Patent Description:
Currently, molten salt is used as heat transfer and storage medium in nearly all Solar Tower CSP plants due to its good thermal properties. For that reason, the applicant has commercialized a Molten Salt Solar Receiver (MSSR) designed to transmit heat from the incident solar power to the molten salt. The heat contained in the molten salt is exploited to generate superheated steam that will be used to produce electricity in a turbine.

However, using molten salt as heat transfer and storage medium also presents some disadvantages. Indeed, under a certain temperature (<NUM> for KNO<NUM>-NaNO<NUM> molten salt), molten salt freezes and can further create some damages to the equipment such as pipe explosion. To avoid this drawback, heat tracing is used to maintain the equipment temperature always above the freezing point. Another limitation of the molten salt is that its operating temperature range is limited: the salt cannot exceed a certain level of temperature otherwise it can deteriorate. Since this operating temperature range is quite limited (usually from <NUM> to <NUM> for KNO<NUM>-NaNO<NUM>), storage density of molten salt is relatively small.

Furthermore, it is known that ceramic particles are not subject to freezing since they are solid at any temperature and their operating temperature range is much broader than the one of molten salt (from <NUM> to <NUM> for bauxite particles). Therefore their storage density is about <NUM> times higher than molten salt storage density. Moreover, due to their higher temperature operating level, ceramic particles can be coupled to high-efficiency thermodynamic cycles, such as supercritical CO<NUM> cycles.

For these reasons, the use of ceramic particles has been investigated and these are considered good candidates for heat transfer/storage media for the next generation of Solar Tower CSP plants.

To anticipate this next generation of particles Solar Tower CSP plants, the applicant has decided to design a particle steam generator, more particularly in the context of a European H2020 Project called "HiFlex", in which the applicant is partnering. This project consists in the development, the building and the demonstration of a complete commercial Solar Tower CSP plant using particles as heat transfer/storage medium.

The applicant is also involved in the "CompassCO2" project, which consists, among others, in the demonstration of a particles-sCO<NUM> heat exchanger.

Otherwise, the applicant already developed a molten salt steam generator comprising a shell-and-tube heat exchanger with helicoidal baffles. As solid particles in a fixed bed are intended to be used in the present invention, the concept will be very different.

So, the following points have to be taken into account:.

Document <CIT> discloses a method and an apparatus for indirect-heat thermal processing of material, such as a dryer or evaporator for treatment of particulate material. A dryer for drying particulate material comprises a plurality of heat transfer plates arranged in spaced relationship for the flow of the material to be dried there between and downward. Each heat transfer plate is provided with an inlet and an outlet for the flow of the heating fluid through the plates, so that particles are heated thanks to the heating fluid passing through the plates. A purge fluid delivery system provides a flow of purge fluid, such as air, gas or steam between the plates in a direction across the direction of flow of the material to be dried, to remove evaporated volatiles generated by the indirect heating of the material.

Document <CIT> discloses a heat exchanger including a housing with an inlet for receiving bulk solids, and an outlet for discharging the bulk solids. A plurality of spaced apart, substantially parallel heat transfer plate assemblies are disposed in the housing between the inlet and the outlet for cooling the bulk solids that flow downwards under gravity from the inlet, between the adjacent heat transfer plate assemblies, to the outlet. In order to protect the parallel plates, a pipe extends along a top end of each heat transfer plate. Cooling fluid that is circulating in these pipes protects the plates from high-temperature particles.

Document <CIT> discloses an indirect-heat thermal processor for processing bulk solids including a housing with an inlet for receiving the bulk solids and an outlet for discharging the bulk solids and a plurality of heat transfer plate assemblies disposed between the inlet and the outlet and arranged in spaced relationship for the flow of the bulk solids that flow downward from the inlet, between the heat transfer plate assemblies, to the outlet. The heat transfer plate assemblies include temperature detectors, heating elements and heat spreaders. These elements enable control and monitoring of the temperature of the solid bulk. There is no secondary heating/cooling fluid.

Document <CIT> discloses a heat exchanger air/sand. Particles are flowing in a wall having a single-piece lattice body comprising straight channels. Particles exchange their heat with air that circulates across the particles channels. The sidewalls of the different particle channels are constituted of porous ceramic or metals. Air is heated/cooled by convective transfer with the particles.

Document <CIT> discloses a vertical tube bundle heat exchanger with a particle moving bed. By moving downward by gravity the particles exchange heat with vertical tubes located in the housing of the heat exchanger. In these tubes is circulating a heating/cooling fluid. With this vertical tube bundle arrangement, there are no void or stagnant zones leading to poor heat transfer.

