The question of spare power has been the most significant problem slowing down the solar revolution.  Where does the electricity come from, when the sun is not shining?  And what shall we do with the extra power, when too much solar electricity is produced?

What became the InSIC network has been promoting an important partial solution to the problem since 2012, already before InSIC was officially established. The concept – which may have been first proposed by the InSIC network of scientists and solar energy activists -- has recently received serious attention in different parts of the world and is now best known as second-generation pumped-storage hydropower.

The concept of pumped-storage hydropower has, of course, been around for a long time. The basic idea is simple: whenever there is extra power, some of it can be used to pump water upstream to reservoirs, constructed for storing water. When there is a need for more power the gates can then be opened and the same water run downstream, through large hydro turbines generating electricity.

The method has some clear benefits. On average 15 per cent of the electricity that was consumed in the pumping of water upstream can be regained when the same water is run down through hydroturbines. Also, when more power is needed, the power-producing capacity of a hydropower plant can be raised from zero to 1 000 megawatts in twelve seconds.

The concept works well in a wide spectrum of scales.  Members of the InSIC team were involved in a pilot-scale project to implement a solar micro pumped storage system in a remote village of Manipur, India. The project provided reliable power to 50 households in the village through the pumped storage powerplant. The concept was to upgrade an existing 12-kilowatt hydro unit to a solar-pumped storage unit. This was done by constructing small reservoirs for storing water, installing a 40-kilowatt solar power plant and a 40-horsepower pumping system, and developing a master controller to automate the entire process. The system increased the plant load factor of the existing hydropower plant, gave it pumped storage capabilities, converted erratic power from solar photovoltaics to firm power from hydro and led to the development of a hybrid renewable energy system which proved to be effective in ensuring reliable power supply in remote areas. It might be a good idea to install millions of similar small hybrid solar power-hydro power systems in the world’s remote rural areas.

Pumped-storage hydropower is by far the cheapest way to store electricity. However, less than 150 gigawatts have been constructed, because it is difficult to find suitable sites for pumped-storage hydropower plants.

Because of this the InSIC network has been pushing for a complete rethink of the concept of pumped-storage hydropower – and hydropower in general.

InSIC has been saying, that instead of constructing new, separate pumped-storage hydropower plants, we should start looking at our whole, already existing hydropower and reservoir capacity with new eyes.

The world’s already existing hydropower plants and reservoirs should be converted to pumped-stored hydropower. In the future the primary function of reservoirs and hydropower plants should not be to produce small amounts of primary power – but to store huge amounts of wind and solar electricity, over and over again.

The nominal power-producing capacity of the world’s existing hydropower plants is 1100 gigawatts, but their average power production is only 400 gigawatts. Most hydropower plants run on less than full capacity for most of the year, because river flows fluctuate. In the rainy season there is more water in the rivers than during the dry season. This means that the existing reservoirs and hydropower plants could also be used as pumped storage plants with minimal additional investment, simply by pumping the water up, from one reservoir to the next, whenever some extra solar and wind power is produced. Even without any significant modifications this would provide us with up to 700 gigawatts of spare power.

If necessary, much more can be achieved by equipping the already existing hydropower plants with larger turbines during their next major overhaul, or by installing a larger number of parallel turbines in each dam.

India was the first country to take the concept seriously, but it has since then aroused some interest in instance in the United States, where the Los Angeles Department of Water and Power is planning to transform the whole Colorado River, including its iconic Hoover Dam, to second-generation pumped-storage hydropower. The 3-billion-dollar project should make it possible for the city of Los Angeles and the State of the California to produce almost 100 per cent of their power with renewables.