Tank Thermal Energy Storage: The asset that makes District Heating more flexible
Updated: 42 minutes ago
Modern district heating plants increasingly combine heat pumps, electric boilers, combined heat and power (CHP), biomass, waste heat, solar thermal and peak-load boilers. This diversification creates options - but only if the plant can choose when to use each source.
That is where tank thermal energy storage becomes valuable. It does not produce heat, but it decouples production from demand: heat can be produced when technical and economic conditions are favourable, stored, and delivered when customers need it.
Diversification needs flexibility
A district heating plant may consist of several heat production technologies.
CHP is often most attractive when electricity prices are high, while combustion-based units may work best at stable loads. A heat pump is preferred when electricity is inexpensive. An electric boiler can respond quickly to favourable prices or power-system needs. Waste heat may be low-cost but available at the wrong time.
Without storage, these assets can appear to compete because each is judged against demand in the current hour. With storage, they can work as a portfolio. The tank accepts heat from the most advantageous source and returns it later, shifting the question from “Which unit should meet demand now?” to “How should we use the complete system over the coming hours or days?”
This is the central value of thermal storage: it creates synergy between heat sources that would otherwise be treated as competing assets.
How a stratified hot-water tank stores energy
Tank thermal energy storage is technically simple: energy is stored as sensible heat in water.
The temperature difference matters as much as the volume. For example, a 5,000 m³ tank operating across a 55 K temperature difference holds roughly 320 MWh of theoretical thermal energy. Allowing for the portion that is practically usable, a typical capacity of about 288 MWh for this case.[1]
In operation, hot water remains at the top and colder water at the bottom, separated by a transition layer called the thermocline.
Good stratification preserves useful supply-temperature water at the top while maintaining a colder return-temperature region at the bottom. Excessive mixing thickens the thermocline and reduces the share of energy available at the required temperature, even if the tank still contains a large amount of heat in total.
This is why diffuser design, inlet velocity, connection levels, flow rate, insulation and operating strategy all matter. The useful capacity of a tank is not determined by volume alone.
What storage changes in plant operation
A well-integrated tank can provide several benefits at once:
Load shifting: move heat production from expensive or constrained hours to more favourable ones.
Peak shaving: discharge the tank during demand peaks and reduce the required operation of peak-load units.
Improved part-load behaviour: allow production units to run for longer periods at stable, efficient loads.
Renewable and waste-heat integration: store heat when it is available, even when demand occurs later.
Operational reserve: provide a thermal buffer during production disturbances, transitions and maintenance.
Sector coupling: give heat pumps and electric boilers more freedom to react to electricity prices and grid needs.
Portfolio optimisation: coordinate the full set of heat sources instead of optimising each unit separately.
A tank must be modelled as part of the system
Simple calculations help with early sizing, but they cannot show how much energy will be available at the required temperature, whether a schedule will preserve stratification, or whether sufficient capacity remains before a price or demand event. For those questions, the tank must be modelled dynamically with the plant.
At EKA, we use a Python-based model of stratified thermal storage. The calculation approach is based on the stratified hot-water tank modelling methodology documented in EnergyPlus, where the tank is represented by vertical temperature layers and their thermal behaviour is evaluated over time. This allows us to analyse how the temperature distribution evolves during charging, discharging, heat losses and mixing.
The model can be combined with heat-demand data, production-unit capacities, supply and return temperatures, operating constraints and electricity prices. For system-level techno-economic analysis and operational optimisation, we can also use energyPRO to model systems with multiple heat and electricity production units, thermal storage, demand profiles and market prices.
This makes it possible to investigate questions such as:
Is the tank large enough for the intended operating strategy?
How much capacity is usable at the required supply temperature?
When should it charge and discharge?
Can power-to-heat units offer grid flexibility without compromising heat supply?
Which combination of sources minimises total operating cost?
Detailed models are valuable for tank design and stratification analysis; faster system-level models suit scenario testing and optimisation. Comparisons show that different platforms can agree well, but discretisation, heat-loss assumptions, thermal bridges and user choices affect results.[3] Tool selection should therefore reflect the timescale, required detail and control problem.[4]
Reading the model output
An example simulated temperature distribution in a stratified hot-water thermal energy storage tank is shown in the figure above. Time is shown on the x-axis, tank height on the y-axis, and colour represents water temperature. The movement and thickness of the transition zone illustrate charging, discharging and the evolution of stratification over the simulated period.
The heatmap makes tank behaviour visible. The warm region near the top represents water potentially available to the network; the colder region below represents return-side water and charging volume. The gradient between them is the thermocline.
During charging, the hot region expands through the tank. During discharging, it contracts. A narrow, clearly defined transition generally indicates good stratification; a broad or irregular gradient can indicate mixing, unsuitable inlet conditions or operating sequences that reduce usable energy.
Unlike a single state-of-charge value, the heatmap shows the state of charge and how plant decisions change its usefulness.
Storage does not produce heat - it makes heat production more valuable
District heating systems need diverse resources to manage fuel risk, electricity-price volatility, renewable availability and changing demand. They also need an asset that coordinates those resources across time. Tank thermal energy storage performs that role.
It allows heat pumps and electric boilers to use favourable electricity hours, gives CHP and other sources greater operating freedom, supports the use of renewable and waste heat, reduces peak-load operation and can enable participation in ancillary-service markets. Above all, it changes a collection of individual production units into an integrated system.
The tank may not produce heat. What it produces is choice - and that choice has real economic and operational value.
References
Danish Energy Agency and Energinet, Technology Data for Energy Storage, version 0011, Chapter 141: Tank Thermal Energy Storage, updated July 2026.
M. Pilotelli, B. Grassi, D. Pasinelli and A. M. Lezzi, “Performance analysis of a large TES system connected to a district heating network in Northern Italy,” Energy Reports, vol. 8, 2022, pp. 1092-1106. https://doi.org/10.1016/j.egyr.2022.07.094
F. Ochs et al., “Comprehensive Comparison of Different Models for Large-Scale Thermal Energy Storage,” Proceedings of the International Renewable Energy Storage Conference 2021, Atlantis Highlights in Engineering, vol. 8, 2022.
L. V. Valentini, S. Forndran, F. Ochs, A. Thür and W. Streicher, “District heating modeling tools: A review based on design and simulation of renewable district heating networks,” Energy Conversion and Management: X, vol. 29, 2026, 101414. https://doi.org/10.1016/j.ecmx.2025.101414
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