SE1851338A1

SE1851338A1 - Pcm storage vessel

Info

Publication number: SE1851338A1
Application number: SE1851338A
Authority: SE
Inventors: Daniel Gloss; Erik Rilby; Henrik Wikström; Torbjörn Lindquist
Original assignee: Azelio Ab
Priority date: 2018-10-29
Filing date: 2018-10-29
Publication date: 2020-04-30
Also published as: SE543298C2

Abstract

A sealed phase change material (PCM) storage vessel for a solar thermal energy storage system, the storage vessel comprising an inverted tapered portion forming a receptacle for a PCM, the tapered portion having a tip portion and a base portion, the tip portion having a diameter less than the diameter of the base portion, the storage vessel further comprising a region for receiving solar thermal energy proximal the tip portion of the inverted tapered portion.

Description

PCM STORAGE VESSEL Field of the InventionThe present disclosure relates to a PCM storage vessel. In particular it relates to a sealed PCM storage vessel comprising an inverted tapered portion.

Background of the invention In concentrated solar power (CSP) generation systems the sun°s radiation isfocused by parabolic mirrors, heliostats or the like on to solar radiation receivers.

The solar radiation receiver absorbs solar radiation as therrnal energy and aconversion system (generally a heat engine) then converts this therrnal energy to electricalenergy for subsequent distribution. There are several classes of systems that comprisedifferent solar receivers and associated electrical energy generation systems. Forexample, a parabolic trough system comprises a large surface area of parabolic troughshaped solar collectors directing solar radiation to a heat transfer ﬂuid (HTF) arrangedalong a focal axis of the parabolic trough. The HTF is then pumped and used to conducttherrnal energy to the conversion system for electrical energy generation. Another systemis a power tower whereby solar radiation is directed by fields of mirrors which track thesun toward a static central tower. The central tower comprises a HTF which can be usedto conduct therrnal energy to a conversion system. A Linear Fresnel reﬂector uses asystem of ﬂat linear mirrors approximating the parabolic shape of a parabolic trough anddirecting the solar radiation to a solar receiver comprising a HTF which can be used forsubsequent electrical energy generation as described.

Another solar receiver design is the parabolic dish system. The parabolic dishsystem comprises a parabolic dish comprising mirror(s) on an inner surface which tracksthe sun in 2-axis during the day. The solar radiation is concentrated at a focal point by themirror(s) where it is traditionally converted directly to electrical energy via a conversionsystem such as a Stirling or Rankine cycle engine.

One aspect of concentrated solar power electricity generation is that energygenerated during hours of sunlight should be stored such that energy can be generatedeven when during hours of less or no sunlight. To this end therrnal energy storage systems for storing therrnal energy generated during periods of sunlight are known in concentrated solar power systems. A therrnal energy storage system may comprise a two-tank systemcomprising for example, three heat exchangers. The first of the two tanks is a hot tankcomprising a medium capable of storing therrnal energy, the second of the two tanks is acold tank comprising a medium for storing therrnal energy. The system further comprisesa heat transfer ﬂuid which is used to indirectly transfer therrnal energy from the hot tankvia a heat exchanger, to a system or device for generating electrical energy via a heatexchanger. The energy extraction of the heat transfer ﬂuid results in the heat transfer ﬂuidhaving a reduced temperature compared with the heat transfer ﬂuid entering the heatexchanger for the electrical energy generator, the therrnal energy remaining in the lowertemperature heat transfer ﬂuid is then transferred to the medium for storing therrnalenergy in the cold tank. Generally this second transfer happens via a third heat exchanger.The existing systems generally use two tanks and a heat transfer ﬂuid as the medium forstoring heat energy in the tanks may be unsuitable for pumping as a heat transfer ﬂuid,and furtherrnore, the respective temperatures of the hot and cold tanks, and the energystorage medium therein, may be so great that the same material may be unsuitable forboth tanks. That is, two different energy storage mediums may be required.

The energy storage mediums may be phase change materials (PCMs). A phasechange material is a material which is capable of storing and releasing large amounts ofenergy when the material changes from a solid to a liquid and vice versa. A PCM isgenerally a material which absorbs energy during heating as a phase change from e.g. asolid to a liquid. The PCM may release energy during the reversed cooling process.During heating of the PCM in a solid phase the solid increases temperature (sensibleenergy storage). During phase change from solid to liquid energy is stored latently. Afterphase change to a liquid, energy is again stored sensibly and the PCM in liquid phaseincreases temperature.

