WO2003021160A1

WO2003021160A1 - Hybrid volumetric receiver assembly and process for producing such assembly

Info

Publication number: WO2003021160A1
Application number: PCT/DK2002/000584
Authority: WO
Inventors: Per Stobbe; Bernhard Hoffschmidt
Original assignee: Stobbe Tech Holding A/S
Priority date: 2001-09-06
Filing date: 2002-09-06
Publication date: 2003-03-13
Also published as: WO2003021161A1

Abstract

A hybrid volumetric receiver unit or assembly for use in solar thermal power plants being heated by concentrated radiation from the sun, said unit or assembly comprises: a volumetric receiver body having an inlet and an outlet and a fluid permeable interior there between for passing an energy-carrying fluid medium such as air, through it, and a highly porous thin material attached to the inlet side of the volumetric receiver body being exposed directly to the solar radiation. Compared to a conventional volumetric receiver unit (without the porous material) a thermal power plant system reaches higher air outlet temperatures ranging from 600-850°C and thermal efficiences above 95% when using such unit. This is directly convertible to a smaller heliostat field with a significant positive influence on a minor investment costs.

Description

Hybrid Volumetric Receiver Assembly and Process for Producing Such Assembly

Field of the invention

Generally, the present invention relates to solar thermal power plants having a receiver surface heated by reflected radiation from the sun, wherein said receiver surface comprises a plurality of identical volumetric receiver bodies each having an inlet and an outlet, through which an energy-carrying fluid medium such as air, is passed and heated to such temperatures that it is capable to generate power or process steam. More particular the invention relates to a hybrid volumetric receiver unit (VRU) for use in such solar power plants.

The basic ceramic VRU has been tested in the confidential project HitRec and further in the SolAir project sponsored by the European Commission and proven durable in the period from late 1990ties.

Background of the invention

Solar power plants are currently used for the conversion of short wave electromagnetic radiation from the sun into electricity. Several technologies may be used in this conversion process. One of these technologies comprises constantly adjustable mirrors, which reflect radiation from the sun and by continuous adjustments concentrate the radiation onto a receiver or absorber surface, which is thus heated. Flowing a liquid or gaseous heat-collecting medium, such as water, molten salt, sodium, a gas or air, against or through the receiver surface, cools it and heats the heat-collecting medium. Once heated the heat-collecting medium may be passed through a heat exchange converter to generate steam, which is passed through a steam engine or turbine connected with an electric generator to produce electricity. The heat-collecting medium, which is cooled by this process, may be re-circulated in order to maximise the energy efficiency of the system. The receiver surface, which is heated by the reflected sun radiation, must be cooled all the time in order to avoid melting or evaporation of the construction material(s). Besides, very high temperature stable materials are needed for the construction of the receiver surface and its support.

Porous solids like extruded monoliths with parallel channels and thin walls made from various oxide and non-oxide ceramics, ceramic foams and metal structures have the objective to act as open volumetric receivers in concentrated solar radiation power plants. In this application, ambient air flows through the volumetric receivers, which is heated by the concentrated solar radiation. A heat exchanger then transfers the energy in a conventional steam turbine process. In all cases, to obtain high efficiencies, high absorptivity in the visible and near infrared range has to be combined with a high porosity to create large surfaces for convective heat transfer from the solid absorber to the fluid, which may be air. Especially high performance absorbers tend to be sensitive to inhomogeneous flux distributions, which may cause local overheating of the material. In various tests with specific kinds of materials flow instabilities occurred, which partly lead to hot spots and a damaging of the receiver.

To some extent also the wind at relatively high speed around the solar thermal power plant top has such an influence that smaller channels of the honeycomb are better than larger cells which increase the short wave to long wave conversion efficiency. The wind changes the air speed down through the individual channels of the volumetric receiver unit by changing the pressure drop over that specific channel. Lower air speed in the specific channel means lower energy passing out through the supporting funnel cup outlet. And the higher front face temperature reflects more energy to the environment.

