WO2015048845A1

WO2015048845A1 - A hybrid solar and chemical looping combustion system

Info

Publication number: WO2015048845A1
Application number: PCT/AU2014/000954
Authority: WO
Inventors: Seyed Mehdi JAFARIAN; Maziar ARJOMANDI; Graham Jerrold Nathan
Original assignee: Adelaide Research & Innovation Pty Ltd
Priority date: 2013-10-02
Filing date: 2014-10-02
Publication date: 2015-04-09

Abstract

A hybrid solar and chemical looping combustion system (10) comprising a fuel reactor (11), an air reactor (12), a looping path (13) for transferring looping material comprising oxygen carrier particles (15) between the fuel reactor (11) and the air reactor (12). The fuel reactor (11) is configured to be heated by concentrated solar energy. First and second reservoirs (41, 42) are provided in the looping path (13) for receiving looping material. The first reservoir (41) is disposed between the air reactor (12) and the fuel reactor (11), and the second reservoir (42) is disposed between the fuel reactor (11) and the air reactor (12). A heat exchanger (45) is provided in the looping path (13) in the between the air reactor (12) and the first reservoir (41) for heat transfer from the looping material (13) to an air flow to the air reactor (12). The heat exchanger (45) may comprise a direct air-particle heat exchanger. A system for generation of power comprising the hybrid solar and chemical looping combustion system (10) is also descried. Further, a method of generating power using the hybrid solar and chemical looping combustion system (10) is also described.

Description

A HYBRID SOLAR AND CHEMICAL LOOPING COMBUSTION SYSTEM TECHNICAL FIELD

[0001] This invention relates to a hybrid solar and chemical looping combustion system.

[0002] The hybrid solar and chemical looping combustion system may be used to generate hot exhaust gas. The hot exhaust gas may be used in a power cycle to generate power.

[0003] The invention also relates to a system for generating power. [0004] Further, the invention also relates to a method of generating hot exhaust gas. [0005] Still further, the invention relates to a method of generating power. BACKGROUND ART

[0006] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0007] The recent increase in the atmospheric concentration of C0₂, due mainly to the combustion of fossil fuels, is widely linked to the simultaneous increase in the global mean temperature. However, despite the urgent need to decrease these C0₂ emissions, the US Department of Energy (DOE) International Energy Outlook reported that fossil fuels are likely to remain the dominant energy source for the short to medium term owing to their relative abundance and their established position worldwide. Therefore, it is necessary to develop technologies that decrease the net C0₂ emissions from fossil fuels combustion at low cost. Two classes of technology under development to mitigate C0₂ emissions are those associated with carbon capture and storage (CCS), which aims to prevent these emissions from being released to the atmosphere, and renewable energy generation from sources such as solar radiation, which aims to avoid them. In following sections the state of art for each technology will be discussed briefly.

[0008] In essence, CCS consists of three main steps: capturing C0₂ from the generator, such as a power plant, compressing it to a supercritical fluid and finally sequestering it in a safe and secure place, such as a deep geological formation. Among the CCS stages, carbon capture is typically the most energy consuming step, so that approximately 75% of the total energy required for CCS from a conventional pulverised fuel power station is typically used for the capture of C0₂. Therefore during the last few years, great effort has been expended to develop technologies that achieve low cost carbon capture. The main options being investigated for this end are: (a) pre- combustion, in which fuel is de-carbonized prior to combustion; (b) oxy fuel combustion, which uses pure oxygen separated from the nitrogen in air; (c) post combustion separation, to remove C0₂ from the flue gas; and (d) chemical looping combustion (CLC), which employs the indirect transfer of oxygen from the air to the fuel by means of a solid oxygen carrier. This avoids direct contact between the fuel and air to generate an industrially-pure stream of C0₂ from one reactor, termed the "fuel reactor", together with hot vitiated air from the other reactor, termed the "air reactor". Different economic assessments performed by the C0₂ Capture Project, CCP, and by the International Panel on Climate Change (IPCC) have shown that Chemical Looping Combustion (CLC) is among the best of the options available for low cost CO2 capture from combustion. Hence, the interest in CLC systems is growing. So far more than 3500 hours of operational experiments in continuous plants of different sizes has been reported. Considering that this technology is less than 10 years old, this development can be considered very successful. Nevertheless, relative to natural gas combined cycle, CLC adds to the cost of power generation, owing to the additional infrastructure, and decreases the efficiency of natural gas utilisation owing to the lower temperature that can be achieved from the air reactor relative to that from a combustor. Hence there is need for alternative approaches to make this technology viable.

[0009] Concentrated solar thermal power (CSP) has the potential to achieve cost- competitive power generation with conventional sources, especially in arid regions where direct sunlight is abundant. However, the intrinsic low intensity along with the intermittent and variable nature of solar radiation are major barriers to realising this potential. To compensate for the intermittent resource, solar (and wind) plants require either supporting generation (from auxiliary sources, the network or as a hybrid) and/or storage capacity to allow the continuous supply of electricity supply when the direct solar flux is below a threshold. Solar thermal power has advantages over solar photovoltaic power in that heat is cheaper to store than electricity. Hence a key driver for the ongoing development of CSP plants is that they are well suited to incorporate thermal energy storage (TES), which has the potential to increase economic viability by allowing it to make better use of time-of-day pricing. However, storage is one of the least developed components of CSP plants and also adds to the capital cost. Three main classes of thermal storage are under development; namely, sensible heat storage, latent heat storage and chemical energy storage. Of these, chemical TES can achieve the highest energy density, but has not yet been developed to the demonstration stage. To date, sensible heat technology, notably using molten salt, is the only commercially viable method for TES at large scale in CSP plants. Molten salt systems typically increase the capital cost of a solar thermal plant by 10-20%, depending on the amount of storage. They also require large quantities of salt, which are vulnerable to solidification, and two large storage tanks which increase cost. Besides, the maximum operating temperature of these systems is about 600 °C, which places a limitation on the thermodynamic efficiency of the power station. While higher temperatures can be achieved through other devices, such as bubbling beds that store heat in ceramic beads, these devices suffer the disadvantage of having a lower heat capacity than molten salts. In addition, as with all sensible heat storage devices, they suffer the key disadvantage that the temperature of the stored heat decreases with the depth of drawdown of the storage device. This further lowers the upper temperature of the power cycle relative to that at which the receiver operates, by typically 50 to 100 °C, which lowers the efficiency of the power station. That is, it introduces an inevitable trade-off between the size, and cost, of the storage device and the efficiency of the plant. Hence there is a need for improved storage technologies that overcome these challenges.

[0010] Importantly, the solid oxide particles employed in CLC systems provide chemical energy storage, combined with sensible storage that can achieve a constant outlet temperature as the energy is drawn down. This temperature is also higher than has been demonstrated for long term operation in any CSP system. Hence, CLC offers to solar thermal technology a form of high temperature storage that has already undergone significant development. For this reason, research has recently begun to explore hybrids between solar thermal and CLC systems. One such proposal involves a hybrid CLC system that incorporates storage. This prior-art hybrid offers shared infrastructure, which can lower capital and operating costs. However, it is also limited to a solar share of about 6.5%. No hybrid system has been proposed to date that achieves a higher solar share, notably in which the contribution of solar energy to the total energy output from the plant is significant. [001 1] Most analyses of future energy sources predict that future energy systems will be powered by a mix of both of fossil fuels and renewable energy technologies due to their complementary nature and stages of development. While it is possible for this mix to be met with a combination of stand-alone renewable and stand-alone fossil plants, it is more efficient and also more cost-effective to integrate these energy sources into a single hybrid plant. For these reasons, the hybridisation of solar thermal and combustion technologies is attracting growing attention. A number of solar hybrid systems have been proposed and tested, with and without the capability of thermal energy storage. However, current state of the art in hybrid systems are typically limited to a relatively low solar share of less than 10%.

