Slightly different question this time – not so much about investigating, but about coming up with some ideas. Because, now that solar energy is being collected here at JeeLabs and winter is over, there’s a fairly obvious pattern appearing:
Surplus solar energy during the day, but none in the evenings and at night for cooking + lighting (it looks like the heater is still kicking in at the end of the day, BTW).
This particular example shows that the amount of surplus energy would be more or less what’s needed in the evening – if only there were a way to store this energy for 6 hours…
Looking at some counters over that same period, I can see that the amount of energy is about 2.5 kWh. The challenge is to store this amount of energy locally. Some thoughts:
- A 12 V lead-acid battery could be used, with 2.5 kWh corresponding to some 208 Ah.
- But that’s a lower bound: let’s assume 90% conversion efficiency in both directions, i.e. 81% for charge + discharge (i.e. 19% losses) – we’ll now need a 257 Ah battery.
- But the lifetime of lead-acid batteries is only good if you don’t discharge them too far. So-called deep cycle batteries are designed specifically for cases like these, where the charge/discharge is going to happen day in day out. To use them optimally, you shouldn’t discharge them over 50%, so we’ll need a battery twice as large: 514 Ah.
Let’s see… three of these 12V 230 Ah units could easily do the job:
Note that the cost of the batteries alone will be €2,000 and their total weight 200 kg!
There’s an interesting article about the energy shortage after the Fukushima disaster, including a good diagram about a somewhat similar issue (lowering evening peak use):
Although driven by a much harsher reality in that article, I wouldn’t be surprised to see new “one-day storage” solutions come out of all this, usable in the rest of the world as well.
For winter-time, I suppose one could heat up a large water tank, and then re-use it for heating in the evening. Except, ehm, that there’s a lot less surplus energy in winter.
Are there any other viable “semi off-grid” options out there? A flywheel in the basement?
PS. New milestone reached yesterday: total solar production so far has caught up with the consumption here at JeeLabs during that same period (since end October, that is).