Thermal Energy Storage for Solar Tower CSP power plants

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Study of strategies for thermal energy storage inSolar Tower CSP Power Plants – Introduction for

“Projecto MEFT” Manuel Nascimento#1

# MEFT – Instituto Superior Técnico

Avenida Rovisco Pais 1, 1049-001 [email protected]

Abstract — This document is firstly an introduction and

discussion on the motivations of the subject of my masters’

thesis, and is also a state-of-the-art review. It was presented in

accordance with the requirements of the “Projecto MEFT”

course.

I.

I NTRODUCTION

A.

C ONCENTRATED S OLAR P OWER (CSP)

Concentrated solar power pertains to the use of solarcollectors to concentrate direct solar irradiation and thussupply solar thermal energy for different media. This thermalenergy can have various applications in industry, but perhapsone of the most important is the electricity generation. CSPhas increasingly proven to provide a high contribution in theelectricity generation markets all over the world. The basic principle for using CSP on electricity generation is to use themedia which received solar thermal energy to power anelectricity generating engine. Four main concepts/designsexist presently: parabolic trough, solar tower (or centralreceiver), parabolic dish and linear Fresnel reflector. Thesecan be seen in fig. 2. Parabolic troughs and linear Fresnel CSP plants have 1-axis solar tracking, while central receivers and

parabolic dishes CSP plants require 2-axis solar tracking.These different concepts present advantages anddisadvantages between them and are in different stages ofmaturity and development. In table 1 a summarizeddescription and status of the different technologies can be seen[2]. Choosing a plant type for a certain location is a verycomplex process that comprises the consideration of multiplefactors of different natures: economic, technical andenvironmental. Reference [3] sets out to evaluate the main

existing collection technologies using the framework of anAnalytical Hierarchy Process (AHP) in order to “weigh in”these multiple factors and try to obtain a result.

Of particular importance in this study is the solar towerdesign. It is important because it allows for high installedcapacities in conjunction with higher temperatures, which canmean higher efficiencies in the electricity generation process.In solar tower plants, the power cycle used for electricitygeneration is the Rankine Cycle in steam turbines, which isthe dominant and very reliable power generation technologythroughout the world. A basic layout of a Solar Tower CSP plant can be seen in fig. 1 [4]. From the same work, currentlyoperating central facilities are listed in table 2. In the future

centrals of up to 500 MW are being commissioned and/or planned [4]. For further detailed information on CentralReceiver Solar Thermal Power Plants [4] is suggested.

B. M OTIVATION FOR T HERMAL E NERGY S TORAGE

This work will focus on strategies for thermal energystorage (TES) systems in electricity-generating Solar TowerCSP plants. Firstly, it is important to realize the importance ofsuch systems and the contributions they can provide for Solar

Fig. 2: The four main CSP concepts

Fig. 1: Basic layout of a Solar Tower CSP Plant


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Tower CSP plants, electricity generation in general, and otheraspects.

Electricity production is essential for mankind. Several production technologies exist, conventional and renewable,with different characteristics and costs. Ultimately, the majordriving force for choosing which technologies to use has beenthe cost factor. Traditionally, fossil-fuel power plants have been less costly. However, they emit large amounts of CO2,which nowadays is obvious that contribute most negatively onthe environment. At least 90% of CO2 emissions come fromthe burning of fossil fuels in the power generation andtransport sectors [4]. Decisive action must be undertaken toreverse this situation.

With the aim of providing incentives to their development,recent renewable energy sources such as Wind Power,Photovoltaics and CSP have benefited from special tariffsystem, such as the Portuguese PRE (Produção em RegimeEspecial – production under special regime), which haveallowed them to operate with LCOEs (levelized cost of energy)

higher than in an competitive market situation. However, it isto be expected that in the future this situation will change. Inany case, it is always in the best interests of a producer ofelectricity to minimize the production costs. Also, it isimportant to notice that in the electricity markets such asMIBEL (Mercado Ibérico de Electricidade – Iberianelectricity market), the energy demand, and thus, clearing price is not uniform throughout the day.

From an electricity generation point of view, CSP ingeneral presents some problems that effectively lower potential CSP plant revenues. Firstly, the number of “solarhours” in a day is limited, even on clear days, which meansthat the plant cannot operate continuously. Another major problem is intermittence: passing clouds can cause variationson the heat flux supplied to the steam generator which notonly reduce the effective working hours of the plant but canalso be incompatible with its specifications. Both these can beaddressed or at least mitigated with thermal energy storage.

Thermal energy storage acts basically as a “buffer”

Table 2: Currently operating central receiver CSP plants

Table 1: Summary of characteristics and status of CSP


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between the receiver system and power block (whereelectricity generation takes place) of a CSP plant. Instead ofheating directly the media that powers the power block, the

media can be stored in a container for later use. This way,when there are passing clouds, the storage system cancontinue to feed the power block and ensure a smoothoperation of the facility. Also, for some hours after the sunsets, the storage media can continue to supply heat to the power block, until its exhaustion, greatly increasing the totalworking hours of the CSP plant. TES also provides the CSP plant with more flexibility - it can store collected solar energyfor later use, especially when demand and/or prices are higher,increasing revenues [5]. This can be seen in fig. 3.Additionally, TES improves the dispatchability of the plant,e.g., its ability to generate electricity on a short notice.

