SE542498C2

SE542498C2 - Concentrated solar power system and thermal energy storage system therefor

Info

Publication number: SE542498C2
Application number: SE1750136A
Authority: SE
Inventors: Erik Rilby; Martin Olausson; Pontus Wettrell; Torbjörn Lindquist
Original assignee: Azelio Ab
Priority date: 2017-02-14
Filing date: 2017-02-14
Publication date: 2020-05-26
Also published as: SE1750136A1

Abstract

A concentrated solar power system comprising a thermal energy storage system (2) comprising a first thermal energy storage tank (200) connected to a second thermal energy storage tank (201) via a bi-directional flow member (202), the first and second thermal energy storage tanks (200, 201) being thermally connected via a first and second thermal flow path (401, 402) to a solar receiver (400), wherein the first and second thermal flow paths (401, 402) form the bi-directional flow member (202).

Description

CONCENTRATED SOLAR POWER SYSTEM AND THERMAL ENERGY STORAGE SYSTEM THEREFOR m The present invention pertains to the field of concentrated solar power systemsand more particularly to thermal energy storage systems for concentrated solar powersystems comprising a first and second thermal energy storage tank connected by a bi- directional ﬂow member.

BACKGROUND 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 toelectrical energy for subsequent distribution. There are several classes of systems thatcomprise different solar receivers and associated electrical energy generation systems.For example, a parabolic trough system comprises a large surface area of parabolictrough shaped solar collectors directing solar radiation to a heat transfer ﬂuid (HTF)arranged along a focal axis of the parabolic trough. The HTF is then pumped and used toconduct therrnal energy to the conversion system for electrical energy generation.Another system is a power tower whereby solar radiation is directed by fields of mirrorswhich track the sun toward a static central tower. The central tower comprises a HTF which can be used to conduct therrnal energy to a conversion system. A Linear Fresnel l reﬂector uses a system of ﬂat linear mirrors approximating the parabolic shape of aparabolic trough and directing the solar radiation to a solar receiver comprising a HTFWhich can be used for subsequent electrical energy generation as described.

An especially efficient solar receiver design is the parabolic dish system. Theparabolic dish system comprises a parabolic dish comprising mirror(s) on an innersurface Which tracks the sun in 2-axis during the day. The solar radiation is concentratedat a focal point by the mirror(s) Where it is traditionally converted directly to electricalenergy via a conversion system such as a Stirling or Rankine cycle engine. As parabolicdish systems can be modular, installed on uneven ground, take less space than the othernamed system and are especially efficient they are particular ideal candidates for cost-effective CSP systems.

However, as opposed to the other static receiver systems generally parabolic dishsystems, Which have a moving solar receiver, do not use a HTF and have no capacity fortherrnal energy storage as the energy conversion system is located directly at the focalpoint of the parabolic dish. Without therrnal energy storage, the parabolic dish systemsonly generate electrical energy during hours of sunlight.

Thermal energy storage systems for storing therrnal energy generated duringperiods of sunlight are known in concentrated solar poWer systems having static solarreceivers but there is a lack of therrnal energy storage systems suitable for use Withparabolic dish systems.

The known therrnal energy storage systems generally operate With a hot storagetank, operating at up to around 390 °C - 560 °C, storing thermal energy from the solar receiver (either storing the HTF, or storing energy from the HTF in a different medium). 2 In such a system, the energy in the hot storage tank is thereafter accessed and distributedto the electrical energy generator Via the HTF. After electrical energy generation, theHTF has a reduced temperature and it, or energy therefrom, is thereafter stored in a coldstorage tank operating at around 290 °C or less. The cold storage tank acts as a receptaclefor the used/cooler ﬂuid. After storage in the cold tank, and When the receiver is againreceiVing sunlight, the HTF is pumped back to the receiVer to be Warmed again. The ﬂuidmust be maintained at a high temperature throughout this entire process and thetemperature of the cold tank is alWays at or below the temperature of the used HTF, andthe used HTF cannot be used for the generation of electrical energy until the receiVer isreceiVing sunlight again.

For efficiency reasons, it is desirable to heat the HTF to as high a temperature aspossible. HoWeVer, this creates complexities in transferring the HTF from the receiVer,Where it is heated, to the conVersion unit generating electrical energy from the therrnalenergy. For example, a ﬂuid haVing a Very high heat capacity may haVe a relatiVely highmelting point and must be maintained at a high temperature throughout the ﬂuidic path.Furthermore, a Very hot ﬂuid is difficult to pump and requires expensive and complexspecially designed pumps. Typically, efforts to oVercome these problems haVe targetedthe proVision of an ideal ﬂuid haVing a loW melting point and high specific heat.

In light of these draWbacks a system Which can store energy for substantialperiods of time and pump the heat transfer fluid more reliably and cost effectiVely is desired.

