Can Clean Energy Eliminate All Rejected Energy?
To understand energy on a large scale in the US, you can’t do better than studying the energy flow charts from the Lawrence Livermore National Lab (part of the US Department of Energy). In the 2023 version above, you see the various fuels we use on the left. On the right you see the outputs. And the thing nearly everyone notices immediately is the huge chunk called rejected energy.
The origin of rejected energy
In a previous article on rejected energy, I explained why it’s there and how resistant it is to reduction as long as we keep our current fossil-heavy mix of fuels. “The first thing to understand is that the great majority of that rejected energy comes from burning fuels,” I wrote. Then we convert the heat of combustion to mechanical work in cars and other machines. Or we go a step further and turn the mechanical work into electricity.
But we can’t ever use all of the heat energy we get from burning those fuels. Here’s what I wrote about that:
As it turns out, we’ve known this limitation for nearly 200 years. A French kid† named Sadi Carnot figured out that there’s a limit on the efficiency of heat engines. He discovered the thermodynamic cycle that yields the maximum efficiency of a heat engine. His work is so important in thermodynamics that his maximum theoretical efficiency is called the Carnot efficiency.
There’s more about rejected energy and the Lawrence Livermore charts in that article, so you might want to click over there and read it.
Can we reduce rejected energy to zero?
Now, the main point of this article is reducing that excess heat we’re not able to use. As I wrote in the previous article, “efficiency isn’t the solution to rejected energy.” Yes, it can cut it down some, but we’re still playing a percentages game.
The only way to make a massive dent in it is to stop burning stuff. Even that would still leave some, though, primarily because of two things: nuclear power production and electrical losses.
Electrical losses also add to the total amount of rejected energy. When electricity travels through the power lines to your house, some of that energy goes to heating the wires. And we lose more in the end uses.
An air conditioner provides a perfect example of end use losses. When we do cooling load calculations at Energy Vanguard, we include the internal loads. When you turn on a light or a fan or an electric oven, all the electricity you use eventually turns into heat. Most of that heat stays in the house, which is why we include it in load calculations.
So the answer is no. We cannot reduce rejected energy to zero.
Do solar and wind add to rejected energy?
When the source of our energy is photovoltaics and wind turbines, they’re not 100 percent efficient. Current photovoltaic (PV) modules, for example, are about 22 percent efficient at converting sunlight to electricity. So should we count the other 78 percent as rejected energy?
No. The excess solar energy doesn’t change form. It doesn’t create pollution. And it doesn’t cost anything. The same is true for wind.
But as soon as we start moving that energy from the modules or turbines through wires, we start getting some rejected energy. Same for running it through inverters to convert between DC and AC or storing it in batteries.
How low can we go?
The most recent LLNL energy flow chart (lead image above) shows that we used 93.6 quadrillion BTUs (quads) of primary energy in 2023. Of that, only 32.1 quads was useful energy that moved vehicles, heated homes, and made stuff. The other 61.5 quads was rejected, mainly as waste heat.
To come up with an estimate of how low we can go with rejected energy if we used only solar, wind, and other renewable sources, I asked claude.ai. The answer was 8 to 12 quads. That’s only 13 to 20 percent of the rejected energy in our current system.
So if we currently need 93.6 quads of primary energy to get 32.1 quads of usable energy, we would need only about 40 to 52 quads of primary energy without the fossil and nuclear. So instead of using only about a third of the energy we’re producing, we’d be using 60 to 80 percent.
The real takeaways
Understanding rejected energy is important. But the solution isn’t finding a way to make better use of all that waste heat. That would mean those thermal power plants (fossil and nuclear) would need to be close to places that could make use of the excess heat for water and space heating. That ain’t gonna happen to any great extent.
What this analysis really means is that we need to focus on the right numbers. To do everything we’re doing now, we don’t need to replace the whole amount of primary energy (94 quads in 2023). We need about half of that, maybe a bit more. Add in nuclear, which isn’t going away, and it’s a bit higher.
Here’s a summary of the important points:
- We cannot take rejected energy all the way down to zero.
- Rejected energy comes from nuclear power generation, too, but nuclear doesn’t come with the same carbon and pollution burden as fossil fuels. (It does still come with the burdens of safety risks, nuclear waste, and a tremendously high price, as Georgia Power proved again.)
- Solar and wind don’t create rejected energy during production, but there are losses with renewably-generated electricity afterwards.
- We don’t need to replace the amount of energy on the input side of the chart. For the same useful energy we got from the inputs in 2023, we may need only 40 to 50 quads with solar and wind instead of the 94 quads it took in our current energy system.
So, if you want to reduce the amount of energy that gets rejected, solar and wind are the way to go.
The Claude analysis
I turned the analysis from claude.ai into an artifact. Click that link to read it.
