Archive for wind

Wind Power: Cool Breeze or Hot Sirocco?

Jamie Bull brought to my attention a paper saying that land based wind turbines are likely to create a small degree of earth surface warming.

Jamie was concerned that there might not be a net surface cooling, even once the effects of reduced greenhouse gas emissions were taken into account. He did some calculations, and showed that the net effect of wind power on surface temperatures was still strongly negative.

He need not have worried. An understanding of the mechanism of this surface heating, and the conservation of energy make it clear that the surface heating effect of wind is smaller than the surface heating effect of thermal electricity generation such as coal, gas, nuclear, and Concentrating Solar Power (CSP) on a per kWh basis.

Conservation of energy tells us that all the energy in wind is eventually dissipated by friction, creating heat. The effect of wind turbines is to take some of this energy that might have become heat in the atmosphere, and create electricity, which will eventually become heat on the earth’s surface, and some will become heat in the turbine itself.

The net heat added to the Earth by wind turbines is zero: heat that would have been created in the atmosphere is now created on the surface instead, and the net effect is zero.

Now consider thermal electricity generation. When a fossil fuel is burned, or when a nuclear power plant or CSP plant makes steam, heat is either created from a fuel or captured from the sun, reducing the amount that would reflect back into space. All this heat is dissipated at some point near the earth’s surface, creating more surface warming than wind power, and also creating net warming where wind power creates none.

The direct surface heating effect from wind is likely to be only a fraction of the direct surface heating effect of thermal electricity, even before the effects of greenhouse gasses are accounted for.

Solar PV and Solar CSP capture heat from the sun that might otherwise reflected into the atmosphere, so more precise calculations are probably needed to determine if their net effect is negative before the effects of greenhouse gasses, but wind is clearly cool by any thermometer.

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Better Software Enables Better Wind Integration

A year ago, I wrote an article about the Dumb Grid, complaining that the reason that many utilities find wind power so hard to integrate is because they aren’t using any brains. I used the infamous Feb 2008 incident when wind power in Texas dropped right as demand picked up because of a cold front to make my case: Both the rise in demand and the drop in wind power were predictable consequences of the cold front, but the ERCOT controllers were not using that weather information in their dispatch planning. Hence, the problem was not wind power or even the cold front: it was failure to use the available information.

Fortunately, things are much better today. There’s an excellent article on Power-Gen Worldwide about the Texas electric grid’s control center two years later. Here’s an excerpt about how they deal with wind variability today:

    The wind resource is more manageable now that ERCOT has wind resource forecasting software at its disposal. [...]

    ERCOT has begun using forecasting tools from AWS Truewind to help it manage wind energy resources. In the coming days ERCOT will begin using a ramping tool, from the same vendor, to improve its forecasting of wind resource ramping events. Just a week before our visit, the AWS Truewind software–operating in a test mode–predicted a 2,000 MW drop in wind resource followed 15 minutes later by a 2,000MW recovery. The predicted ramp event matched the actual event almost perfectly.

    Joel Mickey told me that ERCOT is happy to dispatch as much wind energy as is available.

Thanks to Micheal Giberson over at Knowledge Problem for bringing this article to my attention.

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About 3x as Much Wind Power Available at 80m than at 50m hub heights

A new National Renewable Energy Laboratory (NREL) study, taller wind turbines can produce more power.

This is no surprise to anyone. Trees and other objects on the ground slow the wind, and as you get higher, you enter the region of smooth laminar flow where more energy is available. Laminar flow starts at about 50m.

A wind turbine with a hub height of 50m will have half its swept area above 50m. A wind turbine with 50m blades and a hub height of 80m. See my drawing:

What is interesting is that we may need to revise all our assumptions about how much wind is available for electric power. In Colorado, NREL found 3x as much wind potential at 80m than a previous Colorado study using the 50m hub height assumption. After all, not only is there more swept in the laminar flow, but there are more areas where tall wind turbines would have the 30% minimum capacity factor NREL assumes is enough to make them economic.

Here’s a graph showing the increase in capacity factors going from 80m to 100m hub height.

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Wind Power and Wind Speed

I received serious skepticism to my idea that wind turbines could significantly slow the wind speed on the Great Plains. One of the criticisms came from an atmospheric scientist I asked to weigh in on the matter. The problem is, I did not find either of their arguments convincing, although I concede Daniel knows more about it than I do.