By increasing the effective working hours of a CSP plant,

TES can have a strong future contribution in allowing CSP plants to be intermediate-load, or even base-load plants. Thiswill allow CSP to have an important role in replacing older power stations, namely coal-powered ones, which will be ofgreat importance for reducing CO2 emissions.

C. S OME B ACKGROUND ON C OSTS

As mentioned before, costs are ultimately the major drivingforce when considering CSP power plants. These havevariations, even within the same technology, due to locationdifferences, different components used, etc. Table 3 representsa brief overview of costs for different generating technologiesas of 2013 (taken the from Open Energy Informationdatabase). Technologies are sorted by increasing media LCE.

As can be seen, Solar CSP is still a long way in terms ofcompetitiveness with other sources, and cost reduction is of paramount importance in future research and development. In2009, USA’s De partment of Energy has set a goal to reducethe LCE to 90 $/MWh by 2020. In 2011, the DOE officiallyunveiled the SunShot Initiative, an ambitious research and

development plant that aims to further reduce the LCE to60$/MWh. This is the figure that will allow CSP to be trulycompetitive in the markets. The proposed roadmap can beseen in [6].

Fig. 3: Simulation of usage of TES flexibility [5]

Table 3: Overview of current costs for electricity generation


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II. THERMAL E NERGY STORAGETECHNOLOGIES

A.

I NTRODUCTION TO THERMAL ENERGY STORAGE

Energy storage is the storing of energy of some form inorder to be able to retrieve it at a later time. This is especiallyconvenient and important when the need for energyconsumption occurs at different times and/or places fromwhen and where the energy sources or supplies are available.All energy forms fall into one of these categories: potentialenergy (e.g. gravitational or chemical), kinetic energy,electrical energy or thermal energy. With regards to electricity production, the most widely known forms of energy storageare electrical (e.g. batteries), hydro storage (e.g. in dams) andthermal energy storage. As discussed before, thermal energystorage is one of the most important advantages of solarthermal power over other forms of CSP and other electricity

generation technologies.As mentioned briefly before, the basic functioning principle

of this type of storage is to have the storage material absorbthermal energy from the energy source (the Sun, in CSP),which is also known as charging, keep it stored, and thendischarge the storage system so that it releases the storedenergy to the electricity generation system (also known as the power block). The storage system acts, then, as a “buffer” between the solar field and the power block of traditional CSP plants without storage. Integration between the storage systemand other systems on the CSP plants must be considered fig. 4[2]. The storage system complements the solar receiversystem when there is a low solar irradiation (intermittent

clouds) and can substitute it altogether for a period of timeduring the night or cloudy days, supplying energy to the power block electricity generation.

There are different storage systems for thermal CSP, withdifferent concepts and designs, which have differentcharacteristics that constitute advantages and disadvantagesthat need to be considered. However, they all fall under three basic categories: sensible energy storage (SHS), latent heatstorage (LHS) and chemical heat storage (CHS).

Sensible heat storage pertains to the fact that when thetemperature of a substance increases so does its energycontent. The energy that is absorbed (released) as thetemperature increases (decreases) is called sensible heat. The

amount of energy absorbed or released as the temperaturesuffers a variation depends on the material and can beexpressed by:

∫

where m is the mass of the material, Ti and Tf are theinitial and final temperatures, respectively, and c the specific

heat capacity (given in the SI by J.kg -1.K -1), which usuallyvaries with temperature.

As the temperature of a material varies, the material canchange its phase, e.g. from solid to liquid or from liquid to gas.At the phase change temperature, there is an amount of energythat must be supplied to the material in order for it to undergothe phase change, during which its temperature is constant.That energy is called latent heat, and the amount of energydepends on the material and is called heat of fusion (solid to

liquid) and heat of vaporization (liquid to gas). The latent heatis usually denoted by λ, given typically given by kJ/kg.Usually, when this energy storage principle is used, thematerial not only undergoes a change of phase but also anincrease and decrease of temperature in both the phases, so thesensible heat involved must also be considered. The amount ofenergy absorbed or released, including the sensible heat can be expressed by:

∫

∫

where λ is the latent heat of the phase change and T p is the

phase change temperature for the material (either fusion orvaporization).

The final category of storing thermal energy is based onreversible endothermic chemical reactions. Certain chemicalreactions, called endothermic, require heat in order to occur.By providing this heat at the correct temperature, a chemicalcompound can be dissociated into its products. Later, thetemperature of these products can be lowered, forcing thesynthesis reaction to take place. This synthesis reaction isexothermic, and releases the heat that was absorbed before (allor almost all). The amount of energy absorbed and releaseddepends on the chemical compound and its reaction.

