The invention deals with a process and an apparatus for a process and its relating plant for generating steam, wherein a heat transfer carrier is heated to a temperature between 700 to 1500 °C in a high temperature receiver, wherein the heat transfer carrier is heating steam in at least one steam heating unit directly and/or indirectly, wherein the heat transfer carrier is further passed to at least one steam producing unit to generate steam and wherein the heat transfer carrier is afterwards recirculated to the high temperature receiver.

On our planet, the sun is the most reliable and abundant source of energy. To reduce global warming and safe fossil energy sources a better usage of this energy source is absolutely necessary.

In contrast to photovoltaic systems which are directly converting sunlight to electricity, solar thermal energy allows for storing heat which is currently more efficient than storing electricity. This advantage makes solar thermal energy attractive for large scale power conversion. Since sunlight can be converted to heat during the day and heat converted to electricity during the night, storage systems can significantly improve the availability and economics of solar electricity.

In this context, one possibility is to use Solar Thermal Electric Energy (STE) generation for concentrating sunlight, whereby heat is generated for running a heat engine. The engine in turn drives a generator for producing electricity. Currently, any liquid or gas is used as Heat Transfer Fluid (HTF). Used heat engines are turbines, steam engines or Stirling engines, usually achieving an efficiency of 30- 40%. Solar thermal energy currently is collected by two systems: line focus and point focus. Line focus is less expensive and less efficient. So called, point focus, which means concentrating the sunlight collected by mirrors surrounding a central tower on a point at the top of the tower, has a much larger potential with regard to effi- ciency and storage. Consequently it can reduce the cost per kWh in comparison to line focus and in comparison to energy conversion of photovoltaic and natural gas. The main difference between both systems is related to the maximum theoretical concentration factor: 212 for line focus and 44000 for point focus. The currently achievable concentrations are about 100 respectively 1000, enough for running a steam turbine with 42% efficiency as described in Ma, Z. et al., 2015, Development of a concentrating solar power system using fluid ized-bed technology for thermal energy conversion and solid particles for thermal energy storage, Direct Energy Procedia 69 (2015) 1349-1359, published online under www.sciencedirect.com.

However, conversion of coal is still cheaper. Therefore, cost reduction is needed to achieve a global transition towards solar thermal electricity.

The currently most innovative and most competitive Solar Thermal Electricity (STE) plants with a Central Receiver (CR) on a solar tower and a state-of-the-art two tank molten salt system for six hours heat storage still face a number of limitations: i) the temperature limits of 566 °C for current molten salts used as heat transfer fluids (HTF) and storage media;

ii) as a consequence of the limited temperature the restricted conversion efficiency of turbines,

iii) the high volume and cost of storage systems due to the narrow temperature gradient and

iv) the high cost of molten salts as such. Gradual improvements of salt characteristics, i.e. higher operating temperatures are expected, albeit not allowing disruptive progress, at least not within the commercially viable price range for products. Due to the working parameters of tur- bines and higher temperature gradients needed for effective storage, temperatures above 1000 °C are necessary to get closer to the target Leveled Cost Of Energy (LCOE) of 6 cents per kWh. In addition, molten salts cost about 1000 US$ per ton. Since 30000 tons are used in current plants, molten salts are a relevant CAP EX factor.

Another technical challenge is that molten salts expand when melting. This property poses a risk when salts have been freezing (e.g. during a shutdown) and are again heated to the melting point of about 220 °C - the expansion may destroy piping where solid salts have been sitting. Let alone molten salts may decompose when exceeding the working temperature and require corrosion resistant materials. If increasing the storage capacity of solar fields from 6 hours to 12-15 hours, the capacity factor would increase from 40-53% to 65-80%, up to four times as high as solar fields without storage capacity. The capacity factor (CF) describes the ratio of real annual energy conversion in comparison to its theoretical maxi- mum, i.e. 24/24 continuous conversion throughout the year.

Therefore, it is the object of the current invention to provide a process and a relating plant avoiding all problems caused by a liquid or gaseous heat transfer carrier. This object is solved by the subject matter of claim 1 .

According to the present invention, a heat transfer carrier is heated to a temperature between 700 to 1500 °C, preferably to a temperature between 800 and 1200 °C, in a high temperature receiver. From there, the heat transfer carrier is passed to at least one steam heating unit where it heats steam directly and/or indirectly. The heat transfer carrier is further passed to at least one steam producing unit to generate steam, preferably to produce steam from condensate. After the at least one steam producing unit the heat transfer carrier is recirculated to the high temperature receiver.