As a cool, solidified, PCM generally has a lower therrnal conductivity than theliquid phase PCM existing therrnal energy storage systems have investigated complexencapsulation techniques for the PCM to reduce the effects of solidification onperformance. These may entail encapsulation in small capsules or beads, or encapsulation of the PCM in a matrix of cylinders (heat pipes) inside a storage vessel. Each of these encapsulations techniques result in complex, expensive therrnal energy storage systemsand PCM vessels.Simpler, less costly and more robust systems of therrnal energy storage are required which reduce production, installation and maintenance complexity.

Summary of the invention Accordingly, the present invention preferably seeks to mitigate, alleviate oreliminate one or more of the above-identified deficiencies in the art and disadvantagessingly or in any combination and solves at least the above mentioned problems byproviding A sealed phase change material (PCM) storage vessel for a solar therrnal energystorage system, the storage vessel comprising an inverted tapered portion forrning areceptacle for a PCM, the tapered portion having a tip portion and a base portion, the tipportion having a diameter less than the diameter of the base portion, the storage vesselfurther comprising a region for receiving solar therrnal energy proximal the tip portion ofthe inverted tapered portion.

A method for storing energy in a PCM vessel is also provided.

Further advantageous embodiments are disclosed in the appended and dependent patent claims.

Brief description of the drawings These and other aspects, features and advantages of which the invention iscapable will be apparent and elucidated from the following description of embodimentsof the present invention, reference being made to the accompanying drawings, in which Fig. l is a cross-sectional side view of a PCM storage vessel comprising aninverted tapered portion and a HTF receptacle according to an aspect.

Fig. 2 is a perspective view of a PCM storage vessel comprising an invertedtapered portion and a HTF receptacle according to an aspect.

Fig. 3 is a schematic of a wall of a PCM storage vessel according to an aspect.

Fig. 4 is a cross-sectional view of a HTF receptacle according to an aspect.

Fig. 5 is a partial cross-section of a PCM storage vessel according to an aspect comprising a hatch according to an aspect.

Detailed description Fig 1 shows a phase change material (PCM) storage vessel 100 for a solartherrnal energy plant comprising an inverted tapered portion 101 forrning a receptacle fora PCM 102, the tapered portion 101 having a tip portion 103 and a base portion 104, thestorage vessel 100 further comprising a region for receiving solar therrnal energy 105proximal the tip portion 103 of the inverted tapered portion 101. PCM 102 is enclosablewithin the PCM storage vessel 100.

The PCM storage vessel 100 advantageously uses the effect of gravity on thePCM 102 such that, during warrning by solar therrnal energy 106, the relatively coolerand possibly solidif1ed portions of PCM 102 is amassed at the bottom of the vessel in thetip portion 103 of the tapered portion 101. Whereupon it is heated at the region forreceiving solar therrnal energy 105 and may rise within the vessel 100 to the base portion104. The PCM 102 may thus be warrned within the vessel 100 passively, effectively, andefficiently. As opposed to other heat storage mediums which are maintained in a liquidphase, such as a heat transfer ﬂuid (HTF), in a PCM the relatively cooler portion may bea solid and have significantly reduced heat transfer properties compared to the liquidphase and therefore. The solidif1ed PCM has less volume compared to liquid phase PCM.On solidif1cation, in a PCM storage vessel without a tapered portion, a gap may formbetween the inner wall of the PCM storage vessel and the solidif1ed PCM. The gaps havevery poor heat transfer properties. The tapered portion 101 of the present PCM storagevessel 100 limits the formation of gaps as solidif1ed PCM 102 is directed, due to gravity,downward toward the region for receiving solar therrnal energy 105. Furthermore, in atypical non-tapered vessel, cylinders or columns, having a cross-section approximatingthe cross-section of the vessel, may form within the vessel. The tapered portion 101 ofthe present PCM storage vessel 100 inhibits the formation of such cylinders or columnsas the solidif1ed PCM is directed towards the region for receiving solar therrnal energy 105.

The inverted tapered portion 101 is arranged such that the tip 103, that is theregion of reduced diameter of the cone is below the base 104 of the cone, that is, therelatively wider portion. It is this that is meant by the terrn inverted. In use, the narrowregion of reduced diameter, the tip portion 103, of the tapered portion 101 is beneath thebelow the wider part, the base 104, of the tapered portion 101.