SUMMARY 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 concentrated solar power system comprising a thermal energy storagesystem, a solar receiVer, and an electrical energy generation system, wherein the thermalenergy storage system comprising a first thermal energy storage tank connected to asecond thermal energy storage tank Via a bi-directional ﬂow member, characterized inthat the first and second thermal energy storage tanks are proVided with at least one phasechange material and are thermally connected Via a first and second thermal ﬂow path to atleast one of the electrical energy generation system and the solar receiVer, wherein thefirst and second thermal ﬂow paths form the bi-directional ﬂow member such that a heattransfer ﬂuid can be oscillated from the first thermal energy storage tank to the secondenergy storage tank Via the bi-directional ﬂow member.

A thermal energy storage system is also proVided.

Methods for charging and discharging a thermal energy storage system are alsoproVided.

Further adVantageous embodiments are disclosed below and in the appended 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 Figure 1 is a schematic View of system according to the inVention.

Figure 2 is a perspectiVe View of an arrangement of a system according to theinVention.

Figure 3 is a schematic View of an arrangement of a system according to theinVention.

Figure 4 is a schematic View of an arrangement of a system according to theinVention.

Figure 5 is a perspectiVe cut-away View of the therrnal energy storage systemaccording to the inVention.

Figure 6 is a schematic View on an arrangement of a system according to theinVention showing a separate pre-heating/drainage tank.

Figure 7 is a ﬂowchart for initialisation.

Figure 8a-c shows the results of a simulation of performance in discharging the system.

DETAILED DESCRIPTION For a further understanding of the present inVention, reference should be made tothe following detailed description taken in conjunction with the accompanying drawings, wherein: Fig. 1 shows a therrnal energy storage system 2 comprising a first therrnal energystorage (TES) tank 200 and a second thermal energy storage (TES) tank 201 for receiVinga Heat Transfer Fluid. The first and second TES tanks 200, 201 are ﬂuidically connectedVia at least one bi-directional ﬂoW member 202 such that a Heat Transfer Fluid (HTF)can be reVersib1y displaced from the first TES tank 200 to the second TES tank 201. ByreVersibly displaced it is meant that the HTF can be ﬂoWed in an oscillating manner fromthe first TES tank 200 to the second TES tank 201 and then back again Via the bi-directional ﬂoW member 202.

The bi-directional ﬂoW member 202 has a first end proVided at a loWer region ofthe first TES tank 200. The bi-directional ﬂoW member 202 has a second end provided ata loWer region of the second TES tank 201. The first and second ends of the bi-directionalﬂoW member 202 must be positioned such that, during pumping of the HTF, the first andsecond ends are always immersed in HTF. OtherWise gas bubbles may form in thesystem. During system initialisation the first and second ends do not need to be immersedin HTF.

The HTF is displaced by the provision of a pressure differential in the first TEStank 200 With respect to the second TES tank 201. The pressure differential generates aforce acting on the HTF in the first TES tank 200 and therein actuates the displacement ofthe HTF to the second tank 201. The pressure present at the first end of the bi-directionalﬂoW member 202 is greater than the pressure at the second end of the bi-directional ﬂoWmember 202. This pressure difference means that the HTF ﬂows through the bi- directional ﬂoW member 202.

The inventors have identified that an ideal Way of creating the pressuredifferential between the first TES tank 200 and the second TES tank 201 is to associatethe TES system 2 With at least one pressure vessel system 3.

In Figure 1 A pressure vessel system 3 comprising a first and second pressurevessel 300, 301 is associated With the TES system 2. The first pressure vessel 300 isconnected to the first TES tank 200. In Figure 1 the first pressure vessel 300 is connectedto the first TES tank 200 by a ﬂuidic conduit having a first end at an upper region of thefirst pressure vessel 300 and a second end at an upper region of the first TES tank 200.The second pressure vessel 301 is connected to the second TES tank 201. Each of the firstand second pressure vessels 300, 301 is partially fi11ed With an actuating liquid. In Figure1 the second pressure vessel 301 is connected to the second TES tank 201 by a ﬂuidicconduit having a first end at an upper region of the second pressure vessel 301 and asecond end at an upper region of the second TES tank 200. An actuating liquid ﬂoW path303 is provided between the first and second pressure vessels 300, 301. The actuatingliquid ﬂoW path 303 may be connected at a first end 304 to a lower region 310 of the firstpressure vessel 300. The actuating liquid ﬂoW path 303 may also be connected at asecond end 305 to a lower region 311 ofthe second pressure vessel 301. A pump 302 isprovided in communication With the actuating liquid ﬂoW path 303. The pump is forpumping actuating liquid, it does not need to be capable of pumping HTF.

The total volume of actuating liquid in both the first and second pressure vessels300, 301 is constant. That is, the volume of actuating liquid in the pressure vessel system3 is constant. The remaining volume of the pressure vessel system 3, that is, the volume not filled With actuating liquid may comprise, such as be filled With a ﬂuid, for example 7 an inert gas such as Ng. The remaining Volume may comprise, such as be filled With C02,especially if the HTF or PCM (see below for further information regarding PCM)comprise carbonate salts. Other inert gases such as Argon may also be suitable but cost ofsuch a gas may be prohibitiVe.

