[Stoves] Energy requirement

Wed Oct 16 18:18:16 CDT 2019

Dear Nikhil

I have attached a file from Piet Visser in QuattroPro for DOS format which I don’t have time to convert. Perhaps someone on the list has a converter of a modern version of QPro.

I do not know exactly what’s inside, other than it does include a list of foods and the energy they absorb chemically when cooked.

I don’t recall where it came from – I think I typed it from a list sent by Piet. BTW He lives in Australia and stays several months of the year around Jakarta with his daughter.  He responds to emails.   I lost track of KK Prasad in the Netherlands.

Regards
Crispin

From: Nikhil Desai <pienergy2008 at gmail.com>
Sent: Wednesday, October 16, 2019 22:37

Crispin:

Do you know if that Eindhoven  list is accessible?

I don't know if my offlist reply got to Paul, or my question to Frans.

In useful energy terms, roughly 200 kJ for 1 liter water is the just the energy to raise the temperature for that water. It doesn't say anything about how long at that or higher/lower temperatures. So it is not very useful.

Nor does it adjust for age/size composition of the eating club, number of meals per day, and ambient temperatures, loss of heat from the pots.

Suppose the quantity is six liters a day and total energy input required is 200 kJ per liter per hour for two hours equivalent. That makes 1.2 MJ per day per "family" or,rounding up for the whole year to hundreds, 0.5 GJ per year per "family".

In my view, this is some kind of "basic" for people who do not have much variety and quantity of food, which are limited by economic factors including the cost of fuel and limitations on time.

I would not call it "average" by any means.

Again, look up some tables Seema Patel or someone else at GACC presented at the 2017/8 Ethos meeting. If I remember correctly, there were computations of useful energy, assuming some efficiencies, and the numbers varied hugely.

Cooking is contextual. Any damned "scientific" computation is mere shorthand for thinking or avoiding measuring everything that would interfere with simplistic theories. Eindhoven folks, whom I never met but read and discussed, gave us some direction on how to think.

I think that capacity to think was lost.

Nikhil

------------------------------------------------------------------------
Nikhil Desai
(US +1) 202 568 5831
Skype: nikhildesai888

On Wed, Oct 16, 2019 at 10:12 AM Crispin Pemberton-Pigott <crispinpigott at outlook.com<mailto:crispinpigott at outlook.com>> wrote:
Dear Paul

There was a lot of work done on this matter at Eindhoven University in the early 80’s. They published a list of foods and the energy requirement to cook them.  It is not obvious but as chemical transformations take place in the food, the temperature drops as the reactions are endothermic. So even then energy is going in, they do not rise in temperature some of the time.

You can take foods to be wet, so using water mass is not far wrong. Cabbage requires one of the greatest inputs of energy to cook.

Regards
Crispin

From: Stoves <stoves-bounces at lists.bioenergylists.org<mailto:stoves-bounces at lists.bioenergylists.org>> On Behalf Of Paul Arveson
Sent: Wednesday, October 16, 2019 6:04
To: stoves at lists.bioenergylists.org<mailto:stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] Energy requirement

Thanks for those who responded to my question, how much energy is required to cook a typical meal?

I need more information.  The way we would calculate the basic energy requirement is:
E = q m dT

Where q = heat capacity of food
m = mass of food
dT = temperature change

Example (in metric units):
Let's assume a meal is 1 kg of water.  = m
Water has a higher heat capacity than solid foods, so water is the worst case.

It takes 4186 J to heat 1 kg of water 1 deg. C. = q

Suppose we want to heat the water from 27 to 77 deg. C, or 50 deg. increase.  = dT

So the energy required is 4186 x 50 deg. = 209,300 Joules.

This is only the energy required to heat the food.   But cookers are not 100% efficient, since some energy goes to heat the pot, the stove parts, etc.  Also there is some heat loss during cooking due to conduction, convection and radiation.

Apparently efficiency is low for most stoves, so most of the energy doesn’t go into cooking the food.  So the “steam cooker” was quoted by Frans as 1.2 MJ, which is 17% efficient (assuming he was heating 1 kg of water, which was not stated).   This in turn drives the fuel requirement.

What then are some typical numbers for cookstove thermal efficiency?

Thanks,
Paul Arveson

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