[0012] Gas turbine combined cycles, GTCC, are currently the most efficient commercially available thermal power cycle, achieving some 60% in efficiency. This is very much higher than the current best practice in solar thermal systems, whose efficiency is limited by the maximum temperature of the working fluid to around 35%. Hence there is considerable incentive to seek to develop gas turbines that are heated by solar-thermal energy. However, one of the major technological challenges to achieve this is difficulty in heating pressurised air to the high temperatures of a gas turbine inlet by solar radiation, whilst also achieving a receiver that is efficient, reliable, safe to operate and easy to maintain. Key barriers to achieving high temperatures are the low rate of heat transfer to air, the material constraints of relatively low operating temperature of ductile materials and of poor shock resistance of ceramics for indirect heat transfer designs or, alternatively, the vulnerability of quartz windows to breakage and the poor rates of radiation heat transfer to air. Several receiver configurations have been proposed and tested. Recently, through a French PEGASE project (Production of Electricity by Gas Turbine and Solar Energy) a high temperature air solar absorber based on compact heat exchanger technology was designed and constructed. The experiments measured an output temperature of 739 . More recently, air temperatures of around 1200 °C have been achieved in short-term operation at small scale by ETH Zurich. Importantly though, the only TES technology that is available at these temperatures is that of sensible heat storage, with the disadvantages described above. Furthermore, it is uneconomical to provide sufficient storage with any TES technology to cover for periods of extended cloud, owing to the low utilisation associated with high storage capacity. Hence it is uneconomic for any solar-only technology to be designed to provide true base-load capability; that is, to provide sufficient capacity to avoid the need for a back-up generation in the event of extended periods of cloud. Back-up generation adds to the cost of energy supply to the network. Hence there is a need for technologies that can provide high temperature TES for gas turbine technology, whilst also being configurable to operate from fossil fuels only during periods of low solar resource, in order to enable supply to be maintained from the plant.

[0013] It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.

SUMMARY OF INVENTION

[0014] According to a first aspect of the invention there is provided a hybrid solar and chemical looping combustion system comprising a fuel reactor, an air reactor, a looping path for transferring looping material comprising oxygen carrier particles between the fuel reactor and the air reactor, first and second reservoirs in the looping path for receiving looping material, the first reservoir being disposed between the air reactor and the fuel reactor, the second reservoir being disposed between the fuel reactor and the air reactor, and a heat exchanger in the looping path between the air reactor and the first reservoir for heat transfer from the looping material to an air flow to the air reactor, wherein the fuel reactor is configured to be heated by concentrated solar energy.

[0015] With this arrangement, the first reservoir can store oxygen carrier particles which have been cooled following heat exchange in the heat exchanger. Further, the second reservoir can store oxygen carrier particles that have been heated in the fuel reactor.

[0016] The two reservoirs represent provision for heat storage.

[0017] The fuel reactor may be configured to be heated either directly or indirectly by concentrated solar energy.

[0018] Preferably, the fuel reactor is configured as a direct heated solar fuel reactor. However, the fuel reactor may be configured as an indirectly heated solar fuel reactor, as alluded to above.

[0019] In a direct heating configuration, the solar fuel reactor may comprise a cavity solar receiver. The cavity solar receiver can be of a configuration based on the concepts of a solar vortex reactor or a solar fluidized bed reactor. [0020] In an indirect heating configuration, an intermediate heat transfer medium (such as a working fluid) may be used to transfer absorbed concentrated solar thermal energy from a receiver to heat the particles within the fuel reactor. However, other type of indirect solar reactors may, of course, be used.

[0021] The air reactor may be of known kind.

[0022] In one configuration, the two reactors operate at different pressures. Typically, for a cavity solar reactor, the fuel reactor operates at atmospheric pressure.

[0023] Preferably, heat transfer from the looping material to the air flow provides pre-heated air to the air reactor.

[0024] The heat exchanger serves to cool oxygen carrier particles leaving the air reactor to the storage temperate in the first reservoir.

[0025] Particles from the first reservoir entering the fuel reactor can be exposed directly to concentrated solar radiation (direct heating) to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction, leading to production of reduced particles which are transported to the second reservoir for storage as the hot particles.

[0026] The particles in the second reservoir (the hot reservoir store) comprise both chemical and sensible heat.

[0027] It is a feature of the invention that the heat exchanger allows the temperature of the particles fed to the fuel reactor to be greatly reduced, thereby enabling a much greater contribution of solar energy into the process, stored as sensible heat.

[0028] In the air reactor, reduced hot particles from the second reservoir react exothermically with oxygen in the air introduced into the air reactor. The stored heat in the oxygen carrier particles is released in the air reactor, generating higher temperatures than the storage temperature of the second reservoir (the hot reservoir). The air reactor is also pressurised, leading to generation of hot, pressurised gas within the air reactor.

[0029] By extracting heat from the oxygen carrier particles leaving the air reactor, the heat exchanger has the effect of lowering the temperature of the oxygen carrier particles in the first reservoir (the cold reservoir), whilst also pre-heating the air to the air reactor, so that the outlet temperature from the air reactor is not compromised by increasing the amount of thermal energy storage (TES).

[0030] Preferably, the air reactor comprises an air inlet and an exhaust gas outlet, with the air flow from the heat exchanger communicating with the air inlet.

[0031] The air introduced into the air reactor may comprise not only the preheated air (being the air flow from the heat exchanger) but also supplementary air.

[0032] Preferably, the heat exchanger comprises an air particle heat exchanger. Any other appropriate type of heat exchanger may, of course, be used.

[0033] Preferably, the system further comprises control means for controlling the rate of flow of looping material through the fuel reactor. Such control means may, for example, comprise one or more valves in the looping path for controlling the flow rate of OC particles.

[0034] Preferably, the system further comprises control means for controlling the flow rate of a fuel for a fuel oxidation reaction in the fuel reactor. . Such control means may, for example, comprise a fluid flow control valve.

[0035] The ability to control both the rate of flow of looping material through the solar-fuel reactor and the flow rate of a fuel for a fuel oxidation reaction in the solar-fuel reactor is advantageous as is affords selective control of the operating temperature of the solar fuel reactor.

[0036] The carrier particles may comprise particles of a metal that is easily oxidised. The carrier particles may, for example, comprise Ni.

[0037] One example of such a carrier particle comprises NiO supported on a substrate of N1AI2O4. NiO may be supported on the substrate of N1AI2O4 in a mass ratio of 4 to 6. These particles have been tested experimentally in CLC systems and can provide a high fuel conversion together with low agglomeration, attrition and decomposition rates.

[0038] Other forms of oxygen carrier particles may, of course, also be used, as would be understood by a person skilled in the art; for example, particles comprising Fe and Co. [0039] The system may be configured for operation in two modes.

[0040] In a first mode, the oxygen carrier particles are exposed to concentrated solar radiation in the solar-fuel reactor to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction.

[0041] In a second mode, there is no exposure of the oxygen carrier particles to concentrated solar radiation and the sensible heat of the oxygen carrier particles is used to drive the reduction process. In the second mode, there is provision for bypassing the heat exchanger and also the first and second reservoirs, such that oxygen carrier particles leaving the air reactor are transferred directly to the solar-fuel reactor without the need for a solar energy input so that the particles are reduced using the sensible heat stored in the hot oxygen carrier particles. In this arrangement, the system performs a conventional chemical looping combustion operation.