Afterword
Shortly after publishing this article, I received Lloyd Alter’s latest article in my inbox. It’s about the US Secretary of Energy saying he wants to “unleash” 35 gigawatts of diesel-fueled electric generators to help power America. Ugh!
† Carnot was only 28 when he figured this out.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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Hello Allison,
Thank you for the article. I try to appreciate your optimism.
I think the reason so much “waste” heat is lost in the production of electricity, however it is done, is proportional to the energy quality, economic utility and instantaneous nature of electricity.
It really is not possible to produce and distribute electricity without significant environmental effects. This is true regardless of how it’s done.
This is especially true of the magnitude of electricity production you suggest is needed just to reach the numbers in the LLL sankey chart.
The solar example you used is not a good one as as fortunately, in the natural world, all sunlight not converted to electricity for human consumption, is not instantly reduced to heat; At least in areas not blighted or otherwise desertified by humans. 😉
In a healthy ecosystem, primary producers or green plants convert and store much of the sunlight reaching the planets surface – that process has produced the livable climate over deep time, and release of that sunlight all at once, (ecologically),………. has created the discussion we are having.
Energy we “alternatively” produce or conserve is rapidly replaced with new energy; this to continue to grow the economy with all untoward environmental expenses, “externalized”. *
Ironically perhaps, this is all perfectly natural. See, Maximum Power Principle, sometimes called 4th law of thermodynamics – https://en.wikipedia.org/wiki/Maximum_power_principle
Thanks for putting up with me!
*Economists externalize in a manner similar to how Physicists ignore friction.
Dale:
If you picked up on optimism, I guess that means I did a good job hiding my pessimism. Hmmm.
The amount of rejected energy in our energy system is mostly due to converting thermal energy into mechanical energy and electricity. That puts it in the heat engine category, which subjects it to the Carnot efficiency limits. It doesn’t have anything to do with economics or energy quality or how fast electricity moves. Just basic thermodynamics.
Regarding friction, physicists absolutely do not ignore it. It’s just so complex that developing a comprehensive model to predict it on an atomic scale in more than a few specific scenarios has proven to be too difficult.
When I used to ride my bicycle to work I’d often have to stop at traffic lights next to cars.
In (our mild) winters it was nice to get warmed up by the waste heat from the engines idling while waiting for green.
You don’t get warm standing next to an electric car.
Hello Allison,
I’m sorry you failed to take or investigate the points I tried to make. Given that I won’t bother to defend them except to note that even though they fall outside your frame of reference, energy quality, energy economics, and ecology add dimensions critical to a comprehensive understanding. With respect, reducing this complex subject to Carnot efficiency is so 19th century. 😉
Regarding nuclear produced electricity as carbon clean, please investigate the fuel cycle.
Regarding the use of “AI”. Please don’t. It doesn’t make a good technical reference for a calculated statistic to make a case and only damages any real authors credibility. I’ve decided that for myself, considering the issues we are discussing, it’s use is past my ethics, like consuming red meat.
When my neighbor unleashes his diesel truck and starts idling it for 20 minutes, I run to turn off our ERV (if the wind conditions aren’t favorable). That’s rejected energy that definitely could be eliminated.
Let’s talk about solar energy and Carnot. Solar energy is radiation from the sun which has an equivalent source temperature of about 5800 K. That means that the maximum possible efficiency (Carnot) for converting this to work (e.g., electricity) is (5800-300)/5800 which is about 95%. PV collectors get efficiencies of around 10-20%. The rest is rejected heat. What happens to solar radiation that is not collected on PV’s? If it is absorbed by plants, a small amount of it is converted to chemical energy (photosynthesis) and the rest is rejected heat. If it hits unplanted surfaces, most of it is converted to rejected heat. Some of it might be reflected back to the atmosphere and even back to outer space. What is my point? I just got back from a trip across the midwestern U.S. for Thanksgiving and was amazed at the number of huge solar PV collector fields being installed over very productive farm land. These “solar farms” were covering hundreds of acres. Why not put these solar collectors where they will provide some other useful services like shading buildings or parking lots? The main reason is that midwestern farm land only rents for about $300/acre, so with government subsidies, it is cheaper to install them there then on buildings or parking lots. Perhaps we are not subsidizing this correctly.
One of my favorite graphics to help students see the big picture!!! I also try to explain that just because it landed in “energy services” doesn’t mean it was useful. Just think of how many things are running that provide no “useful service”. Perhaps that should be labeled as “wasted energy”. And next there are the “useful services” that could have been provided with much less energy input due to a lack of energy efficiency. For instance, an incandescent light bulb that is running when nobody is there is wasted. If it was an LED bulb 75% less energy would have been wasted. Of course, as you mentioned in the article if this light bulb is in your home in the winter it could be construed as “useful” — though still inefficient — energy. However, in the summer it would be truly wasted and inefficient!