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| Fig. 1: Schematics of a Pumped Thermal Energy Storage system. (Courtesy of the DOE) |
With the rapid transition towards sustainable energy systems, Long-duration grid storage (LDGS) serves as a key enabler for the efficient and reliable management of variable energy generation and consumption patterns. By providing the capability to store excess energy during peak production periods and release it during times of high demand, LDGS ensures a consistent and stable power supply, mitigating the intermittency issues inherent in renewable energy sources like solar and wind. This capability is instrumental in enhancing grid resilience, reducing reliance on fossil fuels, and ultimately advancing the goals of sustainable development and climate action.
Batteries, pumped hydro storage, compressed air energy storage, and power-to-fuel technologies (including hydrogen and synthetic fuels) are among the most prevalent LDGS solutions, showing great promise for the future. Adding to the mix of solutions, Pumped Heat Energy Storage has the potential to offer low-cost and highly efficient energy storage for the grid. This paper examines its potential as an LDGS solution from an economic perspective.
Pumped Thermal Energy Storage or Pumped Thermal Electricity Storage (PTES) is a technology that uses electricity to store energy as heat, and then converts it back to electricity on demand. It is similar to pumped hydro storage, but instead of pumping water uphill, PTES pumps heat from one reservoir to another. PTES has several advantages over other energy storage technologies, such as batteries, including high energy capacity, long storage duration, high round-trip efficiency, and environmental friendliness. PTES is still in its early stages of development, but it has the potential to play a major role in the transition to a clean energy future.
Fig. 1(a) shows the working principle of a PTES system and Fig. 1(b) shows a process flow diagram of the same system. Fig. 1(a) shows that in the charging cycle, the heat pump (HP) uses electricity as work (Wchg) to extracts heat Qcchg from a low-temperature reservoir and delivers heat Qhchg to a high-temperature reservoir. The efficiency of this step is quantified by Coefficient of Performance (COP), where COP= Qhchg / Qcchg. In the discharging cycle, heat flows from the high- temperature reservoir to the low-temperature reservoir and work Wdis = Qhdis - Qcdis can be extracted via a heat engine (HE) due to the thermal potential. The heat engine efficiency is quantified by η = Qhdis / Qcdis. There is also an efficiency loss due to heat loss at the thermal energy storage, ξ. The round-trip efficiency of such system is therefore χ = COP × η × ξ.
A process flow diagram in Fig. 1(b) shows the practical steps of a Carnot Battery. In a charging cycle, a working fluid such as air, argon, or supercritical CO2 is first compressed to high temperature and pressure (step 1 to 2). Heat generated is then transferred to a thermal energy storage media such as molten salt (step 2 to 3), and the working fluid is cooled down as a result. [1] The cooled and high-pressure working fluid is then expanded to its original pressure in a turbine which further lowers its temperature (step 3 to 4). The cold fluid is used to cool down a different thermal energy storage material (step 4 to 1), which turn the working fluid back to its original temperature. In a discharging cycle, the system is run in reverse. The higher the temperature difference between the hot and cold storage material is, the higher the overall efficiency is. [1]
Laughlin estimated that the maximum achievable round-trip efficiency of PTES is around 72%. [2] Viswanathan et al. compared the LCOS of PTES with other electricity storage technologies based on 2021 cost estimate and 2030 cost estimate. [3] In both 2021 and 2030 cost estimates, the LCOS of PTES is similar to LFP lithium ion batteries in the 100 MW and 1000 MW capacity with a storage duration between 4 hours and 10 hours. It is in 24 hour and 100 hour storage duration that PTES has a cost advantage over LFP lithium ion batteries, especially in the 100 hour case. Due to the heat loss over extended period, PTES may not be most suited for seasonal energy storage.
Twitchell et al. reviewed current uses of energy storage and how those uses are changing in response to emerging grid needs. [4] They identified that two classes of long-duration grid storage will be needed in a decarbonized grid based on electricity surplus and deficit: one class lasting up to 20 hours to manage daily cycles and one lasting for weeks or months to manage seasonal cycles.
In addition to intra-day and seasonal energy storage requirements, Guerra identifies the need for multi-day energy storage as well. [5]
Given the long-duration energy storage requirements and the cost competitiveness of PTES against other storage technologies, PTES could be cost competitive for longer end of intra-day storage (close to 24 hours) and multi-day storage. As a result, PTES as an emerging energy storage solution should be further developed to meet the future energy storage needs.
© Yutong Zhu. 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.
[1] J. Martinek, J. Jorgenson, and J. D. McTigue, "On the Operational Characteristics and Economic Value of Pumped Thermal Energy Storage," J. Energy Storage 52, 105005 (2022).
[2] R. B. Laughlin, "Pumped Thermal Grid Storage With Heat Exchange," J. Renew. Sustain. Energy 9, 0441103 (2017).
[3] V. Viswanathan et al., "2022 Grid Energy Storage Technology Cost and Performance Assessment," Pacific Northwest National Laboratory, PNNL-33283, August 2022.
[4] J. Twitchell, K. DeSomber, and D. Bhatnagar, "Defining Long Duration Energy Storage," Journal of Energy Storage 60 105787(2023).
[5] O. J. Guerra, "Beyond Short-Duration Energy Storage," Nature Energy 6, 40 (2021).