Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
  • F24S20/00—Solar heat collectors specially adapted for particular uses or environments
  • F24S20/20—Solar heat collectors for receiving concentrated solar energy, e.g. receivers for solar power plants
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
  • F24S23/00—Arrangements for concentrating solar-rays for solar heat collectors
  • F24S23/70—Arrangements for concentrating solar-rays for solar heat collectors with reflectors
  • F24S23/79—Arrangements for concentrating solar-rays for solar heat collectors with reflectors with spaced and opposed interacting reflective surfaces
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
  • F24S23/00—Arrangements for concentrating solar-rays for solar heat collectors
  • F24S23/70—Arrangements for concentrating solar-rays for solar heat collectors with reflectors
  • F24S23/81—Arrangements for concentrating solar-rays for solar heat collectors with reflectors flexible
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
  • F24S50/00—Arrangements for controlling solar heat collectors
  • F24S50/20—Arrangements for controlling solar heat collectors for tracking
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S3/00—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic or electromagnetic waves, or particle emission, not having a directional significance, are being received
  • G01S3/78—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic or electromagnetic waves, or particle emission, not having a directional significance, are being received using electromagnetic waves other than radio waves
  • G01S3/782—Systems for determining direction or deviation from predetermined direction
  • G01S3/785—Systems for determining direction or deviation from predetermined direction using adjustment of orientation of directivity characteristics of a detector or detector system to give a desired condition of signal derived from that detector or detector system
  • G01S3/786—Systems for determining direction or deviation from predetermined direction using adjustment of orientation of directivity characteristics of a detector or detector system to give a desired condition of signal derived from that detector or detector system the desired condition being maintained automatically
  • G01S3/7861—Solar tracking systems

Definitions

• determining the error in the orientation may include determining a frequency of changes in the properties of light that forms the optical signal and, based on the frequency and determining that the optical signal was received from a particular one of the multiple retroreflectors. It is also suggested that where the signals are retroreflectors, various techniques can be used to differentiate the signals. For example, it is suggested that the (modulation) frequency of the light reflected by each retroreflector can be different in that the reflected light from the retroreflectors can be optically chopped by placing a rotating partial disk in front of the retroreflector.
• the retroreflectors are comer cube mirrors or prisms where the corner cubes have 90° surfaces and heat resistant material such as ceramic or glass or metal.
• the retroreflectors are suggested to be internally reflective spheres.
• the internally reflective spheres are a multitude of transparent beads of material with a refractive index selected such that incident radiation is internally reflected within the bead and emerges as reflected radiation with a predetermined angular beam spread to the incident radiation (including an angular spread of near 0°).
• the object of the present invention is to provide an improved concentrated solar power facility and methods to operate a concentrated solar power facility.
• a concentrated solar power facility comprising a sun light collector having a sun light collecting area; a plurality of mirrors arrangements for reflecting sun light onto the sun light collector; a plurality of light modulating retroreflectors for reflect- ing modulated light from the sun light collector back to the mirrors; a plurality of light sensors arranged to receive light reflected back to the mirrors, and to output a light detection signal in response to the reception of modulated light reflected back to the mirrors; a light- signal evaluation stage for demodulating the light detection signal and for generating a mirror adjustment signal light signal; is suggested wherein at least a part of the retroreflectors is arranged adjacent to and in front of or within the sun light collecting area and in the optical path of light concentrated from a plurality of mirrors directed towards the sun light collecting area, is made from a material having low absorption for visible sunlight, is preferably at least temporarily shielded from and/or cooled against thermal radiation from the sun light collecting area and is modulating light with marginal or no ab
• the invention allows for a particularly fast and precise control of the mirror alignment. Placing the retroreflectors right in front of the sun light collecting area, which is heated by the incident solar radiation from typically a vast number of mirrors concentrating solar power onto the sunlight collecting area allows to correct for non-ideal alignments even before the spot from a given mirror leaves the sunlight collecting area or, in cases where a temperature is preferred to not exceed a certain limit, more precisely does not leave an assigned area thereof. Accordingly, there is no need to accept a situation where the reflex from a heliostat first has to leave the central or assigned sun collecting area to determine that the mirror has to be adjusted.
• retroreflectors rather than placing the retroreflectors in front of the hot zone, it would also be possible to place the retroreflectors directly into the hot zone, depending on the geometry of the heat collecting device. For example, where a number of fluid-guiding tubes with some degree of spatial separation are used to collect the heat and the array of heliostat mirrors is not very wide, retroreflectors might be placed between such fluid-guiding tubes without the risk of shadowing the retroreflectors by the fluid guiding tubes. However, in most cases, it is preferred to place the retroreflectors in front of, but close to the collector so as to avoid any shadowing.
• Absorption for visible sunlight is considered low if even given the strong irradiation at the sun light collector, the retroreflector will not heat up in a manner compromising the integrity of the optical elements or their mounts. Given this, the acceptable absorption will vary for different concentrated solar power facilities and for different use cases, but generally, it is safe to consider absorption low if the absorption is below 0,05%, preferably below 0,03 % of the radiation between 400 and 1200nm. In the same manner, light modulation with marginal or no absorption does not require that the material used does not absorb light at all at a given wavelength. However, while inherent absorption of material is inevitable, it should not be increased, in particular not periodically for modulation, so that rather than using absorption for modulation, radiation should be redirected for the purpose of modulation.
