thermal energy storage for concentrating solar power ...

1949-8241/12 $90.00 + .00 DOI: http://dx.doi.org/10.3727/194982412X13462021397570 E-ISSN 1949-825X www.cognizantcommunication.com

Technology and Innovation, Vol. 14, pp. 81–91, 2012 Printed in the USA. All rights reserved. Copyright  2012 Cognizant Comm. Corp.

THERMAL ENERGY STORAGE FOR CONCENTRATING SOLAR POWER PLANTS Sarada Kuravi, Yogi Goswami, Elias K. Stefanakos, Manoj Ram, Chand Jotshi, Swetha Pendyala, Jamie Trahan, Prashanth Sridharan, Muhammad Rahman, and Burton Krakow Clean Energy Research Center, University of South Florida, Tampa, FL, USA Thermal energy storage for concentrating solar thermal power (CSP) plants can help in overcoming the intermittency of the solar resource and also reduce the levelized cost of energy (LCOE) by utilizing the power block for extended periods of time. In general, heat can be stored in the form of sensible heat, latent heat, and thermochemical reactions. This article describes the development of a costeffective latent heat storage TES at the University of South Florida (USF). Latent heat storage systems have higher energy density compared to sensible heat storage systems. However, most phase change materials (PCMs) have low thermal conductivity that leads to slow charging and discharging rates. The effective thermal conductivity of PCMs can be improved by forming small macrocapsules of PCM and enhancing convective heat transfer by submerging them in a liquid. A novel encapsulation procedure for high-temperature PCMs that can be used for thermal energy storage (TES) systems in CSP plants is being developed at USF. When incorporated in a TES system, these PCMs can reduce the system costs to much lower rates than currently used systems. Economical encapsulation is achieved by using a novel electroless deposition technique. Preliminary results are presented and the factors that are being considered for process optimization are discussed. Key words: High temperature thermal energy storage systems; Concentrating solar power plants; Novel encapsulation technologies; Economical encapsulation

INTRODUCTION

TES systems can collect energy during sunshine hours and store it in order to shift its delivery to a later time or to smooth out plant output during cloudy weather conditions. Hence, the operation of a solar thermal power plant can be extended beyond periods of no solar radiation without the need to burn fossil fuels. Energy storage not only reduces the mismatch between supply and demand but also improves the performance and reliability of energy systems and plays an important role in conserving energy (5). By extending the hours of usage of the power block beyond the sunshine hours a TES system can reduce the levelized cost of energy (LCOE) for the plant. Several thermal energy storage technologies that have been implemented for CSP plants are mainly

Concentrated solar thermal power (also called concentrating solar power and CSP) systems use mirrors to concentrate sunlight from a large area to a small area where it is absorbed and converted to heat at high temperatures. The high-temperature heat is then used to drive a power block (usually a steam turbine connected to an electrical power generator) similar to the power block of a conventional thermal power plant. A major advantage of CSP plants over solar photovoltaic (PV) power plants is that CSP plants may be coupled with conventional fuels and can utilize thermal energy storage (TES) to overcome the intermittency of solar energy.

Accepted April 3, 2012. Address correspondence to Dr. Yogi Goswami, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA. Tel: 1-813-974-7322; Fax: 1-813-974-2050; E-mail: [email protected]

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two-tank and single-tank systems. In a two-tank system, the fluid is stored in two tanks, one at a high temperature and the other at a low temperature. Fluid from the low-temperature tank flows through the solar collector or receiver, where solar energy heats it to a high temperature and it then flows to the high-temperature tank for storage. Fluid from the high-temperature tank flows through a heat exchanger, where it generates steam for electricity production. The fluid exits the heat exchanger at a low temperature and returns to the low-temperature tank. These systems are called two-tank direct systems. An indirect system, on the other hand, uses different fluids for heat transfer and storage. An indirect system is used in plants in which the heat transfer fluid is too expensive or not suited for use as the storage fluid. The storage fluid from the low-temperature tank flows through an extra heat exchanger, where it is heated by the high-temperature heat transfer fluid. The high-temperature storage fluid then flows back to the high-temperature storage tank. The fluid exits this heat exchanger at a low temperature and returns to the solar collector or receiver, where it is heated back to a high temperature. Storage fluid from the high-temperature tank is used to generate steam in the same manner as the two-tank direct system. The indirect system requires an extra heat exchanger, which increases system cost. Figure 1 shows a two-tank thermal energy storage system integrated into a parabolic trough power plant (11). Single-tank systems, mostly thermocline systems, store thermal energy in a solid medium, most commonly silica sand, in a single tank (Fig. 2). At any time during operation, the top part of the medium is at high temperature, and the bottom part is at low temperature. The hot- and cold-temperature regions are separated by a temperature gradient or thermocline. High-temperature heat transfer fluid flows into the top of the thermocline and exits the bottom at low temperature. This process moves the thermocline downward and adds thermal energy to the system for storage. Reversing the flow moves the thermocline upward and removes thermal energy from the system to generate steam. Buoyancy effects create thermal stratification of the fluid within the tank, which helps to stabilize and maintain the thermocline. Using a solid storage medium and only needing one tank reduces the cost of this

