perforated rib, such as the one represented in
According to the invention, face 6.1 of absorber 6 receiving the luminous flux is flat; a surface treatment can then easily be applied to it to ensure that it collects the luminous flux satisfactorily, the goal being to obtain total absorption of the incident luminous flux, i.e. emissivity in the visible spectrum close to 1, and zero re-emission in the infrared spectrum, i.e. emissivity in the infrared spectrum close to 0. The treatment is, for example, applied to the lower face of the casing after the latter has been assembled, by applying a layer of selective paint, or then before assembling the casing, by accomplishing a bath deposition, for example of the Black Chromium type on the metal plate intended to form lower wall 14.1. The absorber according to the present invention is particularly suitable for operation at a pressure of less than 10 bar, and more specifically at an absolute pressure of between 2 Bar and 6 Bar, for example between 3 Bar and 6 Bar at maximum temperatures of 400° C., and even more particularly for operation at a pressure of the order of 3 Bar and at a temperature of the order of 300° C.
The heat transfer fluid can be water or, more advantageously, thermal oil commonly used in concentrating solar power plants, such as Therminol 66® or Therminol VP1®. The heat transfer fluid can flow in the absorber at a speed of the order of 0.2 m/s to 2 m/s. The casing of the absorber is made, for example, with metal alloy plates of roughly 1 millimetre thick, welded in sealed fashion. Such alloys can be stainless steel 304 or stainless steel 316, or again a pressure-resistant steel such as P265GH and P295GH.
As mentioned previously, solar receivers of high-power solar power plants are generally several hundreds of metres in length. To this end, the casings are preferably manufactured from multiple modules, which are positioned end-to-end, and which are assembled by a seal weld 40 (
By virtue of the invention it is then possible to manufacture standard modules and then to connect them in order to form a casing on to which supply and evacuation collectors are attached, forming an absorber. This production in the form of standard modules enables manufacturing to be automated, and the manufacturing cost prices to be reduced.
As an example, an absorber according to the present invention may measure 50 m in length.
It may be decided to connect several absorbers in series, where the evacuation collector of the upstream absorber is then connected to the supply connector of the downstream absorber. Changing from one absorber to the next may, for example, enable the downstream absorber to be positioned in a direction different to the upstream absorber.
The absorber according to the present invention is particularly suitable for the production of a Fresnel-type solar receiver. In
As a non-restrictive illustration, we shall give a practical example embodiment of a casing according to the present invention, as represented in
A module is 2550 mm in length, and has been manufactured from 316L stainless steel plates 2 mm thick. Two modules forming a casing are welded to delimit a parallelepipede, the external dimensions of which are as follows: length 5,100 mm, width 202 mm and thickness 15 mm. However, it should be noted that the casing may be between 5 mm and 50 mm thick, for example between 10 mm and 25 mm, and may be between 100 mm and 500 mm wide. In the example of
Central rib 30 is 2 mm thick, and is welded to the lower and upper plates. Central rib 6 is initially welded to the lower plate. The upper plate has cut notches into which thickened portions of the rib fit. Rib 30 is then welded to the upper plate by melting the thickened portions.
The central rib comprises vanes obtained by folding after pre-cutting the rib using a laser. The vanes are 30 mm in length; they are positioned every 150 mm and are inclined by 30° relative to axis X of the module. The vanes were produced before the central rib was attached.
We shall now describe the operation of a solar power plant comprising a solar receiver according to the present invention. We shall consider a receiver having a single absorber.
The plant comprises a receiver according to the present invention, mirrors 42 to reflect the solar rays towards the absorber, a system for supplying the receiver with liquid, a system for collecting heated liquid at the outlet of the receiver, and thermodynamic conversion means which comprise, for example, steam turbines, gas turbines, etc.
The solar receiver is suspended above mirrors 42 represented in
Luminous flux F heats lower wall 14.1, particularly if a suitable coating has been applied to it. Since the fluid flowing in the casing of absorber 6 is in contact with the internal face of lower wall 14.1, the latter is heated. On exiting the absorber the heated fluid is conveyed towards the thermodynamic conversion means.
The solar receiver according to the present invention provides improved efficiency compared to receivers of the state of the art, since the luminous flux collector area is increased. Indeed, the surface is flat and continuous, and has no empty spaces. The receivers of the state of the art, conversely, have “gaps” between the pipes. In addition to having a maximum area exposed to the flux, the invention also avoids the need for heat exchanges in the rear of the absorber, as in the case of tubes, where the flux passing in the spaces between tubes heats a surface which then exchanges with the rear face of the tubes. In addition, by virtue of the flat surface, re-emissions in the infrared spectrum are reduced compared to the case of the developed area of tubes of the same width.
The absorber is, furthermore, very efficient in managing varied luminous fluxes on the exposed surface, due to the reblending which takes place within the absorber.
Homogenisation reduces the risks of appearance of hot points, which can appear in zones with very high flux; this homogenisation of the absorber's wall temperature prevents degradation of the thermal oil, which has a limiting film temperature; if this limit is exceeded this causes impaired performance, together with a risk of increased viscosity and the formation of deposits.
It enables a fluid having a uniform temperature to be delivered.
In addition, the lifetime of the surface treatment is increased, since the maximum temperature values are lower.
In addition, the reblending of the heat transfer fluid also allows homogenisation of the temperature of the metal walls, the effect of which is to reduce the deformations and the stresses related to expansion. The lifetime of the absorber, and therefore that of the solar receiver, are increased, since they are subjected to less thermal fatigue, reducing the risks of leakage and failure.
The present invention applies principally to solar receivers of the Fresnel type.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 54067 | May 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP11/58568 | 5/25/2011 | WO | 00 | 11/27/2012 |
