Ultra-high temperature thermal energy storage. part 1: concepts

Abstract

Renewable energy sourced from the sun, wind, waves or tides is clean and secure. Unfortunately, the energy that can be extracted from renewables and the demand for it varies both temporally and spatially. Therefore, energy storage is required to match generation with use. To date grid-scale energy storage has been limited by low energy densities, long-term performance degradation, low round-trip efficiencies or limited deployment locations. Although thermal storage has found uses these have been restricted to lower temperatures by thermal losses resulting in low energy densities and uneconomical electricity generation efficiency. In this paper an ultra-high temperature (1800K) storage system is proposed where heat losses are minimised and recovered to make a higher storage temperature attractive, thus unlocking greater energy densities and efficiencies. Radiation dominates heat losses at ultra-high temperatures but can be minimised through the design of the storage medium container. However, even after energy is lost from storage, heat pumps in the store and charge cycles in addition to preheating during the extraction cycle can be used to recover a significant amount of heat. Collectively loss reduction and recovery techniques can lead to a storage system with a performance and utility that exceeds other energy storage methods. Here the feasibility of the novel storage technique is demonstrated through thermodynamic and thermal analysis in each of the three key states of operation: charge, store and generation. KeywordsThermal grid energy storage electric heat pump co-generation IntroductionCommon renewable generation methods rely on the sun, wind or waves, which vary temporally or spatially and do not follow demand. Grid-scale energy storage will be necessary to fully enable the operation of electricity networks with high penetrations of distributed renewable energy generation and realise targets for CO2 reduction. However, existing energy storage technologies are limited in their uptake and usefulness by low energy densities, long term performance degradation, low round-trip efficiencies or limited deployment locations [1].Thermal storage is a fully reversible process that does not have any of the by-products and degradation over multiple cycles seen in electrochemical storage approaches [2,3]. Until now thermal energy storage has been limited to a temperature of around 800 K [4], making it uncompetitive in terms of energy density and round-trip efficiency from heat to electricity. Consequently, thermal energy storage has only seen widespread deployment to collect heat for later re-use. By storing energy as heat at ultra-high temperatures (>1100K), it is possible to raise energy density and round-trip efficiency to the point where grid-scale thermal storage becomes technically and economically feasible.

Techno-economic analysis of solar PV power-to-heat-to-power storage and trigeneration in the residential sector

“…The evident way to reduce the losses is the use of advanced thermal insulation designs with a very small 𝑎 𝐻𝑇𝐸𝑆 value. Possible options include the use of vacuum insulation or other advance concepts, such as the use of heat pumps [47]. Whether such low 𝑎 𝐻𝑇𝐸𝑆 values are attainable by practical thermal insulation systems is out of the scope of this work.…”

“…However, it is important to notice that, even with such low self-consumption ratios, the PHPS-E system can provide significant electricity savings (> 70%) and reach profitability with reasonably short payback periods. The heat losses through the HTES thermal insulation system could be recovered as low-grade heat to further reduce the amount of fuel consumption, as proposed in [47], [48].…”

Progress and prospects of thermo-mechanical energy storage—a critical review

“…Robinson [141,142] developed a PTES cycle with the aim of using 'ultra-high' temperatures to maximize the round-trip efficiency and energy density. In this system, energy is stored in molten aluminium between 1500 • C and 900 • C and is used to power a gas turbine.…”

A Comparative Study of High-Temperature Latent Heat Storage Systems

“…Six alloys of aluminum and silicon were evaluated to examine their suitability as high temperatures (550-1200 • C) PCM for thermal storage [19]. Karim et al [20] proposed Al-Si-Ni and Mg-Si metallic eutectics as suitable high-temperature (above 900 • C) PCMs. Moreover, Si, Al, and B operating at temperatures >900 • C can have energy densities 20 times higher than commercialized inorganic salts.…”