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.