THERMAL ENERGY STORAGE FOR CONCENTRATING SOLAR POWER PLANTS
Sarada Kuravi, D. 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 33620
Abstract
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 thermo-chemical reactions. This paper describes the
development of a cost effective 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 macro capsules 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.
Keywords: High temperature thermal energy storage systems, concentrating solar power plants,
novel encapsulation technologies, economical encapsulation
1. Introduction
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.
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 [1]. 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 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 lowtemperature 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 hightemperature 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 [2].
Figure 1: Schematic flow diagram of a parabolic trough power plant with two-tank molten salt storage [2]
Single-tank systems, mostly thermocline systems store thermal energy in a solid medium,
most commonly silica sand, in a single tank (Figure 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.
Figure 2: Scheme of installation of a parabolic through power plant, with single-tank storage system [2]
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 [3]:
 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 [4]. Other materials
such as rocks, metals, or concrete [5-8] 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 [9] shows that a TES system with 16 hour storage capacity can
reduce the LCOE for a plant located in Daggett, California by more than 20% (Figure 3). If the
cost of TES goes down to $10/kWhth the LCOE will be reduced by more than 30% (Figure 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 3: Effect of thermal storage cost on Levelized Cost of Electricity (LCOE, cents/kWh) for a thermal
storage system cost of $30/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
2. Thermal Energy Storage Materials
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.
2.1 Sensible heat storage
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 two-tank thermal energy storage system with three hours of full-load storage capacity.
SEGS I operated between 1985 and 1989 [9]. This system used a mineral oil (Caloria) HTF to
store energy. The oil temperature was 3070C in the hot tank and 2400C in the cold tank with the
oil representing 42% of the TES investment cost [3]. 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 4000C, 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 [10] in
California and is now being considered by companies in Spain, Italy and USA. The salts,
however, generally have a high melting point (above 2300C). Therefore, the salt temperature
must remain above 2300C 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 [11, 12]. 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 1000C temperature rise.
Table 1: Some Sensible Heat Storage Media and Heat Stored by them for 100 0C Temperature Rise [12, 13]
Material
Density
Heat
Heat
Type of
(kg/m3)
Stored
Stored
Media
(kJ/kg)
(MJ/m3)
Sand-Rock-Oil
1700
130
221
solid
Reinforced Concrete
2200
85
187
solid
Cast Iron
7200
56
403
solid
Sodium Chloride (NaCl)
2160
85
184
solid
Cast Steel
7800
60
468
solid
Silica fire bricks
1820
100
182
solid
Magnesia fire bricks
3000
115
345
solid
Synthetic Oil
900
230
207
liquid
Nitrite Salts
1825
150
274
liquid
Liquid Sodium
853
130
111
liquid
Silicone Oil
900
210
189
liquid
Lithium Liquid salt
510
419
214
liquid
Dowtherm A
867
220
191
liquid
Therminal 66
750
210
157
liquid
2.2 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 [12, 14-16]. Of the different forms of phase change processes, the solidliquid 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 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 00C and 2000C, 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 1000C temperature rise are shown in
Table 2.
Table 2: Some High Temperature Latent Heat Storage Media and Heat Stored by them for 100 0C
Temperature Rise [13]
Material
Melting
Density
Latent
Heat
Heat Stored
Point
(kg/m3)
heat
Stored
(MJ/m3)
(kJ/kg)
(kJ/kg)
(˚C)
NaNO3
310
2260
172
354
800
KNO3
330
2110
266
388
819
NaOH
318
2100
165
373
783
KOH
380
2044
150
297
607
MgCl2/KCl/NaCl
380
1800
400
496
893
FeCl2
304
2800
266
336
941
KNO3/4.5%KCl
320
2100
150
271
569
60% MgCl2 + 20.4%
380
1800
400
496
893
397
2300
277
442
1017
385-393
1800
410
510
918
576
2700
560
710
1917
KCl + 19.6% NaCl
33%Na2CO3/35%K2CO3
/Li2CO3
42.5%NaCl + 20.5%
KCl + MgCl2
78%Al/12 % Si
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 macro capsules of PCM and enhancing convective heat transfer by
submerging the PCM capsules in a liquid.