[0042] Typically, the system is operated in the first mode when the available solar energy is sufficient to exceed losses. In this mode, the solar fuel reactor is configured to receive concentrated solar energy to provide both sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction.

[0043] In the second mode, the system can perform a conventional chemical looping combustion operation for extended periods of low solar radiation, should this be necessary.

[0044] Preferably, the solar fuel reactor comprises a reaction chamber having an aperture through which concentrated solar radiation can be received for insolating looping material passing therethrough.

[0045] The solar fuel reactor may further comprise an aperture shutter moveable from an open position to a closed position, wherein the shutter in the closed position provides a physical seal for the aperture so as to reduce heat losses.

[0046] Other forms of fuel reactor heated indirectly are possible and may, of course, be utilized.

[0047] In an indirect heated configuration, oxygen carrier particles from the first reservoir entering the fuel reactor can be heated indirectly. The indirect heating of the gas and oxygen carrier particles in the fuel reactor can be achieved using a separating conductive medium, such as an intermediate working fluid (e.g. molten salt), which is heated by concentrated solar radiation in a separate solar absorber. This separating conductive medium functions as an intermediate heat transfer medium, absorbing the concentrated solar thermal energy and transporting it to the reaction chamber. The hot gases and oxygen carrier particles then drive the fuel oxidation reaction, leading to production of reduced particles, which are transported to the second reservoir for storage as hot particles.

[0048] The system according to the invention is estimated to achieve a solar fraction of up to 60% while providing sufficient storage to achieve continuous base-load power generation for the average diurnal fluctuations in solar radiation.

[0049] It is expected that the system can achieve a substantially constant operating temperature and flow rate at the outlet of the air reactor notwithstanding variations in the flows to the solar-fuel reactor associated with variable solar input.

[0050] The hybrid solar and chemical looping combustion system according to the first aspect of the invention is operable to provide a stream of hot exhaust gas from the air reactor.

[0051] The stream of hot exhaust can be used for any appropriate purpose. A particularly suitable application of the hot exhaust gas is in power generation. The power generation may comprise generation of electrical power.

[0052] According to a second aspect of the invention, there is provided a system for generation of power comprising a hybrid solar and chemical looping combustion system according to the first aspect of the invention.

[0053] According to a third aspect of the invention, there is provided a method of generating power using a system according to the first or second aspect of the invention.

[0054] According to a fourth aspect of the invention there is provided a method of generating a stream of hot exhaust gas comprising: transporting looping material comprising oxygen carrier particles between a fuel reactor and an air reactor; exposing the oxygen carrier particles in the fuel reactor to heat generated using concentrated solar radiation to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction; storing the oxygen carrier particles as hot particles; transporting the hot particles to the air reactor to react exothermically with oxygen from air introduced into the air reactor to generate the hot exhaust gas; extracting heat from oxygen carrier particles during transportation from the air reactor; the oxygen carrier particles being stored as cold particles prior to being fed into the fuel reactor.

[0055] The terms "hot particles" and "cold particles" are used in relation to relative temperature conditions, as would be understood by the skilled addressee. The terms are to be taken in context of the invention and are not to be taken as limiting the invention to the literal interpretation of the terms.

[0056] The heat to which the oxygen carrier particles are exposed in the fuel reactor may be generated directly or indirectly from concentrated solar radiation.

[0057] Preferably, the heat extracted from the oxygen carrier particles is used to preheat air introduced into the air reactor.

[0058] Preferably, heat is extracted from the oxygen carrier particles by passing the latter through a heat exchanger.

[0059] More preferably, heat is extracted from the oxygen carrier particles by passing the latter through an air-particle heat exchanger and wherein the air is also passed in heat exchanger relation with the air-particle heat exchanger to receive heat from the oxygen carrier particles.

[0060] According to a fifth aspect of the invention there is provided a method of generating power comprising: transporting looping material comprising oxygen carrier particles between a fuel reactor and an air reactor; exposing the oxygen carrier particles in the fuel reactor to heat generated using concentrated solar radiation to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction; storing the oxygen carrier particles as hot particles; transporting the hot particles to the air reactor to react exothermically with oxygen from air introduced into the air reactor to generate the hot exhaust gas; extracting heat from oxygen carrier particles during transportation from the air reactor; the oxygen carrier particles being stored as cold particles prior to being fed into the fuel reactor; and using the hot exhaust gas from the air reactor in a power cycle.

[0061] According to a sixth aspect of the invention there is provided a method of generating power comprising: transporting looping material comprising oxygen carrier particles between a fuel reactor and an air reactor; directly exposing the oxygen carrier particles to concentrated solar radiation for heating the oxygen carrier particles in the fuel reactor to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction; storing the oxygen carrier particles as hot particles; transporting the hot particles to the air reactor to react exothermically with oxygen from air introduced into the air reactor to generate the hot exhaust gas; extracting heat from oxygen carrier particles during transportation from the air reactor; the oxygen carrier particles being stored as cold particles prior to being fed into the fuel reactor; and using the hot exhaust gas from the air reactor in a power cycle.

[0062] According to a seventh aspect of the invention there is provided a method of generating power comprising: transporting looping material comprising oxygen carrier particles between a fuel reactor and an air reactor; indirectly heating the oxygen carrier particles in the fuel reactor using concentrated solar radiation to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction; storing the oxygen carrier particles as hot particles; transporting the hot particles to the air reactor to react exothermically with oxygen from air introduced into the air reactor to generate the hot exhaust gas; extracting heat from oxygen carrier particles during transportation from the air reactor; the oxygen carrier particles being stored as cold particles prior to being fed into the fuel reactor; and using the hot exhaust gas from the air reactor in a power cycle.

[0063] Preferably, the oxygen carrier particles in the fuel reactor are indirectly heated by concentrated solar radiation using a heat transfer medium to transport the heat from a solar receiver to the fuel reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Further features of the present invention are more fully described in the following description of several non-limiting example embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a schematic view of a first embodiment of the hybrid solar and chemical looping combustion system according to the invention; Figure 2 is a schematic sectional view of a solar cavity vortex reactor forming part of the system shown in Figure 1 ;

Figure 3 is a schematic view of a second embodiment of the hybrid solar and chemical looping combustion system according to the invention;

Figure 4 is a schematic sectional view of a solar cavity fluidised bed reactor forming part of the system shown in Figure 3;

Figure 5 is a schematic view of a third embodiment of the hybrid solar and chemical looping combustion system according to the invention;

Figure 6 is a schematic sectional view of an indirect heated fluidised bed reactor forming part of the system shown in Figure 5;

Figure 7 is a graph depicting calculated normalized hourly average variations of the absorbed solar radiation in the fuel reactor and the input energy of the fuel for the average solar insolation at an example location of use of the hybrid solar and chemical looping combustion system; and

Figure 8 is a hybrid solar- chemical looping combustion combined cycle using the system shown in Figure 1 .

[0065] In the drawings like structures are referred to by like numerals throughout the several views. The drawings shown are not necessarily to scale, with emphasis instead generally being placed upon illustrating the principles of the present invention.

[0066] The figures which constitute the drawings depict various embodiments of the invention. The embodiments illustrates certain configurations; however, it is to be appreciated that the invention can take the form of many configurations, as would be obvious to a person skilled in the art, whilst still embodying the present invention. These configurations are to be considered within the scope of this invention.

