This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2018/055436, filed on Mar. 6, 2018, and claims benefit to European Patent Application No. EP 17163689.7, filed on Mar. 29, 2017. The International Application was published in English on Oct. 4, 2018 as WO/2018/177696 under PCT Article 21(2).
The present invention relates to the technical field of concentrated solar power (CSP) plants or systems. In particular the invention relates to the thermo-mechanical monitoring of a molten salt solar receiver (MSSR) for CSP plants with central tower.
The solution provided by the present invention could be used in any application where thermo-mechanical monitoring is needed.
In the frame of the present invention, the external solar receiver 3 for a CSP tower plant 1 (
The solar receiver panels 30 are made of straight welded tubes 6 (
A supporting system is welded to the back area of the tube at different positions along the tube length. The supporting system allows horizontal and vertical expansion and prevents a bending of the tubes due to a high thermal difference between the front and back faces of each tube.
Thermal monitoring of solar receivers used in CSP tower technology is performed by IR cameras allowing the capture of a thermal image of the receiver surface as already known in other domains like manufacturing process with control and detection of localized over-heating points.
Among the patents on the manufacturing of IR cameras, the following patents can be cited:
Among the patents related to the use of such cameras in manufacturing processes Ircon patent WO 2004/020926 A1 can be cited, related to temperature control during metal melting.
In the solar domain, the following patents related to the heliostat control can be cited: SolarReserve (US 2013/0104963 A1), Brightsource (U.S. Pat. No. 8,360,051 B2, U.S. Pat. No. 9,222,702 B2 and US 2013/0139804 A1), eSolar (WO2010/017415 A2).
Concerning the thermal monitoring of solar receiver panels in CSP tower technology, only two relevant patents have been identified. The first one is the SolarReserve patent filed in 2004 (US 2004/0086021 A1). In this patent, the solution proposed consists in capturing a thermal image of the solar receiver thanks to IR cameras. These IR cameras are located at a distance from the solar receiver and positioned on the ground or at a specific height level. The connection of IR cameras could be a wireless connection. The second one was filed by Brightsource in 2009 (U.S. Pat. No. 8,931,475 B2). This patent is an improvement of that proposed by SolarReserve.
In this latter patent, Brightsource proposes a system and method for directly monitoring the energy flux of a solar receiver, comprising:
Based on patent analysis, the above-mentioned Brightsource patent seems to be the most relevant and complete patent in this domain. However, the proposed solution is exclusively limited to the thermal monitoring of the solar receiver panel tubes. This solution is not capable to:
In summary, the prior art solutions are not satisfying as they are only limited to thermal monitoring of the solar receiver. In fact, there is no specific system and method able to provide the tube temperature and a thermomechanical deformation analysis and for further allowing safe operation condition of the solar receiver.
In an embodiment, the present invention provides a concentrated solar power (CSP) plant, comprising: a plurality of heliostats or a heliostat field; a substantially cylindrical solar energy receiver located atop a central tower and having an external surface covered with receiver panels and a heat shield adjacent the solar receiver, the heliostats being configured to reflect solar energy to the external surface of the receiver, each receiver panel comprising a plurality of heat exchanger tubes configured to transport a heat transfer fluid, which are partly exposed on the external surface of the receiver; a thermo-mechanical monitoring system configured to ensure integrity of the solar receiver panel tubes in operation, the thermomechanical monitoring system comprising at least: a plurality of thermal imaging devices located on ground and mounted each on a securing and orienting device, configured to measure infrared radiation emitted by the external surface of the receiver and to provide a panel temperature-dependent signal in an area of the external surface; for each thermal imaging device, a reference area of interest RAOI located on the heat shield arranged opposite its thermal imaging device and containing one or more temperature sensors configured to measure a reference temperature corresponding to the RAOI, one or more flowmeters configured to measure a flow rate of heat-transfer fluid in the heat exchanger tubes and temperature sensors arranged respectively at inlets and outlets of the exchanger tubes, the measurements provided by the flowmeters and the temperature sensors being intended for calculating an energy balance absorbed by the heat transfer fluid in the receiver as well as mechanical strains experienced by the exchanger tubes; and a data processing system configured to calculate and/or supply respectively a maximum temperature, a temperature profile, and/or an absorbed power profile in each heat exchanger tube and theoretical mechanical strains assigned to each heat exchanger tube as a function of a temperature provided by the imaging devices, taking into account a reference temperature of the RAOI and a temperature of the heat transfer fluid at the inlets and the outlets of the tubes, in order to control if an operating point of an area located on the solar receiver is within an operating envelope in a 2D-space theoretical strain/Tmax defining predefined temperature and strain thresholds and in order to emit alerts upon exceeding predefined temperature and strain thresholds, while being outside the envelope and