The present invention overcomes the above drawbacks and achieves both high efficiencies and a safe operation with an optimised combination of geometrical as well as thermal conductivity and heat transfer parameters. Summary of the invention

The object of the present invention is to provide a high thermal efficiency integrated volumetric receiver unit (VRU), module or assembly, preferably manufactured from SiC based materials, comprising a ceramic volumetric receiver body and a support member for use in a solar thermal power plant at temperatures of about 1000 - 1200 °C. Alternatively the integrated volumetric receiver unit is manufactured from metal or a combination of metal and ceramics.

According to the invention said volumetric receiver body or absorber monolith has an efficiency improving porous material mounted in front of the porous receiver body or monolith which acts as a support member therefore so as to constitute an even more efficient volumetric receiver unit (VRU).

Thus the present invention is depending on a basic VRU as a support for a layer of more porous material mounted in front of the VRU. The front of the volumetric receiver unit faces towards the reflective mirrors concentrating the solar flux thereon and its performance is further optimised for higher short wave conversion efficiency, when supplied with a flow stabilising and/or insulating layer of highly porous material such as fibres formed into a fabric, felt or mat with a thickness of less than 10 mm consisting of fibres being 5 to 250 μm thick. Alternatively a slice of a higher cell density monolith or a super high pore density foam or combinations thereof can be used.

A front material comprising a high specific surface, excellent absorption capacity and high porosity (e.g. Ceramat from Schott Glaswerke) to achieve volumetric absorption of the concentrated solar radiation has been combined with a material of excellent thermal conductivity properties and a quadratic pressure loss characteristic (e.g. SiC or SiSiC honeycomb multi-cell supports such as supplied by HelioTech in Denmark) of 85 mm in diameter as shown in Fig 1. The SiC fibre mesh (Schott Ceramat) was originally developed for pre-mixed natural gas surface burners. It consists of silicon carbide fibres of 25 μm diameter CVD glued together to form a layer of 3,5 mm thickness. The fibres are oriented in directions perpendicular to the direction of the airflow, beneficial for radial heat transport and good heat transfer properties.

An inclined or rounded design of the VRU upper edges and/or corners allows a higher return air/re-circulation air ratio thus further increasing the thermal efficiency of the VRU, the re-circulation air being of a higher temperature than ambient air.

Therefore, an insulation layer, which at the same time is permeable to air and has a high thermal conductivity, increases the total energy efficiency of the system.

Preferably the volumetric receiver unit is exposed to more than 0,1 MW / m² and less than 10 MW / m², more preferably 1 to 2 MW / m².

Design of the invention

As to the technology of solar air receivers, reference is made to US patent No. 5,483,950, which discloses the original Sulzer concept and the possible receiver materials as "modules of wire mesh strips, etc". This type of material is based on alloyed steel and other high temperature metals. Reference is also made to US patent No. 6,003,508, which in more details discloses another solar volumetric receiver.

The present invention is based on the addition of a thin porous layer in front of the VRU disclosed Danish Patent Application No. PA 2001 01328, incorporated herein by this reference.

Preferably the shape or design could be like: - a McDonald burger plastic foam sales cup attached to the monoliths front / power inlet face by anchoring points along the walls

- a flat piece of material attached to the front face of the VRU

- a McDonald burger plastic foam sales cup laminated to the front face of the VRU

- a flat piece of material laminated to the front face of the VRU

Materials suitable for the invention

Ceramic fibrous materials like those manufactured by the German company Schott Glass Werke or Industrial Ceramic Solutions Inc or the Japanese company Nippon Carbon Ltd. among which SiC fibre based felts and mats originally intended as support for low NOx natural gas surface burners are suitable for use in the present invention.

Metallic Cell Foam sheets structures with thickness from less than a few millimetre to 5 mm is available from a supplier like Sumitomo Electric Industries Ltd. Japan, made from Fe, Ni, Cr, Al alloy with 100 to 400 μm pore size, >95% porosity and trade name CelMet.

Ceramic cell foam structures such like SiC, SiSiC, SiN is commercial available on the market in various dimensions and from a variety of sources - also as thin plates 5 to 10 mm in thickness.

