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1
CHAPTER 12
THERMAL ENERGY STORAGE TECHNOLOGIES
Clifford K. Ho and Andrea Ambrosini, Sandia National Laboratories
Abstract
Thermal storage technologies have the potential to provide large capacity, long-duration storage
to enable high penetrations of intermittent renewable energy, flexible energy generation for
conventional baseload sources, and seasonal energy needs. Thermal storage options include
sensible, latent, and thermochemical technologies. Sensible thermal storage includes storing heat
in liquids such as molten salts and in solids such as concrete blocks, rocks, or sand-like particles.
Latent heat storage involves storing heat in a phase-change material that utilizes the large latent
heat of phase change during melting of a solid to a liquid. Thermochemical storage converts heat
into chemical bonds, which is reversible and beneficial for long-term storage applications. Current
research in each of the thermal storage technologies is described, along with remaining challenges
and future opportunities.
Key Terms
Thermal storage, sensible storage, latent storage, thermochemical storage, long-duration storage
1. Introduction
Increasing penetrations of intermittent renewable energy sources (e.g., photovoltaics [PV] and
wind energy) have increased the need for energy storage technologies to accommodate daily
periods of overgeneration and peak loads. These diurnal energy-storage requirements are
categorized in this chapter as short-duration and span periods from seconds to hours with capacities
ranging from kilowatts to gigawatts. Previous studies have suggested that the decreasing costs of
batteries and associated technologies may enable battery systems to meet the short-duration needs
of the grid with high penetrations of intermittent renewable energy systems [1, 2]. However, recent
studies have shown that long-duration energy storage (days to months) will be needed to
accommodate 100% renewable (or carbon-free) energy generation [3]. In addition, long-duration
energy storage will be needed to increase the security and resilience of the electrical grid in the
face of increasing natural disasters and intentional threats.
1.1. Thermal Storage Applications
Figure 1 shows a chart of current energy storage technologies as a function of discharge times and
power capacity for short-duration energy storage [4]. Within the range of short-duration energy
storage capacities, applications include reserve and response services (1–100 kW), transmission
and distribution support grid (100 kW–10 MW), and bulk power management (10 MW–1 GW).
Although thermal storage technology is included in the chart as cryogenic energy storage, hot
thermal storage using sensible, latent, or thermochemical methods [5, 6] is not shown. Commercial
concentrating solar power (CSP) using sensible heat storage has demonstrated the ability to
provide on the order of 100 MW of power capacity over 10 hours (~1 GWh) for both grid support
and bulk power management.
Thermal storage technologies are also being considered for nuclear power plants to increase the
flexibility of these traditionally baseload systems [6]. At times of low or negative electricity prices,
Chapter 12 Thermal Energy Storage
2
heat (or electricity) generated by the nuclear reactor would be sent to thermal storage. At times of
high electricity prices, the heat from the reactor and thermal storage would be used to produce
maximum electricity output (Figure 2). New Generation IV nuclear reactors deliver higher
temperatures to the power cycle relative to water-cooled reactors, which is beneficial for thermal
storage because at higher temperatures, less storage material is required to deliver a desired amount
of thermal power. In addition, the higher temperatures enable more efficient thermal-to-electric
power conversion. Adding thermal energy storage to geothermal power plants to increase
flexibility and dispatchability has also been considered [7].
Figure 1. Discharge time and capacity of various energy storage technologies [4]. Hot thermal
storage technologies are not shown but can provide hundreds of megawatts for many hours
Chapter 12 Thermal Energy Storage
3
Figure 2. Diagram illustrating how thermal storage can increase the flexibility of traditional
baseload power plants that rely on thermal energy [6].
1.2. Technology Overview
The remainder of this chapter provides a summary of thermal storage technologies, which can
include sensible, latent, and thermochemical systems. Sensible storage relies on a temperature
difference within the storage medium to enable useful work to be performed, such as using hot
molten salt to heat water and generate steam to spin a turbine for electricity production. Latent
storage involves storing heat in a phase-change material that utilizes the large latent heat of phase
change, for example, during isothermal melting of a solid to a liquid, which requires heat, and
subsequent freezing of the liquid to a solid, which releases heat, isothermally. Thermochemical
energy storage (TCES) reversibly converts heat into chemical bonds using a reactive storage
medium. When the energy is needed, a reverse reaction combines the reactants, releasing energy.
Table 1 summarizes the different thermal storage technologies and key attributes.