further to require heliostat radiation defocusing on the area, the data processing system comprising first control means configured to control IR cameras, second control means configured to control panel tube integrity in operation, and a distributed control system DCS connected to the first and second control means, to flowmeters, tube temperature sensors, and RAOI reference temperature sensors, the DCS being configured to communicate the measured reference temperature of the RAOI to the first control means, the measured flow rate and inlet/outlet temperatures of the heat transfer fluid to the first control means and/or second control means, and the DCS being configured to receive from the second control means local panel defocusing information for defocusing heliostat radiation on one or more panels, wherein the second control means configured to control panel tube integrity in operation comprise means configured to communicate with the first means and for: calculating a corrected maximum temperature on each exchanger tube by introducing a mathematical correction based on temperatures measured by the IR camera, inlet and outlet panel molten salt temperatures, and molten salt flow in the considered panel; with a dedicated mathematical creep-fatigue model, calculating theoretical strain level in the tubes; comparing a positioning of points defined by the calculated corrected maximum temperature and the calculated strain level within allowed solar receiver operating envelope; and generating an alarm and, according to the actual error level, requesting heliostat defocusing and/or shutdown, when a positioning of the calculated corrected maximum temperature and/or the calculated strain level is outside the operating envelope.
The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. Other features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:
In an embodiment, the present invention provides a solution for ensuring the integrity of the solar receiver panel tubes, by avoiding excessive thermo-mechanical loading on the tubes which compose the receiver surface, when receiving very high thermal flux level.
In an embodiment, the present invention achieves a thermo-mechanical monitoring which will ensure the integrity of the solar receiver panel tubes and being able to generate alarms in case of going outside an operating envelope, a maximum temperature being defined per panel, and defocusing of heliostats based on software calculation, the system acting on the heliostat field either by a local defocusing request or a total defocusing request, or being completely shut down if necessary.
In an embodiment, the present invention provides a complete scope with a simplified guarantee of the whole installation, and thus to propose a similar scope of liability as main competitors.
A first aspect of the present invention relates to a concentrated solar power (CSP) plant comprising a plurality of heliostats or heliostat field, a substantially cylindrical solar energy receiver, preferably a molten salt solar receiver (MSSR), located atop a central tower and having an external surface covered with receiver panels and a heat shield adjacent the solar receiver, the heliostats reflecting solar energy to said external surface of the receiver, each receiver panel comprising a plurality of heat exchanger tubes for transporting a heat transfer fluid, which are partly exposed on the external surface of the receiver and comprising a thermo-mechanical monitoring system to ensure the integrity of the solar receiver panel tubes in operation, said thermomechanical monitoring system comprising at least:
According to preferred embodiments, the concentrated solar power (CSP) plant according to the invention further comprises one of the following features, or a suitable combination thereof:
A second aspect of the present invention relates to the use of the concentrated solar power (CSP) plant as described above, for optimizing the solar power received by the solar receiver and/or the used solar receiver surface.
A third aspect of the present invention relates to a method for thermo-mechanically monitoring a solar energy receiver of a concentrated solar power (CSP) plant, as described above, in order to ensure the integrity of the solar receiver panel tubes and/or to optimize the power loading of the solar receiver, comprising at least the following steps:
Regarding the high thermal loading and the high level of strain of the solar receiver tubes 6, a thermo-mechanical monitoring of the latter is very important for ensuring integrity of the solar receiver surface. An indirect measurement is the only possible solution therefor, opposite to any direct measurement (thermocouple, flux sensor, strain gauge, etc.).
A dedicated thermo-mechanical monitoring system and method are proposed in the present invention to:
The thermo-mechanical monitoring system is based on infrared (IR) cameras 7, 7A, 7B, etc. associated with a dedicated lens installed in a specific supporting system 8 (
Further, a Reference Area of Interest (RAOI) 12 having temperature transmitters or thermocouples 13 (
The thermo-mechanical monitoring method is also based on so-called IR Camera Software (from IR camera supplier) and so-called CMI Software (from applicant) for data analysis and on calculation with an adapted communication protocol between these softwares, the DCS 40 (for Distributed Control System) and the monitoring system (
The following listed points describe criteria example that should be taken into account to set up the main guideline for selecting the camera and associated lens, its position in the solar field, the number of cameras including the camera enclosure (e.g. to ensure a thermal protection of the camera):
Hardware
The proposed solution is consisting of a system and method to ensure the thermo-mechanical monitoring of a solar receiver.
