Solar, wind, and biofuels aren't the alternative' energy sources ' they once were. That's because, in a world turned-on to the benefits of clean energy, they're fast becoming mainstream. Worries over global warming may have birthed the green energy scene, but the dawning realization that all the low-hanging fossil fuel resources have already been plucked - just witness rocketing energy prices over the last decade - has reinforced that our energy future will have to be a renewable one.

But while the newly 'conventional' renewables are beginning to fast-forward us to a low carbon future, they are not without their limits. Electricity from solar and wind power soars and dips with the weather, and the turn of the planet from night to day. Biofuels take away valuable land from being used to feed the swelling global populace, driving up food prices in turn. The clean energy innovators and researchers can't, it seems, rest on their laurels.

Fortunately for the planet, they haven't. Here are three exotic clean energy technologies that could do their part in securing the clean energy revolution.

Storing the sun for the night

India was something of a solar laggard, until the last couple of years. But with the adoption of the Nehru National Solar Mission in July 2009, a massive investment in solar power (some $19 billion) looks to gift sun-lashed India with 20 gigawatts of new solar capacity, by 2020. And India is also moving to the forefront of ideas for developing storage solutions for all of that extra solar energy. An advance in low-cost solar storage was announced at the tail-end of last year, by a team from the Sri Venkateswara College of Engineering and Technology, in Andar Pradesh.

One of the big problems from using solar energy to feed electricity into the grid is that the sun doesn't always shine. It's not so much the cloudy days that are the problem -solar photovoltaic cells (PV) keep on working, even when the light levels are low. It's more that the peaks (around midday) and troughs (at night) of sunlight don't necessarily correspond to the highs and lows of demand for electricity and heat.

What is needed is somewhere to store all that sunlight. And one possible solution, described in the International Journal of Renewable Energy Technology, is as ingenious as it is simple. It involves the use of phase change materials, substances which store and release heat efficiently when they switch between liquid and solid. A mix of paraffin wax, and little fatty beads (stearic acid), it seems, are the material of choice. They readily absorb large amounts of heat, as supplied by solar PV cells, and then slowly release it as they cool and solidify, once the sun goes down.

This sort of heat storage could be of real benefit in India, a country where the smoky stoves used in the evening are often fuelled with expensive fossil fuels or scarce timber. Both materials that make up this new solar 'battery' are relatively cheap. And the team from Sri Venkateswara College has been able to make them even cheaper, by reducing the paraffin content significantly, without their losing effectiveness.

CO2 powered cars?

Man-made carbon dioxide is the pollutant responsible for global warming. But could it also be the source for new carbon-neutral fuels, to power transport, and to feed bio-refineries? Possibly, according to research recently published by a group at UCLA's School of Engineering and Applied Science. The work, published in Science, has turned to the microbial world, to make use of bacteria that can engineer fuels from CO2 - with just a little jolt of electricity.

Ralstonia eutropha H16 is the name of the bacterial strain in question. It has the useful property of being able make use of the so-called 'dark side' of photosynthesis - creating organic compounds without light. This process is happening all around us already in plant photosynthesis, as plants soak up CO2 and sunlight. Normal photosynthesis occurs in two-stages, with the 'light stage' using sunlight to create chemical energy; the 'dark stage' then uses that energy to convert CO2 to make sugars.

But this new bacterial strain, engineered by the UCLA's team led by James Liao, makes use of electrical energy to make organic compounds. And rather than sugars, it creates liquid fuels with a high energy density, like isobutanol. These have the potential to power transport, to act as fuel stores, or to be made into more complex compounds in refineries. By hooking their bioreactor up to solar panels for the electricity source, the team hope that their solution offers a carbon-neutral way to make biofuels. And unlike growing crops for biofuels, there is no pressure on scarce land - solar PV cells can readily be mounted on existing rooftops.

Powering to the future with 'waste batteries'

Two heads are obviously better than one - and it also seems that putting two battery technologies together gets better results than when they are kept apart. That's the conclusion from research conducted by Penn State University, which looked at daisy-chaining salt-powered and waste-powered batteries, for a combined battery whose sum is greater than its parts. And at the heart of one of these new battery technologies is that new workhorse of exotic energy - a community of beneficial microbes.

On the one-hand, scientists have been investigating the potential of microbial fuel cells (MFC), in which naturally occurring bacteria digest waste, producing electricity as they do so. And on the other, research time has been plowed into reverse electro-dialysis battery (RED); here a stack of membranes pull electric charge from the differences in saltiness between fresh water and sea-water.

The problem with RED batteries on their own is that they can clog, due to all the organic matter in sea-water. That's where the microbes in the MFC battery come in. Place an MFC electrode either side of the RED battery, and they help to keep the RED membranes gunk-free. They also add their own buzz of electric charge, which improves the overall efficiency of the combined battery.

The Penn State team, who published their work in Science this month, also found that this two-in-one battery - which they are calling microbial reverse-electrodialysis cells (MRC) - doesn't have to operate on sea-water for its saltiness. By using ammonium bicarbonate as the salt, the MRC battery can be made to work near waste water discharges, far from the coast. The ammonium bicarbonate can easily be recovered, and reused, using waste heat from industrial plants.

Useful amounts of electricity can then be made from something that has only been an energy-consuming disposal problem until now. Difficult-to-remove organic matter consumes up to 60 GW of energy, which could be saved by MRC batteries. And they would themselves be churning out as much as 17 GW of electricity in the process, according to the team's calculations. Of course, those numbers can only be realized if the technology can be scaled up to process all of world's waste water. But if so, that would be a significant boon to the world's difficult to square energy equation.