• any deviations from the ideal can be detected before becoming too large, rapidresponse control signals can be generated.
• the mechanical adjustments necessary to correct the mirror position are also sufficiently fast, additional advantages may be obtained, in particular as a less precise but more frequent adjustment becomes acceptable, reducing the costs of actuators needed for the mirror adjustment and for the overall heliostat construction.
• the increased efficiency also allows to reduce the overall mirror area needed for a given power to be generated, further reducing costs of the concentrated solar power facility.
• the frequent and precise control of the alignment of all mirrors in the concentrated solar power facility according to the invention allows for a less precise initial setup of each single mirror, thus reducing costs.
• the invention in a concentrated solar power facility having a multistage solar radiation concentration arrangement, receiving in a first stage light from the heliostats, and further focusing the light in the second stage onto an area to be heated to particularly high temperatures.
• the retroreflectors might be arranged in front of the first stage and/or between the first and the second stage and then, the light is focused independently on the second stage.
• the overall sun light receiving area is divided into a plurality of different sunlight receiving areas with different target temperatures and/or different heat accepting media
• the overall sun light receiving area is divided into a plurality of different sun light receiving areas
• all different sun light receiving areas are provided with their own retroreflectors and that the modulation of any of the retroreflectors can be dis- tinguished from that of any other retroreflectors.
• This also allows to assign a given heliostat to a different group by simply instructing the corresponding control of the mirror that the mirror is to be aligned to optimize reception of an altered set of modulation patterns.
• the retroreflectors in other areas, in particular in front of other heat collector areas may act as “border” reflectors provided the corresponding sunlight collecting areas are sufficiently close to each other.
• placing some (additional) retroreflectors near the (central) reference region such that the additional retroreflectors are close to, but not directly in front of the sunlight-collecting area may e.g. help to characterize the radiant footprint (which is basically corresponding to the intensity profile, albeit when characterizing the radiant footprint, it may already be sufficient to determine whether light is reflected at all to a given location without simultaneously determining the absolute intensity of light reflected to said given location) of the mirrors or mirror segments in a sub-set.
• intensity information is also easily obtained.
• modulate the retroreflection by rotating the retroreflector as will be explained in more detail below; now, if modulation is effected by rotating a given retroreflector with a given speed, it can be easily checked whether this given retroreflector receives light from a particular mirror if the respective modulation frequency component is found in the light intensity curve detected with the light sensor assigned to the particular mirror. Such check is simple if a fast Fourier transformation is calculated.
• a characterization for each sub-set separately in a serial fashion, e.g. when intentionally directing a mirror onto retroreflectors adjacent to or bordering the actual target area; such characterization need not be part of a daily or continuous routine but helps in setting up the facility.
• each of the plurality of light modulating retroreflectors in a different, distinguishable manner allows to easily distinguish the light reflected back from any single given retroreflector from the light reflected back from other retroreflectors by simply resolving the specific modulation.
• This can be done easily and without expensive hardware e.g. a photodiode by simply detecting a temporal pattern in the retroreflected light; accordingly, only a simple photosensor such as a photodiode capable of generating an output signal modulated in accordance with the modulation of light effected by the retroreflectors is needed.
• At least one light sensor per mirror it is highly preferred and not very expensive to have at least one light sensor per mirror.
• even more than one light sensor per mirror may be provided. This not only creates redundancy, but also allows to determine more precisely any misalignment.
• the light sensors can be placed right in the center of each mirrors, so that light from the entire vicinity of the sensor contributes to the retroreflected light- this is helpful, because for a perfect retroreflector, the retroreflected light would be reflected back onto almost the exact same spot on the mirror whence it came from with only a minute deviation from there due to the parallel displacement caused by the retroreflector.
• a significant part of the data that has to be transmitted for operation of the heliostat array relates to information broadcastable from a central to all or a large number of heliostat mirrors, such as commands to protect the mirror surface against upcoming bad weather conditions and the like.
• the overall communication between a central control and a heliostat may be extremely reduced as the heliostat will keep its alignment by a local closed loop rather than being aligned only in response to commands received from the central control.
• the demodulation and determination of correction signals does not require a particularly high processing effort and can easily be effected on a low power computing device.
• the typical light signal evaluation stage for demodulating the light detection signal and for generating a mirror adjustment signal light signal will may comprise analog signal-conditioning circuitry such as an impedance converter, amplifier and bandpass filter an will furthermore comprise an A/D converter for converting the analog (conditioned) light detection signal into a series of digital values.
• the frequencies used in audio processing will typically be sufficient to provide for oversampling of the analog light detection signals typically modulated with frequencies way below 20kHz; also, the resolution can be as low as 16, 14 or even 12 bit.
