It does look like we can aim for 100% without too many problems. So what's stopping us?
Last year UK Energy Secretary Ed Davey said: "There are some countries with a very large nuclear industry. If they close, we don't have a cat in hell's chance of tackling climate change. I would love to think we can replace that with renewables alone, but frankly we won't be able to."
Is that really true? Over the last few years there have been many studies which have claimed that renewables could supply near 100% of the electricity needs of the EU and indeed the world by 2050, given proper attention to energy saving and the necessary political support.
Some have even suggested that as much as 100% of all energy could be supplied by renewables globally by around then.
Are views like this hopelessly optimistic? Or is a fully sustainable energy future based just on renewables possible? If it is, then we do not have to continue with the seemingly endless debate over nuclear power - it would not be needed.
So can it be done?
For some countries, near 100% renewable electricity is already a reality. That's mostly due to hydro, both large and small, which provides almost all of the electricity on the grids of many developing countries.
Currently hydro produces nearly 100% in Angola, Bhutan, Burundi, Costa Rica, D R Congo, Lesotho, Mozambique, Nepal, Paraguay, Tajikistan and Zambia, as well as 60-90% in 30 other developing countries.
Hydro is also very important in some industrialised countries. It generates nearly all the electricity in Norway, most of it in Iceland, and around 60% in Austria, Canada, New Zealand and Sweden.
Most of this comes from big hydro, and few environmentalists see large hydro as the way ahead, for two reasons: large dams have big and often unacceptable impacts; and most of the best locations for big hydropower have already been developed.
By contrast small hydro has much lower impacts, and offers far greater potential for growth: it has been said that only around 25% of the global potential has been used so far. Globally there are around 85 GW of small projects (under 10MW each), including around 48,000 in China. And it is expanding around the world.
'New' renewables - wind, solar, wave, tidal, geothermal
The 'new' non-hydro renewables - wind and solar especially - are expanding even more rapidly, catching up with the 17% or so of electricity generated by the world's 880GW of so of hydro capacity.
There is now nearly 300GW of wind capacity in place and over 100 GW of PV solar. In addition there is around 260GW (thermal) of solar heating capacity. And wave and tidal are coming along well, as is geothermal.
So where next? The World Energy Council's latest 2050 Global Energy Scenarios report suggests that the share of renewables in electricity generation will be between 31% (in its 'Jazz' market-led scenario) and 48% (in its 'Symphony' policy led scenario).
It does look like we can aim for 100% without too many problems. So what's stopping us?
That is a big jump for the traditionally rather conservative WEC, but still a way off 100% many studies claim is actually possible.
Even so, WEC says: "Electricity generation from renewable energy sources will increase around four to five times by 2050 in comparison to 2010 ... By 2050, globally, almost as much electricity is produced from solar PV as from coal."
No renaissance of nuclear is expected
By contrast it says "No renaissance of nuclear energy is anticipated." While in terms of total primary energy, "the share of renewable energy sources will increase from around 15% in 2010 to almost 20% in Jazz in 2050 and almost 30% in Symphony in 2050 ... nuclear energy will contribute approximately 4% of total primary energy supply in Jazz in 2050 and 11% in Symphony globally - compared to 6% in 2010."
Even the ever-optimistic International Atomic Energy Agency only thinks that, at best, "nuclear energy could account for about 15% of the total GHG reduction in power generation in 2050."
So the view from the establishment seems to be clear: renewables will rip and roar ahead, leaving nuclear well behind. That however still leaves a lot of fossil fuel being used by 2050, as well as some nuclear. Can we do better than that?
Renewables set to soar
The relatively conservative Global Energy Assessment, produced by an international team led by the International Institute for Applied Systems Analysis, concluded: "The share of renewable energy in global primary energy could increase from the current 17% to between 30% to 75%, and in some regions exceed 90%, by 2050."
To go beyond broad brush assessments like that, it may be helpful to look at some national examples. The Zero Carbon Britain scenario, produced by the Centre Alternative Technology, maps out an ambitious expansion of renewables so that they supply nearly all UK energy by 2030.
That may be pushing the pace to the limit. A more cautious High Renewables scenario produced by British Pugwash (on which I worked), makes use of the energy supply and demand model produced by the UK Department of Energy and Climate Change. It has renewables supplying around 80% of UK energy by 2050, but with a possible extension to near 100%.
