New
Development: Tyson Creek: Tapping into the Highest Head in North
America
By Peter Schober
The Tyson Creek project is the third
hydro system developed by Renewable Power Corporation, a British
Columbia company. On the Sunshine Coast of BC, Tyson Creek went online
in December 2009 and has a rated output of 9.3 MW based on 865 meters
gross head and a flow of just 1.3 cubic meters per second.
The “head pond” for the project is Tyson
Lake, a pristine alpine sanctuary at 1,040 meters. Flanked by high
altitude forests and snowcapped peaks, this large lake offers a couple
of significant benefits. First, it acts as a storage reservoir,
allowing Renewable Power to match its output with both short-term and
seasonal demand fluctuations. Second, it acts as a giant settling pond
to prevent dirt and debris from entering the intake. Ironically, this
aspect has turned out be a surprising problem, which this article will
address in more detail.
The Sunshine Coast of BC is wet, steep
and rugged — perfect conditions for small hydro. Along the coastline,
light development is scattered among forested wilderness, and many of
the smaller communities are subject to frequent power outages. The
town of Egmont, a few kilometers from our project, has historically
averaged about 2 weeks per year without power. For this reason,
Renewable Power felt it would be a strong advantage to design a hybrid
system able to operate both grid-tied and islanded. Tyson Creek
supplies power to BC Hydro. But when the grid fails, Renewable Power
continues to operate as a utility to the local community. This has
been especially well received, because Renewable Power is now able to
keep the area’s largest employer running during prolonged outages.
The selection of Tyson Creek
Like all independent power producers, our
objective at Renewable Power is to keep the turbine spinning and
deliver the most power when demand is highest. This can be a challenge
at northern latitudes such as BC, when high power demand in winter
typically correlates with freezing temperatures and low flow. Early
on, Renewable Power made a business commitment to environmental
preservation.
While the company recognized that every
project — hydro or otherwise — will have some impact, it actively
looked for ways to minimize its footprint. This emphasis helps the
company’s projects gain public acceptance, but more directly,
Renewable Power believes it is the right thing to do.
Renewable Power was particularly
interested in finding a way to employ storage without a dam. Larger
utilities like BC Hydro are challenged to meet peak load demands, but
most run-of-river projects have little or no reserve to draw upon when
demand spikes. For utilities, this peak demand problem is exacerbated
by the growth in green power alternatives like solar and wind
production, as output can be highly unpredictable and unlikely to
align nicely with demand.
Renewable Power’s plan was to use a
pre-existing lake for reserve capacity, which could solve a problem
for BC Hydro and allow Renewable Power to tailor its output to take
advantage of peak rates.
Tyson Lake had great potential. At 60
meters deep, it offered plenty of reserve, assuming the company could
find a way to tap in well below the surface. In addition, by bringing
the penstock in below ground, Renewable Power could preserve the
untarnished beauty of the area with no visible signs of our intake
system.
The outlet from the lake, Tyson Creek,
was so small it didn’t have a name before Renewable Power got there.
From Tyson Lake, it flows under boulder fields for a third of its
length, then plunges down sections so steep it would be impossible for
fish to navigate. Near sea level, Tyson Creek empties into the Tzoonie
River, where it then travels a short distance to salt water at Narrows
Inlet.
One major environmental hurdle — fish —
was somewhat less of an issue for Renewable Power, since Tyson Lake
and most of Tyson Creek were barren. The company still had to be very
careful not to impact the Tzoonie River, however, as it is a major
spawning ground for several species of salmon and trout. Water from
our tailrace would return to a short, fish-bearing stretch of Tyson
Creek before joining the river. It was imperative to maintain good
water quality to protect downstream habitat.
The high head was particularly
attractive. It would require a special turbine design to handle
unusually high pressure, but would also save considerably on turbine
cost. Physical turbine size relates directly to flow, not head, so
higher flows require larger, more expensive turbines. In contrast, our
high head system produces almost 10 MW using a single Canyon Hydro
Pelton turbine with a pitch diameter of just 1.3 meters.
High head kept penstock expenses down as
well, allowing us to choose a pipe size of 36 inches (narrowing to 28
inches). The steep terrain helped us limit the penstock length to 4.2
km, not bad considering almost of quarter of this represents vertical
head.