In responding to them, I came up with an approach for calculating the total power of the wind in the Great Plains. Wind is caused by differences in temperature and pressure as a result of uneven solar heating. Hence the total energy of the wind is a small fraction of the total solar flux. I’m guessing that the amount of solar flux that is actually converted into wind energy is below 1%, probably far below that, but I’ll use 1% until someone gives me a better number.

The Great Plains is 1.4 millions square miles in area, including parts in Canada and Mexico. The average solar flux is about 4 MWh/day/m2 (using numbers for Des Moines, IA.) There are 2.6 million square meters per square mile, making the total solar flux on the Great Plains about 14 trillion MWh/day. Using my 1% conversion efficiency into wind, and 24h in a day, we get total average wind power on the great Plains of 6,000 million MW. That energy is currently absorbed by objects on the ground and internal frictional losses in the air. To create significant wind speed drops, a significant fraction of that 6,000 million MW would have to be absorbed by wind turbines.

In my previous article, I used another approach to calculate that 1 million MW of wind turbines would be enough to significantly slow the wind on the Great Plains. Hence, unless my 1% solar-to-wind conversion efficiency is too high by three orders of magnitude, it looks like the skeptics were right.

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Wind Power and Soil

I recently had the somewhat questionable pleasure of driving across most of the Great Plains.  It has been over a decade since I last did a long distance drive across the Plains, and a new feature is starting to pop up: Wind turbines.  Sometimes in ones and twos, sometimes by the tens or hundreds.  I may be biased, but I find modern wind turbines to be among the most beautiful built structures in the world.  They have a slow, graceful motion that belies their Brobdingnagian scale.  They were particularly beautiful on a foggy evening driving as I drove through a wind farm near dusk, when I could see only the bottom half of the giant blades as they swept gracefully down out of the mist in a slow motion appearing and disappearing act.

The other feature of the generally broad and open landscape were lines of trees, sometimes bordering the interstate, and sometimes bordering fields.  I recall from a US history class in high school that these wind breaks were planted in response to the 1930’s dust bowl.  A little web research led me to the Shelterbelt Project, which seems to be what I recalled (somewhat inaccurately) from high school:

Established by President Franklin D. Roosevelt under executive order on
July 21, 1934, the Shelterbelt Project provided for a tree barrier one hundred miles wide extending twelve hundred miles north to south from the Canadian border through the Texas panhandle. It was designed to
reduce wind velocity, which had occasioned severe soil erosion across the Midwest and dust storms to the eastern seaboard.

In some ways, the Shelterbelt project can be seen as an early experiment in geoengineering. I sincerely hope that any future projects are so successful and benign.

Wind turbines, too, reduce wind velocity.  After all, a wind turbine’s function is to take wind energy, and convert it to electricity.  This led me to wonder just how many turbines would it take on the Great Plains to significantly lower the average wind speed in the region?

According to FTExploring, a wind turbine can extract about 35% of the wind energy passing through the swept area of its blades.  A typical 2.5 MW wind turbine from General Electric (GE) has a rotor diameter of 100m.  To get a ball-park figure, imagine two rows of GE 2.5MW turbines were installed from north to south along the Shelterbelt project (1200 miles) with rotor blade tips inches apart.  If the two lines were offset, wind blowing from east to west or west to east would have to pass through one or two rotors, losing 35% to 58% of its energy along the way, and exiting the back of the turbines 15% to 25% slower.

Such a double row of turbines would require about 386,000 turbines, or about 1 million MW of wind.  So, according to this back-of-the-envelope calculation, 1 million MW of wind installed in the Great Plains (even if not installed in a north-south line) should be enough to noticeably decrease the overall wind speeds in the region, and not only reduce soil loss from wind, but also reduce the cost effectiveness of installing more turbines.  Assuming a 35% capacity factor, this equates to about 3,066 million MWh.  In 2008, the US produced 4,119 million MWh of electricity, so 1 million MW of wind represents about a 75% of electricity production from wind in the Great Plains.  Even if there were sufficient transmission to distribute the power across the country, and geographic diversity greatly moderated the the overall variability of wind, such high penetrations would be impossible without prohibitive investments in electricity storage. 