DESCRIPTION OF EMBODIMENT

[0067] Broadly, the example embodiments shown in the drawings are each directed to a hybrid solar and chemical looping combustion system according to the invention in which two reservoirs are provided to store looping material comprising oxygen carrier (OC) particles transported between a first reactor and a second reactor. The looping material is transported between the first and second reactors along a looping path in which the two reservoirs are incorporated.

[0068] The two reservoirs constitute provision for heat storage.

[0069] The first reactor may comprise a fuel reactor and the second reactor may comprise an air reactor.

[0070] The fuel reactor may be configured as a solar fuel reactor or an indirectly heated reactor, and the air reactor may be of known kind. In the arrangement shown, the solar fuel reactor comprises a cavity solar receiver.

[0071] In this system, the operating temperature of the fuel reactor is maintained substantially constant by varying the flow rates of fuel and OC particles in response to variations in the concentration of solar radiation received by the cavity solar receiver.

[0072] The first reservoir is disposed between the air reactor and the cavity solar receiver, and the second reservoir is disposed between the cavity solar receiver and the air reactor.

[0073] The stored heat in the OC particles is released in the air reactor to achieve a higher outlet temperature than the solar fuel reactor. Heat is extracted from OC particles in a heat exchanger positioned between the air reactor and the first reservoir.

[0074] With this arrangement, the first reservoir can store OC particles which have been cooled following the heat extraction, with said particles being hereinafter referred to as cold particles. Further, the second reservoir can store OC particles which have been heated in the cavity solar receiver, with said particles being hereinafter referred to as hot particles. The terms "hot" and "cold" are relative terms in relation to temperature conditions and are to be taken in context of the invention, and are not to be taken as limiting the invention to the literal interpretation of the terms but rather as would be understood by the skilled addressee. As an example only, the hot particles may be at a temperature of about 750 Ό and the cold particles may be at a temperature of about 100 ^*C.

[0075] The extraction of heat allows the temperature of the particles stored in the first reservoir and fed to the fuel reactor to be greatly reduced, thereby enabling a much greater contribution of solar energy into the process. [0076] The extracted heat is used to preheat air for the air reactor. The extraction of heat from the OC particles being transported to the first reservoir also lowers the temperature of the OC particles entering the cavity solar receiver.

[0077] Cold particles entering the cavity solar receiver can be exposed to concentrated solar radiation to provide sensible heating to the OC particles and to drive a fuel oxidation reaction, leading to production of reduced particles which are transported to the second reservoir for storage as the hot particles. As mentioned above, the extraction of heat from the OC particles such that they enter the cavity solar receiver at a lower temperature enables a much greater contribution of solar energy into the process.

[0078] The fuel oxidation reaction is performed in the cavity solar receiver using a hydrocarbon fuel, which typically comprises mostly methane in the form of natural gas.

[0079] The stored hot particles comprise both chemical and sensible heat.

[0080] In the air reactor, reduced hot particles from the second reservoir react exothermically with oxygen from air introduced into the air reactor. The air introduced into the air reactor comprises the air preheated using heat extracted from the OC particles. The air introduced into the air reactor may further comprise supplementary air mixed with the preheated air.

[0081] By extracting heat from the oxygen carrier particles leaving the air reactor, the heat exchanger has the effect of lowering the temperature of the OC particles (the hot particles) in the second reservoir without reducing the oxidation reaction rate in the air reactor.

[0082] The two reservoirs represent provision for heat storage, as mentioned above. Further, the two reservoirs make it technically easier to operate the air and fuel reactors at different pressures than is the case for a conventional chemical looping combustion system. In the arrangements shown, each reservoir comprises an insulated vessel, to achieve low heat losses through the walls, and a hopper configuration to facilitate inflow and outflow.

[0083] In the embodiments described and illustrated, heat is extracted from OC particles being transported from the air reactor to the first reservoir using a heat exchanger incorporated in the looping path along which the OC particles are transported. In the arrangements shown, the heat exchanger comprises a direct air- particle heat exchanger. However, other forms of heat exchange between the OC particles and air may, of course, be used. By way of example, the heat exchanger may comprise a moving grate design of heat exchanger of the type used in cement and lime kilns, which operate reliably in cement kilns at temperatures of up to 1300 Ό. It may also possible to employ cyclones to act both to separate the particles from the air and to transfer heat, as in the preheater towers of cement kilns.

[0084] The hybrid solar and chemical looping combustion system further comprises means for controlling the rate of flow of looping material through the solar fuel reactor. Additionally, the system further comprises for means for controlling the flow rate of a fuel for a fuel oxidation reaction in the solar-fuel reactor.

[0085] The ability to control both the rate of flow of looping material through the cavity solar receiver and the flow rate of a fuel for a fuel oxidation reaction in the cavity solar receiver is advantageous as is affords selective control of the operating temperature of the cavity solar receiver.

[0086] The OC particles comprise particles of a metal that are easily oxidised. In the embodiments described and illustrated the carrier particles comprise Ni. A particularly suitable OC particle comprises NiO is supported on the substrate of N1AI2O4 in a mass ratio of 4 to 6. However, other suitable OC particles may, of course, be used.

[0087] The hybrid solar and chemical looping combustion system may be configured for operation in two modes. In a first mode, the OC particles are exposed to concentrated solar radiation in the cavity solar receiver to provide sensible heating to the OC particles and to drive a fuel oxidation reaction. In a second mode, there is no exposure of OC particles to concentrated solar radiation and the sensible heat stored in the OC particles is used to drive the reduction process.

[0088] In the second mode, there is provision for bypassing the heat exchanger and also the first and second reservoirs, such that OC particles leaving the air reactor are transferred directly to the fuel reactor (represented by the solar fuel reactor operating in a configuration in which it is not insolated), and then the reduced particles are transferred directly to the air reactor. In this arrangement, the system performs a conventional chemical looping combustion operation. [0089] The ability for the hybrid solar and chemical looping combustion system to operate in two modes is advantageous. Typically, the system is operated in the first mode when the available solar energy is sufficient to exceed losses. In this mode, the cavity solar receiver is configured to receive concentrated solar energy to provide sensible heating to the OC particles and to drive a fuei oxidation reaction.

[0090] in the second mode, the system can perform a conventionai chemical looping combustion operation for extended periods of low solar radiation, should this be necessary. As mentioned before, the indirect heating of the fuel reactor with solar thermal energy is also possible and may, of course, be used. In this configuration the fuel reactor is a fluidized bed reactor, of known kind, and a cavity solar receiver is used to absorb the concentrated solar radiation. A heat transfer medium such as a working fluid (e. g. molten salt) circulates between the solar cavity receiver and the fluidised be fuel reactor to transfer the absorbed concentrated solar thermal radiation in the cavity solar receiver to the OC particles within the fuel reactor.

[0091] Several example embodiments of the invention will now be described in more detail.

[0092] Referring now to Figures 1 and 2, there is shown a first example embodiment of the hybrid solar and chemical looping combustion system 10 according to the invention.

[0093] The system 10 comprises the solar fuel reactor 11 in the form of a cavity solar receiver of a configuration based on the concepts of a soiar vortex reactor, as best seen in Figure 2. The second reactor may be configured as the air reactor 12 of known kind. The looping path 13 transports the OC particles between the two reactors 1 , 12. In the drawings the OC particles are depicted schematically as discrete dots and identified by reference numeral 15,

[0094] The solar fuel reactor 1 1 is arranged to capture heat energy from a solar source. In the arrangement illustrated in Figure 1 , the solar source comprises a heliostat field 17 that reflects sunlight 19 towards the solar fuel reactor 11 which is mounted on a tower (not shown) above the heliostat field.