Energy storage can be further categorized as active or passive [7], which can be seen in figure 5. In active systems,

Fig. 4: Component considerations in CSP plants with TES

Fig. 5: Categorization of TES systems


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the heat transfer is characterized by forced convection, wherethe receiver medium is circulated through heat exchangers inthe plant’s collector(s). In contrast, passive systems are “static”

and require a heat transfer fluid (HTF) to pass through thestorage material (via heat exchangers) in order to charge ordischarge the storage. Active systems can be furthersubdivided as direct and indirect. In a direct system, thestorage material used is also the HTF that circulates betweenthe receiver system and the power block, while in an indirectsystem, there is an additional HTF fluid that exchanges heatwith the storage media via heat exchangers. Indirect systemsare preferred when the storage media is incompatible(chemically, pressure-wise, etc.) with the other systems and/or piping, at the expense of potential losses in the additional heatexchangers necessary.

In every category, there are some design and conceptalternatives which will be briefly revised in the followingsections. Also, there is a wide range of materials that can beused, depending on the temperature range and application (foran extensive and commented listing of tested materials, checktable 7 in [2]). Common to all these, however, are somedesirable characteristics which should be pursued wheninvestigating the potential use of a certain design/concept andmaterial TES in CSP. In the storage material it is desirable tohave high energy density (also known as storage capacity),high energy transfer rate to and from the storage material,mechanical and chemical stability (must endure an amount ofcycles of charge and discharge compatible with the power plant’s expected life-time), compatibility and safety in itsintegration with the plant’s systems and components, lowthermal losses, controllability, and others. Besides this, thetotal cost of the power plant must be as low as possible inorder to compete with other technologies and make it an

attractive alternative. Also, a low environmental impact ishighly desirable, as is expected of renewable energy sources.

Finally, in developing a CSP power plant with TES, severalother considerations must be taken into account. These can bedivided into plant level, component level and system leveldesign considerations and are summarized in fig. 6 [2].

TES is available mainly in CSP plants of the trough and thetower (also known as central receiver) types. Remember that

in these CSP plant types use steam turbines performing theRankine Cycle to produce electricity, which is the dominant power generation technology throughout the world.

As such, this also needs to be considered when thinkingabout using TES for CSP plants. Lovegrove et al. [1] presenton table 11.1 a selection of commercial and experimental CSPfacilities and their respective storage systems. Further detailsfor CSP plants can be seen in table 4 of [8]. For moreextensive listing of CSP plants, check Wikipedia(http://en.wikipedia.org/wiki/List_of_solar_thermal_power_stations) As we will discuss in detail, the different categories ofthermal energy storage (SHS, LHS and CHS) are in differentstages of development.

Next we shall discuss in more detail each category ofthermal energy storage with respect to CSP implementation,exploring each configuration and design either available todayor in testing or research.

B.

S ENSIBLE H EAT S TORAGE (SHS) S YSTEMS

SHS systems store energy by heating up the storagematerial, which later will be discharged providing heat for the power block in the absence of solar irradiation. SHS materialscan be liquid or solid. It is important to keep in mind water asa reference substance, since it has the highest specific heatcapacity per mass of all solids and liquids. As such, for a

temperature difference of 100 K, the highest possible storagecapacity for sensible heat storage is of the order of 0.396MJ/kg. Traditional burnable chemical energy sources havemuch higher values (e.g. petrol 42.4 MJ/kg and coal 24MJ/kg), so SHS thermal systems require large masses ofmaterial due to their low storage density. As mentioned, thereare a variety of materials that can be used. Tables 4 and 5 [7]show the main characteristics for various liquid and solidmaterials being used, respectively.

Fig. 6: Overview of TES design considerations

Table 4

http://en.wikipedia.org/wiki/List_of_solar_thermal_power_stationshttp://en.wikipedia.org/wiki/List_of_solar_thermal_power_stationshttp://en.wikipedia.org/wiki/List_of_solar_thermal_power_stationshttp://en.wikipedia.org/wiki/List_of_solar_thermal_power_stationshttp://en.wikipedia.org/wiki/List_of_solar_thermal_power_stationshttp://en.wikipedia.org/wiki/List_of_solar_thermal_power_stations


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1. Liquid media storage

a)

Two tank designThe two-tank design can be seen in fig. 7 [9] where it is

integrated in a central receiver CSP plant. This is an active,direct type storage system: the storage media circulatesthrough the system and is heated up in the receiver directly,

without the use of heat exchangers, which are usuallyexpensive. One of the most recent examples of this kind ofCSP plant is the “Gemasolar” near Seville, belonging to

Torresol Energy. In operation since 2011, it features aninstalled capacity of 19.9 MW with 2300 MWhth (15h)thermal storage, which was proven to be capable of operating24h a day at full load. As the name suggests, this type ofdesign features two tanks that store the liquid media andthrough which the media circulates. When the media passesthrough the receiver system, it absorbs the solar energy andgoes to the “hot” storage tank. It is important to design thefacility to optimize the operation parameters (mass flow rate,solar multiple, receiver power) to assure that the maximum

quantity of storage media reaches the desired operationtemperature. Exiting the “hot” tank, it transfers its energy viaheat exchangers to the power block, by lowering itstemperature to a “cold” temperature (which is still rather hot,at about 290 ºC in Gemasolar). Then, the storage media ischanneled to the “cold” tank where it awaits to be pumped upto the receiver and continue the cycle. The use of two tanks is preferred over one tank due to the large amount of storagemedia needed for high storage energy, as mentioned. The preferred media for these liquid two-tank designs has been, so

far, molten salts. Of these, mixtures of nitrates are the mostcommon. The Gemasolar features 8.500 tons of nitrate salts, amixture of 60% NaNO3 and 40% KNO3 often called “solarsalt” (see also table 4).