A felt-like material of metals is available from Bekaert in Belgium made from alloys such as FeCrAlloy consisting of Ferrum, Chromium, Aluminium and often small amounts of rare earths metal like Cerium, Yttrium is suitable in the VRU improvement according to the present invention. This Bekaert material is a fibrelike material not originating from single fibres assembled into a sheet, but is produced by a process wherein the fibres are cut, scraped from solid material and compressed into a sheet. Available in thickness from less than a millimetre to several millimetres. Honeycomb structures of high cell density, the number of cells per square inch (CPSI) being from 100 to 1600, made from either metal or ceramics are available in metals from Emitec in Germany or in ceramics from HelioTech in Denmark

Otherwise anchoring of the layer of this invention on the monolith surface would be by:

- gluing and firing into one laminated structure

- gluing with ceramic glue as that manufactured by Aremco, by Cotronics, Toagosei or the like

- mechanical anchoring of two different structures / materials

Brief description of the drawings

Fig. 1 is a photo of a VR unit made out of a SiC AFM fibre felt covering the ReSiC (HitRec design principle) material multi-cell circular support of 85 mm diameter.

Fig. 2 is sketch in principle of the solar oven test at DLR in Cologne, Germany used for efficiency measurement testing of the volumetric receiver unit shown in Fig. 1.

Fig. 3 is a graphical picture showing the results of receiver efficiency measurements on a non covered monolith of ReSiC (HitRec design principle) material versus the AFM (advanced fiber material) covered monolith.

Fig. 4 is a cross-section view of a VR unit of the HitRec design with the in front mounted porous material for increase of efficiency.

Fig. 5 is a cross-section of a VR unit of the SolAir design with an inclined edge inlet for better air re-circulation and as a further option the porous material in front. Fig. 6 is a cross-section view of a VR unit of the HitRec design with grooved area to hold the thin layer of flexible porous material

Fig. 7 is a cross-section view of a VR unit of the HitRec design equipped with a thin slice of rigid monolith material of high CPSI number in the front of the support monolith.

Fig. 8 is a graphical picture showing a comparison of the original HitRec design with 90° angled edges versus the optimised SolAir design with 30° inclined edges.

Fig. 9 is a sketch showing in principle the volumetric receiver function of the HitRec design.

Detailed description of the drawings

Fig. 1 shows the porous front material 14, in this example being a 3.5 mm thick Ceramat material from Schott Glass Werke attached to a re-crystallized SiC honeycomb multi-cell support 12.

Fig. 2 illustrates efficiency tests which have been carried out using concentrated radiation within the DLR (Deutches Zentrum fϋr Luft und Raumfart in Cologne, Germany) "Solar Furnace", an installation consisting of a solar movement 40 m² Heliostat and a fixed concentrator. In the focus of the furnace an isolated test- bed is used, in which absorber samples can be placed. A fan forces ambient air to flow through the sample followed by a water-cooled heat exchanger. The power being transferred to the water circuit and the power remaining in the air are calculated from temperature measurements.

Fig. 3 illustrates data measured from experiments elucidating the thermal efficiency increase obtainable with the invention. Using conventional volumetric receiver unit (without the fibre or porous mat) the system reaches air outlet temperatures of 450°C - 750°C and thermal efficiencies ranging from 75% - 82%. For comparison, higher air outlet temperatures ranging from 600 - 850°C and efficiencies above 95 % are reached with the porous material in front of the VR unit according to the invention. This will be directly convertible to a smaller heliostat field with a significant positive influence on investment. Equivalent to the efficiency increase the heliostat field and the installation cost of the solar thermal power plant can be reduced.

Fig. 4 shows a cross section of a volumetric receiver unit consisting of a multi channel monolith 42 in connection with a ceramic funnel inlet arrangement 43, and the porous material 44, in this example a ceramic fibre mat, attached to the monoliths front/power inlet face by high temperature stable ceramic glue. The monolith has a cell density in this example of 90 CPSI, but can be different. The glue may also be applied onto the edges of the monolith. Alternatively the fibre mat may be anchored by high temperature stable thin nails passing down into a few of the channels in the honeycomb monolith. Nails with a small diameter head, like 3 mm in diameter and a nail body of 0,5 mm wave shaped thread being 30 mm long.

