Wind Turbine Hydraulic to Electrical Conversion

[deckart]

Well I wish it was right! :)

I was thinking it would then only take a 1/2 gallon system to store 12 hours of average household kW-hr. Someone on a TOD metering system could buy/store the cheap power all night, and then use it all day, and pay only the cheap rate (but only with net metering, as an inexpensive system could not handle peak loads, so it would need to run the meter backwards when the load was less than average, to make up for peak draw). That might work with a 1/2 gallon system, not so much with a 500 gallon system. I'm curious what a ~ 1 KW continuous duty motor/generator/pump would cost? Hmmm, I guess the pump/motor/generator is a fixed cost, only the tank size would change. Space constraints aside, I suppose a 500 G high pressure tank is far more expensive than a 1/2 gallon tank - that's lots of added area and force.

A hydraulic accumulator used for energy storage is simply a hydraulic cylinder without a cylinder rod. A floating piston separates the gas and hydraulic fluid. Here is a chart showing standard sizes from a supplier that I use, https://www.accumulators.com:

As far as cost for a small unit, you have the accumulator, a standard generator of an appropriate size from Lowe's or Home Depot, and an appropriate hydraulic motor and valve circuit. Remove the gas motor and install the hydraulics. I'll put together a BOM and get quotes together. Originally I was looking something small scale like this but then I saw how well this can scale up and spent my time on that. I believe it gets more cost effective at larger scales.

 

[cjl]

I think a typical Wind Turbine is rated ~ 1 MW (close enough?), the capacity factor might be ~ 30% (close enough?), so on average it is providing 300 KW. If we also round the US home average down to 1 KW average (that would be 24 k-Wh per day), that means a single turbine is supplying energy for ~ 300 homes. So a single turbine would require 300 of those big units the man is standing next to in order to power the homes it supplies for ~ 1 day.

What does one of those accumulator arrays cost?

It's even a bit worse than that - most newer utility-scale wind turbines are more like 2.5MW, and have capacity factors of 35% or so, so you're looking at the better part of a megawatt of continuous power. Really large offshore machines are fast approaching 10MW, and capacity factors of 50% are not unheard of, if you want to look at the extreme upper end of the market.

 

[cjl]

Actually, there are significant differences. One of which is that I'm not adjusting flow (or in your words, "rate"). This is a fixed displacement device. Flow is dependant on RPM, RPM is the variable. The problem this solves is that it captures energy efficiently regardless of how low the RPM is. Electro-mechanical systems do not do this efficiently as you claim. The systems that are used employ a lot of expensive techniques to address this. Some of them are very ingenious from what a colleague has described to me, though I can barely follow the theory behind it.This is a very ambiguous paragraph. There are always losses when you transmit energy, whether electrical, mechanical, or hydraulic. Let’s say there is a total of 10% line and valve loss. Which is high, imo, because the only valves I’m using for the work lines are check valves. Consequently, I may be capturing 30% more energy. I really don’t know yet but I have a system that could be used to find out.

There's nothing particularly hard to understand about modern wind turbine electronics. They fall into a couple of major categories, but they're fairly straightforward. Most wind turbines you see are geared, although a few are direct drive. The geared machines use a ~120:1 ratio gearbox to increase the shaft speed to somewhere in the neighborhood of 1800 RPM at full power. This is the speed at which the generator operates. Direct drive machines obviously have no gearbox, so they have very large diameter, multipole permanent magnet generators designed to operate at ~10-12 RPM at full power. In either case, full power efficiency is usually in the neighborhood of 90%. At low wind speeds, the rotors spin as low as ~4RPM, so the generators need to cover a speed range of about a factor of 3. This is well within the capability of modern generator design, and over the majority of the speed range, the generator efficiency is pretty similar to full power, around 90% (it's usually a bit more efficient at less than full power, but the details aren't important here).

There are also some additional losses associated with converting the electricity to the proper voltage and frequency for the grid, but that's also a very efficient process, whether you use full conversion (all the power goes from AC to DC and back to AC again), or whether you use a DFIG (https://en.wikipedia.org/wiki/Doubly-fed_electric_machine#Double_fed_induction_generator).