The major disadvantage when using molten salts are theirrelatively high freezing temperature, about 120-220 ºC, andlow thermal conductivity, which make re-melting andextremely complex process. The temperature of the storagemedia must be kept above its freezing temperature at all times,otherwise the flow of media would stop, representing acatastrophic breakdown of the facility. This represents a high

risk from the operational standpoint, and means that specialcare must be taken, namely circulating the media through thenight or using auxiliary heating in order to ensure adequatetemperature in the entire system. All this increases the O&Mcosts. Other disadvantage is the inherent corrosivity of thesematerials, which increases with temperature. Because of this,more expensive materials are required for the storage tanks.However, studies have been made to test the resistance ofstainless steel over multiple thermal cycles of nitrate mixtures.In particular, after the plant Solar Two (predecessor of theGemasolar) finished its operation, examinations were made tothe interior of the salt tanks [10]. After 30,000 h of operation,analyses showed that corrosion took place within acceptable

low rates.The use of this two tank system allows for the existence of

a “hot” tank with potentially very high temperatures (as long

as there are adequate storage materials). Remembering thatthe electricity is generating via a Rankine Cycle in the steamturbine in the power block, higher efficiencies can be obtained by using higher temperature steam generation. The mostefficient state-of-the-art steam turbines operate at up to 700ºCsteam inlet temperature, in supercritical conditions (above thecritical point of 22MPa and 374ºC). These kind oftemperatures can only be obtained in tower-type CSP, asthrough and Fresnel concentrators are limited to about 400ºCif thermal oil is used or 500ºC if an alternative HTF is used,like direct steam generation (DSG). The tradeoff when usingthese higher temperatures is the existence of higher heat lossesfrom the solar heat, requiring more expensive piping andmaterials.

If the storage media is expensive, it is possible to adopt asimilar but indirect two-tank design, using an intermediate,less expensive HTF. This is the case in parabolic through plant that uses thermal oil as HTF and a two-tank molten saltstorage system [7]. Notice that this indirect design requiresadditional heat exchangers between the HTF and the storagemedia, which is more costly. One example of such a plant is

Table 5

Fig. 7: Typical 2-tank TES solar tower

CSP plant configuration


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the Andasol-1(http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=3), a 50 MW plant, featuring 1,010 MWh (7.5h) ofstorage via 28,500 tons (14,000 m3) of “solar salt”. ComparingAndasol-1 and Gemasolar, notice that even though Andasol-1has much higher TES content, Gemasolar, due to higher “hot”temperature achievable and greater temperature difference isable to achieve much longer storage periods: 15h vs 7.5h,

which is highly desirable.

b) Single tank designOne other liquid active indirect storage system is the single

tank system, in which the hot and cold fluids (nitrate mixturesmolten salts) are stored in a single tank. In the tank, the hotand cold fluids are separated due to stratification, hot on top,cold on the bottom, and the area between both fluids is calledthe thermocline. To help the thermocline effect, a fillermaterial can be used. At Sandia National Laboratories, silicasands and quartzite rock were demonstrated to withstand themolten salt environment with no significant deterioration [11].The advantage of this approach is the potential cost reduction because only one tank is used. According the mentioned work,this thermocline system would be about 35% cheaper than thetwo-tank storage system. However, maintaining the thermalstratification requires a more complex charging anddischarging procedures and adequate methods to avoid mixingof the hot and cold portions of the storage media. Furthermore,research on the applicability of this concept to tower CSPfacilities is still unavailable.

c)

Steam accumulatorAs mentioned before, water is a reference media for energy

storage, due to its high specific heat capacity per mass of allsolids and liquids. It also has a low cost and highcompatibility. For temperatures above 100ºC (which is thecase for Rankine Cycle Steam Turbines commonly used incentral receiver CSP) water must be kept under pressure inorder to be used in the liquid form as storage media. Steamaccumulators are dynamic 2-phase systems that have beenused for many years in fossil fuel fired power plants and, assuch, benefit from much experience. Also, steamaccumulators are common in process industry fortemperatures between 100ºC and 200ºC. As can be seen in fig9 [8], they consist basically of a pressurized vessel partially

filled with saturated water at high pressure and with a smallvolume of steam. This system can be used in CR CSP byusing water as the HTF and storage media in what iscommonly known as direct steam generation (DSG). Duringcharging, steam generated in the receiver system is blown intothe bottom of the storage vessel. Some of the steam condensesand heats the water, while the remainder will fill the rest of

the volume above the water. This raises the water level toabout ¾ of the volume of the vessel, raising pressure andtemperature. To feed the power block of the facility, steamcan be discharged from the storage system via the discharge pipe on top of the vessel. This relieves the pressure in thevessel which causes more water to vaporize continuing tosupply steam until complete discharge. Temperature will alsogradually reduce. It is important to note that steamaccumulators have fast reaction times. Various operationmethods can be used, namely by controlling the steam massflow rate or by controlling the steam pressure. As such, thereare many operation design considerations to be taken intoaccount. Assuming that the amount of saturated liquid water