During operation, the pressure Vessel system 3 and the TES system 2 are closedand the total Volume of the pressure Vessel system 3 and the connected TES system 2 isconstant. More specifically, the Volume of the first and second pressure Vessels 300, 301,and the first and second TES tanks 200, 201 is fixed. This enables in-direct pumping ofHTF from the first TES tank 200 to the second TES tank 201 and back again Without therequirement of a pump acting directly on the HTF. The pump 302 acts only on theactuating liquid, and the pressure differential created in the system causes thedisplacement of HTF. This Will now be further described.

The actuating liquid can be pumped from the first pressure Vessel 300 to thesecond pressure Vessel 301 Via the pump 302. The displacement of the actuating liquidcauses a pressure differential in the first TES tank 200 With respect to the second TEStank 201. This pressure differential creates a force on any HTF present in the first orsecond TES tanks 200, 201 and displaces the HTF Via the bi-directional ﬂow member202.

The first TES tank 200 may be provided at a height aboVe the second TES tank201. The first TES tank 200 and the second 201 need not be therrnally isolated from oneanother. It may be adVantageous to haVe them adj acent and in therrnal contact such thatany heat energy provided to the first or second TES tank 300, 301 is conducted to the second or first TES tank 301, 300.

A therrnal energy storage system 2 comprising a single pressure vessel is alsoenvisaged. In such a system the HTF is displaced via the actuation of a mechanical pistonacting in the single pressure vessel Which displaces an inert gas. The displacement of theinert gas causes an increase in pressure at the first TES tank 200 and therein forces thedisplacement of HTF from the first TES tank 200 to the second TES 201. The mechanicalpiston may then be retracted, causing a reduction in pressure in the first TES tank 200 andtherein the HTF is ﬂoWed in a reverse direction.

Another, less preferred, method of displacing the HTF is to displace the first andsecond TES tanks (200, 201) relative to one another. For example, by raising the firstTES tank 200 above the height of the second TES tank 201 the force of gravity acting onthe ﬂuid in the first TES tank 200 causes it to ﬂoW through the bi-directional ﬂoWmember 202 in to the second TES tank 201. The second TES tank 201 can thereafter beraised above the first TES tank 200 and the ﬂoW is reversed.

In all of the described methods of pumping the HTF, the pressure of HTF at thefirst end of the bi-directional ﬂoW member 202 is increased relative to the pressure at thesecond end of the bi-directional ﬂoW member 202, and therein HTF from the first therrnalenergy storage tank 200 is displaced to the second therrnal energy storage tank 201.

A valve 305 may be provided between the first and second pressure vessels 300,301 for equalizing the pressure in the first and second pressure vessels 300, 301. Thepressure differential between the first and second pressure vessels 300, 301 can beequalized on opening the valve 305. The valve 305 is generally closed during system operation such that the pressure vessel system 3 is closed and therein that the pumping of the ﬂuids between the first and second pressure vessel 300, 301 causes a pressuredifferential between the first and second TES tanks 200, 201.

The thermal energy storage system 2 may be connected to a solar receiver 400.The solar receiver 400 receives solar radiation and is generally heated, or a portionthereof is heated. ln a parabolic dish CSP system, solar radiation may be reﬂected andfocused on the solar receiver 400 by a parabolic dish 600 as shown in Figure 2. Theparabolic dish 600 comprises a plurality of mirrors 601 reﬂecting and focusing the solarradiation at a focal point 602. In all types of CSP systems, the solar receiver 400 islocated at a focal point where sunlight is reﬂected and focused. Referring again,specif1cally to parabolic dish systems, as the solar parabolic dish 600 moves in 2-axis andtracks the sunlight during the day, the focal point 602 and therein the solar receiver 400also moves during the day. That is, the solar receiver 400 in a parabolic dish 600 CSPsystem is not static. lt is also generally located at a height above ground level. Othersystems for reﬂecting and/or focusing solar radiation on the solar receiver 400 may bepossible to combine with the therrnal energy storage system 2 described herein. Forexample, static solar receiver systems such as parabolic trough, linear Fresnel, and powertower systems could also be used. In such systems, the solar receiver does not move, it isstatic.

The solar receiver 400 is therrnally connected to the therrnal energy storagesystem 2. As is shown in Figure 1, the solar receiver may be connected to the first TEStank 200 and connected to the second TES tank 201. Heat generated at the solar receiver400 is conducted to the first and second TES tanks 200, 201. The solar receiver 400 is connected to the first TES tank 200 via a first therrnal ﬂow path 401. The first thermal ﬂoW path 401 may be a conduit for HTF. The solar receiver 400 is connected to thesecond TES Via a second therrnal ﬂoW path 402. The second therrnal ﬂoW path 402 maybe a conduit for HTF. The first and second therrnal ﬂoW paths 401, 402 may beconnected such that a combined therrnal ﬂoW path is formed from the first TES tank 200to the second TES tank 201. In systems comprising a combined therrnal ﬂoW path andWherein the therrnal ﬂoW paths form conduits for the HTF: HTF can ﬂoW from the firstTES tank 200, through the first therrnal ﬂoW path 401 to the solar receiVer 400, Where itmay be Warrned by solar radiation directed at the solar receiVer 400, through the secondtherrnal ﬂoW path 402 to the second TES tank 201. The HTF is then stored in the secondTES tank 201.

In a system Where the first and second thermal ﬂoW paths 401, 402 form acombined therrnal ﬂoW path and Wherein the thermal ﬂoW paths form conduits for theHTF the combined therrnal ﬂoW path is the bi-directional ﬂoW member 202 describedaboVe. For clarification”s sake, the solar receiVer 400 may be proVided along the bi-directional ﬂoW member 202. HTF may ﬂoW through the solar receiVer 400.

As described previously, the displacement of the HTF from the first/second TEStank to the second/first TES tank 200, 201 can be actuated Via indirect pumping Wherebya Volume of actuation liquid is displaced from the first or second pressure Vessel 300, 301to the second or first pressure Vessel 301, 300 thereby creating a pressure differencebetween the first and second TES tanks 200, 201 and that pressure difference forces theHTF through the first and second therrnal ﬂoW paths 401, 402. The first and second ﬂoWpaths 401, 402 may be ﬂexible pipes forrning conduits for the HTF. The proVision of ﬂexible pipes as the first and second ﬂoW paths 401, 402 reduces the complexity in11 systems comprising a parabolic solar dish as the solar receiver inherently 400 moves withthe dish and therefore rigid pipes require more complex joints and connections.

As is known in concentrated solar power systems, an electrical energy generationsystem 5 is provided to generate electrical energy from the thermal energy generated bythe solar receiver 400, and/ or stored in the therrnal energy storage system 2. Theelectrical energy generation system 5 is an energy conversion system, converting therrnalenergy to electrical energy. The electricity generated by the electrical energy generationsystem 5 can be fed in to an electricity grid, or electricity distribution network. Theelectrical energy generation system 5 may comprise a conversion unit 500 operating onthe Stirling cycle, Rankine cycle, Brayton cycle, or any other heat engine capable ofeff1ciently generating electrical energy from thermal energy to electrical energy.

In Figure l a Stirling engine is shown as the electrical energy generation system 5.The conversion unit 500 is in therrnal connection with the HTF. The conversion unit 500is in therrnal connection with the first and second therrnal ﬂow paths 40l , 402. Due to theuse of a HTF and as opposed to traditional concentrated solar power systems comprisingparabolic dishes, the conversion unit 500 need not be located at the focal point 602 of theparabolic dish 600. The conversion unit 500 may be provided adjacent to the TES system2. Providing the Stirling engine at a location not at the focal point 602 of the parabolicdish 600 greatly improves ease of installation and maintenance as the Stirling engine doesnot need to move with the parabolic dish 600. The solar receiver 400 but not theconversion unit 500 moves with the parabolic dish 600.

The electrical energy generation system 5 may comprise at least one heat exchanger 50l for transferring the therrnal energy from the HTF to the working ﬂuid of12 the conversion unit 500. In Figure 3 a pair of heat exchangers 501, 502 are shown forreceiving energy from the HTF. A first heat exchanger 501 is arranged on the firsttherrna1 ﬂoW path 401, a second heat exchanger 502 is arranged on the second therma1ﬂoW path 402. As can be seen in Figure 3 there is a sing1e therma1 path comprising thefirst TES tank 200, the first therrna1 ﬂoW path 401, the first heat exchanger 501, the so1arreceiver 400, the second heat exchanger 502, the second therrna1 ﬂoW path 402 and thesecond TES tank 201. Idea11y, the sing1e therrna1 ﬂoW path is a1so a ﬂoW path for HTF.That is, the ﬂoW path forrns a conduit for the ﬂoW of HTF.

The e1ectrica1 energy generation system 5 need not be in ﬂuidic connection to theTES system 2. A thermosiphon may be provided between the TES system 2 and thee1ectrica1 energy generation system 5. The thermosiphon is a 1ongitudina11y, andgenera11y vertica11y, extending receptac1e comprising a ﬂuid Whereby heat energyprovided at the 1oWer portion raises under convection and can be extracted at an upperportion. The therrnosiphon may therrna11y connect both the first and second TES tanks200, 201 to the e1ectrica1 energy generation system 5, such as to the heat eXchangers 501,502.

The first TES tank 200 and the second TES tank 201 are provided With at 1eastone phase change materia1 PCM for storing and/or re1easing energy from/to the HTF. Theat 1east one PCM may for example be a sa1t having a me1ting point substantia11y greaterthan the me1ting point of the HTF.

The at 1east one PCM materia1 may be encapsu1ated in members physica11y, butnot therrna11y, iso1ating the PCM from the HTF. The PCM is, as is known in the art, a materia1Which absorbs energy during heating as phase change from e.g. a so1id to a 13 liquid takes and releases energy to the adjacent environment during the reversed coolingprocess. During heating of the PCM in a solid phase the solid increases temperature(sensible energy storage). During phase change from solid to liquid energy is storedlatently. After phase change to a liquid energy is again stored sensibly and the PCM inliquid phase increases temperature.

