Energy from Baking Bread?

Humble looking and a low cost wood fired flat bread ovens like Tandoor ovens are a good source of waste heat energy. Tandoor is used for cooking in developing countries like Azerbaijan, India, Turkey, Iran, Armenia, Pakistan, Uzbekistan, Afghanistan, the Balkans, the Middle East, Central Asia as well as Burma and Bangladesh etc. An affordable source of electricity from heat – even if it is not of very high power – can make a major difference in the quality of the life of the people of these countries.
This project was submitted as part of a NTU IEEE symposium on 8th May 2011 and it aims to harvest waste heat energy from a commonly used Tandoor oven by converting it into electricity using a low cost thermoelectric converting device. The well known Seebeck-effect based thermopiles are used for harnessing the waste heat from the oven and storing it a rechargeable battery. Ceramic based custom built heat spreaders and ceramic wool insulation etc ensure adequate temperature temperature difference at required area without using an external power source.

1. Naan bread being baked on side of tandoor oven with flame.

Waste heat harvesting or converting heat into usable electrical energy is not an
entirely new concept. There are numerous commercially available products that can
convert heat into electricity at various converting efficiencies. However, based on my
limited research knowledge these products may not provide an energy solution to a
common man in a developing country. The heat for a Tandoor is traditionally generated
by a charcoal fire or wood fire, burning within the Tandoor itself. The oldest examples of a Tandoor were found in the settlements of the ancient
Indus Valley Civilization.

2. K Type thermopile

My venture to convert an ordinary Tandoor oven to one which can generate electricity began with finding suitable materials and technology. After some research, the prime thermocouple candidate turned out to be the K type (we can see the construction on the left image 2.), as it gives a steady voltage over 750 °C. Next to prevent the heat from escaping from inner earthen walls of the Tandoor, we need to use a high temperature ceramic insulation material such as mineral wool or calcium silicate wool or Glass wool. Lastly, we need a thermally conductive layer made up of Aluminium Nitride powder or Boron Nitride powder with an aluminium casing and a high temperature adhesive to hold them together. This layer is to disperse the heat carried by the thermocouple to the surroundings and create a temperature difference. We may also need a DC-to-DC converter for the output to be converted into a suitable level of voltage for the rechargeable battery.

Thermopile can be constructed by connecting each component of the thermocouple
wire in series alternatively, forming a circular loop as shown in the figure 2. When we
form a loop of a series of Chromel and Alumel wires we end up with a positive terminal
(Chromel) and a negative terminal (Alumel). For this to happen we should have equal
number of Chromel and Alumel wires. We will need two such loops consisting of five
hundred junctions each to generate about 17W of power [Table 1] (enough to power a bulb). Normal Tandoors have up to 1.5 meter circumference, so it can easily fit thermopile loops of five hundred junctions. These two loops will be connected in series and the positive and
negative ends will be connected to socket. The hot junction will be introduced to to the
burning ember and cold junction will be inserted into a small hole drilled into the thick
Aluminum casing containing a patch of high temperature adhesive. The overall construction is depicted in the figure 3. This construction will help create a temperature deference of up to 350 °C between the outer and inner wall of the Tandoor. Based on Seebeck principle, this temperature difference will induce a current in the thermopile.

3. An artistic representation of Tandoor thermal power generator from 2011
table 1 : Calculation showing the expected output from a Tandoor thermoelectric generator

In developing countries the Tandoor restaurants are often found in remote locations along the major roads that crisscross the country. They are often humble in nature but provides cost effective and nutritious food for travelers. Often they have an open air environment that do not require much illumination etc. If each of the restaurants has a simple thermal Tandoor power generating unit it allows them to set up a restaurant even in road side locations where no traditional power source is available. For example ‘Dabas‘ are one of most popular highway diners in developing countries.

This concept can be adapted to generate power from various other heat sources. In such cases a thermocouple based thermopile could be replaced with a higher efficiency semiconductor technology based thermoelectric power generating device. Most of the such devices can operate upto 200°C. Thermocouple The following sources can be in cooperated:-
Industrial furnace : An industrial furnace or direct fired heater, is an equipment used to provide heat for a process or can serve as reactor which provides heats of reaction. Furnace designs vary as
to its function, heating duty, type of fuel and method of introducing combustion air. A large potential difference can be created inside a furnace due to a large heat source and the power produced from it will be a large amount. But the materials used , should be able to withstand the drastic temperature.
On the base of room temperature deference : In Arctic and Antarctic regions the deference between the temperature inside the house/tents and outside, can be used to generate power .The hot junction 17/20 of the thermocouple can be placed near the fire place or heaters the cold junction can be extended to the outside of the house where the temperature is below 0°C.
Hot water springs: Naturally available hot water springs can be used for generating power. Such a power can be used for proving simple source of power at tourist attraction spots.
Exothermic reactions: Waste heat from exothermic reactions can be used to generate electricity.

If you wish to read more about this project, please click on the download link below.

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