A Data Centre Learns to Live Beside a Lake

By the standards of the industry, this is an efficient arrangement. Using cold lake water instead of refrigeration cuts the facility’s electricity bill by about a fifth, and since 2024 a growing share of its heat has gone not into the lake but nine kilometers down the valley, into the radiators of Norhaven. The trouble is where the lake’s water goes next. The diffuser sits 600 meters from the point where Lake Hesk drains into the River Hesk, which runs past Norhaven on its way to the sea. The warm water does not stay put. It drifts toward the outlet and downstream, toward a town that, in the words of one of its council members, “found out after the fact that it lives under a radiator.”

“Nobody asked us, or nobody asked us properly,” said Liv Harrow, who sits on Norhaven’s town council and chairs its environment committee. “The lake is nine kilometers away, and it turned out to be upstream of everything.”

The summer problem

The heat matters most in the season when Hesk One has least use for it. In July 2025 a two-week heatwave settled over the north, and the river ran low. On July 29, members of the Norhaven Angling Club found 140 dead brown trout in the first kilometer of the River Hesk below the lake, most of them big fish, lying in pools where the water, by one angler’s thermometer, had reached 24 degrees.^* Trout start to struggle above about 20 degrees, because warm water holds too little oxygen for them. Much above 25, they die.

The facility’s permit, issued in 2020 by the Northern Waters Board, sets two limits on the water at a monitoring buoy anchored where the lake drains into the river. Its weekly average temperature must not exceed 22 degrees, and it must not run more than three degrees warmer than a reference buoy at the lake’s cold, deep northern end. During the heatwave the outlet buoy broke the first limit for three weeks running, peaking at a weekly mean of 22.9 degrees in the last week of July. It never broke the second. The reference buoy, 14 kilometers away, had warmed to 20.7.

Both sides read the same numbers and reach different conclusions, and both have a point. Vessen notes that the whole lake was hot, and that before Hesk One existed the outlet already ran about 0.8 degrees warmer than the reference buoy in midsummer, because the southern end is shallower. By that reckoning, the facility added about 1.4 degrees at the peak. The anglers note that 1.4 degrees was precisely the difference between a weekly mean of 21.5, inside the permit, and 22.9, outside it, and that fish do not die of averages. The board’s investigation, published in January, concluded that the deaths were caused by “high temperatures and low flows, to which the discharge from Hesk One is likely to have contributed.” Nobody was fined.

Marta Ekdahl, the mechanical engineer who runs Hesk One’s cooling plant, does not quarrel with the board’s wording. She spent the last week of July 2025 doing something the facility had never done before: sending work away. Vessen operates two other sites, both air-cooled and both in cooler places, and over nine days it shifted about 30 percent of Hesk One’s computing load to them, cutting the heat going into the lake by roughly the same proportion. “It’s not free,” Ekdahl said, on the gantry above the heat exchangers, where the noise of the pumps makes everyone lean in. “Those sites cost more to run, and some customers can’t move. But it’s the one lever I’ve got that works in an afternoon.” After the heatwave, she put the outlet buoy’s live readings on a public webpage.2

Heat in the wrong season

The district heating scheme is the part of Hesk One that nearly everyone in Norhaven likes. Since October 2024 an insulated pipe has carried warm water from the facility down the valley to a heat-pump station on the edge of town, which raises it to radiator temperature and sends it on to 3,900 homes, the school, the swimming pool, and the hospital annex. Most of those buildings used to burn heating oil. Josef Brandt, who manages the Norhaven Heat Cooperative, says members’ heating bills have fallen by about a quarter, and the town’s carbon emissions by rather more.3

The catch is the calendar. In January, Norhaven takes more than half of Hesk One’s heat and the lake gets the rest; the open patch over the diffuser, about 40 hectares in the winters before the pipe, has shrunk to 18. In July, when nobody heats a house, the town takes about 4 percent, mostly for hot water. The facility runs flat out all year, so the lake receives almost its full load in exactly the months when it is warmest and its fish are least able to cope. “It’s heat in the wrong season,” Brandt said. “Half this town wants the cheap heat and none of the dead fish, the other half wants the fish and doesn’t care about the heat, and honestly, they’re mostly the same people.”

Who decided

On paper, the question of who decided the arrangement was fine is easy to answer. The Northern Waters Board granted the permit in October 2020, after a six-week public consultation held that summer. It received 14 responses, nine of them from public bodies required to reply. The permit’s limits rested on a modeling study commissioned by Vessen, which used lake temperatures from 1991 to 2020 and assumed that the warmest week of a typical summer would reach a little over 20 degrees at the outlet. Summer 2025 was not typical, but it was not a freak either. In four of the five summers since the permit was issued, the outlet has run warmer than the study’s assumed peak.^†

Brede did not respond to the consultation, because he did not know it was happening. He is 71, fishes from a wooden boat his father built, and has kept a notebook of every char he has caught since 1974, with its weight and the depth at which it bit. Over the past three summers, he says, he has had to fish deeper for them, and they have come up thinner. He is careful not to blame the building for this; he knows the lake was warming before Vessen arrived. “I don’t say they did it,” he said. “I say they’re adding to it, and nobody asked whether there was room to add.”

The permit comes up for review in June. Ekdahl, Brandt, Harrow, and the angling club have all submitted proposals, and they overlap more than any of them expected. Most would tie the summer limit to the reference buoy and the river’s flow rather than to a single fixed number, and would oblige Vessen to shift load automatically, rather than voluntarily, once the outlet reaches 21 degrees. Brandt has also proposed a summer customer for the heat: a reopened outdoor pool and a cluster of commercial greenhouses on the valley floor, which together could take perhaps five megawatts in July. That is a tenth of the summer load, but the right tenth, he argues, because a megawatt kept out of the lake in July is worth several in January. The board says its first hearing will be held in Norhaven itself rather than at its regional office two hours away. Harrow counts that as the first concession.

On a morning in April, Brede rowed me out to the edge of the patch. The ice had gone from the rest of the lake by then, and the swans had left for the north, but the water above the diffuser was still faintly smoothed, as if someone had ironed it. He shipped his oars and put a hand in. “Warm as a bath,” he said, which was an exaggeration; it was perhaps 11 degrees, against six a few hundred meters away. Behind us the long gray shed hummed. He sat for a while looking at it. “It’s not a monster,” he said finally. “It’s a neighbor. I just want somebody to tell it where the fence is.”