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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.

Cheers.

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  • \$\begingroup\$ Your power monitor sounds good and if a switch-mode power-control circuit could be applied between solar panel and the "system" you could have a control loop that lowered the effective voltage from the cells to the point where you draw only a few watts from the power company. As your household load reduces so would the effective output from the solar panel via the switch-mode control. \$\endgroup\$ – Andy aka May 15 '13 at 12:56
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    \$\begingroup\$ Which country is this? I'm familiar with ones where you're paid for extra electricity to the grid; charging you for it is just insane. \$\endgroup\$ – pjc50 May 15 '13 at 13:00
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    \$\begingroup\$ The other approach is to disconnect the panels individually, by means of relays or similar. \$\endgroup\$ – pjc50 May 15 '13 at 13:54
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    \$\begingroup\$ Power in or out should [tm] be easy enough to detect. Using any excess for electric water heating may be useful. If enough hot water - maybe an electrolysis cell to make gas when spare power is available. Or pump water. \$\endgroup\$ – Russell McMahon May 15 '13 at 17:43
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    \$\begingroup\$ Howabout a bitcoin mining farm to use the excess power, and adjust the clock speed to fine tune its power consumption? \$\endgroup\$ – Bobbi Bennett May 15 '13 at 19:11
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Sounds like he has micro inverters. Micro inverters directly attach to each panel and are wired in parallel. They simply turn DC into AC and feed it to the homes electrical system and to the grid depending on load. The smart meter tracks how much power is consumed from the grid vs fed back to the grid (excess). They also do not work in a blackout, they must see grid power to turn on.

How about storing that excess energy in a battery bank? The higher the excess, the more charge current is delivered to the battery (via PWM switching). The picaxe can monitor the excess and control a charge circuit to compensate for the load. Then you can use an off the shelf inverter to power appliances from the battery bank. The benefit of this system is that not only is it simple, but also gives you battery backup for blackout protection. Your friend can also use relays to switch certain appliances between battery and grid depending on the time of day. So during the day excess is dumped to batteries. At night when there is no more excess, relays can switch certain appliances or lighting circuits to battery and when the batteries run low, switch back to grid. Examples of relayed appliances could be: refrigerator, air conditioner, well water pump (if he has), lights, washing machine etc. The same grid->battery charger can also detect cloudy or low PV producing days and keep the battery bank charged in case of a blackout.

The batteries can get expensive but its a simple way to catch the excess and you have blackout protection.

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  • \$\begingroup\$ The two problems we have with a battery bank are firstly the expense, and secondly they don't help when an excess is being generated and they are fully charged. \$\endgroup\$ – Hoppo May 15 '13 at 18:12

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