Treaty on the appropriateness of thermal energy storage

Summary :

Today reduction of our fossils energetic reserves, the imperative need to preserve our environment under threat to see extinction of the human species, commit us to promote sustainable strategies for energy management.

Furthermore, energy demand is fluctuating, seasonal, unsteady with peaks and slack period consumption.
We hear talk more and more about intermittent energy sources such as solar and wind energy or wind power.

Naturally a serious energy management requires for their rational utilization, a storage phase between the production and use.

It is now recognized that storage has a significant impact in the optimization of the coupling between energy production and use, including minimization of sizes for conversion systems in order to approach their nominal operating speed and so reaching the best performance, the thermal energy doesn't escape this necessity. This is the right strategy for a delayed use, leaving so the possibility to choice the better price period of energy acquisition, that is a real tool for energy management.
Various technical applications storage may take place on the production site or on the site of use, in any cases it is an essential factor for an efficient installation.

Storage devices to heat or cold are therefore essential components for optimizing thermal energy. These are the best tools to ensure consistency of applications and uses energy, intermittent or discontinuous unsynchronized.

It is known that the potential for energy recovery, based on a simultaneous production and use and located on the same place, is extremely small. The non-simultaneity and a significant distance between production and use is the general rule.

This observation leads us to consider studying ways needed to treat most common cases that we identified where production and use are unsynchronized and not localized in one place:

- Thermal storage associated with the production site where the site of use should allow to synchronized the energy resources and the need,
- Transport of heat (or cold) to allow movement of the heat source to the storage or storage for use in conditions where energy costs are minimized: energy circulating pumps, loss / heat input on pipes or transport devices.

Regarding the thermal storage, many technologies have been studied and developed to a certain industrial stage: the diversity of solutions is more related to the principles of storage, than the materials with thermal capacity required, or to the profile of charge and discharge than the only acceptable costs. The principles of thermal energy storage are three in number. This is the sensible heat storage, the latent heat storage and thermo-chemical storage. These are principles that involve specific power and storage temperature range.
Thus, the specific capabilities - ratio of energy stored in the volume of active materials in play in these different methods of storage are shown below and give a sight of the real differentiation between these solutions:

• Sensible heat (solid or liquid): ~ 50 kWh/m3,
• Latent heat (phase change) ~ 100-150 kWh/m3,
• Storage for thermo chemical> 300-500 kWh/m3.

To understand the challenge and complexity of the issue for optimal management of an energy resource as heat, it is interesting to imagine the structure of an energy network starting from the heat source to the use that we know extremely various. Indeed, the chain of the heat source for use is a succession of phases of heat transfer, transport, and energy conversion as the transformation of thermal energy into mechanical or electrical energy and too different heat temperature level.

In this paper, after reminding the adequate context of development of energy storage follow introductive elements about storage. After that we describe several materials, equipments and systems suitable for heat storage energy. Penultimate we describe valuation techniques and heat energy potential and at last we sum up some of technical solutions for storage and heat transport suitable for waste from various sectors.