Image Credit: Walter on flickr. com Customization and complexity are the hallmarks of this fancy hydronic heating system.
Image Credit: Energy Vanguard Sadi Carnot is sometimes called the father of thermodynamics for the ways he looked at heat engines.
Image Credit: Public domain According to Robert Bean, using combustion to heat our homes is like doing backyard gardening with a trackhoe.
Image Credit: Robert Bean Marc Rosenbaum and his nerdy T-shirt. All you need is a little high school science and math to figure this one out. (Hint: It's his alma mater.)
Image Credit: Energy Vanguard
Building Science Summer Camp was last week. That means I was in Massachusetts with 500 of my closest friends, staying up too late, talking building science out the wazoo, and attending some great presentations from leaders in the world of building science.
My big takeaways from Summer Camp this year were Marty Houston’s “hairy hand of quality,” Robert Bean’s three little pigs, and a black toenail. The first was a striking image, the second is the topic of this article, and the third will probably fall off in a few days. (Sorry. If that makes you squeamish, just be glad I didn’t tell you how I relieved the pressure.)
The customization and complexity pigs
The three little pigs that Robert Bean was referring to are combustion, customization, and complexity. I’ll save combustion for last because that’s where he used his most sophisticated arguments, including a term you may not be familiar with. Customization and complexity are similar but independent. A customized building can be simple, and a complex building could be standardized. Both customization and complexity, however, end up making sustainability a harder goal to reach.
Customization, Bean said, is the opposite of standardization. If a mechanical contractor, for example, provides customized heating and air conditioning systems for every house he works on in a 40-year career, he may be leaving a lot of little nightmares for the service contractors who have to go in later and figure the system out. Here’s how Bean described it:
You can take a contractor and plop a box full of parts in front of him, and he will interpret in his own mind with his own creativity how those parts should look when they’re assembled. So what happens is that if you go back twenty years or so, contractor A did it in contractor A’s way for that day on that jobsite. In today’s time, he’s done that for 20 years, and so have millions of other contractors, which means you can’t walk into a mechanical room and find the same system. It’s virtually impossible to go into any mechanical room, if it’s hydronic specifically, and find any standardized method.
If you continue that process into the future — 5, 10, 20 years … over a 40-year period you have this smorgasbord of mechanical systems owned by consumers who haven’t got a clue what this is about. If you’re a contractor and get a call to service this system, where do you even begin?… And the sin in all of that is that the guy who did the customization, when he retires, he walks away from the system and he doesn’t care. He’s retired. The person he did the customized work for, they own it for life.
Complexity, likewise, creates problems for the end users. I thought he was going to talk about how difficult it is to get a good building enclosure or mechanical system with a complex design. More than most of us in the industry, though, Robert Bean has a laser focus on the people who live in, work in, and otherwise occupy buildings, so he sees the problem of complexity all the way down to the occupant. It’s certainly important for all the people who work on the building, but as Bean said about operating the systems in a home, “If it’s so complex the consumer has to actually learn the designer’s profession, they won’t use it.”
The combustion pig
OK, let’s tackle the tough one now. You may be thinking that he said we’ve got to get rid of combustion because of air pollution or because it’s mainly from fossil fuels. You’d be wrong if so. His argument was efficiency — but not energy efficiency. He introduced a quantity that not too many people have heard of — exergy — and said that exergy efficiency is more important than energy efficiency in analyzing how we use energy.
I have to admit I don’t understand exergy well. I’ve seen it mentioned in the past but have never jumped in to see what it’s all about. Since catching the presentation last week, though, I’ve been reading about it more and also spent an hour on the phone with Robert trying to get a handle on it.
A little bit about heat and efficiency
It’s hard to talk about exergy without at least dipping our toes into the thermodynamics pool, but I’ll try to keep this at the 3,000 meter level. (That’s ~10,000 feet for you civilians.) First, exergy is generally defined as the maximum amount of useful work (energy) you can get by moving heat from a higher temperature source to a lower temperature sink.
