WO2014026746A1

WO2014026746A1 - A solar receiver with a heliostat field

Info

Publication number: WO2014026746A1
Application number: PCT/EP2013/002357
Authority: WO
Inventors: Philipp Schramek; Antoine Bittar; Fabian GROSS
Original assignee: SOLAR TOWER TECHNOLOGIES AG
Current assignee: SOLAR TOWER TECHNOLOGIES AG
Priority date: 2012-08-17
Filing date: 2013-08-06
Publication date: 2014-02-20
Anticipated expiration: 2015-02-17
Also published as: WO2014026703A1

Description

A solar receiver with a heliostat field- Description

The invention is in the field of solar central receiver systems with a heliostat field, comprising a solar receiver to collect concentrated solar radiation and a plurality of heliostats form^¬ ing the heliostat field.

Prior Art

In the following the basic functionality of existing and known solar central receiver systems and existing and known receiver technologies for such solar central receiver systems are described.

Fig. 1 shows a solar central receiver system as known from

US 4 172 443 which consists of a tower 120, on which a Receiver 200 is installed, onto which heliostats 110 concentrate direct solar radiation. The heliostat field 120 consists of numerous of such heliostats 110. At some known solar towers more than one Receiver can be installed on one tower, as shown in

EP 2000669 A2. The concentrated solar radiation heats a heat transfer medium which is circulated through the Receiver as shown in [5] - specifically on pages 237ff. The heat captured in the medium can then be used to drive a turbine to produce electrical power through a mechanically connected generator.

Additionally to the shown system, various Receiver technologies suitable for these solar plants are known.

External Receivers

Some receivers are constructed as shown in Fig. 1 where the lateral surface of a cylindrical receiver is the surface that absorbs the incoming solar radiation, called the absorber surface. Such a receiver where the outer surface is the absorber, with a so called external absorber 231 are called external receiver. Other external receivers can have their absorbing external surface oriented with a major orientation as known from Fig. 20 in DE 10 2010 034 986 Al which is shown in Fig. 2. The major orientation in Fig. 2 is downwards but can be as well sideways.

Cavity Receivers

Other receivers are so called cavity receivers where the absorber is concave or placed inside a cavity as known from Fig. 15 in DE 10 2010 034 986 Al which is shown in Fig. 3. The principle of a cavity receiver is also known from patents

US 4 220 140, WO 2010/118276 A2 and WO 2008/153922 Al, respectively.

Receiver Array

A number of receivers can be combined into an array of receivers known from Figure 2 in [1]. In this instance all receivers produce heat at the same temperature.

Multi-section Receivers

Receivers consisting of multiple sections are known. In

US 2009 217 921 Al a receiver is shown with two separate sections of external absorbers for different purposes. The heat transfer medium is in this case water as fluid and as steam. One section is a water economizer and boiler while the second section is a steam superheater. The heliostats of the associated heliostat field are controlled in such a way as to aim the di^¬ rect solar radiation onto the separate sections according to power requirements of the receiver sections.

The Receiver with multiple sections in US 2012 096 859 Al also has sections operated at different temperatures.

Another, different, multi-section receiver is known from

WO 2010/118276 A2 where two cavity sections of the receiver have the same function and are operated at the same temperatures but are oriented towards different parts of the heliostat field. Receiver with external absorber and cavity

Receivers consisting of an external absorber and a cavity are known from [2] and [3] . Fig. 4a) shows the receiver described in [2] where the receiver has a cavity with an opening to one^' side for superheating at high temperature while the remaining outer surface of the receiver is an external absorber oriented towards all other sides and are used for preheating and boiling which takes place at lower temperatures. In Fig. 4b) which is taken from [2] it can be seen that one part of the heliostat field (in this case the northern part of the heliostat field) is directing the solar radiation onto the cavity receiver section 220 while another part is directing it onto the external receiver section 230.

In Fig. 5 a receiver is reproduced from [3] with an external ab^¬ sorber which is described to be used for lower temperatures and a cavity for higher temperatures. The receiver is designed for use in a paraboloidal dish as seen in Fig. 6 where the receiver is mechanically connected to the dish and moves with the dish to track the sun. In this case the receiver, specifically the cavity of the receiver, is basically at all times tilted in some way as the dish follows the movement of the sun. The dish reflector concentrates the solar radiation onto the receiver where the radiation from the outer part of the dish is mainly collected by the external absorber while the radiation reflected from the central region of the dish is collected by the cavity of the receiver.

Direct Steam Receiver

A direct steam receiver is a receiver having one or more receiver sections and with external absorbers or cavity which converts the solar radiation into heat, and thus heats, boils and in some cases superheats water or another heat transfer fluid. This is known from WO 2010/118276 A2. Fluid Receiver

A fluid receiver is a receiver with one or more receiver sections and with external absorbers or cavity which converts the solar radiation into heat which heats a heat transfer fluid, commonly thermal oil or molten salt. This is known from [7].

Gas Receiver

A gas receiver is a receiver with one or more receiver sections and with external absorbers or cavity which converts the solar radiation into heat which heats heat transfer medium in the gas phase such as air.

One special type of gas receivers is the pressurised air receiver known from [1] where compressed air is heated up inside the receiver.

Another special type of gas receiver is the open volumetric receiver known from [6] and DE 19744541 (Al) where ambient air is sucked through a porous absorber surface which typically is an external absorber.

Directly Illuminated Receiver

In a directly illuminated receiver a heat transfer medium is directly illuminated and therefore directly absorbs the concentrated solar radiation. The heat transfer medium can be solid particles like sand or ceramic particle or a coloured fluid or gas. This is known, from US 8109265 (Bl)

Concentrating Photovoltaic (CPV) Receiver

A CPV receiver is a receiver with one or more receiver sections and with external absorbers or cavity which converts the solar radiation directly into electrical power using semiconductor based cells. This is known from [4]. Problem, Solution and Advantage of the Invention

The problem of the invention is to maximise the collection and output of a solar central receiver system by synergistically designing a heliostat field and at least one appropriate receiver to maximise the collection of the concentrated radiation of the heliostat field onto a multi-section receiver and to reduce its losses, especially those due to convection, radiation and conduction .

This problem is solved by a solar central receiver system with a heliostat field as defined in claims 1 or 6, by the use of this solar central receiver system with a heliostat field as defined in claim 11 and by the method of collecting concentrated solar radiation as defined in claim 12 by a solar central receiver system with a heliostat field as defined in any of claims 1 and 6.

The central receiver system with a heliostat field comprises at least one solar multi-section receiver to collect concentrated solar radiation and a plurality of heliostats forming the heliostat field comprising a near field, which may be extended by an intermediate field and a peripheral field, and located on a ground area underneath the solar multi-section receiver.

The at least one solar multi-section receiver comprises at least two receiver sections where any of these receiver sections is selected from the following list: i. a cavity receiver section having a cavity with an aperture, preferably a single aperture, and inside the cavity a thermal absorber, where the aperture of the cavity forms the receiver target area of this cavity receiver section;

ii. an external thermal receiver section comprising an external absorber having a thermal absorber on the outer surface of the external thermal receiver section, where the external absorber is the receiver target area of this external re^¬ ceiver section;

iii. an external photovoltaic receiver section comprising an external absorber having a photovoltaic absorber on the outer surface of the external photovoltaic receiver section, where the external absorber is the receiver target area of this external photovoltaic receiver section;

A plurality of heliostats form the heliostat field located on a ground area underneath the solar multi-section receiver, each heliostat having one reflector or a group of reflectors, whereby each reflector or group of reflectors is pivotable about two axes to reflect solar radiation onto at least one of the at least two receiver target areas.

One of the at least two receiver sections forms a first receiver section which is facing downwards onto the near field of the heliostat field underneath the solar multi-section receiver, where the heliostats are oriented to reflect solar radiation onto the receiver target area _.of this first receiver section.