With current storage technology, a greatly enhanced national grid, and a full roll out of smart grid technology used to better match demand to supply, I would guess that the upper limit for wind penetration would still be only 50% (and considerably lower if any of these things fail to materialize, especially the diversifying benefits of a robust national grid.)   This upper limit (and the fact that only a fraction of wind power is likely to be generated on the Great Plains) means that we’re probably unlikely to need to cut down any trees on the Great Plains in the hope of increasing the wind output of out turbines.

The United States had a cumulative 35,000 MW of wind installed by the end of 2009 (about 3.6% national penetration using the numbers above) so we’re still a long way from slowing the wind significantly on the Great Plains, or anywhere else.

I wonder if farmers who lease some of their land to wind farms notice any local slowing of the wind?  Is that a positive externality worth accounting for?

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Is There a Tradeoff Between Economics and the Environment?

Tom Konrad Ph.D.

California’s RETI process lends insight into the near-term prospects of Solar, Wind, Geothermal, and Biomass.  

In September, California’s Renewable Energy Transmission Initiative (RETI) released their Phase 2A report, which outlined potential transmission corridors to collect renewable energy from Competitive Renewable Energy Zones (CREZ) that had been identified in previous phases.  As part of Phase 2A, they also screened each CREZ for environmental impact, and the potential difficulty of obtaining land for renewable energy development.  

I previously looked at the results from Phase 1A and gained some insight into the cost of renewable energy technologies.  However, what renewable energy projects actually get built has to do with a lot more than just economics.  If it raises too many environmental concerns, such as infringing on endangered Mojave Ground Squirrel habitat, it isn’t going to get built.

Drawing on the spreadsheet "Supplemental Materials, CREZ Data" I put together the following charts, graphing the economics of each type of renewable energy in each CREZ against the expected environmental impact of that CREZ.  

Each circle represents one type of renewable energy at one of 35 CREZs.  Concentric circles in different colors appear where a single CREZ offers multiple types of renewable energy development.  The only difference between the two graphs is the size of the circles.  In the first graph, circle sizes represent the potential annual energy production (GWh/yr) of a CREZ, while circle sizes in the second shows power rating (MW.)  Geothermal and Biomass resources are relatively larger in the first graph because these are typically baseload technologies generating electricity near peak capacity all the time, while solar and wind are variable.

The cluster of circles in the middle right represent resources outside California: they were not rated for environmental concerns, so I assigned them an arbitrary value in the middle of the range in order to display them on the charts.

Economic/Environmental Tradeoff?

I found it surprising that there is little evidence of a tradeoff between economic viability of CREZ’s and environmental impact.  In fact, the circles in the graphs above are generally clustered along a line from the lower left (high environmental impact, bad economics) to the upper right (little environmental impact, good economics).  A tradeoff between economic viability and environmental concerns would manifest itself in a clustering along a line from the upper left (bad economics, little environmental impact) to the lower right (good economics, large environmental impact.)

Considering these four major renewable energy technologies, as they might be deployed in California, there is no real tradeoff between economics and the environment.  The best economics coincide with the least environmental impact.  If we were to include energy efficiency in the analysis, the trend would be even more pronounced: energy efficiency has the best economic profile of all, yet avoids the use of energy and hence does less harm to the environment.

The exception here is biomass.  The small green dots don’t show a pronounced trend in any direction, meaning that there may be some tradeoff for biomass.  Such a tradeoff would not be surprising, because harvesting plant matter on a large scale is bound to have significant ecosystem impacts.  Note that Biomass here does not include such technologies as waste to energy, which can be environmentally benign, or even an improvement compared to land filling.  In this study, the biomass in remote regions that do not yet have transmission, since lack of sufficient transmission was one of the requirements to be a CREZ.

With clean energy, it may actually be possible to do well while doing good.

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The Algonquin Power Income Fund

I recommended the Algonquin Power Income Fund as a renewable energy income investment back in January, and as part of my ten clean energy picks for 2009.

Since then, both the stock and the ten picks have been doing well in comparison to the market, but Algonquin has entered into a couple deals, while I look into in a recent update on the Algonquin Power Income Fund.

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