Giant water heater makes district heating more flexible

In Diemen, near Amsterdam, Vattenfall has taken another step towards reducing the dependence of district heating on fossil fuels. Since its commissioning in March 2026, the electric boiler (e-boiler) has been supplying heat to the district heating networks of Amsterdam and Almere.

The project is an example of so-called sector coupling, which connects different parts of the energy system: renewable electricity is converted into heat for the district heating network, ensuring that available renewable electricity is used as efficiently as possible.

“Our goal is to make all our district heating networks fossil-free,” says Bart Dehue. “To achieve that, we are developing new fossil-free heat sources that help make the energy system more flexible.”

From electricity to heat

The e-boiler is often described as a giant kettle. The comparison helps, although the technology works slightly differently from the kettle in your kitchen. Instead of using a metal heating element, the installation generates heat by passing electricity directly through the water.

In Diemen, the boiler vessels together have a capacity of 150 MW. To put that into perspective, it is roughly equivalent to 150,000 household kettles running simultaneously.

When large amounts of renewable electricity are available and there is sufficient capacity on the electricity grid, the e-boiler converts that electricity into heat for homes. The energy can either be supplied directly to the district heating network or stored in the large thermal energy storage facility located at the Diemen site. This allows Vattenfall to store it for later use, for example when  there is no renewable electricity available for the electric boiler or the grid is at full capacity.

Photo: Vattenfall/Jorrit Lousberg

Why flexibility matters

As wind and solar generation continue to grow, periods of abundant renewable generation are becoming increasingly common. During such periods, electricity prices are often low because supply is abundant. In 2025, for example, the Netherlands recorded 584 hours of negative prices, 28 per cent more than in 2024.* Using that electricity to produce heat or store heat for later use is a smart way to make the most of renewable energy.

“During these periods, a gas-fired power plant needs to run less to provide heat,” explains Bart Dehue. “The e-boiler can respond quickly by converting electricity into heat, helping to reduce both gas consumption and CO₂ emissions. On windy days, together with the heat buffer, the e-boiler can sometimes act like a giant battery for heat.”

The installation provides enough heat for approximately 20,000 homes in Amsterdam and Almere and is expected to cover around 15 percent of the annual heat demand in the connected district heating networks.

“The e-boiler is not designed to operate continuously,” says Bart Dehue. “You can think of it as the flexible link in our heating system: fast, adaptable, and particularly valuable when conditions are favourable.”

A reliable district heating system of the future requires a variety of heat sources. Some, such as geothermal energy or waste heat from data centres, can provide relatively stable heat throughout the year. Others, such as the e-boiler, add flexibility whenever renewable electricity is available. Together, these sources make district heating more robust, more sustainable, and better aligned with the electricity system of the future.

Vattenfall/Jorrit Lousberg

In the foreground is the building housing the electric boilers; in the background, on the left, is the large heat storage tank. Photo: Vattenfall/Jorrit Lousberg

Lessons for industry

The value of the e-boiler extends beyond district heating. The knowledge gained in Diemen can also help other sectors decarbonise.

Many of our industrial clients need to reduce their gas consumption and CO₂ emissions, particularly for process steam and heat that are still produced using gas-fired boilers. E-boilers can also play a role here.

“With the knowledge and experience we have gained in Diemen, we can also support industry in the transition to e-boilers,” says Ruud Stevens, Project Manager at BU Heat. “We understand what it takes to develop, build, operate and maintain such an installation. This enables us to help companies reduce their gas consumption and CO₂ emissions whenever renewable electricity is available whilst reducing their energy costs.”

Valuable insights

The e-boiler in Diemen demonstrates how heat and electricity can work together more intelligently. The installation not only delivers fossil-free heat today but also offers valuable insights into how future energy systems can become more integrated and flexible.

E-boiler at a glance Converts electricity into heat for the district heating network Utilises renewable electricity when it is available Can supply heat directly or store it in thermal energy storage Increases the flexibility of the energy system Reduces the use of fossil fuels Technical specifications Each boiler is approximately 7 metres high. Each boiler has a volume of 36 m3, an unladen weight of 17 tonnes and an operating weight of 31 tonnes. The water volume is 14 m3. The remainder (12 m3) is filled with nitrogen. The water in the boiler forms a closed circuit with a high degree of thermal conductivity. The heat generated in the boiler is transferred to the district heating network via a heat exchanger. The temperature of the water in the boiler is 135 °C when in operation and, when out of operation (‘hot standby’), is equal to the prevailing heating curve temperature to which the district heating network is set at that time. This temperature varies with the season: in summer it is approximately 95 °C and in winter a maximum of 125 °C.

* Source: TenneT, Rising electricity prices, increased electricity exports and stable congestion management costs, 30 april 2026

Photography: Jorrit Lousberg