9 research outputs found
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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
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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
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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
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Modelling and development of thermo-mechanical energy storage
Pumped thermal energy storage (PTES) and liquid air energy storage (LAES) are two technologies that use mechanically-driven thermodynamic cycles to store electricity in the form of high-grade thermal energy, employing abundant materials that are kept in large insulated tanks. Both technologies are free from geographic constraints, providing a significant advantage over competing methods such as pumped hydro or compressed air energy storage. The focus of this thesis is on the analysis, modelling and development of these technologies.
A number of PTES systems have been proposed based on different thermodynamic cycles. A variant based on the Joule-Brayton cycle employing liquid storage media is studied here. An analytical study is presented that reveals how the performance of the cycle varies along a range of operating conditions. Generally, the same strategies that minimise compression/expansion losses also maximise heat exchanger losses, which results in optimal points at certain operating conditions. A numerical model is developed to find these optima while accounting for real fluid properties. Employing a regenerative heat exchanger is found useful to adapt the cycle to the operating temperature ranges of the storage liquids and to increase the performance of the cycle.
A new combined cycle that integrates PTES and LAES is presented. The fundamental advantage is that the cold thermal reservoirs that would be required by the 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. Several configurations are possible and these are studied and optimised. The most advanced configuration reaches a round-trip efficiency of 71 % under nominal conditions, compared to 65 % for stand-alone PTES and 61 % for LAES. A further adaptation of the combined cycle is presented which only employs water and liquid air as storage media, dramatically reducing the cost of energy capacity.
The performance of the heat exchangers is found to have a significant impact on the overall performance of the various cycles. For this reason, an optimisation procedure is developed to obtain heat exchanger designs that minimise entropy generation for a given amount of material. These designs are used when estimating the costs of energy capacity and power capacity of each cycle. Results indicate that the best cycle configurations would be competitive with reported costs for pumped hydro and compressed air energy storage.This PhD project was funded by a Peterhouse Graduate Studentship
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Low temperature sensible PTES with Kalina cycles
This presentation was given at the second International Workshop on Carnot Batteries in September 2020. It details how Kalina cycles could be used in Pumped Thermal Energy Storage, discussing working fluid choice and performance results from a cycle model
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High performance Carnot Batteries based on hybrid cycles
Pumped thermal energy storage (PTES) has seen a rapid increase in research interest and private investment during the last few years. A range of different concepts has been proposed, based on different thermodynamic cycles, and the most promising ones are already being turned into demonstration projects or small-scale storage plants. These include PTES systems based on the Joule-Brayton cycle, the Rankine cycle and the Liquid Air cycle, among others. This presentation will explore how hybridising some of these concepts can result in systems that are more flexible, cheaper, or have superior performance compared with the original cycles. More specifically, two examples will be shown where the Joule-Brayton cycle can be effectively used to support a Rankine battery and a Liquid Air battery. One general advantage of Brayton-PTES systems is that they can use molten salts as liquid storage media. Molten salts are cheap, safe and abundant, and have been used for concentrated solar power (CSP) applications in a large number of commercial plants. Employing the same storage material at similar temperature levels opens the possibility of hybrid “solar-PTES” systems that would require less capital investment than two separate plants. Such a hybrid system could charge the same hot stores using either solar energy or off-peak electricity, becoming both a power plant and an energy storage plant, therefore increasing the capacity factor while employing a single heat engine during discharge. A numerical model has been implemented to study a solar-PTES system where an existing CSP plant (based on the Rankine cycle) is retrofitted with a Brayton heat pump, and several strategies are explored to boost the overall performance. Similar configurations could be employed to transform other kinds of thermal power plant (such as coal power plants) into Brayton-Rankine batteries. In contrast to most PTES systems, liquid air energy storage (LAES) stores most of the available energy cryogenically, by liquefying atmospheric air and storing it at very low temperatures. This is advantageous because liquid air has a very high energy density - and is free. However, the difficulties in reaching full liquefaction during the charge process have a significant impact on the round-trip efficiency of the cycle. It has been found that these difficulties can be greatly minimised by employing the support of a Brayton cycle. A hybrid system was designed where a Brayton-PTES plant operates as a topping cycle and an LAES plant operates as a bottoming cycle. The cooling provided by the Brayton cycle allows the LAES side to achieve full air liquefaction, which translates into a significant boost in performance. Furthermore, the cold thermal reservoirs that would be required by the two separate cycles are replaced by a single heat exchanger that acts between them, therefore saving significant amounts of storage media per unit of energy stored. Results from a numerical study indicate that the hybrid cycle can increase round-trip efficiency by 5-10 percent points compared with the separate cycles, and achieve an even larger increase in terms of energy density
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
Recommended from our members
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