The energy system is rapidly transforming to include more weather dependent energy production. This is a positive development from an environmental perspective, but comes with challenges for the power grid. We have earlier explained why the energy transition requires an increased amount of grid storage to bridge the power between high- and low production hours. This article aims to explain why pumped hydro storage (PHS) is a vital piece of the puzzle and why underground PHS makes both environmental and financial sense.
The power grid in Europe has a frequency of 50 Hz. This must be stable at all times in order not to wreck machines that is connected to the grid as well as maintain its stability. If the power production exceeds the consumption this frequency will increase and vice versa. It is therefore vital that the production and consumption always matches. In a grid where sudden changes in weather conditions can lead to drastic changes in power production, it will be increasingly hard to maintain this balance.
Pumped hydro storage represents more than 94% of the installed storage capacity today. It is therefore a well-established technology with a low technical risk. Furthermore, it has the possibility to store enormous amounts of energy, where the largest PHS-facility can store enough energy to power 1 200 Swedish houses for one year.
However, traditional PHS faces two major challenges in today’s energy grid. Originally it was developed to work with arbitrage and price differences on an hourly scheme, which makes it too slow to balance fast power production fluctuations. It is also very difficult to get return on such an investment as the price differences are too small. Furthermore, the construction of reservoirs will severely affect local ecosystems and water bodies which makes it extremely hard and time consuming to acquire the permits that is required.
By building new PHS underground in abandoned mines, not only could these industrial sores on the landscape be used to facilitate the green energy transformation and consequently minimize the impact on local environments. Modern technical equipment and solutions can also adapt the system for the modern needs of the grid with fast response times. PHS does not only work with the increasing differences in power prices, but also with revenue streams from for ancillary services on markets such as the frequency regulation markets.
Underground PHS will be a lot smaller than the largest PHS facilities, however they will have an energy capacity of several hundreds of MWh (a standard Tesla model S battery is 0,07 MWh). This might not be enough for i.e. seasonal storage, but is enough to balance the production of a wind power park. How underground pumped hydro storage can complement wind power will be further investigated in future articles.