Waste not, want not

December 8, 2006 4:27 PM

Remedial physics and sustainability: if energy is neither created nor destroyed, why do we have an energy crisis? Much more extensive question follows:

The conservation of energy law states that in an isolated system, energy is neither created nor destroyed, simply converted. And though the law only holds true in an ideal (frictionless) environment, I assume that's because the law makes reference to usable energy. Adding back the energy loss from friction in the form of heat, I assume the theory still holds true. (If I'm wrong about this, what happens to the excess energy?)

Applying the law: obviously the Earth is not a closed system; we radiate heat into space, and the sun provides massive amounts of solar energy. Current technologies are fairly inefficient at converting energy into work; heat is often mentioned as a waste byproduct. But if the law of conservation of energy holds, then technically we haven't actually lost any energy, it's just in a form we can't harness.

The question, then, is this: We're doing a lot now to make current processes more energy efficient, but what about reclaiming heat energy (and other waste energy, if it exists) and recycling it? What are the obstacles to doing this on an industrial scale? Could heat reclamation processes greatly increase energy efficiency? Would it be enough to offset the impending energy crisis, or at least soften the impact until alternative energy sources gain more ground?

In other words, why aren't we doing more to harness waste energy? Are there scientific obstacles? Cost obstacles? Or is it just that no one has really bothered?

posted by chrominance to science & nature (17 comments total)

Hey, look, there goes some waste heat. Quick, grab it before it radiates!

You'll get more technical answers, but that's why. The energy crisis is about stuff that can be easily and safely transported and stored until the moment that the energy is wanted. If you liked, you could call it the "energy storage" crisis.
posted by ikkyu2 at 4:40 PM on December 8, 2006

I think Stirling engines are designed to reclaim waste heat. Generally though, you'll probably find that the stored energy we have is very cheap relative to the cost of reclaiming waste energy.

Also, while energy isn't lost, it is converted into mechanical energy (e.g. move this boulder from here to there), so that is energy that is used that we probably won't ever be able to reclaim.
posted by willnot at 4:44 PM on December 8, 2006

At a theoretical level, there is the second law of thermodynamics: The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. The first law says energy is neither created nor destroyed. But the second law says entropy increases, meaning it is more and more evenly dispersed through the system. So, you throw a red-hot brick into a pot of water, after a while the brick and the water are all the same temperature. The energy stored in the brick is still there, but it is much less useful. Your car burns gasoline -- maybe 20% of it drives the wheels and the rest is dissipated as heat and noise. You can capture some of it, but never all of it. The fuel you burn in your furnace or stove heats your house, but eventually it leaks out into the air ouside and can not be recaptured. There are some realistic opportunities to "reclaim heat" as you suggest. For example, the hot water from your shower goes down the drain and into the sewer. A gray-water heat exchanger could reclaim that heat and use it to pre-heat water in your water heater -- very few homes have such a thing. This idea can be applied to some other processes in homes and factories, but not to the majority of the energy being turned into heat. The better approach is to increase energy efficiency (like compact fluorescent bulbs that turn much less of their electricity into heat, versus the incandescent kind), better insulated homes to prevent heat loss, and use of renewable energy sources.
posted by beagle at 4:45 PM on December 8, 2006

You're right, we don't "use" Energy since energy cannot be created or destroyed.

It is more correct to say we use Exergy.
posted by vacapinta at 4:48 PM on December 8, 2006 [1 favorite]

First, the law of conservation of energy makes no reference to whether energy is usable or not: you can have friction, and the energy 'lost' to friction will be converted to heat, but still conserved.

Secondly, How much useful work you can get out of the energy you have is studied in thermodynamics. As things happen, entropy increases, and not as much useful work can be gotten out of it.

Considering that most power plants are heat-to-electricity conversion factories, we can assume that the mechanics of capturing heat are pretty-well advanced (though I'm sure there are still some places where there is waste).

On preview, beagle has an excellent answer.
posted by alexei at 4:51 PM on December 8, 2006

If you're interested in what people are doing to further energy research, you would be well advised to take a look at some of the research done by the various Department of Energy Laboratories.

I personally worked at Argonne National Laboratory for a short time, and there were scores of posters all over the lab describing research that had much to do with your specific question.

If this sort of thing interests you, you also might consider taking your life in a direction that will allow you to make an attempt at solving these problems. Such a trajectory would include College and Grad School.

Maybe you could be the one to find some small portion of the solution to our problems!
posted by zhivota at 4:52 PM on December 8, 2006

"Entropy crisis" just makes people go 'huh'?

The first law is energy isn't created or destroyed; the second is that "
entropy can't decrease.

In a mushy non-scientific way, increasing entropy means a decreasing difference between regions; this difference can be about heat.