The storage vessel 100 comprises a region for receiving solar therrnal energy105. The region for receiving solar therrnal energy 105 is arranged at the bottom of thetapered portion 101, that is, proximal the tip 103 of the tapered portion 101. The regionfor receiving solar therrnal energy 105 may be at the tip 103 of the tapered portion 101such that it is at the lowerrnost face 107 of the tapered portion. The region for receivingsolar therrnal energy 105 may be on the sloping wall 108 of the tapered portion 101proximal the tip 103.

The region for receiving solar therrnal energy 105 may be integral with the PCMstorage vessel 100. That is, the region for receiving solar therrnal energy 105 may be indirect communication, such as directly at, the PCM storage vessel 100. By providing theregion for receiving solar therrnal energy 105 in direct communication with the PCMstorage vessel 100 the PCM 102 is heated more efficiently than if solar therrnal energy106 was first directed on to a receiver not in communication with the PCM storage vesseland then that therrnal energy was subsequently extracted and transferred to the PCM 102.

The region for receiving solar therrnal energy 105 may comprise a surface 109on to which sunlight 106 is directed. Sunlight 106 may be directed on to an extemalsurface of the region for receiving solar therrnal energy 105. Thermal energy may transferfrom the surface 109 of the region for receiving solar therrnal energy to the PCM 102 inthe vicinity of the region for receiving solar therrnal energy 105.

The PCM storage vessel 100 may be substantially sealed such that PCM 102 isnot exposed to the ambient environment in which the storage vessel 100 is located. Thereis no inﬂow nor outﬂow of PCM to or from the PCM storage vessel 100 during provisionand/or extraction of therrnal energy to the PCM 102. The PCM storage vessel 100 maybe sealed at its upper portion 110 and at its lower portion 111. As would be understoodthe sealing of the PCM storage vessel 100, does not preclude the presence of outlets and inlets comprising openable and closeable valves which are openable for the delivery and/or release of ﬂuids such as gasses during operation maintenance. Sealing of the PCMstorage vessel 100 enables better heat energy maintenance within the vessel 100. A sealedPCM storage vessel 100 may also be described as an enclosed PCM storage vessel 100.During the provision or extraction of therrnal energy the PCM storage vessel 100 may beenclosed. The PCM storage vessel may be openable for maintenance.

The region for receiving solar therrnal energy 105 may be sealed. That is, thePCM vessel 100 is not open at the region for receiving solar therrnal energy 105.

As opposed to existing PCM storage vessels, the present PCM storage vessel100 may be substantially free from intemal structures such as a pipes, matrices, or thelike for holding and sectioning the PCM 102. The total mass of PCM 102 is enclosedwithin a single partition. The PCM storage vessel 100 may be considered a single tankfor holding the entire volume of PCM 102 present in the therrnal energy storage system.The intemal volume of the PCM storage vessel 100 may be a single partition.

The PCM storage vessel 100 may be substantially opaque. Preferably, the regionfor receiving solar therrnal energy 105 is opaque. The region for receiving solar therrnalenergy 105 need not be transparent as it is the region for receiving solar therrnal energy105 which first absorbs the solar therrnal energy 106 and not the PCM 102 within thePCM storage vessel 100. This has not been possible in traditional systems as generallyPCM storage vessels comprise insulating walls, not therrnally conductive walls, as in thepresent case. This will be discussed further below. Furthermore, an opaque vessel 100enables a wider choice of materials for the vessel 100.

The term tapered used herein is used in the general sense to describe a vesselhaving a three dimensional shape with a region of greater diameter at an upper baseportion 104 and a region of reduced diameter at a lower tip portion 103. The taperedportion 101 may be a frusto-conical tapered portion 101 wherein the taper is formed by apart of a cone. The tapered portion 101 may be an oblique frusto-conical portion 101wherein the taper is formed by a part of an oblique cone. In Fig. 1 the tapered portion 101is a frusto-conical tapered portion. As will be described below, the tapered portion 101may be a convex protrusion. The tapered portion always forms a receptacle for PCM 102, The tapered portion 101 is always arranged with the lower tip portion 103 beneath the upper greater diameter portion 104 such that heavier particles move, under the force ofgravity, towards the tip portion 103.

The tapered portion 101 is forrned by a wall 108 which encloses the receptaclefor PCM 102.