mliquid is constant in the steam accumulator, the amount ofenergy provided during the discharge process can be estimated by [1]: ()

where cliquid is the average specific heat capacity of liquidwater, Tsat is the saturation temperature (dependent on thetemperature) and Pi and Pf are, respectively, the initial andfinal pressure during discharge. Higher working pressuresmean higher volume specific masses of steam and, thus, moreenergy storage potential. However, these present challengesfrom the technical point of view with regards to the materialsof pressure vessels and also the pipes and valves, which resultin higher costs. Indeed, due to the difficulty to store steam insupercritical conditions (very high temperature and pressure),this approached has been used with relatively lowertemperatures. Because of this DSG type plant system has beenmainly used in parabolic through CSP technology. However,some power tower facilities have used these storage systems.For instance, Europe’s first commercial CSP plant, PS10(http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=38) in fig. 10 [12], near Seville, has begun operation in2007 and produces steam at 40-45 bar and 250ºC. It features 4tanks with a total storage capacity of 20 MWh that enable the

Fig. 8: PS10, a typical DSG solar tower plant with steam storage

Fig. 9: Steam accumulator TES concept

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facility to run the 11 MWel power block during 50 minutes at50% load operation. These are very low values for storage

energy and can only be used as a “buffer” againstintermittency (passing clouds, etc.) rather than as long-termstorage capacity as seen in the previous two-tank molten saltcase.

2. Solid media storage

a) Indirect designs – heat exchangersInstead of using liquid materials in indirect SHS designs,

it’s also possible to use solid media. The configuration issimilar to the Andasol-type plants but with concrete storageinstead of two tanks for molten salts [7]. These types ofsystems are indirect, requiring and HTF to circulate through

the entire CSP system and exchange heat in the variouscomponents. Recalling table 5 solid materials for SHS areusually less expensive than liquid materials, within 10-20% ofthe popular molten salts. Also, the O&M costs are expected to be lower. Furthermore, with solid materials, there is no risk offreezing, leaking or evaporation, as there is in liquid media.Recall, in particular, that the consequences of freezing moltensalts would be catastrophic.Various castable materials (materials that can be easily betransported and placed in a container, usually from a liquid ormolten origin) have been investigated for this purpose. Of particular interest has been the concrete, mainly because itslow cost, availability and ease of use, as well as vast

experience. Laing et al. [13] [14] have studied, and tested amodular concrete system to be integrated in trough plants. Themodules, developed by Ed. Züblin AG and DLR (DeutschesZentrum für Luft- und Raumfahrt – German Aerospace center)are consisted basically of tube registers with 132 tubes withlength 9 m and outer diameter 18 mm, which are then filledwith castable concrete (fig. 11) [13] and then thermicallyisolated. This extensive piping is the main drawback of thesesystems in terms of costs. These models were first tested atPlataforma Solar de Almeria in Spain. In [13], by the end ofOctober 2008 they had accumulated 4 months of thermal

cycles (more than 300) between 300ºC and 400ºC and about50 cycles with a temperature difference of 40K, and havefound that during that time the performance of the storage wasabsolutely constant. It is important to note that, for allindustrial sectors, roughly 30% of process heat requirement is between 100ºC and 400ºC, which fits very well for thisconcrete systems. To implement this on a 50 MW Andasol-type trough plant, with storage capacity of about 1100 MWh,

252 basic storage modules are needed, arranged in 4 groups of63 in series and parallel packed together. The total area would be about 300m x 100m. The investment cost would be in therange of 34.5 €/kWhth. Further studies have been carried out inorder to obtain higher working temperature and reduce costs.To obtain higher working temperatures, different mixtures ofconcrete were investigated. In [14] special type of concretemixture was tested, N4-concrete, mainly based on blastfurnace cement as a binder system, temperature resistantgravel and sand (functioning as aggregates) and a smallamount of polyethylene fibers. Special attention was given tothe initial heating up of the concrete, with regards to masslosses. These are essentially due to loss of free, evaporablewater, dehydration of the hardened cement paste, and masslosses of the aggregates. Also, it is necessary to check ifstrength values (mainly stress) are within acceptable ranges.Overall, results show that, up to 500ºC, mass losses andstrength values for the concrete stabilize after a period of timeand a number of thermal cycles, with no degradation, whichindicates that the use of concrete as a storage medium up to500ºC seems possible. Thermal conductivity between the HTFand the solid media can be further enhanced by usingextended heat transfer areas with fins or similar devices.However, since the amount of piping is so extensive, fins andother structures are not cost effective [8].

b)

Indirect designs – packed bedInstead of using piping to provide contact between the HTF

and the solid storage material, some HTFs and solid materialsallow for direct contact between them. In this case, the particles of the storage material are packed together in acontainer and the HTF flows through the particles inside thecontainer. This direct contact allows for much higher volumespecific heat transfer areas, however, the liquid HTF and solid

Fig. 10: Concrete module of [13] before concrete filling

Fig. 11: Packed bed type solar tower CSP plant


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Table 6: Jülich CSP central storage system specifications

media must be compatible with each other, with no reaction orcorrosion, and work at the same pressure.

