RE: Additional opportunities with Eskom30 Jun 2019 13:31
Good spot Nick,
whilst the document states :-
The current system consists of the following: -
31.kW PV array
3x12kW 3-phase inverters
90kWh lithium battery storage
etc
The document goes on to state:-
The supplier will provide one or more of the following services and products:
..
Storage technology (e.g. supercapacitors, flow batteries)
..
They are clearly opening the door for VRFBs. 90kWh is a useful size battery. Clearly it is a multiple hour battery if it is only coupled with a 31.8kW peak PV array. I'm guessing 4 hours.
For a 14 house community with the quoted average of 1.5KWp per house that would suggest an average load of c. 21kW. A 25kW/100kWh VRFB could provide this for 4 hours, but not all day and all night so we need to think again if this average of 1.5kW per house is really the average over 24 hours.
Backing up .... 1.5kW x 24 hours would be 36kWh per day, or 13,140 kWh per year. As a average household consumption in the UK is only 3,100 kWh per year ( https://smarterbusiness.co.uk/average-gas-electricity-usage-uk/ ) we have to question whether this 'average' value is really the average over 24 hours or just the 'average during the most important hours'.
It is of course possible that the electrical consumption of these 'houses' is much higher than in the UK, if they have significant air-conditioning systems and maybe heat their water just by an electrical immersion heater, however I suspect that this would make these houses rather too top end of Eskom to be working quite so hard for. Instead I am guessing that this use of the term 'average' is not over the full 24 hours and the household consumption is perhaps only 1/4 of the 36kWh per day - i.e. 9kWh per day 3,285 per year, making it similar to the UK average electrical consumption.
9kWh per house x 14 houses would be a total of 126kWh per day - about 1.5 times the overall battery capacity. A full battery (3 hours to charge) plus 2 hours of PV power generation is enough to last a day - thus if you have more than 5 hours of PV a day you can last the whole day.
We don't really have enough information to clearly establish if this is a fully isolated micro-grid, or one which has a backup connection to the main grid though I would suspect the latter.
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