Fig 2 shows the tapered portion 101 of the PCM storage vessel 100 wherein thetapered portion 101 is irregular in that the wall 108 of the conical portion 101 is at leastpartially curved, that is not forrned by a part of a perfect cone. The wall 108 of the taperedportion 101 may be convex such that the tapered portion 101 has the forrn of a convexprotrusion 101, for example an elliptic paraboloid. The apex of the convex protrusionforms the tip portion 103 of the tapered portion 101. The convex protrusion 101 may becoaxial with the central longitudinal axis L of the PCM storage vessel 101. That is, theconvex protrusion 101 may be located centrally on the PCM storage vessel 100,substantially equidistance from the lateral perimeter of the PCM storage vessel 100. Theconvex protrusion 101 may be located laterally offset from the center of the PCM storagevessel 100. That is, the convex protrusion 101 may be located such that distance of thecenter point of the convex protrusion is laterally offset from the center of the PCM storagevessel. An irregular tapered portion 101 has benefits with respect to ﬂow dynamics withinthe PCM storage vessel 100 as there are no or fewer comers where solidified PCM 102may collect.

The tapered portion 101 of the PCM storage vessel 100 may be an irregulartapered portion having a tip portion 103 offset from the center point of the base portion104 of the tapered portion 104. The tip portion 103 of the tapered portion 101 may beextended towards a lateral region of the PCM storage vessel 100. In such an arrangementthe tip portion 103 of the tapered portion 101 extends at a, generally acute, angle from thecentral longitudinal axis L of the tapered portion 101. Such an arrangement may lead toadvantageous positioning of the region for receiving solar therrnal energy 105.

The PCM storage vessel may comprise an upper portion 110 which issubstantially cylindrical. The upper portion 110 and the tapered portion 101 may beconnected at the base 104 of the tapered portion. The upper portion 110 is above thetapered portion 101. Both the upper portion 110 and the tapered portion 101 may forrnthe receptacle for PCM 102. The upper-portion 1 10 and the tapered portion 101 may forrn a single receptacle. The receptacle may have no internal dividers. The upper portion 110may have a volumetric capacity greater than the tapered portion 101. During use, morePCM 102 may be present in the upper portion 110 than in the tapered portion 101. Thismay allow the advantageous passive circulation of PCM 102 whilst maintaining a largevolumetric capacity of the vessel 100.

As stated previously, the PCM storage vessel 100 may be sealed. The covering112, or lid member 112 for sealing the upper portion 110 of the PCM storage vessel 100may be a flat covering 112. The covering 112 may be curved, such as partially dome-shaped as is shown in Fig. 2.

The PCM storage vessel 100 may be located above ground and arranged suchthat the tip portion 103 of the tapered portion 101 is beneath the base portion 104. The tip103 of the tapered portion 101 may be above ground. The PCM storage vessel 100 maybe arranged on a tower.

The PCM storage vessel 100 is formed by a wall 108 enclosing a PCM storageregion thus forrning a PCM 102 receptacle. The wall may comprise tapered portion 101and a non-tapered portion. The wall 108 may comprise a plurality of separate memberswhich are joined to form a single wall 108. For example, the wall may comprise an upperlid 112 or covering 112, at least one lateral wall 114 at the side(s) of the PCM storagevessel 100, and a base 113 or bottom wall 113. Traditionally PCM storage vessels havebeen designed with walls that are therrnally insulating. At least a portion of the wall 108of the present PCM storage vessel may be substantially therrnally conductive, that is, non-therrnally insulating, such that heat may transfer from the PCM 102 in the PCM storagevessel 100, through the wall 108, and in to any elements in therrnal contact with the wallof the PCM storage vessel 100. The plurality of separate wall members may each have adifferent therrnal conductivity. The plurality of separate wall members may have the sametherrnal conductivity. The lateral wall 1 14 of the PCM storage vessel may be the therrnallyconductive portion of the wall 108. The entire wall 108 of the PCM storage vessel 100may be therrnally conductive. The surface 109 of the region for receiving solar therrnalenergy 105 may be made of the same material as the wall 108 of the PCM storage vessel 100.

The Wall 108 of the PCM storage vessel 100 may comprise a metallic layer 181.The Wall 108 may further comprise an additional layer being a ceramic 182. The materialand/or thickness of the metallic layer 181 may be selected such that the metallic layer 181is therrnally conductive. The material and/or thickness of the ceramic layer 182 may beselected such that the ceramic layer 182 is therrnally conductive. The ceramic layer 182may form the intemal layer. The metallic layer 181 may be the outer layer. The ceramiclayer 182 may be arranged on the intemal surface of the Wall 108, such that PCM 102 isin contact With the ceramic layer 182. In such an arrangement the ceramic layer 182 formsan intemal surface of the Wall 108 and the metallic layer 181 forms an extemal surface ofthe Wall 108.

