Project electric cooking for rural zones

Hi David,

Let's say that all of they typical answers to the cooking question are not available (no local wood, fossil fuels not available, etc.). And that a mid day solar oven is either not a good fit (local foods are not oven based) and/or that this does not address the morning/evening cooking/heating of food to match the local workday.

Next, assume that the location is pretty sunny (3 or more hours of noontime equivalent sun per day. Typically closer to the equator/desert regions vs towards the poles/marine environments (like North East, UK).

And since time shifting of heating energy is needed, battery based system required (more or less, adding batteries will increase the cost of the system by 4x or more for batteries, charge controllers, etc.).

Li Ion vs Lead Acid batteries... LiFePO4 batteries are much lighter, more efficient, no maintenance, in some ways more forgiving than Lead Acid (and other ways, much more sensitive to poor operation) and less environmentally hazardous (if there is no local recycling of lead). However, they are 2-4x more expensive per kWH (at least) of storage vs Lead Acid. Li Ion batteries are not very recyclable (going to end up in the local dump). Lead Acid batteries are cheap, heavy (not a problem for a fixed home typically), Flooded Cell batteries need distilled/or very clean rain water (anything else will "poison the cells"). Lead Acid are highly recyclable (assuming local infrastructure available), and can be locally made (low tech manufacturing, although lead is not an environmentally friendly element).

Next, will do a relatively conservative design based on 2.5 kWH per day energy harvest, and 2,000 Watt peak load. This is probably 2x the battery required because conservative design... Make your own adjustments as needed.

Loads drive the battery bank size...:

• 2,500 WH per day * 1/0.85 AC inverter eff * 1/24 volts * 2 days of stored energy * 1/0.50 max discharge for longer battery life = 490 AH @ 24 volt battery bank

Just to give you an idea, using 6 volt @ 220 AH "golf cart" batteries (pretty cheap/forgiving)... 4 batteries in series (for 24 volts) times 2 parallel strings (220AH * 2) gives you an 8x battery bank of 24 volts # 440 AH.

Note that the battery bank also has to be sized for the maximum power draw... Typically that is ~500 Watts per 100 AH @ 24 volts, or:

• 490 AH * 500 Watts * 1/100 AH (at 24 volts) = 2,450 Watts max continuous load (for FLA batteries)

So that will support your peak load requirement.

Then sizing the solar array... Two calculations are needed. One based on the size of the battery bank (larger battery bank needs more charging current). And the second will be based on energy per day needed and hours of sun per day.

For charging lead acid batteries, 10%-13% minimum suggested for a full time off grid system:

• 490 AH * 29.0 volts charging * 1/0.77 solar panel+charge controller derating * 0.010 rate of charge = 1,845 Watt array nominal
• 490 AH * 29.0 volts charging * 1/0.77 solar panel+charge controller derating * 0.013 rate of charge = 2,399 Watt array typical "cost effective" maximum

And sizing on hours of sun per day.. Assuming a fixed array tilted to latitude, 3 hours of sun per day in Winter is a pretty sunny location... You can use this link to explore your location:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Madrid, Spain
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.43	4.19	5.27	5.25	5.46	6.15
Jul	Aug	Sep	Oct	Nov	Dec
6.40	6.26	5.59	4.22	3.35	2.98

So, array sizing based on loads:

• 2,500 WH per day * 1/0.52 off grid AC system eff (lead acid, AC inverter) * 1/3.0 hours per day sun (winter) = 1,603 Watt array "break even" December

Now, several choices here... Reduce amount of energy used during bad weather in Winter, or backup genset/backup cooking fuel, or increase the size of the solar array... Generally, for "base loads", you should only use around 50% to 65% of predicted output (base loads--These that must run every day, such as a refrigerator). So, upwards of an array 2x larger for generator free winter operation would make sense (1,603 WH * 1/0.50 base load fudge factor=3,206 Watt array).

And there are always the question of who will be running the system... The person who bought the system (you for your home in Spain), vs a NGO donation to a poor village elsewhere... The larger the battery bank and solar array, the less day to day work needed to balance energy used vs amount of sun available...

You paying for system, you will watch like a hawk--It is very easy to "murder" a battery bank by undercharging/over discharging. In 3rd world situation, the user will simply use all of the power they can until the system "goes dark"--And start again the next day--A good way to "murder" most battery banks/systems.

For larger systems (this is a larger system), MPPT system are usually chosen. Generally, they are more flexible (matching solar array to battery bank more efficiently). Also, the higher working voltage of the array allows the array to be placed 10s to several hundred meters away from the home/battery bank. PWM system are typically smaller, use more expensive solar panels, and array has to be right next to the battery bank.

I will stop here and let you review and ask more questions. The above is a very conservative/workable system... But not the cheapest solution.

Using resistive heaters vs something like an induction burner is interesting--But Induction burners are not really a good 3rd world solution. AC inverters run resistive heating pretty well. If you could find 24 or 48 VDC electric pots/pans for cooking--Would be nice, but not that common. Many other issues for choosing higher voltage AC vs DC battery power too... Even adding the cost/complexity of an AC inverter.

-Bill