Several methods [14, 18-20] employed by researchers to enhance the heat transfer in PCMs
include using extended surfaces, employing multiple PCM’s, thermal conductivity enhancement
using 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.
Figure 5: Heat transfer Enhancement procedures employed for various PCMs [20]
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.
2.3 Storage using chemical reactions
Storage by means of chemical reactions has also been considered by many
researchers for a wide range of temperatures [21] 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.
Table 3: Thermochemical storage materials and reactions
Temperature (˚C)
Reaction
530 (at 1 bar of
reactant)
505 (at 1 bar of
reactant)
896 (at 1 bar of
reactant)
250-500
MnO2 + ΔH ↔ 0.5 Mn2O3 + 0.25 O2
[22]
Ca(OH)2 + ΔH ↔ CaO + H2O [22]
Material Energy
Density
42 kJ/mol
3 GJ/m3
CaCO3 + ΔH ↔ CaO + CO2 [22]
4.4 GJ/m3
MgH2 + ΔH ↔ Mg + H2 [23]
75 kJ/mol
400 – 500
NH3 + ΔH ↔ 1/2N2 + 3/2H2 [24]
67 kJ/mol
250 – 400
MgO + H2O ↔ Mg(OH)2 [21]
3.3 GJ/m3
180
200 – 300
FeCO3 ↔ FeO + CO2 [25]
Metal xH2 ↔ metal yH2 + (x-y)H2 [26]
2.6 GJ/m3
4 GJ/m3
500 – 1000
CH4 + H2O ↔ CO + 3H2 [26]
n.a.
200 – 250
CH3OH ↔ CO + 2H2 [27]
n.a.
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 cost- effective 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.
3. 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.
Figure 6: Liquid PCM fills the pores during phase transition
Figure 7: Direct contact TES system
3.1 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 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;

Presence of moisture in the case of hygroscopic materials.
Table 4: Phase change materials considered for the study
PCM
NaNO3
KNO3
26.8% NaCl – 73.2%
NaOH
60wt% MgCl220.4%KCl-19.6%NaCl
Melting temperature (⁰C)
Latent heat of fusion (kJ/kg)
Published [28]
307
333
370
Measured
303
334
354
Published [28]
172-177
88 - 266
370
Measured
153
108
138
380
383
400
238
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 [28].
3.2 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 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 which must be non-toxic 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% [28]. Figure 8 shows the NaNO3 pellets made in two different
geometries namely, cylinders and spheres. Cylindrical pellets were made using appropriate dies
in a bench top power press (ranging from 13 to 30 mm diameter). The spherical pellets of
different diameters (2mm to 10mm) were formed using the Mars Minerals disc pelletizer.
Figure 8: Pellets prepared from power press (cylindrical shaped) and pelletizer (spherical shaped)
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% to 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 20-35%) between them. Both spherical and cylindrical shaped
pellets were made (Figure 9). The extra space also allows for expansion of air during melting.
Figure 9: Sodium Nitrate Pellets prepared using Tablet Press Method or Briquetting technique
3.3 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/kWh th. The novel
encapsulation method developed at USF involves encapsulation of PCMs with melting points
between 3000C – 4500C, such as NaNO3 in a metal oxide (SiO2).
The encapsulation process involves the following steps:
1. Coat the pellet with a precursor (stable to 500oC) using dip coating or spray coating,
and cure at temperatures up to 250oC 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. A pellet with the SiO2 coatings with
different precursors is shown in Figure 10.
Figure 10: SiO2 encapsulated PCM pellets with different precursors
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.
4. Conclusions
Thermal Energy Storage can not only overcome the intermittency of solar energy
resource, but it 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 macro-encapsulated 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.
5. Acknowledgements
This work was partially funded by U.S. Department of Energy, E.ON Corporation and the
Florida Energy Systems Consortium (FESC).
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