Last week we asked you to put your toughest energy questions to nine world-leading energy scientists. You responded with more than 350 serious and searching questions on everything from renewable energy to nuclear power. Here are 10 of the best questions, answered by the awards committee of nine scientists on the Global Energy prize.

1. Could we support our current western lifestyle with only "renewable" energy? Asked by Jim Burks

José Goldemberg answers: Presently "renewable" energy accounts for approximately 10% of the energy consumed in Europe. The impressive growth of energy produce from windmills, biomass and other renewables indicates that renewables as a whole could account for "circa" 50% of all energy consumed by 2050.

The present western lifestyle requires the energy equivalent of three tonnes of petroleum per year. Improving the efficiency of energy use (with more efficient automobiles, refrigerators and other end-use appliances as well as better home insulation) could reduce that amount by at least 30%. As is well known considerable efficiency gains have already been achieved in the OECD countries since 1973. Present energy consumption would be 50% higher than it is actually without them. That reduction could give renewables a better chance to replace fossil fuels.

2. Do you agree with the US Joint Forces Command (JFC) that spare capacity in global oil production may very well disappear in 2012 and a shortfall of 10m barrels per day develop by 2015? NoSurrenderMonkey (and others)

Clement Bowman: The word 'may' in the question, and the multiple use of the word "could" in the energy summary statement of the US JFC document, obliges one to accept the possibility that "yes it could". However, I believe that it is highly unlikely that there will be significant oil shortages over the next few decades. Once there is a perceived gap, forces come into play that cause the gap to be filled.

Here are some of the likely forces. Even modest increases in oil prices will convert unproven resources into recoverable reserves. Action on energy efficiency has finally taken hold in response to the need to reduce carbon dioxide emissions. New pipelines are under construction or planned in North America that will bring crude oil to refineries that have unused capacity. The enormous quantities of shale gas that have been discovered will provide part of the energy mix. The Canadian oil sands are just starting to ramp up with new more environmentally acceptable insitu recovery technology. China and India will use a combination of more efficient coal technology, nuclear energy and renewables to help meet their accelerating demand for energy. When I entered the oil industry in the 1960s, the conventional wisdom was that there was only 10 years of oil supply left. Predictions have a habit of failing.

3. The world's population is due to rise to 9 billion people. Can the planet supply the energy needed to achieve that end? ken brookes

Tom Blees: Widespread predictions that energy demand will double by mid-century to meet the needs of an expected 9-10 billion humans are, I believe, too conservative. Billions of people rely on now-shrinking glaciers for much of their water supply, with many areas of the world already lacking adequate water. Increasing human numbers by 50% means that we will have to provide most of the water for some billions of people primarily with desalination, an energy-intensive process. Add to that the fact that the majority of people in the world today use a fraction of the energy used by those in developed countries, and one could easily anticipate at least a tripling of demand in developing countries as they strive to improve their standard of living.

In the book Prescription for the Planet, I explained how a doubling of energy supply could easily be accomplished by 2050 at a rate of deployment even less ambitious than the French employed as they converted to nuclear power in the 1970s and 80s. Given the ability to factory-produce fast reactors of the type described here, a concerted global effort to meet mid-century energy demands should be quite within reach. The fuel is already available and - for all intents and purposes - virtually free.

4. I'm 25 years old. What's your best case scenario for the world's energy supply mix when I'm 75? What's your worst case scenario? And where you you think we'll actually be? Ian Bullock



Tom Blees: While there's widespread agreement that fossil fuels must eventually be abandoned, there seems to be no consensus on which technologies can be expected to take their place. The contenders already available run the gamut from some of the most diffuse energy sources (wind, sunlight) to the most energy-dense. While nearly all of the latter systems currently in use consist of light-water nuclear reactors, fast reactors can extract well over 100 times more energy from uranium, and are seen by most nuclear prognosticators as being the inevitable successors to light-water reactors and the solution to the looming global energy crisis.

All of the energy a person in a developed country today can be expected to use in a lifetime - for electricity, transportation, heating and cooling, and the energy that goes into producing all that they will consume - could be supplied by a single piece of depleted uranium the size of half a ping-pong ball. Despite all the controversy over competing technologies today, this amazing fact - plus the fact that it can supply all that energy safely and without harm to the environment - should eventually carry the day, leaving other energy sources as bit players on the world stage.

