Just how efficient is load shedding really?

[Dave A]

Here is something I've been chewing over in all this load shedding lark - How much are we really saving this way?

Consider this:

• Every rechargeable battery that is being run down during a load shedding has to be recharged.
• Every geyser that cools down during a load shedding has to come back up to temperature.
• Every refrigeration system that has warmed up during load shedding has to be cooled back down.

When they power a section of the grid back up, demand on that sector is probably higher than normal after load shedding.

I wonder by how much?

[murdock]

what is really scary is the initial switching load required when the power comes back on...how long will the switchgear be able to handle this load...bacause i can assure you nobody switches off all the machinery...aircons...geysers...etc.

[Marq]

Does anybody have the big picture of this story?
Total input vs total output - what the shortage really is etc.
And is there any storage of overage - can this be done?
If eskom recons business cuts back by 10% = no shedding - is this the shortfall?
Two people have told me their fridges have packed up as a result of this story already - compressors and intermittent supply - Is this true?

So many questions - so many non electrical hours to meditate on the answers.

[duncan drennan]

And is there any storage of overage - can this be done?

It can be done, and is done. Palmiet is a pumped storage plant in the Western Cape. They can generate up to 200MW, but I don't know for how long. (more info on Wikipedia and Eskom's website)

Typically they use excess energy (when available, i.e. load is down, but don't shutdown coal plant, e.g. at night) to pump water up into a reservoir. When the peak demand surges they let the water run down through the generators. When there is lots of excess energy they also pump water over into the Steenbras dam. The generators can also be used for power factor correction.

I'm pretty sure they are currently using it to its maximum capacity, along with other backup generators (e.g. jet turbines).

[Dave A]

The Drakensberg pump storage scheme is another.

I see Eskom is being rather coy on where they are exactly in terms of power generation. Just how much generator power is down at the moment? I suspect quite a lot.

[SilverNodashi]

it's rather true what you say. You need more power to store power (as-in backup batteries), than to actually use the power.

Geysers & fridges also need more power to get up to "speed", than to run idle while at speed.

We're looking at going gas where possible, it's expensive to buy new stuff, but worth it in the long run