What Is a Tide Mill? Power From Water That Arrives Twice a Day

The arrangement is a pond, a dam and a set of gates. As the tide rises, water flows through gates into an artificial pond behind the dam, and at high water the gates close, either by hand or automatically as the flow reverses and pushes flap gates shut. The sea then falls while the pond stays full, creating a head of water, meaning a height difference across the dam. Once the head is sufficient, the miller opens a sluice and water runs from the pond through a wheel and out to the falling sea, driving the machinery. Milling continues until the head becomes too small to turn the wheel usefully, at which point work stops until the next tide. That gives roughly five to six hours of available working time in each tidal cycle, occurring around twice a day, and the timing shifts by around fifty minutes each day because the tidal cycle is governed by the moon. A miller therefore worked at times that moved steadily through the day and night, which is the defining hardship of the trade.

The design has specific requirements and specific weaknesses:

• •Sufficient tidal range, since the available energy depends on the head and on the volume impounded, and a range under a couple of metres makes a mill impractical
• •A suitable inlet, creek or estuary that can be dammed economically to impound a large area with a short dam
• •A wheel design tolerant of low and falling head, which is why undershot and low breastshot wheels dominate, since the head reduces continuously through each working period
• •Sediment, since impounding water in an estuary causes silt to settle in the pond, which reduces its capacity and requires continual dredging
• •Structural exposure, because the dam and wheel are in salt water with a strong current, so timber decays and iron corrodes far faster than in a freshwater mill
• •Working hours dictated by the tide rather than the clock, including night working whenever the tide falls at night

Tide mills are older than usually assumed, with excavated examples in Ireland dated to the seventh and eighth centuries through dendrochronology of their timbers, which makes them among the earliest documented water-powered machinery in north-western Europe. They spread along the Atlantic coasts, with substantial numbers in Britain, Ireland, France, Spain, Portugal and later in the American colonies, where the tidal creeks of New England suited them. The Domesday Book records mills that are probably tidal. They were built where rivers were absent or unreliable and where coastal settlements needed grinding capacity, and they milled grain principally, along with other industrial uses. Numbers peaked in the eighteenth and nineteenth centuries and declined with steam and then electricity, for the same reason as windmills: a steam mill works continuously and can be sited anywhere. A small number have been restored and operate as museums, including examples at Woodbridge in Suffolk and Eling in Hampshire, which still produce flour on the tide.

The principle returned at enormous scale as tidal power. A tidal barrage is a tide mill with turbines instead of a waterwheel, impounding an estuary and generating on the ebb and frequently on the flood as well, and the Rance barrage in France, operating since 1966, remains the demonstration case alongside a larger installation in South Korea. The advantages over wind and solar are the same as the old mill's disadvantage turned around: tides are entirely predictable years in advance, which is unusual among renewable sources and valuable to a grid. The obstacles are equally consistent, since a barrage alters the tidal regime of the estuary behind it, changing sediment movement, intertidal habitat and fish passage, which has prevented several proposed schemes including repeated proposals for the Severn. Tidal stream turbines, which sit in fast currents without impounding anything, avoid most of those objections and have been deployed commercially at small scale, which is where most current development sits.

A tide mill impounds the rising tide behind a dam, closes gates at high water, and releases the pond through a wheel as the sea falls, giving around five hours of work per tide at times that shift by fifty minutes daily. It needs a decent tidal range and a dammable inlet, and it silts up and corrodes faster than a freshwater mill. Irish examples date to the seventh century, and the same principle returned as the tidal barrage, whose predictability is its advantage and whose estuary impact blocks it.