Small Wind turbines

[Adriaan Kragten]

Recently I watched the video: "Small Wind Turbines: Success or Failure? Do not make those Mistakes".
link: www.youtube.com/watch?v=Gs3y-6CKT0E This video is very useful for someone who wants to buy a small wind turbine.

[kitestrings]

Thanks for sharing this Adriaan.  It's quite good IMO.

There are a few things I wish he would have touched on, many of them listed in the comments (from the few I glanced at).  I always try to caution people, especially those new, & at times enthusiastic, to wind.  I think maintenance and first-costs are very important to consider.  Noise and local ordinance is another consideration.

With few exceptions, I'm not sure as I would recommend wind to someone who already has utility power.  There's probably a case for it - a great rural site, a capable repair person, exorbitantly high utility rates, restrictions on other renewable's (PV not being a suitable option at the site) - but otherwise, I'd probable advise choices with a shorter payback and/or less complications.

For off-grid, and remote communications sites, I think small wind is a perfect supplement to PV.  And, the value of power generated when there my be long, sunless periods is hugely important.  The video spoke to this.  It becomes more of a question of viability in some case, versus purely monetary considerations.  I recall a story I saw years ago filmed in a remote indigenous village, where the 'visitors' were exploring the concept of money.  They used the purchase of a hammock as an example, "I would pay you money, in exchange for you hammock...", to which the local asked, "but where would I sleep."

[Adriaan Kragten]

Finally the only thing which counts is the price per generated kWh and the wanted kWh's at a certain time of the year. The main disadvantage of small wind turbines is the relatively low tower resulting in a low wind speed at the heart of the rotor and a lot of turbulence. The tower must be that high that the lowest part of the rotor is at least 2 m higher than surrounding trees or neighbouring buildings. Even a 12 m high tower is often too low. A higher tower will be too expensive for a rotor diameter of about 3 m. The most energy needed is for heating. The most heat is needed in the winter and in The Netherlands, wind turbines generate maximum power in the winter and therefore match much better with the demand than solar panels.

But if you want to heat a well isolated house with a heat pump, you need a grid connected wind turbine with a rotor diameter of at least 5 m and a tower height of about 18 m. It is very difficult to compete with the grid for such a wind turbine if you want to buy one of good quality. But I think that there is a market for medium size wind turbines with a rotor diameter in between 10 m and 30 m and a tower height of in between 18 m and 36 m. So one wind turbine is used to heat several houses. If the house has a large heat capacity, all generated energy of the wind turbine can be used directly in the winter. However, the wind turbine must be designed such that it is safe, that it has a very low noise level and that flickering by reflection or shadow is limited, otherwise people won't accept it. In my reports KD 727, KD 732 and KD 761, I give examples of such a wind turbine.

[kitestrings]

I generally think of 10m to 30m turbines as being quite large.  Probably one of the most popular US made turbines is the Bergey Excel.  They have a 10 kW, 7m unit that has been around for years, and more recently they have a 15 kW, 9.6m unit.  The larger is only available in grid-tie, the 10 kW can be had for off-grid (48V).  Last I knew these units were selling at $39k and $49 respectively, without towers or install.

Your points on tower height is spot relevant.  Far too often turbines are poorly sited and on a tower that should be taller.

I read with interest your report considering a turbine for heating.  With a grid-tied system - here in VT one can store credits for up to 12-mos. - I think heating, using a heat pump, makes sense.  The overall COP runs about 2.5 (much better than resistance, but not as high as the 4 that you describe for your climate) here in our local.  I have an area friend who has the 10 kW unit and he pretty much runs his house, and maple sugaring operation with the stored credits (via net metering).

For off-grid, generally I don't think heating using a heat pump works too well.  There are some 48V "mini split" systems, but too often the load and available resource just doesn't match well.  Instead, I think pulsing (PWM) available energy, or "opportunity load" to a resistance heat device makes more sense.  This works well with hydronic heating as you can thermally store this energy.

I was recently involved with an electric boiler project at a local ski area.  It is a multi-staged resistance boiler; pretty simple really.  It is supplemental to the heating system, but it operates with a sophisticated software system that manages it around real-time marginal costs and avoiding utility peaks, thus keeping the costs competitive relative to the LPG alternative.  It is a 3 MW system, so fairly significant.

