## 1 Introduction  
Due to the need of reducing emissions of carbon dioxide (CO\({}_{2}\)), which is the major greenhouse gas quantitatively in the atmosphere, most countries have decided to increase the share of solar and wind energy in their energy mix. Unfortunately, those two renewable energies are dependent on meteorological conditions and their production are highly intermittent. Besides energy storage of short and medium durations fulfilling several major functions on national power grids, usually by the means of capacitors, flywheels and mainly batteries, long-duration storage (hereby defined as ensuring continuity of power in a range from 5 to 500 h) is expected to play an increasing role for allowing a higher penetration of intermittent renewables in the energy mix. In a 2018 report \"Global Energy System based on 100% Renewable Energy-Energy Transition in Europe Across Power, Heat, Transport and Desalination Sectors,\" for the Lappeenranta University of Technology in Finland, [PERSON] et al. come to the conclusions that, if the whole energy supply shall depend on renewables by year 2050, the associated worldwide needs for storage shall be in the range of 64,000 GWh of electricity storage of all durations plus 40,000 GWh in the form of thermal storage for pure thermal needs [1]. Consequently, the availability of low-cost storage systems suitable for renewable energy should trigger the reduction of installed capacity of fossil fuel plants, not yet accomplished because of the discontinuous nature of the renewable energy resources. Regarding space cooling, a growing need where the novel storage has unique specific benefits, a 2018 study by the International Energy Agency projects a worrying increase of energy spent to produce cooling from 2000 TWh in year 2016 to 6250 TWh by year 2050 and a dramatic increase from 1.6 billion units of air-conditioners to 6.5 billion units during the same timeframe [2]. PHES (Pumped Hydro Energy Storage) is presently the main worldwide technology for the long-duration storage of intermittent energy such as solar and wind. [PERSON] extends the scope of PHES in the power grid beyond restoring the mere balance of supply and demand, to possibly provide much-needed frequency regulation services, requiring PHES to be operated over a wider range of power and with the shortest possible reaction time. The author concludes that an innovative design approach could overcome the remaining challenge where, under these operating conditions, reversible pump-turbine machines presently suffer from unstable behaviour [3]. Regarding long-duration as well as seasonal energy storage, Power to Hydrogen and Power to Methane are two field-proven technologies, however high Capex (Capital Expenditure) and low RTE (Round Trip Efficiency) of their present development stage presently preclude both of these technologies from being used for mainstream storage of electricity. An alternative technology, CAES (Compressed Air Energy Storage) is making progress by improving the value of its RTE through the development of AA-CAES (Advanced Adiabatic Compressed Air Energy Storage), however no grid-scale AA-CAES is yet available at this moment. A recent CAES concept for localized facilities such as hospitals, hotels, industrial centres, shopping malls and so forth, especially when coupled with production of intermittent renewable energy, is micro-CAES which has the capability to complement a relatively low RTE on pure electricity storage by the supply of cogenerated heat which can be needed for the heating and/or for the hot water consumption of those facilities, resulting in a higher hybrid efficiency for electricity plus heat. [PERSON] et al. provide an economic analysis of such a system in the framework of integration with photovoltaic energy [4].