# Pyro-brainiacs set new record with waste-heat-into-electricity study

Californian scientists have come up with a way of converting waste heat from electronics back into electricity with improved efficiency, according to a study in Nature Materials. The system uses a process called pyroelectric energy conversion, which the study shows can take low-quality waste heat (which it might interest you …

1. #### Waste heat below 100oC...?

I assume we're actually talking about a temperature difference of 100oC relative to the environment? (NOTE, the referenced link in the article doesn't state whether it's 100oC absolute temperature, or a delta T)

a 100oC temperature difference for electronics could easily mean it's sat at a toasty 125oC, which an awful lot of electronics won't like (have a look at some component data sheets...). I'd hazard a guess that for this to be usable in real life applications, we need it to work efficiently at a Delta T of approx 60oC.

1. #### Re: Waste heat below 100oC...?

Quite right, has to be a delta T, presumably across the film.

Also: at a few hundreds of nm thick, unless there is a decent heat sink on one side the film will reach a steady state temp somewhere btw the driving side and ambient - probably a lot closer to the source temp, so your delta T will be less than surface temperature to ambient.

There appears to be a requirement for an external electric field. Assuming we are talking about a static field this may not require much energy... But depending on field strength and dielectric breakdown considerations it may drive up the solution size / decrease mass and volumetric efficiency.

The power density numbers and efficiency for the system are given without any real context, but seem very high for this sort of system. My assumption is that the the volumetric efficiency refers to the film itself... And it takes a hell of a lot of micrometer-scale film to make up the cubic centimeter needed to hit the claimed whole number Joule energies (over what time frame?). The easiest way to make an efficiency number look good is to draw your system boundary as tightly as possible around your magic widget and ignore the balance of plant hardware needed to make it run.

Example: Does the efficiency number include the electric field apparatus? Inefficiencies in the power conversion electronics? Does the efficiency number reflect use of a hypothetical infinite heat sink?

What's the thermal conductivity and heat capacity of the film? Other salient material properties one would need to engineer a material and interface it with others? Guess we will have to wait for and buy the paper to see... But:

Old man rant: the final paper in all likelihood will not provide enough information to make an intelligent decision about any of these real-world factors. Early in my career you could design experiments and systems off of published resilts, at least in the more reputable IEEE, Physical Review, Nature, etc. Now almost nothing is disclosed... The intent of a scientific paper is now to serve as an advertising rag for whatever septic think tank is performing the work rather than a vehicle for effective peer review.

2. #### Re: Waste heat below 100oC...?

Which bit of below 100°C do you not understand? In the paper, they say they're exploring ΔT of 10–90K above a room temperature of 25 °C.

There are a lot of issues. But it's doesn't depend on electronics that can make you a brew.

1. #### Re: Waste heat below 100oC...?

Let's just have some fun and throw some maths at it. So let's assume we're working with a high temp of 75 °C giving a 50 °C delta to room temperature. Carnot efficiency is right about 14.4% and this film operates at 19% of Carnot so overall efficiency is 2.7%. We can also use the 526 W/cc figure along with an average thickness of 75 nm for the film to arrive at an area of 13.33 m2 giving us 39.45 W/m2 or roughly 2.46 W for a sheet of A4 paper which is about the same output as a typical 5 V 500 mA USB port. Finally, using our overall efficiency of 2.7% that A4 sheet works out to about 90 W of waste heat needed to provide that 2.46 W of electricity. Not too shabby but what's it going to cost?

1. #### Re: Waste heat below 100oC...?

No clue what it's going to cost, but I'm trying to get my head wrapped around establishing a steady state Delta t of 50DegC across single digit micrometer thicknesses of any real material. Given any finite thermal conductivity that's a neat trick. Guess you will need a vast number of layers in reality; probably not a big deal. Possibly more difficult is the need is the directional reversal of the hot/cold sinks needed for the ferro material. If I've got alternating temps, why not use a Sterling engine ... I saw a presentation years ago on mems-fabbed Stirling. Wildly immature tech, but maybe some day.

1. #### Re: Waste heat below 100oC...?

A few percent of something that draws 20W like a wireless router isn't worth it, but for something significantly more expensive, drawing more power, and requiring active cooling it might be. Like perhaps the CPUs in a server, drawing 130W or more. If they could recover 4W from each chip they could use that to power the fans. If the whole thing draws 30W less then you save on HVAC too so it would save 50 watts per 8 way server.

If you are nearing your density limits of watts per rack or the cooling limit of your datacenter, saving several hundred watts per rack might be worth it even if it costs \$100 per chip / \$1000 per server.

2. #### Re: Waste heat below 100oC...?

I'm pretty sure my overclocked cpu pushes out more than 90w of heat. I mean the heat sink I've got us treated to 180w of cooling and that gets toasty and loud so quite plausible if you ask me (haven't tried measuring it less I scare myself and burn out the chip)

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1. #### Re: Potential Use in Photovoltaic Cells or Other ???

