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Modelling of thermal energy storage systems for bulk electricity storage

This report was submitted for the First Year Assessment of the PhD course in Engineering at Cambridge University Engineering Department. ABSTRACT Growing concerns about climate change and energy security are increasing worldwide efforts to decarbonize the electrical grids, pushing governments and international institutions to promote the use of Renewable Energies (RE). However, two major RE sources -wind and solar energy- present natural fluctuations, and any grid containing big portions of such sources faces the major challenge of having enough electricity storage available to match supply and demand. A new family of technologies with a high potential for large-scale electricity storage applications is emerging, which in this report are denominated Thermo-Electrical Energy Storage (TEES) systems. Generally, in a TEES system, a heat pump uses electricity to transport thermal energy from one thermal reservoir (cold) into another (hot). Energy may be stored in the form of sensible or latent heat. After storage, a heat engine is used to transform the thermal energy back into electricity. Differently from the two main competing technologies -Pumped Hydro-electric Storage (PHS) and Compressed Air Energy Storage (CAES)-, the implementation of a TEES system does not depend on specific geographical features. Additionally, it normally makes use of cheap and abundant materials and benefits from high values of energy density. The aim of this PhD project is the analysis and comparison of TEES cycles through component and cycle modelling, the identification of their main strengths and weaknesses and the suggestion of novel configurations with improved performance. The first part of this report is mainly concerned with the thermodynamic analysis of a specific TEES technology which is based on the Joule-Brayton (JB) cycle and is known as Pumped Thermal Electricity Storage (PTES). Chapter 2 reviews the fundamentals of the technology and proposes and evaluates new configurations that make use of liquid materials as storage media, substituting the solid reservoirs that have been used until now. It also presents and briefly discusses other TEES technologies that are based on variations of the Rankine cycle. The second part of this report, Chapter 3, is concerned with the modelling of specific components used in TEES cycles, such as a heat exchanger or a reciprocating compressor, and the study of packed-beds of solid particles for thermal energy storage using Computational Fluid Dynamics (CFD). Finally, a summary of the work done up-to-date and the proposed work for the continuation of the project is presented.This project was realised with the support of a graduate studentship from Peterhouse

Effect of heat capacity variation on high-performance heat exchangers for thermo-mechanical energy storage

Counter-flow heat exchangers constitute a major component of several thermo-mechanical energy storage technologies. They are used to transfer thermal energy between the working fluid and the storage fluid, and exergy losses undergone during this process can affect significantly the efficiency of the whole system. The principal sources of loss are irreversible heat transfer and pressure losses, and optimisation is required to find the right balance between them. In this article we focus on the effect that the variation of the specific heat capacity of some fluids has on the thermal component of the loss. First, we assume a linear dependence of the heat capacity with temperature and study the problem analytically, showing that a minimum exergetic loss exists when the variation is different for the two fluids. The effect is negligible in low-performance heat exchangers but it is found to have a critical impact in high-performance devices with a very high number of transfer units. Second, the minimum loss for several couples of real fluids is computed numerically and compared with the prediction of the analytical model. Finally, the effect that this phenomenon has on the optimisation of a flat-plate, counter-flow heat exchanger is studied.Peterhouse Graduate Studentship

Thermodynamic strategies for Pumped Thermal Exergy Storage (PTES) with liquid reservoirs

Pumped thermal energy storage (PTES) is a grid-scale energy management technology that stores electricity in the form of thermal energy. A number of PTES systems have been proposed using different thermodynamic cycles, including the Brayton cycle, the Rankine cycle and transcritical cycles. This talk proposes to employ the Brayton cycle together with liquid storage media (as opposed to packed beds of solid particles), and finds that employing a gas-gas regenerator is useful to adapt the cycle to the operating temperature ranges of candidate liquid materials and to improve the work ratio of the cycle. Because the cycle performance is very susceptible to heat exchanger losses, emphasis is put on employing highly-effective heat exchangers

Solar Radiation Pressure Effects on the Orbital Motion at SEL2 for the James Webb Space Telescope

Due to James Webb Space Telescopes large sunshield, which will always be facing the Sun to protect the observatorys instruments, Solar Radiation Pressure (SRP) has an important effect on its orbital motion around SEL2. Moreover, SRP is highly dependent on the observatorys attitude with respect to the Sun observatory line. This paper explores the impact of SRP for different attitude profiles on the size of a reference orbit

Wide-Field Infrared Survey Telescope and Starshade Formation Flying Dynamics at Sun-Earth L2

The formation flying of an occulter with a telescope at the Sun-Earth L2 (SEL2) Libration Point can be a challenging problem. A good knowledge of the Restricted Three Body Problem dynamics is required to understand how these two spacecraft interact with each other in the SEL2 unstable environment, and how other perturbations such as Solar Radiation Pressure (SRP) affect their mutual trajectories. This paper focuses on the transfer trajectories to achieve specific relative positions between two spacecraft as they fly in formation at SEL2, andanalyzes the relevance of SRP in this formation, using the Wide-Field Infrared Survey Telescope (WFIRST) and the Starshade occulter as an example. Given that WFIRST and Starshade have very different area-to-mass ratios, SRP will affect their motion in different ways, and their relative position can be key to reduce the V cost. In this paper we intend on providing an explanation on how the relative position between both spacecrafts affects the transfer V from one observation to the other using dynamical system theory and Floquet modes

Thermodynamic analysis and optimisation of a combined liquid air and pumped thermal energy storage cycle

Pumped thermal energy storage (PTES) and liquid air energy storage (LAES) are two large-scale electricity storage technologies that store energy in the form of thermal exergy. This is achieved by operating mechanically-driven thermodynamic cycles between thermally insulated storage tanks. Both technologies are free from geographic restrictions that apply to pumped hydro and most compressed air storage. The present paper describes a novel, combined system in which PTES operates as a topping cycle and LAES as a bottoming cycle. The fundamental advantage is that the cold thermal reservoirs that would be required by the two separate cycles are replaced by a single heat exchanger that acts between them, thereby saving significant amounts of storage media per unit of energy stored. In order to reach cryogenic temperatures, the PTES cycle employs helium as the working fluid, while the LAES cycle uses supercritical air (at around 150 bar) which is cooled sufficiently to be fully liquefied upon expansion, thus avoiding recirculation of leftover vapour. A thermodynamic study of a baseline configuration of the combined cycle is presented and results are compared with those of the separate systems. These indicate that the new cycle has a similar round-trip efficiency to that of the separate systems while providing a significantly larger energy density. Furthermore, three adaptations of the base-case combined cycle are proposed and optimised. The best of these adaptations achieves an increase in thermodynamic efficiency of about 10 percent points (from 60 % to 70 %), therefore significantly exceeding the individual cycles in both energy density and efficiency.P. Farres-Antunez gratefully acknowledges Peterhouse for the studentship which is allowing him to develop this research project at Cambridge University Engineering Department