• the time series of digital values can then easily be subjected to processing steps such as fast Fourier transformation to check whether certain modulation frequencies or groups of certain modulation frequencies can be found with an amplitude sufficient to warrant detection of a correspondingly modulated light signal - and to derive mirror adjustment signals where necessary. As all this requires little computational effort and little power, and alignment and as calculations are necessary only during daytime so power can be provided using local photovoltaic solar cells. For example, it is possible to use cheap digital signal processors (DSPs) readily available for the processing and signal evaluation.
• DSPs digital signal processors
• actuators will be provided and used to orient the mirrors according to light signals received.
• These actuators for aligning the heliostat mirrors will typically be electromechanical actuators, but basically, any actuator that can be properly controlled and sufficient for the loads will be suitable for the purpose of the present invention.
• the actuators will be capable of moving the mirrors rather rapidly.
• determining corrections of the focal length can be done e.g. in view of the size of a mirror spot which in turn could be determined either by a sufficiently large number of irradiated retroreflectors or by scanning the reflection of the heliostat over a few retroreflectors in a known manner.
• retroreflectors two types may find use for the present invention: the first type is the so-called comer-cube retroreflector (CCR) and relies on three successive reflections of light from three reflective surfaces set at right angles to each other. These may be front-surface mirrors in a so-called hollow-cube retroreflector, or the interior surfaces of a glass prism shaped as a truncated corner of a cube. In the latter case, reflection may occur due to mirror coatings on the outsides of the reflective surfaces, or due to total internal reflection (TIR).
• CCR comer-cube retroreflector
• the second type of retroreflector also know as ‘ cat-eye retroreflector’ (CER) uses a piece of transparent material such as glass with two semi-spherical surfaces, spaced from each other such that light entering through one, first, such surface, and refracted at it, is focused onto the interior side of the other semi- spherical surface, from whence part of the light is reflected due to the refractive-index contrast. This reflected light is then re-collimated on the first semi-spherical surface and returned to its source. Due to spherical aberrations, the returned light is slightly divergent.
• CER cat-eye retroreflector
• the spacing between and the radii of curvature of the two semi- spherical surfaces are related to each other through the refractive index n of the glass or other transparent material.
• n the refractive index
• the cat-eye retroreflector is simply a sphere.
• the mechanical elements associated with the retroreflectors will typically comprise at least mounting elements for positioning the retroreflectors right in front of the sunlight collecting area; where the retroreflectors modulate light by a mechanical movement e.g. a rotation of the retroreflector, further mechanical elements might be necessary to transmit a driving torque, although the retroreflectors could be mounted on a spindle driven by a motor outside the light collecting area.
• Some of the mounting elements might be placed outside the sun light collecting area and thus can be kept at lower temperatures, but what has to be kept in mind is the typical size of the sun light collecting area which in a midsized to large concentrated solar power facility has an extension of e.g. up to about 20m x 20m.
• the (single spot) sun can be easily distinguished from a resolvable multitude of retroreflectors close to each other and spaced apart from the sun.
• an overall orientation can be determined using the DIS.
• the mirrors focus light onto the light collecting area in an adjustable manner and the light signal evaluation stage is adapted to determine a quality of focusing of light onto the light collecting area, in particular in view of the signals from a plurality of retroreflectors.
• the light signal evaluation stage is adapted to determine a quality of focusing of light onto the light collecting area, in particular in view of the signals from a plurality of retroreflectors.
• Another way of obtaining said radiant footprint information relating to a current distribution would be to move the mirror in a manner such that the reflection thereof “scans” over a given area, in particular an area where the density of retroreflectors, in particular a line density orthogonal to the direction of scan movement is higher than in other light reception areas.
• the mirror may be moved towards such region of high retroreflector line density and that therefore, it is preferred to have the area over which the light reflected from the mirror should best be placed within the actual sun light collecting area thus reducing the excursion needed for scanning.
• evaluating the intensity profile of a heliostat reflection are also helpful in determining whether certain mirrors have been damaged, e.g.
• coating is considered highly absorptive if at least 5% preferably 10% and preferably at least 15% improvement over an uncoated surface is achieved.
• the coating should be highly absorptive at visible and near-infrared wavelengths as there, most of the radiant energy is incident. Reducing the amount of the light energy that is reflected instead of being absorbed not only improves the efficiency of the arrangement but also reduces the thermal load on the retroreflectors. Thus, such coating contributes to the potential increase of operating temperatures that become possible due to the better alignment. It should be noted that it is possible to define different temperature zones on the sunlight collecting area and to coat only a part of these zones, e.g. where a zone so hot is needed that coatings are not durable enough. Note that it is in particular possible to provide different heat zones on different sides of a central light collection tower and/or at different heights thereof.
• the retroreflectors are contained in one or more cooled housings having highly light-reflective or highly transmissive, but not absorbant, walls and/or a heat shield between the reflector and the collector to protect the retroreflector from radiant heat due to concentrated sunlight directed towards the collector and/or due to the (infrared) heat radiation from the collector.
• a gaseous cooling fluid is typically preferred as the refractive index of gases or gas mixtures such as air is close to 1 and a total internal reflection of a corresponding retroreflector will not be disturbed.