For example, you can burn a fuel like coal to create a high temperature, converting chemical to thermal energy. Then you can use the heat to make high-pressure steam, converting the thermal energy to mechanical energy that can be used to turn a turbine that generates electricity. As the energy moves through the system, it does work and the temperature drops. A real power plant doesn’t extract the maximum amount of useful work from the energy because real systems are always less efficient than the ideal.
And that, of course, brings us to Sadi Carnot and the maximum theoretical efficiency of heat engines. Carnot came up with the idea of an upper limit for energy efficiency, which is now called the Carnot efficiency. That theoretical efficiency depends only on the temperatures of the source and the sink. (For the record, it’s calculated as 1 – [TC/TH], where TC is the sink temperature and TH is the source temperature.) The bigger the temperature difference between source and sink, the higher the theoretical efficiency.
When you multiply the Carnot efficiency by the amount of heat available, you find the maximum amount of useful work you can get from those two temperatures. Go back to the first paragraph of this section and you’ll find that this is exactly what we defined as exergy.
In his presentation (which you can download from the BSC website), Bean showed calculations of exergy efficiencies for different fuels with different temperatures. For example, natural gas combustion results in an exergy efficiency of 6.1% (slide 172), whereas using solar thermal energy can be done at an exergy efficiency of 20.1% (slide 174). Those are all based on the temperatures of the source energy: 3,400°F for natural gas and 220°F for solar thermal.
Based on those calculations, Bean says that we should opt for lower temperature sources of fuel. The way he put it is that it doesn’t make sense to create heat at a temperature of 3,400°F when we’re trying to heat our homes with fluids at a temperature on the order of 100°F. If we used sources with temperatures closer to 100°F, we’d be doing the job with a much higher exergy efficiency.
According to Bean, using combustion to heat our homes is like doing backyard gardening with a trackhoe. It’s like hammering in finishing nails with a sledgehammer. It’s like using a Turbo-Thermo-Encabulator Max to harvest dental floss! (OK, he didn’t really say that last one.)
My take on Bean’s take is that the temperature of the fuel is the main thing you need to look at because it governs the exergy. Rather than using high-temperature sources of energy, he thinks we need to leave the combustion for industrial processes and let the lower-temperature “waste” heat filter down to the low-grade uses like space heating.
My difficulties with the exergy analysis
I’m far from the smartest person who goes to Summer Camp. In fact, I was in the bottom half of my class in graduate school and usually have to work hard to understand the more abstract concepts. If I were a Richard Feynman or a Lise Meitner, the deep remifications of exergy would probably be immediately obvious to me. But I’m not and they aren’t, so I’m still sitting here trying to figure it all out nearly a week after Robert gave his presentation.
One thing Robert and I went back and forth on when I spoke with him about this was his use of temperatures to draw conclusions. The trackhoe versus trowel contrast above works because of the vastly different capacities of the two tools. But temperature isn’t energy. A burning match at 1,400°F has a lot less energy available for heating than a 10,000 gallon tank of water at 100°F.
Who cares, I said to Robert, that a gas furnace burns at a high temperature if it’s a condensing furnace and you’re extracting 96% of the BTUs and using them to heat the building? After spending an hour talking with him on the phone, the best I could make of this is that an exergy analysis doesn’t really help you when you’re looking at a single building. Its best use if for deciding how to use energy on a large scale.
In Bean’s view, the best use for high-temperature fuels is for industrial purposes. Then you use the moderate temperature “waste” heat for processes that can’t use lower temperatures. Only at the bottom of the chain do you use what’s left for heating buildings.
Now my mind is wandering to entropy and air conditioning and the distribution of electricity. I’m thinking about thermodynamic potentials, statistical mechanics, and the words of David Goodstein:
Ludwig Boltzmann, who spent much of his life studying Statistical Mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study Statistical Mechanics. Perhaps it will be wise to approach the subject cautiously.
My brain is hurting and my self-esteem is waning. But at least I can read the shirt Marc Rosenbaum was wearing on the last day of Summer Camp!
Can you? (Hint: It’s his alma mater.)
Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.
