Everything you ever wanted to know about Guri but were afraid to ask

The lower the water level at Guri goes, the more water you need to produce the same amount of power, and the more you risk damaging the actual turbines.

“The other day I was at the dentist and he is getting ready with his electric drill to make holes, and I thought I better think of something fast or else it is gonna hurt. Then I thought about this little motor going around and what was it that made it turn. And what was going on, and what’s going on is that there is a dam some distance away, and water going over the dam turns a great big wheel, alright. And this wheel is connected with long thin pieces of copper, which split up into other pieces of copper, which split up into other pieces of copper and spread all over the city. And they they are connected back thru another little gadget and makes the wheel turn. All the wheels in the city are turning because this other wheel turns. If this wheel stops all the wheels stop…”

Richard Feynman

Feynman had a knack for explaining things simply and beautifully. He paints a dramatic picture of what will happen when water tumbling over Guri’s great wheels no longer flows and motion stops. Every wheel in every city, every light bulb, every compressor, pump, radio, TV, everything will stop.

Guri is by far the biggest power generating wheel we have. Chavez demonized hydropower and our dependance on it and shelled out cash to nearly double the country’s thermoelectic generation capacity. The plan was to go from 19,000 MW in 1998 to 34,500 MW in 2015.

Right now, Guri is just three meters from the lowest point ever recorded.

After all was said and done, today we have today less generating capacity than in 1998 with 17,000 MW available. That’s some 1,700 MW less than the actual demand. We’ve proven utterly incapable to get thermoelectric plants old and new in operation. Hydropower continues to be our most reliable source of energy.

Hydropower is simple. You dump buckets of water from a certain height onto a wheel. Power is proportional to the size of the bucket you drop per unit of time (flow) and the elevation (known as head). Bigger buckets dropped from higher up yield more power. In a hydropower plant, the effective drop is the difference in elevation from the upstream reservoir level to the downstream reservoir level.

This week the upstream water level at gury fell to just 247 meters above sea-level, from a maximum level of 271 meters.

Why is this a big deal?

For a few reasons. Our water buckets are falling from a lower elevation, which means that we have to consume more water to generate the same amount of power. So reservoirs have to be depleted faster and faster just to stand still, in terms of the power availale.

At this rate, we are going to reach the historic minimum this month and fall all the way to the critical 240 m mark, where half the plant has to shut down, sometime in late April.

But it’s not just that. You can think of the reservoir as a martini glass. Water available is proportional to the square of elevation. Up higher to the rim, there’s a lot more liquid per cm. elevation than down towards the bottom.

Another key point is that you need to keep a certain minimum pressure to keep the turbines working properly. Without it, you get cavitation. And that’s bad news.

Cavitation is what happens when water goes so fast around the turbine blades that local drops in pressure turn it into vapor globules. When it slows down again, pressure rises again and the vapor bubble becomes water again in a process that creating tiny implosions. Those implosions chip the metal away. This slowly damages the turbines, which undermines their efficiency.

Of course, when the reservoir is low, the last thing you need is anything that requires you to use still more water to generate the same amount of power…but that’s just what cavitation does.

We’ve heard reports that the turbines already suffered some cavitation damage after the last dry spell in 2010, and they were never repaired.

The bigger deal about this 247 meters above sea level number is that, at 240 meters the inlet to nearly half the turbines starts to get exposed, and that forces you to shut them down altogether. That’s 5,000 MW off, what it takes to power maybe 20 mid-size venezuelan cities. It’s about 30% of the total power generation capacity in the whole country: hitting 240 meters would be a proper disaster.

Is that in the cards, really?

Oh yes.

Right now, Guri is just three meters from the lowest point ever recorded. At the moment, the reservoir is dropping at around one meter per week. At this rate, we are going to reach the historic minimum this month and fall all the way to the critical 240 m mark 49 days from now. By then, we’ll have damaged the turbines further through cavitation.

And remember, because of the Martini-glass effect, we shouldn’t expect the rate-of-decline to be quite linear: at least, if the water continues to drop at a linear rate, Guri will be producing less and less power every day. Even so, we have enough water until the end of April, more or less.

The rainy season usually start in May…but not always. Sometimes the rains start a bit earlier, sometimes later. It’s an El Niño year, when the normal patterns are upended.

What can we done today? Brace yourself, buy candles and pray for rain.


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