The Hidden Battery in Your Walls and Floors: Thermal Mass and the Renewable Grid

As the world races to decarbonise the energy grid, the spotlight often falls on “Long Duration Energy Storage” (LDES)—technologies like hydrogen, gravity storage, and flow batteries designed to power the grid during the dunkelflaute (dark, windless periods) of winter.

However, recent market analysis suggests the window for these expensive, specialised technologies may be shrinking. Lithium-ion batteries are rapidly conquering the 8–12 hour storage window, and the need for massive amounts of “seasonal” storage may be overstated. It is increasingly argued that grids could reach 90–98% renewable penetration just by optimising solar, wind, and existing battery tech, leaving the rare remaining gaps to be filled by existing reserves rather than new, unproven infrastructure.

This is where a low-tech, often overlooked solution comes into play: Thermal Mass. Instead of waiting for complex grid-scale tech, we can turn our buildings into distributed energy storage devices right now.

What is Thermal Mass?

Thermal mass refers to the ability of a material to absorb, store, and release heat energy. Heavy, dense materials—such as concrete slab floors, brick walls, stone, and tile—have high thermal mass.

• In Winter: Sunlight or heating systems warm the floors and walls during the day. These dense elements hold the heat and slowly release it at night, keeping the building warm without drawing extra power from the grid.

• In Summer: The mass acts as a heat sink, absorbing excess heat during the day (keeping the air cool) and releasing it at night when ventilation allows it to escape.

The “Virtual Battery” for the Modern Grid

If lithium-ion batteries can effectively handle overnight shifts (8–12 hours), thermal mass acts as the perfect partner to extend that capability. It functions as a thermal battery, drastically reducing the electrical load required to maintain comfort.

Here is how using thermal mass supports this high-renewables grid strategy:

1. Flattening the Peak Lithium-ion batteries are excellent at handling short bursts, but they are expensive to scale for heating millions of inefficient buildings. Structures with high thermal mass, such as floors and walls, maintain their temperature for hours or even days. This drastically reduces the “peak demand” for electricity in the evening—precisely when solar generation drops off, and the grid is most vulnerable.

2. Bridging the Gap While thermal mass isn’t “seasonal” storage, it extends the effective duration of short-term storage. By shifting heating and cooling loads by 12–24 hours, thermal mass reduces the pressure on the grid to provide immediate power. This helps bridge the gap that might otherwise require firing up gas reserves or future fuels.

Historic Buildings as Grid Assets: The ECS Solution

One of the most compelling applications of this technology lies in the retrofit of historic buildings, a sector often considered difficult to decarbonise due to preservation restrictions. ECS specialises in unlocking the latent energy potential of these heritage structures, proving that we do not need to choose between preserving the past and powering the future.

By combining NZEB (Nearly Zero Energy Building) standards with the immense, existing thermal mass of historic stone and brickwork, ECS creates highly efficient energy systems without compromising architectural integrity. Using their proprietary systems, they transform the fabric of historic buildings into a Distributed Local Energy Storage (DLES) thermal store.

The technology actively charges the building’s mass (floors and walls) using renewable energy when it is abundant and cheap. Because historic masonry is significantly denser than modern lightweight materials, these buildings effectively become massive LDES assets, capable of storing heat for longer periods than standard structures.

This approach highlights a crucial shift in how we view the grid: we should stop looking exclusively at the supply side for storage solutions. By treating our building fabrics—whether modern concrete or historic stone—as thermal storage devices, we reduce the size of the “dunkelflaute” problem. We don’t necessarily need a hydrogen plant to keep us warm through a windless night if our walls and floors are doing the work for us, prioritising emission reductions today using the materials we already have.