Definition of sector coupling and city storage

While the share of renewable energy in the electricity sector is growing continually with the integration of i.e. PV panels and wind turbines, other sectors, such as transport, buildings and industry, still depend largely on fossil fuels. Apart from decarbonisation of each sector individually such as the heating sector, they can either be electrified via sector coupling or the fossil fuels can be substituted by other forms of renewable energy. Apart from energy production technologies, decarbonisation can be furtherly fostered by integration of energy storage which increases the possibility of utilizing renewable energy and positively affects on mitigation of intermittency. 

Within the project CSSC Lab, it is necessary to furtherly develop the definition of both City Storage and Sector Coupling (CSSC) in order to clear the path for future activities such as identification of good practices, development of model solutions and creation of effective training materials. 

Sector coupling presents an effective way of fossil-fuel phase out in multiple sectors such as electricity, heat and transport by connecting sectors and using decarbonized electricity or energy carriers produced from decarbonized sources. Within the definition, two approaches can be followed: 

  1. end-use sector coupling which focuses on electrification of energy use in the end-use sector where currently fossil fuel plays the dominant role – i.e. in transport by using electric and hydrogen-powered vehicles and in heating sector by using electric heat pumps, electric boilers or other available technologies 
  2. cross-vector integration (power-to-X) which involves indirect electrification on larger scale by using renewable energy to produce other forms of energy (heat, gaseous or liquid energy carriers). In other words, electricity is used in converters (as an input) for energy transformation in other usable forms.

Energy storage plays an important part in energy sector since its development. Currently, fossil fuels come in various forms and can be stored in well-developed storage infrastructure. In a process of decarbonisation, electricity and other forms of renewable energy cannot be stored following the same methodology since i.e. electricity production must always be equal to consumption and renewable energy has intermittent nature. Therefore, electricity must be converted to a different form (explained below) in order to store it successfully or specialized energy storage has to be built (i.e. thermal storage). 

Chemical form – Batteries 
Batteries (lithium-ion, salt water…) can provide valuable services for the electricity systems such as frequency control, temporal shift of production and flattening of demand peaks. Also, they can act as buffers to store variable renewable energy near the production site and use it in times of high demand and high price. Best selection of battery depends on characteristics and uses, as well as necessity for short- and/or long-term storage. 

Potential energy – pumped hydro or compressed air
Pumped storage is most widely used large-scale electricity storage technology (97% of EU) where potential energy of water is used. In periods of excess of electricity production, water is stored on a higher altitude level (reservoirs) and then released to drive turbines and generators. 

Thermal energy 
Thermal storage can be achieved by storing heat produced by i.e. solar thermal collectors (direct production of heat) or by sector coupling (direct/indirect usage of electricity to produce heat via i.e. heat pumps and electric boilers). There are various application of thermal storage in heating sector, focusing on four main applications using water: tank thermal storage, Pit Thermal Energy Storage, Borehole Thermal Energy Storage and Aquifer Thermal Energy Storage . Thermal storage can be used on a daily, weekly or even seasonable basis – to i.e. store heat in building structures or to couple waste heat and district heating systems.

It can be used to produce gases or liquid fuels (cross-vector integration)
This option presents adjustment of storage infrastructure in terms producing gas or liquid fuels using electricity and storing it for later usage. In this way, existing storage infrastructure (and same distribution network) can be used. 

Based on presented, city storage and sector coupling for the purposes of the CSSC Lab project should focus on possibilities of 1) connecting all three sectors (or at least two should be coupled) where renewable electricity produced in any sustainable way should be used in other sectors via energy converters (and stored in other viable energy storages), or 2) achieving integration of other forms of renewable energy in those sectors (heating, transport) such as solar thermal collectors which will be boosted by energy (thermal) storage. This will enable larger penetration of renewable energy in the system. Apart from above-mentioned ways of achieving CSSC, the project should aim at a societal value in each region given the fact of different levels of development in terms of viable technical solutions and current focus on energy sector development.

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