The PCM in the TES tanks 200, 201 acts as a reliable and predictable thermalenergy storage medium. Therein a substantial amount of therrnal energy can be stored inthe TES system 2 beyond that Which could be stored in only the HTF. The PCM maycomprise any media capable of storing therrnal energy such as a salt, or mixture of salts, aparticle such as sand, concrete or combinations of the above. The PCM may have amelting point from 500 °C to 900 °C, such as from 650 °C to 810 °C. During operation,the temperature of the first and second TES tanks (200, 201) is from about 500 °C toabout 900 °C.

The HTF has a melting point of from 200 °C to 500 °C, such as from 300 °C to400 °C. The HTF may comprise a molten salt, molten glass and/or liquid metal(s). Forexample, suitable molten salts may be eutectic salts of carbonates, ﬂuorides, or chloridesand/ or nitrate salts. The HTF may comprise oxides. The HTF may also comprise mixturesof the above in various fractions. Whilst named a ﬂuid it is obvious that the HTF issimply a substance Which is a ﬂuid, i.e., is at a temperature greater than melting point, atits normal operating temperatures.

As described above, the PCM is comprised Within a plurality of members beingPCM receptacles 220. Each PCM receptacle 220 comprises a portion for storing PCM and a Wall Which physical separates the PCM in the portion for the storing the PCM from14 the HTF. The PCM receptacles 220 may be longitudinally extending members having asubstantially vertical alignment. The PCM receptacles 220 may be cylindrical members,having a circular cross-section. They may also be longitudinally extending members havesquare, hexagonal, or oval cross-sections. The PCM receptacles 220 may have an openingin an upper portion such that the PCM can expand freely on phase change without apressure build-up occurring within the receptacle. The PCM receptacles 220 are sealed atthe bottom portion such that HTF present in the first and second TES tanks 200, 20lcannot directly contact the PCM. The PCM receptacles 220 may be arranged in a grid asshown in Figures l, 3 and 5. The PCM receptacles 220 are arranged substantially parallelto each other.

Figure 5 shows a cut-away view of the thermal energy storage system 2 showingan arrangement of PCM receptacles 220 in both the first and second TES tanks 200, 20l.The plurality of PCM receptacles 220 are arranged parallel and adj acent to one another ina grid. The HTF can ﬂow around the PCM receptacles 220 and therein heat the PCM inthe PCM receptacles 220.

The first and second TES tanks 200, 201 may be provided with a second PCMhaving a melting point substantially higher than the melting point of the HTF butsubstantially lower than the melting point of the first PCM. This second PCM may beconsidered a low-temp PCM, with the first PCM being considered a high-temp PCM. Thelow-temp PCM may have a melting point from 0 to l00 °C higher than the melting pointof the HTF. The TES system 2 may comprise from 0 to 30% by mass of the low-tempPCM. One advantage of the low-temp PCM is that the HTF can be maintained in liquid state for longer compared to a TES system 2 not comprising a low-temp PCM. The low- temp PCM is not used for maintain the HTF at a temperature suitable for energygeneration in the energy conversion system 5 but rather maintains the HTF in a liquidstate.

As shown in Figure 1 the TES system 2 may comprise a pre-heater 700 forheating the HTF and TES system 2. The pre-heater 700 may be in therrnal connectionWith the first or second TES tank 200,201 and is arranged for heating the HTF on systeminitialization.

As is shoWn in Figure 6 it is also possible that the pre-heater 700 may be arrangedin communication With a separate HTF storage/drainage tank 800. The separate HTFstorage/drainage tank 800 is provided in ﬂuidic communication With the pressure vesselsystem 3, for the provision of pressure to displace the HTF, and the first or second TEStank 200, 201, for the transfer of HTF.

With reference to Figure 7 a method for initializing a TES system 2 Will noW bedescribed.

With the TES system 2 off and prior to starting the Valve 305 can be opened 1001and the volume of actuating liquid in each of the first and second pressure vessels 300,301 is equalized 1002 by the pump 302. As the valve 305 is open the HTF in the TESsystem 2 Will ﬂoW, due to the gravity, to the loWest point in the TES system 2. ln Figure1, the loWest point is the second TES tank 201. By altering the arrangement of the tanksthe first TES tank 200 could be positioned loWer than the second TES tank201 .lrrespective of Which tank is at a higher or loWer point, all HTF Will be present in a single tank. 16 The pre-heater may then begin to pre-heat 1003 the HTF in the second TES tankto or above its melting point. Super heating the HTF above its melting point prevents theHTF from freezing if there are cold spots in the system that haven”t been pre-heatedsufficiently. The super heating could be from 1 to 100 °C above the melting point of theHTF, such as from 10 to 50 °C. As stated previously, the first and second TES tanks 200,201 may be therrnally connected to each other, and in such a Way, heating the secondTES tank 201 may, via conduction, heat the first TES tank 200.

