Saturday, March 23, 2013

The Merrimack River - Renewable Energy

We all know that hydroelectric power is renewable energy. As an engineer of a sort and historian of a different sort, I've been asked a few times what the actual hydroelectric power potential of the Merrimack River at Lowell is. Many months ago I did a post that covered some of this. However, I don't quite trust my math and that wasn't the pure focus of that post, so I decided to do another one.

The questions are:

  1. How much power - at peak - can the Merrimack theoretically provide? Because of large seasonal variations in flow, this number can vary by a magnitude. So, we are also interested in how much power, in dry spells, it can reliably provide.
  2. How much water can go through the canal system? This number I figure should be very close to the historical figures. We've all seen in the summer that the river underneath the Pawtucket Falls Bridge is bone dry as it is all diverted into the canals. In the spring, there's a ton of water down there that is not being used for any mechanical purposes. 
  3. How much electricity does Lowell currently produce off of the Merrimack, at various sites around the city?
  4. Are there ways to produce more power we haven't implemented?
The first figures aren't hard to get. We just need to go to the USGS website and look at some flow figures for Lowell, since waterpower is a simple formula of volume times drop times weight of the water.

Total Power Available

So, we'll start here. This is the statistics for the amount of water in the Merrimack River below the Concord River. So, this isn't really at the point we are interested in, but for our purposes, it should be fine. You can surf around the site if you'd like, but it seems like the Concord River contributes maybe 10% of the water volume at this point, so we'll be within reason to use these figures and maybe round down as appropriate.

Two numbers in this chart are the most important: High flow and low flow.

Low flow, in August, is about 2,500 cfs (cubic feet per second). This is the reliable amount of energy available to the city. We can assume that due to the dam and lock system, the height of the canals, that is, the drop part of the equation, is constant.

High flow, in April, is nearly ten times that: 22,000 cfs. Again, the canal and dam system controls the level of the water in the system.

Now, that formula. I'm going to mention Patrick Malone's excellent Waterpower in Lowell book, which, in addition to having extensive data on the water the mills used (we'll get back to that), and the way it was captured, utilized, and researched, contains the formula I mentioned above. It also mentions, as we are often told, that the Merrimack River drops about 30 feet at Lowell and water weighs 62.4 lbs per cubic foot.

So, on the low end, we're talking about

2,500 cfs * 62.4 lbs per cf * 30 ft = 4,680,000 lb-ft per second.

Now, let's step back to high school physics. What is a pound-foot per second?

Well, a foot-pound is a unit of energy, equal to 1.356 Joules. A Joule per second is a Watt, a measure of power. So...let's convert foot-pounds to joules:

4,680,000 lb-ft * 1.356 = 6,345,228 joules

Since we're interested in joules per second, the number is directly equivalent to a watt. Our math says that at lowest flow, Lowell can expect 6.35 MW of power from the river before we factor in the energy efficiency of a turbine (which can be as high as 90%). That's a lot of lightbulbs! A horsepower is 746 watts so we're talking 8,509 horsepower, or a few dozen cars. Let that sink in as you think about gasoline: If you drive a 250hp car, your car produces about 1/30 the guaranteed power available from the entire Merrimack River at one of the best sites for an industrial city in the nation.

Now, sometimes the drop in the river is listed as 32 or even 34 feet. What's that do to these numbers? 34 feet is 7.19 MW.

Now, high flow:

22,000 cfs * 62.4 lbs per cf * 30 ft * 1.356 = 55.85 MW. Much better!

How much power is this?

Well...the US Energy Information Administration says that the average Massachusetts house uses 618 kilowatt hours a month. Let's assume that's ballpark for Lowell. Note that electricity usage is low in the Northeast because of the lack of electric heat and heavy air conditioning use. If you're using 618 kw/h a month, you're using 20.6 kw/h a day, or about 0.85 kw/h an hour. The hours cancel out so that's an instantaneous draw of 0.85 KW, or 850 Watts. You could run 850 homes on a Megawatt then, or over 5,000 homes off of the reliable energy the river can produce. Multiply by 10 for the high flow data. There are about 106,000 people in Lowell, and it looks like reasonable estimates are 30,000 households (which isn't exactly a house but close enough for us). That's one in six we could power, and that includes no other electricity uses from businesses and factories to street lights.