The metallic layer 181 may comprise, such as be composed of, stainless steel,such as an austenitic chromium nickel stainless steel alloy comprising nitrogen and rareearth metals The metallic layer 181 may be designed to be used at temperatures greaterthan about 550 °C. The metallic layer 181 may comprise for example stainless steel oftype EN 1.4835. The metallic layer 181 may have a thickness of from about 0.5 mm toabout 10 mm, such as from about 1 mm to about 5 mm, such as about 3mm. The metalliclayer 181 is substantially non-Wetting, that is, the PCM 102 is not in contact With themetallic layer 1 8 1 .

The ceramic layer 182 may comprise, such as be composed of, boron-nitride,aluminum oxide (AlgOg), and/or another ceramic material having a suitable therrnalconductivity. The ceramic layer 182 may have a thickness of from about 0.01 mm toabout 1 mm, such as from about 0.2 mm to about 0.4 mm. A thicker ceramic layer 182 isnot advantageous as it may split or crack. The ceramic layer 182 may have a therrnalconductivity greater than the therrnal conductivity of the metallic layer 181. The ceramiclayer 182 may have a heat transfer rate of 100 - 200 times that of the metallic layer 181.The ceramic layer 182 is non-insulating. The ceramic layer 182 is in contact With thePCM 102. A therrnally conductive ceramic layer 182 increases the therrnal conductivityof the Wall of the PCM storage vessel 100. The ceramic layer 182 furtherrnore enablesthe use of a PCM Which may otherwise react With a metallic Walled vessel. The ceramiclayer 182 may comprise a plurality of sub-layers, Where each sub-layer comprises, or is composed of a ceramic material.

The above described PCM storage vessel 100 Wall 108 design is furthermorelightWeight Which reduces installation and maintenance complexity and cost.Furthermore, a therrnally conductive PCM storage vessel 100 Wall 108 enables theefficient extraction of therrnal energy compared to systems Where therrnal energy fromthe PCM 102 is extracted via separate heat exchangers and requires ﬂuid transfer Withassociated losses.

The PCM 102 may be a known phase change material. The PCM may be amolten salt, a metallic alloy, or the like. Preferably, the PCM 102 for use in the presentPCM storage vessel 100 is a composition comprising aluminum and silicon, such aseutectic Aluminum Silicon Alloy, AlSi12. The PCM 102 may be an aluminum-siliconcomposition comprising silicon at a ratio of from about 10% to about 13% by Weight,such as about 12.6%. The temperature at Which the PCM 102 melts may be from about570 °C to about 590 °C, such as about 580 °C. As is the case With a PCM the PCMundergoes phase changes from solid to liquid, and liquid to solid, during therrnal energystorage and therrnal energy extraction. The PCM 102 may be present in both solid andliquid phases throughout the PCM storage vessel 100. The PCM 102 may be initiallyprovided to the PCM storage vessel 100 in a solid phase. The PCM may undergo a solid-liquid phase change at temperature of greater than 100 °C, such as greater than 200 °C.Due to the high temperatures at Which the PCM storage vessel operates, and the therrnalenergy storage requirements Water is not a suitable PCM.

Molten and/or solid PCM 102 may be free to circulate passively throughout thePCM storage vessel 100. The PCM 102 may be un-encapsulated, that is, it may be freefrom any form of encapsulation separating portions of PCM 102 from each other.

The PCM storage vessel 100 may comprise a volume of PCM 102 greater thanabout 5 L, such as greater than about 50 L of PCM. The PCM storage vessel may comprisefrom about 500 L to about 2500 L of PCM, such as from about 1000 L to about 2000 L,or more speciﬁcally, from about 1600 L to about 1700 L, or about 1630 L of PCM 102.

A receptacle 200 for heat transfer ﬂuid (HTF) 202 may be provided adjacent toand in therrnal contact With at least a portion of the PCM storage vessel 100. Thereceptacle for HTF 200 abuts at least a portion of the PCM storage vessel 100. An 11 assembly comprising the PCM storage vessel 100 and the receptacle for HTF 200 is thusprovided.

