The hill made of rubbish that pays rent.

There is a grassy mound beside the highway on the edge of New Haven, Connecticut. For decades it took the city's rubbish. Then it was sealed, planted over and left alone, which is what happens to a landfill when it is full. In July a crew finished bolting about 1,900 solar panels along the slope and switched them on.

Nobody dug a hole to do it, and that is the whole story.

Why almost nothing can be built on a tip

A closed landfill is not empty land, it is a sealed system. The cap laid over the waste is the engineering. It keeps rainwater from getting in and leachate from getting out, and it has to go on doing that for decades after the last lorry left. Break the cap and the whole arrangement fails, which is why you cannot put footings, basements or drainage through one. Councils and their insurers know this, so the standard answer to any proposal on a capped tip is no.

Solar is one of the few things that fits, because it does not need to go through anything. The arrays sit on ballasted supports that press down on the surface rather than driving foundations into it. Nothing punctures the seal, nothing disturbs what is buried, and if the cap ever needs work the array can be lifted off and put back. It is a use that respects the constraint instead of fighting it.

The numbers from New Haven

The developer, Greenskies Clean Energy, leases the land from the city and owns the array. New Haven paid nothing to build it and takes roughly $1.5 million in rent across a twenty year lease, on ground that previously earned nothing and could not be sold. Six more arrays are planned across the city, about four megawatts, and another quarter of a million dollars a year in rent.

The mayor, Justin Elicker, put the site's old value plainly. "We can't use it for much of anything, so really it was just a large mound that people drove by every day on the highway," he said, "and now we have on top of it an actual production of energy."

A hundred and twenty acres of anything is a fight. A capped landfill is not, because nobody else wants it. That is the asymmetry worth understanding.

The bit that made it happen was not the panels

Connecticut has quietly stacked the deck. Every round of its shared clean energy procurement carries a bid preference for projects sited on landfills and brownfields. A scarred site can therefore beat an open field on score even when it loses on price, and a developer who would otherwise take the easy greenfield option has a commercial reason to take the hard hill.

That is the transferable part. The engineering of ballasted racking has been solved for years. What was missing was a reason to prefer the awkward site, and Connecticut wrote one into procurement.

Ten thousand more hills

New Haven is one dot. The United States has more than ten thousand closed landfills, capped, fenced, mown occasionally and otherwise doing nothing. A research group went through the subset with usable data, a little over four thousand three hundred sites, and added up what would physically fit: about 63 gigawatts of solar, roughly 83 terawatt hours a year.

Against that, only about five hundred megawatts of landfill solar had actually been built when the count was made, and three quarters of it sat in four north-eastern states. That is not a forecast. It is a map of what has not happened.

Now translate it to Britain

England alone has on the order of twenty thousand historic landfill sites on the Environment Agency's register, most of them closed long ago, many of them capped, fenced and sitting in exactly the condition that makes them useless for everything except this. Some are on the edge of towns, which is to say near demand, near substations, and near the roads a construction crew needs.

We should be honest about the three things that stop a given hill working, because they are real and they kill projects.

The first is settlement. Waste consolidates unevenly for years after capping, and a surface that moves differentially will pull a ballasted array out of alignment and load the frames in ways nobody designed for. Some caps are stable enough. Some are not, and the survey has to come before the enthusiasm.

The second is the grid. A tip is often exactly where the network is thinnest, and a site with no connection worth having is not a site. This is the same question we ask on any field, and it is answerable early, cheaply, before anyone spends money on design.

The third is the permitting overhead. Work on contaminated ground brings the environmental permit, the gas management system and the landfill operator into the room, and every one of them can add months. Not fatal, but it has to be in the programme from day one rather than discovered in month six.

And then there is the fourth, which is ours rather than the ground's. Nobody in Britain has written a bid preference for brownfield and capped landfill into any procurement that matters. A former tip competes with a flat green field on price alone, and it loses, because the field is cheaper to build on and easier to consent. Until someone tilts that, these hills stay mown.

Why we care about it here

We have argued before that Britain does not have a land problem, it has a coordination problem, and that roughly one per cent of the country would do the whole job. Landfill sites are the purest version of that argument. Every acre of ruined ground carrying panels is an acre of farmland nobody has to defend at a parish meeting, an acre of hedge nobody has to justify, an objection that never gets written.

Solar on council land, closed tips included

Most councils own more ground than they think of as an asset: a closed tip, a depot roof, a park-and-ride, a surplus yard behind a leisure centre. The structure that works on a capped landfill works on all of them. The council keeps the freehold and grants a lease, usually twenty to forty years. A developer funds, builds, insures and operates the array, and the council takes ground rent, or takes the power itself at a fixed price and stops buying it at retail. New Haven paid nothing to build and takes roughly $1.5 million over its lease.

The order of the questions matters more than the answers. First the grid, because capacity at the boundary sets the size of everything that follows, and you can check the network at a postcode before spending a penny. Then the cap: how much has it settled, and is the survey record good enough to design ballast against. Then the permit, the gas management system and the landfill operator, all of which belong in the programme from day one. Only after those three does the money question mean anything.

A hill made of rubbish is generating electricity, and the rubbish underneath it has not moved an inch.

If you sit on a closed tip, or a council that owns one, the first question is not what it would cost. It is what the grid looks like at the boundary and how much the cap has settled. Both are answerable in a few weeks. We are happy to do the looking.

Sources: the Greenskies press release on the completed array, and Ecoportal's account of the switching ceremony and the national landfill solar potential figures.