Another of the at least two receiver sections forms a second receiver section, which surrounds the first receiver section and where the receiver target area of the second receiver section collects reflected solar radiation which does not hit the re^¬ ceiver target area of the first receiver section and wherein some heliostats of the intermediate field, and some heliostats of the peripheral field of the heliostat field may be oriented to reflect solar radiation onto parts of the receiver target area of the second receiver section and wherein other heliostats of the intermediate field and other heliostats of the peripheral field may be oriented to reflect solar radiation onto parts of the receiver target area of the first receiver section.

The first receiver section is the hottest receiver section dur^¬ ing operation and is formed by a cavity receiver section where the internal surface Ai_S of its cavity is larger than the area A_ai of the cavity aperture of its cavity by the factor Ai_S/A_ap > 2.

The second receiver section is formed in one embodiment by an external thermal receiver section and in another embodiment by an cavity receiver section .

In a further embodiment the first receiver section is the hottest receiver section during operation and is formed by an external thermal receiver section whereby in another embodiment the first receiver section is formed by an external photovoltaic receiver section .

Each receiver section of a solar multi-section receiver is operated at different temperatures to reduce the losses of the hot^¬ test first receiver section. In a further embodiment solar ra^¬ diation is collect with a second and third receiver section which does not reach the receiver target area of the first re^¬ ceiver section. The second and third receiver sections are oper^¬ ated at lower temperature levels than the first receiver sec^¬ tion.

The first receiver section is operated at the highest tempera^¬ ture and receives concentrated solar radiation from the helio- stat field at the highest level of concentration. The receiver target area of the first receiver section is downward facing to be oriented optimally to receive radiation from the nearest heliostats of the heliostat field underneath the receiver, the near field, which has the highest level of concentration, thus reducing the convection losses. Typically, the first receiver section is implemented as a cavity receiver section as in to ac^¬ cept the highest level of concentration onto the first receiver target area; the reduced area of a cavity aperture of the cavity receiver section, leads to a reduction of the radiative losses of the first receiver section which are directly proportional to the size of the aperture. Further, the downward facing aperture of a cavity has much reduced convective losses. Also, the ab- sorptance and hence collection of the concentrated solar radiation of a cavity receiver with an absorber inside the cavity is higher than those of an external absorber when the absorbers 224 and have the same absorptance and emittance properties. Addi^¬ tionally a cavity aperture can withstand much higher levels of concentration of solar radiation than an external absorber since at the aperture of the cavity the concentrated solar radiation does not interact with any solid materials. Instead the concen^¬ trated solar radiation interacts inside the cavity with an absorber where the concentration is already reduced because the radiation is spreading out behind the point of highest concentration. Therefore the absorber in the cavity is placed in such a way that the level of concentration is acceptable for the ma^¬ terials of the absorber. Therefore the internal surface A_s of its cavity may be selected to be larger than the area A_ap of the cavity aperture of its cavity by the factor A±_s/A_ap > 2.

The second receiver section collects the solar radiation which cannot be collected by the first receiver section and would oth^¬ erwise be lost as spillage. This allows further reduction of the size of the aperture of the first receiver section to optimise the collection of high concentrated radiation. This results in reduction of the radiative and convective losses of the first receiver section without reducing the overall collection of ra^¬ diation through spillage. The second receiver section is oper^¬ ated at a lower temperature and therefore the losses per re^¬ ceiver target area, which strongly depend on the temperature, are much lower. The second receiver target area can be oriented and shaped to collect radiation from the heliostats further away from the receiver and not necessarily those directly under the receiver, the intermediate field, while still collecting the spillage of the first receiver section. Fig. 8 a) and b) shows the intersection of two such possible implementations where the external absorber of the second receiver section 212 is oriented outwards .

The second receiver section may be a cavity receiver section.

In some cases, e.g. if the heliostat field is smaller than three times the optical height of the receiver above the heliostat field, the second receiver section has a downward facing receiver target area.

For heliostats further away from the receiver a vertical receiver target area is more suitable than a horizontal one.

Therefore for heliostat fields which have a large number of heliostats with a horizontal distance from the receiver greater than two times the optical height (difference of height of the pivot point of the heliostat to the centre of the receiver) of the receiver over the heliostat field a third receiver section 213 can be foreseen which is curved and oriented towards the pe^¬ ripheral field of the heliostat field.

For a solar central receiver system with a heliostat field where the peripheral field and the intermediate field are partly stretched out to one side of the multi-section receiver, typically on the side of the multi-section receiver closer to the nearest geographic pole (North in the northern hemisphere, South in the southern hemisphere) , a multi-section receiver with an additional fourth receiver section may be used as in. The fourth receiver section is located below the receiver target area of the second receiver section. The receiver target areas of the third receiver section and the fourth receiver section are placed and oriented in such a way that they collect solar radia^¬ tion which is not collected by the first and second receiver sections. This is basically concentrated solar radiation from the heliostats from the peripheral field and the intermediate field. The fourth receiver section may be a cavity receiver sec^¬ tion as in instead of an external receiver section to take ad- vantage of the lower losses of a cavity receiver section compared to an external receiver section.

For some applications the first receiver section is an external receiver section which is especially the case if the first re^¬ ceiver section has an external photovoltaic receiver section.

In a further embodiment a solar multi-section receiver has volu^¬ metric absorbers and would be designed such that it has a cavity receiver section as first receiver section with volumetric absorbers at the ceiling of the cavity. The lower levels of the cavity walls surrounding the aperture of the cavity receiver section is the second receiver section and the outer sides of the cavity the third receiver section. Both, the second and third receiver sections have volumetric absorbers as external absorbers sucking the gaseous heat transfer medium, e.g. ambient air, through the irradiated and heated volumetric absorbers of the second and third receiver sections and preheating the air before it enters the cavity. In the cavity the air is heated to a higher temperature by sucking it through the cavity absorber inside the cavity which is formed by a volumetric absorber.

It is advantageous to use a solar multi-section receivers with a heliostat field with a downwards facing receiver as defined in DE 10 2010 034 986 Al or with a heliostat field which has a region under at least one multi-section receiver which has a reflector area density p > 50%; the reflector area density p is defined as the ratio of the total reflector area of all helio- stats in a defined region to the total ground area of the defined region.

Typically the multi-section receiver is used for solar central receiver systems where the power of the concentrated radiation collected by the multi-section receiver is greater than 1 MW. To minimise the convection losses from the cavity receiver section of the first receiver section 211 it is oriented downwards. However in some applications the orientation may be tilted away from this orientation by up to 30° to optimise the collection of solar radiation from the heliostat field.

The invention allows arbitrary selection of the inner volume of the cavity, ensuring acceptable peak flux values at the cavity absorber and internal cavity walls adapted to physical and thermal properties of the materials used for the cavity absorber and internal cavity walls.

The invention allows the position and structures of the second receiver sections to be independently chosen in order to collect a maximum of the flux distribution not collected by the first receiver section.

The invention allows for either receiver section to be sized independently of the others.

The invention allows for the more than one receiver sections to be independently selected and designed to collect and reduce spillage with lower concentration which can be used by receivers sections operated at accordingly lower temperatures.

The invention allows the design of optimized heliostat fields for such multi-section receivers where the heliostat field has different regions reflecting their concentrated ^' solar radiation onto different receiver sections.

Advantage of the invention over the prior art :

The multi-section receiver differs from commonly used receivers, cavity or external receivers, by the fact that it has more than one receiver section where each receiver section is operated at a different temperature. In this way the design of each receiver section can be optimised for a specific purpose. Therefore a multi-section receiver usually consists of a combination of cav^¬ ity receiver sections and external receiver sections.

The multi-section receiver, with a cavity receiver section as first receiver section, is typically used to achieve the highest possible levels of concentration at the aperture of the cavity receiver section while the second receiver section surrounds the first receiver section and therefore collects the radiation which is not collected by the first receiver section and which would be otherwise lost by spillage.

Multi-section receivers for solar central receiver systems are known from US 2009 217 921 Al, US 2012 096 859 Al and

WO 2010/118276 A2. They all differ from the invention. The receiver sections of the known multi-section receivers are placed next to each other and therefore are not constructed such that the one section is collecting the radiation not collected by the other receiver section. Additionally these multi-section receivers do not have a consistently downward facing first receiver section which is operated at a higher temperature than the second receiver section which surrounds the first receiver section. Especially those multi-section receivers differ from the invention of the multi-section receiver in that they have either two external receiver sections or two cavity receiver sections whereas the present multi-receiver may have at least one cavity receiver section and at least one external receiver section.