The receptacle for HTF 200 may surround at least a portion of the PCM storagevessel 100. The receptacle 200 may forrn a sleeve around a portion of the PCM storagevessel 100. The HTF receptacle 200 may surround a portion of the tapered portion 101and/or the upper portion 110 of the PCM storage vessel 100. The HTF receptacle 200may completely surround the PCM vessel 100. The HTF receptacle 200 may comprise anopening 201 such that a portion of the PCM vessel 100 is not surrounded by the HTFreceptacle 200. This may ease installation of the HTF receptacle 200 at the PCM storagevessel 100.

Thermal energy stored in the PCM 102 may be transferred to the HTF 202 viathe Wall 108 of the PCM storage vessel 100. In Figs. 1 and 4 the level of the HTF 202 isshown via dotted lines. In Fig. 3 the level of the HTF is shown via the Wavy line.

The HTF receptacle 200 may be a defined as a annular receptacle having acentral aperture 203 for receiving the PCM storage vessel 100, an inner Wall 204arrangeable adjacent the Wall of the PCM storage vessel 100, an outer Wall 205, and abase 206 connecting the inner Wall 204 to the outer Wall 205, all of Which define thereceptacle for HTF 200. The HTF receptacle may comprise an upper cover 207 Whichsubstantially seals the HTF receptacle 200.

The inner Wall of the HTF 204 receptacle may be therrnally conductive such thattherrnal energy is transferred from the PCM 102 to the HTF 202, via the Wall 108 of thePCM storage vessel 100 and the inner Wall 204 of the HTF receptacle 200. The outer Wall205 of the HTF receptacle 200 may be therrnally insulating.

The HTF receptacle 200 may be in the form of an annular cylinder, that is acylinder having a central aperture 203, if the HTF receptacle 200 is arranged adjacent acylindrical portion of the PCM storage vessel 100. The HTF receptacle 200 may be in theform of an annular tapered portion, that is a tapered portion having a central aperture 203,if the HTF receptacle 200 is arranged adjacent the tapered portion 101. For example, theHTF receptacle 200 may have the form of an annular frusto-cone having a central aperture 203 if the tapered portion 101 of the PCM storage vessel 100 is a frusto-cone. 12 The HTF receptacle 200 may comprise at least one, such as a plurality ofpartitions 208 forrning a single HTF receptacle 200. Each partition 208 may be a sectionof the entire of shape of the HTF receptacle 200. For example, each partition 208 may bea section of an annular cylinder having a central aperture 203. If, for example, the HTFreceptacle 200 is formed to engage With the tapered portion 101 of the PCM vessel 100then the each partition 208 may have the form of a portion of an tapered portion having acentral aperture 203.

The HTF receptacle 200 may comprise at least one opening for the provisionand/or extraction of HTF 202. The opening may also be used for the emptying of HTF,during maintenance. The HTF receptacle 200 may comprise an aperture for the provisionof a pump. The HTF receptacle 200 may comprise a plurality of openings such as anaperture for a pump, and an outlet for pumped HTF 202.

The HTF receptacle 200 may be manufactured from a metal, such as stainlesssteel, such as an austenitic chromium nickel stainless steel alloy comprising nitro gen andrare earth metals. The metal may be designed to be used at temperatures greater thanabout 550 °C, the metal may for example be of type EN 1.4835. The inner 204 and/orouter 205 Walls of the HTF receptacle may comprise, such as be composed of stainlesssteel.

The HTF 202 may be a molten salt solution. Preferably the HTF 202 is moltenmetal such as molten Sodium. Due to the high temperatures at Which the HTF receptacleoperates, and the therrnal energy storage requirements Water is not a suitable HTF.

The HTF receptacle 200 may be provided With a ﬂuid such as an inert gas, suchas a nitrogen (Ng). A portion of the HTF receptacle 200 may be filled With the HTF 202,the remaining portion of the HTF receptacle 200, not filled With HTF 202, may be filledWith the inert gas.

The HTF receptacle may be substantially gas tight at its upper portion 209 suchthat any gas leakage from the HTF receptacle 200 is minimized.

As shown in Fig. 5 the PCM storage vessel 200 may be at least partially enclosedin an outer Wall 300 Which is therrnally insulating. The HTF receptacle 200 adjacent thePCM storage vessel 100 may be at least partially enclosed in the outer Wall 300. The outerWall 300 may surround both the PCM storage vessel 100 and the HTF receptacle 200. 13 The outer wall 300 may fully enclose the PCM storage vessel 100 and HTF receptacle200. The outer wall 300 is non-wetting, that is, it is not in contact with the PCM 102and/or the HTF 202. The outer wall 300 may therrnally insulate the PCM storage vessel100 and the HTF receptacle 200 from the ambient environment.

