In our final installment this week, here is a rather unconventional solution for storing surplus energy. If only Sisyphus had known about this...
Batteries are rapidly becoming less expensive and might soon offer a cheap short-term solution to store energy to meet daily demand. However, the long-term storage capabilities of batteries, for example, in a yearly cycle, will not be economically viable. Although pumped-hydro storage technologies are an economically viable choice for long-term storage of large amounts of energy storage – over 50 megawatt hours – it becomes expensive for locations where the demand for energy storage is often smaller than 20 MW with monthly or seasonal requirements, such as small islands and remote locations.
In a study published in the journal Energy, researcher Julian Hunt of the International Institute for Applied Systems Analysis (IIASA) and his colleagues propose Mountain Gravity Energy Storage (MGES) for closing the gap between existing short- and long-term storage technologies. MGES works by setting up cranes at the edge of a steep mountain with enough reach to transport sand (or gravel) from a storage site located at the bottom to a storage site at the top. A motor/generator moves storage vessels filled with sand from the bottom to the top, similar to a ski lift. This process stores potential energy, while the reverse, transporting sand from the upper storage site back to the bottom, causes electricity to be generated. The presence of rivers or streams on the mountain allows the combination of MGES with hydropower: In times when water is abundant, it can be used to fill the storage vessels instead of the sand or gravel, thus generating energy. MGES systems have the benefit that the water could be added at any height of the system, thereby increasing the possibility of catching water from different heights on the mountain, which is not possible in conventional hydropower.
Not a replacement for current storage
“One of the benefits of this system is that sand is cheap and, unlike water, it does not evaporate – so you never lose potential energy and it can be reused as often as necessary. This makes it particularly interesting for dry regions,” notes Hunt. “Additionally, PHS plants are limited to a height difference of 1,200 meters, due to very high hydraulic pressures. MGES plants could have height differences of more than 5,000 meters. Regions with high mountains, for example, the Himalayas, Alps or Rocky Mountains, could therefore become important long-term energy storage hubs. Other interesting locations for MGES are islands, such as Hawaii, Cape Verde, Madeira and the Pacific Islands with steep mountainous terrain.”
In their study, the IIASA researchers propose an example of a future energy matrix for the Molokai Island in Hawaii, using only wind, solar, batteries and MGES to meet the island’s energy demand. Hunt emphasises that the MGES technology should not be used for peak generation or storing energy on a daily basis – instead it fills a gap in the market for locations with long-term storage. MGES systems can, for instance, store energy continuously for months and then generate power continuously for months or when there is water available for hydropower, while batteries deal with the everyday storage cycles.
“It is important to note that the MGES technology does not replace any current energy storage options, but rather opens up new ways of storing energy and harnessing untapped hydropower potential in regions with high mountains,” Hunt concludes. (mfo)
Here, in case you missed them, are Part 1, Part 2, Part 3 and Part 4 of our current series. Next week we will talk about something more directly related to money.