A multi-section receiver which has a single external receiver section and a single cavity receiver section which is operated at a higher temperature is known from [2] and shown in Fig. 4a. The cavity receiver section of the known multi-section receiver has an aperture which is essentially oriented to the side facing a slice of the heliostat field which is indicated in Fig. 4b. The cavity aperture which is essentially oriented to the side leads to higher convection losses than a downwards facing cavity receiver section as in the invention. Convection losses from a cavity increase strongly the further the aperture of a cavity receiver section is rotated from the horizontal downwards facing orientation. Therefore the invention has a cavity receiver section where the receiver target area, typically the aperture, is essentially horizontal, facing downwards, within a range of maximum angle of 30°.

Additionally in the multi-section receiver known from [2] the receiver target area of the cavity receiver section is not fully surrounded by the receiver target area of the external receiver section. The external receiver section is oriented in such a way as to collect radiation from the other parts of the heliostat field shown in Fig. 4b) which are not part of the first slice.

The multi-section receiver with a cavity receiver section 220 and an external receiver section known from [3] and shown in Fig. 5 is designed for a paraboloidal dish where the receiver must be located in the plane formed by the rim of the dish reflector Fig. 6, and is designed for thermal power levels of less than 50 kW to heat water at temperatures below 100 °C. The cavity of this receiver also differs from the invention in the above mentioned aspect; it is typically not facing downwards since the receiver rotates with the paraboloidal dish as the latter follows the sun's path.

Further this receiver is positioned at the plane of the rim of the paraboloidal reflector and cannot operate with a heliostat field where the plane of the heliostat field is, perforce, substantially below the receiver plane

Finally, this receiver is designed and operated for thermal power levels below 50 kW to heat water at temperatures below 100°C whereas the multi-section receiver of the invention addresses operations at temperatures of more than 300°C and with solar radiation from the heliostat field of more than 1-600 MW power rating.

Short description of the figures

In the Figures a numbering is applied as follows:. The figures show:

Fig. 1 Prior Art. A central solar receiver system with a receiver 200 which is held by a tower 105 in position with an optical height of H_R over the heliostat field 120 with heliostats 110 directing reflected solar radiation 130 onto the receiver 200 atop the tower 105. Known from

US 4 172 443

Fig. 2 a) Prior Art. A cross-section drawing and b) a perspec^¬ tive drawing of a downward facing external receiver section 230 with an external absorber 231 as known from DE 10 2010 034 986 Al .

Fig. 3 a) A cross-section drawing and b) a perspective drawing of a downward facing cavity receiver section 220 with in^¬ ternal cavity absorbers 224 as known from

DE 10 2010 034 986 Al . Fig. 4 Prior art. A multi-section receiver 205 with external re^¬ ceiver section 230 and a cavity receiver section 220 with an aperture open to the side known from [2] b) sketch of the related heliostat field 120 and aiming strategy for said receiver system where the heliostats 110 indicated in the slice of the heliostat field 120 in the north are aiming onto the cavity receiver section 220 while the parts of the heliostats field 120 east, west and south of the receiver are aiming onto the external receiver sec^¬ tion . Prior art. A dish receiver with cavity 221 and an external absorber 231 for a liquid heat transfer fluid 280 as known from [3] . Reflected rays intercepted by the cavity or by the external absorber on their way to focal point.

Prior art. The multi-section receiver 205 from Fig. 5 system together with paraboloidal dish reflector 111.

An embodiment of the invention showing a solar multisection receiver with first receiver section 211 as cavity receiver section 220 with a second receiver section 212 with a downwards facing external absorber 231 fully surrounding the aperture 222 of the cavity receiver section 220: a) cross-section drawing, b) perspective drawing,

An embodiment of the invention showing a cross-section drawing of solar multi-section receiver 205 with first receiver section 211 as cavity receiver section 220 and second receiver section 212 with external absorber 231 with a) cylindrical external absorber 231 and b) conical external absorber 231. ^■

An embodiment of the invention showing a solar multisection receiver with where both receiver sections are cavity receiver sections 220 with adapted irradiation stemming from heliostats with different horizontal distance: a) apertures 222 in the same plane and b)and c) two variation of a first cavity 221 above the second cavity 221

An embodiment of the invention showing a solar multisection receiver with first receiver section 211 as cavity receiver section 220 with a second receiver section 212 with a downwards facing external absorber 231 fully surrounding the aperture 222 of the cavity receiver sec- tion 220 and a third receiver section 213 with a cylindrical external absorber 231. An embodiment of the invention showing zones of a 260,000 m² heliostat field 120 with a near field 121, an intermediate field 122 and a peripheral field 123. An embodiment of the invention showing a solar multisection receiver with where the first and second receiver section (211 & 212) are cavity receiver sections 220 with adapted irradiation stemming from heliostats with different horizontal distance: a) apertures 222 in the same plane and b)and c) two variation of a first cavity 221 above the second cavity 221 and where in all cases a third receiver section 213 has a cylindrical external absorber 231. An embodiment of the invention showing a solar multisection receiver with first receiver section 211 as cavity receiver section 220 with a second receiver section 212 with a downwards facing external absorber 231 fully surrounding the aperture 222 of the cavity receiver section 220 and a third receiver section 213 with a half cylindrical external absorber 231 and a fourth receiver section 214 with a flat tilted external absorber: a) cross-section and b) perspective view. An embodiment of the invention showing a cross-section of a solar multi-section receiver with first receiver section 211 as cavity receiver section 220 with a second receiver section 212 with a downwards facing external absorber 231 fully surrounding the aperture 222 of the cavity receiver section 220 and a third receiver section 213 with a half cylindrical external absorber 231 and a fourth receiver section 214 with a tilted cavity 221. An embodiment of the invention showing a cross-section of a solar multi-section receiver with first receiver section 211 as cavity receiver section 220 with a second receiver section 212 with a downwards facing external absorber 231 fully surrounding the aperture 222 of the cav^¬ ity receiver section 220 and a third receiver section 213 with a half cylindrical external absorber 231 and a fourth receiver section 214 with a flat tilted external absorber where the first and second receiver target area are tilted out of the horizontal.

Preferred embodiment 2 of the invention showing a cross section of solar multi-section receiver 205 with a) two receiver section and b) three receiver sections where the cavity absorber 224 and the external absorbers 231 are volumetric absorbers 244, where the ambient air 285 is first sucked through the external absorbers 231 of the second and third receiver section (212 & 213) into the cavity and then through cavity absorber and heated up.

Preferred embodiment 1 of the invention showing a solar multi-section receiver 205 as direct steam receiver with a first receiver section 211 as cavity receiver section 220 with a cavity absorber 224 as superheater 243 and a cavity absorber 224 as a boiler 242 and a second receiver section 212 with external absorber 231 facing downwards as preheater/economiser 241: a) principle perspective drawing and b) cross-section which shows the functions and the steam drum 273 for preferred embodiment.

Preferred embodiment 3 of the invention showing a solar multi-section receiver 205 with a cavity receiver section 220 as first receiver section 211 where the cavity absorber 224 is a cylindrical and a second receiver section 212 with a cylindrical external absorber 231. Fig. 19 Preferred embodiment 4 of the invention showing a solar multi section receiver with two cavity receiver section 220 above each other and a cylindrical external absorber 231.

Fig. 20 Preferred embodiment 5 of the invention showing a solar multi-section receiver 205 with a cavity receiver section 220 as first receiver section 211 where the cavity absorber 224 is at the ceiling of the cavity 221 and a second receiver section 212 with a downwards facing external absorber 231 and a third receiver section 213 with a cylindrical external absorber 231.

Fig. 21 Multi-section receiver with an external receiver section

230 as first receiver section 211 and a cavity receiver section 220 as second receiver section 212.