Choosing the right turbine
The unique characteristics of our project
created some special requirements for the turbine. 865 meters head
translates to more than 1,200 pounds per square inch (psi) at the
turbine. Renewable Power was concerned a conventional Pelton design
would disintegrate under load. Likewise, operating in islanded mode
creates an entirely new set of issues relating to load compensation
and frequency control. It is no longer possible to simply rely on the
grid to keep everything in sync.
Renewable Power talked with several
turbine suppliers and eventually selected Canyon Hydro. The company
appreciated Canyon’s attention to detail on a previous project, and
Canyon had extensive experience with islanded systems. But Renewable
Power still needed to address concerns about turbine reliability under
very high head. Only a few suppliers have had direct experience with
such high pressures.
I contacted Dan New, president of Canyon,
and suggested he join me on a trip to Switzerland to visit a project
similar to ours, with just a couple more meters head. We learned a
great deal about how water behaves at those pressures, and upon our
return, Canyon Hydro designed and fabricated a heavily gusseted Pelton
turbine that continues to run flawlessly.
Islanded operation requires control
systems that, while common on larger hydro systems, are unusual for a
10-MW project. Instantaneous changes in load are compensated by the
jet deflector shield, which immediately adjusts to deflect more or
less of the water jet to the turbine runner. Canyon Hydro integrated
controls from Unit Electric with their jet deflector design to
maintain constant frequency under rapidly changing load conditions.
Intake & penstock challenges
Getting the water to the turbine was
tricky, given the combination of steep terrain, the use of a lake as
our intake system, and our objective to preserve the natural
landscape.
Pictured is the lower end of the
tunnel during construction. In addition to the main penstock, a
smaller return pipe is used to pump water back up to the source of
Tyson Creek to maintain constant flow in the creek bed. The curved
white pipes vent the tunnel.
We decided to underground the penstock on
all but the steepest section of terrain. This would minimize potential
damage in the years ahead, and allow nature to gradually hide any
evidence of a pipeline.
On the uppermost 300 meters of the route,
the surface was not disturbed. Instead, the company tunneled through
contiguous bedrock, including a few small bends to ensure the penstock
remained in stable rock. The penstock reaches the lake about 30 meters
below the surface.
Before the final route for the tunnel was
determined, Renewable Power had to decide where to tap into the lake.
To help with this, we used an unmanned submarine to navigate around
sunken logs and boulders to chart the bottom of the lake. Tyson Lake
was once fed by a now-extinct glacier, and there was evidence of a
thick layer of fine, glacial flour along the bottom surfaces. We
concluded that if we tapped into the lake near any horizontal surface,
the suction would pull this muddy, abrasive layer into the turbine.
But luck was with us. We discovered a nearly vertical, underwater rock
wall that offered an excellent point of intake. With appreciable
distance to any horizontal surface, it would allow the company to draw
water without disturbing glacial flour.
Or so we thought.
Building the steep section of pipe
A steep hillside is great for keeping a
penstock short, but getting those pipe sections into place was a
lesson in creative logistics. Unlike the lower sections of penstock,
we couldn’t simply clear a path, dig a trench, set a length of pipe
and cover it up. Faced with a 400-meter stretch of grade angled at
more than 45 degrees, we knew the use of trucks and mobile cranes
would be out of the question.
The company considered winching the pipe
sections up the slope, but realized it would still have no way of
manipulating the pipe position when it got there. A method to
precisely control the small adjustments necessary to align the
sections was needed. On a mountain as steep as this, gravity is not
always your friend.
Renewable Power solved the problem with a
skyline crane. Originally designed as a system for the logging
industry, a skyline is a suspended cable running up the mountain,
anchored at towers (or trees) on each end. A carriage with a winch
travels along the length of the skyline. In logging operations, the
carriage rides up the skyline cable and the winch lifts the logs off
the hillside for the ride back down.
The same principles would be used, but
the company demanded a lot more precision. Our skyline would require
two carriage/winch units so each end of a pipe section could be raised
and lowered independently. Alignment of the skyline cable was also
critical.
Even with the skyline, managing 39-meter
sections of 36-inch steel pipe is tricky. Unlike a steel-beam crane, a
skyline cable swings and bounces. Every small positional adjustment
triggers new motion that requires time to settle. With practice,
however, we found we manipulate the carriage and both winches to set
the pipe sections on their supports, align the ends, and hold back the
forces of gravity while we welded them into place.
At 865 meters above the turbine,
Tyson Lake acts as a storage reservoir. The lake is 60 meters deep and
offers plenty of reserve.
