Efficiency: the many conflicting numbers

If I told you that nuclear energy had the worst fuel efficiency of any energy production system would you believe me? What if I then said that it had the best operational efficiency of any electricity generating system? I would think that you would be starting to either get confused or think that I was just making shit up. And that doesn’t even touch on the thermal efficiency of the turbines or the generating efficiency of the generator. 

Trying to navigate the simple question of how efficiently do we generate electricity is, frankly, incredibly annoying and complicated. But the easiest way to do it is to break it down into 3 main numbers.

  1. Thermal or Generator or Conversion efficiency: This is how efficient the system is at converting the energy source into electricity and is usually dependent on the equipment used like turbines and heat exchangers. In something like a windmill, this is the conversion of mechanical rotation of the blades into electrical energy via a generator, while in a photovoltaic solar panel this is the efficiency of converting sunlight directly into electricity. And in a nuclear reactor, this is the total efficiency of converting nuclear heat into steam, to turn a turbine, and then the conversation of rotational energy of that turbine into electricity via a generator. 
    (you can already see that there are many factors which can affect this kind of efficiency so it is very important when reading about comparisons to know what the components of the comparison are)
  2. Fuel Efficiency: This is how much of the energy that is in fuels like Oil or Coal or uranium that we currently extract in our best facilities. think of it as another modifier on top of the previous kind of efficiency as it takes into account how much is wasted turning the fuel into energy in the first place (Curse you Laws of Thermodynamics!)
  3. Finally, we have Capacity Factor. (This one is my favourite because it is a purely technological measure of efficiency that has nothing to do with fundamental universal laws and instead is a measure of how good we are at using these energy generating technologies.) What Capacity Factor is is a comparison of the absolutely Ideal case of the operation of a generating facility compared to how much energy it Actually produces. For those of you who work in any kind of industrial production facility or are sysadmins, this is the measure of operating time or server uptime respectively as compared to total available time. 

Conversion efficiency is the most standard, and easiest to find values for almost any field. Turbine efficiencies depend on temperature and the thermodynamic cycle they use (ugh flashbacks of university thermodynamics classes). Windmills use things like blade sweep and drag forces to calculate their efficiency turning wind force into rotational energy, and solar cells have a nice neat direct conversion efficiency (it actually can get pretty damn complicated based on temperature but that’s more than I want to explore right now.) 

Fuel efficiency is a little trickier to get a handle on but honestly, the fuel efficiency of fossil fuel plants isn’t the most interesting part of talking about fuel efficiency. What do you think that the fuel efficiency of the vast majority of nuclear plants is? 50%? 70? Maybe you’re really anti-nuclear and want it to be really low so you say 10%? The funny part is that the pessimistic people are actually the closest. Non-breeder style nuclear reactors are generally about 3-4% fuel efficient. That’s right, they only use about 3-4% of the energy stored in those little fuel pellets before they have to remove them and store them. Here’s an article that touches on it in Figure 1 if you don’t believe me!

Now there are several reasons for this but they generally boil down to safety and why Xenon is my least favourite element. If that comment doesn’t make any sense to you I will explain in another post. Now there are several ways to combat this enormous inefficiency, breeder reactors, fuel reprocessing, and my favourite, Liquid fuel reactors. But lots of people make lots of money by not only making these barely used fuel pellets but also by being able to be outraged over all this spent nuclear fuel that we have to keep stored but aren’t allowed to actually dispose of or use. So we’ve been stuck with this shitty 3-4% efficiency for about 4 decades. Before I get even more angry about this, let us move on to the final topic exploration.

Capacity Factor, the bane of renewable energy sources everywhere and the absolute goldmine for combined cycle natural gas. It’s the simplest to put a number on out of the three kinds of efficiency mentioned here, how many hours did the plant operate out of all the hours in the day/week/month/year. This is where nuclear really shines. Nuclear power plants have basically all of the records for the longest amount of time running at full power. The current record for the longest operational period of a nuclear powerplant is held by the Unit 8 of Heysham 2 nuclear power station which operated for 940 continuous days. The previous record was 894 days held by Unit 7 of the Pickering station in Canada. That means that for those almost 3 years each respectively, those reactors had capacity factors of 100%. Nuclear reactors also don’t care about the weather. In fact the average capacity factor of a nuclear plant in the US since 2013 has been approximately 91.7%! The next most reliable power generation is geothermal at 74.4%. Everything else of significant amounts of energy production like coal, natural gas and oil is below 60%, and it only gets worse when you get into wind, and solar (*cough* 20%-35% *cough*). 

So next time someone starts throwing out figures of efficiency, remember that there are different kinds that they might be using or mixing up and make sure that what gets talked about is clarified. Don’t let someone mix fuel efficiency and thermal efficiency to cloud a point and promote their opinion at the expense of truth. And also take to heart that theory numbers don’t stack up to capacity factors. Hard, proven data always should win out when attempting to make informed decisions that can influence things like the answer to the question “Will I have electricity for me and my family in the middle of the winter this year?”

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