Blogue sobre Diversion-Type Hydropower Stations: Borrowing a Shortcut Through the Mountain

Diversion-Type Hydropower Stations: Borrowing a Shortcut Through the Mountain

How a gentle canal or a long tunnel can concentrate a river's steepest drops into clean electricity

Stand beside a young, fast mountain river and you can hear the energy: water thundering down a slope that sometimes falls hundreds of metres over a single winding valley. The trouble is that a turbine can only use a steady, concentrated head - the vertical distance between the water entering it and the water leaving it - not the scattered drops of a zigzag river. The engineering answer is beautifully simple: don't follow the river; shortcut it.

On the middle and upper reaches of rivers, or on steep mountain streams, engineers often build a gently graded conveyance - an open canal, a tunnel, or a pipeline - that carries water around the outside of a sharp river bend and delivers it far below. The conveyance itself has a mild slope, so almost the entire natural drop of the bypassed reach is converted into head at the powerhouse. This way of developing a river is called diversion-type development, and a station built this way is a diversion-type hydropower station.

Depending on whether the water in the conveyance flows under pressure or with a free surface, diversion stations come in two families: pressurized and unpressurized.

1. The Pressurized Diversion Station: a River Rerouted Through Rock

Let's build a thought experiment. Imagine a steep, meandering reach of river where the water wanders for tens of kilometres, yet the two ends of the reach are only a few kilometres apart as the crow flies - and differ in elevation by a full 500 m (a dam holding the water at 850 m, a valley floor at 350 m). Figure 1 shows the plan view of this model station.

Figure 1 - Plan layout of a pressurized diversion-type hydropower station: the river loop is ~50 km long, but the dam and powerhouse are only ~8 km apart in a straight line, with a 500 m fall between them (850 m to 350 m).

The recipe is simple. At the upstream intake we build a dam to create a small reservoir. From the reservoir, we drive a headrace tunnel straight through the mountain and lay a penstock (a steep pressure pipeline) down to the powerhouse near the downstream river. In this model, the tunnel pierces about 3 km of rock and the penstock runs another 3 km down the mountainside - and together they replace roughly fifty kilometres of rushing river.

Because the mountain and the powerhouse are far apart, the profile in Figure 2 is drawn in two pieces: the reservoir on the upper mountain, and the powerhouse at its foot. The blue arrows mark where the upper pressure conduit and the lower pressure conduit are actually one continuous pipe.

Figure 2 - Vertical profile of the pressurized scheme. The headrace tunnel crosses the upper mountain; a penstock carries the water down to the turbine. The blue arrows indicate that the upper and lower conduits are the same pipe.

Here is the defining feature: from the intake to the turbine, the water fills the tunnel and the penstock completely and flows under pressure - every metre of that 500 m elevation difference is locked in as pressure head. That is why this arrangement is called a pressurized diversion-type station.

For small plants, the conveyance is often a long surface penstock, as in the photo below: hundreds of metres of pressure pipeline marching down the valley toward the powerhouse. In many large stations, by contrast, the entire conveyance is underground - nothing but pressure tunnels all the way.

Figure 3 - The penstock and powerhouse of a small diversion-type station (photo: internet).

2. The Unpressurized Diversion Station: Letting the Water Ride a Canal

Now a second thought experiment. The river bend is even bigger this time: about 50 km of winding channel versus only 8 km in a straight line, again with a 500 m fall. Geographically, there is one extra gift - along the route toward the downstream outlet, the terrain passes through a belt of comparatively gentle ground.

Figure 4 - Plan layout of an unpressurized diversion-type station: a low weir, a 7 km open canal to a daily regulating basin (800 m), then a 2 km penstock down to the powerhouse (350 m).

This time the engineering is different. At the upstream intake we build only a low diversion weir to split off part of the flow. Then we dig a 7 km open canal across the gentle ground, delivering the water to a pond - a daily regulating basin - not far above the powerhouse site. From the basin, a 2 km penstock carries the water down into the turbines. Because the canal crosses soft, flattish terrain, the water surface only drops about 50 m from intake to basin, leaving 450 m of head for the penstock to deliver - nearly the whole natural fall, harvested by a canal that barely slopes at all.

Figure 5 shows the vertical profile, again drawn in two pieces with the blue arrows marking the same continuous pipe above and below.

Figure 5 - Vertical profile of the unpressurized scheme: reservoir and low dam, open diversion canal, daily regulating basin, penstock, powerhouse.

The main conveyance here is an open channel with a free water surface - the water is not pressurized inside it - which is exactly why this arrangement is called an unpressurized diversion-type station.

High head, small flow - and a river that must not run dry. Rivers that plunge steeply through tight bends usually carry modest flows, so diversion-type stations are typically high-head, low-flow plants. The heads can be spectacular: China's Tianhu station in Guangxi works under 1,074 m, and Austria's Reisseck plant in the Alps approaches 1,767 m - among the highest heads of any developed hydropower in the world.

