The Heat Beneath The Streets

AHeat, not electricity, accounts for about half of the energy consumed in the buildings of northern Europe, and almost all of it is still produced by burning something. Geothermal heating promises an alternative that is neither weather-dependent nor visible from the street: water is drawn from a permeable layer a kilometre or two underground, its warmth is stripped out through a heat exchanger at the surface, and the cooled water is put back into the layer it came from through a second borehole, not the one it was drawn from. The pair of wells is known as a doublet, and the technique is not new. Reykjavik has been heated in this way since the 1930s, and boreholes sunk into the limestone beneath Paris have supplied housing estates since 1969. What remains contested is whether these are early instances of a general solution or the exceptional cases of a technique that works only where the geology is unusually obliging. Over the past decade the argument has narrowed to a disagreement about which part of the system runs out first.

BHalvard Ness, a hydrogeologist at the Nordvik Institute of Subsurface Energy, holds that the constraint lies underground and that it has been consistently understated. Between 2004 and 2019 his group monitored 38 doublets operating in the Verron Basin, a sedimentary formation whose thickness and porosity resemble those beneath Paris, taking readings every six weeks. Across the sample the water arriving at the surface cooled by a median of 0.9 °C over the fifteen years, a modest figure that the industry has cited many times as reassurance. Ness regards that median as the least interesting number in his dataset. The distribution behind it is heavily skewed: 9 of the 38 wells lost more than 3 °C, and in four of those the operator had already been obliged to drill a replacement. What the rock holds, he argues, is not a stock of heat waiting to be spent but a rate at which heat creeps back into the water, and nothing done at the surface alters that rate.

CThe claim yields a prediction, and Ness has tested it. If cooling is governed by how quickly the chilled water leaving the injection well finds its way to the production well, then the spacing of the two boreholes, rather than the volume pumped, should determine which systems decline. In the Verron sample every one of the nine badly affected doublets had a separation of less than 1.2 kilometres, whereas among the wells drilled more than 1.5 kilometres apart none had lost more than 1 °C, and that group included several of the hardest-worked systems in the survey, pumping at the highest flow rates recorded. Severe drawdown was confined to the closely spaced installations. The pattern is awkward for planners, since the surface land needed to spread boreholes that far apart is precisely what dense cities lack. Ness is nonetheless careful to present the finding as a constraint on how fields are laid out rather than as a verdict on the resource itself.

DEsther Mwangi, an energy systems engineer at the Calder Hill Centre for Urban Energy, considers the diagnosis misdirected. A resource looks scarce, she observes, only in relation to what is asked of it, and what has been asked of geothermal water is that it feed pipe networks designed in the 1960s to circulate water at 80 °C. In four municipal networks that her team rebuilt between 2016 and 2022, replacing radiators, insulating the worst-performing blocks and bringing the circulating temperature down to 55 °C, the same wells covered 71 per cent of annual demand, against 34 per cent before the work began. Redesign of this kind is now written into procurement rules across her region, although it was thought eccentric when the first network was converted. She concedes two things. All four schemes lie in a single country with unusually generous retrofit subsidies, and the cost figures were supplied by the operators themselves rather than audited by an outside body.

EThe argument is no longer confined to journals. A 2019 directive obliged municipalities above a certain size to publish heating plans, and several national schemes now insure developers against the risk of drilling a well that yields too little; a single failed borehole can cost more than the network it was meant to serve. Public consent has proved harder to insure. Deep projects that fracture hot granite have set off tremors, Basel in 2006 being the case everyone cites, and shallow doublets in sedimentary rock, which do nothing of the sort, have inherited the suspicion. Ness points out, awkwardly for his own argument, that nothing in his data speaks against geothermal heating as such, since a resource that must be laid out sparsely is not a resource that is absent. Mwangi replies that the distinction rarely survives contact with a council committee. Both regret that the field advertises its showpieces and files its failures quietly.

FA truce of sorts is emerging in which the two accounts are assigned to different questions rather than ranked against each other. Where a city has land at its margins and a network it is willing to rebuild, the surface fix appears to lift the ceiling Ness identifies, since a network running cool draws less water for the same warmth. Where boreholes must be crowded beneath existing streets, no amount of retrofitting removes the thermal interference between them. The unresolved question is how much of the heat now drawn from sedimentary basins could survive fifty years of continuous extraction, and the monitoring records that might answer it barely reach two decades. What both researchers reject is the assumption that made the early estimates so generous: that heat measured in the ground is heat delivered at a radiator.