The traditional solution to intermittency of renewable energy from wind and solar in the form of fossil fueled backup contains the contradiction of a need for fossil fuels in the climate action effort to replace fossil fuels with renewable energy. Therefore, from the anti fossil fuel position of climate science, it is necessary to find alternative solutions to the intermittency issue in wind and solar renewable energy. Although pumped hydro has been traditionally used for this purpose, this solution is location specific and is not a global solution. In the papers reviewed above and those listed in the bibliography below, are described new technologies currently under development that may offer a purely renewable energy storage solution to address intermittency in wind and solar without the contradiction of the need for fossil fuels to move the global energy infrastructure away from fossil fuels in a climate action effort to limit the temperature rise in anthropogenic global warming to 2C since pre-industrial.
Foremost among these energy storage technologies proposed in the the last decade, 2010-2020, are Thermal Energy Storage (TES), Pumped Heat Electricity Storage (PHES), and thermal energy storage with Phase Change Material (PCM). These alternatives to pumped hydro are currently under development with pumped hydro being currently the only storage technology available to address the intermittency issue in renewable energy. The Henry (2017) paper presented above addresses two TES technologies citing the Caleb (2017) and Laughlin (2017) where energy storage is envisioned in molten metals and in molten salt. Advances in TES technology and engineering as well ongoing developments in PCM and PHES are described in the bibliography below over the period from 2011 to 2020. Advances in the science, technology, and engineering of such energy storage systems are encouraging and may one day lead to a solution to intermittency of wind and solar power in the absence of pumped hydro.

PHES. PCM, AND TES BIBLIOGRAPHY

1. Thess, André. “Thermodynamic efficiency of pumped heat electricity storage.” Physical review letters 111.11 (2013): 110602.  Pumped heat electricity storage (PHES) has been recently suggested as a potential solution to the large-scale energy storage problem. PHES requires neither underground caverns as compressed air energy storage (CAES) nor kilometer-sized water reservoirs like pumped hydrostorage and can therefore be constructed anywhere in the world. However, since no large PHES system exists yet, and theoretical predictions are scarce, the efficiency of such systems is unknown. Here we formulate a simple thermodynamic model that predicts the efficiency of PHES as a function of the temperature of the thermal energy storage at maximum output power. The resulting equation is free of adjustable parameters and nearly as simple as the well-known Carnot formula. Our theory predicts that for storage temperatures above 400 °C PHES has a higher efficiency than existing CAES and that PHES can even compete with the efficiencies predicted for advanced-adiabatic CAES.
2. Howes, Jonathan. “Concept and development of a pumped heat electricity storage device.” Proceedings of the IEEE 100.2 (2011): 493-503.  This paper addresses the early conceptualization of a system for reversible heat/work conversion based upon the heat engine cycle, developed in 1833 by John Ericsson, in combination with utility scale thermal energy storage in particulate mineral (e.g., gravel) and the development and test of the first prototype. Using these test results, mathematical modeling of the engine/heat pump has yielded improved second and third prototypes. Design of the second prototype and its behavior under test is discussed. Extant test results are used to extrapolate to the predicted performance of utility scale equipment.
3. Ni, Fan, and Hugo S. Caram. “Analysis of pumped heat electricity storage process using exponential matrix solutions.” Applied Thermal Engineering 84 (2015): 34-44.  Pumped heat electricity storage (PHES) is a recently proposed competitive energy storage solution for large scale electrical energy storage (EES). It is especially valuable for regions where specific geological structures are not available. The performance of PHES depends on two factors: the operations of turbomachines and the thermal storage system. The former is characterized by pressure ratio, polytropic efficiency and gas heat capacity ratio. The latter contains the parameters of heat regenerators that can be summarized into two dimensionless numbers: length Λ and step time π. The overall process operation can be described by temperature difference representing the energy stored per unit heat capacity, the storage bed utilization ratio and the turn-around efficiency. Exponential matrix solutions are obtained for a discretized heat transfer model of a typical pumped heat electricity storage process. Using the cyclic steady state and transient state solutions, we are able to analyze how dimensionless length and step time affect the storage bed utilization ratio as well as the turn-around efficiency. This model provides basic guidance for further development of such processes.