Fig. 21 Multi-section receiver with an external receiver section

230 as first receiver section 211 and a cavity receiver section 220 as second receiver section 212 and an external receiver section 230 as third receiver section 213.

Fig. 23 Two cases for a multi-section receiver with external receiver sections 230 as first and second receiver section (211 & 212) .

Description of the Invention

The aim of the invention is to maximise the collection and out^¬ put of a solar central receiver system by synergistically designing a heliostat field 120 (Fig. 1) and at least one appropriate receiver 200 (Fig. 1) to maximise the collection of the concentrated radiation 130 of the heliostat field 120 onto the at least one receiver 200 and to reduce its losses, especially those due to convection, radiation and conduction.

This is realised by having a multi-section receiver 205 (Fig. 7 and others) where each receiver section 210 is operated at dif- ferent temperatures to reduce the losses of the hottest first receiver section 211 while collecting solar radiation 130 which does not reach the receiver target area 251 of the first receiver section 211 with the second and third receiver section (212 & 213; Fig. 10) which are operated at lower temperature levels .

The first receiver section 211 is operated at the highest temperature and receives concentrated solar radiation 131 from the part of the heliostat field 120 which is near under the receiver, the near field 121 at the highest level of concentration. The receiver target area 251 of the first receiver section 211 is downward facing to be orientated optimally to receive radiation from the nearest heliostats 110 of the heliostat field underneath the receiver, the near field 121, which has the highest level of concentration, thus reducing the convection losses. Typically, the first receiver section 211 is implemented as a cavity receiver section 220 to accept the highest level of concentration onto the first receiver target area 251, the aperture 222 of the cavity receiver section 220, to be able to reduce the size of the aperture 222 to reduce the radiative losses of the first receiver section 211 which are directly proportional to the size of the aperture 222. Especially a downward facing aperture of a cavity 221 has much reduced convective losses. The ab- sorptance and hence collection of the concentrated solar radiation of a cavity with an absorber 224 inside the cavity is higher than those of an external absorber 231 where the absorbers 224 and 231 have the same absorptance and emittance properties. Additionally a cavity aperture 222 can withstand much higher levels of concentration of solar radiation than an external absorber 231 since at the aperture 222 of the cavity receiver section 220 the concentrated solar radiation does not interact with any solid materials like at an external absorber 231. Instead the concentrated solar radiation interacts inside the cavity 221 with an absorber 224 where the concentration is reduced because the radiation is spreading out behind the point of highest concentration. Therefore the absorber 224 in the cav^¬ ity 221 is placed in such way that the level of concentration is acceptable for the materials of the absorber 224.

The second receiver section 212 collects the solar radiation which cannot be collected by the first receiver section 211 and would otherwise be lost as spillage. This is radiation 131 from the near field 121 but especially radiation 132 from the helio- stat field further away from than the near field the, intermedi^¬ ate field 122 (Fig. 11). This allows further reduction of the aperture size of the first receiver section 211 to optimise the collection of high concentrated radiation. This results in reduction of the radiative and convective losses of the first re^¬ ceiver section without reducing the overall collection of radia^¬ tion through spillage. The second receiver section 212 is oper^¬ ated at a lower temperature and therefore the losses per re^¬ ceiver target area are much lower which strongly depend on the temperature. The second receiver target area 252 can be oriented and shaped to collect radiation from the heliostats further away from the receiver and not necessarily those directly under the receiver, the intermediate field 122-, while still collecting the spillage of the first receiver section. Fig. 8 a) and b) shows the intersection of two such possible implementations where the external absorber 231 of the second receiver section 212 is ori^¬ ented outwards.

If the heliostat field is smaller than three times the optical height of the receiver above the heliostat field it may be suf^¬ ficient to have a second receiver section 212 which has a down^¬ ward facing external absorber 231 as shown in Fig. 7 and suffi^¬ ciently sized to collect the lower concentrated radiation from the heliostat field which typically comes from the intermediate field 122. Fig. 9 a) to c) shows cross-sections of cases where the first receiver 211 is a cavity receiver section 220 and the second receiver section 212 surrounding the first receiver section is as well a cavity receiver section 220.

For heliostats 110 further away from the receiver a vertical receiver target area is more suitable than a horizontal one.

Therefore for heliostat fields which have a large number of heliostats with a horizontal distance from the receiver greater than two times the^' optical height (difference of height of the pivot point of the heliostat reflectors 111 to the centre of the receiver) of the receiver 200 over the heliostat field a third receiver section 213 can be foreseen which is curved and oriented towards the periphery 123 (Fig. 11) of the heliostat field. Then, the heliostats from the periphery 123 of the heliostat field may reflect the solar radiation 133 onto parts of the receiver target area 253 of the third receiver section 213. The third receiver section 213 is as well operated at a lower tem^¬ perature than the first receiver section 211.

Fig. 10 shows a multi-section receiver 205 with a cavity receiver section 220 as first receiver section 211 and the second receiver section 212 and third receiver section 213 each with external absorbers 231.

Fig. 12 a) to c) shows cross-sections for examples where the first receiver section 211 and the second receiver section 212 are both cavity- receiver sections 220 and the third receiver section 213 is an external receiver section 230 with an external absorber 231.

For a solar central receiver system with a heliostat field where the peripheral field 123 and the intermediate field 122 are ba^¬ sically stretched out to one side of the multi-section receiver 205, typically on the side of the multi-section receiver 205 closer to the nearest geographic pole (North in the northern hemisphere, South in the southern hemisphere), a multi-section receiver 205 with an additional fourth receiver section 214 (Fig. 13) is used. Additionally to the third receiver section 213 which is located above the receiver target area 252 of the second receiver section 212 another fourth receiver section 214 is located below the receiver target area 252 of the second receiver section 212. The receiver target areas (253 & 254) of the third receiver section 213 and the fourth receiver section 214 are placed in such way that they collect solar radiation which is not collected by the first and second receiver section (2ΐΊ & 212). This is basically concentrated solar radiation (132 & 133) from the heliostats from the peripheral field 123 and the intermediate field 122. Accordingly the receiver target areas (253 & 254) of the third and fourth receiver section are basically orientated to collect solar radiation (132 & 133) from the region where most part of the intermediate field 122 and peripheral field 123 are located. In case of multi-section receiver with four receiver sections where the intermediate and peripheral field (122 & 123) are basically stretched to the polar side of the multi-section receiver 205 all heliostats 110 of the complete heliostat field 120 are oriented to reflect solar radiation 130 on parts of the first receiver target area 251. On this way radiation which cannot be collected by the multi-section receiver 205 can minimised while maximising the solar radiation collected by the first receiver section 211.

Fig. 13 shows the a) a cross section and b) a perspective view of an example for a multi-section receiver 205 with four receiver sections where the first receiver section 211 is a cavity receiver section 220 while the second, the third and the fourth receiver section (212, 213 & 214) are external receivers sections 230.

Fig. 14 a cross section of an example for a multi-section re- ceiver 205 with four receiver sections where the first and fourth receiver section (211 & 214) are cavity receiver sections 220 while the second and the third receiver section (212 & 213) are external receiver sections 230.

The first receiver target area 251 and the second receiver target area 252 may be tilted by 30° out of the horizontal orientation as shown in Fig. 15 for a multi-section receiver 205 with four receiver sections. This is especially of interest if parts of the intermediate and peripheral field (122 & 123) are basically located on the polar side of the multi-section receiver 205.

If a multi-section receiver 205 has three or four receiver sec^¬ tions in some embodiments the second receiver section 212 is operated at a higher temperature than the third receiver section 213 which is operated at a higher temperature than the fourth receiver section 214, in some embodiments the receiver sections are operated at temperatures in a different order and in some embodiments some receiver sections are operated at the same temperature. In any case in normal operation the first receiver section 211 is operated at highest temperature.

Each receiver section may have subsections which operate as the receiver section they belong to as described. An example of a multi-section receiver 205 with receiver sections with subsec^¬ tions is the direct steam receiver where the cavity receiver section 220 has a cavity absorber 224 which is operated as boil- ser 242 at boiling temperature while another cavity absorber 224 is operated as superheater 243 at temperatures higher than the boiling temperature.

