Hacker News Reader

Comment by jumpingmice | Hacker News Reader

The university of California at Berkeley has ample cogeneration capacity but they are tied to PG&E grid and so PG&E can still turn them off.

View on HN

I recall some time back, the power generated with cogeneration needed main power with the right frequency to condition(?) it. The cogenerator couldn’t run during a blackout.

For a blackout, the diesel generators kicked in (and could completely power the platform).

Power plants need to be connected to other sources so their AC frequencies can be in phase. If the multiple AC waves were out of phase, you’d cause all sorts of issues. It wouldn’t surprise me if the cogen plant was designed to only work in concert with external power, but not in place of it.

Emergency power would be expected to work independently of mains power.

True for inverter based generation (most renewables like wind and solar), but generally not for rotating machines like most Co-gen plants. Some rotating machines (steam turbines generally) won’t have governors for frequency control, but usually in a CHP plant they would have a linked combustion turbine that does. Rotating machines that are paralleled with the rest of the grid are by default in sync with all of the other rotating machines if there is not some sort of fault situation at the plant or on the grid. If you try to parallel a generator with the grid that is out of sync (frequency or phase), the grid will always win and pull the generator into sync (potentially destructively for the generator).

The most likely limitation of a typical power plant operating disconnected from the grid would be if it could “black start” or not. Cogen plants would normally have this capability. Essentially the problem is whether you have enough power available from backup generation to bring all of the plant auxiliary systems online and get the generators initially started and producing power. Once started you set the governor to maintain 60.0Hz (if in North America) and begin slowly bringing on load so you can ramp the generation to match it.

The easiest solution to a future blackout would be to match the load in your island area to the output of your plant and then disconnect from the rest of the grid before or at the time the blackout occurs. This way you have avoided the black start situation completely. The blackout detection, disconnection, and load balancing steps can be automated to occur nearly instantly if the system is designed well.

The main issue for Berkeley is that the cogeneration plant doesn’t seem to be sized to accommodate their islanded load. This is especially problematic if you lose a generator while islanded because then you will have instant frequency collapse if you were already on the edge of your plant’s capabilities.

There are different types of generators: some supplement, and others replace, the grid.

Cogeneration is designed to produce heat (in chillier climates) and the process makes electricity as well while doing so, because why not?

* https://en.wikipedia.org/wiki/Cogeneration

Emergency / standby generators do nothing most of the time, and only kick in when needed:

* https://en.wikipedia.org/wiki/Standby_generator

* https://en.wikipedia.org/wiki/Emergency_power_system

Wow, that's annoying, too bad that they wouldn't have a way to disconnect. IIRC, Princeton was able to operate some buildings off of the cogen plant during Hurricane Sandy, so I guess they had things engineered differently.

All grid tie inverters shut off if power in the lines are down for safety reasons. You would need something special to switch over.

Yes of course, I just would have assumed the cost of that is pretty minor compared to the cost of the (apparently) 26 megawatt cogeneration plant

I don’t know — just imagine how many places the mains power is tied into all of the buildings. Each of those places would also need a mains cutoff, and they would need to all be thrown/active at the same time before the cogen plant could be activated independently. At a place the size of UC Berkeley, that would be a pretty big cost (and logistical nightmare).

If the system wasn’t designed for the switchover from the beginning, I can see how it would be considered impossible/not practical to do. Especially when this type of “blackout” was thought to be rare.

I’m sure the prior assumptions would be reconsidered today.

> just imagine how many places the mains power is tied into all of the buildings

Not many? As in, I don't think many buildings would have two separate power feeds. I'd expect a small number of very large connections from the cogen plant and the outside word into local distribution network(s), and then everything is single-path from there on.

But isn't that just because of legislation? As far as I can tell the system could just as easily throw a switch, test to make sure the switch was thrown, and then safely power locally. I know in California this is law, but I have never heard of a very good reason why it is technically difficult or inherently more dangerous to "air gap" your local power at the building rather than at the battery/power source.

