The electricity grid where I live is well maintained; grid outages are rare and when they do happen they are usually brief. But like everybody we have a freezer and a central heating pump so the consequences of a power failure are not without reckoning: the possibility of food lost or the inability to keep warm in the middle of winter.
A more subtle pressure to consider a particular type of back-up comes from having a Powerspout contributing to house electricity 24 hours a day; though the turbine will remain operational during an outage, it cannot deliver its output to the house unless its inverter is working, and when the grid goes down, the rules are that a grid connected inverter must stop working; how frustrating is that ! - to have hydro power available and yet not be able to make use of it !
So the subtle pressure is to opt for a particular type of back-up which would allow the Powerspout to continue supplying the house.
The differing factors to be considered make the decision a delicate matter; it's delicate because the added expense and complexity are considerable for a system that would see the Powerspout continue to supply the house.
As often happens, the factors which sway a decision are in the detail; not all grid outage back-up systems are the same and household electricity safety requirements rule out some configurations which might otherwise be possibilities.
So what are the details of the different options, and what operational technicalities have to be thought about?
Back-up by way of a secure outlet.
This is the simplest option because it doesn't require the house distribution system to be islanded (ie: isolated) from the utility grid, - but in not requiring islanding the option doesn't allow the Powerspout to supply the house in an outage; it is also simple in that it is not automated and would require manual switching, both for switching on and off.
The way it works is that power is supplied from the battery, via the inverter, to a single socket which is not a socket like any others in the house distribution system. It stands alone outside from any house ring main and has no connection to the house fuse board (consumer unit).
Into this socket, either directly or via an extension cable, can be plugged those loads which are deemed essential. The maximum power that can be drawn is limited; for SMA inverters it is 3.68 kW. How long power will be supplied depends on the size and state of charge of the battery, as well as the size of the load connected.
The position where the secure outlet is located is clearly of importance in order that loads deemed to need power can be connected to it.
Manufacturers of battery charger / inverters offering a secure outlet include SMA in their SunnyBoy Storage 3.7/5.0/6.0 range.
Back-up by way of islanding.
This is the option which is the attractive one because it allows the Powerspout to continue to contribute during an outage.
To island a house means to completely separate it from the utility grid; in a typical house supplied by single phase power, this entails interrupting both the live and neutral conductors where they first come into the house; the isolation has to take place between the utility company's meter and the house consumer unit.
For safety reasons, because the neutral conductor is always made to be at zero potential (or at least near zero) by being earthed outside of the house by an earth connection constructed by the electricity company, interruption of the neutral as it enters the house will mean the neutral conductor loses its earth connection and will need to be re-connected to earth on the house side of the dis-connect; this has to happen at the same moment it is dis-connected from its outside earthing and at the same moment the live conductor is disconnected.
The new earthing of the neutral can be to the earth rod the house will already have, or it can be to a new and separate earth rod. Details about this can be found in this very informative on-line document.
The switching necessary to accomplish everything is achieved easily enough, and automatically, using relay contacts housed in a secure box, and it is best if this is sited near the point where the utility supply first comes to the house.
Many manufacturers offer this type of hardware, for example Tesla, Sonnen and SMA.
Whether power from the battery supports the whole house or only some circuits within the house is a question which can only be answered by estimating the load for each of these two options and weighing that against the ability of the battery and inverter to meet it;
Battery inverters are not all equal in how much power they can output, and not all batteries are equal in the maximum power they can deliver, so careful consideration needs to be given to ensure that battery and inverter have matching power output ratings sufficient to meet the load of either the whole house, if that is chosen, or just selected circuits such as lights / freezer / central heating pump.
A point of detail: "What would happen if..."
Attention to the detail of how my system would operate if I choose the option of 'islanding' and continuing to have the Powerspout contribute to the islanded grid is the question "what would happen if too much power is being generated into the islanded grid"; it is a question to which as yet I'm not sure of an answer !
The situation could arise (a worst case scenario which might never actually arise but which needs to be catered for) where there is a grid outage, the house is islanded and being powered by the battery, nobody is at home so house consumption is very low, say < 300 W, the state of charge of the battery is 100% and so can receive no further energy input, and yet both solar and hydro 'generators' are feeding into the islanded grid their outputs, of perhaps over 2 kW; a surplus of 1700 W exists in the system.
Where does the surplus power go ? what happens to ac voltage and frequency in a situation like this ? what can be done to manage the situation ?
These are all questions which still need answers and solutions; until I find them, for me back-up by secure outlet rather than islanding may prove to be the safest, maybe even the only way to go.
Any suggestions are welcome !