There is a thought-machine called Carnot's heat engine that describes the maximum amount of mechanical work you can get from a given difference in the temperatures of a "source" and "sink". As regions of high heat and regions of low heat become less differentiated - in other words, as the difference in temperatures decreases - the payoff in mechanical work of "harnessing" the difference gets lower per amount spent on the hardware.

For any degree of hardware sophistication there is a limit beyond which it doesn't pay to go after that "waste" heat.
posted by jet_silver at 4:52 PM on December 8, 2006

And though the law only holds true in an ideal (frictionless) environment...

This is not true. A law is true for all situations. Energy is neither created nor destroyed. Always.
posted by odinsdream at 5:19 PM on December 8, 2006

Hmm, a question I am well-qualified to answer.... There is no lack of energy on Earth, but rather a huge problem of energy conversion. As you correctly deduce, our inefficient "machines" produce a lot of waste heat.

I actually work on thermoelectric energy conversion, which is a way of converting waste heat into electrical energy. There are a few reasons why thermoelectrics aren't more common.

The first is that they aren't generally terribly efficient. A very good thermoelectric is ~10% efficient, and that's with efficiency given as a percentage of Carnot efficiency (see jet_silver's comment). See, you can't turn thermal energy into work, but rather thermal gradients, and the theoretical maximum efficiency (Carnot) is T_H-T_C/T_H. Which means there's not a lot of energy to get out of waste heat.

The second reason is cost; thermoelectrics, solar, etc. remain substantially more expensive than burning fossil fuels. The main costs are production, materials that go into devices, and research costs (my research is expensive, and while my salary is but a blip, I do have to eat, y'know).

The third reason is closely related to the first two; in order for a technology like thermoelectrics (or a Stirling engine or whatnot) to really matter on a global scale, you would need massive amounts of area converting waste heat. It's simply not feasible or viable with current technologies. Give me a few years, and maybe I can help improve that.

If you'd like more information about thermoelectrics (or solar!), feel free to let me know. I love talking about it, but I doubt people here want to read much more than this comment. (And, to be honest, it's possible most don't want to read this much!)
posted by JMOZ at 5:29 PM on December 8, 2006 [1 favorite]

In other words, why aren't we doing more to harness waste energy? Are there scientific obstacles? Cost obstacles? Or is it just that no one has really bothered?

Approach the question from an economic angle.

Why?

Because. You. Have. To.

Science is involved, but so is human effort, and human effort involves transfer of wealth in exchange for goods and services.

It is just plain cheaper and easier to burn fossil fuels and release gobs of heat. Because fossil fuels are cheap and easy and has a huge payoff in usable energy right now. For all their inherent badness, fossil fuels are still a helluva bargain.

If said fuels (or any other means of generating power) were more expensive in terms of total cost (extraction, refinement, delivery, deployment, etc), there would be far more incentive (and results) in attempting to capture waste heat.

But it's not so there isn't.
posted by frogan at 6:18 PM on December 8, 2006

Actually, according to John Pi�a Craven, the big problem is not that we don't have enough energy, but that we're running out of cold sinks. For reasons elucidated by beagle and jet_silver, the diffusion of waste energy into the environment is what is becoming problematic, since a world moving towards an entropy equilibrium is going somewhere that doesn't conveniently include 6+ billion humans.

Once the permafrost isn't any more, we probably lose what little control we think we have of the Earth's carbon cycle. That this is happening concurrently with rapid ocean warming is just our bad luck as a species. The world as we know it will not end in our lifetimes, but the cold is going fast, and with it, our hopes as a species. Our great-granchildren or our great-great-grandchildren will surely curse our stupidity and selfishness, and they may be the last to do so.
posted by paulsc at 7:39 PM on December 8, 2006

So if I understand correctly, the reason why waste heat is so hard to harness as a source of energy is because work only occurs when there's a temperature gradient, and since entropy always increases, eventually that gradient disappears. Therefore there's an upper limit on the amount of work we can get out of any energy source based on waste heat. Fossil fuels and the like are much easier to exploit and since extracting energy from waste heat is so difficult, no one's really bothered with it yet.

I'm still trying to wrap my head around this—it's clear that for all the high school physics I studied, I didn't really absorb much beyond kinematics. For some reason I never really paid much attention to entropy, but the explanation of why entropy always increases helps with the question of why it seems impossible to completely reclaim waste heat and convert it back into usable energy.

As to zhivota's comment, I think it's a bit premature for me to go back and get an undergrad physics degree (let alone a PhD) when I'm still mired in student debt from my last degree, but you never know...
posted by chrominance at 10:41 PM on December 8, 2006

We have an energy crisis because we're accustomed to getting our energy from fossil fuels, and they're not as plentiful (or as widely distributed geopolitically) as they used to be.