With our modest wind/PV system, we do a similar thing though, which is to preheat dhw whenever there is surplus PV or wind.  Our charge controllers allow for settings relative to the 3-stage charging parameters.  The elements may be lightly tickled on and off, or connected for several hours as conditions vary.  LPG supplements as needed.  This has worked really well for us.

Sorry to be so long-winded, ~ks

[Adriaan Kragten]

In The Netherlands, almost all houses are connected to the natural gass grid but because of earth shocks, our biggest natural gass field is closed. So almost all natural gass has to be imported which is very expensive. New houses therefore get no longer a connection to the gass grid. All new houses have a heat pump and solar panels on the roof. But the generation of energy of solar panels in the winter is much too low and therefore in winter, one has to buy electrical energy from the grid. Up to now we have the so called "salderingsregeling" which means that you get the same amount of money for energy supplied in the summer as bought in the winter. However, this methode will be stopped soon as it is payed by those who don't have solar panels. This has given big problems for the solar industry and several companies have already gone bankrupt.

I would have expected that this would stimulate the wind industry but nobody in my country is developing medium size wind turbines suited to use close to houses. Most people don't realize that the energy they use must have been generated somewhere and that too strong resistence against wind energy will finally result in energy cut down for some hours during peak demands. There are countries like India and some countries in Africa where this is already a very standard procedure. I expect that within some years, every house will get energy only up to a certain limit. Using less energy is a sake of attention and preventing the use of equipment with high power during long periodes.

The resistance against megawatt turbines on land is very high. Projects with three groups of three 3 mW turbines in my city are cancelled because they would give too strong influence on the radar of the military airfield of Volkel. But almost all political parties in the city are against very big wind turbines. So the radar problem is used to cancell the projects. Medium size wind turbines have no radar problems because the highest point of the rotor stays below the critical radar area but these turbines are not taken into consideration because of the strong resistance of most people against wind turbines in their backyard. I have written a Dutch note about a 4-bladed wind turbine with a rotor diameter of 30 m and a design tip speed ratio of 5 which can supply the needed energy for the heat pumps of 60 well isolated houses in December. Almost all realistic points of resistance against wind turbines are cancelled for this turbine but nobody takes this option as a serious alternative for megawatt turbines. The note is given at my website at the bottom of the list with KD-reports. 

[mbouwer]

Let's accept that there are no easy answers right away and continue developing a small DIY mill in our garden.

[MattM]

New technology is always chased when older solutions can work affordably.

Steam distribution is probably feasible for areas of the netherlands.  And wind can generate heat well, the precursor to steam, better than just harvesting electricity alone.

It is older technology that people forget was important in cities around the early 1900s.

[kitestrings]

We live in a State that could be described as being pretty "green".  The goals the current energy plan call for 100% carbon free resources by 2032 with at least 75% from renewables.  There are similar goals in the thermal and transportation sectors.  While wind is part of the mix - there is some ~150 MW now IIRC - it will be difficult for any new developments based on public sentiment.  Several even 1, 2, 3 turbine sites have already baled on proposed projects.  There is little or no appetite for large-scale wind I'd say -

[Adriaan Kragten]

The biggest problem with renewables like solar and wind is the fluctuation of the output. Solar gives the largest output at summer at the middle of the day when the demand is low. Wind gives the largest output at winter when the demand is high. It can also have output at night. Only water power has a steady output if the reservoir is large enough and if there are no large periods with drought. So very large ways of storage are needed to balance the fluctuations. At this moment alsmost all fluctuations are balanced with fossil fuels.

The biggest problem with wind is that the wind speed depends very much on the height and that the power is proportional to the cube of the wind speed. So big power is only available at large heights and at large rotor diameters. This is the main reason why megawatt turbines with very high towers have won the game and why small wind turbines are only successfull at places where no grid is available. However, the increase of the wind speed by height goes logaritmic. The wind speed is normally measured at a height of 10 m at open terrain. The real wind speed at a height of 10 m depends very much on the terrain roughness. The increase of the wind speed at height also depends very much on the terrain roughness. This effect is given in figure 2.5 at page 22 of the report "Introduction to Wind Energy" of Erik Lysen. I have scanned this page and added it as an attachment. This means that for open terrain, the wind speed increases strongly if you go from 10 m up to 20 m but that the increase is much less if you go from 20 m up to 30 m and even more less if you go from 30 m up to 40 m. So once you have reached a tower height of about 35 m, the increase of the wind speed by a higher tower is only little but the costs of a higher tower are enormous.

