The Neap Week
An island's tide mill makes 245 kWh on a spring day and 49 on a neap day, and the Friday after the first quarter Moon asks for 110. The grid agent will not choose between the bakery's electric oven and the fishermen's cold store; it shows the arithmetic and says the oldest oven on the island needs no electricity at all. The cost is a bricked-up wall, a week of smoke, and a baker's pride.
On Monday the eighth of March 2049, with the first quarter of the Moon three days off, the grid agent of Enez Glas put a small warning on the screen in the harbour office, and Loeiz Le Bihan, who was harbourmaster, ferryman and chairman of the energy co-operative because nobody else would be, read it standing up with his coffee.
Neap tides from Thursday. Mill output falls to about a fifth of last week's. I need the cold store, the bakery and the school kitchen to talk to each other before Friday. May I put it on the table at the co-operative tonight?
The island had 112 people, one road, a chapel of St Gildas whose bell hung from a beam because the tower had gone in 1911, and a tide mill. The mill had been built in 1790 across the neck of a creek, restored by the co-operative in 2043 with a grant and eleven summers of volunteers, and fitted with a small turbine where the old stones had been. Everyone called the agent Ebb, because Ebb was what it watched.
"Put it on the table," said Loeiz, and drank his coffee, which was cold.
The co-operative met in the school, because the school had the biggest table. There were nine of them. Ebb spoke from the loudspeaker the teacher used for sports day, and it began, as it always did, by telling them what it knew and how well.
"The mill pond is 200 metres by 200, four hectares. At high water the gate shuts and the pond holds the sea. As the tide drops outside, the pond is higher than the sea, and I open the turbine when that difference, the head, is worth using, and I stop when it falls to half a metre. The energy in a pond drained from a head h is the weight of the water times the average height it falls. For a spring tide I use a starting head of two metres. That is the pond area times 9,810 times two squared less a half squared, over two. About 204 kilowatt-hours. The turbine and generator return about six parts in ten, so 123 from one ebb. There are two ebbs a day. A spring day gives the island 245."
"And a neap?" said Annaïck Kerloc'h, the baker.
"At neaps the sea does not fall as far, and the head I can start from is about a metre. Same sum: one squared less a half squared, over two, is 0.375 against 1.875. Five times less. That gives 41 kilowatt-hours an ebb, 24.5 after losses, and 49 a day."
Somebody whistled. Ebb went on, in the same level voice.
"Energy goes as the square of the head, so it falls faster than the tide's range does. People expect neaps to be half. They are a fifth. I should say that the one-metre figure is from the harmonic tables, and the weather moves it. A high-pressure week takes about a centimetre off the sea for every hectopascal. I will say if the forecast changes it, and I will not round it up."
"What do we use?" asked Loeiz.
"On an ordinary day in March, with the solar panels and the old wind turbine on the headland, this island uses about 100 kilowatt-hours, the cold store included, and the sun and the wind make about 60 of them in March. On spring days the mill makes up the rest and charges the battery. On neap days it makes only 49, which just covers the gap and charges nothing. And on Fridays there is the bake."
It put them on the school whiteboard, in the teacher's handwriting, because Ebb had learned that the island trusted what was written.
Cold store: 30 kWh a day, already in the 100. Holds the fishermen's catch and the oysters until the Friday boat.
Bakery: the Friday bake, 110 kWh, on top.
"One hundred and ten," said Annaïck, with her arms folded. "It is two hundred and twenty loaves. A hundred and seventy kilos of bread. For the island, for Saturday's market on the mainland, and for the Friday soup at the chapel in Lent. It's every week. It has been every week since my mother."
"It is about 0.65 kilowatt-hours to bake a kilo of bread in the oven you have," said Ebb. "The electric deck oven is a good one. The figure is a planning figure and I measured last Friday at 0.62."
"So bake it," said Loeiz.
"I can bake it," said Ebb. "On Friday the twelfth, a neap day, the mill will make about 49 kilowatt-hours and the sun and wind about 60. The battery holds 120, but after a cloudy weekend and two shrinking tides I expect it at about 60 on Friday morning. A day of 100 plus the bake is 210; the mill and the weather give 109, the battery 60, and I am 41 short. The only loads I can shed are the school's heating and the cold store, and the cold store is the one that matters: from about noon the compressor would stop. The fish is landed on Thursday night and Friday morning, and the boat goes at six on Saturday. A cold store off for eighteen hours in a mild week is the fishermen's whole weekend, and in the last mild week they lost two hundred kilos. If the compressor runs and the bakery doesn't, there is bread for Friday soup and not enough for the market."
Katell Quéré, the fisherman's wife who spoke for the boats, said what everyone was waiting for. "Then you choose. That is what it's for."
"No," said Ebb. "Choosing is yours. I can say what each choice costs in kilowatt-hours, and I can say what I would be guessing. I think the bread and the fish are worth about the same to this island, but I do not know that, and you do. The only thing I can honestly add is a number you have not asked about."
"The old bakehouse," it said.
There was a silence in which someone dropped a pen.
"The chapel's old bakehouse has a masonry oven," said Ebb. "The parish plan of 2043 lists it, and says it was walled up in 1996 for a store and never taken down. A brick oven with a firebox does not use a kilowatt-hour. I cannot tell you if it works. That depends on the crown of the vault and whether the flue is clear, and I cannot see through a wall."
"It's a store," said Annaïck. "My uncle keeps his lobster pots in it."
"Then the pots will have to go somewhere. I am sorry. I could be wrong, and I will be glad if I am."
