The Units Commonly Applied to Renewable Energy Are WRONG!
by Scott Sklar
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 (MW), and gigawatts (GW) 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 than 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., New Hampshire
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 (PV) 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 PV 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 the recent article, Less Backup for Wind Power May Be Required,
"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, 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
emphasizing 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 cataloged, 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 end-users using established
technologies in mature projects.
Reader Comments
While you agree with Rob on some points, you seem to have missed his
point almost entirely. He did not say that renewable energy sources were
unpredictable, yet you went after that like a strawman.
It doesn't matter if you could predict solar output of PV system with 100%
accuracy a day ahead or even 1 million years ahead. It's capacity factor on
earth will still be about 20%, maybe 25% at best in the Mohave desert. So it
can be misleading to compare the rated peak capacity of a PV plant to the
peak rated capacity of a coal plant that runs at closer to 70% capacity
factor.
A case in point, the New York Time says in http://www.nytimes.com/2007/12/18/technology/18solar.html?_r=1&ref=business&oref=slogin
Nanosolar's founder and chief executive, Martin Roscheisen, claims to be
the first solar panel manufacturer to be able to profitably sell solar
panels for less than $1 a watt. That is the price at which solar energy
becomes less expensive than coal.
It is true that if you could build a PV plant for $1/W, the plant would be
cheaper than a coal plant of the same rated capacity. However, the PV plant
will typically produce less than one third the energy in KWH in a year than
the coal plant, and the cost of the electricity in cents/KWH will be
significantly higher for the PV plant than for the coal plant. This is the
frustration.
-Carolyn L,
proud rooftop PV owner
Interesting...
But poorly stated, Rob L. has a 'little' trouble with units. He fails to understand that we have to rate our systems in terms of peak POWER, otherwise we risk damaging the infrastructure. Our 200kW can deliver 200kW under full sun, but It I called it a "35kW average", I'd find myself at the pointy end of a lot of lawsuits. The only real purpose of the peak rating is to determine the initial cost of the hardware.
Now when I quote ENERGY, he's correct the numbers aren't nearly as impressive... Blame the 19th century scientists on that. Solar can operate about 4-6 hours per day, and on average we expect about 2300J of total 'flux' per m^2 per day. That's an awfully boring number. Worse yet, when you multiply by the system efficiency you get electricity at 1/3 kWh * m^-2 * day^-1... Woot woot!
So what can we do? Educate the public! What else?
Paul Passarelli
(to join in on this conversation go to: http://www.renewableenergyaccess.com )
To subscribe or visit go to:
http://www.renewableenergyaccess.com