Fig. 5 shows a cross section of a volumetric receiver unit consisting of a multichannel monolith 52 having inclined upper edges 56 and being connected with a funnel inlet arrangement 53. The inclined edges 56 of the front of the monolith 52 allow for a better air re-circulation thus improving the total system efficiency. The porous material 54 exposed to the solar power is shaped like a McDonald burger plastic foam sales cup attached to the monoliths front/power inlet face by anchoring points 57 along the walls. The porous material may be of a ceramic material or of a metallic material. This version of the inventive features has the advantage that the porous material cup may be easily exchanged or replaced when they are worn out or damaged. Fig 5 is similar to Fig. 8 with respect to the graphical results of the inclined edges of the VRU monolith. The angle of the inclined edge faces is at present selected to be about 30°, which gives 3 channels being shorter in length at the circumference, though the angle and number of selected channels may be different from 30° and 3 channels. Thus the 150 °C hot return air is returned through the 2-5 mm wide gap between each of the VRU, re-directed 180° and sucked back into the 2-3 short channels at the upper circumference.

Fig. 6 shows a cross section of a volumetric receiver unit consisting of the primary monolith 62 connected with a funnel inlet arrangement 63. The primary monolith has elongated edges 66 at the upper circumference extending 4 mm further up from the main surface. Hereby a lower area in between the four walls appear suitable for fixing the porous felt 64 by gluing the edges of the felt to the inside of the walls.

Fig. 7 shows a cross section of a volumetric receiver unit consisting of the primary low number channel monolith 72 connected with a funnel inlet arrangement 73 and further a secondary thin slice of a high number channel honeycomb monolith 74 attached to the primary monoliths front 75. Preferably the primary monolith has a channel density ranging from 30 to 100 CPSI with a cell size ranging from 2 x 2 to 10 x 10 mm. This body has a height ranging from 25 to 100 mm, preferably 40 to 60 mm being at least twice the thickness of what it is intended to support. The secondary thin slice high number channel honeycomb monolith has a cell density ranging from 100 to 600 CPSI with a cell size ranging from 0.5 x 0.5 to 2 x 2 mm. The channels may in both or just one monolith be square, triangular, hexagonal, circular or combinations hereof.

The principles will be applicable with materials made from both metals and ceramics or combinations thereof. Fig. 8 shows graphically the results of the calculation with the 3D computer analysing program FLUENT of CFD (Computer Flow Dynamics) type. The improvement is a return air/re-circulation air increase from 55 to 75%.

Fig 9 shows in details the principles of the volumetric receiver functioning. Incident solar flux <2 MW/m² thermal power impinging onto the surface of the VR units is concentrated solar radiation by many heliostats on the ground. The Double Membrane on top of the plant tower carries the VR units each in a metal tube, carries the insulation and separates the 800°C hot air from the return air, i.e. the re-circulation air having passed the heat exchanger and been cooled down to ~150°C. The double membrane as cooled by the return air may be manufactured from less advanced metal alloys compared to a non-cooled double membrane.

Description of preferred embodiments

Example 1

A 140 x 140 mm standard Silicon Carbide slip cast cup assembled with an extruded 140 x 140 mm and 60 mm long honeycomb monolith into a volumetric receiver unit was designed with a 30 degree inclined inlet at its upper surface circumference for obtaining a better air re-circulation. The inclined sides of the honeycomb monolith was further equipped with slots, grooves parallel to the front face approximately 4 mm wide and 3 mm deep on all four sides as one long cut. A cup having a 3 to 4 mm wall thickness was formed from the porous SiC fibres into a so-called McDonalds burger foam cup shape. The cup has inside bulbs corresponding the grooves on the outside of the honeycomb monolith inclined faces. Thus anchoring the fibre cup unto the monolith was secured.

Example 2

A 140 x 140 mm standard Silicon Carbide slip cast cup funnel was assembled with an extruded or even cast 140 x 140 x 60 mm honeycomb monolith into a volumetric receiver unit designed with a 30 degree angled inlet at its upper surface circumference for obtaining a better air re-circulation.

