59487 - A Novel Energy Storage System Based on Carbon Dioxide Unique Thermodynamic Properties 

The common goal of progressively decarbonise the power production sector will result in next years in a strong increase of renewable energy contribution in the energy market. The most competitive renewable energy technologies are today solar PV and wind turbines. Their installed capacity has been increased dramatically in the last 5-10 years in many countries due to their significant cost decrease as well as to the availability of relevant economic incentives. Main drawback of these renewable energy technologies is represented by the non-perfect forecasting and the intrinsic non-dispatchability that already today involves issues on the transmission grids management. The need of an affordable, efficient and sustainable energy storage technology is mandatory for the next generation energy market in order to allow large share of renewable energy but also ensure grid stability. Unfortunately, energy storage technologies available today lack in meeting these ambitious targets. Pumped hydro energy storage is already mostly exploited and the possibility of new installations is strongly related to the availability of a proper site orography. Electrochemical batteries are available in several different models but their RTE (Round Trip Efficiency) , considering all the auxiliary consumptions plus performance decay due to wearing and ageing, is easily below 70% in real operation; moreover they are characterized by short lifetime, relevant replacement cost and issues related to disposal of equipment. Compressed Air Energy Storage (CAES) can reach competitive RTE (around 70%) but it requires the availability of large underground caves or deep seabed and so it cannot freely located close to renewable energy production sites. Liquefied Air Energy Storage (LAES) is a promising solution that can limit the system footprint; however maximum RTE is expected to be lower than 60%. Other technologies like pumped heat energy storage is still at low technology readiness levels and far from a realistic penetration on the market. In this scenario EnergyDome Srl has patented an innovative energy storage technology that can meet all the requirements of affordability, efficiency and sustainability. This non-conventional energy storage solution is based on the close looping of CO₂ between gaseous form at ambient pressure and high pressure liquid status. Different plant configurations have been proposed and patented. In this work the most simple is analysed in detail: it consists in a charging phase when CO₂ from ambient pressure inventory is compressed, cooled, condensed and eventually stored in liquid form and a discharging phase when high pressure CO₂ is vaporized and heated before expansion down to ambient pressure. The system requires a Thermal Energy Storage in order to store the thermal power released by the cooling of the compressed stream of CO₂ during the charging mode and let it available for CO₂ heating before expansion in discharging mode allowing to maximize the system RTE. Independent calculations from Politecnico di Milano demonstrated that this system can reach a realistic roundtrip efficiency higher than 75% while ensuring long life time of the components (>25 years) and limited performance decay. Moreover, this energy storage system adopts components that are already available, resulting in a technology that can rapidly penetrate the market being able to exploit technologic transfer from other industrial fields. The paper describes the plant arrangement, the energy and mass balances, the selection of the main components and the preliminary design of a first-of-a-kind prototype of a 20 MW capacity.