Residential Solar PV vs Residential Wind Turbine

This post originally appeared on Mapawatt Blog and is a great comparison of residential solar vs. wind. 

 

If you're a homeowner interested in creating your own clean energy, chances are you are considering installing a solar pv array or a wind turbine.  Which is the better option?  Luckily Spokesman-Review did a story on Inland Power and Light in Spokane, WA and their efforts to see whic....  They installed a solar array and a wind turbine that were comparable in cost, and then sat back to see which one came out ahead.

The solar panels have produced about five times as much electricity as the wind turbine over the past 14 months. The sun’s ability to generate more electricity than the wind – even during short winter days – has surprised the utility’s engineers. The utility bought a 35-foot wind turbine and a bank of solar panels. The systems are representative of technology scaled to individual homeowner use, Damiano said. Each cost from $22,000 to $24,000 to install.

The results were surprising to them because conventional thinking is that the Northwest would have more wind than it does sun:

But wind is more erratic than people realize, he said. The wind dies down, for instance, during hot weather and cold spells. Inland Power’s turbine is similar to the larger ones installed in the Columbia River Gorge. It needs a stiff breeze of around 12 miles per hour to start producing electricity. Solar panels, on the other hand, generate a certain amount of electricity even on cloudy days.

This is a great example of solar vs. wind, but aside from understanding some of the basics about solar and wind power, it is really only applicable if you live in or around Spokane, WA.   At Mapawatt, we like to do the analysis ourselves and help show you how to your own analysis....so here we go! (Update - 3/28/11 - I contact Richard Damiano, Inland's chief engineer, after I wrote the post and he was gracious enough to provide me with more information on their installation. If you don't want to see my analysis, skip down to the bottom to see the actual data!)



We covered how to calculate solar power production in your area on our post "Solar Power Payback".  For some reason, the utility did not share the actual sizes of the solar array or wind turbine, so we can only estimate those sizes given the price they paid.  Based on solar installer friends I have in Atlanta, I know solar installation prices (including panels, mounting equipment, inverters, labor, etc.) can be had for around $5/watt (since their system has been installed for awhile, and labor is more expensive on the west coast, I'll use $6/watt) .  Since their prices ranged from 22k to 24k, I'll take the high-range at $24,000.   Divide that by $6 and we can estimate that their solar installation was 4 kW.   Entering this value into the BP Clean Power Estimator, I get the yearly solar power energy produced for Spokane, WA at 4,856 kWh.

Now, the news article on the Inland Power pilot project states:

In the Inland Power pilot project, the solar panels produced about 15 percent of a typical household’s electric needs over the course of a year. The wind turbine produced less than 3 percent.

But this is a very poor way at analyzing clean energy because what is a "typical household's electric needs"?  One home that is 5,000 sq. feet and has a family of six who don't really try to save energy is going to use way more energy than a 2,000 sq. feet and family of three who tries to save energy.  This isn't just a few percent, but could be double or even quadruple.  The only fair way to present this information is to give the yearly kWh produced, and recommend that people look at their utility bills from last year and see what percentage this would make up.  Or give the range that this could make up for different households, i.e. "This solar panel array would produce 50% in a smaller home that already conserves energy, but only 15% in a large home that doesn't do a good job conserving energy".  I firmly believe that when reporters try and over simplify energy data it only serves to confuse the reader more, or at worst give them false information that they then base their thought process off of.

Update - 3/28/11 - Mr. Damiano sent me an excellent flyer describing how much the solar panel system would produce as a percentage of a home's energy consumption. He also explained that the 15% number was based on an average annual electricity consumption of 18,000 kWh.  This may be higher than what you would expect but most of their customers use electric furnaces to heat with.  Here is the  Inland Power Residential Solar Flyer.

Trying to estimate how much clean energy you can produce at your home is much easier with solar power than wind power.  This is because with solar, we know approximately how much solar insolation you will receive in a year based on your latitude and longitude.  This obviously varies due to cloud cover, but it can be reasonably estimated based on past history.  The only real variable for homeowners is shading by trees, buildings or other obstructions, but those can also be accounted for.

Wind power is not so easy.  I did tell homeowners how to find wind speed data for their homes in our post "Wind Speed data for Residential Wind Turbines".  But I also pointed out the flaws in this method:

"So this data is helpful,  but unfortunately it’s not perfect.  Unlike the sun which shines straight down and is easy to predict (“Is there a tree in the way?”) wind blows from the side and is affected by the landscape, trees,  buildings, other houses, etc.  The only real way to predict how a wind turbine will perform at your house is to buy an anemometer.  Because wind can be so fickle, you can’t just rely on a computer program to predict if a wind turbine is going to be a good investment.  You should really do the measurement yourself, or have a reputable dealer help you do it.

