An experiment is under way that will soon see heat from tube train tunnels being used to heat homes and businesses around parts of Islington.

It’s all part of the Bunhill Energy Centre, an experiment in modern electricity power plants that avoids dumping surplus heat into the atmosphere as waste, and instead pumps it via underground pipes to local homes.

Completed in the winter of 2012, the 2km of pipes under the streets links a number of local council estates and a few businesses up to a single gigantic hot water tank.

By making use of what is known as a Combined Heat and Power plant, the council is currently using a gas powered electricity generator to produce electricity, and rather than venting the heat away, they use it to heat the hot water tank and distribute it to the local residents.

The aim is to maximise the efficiency of the power plant by using what is in most power plants a waste product — heat. Modern power plants tend to be far from where people live, thanks to the National Grid making that possible, but it also means that the heat created by turning fuel into electricity is wasted.

All those huge cooling towers filling the rural skies with steam are a symbol of heat being thrown away.

What if you put the power plant next to where people lived though, could the heat be useful? And indeed that’s what’s been built at Bunhill in Islington. Electricity is generated for sale to the National Grid, and the heat used to sell cheap hot water to the locals.

This is in part thanks to the fact that the local council blocks were built at the time with a centralised heating network instead of individual boilers in each flat.

That made it much easier to pump hot water to the council blocks, where it would be easily distributed to the residents. Had they tried to wire up each individual flat, it would have been vastly more expensive to build.

The plant, with a very distinctive wood clad hot water tank was installed in 2012, and was largely built off-site then craned into place. The decision for off-site construction was to reduce disturbance to residents, but the modular design also makes it easier to swap out faulty components.

While the tall hot water tank acts as the local landmark, the real power comes from the gas power plant hidden behind the high walls, and which looks not unlike a giant car engine. It also has to be housed in a special building to reduce its noise levels.

Elsewhere fans can vent away any heat not needed for the hot water tank. Although the aim is to reduce the cost of heating homes, it’s also an eco-project to reduce CO2 emissions, so venting heat away that’s unused is seen as wasteful. That does mean that opportunities to switch on the electricity generator when the National Grid are paying a premium for electricity are not exploited as often as a commercial power plant might.

However there are times when they are paid enough for electricity that the hot water is essentially being provided free of charge. At other times they buy in electricity when it is particularly cheap to avoid using their own gas supply.

What is much more interesting though are the plans for the future.

Rather than burning gas to generate electricity, they are looking around for other sources of heat that can be used to at least warm up the hot water supply before being topped up conventionally.

And just up the road, London Underground is currently converting the former City Road tube station site into a new ventilation shaft to cool the Northern line as part of the cooling the tube project. Suddenly lots of hot air is going to be pumped up to the surface, and within reach of the power plant.

The council is installing a warm air heat exchange in the shaft with London Underground, and that will be used to top up the heat in the pipe network, which is also currently being extended northwards to the new heat source.

The expectation that a 2-stage ammonia heat pump installed in the shaft along with a local CHP generator should be able to pump an additional 1MW of energy into the network, increasing total capacity by a half again.

In addition, the currently rather shabby building will be replaced with a very striking bronze clad structure.

They are also looking at tapping into other local heat sources, such as an electricity sub-station on City Road, and a local computer data centre at Moreland House which pumps out an awful lot of heat via roof fans.

That meets their goal of reducing the local area’s carbon emissions by recycling otherwise wasted heat, but also broadens the supply of heat so that they are less dependent on one point of failure.

In time, councils could return to a role they once occupied before the nationalization of electricity generation — providing heat and electricity to their own residents from small localized power plants.