Acton Hydro: Powdermill Dam


The first dam on the Assebet River was located in the town of Concord at the Damonmill site as early as the mid 1600s. The first dam at the Powdermill site, along what is now Route 62, was circa 1800 and operated as a sawmill.

Around 1835, the dam was purchased by Nathan Pratt to manufacturer black powder. The site grew to about 400 acres and 40 buildings and eventually became the Massachusetts Powder Works. The Works used at least twelve separate water wheels to utilize the river’s energy to drive the machinery used to crush the dried saltpeter into powder. Eventually the saltpeter was mixed with sulphur and charcoal and pressed into cakes and sold as black powder.

The manufacturing process was very dangerous and explosions were a constant hazard. At least eleven different explosions were recorded at the site. Henry David Thoreau mentioned in his diary in 1853 that he visited the powder mill after an explosion that killed four men.

Around 1900, black powder was replaced with smokeless powder and the need for black powder was reduced to sporting purposes.

In 1923, hydroelectric generating equipment was installed in the present brick powerhouse. The facility was equipped with a 240 horsepower, vertical shaft turbine and a synchronous generator to manufacturer electricity.

In 1946, the property owners began to sell off the property but continued to operate the hydro plant until 1964 when the turbine sustained some major damage. The plant was shut down initially but did operate periodically until 2000 when further equipment damage occurred.

Mike Coates, who now owns the plant, purchased the facility several years ago and has been fixing and replacing equipment so he can operate the plant and sell the electricity to Concord Light. We anticipate the plant will begin operating sometime early next year.

The Powdermill is a “run of the river” hydro plant meaning there is no way to back up the Assabet River and store the water. Thus the plant’s output is in direct proportion to the flow of the river. The flow of the river is directly impacted by the amount of rain we receive and as it does not rain enough during the summer months in this area it is not practical to run the plant during those months. However, the rains historically begin increasing in October and by November there is typically enough flow in the Assabet to generate electricity. After the spring rains the plant is typically shut down during the low flow summer months.

You may be interested in the environmental impact the plant will be making once it begins operation. The typical generating plant in downtown Boston has a heat rate (efficiency) of 10,000 British Thermal Units (BTUs). This means they must burn 10,000 BTUs to generate one kilowatthour of electricity. So for every 100 kilowatthours of green power generated by Acton Hydro, the need to burn one million BTUs of fossil fuel, or seven gallons of oil, is eliminated along with all the related pollution.

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