This is one reason why I think that a medium size wind turbine with a rotor diameter of 30 m and a tower height of 35 m is a realistic alternative for megawatt turbines. The other reason is that the generated energy is used directly for mainly heating by the group of people which own the turbine and therefore the generated energy has a much higher economic value than the energy which is feed into the high voltage grid by megawatt turbines.



For better understanding of figure 2.3, I have also added page 21.

[Adriaan Kragten]

I will now give two examples how to calculate the wind speed at a certain height if the windspeed at a height of 10 m is given. Assume that the wind speed V at a height z = 10 m is 6 m/s for both examples. On the y-axis of figure 2.3 we see the height z. On the x-axis we see the ratio with which the wind speed is increased.

1)  Assume we have open terrain with high grass. This gives z0 = 0.10. Assume we want to know the wind speed at z = 20 m. In figure 2.3 we can read that V is increased with about a ratio 1.145 and it becomes 1.145 * 6 = 6.87 m/s. The power increases with V^3 and so the power at z = 20 m is a factor 1.145^3 = 1.5 higher than at z = 10 m.

2)  Assume we have closed terrain with villages. This gives z0 = 1.0. Assume we want to know the wind speed at z = 20 m. In figure 2.3 we can read that V is increased with about a ratio 1.29 and it becomes 1.29 * 6 = 7.74 m/s. The power increases with V^3 and so the power at z = 20 m is a factor 1.29^3 = 2.15 higher than at z = 10 m. 

[MattM]

The reason I brought up steam is because sand batteries are so cheap to both make and to power from renewables.  You can get by with a wide ranges of voltages.  Municipalities found steam easy enough to distribute on a large scale through buried pipes.  The earth insulated the piping.  And steam generators were fairly easy to maintain.  While it can be explosive, the science was well understood over 100 years ago.  Portability was its downfall.  But for homesteading that should be less of a concern.  Its not as sexy as the other renewables so its unpopular to explore.

[Adriaan Kragten]

The reason I brought up steam is because sand batteries are so cheap to both make and to power from renewables.  You can get by with a wide ranges of voltages.  Municipalities found steam easy enough to distribute on a large scale through buried pipes.  The earth insulated the piping.  And steam generators were fairly easy to maintain.  While it can be explosive, the science was well understood over 100 years ago.  Portability was its downfall.  But for homesteading that should be less of a concern.  Its not as sexy as the other renewables so its unpopular to explore.

I don't think that using steam and storage of energy in sand is a good solution. You should make the proper calculations to prove on paper that it works. Steam gives a high pressure which is dangerous and it will all depend on how well the sand is isolated if the energy can be stored long enough. You can't expect that the earth isolates the pipes well enough. You will need a very thick layer of isolation material around the pipes from the energy source to the storage drum and from the storage drum to the house.

I have done some research for storage of energy in hot water. This is described in my Dutch public report KD 713. Even for a rather big reservoir and very good isolation, the heat losses are too large to store energy generated by solar panels in the summer to use for heating in the winter. It might be an option for storage of energy generated by a wind turbine in the winter for use in the winter because then the required storage time is much shorter. Generation of heat by a wind turbine is described in my Dutch report KD 709. These reports are written in Dutch because I have used them in the discussion with my municipality about the energy transition.

[electrondady1]

Once Canada replaces our present prime minister liquid natural gas faiciities will come into existance . we will ship as much liquid natural gas as our friends in Holland need. Natural gas, Methane , Ch4. these are all the same thing . facilites can be built that can convert every source of waste (including human waste) into gas. the beautiful farms in you native land can produce a feed stock for gas production . Almost every form of plastic waste can be heated and reduced to gas. Of course every wind mill  can create hydrogen and that can be fed into a natural gas grid. the future is going to be great with energy abundance

[esposcar]

A small portable micro or small wind turbine, one of the uses can be when you are doing outdoors activities or you stay in a camping or in a small ship, but of course with lot of wind, miracles in physica does not exist!!!

