A friend of mine is fortunate to have moved to a very warm and sunny country, and has installed a photovoltaic system on his property. This is grid tied and any excess that he generates flows into the grid. Whilst this is a great system his only choice of electricity suppliers have taken it up on themselves (or between themselves) to now start charging for any excess power that is generated and goes into the grid. They have now changed all the meters and it now seems that the undesired solution to avoid these charges, is to disconnect the photovoltaic system and run it separately. Obviously this would be annoying as he’d like all his excess power to be available to all his appliances in the home, at any time there is any excess power, and for them to use this rather than power from the grid.
In trying to figure a solution we have worked out a way to measure the power flow by monitoring voltage and current at the grid injection point and doing some maths within an mcu (picaxe) to work out if power is being imported or exported using a CT sensor and AC transformer.
The problem lies with how we control the useage of excess power. We have figured out a way of doing it ‘digitally’ where we have excess generation of say 40w we use the mcu to turn on a 40w bulb. If the excess increases we can turn on say more bulbs or a heater etc. So If we had a range of appliances at various wattages 100w, 200w, 400w , 800w etc we could utilise the excess energy by turning on the appliances to match the excess, e.g if we had a 600w excess we could turn on the 400w and 200w device.
This seems a bit of a ham fisted way of doing things, meaning we would be turning things on we didn’t necessarily need and also if we were generating an excess of less than 100w it would go back into the grid. (I realise we could go down to 1w using the same idea) but ideally I’m trying to figure out a more elegant solution. We have looked at using a triac to chop the waveform but it looks like this may upset the smart meters that the utility companies have fitted, and possibly alert them and change the charges to suit. I don’t think we are doing anything wrong as it is only the same as someone reading the power reading and responding by turning something on to prevent power flowing into the grid, we just want to be able automate that process. Therefore the question we have is how to ‘linearly’ utilise the excess power to ensure minimal injection back into the grid? Is there an easy way to do this? Do things such as 1kw voltage controlled variable resistors exist? ;) Ultimately we are trying to waste any excess power as closely as possible until the electric companies change their tariffs to be fair and make sense.