Desirable but not simple

Feeding power back into the grid

Desirable but not simple

Postby turning green on Fri May 08, 2009 6:37 pm

Most people would respond that "Reverse Metering" is good and I'd go along with that view. I also decided that it was theoretically not so difficult and I was happy with my point of view until someone inserted a bit of reality.

It is being done so the mechanics of the problem are known. There is a lot of scope to get it wrong but that is eliminated in the USA (for instance) by having to get the system inspected rigorously by inspectors before commissioning. I know it involves pure sine wave inverting. Go to any site selling inverters and compare modified sine wave inverters to pure sine wave inverters and you'll see that pure sine wave inverters are at least 3 times more expensive and that is before the synchronizing problem is even tackled. At the level of small turbines of less than 1 KWatt I would think it would be virtually impossible to make it worthwhile from anybody's standpoint ie the utility or the private generator.

However, and this surprised me, the mechanics of the process are not the biggest problem. There are three problems that I can remember - there are more.

The first concerns grid maintenance. In the conventional system that excludes reverse metering when a switch is thrown everything below that switch is isolated and can be worked on. With reverse metering that no longer holds and maintenance becomes difficult and where an end node is not obvious virtually impossible. There are ways around this problem and they are all expensive - for instance duplicate lines. This topic is the subject of major research and for new distributions new workable solutions could be designed into the system. Not easy.

The second is costing. By and large the biggest problem facing a utility is matching power production to power consumption with the production exceeding consumption by the smallest margin practicable. The excess production is lost and the costs involved are simply part of the production costs. However with Reverse Metering the utility is forced to "repurchase" any and all production by the private generator - this is not fair on the utility. The private generator suffers no losses due to overproduction and is not fair "competition" to the utility. One supposes that somehow the utility could control when it will repurchase - another level of complexity !?

The tail can wag the dog. If the utility is not many times stronger/more dominant then the sum of private generation then the utility cannot predictably make production slightly exceed consumption and essentially control is lost over the grid and can lead to massive under-production, a very disagreeable situation. Incidentally it can also lead to massive over-production and who would foot the bill ?

For these reasons alone it is already obvious that although desirable it is simply not simple. I'm not sure that if I represented the utilities I would allow it at all and paradoxically I'm firmly on the side of the private generator.
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Re: Desirable but not simple

Postby windgat on Sat May 09, 2009 9:19 am

First concern: the solution to this is not hard, and is already widely implemented: generating (feed in) site have a lockable isolate switch accessible from the street, and when maintaining the lines, these switches are locked off. Parallel lines sounds crazy! And you still need to be able to maintain them so that doesn't even solve the problem.

Second: This only becomes an issue when the feed in power starts to get to around 10%. We are sooo far away from that, so lets not worry about succeeding beyond that for now! But since you raise it... what is needed is power storage, then the excess if stored, and used to even out future shortfalls. The most promising are thermal storage, using concrete, oil, or molten salt as a 'heat battery'. It also means the grid must be able to feed power from/to the storage facilities etc. If only Eskom has the wisdom to abandon the Nuclear route, especially the PBMR, and divert the BILLIONS of rand being spent on that to upgrading the grid and developing power storage facilties!

Dog wagging: again, we are soooo far away from this, and Eskom will remain powerful and dominant for many years to come.

By the way, Eskom and DME (Dept. Minerals & Energy) made a pathetic attempt recently to derail the feed in tariff, by proposing a 'tender' system that would have given Eskom the right to refuse to buy power whenever it felt like it. The mining industry and its lobby groups absolutely HATE renewable energy and FITs - it is what will gradually erode their revenue, and make their huge capital investments in machinery, trucks, etc more and more useless.

Forunately it looks like that has been buried in embarrassment (after no doubt a huge waste of tax payers money), and the FITs are through, well done NERSA! I hear domestic FITs may be finalised in September...
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