This packed bed concept can also be used with flue gases(exhaust gases, rich in Nitrogen, sometimes up to two thirds,and CO2; often from power plants) or air as heat transfer

“fluids”. There is already usage of these gases in existingindustrial processes, such as in the steel industry (hot blaststoves or “Cowper” stoves), glass industry and industrial air

purification systems. Zunft et al. [15] have tested this possibility for CSP in the Jülich Solar Power Tower (figure 10)[15]. This tower plant uses air at atmospheric pressure,heating up the air up to about 700ºC. The storage system iscomposed of ceramic material, cycled between 120ºC and680ºC (table 6). In particular, it was shown that the facilityoperated satisfactorily through thermal cycling of the storage,with almost constant temperature of the storage outlet duringdischarge, followed by a sharp decrease near depletion. This iscompatible with an effective turbine operation. Further

investigation needs to be developed in order to increase thestorage duration, which was of about 1.5h.

Furnas [16] studied the heat transfer from air to a bed ofiron ore pellets at up to 750 ºC, concluding that the coefficientof heat transfer varies linearly with the air flow rate. Nsoforand Adebiyi [17] performed experimental studies andmodelling on a packed bed of zirconium oxide pellets, withflue gas as the charging fluid and ambient air as thedischarging fluid. Temperatures up to 1000ºC were achieved.Tower CSP plants alone could provide these kind oftemperatures.

c)

Direct designs – solid particles

One final design which is in its early stages of conception isthrough the direct absorption by solid particles of concentratedsolar radiation in the receiver. It is considered to be a potentialsystem for chemical applications requiring high temperatures,and could potentially be used for electricity generation in CSP.Several issues have to be addressed before test-scale andcommercial-scale CSP is possible. Firstly, thehandling/pumping of the material from the ground level to thetop of the tower where the receiver is placed will require piping that will be subjected to significant mechanical loadsand at high pressures, presenting technical challenges. Also,

the heat transfer between the particles and some working fluidis a complex procedure [18].

C. L ATENT H EAT S TORAGE (LHS) S YSTEMS

Latent heat storage systems use the enthalpy change of asubstance that undergoes a phase change to store energy.Usually, the solid to liquid transition is used, and thus the heat

of fusion of the used substance is the relevant quantity. Themain advantage of this concept is that the change of phase is anearly isothermal process, which allows for energy transferwithin a narrow range of temperatures, close to the phasechange temperature. This can be important for maintaining theHTF or the inlet temperature to the power block at acontrolled temperature. Furthermore, because the heat offusion is very high, large amounts of energy can be store withrelatively low volumes when compared to other types ofenergy storage. This results in some of the smaller volumesand lowest storage media costs of any storage concepts. Themajor design concern when employing this storage system isthe choice of operating temperature, and thus of material used,

which is commonly called “phase change material” (PCM).All LHS systems are indirect, requiring HTF, and so thechosen PCM must be compatible with the chosen HTF. Also, potential materials must involve a phase change that is both physically and chemically reversible, that is, the meltingtemperature and melting enthalpy should not change overmany phase change cycles. There have been studies over awide range of materials, over a temperature range up to about1000ºC, in a number of categories, such as inorganic (salts)substances, inorganic euctetic salt mixtures, organicsubstances and metals and metal alloys. The variety ofsubstance is very extensive, and [19] presents an exhaustivereview and extensive tables with materials that have been

tested. It is important to notice that most of the pure inorganicsalts and euctetic salt mixtures are commonly seen in nitrates(NaNO3 is the most investigate medium), carbonates andchlorides, which have low costs. Table 7 [2] presents a brieflisting of some potential latent heat storage materials.

Usually, the thermal conductivity of these materials (except

Table 7: Potential latent heat storage materials


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metal and metal alloys) is the main obstacle presented by LHSsystems. Consequently, the main challenge when using thesesystems is the development of cost-effective heat transferconcepts, in order to provide adequate heat transfer between

the PCM and HTF, while maintaining the cost attractive.Because of this, thermal energy storage systems using PCMsare often categorized according to the heat transfer conceptemployed [20].

1. LHS with extended heat transfer area

With the objective of increasing the thermal conductivity between the PCM and the HTF, instead of using a greateramount of HTF pipes, which is expensive, it is possible toextend the effective surface area with fins which can also

be configured in a “sandwich” type arrangement.Reference [19] presents an aluminium heat exchanger withlong fins for usage with PCMs. Because they are in directcontent with the PCM, special care must be taken for thematerials and design of the fins. Usually, parallel tubes aremore distanced from each other than in conventional heatexchangers. The material used must be resistant to potentialcorrosion from the PCM and also be capable of sustainingthermomechanical stress caused by the volume variation ofthe PCM during the change of phase. In table 8 [19] severalmaterials for extending the heat transfer area in PCMs are presented. Various research projects carried out by DLRusing the “sandwich” concept have demonstrated thefeasibility of this finned tube concept. The contribution ofthe finned heat exchangers was shown to be significant[19].

