Your burning energy questions answered: Part II

Hybrid wind-hydro power station Ringwallspeicher
Ringwallspeicher-Hybrid power station: an example of a hydropower project using wind energy for pumping proposed by German engineer Dr Ing. Mattais Popp. Image: Stefan Schiessl, Dachau, Germany.

Is pumping water for hydroelectric dams a good way of storing solar energy?  And what is stopping humankind from harvesting more solar energy in the first place?

Iceland’s Thorsteinn I. Sigfusson, one of the country’s top experts on alternative energy, answers these questions from Eco-Business readers in the second part of the Global Energy Prize series.

Reader Ger Groeneveld asked: Trusting that rain and melting glaciers and snow will provide enough water to keep large dam hydro up and running is not very likely in areas with heavily changing climate patterns. How much water could be pumped back by intermittent generated renewables like solar installed on lake floating barges?


In many hydroelectric stations around the world the energy producing company makes use of the possibility to utilise the excess generation capacity to pump water into the dam at times of low demand. At times of high demand, and high prices of electric energy, the stored water is used to run the turbines of the hydroelectric generation. If the price fluctuations are large enough this can be a very profitable method of storing energy.

Wind energy is a very suitable method for this purpose and a number of installations are in use all over the world.

In your question you ask specifically about solar installation on floating barges. What you really need is a lake which is situated lower in altitude from your dam. By pumping the water from the low reservoir to the high reservoir you change the potential energy of the stored water.  A difference of 100 metres will give a kilogram of water an extra kiloJoule of energy. Releasing such a kilogramme of water in a second yields a kilowatt of electric power. This is of the order of magnitude similar to your tea kettle.

In your question you ask how much water could be pumped back by intermittent renewables, there are almost limitless possibilities. What will limit the exercise is perhaps the available water at the lower reservoir. If you could select the ocean as your lower reservoir, the balancing act will depend upon the price range of your electricity.  Let us assume that you have a low  night-price and a high day-price of electricity. If the difference is large enough you could in principle increase your daily output from the hydroelectric station to the limit allowed by the construction of the station involved.

However, before embarking on the project, you may consider using the solar energy directly and saving the about 20 per cent losses you are bound to suffer in efficiency with the pumping exercise. Power engineering is a challenge.

Another reader, Nicole Witherbee of the United States posed this question: If annual insolation is so large, why don’t we utilize solar conversion technologies more extensively?


Over the course of a year the average solar radiation arriving at the top of the Earth’s atmosphere at any point in time is roughly 1,366 watts per square metre. This is often called the solar constant.

The radiant power is distributed across the entire electromagnetic spectrum although most of the power is in its visible light portion. But this is not all; the Sun’s rays diminish as they pass through Earth’s atmosphere thus reducing the insolation at the Earth’s surface to approximately 1,000 watts per square metre for a surface perpendicular to the Sun’s rays at sea level on a clear day.

In reality, the insolation also varies because of haze and cloud cover and may on the average only be 250 watts per square metre.

Utilizing this in space and bringing the harvested energy down to ground would be expensive.

Satellites can do this locally and make use of the power thus produced. Beaming such power to the surface would be cumbersome. So we are left with installations on the ground facing the sun.

Direct solar conversion:

Let us start with the easiest part – to collect the sun rays with mirrors and direct them to a central power generator – let us assume a steam generator. This is possible but has proven expensive due to cost of construction, materials etc.

Such, so-called direct solar conversion, was first tested by Auguste Mouchout in 1866. The first plant was built near Genoa in Italy in 1968.

The cost of installed power and the cost of the generated power are still much more expensive than the least expensive renewable energy power stations, large hydroelectric ones.

Photon energy conversion into electric energy:

OK, let us use physics and convert the photon energy directly into electric energy by the use of the photovoltaic effect where electrons are pumped between energy bands in the crystal and resulting in the buildup of voltage difference between two electrodes.

The efficiency of devices made for photovoltaic conversion varies. The top of the line is above 20 per cent. To answer your original question and with our 250 watts per square metre you are down to about 50 watts per square metre.

Cost of materials, systems and the like are still likely to be very high compared with simpler renewable energy methods such as hydroelectric. But constant improvements in technology bring us closer to economic feasibility. Until now, only 40 GW of electric power has been installed in photovoltaic systems worldwide.

About The Global Energy Prize

The Global Energy Prize is one of the world’s most respected awards in energy science, awarding over US$1million every year for outstanding energy achievements and innovations.

Thus far, the Prize has been granted to 24 scientists from around the globe, including past Laureates from the US, Great Britain, Canada, France, Germany, Iceland, Ukraine, Russia, and Japan. The President of the Russian Federation participates in each year’s award ceremony held at the conclusion of a week-long celebration of the awardees’ work, Laureates’ Week.  Other world leaders who have supported the prize include the former US President George W. Bush, former British Prime Ministers Tony Blair and Gordon Brown, former French President Jacques Chirac and current Canadian Prime Minister, Stephen Harper.

The Global Energy Prize rewards innovation and solutions in global energy research and its concurrent environmental challenges. The degree to which a development contributes to the benefit of humanity is a key driver in deciding the recipient of the Prize.

The panelists

The award-winning scientists on the panel provide a global perspective on a wide range of topics that directly affect Asia, for example, renewable energy industries, national energy policies and climate science. They are all members of the International Award Committee chosen to determine this year’s Global Energy Prize.

Thorsteinn I. Sigfusson is a pioneer of the energy research in Iceland, President of Icelandic New Energy Ltd., Chairman of the thermoelectric company Genery-Varmaraf Ltd. and Professor of physics at the University of Iceland. He has been a key figure in the introduction of new ideas and opportunities in the further greening of Icelandic society through the energy industry. Dr. Sigfusson was awarded Global Energy Prize in 2007 for his extensive work in the development and research in the field of hydrogen energy generation in Iceland.

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