According to preferred embodiments, the above system is composed of:
1. IR Cameras with Specific Lenses
Eight (8) infrared (IR) cameras, each with a dedicated lens, are located in the heliostat field at ground level and distributed approximately at every 45° of angle spacing around the tower to get a global image of the solar receiver panel surface. Every panel is in the Field Of View (FOV) of two cameras (
This IR camera solution with dedicated lens is preferred to IR cameras with higher resolution to ensure a reasonable budget and also to ensure the adequate Field Of View (FOV) and avoid to see the sky (i.e. risk to see the sun).
The angle between the surface to monitor and the lens axis is recommended to be included between 45° and 90°, the optimum being 90° (but not possible in the current application). An angle smaller than 45° will imply that the thermal information received by the camera starts to be influenced by the “sky temperature” (reflection on the solar receiver surface) and then an accuracy degradation is foreseen.
For one pixel of the sensitive area, the IR camera gives an average of the energy emitted by the corresponding surface on the solar receiver surface and then a temperature average.
The distance between the IR camera and the solar receiver surface impacts the precision/accuracy of the measurement due to the air pollution: dust, humidity, CO2, etc., which justifies the use of RAOI to correct the IR camera measurement.
2. A Dedicated System to Support and Protect the Set of IR Camera and Associated Lens
As represented in
3. RAOI (Reference Area of Interest)
The RAOI is a temperature reference zone for IR camera calibration installed on the thermal shield 2. A distinct RAOI zone is associated with each camera. The temperature reference is given by thermocouples, e.g. 5 thermocouples installed on each RAOI, which will provide temperature data to be compared to IR camera measurements. Calibration of IR cameras will be done by adjusting transmissivity and/or emissivity parameters. Each RAOI has to be located in front of each IR camera. Each RAOI will be similar to the tube surface in terms of material and coating. The RAOI being located on the thermal shield, it is submitted to temperature values equivalent to that of the tubes (500 to 700° C.).
4. IR Camera Software (Provided by IR Camera Supplier)
This is a specialized software for permanent infrared hotspot detection on a solar receiver with defined areas of interest (AOI fields) having, among others, the following features:
The IR camera software will:
5. CMI Software
This is a software developed by the applicant in view of ensuring the solar receiver integrity, and which is complementary to the software from IR camera supplier.
Owing to the resolution of the IR camera (1 pixel=21 mm2) and keeping in mind that the diameter of one tube is 50.8 mm, IR camera software is not able to detect the maximum temperature on the tube temperature profile, but only an average temperature.
The temperature or absorbed power profile on the tube exposed to the sun rays is a sinusoidal profile 14, 15 (
An improvement according to the present invention in that respect is to introduce a mathematical correction to calculate the maximum local temperature on the tube. This correction is based on:
As the sole knowledge of the maximum temperature on the tube is not enough to ensure the integrity of the receiver, the applicant developed a mathematical model (so-called CMI software) able to calculate theoretical strain level in the tubes. The evaluation of both temperature and strain level in the tubes will allow ensuring the integrity of the receiver by comparison to the solar receiver operating envelope 16 that has been validated by a lifetime analysis of the tube under several thermo-mechanical loadings (fatigue-creep evaluation) (
Whenever the defined limits are exceeded, CMI software will generate an alarm and will require an area defocusing (
CMI software is therefore an add-in to IR supplier software which is installed on same master/slave computers, PC143 and PC244 respectively.
6. DCS (Customer) or Distributed Control System of the Plant (
In a DCS, a hierarchy of controllers is connected by communications networks for command and monitoring. Two registers for storing for example a 4 bytes number are used (may be adapted depending on DCS data structure).
IR camera software needs address from DCS to get temperature measurement given by the thermocouples installed on the RAOI.
CMI software also needs address from DCS 40 to get inlet and outlet salt temperature for each panel. A number of registers are necessary respectively for the inlet and outlet salt temperature. Information in these registers is read from DCS 40 by the CMI software in PC143 and PC244. Moreover, CMI software needs mass flow measurement given by flow transmitters of the two solar receiver circuits to DCS 40.
CMI software also needs to be able to write defocus information in DCS registers such as the power decrease rate and the localization of the critical area.
CMI software also needs registers information of DCS 40 to be able to write therein the alarm status. Each of this information needs 1 bit (0 or 1) to be written in the registers.
In normal operation, CMI software in the PC master 43 (PC1) writes these data in the DCS 40 while the PC slave 44 is in standby. PC slave 44 will overtake PC master's role in case of failure of PC master 43 or failure of the IR software in the PC master 43 (
IR supplier software needs register address of DCS to be able to write information such as the min/max/average temperature with its coordinates per AOI and the min/max temperature of all AOI per panel.
IR camera software from the PC master 43 writes the above data into and from the DCS 40 registers. If this PC fails, PC244 will write this data instead of PC1.
7. Computers PC1 and PC2 (
These computers are necessary to post-treat and manage the data of the 8 IR cameras. Each PC 43, 44 respectively post-treats raw data of 4 cameras to ensure the redundancy.
IR camera post-treated data exchange, between PC143 and PC244, is only done through IR software of each PC. Both PC communicate data to each other from their 4 respective IR cameras.
CMI software is running on PC143 and PC244 individually and treats the data of the 8 IR cameras coming from the IR software through the DDL 45, respectively located in the same PC.
IR camera software and CMI software on PC1 (PC master 43) write and read information into and from DCS 40 respectively.
8. HTF Temperature Transmitters
Temperature transmitters are available for the 16 panels. At their inlet and outlets, for each measurement point, 3 thermocouples are available and the rule “2 out of 3” is applied, so ensuring a reliable temperature measurement of the heat transfer fluid (molten salt).
9. Flow Transmitters
Flow transmitters are available for the 2 circuits. Moreover a certain redundancy also exists thanks to the flow meter design that includes two pairs of ultrasonic probes (intrinsic redundancy).
Functional Aspects (Software)
To ensure the integrity of the molten salt solar receiver of a CSP tower plant, a thermo-mechanical monitoring method is proposed based on a mathematical model. The mathematical model is based on a thermal balance applied to a discretized tube.
A tube of the solar receiver panel is discretized. The position of these segments are identified by the angle α ranging from 0 to 180° (
The thermal balance takes into consideration the following heat transfer and mode of transfers:
The thermal balance of the tube can be constructed and used according to a direct method or an indirect method. The direct method and indirect method mainly used the same classical thermal equations but not in the same order, and do not use the same inputs and do not generate the same outputs.
The classical thermal equations are based on the heat transfer theory and used also some empirical equation based on VDI Heat Atlas, Second Edition, Part G1, Section 4.1.
The originality of the software developed by the applicant is based on the possibility:
The two following sections will define more precisely the originality of the CMI software with the direct and indirect methods.
1. Direct Method of CMI Software
The direct method is mainly used to define and calculate:
The direct method is based on the following architecture and steps, depicted in Table 1, which is shown in
In the direct method, the procedure is:
As it is shown in
Every Camera is Dedicated to Four Panels
The calculation will be performed in parallel for two configurations:
The obtained results depend on the following conditions:
From the above-mentioned simplified equation, a value of Tcamera,tube
Another important point to notice is that the camera temperatures have to be considered at the “cell level” or the discretization level as defined above (and not pixel level or tube level) and have to be representative of the radiative transfer from the tube to the IR camera taking into consideration the Tcamera at pixel level.
To simplify the relations, taking into consideration cell discretization, small variation of thermal flux from one cell to another one, and then from one tube to the others, and both configurations (see
So we can define Tcamera at cell level that respects the global power balance between the cell and the camera. Tcamera at cell level can be evaluated using the same discretization method as previously but integrated at the cell level and taking into consideration the Tcamera at pixel level coming directly from the camera.
Finally, in the (x, y) axes, the above-mentioned relation can be written at the cell level:
where, Tcamera,pixel_i,j are the pixel temperatures in the considered cell.
As far as evaluation of A and B coefficients is considered, a linear least squares method is used on the temperature values found with the direct method:
The maximum external wall temperature of the tube is a very important parameter in order to perform thermo-mechanical monitoring and can be evaluated using A and B coefficients coming from the direct method (see above):
To,max
The external wall temperature on a half tube exposed to the normal solar flux are following a sinusoidal profile with a maximum on the tube crest:
To
2. Indirect Method of CMI Software
Based on the camera temperature map, the panel in/out salt temperatures, the salt mass flow, A and B coefficients, and the operating envelop, indirect method is used to
To be able to make this comparison, the following parameters have to be calculated:
The indirect method used in the CMI software is based on the architecture and steps depicted in Table 2, which is shown in
Step 5 is calculated, cell by cell, several times until convergence of absorbed power. Then, based on this power value, the direct method (step 6) can be used.
When a panel (19×92 cells) has been calculated, its outlet average salt temperature is compared to the process value coming from the DCS. If the values do not match, the calculation is performed again for each cell of this panel with a corrected value of inside heat transfer coefficient, a corrective factor being applied.
