What is Thermal Energy Storage (TES)?

Thermal Energy Storage is a collection of technologies used to store energy by heating or cooling a storage medium. The goal is to use that stored energy at a later time for heating, cooling, or power generation. For a founder entering the hardware or energy space, it is helpful to think of this as a thermal battery. While the world is currently focused on chemical batteries like lithium-ion, heat remains one of the most significant energy challenges and opportunities in the physical world.

Energy storage is usually discussed in terms of electricity. However, a massive portion of global energy consumption goes toward changing the temperature of things. This includes keeping buildings cool, heating industrial furnaces, or pasteurizing food. Thermal Energy Storage allows a business to decouple the moment of energy generation from the moment of energy consumption. This shift is critical for efficiency and cost management.

In a startup context, understanding these systems is about more than just physics. It is about capital expenditure and operational stability. If you can store heat when energy prices are low and use it when prices are high, you change the fundamental economics of your operation. This is often referred to as load shifting or peak shaving. It is a practical way to manage the intermittent nature of renewable energy sources like wind and solar.

There are three main ways that engineers and founders categorize thermal storage systems. The first and most common is sensible heat storage. This method involves raising or lowering the temperature of a solid or liquid medium. Water is the most frequent choice due to its high specific heat and low cost. Other materials include molten salts, rocks, or even specialized bricks. The energy is stored simply by making the material hotter or colder without changing its physical state.

The second method is latent heat storage. This involves phase change materials, or PCMs. These materials store energy by changing their state from solid to liquid or liquid to gas. When a material melts, it absorbs a large amount of energy at a constant temperature. This is known as the heat of fusion. When it solidifies again, it releases that energy. Paraffin waxes and salt hydrates are common examples of PCMs used in these systems. This method often allows for much higher energy density than sensible heat storage.

The third and most complex method is thermochemical storage. This relies on reversible chemical reactions. You use heat to trigger a chemical reaction that separates two components. You then store those components separately. When you want the heat back, you allow them to recombine. This process has the potential for very high storage density and can hold energy for long periods with almost no loss. However, it is currently the least mature of the three technologies from a commercial standpoint.

When a founder evaluates storage options, they often default to lithium-ion batteries. While chemical batteries are excellent for high-density power and fast discharge, thermal storage offers several distinct advantages for specific use cases. One of the most important factors is degradation. Lithium-ion batteries lose capacity every time they are charged and discharged. Most thermal storage systems, especially those using water or rocks, do not suffer from this same type of chemical wear.

Cost is another significant differentiator. On a per kilowatt-hour basis, thermal storage is often significantly cheaper than electrochemical storage. This is particularly true for large-scale applications where the storage medium is an abundant material like gravel or water. If your business model involves long-duration storage or high-capacity heating and cooling, the lower capital requirements of thermal systems can provide a more sustainable path to profitability.

Safety profiles also vary between these technologies. Chemical batteries carry risks of thermal runaway and fire. While high-temperature thermal storage systems require careful insulation and containment, they do not generally pose the same risk of explosive failure. This can simplify the regulatory and insurance hurdles for a startup building physical infrastructure. However, thermal systems are often much larger and heavier, which makes them less suitable for mobile applications or small-scale residential use.

There are several scenarios where a startup might find thermal storage to be the optimal choice. One common use case is in commercial HVAC systems. A building can use electricity at night when rates are low to freeze a large tank of water. During the day, the melting ice provides cooling for the building, reducing the need for expensive daytime electricity. This is a proven way to reduce operational costs and improve the carbon footprint of a facility.

Industrial heat is another major opportunity. Many manufacturing processes require high-temperature steam or direct heat. Using renewable electricity to heat molten salt or ceramic bricks allows a factory to run 24/7 on clean energy. This decouples the factory from the fluctuations of the power grid. For a founder, this represents a chance to build a resilient supply chain that is less vulnerable to energy price spikes.

Grid-scale storage is perhaps the most ambitious application. Concentrated solar power plants use mirrors to heat molten salt during the day. That salt is then used to produce steam and turn turbines long after the sun has set. As the world moves toward 100 percent renewable grids, the ability to store energy for twelve hours or more becomes essential. Thermal storage is currently one of the few technologies capable of meeting this need at a reasonable cost.

Despite the potential, there are several unknowns that provide opportunities for new research and business ventures. One of the biggest challenges is round-trip efficiency. Every time you convert energy from electricity to heat and back to electricity, you lose a significant portion of it. Scientists are still searching for ways to minimize these losses and improve the thermal conductivity of storage mediums. This is a wide-open field for material science startups.

Longevity and corrosion are also major concerns. Molten salts are highly corrosive and can damage the tanks and pipes that hold them. Finding cost-effective materials that can withstand these environments for thirty years is a difficult engineering problem. Furthermore, we do not yet fully understand the long-term behavior of many phase change materials when they undergo thousands of cycles. For a founder, these gaps in knowledge are not just obstacles; they are the areas where the most valuable intellectual property will likely be created.