Table 1. Summary of thermal storage technologies
Sensible Heat Storage
[5, 8-12]
Latent Heat Storage
[5, 9, 10, 12, 13]
Thermochemical Storage
[9, 11, 13]
Storage
mechanism
Energy stored as
temperature difference in
solid (e.g., concrete, rock,
sand) or liquid media
(molten salt)
Energy stored using phase
change materials (e.g., salts,
metals, organics)
Energy stored in
chemical bonds
Energy
Density
• ~200 – 500 kJ/kg (for
~200 – 400 °C
temperature differential)
• ~100 – 200 kJ/kg for
nitrate salts; ~200 – 500
kJ/kg for metals; ~1000
kJ/kg for fluoride salts
• ~300 – 6,000 kJ/kg
Chapter 12 Thermal Energy Storage
4
Sensible Heat Storage
[5, 8-12]
Latent Heat Storage
[5, 9, 10, 12, 13]
Thermochemical Storage
[9, 11, 13]
Advantages
• Demonstrated large
energy capacity (~GWh)
• Inexpensive media
• Solid media does not
freeze and can achieve
>1000°C
• Good for isothermal or
low T applications
• Can provide large energy
density with combined
sensible and latent heat
storage
• Large energy densities
• Small heat losses
• Potential for long-
term storage
• Compact storage
system
• Oxide TCES Stable at
high temperatures (>
1000°C)
Challenges
• Requires insulation to
mitigate heat losses
• Lower energy density
requires larger volumes
• Molten salts freeze at
~200 °C.
• Potential for corrosion
• For larger T, may need
cascaded systems (adds
costs and complexity)
• Low maturity
• Higher complexity
• Low maturity
• Higher capital costs
• May require storage
of gaseous products
Maturity
High
Low
Low
Cost
• ~$1/kg for molten salts
and ceramic particles
• ~$0.1/kg for rock and
sands
• ~$1/MJ – $10/MJ
(system capital cost)
• ~$4/kg – $300/kg
• ~$10/MJ – $100/MJ
(system capital cost)
• ~$10/MJ – $100/MJ
(system capital cost)
2. State of Current Technology
2.1. Sensible heat storage
Sensible heat storage consists of heating a material to increase its internal energy. The resulting
temperature difference, together with thermophysical properties (density, specific heat) and
volume of storage material, determine its energy capacity (J or kWh):
()
H
C
T
sensible p
T
E V c T dT
=
(1)
Desirable features of sensible storage materials include large densities, (kg/m
3
), large specific
heats, c
p
(J/kg-K), and large temperature differences between the hot and cold states, T
H
– T
C
(K).
Key advantages include a low cost of sensible storage materials, high maturity level, and large
energy capacities. Table 2 provides a summary of thermophysical properties of various sensible
solid and liquid storage media.
Chapter 12 Thermal Energy Storage
5
Table 2. Thermophysical properties of sensible storage media (adapted from [5]). Calculation of
volumetric and gravimetric storage densities assume a temperature differential of 350°C.
Storage Medium
Specific
Heat
(kJ/kg-K)
Density
(kg/m
3
)
Temperature
Range (°C)
Cold Hot
Gravimetric
Storage
Density
(kJ/kg)
Volumetric
Storage Density
(MJ/m
3
)
Solids
Concrete
0.9
2200
200
400
315
693
Sintered bauxite particles
1.1
2000
400
1000
385
770
NaCI
0.9
2160
200
500
315
680
Cast iron
0.6
7200
200
400
210
1512
Cast steel
0.6
7800
200
700
210
1638
Silica fire bricks
1
1820
200
700
350
637
Magnesia fire bricks
1.2
3000
200
1200
420
1260
Graphite
1.9
1700
500
850
665
1131
Aluminum oxide
1.3
4000
200
700
455
1820
Slag
0.84
2700
200
700
294
794
Liquids
Nitrate salts
(ex. KNO
3
-0.46NaNO
3
)
1.6
1815
300
600
560
1016
Therminol VP-1 ®
2.5
750
300
400
875
656
Silicone oil
2.1
900
300
400
735
662
Carbonate salts
1.8
2100
450
850
630
1323
Caloria HT-43 ®
2.8
690
150
316
980
676
Sodium liquid metal
1.3
960
316
700
455
437
Na-0.79K metal eutectic
1.1
900
300
700
385
347
Hydroxide salts (ex. NaOH)
2.1
1700
350
1100
735
1250
Silicon
0.71
2300
1900
2400
250
575
Commercial CSP plants that employ sensible thermal storage with over 1 GWh of storage have
been deployed worldwide. For comparison, Figure 3 shows the total number of large-scale battery
demonstration facilities in the United States at the end of 2017 along with two CSP plants. Each
CSP plant provides more energy storage capacity than all ~100 PV demonstration facilities
combined.
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