Is minimising your power's CO2 emissions the same as reducing your bills?
So you have some sort of (grid-tied) microgeneration such as solar PV or wind at home (or for a small business for example). Partly to save the planet, and to be part of the solution.
You are already being efficient and cutting out waste, so you may not have a lot of scope for further reducing demand significantly. Maybe you are also being careful to minimise use at times of peak grid demand out of the goodness of your heart or because you have time-of-day metering where the cost goes up to reflect extra system costs (and CO2 intensity).
Is minimising your overall CO2 emissions (for the electricity you use) always the same as minimising the bill that you pay your supplier each month?
And how does that depend on the payment regime you have for import and export, eg net-metering, feed-in tariff, ROCs/RECs (tradeable green power certificates), time-of-day (or half-hourly) metering, etc, etc?
Would it be useful to have a system that netted or shifted your power use and could provide backup in the event of a power cut? And would such a system save CO2 or money or significant disruption if so? The aim is not autarky, simply a little less absolute dependency, and the ability to maximise the CO2-free microgenerated power.
It seems to me that there may even be some perverse incentives that encourage increased CO2 emissions in return for more money (eg when you are paid more for export per kWh than you pay to import).
I want to explore some of these issues as I have time...
If your microgeneration produces more than you use (in a day, say), and especially if you don't have a net metering arrangement, and you get paid less for exported power than imported power, then you may wish to net off your consumption to arrange that you never import, even for spikes of demand higher than your microgeneration output at any moment. This might happen, for example, when your diswasher turns on its heater but a cloud is just passing over your solar PV collectors!
If your microgeneration produces less than you use then you will still have to import at least the difference however well you net, but it might be worth load-shifting, ie moving power demand to times when it is cheaper and/or the CO2 intensity of generation is lower.
And even if you do have net metering and produce more than you use, you might want to shift the time that you export that excess to the grid to be paid more and/or to displace maximum CO2 for your exported kWh.
Given that most domestic-scale microgeneration is non-dispatchable, ie you cannot control when it generates power, netting and load-shifting implies the use of batteries (or other storage), and to complicate matters getting energy in and out of batteries loses ~20%.
The core of the system could be something like SMA's Sunny Backup-System which can effectively act as UPS (Uninterruptable Power Supply) for an entire house if the mains electricity fails.
But some useful modifications might be to only allow the storage batteries to be charged when:
Once the unit is capable of monitoring the direction of flow of power then it can potentially ramp up its output to cover local demand spikes which exceed current on-site generation so as to avoid importing from the grid. If the demand is too high for the unit to cover or the batteries are too low then power will be imported from the grid as usual.
We might want to prevent any import of power during high-cost/high-intensity times (eg winter evenings in the UK) unless the batteries are especially low. This could be driven by a simple timer/calendar. If billing is time-of-day related then blocking imports could be driven by high prices instead (eg higher than average for the last day or week).
This load-shifting is potentially worthwhile for the householder when the price or CO2-intensity difference of imported units is bigger than the storage losses, and if imported power has a lower price per unit than exported power. If a feed-in tariff pays only for exported power, and at a much higher price per unit than imported power, then there are perverse incentives to avoid otherwise beneficial load-shifting. (Note that UK ROCs can be paid regardless of how the energy is used, ie whether consumed on-site or exported, and so doesn't provide perverse incentives in this case.)
The system's high capital cost is likely to prevent such a system being financially worthwhile for a householder unless, for example, there are frequent disruptive power cuts that make the UPS aspect enough.
If exported power to the grid is paid for on a time-of-day basis, then at times of expecially high prices a householder may earn some useful cash by exporting from their battery store to the grid whether their microgeneration is producing or not. This would help stabilise the grid too, and is similar to the various V2G (Vehicle-to-Grid) schemes proposed.
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