Agreed. When I lived in Wisconsin the power would go out frequently during storms. Immediately I'd hear some neighbor's natural gas generators kick in and then a few minutes later I'd hear a few neighbors fire up their gasoline generators.

In both cases, they would flip over their home (or a sub panel) to the alternate power source (and disconnect it from the upstream grid). I looked into these systems and the natural gas ones could do this automatically using a kit available at Home Depot, and the gas generators required a manual switch, but were still very simple.

I looked into Solar in California and when I found out that most people can't use their systems during blackouts I asked about these generator systems that switch over and they basically said they had not seen equipment that could it and they didn't believe it was allowed anyway.

Solar is different from a generator, in that a generator is already producing AC, and as long as it produces a reasonable frequency and waveform you're good. Solar produces DC, which you then need to convert to AC, and the inverters that are typically installed are not able to do their own waveform shaping, depending on the grid to do it for them. It's possible to get inverters that handle this correctly, but they're more expensive, so typical home solar installs don't use them.

Or is it a horse and cart problem? Since there is legislation against it, there is not a demand, and prices have stayed high to sell them as "off grid" tech only? Most anybody would spend $500 extra on a 30k system for guaranteed power during blackouts.

There is no legislation against it where I am (Massachusetts), and there is still very limited demand for it because of the pricing.

> Most anybody would spend $500 extra on a 30k system for guaranteed power during blackouts.

To be clear, to get "guaranteed power during blackouts" out of solar, you need both a more expensive inverter ($1k or so last I checked) and batteries ($5-15k last I checked). If you're OK power only when it's sunny _and_ a blackoutm you might be able to do just the more expensive inverter, maybe. I'm not actually sure whether that work without a battery sink for your unused current.

> I'm not actually sure whether that work without a battery sink for your unused current.

What happens to unused current now when the panels are disconnected during a blackout?

There isn't any. That's not how electric current works. If you leave a disconnected solar panel out in the sunlight it doesn't explode or whatever.

Nothing, since they are disconnected, so there's a voltage across the gap but not enough to spark. And you don't care precisely what the voltage is, because there's nothing happening with it.

But if you don't disconnect them, now you are maintaining a voltage across whatever your load is, but that voltage depends, for solar panels, on the current being pulled from them, as I understand. And you need to maintain the voltage in some nominal range. Maybe it all works out, but I'd have to do a lot of digging to be sure...

Any inverter is expensive. I am currently pricing out a small system and just found out today the battery ready inverters don't cost more. In fact I am going without batteries due to a niche use case, but the inverter will be battery ready just because it (SolarEdge) is the best deal overall for the system. You are right about battery cost, but just having one that can get you (frugally) through the night would be a world different than just being shafted.

As to battery requirement, I can't see how attaching a wire to a panel would cause it to burn up. The wire would charge and then it would be the same thing as if the panel was disconnected with the same amount of latent charge in the panel.

Inverters are only used for DC to AC conversion, cogeneration sets are conventional prime mover setups that output 3-phase AC directly. The problem likely has to do with islanded grid stability, without the grid you have to take care of power factor correction and grid voltage regulation yourself, this can get pretty dicey if you don't have direct control or knowledge of the type of loads you'll have.

In an islanded mode of operation you also need to match load and generation to maintain system frequency, which is tough to do with cogen and non-controlled loads.

Basically, it can work, but the whole system is going to need to be engineered for it from the outset.

I work for a significant tech company in the Valley and we are in the PG&E grid, but we have diesel backup generators that we immediately fired up when power was cut to a few of our buildings. “Tied” to the grid doesn’t preclude backup power. As long as we can get fuel trucks to our buildings, we can run indefinitely.

Thanks for the clarification; I was wondering what had happened to their cogeneration plant.

I remember some time ago when the cogeneration plant failed and took out power in one building where many biological samples were stored at -80.

Reply on news.ycombinator.com