But diversion development has a serious ecological price tag. Because the conveyance carries the water straight from the dam to the powerhouse, the river reach in between is starved of flow. In the dry season that bypassed reach can run completely dry, damaging the aquatic ecosystem along it. Modern projects therefore reserve an ecological discharge for the bypassed reach - a point that deserves every bit as much attention as the turbine selection itself.

3. Two Giants of Diversion Development in China

China is home to many diversion-type stations. Two of the largest show just how far this idea can be pushed.

3.1 Jinping II on the Yalong River - 4,400 MW underground

The Jinping II hydropower station sits on the great horseshoe bend of the Yalong River (the "Jinping Grand Bend") in Liangshan Yi Autonomous Prefecture, Sichuan Province. Over its 150 km loop, the river falls a natural 310 m. The project simply cuts the corner: four giant headrace tunnels drive straight through the mountain that the river loops around, as shown in Figure 6, which also marks the upstream sluice and the downstream powerhouse.

Figure 6 - Geography of the Jinping Grand Bend: the green lines are the headrace tunnels between the upstream sluice and the downstream powerhouse (image: internet).

The headworks is a sluice barrage at Maomaotan on the western (upstream) Yalong River: maximum height 37 m, crest length 162 m, with 5 gate bays of 12 m each (Figure 7).

Figure 7 - The Jinping II sluice barrage at Maomaotan (photo: internet).

The conveyance system uses a "one tunnel, two machines" arrangement: four headrace tunnels, excavated at 12 m diameter and lined down to 11 m, each averaging 16.6 km long. They run through up to 2,525 m of rock cover (typically 1,500-2,000 m) under a rated head of 288 m. The powerhouse is buried beneath the mountain beside the river at Dashuigou on the eastern Yalong: three huge caverns - the main and auxiliary powerhouse, the main transformer hall, and the tailrace gate chamber - laid out side by side. Each of the four tunnels reaches an upstream surge chamber, then splits in two to feed eight turbines, whose draft tubes return the water to the river. Add the access, ventilation, drainage, cable, busbar and safety-and-construction tunnels, and the whole complex resembles an underground city (Figure 8).

Figure 8 - 3D cutaway of the Jinping II underground powerhouse complex: diversion tunnels, upstream surge shafts, main powerhouse cavern, transformer cavern, draft tubes and tailrace gate cavern beside the Yalong River (image: internet).

Those surge chambers deserve a word. A long pressurized tunnel full of moving water resists any sudden change: close the turbine gates and the inertia of kilometres of water sends a pressure wave - water hammer - surging back up the tunnel. A surge chamber is a vertical cavern with a captive cushion of air near the powerhouse that absorbs these shocks, whether caused by gate operation, load swings on the turbines, or other transients. In any long-conveyance scheme it is not an accessory; it is essential protection for the machines.

The main cavern houses 8 x 550 MW turbine-generator units - a total installed capacity of 4,400 MW, delivered by nothing more than the geometry of a river bend.

3.2 Tianshengqiao II - 1,320 MW on the Nanpan River

Tianshengqiao II ("Natural Bridge II") stands where Longlin County in Guangxi meets Anlong County in Guizhou, on the upper Nanpan River of the Hongshui (Red Water) system. Over a 14.5 km reach the river concentrates a fall of 181 m.

The headworks sits at Basuo, at the exit of the Tianshengqiao gorge. It comprises non-overflow gravity dams on both banks, an overflow spillway section, a sand-flushing sluice and the power intake, with a dam axis 470 m long. The dam is a roller-compacted concrete (RCC) gravity dam, 60.7 m at its maximum height with a 471 m crest; nine spillway bays of 9 m each occupy the middle, with the sand-flush sluice and intake on the right. Figure 9 shows the dam as seen from the reservoir.

Figure 9 - The Tianshengqiao II barrage viewed from the reservoir (photo: internet).

The conveyance system consists of the intake, headrace tunnels, surge shafts and high-pressure penstocks: three headrace tunnels, each 9,776 m long with an internal diameter of 9.8 m. The powerhouse stands at the foot of the Xiashanbao hillside by the river, backed by a 380 m high artificial slope that had to be stabilized with a combined system of anti-slide piles, rock-anchor piles, anchor cables and concrete frames. Figure 10 is a bird's-eye view of the powerhouse from the downstream hills.

Figure 10 - The Tianshengqiao II powerhouse seen from the hills downstream (photo: internet).

Inside the powerhouse are 6 x 220 MW units, for a total installed capacity of 1,320 MW.

The Takeaway

A diversion-type station is, at heart, a bet on geometry: the energy in a mountain river is not only in its flow, but in the path it takes. Wherever a river draws a great lazy loop around a mountain, a canal or a tunnel can draw a straight line - and every metre of elevation that the loop wastes slowly, the line harvests all at once. The engineering challenge is to collect that height gently, protect the machines from the water's temper (hence surge chambers), and leave enough water in the bypassed reach for the river itself. Done well, it is one of the most elegant tricks in hydropower.

Figures 1, 2, 4, 5 and 8 are schematic illustrations; photos of real plants are credited as from the internet. Technical data are for popular-science reference; final design values must be confirmed by the design institute and the plant owner.