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| Fig. 1: Schematic of a two-tank molten-salt thermal energy storage system with cold and hot storage tanks, solar receiver, heliostat field, and steam-generation. (Image source: N. Ly, after Yahya et al. [11]) |
Renewable power generation introduces timing challenges for modern grids. Solar and wind energy fluctuates according to weather and time of day, often producing power when demand is low and falling short during peak times. Measured PV output can fluctuate sharply over short intervals with residential systems ramping up to 13% in just five minutes while medium and large commercial arrays can vary by over 60% in the same period, showing how quickly solar generation can rise or fall in response to demand. [1] Lithium-ion batteries have emerged as a technology to address the discrepancies as it has fast response times and high round-trip efficiencies. However, their performance slows down over repeated cycling, their cost increases with longer storage durations, and limitations with large installations.
Molten-salt thermal storage can store large amounts of energy at lower cost and discharge over many hours. Molten-salt thermal storage is already commercially deployed in utility applications such as parabolic-trough solar power plants. Molten salts possess a high volumetric heat capacity and are capable of storing very large amounts of thermal energy. Standard two-tank molten-salt systems can provide between 6 and 15 hours of storage capacity, allowing them to discharge over many hours when renewable generation declines. [2] Here we compare lithium-ion battery systems and molten-salt thermal storage to evaluate which technology offers the greater viability for large-scale renewable integration.
Thermal energy storage using molten salts is a proven method for providing electricity supply from renewables that do not produce power continuously. In this system, energy is stored as sensible heat, meaning the temperature of the molten salt increases during charging and decreases during discharging. [3] Most molten-salt formulations used for thermal energy storage operate between 240-600 °C, with standard solar salt melting at ~240C and remaining stable up to 600°C. [3]
The most common working fluid is a nitrate mixture known as solar salt, typically composed of 60% sodium nitrate (NaNO3) and 40% potassium nitrate (KNO3). [4] Solar salt is used because it is non-flammable, low cost, and has a high heat capacity. Their low vapor pressure also allows molten-salt tanks to operate unpressurized, reducing overall system cost. [3]
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| Fig. 2: Thermal energy storage cost as a function of operating temperature difference for multiple molten-salt configurations. (Image source: N. Ly, after Jacob et al. [12]) |
A molten-salt system typically is made up of a cold tank and hot tank. The cold tank holds salt at the lower end of the operating temperature and a hot tank stores salt after heating. During charging, cold molten salt is pumped upward to a heat source that raises the temperature. The heated salt then flows down into the hot tank for storage. [5] Hot molten salt from the hot tank is pumped into the evaporator to heat the water for producing steam, which runs the turbine to generate electricity. This dual tank configuration ensures the continuous power generation even during nighttime and cloudy days. The molten salt stored in the hot tank can sustain operation for hours. [5]
Molten-Salt TES systems can be built in direct or indirect configurations. Direct systems use molten salt as both the heat transfer fluid and the storage medium. Indirect systems use a separate heat transfer fluid and transfer heat to molten salt through an exchanger, which can contribute to heat loss. An alternative to the two-tank system is the thermocline configuration, which stores hot and cold molten salt in a single tank. This reduces costs and the thermocline systems demonstrate up to 35% lower cost. [2]
Lithium-ion batteries have become the dominant technology for grid-scale energy storage because of their high energy density, long cycle life, and rapid charge-discharge capability. Li-ion cells can store large amounts of energy due to their reversible lithium-ion intercalation chemistry, where ions shuttle between layered transition-metal oxides and anodes. [6] Experimental data from a full-scale grid connected 822 kWh NMC lithium-ion system shows that lithium batteries deliver high round trip efficiency but experience performance at lower power and losses over time. When auxiliary systems such as HVAC and controls are used, the global system efficiency (true measure relevant to grid integration) averages 85% at high power, but falls to 60% during slower rate operation. [6] Their rapid discharge capability is essential for high-power output with fast response times. [7] However, Li-ion systems typically deliver 2-4 hours of economically viable storage, with costs increasing beyond this. [7]
Degradation plays a major role in performance. The lithium-ion batteries state of health declined an average annual capacity loss of 1.37%. [6] Li-ion cells are susceptible to thermal runaway, which is an exothermic process that causes internal temperature to increase rapidly, so the system requires many regulators. [6] Overall, lithium-ion batteries have high efficiency and fast response but are limited by degradation, reduced efficiency at low power, and higher costs.
Dersch et al. provided a specific investment cost for the thermal storage block of a molten-salt tower plant of $23.1/kWhth. [8] They accounted for net thermal storage capacity, salt, melting, insulation, tanks, pumps, and other markups. As shown in Fig. 2, the cost of the two-tank molten-salt thermal storage falls with increasing temperature difference, reaching the value we chose at higher temperature.
To convert thermal storage cost into a cost per delivered kWh of electricity, we shall use the round-trip efficiency on the low end of 36.25% (ηMS = 0.3625) from the simulation of a two-tank molten-salt heat storage system integrated with a 600 MW plant. [9]
The effective cost per kWh delivered is
| $23.1 kWh-1 0.3625 |
= | $63.724 kWh-1 |
Under optimized operating temperatures (309C), the round-trip efficiency can reach 69.88%, reducing the cost to:
| $23.1 kWh-1 0.6988 |
= | $33 kWh-1 |
For lithium-ion batteries, we use the cost trajectory analysis by Orangi, which reports a 2020 average Li-ion cell cost of $102.5/kWh. [10] The round trip efficiency of lithium-ion batteries averages 85% at high power. [6] The effective cost per delivered kWh is thus
| $102.5 kWh-1 0.85 |
= | $120..59 kWh-1 |
Even though molten-salt has a lower round-trip efficiency, its much lower capital cost per kWh of storage capacity results in a lower overall cost per kWh delivered. Molten salt is advantageous for long duration and bulk energy, while lithium-ion batteries remain better for short duration and high efficiency.
The nitrate salts used as a storage medium are the same compounds in global fertilizer production. Their high abundance stands in contrast to the supply chains of lithium, nickel, etc that dominate electrochemical storage technologies. Molten salt relies on available materials that can be produced at industrial scale without specialized mining, so it offers a pathway for long duration renewable energy storage in areas that lack access to more advanced processes.
Molten-salt thermal storage offers a path toward long-duration energy storage that supports renewable power. Its ability to store heat inexpensively over many hours allows for energy generation even during fluctuations in supply and demand. Lithium-ion batteries will play an important role in fast response and short duration applications, but their higher effective costs, degradation, and materials limit overall energy generation. As renewable energy becomes a larger share of the energy supply, molten-salt systems will provide a stable power delivery system.
© Nathan Ly. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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