system relative to the two-tank systems. This system was demonstrated at the Solar One central receiver CSP system in California, where steam was used as the heat transfer fluid and mineral oil was used as the storage fluid. A thermal energy storage system is a key component of CSP plants; however, it is one of the least researched elements with only a few tested high-temperature thermal energy storage systems in the world. Important criteria for designing a thermal energy storage system include the cost of the system, operating range of temperatures, maximum load or energy to be stored, operational strategy, and the ease with which the system can be integrated into the power plant. Technically the TES system should meet the following criteria (9): • have high energy density or heat storage capacity; • a mechanically and chemically stable storage medium (must be able to store and give heat for number of cycles without any degradation); • good heat transfer between the heat transfer fluid (HTF) and the storage medium (for fast heating and cooling of the system); • compatibility between the heat exchanger, heat transfer fluid, and storage medium; • complete reversibility or be able to store maximum energy and provide that energy for reuse and revert to the initial state; • minimum thermal losses. The present CSP power plants that incorporate energy storage use a two-tank sensible heat storage system based on synthetic oil or molten salts. The current near-term TES option has a unit cost of more than $30 to $40/kWhth depending on storage capacity (26). Other materials such as rocks, metals, or concrete (3,6,15,16) have been proposed but some are too expensive while others have technical problems at very high temperatures. The US Department of Energy has established a target to reduce the capital cost TES for CSP to less than $10/kWhth. An LCOE analysis of a CSP plant based on current CSP costs and $30/kWhth cost of TES using the SAM model developed by NREL (20) shows that a TES system with 16-h storage capacity can reduce the LCOE for a plant located in Daggett, California by more than 20% (Fig. 3). If the cost of TES goes down to $10/kWhth the

THERMAL ENERGY STORAGE

Figure 1. Schematic flow diagram of a parabolic trough power plant with two-tank molten salt storage (10,11).

Figure 2. Scheme of installation of a parabolic through power plant, with single-tank storage system (10).

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THERMAL ENERGY STORAGE A major classification of TES systems is based on the type of storage material used. Heat can be stored in the form of sensible, latent and thermochemical energy. Sensible Heat Storage

Figure 3. Effect of thermal storage cost on levelized cost of electricity (LCOE, cents/kWh) for a thermal storage system cost of $30/kWhth.

LCOE will be reduced by more than 30% (Fig. 4). This article presents a summary of current thermal energy storage systems being used or researched and a novel approach that is being researched at the University of South Florida in the development of low-cost thermal storage systems with a target cost of less than $15/kWhth.

Figure 4. Effect of thermal storage cost on levelized cost of electricity (LCOE, cents/kWh) for a thermal storage system cost of $10/kWhth.

Sensible heat storage is achieved by raising the temperature of a material: liquids such as water, oil-based liquids, molten salts, etc., or solids such as rocks, metals, and others. The amount of heat stored is a function of the medium’s heat capacity and is linearly dependent on the temperature increase. The larger the difference between the high-temperature and low-temperature system, the higher is the heat stored by the material. All of the currently installed thermal energy storage systems in solar thermal electric plants store energy use sensible heat. The current systems use two tanks with either oil or molten salt. Both oil and molten salt systems were found to be technically feasible. The SEGS I (Solar Electric Generating System I in California) storage system included a direct twotank thermal energy storage system with 3 h of fullload storage capacity. SEGS I operated between 1985 and 1989 (10). This system used a mineral oil (Caloria) HTF to store energy. The oil temperature was 307°C in the hot tank and 240°C in the cold tank with the oil representing 42% of the TES investment cost (9). The mineral oil used in SEGS I was very flammable, which was the main handicap for use as HTF in plants with higher operating temperatures. When the operating temperatures were increased to 400°C, the cost of the synthetic oil (that was stable at such high temperatures) was approximately eight times higher than the SEGS I oil. The cost and other important considerations like total system investment, very large tank size requirements, and inflexibility, compared to a back-up system, made oil usage infeasible in later SEGS plants. Use of molten salts has been demonstrated in the 10 MW Solar Two project (21) in California and is now being considered by companies in Spain, Italy, and US. The salts, however, generally have a high melting point (above 230°C). Therefore, the salt temperature must remain above 230°C at all times


