Green conversion of heat to electricity

May 21, 2013 by Cécilia Carron, Ecole Polytechnique Federale de Lausanne
Green conversion of heat to electricity
Credit: 2013 Alain Herzog

Soon, it will be possible to produce electricity from heat over 30 degrees emitted from a waste incinerator, refinery, or data processor. The start-up Osmoblue has just confirmed the feasibility of this new concept.

A large proportion of the energy consumed – between 20% to 50%, according to some studies – is dispersed as heat. Although it is already possible to recycle heat at temperatures over 150 degrees to produce electricity or to heat homes, the rest is simply released into the environment. At a time when companies are forced to be concerned with their , this deficiency must be remedied. The start-up OsmoBlue, based in EPFL's Laboratory of Microsystems, developed a process based on the principle of osmosis to convert heat over 30 degrees into electricity.

Osmosis is a that occurs when the concentration between two solutions separated by a membrane differs, for example between saltwater and freshwater. A stream flows from the less concentrated to the more concentrated solution, which tends to balance the concentrations on each side of the membrane. The of this stream may be converted into by a turbine and an alternator. Heat is again used to separate the fluid into two separate solutions, one of which is more concentrated than the other. It is, therefore, a closed circuit (see image) that does not consume water. Though this concept has attracted significant investment, it has struggled to become a reality due to low yields.

The OsmoBlue technology is advantageous because it can be implemented with any : air, water, gas, etc. The efficiency of the machine is both dependent on temperature and the nature of the hot and cold sources (air, water, gas, or steam). Connected on one side to the heat source and the other to the , modular systems could eventually be installed in existing structures, near the company's .

With a team of seven people, the young entrepreneur has completed a digital laboratory demonstrator and a digital model for evaluating the performance of the product. "For example, it allowed us to estimate that 10 megawatts of heat could produce between 100 and 600 kilowatts of electricity, the consumption of one hundred homes."

It was during a postdoctoral fellowship at Harvard University in the United States that Elodie Dahan had the idea to revisit the method of osmosis. Components have been revised in light of recent advances in materials engineering and microtechnology. A first prototype is currently being manufactured at EPFL. A pilot unit on a larger scale could then be installed in a regional waste incineration company in 2014.

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1 / 5 (4) May 21, 2013
Imagine how this could increase efficiency in an electric vehicle- recharging batteries from heat generated from its motor, transmission, axles, and the overweight driver's seat.
2 / 5 (5) May 21, 2013
100 kw out of 10mw....1% increase....keep trying.
3.2 / 5 (6) May 21, 2013
Imagine how this could increase efficiency in an electric vehicle- recharging batteries from heat generated from its motor

Not much, since these motors are already very efficient. EVs use the on-board stored energy to about 80%. And most of the losses comes from the batteries and not the motor.

This is more useful for largeish powerplants where a percent (or even a tenth of a percent) in gains quickly translates into big bucks which merits the added installation cost/complexity/maintenance of such systems.
2 / 5 (4) May 21, 2013
What about the high temperatures from the vehicle's batteries? Wouldn't it be wise to convert that waste heat while cooling them- critical to their lifespan?
3.5 / 5 (4) May 21, 2013
Since you have to get to a temperature differential of 30 degrees above ambient temperature for this to work...I really doubt it makes much sense in an EV.
While one could say that in winter that would work it's rather advantageous to keep the batteries at temperature during cold days (as too low temperatures reduce battery efficiency even more)

Especially taken into consideration the added weight and complexity of the system - which translates into added purchasing and mainenance costs.

I'm sure they will do a calculation for that, but my gut feeling is that it isn't worth it. Even if it delivers 1% more current EV ranges that'd be 1-2 extra kilometers.

I'm think the avenue Ford is taking with the more efficient AC has a lot more potential in that regard.
2.3 / 5 (3) May 21, 2013
There wouldn't be any cumbersome equipment adding weight or complexity considering that thermoelectric materials today are remarkably simple and efficient :

1 / 5 (1) May 22, 2013
30 degree differential to produce a current? Sounds like it could become ubiquitous.
1 / 5 (1) May 22, 2013
There wouldn't be any cumbersome equipment adding weight

Their setup require a closed liquid system. The amount of weight you can add before killing a 1% increase in range is around 25 pounds (and you only get a 1% increase with optimal conditions for this system. Real life conditions aren't optimal most of the time).

Until I see a prototype in a car under real conditions this doesn't sound very promising for such small scale applications. For powerplants it's all good, though (maybe even for home solar thermal setups, where excess warm water can be usd to make some electricity. But there the power output is likely too low to merit the installation cost).

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