The cavity absorbers 224 or the external absorbers 231 may be pipes, photovoltaic absorbers 245 or volumetric absorbers 244 or the heat transfer medium 280 itself can be the absorber either as a falling curtain of particles 281 or fluid 282 or particles or a fluid moving or rinsing over a surface or a heat transfer medium 280 moving through translucent pipes. In a multi-section receiver 205 designed with pipes as absorbers 240 with a fluid or gas as heat transfer medium, the fluid or gas will pass through the absorbers 240 of the different re^¬ ceiver sections 210 with the first receiver section 211 as last hottest part.

A multi-section receiver for direct steam generation may be such that the fourth, third and second receiver section (212, 213, 214), or only the third and the second receiver section (212, 213) , or only the second receiver section 212, are used as economiser to preheat the water or the heat transfer fluid. The first receiver section 211 would preferably have two absorbers 224 inside the one cavity 221 where one absorber 224 formed by vertical pipes inside the cavity 221 of the first receiver section 211 would be a boiler 242 and the other absorber located at the top ceiling of the cavity 221 would be formed by horizontal pipes as superheater 243 of the steam. See Fig. 17. The wet steam moves from the boiler 242 to the steam drum 273 where the steam and the fluid is separated before the steam is superheated in the superheater 243 and the fluid is fed back into the economiser 241 and boiler 242.

A multi-section receiver 205 using volumetric absorbers 244 is shown in Fig. 16 a) and b) .It has a cavity receiver section 220 as first receiver section 211 with volumetric absorbers 244 at the top ceiling of the cavity 221. The lower levels of the cavity walls surrounding the aperture 222 of the cavity receiver section 220 is the second receiver section 212 and the outer sides of the cavity the third receiver section 213. Both, the second and third receiver section (212, 213) have volumetric ab^¬ sorbers 244 as external absorbers 231 sucking the gaseous heat transfer medium 283, in this case ambient air 285, through the irradiated and heated volumetric absorbers 244 of the second and third receiver section (212, 213) and preheating the air 285 be^¬ fore it enters the cavity 221. In the cavity 221 the air 285 is heated to a higher temperature by sucking it through the cavity absorber 224 inside the cavity 221 which is formed by a volumetric absorber 244. In a two section receiver the area around the aperture 222 may be the second receiver section 212. In a three section receiver the third receiver section 213 may be the outer walls of multi-section receiver 205 preheating the air entering the cavity 221 through the volumetric absorber of the external absorbers 231.

Though in most preferred embodiments the different receiver sections will part of a single cycle of heat transfer medium there are as well applications where one or more receiver sections are part of a separate cycle. Namely this is the case for applications where the part of concentrated solar radiation 130 which would be usually lost as spillage since it is not collected by any other receiver section is used for applications of lower temperatures than used for most applications for solar central receiver systems. As an example the low concentrated radiation can be used to heat up a fluid to a temperature below 200°C to run thermal desalination processes or absorption chillers for air conditioning.

The first receiver section 211 can be as well an external receiver section 230. Fig. 21 shows a multi-section receiver with an external receiver section 230 as first receiver section 211 and a cavity receiver section 220 as second receiver section 212. Fig. 22 shows a multi-section receiver with an external receiver section 230 as first receiver section 211 and a cavity receiver section 220 as second receiver section 212 and an external receiver section 230 as third receiver section 213. Fig. 23 a) and b) shows two cases for a multi-section receiver with external receiver sections 230 as first and second receiver sec^¬ tion (211 & 212) . Preferred Embodiments

Preferred Embodiment 1

A Preferred Embodiment of a multi-section receiver 205 with two receiver section 210 shown in Fig. 17 is based on this patent has the following properties:

Heliostat Field

The heliostat field 120 belongs to a solar central receiver system with a reflective area of approx. 50,000 m² and a tower height of ca. 100 m. The heliostats 110 are positioned around the tower with some elongation towards the pole (e.g. North in the northern hemisphere) . The heliostat field 120 reflects up to 40 MW of concentrated radiation 130 onto the multi-section receiver 205. The aiming point for all heliostats is within the aperture 222 of the cavity receiver section 220, minimizing spillage.

Receiver

The receiver 200 is a multi-section receiver 205 that produces superheated steam directly. It has two receiver sections 210: the second receiver section 212 is an external receiver section 230 which comprises an external absorber 231, and the first receiver section 211 is a cavity receiver section 220 which comprises two cavity absorbers 224, one of these cavity absorbers 224 is vertical and forms the vertical walls of the cavity receiver section 220 while the other is horizontal and forms the ceiling of the cavity receiver section 220. The external absorber 231 forms an annulus around the aperture 222of the cavity receiver section 220 and is used as an econo- mizer/pre-heater 241 for the water heat transfer fluid 280. It collects concentrated radiation 130 with peak value of

300 kW/m². Due to its relatively low operating temperature, the radiative loss from this external absorber 231 is reduced. The aperture 222 of the cavity receiver section 220 has a diameter of about 4 m. The cavity receiver section 220 is a capped vertical cylinder with a lower aperture 222. The inner diameter (~6 m)of the cavity receiver section 220 cylinder is larger than the aperture 222 of the cavity receiver section 220. The cavity receiver section 220 capped cylinder is displaced with respect to the cavity aperture 222 by 1 m towards the equator. The inner wall 223 on the polar side of the cavity receiver section 220 then coincides with the internal rim of the aperture 222 on the polar side of the aperture 222. The concentrated radiation 130 on the opposite, equatorial wall of the cavity receiver section 220 is thus diluted, is more uniform, and reaches a value less than 700 kW/m².

The cavity receiver section 220 is ~4 m high. The concentrated radiation 130 captured by the vertical cavity absorber 224 is used to effect the boiling of the water heat transfer fluid. A steam drum 273 connected to the vertical cavity absorber 224 separates the steam from the water. The separated water is recycled into the external receiver section 230. The steam is sent to the horizontal cavity absorber 224 which is used as superheater 243 to superheat the steam.

Summary of the main characteristics of Preferred Embodiment 1

• First receiver section 211: Cavity receiver section 220 with aperture 222 of 4 m

• Cavity receiver section 220with height of the cavity 221 of

4 m with vertical cavity absorber 224 as boiler 242 for boiling the water heat transfer fluid 280.

• Cavity receiver section 220 with a diameter of the cavity 221 of 6 m - with horizontal cavity absorber 224 as superheater 243 for superheating the water heat transfer fluid 280.

• Second receiver section 212: External receiver section 230 with external absorber 231 of 8 m diameter concentric with a 4m diameter aperture 222for preheating as preheater 241 the water heat transfer fluid 280.

Preferred Embodiment 2

A second preferred embodiment of a multi-section receiver 205 shown in Fig. 16a) based on this patent has the following properties :

Heliostat Field

The heliostat field 120 belongs to a solar central receiver system with a reflective area of approx. 50,000 m² and a tower height of ca . 100 m. The heliostats 110 are positioned around the tower with some elongation towards the pole (e.g. North in the northern hemisphere) . The heliostat field 120 reflects up to 40 MW of concentrated radiation 130 onto the multi-section receiver 205. The aiming point for all heliostats 110 is within the aperture 222 of the cavity receiver section 220 which is the first receiver section 211, minimizing spillage.

Receiver

The multi-section receiver 205 uses air as the heat transfer fluid. It has two receiver sections 210, a second receiver section 212, an external receiver section 230 and a first receiver section 211, a cavity receiver section 220. The external receiver section 230 has an external absorber 231 made of a volumetric absorber 244 placed around the aperture 222 of the cavity receiver section 220. It collects concentrated radiation 130 and captures most of the heat radiation from the cavity receiver section 220. The concentrated radiation on the volumetric absorber 244 reaches peak levels of 300 kW/m². Due to its relatively low operating temperature and its inherent properties, the radiative loss from the volumetric absorber 244 is modest. The air heat transfer fluid passes through the volumetric absorber 244 and enters the cavity receiver section 220. The aperture 222 of the cavity receiver section 220 has a diameter of about 4 m. It allows highly concentrated radiation 130 to enter the cavity receiver section 220.