4. Roskosch, Dennis, and Burak Atakan. “Pumped heat electricity storage: potential analysis and orc requirements.” Energy Procedia 129 (2017): 1026-1033.  The rising share of renewable energy sources in power generation leads to the need of energy storage capacities. In this context, also some interest in thermal energy storages, especially in a concept called pumped heat electricity storage (PHES), arises. One possible design of such a PHES system consists of a compression heat pump, a thermal storage and an organic Rankine cycle (ORC). The present work analyses the general thermodynamic potential and limits of such a system and deals with the unusual requirements for the ORC. The potential analysis starts with the optimal case of combining two Carnot cycles with irreversible heat transfer. It is found that the efficiency of the entire process increases with increasing storage temperature and in general roundtrip efficiencies up to 70 % are predicted. Afterwards the cycles are transferred to cycles that are more realistic by considering technical aspects and a hypothetical working fluid which is optimized by an inverse engineering approach. This leads to lowered roundtrip efficiencies, which now, decrease with increasing storage temperatures. In a second step the specific ORC requirements as part of a PHES are considered, emphasizing the working fluid parameters. Especially, the use of a latent thermal energy storage leads to an ORC design differing from common (e.g. geothermal) applications. It is shown that the efficiency of the ORC and of the entire process strongly depends on the superheating at the expander inlet; here, the superheating must be held as small as possible, contrary to ORCs using common heat sources. [FULL TEXT PDF]
5. Dietrich, Axel, Frank Dammel, and Peter Stephan. “Exergoeconomic Analysis of a Pumped Heat Electricity Storage System with Concrete Thermal Energy Storage.” International Journal of Thermodynamics 19.1 (2016).  Within the last 25 years the share of renewable energy sources in electrical energy production in Germany has been rising considerably. The volatility of renewable energy sources results in an increasing mismatch between supply and demand of electrical energy creating the need for storage capacities. The storage of electrical energy via the detour of thermal energy can be realized by a relatively new technology known as Pumped Heat Electricity Storage systems. This paper examines the exergoeconomic performance of such a storage system. A sample system comprising a concrete thermal energy storage is introduced; unsteady operations are simulated and analyzed. Although the achieved efficiencies are reasonable economical operations of the analyzed Pumped Heat Electricity Storage System are currently not possible. For the analyzed operation scenario the exergetic system efficiency, electrical energy output to electrical energy input, amounts to 27:3%. Considering the storage capacity and the lack of geological requirements the Pumped Heat Electricity Storage system can compete with pumped hydro and compressed air energy storage. However, prices of the order of 60 ct/(kWh) are not competitive considering current energy prices. Based on improved system designs as well as rising energy prices we assess Pumped Heat Electricity Storage Systems as a potential alternative to established storage technologies.
6. Levelised Cost of Storage for Pumped Heat Energy Storage in comparison with other energy storage technologies, AndrewSmallbonea, VerenaJülchb, RobinWardlea, Anthony PaulRoskillyaa.  Sir Joseph Swan Centre for Energy Research, Newcastle University, Newcastle upon UK, Fraunhofer Insitute for Solar Energy Systems ISE, Freiburg, Germany. 2017.  ABSTRACT:  Future electricity systems which plan to use large proportions of intermittent (e.g. wind, solar or tidal generation) or inflexible (e.g. nuclear, coal, etc.) electricity generation sources require an increasing scale-up of energy storage to match the supply with hourly, daily and seasonal electricity demand profiles. Evaluation of how to meet this scale of energy storage has predominantly been based on the deployment of a handful of technologies including batteries, Pumped Hydro, Compressed Air Energy Storage and Power-to-Gas. However, for technical, confidentiality and data availability reasons the majority of such analyses have been unable to properly consider and have therefore neglected the potential of Pumped Heat Energy Storage, which has thus not been benchmarked or considered in a much detail relative to competitive solutions. This paper presents an economic analysis of a Pumped Heat Energy Storage system using data obtained during the development of the world’s first grid-scale demonstrator project. A Pumped Heat Energy Storage system stores electricity in the form of thermal energy using a proprietary reversible heat pump (engine) by compressing and expanding gas. Two thermal storage tanks are used to store heat at the temperature of the hot and cold gas. Using the Levelised Cost of Storage method, the cost of stored electricity of a demonstration plant proved to be between 2.7 and 5.0 €ct/kW h, depending on the assumptions considered. The Levelised Cost of Storage of Pumped Heat Energy Storage was then compared to other energy storage technologies at 100 MW and 400 MW h scales. The results show that Pumped Heat Energy Storage is cost-competitive with Compressed Air Energy Storage systems and may be even cost-competitive with Pumped Hydroelectricity Storage with the additional advantage of full flexibility for location. As with all other technologies, the Levelised Cost of Storage proved strongly dependent on the number of storage cycles per year. The low specific cost per storage capacity of Pumped Heat Energy Storage indicated that the technology could also be a valid option for long-term storage, even though it was designed for short-term operation. Based on the resulting Levelised Cost of Storage, Pumped Heat Energy Storage should be considered a cost-effective solution for electricity storage. However, the analysis did highlight that the Levelised Cost of Storage of a Pumped Heat Energy Storage system is sensitive to assumptions on capital expenditure and round trip efficiencies, emphasising a need for further empirical evidence at grid-scale and detailed cost analysis. [FULL TEXT PDF]
7. Arce, Pablo, et al. “Overview of thermal energy storage (TES) potential energy savings and climate change mitigation in Spain and Europe.” Applied energy 88.8 (2011): 2764-2774.  Thermal energy storage (TES) is nowadays presented as one of the most feasible solutions in achieving energy savings and environmentally correct behaviors. Its potential applications have led to R&D activities and to the development of various technologies. However, so far there is no available data on a national scale in Spain and on a continental level in Europe, to corroborate the associated energetic and environmental benefits derived from TES. This is why, based on a previous potential calculation initiative model performed in Germany, this work intends to provide a first overview of the Spanish TES potential as well as an European overview. Load reductions, energy savings, and CO₂ emissions reductions are tackled for the buildings and industrial sector. Results depend on the amount of implementation and show that, in the case of Europe for instance, yearly CO₂ emissions may get to be cut down up to around 6% in reference to 1990 emission levels.