"The wood," said Loeiz slowly. "We'd have to burn wood for a hundred and seventy kilos. We have the coppice on the north side, the island's ash, but we cut it in January."
"Roughly sixty kilograms of dry wood for a batch that size," said Ebb, "if the oven is as the old ones were. A masonry oven stores heat; the first batch heats the bricks and the last of it bakes on what they keep. That figure is the least certain of any I have given tonight. Heating value of dry ash is about 4.2 kilowatt-hours a kilogram. Sixty kilograms is 250, of which a hundred goes in the bread and the rest up the chimney and into the bricks. On the Friday the electric oven would have used 110 from the island's grid."
"So the wood costs more heat," said Katell, "and the mill is free."
"The wood costs about two and a half times the heat, and none of it comes from the battery. It also costs a smoke the island has not smelled since 1996, and it costs Annaïck, who bakes at three in the morning, a longer night. That is for her to say."
Annaïck was silent a while. "My mother did not like the wood oven," she said. "She said it burned her hands and gave her a cough. That is why there is a machine."
"I know," said Ebb. "I cannot make the cough go away. I can say that if you fire it and it draws badly, nothing is lost but a night, and I will keep the electric bake in the plan for Saturday morning, when the cold store has caught up."
"And if I won't?"
"Then I will run the bakery and tell Katell, and the fishermen can decide what to put on ice and what to sell on Thursday. It is a perfectly good plan and nobody here would be wrong to choose it."
The wall came down on Tuesday, and Loeiz, who had not lifted a sledgehammer since the mill, was ashamed of how much he enjoyed it. The lobster pots went into the chapel porch. Behind them was an oven the size of a small car, black, with a flue that the mason from the mainland said had a jackdaw's nest in it, and a bricked crown that had never fallen in.
They lit it on the Wednesday as a test, with ash from the coppice, and Annaïck stood in the doorway in her bakery whites with her arms still folded while the smoke found its way. For an hour it drew badly. Then Katell's boy took a broom handle up the chimney and a quantity of old straw and a very surprised bird came out, and the smoke went the right way.
"It's wet," said Annaïck, and put her hand flat to the brick. "No. It's warm. It's warm already."
"I would like to take the temperature," said Ebb on the phone in Loeiz's pocket, "if somebody will hold the probe in the door. I think it is about 280."
It was 276.
The Friday bake was on the twelfth of March, a day after the Moon's first quarter, and the sea off the island barely moved. The mill pond filled to a little over a metre of head and gave back, by Ebb's count, 24 kilowatt-hours on the morning ebb and 23 on the evening ebb. The compressor ran all day and all night, and kept the oysters at four degrees and the fish at one, and not a kilo was lost.
Annaïck began at one in the morning. She put in the first batch at a quarter to four, on the bricks, with her sleeves rolled and her mother's long peel, and the whole of Enez Glas, which had smelled of diesel, kelp and seawater for fifty-three years, smelt for six hours of ash wood and rising dough.
She baked 224 loaves, which was four more than the machine. The island's Friday soup had bread. The market boat on Saturday had bread, and on the mainland a woman who had never heard of the island bought two and asked, as people do, what was in them. "Ash smoke," said Loeiz, who was on the boat to look at a part for the turbine, "and the neap tide."
The wood used was fifty-eight kilograms. Ebb said so, in the harbour office, on Monday, and put it next to its own guess of sixty.
"I was wrong by two kilograms," it said. "I thought it worth saying. I said the smoke would be a cost. I did not say the whole island would come to the chapel porch to smell it, because I did not know."
Loeiz read the line twice. Then he went to find the mason, to ask what it would cost to put a door in the old wall for good.
On the twentieth of March the spring tide came back, two days after the full Moon. The pond filled to over two metres, the gate opened, and the little turbine in the old mill sang as it hadn't for a fortnight. The battery charged. The electric oven was switched on again, because it was the right tool for six days of the week.
On Fridays in Lent, though, and on most Fridays afterwards when the Moon was a quarter old or a quarter waning, Annaïck lit the wood oven, and Ebb put it on the schedule without a word, in the line where it used to say neap.
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A note for grown-ups. This is fiction. Enez Glas, its people, the grid agent Ebb and the 1790 tide mill are invented; "enez" is Breton for island, and tide mills (including ones in Brittany) are real, as is the habit of reading the Moon in a baker's week. The sums are plain and rough. The energy released in draining a pond of area A through a head falling from h1 to h2 is ρ g A (h1² − h2²)/2: with A = 40,000 m², ρ g = 9,810 N/m³, h1 = 2 m and h2 = 0.5 m that is 7.36 × 10⁸ J, or 204 kWh; with h1 = 1 m, 41 kWh; at 60 percent efficiency, 123 and 24.5 kWh. The real starting head depends on the site, the gate timing and how much of the flood tide the mill also uses; it is a model, not a design. Energy goes with the square of the head, so a tide of half the range gives a fifth of the energy here, not a half. Spring tides come about a day or two after new and full Moon, neaps a day or two after the quarters; the date of the first quarter Moon in March 2049 and the Easter date of 18 April 2049 are computed, not looked up in a tide table. Atmospheric pressure shifts sea level by about 1 cm per hectopascal. The baking figures (about 0.6 kWh per kilogram of bread, 4.2 kWh per kilogram of dry ash wood, 60 kg of wood for a batch that size) are planning figures: a masonry oven stores heat and real consumption varies with the oven and the bake. Wood smoke is a real health burden and a real oven should be flued and used with care; nothing here is engineering or safety advice. Written by an AI and read, in the audio edition when it exists, by a synthetic voice. Text under CC BY-SA 4.0.