[topspeed]

What you think of this ?


https://www.youtube.com/watch?v=83smlFegbCM

[topspeed]

What you think of this ?


https://www.youtube.com/watch?v=83smlFegbCM

[Adriaan Kragten]

What you think of this ?


https://www.youtube.com/watch?v=83smlFegbCM

This is a Savonious rotor with twisted blades. A normal Savonious rotor has a very fluctuating starting torque. So it can give starting problems if the generator has a high peak on the cogging torque. To flatten the starting torque, one can put two Savonious rotors on top of each other for which the blades are rotated 90° with respect to each other. This is called a 2-phase Savonious rotor. The other option is to twist the blades over 90°. Twisting of the blades is done on a special press and so it is not something you can do yourself. The maximum Cp of a well designed Savonious rotor is about 0.22 (see my public report KD 599) so much lower than for a well designed HAWT. However, for this Cp, the rotor must have end plates at the top and the bottom of the rotor to prevent tip losses. The rotor shown in the photo has no end plates and so the maximum Cp will be lower than 0.22.

A Savonious rotor works partly on lift and partly on drag. The maximum Cp is therefore much higher than for a pure drag machine which has a maximum Cp of only about 0.05 (see my public report KD 416). If you would put the material which is used for a Savonious rotor in a slow running HAWT with cambered sheet blades, you will get a swept area which is at least a factor four larger (see for instance the 3-bladed VIRYA-1.38 rotor rotor which is described in chapter 6 of my public report KD 745). As the maximum Cp is also about a factor two larger, you get about a factor eight more energy for the same amount of material. So to generate a certain amount of energy with a Savonious rotor, you will have much higher investment costs than for a well designed HAWT. That is why any commercial wind farm uses HAWT's.

[Adriaan Kragten]

To prove that the required amount of material of a Savonious rotor is so much higher than for a HAWT with a low design tip speed ratio, I have used the VIRYA-1.38 for comparisson. A drawing of this rotor is given in figure 10 of KD 745. The 3-bladed rotor has a rather low design tip speed ratio of 2.5 and the starting torque coefficient is therefore 0.109 which is very high for a HAWT but this was needed because the used Sparta Ion wheel generator has a very high peak on the cogging torque.

A blade is made out of a 1 mm thick stainless steel sheet with a lenght of 0.5 m and a width of 0.3333 m. The mass is 1.3 kg. A blade is connected to the generator by a spoke which is made out of 2 mm thick stainless sheet with a lenght of 200 mm and a width of 200 mm. The mass is 0.62 kg. So the total mass of the rotor is 3 (1.3 + 0.62) = 5.76 kg. The swept area of the rotor is pi/4 * 1.38^2 = 1.5 m^2.

Now assume that we want to make a 2-phase Savonious rotor out of 1 mm stainless steel sheet. Assume that the diameter D is 0.5 m and that the height H = 1 m. So the swept area is 0.5 * 1 = 0.5 m^2. So the swept area is only 1/3 of the VIRYA-1.38 rotor. For a 2-phase Savonious rotor we need three end plates with a diameter of 0.5 m. This requires three square sheets size 0.5 * 0.5 m with a total mass of 5.85 kg. Assume that a bucket is made out of a sheet size 0.5 * 0.5 m. Four of those buckets are needed and the total mass is 7.8 kg. And so the total mass of the buckets and the end plates is 5.85 + 7.8 = 13.65 kg. But we also need a central shaft which connects the three end plates together and a bent angle iron to connect the buckets to the end plates. So the real mass will be at least 15 kg. This is a factor 15 / 5.76 = 2.6 larger than for the VIRYA-1.38 rotor. So for the same mass as the VIRYA-1.38 rotor, the diameter must be much smaller than 0.5 m and the height must be much smaller than 1 m. And then there is still the large difference in maximum Cp which is about 0.22 for a Savonious rotor and about 0.39 for the VIRYA-1.38 rotor. So this proves that my assumption that the amount of material needed for a Savonious rotor with the same power at the same wind speed is about a factor eight larger is very conservative; a factor 2.6 * 3 * 0.39 / 0.22 = 13.8 is closer to the reality.

Another very important difference is that the VIRYA-1.38 is provided with the hinged side vane safety system and so the rotational speed and the thrust are limited at high wind speeds. A Savonious rotor will turn faster as the wind speed is higher and so at very high wind speeds, the rotational speed will be very high resulting in strong centrifugal forces in the blades. It can be questioned if 1 mm stainless steel is strong enough. The thrust on the lower and the upper part of a 2-phase Savonious rotor will vary strongly during one revolution and this will result in strong vibrations at high rpm. The total thrust of a Savonious rotor with twisted blades will be about constant but the force moves upwards and downwards and this will also give vibrations. This makes a Savonious rotor dangerous at high wind speeds. I hope that this shows that building or buying a Savonious rotor isn't a good idea.