On the flat front face a flat sheet of Schott SiC fibre type CeraMat was glued onto the honeycomb front with ceramic glue like the ones obtained from Cotronics Corp. and Aramco Corp., USA. The glue was added onto the edges of the monolith hereafter the felt was secured under slight pressure for 24 hours. No further heating was required.

Example 3

A thin wall sheet metal or cast metal cup alloyed from temperature stable metals combined with a corrugated metal foil honeycomb monolith into a Volumetric Receiver. The alternatively hydro formed metal cup has sufficient strength with wall thickness of 2-3 mm. The two parts were assembled by welding or brazing. On front of the honeycomb monolith a further metal based porous structure is mounted either mechanically or by welding or brazing.

Example 4

A 140x140 mm Silicon Carbide slip cast cup assembled with an extruded or potentially cast 140 x 140 x 40 mm multi cell honeycomb monolith with 90 CPSI into a volumetric receiver unit.

On the flat front face a 20 mm thick multi cell honeycomb monolith with CPSI of

200 designed with a 30 degree angled inlet for better air re-circulation was anchored so as to take advantage of the invention. Alternatively the honeycomb monolith may be fabricated by the "freeze casting" technique. Or by the "direct typing process" to which reference is made EP

0927983.

Test results Efficiency tests have been carried out in concentrated radiation using the DLR (Deutches Zentrum fϋr Luft und Raumfart in Cologne, Germany) "Solar Furnace", an installation consisting of a 40 m² Heliostat and a fixed concentrator. In the focus of the furnace an isolated test-bed is used, in which absorber samples can be placed. A fan forces ambient air to flow through the sample followed by a water heat exchanger. The power being transferred to the water circuit and the power remaining in the air can be calculated from the temperature measurements shown in Fig 2.

In the experiments behind this invention, the hybrid / integrated and improved volumetric receiver unit system, a SiC Fibre felt addition was made to the conventional VR unit and compared. The results are presented in Fig. 3. In the conventional volumetric receiver unit (without the fibre mat) the system reaches air outlet temperatures of 450°C - 750°C and efficiencies ranging from 75% - 82%. This is mainly caused by excellent absorption properties and a high amount of specific surface. In contrast, the combination with the enormous fibre mat surface area on top of the volumetric receiver unit enables an improved radial heat transfer due to the good thermal conduction properties of the SiC monolith. Because the monolith also dominates the flow resistance properties of the system, a more homogeneous temperature distribution is reached, which generally leads to lower peak temperatures at the front part of the absorber material. Consequently, higher air outlet temperatures ranging from 600 - 850°C and efficiencies above 95 % are reached.

As a further new design of the present invention the Hybrid volumetric receiver unit was assembled from the following 3 parts: - a ceramic funnel

- a parallel multi cell ceramic honeycomb monolith body with 90 CPSI, 2 x 2 mm cell size and 0.8 mm wall thickness, physical dimension of 140 x 140 mm and 50 mm thickness was mounted as the primary monolith onto the funnel inlet side - the solar exposed honeycomb monolith with 200 CPSI, 1 x 1 mm cell size and 0.5 mm wall thickness, physical dimension of 140 x 140 mm and 10 mm thickness was mounted as the secondary monolith onto the primary monolith acting now as a mechanical support

The secondary monolith slice will with its higher cell density offer higher absorption, more stable airflow and improved temperature homogeneity. The two in design different SiC ceramic monolith pieces, but with the same outside dimension of 140 x 140 mm was cut to length and attached to each other in partly wet stage and fired at a temperature higher than 2000°C in Argon for one hour to form re-crystallized Silicon Carbide. This formed a laminated block of two different honeycomb bodies in total 60 mm high.

The edge design of the VRU has been calculated with the 3D computer analysing program FLUENT of CFD (Computer Flow Dynamics) type. The results as seen in fig 8 show a return air / re-circulation air increase from 55 to 75%. Thus increasing the thermal efficiency of the VRU. The re-circulation air when being returned back to the VRU and sucked into the angled corners being typically 150°C and ambient air typically of 30°C.