Herein lies a big long-term problem, it seems to me: fossil fuels are chemical energy, which we convert to kinetic (combustion engine) or electrical (power plant) energy. Yes, much heat is expended during that process, which could potentially be partially reclaimed to generate more energy, but the imbalance is not due to allowing the heat to just radiate out. The imbalance in the system is that we're rapidly converting millions of years of stored-up solar energy (which made the plant and animal matter that died to become those fossil fuels) to do work for us.

Mankind grew up on Earth and inherited the concentrated chemical energy from countless biological organisms in history. The petrochemicals in fossil fuels have a ton of chemical energy, and we're using it to move stuff around. As we convert the hydrocarbon fossil fuels to other energy types, all the carbon that had been so nicely "sunk" into the ground by all those plants and animals is released into the atmosphere again. I think (though I could be wrong) that this is real problem, not just that engines release heat. We've always had hot things on Earth, only now the atmosphere is becoming less able to release energy due to the increase in carbon, so it's getting hotter.

When we use up all the fossil fuels, the only way to get the amount of energy that we currently use will be to go right to the sun with a big solar collector and send the power back to Earth from space.
posted by dammitjim at 11:07 PM on December 8, 2006

Well, there are two separate issues at play here. One, the energy crisis, is a purely economic problem caused bv increasing demand for energy and decreasing supply of fossil fules. Two, climate change, is a consequence of human over-industrialization, pollution, etc, and is not, I believe, what the questioner is asking about. Which one is a bigger problem depends on your perspective.

As to the question - it is most certainly an engineering problem. I wouldn't say that no-one's bothered extracting energy from waste heat. Believe me, anywhere there are engineers, there are people looking for ways to increase power efficiency (if that's what the market demands). For example, hybrid cars use regenerative braking, converting the motion of the car into power for use in the engine and thus slowing it down while recharging the battery. In a conventional car the motion is dissipated purely as heat through the braking system. This is a relatively old idea.

Furthermore some new 'green buildings' used new heating and cooling systems to store energy more efficiently. I read about one in Japan that had some kind of seasonal exchange system whereby snow was collected and stored in the winter, and then used to cool the building in the summer, though I can't find the link. Also in Toronto there is a project to use the cold water from the depths of Lake Ontario to cool buildings in the summer. This falls more under 'sustainable energy' than 'making current processes more efficeint' but it sounds like the sort of thing you had in mind. Look up green buildings and sustainable architecture for more.
posted by PercussivePaul at 12:02 AM on December 9, 2006

Oh, and after re-reading, I thought maybe you were imagining vast heat-recapturing plants. And it turns out that JMOZ actually works on such things, which I didn't even know existed. Neat. (for the record I don't mind if you talk more about them.)

In turns of wrapping your head aroudn the temperature gradient thing - I would think of it as working kind of like water flowing over a dam. If you stick a turbine in the middle of a lake it's not going to spin. but if you stick it inside a waterfall (i.e. a water pressure gradient, where water flows from an area of high pressure to an area of low pressure), you've got energy moving past you that you can recapture. Something like that.

The lake in this analogy is like air - the ambient - where everything is a flat 22 degrees. You can imagine entropy working because as soon as you put something warm outside in the cold it is not too long before all of that heat has escaped into the ambient.

I'm trying to think of good places for a thermoelectric plant. The natural equivalent of a heat waterfall is something like a hot spring or a volcano and that's not very practical. There are many heat gradients on a small scale in industrial settings - in places like manufacturing plants, and office buildings, etc. And on an even smaller scale, the package that holds the chip inside your computer, though very small, has a ridiculous temperature gradient. maybe JMOZ has more.

(note that the turbine is just an analogy. a thermoelectric would work with a completely different mechanism, not to be confused with hot air spinning a turbine, which relies on an air pressure gradient.)
posted by PercussivePaul at 12:25 AM on December 9, 2006

The question, then, is this: We're doing a lot now to make current processes more energy efficient, but what about reclaiming heat energy (and other waste energy, if it exists) and recycling it?

It is called cogeneration.

In Toronto, we use deep lake water to cool some downtown buildings. I have often said, only half joking, that I'd be okay with new nuclear plants so long as they were built downtown where the excess heat could be used to provide steam for heating (NIMBYism really pisses me off!). Also, check out OTEC at that wiki article for another perspective on the question of heat vs. useful work.

And, I can't believe nobody has mentioned the impending heat death of the universe yet! I could swear I've read a Larry Niven essay on the topic, but I can't figure out what it was called..
posted by Chuckles at 3:06 AM on December 9, 2006

Don't worry too much about the overall entropy thing. Ever-increasing entropy is a property of a closed system; as you correctly pointed out in the question, the Earth isn't one of those. It has massive energy flows both inward and outward. Energy supply is not the problem.

