As nations install more and more renewable energy sources in pursuit of the “green revolution”, so the need increases for other sources which will ensure a steady supply of energy when the sun is not shining or the wind is not blowing. At present such backup power is commonly supplied by gas turbine generators powered by natural gas, which are quick-starting and thus more flexible than traditional coal-fired power stations. While natural gas is a fossil fuel, it burns much cleaner than coal and oil, emitting 50-60% less CO2 when used for power generation, and for this reason it is regarded as a “transition fuel”, providing backup and grid stabilisation until such a time as “greener” alternatives are available.

Non-fossil fuel burning power sources such as hydroelectric and nuclear are not widely available as backup, so there is an increasing need for some form of power storage. Grid-scale lithium ion or lithium iron phosphate batteries are beginning to be used. but are best suited for short-duration storage (under 8 hours), due to their sensitivity to degradation at high states of charge. A promising development is the use of vehicle-to-grid (V2G) storage in which electric cars are connected to the grid so that their batteries can act as back-up, but its viability will depend on a much greater uptake of electric vehicles by the general public.

For larger-scale and especially longer-term power storage, various techniques are available. Chemical storage involves the use of gases or liquids such as hydrogen or ammonia. For example green hydrogen can be produced from the electrolysis of water, stored and converted back into electricity via an internal combustion engine or a fuel cell, with a round-trip efficiency of roughly 41%.

Mechanical storage includes the use of flywheels to store power in the form of kinetic energy – not commonly used – and pumped hydro storage, which transfers a large body of water from a lower to an upper level when surplus electricity is available, and releases it through turbines to generate power when needed. According to Wikipedia, “As of 2023, pumped-storage hydroelectricity was the largest form of grid energy storage globally, with an installed capacity of 181 GW, surpassing the combined capacity of utility-scale and behind-the-meter [i.e. domestic or commercial consumer] battery storage, which totalled approximately 88 GW.”

Crete, with its mountainous centre, is well suited to the use of pumped storage systems, and if they have not been used hitherto, the high cost and the need for long-term planning are two factors which may have prevented it, in a country only recently emerged from a severe financial crisis. However, as Giannis Vourdoubas – a chemical engineer and regular contributor to Haniotika Nea – points out in a recent article, the pressure for “green” solutions has led to an upsurge of interest in the subject. An edited version of the article follows.

A pumped storage project in Crete

Europe is at a critical stage in the energy transition which aims to reduce carbon dioxide emissions and achieve independence from fossil fuels. The rapid development of renewable energy sources, chiefly wind and solar, is creating new challenges for the stability of electrical grids. Given that power production by these sources is intermittent and dependent on weather conditions, there is a need for effective large-scale storage systems.

Pumped hydro storage is the most mature and reliable technology in this field. It offers the possibility of storing enormous quantities of energy with high output and for a long periods of time. In addition it can respond immediately to fluctuations in demand, ensuring the stability and security of the European electrical system.

The need for such projects in Europe is especially acute since the continent is pursuing energy independence and the reduction of dependence on imported fuels. Pumped storage systems can act as a “bridge” between production and consumption, balancing surpluses and shortages of energy. At the same time they contribute to reducing the cost of electrification, since they reduce the need for expensive auxiliary generating units. Altogether, the development of pumped storage systems in Europe is not just a technical choice, but a strategic necessity for the successful transition to a cleaner, sustainable and resilient energy future. …

Pumped hydro storage — a mature technology
Pumped storage is the most basic large-scale energy storage technology in Europe and already covers around 90% of the continent’s total storage capacity. Today its development is receiving new impetus because of the increased penetration of renewable energy sources and the need for stability and security in electrical grids. …

Pumped storage projects in Europe will result in:
a) speeding up the development of wind and solar voltaic technologies, protecting these systems from interruptions and ensuring their commercial viability in the long run;
b) ensuring the resilience of the electricity system and reducing the danger of blackouts, as well as contributing to the process of recovery from major faults;
c) boosting energy security and reducing dependence on imported fossil fuels;
d) reducing the effects of climate change, offering productive and effective solutions to the management of vital water resources;
e) creating large power storage projects with major local added value;
f) offering the possibility of installing further photovoltaic systems on the surface of reservoirs with the possibility of simultaneous production of green electricity;
g) creating tens of thousands of new jobs and boosting national economies including in agricultural areas.

The technology of pumped hydro storage
The basic principle of pumped storage rests on the possibility of storing surplus electrical energy in the form of dynamic water energy. Specifically when there is surplus electricity production which is not used on the grid – for example from wind farms or photovoltaic installations – it is used to pump water from a low reservoir to a higher one. When the demand for electrical power increases, the water flows back into the lower reservoir, turning turbines which produce electricity.

This process can be repeated thousands of times with relatively low losses of energy. The total output of pumped storage systems varies between 70% and 80%, an especially satisfactory amount by comparison with other large-scale storage technologies. In addition, the lifespan of these installations is exceptionally long, often reaching five or six decades, which is economically advantageous in the long term.

The advantages of pumped storage are multiple. First of all it offers the possibility of storing enormous quantities of energy, something which is not possible with today’s battery technologies. In addition it can supply power to the grid very quickly, within a few minutes, contributing to stability and to coverage during peak demand. At the same time it facilitates the incorporation of renewable sources, ironing out the fluctuations in their output.

However there are some particular challenges. Installation of pump storage systems requires suitable geomorphology, that is to say the availability of two reservoirs with different heights. They often require major infrastructure works and significant investment, which can delay or restrict their application. At the same time they need careful environmental management, so that they do not negatively affect local ecosystems and water resources.

To summarise, pumped storage is a reliable, effective and tested technology, which offers solutions to the need for large-scale energy storage and stabilisation of networks, facilitating effective and sustainable exploitation of renewable energy sources. …

The pumped storage system at the Potamoi Dam, at Amari, in the Nomos of Rethymnon
The pumped storage system to be constructed at the Potamoi Dam at Amari in Crete is one of the most emblematic examples of the use of this technology in Greece. Crete has ample resources of wind and solar power, however the intermittent nature of power generation from renewable sources creates a problem of stability in the network. The pumped storage system at Amari will provide a solution, affording the possibility of storing surplus renewable energy and using it in periods of increased demand.

This solution will not only contribute to the island’s energy stability and security, but also to the reduction of Crete’s dependence on conventional fuels, aiding its progress towards energy self-sufficiency. At the same time the project will create significant benefits for the local community, both through employment, and through boosting sustainable development with substantial local added value. The planned system at Amari constitutes a prototype of how pumped storage can be incorporated harmoniously in island areas of the country with a high output from renewable energy sources. It could be said that the geomorphology of the island favours the development of further and pumped storage systems in the future.
(Haniotika Nea, 01/10/25)