The cavity receiver section 220 comprises a vertical cylinder whose inner diameter (~6 m) is larger than the aperture 222 of the cavity receiver section 220. The concentrated radiation 130 on the wall is less than 700 kW/m² and substantially uniform. The cavity receiver section 220 is ~4 m high.

The cavity receiver section 220 also comprises a cavity ab- sorber 224 at its ceiling formed by a volumetric absorber 244.

The air passes through the volumetric absorber 244 and reaches the required operating temperature.

Summary of the main characteristics of Preferred Embodiment 2

• Cavity receiver section 220of height 4 m

• Cavity diameter 6 m

• Second receiver section 212: Volumetric absorber 244 surrounding the cavity aperture 222

· Volumetric absorber 244 at the ceiling of the cavity receiver section 220

Preferred Embodiment 3

A Preferred Embodiment of multi-section receiver (205) shown in Fig. 18 based on this patent has the following properties: Heliostat Field

The heliostat field 120 belongs to a Solar Tower Plant with a reflective area of approx. 260,000 m² and a tower height of ca. 175 m. The heliostats 110 are positioned around the tower with some elongation towards the pole e.g. North in the north- ern hemisphere. The heliostat field 120 reflects up to 200 MW of concentrated radiation 130 onto the multi-section receiver 205. The aiming point for each heliostat 110 depends on its distance from the tower.

Receiver

The receiver 200 is a multi-section receiver 205that uses a heat transfer fluid that remains, in liquid form throughout (e.g. molten salt, sodium or other). The multi-section receiver 205 has two co-axial receiver sections 210. The third receiver section 213 is an external receiver section 230 with an external absorber 231 and the first receiver section 211 is a capped cylindrical cavity receiver section 220 with a cylindrical cavity absorber 224. The cavity receiver section 220 fits into the external receiver section 230; the multi-section receiver is then a cylinder with 8 m diameter and 8 m height. The external absorber (~200 m² ) collects concentrated radiation 130 (<800 kW/m²) from the heliostats 110 in the peripheral field 123 to preheat the heat transfer fluid.

The pre heated heat transfer fluid then enters the cavity absorber 224 in the cavity receiver section 220 where it is heated to the required operating temperature by the near field 121 and intermediate field 122. The concentrated radiation 130from the near field 121, which reaches peak values of

900 kW/m², is absorbed by the cavity absorber 224 and increases the temperature of the heat transfer fluid. Due to relatively lower temperatures close to the cavity aperture 222 and the cavity shape, the radiative loss is modest.

The concentrated radiation from the near field 131 travels furthest into the cavity receiver section and heats up the highest part of the cavity absorber 224 thus delivering the heat transfer fluid at the required operating temperature. The ceiling of the cavity receiver section may contain a fur^¬ ther cavity absorber 224 or be adiabatically reradiating onto the vertical cavity absorber 224.

Summary of the main characteristics of Preferred Embodiment 3

• Multi-section receiver 205 made of co-axial external receiver section 230 and internal cavity receiver section 220both cy^¬ lindrical and of height 8 m

• cavity receiver section aperture 222 diameter 8 m

• external receiver section 230 comprises a cylindrical external absorber 231

• cavity receiver section 220 comprises a cylindrical cavity absorber 224.

• Heat transfer fluid travels once through the external absorber 231 and inside the cylindrical cavity absorber 224 in the cavity receiver section 220

Preferred Embodiment: 4

A Preferred Embodiment of multi-section receiver shown in Fig. 19 based on this patent has the following properties: Heliostat Field

The heliostat field 120 belongs to a Solar Tower Plant with a reflective area of approx. 260,000 m² and a tower height of ca. 175 m. The heliostats 110 are positioned around the tower with some elongation towards the pole e.g. North in the north- ern hemisphere. The heliostat field 120 reflects up to 200 MW of concentrated radiation 130 onto the multi-section receiver 205. The several aiming points of the heliostats 110 depend on their distance from the tower and include the aperture 222 of the first receiver section 211, the aperture 22 of the second receiver section 212, and the third receiver section 213, re^¬ spectively. Receiver

The receiver 200 is a multi-section receiver 205that uses a heat transfer fluid that remains in liquid form throughout ( e . g . mol^¬ ten salt, sodium or other) .

The multi-section receiver 205 has three sections. The first receiver section 211 is a cavity receiver section 220 compris^¬ ing a cavity absorber 224; the second receiver section is also a cavity receiver section 220 comprising a cavity absorber 224 and the third receiver section is an external receiver section 'comprising an external absorber 231. The overall shape is a cylinder with 10 m diameter and 10 m height. The second receiver section cavity 221 has an aperture 222 of up to 10 m diameter and 5 m height and comprises a cavity absorber 224. The first receiver section cavity 221also comprises a cavity absorber 224, is 5 m high, up to 5 m in diameter and has an aperture 222 of 4 m situated at the top of the second receiver section cavity.

The external absorber 231 collects radiation (< 600 kW/m²) from the peripheral field 123 to preheat the HTF.

The second receiver section 212 cavity absorber 224 collects concentrated radiation 130, with a peak of 900 kW/m² , from the intermediate fieldl22 and increases the temperature of the heat transfer fluid. Due to its relatively low temperature, the radiative loss is reduced.

The first receiver section 211 cavity absorber 224 collects concentrated radiation 131, from the near fieldl22 and increases the temperature of the heat transfer fluid further.

Summary of the main characteristics of Preferred Embodiment 4

• Multi-section receiver height 10 m • second receiver section212 cavity aperture 222 diameter 10 m and height 5 m

• first receiver section 211 cavity height 5 m, diameter 5 m and aperture 4 m

• First receiver section 211 positioned above second receiver section .

• The heat transfer fluid travels into the external absorber 231 where it is initially raised in temperature, through to the second receiver section 212 cavity absorber 224 and finally through the first receiver section 211 cavity absorber 224 reaching the required operating temperature before exiting the multi-section receiver 205.

Preferred Embodiment 5

A Preferred Embodiment of multi-section receiver shown in Fig. 20 based on this patent has the following_^ properties :

Heliostat Field

The heliostat field 120 belongs to a Solar Tower Plant with a reflective area of approx. 260,000 m² and a tower height of ca . 175 m. The heliostats 110 are positioned around the tower with some elongation towards the pole e.g. North in the northern hemisphere. The heliostat field 120 reflects up to 200 W of concentrated radiation 130 onto the multi-section receiver 205. The several aiming points of the heliostats 110 depend on their distance from the tower and include, variously, the aperture 222 of the first receiver section 211, the second re^¬ ceiver section 212 and the third receiver section 213. Receiver

The receiver 200 is a multi-section receiver 205that uses a heat transfer fluid that remains in liquid form throughout (e . g. molten salt, sodium or other) .

The multi-section receiver has three sections.

The first receiver section 211 is a cavity receiver section 220 comprising a cavity absorber 224; the second receiver section is an external receiver section 230 comprising an external absorber 231 and the third receiver section is an external receiver section 230 comprising an external absorber 231. The overall shape is a cylinder with 10 m diameter and 10 m height. The first receiver section 211 is a cavity which is 10 m high and has an aperture 222 5 m in diameter; it comprises a cavity absorber. The second receiver section 212 is a planar horizontal annulus and is co-planar and co-axial with the aperture 222 of the first receiver section 211. The third re^¬ ceiver section is a vertical external receiver section 230 with a vertical cylindrical external absorber 231 10 m in diameter and 10 m high.

The first receiver section collects concentrated radiation 130from the whole Heliostat field 120 passing through its ap- erture222.

The external absorber 231 of the second receiver section 212 collects concentrated radiation 130 that does not fall onto the first receiver section's aperture222. This external absorber is used to pre-heat the heat transfer fluid. The exter^¬ nal absorber 231of the third receiver section 212 collects concentrated radiation 130, with a peak value less than

600 kW/m², originating at the peripheral field 123 and that does not fall onto the first receiver section's aperture222 or the second receiver section 212. It is used to increase the temperature of the heat transfer fluid. The cavity absorber 224 of the first receiver section 221, which collects the main concentrated radiation 130, then heats the heat transfer fluid to its required operating temperature.

