As you can imagine, the energy demand does often not occur at the same time energy is produced, especially in the case of renewable energy supply. A very clear example is the seasonal mismatch between heating demand in a relatively cold climate and the solar energy potential in the same climate. But there are also hourly differences between demand and supply occurring each day, as is illustrated in this figure. Thermal energy storage can be used to overcome this mismatch and make better use of energy potentials.
In general, it can be used to store thermal energy for later use. This can have many advantages, such as using energy when the price is low, or reducing the capacity of the equipment needed, sine the peak demand can be supplied with the thermal energy stored.
Thermal energy can be divided into three types: 1-Sensible thermal energy, which is related to the temperature.2- Latent thermal energy, which is the energy needed to achieve a phase change of a material and lastly 3-thermochemical energy, which is the storage of thermal energy in chemical bonds. In all these cases the energy is transferred to and from the storage system by heat. Today we will mainly discuss sensible thermal energy.
Sensible heat storage is achieved by changing the temperature of storage medium. This is usually water, but can also be sand or concrete. Also in the thermal mass of buildings – for example in concrete or brick walls – energy can be stored. The amount of sensible heat stored is calculated by multiplying the related mass with the specific heat capacity and the temperature difference before and after the storage is charged or discharged.
As an example the thermal energy stored in a hot water buffer is shown: The boiler has a volume of 200 , and the temperature is raised with 30 degrees Celsius. The specific heat capacity of water is 4,2 kJ/kgK.
The thermal energy stored is thus 200 times 30 times 4,2, which equals 25.200 kJ. If you divide this value by 3600, you will get the energy content in the unit kWh, resulting in a value of 7 kWh of thermal energy stored. This is enough for taking 7 showers of 5 minutes
The same principle is used for larger thermal energy storage, as can be used for smaller or larger multifamily buildings or non-residential buildings, and even for neighbourhoods. The very large tank shown on the left can even be used for seasonal storage.
As you can see, enormous volumes are needed for seasonal thermal energy storage. Therefore, also the subsurface is often used for seasonal thermal energy storage. Subsurface aquifers – underground layers of sand with ground water – can be used to store heat. These systems are called ATES systems – Aquifer Thermal Energy Storage. In these systems two wells are drilled, which will become a cold well and a warm well. In summer, ground water is extracted from the cold well. passes through a heat exchanger in order to cool a building. The heat from the building is transferred to the ground water, which is then injected into the warm well. In summer, the reverse happens: warm water from the warm well is extracted to be used for heating the building. Since the water stored usually has a maximum temperature of 25 degrees, a heat pump is used to further upgrade the temperature. The cooled ground water is injected in the cold well again. On an annual basis, the heat and cold extracted must be in balance.
Also ground heat exchangers can be used to cool and heat buildings, combined with a heat pump. These systems are called BTES systems, ‘Borehole Thermal Energy Storage’. Vertical boreholes are the most common, but also horizontal ground heat exchangers can be applied. As you can see below The figure on the right shows a scheme of a dwelling with a heat pump using a horizontal ground heat exchanger as source for the heat pump.
The feasibility of an ATES system depends on the properties of the subsurface. The following map shows the worldwide suitability for applying ATES systems.
Hence, as you have seen sensible thermal energy storage can be found in many sizes and for different storage periods, from short term to long term seasonal storage.
Now we would like to briefly mention latent thermal energy storage. In the change of phase of a material – for example from liquid to solid or from solid to liquid – a lot heat is required or released. This phenomenon can be used to store energy. The most well-known phase change material is obviously water: in the phase change from water to ice, the energy released it 80 times as much energy released when cooling down the same amount of water with one degree Celsius. For the built environment various other types of Phase change materials have been developed and are still being developed. These usually have a higher melting point, which means much energy can be stored at these temperatures.
