SOLAR TODAY - March 2010

because our electric grid is so interconnected, electricity is continually bought and sold between different operating regions.

While variable wind and photovoltaic (PV) technologies do present additional challenges to grid operators, the need to deal with rapidly changing conditions is by no means new. To accommodate the variable output of more renewable generators, operators can plan on greater use of natural gas combustion turbines or they can run two combined-cycle gas plants each at a partial load. And the argument that using natural gas to back up renewables defeats the purpose of reducing carbon emissions is a specious one. Enabling a larger percentage of our kilowatt-hours to be produced by carbon-free renewable energy clearly reduces total carbon emissions.

The strategy for deploying renewable energy technologies also affects how easily they can be integrated. When wind and solar are deployed together, they are generally complementary. That’s because the wind resource is usually greater in the winter and at night — the opposite of the solar resource. Also, while the wind may die down, or scattered clouds may temporarily reduce available sunlight at one location, other locations can help carry the load. Thus, expanding an operator’s region to achieve a wide spatial diversity of renewable electricity sources will steady the electric output.

A utility’s ability to utilize renewable energy depends a great deal on how well the availability of the renewable resources can be forecast an hour ahead and a day ahead. The National Oceanic and Atmospheric Administration (NOAA) provides the public with information about wind and solar energy resources on these short time scales through the National Weather Service. Dr. Melinda Marquis of NOAA in Boulder, Colo., has championed a joint effort by NOAA and the National Renewable Energy Laboratory to provide improved renewable energy forecasts across a range of time scales, thus making the job of grid integration easier.

Numerous utilities have conducted grid-
integration studies to determine the additional
costs of integrating wind energy into the grid.
These studies generally show that even at pene-
trations as high as 25-30 percent of peak power
(we are only at about 4 percent penetration by
non-hydro renewables today), the additional
cost associated with utilizing existing reserves
adds no more than about a half-cent per kilo-
watt-hour to the cost of generating electricity,
or about 10 percent of the wholesale value of
wind power. Improved forecasting can signifi-
cantly reduce that cost. And combining wind
power with steady biomass power, geothermal
power and concentrating solar power with
built-in thermal storage can allow for still higher
total renewable energy penetration before one
needs to build advanced electrical storage sys-
tems such as compressed air energy storage.

Grid operators have at their disposal a wide range of options to ensure that electric demands are reliably met.

coal.) But it is misguided to conclude that we must resort to new large-scale, expensive electric storage systems before renewables can make a large contribution. At some value of renewable grid penetration substantially greater than 25 percent, economics will favor the construction of advanced electric storage systems, so it makes sense to be developing such systems for the future. But before we ever reach that point, we can deploy a great deal of renewable energy (along with efficiency) and make an immediate impact on the climate change problem. ST

Site evaluation just got easier. The new SunEye 210. One-handed operation makes your shade measurements a snap. Preview mode shows the sun path overlay adjusted to the device’s orientation. Pitch and azimuth measurements are built-in.