Thermal energy storage has the potential to be an important enabler of increased renewables penetration in energy systems.Solar and wind generation is variable across daily and seasonal timescales. Energy system operators can match supply and demand of energy through forms of flexibility such as energy storage. This helps to make energy systems more stable, flexible, and cheaper to build and operate.

during the colder seasons. Also, the other way around, chilled water tanks and UTES can be used on a seasonal basis to provide district cooling. This helps to cover electricity demand peaks when consumers require the most heating or cooling.

The energy transition Globally, energy systems are undergoing a significant transition driven by decarbonisation.The manner in which energy is generated, stored, transmitted, distributed and used is changing throughout the world. Various factors are driving these changes, including moves to widen access to energy, to make energy supplies more affordable and secure, and crucially to reduce the emissions of greenhouse gas emissions associated with our energy systems. International efforts to mitigate climate change have most recently been reaffirmed in the Paris Agreement, which sets a goal to keep the global average temperature rise to well below 2°C. At the time of writing, France, Norway, Sweden and the United Kingdom have enshrined in law a commitment to achieve net-zero greenhouse gas emissions by 2050,4
and other countries are looking to follow suit.

Decarbonising the heating and cooling sectors will be another significant challenge Globally, half of final energy consumption is for heat, split evenly between space heating and industrial
processes. Only 9% of global heat demand (including water heating) is currently met by renewables, compared with 26% for electricity generation in 2017 (Collier, 2018). Thus, it is important to find a sustainable and affordable way to decarbonise the supply of heat.

Market assessment of TES
A growing number of solutions are available to smoothly incorporate the increased share of VRE into energy systems. Electricity storage technologies have emerged as a critical source of flexibility, particularly for the power, buildings and transport sectors. However, the energy storage sector needs to diversify to avoid potential bottlenecks that might arise in the production supply chain, and to cater to a variety of end uses. Therefore, TES as a thermal solution has a clear role in providing flexibility to the power sector as well as for heating and cooling applications. Section 2.3 explains the advantages of integrating TES into energy systems.

IRENA estimates the current installation costs for molten-salt TES to range between USD 26.1/kWh and USD 40/kWh. Assuming an 8-11 hour nominal operating capacity for TES, the cumulative investment needed in molten-salt TES in the next 10 years to match CSP capacity (according to IRENA’s Transforming Energy Scenario) is between USD 12.3 billion to USD 24.4 billion, depending on the CSP technology used.

Heating for buildings, district heating and industry The efficiency of heating systems (space and water heating) has increased rapidly in recent years, slowing the global increase in heating demand. Global heating demand reached 212 EJ in 2018 (IEA, 2019).

The role of TES in integrated energy systems As end-use sectors in the energy system are electrified, and as renewable generation technologies are more widely deployed in sectors other than power, enhanced sector integration will contribute to realising ever more
efficient energy systems. One example of coupling heat and power is the “power to heat” (P2H) concept in which demand for heat is met by a range of decentralised electrified heating and storage technologies (Bloess, Schill and Zerrahn, 2018). Such approaches are sometimes referred to as “smart energy systems”, in which electricity, thermal and gas grids are linked and co ordinated to leverage synergies and deliver optimal outcomes for each sector, as well as maximising efficiency for the overall system (Lund et al., 2016).

TES can help reduce curtailment and improve renewable energy utilisation via sector integration. This refers t linking power generation to demands in other sectors such as heat by converting excess power to heat, significantly increasing the flexibility of the energy system.
As thermal demand is usually far higher than electricity demand, particularly for end-use heating applications, it is more efficient to store energy as thermal energy rather than electricity. Given the high cost-effectiveness and efficiency of TES technologies (Lund et al., 2016), deploying TES could help to decarbonise the power system by enabling sector integration.

UTES can be used to store the heat from solar collectors or industrial processes, or the cold from the winter air. Then the thermal energy is used for space heating in winter or cooling in summer. Some systems use heat pumps to help charge and discharge the storage during part or all of the cycle. For cooling applications, normally only circulation pumps are used (BEIS, 2016). ATES is used to provide buildings with heating in winter and cooling in summer by using the underground water from naturally existing aquifers. ATES consists of a hot and a cold well,
while PTES systems utilise underground pits insulated to reduce heat losses, and filled with gravel and water. PTES has the lowest specific cost, along with ATES. The system can be charged and discharged with heated water by direct contact or by using pipes along the gravel. PTES needs a greater volume than ATES, but has almost no geographical constraints. Finally, BTES is based on vertical heat exchangers that charge or discharge a soil mixture that presents a high specific heat, high thermal conductivity and a very low hydraulic conductivity (Gao, Zhao and Tang, 2015)

Salt hydration Salt hydration is a reversible process that absorbs and releases energy through the hydration and subsequent dehydration of a solid salt. The interest in using hydration reactions for heat storage applications mainly focuses on the hygroscopic salts such as magnesium chloride (MgCl2), sodium sulphide (Na2S), strontium bromide (SrBr2) and magnesium sulphate (MgSO4) (Yu, Wang and Wang, 2013). When heat is added the salt dehydrates, releasing water molecules that can be stored separately from the salt. When there is demand for heat, water is added to the salt, which absorbs it and releases heat. Heat batteries that exploit this process can store heat in small volumes with a minimal loss of energy over long periods of time.

This section provides an overview of how thermal storage could be used to facilitate the introduction of higher shares of renewables across five key sectors in which energy is consumed: power, industry, district heating and cooling, the cold chain, and buildings. The
sector refers to the location at which TES is deployed, as the benefits are often shared across the system. TES can be a form of both supply-side and demandside flexibility. The power sector sub-chapter focuses only on TES as a supply-side flexibility enabler, whereas the other sub-chapters (industry, district heating and cooling, the cold chain, and buildings) focus on TES as an enabler of demand-side flexibility and of integrating on-site renewable energy generation.

Cold energy generated from renewable thermal energy sources (such as solar and biomass co-generation) could be stored using absorption systems. Due to the high energy density and minimal thermal losses seen for these systems, cold could be stored for both short and long timescales (such as inter-seasonally) for space cooling in the cold chain.

Solid materials as sensible TES can be utilised from cryogenic temperatures up to 1 000°C (Xu and Chung, 2000). Both natural and artificial substances have been studied as storage media, such as rocks, pebbles, concrete and ceramic bricks. For small-scale applications (domestic and commercial) ceramic bricks working at temperatures up to 700°C are used. The system components are: the heat storage material, high-performance insulation (to avoid heat losses) and a fan to drive heat from the storage medium to point of use. These systems are typically charged overnight, and have a smart control to manage the level and timing of charge/discharge processes.

Adiabatic compressed air energy storage (A-CAES) systems have been proposed to improve the overall efficiency of CAES by adding a high-temperature TES unit that stores compression heat, which would have otherwise been lost during the gas compression stage, for later use during the expansion process.

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