Thursday, October 13, 2011

Hydropower at the Woolen Mill

Vertical drop in feet times flow in gallons per minute divided 
by 10,000 equals kilowatts, and kW x 1.34 equals horsepower.

Deep in the bowels of Clock Tower Place there is a space where an antiquated monster once sat – the turbine which converted water power to electrical power. This machine was the last of several generations of hydropower generating engineering in the mill.

Hydropower is all about high school algebra. Vertical drop in feet times flow in gallons per minute divided by 10,000 equals kilowatts, and kW x 1.34 equals horsepower. For the metric-minded, vertical drop in meters times flow in liters per second times 9.81 divided by 1,000 also equals kilowatts. By this math, ten gallons of water dropping 100 feet yields the same power as 100 gallons of water falling ten feet.

Actual power yield is always less than theoretical due to friction and other inefficiencies. An “undershot” waterwheel describes a design with water running under the wheel, pushing the bottom blades forward. For wooden wheels of this design capture of the water’s energy was on the order of 20 to 30 percent.

Wherever greater vertical drop allowed, water was led over the top of the wheel to pull blades downward by force of gravity. This style, referred to as “overshot,” traditionally captured 50 to 70 percent of available energy. By the 1830s wheels would have had wooden blades, rims and spokes attached to iron hubs and axles. Late in the 1800s the Fitz Waterwheel Company was selling all-steel waterwheels, with the advantages of nearly 90 percent efficiency plus resistance to icing in winter.      

Ben Smith dam stored water for the woolen mill

Back to Maynard, or what at the time was known as Assabet Village. Construction of the Ben Smith Dam, the mill pond and the canal between the two in 1847 resulted in a large, year-round water reserve at an altitude above sea level of 175 feet. Outflow from the waterworks would have reentered Assabet River below the mill at an altitude of 155 feet. Flow rate through the system is not known, nor design, but a good guess is two overshot wheels each with a 15 foot drop and a combined flow of up to 45,000 gallons per minute.

That flow is equivalent to 100 cubic feet of water per second (cfs). For comparison, the Assabet River has a year-round average of 200 cfs, but summer months average under 100 cfs. Keep in mind that water power production was never around the clock. Flow through the mill was stopped at the end of the workday to back up as much water as possible in the millpond for the next day.

Power production was approximately 50 horsepower. As Maynard’s mill operations grew, water power was supplemented by coal-fueled steam power, hence all the historic images with smokestacks. The book “Assabet Mills” states that by 1879 nearly 40 percent of power was from steam engines. A different source states that the first steam engine was installed in 1862, when Amory Maynard was increasing capacity to meet Union Army contracts for wool goods. 

At some point the waterwheel complex was replaced by a turbine. Turbines are much more compact than wheels. Water drops down through a progressively narrowing pipe. This water, now under high pressure, jets into the turbine chamber at high speed, spins the turbine blades, and exits out the bottom. Turbine efficiencies rival the best wheels.

Records show a hydroturbine used to generate electricity from 1902 to 1968, with a hint that Digital Equipment Corporation may have refurbished the power plant in 1983 and run it until 1992. The Clock Tower Place 2002 petition to surrender electricity generating rights described the last operative turbine as having a pass-through of 128 cfs and production of 125 kilowatts of energy.

CTP has recently had some second thoughts about alternative and renewable energy sources, and according to Joe Mullin, CTP’s Public Affairs Director, is tentatively exploring the process of restoring hydroelectric power generation. Currently, the turbine is long gone, the smokestack is merely a support for cell phone antennae, and all of the kilowatt-hours used to power CTP by day and light up the windows by night are wired in from elsewhere.

Coda: According to an article in the March 14, 1902 issue of The Maynard News, the switch to electric power included the installation of two vertical compound engines directly connected to electric generators. The engines would have been steam engines powered by either coal or fuel oil. The power of the two engines was 2,500 and 800 horsepower. From this time forward hydropower was clearly a minor portion of total energy production at the mill complex. Employment at the newly enlarged complex owned by the American Woolen Company was 2,000.