• the aforementioned slight divergence of retro-reflected light from a cat-eye retroreflector may be advantageous, as it will result in the photosensor receiving retroreflected light not just from the small mirror area in the immediate vicinity of the photosensor, but also from mirror areas more distant from the photosensor.
• the retroreflectors are provided with a defocusing lens arrangement on the entrance/exit window of each retro -reflector.
• the standard comer-cube retroreflectors are rather precise optical elements, up to the degree where the sensing of light is impaired.
• a defocusing lens in particular obtained by suitably polishing a retroreflector entrance/exit window might help to spread the retroreflected light and ensure that detection of the retroreflected light at the heliostat is not impaired by the perfection of the retroreflector cube.
• each retro-reflector or each of a plurality of retro-reflectors should be defocusing to such a degree that it slightly spreads the back-reflection over an area the size of a mirror segment, so that samples of light reflected by any part of the respective mirror segment and back by the retro-reflector reach the photosensor on or by the respective mirror segment.
• Providing a defocusing lens arrangement by suitably polishing the entrance/exit window rather than using a separate element is helpful because the retroreflector arrangement will then be more stable even under extremely adverse operating temperature conditions.
• LCLM liquid-crystal light modulator
• the retroreflector arrangement can in certain instances be retrofitted to existing concentrated solar power facilities in order to obtain a concentrated solar power facility according to the present invention. Therefore, protection is also sought for a retroreflector arrangement adapted for use in a concentrated solar power facility as claimed, the retroreflector arrangement in particular being adapted in certain embodiments for such use in that it comprises a plurality of retroreflectors that are adapted to be arranged adjacent to and in front of the sun light collecting area and in the optical path of light concentrated from a plurality of mirrors directed towards the sun light collecting area, and in that they are made from a material having low absorption for visible sunlight, being provided with a shield for at least temporarily shielding from and/or cooled against thermal radiation from a sun light collecting area of a heat collector of the concentrated solar power facility and in that they are adapted to modulate light with marginal or no absorption.
• an optimal reflection geometry can be maintained where heliostats periodically relocate themselves, so that the angle between the optical axis of each respective mirror and the incoming and out- going sunlight is as small as possible, i.e., where the re- flection geometry is as close as possible to back-reflection.
• optical aberrations are minimized and the cross section of the beam of captured sunlight is maximized.
• the mirrors need to move around to always be on the opposite side of their assigned collectors, relative to the sun. This method makes use of the capability of mirrors to rapidly re-acquire lock after moving.
• heat collectors may also and or alternatively be mounted on mobility platforms to move them into optimal alignment with mirrors.
• the retroreflector arrangement may comprise a rotating or otherwise modulated retroreflector as disclosed above together with suitable mountings, although a person skilled in the art will understand in view of the disclosure in its entirety that other ways to implement a retroreflector arrangement according to the invention exist and can be implemented.
• a light sensor arrangement might be needed which is adapted for such use, in particular in some embodiments in that the light sensor arrangement comprises a plurality of light sensors arranged to receive light reflected back to the mirrors and to output a light detection signal in response to the reception of modulated light reflected back to the mirrors; the light detection signal being adapted to be put into a light signal evaluation stage for demodulating the light detection signal and for generating a mirror adjustment signal light signal.
• a concentrated solar power facility as claimed can be operated in that the reflectivity of the retro-reflectors is based on total internal reflection and is modulated by altering the efficiency of the total internal reflection, in particular by at least one of altering the width of a gap between a total internal reflection -active surface and a matching surface of a piece of a material with a refractive index greater than unity or and/or by altering the refrac- tive index within a gap between a total internal reflection -active surface and a matching surface of a piece of a material with a refractive index greater than unity.
• the retroreflector will at all times either back-reflect to a mirror segment, or not back-reflect at all. This is different from e.g. a lighthouse beam sweeping continuously across the horizon. It is interesting to note that in the transition period between reflecting and nonreflecting, only a part of the retroreflector will participate in retroreflecting incident light.
• Fig.2 mobile heliostats with adjustable focus
• Fig.4d a plurality of placement options of retroreflectors for footprint determination for different forms of heliostat mirror arrays around a central tower;
• Fig. 5 a retro -reflector (here in two dimensions) in different phases PI... P6 of its rotation about an axis perpendicular to the graph plane, where in phases PI... P3, an incident light ray is back-reflected with constant pointing into itself and in phases P4 the incident light is not reflected at all, so that the back-reflection will, in the course of the rotation, be periodically modulated;
• Fig. 10 retroreflector with added interference filter in front thereof;
• Fig. 11 a sketch of a photosensor movable in an eyeball-like manner
• Fig. 12e a detail of the Fig. 12d embodiment.
• retroreflectors might also be arranged directly within the sun light collecting area; these reflectors would still be in the optical path of light concentrated from a plurality of mirrors as they are at the end of such path directed towards the sun light collecting area.
• a mirror unit 101, a mirror unit 102 and a mirror unit 103 each direct light from the sun 104 onto a collector 105 atop the tower 106.
• a typical concentrated solar power facility may comprise hundreds or thousands of such mirrors arranged around the tower.
• each mirror unit, such as mirror unit 103 is mounted on a mobility plat-form 107, which allows free motion 108 on a field 109.
• the mirror units are at least temporarily assigned to specific ones thereof which can be done according to an optimization procedure for the entire facility.
• a maintenance shop 111 or a plurality of maintenance stations is situated on or near the field 109 for mirror units to visit for periodic maintenance or repairs.
• the maintenance shops can also serve as a factory for producing mirror units during buildup of the facility, or for replacement of worn-out or damaged mirror units during operations.
• the mirror units can position themselves as they are mobile, so they can be placed optimally with respect to the sun 104 and with respect to the collector 105 atop the tower to which they are directing the sunlight to, so that the angles of incident and reflected sunlight relative to each mirror axis are minimized, and optical aberrations thus mitigated.
• such mobility is not necessary and mirrors and their alignment devices can be kept at a fixed position in the field.
• the mirrors have to keep directing sunlight onto their assigned collectors, even while moving on the field 109, or at least after moving to a given position.
• the pointing and in certain instances the focusing of each mirror is controlled in a feedback loop which is schematically referred to with reference sign 112.
• the contribution of each mirror unit to the light reaching the respective collector 105 is analyzed or it is at least determined whether the contribution needs to be amended by realignment and hence, corrective information is provided by the feedback loop 112 to the respective mirror unit 101 so as to allow for adjustment of its pointing and, where applicable, its focusing.
• the feedback loop is active at least during a part of the day to periodically correct the motion of the otherwise free-running actuators and in certain instances, other controls on the mirror units.
• the light is labeled by way of modulation using retroreflecting elements ; such modulation may refer to wavelength- spectral features, temporal modulation of the intensity or a combination of both; in particularly simple implementations, only a temporal modulation is provided.
• an infrared-reflecting, yet shorter- wavelength-absorbing coating may be applied to the collector 105 which, by the Einstein coefficients of thermal absorption and emission, proportionally reduces the infrared emission.
• This coating has two purposes: The first is to reduce thermal emission losses, i.e., better utilizing the collected radiant power reaching the col- lectors in the shorter-wavelength, i.e., mainly visible, parts of the spectrum.
• the second purpose is relevant for implementations of the invention where the suppression of thermal emission reduces the background and thermal load on the retroreflectors as will be understood hereinafter.
• Coatings that preferentially enhance infrared reflection, while maintaining a high degree of shorter-wavelength absorption have been developed for multiple purposes, including architectural ones. The latter offer robustness in open-air operation, which is relevant for solar-energy collection.
• FIG. 2 shows a schematic cross section of one mirror unit with a mirror 201 mounted on a support structure 202 which, to implement the mobility platform, may move autonomously or may be moved by a drive unit temporarily attached.
• the mirrors may be directed by pointing actuators 203.
• the reflective side of the mirror 201 i.e., the side on which the sunlight is incident 204 and from which it is reflected 205, may be protected from the environment by an enclosure 206.
• the opposite side of the mirror is a ‘non-reflective’ side.
• the mirror in the embodiment shown is a focusing mirror 201 having adjustable focus and the focusing properties may be controlled in several ways, examples of which are given here. These examples are meant for illustrative purposes only and are not to be understood to constrain the scope of the preferred embodiment relating to mirrors allowing for focal adjustment. Such preferred embodiments may encompass any means of adjusting the focusing properties of the mirror 201.
• One type of focusing adjusters comprises motor-driven screw actuators 207.
• Another way of adjusting the focal properties of the mirror relies on flexing of the mirror and is based on differential pressure between the inside of the enclosure 206, i.e., on the reflective side of the mirror 201 and a different overall pressure on the non-reflective side, causing the mirror to flex.
• More detailed control of the mirror shape can be obtained with several independently pressure-controlled gas-filled bags 208 pushing on the non-reflective side.
• Yet another way of Controlling the focal strength of the mirror unit is based on control of the gas pressure, and thus the refractive index inside the enclosure 206 in conjunction with a curved, for example hemispherical shape of a dome 209 on the outward-facing side of the mirror unit. This refractive effect is independent of focusing due to pressure-induced flexing of the mirror and may be used additionally or alternatively to it.
• the preferred embodiment may include a variant where the mirror is entirely contained inside the pressurized volume, so that there is no flexing, and where all of the focusing power is due to the refractive effect on the dome 209.
• An alternative Option is to place a gas volume with curved surfaces, such as a balloon 210 inside the enclosure 206, which then acts as a gas lens with a pressure-controlled refractive index.
• the spherical shape of the gas lens is meant as an example only, and the embodiment shall comprise any gas volume with curved surfaces used as a variable lens through control of the pressure internal to it.
• a good way of achieving high and variable densities for a strong refractive effect is to use a gas with a critical point near ambient conditions.