8. Zhou, Zhihua, et al. “Phase change materials for solar thermal energy storage in residential buildings in cold climate.” Renewable and Sustainable Energy Reviews 48 (2015): 692-703.  Heating accounts for a large proportion of energy consumption in residential buildings located in cold climate. Solar energy plays an important role in responding to the growing demand of energy as well as dealing with pressing climate change and air pollution issues. Solar energy is featured with low-density and intermittency, therefore an appropriate storage method is required. This paper reports a critical review of existing studies on thermal storage systems that employ various methods. Latent heat storage using phase change materials (PCMs) is one of the most effective methods to store thermal energy, and it can significantly reduce area for solar collector. During the application of PCM, the solid–liquid phase change can be used to store a large quantity of energy where the selection of the PCM is most critical. A numerical study is presented in this study to explore the effectiveness of NH₄Al(SO₄)₂·12H₂O as a new inorganic phase change material (PCM). Its characteristics and heat transfer patterns were studied by means of both experiment and simulation. The results show that heat absorption and storage are more efficient when temperature of heat source is 26.5 °C greater than the phase transition temperature. According to heat transfer characteristics at both radial and axial directions, it is suggested to set up some small exchangers so that solar energy can be stored unit by unit in practice. Such system is more effective in low density residential buildings.
9. Zhou, Dan, Chang-Ying Zhao, and Yuan Tian. “Review on thermal energy storage with phase change materials (PCMs) in building applications.” Applied energy 92 (2012): 593-605. Thermal energy storage with phase change materials (PCMs) offers a high thermal storage density with a moderate temperature variation, and has attracted growing attention due to its important role in achieving energy conservation in buildings with thermal comfort. Various methods have been investigated by previous researchers to incorporate PCMs into the building structures, and it has been found that with the help of PCMs the indoor temperature fluctuations can be reduced significantly whilst maintaining desirable thermal comfort. This paper summarises previous works on latent thermal energy storage in building applications, covering PCMs, the impregnation methods, current building applications and their thermal performance analyses, as well as numerical simulation of buildings with PCMs. Over 100 references are included in this paper.
10. Soares, Nelson, et al. “Review of passive PCM latent heat thermal energy storage systems towards buildings’ energy efficiency.” Energy and buildings 59 (2013): 82-103.  This paper aims to explore how and where phase change materials (PCMs) are used in passive latent heat thermal energy storage (LHTES) systems, and to present an overview of how these construction solutions are related to building’s energy performance. A survey on research trends are firstly presented followed by the discussion of some physical and theoretical considerations about the building and the potential of integrating PCMs in construction elements. The different types of PCMs and main criteria that govern their selection are reviewed, as well as the main methods to measure PCMs’ thermal properties, and the techniques to incorporate PCMs into building elements. The numerical modeling of heat transfer with phase-change and heat transfer enhanced techniques are discussed, followed by a review of several passive LHTES systems with PCMs. Studies on dynamic simulation of energy in buildings (DSEB) incorporating PCMs are reviewed, mainly those supported by EnergyPlus, ESP-r and TRNSYS software tools. Lifecycle assessments, both environmental and economic are discussed. This review shows that passive construction solutions with PCMs provide the potential for reducing energy consumption for heating and cooling due to the load reduction/shifting, and for increasing indoor thermal comfort due to the reduced indoor temperature fluctuations.
11. H Abedin, Ali, and Marc A Rosen. “A critical review of thermochemical energy storage systems.” The open renewable energy journal 4.1 (2011).  Thermal energy storage (TES) is an advanced technology for storing thermal energy that can mitigate environmental impacts and facilitate more efficient and clean energy systems. Thermochemical TES is an emerging method with the potential for high energy density storage. Where space is limited, therefore, thermochemical TES has the highest potential to achieve the required compact thermal energy storage. Thermochemical TES is presently undergoing research and experimentation. In order to develop an understanding of thermochemical TES systems and to improve their implementation, comprehensive analyses and investigations are required. Here, principles of thermochemical TES are presented and thermochemical TES is critically assessed and compared with other TES types. Recent advances are discussed.