The pumped hydro storage potential of the Great Lakes

From Euan Mearns’ Energy MattersThe pumped hydro storage potential of the Great Lakes

The potential energy contained in the waters of the Great Lakes amounts to approximately six thousand terawatt hours, enough to supply the US and Canada with electricity for an entire year were the lakes to be drained to sea level. This of course will never happen, but there may be potential for partial utilization of the resource. A pumped hydro system that uses Lakes Huron and Michigan as the upper reservoir and Lake Ontario as the lower could theoretically generate 10 terawatt-hours, or more, of seasonal energy storage without changing lake levels significantly. The most likely show-stopper is the increased likelihood of flooding in the lower St. Lawrence River during pumped hydro discharge cycles. (Inset: Niagara falls runs dry in 1969).
The idea of using the Great Lakes for pumped hydro storage isn’t new – I remember reading about it once before but can no longer find the article. What brought it back to mind was a comment posted by Alex on the recent 100% renewable California thread in which he agreed that while there were indeed no fresh water lakes that no one cared about there were some that could perhaps be adapted for pumped hydro without anyone noticing:

Alex says:
January 18, 2018 at 5:02 pm

“The only existing fresh-water lakes that would be feasible targets for large-scale pumped hydro are in fact those that no one cares about.”

Or perhaps those that are so big you won’t notice the change. Here is a modelling challenge: Lake Ontario and either Lake Erie or Lake Huron.

I estimate 6TWh per metre elevation change in Lake Ontario.

A one-meter change in the level of Lake Ontario would in fact be noticed, but on the other hand 6 TWh of energy storage is worth shooting for, particularly in high-consumption areas that have grandiose plans to expand intermittent renewable energy, such as the Northeast US.

We begin with a couple of graphics. First a location map:

Figure 1: The Great Lakes

The map shows five lakes, but from the standpoint of pumped hydro potential there are effectively only four. Lake Michigan, Lake Huron and Georgian Bay (the large body of water to the northeast of Lake Huron) are interconnected and form a single lake with the same surface elevation. Flow out of Lake Superior is constrained by the Soo Locks, flow out of Lake Huron by the St. Clair River, and between Lakes Erie and Ontario we of course have Niagara and Horseshoe Falls.

Elevation details are shown in Figure 2. Pumped hydro storage capacity is a direct function of head, and the ~100m head differential between Lakes Erie and Ontario is the only elevation change large enough to offer any significant pumped hydro potential (the 74m fall between Lake Ontario and the Atlantic Ocean extends irregularly over hundreds of kilometers):

Figure 2: Great Lakes/St. Lawrence River elevations. A number of large dams, including the Robert Moses and Adam Beck dams near Buffalo, are not shown. Image from the University of Michigan.

Three major electric utilities presently generate hydropower from the Great Lakes/St. Lawrence river. Cumulative generation amounts to about 240 TWh of electricity a year from ~50 GW of installed conventional reservoir and run-of-river hydro capacity:

  • Hydro Quebec: 172 TWh from 36 GW of installed capacity in 2016, representing effectively 100% of Quebec’s electricity generation.
  • Ontario Power Generation: 38 TWh in 2017, representing 26% of Ontario’s electricity generation.
  • New York Power Authority : 28 TWh in 2016, representing 21% of NYPA’s electricity generation

The similarity between Quebec and Norway, which generated 144 TWh from 32 GW of conventional hydro capacity in 2016, is evident. And in the same way as European countries want to use Norway’s hydro to balance their intermittent wind/solar output so too do US states and Canadian provinces (including Ontario, which gets over 60% of its electricity from nuclear) want to use Quebec’s hydro to balance theirs. They are further encouraged by the supposedly immense storage capacity of Quebec Hydro’s reservoirs (176 TWh according to Quebec Hydro), but this isn’t really storage. It’s the amount of water Hydro Quebec has to run through its turbines to keep the lights on in an average year – in short, it’s a stockpile. And if Quebec exports enough of it the lights will eventually go out, unless of course enough water comes in to replenish the losses. This is a questionable proposition given the numerous externalities involved, which include environmental impacts, recreation, lake levels, river flow rates, navigation, flood control and, of course, the weather, which does not always cooperate. (As shown in Figure 3, Norway’s reservoir capacity once shrank to less than 20% of maximum, although the graphic doesn’t say when. A series of drought years could in fact dry hydro reservoirs up altogether. California came close recently.)

Figure 3: Reservoir Content for Norway. The figures represent 97.5 percent of the total reservoir capacity. Norway’s total reservoir capacity is 84.3 TWh. Data from Nord Pool.

Having established that we need storage rather than stockpiles, how much pumped hydro storage is presently installed on the Great Lakes/St. Lawrence? According to my estimates it amounts to only 7 GWh, all of it at the New York Power Authority’s Lewiston plant next to Niagara Falls, which acts as a peak-load facility for the Robert Moses dam. Lewiston is now undergoing an upgrade which in the words of New York governor Andrew Cuomo represents:

an investment in our future in sustainable, affordable energy across the state. With the halfway milestone of this critical project now met, businesses and residents in Western New York and beyond will enjoy improved performance and reliability in their power while New York takes yet another step toward achieving our long-term clean energy goals.”

And what are New York state’s long-term clean energy goals?

New York’s Clean Energy Standard is designed to fight climate change, reduce harmful air pollution, and ensure a diverse and reliable low carbon energy supply. To help achieve these goals, the CES requires that 50 percent of New York’s electricity come from renewable energy sources such as solar and wind by 2030.

Obviously Lewiston’s ~7 GWh of storage, which is enough to keep New York state in electricity for all of four minutes after the sun sets and the wind dies, is not going to get the job done. Gov. Cuomo is going to have to start thinking bigger. And so are his opposite numbers in neighboring jurisdictions that are also targeting major wind and solar expansions. Exactly how much bigger they are going to have to think is a question I have not looked into, but since this post is more of a theoretical exercise than an engineering appraisal I have settled on 10 TWh of storage as a test case. And here it’s important to note that the purpose of this storage is purely to smooth out long-term seasonal fluctuations. Daily and other short-term supply-demand imbalances can be handled by more agile technologies, such as varying the water flow through conventional hydro dams, batteries, CAES or maybe even dropping weights down mine shafts.

Going back to Figure 2, the obvious first choice is to construct a pumped hydro plant that takes advantage of the ~100m head differential between Lakes Erie and Ontario, with Erie forming the upper reservoir and Ontario the lower. Complications are introduced by the fact that the Moses and Beck dams are already there, generating ~20TWh/year between them from a combined capacity of 4.7GW, and I have assumed that these dams will be used as pumped hydro impoundments while continuing to generate as before.

Another question is plant operation. Since the plant is there to smooth out seasonal variations I have assumed that it will generate electricity by releasing water from Lake Erie into Lake Ontario for six months of the year and recharge itself by pumping the water back up again during the other six. In practice it won’t work out this way because Lake Erie is continuously being recharged from upstream, but coming up with a real-life scenario would require hydrologic modeling that I’m not in a position to conduct, so I have considered Lake Erie as a closed rather than an open system.

The basic requirement is to connect Lakes Erie and Ontario with a ~40km tunnel (or tunnels, or maybe even a canal) large enough to accommodate 10 TWh of water flow from Lake Erie down into Lake Ontario during the six-month generation cycle and to pump it back up again during the six-month recharge cycle. How much water do we need to generate 10 TWh? According to the formula provided by Stanford University :

E = (ρ g h v η)/3600

Where E is the energy storage in watt-hours, ρ the density of water in kg/m3 (1,000 for fresh water), g the acceleration of gravity (9.81 m/s2), h the head in meters, v the water volume in cubic meters, η the round-trip efficiency of the pumped hydro cycle and 3600 the number of seconds in an hour.

We need 44 billion cubic meters.

And what would the withdrawal of 44 billion cubic meters of water do to Lake Erie? (We will look at Lake Ontario later.) Lake Erie has a surface area of 25,700 square kilometers, or 25.7 billion square meters (have to be careful with zeroes here), so if we begin by withdrawing 44 billion cubic meters we would lower the lake level by 1.6 meters. The impacts are summarized in Figure 4. Depending on timing a 1.6m decrease could cause the lake to fall well below the historic low of 173.2m recorded during the dust bowl years of 1934-36, effectively replicating “megadrought” conditions:

Figure 4: Monthly water levels since 1918, Lake Erie. The vertical red bar analogs a 1.6m decrease below mean lake level. Data from the Watershed Council

With enough recharge from upstream the impacts may not be this large and may therefore not pose an insuperable obstacle to an Erie/Ontario pumped hydro plant, but it’s nevertheless a good idea to have a Plan B just in case. Plan B bypasses Lake Erie altogether by using Lakes Michigan and Huron as the upper reservoir and constructing a tunnel linking them directly with Lake Ontario via Georgian Bay (Figure 5):

Figure 5: Plan B, with Huron/Michigan as the upper reservoir and Ontario as the lower

The obvious disadvantage of Plan B is that it requires a much longer tunnel (110 vs. 45km). The advantage is that withdrawing 44 billion cu m of water from Lakes Huron and Michigan, which have four times the surface area of Lake Erie, lowers water levels by barely more than a foot, hardly enough to be noticed and probably not enough to affect water levels in Lake Erie either. As illustrated in Figure 6 it also won’t result in any record-setting low water levels:

Figure 6: Monthly water levels since 1918, Lakes Michigan & Huron. The vertical red bar analogs a 0.35m decrease. Data again from the Watershed Council

So far so good. But what about Lake Ontario, the lower reservoir in both cases? With an area of 19,000 sq km Lake Ontario is the smallest of the Great Lakes, and adding 44 billion cu m of water to it during the Huron/Michigan discharge cycle could increase its level by well over 2 meters. The implications would be undesirable to say the least:

Figure 7: Water levels since 1918, Lake Ontario. The vertical red bar analogs a 2.3m increase. Data again from the Watershed Council

So what Plan B essentially comes down to is how much outflow Lake Ontario can handle. Outflow from the lake has historically been around 7,000 cu m/sec, or 220 billion cu m/year, and adding 44 billion cu m to that during a six-month discharge cycle would raise outflow rates by up to 40%. Can such increases be managed by downstream flood control dams? When one reads that letting out enough water to lower Lake Ontario by 1 inch would raise the level of the river at Montreal by nearly a foot one has to wonder. The difficulties encountered in controlling the 2017 flooding along the lower St. Lawrence also do not inspire confidence. Flood control is something I have not been able to check into in any detail, but it strikes me as the most likely show-stopper for my 10 TWh project, of for that matter any pumped hydro project in the Great Lakes catchment.

On the other hand, 10 TWh of energy storage is no great loss in the context of the planned global transition to renewable energy, particularly when at least as much sea water pumped storage reservoir potential is available in glacially-sculpted valleys just northeast of the city of Quebec (details on request). Besides, it’s only appropriate that we should use landforms left by the last episode of climate change to protect us from the impacts of the next, always assuming there are any.

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Click here for the original piece at Energy Mattershttp://euanmearns.com/the-pumped-hydro-storage-potential-of-the-great-lakes/

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