Now, it's true that below 10% of rated power or so, the efficiency drops, but the amount of additional energy you could gain is really just not worth it. Say you change the turbine from being 66% efficient at 5% of rated power to 90%. You've only increased power output by about 2% of the overall turbine rating, which will have a fairly small impact on annual energy production. I also suspect you would have a hard time with the hydraulic drivetrain concept in getting the full power efficiency up to 90%, so you'll probably lose more energy during high power production than you'll ever gain back in low wind, especially given that you would never want to put a wind turbine on a site where it spends most of its time at <10% power anyways.

 

[deckart]

There's nothing particularly hard to understand about modern wind turbine electronics. They fall into a couple of major categories, but they're fairly straightforward. Most wind turbines you see are geared, although a few are direct drive. The geared machines use a ~120:1 ratio gearbox to increase the shaft speed to somewhere in the neighborhood of 1800 RPM at full power. This is the speed at which the generator operates. Direct drive machines obviously have no gearbox, so they have very large diameter, multipole permanent magnet generators designed to operate at ~10-12 RPM at full power. In either case, full power efficiency is usually in the neighborhood of 90%. At low wind speeds, the rotors spin as low as ~4RPM, so the generators need to cover a speed range of about a factor of 3. This is well within the capability of modern generator design, and over the majority of the speed range, the generator efficiency is pretty similar to full power, around 90% (it's usually a bit more efficient at less than full power, but the details aren't important here).

There are also some additional losses associated with converting the electricity to the proper voltage and frequency for the grid, but that's also a very efficient process, whether you use full conversion (all the power goes from AC to DC and back to AC again), or whether you use a DFIG (https://en.wikipedia.org/wiki/Doubly-fed_electric_machine#Double_fed_induction_generator).

Now, it's true that below 10% of rated power or so, the efficiency drops, but the amount of additional energy you could gain is really just not worth it. Say you change the turbine from being 66% efficient at 5% of rated power to 90%. You've only increased power output by about 2% of the overall turbine rating, which will have a fairly small impact on annual energy production. I also suspect you would have a hard time with the hydraulic drivetrain concept in getting the full power efficiency up to 90%, so you'll probably lose more energy during high power production than you'll ever gain back in low wind, especially given that you would never want to put a wind turbine on a site where it spends most of its time at <10% power anyways.

Good stuff, thank you. Looking at the graphic below for a 5MW wind-turbine, there is a great deal of cost getting from the turbine to the generator. And, according to this article, the transformer should not be something off-the-shelf but constructed to deal with the variable velocities of the generator.

I propose that using hydraulics as the interim transmission and driving a standard generator at a constant RPM would have similar, if not better efficiency, and reduce much of the upfront cost. 120:1 gearbox alone is between 80-90% efficient.

18-20% of a 5-6 million dollar system is a lot of room to work with.

And there are other things that can be done easily such as regenerative dynamic braking to limit high speeds rather than simply stopping the whole system. That alone could increase overall output in areas that have a lot of extreme wind conditions that require the turbine to be shut down.

It is definitely worth exploring.

 

[Baluncore]

Originally I was looking something small scale like this but then I saw how well this can scale up and spent my time on that. I believe it gets more cost effective at larger scales.

There is not much energy in those typical hydraulic accumulators. Given that 1 unit = 1 kW∙hr = 3.6 MJ
5 gal(us) = 0.018927 m³, 20,000 psi = 137.895 MPa → 2.61 MJ = 0.725 kW∙hour
20 gal(us) = 0.075708 m³, 10,000 psi = 68.947 MPa → 5.22 MJ = 1.450 kW∙hour
50 gal(us) = 0.18927 m³, 3,000 psi = 20.684 MPa → 3.915 MJ = 1.087 kW∙hour

And there are other things that can be done easily such as regenerative dynamic braking to limit high speeds rather than simply stopping the whole system. That alone could increase overall output in areas that have a lot of extreme wind conditions that require the turbine to be shut down.

The hardware will need to be designed to withstand an operational envelope. The maximum energy flow that any part can handle is specified as a power rating in say kilo or megawatts. That specification must include any intended dynamic braking.

During a wind-storm the blades must be feathered, and/or the head rotated side-on, to minimise the rotational speed and total dynamic wind pressure applied to the blades and tower structure. Those peak pressures and speeds will be increased significantly if any attempt is made to oppose them using braking.