What emerges from studies like this is that there is no real problem in term of finding enough green energy. Certainly in the case of the UK, we have an excess of riches, especially offshore.
Balancing supply and demand
The real problem is balancing the variable inputs and variable demand. That didn't turn out to be a major issue for the Pugwash scenario, given the use of supergrid links to the continent and smart grid management techniques - delaying demand peaks.
Also important in the Pugwash scenario is the use of biomass / gas fired Combined Heat and Power plants, linked to district heating networks and large heat stores.
For the 100% renewables version, some of the surplus energy would be converted into hydrogen (by electrolysis) and then possibly methane (using captured CO2) and used to meet peak demand when wind etc was low, eliminating the need for fossil-gas fired back-up plants. Some of the excess would also be used to power vehicles, eliminating residual oil use.
But won't all this cost more? The situation varies around the world. Wind and PV solar now being seen as competitive in some parts of the EU, while the US National Renewable Energy Lab says they will be cost competitive with fossil fuels without recourse to federal subsidies by 2025 - even when transmission and integration costs are taken into account.
Moreover, even for large contributions from renewables - up to 35% - NREL's study of wind and solar grid impacts in the Western USA says the extra cycling of the already existing backup plants only increases the their operating cost by 2-5% on average. Also the use of long distance supergrid transmission exchanges can significantly reduce the need for fossil fuel balancing.
NREL also carried out a meta-analysis comparing the various quantitative scenarios that have emerged with very high penetrations of Renewable Energy in the power system for a range of countries and regions around the world. It concluded that renewables "can supply, on an hourly basis, a majority of a country's or region's electricity demand."
100% renewable energy 'feasible'
Yet another paper has emerged saying that it is feasible to get to near 100% of global electricity, and possibly near 100% of energy, from renewables by 2050. US academics Mark Delucchi and Mark Jacobson extend the results from their earlier studies and say, bluntly: "energy systems worldwide can be run entirely on wind, water, and solar power."
There will clearly be a lot of details and indeed some major strategic issues to thrash out. For example Delucchi and Jacobson excluded bioenergy from their list, since they felt there were irresolvable environmental problems: "getting energy from wind, water, or the sun rather than from bioenergy allows society to put land to better use than growing energy crops."
That may seem a little sweeping. Their focus is big bio-ethanol schemes and there are certainly major problems there. But what about bio-wastes? For example, AD biogas production from food scraps collection, farm wastes and forestry residues?
That involves no extra land use, and although its combustion does produce CO2, that's at least partly offset over time by the CO2 absorbed when the biomaterials are produced.
And it's better than letting this material rot in the open, releasing methane, a much worse greenhouse gas. Moreover with CCS it could be possible to have negative net emissions.
While there is plenty of room for debate, as I have argued in a new book, the basic situation seems clear: if we wanted we could head for a fully sustainable future without fossil fuel or nuclear power.
UK has huge renewable resources
It may be harder in some countries than others, but most countries have good renewable resources, including many with large offshore or sunny desert areas available, and in some cases (e.g. Russia), high windy steppes. Or failing all that, opportunities for supergrid links.
The UK has some of the best resources, especially offshore. It does look like we can aim for 100% without too many problems. So what's stopping us?
It's not the cost - surely? Using DECC figures, the Pugwash study found that the cost of the UK high renewables mix was actually slightly less than that of a mix with mainly nuclear and CCS.
It's not the size or availability of the resource. The UK offshore wind, wave and tidal resource could supply around six times more than our current electricity use.
It's not because others aren't doing it: Germany is aiming for 80% of electricity by 2050, Denmark for 100% of all energy by then.
Maybe it's just because we are told so often that we can't do it. Clearly we are battling against powerful forces, locked into the old model of centralised energy provision.
But on the ground it's changing fast, with for example, consumers and energy co-ops in Germany and elsewhere in effect taking control of the energy market and laying the basis for the new decentralised system.
David Elliott is Emeritus Professor of Technology Policy at the Open University, where he pioneered courses and policy research on sustainable energy. His latest book 'Renewables: a review of sustainable energy supply options' is available at IOP Science: