Thermal energy storage

We can save heat for later.

Heat can stay in big tanks. We can use it to warm homes.

It helps us when the sun goes down. It can even save summer heat for winter. We can store heat in water or ice. Some people use hot rocks or sand too. This is a smart way to use energy.

We can save heat to use later. This is called thermal energy storage.

It helps us balance energy needs. We can save sun heat for the night. We can even save summer heat for winter.

One way to store heat is sensible heat storage. This means we change the temperature of a material. Water tanks are a very common choice. They are cheap and safe to use.

Some systems use molten salts. These are salts that have melted into a liquid. This works well with solar power. The liquid salt gets very hot in the sun. It stays hot in an insulated tank. Later, we can use that heat to make steam. That steam can turn a turbine to make electricity.

Other ways use different materials. Some people use hot rocks, sand, or concrete. These can get very hot. However, they need a lot of space.

This is because they do not hold heat as well as water. Scientists are also studying silicon. It can hold a lot of power at very high heat.

Thermal energy storage is a way to save heat for later use. This process helps us balance energy needs throughout the day and night. We can store summer heat to use for heating in the winter. We can also save winter cold to help with cooling in the summer. This is often called seasonal thermal energy storage. It is a vital tool for using renewable energy like wind and sun. These sources can be variable, so storing their extra energy is very helpful.

One common way it works is called sensible heat storage. This method simply changes the temperature of a material. We can use water tanks because they are safe and inexpensive. Some systems use molten salts, which are salts melted into a liquid. The liquid salt is pumped through solar collectors to get very hot. Then, the salt goes into an insulated tank to stay warm. When we need electricity, the hot salt makes steam to turn a turbine.

Scientists have studied many ways to store this heat over time. The Solar Two project tested molten salt technology from 1995 to 1999. In 2013, a plant in Spain called Gemasolar produced electricity for 36 days straight. The Cerro Dominador plant in South America started in June 2021. It can store heat for 17.5 hours. Other projects use large underground spaces to hold heat. For example, a water cistern in Finland has an 11.6 GWh capacity.

Different materials offer different strengths for these big jobs. Water is great because it has a high thermal capacity. However, solid materials like rock, sand, or concrete can get much hotter. Concrete can reach temperatures of 1200 degrees Celsius. Researchers are also looking at molten silicon for even higher heat. Silicon can store a lot of energy at 1400 degrees Celsius. Even recycled aluminum is being studied for its high energy density.

These technologies connect to the world we live in every day. Many towns use district heating networks to share warmth. In Germany, a concrete store helps 570 houses with heat. In the United States, the Solana Generating Station uses molten salt. This plant can store 6 hours of energy to use later. These systems help us use more clean energy from the sun and wind. They make our energy systems much more reliable for everyone.

Thermal energy storage, or TES, is the process of saving heat for later use. This technology is vital for modern energy systems. It allows us to balance energy demand between different times, like day and night. We can even store summer heat to provide warmth during the winter. This is known as seasonal thermal energy storage. By saving excess energy, we can better integrate renewable sources like wind and solar power. These sources change constantly, so storage helps keep the energy supply steady.

There are three main categories of thermal energy storage. These are sensible heat, latent heat, and thermo-chemical heat storage. Each method has different advantages and disadvantages. Sensible heat storage is the most common and commercially available method. It works by simply changing the temperature of a substance. This substance is called a storage medium. Other methods are still being developed for specific uses. The choice of medium depends on the temperature needed and the space available.

Sensible heat storage (SHS) is a very straightforward process. You increase or decrease the temperature of a medium to store energy. Water is a very popular choice for this. Water has a high thermal capacity of 4.2 kJ/(kg·K). This means it can hold a lot of heat. However, SHS often has a low energy density. This means you need very large volumes or tanks to store much energy. Some systems use underground storage, known as UTES. This can involve pipes in boreholes or trenches filled with heat-transfer fluids.

Molten salt technology is a specialized type of sensible heat storage. It is used frequently in concentrated solar power plants. In this process, various salts like sodium nitrate or potassium nitrate are melted into a liquid. The liquid salt is kept in a cold storage tank at about 260 °C. It is then pumped through solar collectors. The focused sun heats the salt to about 565 °C. The hot salt is moved to an insulated hot storage tank. This salt can stay hot for up to a week.

When electricity is needed, the hot molten salt is pumped to a steam generator. This creates superheated steam to drive a turbine and generator. This process can turn heat into electricity long after the sun has set. The Solar Two project demonstrated this from 1995 to 1999. In 2013, the Gemasolar plant in Spain achieved a major milestone. It produced electricity continuously for 36 days. The Cerro Dominador plant, opened in June 2021, can store heat for 17.5 hours.

Researchers are exploring even hotter materials for higher efficiency. Molten silicon is one such possibility. Silicon can store more than 1 MWh of energy per cubic meter at 1400 °C. It is also very abundant in nature. Another material being studied is molten aluminum. A Swedish company called Azelio developed technology using aluminum heated to 600 °C. Aluminum has a high energy density, which is helpful. However, it is difficult to handle because it can react easily or solidify unexpectedly.

Solid materials like rock, sand, and concrete are also used for storage. These materials have a heat capacity about one third that of water. This means they require much larger volumes to store the same amount of energy. However, they can reach much higher temperatures. Concrete can be heated to 1200 °C. In Germany, the Wiggenhausen-Süd development uses a 1,000 m³ concrete store. This system helps 570 houses with their heating and hot water. In Finland, large water cisterns and rock caverns help manage seasonal heat.

Thermal energy storage connects many different parts of our world. It links the electricity sector with the heating sector. This is especially important for towns using district heating networks. Large-scale projects, like the 90 GWh facility planned for Vantaa, show the growing scale of this technology. These systems help us use more renewable energy. They turn variable sources like the sun into a reliable, constant supply for homes and industries.