[0095] The solar fuel reactor 11 comprises a body 21 defining a reaction chamber 23 having an aperture 25 through which concentrated solar radiation can be received from the heliostat field 17. The body 21 further comprises an aperture shutter 27 moveable from an open position to a closed position, wherein the shutter in the closed position provides a physical seal for the aperture 25 so as to reduce heat losses.

[0096] The body 21 incorporates an inlet 31 and an outlet 33 communicating with the looping path 13 for introduction of OC particles as cold particles 15a into the reaction chamber 23 and subsequent removal of OC particles as hot particles 15b from the reaction chamber 23. Particles 15c within the reaction chamber 23 are heated to transform them from cold particles 15a to hot particles 15b.

[0097] The body 21 also incorporates a fuel inlet 35 for introduction of a fuel stream which is depicted schematically and identified by reference numeral 36, and which is for the purpose of performing a fuel oxidation reaction with the OC particles 15c in the reaction chamber 23 in known manner. In this embodiment the fuel stream comprises CH₄, although of course other appropriate fuels may be used.. With this arrangement, the concentrated solar radiation received in the reaction chamber 23 provides sensible heating of the OC particles and drives the fuel oxidation reaction (4NiO + CH₄→ 4Ni + CO2 + 2H2O). The temperature of the solar fuel reactor 1 1 can be kept constant by varying the respective flow rates of fuel (CH₄) and the OC particles, as will be explained in more detail later.

[0098] Further, the body 21 incorporates an exhaust outlet 37 for discharge of an exhaust stream which is depicted schematically and identified by reference numeral 38, and which is for the purpose of extracting products from the reaction process within the in the reaction chamber 23. In this embodiment, the exhaust stream 38 comprises H₂0, C0₂, H₂, CO and un-reacted fuel.

[0099] The first reservoir 41 is incorporated in the looping path 13 between the air reactor 12 and the solar fuel reactor 1 1 , and the second reservoir 42 is incorporated in the looping path 13 between the solar fuel reactor 11 and the air reactor 12. In the arrangement shown, each reservoir comprises an vessel insulated to achieve low heat losses through the walls and having a hopper configuration to facilitate inflow and outflow of OC particles 15.

[00100] The heat exchanger 45 is incorporated in the looping path 13 between the air reactor 12 and the first reservoir 41. As mentioned above, in the arrangements shown, the heat exchanger 45 comprises a direct air-particle heat exchanger. The direct air- particle heat exchanger 45 is configured to receive an intake air stream which is depicted schematically and identified by reference numeral 46, and an outlet air stream which is depicted schematically and identified by reference numeral 47. In the arrangement shown, the intake air stream 46 comprises ambient air, and the outlet air stream 47 comprises air heated in the heat exchange process to provide the preheated air delivered to the air reactor 12.

[00101] As also mentioned above, the air reactor 12 is of known kind. The air reactor 12 may, for example, comprise an air reactor of the type used for conventional chemical looping combustion systems, such as the fluidised beds developed at the Chalmers University Technology.

[00102] In the arrangement shown, the air reactor 12 comprises a body 51 defining a reaction chamber 53 having an inlet 55 and an outlet 57 communicating with the looping path 13 for introduction of reduced OC particles as hot particles 15b from the second reservoir 42 into the reaction chamber and subsequent removal of oxidised OC particles 15d from the reaction chamber 53.

[00103] In the reaction chamber 53, the incoming reduced hot particles 15b react exothermically with oxygen in the air (4Ni + O2→ 4NiO) in known manner to transform them into the oxidised particles 15d which discharge through the outlet 57 for transportation along the looping path to the first reservoir 41 via the direct air-particle heat exchanger 45.

[00104] The air reactor body 51 also incorporates an air inlet 61 for introduction of an intake air stream which is depicted schematically and identified by reference numeral 63.

[00105] Further, the body 21 incorporates a hot gas outlet 65 for discharge of an exhaust gas stream which is depicted schematically and identified by reference numeral 67. The exhaust gas comprises N₂ and excess 0₂.

[00106] The intake air stream 63 delivered to the air inlet 61 comprises the outlet air stream 47 from the direct air-particle heat exchanger 45 mixed with ambient air, the mixing process being provided to control the temperature of the intake air stream 63 delivered to the reaction chamber 53. In the arrangement shown, the mixing process is performed by a mixer 68 and the ambient air intake stream is depicted schematically and identified by reference numeral 69. [00107] The stored heat in the OC particles is released in the air reactor 12 at a higher temperature and pressure, leading to generation of hot gas within the air reactor to achieve a higher outlet temperature than the solar fuel reactor 1 1 . The higher temperature of the air reactor 12 is achieved through the storage of the solar thermal energy as chemical heat in the OC particles within the fuel reactor 1 1 . By way of example only, operating temperatures of 750 and 950 °C were co nsidered for the fuel and air reactors 1 1 , 12 respectively, based on the experimental tests performed on the NiO-NiAI₂04 OC particles referred to above. However, the system 10 is not restricted to these temperatures. The temperature of the exhaust gas stream can be in the range 950-1200Ό, thus potentially suitable for gas-turbine combined cycle.

[00108] The looping path 14 incorporates various valves for controlling the flow of OC particles. In the arrangement shown, the valves comprises valve 71 between the air reactor 12 and the direct air-particle heat exchanger 45, valve 72 between the first reservoir 41 and the solar fuel reactor 1 1 , valve 73 between the solar fuel reactor 11 and the second reservoir 42, and valve 74 between the second reservoir 42 and the air reactor 12.

[00109] Furthermore, the hybrid solar and chemical looping combustion system 10 has provision for bypassing the direct air-particle heat exchanger 45, and also the first and second reservoirs 41 , 42, such that OC particles leaving the air reactor 12 are transferred directly to the solar fuel reactor 1 1 operating in a configuration in which it is not insolated, and the reduced OC particles then transferred directly to the air reactor. In this arrangement, the system performs a conventional chemical looping combustion operation.

[001 10] For this purpose, the hybrid solar and chemical looping combustion system 10 incorporates two bypass lines, one being bypass line 81 incorporating valve 83, and the other being bypass line 85 incorporating valve 87.

[001 1 1] Bypass line 81 extends between the section of the looping path 14 between the air reactor 12 and valve 71 , and the section of the looping path between the first reservoir 41 and valve 72.

[001 12] Bypass line 85 extends between the section of the looping path 14 between the solar fuel reactor 1 1 and valve 73, and the section of the looping path between valve 74 and the air reactor 12. [001 13] With this arrangement, the direct air-particle heat exchanger 45, and also the first and second reservoirs 41 , 42, can be bypassed to perform a conventional chemical looping combustion operation by closing valves 71 , 73 and 74, and opening valves 83, 87 in bypass lines 81 , 85 respectively. In this way OC particles leaving the air reactor 12 are transferred directly to the solar fuel reactor 11 operating in a configuration in which it is not insolated, and the reduced OC particles then transferred directly to the air reactor 12.

