3 ms·

Wikipedia claims an average power consumption of ~12kW per person in the US (but it's marked as needing a citation). Assuming a wind power density rating of goo

by what 17y ago

Wikipedia claims an average power consumption of ~12kW per person in the US (but it's marked as needing a citation). Assuming a wind power density rating of good (500W/sq m), you would need to harness all of the energy in a 24sq m area per person. There are >300M people in the US. Plastering turbines over that 30% of the landmass would serve only half the population. (Note: I'm no expert and maybe there is some glaring oversight in my quick calculation.)

jacquesm 17y ago
12kW is not a power consumption but a momentary rate, power consumption per person is rated in KWh per unit of time, or in oil equivalents, about 8 tons of oil per US citizen per year. Also, wind power is not delivered in cross section on a surface parallel to the ground but on one at a right angle to the ground. There are other forms of renewables besides wind, such as hydroelectric dams, geothermal power and solar. A combination of those would be called for anyway. And since it is much cheaper to save a Watt-hour than it is to make one conservation is where it all begins, currently the US uses about double the amount of energy per person compared to the rest of the developed world. I'm fairly sure the US could increase its 7% in renewables to a much more significant fraction without any breakthroughs in technology, say up to 20% or so. That alone would be a major relief. Right now there are iirc about 35GW of installed power in wind turbines in the US, that's about 25,000 turbines. I've been out of the wind energy scene for 5 years now, I was mostly interested in the smaller scale anyway, but there is a surprisingly large amount of energy in wind power and we are using only a very small fraction of it. Keep in mind too that the total energy consumption of a country is used to power lots of different stuff, transportation is going to be the hardest to convert, but anything that runs off the grid is fair game for being powered by renewables, and in all those per-capita energy uses all those different categories are lumped together.
- what 17y ago
  Ok, you seem to be more knowledgeable on the subject than I am and I probably misinterpreted the chart as well. I was just trying to say turbines aren't the end all solution since most places don't seem to have enough wind. But there should definitely be push for using more renewable energy sources and improving the associated technologies. Also, maybe you can give me some numbers so I can plug them into this formula. What is the wind conversion efficiency for a turbine? What is the typical rotor diameter? And how many turbines per unit area on a typical wind farm? energy generated per turbine = efficiency * (power per unit area) * area * time total energy generated = (available area) * (turbines per unit area) Then I can redo my calculation using 29 PWh as the total energy consumption for the US (http://en.wikipedia.org/wiki/Energy_in_the_United_States http://en.wikipedia.org/wiki/Energy_in_the_United_States). Just curious now, don't want to start an argument or anything.
  - jacquesm 17y ago
    The biggest factor is 'Betz', the guy that figured out that a windmill has a theoretical upper limit of efficiency of ~59%, the easiest explanation for that is that in order for the windmill to work you have to slow down the moving air but the air should still be able to escape to allow the cycle to continue. The practical upshot of that is that you can't slow the air down to a standstill, and hence you can only extract so much power. The next step is the difference between the theoretical maximum and the actual rotor used to extract the energy, this will always be a compromise, but real world rotor efficiencies of about 50% can be reached. Then there's the generator/alternator (depending on the type of machine) losses and gearbox losses if a gearbox is used (there are very large direct drive models, specifically the Enercon series). After all is said and done figure about 35 to 48% of the wind 'input' energy can be extracted and applied to do something useful, assuming the wind is blowing at 'rated speed' (when the machine produces its maximum design power). If the wind goes over that speed you will lose power because the blades will be 'furled' to take them out of the wind to limit the rotor speed to something safe, and to avoid burning up the powertrain or wrecking the gearbox. Then there are still transmission losses but we can leave those out because they apply to any power source connected to the grid. Typical rotor diameters for 2MW machines are in the 80 to 90 meter range depending on how efficient the machine is. There is no 'typical wind farm', it all depends on the effects of the terrain and the windmills on each other and the amount of investment. A single bladed rotor is the most efficient design, and a single windmill has the best extraction efficiency, multiple blades (usually 3, but 2 is reasonably common in smaller machines (even though it suffers from 'tower thump'), and there are some experimental large machines using only 1 blade), terrain details, local variations in wind consistency, wind shear and a whole bunch of other factors including humidity and so on play havoc with any simple 'back of the envelope' calculations. A 7 meter per second wind in the desert versus a 7 meter per second wind at sea level can make a huge difference. If you really want to dig in to this I'd advise you to go and read the http://www.awea.org/ http://www.awea.org/ site as well as a community for homebrew wind enthousiasts called fieldlines (http://www.fieldlines.com/ http://www.fieldlines.com/), I used to be an active member of the second but since I've left Canada I have not worked on anything renewable energy related. I spent a good 2 years studying this stuff before being able to make a very small machine, the forces involved in even such a 'toy' (2.4 KW) machine are very impressive and the amount of knowledge you need to pull it off was far beyond what I ever had expected. Windmills for power generation are right at the crossroads between many different disciplines, including aerodynamics, magnetics, electrical theory, semiconductors and structural engineering, coupled with a very powerful and unpredictable adversary, the wind. If anything that journey taught me great respect for the people that design those large machines and one the whole get it right most of the time. When they mess up it looks like this: http://home.wxs.nl/~hzwarber/wind/plaatjes/crash_turm2.jpg http://home.wxs.nl/~hzwarber/wind/plaatjes/crash_turm2.jpg