Claims

1. A hybrid volumetric receiver unit or assembly for use in solar thermal power plants being heated by concentrated radiation from the sun, said unit or assembly comprises:

- a volumetric receiver body having an inlet and an outlet and a fluid permeable interior there between for passing an energy-carrying fluid medium such as air, through it, and

- a highly porous thin material attached to the inlet side of the volumetric receiver body being exposed directly to solar radiation

2. A ceramic hybrid volumetric receiver unit according to claim 1 characterized by that:

- the in front mounted honeycomb monolith body of the volumetric receivers unit is of a ceramic material with less than 70% open porosity and more than 10 mm thickness

- the solar exposed ceramic material has more than 70% open porosity

- the volumetric receivers unit air outlet temperature being more than 600°C and less than 1300°C.

3. A metallic hybrid volumetric receiver unit according to claim 1 characterized by that:

- the in front mounted honeycomb monolith body of the volumetric receiver unit is a metallic material with less than 70% open porosity and more than 10 mm thickness - the solar exposed metallic material has more than 70% open porosity

- the volumetric receiver unit air outlet temperature being more than 500°C and less than 1100°C.

4. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel where the in front mounted porous fiber felt has been glued onto the front face of the monolith.

5. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel wherein the in front mounted porous fiber felt has been glued onto angled faces of the monolith.

6. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel wherein the in front mounted porous fiber felt has been glued onto the parallel faces of the monolith.

7. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel wherein the in front mounted porous fiber felt has been mechanically attached onto the front face of the monolith.

8. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel wherein the in front mounted at least 70% porous fiber felt has been mechanically attached onto angled faces of the monolith.

9. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel wherein the in front mounted porous fiber felt has been mechanically attached onto the parallel faces of the monolith.

10. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel having further a slice of a honeycomb structure being less than 30 mm thick, preferably 5 to 10 mm thick and with CPSI numbers higher than 150, mounted onto the monolith.

11. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel having further a slice of a honeycomb structure being less than 30 mm thick, preferably 5 to 10 mm thick and with CPSI numbers higher than 250 mounted onto the monolith.

12. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel having further a slice of a honeycomb structure being less than 30 mm thick, preferably 5 to 10 mm thick and with CPSI numbers higher than 400 mounted onto the monolith.

13. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel having a thin slice of a high-density cell foam structure with CPI numbers higher than 40 mounted onto the face of the monolith, which is attached to the monolith in the green stage and anchored permanently to it during the firing process.

14. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith and a funnel having a thin slice of a high-density cell foam structure with CPI numbers higher than 40 mounted onto the face of the monolith,

15. A hybrid volumetric receiver unit according to any of the above claims, consisting of a monolith mounted onto a funnel wherein the monolith has one flat side and the opposite side being of a different shape than flat.

16. A volumetric receiver unit according to any of the above claims, incorporating edges, facets on the circumference of the volumetric receiver body with angles different than 90° on the in front mounted monolith body of the volumetric receivers unit. KR EVER NOGEN BEARBEJDNING omkring snakken om vinkler - se figur 5 der viser de skra flader - virker ogsέ fint uden matten monteret.

17. A hybrid volumetric receiver unit according to any of the above claims, consisting of an in front mounted monolith exposed to the solar flux and mounted directly on the double membrane metal support structure.

18. A hybrid volumetric receiver unit according to claim 17, consisting of the in front mounted primary monolith behind a porous media exposed to the solar flux and mounted directly on a double membrane metal support structure.

19. A hybrid volumetric receiver unit according to claim 17 - 18, consisting of an in front mounted primary monolith behind a porous media exposed to the solar flux with return air facets and mounted directly on a double membrane metal support structure.

20. A process for producing a hybrid volumetric receiver unit or assembly for use in solar thermal power plants, comprising combining:

- a highly porous thin material attached to the inlet side of the volumetric receiver body being exposed directly for solar radiation

21. A process for producing a hybrid volumetric receiver unit or assembly according to claim 20, wherein the combination of the volumetric receiver body and the highly porous material is effected by gluing and firing into one laminated structure, gluing with ceramic glue as that manufactured by Aremco, by Cotronics, Toagosei or the like, or by mechanical anchoring of the two structures