Paul Denholm,
a Senior Analyst at the National Renewable Energy
Lab (NREL), sees an upcoming struggle between renewable sources of
electricity such as photovoltaics (PV) and wind with low-carbon baseload
alternatives for space on the low carbon grid of the future. These
baseload alternatives are nuclear and Internal
Gasification Combined Cycle coal plants with Carbon Capture and Sequestration
(IGCC w/ CCS, refereed to by advocates as "Clean Coal).
This may come as a shock to advocates
of the idea that Global Warming is such a big problem that we will need all
forms of low carbon electricity, because the heart of that argument is that
the choice between nuclear and IGCC w/CCS (if and when it's available) is that the
decision is not one of "either-or" but "and."
PV Curtailment
In his seminar
at NREL's Strategic Energy Analysis and Applications Center, Dr. Denholm showed
what could happen with only an 11% penetration of photovoltaics on a cool, sunny
Spring day.
Electricity demand in the Spring is typically low, and likely to be lower on
weekends, because there is no need for air conditioning, nor much
lighting on a sunny day. The graph above shows a day where PV (even at
only 11% penetration) would actually be producing more energy than the total
demand on the grid. While worries about the cost
of integration and even curtailment
are well known and studied
[pdf] in the context of wind power, integration has typically not been
considered a problem with photovoltaics. In fact, PV is generally
considered to bring integration benefits, given the good correlation of PV
output with summer peak loads.
The above chart is just a simulation, and PV is a long way from 11%
penetration anywhere, but PV curtailment lurks in our future. A
combination of wind and photovoltaics will simply reach that limit sooner
(as wind already has.)
The Problem With Baseload
If PV curtailment is not yet a problem because of low penetration, the more
baseload power we have, the more difficult it will be to integrate intermittent
power sources into the grid. Unlike Natural Gas Combined Cycle plants,
IGCC and Nuclear, like current coal plants, have very little flexibility in how
much power they generate. This means that the more baseload generation
there is on the grid, the less "room" there will be between baseload
power and current demand for electricity from intermittent sources. So
while higher penetrations of dispatchable power such as natural gas aid the
profitability of wind and solar, higher penetrations of coal and nuclear power
reduce their profitability. And vice-versa: a grid with high penetration
of intermittent sources will make proposals for new baseload power less
attractive, since intermittent power lowers the minimum electricity demand on
the grid, effectively reducing the market for baseload power.
Why IGCC is Baseload
I was a bit surprised that Dr. Denholm spoke of IGCC woth CCS as a baseload
technology. Although I consider "Clean
Coal" an expensive distraction, I had thought that one advantage of the
technology would be to make coal dispatchable. My thought was that, since
IGCC first involves producing syngas (a mix of carbon monoxide and hydrogen) by
gasifying coal, and then burning the syngas in a turbine very similar to the
natural gas turbines used today (which are dispatchable), it would not be too
difficult to separate the steps, and store the syngas for later
combustion. When I asked, he replied that adding storage is an
"extremely tricky" problem, because the current technology relies on hot
gas cleanup to achieve high efficiencies.
Squaring the Circle
It's not surprising that we hear about the baseload-intermittent power
conflict from Dr. Denholm: his expertise is energy storage. Energy storage
has the potential to mediate between the fluctuations of load and
intermittent power, and difficulty of shutting down baseload power plants for
short periods. I've discussed investments
in large scale energy storage, but there are also opportunities for smaller
scale and shorter time span operations which I will explore in later articles.
Tom Konrad
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