Put Your Waste Heat To Work With a Green Machine
by Lloyd Alter, Toronto 
on 05.29.08
Recently we discussed how It's the efficiency, stupid!- how 56% of the energy created is wasted. Electratherm goes after low-grade heat that is usually wasted with its Green Machine- making electricity from water that is only 200 Degrees F. (96 C) at a cost of under 4 cents a kilowatt-hour. (3 cents per horsepower-hour for those who don't use the metric system).
It is based on the organic Rankine cycle, where a high molecular mass organic fluid (in this case an EPA/Kyoto approved chemical) is vaporized, runs a turbine and then condensed in a closed loop, creating no emissions. How much waste heat is there that could run these things? Probably thousands of industrial and commercial sites. Hook them up to warm water coming from geothermal sites across America and you have both power and heat. Perhaps a bank of solar hot water heaters.
The smallest unit at 30 kW costs $ 81,500, but produces $25,500 of electricity per year at 10 cents per kWh, so it pays for itself pretty fast. This is the kind of thing we talk about when we say go for efficiency and go for the low-hanging fruit- we are just throwing energy away when we could be making it into electricity for next to nothing. ::Electratherm via ::Jetson Green
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I've seen similar machines being used by the US Army.
They use a machine that converts garbage into energy.
Here's a link:
http://earth2tech.com/2008/03/14/military-green-tech-trash-to-power/
Nifty devices. They look to be about 10% efficient, that is the 30kW liquid driven unit takes 1,000,000 btu/hr of heat, which comes out to about 10%. This is good, considering that you are starting with waste heat. The down side, is that you only get to capture 10% of the 56% of the energy that is wasted.
Look at the graph on this page. I think you are looking at the total available waste heat (of which 10% could be recovered if it was all suitable for these units) and comparing against actual energy production of the other bars.
They also use 10 cents/kwHr to frame a pay back. I'm out in coal/nuclear land for electric and industry pays 4 cents/kwHr out here. That tips the equation to "never pays for itself". Places or futures with higher energy costs will be different.
I don't suppose the concept scales down. I'd love to get 40 watts off of a 20F temperature differential.
What I want to know is what does it sound like? Are we looking at a whir, a hum, or the constant pounding of a Stirling engine? How much heat is extracted? I mean does the fluid go in at 200° and come out at 150°? I'd still want to use the heat for a hot shower and an evaporative sewage concept I'm toying with, of course those could just be on a separate loop I guess.
Here in Texas, a big array of solar water heater banks could probably power this thing. The 30kW geny probably provides about 2x-3x my average electrical use. Getting fluid heated up to >200° isn't too hard, but that 100gpm flow rate might be tricky. That's 1.7 gallons per second. It all depends on how much heat is left in the fluid when it leaves the generator. If it's still like 150° then you just have to heat it back up 50° to get it to where the system could use it again. If it chills it down to 90°, then you'd have to pump it up by 110° which would probably be too much for what I have in mind.
Energy isn't created, it can only be transformed.
The first law of thermodynamics: "The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings."
Could they downsize it to work for a semi to run a refrigerator unit on the trailer?
Cars and trucks have huge heat sinks (aka radiator) to dump heat at ~180F, so seems like a sensible application.
I suppose when your talking 80,000 pound truck, downsizing may not matter much.
I just heard back from Electratherm about some of my questions. They say the fluid coming out of the system is about 15-20°F cooler than what it is when it goes in. A solar collector loop could easily heat that back up.
The system has 2 inputs. One feeds the hot fluid/gas. The other one feeds the coolant (liquid or air cooling) which I'd feed with fluid cooled in a geothermal ground loop. That should keep the coolant at about 50-60°F.
As I understand it, the greater the temperature differential between the two fluids, the better the system should work.
They didn't say anything about how much noise the system makes.
I just wish the system wasn't the size of a cargo van.
t isn't the first law of thermodynamics that matters here, it is the second. The reason you can't turn all the heat into electricity (if we could, our power plants would certainly use that ability) is that entropy must increase or remain constant in any transformation, and this sets a strict limit on the maximum efficiency of any power generation that creates electricity from a temperature gradient. Note that other forms of power generation that do not depends on temperature differences, like discharging a battery or photovoltaics, have different maximum efficiencies but they have them nonetheless.
Numerically, the maximum efficiency is 1-T(cold)/T(hot) with temperature in kelvins
200F=366 K
ambient temperature ~300K
Maximum efficiency=1-300/366=18%
So out of 56% of power wasted the most we can recover is 56*.18=11%
The 10% we actually get doesn't seem so bad then, does it?
The footprint is 4x8, the size of a sheet of plywood, and the noise is a loud whirring sound, a combination of the expander and the generator turning. Without a sound dampening box, protecting your ears with plugs is a good idea.