[001 14] As mentioned above, the hybrid solar and chemical looping combustion system 10 has provision for selective control of the operating temperature of the solar fuel reactor 1 1. In this embodiment, the temperature of the reaction chamber 23 within the solar fuel reactor 11 is maintained substantially constant. This is achieved through controlling the rate of flow of OC particles 15 through the solar fuel reactor 1 1 and also controlling the flow rate of CH₄ for the fuel oxidation reaction in the solar-fuel reactor. The rate of flow of OC particles 15 through the solar fuel reactor 11 may be controlled by operation of valve 72 to regulate flow of cold particles 15a from the first reservoir 41 to the solar fuel reactor 11. The rate of flow of CH₄ for the fuel oxidation reaction in the solar fuel reactor 1 1 can be controlled in any known matter, such as an appropriate fluid flow control valve.

[00115] Operation of the hybrid solar and chemical looping combustion system 10 will now be described.

[00116] When the available solar energy is sufficient to exceed the losses, the solar aperture shutter 27 is opened to allow the concentrated solar radiation from the heliostat field 17 to insolate the reaction chamber 23 to provide sensible heating of the OC particles 15c and to drive the fuel oxidation reaction (4NiO + CH₄→ 4Ni + C0₂ + 2H₂0). The temperature of the reaction chamber 23 is maintained substantially constant by controlling the flow rates of CH₄ and of the oxidised cold particles 15a from the first reservoir 41. The reduced hot particles 15b produced in the solar fuel reactor 11 are stored in second reservoir 42 and fed to the air reactor 12 in accordance with demand. This allows the flow rate of the cold particles 15a from the first reservoir 41 into the reaction chamber 23 of the solar fuel reactor 1 1 to be controlled and maintained constant despite variation in the solar energy input, as the feed rate of the cold particles 15a from the first reservoir 41 into the reaction chamber 23 need not necessary correspond to the feed rate of reduced hot particles 15b from the second reservoir 42 into the air reactor 12. The storage capacities of the two reservoirs 41 , 42 can accommodate differences in the feed rates. The differences in the feed rates may be accommodated, for example, by differences in the residence times of the particles 15 in the respective reservoirs 41 , 42.

[001 17] In the air reactor 12, the reduced hot particles 15b react exothermically with oxygen from the air (4Ni + 0₂→ 4NiO) in known manner to transform them into the oxidised particles 15d which discharge through the outlet 57 for transportation to the first reservoir 41 via the direct air-particle heat exchanger 45. The direct air-particle heat exchanger 45 serves both to lower the temperature of the OC particles stored in the first reservoir 41 without reducing the oxidation reaction rate in the air reactor 12 and to preheat the air stream 47 which provides a component of the intake air stream 63 delivered to the air inlet 61 of the air reactor 12. The ambient air intake stream 69 at mixer 68 is also used to control the operating temperature of the air reactor 12.

[00118] As described above, the hybrid solar and chemical looping combustion system 10 can also be operated as a conventional chemical looping combustion system, when solar radiation is not available at sufficient fluxes to exceed the required threshold.

[00119] The hybrid solar and chemical looping combustion system 10 can achieve a constant temperature and mass flow rate of the exhaust gas stream 67 from the air reactor 12 with and without concentrated solar thermal energy for base-load power generation.

[00120] Alternatively, the hybrid solar and chemical looping combustion system 10 can be also achieve flexible operation (such as for load following), since the stored energy of the OC particles in the solar fuel reactor 11 is released in the air reactor 12, which can be operated independently. By way of explanation, the air and fuel reactors are separated by the two reservoirs 41 , 42. Consequently, the air reactor 12 is fed by the particles stored in the second reservoir 42, not directly by those coming from the fuel reactor 1 1. This results in independency of the operation of the air reactor 12 for so long as there are enough particles in the second reservoir 42.

[00121] Furthermore, the solar fuel reactor 1 1 can be "over-sized" for those periods of high solar insolation to achieve longer term storage of solar energy in the second reservoir 42 for use during periods of lower insolation. [00122] The first and second reservoirs 41 , 42 represent provision for heat storage. Further, the two reservoirs 41 , 42 make it technically easier to operate the fuel reactor 11 (the cavity solar receiver) and the air reactor 12 at different pressures than is the case for a conventional chemical looping combustion system. This is significant because the difference in the operating pressures of the fuel and air reactors can lead to gas leakage in conventional chemical looping combustion system, lowering the efficiency of C0₂ separation. The potential to operate the solar fuel reactor 11 at a much lower pressure than the air reactor 12 is highly desirable. This is because the solar fuel reactor 11 has a transparent window, as is commonly the case with solar reactors. Operation of a solar reactor at a much lower pressure than the air reactor 12 significantly reduces the risk of damage to the transparent window, which is vulnerable to high pressures and harsh environments. In particular, operation at atmospheric pressure allows window thickness to be minimised for efficient transmission of solar energy.

[00123] Referring now to Figures 3 and 4, there is shown a second example embodiment of the hybrid solar and chemical looping combustion system 10 according to the invention. The second embodiment is similar in many respects to the first embodiment and so corresponding reference numerals are use to identify corresponding parts.

[00124] A difference in the second embodiment from the first embodiment is that the solar fuel reactor 11 comprises a cavity solar receiver of a configuration based on the concepts of a fluidise bed reactor, as best seen in Figure 4. The second reactor is configured as the air reactor 12 of known kind, as was the case in the first embodiment.

[00125] In this second embodiment, a reflector 90 is used to reflect the concentrated beam 17b, as a downward beam 17c, to the cavity reactor 11 , as shown in Figure 3.

[00126] As mentioned above, the hybrid solar and chemical looping combustion system according to the invention can also work with an indirectly heated fuel reactor. The next embodiment is directed to such a system.

[00127] Referring now to Figures 5 and 6, there is shown a third example embodiment of the hybrid solar and chemical looping combustion system 10 according to the invention. The third embodiment is similar in many respects to the previous embodiments and so corresponding reference numerals are use to identify corresponding parts.

[00128] In this third embodiment, all components of the hybrid solar and chemical looping combustion system are the same as the directly heated system described in relation to the previous embodiments, except that the solar cavity reactor is replaced by an indirectly heated reactor 11.

[00129] The indirectly heated reactor 11 comprises the body 21 defining the reaction chamber 23, and a system for indirectly heating the reaction chamber 23. The system for indirectly heating the reaction chamber 23 comprises a solar absorber 200 and a circulating loop 250 of a heat transfer medium such as working fluid in the form of molten salt 251 . A suitable heat transfer medium other than molten salt can also be used, as would be understood by one skilled in the art.

[00130] Importantly, the solar absorber 200 can comprise any suitable type of solar receiver design. In this embodiment, the solar absorber 200 comprises an absorber mounted in a cavity configured to receive concentrated solar radiation, thereby forming a solar cavity absorber as shown in Figures 5 and 6. However, it should be understood that any other type of receiver design can be used, such as an externally-heated tubular design, as would be understood by one skilled in the art.

[00131] In the arrangement shown, the solar cavity absorber 200 comprises a body 201 defining an absorber chamber 202 having an aperture 203, thereby defining a solar cavity. The solar cavity accommodates an absorber in the form of a heat exchanger configured as a tube coil 204 incorporated in the circulating loop 250.

[00132] The circulating loop 250 of the molten salt 251 comprises the tube coil 204 within the solar cavity absorber 200and a further heat exchanger configured as heating coil 205 within the reaction chamber 23. The circulating loop 250 further comprises a pump 170 for circulating the molten salt 251. The molten salt 251 heated within the solar cavity absorber 200 is transferred to the heating coil 205 within the reaction chamber 23, by the circulating pump 170, to provide the required heat for the sensible heating of the OC particles.