For comparison, the Natural Gas plant on Tanner Street, L'energia, produces 80 MW peak. The Coal/Petroleum plant on Salem Harbor is 750 MW peak. Seabrook Nuclear Power Station is 1,200 MW peak!

So, how much can the canals harness?

Again, as far as we've gotten is that the low flow for the river is 2,500 CFS. We know all of that water can get into the canal system. We know when there is a lot more than that, it flows over the top of the dam unused. We don't know how much the system can actually produce. The best way to find out the answer to this question is to look at historical data: back to Malone's book.

The thing to remember here is that in early Lowell, water was a utility. James B Francis, the most famous of the Locks and Canals engineers, the "Chief of Police of Water", really had to know how much water he had and could provide each mill so he could provide it to them reliably - and charge them for it. There was a standard unit for sale called the Mill Power, which was equivalent to 25 cubic feet per second of water falling 30 feet, or 85 theoretical horsepower. Malone has all this data. At the build-out of the canal system, Lowell produced a reliable 140 "Permanent" Mill Powers, which would be 11,900 horsepower, or 15.95 MW. This is three times the expected minimum (but under a third of spring flow), which is why the dam systems were needed to keep the water levels high enough during the day - we turn the river into a giant mill pond that refills overnight if need be. Mill companies were allowed to buy extra water when it was available, and in 1859 (after the system was completed and before heavy use of steam power), Lowell was often using an extra 40 Mill Powers. Let's call that an easy extra six Megawatts for 22 MW. There is complex engineering related to flow rates lowering head and becoming counterproductive, plus backed-up water slowing wheels when the river was high - for the book.

How much is installed?

Well, Locks and Canals, the company that provided the water, eventually ended up being owned by Italian energy conglomerate Enel. Enel, as Boott Hydroelectric, has a few sites in Lowell's mills as well as the main plant on the Northern Canal by the University Ave Bridge. Their statistics are surprisingly missing from the EIA map above, but they provide it here.

Their main plant uses 3300 cfs of water and drops it 37 feet. Using our formula to check their math, we get 10.33 MW. This is considerably lower than the 17.3 MW installed they claim here and there is no explanation for the gap. However, adding in the supplemental plants in the old mills, we get to 20.7 MW installed. If we take Lowell's historic max with surplus mill powers of 22 MW and we multiply by an efficiency factor of 90%, we get 19.8 MW... which is essentially what Enel says they can produce at maximum.

What I'm seeing is that if they are producing this power regularly, we are using all of the available water Lowell historically has had and this is not enough electricity, even in high water scenarios, to run the residential sector of the city.

Is there more somewhere?

An interesting fact about hydro power is that the technologies, because turbines are so efficient, haven't changed much since Francis was doing his experiments 150 years ago. Is there anywhere else we can squeeze out any more power at any price? Raising the dam would work, but we would flood out our neighbors upriver. There probably isn't much extra summer water, as we're already in control of the water all the way up the Merrimack's watershed to its source at Winnipesaukee.

The only way to get power out of water is to utilize its kinetic energy - it has to be moving. The Wikipedia article on hydraulic head provides a good graphic showing how a turbine, like those in Lowell, work. If you've ever seen a boat on the Merrimack in Tyngsboro or up by the Rourke Bridge up to the buoys above the dam, you know there isn't much motion to that water - it's being backed up by the dam. Go too much downstream, and you hit the 17 MW Lawrence dam installation - again, the water is not moving much above this point.

However, you have to wonder if during the summer months if it would be cost effective to install small run-of-the-river installations to catch the more-than-half of the power potential the river has. 

However, I'm not going to pretend to know anything about the math on that ;-)

In conclusion...

It would appear we are using the majority of water that Lowell has ever had available to it for power generation purposes already, and, while not an insignificant amount, is not anywhere near the electricity needs of the modern city...never mind its resource-poor suburbs. Always, always be conscious about claims of renewables replacing our dwindling fossil fuel reserves.