My friend Bevan asked me recently why we don’t see more hydroelectricity projects being developed. I replied, “All available hydroelectricity that can be developed has already been developed”, hoping it didn’t sound too much like the unfortunate 19th century quote attributed to Charles Duell about how everything that can be invented has already been invented. Despite my interest in renewable energy, I had not thought very much about hydroelectric power since I have long assumed that if someone could have built another dam on any existing waterway and placed a generating turbine at the bottom of it, it would have been done decades ago.
Bevan responded, “Well, how about the Big Thompson River which flows unimpeded down a canyon for many miles and has no hydro generating stations along it?” I began to wonder if we had been avoiding employing a source of clean and renewable energy simply because of its environmental impacts, as often seems to be the case. I thought, “If the Big Thompson canyon could be dammed, how much hydroelectric power could it produce?” Please note I’m not suggesting that anyone actually do this. It’s one of the most naturally beautiful and accessible canyons in Colorado and even if it could supply enough energy to solve the entire world’s energy needs, it probably still would never be approved due to opposition by those who would like to keep the canyon and river in their natural states. I am just trying to satisfy a curiosity I have.
Often times, putting up dams on rivers is resisted even more vehemently by environmentalists than constructing fossil fuel-burning plants. The concerns range from the people who are displaced by lakes that are created by the dams to fish species that can no longer reach the river’s head waters to breed. Dams have some other benefits such as flood and drought control, but those may be overshadowed by the safety concerns of a dam breaking and causing flooding and destruction. So despite its environmentally-friendly electric power generation, hydroelectric power has other environmental impacts that can limit its acceptance.
A few months ago I took a clean energy class through CSU. One of the classes met at the offices of the Platte River Power Authority, a co-op power company that supplies electricity to the cities of Fort Collins, Loveland, Longmont and Estes Park. The PRPA gets about 20% of its power from hydroelectricity. While touring the grid control facility, I saw a lot of generating stations on the large control board. Among them were several hydro generating stations located west of Fort Collins. I realized that I had somehow overlooked these generating stations in my travels, although I’m sure I had driven by them or flown over them on many occasions. Upon doing some research into the topic, I found that the hydroelectric generating stations monitored by the PRPA are powered with water that is diverted from the western slope. Through a series of tunnels, canals, and siphons, nearly all of the 260,000 acre feet of water diverted annually from the western slope has its energy converted to electricity by a series of power hydroelectric generating facilities, most of which are easy to overlook.
The means to divert the water from the western slope is called the Colorado Big Thompson (C-BT) Project. It is one of the largest and most complex natural resource developments ever undertaken by the Bureau of Reclamation. There are more than 100 structures comprising this project which you can read about here.
The most important part of the project is the Alva B. Adams tunnel that directs water from Grand Lake on the western slope to the eastern slope of the Rocky Mountains. The tunnel is 13 miles long and goes through solid rock under the continental divide. The water eventually finds its way into several large man-made reservoirs, the largest two being Carter Lake and Horsetooth Reservoir which are situated in the foothills along the Front Range. Prior to arriving at these reservoirs, the water flows through 5 hydroelectric generating stations. I’ve listed them and their capacities in this table:
|Generating Station||Penstock Head (ft)||Capacity (MW)|
Unlike many of the hydro generating stations you find on large rivers, the ones that are part of the Colorado Big Thompson Project are small stations located at the end of long pipes called penstocks, which are large diameter tubes connected to the upstream water source.
Energy contained in water is proportional to both the water’s pressure and flow rate. This energy is converted to electric power through a turbine-generator. Available water flow is usually dictated by nature by the region’s snowfall and rainfall. Pressure, however, can be adjusted. To increase the pressure, it’s necessary to increase the depth of water, also known as its ‘head’. Water pressure increases about 1 psi per 2.3 foot of head, and as a result, you can see that the generating stations with the higher penstocks in the table above produce much more power. One way to get more pressure is to construct a very tall dam. Another way is to construct a relatively shallow dam and put the generating facility at lower elevation and connect the dam and generating station with a penstock. You need to constrain the water in the penstock that runs downhill to the generating station which has the effect of greatly increasing its pressure. Sometimes these large diameter pipes are buried, and other times they are exposed. You can see them exposed in several places in Colorado Big Thompson Project, such as above and below Mary’s Lake just south of Estes Park. There you can see a long sections pipes running down a mountainside. At the bottom of the upper penstock, you’ll find Mary’s Lake generating station. A similar penstock runs partially underground and partially above ground from Mary’s Lake to the east side of Estes Lake, where you will find Estes Park generating station.