[00133] With this arrangement, the concentrated solar energy received in absorber chamber 202 provides sensible heating of the molten salt 251 passing through the tube coil 204 to transform the molten salt from cold molten salt 251 a to hot molten salt 251b. The operating temperature of the solar cavity absorber 200 is maintained constant by varying the flow rate of molten salt by the pump 170 in response to variations in the intensity of concentrated solar thermal energy 17.

[00134] The body 21 defining the reaction chamber 23 incorporates fuel inlet 35 for introduction of a fuel stream 36, which is for the purpose of performing fuel oxidation with the OC particles.

[00135] The body 21 also incorporates inlet 31 and outlet 33 communicating with the looping path 13 for the introduction of OC particles as cold particles 15a into the reaction chamber 23 and removal of OC particles as hot particles 15b from the reaction chamber 23. Particles 15c within the reaction chamber 23 are heated to transform them from cold particles 15a to hot particles 15b.

[00136] In this embodiment the fuel stream comprises CH₄. The hot molten salt 251 b coming from the solar cavity absorber 200 provides sensible heating of the OC particles and drives the fuel oxidation reaction (4NiO + CH₄ → 4Ni + C0₂ + 2H₂0). The temperature of the molten salt consequently decreases and the resultant cold molten salt 251a returns back to the solar cavity absorber 200. In this configuration the temperature of the reaction chamber 23 is maintained constant by varying the flow rates of CH , and of the OC particles.

[00137] The various embodiments of the hybrid solar and chemical looping combustion system 10 described above are each estimated to achieve a solar fraction of up to 60% while providing sufficient storage to achieve continuous base-load power generation, for the average diurnal fluctuations in solar radiation. This may be better understood by the following example which is provided for illustrative purposes only and which is discussed with reference to Figure 7.

[00138] Fig. 7 presents the calculated average diurnal variations of the absorbed solar energy, Q^s--^ahs , and input fuel energy, ^p , into the solar fuel reactor 11 . These parameters are normalized to the average maximum solar heat input to solar fuel reactor, ^V^K,™* f_or p_ort Augusta, South Australia. The normalized absorbed solar energy in solar fuel reactor 11 per maximum solar heat increases from 1.8x10-2 at 6:30 to 95x 10-2 at 12:30 and then decreases to 15x 10-2 at 18:30, when the aperture is closed. The normalized energy of fuel to maximum solar heat also shows the same trend of variations. It increases from an initial value of 1 .2x10-2 at 6:30 to reach a peak value of 62.8x10-2 at 12:30 and then decreases to a final value of 1 x10-1 at 18:30.

[00139] With reference to Figure 7, the area under the curves 'βν«,π™ _{a n c}|

Q^F 1 2 „,max corresponds respectively to the total solar energy absorbed in solar fuel reactor 1 1 and the total fuel energy input relative to the maximum solar input. That is from the total absorbed solar energy and fuel energy to the system 10, 60 % comes from the input solar energy, which means a total solar share of 60%, for the system.

[00140] Referring now to Figure 8, there is shown diagrammatically an implementation of the hybrid solar and chemical looping combustion system 10 in a power cycle 100, thereby providing a hybrid solar-CLC combined cycle. The power cycle 100 comprises of two main sections: (i) a hot gas generator 101 corresponding to the system 10 and (ii) a combined power generation system 103.

[00141] In Figure 8, relevant parts of the hot gas generator 101 which correspond to the hybrid solar and chemical looping combustion system 10 are identified with corresponding reference numerals with further description. While in the arrangement shown the hot gas generator 101 is configured as the first embodiment shown in Figures 1 and 2, it should be understood that the hot gas generator 101 may be of any other appropriate configuration, including either the second embodiment shown in Figures 3 and 4, or the third embodiment shown in Figures 5 and 6.

[00142] The hot gas generator 101 operates in the manner previously described in relation to the system 10 to provide the hot exhaust gas stream 67 at the gas outlet 65 of the air reactor 12 for delivery to a gas turbine 105. The gas turbine 105 is a three stage gas turbine in this example.

[00143] An air compressor 107 driven by an output from the gas turbine 105 is provided to pressurize the intake air stream 63 delivered to the air inlet 61 of the air reactor. Specifically, the air compressor 107 pressurizes the intake air stream 46 passing through the direct air-particle heat exchanger 45 such that the outlet air stream 47 is pressurized. The air intake stream 69 may also comprise pressurized air received from the air compressor 107.

[00144] An air heat exchanger 1 1 1 is provided to cool the pressurised intake air stream 46 delivered to the direct air-particle heat exchanger 45 for the cooling of the OC particles and to further produce steam for power generation. The use of valves 72 and 74 allows the air reactor 12 to be pressurised while the solar fuel reactor 1 1 is at atmospheric pressure. In addition to making provision for heat storage, the use of the first and second reservoirs 41 and 42 makes it technically easier to operate the air and fuel reactors at different pressures than is the case for a conventional CLC system, as discussed previously.

[00145] An after-burner 121 can be used to increase the temperature of the pressurized hot exhaust gas stream 67 from the air reactor 12. Under conditions in which the excess oxygen from the air reactor 12 is not sufficient to burn all of the fuel supplied to the after-burner 121 , supplementary pressurized air is provided by the air compressor. The flow rate of supplementary pressurized air is adjustable using valves 123, 124 and 125.

[00146] The cycle can also operate without the after-burner 121 , using valves 127 and 129, in which case the exhaust gas stream 67 from the air reactor 12 is directly introduced to the gas turbine 105.

[00147] The hot and pressurised exhaust gas stream 67 leaving the air reactor 12 is used to generate power by means of the three-stage gas turbine 105. The heat recovered through the heat recovery steam generators 131 , 132 is also utilized to produce additional power with the gas turbine 105 and steam turbines 133, 134. The steam turbine 133 has two stages, the high pressure, HP, and low pressure LP, respectively. The after-burner 121 is optionally used to increase the temperature to the gas turbine inlet using valve 127 and 129.

[00148] The steam turbines 133, 134 are fed by their own heat recovery steam generators 131 , 132 respectively, as mentioned above.

[00149] The C0₂-rich exhaust stream 38 from the solar fuel reactor 1 1 may be treated to render it suitable for transport and geological sequestration. In the arrangement shown, the CCVrich exhaust stream 38 is cooled by heat exchange in steam generator 132 and fed to a C0₂ dehydrator 143 and compressor 145. The compression by compressor 145 represents only a small a parasitic loss for the cycles with and without the integration of the after-burner 121 , which is approximately 1.6% and 2.6 respectively. These losses could be decreased with multi-step compression and inter-cooling. [00150] An option to the use of the heat exchange together with the compression of the CO₂ for transport and geological sequestration is to employ mineral sequestration of C0₂. One such process involves the endothermic conversion of magnesium silicate to magnesium hydroxide before the exothermic carbonation reaction with C0₂ to produce a stable magnesium carbonate. In the arrangement shown, the enthalpy in the hot exhaust gas stream leaving the air reactor has potential to drive this process.

[00151] With this arrangement, the power cycle 100 achieves a high level of shared infrastructure between the hot gas generator 101 and the combined power generation system 103.

[00152] From the foregoing, it is evident that the present embodiments each provide a hybrid solar and chemical looping combustion system according to the invention which provides thermal energy storage chemical and sensible heat storage. The relative high temperature of the exhaust gas at 950 - 1200Ό makes the system suitable for high thermal efficiency of the power plant; for example, by way of a gas turbine or a super critical Rankine cycle. The system also affords capacity to pressurise the hot exhaust gas at this relatively high outlet temperature. The system can also deliver a relatively high radiation efficiency of the solar fuel reactor by achieving a lower temperature in the reaction chamber than the final temperature of the hot exhaust gas discharging from the air reactor. Importantly, the system also offers a high solar fraction; for example a solar fraction of about 65% with continuous operation.

