All these things are happening to decarbonize the electric energy production sector, which is the major net contributor to pollutants and CO

Abstract

Electric energy production systems all around the world are currently experiencing a major revolution caused by the massive introduction of non- programmable Renewable Energy Sources (RES). The widespread diffusion of such technologies is changing both the electric energy production and consumption paradigms. As a matter of fact, the systems are now shifting towards a less centralized configuration, and the consumers are assuming a more active role as they are becoming able to produce at least a part of the energy they need.

All these things are happening to decarbonize the electric energy production sector, which is the major net contributor to pollutants and CO

emissions. To reach this ambitious goal, the RES penetration must be greatly increased, but this arises technical issues that must be solved before to continue the RES development. After a rapid initial RES growth, in which the electric systems demonstrated their ability of accommodating the RES, it is now starting a phase in which additional electric storage capacity must be deployed on the grids to balance the RES production fluctuations. As the sites suited for pumped hydro storage plants are already exploited at most, a strong research effort aimed at developing alternative storage technologies is taking place. Several alternative technology concepts have been proposed, ranging from electrochemical devices, to thermo-mechanical applications. However, which one could be the successor of pumped hydro, or even if there will be just one or more successors, is still an open question. To contribute to the outlined research line, in this dissertation an innovative electric energy storage technology potentially suited for grid scale applications is proposed and analyzed. The investigated system belongs to a broader technology family which comprises an heterogenous group of technologies based on the idea of storing electric energy as thermal energy. Such group of technologies is known in literature by several names, with Pumped Thermal Electricity Storage (PTES) being the most common. Notwithstanding, the name “Carnot Batteries” is lately growing in popularity and may be found as well in literature. In PTES systems, the charge phase is usually performed by using a Heat Pump (HP), the energy conservation is entrusted to a Thermal Energy Storage (TES), which can be either a latent or a sensible one, and the discharge phase is performed by means of a Heat Engine (HE). In this dissertation, a system based on Vapor Compression HPs (VCHPs) and Organic Rankine Cycles (ORCs) is investigated. The classical configuration which may be found in literature has been modified to allow the heat pump being powered also by low-temperature heat sources, such that the system performance is improved. The use of additional low temperature heat sources has been called Thermal Integration (TI), which led to the development of a TI-PTES system.

Compared to standard PTES configurations, the VCHP does not move the

thermal energy between two thermal reservoirs (hot and cold). In fact, by

exploiting additional thermal energy inputs, here the VCHP performs an

upgrading of the provided thermal energy, which is then stored in the TES. This

technique is known in literature, since it may have also some waste heat recovery

applications in industry. In the industrial context, the systems performing the heat

upgrading are called High Temperature HPs (HTHPs). Therefore, in this

dissertation a TI-PTEs system based on HT-VCHPs and ORCs is proposed and

investigated. Several TI-PTES design aspects are analyzed in detail, ranging from

the working fluids choice, to the thermodynamic cycle design optimization. In

the dissertation a multi-criteria approach is often assumed to characterize the TI-

PTES and HT-VCHP systems in respect to several performance metrics. Part of

the dissertation is focused on HT-VCHP design and economic analysis, as this

component is the most critical and the less studied of the proposed TI-PTES

system. In conclusion, in this dissertation a detailed theoretical analysis of a TI-

PTES system is provided. Several aspects are investigated and the trade-offs

between different performance parameters are characterized by means of a multi-

criteria analysis approach. Special attention is dedicated to the thermodynamic

design optimization of the TI-PTES sub-systems and to the economic analysis of

the HT-VCHP. A cost model for this component is developed and prosed as a

first step towards a complete TI-PTES cost model.