An opening 301 may be present in the outer wall 300 in the Vicinity of the regionfor receiVing solar therrnal energy 105, such as adjacent the region for receiVing solartherrnal energy 105. The opening 301 may coVered with a hatch 302 being openable andcloseable. The hatch 302 may be therrnally insulating. When closed, the hatch 302 maylimit therrnal energy losses from the PCM storage vessel 100 and/or HTF receptacle 200,and therein, from the PCM 102 and HTF 202, Via the opening 301. When open, the hatch302 may allow the transmission of solar therrnal energy 106. When open, the hatch 302may allow the transmission of sunlight 106 to the region for receiVing solar therrnalenergy 105. When closed, the hatch may inhibit, such as stop, the transmission of sunlight106 to the region for receiVing solar therrnal energy 105. The hatch 302 may be providednear the tip 103 of the tapered portion 101.

The hatch 302 may be actuated automatically Via a control means. The controlmeans may comprise an actuator and a sensor. The actuator opens and closes the hatch302 depending on output from the sensor. The sensor may be a temperature and/or lightsensor or another type of sensor suitable for deterrnining a condition at which the hatch302 should be opened or closed. The hatch may be openable dependent on the temperatureat the hatch 302. The temperature at the hatch 302 may be dependent on the presence orintensity of light 106 directed towards the hatch 302. As the hatch 302 is positioned inthe Vicinity of the region for receiVing solar therrnal energy 105 the hatch 302 may beopenable depending on the temperature of the hatch 302 due to light directed towards theregion for receiVing solar therrnal energy 105. The sensor may be a light sensor adaptedto detect the presence and/or intensity of light. The hatch 302 may be configured to openand/or close dependent on the output from a sensor for detecting the presence and/orintensity of light being directed toward the region for receiVing solar therrnal energy 105.The control means may comprise a clock or timer. The hatch 302 may be controlleddependent on the time-of-day. For example, the hatch 302 may be opened during hours of daylight and closed during hours of no or low sunlight. The control means may 14 comprise both a sensor and a clock or timer, or the control means may comprise one of asensor or clock/timer.

The hatch 302 may be opened or closed only partially. That is, the hatch may beadapted such that it is partially open such that a portion of the total sunlight available istransmitted to the region for receiving solar therrnal energy 105, and that the opening issmall enough to limit therrnal losses. The degree to which the hatch is opened may dependon the intensity of light directed towards the region for receiving solar therrnal energy105.

The PCM storage vessel 100 may be used for therrnal energy storage system ina concentrated solar polar system. Thermal energy may be provided to the PCM 102which is stored and subsequently extracted. The therrnal energy is generally extracted viathe HTF 202. The concentrated solar power system comprises an electrical energygeneration system. The electrical energy generation system converts therrnal energyprovided by the HTF 202 to electrical energy. The generated electrical energy generatedby the electrical energy generation system may be fed in to an electricity grid or electricaldistribution network. The electrical energy generation system may comprise a conversionunit operating on the Stirling cycle, Rankine cycle, Brayton cycle, or any other heatengine capable of eff1ciently generating electrical energy from therrnal energy toelectrical energy.

The conversion unit is in therrnal connection with the HTF. A heat exchangermay be used to transfer therrnal energy from the HTF to the conversion unit. The heatexchanger may transfer therrnal energy from the HTF to the working ﬂuid of theconversion unit.

As described above, therrnal energy may be provided to the HTF 202 via thePCM 102, and specifically via the PCM 102 through the PCM storage vessel 100.

A concentrated solar power system may comprise an array of solar receiverswhich receive, reﬂect and concentrate sunlight at the solar energy receiver of the PCMstorage vessel 100. The solar receivers may be heliostats arranged at a height above orbelow the PCM storage vessel 100. As stated previously, the PCM storage vessel 100,and therein, the region for receiving solar therrnal energy 105, may be located above ground. For example, the PCM storage vessel 100 may be located on a tower. If the solar receivers are placed below the height of the PCM storage vessel 100 then the sunlightwill be directed upwards towards the region for receiving solar therrnal energy 105.

A description of the process for therrnal energy storage and retrieval will now bedescribed with respect to the PCM storage vessel 100, an assembly comprising the PCMstorage vessel and HTF receptacle 200, and solar receiver(s).