The solar receiver 400 and therrnal ﬂoW paths 401, 402 are also pre-heated 1004.The valve 305 may be closed 1005 such that the f1rst and second pressure vessels 300,301 are only connected via the actuating liquid ﬂoW path 303. The actuating liquid ispumped 1006 via the pump 302 to the pressure vessel in connection With the TES tankcomprising the HTF. According to the system in Figure 1, the actuating liquid is pumpedvia the pump 302 to the second pressure vessel 301, Which is connected to the secondTES tank 201. An inert gas, such as Ng, is then provided 1007 to the first TES tank 200.The inert gas acts to charge the system and to aid in pre-heating. The inert gas flowsthrough the bi-direction ﬂoW member 202 in to the HTF in the second TES tank 201. Theinert gas may be provided to the first TES tank 200 such that a predeterrnined pressure inthe pressure vessel system 3 is reached. The actuating liquid may then be pumped 1008,via the pump 302 from the second pressure vessel 301 to the f1rst pressure vessel 300such that the inert gas is displaced and ﬂoWs through the bi-directional ﬂoW member 202and eventually to the second TES tank 201.

After the actuation liquid has been pumped in to the first pressure vessel 300, the pressure vessel system 3 is reset. 17 The Valve 305 is then opened and the actuating liquid is pumped 1009, Via thepump 302, to the first pressure Vessel 300. The actuating liquid is pumped to a heightsuch that pressure provided by the actuating liquid when the pump 302 ceases pumping issufficient to oVercome the static pressure of the HTF at the highest point, aboVe thelowest system 1eVe1, in the bi-directional ﬂow member 202. As shown in Figure 1 thehighest point in the bi-directional ﬂow member is generally the solar receiVer 400. TheValve 305 can now be closed 1010 and the pressure Vessel system 3, and TES system 2now have a fixed total Volume.

On pumping the actuating liquid from the first pressure Vessel 300 to the secondpressure Vessel 301 the HTF ﬂows from the second TES tank 201 to the first TES tank200. The Volumes of actuating liquid in each of the first and second pressure Vessels 300,301 are equalized such that the Volume of HTF in each of the first and second TES istherein equalized. As the pressure Vessel system 2 and TES system 2 are closed the pump302, to displace the HTF the pump 302 no longer needs to oVercome the static pressure ofthe system. It has to only oVercome the intemal pressure losses in the HTF conduits.

The indirect pumping method can then be used to oscillate the HTF from the firstTES tank 201 to the second TES tank, Via the solar receiVer 400. Therein a 2 hot-tankTES system 2 with indirect pumping is achieVed. The system has a great therrnal storagecapacity as two tanks are utilized for therrnal energy storage. As the pump 302 does notneed to pump HTF, but only the actuating liquid, operational costs are significantlyreduced.

Charging the TES system 2 is when the therrnal energy stored in the TES system 2 is increased such that it can be accessed later for electrical energy generation. Charging 18 the TES system 2 occurs during hours of sun1ight When the solar receiver 400 is receivingsun1ight. The HTF is osci11ated from the first TES tank 200 to the second TES tank 201via the bi-directiona1 ﬂoW member 202. The so1ar receiver 400 is provided at a pointalong the bi-directiona1 ﬂoW member 202. The HTF is heated via the so1ar radiation atthe so1ar receiver 400. The HTF may pass through the so1ar receiver 400. Duringcharging, the temperature of the HTF on entering the so1ar receiver 400 is 1oWer than thetemperature of the HTF 1eaving the so1ar receiver. The therrna1 energy in the HTF istransferred to the PCM, or PCMs, in the first and second TES tanks 200, 201. Thetherma1 energy is therein stored in the first and second TES tanks 200, 201.

Discharging the TES system 2 is When energy stored in the TES system 2 isaccessed and therein that the therrna1 energy present in the system is decreased.Discharging the TES system norrna11y occurs When e1ectrica1 energy is to be generated bythe energy conversion system 5. One Way of discharging the system is to provide anenergy conversion system 5 in therrna1 connection With the bi-directiona1 ﬂoW member202. HTF is osci11ated from the first TES tank 200 to the second TES tank 201 via thebidirectiona1 ﬂoW member 202. The energy conversion system 5 genera11y comprises afirst heat exchanger 501, provided between on the first therrna1 ﬂoW path 401 of the bi-directiona1 ﬂoW member 202, arranged to receive therrna1 energy from the HTF. Thetherma1 energy is used to drive the energy conversion unit 500 and can thereafter beconverted to e1ectrica1 energy. A second heat exchanger 502 can be provided on thesecond therma1 ﬂoW path 402 and arranged to receive therma1 energy from the HTF. Thesecond heat exchanger 502 provides the capacity for improved energy conversion. As described previous1y the HTF is osci11ated from the first TES tank 200 to the second TES19 tank 201 and back again. During discharging the temperature of the PCM drops as energyis removed from the TES system 2. Once the temperature of the PCM is below its meltingpoint the TES system 2 is considered empty and discharging for electrical energygeneration ceases. If, as described previously, the TES system 2 comprises a second low-temp PCM, haVing a melting point substantially lower than the melting point of the firstPCM, then the system is considered empty when the first high-temp PCM has droppedbelow its melting point.

When the TES system 2 is neither charging nor discharging, that is, when thesystem is at idle, the HTF can be maintained in both the first and second TES tanks 200,201 such that HTF is maintained aboVe its melting point by the therrnal energy stored inthe PCM. If the TES system comprises a second low-temp PCM then the HTF ismaintained aboVe its melting point predominantly by energy stored in the low-tempPCM.

During charging, discharging and idle, the PCM in each of the first and secondTES tanks 200, 201 are maintained at a temperature higher than the melting point of theHTF.

Figure 4 shows a schematic of the system. The system comprises a TES system 2,a solar receiVer 400 and an electrical energy generation system 5. A first heat exchanger501 is proVided on the first therrnal ﬂow path 401. A second heat exchanger 502 isproVided on the second therrnal flow path 402. The therrnal ﬂow paths 401, 402 form thebi-directional ﬂow member 202. The different temperatures at different points of thesystem will now be described. As an example, after initialization and during charging the temperature of the TES system 2 may be about 714 °C. The temperature of HTF ﬂowing in the first therrnal energy ﬂow path 401 between the TES system 2 and the first heatexchanger 501 may therefore in the ideal case be about 714 °C. The temperature of theHTF in the first therrnal energy ﬂow path 401 after leaVing the first heat exchanger 501and before reaching the solar receiver 400 may be about 704 °C (10 °C drop during heatexchanging). The HTF is then heated in the solar receiVer 400 by about 40 °C. Thetemperature of the HTF in the second therrnal ﬂow path 402, between the solar receiVer400 and the second heat exchanger 502 may therefore be about 744 °C. the temperatureof HTF in the second thermal ﬂow path 402 after leaVing the second heat exchanger 502may be about 734 °C (10 °C drop during heat exchanging). As the temperature of theTES system 2 was about 714 °C, and the temperature of HTF entering the TES system 2is about 734 °C then the HTF may slightly increase the temperature of the TES system 2.As an example, after initialization and during discharging the temperature of theTES system 2 may be about 714 °C. The temperature of HTF ﬂowing in the first therrnalenergy ﬂow path 401 between the TES system 2 and the first heat exchanger 501 maytherefore be about 714 °C. The temperature of the HTF in the first therrnal energy ﬂowpath 401 after leaVing the first heat exchanger 501 and before reaching the solar receiVer400 may be about 704 °C (10 °C drop during heat exchanging). During discharging theHTF is not heated in the solar receiVer 400. The temperature of the HTF in the secondtherrnal ﬂow path 402, between the solar receiVer 400 and the second heat exchanger 502may therefore be about 704 °C. the temperature of HTF in the second therrnal ﬂow path402 after 1eaVing the second heat exchanger 502 may be about 694 °C (10 °C drop during heat exchanging). As the temperature of the TES system 2 was about 714 °C, and the 21 temperature of HTF entering the TES system 2 is about 694 °C then the HTF mayslightly decrease the temperature of the TES system 2.

In the above description the HTF ﬂowing from the TES system 2 ﬂoWs from thefirst TES tank 200 to the second TES tank 201. As has been described previously it mayalso ﬂoW in a reciprocal or oscillating manner, from the second TES tank 201 to the firstTES tank 200. This oscillating ﬂoW of HTF is used in both charging and discharging theTES system 2. The oscillating ﬂoW alloWs the system to charge and discharge the TESsystem Without needing to have a pump acting directly on the HTF as Would be the caseWith a traditional system. This reduces system costs substantially.

Also provided is a method of pumping a heat transfer ﬂuid in a thermal energystorage system 2, the therrnal energy storage system 2 comprising first therrnal energystorage tank 200 connected to a second thermal energy storage tank 201 via a bi-directional ﬂoW member 202 having a first end provided at the first therrnal energystorage tank 200, and a second end provided at the second therrnal energy storage tank201, the method comprising: increasing the pressure of the heat transfer ﬂuid present atthe first end of the bi-directional ﬂoW member 202 relative to the pressure at the secondend of the bi-directional ﬂoW member 202, and therein displacing heat transfer ﬂuid from the first therrnal energy storage tank 200 to the second therrnal energy storage tank 201.

SimulationFigure 8a-c shows results of simulation of the expected performance duringdischarging the TES system 2 described herein.

Parameters 22 In the simulation the PCM was 2200kgs of salt. The PCM receptacles 220 weremodelled as having walls comprising steel. The HTF was modelled as filling up the f1rstand second TES tanks 200, 201 to the total height of PCM receptacles 220. Mass ﬂow ofHTF was 2.5 kg/s. This equals a temperature delta of ll °C for a heat load of 40 kW. Thecycle time, that is the time taken to displace the HTF from the first TES tank to thesecond TES tank and then back again was 84 seconds.

Results Figure 8a and 8b show that the extracted power from the system over the first 8hours is equal. Figure 8c shows that the temperature from the system decreases over time.The current system design as described herein provides sufficiently high therrnal energystorage temperatures for about 6 hours.

While the invention has been described in conjunction with a number ofembodiments, it is evident that many altematives, modif1cations and variations would beor are apparent to those of ordinary skill in the applicable arts. Accordingly, Applicantintends to embrace all such altematives, modif1cations, equivalents and variations that are within the scope of the invention. 23

Claims

What is claimed is:

1. A concentrated solar power system comprising a therrnal energy storagesystem (2), a solar receiver (400), and an electrical energy generation system (5),wherein the therrnal energy storage system (2) comprising a first therrnal energystorage tank (200) connected to a second therrnal energy storage tank (201) via a bi-directional flow member (202), characterized in that the first and second therrnalenergy storage tanks (200, 201) are provided with at least one phase change materialand are therrnally connected via a first and second therrnal ﬂow path (401 , 402) to atleast one of the electrical energy generation system (5) and the solar receiver (400),wherein the first and second therrnal ﬂow paths (401, 402) form the bi-directionalﬂow member (202) such that a heat transfer ﬂuid can be oscillated from the firstthermal energy storage tank (200) to the second energy storage tank (201) via the bi- directional ﬂow member (202).

2. The concentrated solar power system according to claim 1, wherein the bi- directional flow member (202) is a conduit for heat transfer fluid.

3. The concentrated solar power system according to c1aims 1 or 2, whereinthe therrna1 energy storage system is connected to a pressure Vesse1 system (3) forindirect1y disp1acing the heat transfer ﬂuid, comprising a first and second pressureVesse1 (300, 301) each first and second pressure Vesse1 (300, 301) being partia11y f111ab1e with an actuating liquid.

4. The concentrated so1ar power system according to any of c1aims 1 to 3,further comprising a parabolic dish (600) for reﬂecting and directing sun1ight at afoca1 point (602), and wherein the so1ar receiVer (400) is proVided at the focus point (602).

5. The concentrated so1ar power system according to any of c1aims 1 to 4,wherein the e1ectrica1 energy generation system (5) is stationary with respect to the therrna1 energy storage system (2).

6. The concentrated so1ar power system according to any of c1aims 1 to 5,wherein the so1ar receiVer (400) is moVeab1e with respect to the therrna1 energy storage system (2).

7. The concentrated so1ar power system according to any of c1aims 1-6, wherein a first heat exchanger (501) is proVided at the first therma1 ﬂow path (401) and a second heat exchanger (502) is proVided at the second therrna1 ﬂow path (402).

8. The concentrated so1ar power system according to any of c1aims 2 to 7,wherein the heat transfer ﬂuid is a ﬂuid haVing a me1ting point from about 200 °C to about 500 °C.

9. The concentrated solar power system according to c1aims 1 to 8, whereinthe therma1 energy storage tanks (200, 201) of the therrna1 energy storage system (2)comprises a first phase change material haVing a me1ting point from 500 °C to 900 °C.

10. The concentrated so1ar power system according to c1aim 9, wherein theenergy storage tanks (200, 2001) of the therma1 energy storage system (2) comprisesa second phase change materia1haVing a me1ting point substantia11y 1ower than the me1ting point of the first phase change materia1.

11. A therrna1 energy storage system (2) for a concentrated so1ar power systemcomprising a first therma1 energy storage tank (200) connected to a second therma1energy storage tank (201) Via a bi-directiona1 flow member (202), characterized inthat the first and second therrna1 energy storage tanks (200, 201) are proVided with at1east one phase change materia1 and are therma11y connected Via a first and secondtherma1 ﬂow path (401 , 402) to at 1east one of an e1ectrica1 energy generation system(5) and a so1ar receiVer (400), wherein the first and second therrna1 ﬂow paths (401,402) form the bi-directiona1 ﬂow member (202) and a heat transfer ﬂuid can beosci11ated from the first therrna1 energy storage tank (200) to the second energy storage tank (201) Via the bi-directiona1 ﬂow member (202).

12. The therrna1 energy storage system (2) according to c1aim 11, Wherein thetherma1 energy storage tanks (200, 201) of the therma1 energy storage system (2)comprises a first phase change material having a me1ting point from 500 °C to 900 °C.

13. The therrna1 energy storage system (2) according to c1aim 12, Wherein theenergy storage tanks (200, 2001) of the therma1 energy storage system (2) comprisesa second phase change materia1 having a me1ting point substantia11y 1oWer than the me1ting point of the first phase change materia1.

14. A method of charging a therrna1 energy storage system (2) according to any of c1aims 11 to 13, the method characterized by: receiving so1ar radiation at the so1ar receiver (400), osci11ating heat transfer ﬂuid from the first therma1 energy storage tank (200) tothe second therrna1 energy storage tank (201) via the bi-directiona1 ﬂoW member (202) and the so1ar receiver (400), and therein providing therrna1 energy to the heat transfer ﬂuid.

15. A method of discharging a therrna1 energy storage system (2) according toany of c1aims 11 to 13, the therma1 energy storage system (2) being in therma1connection With an e1ectrica1 energy generation system (5) comprising an energy conversion unit (500), the method characterized by: oscillating heat transfer ﬂuid from the first thermal energy storage tank (200) tothe second therrnal energy storage tank (201) Via the bi-directional ﬂoW member (202), receiVing therrnal energy at the electrical energy generation system (5), and Via a conVersion unit (500), conVerting the therrnal energy from the heat transfer ﬂuid to electrical energy.