It was a small but chilling symbol of 20th century consumerism. Obsessed with choice and convenience at any cost, supermarkets for years refused to put doors on fridges because they might get in the way of customers reaching in.

Now a small but very 21st century revolution is under way. On Monday Asda will open what it claims is Britain's first superstore with doors on every freezer, fridge and chill cabinet. The initiative is part of a £27m new "low carbon" store in Bootle, Liverpool, and this one measure alone is expected to save 8% of the building's electricity bill - and emissions. Asda is not alone.

With electricity bills rising fast and campaigners and regulators turning up the heat on companies to cut their greenhouse gas emissions, supermarkets are opening ecostores to test a hotchpotch of improvements, from renewable energy to recycling plastic bags into kerbstones, animal-friendly products, and seeking out the green pound of more environmentally-friendly shoppers. With tens of millions of shoppers a week and £85 out of every £100 spent on food in the UK, the supermarket chains could be a force for transforming the way people shop and encouraging suppliers around the world to reduce energy use, waste and exploitation of people and land.

"It's a healthy competition which is moving us all faster and faster," said Katherine Symonds, Tesco's sustainability manager. Others claim the very notion of an "ecosuperstore" is a contradiction in terms. "While green technology can improve the environmental footprint of a building, this is a small part of a supermarket's impact, which includes emissions from food freight and customer travel to stores," said Helen Rimmer, campaigner for Friends of the Earth.

Walking around the huge (recycled) brick and (sustainable) wood Bootle store, Bob Simpson, Asda's head of project development, pointed out two geothermal pumps to bring up hot water from underground aquifers and a wood pellet-burning boiler to help heat the store. Passing through wafts of warm bread from the bakery test on to a shopfloor busy with shelf stockers, Simpson showed off ventilators recycling "ambient air", sun pipes in the roof, and translucent bricks to bring in natural light but keep out the heat of the sun - and, of course, the omnipresent fridge doors. The fridge doors are, admitted Simpson, a leap of faith. At one store the electricity bill fell 14%, but in two sales dipped.

"I think here it will be slightly different, and that's why we've not changed them; also it's a bit late [to change] because we open on Monday," he said. The mantle of Britain's greenest supermarket changes almost monthly: in August Sainsbury's opened what it claimed was the greenest store in Dartmouth, Devon, promising to reduce emissions by 40%; Asda says Bootle will cut energy use and emissions by up to half; but Tesco claims to have already trumped that, with a 60% cut at Shrewsbury. Nor is carbon the only battleground: all the supermarkets have targets to cut the distance goods travel, emissions from vehicles, waste and a range of other unpopular byproducts of their business.

One key driver has been rising electricity bills, which account for about 40% of store costs and 60% of the companies' direct emissions. "We have seen a dramatic upturn in price that's changed the energy environment in the UK ... that's driving a totally different set of behaviours than even was the case two years ago," said John Ashford, head of engineering for Sainsbury's. The other is customers, though evidence that shoppers are lobbying for greener goods is hard to find, especially since the threat of recession.

But supermarkets see green values as a way of winning or at least retaining customer loyalty, said Ronan Hegarty, news editor of The Grocer industry magazine: "It's a bit of a trump card to say 'who's the greenest'." At the same time, retail bosses see the opportunity to use the cost savings to win more customers by bringing down prices, said Julian Walker-Palin, head of corporate policy for sustainability and ethics for Asda. "Anything a supermarket does, we do for our customers. "For 17 million people who shop [in Asda] each week, the majority don't understand climate change or carbon, and they don't want to. But they understand there's this issue we need to do something about; they very much [say to] their supermarket 'you have the experts, I want you to help me do the right thing'," she said.