It is the essential part of the idea underlying the invention that the heat transfer carrier is a stream of preferably partly fluidized, solid particles with a melting point of more than 1200 °C while the heat transfer carrier is heated in the high temperature receiver by solar radiation. It is most preferred that for a more easy transport these solid particles are fluidized during the whole process.

Thereby, it is possible to avoid all disadvantages coupled to a liquid or gaseous heat transfer carrier. In a preferred embodiment of the invention the heat transfer carrier has a temperature between 200 and 500 °C after leaving the at least one steam generating unit. Thereby, the process is very effective due to the high temperature difference and, therefore, high energy transfer. In another preferred embodiment, the heat transfer carrier is not only heated but also pressurized in the high temperature receiver. Preferably, it is pressurized to a pressure between 200 and 350 bar, preferably between 250 and 300 bar. So, a higher energy absorption rate of the heat transfer carrier is achieved. It is preferred that electrical energy is produced from at least part of the steam. In a preferred embodiment, at least part of the heat transfer carrier is alternatively or additionally stored in a storage silo, which is preferably located downstream of the reactor. Preferably, the hot heat transfer carrier is stored in the storage silo. In accordance with a preferred embodiment, the storage silo is thermally isolated such that the temperature decrease of the stored heat transfer carrier is less than 100 °C per hour, preferably less than 50 °C per hour, most preferred less than 10 °C per hour. Thereby, the stored material keeps most of its thermal energy and a continuous process even during night time is possible. Preferably, any solid particles sharing the properties of high density (2500- 4000 kg/m ³ ) high thermal stability (>1000 °C) and high capacity (600 to 1500 preferably 700-900 J/kg °C) could be used as a heat transfer carrier. Even more preferred the material of the solid particles is selected from the group comprising S1O2, AI2O3, CaF ₂ and/or CaSO ₄ . Mixtures of these compounds may be used as well. S1O2 as a very cheap and easy available material is most preferred.

In one embodiment of the invention the heat transfer carrier is passed from the high temperature receiver to at least one heat exchanger to superheat steam. After the superheating, the heat transfer carrier is fed to a fluidized bed heat exchanger wherein it is used as bed material . Thereby, it is possible to evaporate condensate in a direct or in an indirect heat transfer. Afterwards the heat transfer carrier is recirculated to the high temperature receiver. In a preferred option the used condensate is recycled for energy production. In another embodiment, the heat transfer carrier in the high temperature receiver is fed to at least one steam heating stage in which the heat transfer carrier comes in direct contact with a stream at least partly consisting of steam. Afterwards the combined stream of heat transfer carrier and steam is passed to a relating separating stage to separate the heat transfer carrier from the heated steam.

Preferably at least two units in the steam heating stage are sequentially switched such that the heat transfer carrier separated in a first unit of the steam heating stage is used as the heat carrier for a second unit of the steam heating stage. Thereby, it is possible to arrange a number of steam heating units for a further increase of the heat transfer.

In an even more preferred embodiment said at least two contact units and two separating units are sequentially switched such that steam produced in the fluidized bed is passed to the contact unit passed as second by the heat transfer carrier. The steam separated in the relating separating stage is passed to the contact unit passed before. Thereby, the steam flows countercurrent to the heat transfer unit.

In another preferred embodiment, a bypass stream is split off from at least one stream of steam and/or heat transfer carrier and recirculated to the high temperature receiver. So, process parameters, e.g. temperature or pressure can be controlled and/or regulated.

In each embodiment a fluidized bed is used, the heat transfer can be performed by direct heating wherefore the condensate is sprayed into the bed or by indirect heating, e.g. having cooling coils or plates in the fluidized bed. A direct heating is preferred since abrasion on the cooling devices in the fluidized bed can be avoided. Further, in direct heating system the heat transfer rate is higher.

The invention also is directed to a plant for producing steam with the features of claim 1 1 . The plant is preferably configured to perform the process described above.

Such a plant comprises a high temperature receiver to a temperature between 700 to 1500 °C, at least one steam heating unit wherein steam is directly and/or indirectly heated by a heat transfer medium, at least one steam generating unit wherein steam is generated by heating with the heat transfer medium and a recirculating line wherein the heat transfer carrier is recirculated to the high temperature receiver.

As it is the basic inventive idea, the high temperature receiver is designed for heating the heat carrier by solar radiation and the whole plant features a construction design for using, preferably at least partly fluidized solid particles as a heat transfer medium. This leads to a very effective system using solar power.