Summary of the main characteristics of Preferred Embodiment 5

• Multi-section receiver 205 height: 10 m

• First receiver section aperture diameter: 5 m

• Second receiver section 212 outer diameter: 10 m, second receiver section212 inner diameter: 5 m.

• height of first receiver section 211 cavity 221: 10 m

• diameter of third receiver section 213: 10 m

Definitions :

The following definitions are used in this application:

Heliostat 110: A heliostat is a reflector which is movable around two orthogonal axes to reflect solar radiation onto a target point or area. The two-axes tracking allows to irradiate the fixed target although the Sun's position is changing over time .

Heliostat field 120: A heliostat field is an arrangement/array of heliostats which reflects and concentrates the solar radiation onto a target point or area of a receiver (system) . A heliostat field may be divided into a near field 121, an intermediate field 122 and a peripheral field 123 depending on the heliostat horizontal distance from the receiver, relative to the receiver height, H_R

Near field 121: The near field or central region of the heliostat field 120 is defined in this invention as the part of the heliostat field which lies beneath the receiver system and is horizontally not further than two receiver heights. i¾ away. The heliostat field 120 may consist just of a near field 121. Intermediate field 122: The intermediate field is defined as the part of the heliostat field 120 which lies in a zone with horizontal distance between 1 and 2 H_R from below the receiver system. It may be polar or surround the receiver 200. The heliostat field 120 may consist just of a near field 121 and an intermediate fields 122.

Peripheral field 123: The peripheral field is the part of the heliostat field 120 which has a horizontal distance from the receiver system of at least 2 ¾ when an intermediate field is defined. If an intermediate field 122 is defined the peripheral field 123 is further away than the intermediate field 122. It may be polar or surround the receiver 200.

Receiver 200: A receiver is a system which converts solar radiation into heat or - in the case of a photovoltaic receiver - di^¬ rectly into electricity. The heat of the receiver is transferred to and carried away by a heat transfer medium. Typically, the target point of the reflectors of the heliostat field is on the receiver. The principle of a receiver is known e.g. from Patent US 4 172 443 and US 4 220 140, respectively.

Receiver support structure 105: A receiver is held by a receiver support structure or above the heliostat field 120.

Orientation of the receiver (section) : The vector normal to the aperture or absorber area describes the orientation of the receiver (section). For curved surfaces/areas it is locally defined as the direction in which the receiver (section) is facing and from where it receives the dominant part of its irradiation.

Receiver section 210: A receiver section is a part of the receiver 200 or multi-section receiver 205. It is characterized by its position, the orientation of its receiver target area 250 and the radiation 130it is supposed to collect. A receiver section 210 can be a cavity receiver section 220 or an external re- ceiver section 230 which again can be an external thermal re- ceiver section 234 or an external photovoltaic receiver section 236.

Receiver Target Area 250: The area of the receiver which is il^¬ luminated by the reflected radiation of the heliostats to convert the solar radition 130 to thermal power or an electrical current .

Cavity receiver section 220: As shown in Fig. 3, a cavity re^¬ ceiver is a receiver where the solar radiation absorbing sur^¬ face, the cavity absorber 224, lies within a cavity 221. The radiation 130 enters the cavity 221 via the aperture 222. The principle of such a cavity receiver is e.g. known from Patents US 4 220 140, WO 2008/153922 Al and as well from [1] .

Aperture 222: The aperture is the opening of a cavity receiver section 220 which allows solar radiation 130 to enter the cavity 221 as known from e.g. [1] or WO 2008/153922 Al [ref.] and shown in Fig. 3. The receiver target area 250 and the aperture 222 of a cavity receiver section 220 are identical.

Absorber 240: The absorber is the part of the receiver 200 which is hit by the concentrated solar radiation 130 and converts the radiative power. In the case of thermal absorber the power is converted into heat and delivered to a heat transfer medium as known from Patent US 4 220 140. In the case of a photovoltaic absorber 245 made out of photovoltaic cells the solar radiation is converted directly into electricity. Absorbers 240 may be pipes or panels, photovoltaic absorbers 245 or volumetric absorbers 244.

Heat transfer medium 280: The heat transfer medium is a medium, often liquid or gaseous, that is circulated in a thermal absorber to collect and transport heat; it may be a gas 283 such as air 285; a liquid 282 such as water, a molten metal or com- pound e.g. molten salts; or solid particles 281 such as sand or ceramic particles. In the latter case the heat transfer medium does not necessarily circulate inside a solar absorber, but may be in the form of a falling curtain of small particles which is then both the heat absorbing and heat transfer medium.

External absorber 231: An external absorber faces directly the heliostat field. There is no housing visible in the half space above the absorber. The receiver target area 250 and the absorber surface are identical.

Cavity Absorber 224: A cavity absorber is the absorber 240 of a cavity receiver section 220 which is inside the cavity 221.

External receiver section: An external receiver section with an external absorber is a receiver where the absorbing area is part of the outer surface of the receiver system e.g. the. lateral area of a cylindrical receiver as known from Patent

US 4 172 443.

Receiver (or optical) height H_R: The receiver height H_R is defined, as shown in Fig. 1, as the vertical distance between the center of the aperture of first section of the receiver and the plane in which the centers of the heliostats lie. The receiver height is used as unit of length to compare the scale of e.g. size of the heliostat field or parts of the heliostat field such as near, intermediate or peripheral field.

Volumetric absorber 244: A volumetric absorber is an absorber 240 to heat a gaseous heat transfer fluid. It consists of a porous material which absorbs concentrated radiation 130 inside the volume of a structure and transfers the absorbed heat to a fluid passing through the structure. The internal surface of the absorber interacting both with the radiation and the heat transfer fluid is much larger than the receiver target area 250 due to its internal channels or porosity. Concentrated Solar Radiation 130: Concentrated solar radiation is the radiation which was concentrated by heliostats 110 of he- liostat field 120.

Spillage : Spillage is concentrated solar radiation 130 which is not collected by a receiver section 210 since it does not hit the receiver target area 250 but a region next to the receiver target area.

References

[1] Buck, R. ; Brauning, T.; Denk, T.; Pfander, M. ; Schwarzbozl, P.; Tellez, F; Solar-Hybrid Gas Turbine-based Power Tower Systems (REFOS); Journal of Solar Energy Engineering, Vol. 124, February 2002; pp. 2-9

[2] Ben-Zvi R. , Epstein M. , Segal A.; Simulation of an integrated steam generator for solar tower; Solar Energy 86 (2012) 578-592.

[3] Hernandeza, N.; Riveros-Rosasb, D. ; Venegasa, E. ; Dorantesc, R.J.; Roj as-Morind, A.; Jaramilloa, O.A.; Arancibia-Bulnesa , C.A.; Estradaa, C.A. (2012); Conical receiver for a parabol- oidal concentrator with large rim angle; Solar Energy Solar Energy 86 (2012) 1053-1062.

