Hydrogen is the least talked-about renewable energy but has the greatest potential to replace fossil fuels, both to heat homes and to provide fuel for road transport. The possibility of using hydrogen has been known about for generations, but only in the last two years has it become both practical and financially viable to see it as a large-scale competitor to both gas and oil. Networks of hydrogen filling stations are now being opened in Europe and parts of the US. Batteries for road transport have attracted most of the recent publicity around renewables and have become the focus of many governments with targets for switching away from petrol and diesel, particularly in cities with air pollution problems. But hydrogen has even greater potential because its only emissions are pure water and warm air. What has made hydrogen so attractive is that it can use surplus renewable electricity from wind and solar farms by using electrolysers to produce hydrogen. This is a process of passing an electric current through water and converting it into oxygen and hydrogen. The hydrogen can then be stored. The highly combustible gas can be used as a fuel to drive cars, lorries and trains, be fed into gas pipelines along with natural gas to heat homes, or simply burned to produce electricity when power demand is high. Hydrogen also solves a problem for the renewable energy industry – surplus production when demand is low. The large-scale introduction of both wind and solar power has meant that increasingly, when the wind blows and the sun shines and demand for electricity is low, there is no use for the power. However, if this surplus power is diverted to produce hydrogen that can be stored, then the cost of the valuable gas produced is low. The hydrogen can then be used as a power source when demand is great and sold at a much higher price than the cost of production. One of the pioneers of the process is ITM Power, which believes that hydrogen will become the major fuel for long-distance transport – lorries, trains, buses and cars – while battery-driven vehicles will be for cars on the school run or for local shopping. Already fuel cell cars using hydrogen fuel have a 300-mile range and take less than five minutes to fill up. The company has opened a number of filling stations in the UK, including three in London. ITM Power has teamed up with the oil giant Shell in a venture in Germany to make hydrogen for industrial processes. Among the projects that will make a big difference to emissions from home and office heating is the programme of blending hydrogen with natural gas. In theory, with little modification, the entire gas network could eventually be converted to hydrogen. This would require a massive increase in hydrogen production from renewables, but the growth of wind and solar power in Europe and the US makes this a credible idea.