• a mirror unit Other components in a mirror unit include: the mobility platform 211 on wheels 212 or other means of providing mobility, a control computer 213, local power generation and storage, for example with a small photovoltaic module 214 and batteries 215, wireless Communications 216 with a local- area network for the entire facility, or wireless Communications 217 with local antennas built into the facility platform to enable more targeted and secure Communications.
• Fig. 4a two retroreflectors are shown right in front of the heat collecting area and thus within the path of concentrated sun light, receiving light from at least 10, preferably at least 50 and typically more than 100 mirrors in operation and hence irradiation densities of more than 10kW/m2, typically even more than 50kW/m2 and preferably even above 100kW/m2.
• the retroreflector will be so close to the heat collecting area that at least for a large number of mirrors, retroreflected light is only received if the mirror also directs its light onto the heat collecting area.
• the reflector Given the sometimes oblique angle of incident, this requires the reflector to be not more than 10m in front of the heat collector front surface, typically less then 5m in front of the heat collector front surface and typically at a distance of no more than 2m from the heat collector front surface. It is advantageous to have the retroreflectors as close to the heat collector front surface as possible, so typically, distance may be as low as Im or below. Note that space must be allowed for a housing, drive units and so forth, imposing a minimum gap requirement. In more detail, FIG.
• a particularly preferred way of footprint determination is to scan the mirror across the heat collector by altering the alignment angles and to determine for a plurality of scanning angles the retro-reflected intensities from the retroreflectors already there.
• This method might require that some mirrors in an array undergoing such a footprint check might direct all or a major part of their reflected light to an area different from e.g. a central target zone.
• focusing mirrors will typically have some sort of optical imperfection such as spherical aberrations and such imperfections will vary with the angle of incident light- however, if a specific incident angle is needed at a specific time so that the heliostat mirror directs its reflection into a target zone, the scanning method suggested necessitates that the alignment angle and hence also the angle of incidence is changed, resulting in an altered spherical aberration pattern. Nonetheless, such changes to the spherical aberration pattern or other errors due to a scanning motion will typically be negligible compared to the overall imperfections of a heliostat mirror, even if facetted.
• the radiant footprint will move, 448, across the collector area and, specifically, across the retroreflectors contained therein, such as the one 449, shown in the figure.
• this is equivalent to scanning the retroreflector 449 across the footprint 443, as both will provide an intensity profile of said radiant footprint in the form of the retroreflected light in dependence on the scan parameters, i.e., angles 446 and 447.
• footprint pattern determination it is also noted that several options exist to place heliostat mirrors around a central tower and that different ways of retroreflector placement can be used for footprint determination. Fig.
• FIG. 4d depicts several of these options. Shown on the left side of FIG. 4d is the placement of one or more retroreflectors in a reference region 461 below the collector 462, both on a tower 463; a batch of mirrors 464 steers sunlight onto the reference region 461 where one or more retroreflectors 465 are situated. Meanwhile the other mirrors reflect light onto the collector 462 (where additional retroreflectors for keeping the reflected light in the central hot zone may be arranged in front of the sun light collecting area as described in this document). In the center of Fig.
• retroreflectors (with only one, 466, being shown) are arranged in front of the collector and rays from all mirrors are directed towards the collector while alignment is characterized with only such retroreflectors - this will be sufficient for a sufficiently large number of retroreflectors in front of the sunlight collecting area, e.g. because different temperature zones need to be defined within the sunlight collecting area or because the number of mirrors is so large that overheating of the heat collector structure by inadvertently directing light reflections of too many mirrors onto the same target area must be avoided.
• retroreflectors are arranged in front of the collector and rays from all mirrors are directed towards the collector while alignment is characterized with only such retroreflectors - this will be sufficient for a sufficiently large number of retroreflectors in front of the sunlight collecting area, e.g. because different temperature zones need to be defined within the sunlight collecting area or because the number of mirrors is so large that overheating of the heat collector structure by inadvertently directing light reflection
• the retroreflectors When performing the footprint characterization simultaneously for all heliostats of the facility with retroreflectors in front of the sunlight collector itself, the retroreflectors obviously need to withstand the full heat load due to the incident sunlight, but the present invention teaches how this can be achieved.
• Such a configuration allows continuous simultaneous feedback control of heliostat and may be advantageous if a sufficient number of retroreflectors can be provided.
• the first two options could be implemented with a single retroreflector being scanned across the pertinent area, or with several retroreflectors whose back-reflections that are disambiguated by intensity modulation or other light- labeling techniques.
• the second option requires modulation even with a single retroreflector in order to make the back-reflection stand out against the bright scattered sunlight from the collector.
• the third option inherently uses several retroreflectors. Due to the limited angular range of heliostat placement, it is most suitable to locations within a valley or other geographic constraint oriented roughly in the north- south direction.
• All three options allow characterization and correction of heliostats footprints under operating conditions, using the sunlight itself.
• heliostat figure (footprint) inaccuracies show large diurnal variations, e.g. due to diurnal thermal cycling, it might be advantageous to characterize mirrors repeatedly at different times of the day.
• all heliostats in a batch can be characterized within about a minute, i.e., the time required to steer the heliostats by a few milliradians to the reference region, acquire samples of the footprint of each mirror segment of each heliostat simultaneously for all mirror segments and in simultaneously shapes of the footprint, and then steer back to the collector.