to avoid solidification. To keep the salts liquid at night or during low insolation periods or during plant shutdowns, extra heating is required. Maintaining such high temperatures at all times also means heat losses, requiring more expensive piping and insulation materials. Alternate concepts that were considered by researchers include sensible heat storage in solid media like concrete (8,25). The advantage of concrete systems is low cost of the storage medium. The disadvantages are large volumes, increased costs of heat exchangers, and engineering. Table 1 shows some commonly used solid and liquid sensible heat storage media and the heat stored in them for a 100°C temperature rise. Latent Heat Thermal Storage (LHTS) Storage systems based on PCMs can be smaller, more efficient, and provide a lower cost alternative to sensible thermal storage systems. There have been many studies on solar TES systems using PCMs (8,19,23,28). Of the different forms of phase change processes, the solid–liquid transition is efficient in terms of low volumetric expansion compared to the liquid–gas transition and high latent heat compared to the solid–solid transition. During phase change, heat is stored in the medium without an increase in the temperature. Due to this reason, PCMs can store larger amounts of heat

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compared to sensible storage media, for the same operating temperatures. Several types of PCMs are available based on the type of application. For example, for melting ranges between 0°C and 200°C, PCMs such as paraffins, fatty acids, polymers, salt hydrates, and sugar alcohols may be used. For higher melting temperatures, salts, salt eutectics, high-performance polymers, metal alloys, and carbonates are available. Some high-temperature PCMs, their melting point, and the heat stored by them for a 100°C temperature rise are shown in Table 2. It is important for the TES system to have good heat transfer between the HTF and storage media, and also to have fast charging and discharging capability. Though PCMs can store large amounts of heat, most of them have low thermal conductivity, which leads to slow charging and discharging rates. This problem can be overcome by improving the effective thermal conductivity of the PCMs, which can be achieved by (17): 1) adding materials with high thermal conductivity to pure PCM; 2) forming small macrocapsules of PCM and enhancing convective heat transfer by submerging the PCM capsules in a liquid. Several methods (1,2,13,19) employed by researchers to enhance the heat transfer in PCMs include using extended surfaces, employing multiple PCMs, thermal conductivity enhancement using

Table 1. Some Sensible Heat Storage Media and Heat Stored by them for 100°C Temperature Rise (2,8)

Material Sand–rock–oil Reinforced concrete Cast iron Sodium chloride (NaCl) Cast steel Silica fire bricks Magnesia fire bricks Synthetic oil Nitrite salts Liquid sodium Silicone oil Lithium Dowtherm A Therminal 66

Density (kg/m3)

Heat Stored (kJ/kg)

Heat Stored (MJ/m3)

Type of Media

1700 2200 7200 2160 7800 1820 3000 900 1825 853 900 510 867 750

130 85 56 85 60 100 115 230 150 130 210 419 220 210

221 187 403 184 468 182 345 207 274 111 189 214 191 157

solid solid solid solid solid solid solid liquid liquid liquid liquid liquid liquid liquid

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Table 2. Some High Temperature Latent Heat Storage Media and Heat Stored by Them for 100°C Temperature Rise (29)

Material

Melting Point (°C)

Density (kg/m3)

Latent heat (kJ/kg)

Heat Stored (kJ/kg)

Heat Stored (MJ/m3)

NaNO3 KNO3 NaOH KOH MgCl2/KCl/NaCl FeCl2 KNO3/4.5% KCl 60% MgCl2 + 20.4% KCl + 19.6% NaCl 33% Na2CO3/35% K2CO3/Li2CO3 42.5% NaCl + 20.5% KCl + MgCl2 78% Al/12% Si

310 330 318 380 380 677 320 380 397 385–393 576

2260 2110 2100 2044 1800 2800 2100 1800 2300 1800 2700

172 266 165 150 400 266 150 400 277 410 560

354 388 373 297 496 336 271 496 442 510 710

800 819 783 607 893 941 569 893 1017 918 1917

metallic structures, PCM impregnated foams, dispersion of highly conductive particles, and encapsulation of PCM. Figure 5 shows some methods used for enhancing the heat transfer in PCM thermal storage systems. Other problems with using PCMs include supercooling, large volumetric changes during phase transition, and incongruent melting. Moreover, some of the PCMs such as salts are corrosive in nature. Storage Using Chemical Reactions Storage by means of chemical reactions has also been considered by many researchers for a wide range of temperatures (14) using reversible endothermic/exothermic reactions. Drawbacks may include complexity, cyclability, uncertainties in the thermodynamic properties of the reaction components and of the reaction kinetics under a wide range of operating conditions, high cost, toxicity, and flammability. Table 3 lists some thermochemical reactions that can be used for thermochemical heat storage. Implementation of storage technologies in CSP plants suffers from these bottlenecks. Hence, there is a need for developing an efficient TES system for achieving a cost-effective solution for successful implementation in solar power plants on a large scale. Based on the review of the available options, PCM storage can provide a costeffective solution, provided that industrially scalable containment is developed and the heat transfer enhancement can

