Tide mills old and new

A convergence of factors has had me thinking about tidal power and the history of Georgetown Island recently.  First, the additional clearing for orchard expansion is opening up a vista almost from the height of land on the eastern peninsula down to the cove (at this time of year at least, when the leaves are off the trees).  Though an unusual sight nowadays, open land was the rule rather than the exception in the 1800s into the early 1900s, as can be seen in the image below, sent to me by Will Ansel of Georgetown Center:

This is a photo of the Trafton Mill ca. 1900; it was located on a rock dam across the smaller west arm at the head of Robinhood Cove.  A couple of posts ago I reported that Will and the students of the Georgetown elementary school are planning to build a demonstration tidal mill at the site of the former Trafton Mill, and though I knew from paddling around the remnants of the dams that these mills existed, the photo brought home to me how significant these structures were, and how recently they were in use and in apparent good repair.

As best I can tell from the photo, (at least some part of the house on the hillside is still intact, belonging to a one Dick Green whose son I went to school with) the mill building was actually located over the cove, extending from the breach upstream = to the south (presumably on wood piers, as rock supports would still be intact), with its long axis oriented with that of the cove.  Of course the next question to a renewable energy engineer is, how much energy could this mill have potentially extracted?

I did a bit of modeling in order to help out Will and the kids, and now I’ve extended it to an estimate of the potential energy available in the impound above the dam.  Will gives the dimensions as 1600′ long, 133′ wide, and 8′ deep.  But I measure it on google maps as 1800 by an average of 320 feet.  So I’ll use these numbers.  Next I need a model of the cross section (since the volume of the cove is not a rectangular prism, but rather tapers significantly as it dries out on the ebb tide).  Based on nothing more than recollection, I generated the model below:

I think if anything this under-estimates the volume of the impound (ie. the slope is initially steeper, and the mudflats are relatively flatter) – keep in mind that the scale is off by about a factor of 10.

I converted to SI units and calculated the potential energy of the water in the impound, in sixteen vertical slices.  The number that results is approximately 1.4GJ, or about 390 kWh – about the energy content of 10 gallons of gas.  Of course, a primitive tide mill probably isn’t going to extract more than about 30% of the potential energy that’s available, on the other hand, 30% is probably a generous estimate for the thermodynamic equivalent of a 10hp gas engine as well.  The amount of energy in the impound could be considered impressive or pathetic, depending on how you think about it.  On the one hand, the Trafton Mill would surely be a million-plus dollar facility if built today (by any but the most crafty scavenging hackers), and accesses an energy stream worth at most $35 per day (assuming 30% conversion, 2 tides a day, 15 cent electricity) – that’s a measly ~1% simple gross annual return on capital.

But in historical context, by the standards of the day, the energy stream corresponds to 289 man-days of human labor per day (assuming 30% conversion, 2 tides a day, 100W useful human effort 8 hours a day).  In a time when the alternative was human or animal labor, a tide mill accessing the equivalent of 300 laborers would be a pretty sweet thing to have.  And I recall from working on old houses around town as a kid, there were quite a lot of boards in them with the coarse linear saw marks characteristic of the sort of gang saw that would likely have been used in such a mill.

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