• the 10000 heliostats of a typical facility can then be processed within less than 2 hours, i.e., allowing characterization multiple times a day.
• the first option may take a somewhat longer to allow for scanning the retroreflector across the reference region.
• the second option with several retroreflectors works continuously on all heliostats; depending on the details of the modulation scheme used for disambiguation, reaction times can be less than a second to about a minute, thus allowing for inexpensive heliostat mounts and actuators with limited mechanical stability.
• the response is also almost instantaneous, but providing less or less precise detail than the second option. Nonetheless, it still allows the use of rather inexpensive mounts and actuators and still obtain both good alignment and footprint control.
• the determination of the radiant footprint can be used to improve the operation of the concentrated solar power facility in a variety of ways. If suitable actors are provided as per se known e.g. from large astronomical telescopes, warping of the mirrors could be corrected and could be corrected locally. However, this requires additional controlled actuators and might not be preferred due to the additional hardware needed. Another possibility would be to determine the radiant footprint for different positions and use any given mirror to reflect light into that part of the sunlight collecting area where the “best” - i.e. smallest- radiant footprint would be obtained. Then, a check could be made whether the quality e.g.
• the size of the radiant footprint of a given is significantly worse than the radiant footprint obtained by other mirrors, which might indicate that the specific mirror is particularly warped and might be damaged. Furthermore, as the radiant footprints obtained from different heliostat mirrors are expected to show noticeable variations from heliostat mirror to heliostat mirror, efforts could be taken to optimize not the specific radiant footprint of each single heliostat mirror but rather to obtain an overall irradiation pattern of the solar light collecting (hot) zone. To this end, once a mirror has finished determination of its radiant footprint, the pattern observed can be transmitted to a central control center common to all mirrors or all mirrors of a group of mirrors.
• FIG. 5 shows one possibility of modulating the reflectivity from a corner-cube retro- reflector, namely by rotating it. This is a modulation method particularly easy to implement.
• a retro- reflector for the sake of graphical simplicity in two dimensions, is shown in different phases during one turn of its rotation, namely phase Pl 01 to phase P6 502 , and again in phase Pl 303.
• An incident ray 504 is returned in a constant direction opposite to the incident light as a back-reflected ray 505, but only during rotation phases PI... P3 when it hits the reflective side of the retro -reflector, i.e., within the legs of the ‘L’ denoting a retro-reflector.
• P4... P6 in the graphic the incident light does not hit a reflective surface, and is thus not returned to the respective mirror segment. Under continuous rotation, this results in a periodic on-off modulation of the light back-reflected from the retroreflectors.
• this modulation occurs for light incident from all directions onto the retroreflector. Accordingly, it is possible to use the same retroreflector for a plurality of different mirrors. This is shown by way of example in Fig. 6, although it will be understood that in practice, a very large number of heliostat mirrors (rather than only 2 as depicted in Fig 6) will irradiate the solar light collecting area. It will also be understood that Fig. 6 is for explanatory purposes only, namely for showing that a retroreflector properly positioned as suggested is useful for controlling alignment of many, typically all heliostat mirrors.
• a plurality of retroreflectors as shown will be arranged in the path of concentrated sunlight, in front of and adjacent to the sun light collecting area heat as described shall be used for the purpose of this invention.
• Fig. 6 shows light from the sun 61 as a first incident ray 62 and a second incident ray 63 reflected respectively by a first mirror 604 and a second mirror 605 towards the sunlight collector 606.
• a retro-reflector 607 is placed on a support 608 in front of the solar light collector 606 and within the region illuminated by all heliostat mirrors of the facility. Accordingly, the retro -reflector 607 is arranged in the optical path of light concentrated from a plurality of mirrors directed towards the sun light collecting area.
• the distance between the surface of the sun light collecting area - as for example defined by the closest part of tubes guiding fluid to be heated by the concentrated sunlight is a small compared to the linear extension of the sun light collecting area in a given direction.
• the distance between the closest part of tubes guiding fluid or other physical surface will be less than 25%, typically less than 10% of the distance and preferably less than 5% of the linear extension of the sunlight collection area.
• the retroreflectors will be spaced from the sunlight collection area no more than 5m, preferably less, e.g. 2m or even only Im.
• Fig. 7 relates to the reception of light from a plurality of retroreflectors each modulating the respective retroreflected light with a different modulation frequency.
• a plurality of retroreflectors each modulating the respective retroreflected light with a different modulation frequency.
• the light retroreflected by the different retroreflectors will be superimposed on the receiver of that heliostat.
• the intensity coming from this direction is constant; it is however different from zero, as background light from the area around the retroreflector, which area might be brightly illuminated by other heliostat mirrors, is received at the light receiver 72.
• retroreflected light is superimposed resulting in an intensity 711 curve over time as depicted in 713.