The next points define more precisely some steps of the indirect method described previously:
Camera Temperature Map at all Level
The camera temperature map (
Guesses Values—Absorbed Power
For the first run, the power absorbed by the salt for each cell is defined as follow:
The enthalpies are calculated in accordance with the process values coming from the DCS.
Maximal External Wall Temperature (See Direct Method)
Classical Thermal Equations
The classical thermal equations based on the heat transfer theory and also some empirical equation based on VDI Heat Atlas, Second Edition, Part G1, Section 4.1. are used to calculate:
The inside heat transfer coefficient is calculated according to the direct method. However, if the result obtained does not match with the reality, this inside heat transfer coefficient has to be corrected. Therefore, the inside heat transfer is defined by the following modified equation:
where,
This correction factor is updated at the end of each panel calculation. The same correction factor is used for every cells of one panel.
Direct Method Based on the Absorbed Power
Once the power absorbed has been evaluated, the direct method can be used.
Theoretical Strain Vs. Temperature Envelop
The theoretical strain associated to the maximum wall external temperature is calculated to be compared to the validated envelop.
Once the external temperature profile is known around a representative half tube of the cell, the metal properties of the tube alloy (which can be for example a nickel-based or a stainless steel alloy) can be evaluated and the theoretical strain on the cell is calculated. The calculation applies to different operating flux maps gives the chart of
A dedicated study based on a lifetime evaluation has been performed to validate the envelop identified on
Every cell needs to comply with the following criterion evaluated at To,max
εalloy
If one or more cells do not comply with that criterion, the information is sent to DCS to perform selective defocusing.
The dεalloy230 is defined in accordance with the representation presented on
dεalloy=εalloy
It can be determined by calculating the intersection between the thermo-mechanical trend line and the operation envelop (see
The thermo-mechanical trend line is given by:
while the operation envelop is given by:
εalloy
Power Decrease Rate
A correlation between the theoretical strength decrease and the power variation has be found to evaluate the power decrease rate.
One can show that the power decrease on tube crest for one cell is a function of the theoretical strength variation, of the form given below:
Criteria and Defocusing Request to the DCS
Local defocusing signal is generated by CMI software when limits in terms of maximum temperature and strain inside the tubes are exceeded in comparison to the operating envelope. An alarm is generated as well.
According to a preferred embodiment, criteria on strain Δε are defined in accordance with an order of magnitude related to the measurement uncertainly:
The percentage of defocusing given by CMI software to DCS will trigger one of three alarms based on the following conditions:
1) SR PANEL MAX 1: ε1=Nominal envelope+0.015%; t1>15 sec
→Alarm to DCS;
2) SR PANEL MAX 2: ε2=Nominal envelope+0.03%; t1+t2>30 sec
→Alarm to DCS;
3) SR PANEL MAX 3: ε3=Nominal envelope+0.05%; t1+t2+t3>45 sec
→Alarm to DCS+shutdown request DCS.
The defocus is activated if Δε>0.015%:
The information sent to DCS are the problematic area location and the incident power decrease rate:
Still in the frame of the present invention, IR cameras with higher resolution could be used. However, this solution is not recommended for the following reasons:
A further perspective related to the present invention is to provide a simplified guarantee (for example for molten salt flow (kg/s) at 565° C.) and heliostat control optimization with a close-loop.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
Number | Date | Country | Kind |
---|---|---|---|
17163689 | Mar 2017 | EP | regional |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/EP2018/055436 | 3/6/2018 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2018/177696 | 10/4/2018 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
5689734 | Bauer et al. | Nov 1997 | A |
8360051 | Gilon et al. | Jan 2013 | B2 |
8931475 | Gilon et al. | Jan 2015 | B2 |
9222702 | Goldberg et al. | Dec 2015 | B2 |
20040086021 | Litwin | May 2004 | A1 |
20080265162 | Hamrelius et al. | Oct 2008 | A1 |
20090250052 | Gilon | Oct 2009 | A1 |
20100006087 | Gilon | Jan 2010 | A1 |
20130088604 | Hamrelius et al. | Apr 2013 | A1 |
20130104963 | Cap et al. | May 2013 | A1 |
20130139804 | Goldberg | Jun 2013 | A1 |
20160025383 | Shinozaki | Jan 2016 | A1 |
Number | Date | Country |
---|---|---|
WO 2004020926 | Mar 2004 | WO |
WO-2009152571 | Dec 2009 | WO |
WO 2009152571 | Dec 2009 | WO |
WO 2010017415 | Feb 2010 | WO |
Number | Date | Country | |
---|---|---|---|
20200386211 A1 | Dec 2020 | US |