Background

The English title of the study is "Compatibility of renewable energies and nuclear power in the generation portfolio - Technical and economical aspects". Thus, it is a radical deviation from the standard renewables-or-nuclear framing used by most people on both sides of the debate in Germany (and more fitting for the future mix favoured by many people on this blog, though not me).

The study was prepared for E.ON last year at the Energy Markets and Macroeconomic Analyses (EGA) section of the Institute for Energy Industry and Rational Energy Use (IER), which belongs to Stuttgart University. EGA and its head Alfred Voß (who is among the authors) appear in public debates on the decidedly pro-nuclear side. Thus the EGA/IER paper can be seen as the nuclear lobby's answer to claims made in studies by three sources that may have an opposed bias: the German Federal Environment Ministry, the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES), and the Council of Experts on the Environment (SRU) advising the government. As such, it's only fair.

Variable output from nuclear

The starting point of the study is that, although nuclear plants are operated in Germany as baseload, their specifications allow regulation with rapidly changing outut in the upper power range:

1. types of pressurized water reactors (PWR) in Germany can move between 50% and 100% nominal power in 15 minutes (3.3% per minute), and can be regulated down to 20% nominal power using the control rods;
   
2. between 80% and 100% nominal power, PWRs can even move at 10% per minute;
   
3. types of boiling water reactors (BWR) in Germany are comparable to PWR between 90% and 100% power, though limitation to 1% per minute can be necessary, and can be regulated down to 60% nominal power by changing the steam bubble quotient;
   
4. for all German nuclear plants, they calculated a sustainable load-following capacity of 9.6 GW in 15 minutes, at speeds of 3.8-5.2% per minute.

The central aim of the study was to test how intermittency can be balanced in Germany in 2030. Two scenarios were tested. The shared assumptions were:

• renewables increase to 42% of generated power,
  
• net demand stabilises at the 2007 level,
  
• coal contracts, gas continues to expand,
  
• renewables have priority in production.

With the above assumptions, this is how the 2020 and 2030 power plant capacities look in the two scenaros:

Legend translated:

• orange: photovoltaics
  
• light blue: wind on-shore
  
• extra-light blue: wind off-shore
  
• light magenta: pumped water storage
  
• magenta: mobile electrical storage [batteries etc]
  
• purple: pumped air storage
  
• blue: hydro
  
• green: biofuels
  
• dark blue: other
  
• red: oil
  
• light orange: natural gas, gas turbine
  
• light green: natural gas, steam turbine
  
• yellow: natural gas, combined cycle
  
• black: anthracite or bituminous coal
  
• brown: sub-bituminous coal [terminology warning: in English, 'brown coal' is a rarely used synonym for lignite, but the German literal translation is used for a higher grade]
  
• grey: nuclear fuel[sic!]

...and here is the total annual generated electricity in 2030 in the two scenarios:

Balancing intermittency in the scenarios

To test intermittency, the study scaled up actual measured power curves for wind power and photovoltaics, and then let other modes do the balancing.

In the nuclear phaseout scenario, as today, most of the variable power is provided by gas and anthracite coal fired condensing power plants. (Though this study doesn't make the distinction, it is worth to note another terminology issue. In contrast to the simple baseload-peak load duality in English, German terminology subdivides variable power in intermediate load -- the pre-scheduled part meant to balance the expected daily variation in demand and baseload -- and peak load -- the unscheduled, active response to the actual momentary load. In Germany, the anthracite coal plants do the bulk of intermediate load, and gas that of peak load.)

In the extended nuclear lifespans scenario, the nuclear plants can do their share of the balancing, alongside fossil fuels:

As you can see, the balancing goes beyond 50% nuclear capacity: this is achieved in the simulation by controlled shutdowns of individual plants.

The main conclusion is that balancing intermittency is possible either way. The study goes on to compare the cost and CO2 emissions in the two scenarios, of course favouring the second.

Critical notes

• The study is specific to Germany. For our purposes, the main resulting limitation is that nuclear is considered only at a relatively low grid penetration. It would have been interesting to see two more scenarios tested: one with renewables and nuclear only, with the latter doing all the load-following to balance changing demand and renewables intermittency; and another with nuclear only, doing the balancing of changing demand.

• The power output variation in this study is theoretical. Though the authors claim that they assumed levels of variable operation sufficiently short of the specifications to be equipment-protecting, I'm not sure one can make easy claims about the stable level of long-term quasi-continuous operation for control rods and operating personnell, and the workability of regular controlled shutdowns (small accidents -- ones not affecting safety but affecting production -- are most common during power-up).

• On the other hand, an international outlook could show up the potentials of power output variation in practice. Though countries with a high grid penetration for nuclear are also major exporters, thus foreign fossil fuel plants 'contribute' to balancing, in a half-sentence, this study confirms Jérôme that the whole French nuclear plant portfolio is operated in a load-following way. It would be good if readers could find details on that; and also on possible load-following operation in Sweden or Spain.

• Two apparent omissions of the study might make load-following easier. One is the simple scaling up of present-day wind intermittency. However, off-shore wind blows more evenly, leading to less intermittency especially if both North Sea and Baltic Sea locations are utilised.

• The other omission was to consider the power balance of Germany isolated. But cross-border flows would serve to spread out the storm-related peaks in wind generation in time, thus reducing the level and rate of load-following output needed.

• UPDATE A third possibility to reduce renewables intermittency is 'natural balancing' between solar and wind. This would require a solar output much closer to wind's, that is much higher than the annual 19 TWh assumed in the study. (Which was definitely conservative: photovoltaics fed 6 TWh into the grid in 2009 already.)

• It must be noted that the study misses the point of the political-business battle for market share: the power plant owners would first have to be convinced to accept the lower profits and extra work after conversion to power-following operation, and not insist that they can only operate as baseload.

All in all, an interesting study.