be achieved at low cost. With this motivation, an innovative method to produce low-cost, high-temperature phase change material capsules is being developed at the USF Clean Energy Research Center, University of South Florida. These capsules are formed by encapsulating PCMs of interest in a coating material that melts at much higher temperature than the PCMs and which can be used as storage media for thermal energy storage in CSP. The following section describes the methodology being employed for the development of these capsules. The developed encapsulated PCMs have the potential to reduce the TES system costs to less than $15/kWhth. The proposed technique can also be used to produce encapsulated PCM capsules of different sizes and melting ranges for use in several energy storage applications such as space heating and cooling, solar cooking, solar water heating, industrial process heat, greenhouse, and waste heat recovery systems. DEVELOPMENT OF ENCAPSULATED PCM CAPSULES Figure 6 shows the pictorial representation of the capsules being developed. Initially porous pellets of PCMs with certain void space are fabricated. The void space will allow for the volumetric expansion during PCM melting and hence impose less stress on the shell material. The porous pellets are coated with the encapsulation material (or coating material) using low-cost coating techniques.


The developed capsules can be used in one-tank TES systems as shown in Figure 7. The heat is transferred to or from a heat transfer fluid as the heat transfer fluid flows through the space between the capsules. During charging, the hot fluid from the solar field is circulated through the tank. The PCM inside the capsules absorbs latent heat and melts. During the discharging mode, cooler heat transfer fluid is circulated through the tank to absorb heat from the PCM resulting in freezing of the encapsulated PCM. The heated fluid is then used to heat the power block working fluid through a heat exchanger.

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Selection of PCMs The PCM material can be selected depending on the melting temperature, latent and sensible heat capacities, thermal stability, mechanical stability, cyclic property degradation, heat transfer characteristics, and cost. It is important that the thermophysical properties be measured accurately as they will affect the heat stored in the system and its performance. The properties that were measured are the melting point, latent heat and thermal conductivity of individual PCMs, the porosity of the fabricated pellets, coating thickness of the encapsulated

Figure 5. Heat transfer enhancement procedures employed for various PCMs (2).

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Table 3. Thermochemical Storage Materials and Reactions

Temperature (°C) 530 (at 1 bar of reactant) 505 (at 1 bar of reactant) 896 (at 1 bar of reactant) 250–500 400–500 250–400 180 200–300 500–1000 200–250

Reaction

Material Energy Density

Reference

MnO2 + ∆H ↔ 0.5 Mn2O3 + 0.25 O2 Ca(OH)2 + ∆H ↔ CaO + H2O CaCO3 + ∆H ↔ CaO + CO2 MgH2 + ∆H ↔ Mg + H2 NH3 + ∆H ↔ 1/2N2 + 3/2H2 MgO + H2O ↔ Mg(OH)2 FeCO3 ↔ FeO + CO2 metal xH2 ↔ metal yH2 + (x − y)H2 CH4 + H2O ↔ CO + 3H2 CH3OH ↔ CO + 2H2

42 kJ/mol 3 GJ/m3 4.4 GJ/m3 75 kJ/mol 67 kJ/mol 3.3 GJ/m3 2.6 GJ/m3 4 GJ/m3 n.a. n.a.

22 22 22 4 18 18 27 7 7 24

pellet, and chemical compatibility of the coating material and the PCM. Different high-temperature salts/eutectic salt mixtures that melt that were considered and their latent heat and melting points were measured using differential scanning calorimetry (DSC) and thermal gravimetric analyzer (TGA) (SDT-600 from TA Instruments). Characterization results for some of the salts considered are shown in Table 4. It can be observed from the table that there is a large difference between the measured and published values of latent heats of the materials, which can be due to: purity of the sample, or presence of moisture in the case of hygroscopic materials. Hence, for accurate characterization of PCMs such as salts and salt eutectics, proper procedures must be followed, such as dehydration of individual components before preparing the eutectic, elimination of moisture from the sample, and DSC characterization in inert environment.

