Generating electricity out of waste heat.

EDC Gear for Flashlight Carriers USB Power Devices
1

I’m not even sure what the name is of what I’m looking for. I think it’s called a TEC.

A a way to have something on my hands during the lockdown I’ve started researching preserving energy when cooking.
Haybox cooking, rocket stove, that sort of stuff.

When cooking there’s always heat wasted.
I’ve read about the possibility to turn waste heat into electricity.

I know virtually nothing about this technology so this will be a Q&A thread.

How much power can expect to generate?
Talking about heat of boiling water or warmer

I’ve read it requires a hot and cold side. How do I provide for the cold side?

2

The efficiencies of these are pretty low.

In colder climates the heat helps keep your house/apartment warm.
In hot climates it adds to the AC load.

3

Here you have an example for camping. : )

4

Not worth the effort.

5

I’d have thought a combination of induction hob and haybox style cooking would be most efficient.

To get a good delta of temperature, seems like DIY thermoelectric generator people use ice (which, obviously takes electricity to make….)

Air to air heat exchange for a kitchen extractor fan would likely save more energy if you were trying to keep your kitchen warm.

Edit to add: Perhaps a pressure cooker would also help you save energy.

6

You would be way better off finding a way to reuse the waste heat than trying to generate electricity with it. For example, find a way to use the exhaust from the rocket stove to preheat the combustion air coming in. As for household cooking, induction cooktops produce way less waste heat than gas or conventional electric. Pressure cookers cook the food much faster, thereby saving a lot of energy.

7

A pressure cook is on my list :slight_smile:

I’ve nothing to measure the temp with but the pot contains boiling water. But through the skirt of the pot there’s even more heat going up from the fire.
So the heat difference is there for sure.
Sinking the heat into the colder temperature may be the bigger challenge.

But the question that should be asked before all other questions, what’s the theoretical yield?
If I have to burn a cord of wood to charge a 18650, then I’ll instantly abandon the idea :slight_smile:

8

Wikipedia says 5-8%…

If I were you, I’d save the money you want to spend on a TEC and put it towards a pressure cooker or thermal cooking pot…

Edit to add: I did some wild calculations based on some numbers from wikipedia/google…

Wikipedia gas stove outputs: Medium output burner is 5000 - 10,000 BTU, so let’s assume 7,500 BTU. You run the stove for an hour cooking a delicious stew, 7500 BTU/hr is 2200Wh…

Gas stove/hob efficiency is hard to find, I found following numbers touted: 32, 33, 35, 38, 40. efficient, so, average 37, means you’re losing 1386W

Efficiency of TEC: (midpoint of number above) 6.5%… so even if you captured ALL of that energy, you’d have 90W…

I hope my very rough calculations are correct!

9

How about use the excess heat to run a Stirling engine.

Then put a small permanent magnet motor (either brushed or brushless 3-phase) on the output shaft and back-drive it as a generator.

The generator voltage will be proportional to the speed (use some small gears if desired to raise the rpm).

Collect this energy by charging up a large capacitor or lead acid battery.

Depending upon the voltage level use either a buck or boost converter to regulate the output and charge the 18650.

10

Too bad it’s not more efficient. If it were you could mount one to the back of the shelf on a flashlight to recycle some of the waste heat from the LED.

11

Best most efficient path — don’t generate heat waste. Find ways to mitigate it. If you’re going to generate heat, try to use it for its best facility, which is warming up the environment.

If you can’t avoid it, there are some very simplistic things you can do, if you really want to squeeze the max potential. Some use heat sensitive fans, to help with air circulation.

The Japanese developed thermal recharging for watches. Citizen either invented it or utilized a licensed concept. A number of watch models they created mostly for the JDM only, used the heat from your wrist to help recharge a secondary battery.

Back in 2014, this was published: Stanford, MIT scientists find new way to harness waste heat. I’ve no idea whatever came of it…

How Stuff Works: Is it possible to generate electricity directly from heat?

This is an interesting approach:

12

In industry the recovery of excess heat from refining and chemical processing is called 'waste heat recovery' and is accomplished with heat exchanger(s) or what is called 'waste heat boilers'. Google will be your friend on this subject reclaiming waste heat and the many methods of implementation.

Peace.

13

So for maximum efficiency we should attach a small boiler to the back of the shelf in our flashlights. Then use the boiling water to power a small turbine generator to recharge the light while running on turbo. :disappointed:

14

If you can find a way to make it economically feasible you'll be a millionaire. Good luck.

In industry we never tried to stifle creativity..... but we always had to do an economical analysis to justify any expense in hardware.

15

My induction burner ($50 from Ikea) uses 800W to keep water boiling. If you are making a large meal then 1000-1200W (in my experience larger cookware needs a bit more power to do the same thing).

The most cost effective way to generate power on a small scale is a solar panel.
That said if this is a hobby do buy a small thermoelectric device to play with. Like flashlights it can be fun. It does not have to be hyper efficient.

However you can buy a 50-100W solar panel and a small 100 or 200Wh or more Jackery or Renogy battery that accepts solar and have yourself a tiny power station that can charge phones and run small appliances (laptops, fans, LED light bulbs, cassette players, etc.)
I have one of these in my thread, second pic, 100Wh

:smiley:

While i completely agree with you in a business context in a personal one i think its a good idea to explore. Not just to know what doesn’t work but stifling creativity means no innovation.

I was where the OP is many years ago, while in retrospect my first post in this thread was more cynical than necessary i now support the OP trying it out if they wish becasue i have learned by experience many times and not by people giving their opinions (even though its correct that its not efficient enough today).

By experience i learned a lot. It of course took a lot more than a thermoelectric gizmo to learn all the lessons, it was a piece of my journey. Today i have expertise in renewable energy.