[4] Frohberger, D. ; Jaus, J.; Wiesenfarth, M. ; Schramek, P. ;

Bett, A.W. (2010); Feasibility study on high concentrating photovoltaic power towers; 6TH INTERNATIONAL CONFERENCE ON CONCENTRATING PHOTOVOLTAIC SYSTEMS: CPV-6; AIP Conf. Proc. 1277, pp. 194-197;

[5] Winter, C.-J.; Sizmann, R. L.; Vant-Hull, L. L.: Solar Power Plants, Berlin, Heidelberg, New York: Springer, 1991

[6] Hoffschmidt, B. ; Tellez, F.M.; Valverde A.; Fernandez, J. and Fernandez, V. (2003) Performance Evaluation of the 200- kWth HiTRec-II Open Volumetric Air Receiver; Journal of Solar Energy Engineering, ASME 2003, Vol. 125, pp.87-94 [7] Ortega, J.I; Burgaleta, J.I and Tellez, F.M. (2008), Central Receiver System Solar Power Plant Using Molten Salt as Heat Transfer Fluid; Journal of Solar Energy Engineering, ASME 2008, Vol. 130, pp.024501-1 to 6 List of reference signs:

105 Receiver support structure/Tower

110 Heliostat

111 Reflector

120 Heliostat field

121 Near field

122 Intermediate field

123 Peripheral field

124 Boundary between near field and intermediate field

125 Boundary between intermediate field and peripheral field

130 Concentrated radiation

131 Radiation from near field

132 Radiation from intermediate field

133 Radiation from peripheral field

200 Receiver

205 Multi-section receiver

210 Receiver section

211 First receiver section

212 Second receiver section

213 Third receiver section

214 Fourth receiver section

220 Cavity receiver section

221 Cavity

222 Aperture 223 Internal cavity walls

224 Cavity Absorber

230 External receiver section

231 External absorber

234 External thermal receiver section

236 External photovoltaic receiver section

240 Absorber

241 preheater or economiser

242 boiler

243 superheater

244 Volumetric absorber

245 Photovoltaic absorber

250 Receiver target area

251 First receiver target area

252 Second receiver target area

253 Third receiver target area

254 Fourth receiver target area

270 Receiver components

271 Inlet of heat transfer medium

272 Outlet of heat transfer medium

273 Steam drum

274 Insulation

280Heat transfer medium

281 Solid heat transfer medium

282 Liquid heat transfer medium

283 Gaseous heat transfer medium

284 Heat transfer medium at phase change from fluid to gas

285 Ambient air

Claims

A solar central receiver system with a heliostat field (120), comprising at least one solar multi-section receiver (205) to collect concentrated solar radiation (130) and a plurality of heliostats (110) forming the heliostat field (120) comprising a near field (121), which may be extended by an intermediate field (122) and a peripheral field

(123), and located on a ground area underneath the solar multi-section receiver (205) ;

(a) wherein the at least one solar multi-section receiver (205) comprises at least two receiver sections (210) where any of these receiver sections is selected from the following list:

i. a cavity receiver section (220) having a cavity (221) with an aperture (222), preferably a single aperture, and inside the cavity a thermal ab- · sorber (224), where the aperture of the cavity forms the receiver target area (250) of this cavity receiver section;

ii. an external thermal receiver section (234) comprising an external absorber (231) having a thermal absorber on the outer surface of the external thermal receiver section (234), where the external absorber (231) is the receiver target area (250) of this external receiver section; iii. an external photovoltaic receiver section (236) comprising an external absorber (231) having a photovoltaic absorber on the outer surface of the external photovoltaic receiver section (236) , where the external absorber (231) is the receiver target area (250) of this external photovoltaic receiver section; wherein a plurality of heliostats (110) forming the heliostat field (120) located on a ground area under neath the solar multi-section receiver (205), each heliostat having one reflector or a group, of reflectors, whereby each reflector or group of reflectors is pivotable about two axes to reflect solar radiation (130) onto at least one of the at least two receiver target areas (250);

wherein

i. one of the at least two receiver sections (210) forms a first receiver section (211) which is facing downwards onto the near field (121) of the heliostat field (120) underneath the solar multi^¬ section receiver (205), where the heliostats (110) are oriented to reflect solar radiation (131) onto the receiver target area (251) of this first receiver section;

i. and another of the at least two receiver sections (210) forms a second receiver section (212), which surrounds the first receiver section (211) and where the , receiver target area (252) of the second receiver section (212) collects reflected solar radiation (130) which does not hit the receiver target area (251) of the first receiver section (211) and wherein some heliostats (110) of the intermediate field (122, Fig. 11) and some heliostats (110) of the peripheral field (123, Fig. 11) of the heliostat field (120) may be oriented to reflect solar radiation (132 & 133) onto parts of the receiver target area (252) of the second receiver section and wherein other heliostats (110) of the intermediate field (122, Fig. 11) and other heliostats (110) of the peripheral field (123, Fig. 11) may be oriented to reflect solar radiation (132 & 133) onto parts of the re^¬ ceiver target area (251) of the first receiver section (211 ) ;

(d) wherein the first receiver section (211) is the hottest receiver section during operation and is formed by a cavity receiver section (220) where the internal surface A±_s of its cavity (221) is larger than the area A_ap of the cavity aperture (222) of its cavity by the factor Ai_s/A_ap > 2;

wherein the second receiver section (212) is either formed by an external thermal receiver section (234) or a cavity receiver section (220).

A solar central receiver system with a heliostat field according to claim 1, wherein the receiver target area (252) of the second receiver section (212) is facing downwards. A solar central receiver system with a heliostat field according to one of the claims 1 or 2,

(f) wherein the solar multi-section receiver (205) has a third receiver section (213) being an external thermal receiver section (234) with the receiver target area (253) located on a lateral surface around the receiver target area (252) of the second receiver section (212), and wherein the external absorber (231) of the third receiver section (213) is shaped and orientated outwards to receive solar radiation (130) from the heliostats (110) in the peripheral field (123, Fig. 11) of the heliostat field (120) be^¬ ing oriented to reflect solar radiation (133) on parts of the receiver target area (253) of the third receiver section (213) .

A solar central receiver system with a heliostat field ac^¬ cording to claim 3, wherein the solar multi-section receiver (205) has a fourth receiver section (214; Fig. 13; Fig. 15) formed by an external thermal receiver section (234) with the receiver target area (254) located below the receiver target area (252) of the second receiver section

(212), and wherein the external absorber (231) of the fourth receiver section (214) is shaped and oriented to receive solar radiation (130) from the heliostats (110) in the peripheral field (123) of the heliostat field (120), where all heliostats (110) of the heliostat field (120) are oriented to reflect solar radiation onto the receiver target area (251) of the first receiver section (211).

5. A solar central receiver system with a heliostat field according to claim 3, wherein the solar multi-section receiver (205) has a fourth receiver section (214; Fig. 14) formed by a cavity receiver section (220) with the receiver target area (254) located below the receiver target area (252) of the second receiver section (212), and wherein the aperture (222) of the fourth receiver section (214) is shaped and oriented to receive solar radiation (130) from the heliostats (110) in the peripheral field (123) of the heliostat field (120), where all heliostats (110) of the heliostat field (120) are oriented to reflect solar radia^¬ tion onto the receiver target area (251) of the first receiver section (211).

6. A solar central receiver system with a heliostat field according to any of the claims 1 to 5, wherein the feature (d) in claim 1 is replaced by the following feature (d' ) :

(d¹) wherein the first receiver section (211) is

either the hottest receiver section during operation, which is formed by an external thermal receiver section (234; Fig. 21; Fig. 22;Fig. 23),

or an external photovoltaic receiver section (236) .

7. A solar central receiver system with a heliostat field according to any of claims 1 to 6,

wherein the thermal absorber (224) of the cavity receiver section (220) and/or the external absorber (231) of the ex- ternal thermal receiver section (234) is formed by a volumetric absorber (244; Fig. 16) comprising gas channels or porous material for sucking a gaseous heat transfer fluid through the volumetric absorber (244).

8. A solar central receiver system with a heliostat field according to any of the claims 1 to 7, wherein the heliostat field (120) has a region (121) under the at least one solar multi-section receiver (205) which has a reflector area density p of p > 50%; the reflector area density p is defined as the ratio of the total reflector area of a all heliostats (110) of a defined region to the total ground area of the defined region.

9. A solar central receiver system with a heliostat field according to any of the claims 1 to 8 wherein the contour of the aperture (222 ) of the cavity receiver section (220) of the first receiver section (211) is essentially parallel to a plain which is tilted in relation to the ground area of the heliostat field (120) with a angle less than 30°.

10. A solar central receiver system with a heliostat field according to any of the claims 1 to 9 wherein the contour of the aperture (222) of the cavity receiver section (220) of the first receiver section (211) is essentially parallel to the ground area of the heliostat field (120).

11. Use of a solar central receiver system with a heliostat field according to any of the claims 1 to 10 for a concentrated solar radiation (130) having a power greater than

1 M .

12. Method of collecting concentrated solar radiation (130) by a solar central receiver system with a heliostat field (120) as defined in any of claims 1 to 10.