Renewable energy is getting better all the time, but no-one's yet worked out a good way to store the juice for those cloudy, windless days. Batteries do work, but given the high cost of swapping out the cells for your EV, we doubt that Duracell will start making a five-megawatt version any time soon. That's why British company Isentropic is developing a system that could store energy in cells that are filled with gravel. Using Argon gas and a heat-exchange pump, the electrical energy from solar panels or wind turbines is used to warm the gravel. When the energy is required, the process is reversed, and the heat is used to drive an engine that creates power ready for the grid. It's nowhere near as efficient as other power storage systems, since you'll only get out about 75 percent of what you put in, but the low cost and ease of installation means that it's not so outrageous as you think. It's a shame that the gravel has to be stored in a tank full of Argon -- we quite liked the idea of filling the boot of an EV with rocks, or even just parking it on an energy-storing driveway, before driving away.