Another option is to use micro-encapsulation of thePCM, that is, distribute the PCM over an amount of smallersealed vessels or “capsules” and have the HTF flowthrough them. However, studies carried out in the DISTOR project [20], have determined that it was not economicallyattractive to pursue this concept.

2.

LHS using composite material with increased

thermal conductivity

Another method for improving the effective thermalconductivity of a PCM material is by homogenouslyadding a different material with higher thermalconductivity. This can be achieved by either dispersinghighly conductive particles in the PCM material or byintegrating the PCM in vessels with “matrices” or “nets” of,

for instance, graphite or aluminium. As in the previous case,corrosion can occur and must be considered. In both cases,significant amounts of these better conductive materials

need to be added, increasing the costs, making thisapproach less attractive.

3.

Taking advantage of LHS for DSG The property of isothermal energy storage can be used to

great advantage in CSP technology in DSG (direct steamgeneration) as can be seen with figure 12 [2]. Steamgeneration is an isothermal process. When using steam forcharging and discharging a sensible heat storage, thecharging steam must be in a much higher saturationtemperature than the discharging steam so that the heattransfers occur. Because of this, the discharged steam, thatis, the steam generated by discharge the storage system,will have lower exergy, and thus lower power generation potential. In contrast, if a LHS system is used, the chargingand discharging steams will have approximately the same

temperature, and no exergy is lost.These systems have mostly been used in trough CSP

plants, but simulations have also been made for tower power plants [9]. A proposed implementation of LHSstorage systems in DSG has been through a combinationwith two other storage systems, in parallel [21]. A sensibleheat storage system is used for preheating, then a LHS forthe steam generation and, after that another SHS system forsuperheating of generated steam. Obviously, the integrationof 3 storage systems is more challenging with respect tooperation control. The PCM used was 140 kg of NaNO 3.

D.

C HEMICAL H EAT S TORAGE (SHS) S YSTEMS

Chemical heat storage systems use the property that certainchemical reactions are reversible. The heat collected in the

Table 8: Materials considered for heat exchan er im lementation in LHS s stems

Fig. 12: Typical Rankine Cycle of a steam turbine for CSP


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receiver system of a CSP plant can be used to excite anendothermic chemical reaction, typically a dissociation of themedia in less complex chemical structures. If this reaction iscompletely reversible, the reverse reaction can be triggeredlater, usually in a lower temperature, which will release thestored energy. This operation is an analogous fashion to latentheat storage systems, except in the fact that chemical reactionsmay have a wider temperature range. Design-wise, thesesystems could be similar to two-tank system, each tank foreach reaction. CHS systems are the least investigated anddevelopment of reversible thermochemical reactions is in thevery early stages. As such, economic issues and systemaspects warrant further investigations. However, the mainadvantage of using reversible chemical reactions as an energystorage system is the potentially high energy density. Alsoattractive is the possibility to store the reactants inatmospheric temperature, preventing against thermal losses.Again, corrosion in the containers/piping must be taken intoaccount, as well as complete reversibility of the reactions. Intable 9 [7] some reactions that have been investigated to beused as chemical storage system are presented. Take note ofthe variety of temperature ranges available, much similar tothe case of PCMs. According to [ref new_6], the most relevantchemical processes for CHS systems at present are the metaloxide/metal and ammonia.

1.

Metal Oxide/Metal

Foster et al. [22] studied these materials for CHSsystems and deemed them technically feasible. Thegoverning reactions are:

A schematic for a corresponding solar reactor was proposed, as can be seen in figure 13 [22]. The dissociationreaction takes place at about 980K in the reactor using theconcentrated solar radiation, reducing the SnO2 with CH4.At these temperatures the SnO2 is a solid dust floatingabove the liquid Sn. This liquid Sn is stored after beingformed. When heat from the storage system is needed, theSn is cooled via heat exchangers until it reaches the reversereaction temperature and is passed to a secondary tank.There, water vapor is added, forcing the reverse reaction, producing SnO2 and thus effectively recovering the initialmaterial. Although technically feasible, development anddemonstration of this configuration in CSP technology isstill pending.

2. AmmoniaThe main industrial process for ammonia synthesis (NH3)

is the well-known Haber Bosch process. This process iswidely used around the world for the production offertilizers and explosives. An excess of 125 million tonnesof ammonia are produced every year. The synthesis occursvia an endothermic reaction, and thus the reverse reaction(dissociation of ammonia) can be used for chemical energystorage. The governing equations are:

In Lovegrove et al. [23] the Solar Thermal Group of the

Australian National University have proposed andextensively investigated a parabolic dish configuration forthis effect, operated at 15 kW, at 10MPa. The volumetricstorage capacity obtained was about 40 kWhth/m

3.

Table 9: Potential reactions to be used in CHS systems

Fig. 13: Proposed configuration for a CHS system with CSP for

the SnO2 dissociation reaction


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III. MATURITY, SUMMARY AND FURTHERDEVELOPMENTS FOR TES

As we have seen, there are multiple technologies, designsand materials for thermal energy storage. A brief overview ofthe maturity of the different systems and their workingtemperatures is shown in table 10 [24].

