The sun is an energy source with low density and variable intensity depending on weather conditions. It is therefore essential to develop techniques to overcome the discontinuity of the resource. In the case of Solar Heat for Industrial Processes (SHIP) using parabolic solar concentration panels, the preferred form of energy storage is energy storage in thermal form.
Solar Parabolic Concentration: the best option for industrial solar heat
Developed at the beginning of the 1980s to produce electricity on a larger scale (notably with the SEGS projects in California), it is now known that given its high thermal efficiency; solar parabolic concentration offers the most advantages in the production of local heat in industrial environments.
The industry traditionally uses fossil fuels to produce heat and the use of fossil fuels is increasingly regulated. Parabolic solar concentration is therefore becoming an increasingly attractive option, especially with government subsidy programs and economic incentives that have been in place for several years to change our energy production methods and limit climate change. But technology must overcome the energy storage issue before it becomes truly unavoidable in an industrial environment.
The thermal storage issue
In order for a parabolic solar field to provide peak heat at night or early in the morning, thermal energy storage technology becomes a crucial element in returning this heat outside of daylight hours.
The different technologies developed
Solar concentration technologies typically use synthetic oil as a heat carrier fluid at operating temperatures below 350 degrees Celsius. The oil is therefore used to transport heat from concentrated solar rays. The oil thus makes it possible to produce a non-pressurized circuit unlike with the use of other heat transfer products.
One of the first storage technologies consisted of accumulating hot oil in an insulated tank, with another tank for cold oil and for drawing from the hot oil, which becomes a heat reservoir. Another technique consists of using a tank of thermocline oil; the hot oil accumulating at the top of the tank, the cold at the bottom. The hot oil can then be pumped from the top of the tank as required.
However, these two types of storage require a lot of oil which is very expensive. In addition, such an amount of oil can cause environmental problems in the event of an accidental leakage. The developed storage beds subsequently have the advantage of considerably reducing the oil requirement in a thermal storage system.
Thermal storage beds
For storage temperatures below 350 degrees Celsius, as in the case of a heat transfer circuit using synthetic oil, the thermal storage bed is a very effective solution. It represents the circulation of oil in a bed of dense material which is insulated in an adequate way. The material chosen for its density then accumulates the heat in its mass and can restore it later on. Bricks, steel or glass balls can be used and the oil circuit is circulated through this material to charge it thermally. This bed of material is then isolated in order to limit thermal losses, but generally these losses are minimal: approximately 1 to 3% daily.
For this compacted bed storage technique, the material chosen will be a critical parameter for the efficiency of the system. The simplest and least expensive material to use is stone or brick, but metal or glass beads can also be used, which offer higher thermal storage capacities. It is also possible to use phase change materials to store energy, such as paraffin, but the industrial applications of these phase change materials do not yet seem to have been very conclusive.
Pressurized systems and very high temperature thermal storage
It is also possible to produce steam from certain systems using the concentration of solar energy; the vapour becoming the heat transfer fluid. In this case, the system is under pressure and the storage of thermal energy is a greater hazard and can be much more complex to set up. Similarly, for solar energy concentration systems on a larger scale that produce very high temperatures (above 400 degrees Celsius); the heat transfer fluids used are often liquid salts such as sodium / potassium compounds. At this point, energy storage becomes more complex and costly because temperatures are very high and storage systems are much more complex and sophisticated.
The technological challenges associated with thermal storage beds
For temperatures below 350 °C, the thermal storage bed seems to be the most suitable and efficient storage system for industrial solar heat production systems, but there are several challenges that arise with using this technology.
Problems with charge-discharge time
In a heat storage system, one of the potential problems is the rate of thermal charge and discharge, meaning the time that the system needs to store and then recover the accumulated heat back into the system when the need is present. Presently, there are few solutions to improve the speed of this charging-discharging time which greatly depends on the physical properties of the materials used. This problem is especially present for very specific needs of clients, who want to use the heat of the storage system during very particular and precise hours. For storage purposes in operating the system continuously over long periods of time (such as being able to access the heat at night), the charging-discharging time does not pose any problems at the industrial level.
Problem with an automatic management in a transitional system
In an automated energy storage system, the problem of deciding whether to store energy can become very complex. In a system combining a solar field of parabolic panels, a thermal storage unit and an industrial process, several valves may or may not be operated, depending on the system data. In day mode and under optimal solar conditions, the solar field provides heat to the process and part of the heat to the storage system, via the storage system valve that partially opens. In night mode, the solar field valve closes, and the storage system valve opens to take over and continue to supply heat to the industrial process.
But in a transitional system, meaning a day with intermittent rain and sun for example, these choices of automatic opening of the valves become more complex. If the choices only rely on the local meteorological station of the system, the system may lose a lot of efficiency because it would then lack predictive capabilities. For example, if a storm of several hours is expected at the end of the day, the energy in the storage module should be predicted and stored quicker and earlier than in a normal day.
Increasingly, industrial solar technologies are turning to artificial intelligence applications in order to facilitate predictive decision making under such conditions and learn from past experiences.
In summary, energy storage for solar heat from processes has already evolved considerably and can already be applied in an industrial context with an installation comprised of parabolic solar collectors. With industrial temperature requirements of less than 350 °C, thermal storage in materials is currently the best solution for this type of technology. But there are still many areas that can be improved on in terms of the chosen materials as well as the automatic management of the loading times of such a storage system.
For more information:
Christian Dubuc, Technical Director at Rackam
Experimental results and modeling of energy storage and recovery in a packed bed of alumna particles. Anderson et al, Applied Energy (2014) 521-529.
Experimental and numerical investigation of a pilot-scale thermal oil packed bed thermal storage system for CSP power plant. Bruch et al, Solar Energy 105 (2014) 116-125
State of the art on high-temperature thermal energy storage for power generation, part 2- case studies. Medrano et al. Renewable and sustainable Energy reviews 14 (2010) 56-72