A wind-storm may only last for an hour or two but the power rating needed to fight it can be 10 or more times the designed operating envelope. It is better to build 10 units than can duck for cover, than one that will survive a head to head fight with a wind-storm.

 

[deckart]

There is not much energy in those typical hydraulic accumulators. Given that 1 unit = 1 kW∙hr = 3.6 MJ
5 gal(us) = 0.018927 m³, 20,000 psi = 137.895 MPa → 2.61 MJ = 0.725 kW∙hour
20 gal(us) = 0.075708 m³, 10,000 psi = 68.947 MPa → 5.22 MJ = 1.450 kW∙hour
50 gal(us) = 0.18927 m³, 3,000 psi = 20.684 MPa → 3.915 MJ = 1.087 kW∙hourThe hardware will need to be designed to withstand an operational envelope. The maximum energy flow that any part can handle is specified as a power rating in say kilo or megawatts. That specification must include any intended dynamic braking.

During a wind-storm the blades must be feathered, and/or the head rotated side-on, to minimise the rotational speed and total dynamic wind pressure applied to the blades and tower structure. Those peak pressures and speeds will be increased significantly if any attempt is made to oppose them using braking.

A wind-storm may only last for an hour or two but the power rating needed to fight it can be 10 or more times the designed operating envelope. It is better to build 10 units than can duck for cover, than one that will survive a head to head fight with a wind-storm.

Accumulators are really just capacitors. As shown with that calc sheet, they aren't suitable for that scale of storage. Their strength is in that they can absorb and release energy quickly and make hydraulic power available for auxiliary functions.

I think you are right, being that there is a structural capacity of the whole tower assembly and the opposing torque has to be kept safely below that value.

Up to that point, however, turbine speed can be kept at lower speeds than is typical by using hydraulic regenerative braking without any loss of power. A combination of blade pitch and pump control, by bringing individual cylinders on and offline, can keep blade rotation at a low speed throughout much of the higher wind-speed range. Increasing the service life of the main turbine bearing. It may make them quieter too. Not to mention, be safer for wildlife navigating through the blades.

 

[Baluncore]

Up to that point, however, turbine speed can be kept at lower speeds than is typical by using hydraulic regenerative braking without any loss of power. A combination of blade pitch and pump control, by bringing individual cylinders on and offline, can keep blade rotation at a low speed throughout much of the higher wind-speed range. Increasing the service life of the main turbine bearing.

I think you are kidding yourself. The wind energy would need to move along the blades, pass through the pump, twist and lean on the tower, before being removed in the hydraulic oil. The oil would then need to be cooled in a massive radiator. If that was not the case, then it would be operating within the design envelope and generating useful power.

If you are relaxed when you fall, your injuries will be reduced. Likewise, a free-wheeling wind turbine will suffer less damage during a wind-storm than one under any unnecessary load.

It leaves me to question what exactly might you mean by “hydraulic regenerative braking without any loss of power”.

 

[deckart]

I think you are kidding yourself. The wind energy would need to move along the blades, pass through the pump, twist and lean on the tower, before being removed in the hydraulic oil. The oil would then need to be cooled in a massive radiator. If that was not the case, then it would be operating within the design envelope and generating useful power.

If you are relaxed when you fall, your injuries will be reduced. Likewise, a free-wheeling wind turbine will suffer less damage during a wind-storm than one under any unnecessary load.

It leaves me to question what exactly might you mean by “hydraulic regenerative braking without any loss of power”.

Regenerative meaning that the energy is not wasted and expressed as heat. It is put back into the system by increasing the torque of the pumping mechanism, as I have described, to reduce velocity. There are additional methods that can be utilized but that is regenerative braking without any loss of power.

No need to get excited, no one is going to fall, if you don't understand what I'm describing just ask.

 

[cjl]

Good stuff, thank you. Looking at the graphic below for a 5MW wind-turbine, there is a great deal of cost getting from the turbine to the generator. And, according to this article, the transformer should not be something off-the-shelf but constructed to deal with the variable velocities of the generator.

True, the converter is a large chunk of the cost, which is part of the reason for the use (in some cases) of the doubly-fed induction generator I mentioned above. They reduce the cost of the converter considerably, at the expense of a small amount of efficiency and a reduction in flexibility when it comes to reactive power and grid support.

I propose that using hydraulics as the interim transmission and driving a standard generator at a constant RPM would have similar, if not better efficiency, and reduce much of the upfront cost. 120:1 gearbox alone is between 80-90% efficient.

120:1 gearbox alone is between 95 and 97% efficient. There are nowhere near the magnitude of losses you're supposing here. Overall system efficiency is between 85 and 90%, from input shaft power to electrical output.

18-20% of a 5-6 million dollar system is a lot of room to work with.

You're high by about a factor 2 on the cost of a modern 2-3 megawatt machine.

And there are other things that can be done easily such as regenerative dynamic braking to limit high speeds rather than simply stopping the whole system. That alone could increase overall output in areas that have a lot of extreme wind conditions that require the turbine to be shut down.

It is definitely worth exploring.
View attachment 224574

Increasing high wind operational capabilit is worth a negligible quantity of annual energy production, and is severely detrimental to turbine loads. Modern wind turbines already operate out to about 22-27 meters per second wind speed (sustained, not gust), and the time spent above this at most wind generation sites is small enough that it just makes more sense to shut down rather than try to harvest the very small amount of energy at these high speeds. In addition, there's no generator limitation that prevents operation at these high wind speeds - it's purely a loads concern. Speed is already regulated through blade pitch, so excess torque isn't a problem, and most turbines run at a fixed RPM from about 10m/s up to high wind cutout using this blade pitch control method.

 

[cjl]

If you are relaxed when you fall, your injuries will be reduced. Likewise, a free-wheeling wind turbine will suffer less damage during a wind-storm than one under any unnecessary load.

A free wheeling wind turbine will fail in high wind, due to a number of reasons. Higher than nominal blade speed is an emergency, and exceedences of only 30% or so can cause permanent damage. However, modern turbines simply pitch the blades to full feather during high wind, so their rotational speed is very small and there is near zero torque when wind is above about 25m/s or so.

 

[deckart]

All I have to go on is what I can find through searches.

This is where I found the original .8-.9 gearbox efficiency: https://wind.globecore.com/wind-turbine-gearbox-efficiency.html

This article puts them in the range of 90-95%: http://people.bu.edu/dew11/turbineperformance.html.

cjl, you say that overall efficiency is 85-90%. I cannot find any good numbers on that. Can you cite something for me?

I see that you are a Wind Turbine Engineer! I'm going to be picking your brain if you don't mind. Is that graphic of the turbine breakdown scalable to the 2-3 MW turbines?

 

[deckart]

I found an abstract of a thesis written about replacing wind turbine gearboxes with hydraulic transmissions here: https://scholarworks.iupui.edu/handle/1805/3800

I like where she is going but again, it looks like she is considering the use of conventional pumps as opposed to a more efficient radial cylinder design. I'm going to see if I can look her up.

 

[cjl]

All I have to go on is what I can find through searches.

This is where I found the original .8-.9 gearbox efficiency: https://wind.globecore.com/wind-turbine-gearbox-efficiency.html

To me, that reads as if it's quoting an overall system efficiency of 0.8 to 0.9, which is in line with what I'd expect.

This article puts them in the range of 90-95%: http://people.bu.edu/dew11/turbineperformance.html.

As I said, I'd put them closer to 95%+, but that's really just quibbling over a few percent, so this number is pretty close. The 80% they quote for a modern generator is certainly on the low end though - I'd expect those to be more in the 90% range for the most part.

cjl, you say that overall efficiency is 85-90%. I cannot find any good numbers on that. Can you cite something for me?

Your first source seems to indicate overall of 80-90% to me, based on how I read it, so there's one source. I can't find anything with some quick googling other than that, but I don't have a lot of time right now.

I see that you are a Wind Turbine Engineer! I'm going to be picking your brain if you don't mind. Is that graphic of the turbine breakdown scalable to the 2-3 MW turbines?

I don't know about the cost breakdown. I know that current US market turbines in the 2-3MW range are running a bit under $2MM each right now (so I was actually even still estimating high earlier), but I couldn't tell you where in the turbine that cost is going.