[00153] It should be appreciated that the scope of the invention is not limited to the scope of the embodiments described as examples.

[00154] While the present invention has been described in terms of a preferred embodiments (and a system for implementation of the preferred embodiments in a power cycle) in order to facilitate better understanding of the invention, it should be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications within its scope.

[00155] Reference to positional descriptions, such as "upper", "lower", "top" and "bottom", are to be taken in context of the embodiments depicted in the drawings, and are not to be taken as limiting the invention to the literal interpretation of the term but rather as would be understood by the skilled addressee. [00156] Additionally, where the terms "system", "device" and "apparatus" are used in the context of the invention, they are to be understood as including reference to any group of functionally related or interacting, interrelated, interdependent or associated components or elements that may be located in proximity to, separate from, integrated with, or discrete from, each other.

[00157] Further, reference to the relative terms "hot", "cold", "higher" and "lower" in relation to temperature conditions are to be taken in context of the invention, and are not to be taken as limiting the invention to the literal interpretation of the terms but rather as would be understood by the skilled addressee

[00158] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A hybrid solar and chemical looping combustion system comprising a fuel reactor, an air reactor, a looping path for transferring looping material comprising oxygen carrier particles between the fuel reactor and the air reactor, first and second reservoirs in the looping path for receiving looping material, the first reservoir being disposed between the air reactor and the fuel reactor, the second reservoir being disposed between the fuel reactor and the air reactor, and a heat exchanger in the looping path in the between the air reactor and the first reservoir for heat transfer from the looping material to an air flow to the air reactor, wherein the fuel reactor is configured to be heated by concentrated solar energy.

2. The combustion system according to claim 1 wherein the fuel reactor is configured to be heated either directly or indirectly by concentrated solar energy.

3. The combustion system according to claim 1 or 2 wherein the fuel reactor comprises a direct heated solar fuel reactor.

4. The combustion system according to claim 1 or 2 wherein the fuel reactor comprises an indirectly heated solar fuel reactor.

5. The combustion system according to any one of the preceding claims wherein the air reactor comprises an air inlet and an exhaust gas outlet, with the air flow from the heat exchanger communicating with the air inlet.

6. The combustion system according to claim 5 wherein air introduced into the air reactor comprises the air flow from the heat exchanger and also supplementary air.

7. The combustion system according to any one of the preceding claims wherein the heat exchanger comprises an air particle heat exchanger.

8. The combustion system according to any one of the preceding claims further comprising control means for controlling the rate of flow of looping material through the fuel reactor.

9. The combustion system according to any one of the preceding claims further comprising control means for controlling the flow rate of a fuel for a fuel oxidation reaction in the fuel reactor.

10. The combustion system according to any one of the preceding claims wherein the system is operable in a mode in which the oxygen carrier particles are exposed to concentrated solar radiation in the solar-fuel reactor to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction.

11. The combustion system according to any one of claims 1 to 9 wherein the system is operable in a mode in which there is no exposure of the oxygen carrier particles to concentrated solar radiation and the sensible heat of the oxygen carrier particles is used to drive the reduction process.

12. The combustion system according to claim 11 further comprising provision for bypassing the heat exchanger and also the first and second reservoirs, whereby oxygen carrier particles leaving the air reactor are transferred directly to the solar-fuel reactor without the need for a solar energy input so that the particles are reduced using the sensible heat stored in the hot oxygen carrier particles.

13. The combustion system according to any one of claims 1 to 9 wherein the fuel reactor is operable in an indirectly heated configuration, whereby oxygen carrier particles from the first reservoir entering the fuel reactor are heated indirectly.

14. The combustion system according to claim 13 wherein indirect heating of gas and oxygen carrier particles in the fuel reactor comprises a separating conductive medium heated by concentrated solar radiation in a separate solar absorber.

15. A system for generation of power comprising a hybrid solar and chemical looping combustion system according to any one of the preceding claims.

16. A method of generating power using a system according to any one of the preceding claims.

17. A method of generating a stream of hot exhaust gas comprising: transporting looping material comprising oxygen carrier particles between a fuel reactor and an air reactor; exposing the oxygen carrier particles to in the fuel reactor to heat generated using concentrated solar radiation to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction; storing the oxygen carrier particles as hot particles; transporting the hot particles to the air reactor to react exothermically with oxygen from air introduced into the air reactor to generate the hot exhaust gas; extracting heat from oxygen carrier particles during transportation from the air reactor; the oxygen carrier particles being stored as cold particles prior to being fed into the fuel reactor.

18. The method according to claim 17 wherein heat to which the oxygen carrier particles are exposed in the fuel reactor comprises heat generated directly or indirectly from concentrated solar radiation.

19. The method according to claim 17 or 18 wherein the heat extracted from the oxygen carrier particles is used to preheat air introduced into the air reactor.

20. The method according to claim 17, 18 or 19 wherein heat is extracted from the oxygen carrier particles by passing the oxygen carrier particles through a heat exchanger.

21. The method according to claim 17, 18 or 19 wherein heat is extracted from the oxygen carrier particles by passing the oxygen carrier particles through an air- particle heat exchanger and wherein the air is also passed in heat exchanger relation with the air-particle heat exchanger to receive heat from the oxygen carrier particles.

22. A method of generating power comprising: transporting looping material comprising oxygen carrier particles between a fuel reactor and an air reactor; exposing the oxygen carrier particles in the fuel reactor to heat generated using concentrated solar radiation to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction, storing the oxygen carrier particles as hot particles; transporting the hot particles to the air reactor to react exothermically with oxygen from air introduced into the air reactor to generate the hot exhaust gas; extracting heat from oxygen carrier particles during transportation from the air reactor; the oxygen carrier particles being stored as cold particles prior to being fed into the fuel reactor; and using the hot exhaust gas from the air reactor in a power cycle.

23. A method of generating power comprising: transporting looping material comprising oxygen carrier particles between a fuel reactor and an air reactor; directly exposing the oxygen carrier particles to concentrated solar radiation for heating the oxygen carrier particles in the fuel reactor to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction; storing the oxygen carrier particles as hot particles; transporting the hot particles to the air reactor to react exothermically with oxygen from air introduced into the air reactor to generate the hot exhaust gas; extracting heat from oxygen carrier particles during transportation from the air reactor; the oxygen carrier particles being stored as cold particles prior to being fed into the fuel reactor; and using the hot exhaust gas from the air reactor in a power cycle.

24. A method of generating power comprising: transporting looping material comprising oxygen carrier particles between a fuel reactor and an air reactor; indirectly heating the oxygen carrier particles in the fuel reactor using concentrated solar radiation to provide sensible heating to the oxygen carrier particles and to drive a fuel oxidation reaction; storing the oxygen carrier particles as hot particles; transporting the hot particles to the air reactor to react exothermically with oxygen from air introduced into the air reactor to generate the hot exhaust gas; extracting heat from oxygen carrier particles during transportation from the air reactor; the oxygen carrier particles being stored as cold particles prior to being fed into the fuel reactor; and using the hot exhaust gas from the air reactor in a power cycle.

25. The method according to claim 24 wherein the oxygen carrier particles in the fuel reactor are indirectly heated by concentrated solar radiation using a heat transfer medium to transport the heat from a solar receiver to the fuel reactor.