Thermal Energy Storage

Thermal Energy Storage can be coupled with mechanical energy storage technologies providing complementary capabilities from both technologies. Today Thermal Energy Storage is tested and deployed in a variety of applications, such as utility-scale power generation, industry, district heating and cooling, buildings and cold chain logistics.

Figure 3: Source – IRENA Innovation Outlook

Energy demand can be shifted in time using thermal storage to better match VRE supply and reduce system strain. For example, high peak coincident loads like building space heating and/or cooling can be moved into off-peak times by charging up the thermal storage during off-peak times and then discharging when required. This enables the on-site demand pattern to stay the same while moving production of the heat or cold to more favorable times (e.g. low grid congestion, high renewable energy, lower price periods). Additionally, system efficiency can also be increased by charging thermal storage during times of high renewable availability and low demand, which can then be discharged at high demand periods to improve utilization. Managing excess renewables production with storage is more efficient from a systems perspective than curtailment as it avoids energy wastage and improves the utilization of generators, thus reducing the overall cost to consumers. Demand shifting is also a critical enabler of efficient sector integration, which otherwise would require significantly increased overall supply and network capacity to meet the same demand.

Molten-Salt Thermal Energy Storage systems can be used to regulate the outputs from variable energy sources. This is sometimes referred to as capacity firming. It is possible to mitigate rapid dips or spikes in output, as well as longer-term variations in supply such as those which occur overnight or throughout the day. Given that solar irradiation and wind are not consistent every minute, electricity generated from these sources currently needs to be supplemented with appropriate reserves from conventional generators such as coal, gas or pumped hydro to fill in shortfalls against demand. System operators use a range of balancing tools to manage fluctuations over timescales from sub-seconds to minutes and hours. Thermal storage is not suited to providing services to meet sub-minute demands yet, such as frequency management, which electrical storage can address at much faster rates. From technical and economic perspectives, thermal storage is suited to delivering power system balancing services across timescales of minutes/hours, and thermal demand shifting across hours. Molten-Salt Thermal Energy Storage systems have long cycle lifetimes and relatively small degradations in efficiency over time compared with batteries, which reduces overall lifetime cost.

Molten-Salt Thermal Energy Storage systems can help reduce curtailment and improve renewable energy utilization via sector integration. This refers to 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 Molten-Salt Thermal Energy Storage systems, deploying TES could help to decarbonize the power system by enabling sector integration.

Heat and transport electrification will add a significant load to the power system, and relying solely on power sector assets may stretch energy system resources and increase the overall cost. Molten-Salt Thermal Energy Storage systems can help to decouple heat demand and to a lesser extent that of the transport sector (through lower cooling/heating loads in vehicles) from power generation with a high share of variable generation. Molten-Salt Thermal Energy Storage systems is also a critical enabling system component for effectively deploying technologies such as heat pumps, allowing their size to be optimized and efficient full load operation at lower costs. This helps to improve the potential of strategies such as power to heat (which already improves renewable integration) and so facilitate whole-system approaches.

Thermal storage can store energy for days or even months to help address seasonal variability in supply and demand. This is of particular benefit to energy systems in regions that have a significant difference in thermal loads between seasons. Surplus heat produced with renewables like solar PV or wind in the summer can be stored in TES, and then be used to supplement or meet winter heating demand. Such an initiative would reduce the need for non-renewable sources of heat during peak times. Thermal storage can also be used to store natural cold in the winter to supply space cooling during the summer season. While this particular use case does not directly aid renewables integration, it helps reduce electricity demand during peak times in the summer.

Load shifting not only helps to improve utilization of renewables and allows them to meet a higher share of demand, but also helps defer or avoid the need for costly electricity network reinforcement. Distributed generation is putting pressure on network operators due to challenges associated with periods of high supply and low demand. Without reinforcement or increased network capacity, power must be exported out of local networks at times of peak supply. Additionally, networks are built to meet peak demand; heat and transport electrification could increase it, triggering additional investment to increase head room availability.

Network capacity is thus a limiting constraint determining the local viability of greater deployment of renewable generation assets, heat pumps and air conditioning. Without storage or other forms of demand management, networks globally will require significant reinforcement. Demand peaks driven by heating and cooling loads can be managed effectively by thermal storage systems. This is because the final demand is heat or cold rather than electricity.

As an example, analysis of the Latvian power system suggests that material network reinforcement could be required even under scenarios of incomplete heat electrification. This is significantly reduced when Thermal Energy Storage system is used in a highly coordinated and controlled manner to reduce the peak load. This finding is case-specific but offers an insight into the potential role for Thermal Energy Storage systems in network management.