Peltier heaters and coolers

[R791945]

I have a 30 litre cooler box. It is not highly insulated. It is the type with a handle and thick plastic walls for carrying food for a picnic. I want to use it for bean fermentation. This means that the temperature has to be raised to 40C at the start and then after 20 hours gradually reduced over the last 4 hrs down to 10C.

I have an ultrasonic mister which I hope will increase the humidity to 85-90% for the first 5 hours and then gradually bring it down to 55% by hour 24. To reduce the humidity I will have an extractor fan in the lid and in taking out the humidity I will extract heat, and so the heat/ cooling will need to be that bit more than without any need to reduce the humidity. I assume it will be necessary to over specify the heating / cooling power of the peltiers.

Does anyone know how to calculate how many peltier elements I will need? Guess work is allowed! I am in the UK and so I assume that the highest ambient temperature would be 30c.

 

[CCurmudgeon]

I thought at first this would be easy to research, but I was wrong. Physics is a mean master.
First you have to know the specific heat of beans( water is about 4200 J/(kgK).
How big are they? What is the packing density (how much air space between beans in a pile)?
How many kg of beans?
What is the specific heat of the interior plastic wall material? How many kg of it?
Answer these and you have a chance of figuring out how many watts of heating/cooling you need to achieve the bean temps needed in a good amount of time.

Peltier modules are rated in watts of heat transfer possible, assuming you have infinite thermal sinks on both sides. This is never the case so now it's necessary to guess the efficiency of your heat sinks that transfer the heat into/from the air.

You know what? Much easier to build a small prototype and empirically test and measure, then try scaling to the full size version.

Anyway, that's my guess. Good luck!