I am missing one C-BT power plant in my list above because it is located on the western slope, a 21.6 MW generating facility on the Green M
ountain Reservoir. Although it is technically part of the C-BT project, I didn’t include it because I was interested in figuring out how much power is generated from this diversion project from water flow to the east side of the continental divide. I was also curious about how much water flowed to the east side of the continental divide through the tunnel and how it compared with the normal Big Thompson River flow. In other words, just how much water do we import from the western slope?
Wikipedia lists the average flow rate of the Big Thompson River to be 72.5 cu. ft/sec where it exits the Big Thompson Canyon. The Alva B. Adams tunnel can handle a flow rate of 550 cu. ft/sec. It runs at about 84% capacity on an annual basis which means that it has an annual average flow of 460 cu ft./sec. I was very surprised by this number. This means the tunnel provides more than 6 times as much water flow as the Big Thompson River provides on an annual basis.
The total maximum generating capacity of the 5 hydro plants that I’ve listed in the table is 162.5 MW. A reasonable capacity factor for hydroelectric generating plants is 50% so we can assume that the average annual generating capacity of these stations is about 81 MW or 710 million kWh per year.
So, where am I going with all this? I’m trying to figure out how much energy could be obtained if we were to convert all the flow of the Big Thompson canyon using several large dams or a series of smaller dams with penstocks connected to generating stations. Based on the differences in flow rates between the Big Thompson River and the diverted flow through the Alva B. Adams tunnel, which is a factor of 6.4 greater, I would estimate that the amount of power available from the Big Thompson River would be around 12 MW, again assuming a capacity factor of 50%. I figure that the head of each flow is the same, and thus the flow rate difference means that the Big Thompson would generate about 16% (1/6.4) of the capacity of the C-BT generating stations if it were to all be converted to electricity. That’s enough to power 12,000 homes. This sounds like a lot, but to really put it in perspective, this is only around 2% of the capacity of a typical fossil fuel generating plant. For example, the Rawhide Power Plant north of Fort Collins which is a coal/gas plant can generate 522 MW. The Fort St. Vrain power plant south of Greeley which uses natural gas can generate 720 MW. Nuclear plants typically have capacities of 1000 MW or more, with the largest one capable of generating more than 8000 MW. When you talk about power in those quantities, 12 MW seems like a drop in the bucket.
It would appear that the amount of power available from the Big Thompson River is so small as to not make it worth the investment even if there were no environmental concerns. The construction costs would be quite substantial, considering the impact to the region such a project would cause. It would require moving many homes and businesses as well as the highway that runs through the canyon.
Rivers in the Colorado are much smaller than the ones you find in the eastern and pacific northwestern parts of the United States. In fact, based on flow rate, they would probably only qualify as creeks in other parts of the country. On the lower section of the Susquehanna River in Pennsylvania, there are 3 hydroelectric dams in a span of 21 miles that combine to generate more than 1,000 MW of hydroelectric power. But with an average flow rate of 40,000 cu ft/sec, the Susquehanna River has 500 times the flow of the Big Thompson River.
The largest hydroelectric generating project in the world is currently under construction in China. The Three Gorges Project will have an output of 22,000 MW and will thus qualify as the largest electrical generating plant in the world of any kind. It’s not without its environmental impacts, however. Nearly 1.4 million people had to be moved in order to fill the lake behind the dam. I suppose eminent domain may work a little differently in China than it does in the U.S.. I can’t conceive of any public works program that could displace people on that scale in the U.S. or any other country for that matter.
Hydroelectric power is one of the oldest and largest sources of renewable energy available today. Its output doesn’t vary as much as other renewable energy sources like wind or solar. It even offers the potential for energy storage to allow for peak demand-shifting. However, I don’t think that it can be expanded significantly from its current state, except perhaps in a few geographies around the world that are underdeveloped by western standards.