Concentrate on the Carnot aspect of entropy, and think about the quality of your energy sources. This is what usually makes energy recovery from waste heat too expensive to bother with: waste heat, though abundant, is typically found at quite low temperatures, meaning that the useful work that can be extracted from it doesn't often justify the cost of the machinery required to do so.

What we like to have, and what waste heat generally isn't, is high-quality forms of energy. These allow for very efficient - which generally means cheap - conversions into other forms of energy. Mechanical energy and electricity are about the highest-quality forms you can have: you can convert mechanical energy to electricity, or the other way around, at better than 95% efficiency.

Controllable mechanical energy is what we need, to get us and our stuff to the places we want it; controllable electrical energy is what we want to run our assorted gadgets. So the trick is to find cheap ways of getting those high-quality forms of energy on tap.

Heat engines need high input temperatures if you're going to get reasonable conversion efficiencies out of them. Fossil fuels are priced according to their extraction cost (relatively low per available kilojoule) rather than their replacement cost (incredibly high), so the strategy of sourcing the controllable energy we need by burning cheap fossil fuels at moderately high temperatures in not-too-outrageously-inefficient heat engines has pretty much dominated for the last century or so.

There is so much capital invested in fossil-fuel-burning infrastructure that people generally consider the fossil fuel inputs to be energy; and now that we're past the peak of global oil production, meaning that fossil fuel prices will trend inexorably upward over time, it's natural to describe the resulting financial pain as an "energy crisis".

The crisis is not about energy itself being in short supply. It's just that energy in convenient and controllable forms is starting to cost more than we're accustomed to.

There are, of course, many other things we could be doing to reduce this pain. If you go and have a poke around on the Rocky Mountain Institute's website, you'll find many useful and practical suggestions.

I have been spouting the RMI "end-use least-cost" line to anybody who will listen for a hell of a long time, and have recently got around to putting my money where my mouth is. I turned off my hot water service's electricity supply last week, having commissioned my shiny new retrofit solar hot water heater. Cutting my household's electricity use in half at one fell swoop felt really good, as does knowing that over the system's expected twenty-year service life it's going to save me at least five thousand dollars. There is no longer any sign of an energy crisis in my shower recess!

The Sun is, of course, the most immediately obvious energy source we have access to. On clear days, we get roughly a kilowatt of radiant energy per Sun-facing square metre, so there's plenty there to use. The trouble with the Sun is that there's often a bunch of stuff (air, clouds, the entire Earth at night-time) between us and it, and we have no way to regulate its output. If we're going to use solar energy effectively, we need ways to store and release it as required.

In the case of my hot water service, that's easy: my end-use energy form is low-temperature heat, so all I have to do is collect it during the day and keep the hot water in an insulated tank so it stays hot until I want to use some. But if the intended end-use is electricity or mechanical energy, the problem is harder.

It comes back to this business about energy quality. The easiest form in which to store solar energy is as heat. Expose something to the sun in the daytime, and it will get hot. If you use concentrating mirrors to increase the inward radiant power per square metre, you can make things very bloody hot indeed - hot enough that conversion to mechanical energy by familiar heat-engine means becomes practical. But the hotter you make something, the harder it is to keep it thermally insulated and the higher your storage losses will be.

Another approach is to do the storage post-conversion. This is the typical approach used in remote-area power systems, which typically have solar photovoltaic panels to convert radiant solar energy to electrical energy, and batteries that convert electrical energy to stored chemical energy and back.

Then again, we could just say: bugger efficiency! Efficiency is overrated! Let's just trap a shitload of solar energy at quite low temperatures, and convert it to electricity at the inevitably piss-poor efficiency that this will involve. As long as our collector can be made really big for really cheap, and our converter isn't too outrageously expensive, we'll get away with it; and we can use cheap and simple ground and water heat banks to keep us running overnight. This is the principle behind Enviromission's proposed solar tower; which I still have shares in, despite the fact they've been saying they'll start building it next year for the last four years :-)

Yet another approach is to deliberately design systems to emit their waste heat at temperatures that are high enough to be useful. This is the principle behind combined-cycle gas-fired generation plants, which burn natural gas in a gas turbine to extract mechanical energy, then use the "waste" heat from the turbine's exhaust gas to run a standard steam turbine such as you might find in a coal-fired generation plant.

And if there's a way to capture and store the waste heat from the steam turbine, to produce (for example) hot water or relatively low-temperature steam for industry process heat or domestic heating: so much the better! It's a lot easier to use low-temperature waste heat for actual heating than it is to try to make more electricity or motion out of it.
posted by flabdablet at 4:31 AM on December 9, 2006