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

Abstract: The share of electricity generated by intermittent renewable energy sources is increasing (now at 26% of global electricity generation) and the requirements of affordable, reliable and secure energy supply designate grid-scale storage as an imperative component of most energy transition pathways. The most widely deployed bulk energy storage solution is pumped-hydro energy storage (PHES), however, this technology is geographically constrained. Alternatively, flow batteries are location independent and have higher energy densities than PHES, but remain associated with high costs and short lifetimes, which highlights the importance of developing and utilizing additional larger-scale, longer-duration and long-lifetime energy storage alternatives. In this paper, we review a class of promising bulk energy storage technologies based on thermo-mechanical principles, which includes: compressed-air energy storage, liquid-air energy storage and pumped-thermal electricity storage. The thermodynamic principles upon which these thermo-mechanical energy storage (TMES) technologies are based are discussed and a synopsis of recent progress in their development is presented, assessing their ability to provide reliable and cost-effective solutions. The current performance and future prospects of TMES systems are examined within a unified framework and a thermo-economic analysis is conducted to explore their competitiveness relative to each other as well as when compared to PHES and battery systems. This includes carefully selected thermodynamic and economic methodologies for estimating the component costs of each configuration in order to provide a detailed and fair comparison at various system sizes. The analysis reveals that the technical and economic characteristics of TMES systems are such that, especially at higher discharge power ratings and longer discharge durations, they can offer promising performance (round-trip efficiencies higher than 60%) along with long lifetimes (>30 years), low specific costs (often below 100 $ kWh−1), low ecological footprints and unique sector-coupling features compared to other storage options. TMES systems have significant potential for further progress and the thermo-economic comparisons in this paper can be used as a benchmark for their future evolution

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

Abstract: The share of electricity generated by intermittent renewable energy sources is increasing (now at 26% of global electricity generation) and the requirements of affordable, reliable and secure energy supply designate grid-scale storage as an imperative component of most energy transition pathways. The most widely deployed bulk energy storage solution is pumped-hydro energy storage (PHES), however, this technology is geographically constrained. Alternatively, flow batteries are location independent and have higher energy densities than PHES, but remain associated with high costs and short lifetimes, which highlights the importance of developing and utilizing additional larger-scale, longer-duration and long-lifetime energy storage alternatives. In this paper, we review a class of promising bulk energy storage technologies based on thermo-mechanical principles, which includes: compressed-air energy storage, liquid-air energy storage and pumped-thermal electricity storage. The thermodynamic principles upon which these thermo-mechanical energy storage (TMES) technologies are based are discussed and a synopsis of recent progress in their development is presented, assessing their ability to provide reliable and cost-effective solutions. The current performance and future prospects of TMES systems are examined within a unified framework and a thermo-economic analysis is conducted to explore their competitiveness relative to each other as well as when compared to PHES and battery systems. This includes carefully selected thermodynamic and economic methodologies for estimating the component costs of each configuration in order to provide a detailed and fair comparison at various system sizes. The analysis reveals that the technical and economic characteristics of TMES systems are such that, especially at higher discharge power ratings and longer discharge durations, they can offer promising performance (round-trip efficiencies higher than 60%) along with long lifetimes (>30 years), low specific costs (often below 100 $ kWh−1), low ecological footprints and unique sector-coupling features compared to other storage options. TMES systems have significant potential for further progress and the thermo-economic comparisons in this paper can be used as a benchmark for their future evolution

Introduction of PCM Flux as a Dynamic High Temperature Latent Heat Storage Concept

In times of a rising share of fluctuating renewables in the worldwide energy mix, there is a great need for energy storage units allowing harmonization of electricity production and consumption. Using phase change materials (PCM) to store heat for a flexible electricity generation with direct steam generating solar thermal power plants promises a good overall efficiency due to small temperature gradients that occur. This good efficiency, however, is associated with the need for expensive heat transfer structures (HTS) because of a poor thermal conductivity in PCMs. The required ratio of HTS and PCM to achieve an acceptable effective thermal conductivity is constant, and upscaling a storage unit is not expected to reduce costs significantly. With respect to the local fixation of the PCM, the system is defined as a static system. While discharging, the maximum heat flux decreases with time due to a growing and isolating layer of solidified PCM on the heat exchanger modules. One possibility to overcome this problem is the development of new dynamic concepts of PCM storage systems with moving PCM. In such systems, the isolating layer of PCM is slid away from areas with the highest heat transfer. The heat flux can thereby be controlled accurately by the PCM`s velocity. In this paper, PCM Flux as a completely new PCM storage concept is presented. PCM Flux is compared to other dynamic concepts and to the state-of-the-art static PCM storage system regarding possible heat flux and performance. The comparison outlines the outstanding properties of PCM Flux. Simulations with a specifically developed finite difference method tool modeling the movement of PCM, aluminum and steel, show a fully controllable heat flux with a peak power of 10 kW/m2 at a ΔT of 10K. Additionally, a negligible parasitics share of 0.13% arises in this storage system. The heat exchanger with its HTS only has to be designed for the nominal power output independent of the storage capacity. That is why the relative costs decline is inversely proportional to the storage size. The PCM Flux concept represents a PCM storage system with strict separation of power and capacity reducing the specific electricity costs of direct steam generating solar thermal power plants significantly

Proof-of-Concept and Advancement of the CellFlux Concept

The CellFlux storage system is a new concept for reducing the costs of medium to high temperature thermal energy storage. Initially designed for solar thermal power plants, the concept is suitable for industrial processes and power to heat applications as well. This paper gives first results of a new pilot scale plant set up at DLR in Stuttgart as a proof of concept. Experimental results are used for the validation of a simplified model. The model is apllied to calculate pareto optimal storage configurations in terms of necessary storage mass and exergetic efficiency, suitable for two types of solar thermal power plants. Particularly for applications having larger temperature differences, high exergetic efficiencies at low costs for the storage material can be achieved

Experimental demonstration of an active latent heat storage concept

Latent heat storage allows efficient energy storage in systems with isothermal processes. The low thermal conductivity of cost-effective storage materials is the main challenge in the development of latent heat storage systems. Most of these systems developed so far use extended heat transfer surfaces to ensure sufficient heat transfer rates. The PCMflux concept described in this paper is based on the transport of the storage material across the heat transfer surface. The aim of this approach is to avoid the blockage of the heat transfer surfaces by solidified storage material. The paper gives an overview of the current development of the PCMflux concept including the theoretical analysis and the experimental proof-ofconcept

Advanced high temperature latent heat storage system - design and test results

Processes with a two-phase heat transfer fluid (e.g. water/steam) require isothermal energy storage. Latent heat storage systems are an option to fulfil this demand. For high temperature applications nitrate salts are suitable materials for phase change storage. The main drawback of these materials is the low thermal conductivity, limiting the power density during the charging/discharging process. At DLR the so called sandwich-concept has been developed to realize latent heat storage with high power densities for applications in solar thermal power plants and process industry. This concept has already been demonstrated successfully for three different storage units ranging from 2-100 kW at melting temperatures of 142 °C and 222 °C. In 2008, a test storage using sodium nitrate as phase change material (PCM) with a melting temperature of 306 °C was operated in a 5 kW laboratory loop. The designed heat transfer rate was achieved and after 172 cycles no degradation was observed