Cities constantly reject huge amounts of low-grade heat: warm air pushed out of metro tunnels by braking trains, hot exhaust air from data-centre server racks, and flue heat from industrial processes. That heat is too cool to use directly for space heating, so it is normally wasted. Electric heat pumps can capture it and "lift" it to a useful temperature, then a network of insulated pipes — a district heating network — carries the hot water to buildings, where it displaces gas or oil boilers.
London's Bunhill 2 heat network captures warm air rejected from a London Underground ventilation shaft and uses a heat pump to supply over 1,000 homes — waste heat that used to simply vent into the street.
A cutaway city block where waste heat from a metro tunnel, a data centre, and an industrial plant is drawn into a central heat pump and pumped out through a district heating network to warm surface buildings.
Low-grade heat sources feed a heat pump that lifts them to a useful supply temperature; the heat pump's coefficient of performance (COP) sets how much electricity that lift costs, and the network's operating temperature sets how much fossil heating it can displace.
Pick which waste-heat sources are connected, adjust the heat pump's COP and building demand, and switch between a low-temperature 5th-generation network and a hot legacy network. Watch the flow particles, radiator glow colour, and live stats respond.
Because 5th-generation networks run only a few degrees above ground temperature, pipe heat losses are minimal and the heat pump does far less "lifting" work than in a legacy 80°C network — often doubling the effective COP.