by what 17y ago

jacquesm 17y ago

12kW is not a power consumption but a momentary rate, power consumption per person is rated in KWh per unit of time, or in oil equivalents, about 8 tons of oil per US citizen per year. Also, wind power is not delivered in cross section on a surface parallel to the ground but on one at a right angle to the ground. There are other forms of renewables besides wind, such as hydroelectric dams, geothermal power and solar. A combination of those would be called for anyway. And since it is much cheaper to save a Watt-hour than it is to make one conservation is where it all begins, currently the US uses about double the amount of energy per person compared to the rest of the developed world. I'm fairly sure the US could increase its 7% in renewables to a much more significant fraction without any breakthroughs in technology, say up to 20% or so. That alone would be a major relief. Right now there are iirc about 35GW of installed power in wind turbines in the US, that's about 25,000 turbines. I've been out of the wind energy scene for 5 years now, I was mostly interested in the smaller scale anyway, but there is a surprisingly large amount of energy in wind power and we are using only a very small fraction of it. Keep in mind too that the total energy consumption of a country is used to power lots of different stuff, transportation is going to be the hardest to convert, but anything that runs off the grid is fair game for being powered by renewables, and in all those per-capita energy uses all those different categories are lumped together.

what 17y ago
Ok, you seem to be more knowledgeable on the subject than I am and I probably misinterpreted the chart as well. I was just trying to say turbines aren't the end all solution since most places don't seem to have enough wind. But there should definitely be push for using more renewable energy sources and improving the associated technologies. Also, maybe you can give me some numbers so I can plug them into this formula. What is the wind conversion efficiency for a turbine? What is the typical rotor diameter? And how many turbines per unit area on a typical wind farm? energy generated per turbine = efficiency * (power per unit area) * area * time total energy generated = (available area) * (turbines per unit area) Then I can redo my calculation using 29 PWh as the total energy consumption for the US (http://en.wikipedia.org/wiki/Energy_in_the_United_States http://en.wikipedia.org/wiki/Energy_in_the_United_States). Just curious now, don't want to start an argument or anything.
- jacquesm 17y ago
  The biggest factor is 'Betz', the guy that figured out that a windmill has a theoretical upper limit of efficiency of ~59%, the easiest explanation for that is that in order for the windmill to work you have to slow down the moving air but the air should still be able to escape to allow the cycle to continue. The practical upshot of that is that you can't slow the air down to a standstill, and hence you can only extract so much power. The next step is the difference between the theoretical maximum and the actual rotor used to extract the energy, this will always be a compromise, but real world rotor efficiencies of about 50% can be reached. Then there's the generator/alternator (depending on the type of machine) losses and gearbox losses if a gearbox is used (there are very large direct drive models, specifically the Enercon series). After all is said and done figure about 35 to 48% of the wind 'input' energy can be extracted and applied to do something useful, assuming the wind is blowing at 'rated speed' (when the machine produces its maximum design power). If the wind goes over that speed you will lose power because the blades will be 'furled' to take them out of the wind to limit the rotor speed to something safe, and to avoid burning up the powertrain or wrecking the gearbox. Then there are still transmission losses but we can leave those out because they apply to any power source connected to the grid. Typical rotor diameters for 2MW machines are in the 80 to 90 meter range depending on how efficient the machine is. There is no 'typical wind farm', it all depends on the effects of the terrain and the windmills on each other and the amount of investment. A single bladed rotor is the most efficient design, and a single windmill has the best extraction efficiency, multiple blades (usually 3, but 2 is reasonably common in smaller machines (even though it suffers from 'tower thump'), and there are some experimental large machines using only 1 blade), terrain details, local variations in wind consistency, wind shear and a whole bunch of other factors including humidity and so on play havoc with any simple 'back of the envelope' calculations. A 7 meter per second wind in the desert versus a 7 meter per second wind at sea level can make a huge difference. If you really want to dig in to this I'd advise you to go and read the http://www.awea.org/ http://www.awea.org/ site as well as a community for homebrew wind enthousiasts called fieldlines (http://www.fieldlines.com/ http://www.fieldlines.com/), I used to be an active member of the second but since I've left Canada I have not worked on anything renewable energy related. I spent a good 2 years studying this stuff before being able to make a very small machine, the forces involved in even such a 'toy' (2.4 KW) machine are very impressive and the amount of knowledge you need to pull it off was far beyond what I ever had expected. Windmills for power generation are right at the crossroads between many different disciplines, including aerodynamics, magnetics, electrical theory, semiconductors and structural engineering, coupled with a very powerful and unpredictable adversary, the wind. If anything that journey taught me great respect for the people that design those large machines and one the whole get it right most of the time. When they mess up it looks like this: http://home.wxs.nl/~hzwarber/wind/plaatjes/crash_turm2.jpg http://home.wxs.nl/~hzwarber/wind/plaatjes/crash_turm2.jpg