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Burning The Future: Coal in America
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Wind /Solar variability
Written by Administrator   
January 16, 2008

Q:The units commonly applied to renewable energy are WRONG!  It is wrong and tremendously misleading, albeit politically favorable to use PEAK POWER units to describe the capacity of energy conversion devices applied to diffuse and intermittent sources such as wind, solar, ocean etc. The units of Watts, kiloWatts, MegaWatts, and GigaWatts are units of POWER and for an intermittent and constantly varying source they are only instantaneous values.  Power units are for the most part only of value in sizing electrical hardware for peak capacity. These large and impressive numbers are no more meaningful that claiming "the enormous solar / wind array provides ZERO Gigawatts" for much of the time. To be meaningful, renewable energy must be described in time averaged ENERGY Units (it's renewable ENERGY, not renewable POWER).  So a solar array for example, might be able to provide 100 kWatts/hr when averaged over time.  An even better idea is to use kW-hr/Square Meter of whatever...

Rob L, NH

 

A:

Rob,

I sense your frustration, and agree on some points, but don't accept your premise fully. Water technologies are quite predictable, particularly tides, waves and ocean and river currents.  Most photovoltaics is measured by watts per square meter at noon. So I agree with your point that energy should be measured as an actual output not as a nameplate. Actually this holds true in conventional energy generation as well - most generators do not live up to their nameplate predictions.

But while wind and solar are intermittent, it is rather blown out of proportion (pun intended). Solar radiation for the concentrated solar plants in the Southwest are quite predictable, and the Solar One 64 MW concentrated solar plant, for instance, has been operational everyday since the generation facility came on line mid-2007.:   As for photovoltaics in non-desert settings, even on the cloudiest days I am generating a third of the electric "nameplate", so if I finance the system incorporating for that lowered output for a third of the year, then any extra power is 'gravy'. Sunlight is not as unpredictable as one might expect.

Wind always is tarred with this unpredictability label as well.

According to a recent article, “The varying nature of wind power means that it is harder to forecast than the fluctuations in electricity demand. Adding large quantities of wind power to power systems is therefore challenging. The results of a recent study coordinated by VTT Technical Research Centre of Finland, an international collaboration within the International Energy Agency (IEA) has been published in a report entitled: Design and Operation of Power Systems with Large Amounts of Wind Power.” The report contains a summary of the wind power impact assessments performed in 11 countries. with assessments divided into three categories: 1. Additional costs arising from the balancing of wind power fluctuations, 2. Grid reinforcement needs due to wind power, 3. Capacity of wind power to replace other power plant capacity.

The bottom line of the study, “With wind power penetrations amounting to 10-20% of the gross electricity demand, the additional costs (per MWh of wind power) arising from the balancing of wind power fluctuations are estimated to range between 1-4 €/Mwh (US $1.46-$5.88/MWh). This is less than 10% of the long-term market value of electricity.”


Ken Westrick, CEO and cofounder of 3Tier (WA), tracks solar, wind and hydropower future capacity and performance for electric utilities. He says, “While it is true that wind is intermittent it doesn't mean it is unpredictable, nor that it is unreliable. With regard to unpredictability, even several days in advance energy forecasts that are much better than climatology or persistence are available with a state-of-the-art forecasting system. Errors of less than 20 percent of nameplate capacity can be achieved regularly and with multiple wind farms that are geographically dispersed, the grid impacts become even less of an issue. Regarding reliability, when averaged over a year a wind project is quite reliable with regard to capacity, with average annual capacity factors within 10 percent of a long term average, which is much better than many hydropower systems.”

In the January 4, 2008 RE Access article, “Keeping the Electrons Flowing” by John Dunlop, “In spite of that complexity, the wind turbine must perform those functions day in and day out. A wind turbine commonly is producing electricity up to 80% or 90% of the time” and further emphasizes performance, “. He goes on to state, “An owner receives no tax credit from the federal government until the turbine begins to operate, and then it only receives the credit based on the quantity of electricity produced-over a long period of time. Financial institutions, insurers, and project owners now depend on a continued flow of electrons from the wind turbine to justify their investments, which ultimately provide a reliable supply of electricity to power-hungry customers.”

Economics of all energy systems are based on MWh outputs (electricity generated by hours per year)– not nameplate capacity of the system. This is true for traditional thermal systems (coal, natural gas, nuclear, diesel) which have heat losses, more moving parts, and greater O&M downtimes that have to be catalogued, tracked, and incorporated. That’s what project financing due diligence is all about – pretty well sophisticated – and so far I haven’t heard much complaints from endusers using established technologies in mature projects.

Scott Sklar

Last Updated ( May 02, 2008 )
 
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Energy Star is the national program that rates appliances and promotes energy savings activities. Experts at the US Environmental Protection Agency and US Department of Energy have also sorts of programs and tips. Here are three sites: