Energy storage is the way to conserve energy in one form and release it when needed in the same or another form. Energy storage is used to store both thermal (heat and cold) and electrical energy by electrochemical, electrical, mechanical and thermal methods. Conventional technologies used to store energy are alkaline batteries, capacitors, hydroelectric and sensible heat storages. Energy storage has always been closely associated with solar installations, including both solar heating and no grid connected
photovoltaic applications.

One of the problems of the energy systems is the way to match the demand and the supply of the energy, when we focus on buildings and other industrial sectors, a variation on the demand occurs by different cycles, as well, the utilities must produce an amount of energy greater than the demand at every moment in order to guarantee the energy supply and to have a sufficient safety margin.
Furthermore, the power plants of energy production do not react adequately well to the changes of load in the network, reason why the margin still turns out to be greater at some moments. Likewise, the renewable energies (that are used more and more due to the exhaustion of fossil fuels) are very variable in the time; it can have moments at which they produce large amount of energy and just a moment later, the production can fall abruptly. This is because they depend on external factors that, in addition, are not yet fully controlled: in a day with cloudy intervals the solar energy will be irregular, and along the year more still; with the wind it is exactly the same. Evidently these moments of fall in the production are not generally going to coincide with the demand, reason why becomes necessary a system that should able to solve this problem.

Thermal energy storage
The energy sources normally used for heating and cooling are oil, gas, coal and electricity. However, it is not entirely logical, nor efficient, to burn fossil fuels at temperatures up to 1000ºC in order to create an indoor climate at 20-25ºC. . Neither is it efficient to use electric power, a form of highly processed energy, only for resistance heating. There are basically three concepts applied to thermal storage devices which are being investigated at present by the international research society and some industrial players:

• specific (sensible) heat storage,
• latent heat storage (phase change materials),
• thermochemical heat storage.

The main technological developments for thermal energy storage (heat/cold) are:

• underground thermal energy storage (UTES),
• water tanks above ground,
• rock filled storage with air circulation,
• phase change materials (PCM),
• thermochemical storage.

The principal benefit of MESSIB thermal storage capacity is the increase of the energy efficiency and indoor comfort in buildings by the reduction of the energy demand and the decrease of thermal gradients and temperature variations. Thermal energy will be moved within the building through space and time to take advantage of this energy that otherwise would be wasted.
Most heat storage concepts with the exception of latent storage have one basic challenge in common.
When heat or cold is charged into or discharged from the store, there will be temperature differences in different parts of the storage volume. It is then of the utmost importance that the storage medium can maintain a stratified layer, for instance with the warmest water on the top, and the coldest at the bottom.
The effective storage capacity will be drastically reduced if mixing occurs and the overall temperature approaches some sort of average value over the whole volume. Thermal energy storage systems themselves do not save energy. However, energy storage applications for energy conservation enable the introduction of more efficient, integrated energy systems and reduce the energy demand.pe of energy source: Type of stored ene

Electrical energy storage
Electrical energy storage enables the decoupling of electricity generation from demand. This is of particular importance to the electricity industry since electricity demand is subject to substantial hourly, daily and seasonal variation. Also, electricity generation, particularly from renewable sources, is also subject to significant variability, both short term (over a few seconds) and longer term (e.g. hourly, daily and seasonal).
Many electrical storage technologies have been considered in the context of utility-scale energy storage systems. These include:

• pumped hydroelectric storage,
• batteries (including conventional and advanced technologies),
• superconducting magnetic energy storage (SMES),
• flywheels,
• fuel cell/electrolyser systems,
• conventional capacitors,
• supercapacitors/ultracapacitors.

Each technology has its own particular strengths and operational characteristics. For example, pumped hydro is best suited for large-scale bulk electrical energy storage. For short-term energy storage, rapid power delivery is often more important than energy capacity and so SMES have beneficial characteristics. Battery energy storage tends to be the most flexible storage medium, finding use in both bulk energy storage and short-term power quality applications, but have usually associated environmental and durability problems. Hydrogen is an energy vector, that must be converted in energy,
the H2 fuel cell technology is a more or less mature technology, demonstrated in many cases in building with the integration with RES like the “aeropila” demonstrator project in Spain, but has a lot of weak points: Hydrogen is a very light gas, which is a handicap for storage and transmission9 and related safety problems due its combustion in air.
The new use of electrical storage capacity in buildings of MESSIB will permit to fit the energy demand with the generation, optimizing the power use from the grid. Furthermore, this will favour the penetration of RES in buildings, microgrids and distributed energy concepts.
Current electrical energy storage in buildings is only applied to UPS systems in banks or big offices to increase the data protection.

 Smart grids