The Limitations Of Renewable Power (Part 3)

Posted on Thu 08/27/2009 by



Large Wind Turbine at Nine Mile Beach Wind Farm. Image courtesy of Verve Energy.

Large Wind Turbine at Nine Mile Beach Wind Farm. Image courtesy of Verve Energy.

The image at left shows a single large wind turbine. This is one of the turbines at the Nine Mile Beach Wind Farm near Esperance in Western Australia. Again I have left the image very large, so for greater detail, click on the image to open it in a new and larger window, and then click on the image again to see it in even greater detail. For scale of size, notice the Toyota Land Cruiser at the base of the tower.

These towers are now starting to appear in quite prolific numbers across the U.S. and also across the rest of the World. They have to be necessarily quite tall, so that the very large three bladed propeller can clear the ground by a significant amount during rotation. That propeller rotates what looks to be quite slowly, but the tip speed is quite high. The wind provides the process, and the actual generation of the electrical power is achieved inside the large structure immediately behind that propeller. This structure is called the ‘nacelle’.

Inside that nacelle is a generator that uses the same principle of a rotating magnetic field inducing a current flow to produce the electrical power.

If you refer back to the large image of the generator deck at Daiblo Canyon, shown at this link, that huge complex has a weight that is quite large, in all, probably ore than 1500 tons when the turbine and the generator are both considered, and just the steam turbine driving the generator weighs almost 400 tons. Something of that weight and size would be physically impossible to mount off the ground. To that end, the generator in a wind tower’s nacelle is considerably smaller, and because of that the power output is also considerably lower. The tower you see in the image might typically have a power output approaching 5MW. Considerations regarding size are all taken into account, and the units most commonly in use have generators typically producing around 3 to 4MW. Technology has improved considerably over the years, because some of the earlier towers contained generators producing only around 500 KW to 1MW at best. As that technology has improved, greater magnetic fields can be achieved with smaller units, and because of that, the power output of these units has steadily increased.

Wind Turbine Nacelle. Image courtesy of Siemens.

Wind Turbine Nacelle. Image courtesy of Siemens.

At the left here is a typical nacelle with an exploded view of what is inside the structure. Again, I have purposely left the image quite large, so click on the image to open it in a new and larger window and then come back here for the text. You’ll see that the exploded view has a series of numbers indicating the parts, and there’s no real need to know all of them, because the two most important parts of the structure are indicated at the arrows numbered 8 and 12.

The wind is the process here. It causes the blades to rotate, and as they rotate, this in turn rotates the main driving shaft. This is connected to the Constant Speed Drive (CSD), indicated by arrow number 8. This CSD is basically just a gearbox, but just calling it a plain old gearbox does not give the best impression. The blades can rotate at varying speed, and what this CSD does is to step up the rotational speed at the opposite side to that of the propeller. As the speed of the rotating blades varies, this CSD keeps the speed constant at the one driving speed, no matter if the blades slow or speed up. This in turn drives the generator at arrow number 12 at a constant speed, basically just a rotating magnetic field as in all other generators in conventional power plants, and again, that also describes it in the most simple of terms. It is more intricate than larger generators, mainly because it has to be, because weight here is the main limiting factor, and it has to be light enough to be able to be supported by the tower itself, and in turn, balancing out with the weight of the blades on the opposite side of the Tower to that nacelle. Even though the blades may seem to be rotating at such a relatively slow speed, that CSD steps up the speed and keeps it constant throughout the speed range of that propeller. So, for all the times that those blades are rotating, the generator is producing its maximum amount of power.

The whole nacelle can rotate around the tower so that it can face in the direction of the prevailing breeze, something it does automatically, again adding further intricacy to the internals of that nacelle. The blades can also be feathered, and what I mean by that is that each blade can individually be rotated in the hub, so that the wind does not force them to keep rotating. This is most important because if the wind becomes too strong, then the blades have to be stopped so that they do not speed up beyond a certain speed, one of the limitations of this type of design. As you might guess the tip speed is very high, and if it becomes too high, the blade can be destroyed.

These wind plant towers are specifically designed to operate in a fairly narrow wind speed band. At speeds below around 8MPH it is not feasible to keep the plant in operation, and at speeds above around 16MPH, the blades are feathered, and then locked, so the plant again stops moving.

So even before considering construction of a plant of this nature, which is a series of these towers, then the specific area to place them must be very carefully researched for the best wind conditions over the full course of the year, and even longer term than that over the projected life time of the towers.

This again further limits the placement of these huge towers. You just cannot just construct them in any old place.

The power produced is also quite variable. This is referenced as the Capacity Factor (CF) of the plant.

Theoretically, the best case scenario for actual power delivery (that CF) from a wind tower is 38%

Best claims from some Wind Farms may have this total power delivered sometimes as high as 30%, but plants that have been in operation in Europe for some years now are delivering only around 20% efficiency. Again, this can be better explained as the plant delivering its power for only 30% of the time, or, on average, seven and half hours out of every 24.

Put aside the limitations and let’s go ahead and construct one of these plants, and again, for the sake of comparison, I will use the Diablo Canyon Nuclear power plant facility. As I explained yesterday, the cost of that plant when viewed in isolation seemed quite high at $5.6 Billion.

So, let’s take that same amount of cash, and construct environmentally friendly, renewable Wind plants. For comparison, we have an example being mooted for construction, that of the Cape Wind Facility in Nantucket Sound in Massachusetts. This plant has 140 of these huge towers, each higher than the Statue Of Liberty. Each is topped with a 3MW nacelle producing 420MW of power when all towers are producing at their maximum. This plant costs $1.1 Billion, so for the same money as it costs for Diablo Canyon, we can get five of these wind plants. That’s 700 of these huge towers. We cannot put them side by side in general proximity, so we’re looking at covering an area of perhaps hundreds of square miles.

Forget the huge construction of factories to construct the nacelles, the internals, the towers, the propellers. Forget even the huge workforce that will need to be trained to do that. Forget the vast infrastructure required to construct all this together at the huge site where the towers will stand. Forget the transport required to move it to that area, Forget even the time this will take to actually construct, and the workforce required for that, keeping in mind this is 700 of those towers. Forget the huge infrastructure to connect all these towers together. Forget the infrastructure to get that power to the grid. Forget even the complex negotiations for all that land.

Forget all those things.

Let’s just look at the actual power produced.

700 towers each producing 3MW. We now have 2100MW of Nameplate Capacity, and by surprise more than anything this is around the same Nameplate Capacity as for Diablo Canyon.

However, in this case, there is one huge difference. Diablo Canyon can deliver its power 24/7/365. Let’s believe the hype of a promise that these wind towers can actually keep to that 30% efficiency. The amount of power that these towers will actually deliver to the consumer is still only just under one third of the power that Daiblo Canyon can deliver, and this huge wind plant can only deliver it for around seven and  half hours of each day.

Let’s then construct enough towers to actually deliver that same amount of power that Diablo Canyon can deliver. Now we are looking at constructing 2,200 of these Wind towers. The cost for all that. $18 Billion, and you still only have power being delivered for seven and a half hours out of every 24.

The average life span of a wind powered nacelle is also around 20 to 25 years, so realistically, long after these wind towers have stopped turning, Diablo Canyon will still be humming along, in fact almost three times longer, if the life of the plant is extended, as most of them are.

Now, go back to the top of the page and look at that huge tower and three bladed propeller. Just the one of them there in that image.

Imagine 2,200 of them. Imagine even the 700 of them for the same money as the large nuclear plant.

I’ve barely even considered a fraction of the things that cause wind power to be so limited, but just wonder at this as another. If plants like this cost so much, how much will the cost of that produced electrical power be for you, the consumer, and will you be willing to go without electrical power for 17 hours out of every day.

Tomorrow, I will canvass the limitations of Concentrating Solar Power, also called Solar Thermal power.