• the corresponding light reception signal can be conditioned such as impedance converted, amplified and bandpass-filtered and digitized. Thereafter, a fast Fourier transform (FFT) can be effected as shown by 714, and the respective modulation components 715, 716, and so on, can be identified thus identifying from which retroreflector light is received. From this, a correction signal for re-aligning the heliostat mirror can be easily determined, keeping the heliostat mirror reflection in a desired target zone.
• FFT fast Fourier transform
• a set of mutually orthogonal binary sequences could be employed, for example pseudorandom sequences, similarly to the CDMA technique widely used in digital radio communications such as cell phones.
• BINS mutually orthogonal binary sequences
• the BINS are preferably chosen such that, while satisfying the orthogonality requirement, they optimize use of the modulation-frequency spectrum to minimize interference from other influences on the intensity on the collector, such as due to atmospheric turbulence, mirror vibrations, etc.
• a scheme of an arrangement is shown in Fig. 9.
• Fig. 9 closely corresponds to the arrangement shown in Fig.
• Another way to introduce a wavelength dependency would be to place interference filters 1001, 1002 in front of the retroreflector or in front of a part of the entrance window into a retroreflector 1003 as shown in Fig. 10.
• any glass used for the retroreflectors or a housing therof might be uncoated, e,g. uncoated quartz glass, or might only be coated to block thermal radiation in the far IR range.
• the photosensors will typically have optics to image the reference region and/or the entire sunlight collecting area and/or parts thereof. If the photosensors are placed on a heliostat mirror - as is preferred- and is oriented such that it receives light from in a direction perpendicular to the heliostat mirror surface, it will not “look” into the direction from which the retroreflection is received if the sun is incident on the heliostat mirror with a non-perpendicular an- gle. There are several ways to receive retroreflected light at the photosensors nonetheless. The first is to provide the photosensor with a wide angle- entrance optic so that light is received from any angle or at least any relevant angle without additional measures. To this end, a wide angle lens such as a fish eye lens can be placed in front the photosensor or no lens could be provided at all as long as the photosensor is sufficiently sensitive for light incident under an oblique angle.
• a wide angle lens such as a fish eye lens can be placed in front the photosensor or no lens could be provided at all
• Such counter- movement would need to be accurate within the angle subtended by the collector as seen from a heliostat, i.e., several milli -radians if a very narrow field of view is provided by a photosensor entrance optics, but the precision can be significantly relaxed if the field of view of the photosensor is made somewhat wider, even when not providing for a wide- angle view.
• the accuracy need not be as high as that of the actuators moving the photosensor assembly, as the signal from the retroreflectors, particularly in view of the signal modulation, will stand out clearly enough to discriminate against stray light from the collector or a much darker (but still bright) background sky, even when cloudless.
• the motion will be rather slow, of the order of radians per day.
• a possible low-cost approach is to use piezo motors similar to those in autofocusing camera lenses, but with much lower performance requirements.
• a corresponding design example is shown in Fig. 1 la where a spherical body 1101 (photo) containing a lens 1102 opposite from a photodiode 1103 and other electronics sits like an eyeball inside a socket 1104, imaging 1105 the tower 1106 with sunlight collector onto the photodiode. It is driven in two- dimensional angular motion by the aforementioned piezo motor 1107, such as a stick-slip device, ultrasonic motor, or a piezo- walker. These devices are usually designed for one-dimensional motion, but can, in principle, also be made for two dimensions.
• a photodiode array here represented by photodiodes 1021, 1022, and 1023, but typically made up of many more photodiodes, that is fixed relative to its heliostat frame
• a lens 1024 is placed such that an image 1025 of the solar light collecting area 1026, is projected onto one or a few of the diodes, here photodiode 1022 of the photodiode array.
• the photosensors each comprise an optical tube 1041 with a lens 1042 on one end and a photodiode 1043 on the other end, preferably directly connected to associated electronics 1044, cmp. Fig. 11c and Fig. l id.
• a plurality of such photosensors such as the one referenced as 1045 in Fig. 11c, are each attached by one end to wires 1046 running in one direction, and by their respective other ends to wires 1047 running in another direction, typically perpendiclar to that of the wires 1046.
• These sets of wires 1046 and 1047 are strung on a frame 1048 which contains actuators to collectively pull along wire sets 1046 and 1047, thus inducing a collective tip-tilt motion on all photosensor tubes 1045 attached by their ends to the wires.
• a frame 1048 is then mounted to each heliostat, the angular motion of which is compensated by the tip-tilt motion of the photosensors, so that they maintain alignment towards the sunlight collector.
• the rotation of the retroreflector can be effected by an electrical motor 121.
• an electrical motor driving a spindle 122 which in turn carries the retroreflector (not shown in Fig. 12a, see 1210 in Fig. 12d) is provided along with bearings 123, 124 holding the spindle in a desired, here upright orientation.
• the housing is made of quartz glass tubes. Also, the housing can be placed on support struts 1211 also made of quartz glass (see Fig. 12c).
• the retroreflector is placed on an air bearing and driven by the air streaming through said bearing and /or quartz glass made fans. In this manner, all relevant parts for the driving mechanism that need to be subjected to the high radiation intensities can be made of low absorbing material.

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