In a preferred embodiment, a plurality of heliostats is used to direct the solar radiation to the high temperature receiver, such that the heat carrier is heated by the solar radiation inside the high temperature receiver. As discussed above, this configuration leads to an extraordinary high rate regarding efficiency and storage. In a further preferred embodiment, the plant comprises at least one storage silo for receiving the hot heat carrier, preferably directly from the high temperature receiver, and storing of said heat carrier for an adjustable duration. In other words, the storage silo is configured to receive and store hot heat carrier. Preferably, the storage silo comprises a thermal insulation in order to maintain the temperature of the stored heat carrier and to minimize heat losses. In case the solar radiation is not sufficient for heating the heat carrier to the required temperature, stored hot heat carrier can be removed from the storage silo and be subjected to the reactor in order to maintain the required calcination temperature therein. Most preferred the storage silo's size is adjusted such that the plant can be operated continuously, even at night times. Consequently, the production of electricity is independent from solar irradiation, which increases the capacity factor of the plant to 65 to 85%.

In a preferred embodiment the at least one steam generating unit is a venturi, wherein steam and the heat transfer carrier is mixed. Thereby a direct heat transfer and, therefore, a higher energy transfer rate is achieved.

A typical separation device to separate steam and heat transfer carrier after the mixing these two components is a cyclone. However, also a filter or the like is possible.

Preferably, the at least one steam generation unit can be a fluidized bed reactor having the advantage of a very good heat transfer. All types of fluidized bed reactors, especially stationary fluidized bed reactors, annular fluidized bed reactors and circular fluidized bed reactors are possible.

To produce electricity from the steam, it is preferred to use an expansion turbine to produce electricity from the steam. Typically, high efficiencies of 80 % or even more are achieved.

Developments and advantages in application possibilities of the invention also emerge from the following description of the process. All features described and/or illustrated in the drawings form the subject matter of the invention are per se or in any combination independent of their inclusion in the claims or their back references.

In the figures:

Fig. 1 a first embodiment of the inventive concept,

Fig. 2 a second embodiment of the inventive concept and

Fig. 3 a variation of the second embodiment of the inventive concept Fig. 1 shows a first embodiment of the invention directed to a direct heat transfer from the heat transfer carrier (HTC).

Via conduit 131 , a heat transfer carrier consisting of solid particles from a heat transfer circle is fed to a high temperature receiver (HTR) 1 10, preferably at the top of a tower irradiated by concentrated solar power. In the high temperature receiver 1 10, the heat transfer carrier is heated to a temperature between 800 and 1200 °C and pressurized to 250-320 bar. Additionally, a make-up may be fed to the high temperature receiver 1 10 via line 1 1 1 to compensate for losses in the system flow of the heat transfer carrier.

The heated heat transfer carrier, e.g. featuring a temperature of 1000 °C, is fed via conduit 1 12 to the steam heating unit 120, e.g. a shaft heat exchanger where it superheats steam for example to a temperature of 700 °C. Thereby, the heat transfer carrier cools down. For the given value, its temperature will fall to 600 °C.

From the steam heating unit 120 the partly cooled heat transfer carrier is carried via conduit 121 into a steam production unit 130 where it heats condensate to steam. Here, the steam production unit 130 is a fluidized bed. The steam tempera- ture for the given example will be 370 °C. The heat transfer carrier leaving the fluidized bed heat exchanger 130 is fed back via conduit 131 to the high temperature receiver 1 10, preferably by a not shown bucket elevator. A typical temperature of the heat transfer carrier in conduit 131 is 300 °C. The superheated steam, here with a temperature of 700 °C and 250 bar, is fed via conduit 122 to turbine 140 to generate power. For an efficiency of 85 %, the given values will lead to 240,5 MW.

After the turbine 140, part of the steam, e.g. with a temperature of 105 °C, is fed via conduit 141 to a degasser 142. From the degasser 142, a first stream is fed via conduit 143, 143' and a quench water pump 144 into the shaft heat exchanger 120, where it is superheated as described above.

A second stream of the condensate is fed via conduit 145, 145' and boiler conden- sate pump 146 into the fluidized bed heat exchanger 130 for producing steam by heat transfer from the heat transfer carrier. Afterwards, the steam from the fluidized bed heat exchanger 130 is passed into the shaft heat exchanger 120 via conduit 131 where it is also superheated. Another part of the steam is fed via conduit 151 from the turbine 140 into a condenser 150 where the steam is condensed by cooling with cooling water passed through the condenser 150 via conduits 152 and 153.The condensate from the condenser 150 is fed via conduits 154, 156 and pump 155 into the steam production unit 130.