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Exergy is a useful concept, but it should be our final objective. When you consider it for heating a building, it is useful in order to point in two directions. One is to do CHP (cogen): use the high temperature combustion process to generate electricity first. That's only perhaps 25% efficient, so you still have plenty of waste heat left to heat the building. That waste heat is at a lower temperature, so it has lower exergy and is a better match to heating the building. The other thing that considering exergy can lead you to is using heat pumps, which have >100% energy efficiency but are limited to <100% exergy efficiency by theory, more like 50% in practice. Exergy efficiency does a better job of bounding what's possible than energy efficiency does in that case.
If you consider the exergy of solar thermal, well, there are solar furnaces that achieve over 6000 F, so that's actually higher exergy than combustion, per unit energy. So using solar energy to heat water to 200 F is lower exergy efficiency than using a natural gas flame. It's true that the output of a solar water heater is low exergy, but so is the output of a natural gas water heater. The bottom line is that considering exergy, though useful, doesn't really tell you what's important. You have to consider the cost and environmental impacts of the energy source and the cost of the equipment. Same as people are already doing.
The heat pump thing...
Natural gas burned in a best-in-class combined cycle powerplant has a source to load efficiency of about 50%. So a heat pump with a paltry COP of 2 is already "100% efficient", and a heat pump that averages a COP of 3 would be 150% efficient, beating a 98% AFUE gas-burner.
A heat pump with a COP of 2 or 3 powered by a sub-critical coal plant at 30% source to load thermal efficiency would be worse off.
A heat pump with a COP of 2 or 3 powered by a super-critical coal plant at 40% source to load thermal efficiency would be better off, despite the super-critical operating temperatures.
A molten salt nuke runs at about 1000-1300F, substantially higher temps than a third generation pressurized water reactor (600-700F), but it can make use of 15-20x more of the source fuel's energy than a PWR. There is enough spent fuel rod currently sitting in cooling ponds and storage casks worldwide (much of it in the US) to run the entire world grid at current levels for 75-100 years, if it were recycled for use in a molten salt reactor rather than mining new fuel ores. It would also reduce the radioactivity of what's left to where it's safe to handle in only a few hundred years rather than tens or hundreds of thousands of years. Exergy schmexergy- does it really matter that the molten salt reactor would have less favorable exergy numbers than a LWR? Really?
Exergy is great on paper, and can is useful to keep in mind, but as a central defining goal for the world's energy systems it's kind sketchy, with lots of large scale exceptions to prove the general rule. Yes, it's ridiculous to run any sort of high-temp source to raise or lower the temperature of a building a few degrees, but it's certainly not everything. The financial & material cost of low delta-T heating/cooling systems are high, and it's unlikely that there's a sustainability or cost case to be made for low temp active solar thermal vs. heat pumps + local grid connected PV, even if in the near term the exergy numbers are lousier when hooked up to a nuke or coal plant.
Assuming Leslie Dewan's clever reactor design ( http://www.transatomicpower.com/whats-next/ ) gets fully funded, is it really better to better for the world to be heating space and hot water with higher-capital-cost distributed solar thermal than to use heat pumps running off a high-temp molten salt reactor that is simultaneously solving a legacy nuclear waste problem, using orders of magnitude less metal & other resources per delivered kwh/BTU (delivered at any temperature) than solar thermal?
An exergy focused approach works best in the abstract, when it's still in soft-focus. Details matter (a lot!)
another human factor
One significant human factor that often runs counter to efficiency is
the *rate* at which their perceived comfort can get changed, e.g.
someone who's feeling cold wants it warmer *now* and is unwilling to
wait for the efficient heat pump to do its thing in the home she
had on setback while traveling last week. Maybe that's the ideal
target "nest" or "ecobee" customer, who can get that going in advance
over the internet before arriving, but in general lower delta means
lower transfer rate and that still doesn't sit well with many people.
Especially in countries where we have a disproportionate sense of
entitlement as far as energy use.
If we were generally more willing to throw on a few more clothes and
find something to do with a higher activity level until the house
finally warms up, we'd be better off ... all this catering to the
"seated human" at a lazy MET 1.0 seems a little over the top when
plenty of alternatives exist.
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