Not sure why you got the downvote there, your questions seem legit to me.

Since this tech is still pre-publication, and the press release raises more questions than answer, I'd say there will be nothing to apply to the back of your PV for a long time. For PV usually a quick win on the materials side is to apply a decent antirelfective coating, might get you another percent effcy, for a price (*). Another thing to look at is new nanomaterial coatings that are less susceptible to loading up with dust and crud than glass or polycarbonate. Again, maybe a percent.

What type of bore hole? If we are talking oil exploration you've got a lot of vibrational energy you can harvest. Temps are very high but downhole it's tough to get a delta T big enough to be interesting.

* As with anything there is a cost/benefit trade-off you've got to work. To include procurement, maintenance, disposal costs...

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1. #### Re: Potential Use in Photovoltaic Cells or Other ???

For home HVAC I'm using geothermal, in a closed loop heat exchanger system. Demonstrated COP is about 3.5 to 4.0. I live close to latitude 40N, in a heavily forested area with significant cloud cover so PV or solar heat concentration is not economical.

The economics of the proposition are neutral; the high cost of this installation will only just be offset by energy cost savings over the 15yr life expectancy of the system. That said I'm insulated from radical shifts in heating fuel prices...

1. #### Re: Potential Use in Photovoltaic Cells or Other ???

That said I'm insulated from radical shifts in heating fuel prices...

You're insulated from radical increases in heating fuel prices. If they drop a lot (unlikely), you've paid out a lot more money than you needed to.

1. #### Re: Potential Use in Photovoltaic Cells or Other ???

Quite right! Our fracking craze in the US has provided quite a glut of natural gas; if you're on the distribution net the prices can be compelling. One has to ask whether the toxic contamination of groundwater is worth it, not to mention our ever increasing carbon footprint.

Personally I think it's nuts for another reason, too - fast forward to a time of greater petroleum scarcity: LNG makes a fairly decent vehicle fuel. Why burn it all up in stationary applications?

1. #### Natural gas vs renewable energy

While natural gas is indeed cheap in the US, wind power is cheaper still - even without subsidies it would be cheaper (and people shouldn't fool themselves into think that natural gas production isn't subsidized, gas & oil tax breaks have just existed a lot longer so the industry likes to cry foul on wind/solar tax breaks and change the subject when their own are mentioned)

This is the reason Trump can't bring back coal no matter what promises he makes - it is more expensive than natural gas, and more expensive than wind. The only reason to mine coal is to burn in existing plants. There probably won't be any new coal plants built in US, and environmentalists won't even have to protest to make that happen - the utilities' beancounters will protest for them!

The only place where new coal plants could even potentially make sense would be in the NE where there isn't much of a natural gas grid and not many areas good for large wind turbines. But more likely they'd just build more power lines to import the growing excess of wind power from the plains states - MidAmerican Energy (owned by Berkshire Hathaway) will be generating 95% of the annual consumption of its customers via wind by the end of next year.

1. #### Re: Natural gas vs renewable energy

> not many areas good for large wind turbines

This is a political fact, not a wind-energy fact. In Massachusetts, politicians passed laws to encourage wind turbines, and the big blades are all over the place (except off the shore, where wind energy is best, due to liberals who suddenly became ulta-conservatives and blocked them). In New Yawk, politicians have not encouraged wind turbines, so there are practically zero. And in New Yawk, they are shutting down the big Nuke plant, and replacing it ... with multiple gas-powered generation plants. And also, New Yawk politicians are also preventing gas fracking, so the gas for those replacement generators will come from out-of-state fracking. Energy in N.E. US has little to do with reality, a great deal to do with politics. 'Nuff said.

1. #### Re: Natural gas vs renewable energy

except off the shore, where wind energy is best, due to liberals who suddenly became ulta-conservatives and blocked them

This isn't a liberal or conservative thing, it is a "I paid big \$\$\$ for my oceanfront property, and I'll be damned if I have to look at a wind turbine a few miles offshore, let the farmers and poor people look at them" thing.

1. #### Re: Natural gas vs renewable energy

"I paid big \$\$\$ for my oceanfront property,"

"and I'll be damned if I have to look at a wind turbine a few miles offshore, let the farmers and poor people look at them" thing."

Oh, so who did you pay for it then, just another toff who didn't need the money, not the poor of farmer, not even the Gov, most likely, and it didn't include the view that's not for sale, neither is subsurface or airspace.

I'd rather look at a bit of life moving around than an almost flat horizon with end to end ocean and nothing in between.

1. #### Re: Natural gas vs renewable energy

I was stating that as a 'what they say' thing. I didn't say I paid a lot for my oceanfront property, as I neither own oceanfront property in Massachusetts nor likely could afford it if I wanted it.

2. #### Re: Potential Use in Photovoltaic Cells or Other ???

" I live close to latitude 40N, in a heavily forested area with significant cloud cover so PV or solar heat concentration is not economical."

Hi Charmian,

(In the new popular beat lingo)

latitude 40n has decent PV potential, you might be surprised. I'm at 53n in a moistened country and my 6kw PV has considerably reduced my bills (also using closed loop geothermal that might have paid for itself by now, except the compressor blew :|). The PV won't pay for itself in 8 years, but should do in 12 (even if we keep having rubbish summers).

Besties,

YM.

1. #### Re: Potential Use in Photovoltaic Cells or Other ???

I think being "heavily forested" is his problem moreso than being 40N.

2. #### Re: Potential Use in Photovoltaic Cells or Other ???

Thanks YM! I think I will have another look at the economics. Built this house in the late 90's and much has probably changed. Now if I could just get the wife to let me clear the treeline...

Vr

CB

2. #### Re: Potential Use in Photovoltaic Cells or Other ???

Would it therefore be possible to add this layer to the rear of the solar cell

In theory yes, but the temperature differential is the challenge - if you modify the panel to emphasise the temperature gradient, the panel runs hotter, which generally speaking shortens the life. The lower the temperature gradient, the less efficient the heat-to-electricity conversion is.

Sadly low grade heat remains a very difficult to use resource. There's plenty of it (eg a coal power station like Ratcliffe on Soar can waste about 8 TWh of low grade heat annually, enough to meet the entire heat demands of a large city), but recovering it is expensive, and unless converted to easy-to-use high grade heat, or electricity then it is also expensive to use.

As the article notes, heat-to-electricity has been a known technology for many years - problem is that it doesn't scale well due to the capex needs, and the efficiency is generally low, so you're still not recovering much of the waste heat.

1. #### Re: Potential Use in Photovoltaic Cells or Other ???

In some countries like Denmark such low grade waste heat is recovered for use in homes for instance for heating hot water tanks or room heating. On a more industrial scale you can heat hot houses in winter (in summer there is enough heat in sun light anyway) or heat swimming pools. One research institute used waste heat to heat a nice outdoor swimming pool.

1. #### Re: Potential Use in Photovoltaic Cells or Other ???

@AC- totally sensible approach! I especially like the pool idea.

On a little larger scale I know that at least in the City of Baltimore in the late 70s and early 80s, waste heat from a massive waste incinerator was used for district heating. I thought it was really neat at the time ... Use waste from a waste stream for something useful.

3. #### Geting there

The biggest untapped energy reserve on Earth is the tropical thermocline. For that you need to be able to do 15-25 degree difference.

While this is still way off, it is a step in the right direction :)

1. #### Re: Geting there

>The biggest untapped energy reserve on Earth is the tropical thermocline. For that you need to be able to do 15-25 degree difference.

It is a hard problem with a huge energy budget with a relatively small output. Vast amounts are used to pump sea water from different depths and to boil the water at reduced pressure. The net energy output is tiny in comparison. Also boiling sea water is rather corrosive. Using heat exchangers rather than seawater directly will reduce efficiency which is already small.

One important advantage is that you can extract a lot of freshwater.

4. According to a paper on pyroelectric relaxors I found, you get the energy not by applying a thermal gradient to the relaxor element but by alternately heating and cooling it.

1. You could probably set up some system to do this. In my mind it works similarly to the piston in a steam engine with the pyroelectric* material around the cylinder.

*Man, that word could do with removing the first "e". Pyrolectric rolls off the tounge so much more smoothly.

1. What's a tounge?

2. so set the cooling fan to blow for 5 seconds stop for for 5 seconds blow for 5 seconds....

5. #### Excellent!

Multigeneration space travel running on energy scraps is nearer!

6. #### So, next step, perpetual motion!

Or possibly Nuclear Fusion, real soon now!

1. #### Re: So, next step, perpetual motion!

Economically viable Nuclear Fusion will arrive about 5 years after IPv4 depletion!

7. #### Terminology

What is a 'scientist'? Is it a sort of Boffin?

;o)

(I know, I shouldn't object to the use of new-fangled terminology to describe people, or the evolution of language.)

8. #### Wonderful

And even though they are still developing this idea now I'm sure we can recover more energy from waste heat, more effectively than we have been, prices of this tech will drop as it develops and is utilised.

Just amazing what they were understanding over 2000 years ago, we miss the import of this.

i.e. the reason Archimedes had to discover displacement as a solution for the "is that all Gold" problem is due to the fact they were electroplating gold onto copper back then, and substituting electroplated gold statues for full gold statues.

Electroplating and batteries have been found in Nineveh around 600BC.

9. So basically you could make a heat sink that produces some extra leccy? Cool.

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