Sunlight incident on at least one, such as a plurality, of solar receiver may bereﬂected, directed toward, and focused at the region for receiving solar therrnal energy105. The reﬂected, directed and focused sunlight 106, that is the solar therrnal energy106, warrns the region for receiving solar therrnal energy 105. A therrnal energy transferoccurs at the region for receiving solar therrnal energy 105 such that the PCM 102 withinthe PCM storage vessel 100 is warrned. The temperature of the PCM 102 in the vicinityof the region for receiving solar therrnal energy 105 may be greater than 500 °C, such asgreater than 580 °C, such as about 590 °C. The maximum temperature at the surface 109of the region for receiving solar therrnal energy 105 may be greater than 650 °C, or about700 °C. The PCM 102 in the vicinity of the region for receiving solar therrnal energy 105is warrned by the therrnal energy applied at the surface 109.

The warrned PCM 102 at the region for receiving solar therrnal energy may beless dense than the relatively cooler PCM 102 present in the PCM storage vessel 100. Itmay have undergone a phase change to liquid. The warrned PCM 102 may rise within thePCM storage vessel 100, the cooler, denser, possibly solidif1ed PCM 102 may ﬂowtoward the region for receiving solar therrnal energy 105 in the tapered portion 101. Thisprocess of free het convection, or natural heat convection, continues whilst the region forthe receiving solar therrnal energy 105 is receiving sunlight 106.

The therrnal energy present in the PCM 102 may be extracted via a HTF 202 intherrnal communication with the PCM storage vessel 100. HTF 202 in the HTF receptacle200 is warrned through the wall of the PCM storage vessel 100, and through the wall ofthe HTF receptacle 200. The warrned HTF 202 may be pumped to a conversion unit forconverting therrnal energy in to electrical energy. The warrned HTF 202 may then warrnthe working ﬂuid of the conversion unit. For example, the warrned HTF may be pumpedto a Stirling engine. The Stirling engine may thereby convert the therrnal energy extracted from the PCM to generate electricity. 16 Although, the present invention has been described above With reference tospecific embodiments, it is not intended to be limited to the specific forrn set forth herein.Rather, the invention is limited only by the accompanying claims.

In the claims, the terrn “comprises/comprising” does not exclude the presence ofother elements or steps. Additionally, although individual features may be included indifferent claims, these may possibly advantageously be combined, and the inclusion indifferent claims does not imply that a combination of features is not feasible and/oradvantageous. In addition, singular references do not exclude a plurality. The terms “a”,“an”, “f1rst”, “second” etc do not preclude a plurality. Reference signs in the claims areprovided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any Way.

Claims

1. A sealed phase change material (PCM) storage vessel (100) for a solartherrnal energy storage system, the storage vessel (100) comprising aninverted tapered portion (101) forrning a receptacle for a PCM (102), thetapered portion (101) having a tip portion (103) and a base portion (104), thetip portion (103) having a diameter less than the diameter of the base portion(104), the storage vessel (100) further comprising a region for receiving solartherrnal energy (105) proximal the tip portion (103) of the inverted taperedportion (101).

2. The sealed PCM storage vessel (100) according to claim 1, Wherein thevessel (100) is sealed such that there is no inﬂow nor outﬂow of a PCM (102)during the provision and/or extraction of therrnal energy to/from the PCM (102).

3. The sealed PCM storage vessel (100) according to claim 2, Wherein the intemal volume of the storage vessel is a single partition.

4. The sealed PCM storage vessel (100) according to any of claims 1 to 3,Wherein the PCM vessel (100) comprises a PCM (102) comprising Aluminium and Silicon.

5. The sealed PCM storage vessel (100) according to any of claims 1 to 4,Wherein the PCM storage vessel (100) is opaque.

6. The sealed PCM storage vessel (100) according to any of claims 1 to 5,Wherein the tapered portion (101) of the PCM storage vessel (100) is a frusto-conical tapered portion (101).

7. The sealed PCM storage vessel (100) according to any of claims 1 to 5,Wherein the tapered portion (101) of the PCM storage vessel (100) is a convex tapered portion (101). 18

8. A method for storing therrna1 energy in a PCM storage vesse1 (100)according to any of c1ain1s 1 to 7, the PCM storage vesse1 (100) coniprising a PCM, the method coniprising: -directing sunlight towards the region for receiving solar therrna1 energy(105), -Warrning so1idif1ed PCM (102) at the region for receiVing so1ar therrna1energy such that it n1e1ts, and rises due to buoyancy Within the tapered region (101).