5. Is energy storage - ie battery technology - one of the biggest things holding back renewables and widespread energy efficiency? Look at the intermittancy of wind power, the requirements of a "smart grid city", electric cards etc - surely decent energy storage could transform the economics of these industries. And when might/how the problem be solved? Mago Salas (and others)

Alvin Trivelpiece: An excellent source of technical information regarding batteries is the Wikipedia site.

Even so, it does not answer the implicit theme of the question. Namely, why not large-scale energy storage in batteries to capture energy from intermittent sources such as wind or solar for use at times when the energy is required by a consumer?

The use of batteries for energy storage is a matter of the application and its need for a source of energy. Standard small batteries for toys and other convenience devices such as flashlights are examples where the cost per kilowatt-hour is irrelevant. The consumer pays the asking price and discards them without additional cost. Some solar applications using battery storage make great sense. Remote applications in the middle of desert where the cost of transmission lines is greater than the cost of a solar panel with some battery storage system. Same reasoning applies for spacecraft applications.

For other applications, the three laws of thermodynamics and the rules of economics must be taken into account. A simplified version of thermodynamics is: (1) You can't win, (2) You can't even break even, and (3) You can't get out of the game.

This means that you have to take all costs from cradle to grave into account and see if you make money selling the energy at competitive price. If you can do this without any subsidy, then you don't have a sustainable situation.

Unfortunately, when this is done for batteries, with all factors taken into account, it doesn't seem to come out favorably. That is, taking into account the cost of the raw materials including whatever environmental remediation might be needed, the transport of these materials to the location where fabrication takes place, cost of fabrication and distribution, the cost of disposing of the batteries, including the cost of maintenance during their useful life, etc.

Any energy storage or distribution scheme that doesn't make net energy, without subsidies, is not likely to be sustainable. Subsidies are a good way to get some products developed and deployed, but at some point it is usually assumed that the subsidy can eventually be eliminated, or justified on some non-economic basis.

6. How far away is nuclear fusion? Is it a realistic goal? Mischa Hewitt (and others)

Robert Aymar: There is a popular view that fusion energy has been just over the horizon for decades and it has failed to deliver. This is false.

Fusion has always been a long-term project; scientific progress in magnetic confinement of plasmas has been impressive and quantitative performances, achieved in the successive experiments, have from 1975 done better than the well known Moore's law of digital technologies.

On this ground, seven among the largest countries in the world ( China, Europe, India, Japan, Russia, South-Korea, US) have decided on the strategy to pursue the development of fusion through international collaboration and are building together the large facility, called "ITER", the first burning plasma to produce after 2025 half a gigawatt of fusion power.

This device , a physics experiment and an experimental reactor, should demonstrate the scientific feasibility of fusion as an energy source; it should validate and optimise the parameters and develop the technologies for the following strategical step, an electricity generating demonstration reactor to evaluate economics of fusion, before a commercial power reactor can be designed. Each of these steps requires around 40 years for its design, construction and enough operation time to capitalise on its results. Unless there is an urgency to provide a faster track (and pay for more risk), it is unreasonable to assume a sensible amount of fusion generated electricity in the grid before the middle of this century.

The need for new energy sources by the end of the century is undisputed. Besides coal burning plants, with total sequestration of the CO2 produced, large electrical power plants will possibly rely only on nuclear fission or fusion. Magnetic fusion has many appealing features (unlimited fuel reserve, safety and environmental characteristics), and from present analysis, its potential for energy generation is real and ITER will bring an experimental confirmation.

7. Why has tidal energy not been employed on a large scale (similar to hydro) anywhere in the world ? Is it down to cost or lack of efficiency? The Doc (and others)

Klaus Riedle: Tidal power differs from other renewable sources, in that it offers predictable though still intermittent power with decent power densities at certain preferred locations like estuaries or tide channels. One of the main barriers to large scale use is the cost of the back-up needed due to the intermittency.

Tidal range technologies make use of large tide differences by blocking off an estuary or forming a tidal lagoon and using a conventional water turbine in the dam to generate power from the tides going in and out, much like in a river. A large plant in Brittany, La Rance, has been operating successfully since 1966. Specific cost can be taken from planned projects on the Severn, which have an estimated 120-year lifespan, with commercial discount rates to €0.1-0.2/kWh stated by the UK Sustainable Development Commission [Last month, the UK government scrapped the Severn barrage tidal project on financial grounds]. Environmental concerns, related to whether the barrage causes harm to the estuary, will be a significant obstacle to their implementation.