Document <CIT> discloses the features of the preamble of claim <NUM>, namely a heat exchanger including a housing with an inlet for receiving bulk solids, and an outlet for discharging the bulk solids, a plurality of spaced apart, substantially parallel heat transfer plate assemblies disposed within the housing, between the inlet and the outlet for heating the bulk solids that flow from the inlet, through spaces between the heat transfer plate assemblies, and at least two gas handling zones including a first gas handling zone and a second gas handling zone spaced from the first gas handling zone, the two gas handling zones disposed between the inlet and the outlet for entry of a pulsed flow of air into the housing and around the bulk solids and for exit of the pulsed flow of air from the housing.

Document <CIT> discloses a device for cooling the solid matter from a coal gasification. The device includes a container with a feed part, a cooling part and a venting part. Lines arranged transverse to the flow direction are located inside of the cooling part that are grouped in two kinds, the one carrying liquid and the other carrying gas. The liquid carrying lines are closed in the interior of the cooling part and are provided for the heat exchange. The gas carrying lines are gas permeable into the interior of the cooling part in such a way that solid matter comprising primarily cooled slag, ash and flue dust is cooled and the remaining gas present in and between the solid matter particles is exchanged. A method for cooling down the solid matter and for removing the remaining gas from the particles is also disclosed.

The present invention aims to implement a new design for a compact, low-cost and highly efficient particle heat exchanger, whenever used with water steam, supercritical CO<NUM> or other heat exchange media, in order to commercialize it in large-scale CSP power plants.

A particular goal of the invention is to provide an installation designed to serve high-efficiency power cycles that can withstand temperatures that can approach about <NUM>, i.e. much higher than what can be achieved using heat exchange fluids such as oil or molten salt and ordinary metals or alloys for tubing.

Another goal pursued by the invention is to operate the particle heat exchanger at least during <NUM> operation without breaking. However, in this design, an assumption of <NUM> lifetime will be made to be closer to the lifetime requested by large- scale power plants.

A first aspect of the present invention relates to a heat exchanger comprising a casing, said casing including a top inlet part for receiving hot solid particles and a bottom outlet part for discharging the solid particles after cooling ; and a plurality of spaced apart adjacent heat transfer tubes disposed in the casing between the top inlet part and the bottom outlet part, assembled in at least one tube bundle and intended for the circulation of a coolant fluid, for cooling the solid particles that flow downwards by gravity from the inlet, between the spaced apart adjacent heat transfer tubes, to the outlet ; wherein the spaced apart adjacent heat transfer tubes are arranged in rows substantially transverse to the flow direction of the solid particles, wherein the tube bundle is supported in the casing by at least two tubesheets disposed vertically and laterally near each of both ends of the tubes and wherein each tubesheet is composed of independent plates corresponding to the individual tube rows, adjacent plates being stacked on top of each other thanks to insulated sliding pieces.

According to preferred embodiments, the heat exchanger further comprises at least one of the following characteristics or a suitable combination thereof :.

A second aspect of the invention relates to a solar tower CST plant comprising :.

Alternately, in the solar tower CST plant, the particle heat exchanger is replaced by a heat exchanger in which the tube bundle contains, instead of water, supercritical CO<NUM> operated according to a recompression Brayton cycle.

A general structural arrangement of the particle heat exchanger according to the present invention is shown on <FIG>, <FIG>, <FIG> and <FIG>. The particle heat exchanger <NUM> is composed, fully or partly, of the following parts:.

Functionally, the operation of the particle heat exchanger according to the invention in the example of a particle solar tower power plant <NUM> is shown on <FIG> and discussed here below. Solar energy is reflected by the mirrors of the heliostat field <NUM> and concentrated onto a direct absorption particle receiver <NUM> located on top of a solar tower <NUM>. Hot particles are stored in a hot storage tank <NUM> at high-temperature (<NUM>-<NUM>) and connected to the receiver <NUM>. The particles are directed to the heat exchanger <NUM> inlet thanks to an entry pipe and the fixed bed of particles goes downward very slowly in the casing of the heat exchanger <NUM> under the action of gravity. By moving down in the heat exchanger, the particles transfer their heat to the tube bundle(s) of the heat exchanger <NUM>, in which a coolant fluid, for example water, is flowing. Steam is then generated in the heat exchanger/steam generator circuit connected to a steam turbine <NUM>. The superheated steam at the exit of the heat exchanger <NUM> is for example at a temperature of <NUM>. After passing the steam turbine <NUM> and condenser <NUM>, liquid water returns to the heat exchanger at a temperature of <NUM> for example. The heat exchanger <NUM> is preferably countercurrent: the cold coolant fluid arrives in the header located at the bottom of the heat exchanger and leaves, after heating, through the outlet header located at the top thereof. After having crossed and contacted the tube bundle, the cold particles are directed to the cold storage tank <NUM> through the outlet conical piece of the heat exchanger and then further directed to the particle receiver <NUM> using a particle transport system <NUM>, preferably a mechanical system such as a worm screw.