The common requirements that should be pursued when

considering any TES system are: high storage capacity; goodheat transfer between the HTF and the storage media;mechanical and chemical stability of the storage material;compatibility between the HTF and the storage material;compatibility between the storage material and the storagevessel (avoiding corrosion and degradation); completereversibility over a number of charging/discharging cycles(longevity); low thermal losses; ease of control; ease ofintegration into the power plant. Plant design considerationmust take into account operation strategy, maximum load andnominal temperatures. The different storage concepts havevarious advantages and disadvantages between them. Aconsiderable exhaustive summary of them can be seen in [8].

The current leading storage system is SHS with moltensalts. As can be seen in [5], about 50% of the costs with thesesystems concern the molten salts, and, as such, research incost reduction of these salts will be very important. Steamaccumulator systems have been extensively used as well, butsince the costs for the pressure vessels at temperatures higherthan about 250 ºC are very high, future growth of thistechnology is probably limited. Concrete SHS systems show promise in the near future as they have been recently tested.They show a potential for cost reduction of the storage system,when comparing with the molten salts storage. This has to be balanced by the additional cost of the heat exchangersemployed, however, which dominate the storage system’scosts [1]. Further research and development of cost-effectiveheat transfer systems should be pursued in the future. Packed bed solid ceramic storage with air or flue gas HTF has the potential to reach very high temperatures, which is of particular interest to tower CSP. Latent heat storage systemswith their PCMs may be particularly interesting in the futurefor DSG plants, taking advantage of their higher energydensity and nearly constant temperature. This can be achieved by integrating a 3-storage design, as proposed in [21]. Aninteresting comparison between plants using two-tank moltenstorage systems and this 3-storage DSG concept can be seenin [9] and is explained in the following section. Chemical

storage is still in its earlier development stages, requiringfurther research especially since the long-term reversibility of

reactions must be ensured and overall energetic and exergeticanalysis should be performed.

IV. THE IMPORTANCE OF COMPUTERSIMULATIONS

Computer simulations can be used and are very importantwhen studying, designing and optimizing CSP plants and/orTES systems. At the end of the day, perhaps the mostimportant criteria for analysis of a CSP plant is the LEC orLCOE – levelized cost of energy. Still, obviously, themodelling of the physical behavior of CSP plants and/or TESis of extreme importance when performing research on thosetopics.

Innumerous simulation tools have been developed fordesigning CSP plants, comprising different software/code fordifferent components. In table 3 of [2] a summary of some ofthese are presented. In section 4 of [7] a more detailed reviewof various studies and respective software used to investigate

different CSP and TES configurations.In [5] a MIP software package for analyzing CSP plants

with and without TES. That software is based on SAM,developed by NREL (National Renewable Energy Laboratory- USA), which maximizes revenues from energy sales, takinginto account various real-world market considerations, ratherthan by using heuristic rules. This work showed that with TESin CSP plants it is possible to achieve return on investmentsabove 100%, providing a strong motivation for theirimplementation and further research and development.

Reference [25] provides a very interesting study of the benefits of using CSP facilities with TES in conjunction withcombined cycle gas burners. This is called hybridization. Five

different CSP facilities (and 3 different solar multiples) wereconsidered and compared with a reference conventional fossil-fired combined cycle plants. In particular, reductions on CO2emissions and LEC were investigated. The results show thatthe potential to reduce CO2 emissions is high, particularlywith large solar fields and high storage capacity. However, allsolar-hybrid plants show an increase in LEC with increasingfield sizes and storage capacities, which corroborate thecontinuing the need for cost reduction and/or efficiencyenhancement in CSP plants in general.

Avila-Marin et al. [9] have performed a parametric analysisfor medium to large size (290-500MW) central receiver CSP plants, comparing both two-tank molten salts storage systemsand DSG plants with a combination of 2 storage systems:PCM for the steam generation and either concrete or moltensalts for sensible storage. The analysis comprises not only thedifferent technologies, but also different locations, different plant sizes and different costs for components. The mainresults are that both technologies demonstrate that, despite thelocation, the larger the plants, the lower LEC. This suggeststhat the, in the future, CSP plants with increasingly higherinstalled capacity should be preferred. Two tank molten saltsstorage minimizes the LEC of the plant using lower values ofgross turbine power (33-78 MWe) and very high TES (14-

Table 10: Brief summary of maturity status on CSP TES

technologies


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16h). On the other hand, DSG plants minimize their LEC forhigher gross turbine power (67-122 MWe) and low values ofTES (2-5h). This provides insight in design choices andoperation strategies when considering these CSP plants. Also,and in particular, it shows the potential and desirability forusing CSP plants with TES as base-load or at leastintermediate-load, rather than peak-load, further contributingto power grid interconnection.

ACKNOWLEDGMENT

I would like to thank Professor Luís Filipe Mendes at ISTand Engº João Farinha Mendes and Engº João Cardoso atLNEG for giving me this opportunity to do my masters’ thesison CSP with LNEG. I would also like in particular to thankthem for their patience and understanding.

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