In this example, the steam production unit is designed as a fluidized bed reactor, wherein the solid particles of the heat transfer carrier are fluidized via a fluidizing gas, e.g. air, via conduit 127, 127' and blower 128. The heat transfer carrier can transfer the heat directly by direct injection of the condensate. Also an indirect heating is possible, whereby the condensate is recirculated through the fluidized bed, for example in cooling coils or plates

Fig. 2 shows a second embodiment of the invention directed to a direct steam and electricity generation.

Recycled heat transfer carrier enters the high temperature receiver 210. For example, the entering heat transfer carrier has a temperature of 300 °C, is heated to 800 °C and pressurized to 260 bar. Via conduit 21 1 , a make-up feed of the heat transfer carrier may be fed to the high temperature receiver 210 to compensate for losses in the system flow. The system is pressurized either by steam from a splitter 213 or by the condensate from a degasser 242.

The heated heat transfer carrier flows via conduit 212, splitter 213 and conduit 214 to a venturi 220 where the heat transfer carrier gets in contact with steam, e.g. at 750 °C. The heat transfer carrier/steam flow is fed to a cyclone 222 where the main heat transfer carrier fraction is separated from the steam. The steam flows via conduit 223 to a splitter 224 from which steam is fed to the high temperature receiver 210 for pressurizing the heat transfer carrier. For the given values 6 wt-% of the steam are fed into the high temperature receiver 210. The remaining steam is passed via conduit 241 into a splitter 243. From there, it passes a filter 244 before being fed into a turbine 240.

Further parts of steam flows via conduits 227, 247 in a second filer 248 and is also fed to the turbine 240. Both filters typically operate at a temperature of 500 - 800 °C

Also steam used to pressurize the heat transfer carrier can be fed via conduits 235' and 235" to a degasser 242 and or via conduit 235, 235' into the splitter 224. From splitter 224, it flows further via conduit 247 to a filter 248 where contained dust is removed so the steam can be also fed via conduit 249 into the turbine 240.

A not shown process option allows feeding the steam to a second mixer from where superheated steam (e.g. 300°, 85 bar) is fed to a filter operating at normal temperatures for removing dust.

As described, cleaned steam is fed via conduits 245 and 249 to an expansion turbine 240. Such a turbine has e.g. 85% efficiency. The outgoing steam is fed via conduit 251 to a water cooled condenser 250 cooled with water via conduits 252 and 253. From the condenser 250 the condensate stream is fed to the upstream degasser 242 via conduit 254. From the degasser 242 the condensate flow may goe though conduit 281 to a not-shown pressure pump (e.g. 150 bar) and to a mixer 280, where it is pre-heated for example to 320 °C by the heat transfer carrier/steam flow coming via conduit 274 from the last stage cyclone 272. Further, heat transfer carrier is recycled into the high temperature receiver 210 via conduit 246.

After pre-heating, the condensate is fed via conduit 282 to a hydrocyclone 230 from where the slurry may go to another not shown pump, where it is pressurized e.g. to 260 bar. From the hydrocyclone 230 most of the heat transfer carrier is recycled via conduit 232 into a pressure relief system 233 and from there via conduit 234 into the high temperature receiver 210. A typical temperature of the recycled heat transfer carrier is 300 °C. Further on the slurry comprising heat transfer carrier and steam is fed via conduit 231 to a venturi 270.

In the venturi 270, heat transfer carrier from the high temperature receiver 210 is passed via splitter 213 and conduit 215 and contacted with the heat transfer carrier (together with contained steam) from a cyclone 272. For the given example values here the steam is superheated to 470 °C. The mixture of heat transfer carrier and steam leaving the cyclone 272 is passed via conduit 273 into a venturi 260, where it is contacted with heat transfer carrier out of conduit 224. For example, this results in a mixture temperature of 750 °C. In the given example, the steam separated from the heat transfer carrier in cyclone 262 is further passed to the venture 220, which has already been described above. In principles, the cycles comprising one venturi and a relating cyclone can be repeated N-times for optimal preheating and high conversion efficiency, whereby N is preferably a number between 3 and 10 In principle, the heat transfer carrier can be either directly used for conversion of heat to steam and electricity or safely stored in simple concrete silos for use during the night enabling continuous (24/24 hours) plant operation. Consequently, the conversion of heat to steam and electricity is independent from solar irradiation increasing the capacity factor of concentrated solar power plants to 65-85%. Fig. 3 shows in principle the same structure already known from Fig. 2. The heat transfer carrier (HTC) is fed to a high temperature receiver (HTR) 310 at the top of a tower irradiated by concentrated solar power. In the high temperature receiver 310, the heat transfer carrier is heated and can be either directly used for conversion of heat to steam and electricity or safely stored in simple concrete silos for use during the night enabling continuous (24/24 hours) plant operation. Consequently, the conversion of heat to steam and electricity is independent from solar irradiation increasing the capacity factor of concentrated solar power plants to values between 65-85%. The heat transfer carrier is in a continuous circulating flow in the system. Typically, it enters the high temperature receiver 310 at temperatures between 250 and 350°C, is heated to 800 to 1 100 °C and pressurized to 250-320 bar.

The heat transfer carrier is fed to the high temperature receiver 310 from i) the internal recycling loop via conduit 362 or ii) the not shown cold silo. From the high temperature receiver 310 the heat transfer carrier to i) the first of a cascade of Venturis 320, 360, 370 or ii) a not shown hot silo.

Turning to the normal operation mode, the heat transfer carrier flows to a ven- turi 320 where the heat transfer carrier gets in contact with the flow of heat transfer carrier in steam, e.g. at 750 °C. From venturi 320 the heat transfer carrier/steam flow is fed to a cyclone 222 where the main heat transfer carrier fraction is separated from steam. The steam comprising some solid particles of the heat transfer carrier flows to a splitter from which preferably parts of the steam (e.g. 6 wt-%) are fed to the high temperature receiver 310 for heat transfer heating and pressurizing. The remaining parts are fed to a mixer 343 where it is quenched with condensate of a condenser 350, e.g. to a temperature of 700 °C. The quenched steam is cleaned in a hot gas filter 344 to remove dust from the solid heat transfer carrier.

The clean steam is fed to an expansion turbine 340 and converted to electricity, typically with 85% efficiency. The outgoing steam is fed to the above mentioned water cooled condenser 350 from where a condensate stream is fed to the upstream mixer 343.

Another condensate stream is fed to a mixer 432 where the condensate is contacted with steam and pre-heated. From the degasser 342, the condensate in injected into a fluidized bed heat exchanger 330 where it is further pre-heated by the heat transfer carrier coming from the last stage cyclone 372 and the depres- surizing unit 380 where the heat transfer carrier flow pressure is reduced, e.g. from 45 bar to 3 bar at a temperature of 420 °C.

Similar to Fig. 2, the condensate is fed to a venturi 370 and contacted with the heat transfer carrier which passes this venturi 370 as the last one of all Venturis 320, 360 and 370. Thereby, the condensate is heated by the heat transfer carrier in counter current flow.

The heat transfer carrier is fed from the fluidized bed heat exchanger 330 back to the high temperature solar receiver 310. Reference numbers

110 high temperature receiver 111,112 conduit

120 steam heating unit

121,122 conduit

130 steam production unit

131-133 conduit

134 filter

135-137 conduit

138 blower

140 turbine

141 conduit

142 degasser

143 conduit

144 pump

150 condenser

151-154 conduit

155 pump

156 conduit

210 high temperature receiver

211,212 conduit

213 splitter

214,215 conduit

220 venturi

221 conduit

222 cyclone

2234 splitter 225-228 conduit

230 hydrocyclone

231 -232 conduit

233 pressure relief system 234,235 conduit

240 turbine

241 conduit

242 degasser

243 splitter

244 filter

244' conduit

245-247 conduit

248 filter

248' conduit

250 condenser

251 -254 conduit

256 conduit

260 venturi

261 conduit

262 cyclone

263,264 conduit

270 venturi

271 conduit

272 cyclone

273,274 conduit

280 mixer

281 ,282 conduit 310 high temperature receiver 31 1 ,312 conduit 313 splitter

314,315 conduit

320 venturi

321 conduit

322 cyclone

323 conduit

324 splitter

325-328 conduit

330 fluidized bed heat exchanger

331 -332 conduit

333 pressure relief system

334,335 conduit

340 turbine

341 conduit

342 degasser

343 splitter

344 filter

345,346 conduit

350 condenser

351 -356 conduit

360 venturi

361 conduit

362 cyclone

363,364 conduit

370 venturi

371 conduit

372 cyclone

373,374 conduit

380 mixer

381 -383 conduit