[Stoves] Dhan Hurley..... TLUD enhancements....Whyaren'tthereany comments, except from what it seems is a Venture Capital Company?

[Kirk H.]

Good answers from both Andrew and Crispin.

Thank You,

Kirk H.

Sent from Mail for Windows 10

From: Crispin Pemberton-Pigott
Sent: Wednesday, June 12, 2019 3:03 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Dhan Hurley..... TLUD enhancements....Whyaren'tthereany comments,except from what it seems is a Venture Capital Company?

Dear Kirk

There are many recent advances in the production of more reliable TEGs. Recent technologies area vailable that will operate at 300C, 340 C, 700C and 1000C.

Efficiencies are now as high as 7%. There is a new technology rumoured to be coming to market in 2 years which is 35% efficient. That will be a paradigm shifter as it competes directly with thermal power stations (except combined cycle gas generators).

One site to purchase from that tends to use only the latest, most reliable types is Thermal Electrics Corporation, also known as TECTEG.  https://tecteg.com/about-us/ 

He shows some water cooled units with 200 Watts output. This interesting thing for small stove builders is the rapid increase in the maximum temperature they can withstand. 

The earlier ones to which Andrew refers were limited by the melting temperature of the solder. That style (often designed for cooling, not power generation) is found in two ranges: about 180 C and about 220 C indicating the different solder used.

It seems the new ones are welded, maybe using lasers. An experimental 700 degree ceramic and stainless steel TEG was demonstrated at ETHOS one year. It was extremely fragile, a ceramic foam that broke easily even if picked up very gently. But it worked. Maybe those have come to fruition.

Regards
Crispin



On Tue, 11 Jun 2019 at 20:03, Kirk H. <gkharris316 at comcast.net> wrote:
>

>
> Your response describes how the Joule Thief works.  Could you continue further to describe how it might work with a thermocouple (or pile) to get electricity from a wood stove, perhaps for an led light or a radio.  This might be useful information for Dhan.
>

Kirk my experience with electronics is nearly 50 years old and field effect transistors were just discovered, these are  the very low resistance devices which enable low loss  circuits which boost or reduce voltage (known as buck or boost circuits as Crispin mentioned).

On the thermoelectric side I would refer you to a post, in the archive at
https://eur02.safelinks.protection.outlook.com/?url=http%3A%2F%2Flists.bioenergylists.org%2Fmailman%2Fprivate%2Fstoves_lists.bioenergylists.org%2F2016-June%2F011717.html&data=02%7C01%7C%7C60c69bbc30284005fd5a08d6ef1794f8%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636959281893620996&sdata=xovIaF1b7xIiGZEvXKKTH6Dk6V6%2FMaW1bB3H3TFqxGg%3D&reserved=0

by Paul Van Sluis who worked on the Phillips stove. This used a TEG at 2% conversion to electricity, the heat being fed back into the stove so at little heat cost.

He points out that semiconductor TEGs have a limited life because they fail  if cycled many times. I only have one semiconductor  TEG device which runs a fan atop my wood stove and that has survived 3 heating seasons  so far but uses a TEG derived from a mass produced cooling chip.

Back to the basics:

The voltage developed by the Seebeck effect "the conversion temperature difference between two dissimilar conductors into a voltage" is proportional to the temperature difference between the hot and cold junctions and is tiny, so in order to work in an electric circuit with transistors the voltage has to be multiplied by stacking a number of these junctions into a thermopile. Once you generate a voltage above a Volt or so then the boost devices, like Joulethief can increase the voltage to what we need.

Metal to metal junctions produce a very small voltage difference per degree of heat  difference between hot and cold side, they are used in flame sensors as they withstand high temperatures but normally have an electrically powered amplifier to operate a solenoid. Honeywell
produce(d) a metal thermopile flame sensor that produced 0.75V and would operate a gas solenoid directly.

Semiconductor thermopiles produce higher voltages but have to be kept below 200C, with my stove top fan this is managed by the base having a bimetallic strip which lifts one side of the hot plate away from the stove top slightly.

That's the voltage side but to do work you need to produce current and this is dependant on the heat flux through the thermopile, for a usable current you need a large thermopile and a lot of heat going from the hot side and being removed from the cold side.

Often we already preheat combustion air in a stove and this also cools the metal sides of a combustion chamber so there need not be a  heat loss involved by using combustion air to cool the cold side.

In his post Philip points out that  you still need to get the stove up to temperature and he talks of a hand cranked generator and reliability issues, he also points out Phillips moved over to a battery powered device charged by solar PV  as being more reliable.

If it weren't for the fact I am so ham-fisted that I fail to make gadgets very well I would have liked to try and make a combustion chamber made entirely from metal to dissimilar metal junctions formed into a large circumferential pile running a 500C temperature difference.

I hope that helps answer your question in part.

Andrew

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