For encapsulation and other thermal measurements, sodium nitrate was used as the PCM. Thermal conductivity of this PCM (NaNO3) was measured using Linseis XFA500 and the result obtained was 0.56 W/m.K, which was comparable to literature value of 0.6 W/m.K (12).

Figure 6. Liquid PCM fills the pores during phase transition.

Figure 7. Direct contact TES system.

Preparation of PCM Pellets Once the PCM is selected, fabrication of pellets with required void space is important. Two different manufacturing methods were considered for


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Table 4. Phase Change Materials Considered for the Study

PCM NaNO3 KNO3 26.8% NaCl-73.2% NaOH 60wt% MgCl2-20.4% KCl-19.6% NaCl

Melting Temperature (°C)

Latent Heat of Fusion (kJ/kg)

Published (12)

Measured

Published (12)

Measured

307 333 370 380

303 334 354 383

172–177 88–266 370 400

153 108 138 238

preparation of porous pellets. The first procedure is called wet granulation through agglomeration that can produce pellets of different sizes. In this process, a liquid solution or a granulating fluid is added to powders resulting in the massing of a mix of dry primary powder particles to form large sized particles. The fluid contains a solvent that must be nontoxic yet volatile so that it can be removed by drying. Typical liquids include water, ethanol, and isopropanol either alone or in combination. The second procedure used for preparation of pellets is the table press method, which is similar to the briquetting process in which dry powder is pressed between dies to produce pellets of required shape and size. One advantage of this method is that it does not require binders and hence is suitable for PCMs that are hygroscopic. For pellet fabrication, sodium nitrate is used as the PCM and its volume expansion coefficient is around 10.7% (12). Figure 8 shows the NaNO3 pellets made in two different geometries: cylinders and spheres. Cylindrical pellets were made using

Figure 8. Pellets prepared from power press (cylindrical shaped) and pelletizer (spherical shaped).

appropriate dies in a bench top power press (ranging from 13 to 30 mm diameter). The spherical pellets of different diameters (2 to 10 mm) were formed using the Mars Minerals disc pelletizer. Porosity of the pellets fabricated using the above two methods was measured with the aid of the Sartorius YDK01MS apparatus. The porosity of the spherical pellets obtained from the pelletizer varies from 22% to 29%. Porosity of the pellets made using the tablet press method was found to be between 1% and 5% depending on the pressure applied in the press and the diameter of the die. Since this porosity is not sufficient to account for PCM expansion during phase change, the procedure was modified and two different pellets were joined to form one pellet with the required void space (between 20% and 35%) between them. Both spherical and cylindrical shaped pellets were made (Fig. 9). The extra space also allows for expansion of air during melting. Encapsulation There are several ways to encapsulate materials, such as precipitating the hard coat from the continuous phase via suspension polymerization, interfacial polymerization, spray drying, membrane emulsification, electroless metal coating, and ceramic coating. At present, high-temperature PCMs can be encapsulated in metallic cans; however, the costs of such encapsulation are high and unlikely to yield a system cost of less than $21/kWhth. The novel encapsulation method developed at USF involves encapsulation of PCMs with melting points between 300°C and 450°C, such as NaNO3 in a metal oxide (SiO2). The encapsulation process involves the following steps:

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Figure 9. Sodium nitrate pellets prepared using tablet press method or briquetting technique.

1. Coat the pellet with a precursor (stable to 500°C) using dip coating or spray coating, and cure at temperatures up to 250°C so that the pellet becomes insoluble in water as well as organic solvents (methanol, isopropyl alcohol, ether, acetonitrile, or acetone). 2. Coat the metal oxide (SiO2) over the pellet using self-assembly, hydrolysis, and simultaneous chemical oxidation at various temperatures.

can reduce the LCOE by as much as 20% with the current costs of TES systems and by more than 30% if the costs of TES is reduced to $10/kWhth. Ongoing research at USF is developing a system based on macroencapsulated PCM, which has the potential to reduce the cost of TES to less than $15/ kWhth. This development is based on an industrially scalable encapsulation technique. The process is being optimized in order to implement it successfully at a large scale.

A pellet with the SiO2 coatings with different precursors is shown in Figure 10. Capsules were made using the above procedure and tested for cyclic stability. The process is being modified to achieve uniform coatings and improve cyclic stability of the capsules.

ACKNOWLEDGMENTS: This work was partially funded by U.S. Department of Energy, E.ON Corporation, and the Florida Energy Systems Consortium (FESC). The authors declare no conflict of interest.

CONCLUSIONS

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Figure 10. SiO2 encapsulated PCM pellets with different precursors.

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