Slowly rotating, large axial turbines make use of tide stream velocities above 1 metre per second. Like offshore windmills, such turbines are fixed to the seabed or even to masts, to be lifted out of the water for maintenance. Several prototypes have being tested in recent years; some projects around the UK are under planning. Little information so far is available about generating cost; a UK Carbon Trust study gives a range of €0.12-0.18/kWh. Environmental concerns and the impact on fishing and sea transport have to be addressed, as for offshore windfarms.

As with the other renewables, public support for tidal power should go into further development and testing of prototypes allowing them to find their niche in the market, rather than continuously subsidising power generation.

8. What are the barriers in getting our reliance on oil and petrol transferred over to electricity/hydrogen? Matt Flynn

Marta Bonifert: Fossil fuel reserves - like oil, petrol and coal - have been depleted quite rapidly in the recent years. This fact and the need to reduce the green house gas emissions of anthropogenic origin (global climate change) drive the business and the governmental sector to utilise renewable energy resources on a much wider scale. The transfer is not easy – there are technical, political and last but not least economical barriers. The efficiency of the new technologies has to be improved, there is a need for moderating the costs and legislation should support these new energy resources. Further more we cannot forget that oil is not only an energy resource but we use it in various forms – even in the human heart as artificial plastic valve – when needed.

So it is a very complex question which has implication on the economy, the environment and the society at the same time. Electricity and hydrogen will most probably be used at larger increasing extent substituting traditional energy carriers but again the question is their resources: whether they will be produced from fossil or renewable.

But there is a much easier and available solution which immediately helps to combat climate change: energy saving and energy efficiency. Changing our way of life, taking actions perceived as difficult however with a simple move – eg switching off the lights when they are not needed – we together can do a lot for the environment and the future generation.

9. Is it really possible to justify the legacy of nuclear waste for countless generations while we continue to waste electricity so carelessly, on things like flashy advertising and keeping buildings lit at night? Surely this should only be considered as a very last resort, when we have finally given up all such inessential useage of energy? greghaddock (and others)

Pius N'gwandu: Something must be done now. We cannot afford to add to our plight the luxury of the proliferation of nuclear waste. Yet the evidence of the threat from nuclear waste does not show that it comes from the generation of nuclear energy. The threat rather comes from the stockpile of the arsenal of nuclear weapons accummulated by nations with such weapons. Moreover the data on the utilisation of World Energy Resources nuclear energy accounts for only 16%.

With the development of nuclear science and advances in nuclear reactor technology and the international jurisprudence developed by the IAEA safeguards and safety measures have been developed for the peaceful use of nuclear power. In Africa we we have substantial uranium resources which are being mined and exported by large transnational corporations. Yet Africa suffers chronic shortages of energy which factor is a serious constraint to her development. International cooperation through the IAEA could reduce the danger for nuclear proliferation and dumping of nuclear waste by offering the latest technology in mining and mangement of thentire nuclear fuel cycle. Depleted uranium could be used to produce clean and safe energy Other uses of nuclear technology would include nuclear medicine, eradication of pests and vectors such as mosquitoes and tsetse flies which spread diseases such as malaria and tripanosomiasis (SIT). With the ominous prospects of mutual terror and extermination nations have no other rational choice but to learn fast to cooperate for the survival of the human species. Time is running out. We must move away from the self inflicted fear of nuclear energy. Let us combine knowledge, technology and the collective will to survive. Fifty years after President Eisenhower's speech on "atoms for peace", we must build the will to tame these atoms for peace, development and development.

10. What exactly is the carbon footprint of nuclear power (including uranium refining)? Dr Tim (and others)

Tom Blees: The IPCC has studied this and put nuclear in about the same category as wind and solar. See the IPCC Fourth Assessment Report section entitled Climate Change 2007: Mitigation of Climate Change. On page 293 of this report there is a chart that describes both non-biomass renewables and nuclear in terms of their carbon output as "small amount." The text of the report (on page 269) states: "Total life-cycle GHG emissions per unit of electricity produced from nuclear power are below 40 gCO2-eq/kWh (10 gC-eq/kWh), similar to those for renewable energy sources. Nuclear power is therefore an effective GHG mitigation option…" Cynics may point out that they mention a thoroughly debunked report that claims much higher life-cycle emissions, but the IPCC clearly found it unpersuasive.

It's important to note that the vast majority of CO2 emissions in the nuclear life cycle arise from uranium mining and enrichment. Deployment of integral fast reactors, however, will eliminate the need for both mining and enrichment for nearly a millennium, so the life-cycle carbon cost will be virtually nil, especially if the concrete used in the new plants is of the magnesium silicate variety that actually is carbon negative.