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."
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.
PV Coincidence With Load – Spring
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.
UPDATE: As promised: Investments to mediate between intermittent loads, and intermittent power.