Powerhouse construction, turbine
installation and the transmission system all went according to plan,
with great coordination between our many contractors and suppliers.
The quality and support we received from Canyon Hydro was second to
none. On several occasions, senior managers and engineers from Canyon
were on site, often providing expert advice on non-turbine issues such
as control programming and tailrace design.
The use of Tyson Lake as a storage
reservoir allows the project tremendous flexibility as to how much
power it generates and when. Storage capacity is significant, enough
to run at full output 70 to 80 percent of the time. The freezing
winter temperatures at lake altitude halt water flow into the lake,
but a full lake gives us sufficient reserves.
A suspended cable “skyline” moves a
26-meter penstock section into position on a grade too steep for
conventional machinery.
In spring and summer, runoff from
snowmelt will be adequate to run the hydro system at full capacity and
refill the lake for use the following winter. Because full time
production began in December, Renewable Power was able to test the
storage aspect immediately.
Good water gone bad
The news is not all good, however. As
mentioned earlier, the bottom of Tyson Lake has a layer of fine,
glacial flour, deposited long ago by an old glacier. We were fortunate
to find the nearly vertical, underwater cliff for our intake, but we
were not expecting the problem that came winging in from left field.
As the lake level was drawn down, a
previously submerged delta of glacial slur became unstable and slid
into the lake. The delta had once been the base of the glacier, and as
it dislodged it caused a significant release of mud and glacial flour
into the lake water.
This immediately triggered two very
serious problems. First, glacial flour is extremely abrasive. The
combination of abrasive particles and exceptionally high water
pressure quickly caused severe surface degradation around the turbine
jet, particularly on the highly polished stainless needle nozzles.
Just a few weeks into production, we were already facing wear issues
that should have taken years.
Worse, a milky trail of turbid water
stretched from our tail race down Tyson Creek, along the Tzoonie
River, and out into Narrows Inlet. Understandably, this created alarm
within the local community and within hours, the Ministry of
Environment was asking tough questions. They agreed with Renewable
Power’s proposal to drastically reduce output, but to retain enough
flow to prevent the entire system from freezing up. During this time,
Renewable Power worked closely with Ministry officials to test the
water for environmental impacts, and installed a turbidity monitoring
system that would automatically shut down the plant if a similar
situation occurred again.
The heavier particles from the slide
settled to the lake bottom fairly quickly, but the extremely fine
glacial flour can remain suspended for months. Once the mud cleared,
the Ministry concluded that these microscopic particles were not an
environmental threat and allowed us to return to limited production.
Renewable Power is taking steps to
prevent a similar incident. In the meantime, the company continues to
deal with elevated levels of glacial flour passing through the turbine
while waiting for the lake to clear.
A plan for environmental restoration
Despite our unforeseen problem with
turbid water, Renewable Power remains committed to preserving both
habitat and scenery, and we are well into the four-year plan to
restore the area as closely as possible to its original state. There
will always be some visible evidence of the hydro project such as the
powerhouse and one steep section of penstock.
Over the next few months, Renewable Power
will remove the minor traces of its work at Tyson Lake. When the lake
is full, it will be difficult to detect that a hydro system is in
operation. Most of our penstock is hidden underground and, while it
was necessary to clear a path to install portions of it, Renewable
Power is working on a reforestation program. And since the level of
Tyson Lake will periodically fall below the threshold to feed Tyson
Creek, the company pumps water back up to the original creek source to
maintain minimum flow on all sections of the creek.
The Tyson Creek project has its
detractors. To help mitigate potential problems, Renewable Power
proactively communicates. During construction, the company invited a
local environmental group to the site to show them what was really
happening and to answer their questions and concerns.
Not surprisingly they found a few things
that didn’t pass muster (such as water runoff during heavy rain), and
the company followed their recommendations to resolve those issues.
When the release of muddy water occurred, Renewable Power issued a
press release outlining exactly what happened and what it was doing
about it.
The company also maintains a website for
all of our projects, focused on helping the general public understand
what we do and why clean hydropower is good for all of us.
Peter Schober is Senior
Engineer for Renewable Power Corporation. He was Project Engineer for
the Tyson Creek project, and has directed the development of several
other hydro systems.
Green Play Ammonia™, Yielder® NFuel Energy.
Spokane, Washington. 99212
www.exactrix.com
509 995 1879 cell, Pacific.
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