Must the 3 Little Pigs Die?
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 & complexity pigs
The 3 little pigs 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 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 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. Since Bean is known as "The Exergist" in the building science world, I know of no one better to learn this stuff from. (Well, all right. He's not really known as The Exergist...yet. With your help, though, we can make it happen!)
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 3000 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: 3400° F for natural gas and 220° F for solar thermal.
Based on those calculations, Bean says 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 3400° F when we're trying to heat our homes to temperatures 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 ramifications 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 1400° 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.)
Photo of 3 little pigs by liz west from flickr.com, used under a Creative Commons license. Sadi Carnot image in the public domain. Trackhoe and gardening image from Robert Bean's 2015 Building Science Summer Camp presentation. Other images by Energy Vanguard.
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