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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Hi guys..
I just had an interesting idea regarding energy recycling.
It occurs to me that a "useable" bath is between 55 and 70 Celsius (depending on tolerance) but once it drops too far the heat is wasted.
My proposed device includes a Peltier module, a suitable heat exchanger on the "hot" side, a heatsink and fan on the "cold" side, a peristaltic pump and a suitable length of aquarium tubing.
i estimate the energy salvaged on each "cycle" to be well above the energy used to run the fan and pump.
is this any use to anyone for charging batteries or something?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It's quite common to use the heat in wastewater like this, in low-energy building design.
It's either used to preheat incoming water destined for the hot water system, or as fodder for the cold side of a heat pump.
One simple trick is to make a heat exchanger that passes the warm water from the shower drain over the cold water pipe feeding your shower. You get a hotter shower for the same energy cost.
As for upgrading it to electrical energy: If you look at the Carnot efficiency of a heat engine running between 40 degrees and 20 degrees, it's not good. And Peltiers don't get anywhere near this theoretical bound. The question you have to ask is how long the salvaged energy will take to pay the cost of the Peltier module and associated electronics.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I have a pretty nice peltier that can manage 25% of theoretical maximum efficiency. If we assume that the maximum efficiency you can get from a sensible bath temperature is 8% then we take 25% of that and end up at 2% efficiency then we have to subtract numerous losses and it seems impossible to reach more than 1% efficiency using a peltier.
So making some random assumptions you might neeed to charge 100 000 AA batteries to get your money back. Except that if you factor in the risk of water leaks and other problems you will always end up paying more than you gain.
The idea of a simple heat exhanger on a shower is much more sensible since the efficiency will be enormous in comparison. It will be a bit more complicated on a bathtub.
There are a few people that live where electric power is not available that use peltiers on wood burning stoves to charge batteries, in that case the efficiency is a lot higher and there are few other reliable sources so it can be competetive.
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Energy recycler idea