Another thing to remember with wind-speed is that the “average wind speed” can be misleading when it comes to determining how much power a wind turbine will produce.  As I said in our post on the Anemometer, wind speed is so important because it is cubed in the equation above (P = .5 • ρ • A • V³; where P = power and V= wind speed).  If you double your wind speed you don’t double your power output, you increase it by a cubic factor! For instance, increasing wind speed from 4 mph to 8 mph is not double the power output, but increases it 8 times (8³/4³=512/64=8)!"

Using data from a weather station will put you in the ballpark for determining if wind may work, but it wont get you all the way there.  For instance, if the wind speed is low at the weather station near you, the wind speed will probably be low near you, but a high wind speed at a weather station may not mean a high wind speed at your home.  The only way to accurately gauge how much power a wind turbine will produce at your home is to use an anemometer and a data recorder over a several month study.

But if we were to try to predict the yearly energy output from the wind turbine, we could follow my instructions to find the average wind speed for Spokane.  Doing that (see Wind Speed data link above) we get the following result:

At the bottom you can see that the average wind speed in Spokane over the course of a year was 8 mph.  One of the more popular wind turbines for residential use out there is the Skystream 3.7 sold by Southwest Wind Power.  This turbine is rated at 2.4 kW.  Even though this site says the Skystream wind turbine costs $14,000 to $18,000 to have it installed, let's just assume this is the unit Inland installed for comparison's sake.  Using the graph below on the Skystream wind turbine's power output against wind speed, you can see that the Skystream produces 100 kWh per month wind the wind speed is 8 mph.

Using this information, one would deduce that this wind turbine would produce 1,200 ( 100 kWh per month * 12 months) kWh per year.  This is only a quarter of what we calculated the solar panels would produce (4,856 kWh) and this is very close to the real world results that Inland is seeing where the solar panels are producing 5 times more power than the wind turbine!  Of course I made many assumptions in my analysis, but I'm in the ball park and hopefully you can see how to do some of this analysis on your own.

As you can see, it is important to do your own due diligence when deciding between solar panels or wind turbines and the need to speak to many experts.  Don't believe the turbine dealer who says "The wind always blows 'round here so you'll produce a ton of power!"  Now you can do the analysis yourself!

Keep in mind, that this study is only for residential installations.  For utility scale installations, the prices change drastically as do the operating characteristics, mostly for wind due to the height of the turbine.

The story on Inland ends with a note that the up front costs of clean energy installations are out of reach for many, but I like the conclusion:

From a cost-benefit standpoint, erecting a wind turbine or putting in solar panels is still a reach for most homeowners, Damiano said. Those who take the plunge are making a lifestyle choice to reduce their carbon footprint, he said. Recovering the installation costs for turbines or solar panels can take years, even with the 30 percent tax subsidy available to homeowners. “It will take you a chunk of time,” Finney acknowledged. But she encourages people to think about the long-term benefits.

“Some people spend $25,000 on a new car,” she said. “We decided this is how we wanted to live and how we wanted to spend our money.”

------------------------------------

Here are the project specs from Richard Damiano, Chief Engineer at Inland:

Solar System:

• 2.24 kW, fixed array, no adjustment for season or ability to track the sun

• Each panel has an inverter

• Installed in the fall of 2009 at a cost of about $10/W (total installed cost)

Wind System:

• 2.4 kW turbine (Proven Wind) - (Mapawatt Note: I was close with my guess of the Skystream in that they are both rated at right around 2.4 kW!)

• Installed in the fall of 2009 but was about 2 years old when installed

• A member of Inland Power’s had the wind turbine installed, the unit was chosen based on wind maps showing 12 mph average at the location of their home, this did not prove to be true.

Richard went on to explain why the solar panels are outperforming wind:

One thing that most people do not take into account is the power factor of the system.  Our solar panels are rated in the mid-90% range for Power Factor, this has proven out with actual peak production of slightly over 2.0 kW, exactly what I would expect given the power factor and efficiency of the panels and inverters.  The wind turbine is rated in the lower 80% range for power factor.  When you take this power factor and combine it with the efficiency of the inverters we see an actual peak production from the wind turbine of only slightly more than 1.0 kW at winds of 40 mph.  So even though the wind turbine is nominally larger than the solar system, the efficiency of the system is much lower resulting in about half of the peak production capability of the solar.  Combined with the erratic nature of wind it is easy to see why solar is outperforming wind in our area.