1.4.7.Thermal energy storage

thermal energy storage (tes) relates to the temporary holding of thermal energy by adding heat or cold in substances to be used later, when it is needed. This technology is useful in increasing the efficiency of energy consumption by correcting the mismatch between the supply and demand of energy and in facilitating large-scale switching. there are various thermal energy storage technologies which correspond to different applications. Their selection for a particular application is based on many factors such as storage duration, storage capacity, economics, supply and utilization temperature requirements, available space etc.

thermal energy storage systems can be classified according to the process used for energy storage as follow:

• sensible heat storage systems;

• latent heat storage systems;

• chemical heat storage systems (sorption and thermochemical).

thermal energy storage systems can be classified according to the storage concept as [8]:

• active systems:

  • direct systems;

  • indirect systems.

• passive systems.

An active system is a forced convection heat transfer into the storage material. In this system, the storage material circulates through a heat exchanger and uses one or two tanks for material storage. In a direct system, the heat transfer fluid serves also as storage medium, while in an indirect system, the heat transfer fluid changes heat with the storage medium.

An example of active direct storage system is shown in Fig. 10. The hot heat transfer fluid (molten salts: NaNO₃ and KNO₃) is directly stored in the hot tank to be used during cloudy periods or nights. The cooled heat transfer fluid is stored in the cold tank, where it remains waiting to be heated when the power demand is low.

Figure 10. operating diagram of the solar power plant Solar Tres/Gemasolar, spain [12].

Figure 11 shows the operating scheme of a solar power plant which uses active indirect storage of heat. The energy is stored not directly by the heat transfer fluid, but by a heat storage media (phase change material), heated by the heat transfer fluid pumped through the heat exchanger during the low power demand. The stored heat is taken by the heat transfer fluid during cloudy periods or nights.

Figure. 11. Scheme of a parabolic through power plant, with phase change material as storage system [13].

In a passive storage system the circulation of heat transfer fluid which exchanges heat with storage medium occurs without using mechanical equipment. An example of passive heat storage system is shown in Fig. [12]. In winter, the heat storage mass absorbs solar radiation during its exposure to direct sunlight and transfer that heat by convection to natural circulating air and by radiation into the space during the cooler night. In summer, the heat storage mass is prevented from receiving direct sunlight and absorbs the heat from the room.

The amount of thermal energy which can be stored by thermophysical and thermochemical processes can be expressed by the following equations:

[J] (5)

[J] (6)

[J] (7)

where: m – mass of the storage material, kg;

c_p – specific heat at constant pressure of the storage material, J/(kg·K);

t_i, t_f – initial and final temperature of the storage operation, K;

t_mf, t_r – average temperature of the phase change process (fusion for solid–liquid transition; vaporization for liquid–vapour transition), K.

Summer Winter

Figure 12. Passive heat storage system.

the quantity of sensible energy storage, Q_sensible, depends on the mass of the storage substance, m, specific heat of the storage substance, c_p, and the temperatures of the storage substance at the beginning, T_i, and final storage process, T_f.

The most common substances used for sensible energy storage are given in Tab. XVIII.