In an alternate embodiment, the particle heat exchanger according to the invention contains supercritical CO<NUM> operated according to a recompression Brayton cycle (not shown).

Firstly, while presenting some similarities with the heat exchangers of prior art (<CIT>, <CIT>), the heat exchanger of the present invention is based on a transverse tube design and not on a concept of plates or vertical tubes/channels, characterized by the absence of stagnant zones or slowing down zones for the particles.

One advantage of a heat exchanger designed with a tube bundle is that it is more suitable to resist to high-pressure fluid circulating inside the tubes, which would be the case for example if this heat exchanger is operated with supercritical CO<NUM>. In addition, the tube-in-shell design of the invention is superior with respect to severe thermal transients and temperature ramps. Further the sCO<NUM> system does not use water, which is significant knowing that CSP plants are typically located in hot and dry climates where water is scarce.

Additionally, heating sCO2 directly delivers benefits that are most pronounced with smaller systems that have a few hours of thermal storage, including the ability to place the power cycle in the receiver to minimize the length and cost of high pressure piping. Both novel power receiver approaches (particles and sCO<NUM>) have numerous potential advantages that void the current limitations of CSP systems using molten salt as a heat transfer fluid (HTF) and/or for thermal energy storage (TES).

Secondly, as shown on <FIG>, the design of specific tubes with elbows is such that a small spacing between tubes (about <NUM>, corresponding to <NUM> times the diameter of bauxite particles) is possible without exceeding the allowable maximum elbow bending radius (according to manufacture constraints). This specificity enables high compactness of the tube bundle of the particle heat exchanger. The small spacing between tubes has a very positive impact on the heat transfer efficiency between the particles and the tubes.

Thirdly, a tubesheet design is originally provided to support the tubes, under the form of a set of plates <NUM> (one for each tube row) that are stacked on top of each other without connection (<FIG>) and further maintained by an insulated sliding pieces (see <FIG>). The tubesheet plates <NUM> are supported by the heat exchanger bottom and the sliding pieces are welded to the casing <NUM>. This design enables to break the important thermal gradient between the top and the bottom of the heat exchanger in smaller ones seen by each part. As a result, thermal stress in strongly reduced. In order to resist to particles at the highest temperatures, nickel superalloy (e.g. H230) is advantageously used for a few plates located at the top of the heat exchanger, the other plates being in refractory alloy. Furthermore sliding pieces are made for example of nickel superalloy H230 in the case of the two sliding pieces <NUM> exposed to the highest temperatures while the sliding pieces <NUM> exposed to lower temperatures are made of <NUM> stainless steel (see <FIG>).

Fourthly, in order to reduce the large thermal gradient (and therefore of additional thermal stresses) induced by the insulant <NUM> that covers the sliding part in current design, a sliding piece design will be a male part <NUM> with retracted insulation, while tubesheet plates will be a female part <NUM> (<FIG>).

Claim 1:
A heat exchanger (<NUM>) comprising a casing (<NUM>), said casing (<NUM>) including a top inlet part (<NUM>) for receiving hot solid particles and a bottom outlet part (<NUM>) for discharging the solid particles after cooling ; and a plurality of spaced apart adjacent heat transfer tubes (<NUM>) disposed in the casing (<NUM>) between the top inlet part (<NUM>) and the bottom outlet part (<NUM>), assembled in at least one tube bundle (<NUM>) and intended for the circulation of a coolant fluid, for cooling the solid particles that flow downwards by gravity from the inlet, between the spaced apart adjacent heat transfer tubes (<NUM>), to the outlet, wherein the spaced apart adjacent heat transfer tubes (<NUM>) are arranged in rows substantially transverse to the flow direction of the solid particles, wherein the tube bundle (<NUM>) is supported in the casing (<NUM>) by at least two tubesheets (<NUM>) disposed vertically and laterally near each of both ends of the tubes (<NUM>) and characterised in that each tubesheet (<NUM>) is composed of independent plates corresponding to the individual tube rows, adjacent plates being stacked on top of each other thanks to insulated sliding pieces (<NUM>).