Why a Typical Home Solar Setup Does Not Work with the Grid Down

https://syonyk.blogspot.com/2018/05/why-typical-home-solar-setup-does-not-work-off-grid.html

by cesarb • 8 years ago

443 269 8 years ago

syonyk.blogspot.com

gnicholas2• 8 years ago

Super useful. Pacific Gas & Electric (which powers much of CA) recently announced that they may pre-emptively shut off power to certain parts of Silicon Valley out of concern for potential fires. In the article I read, they mentioned that even home solar wouldn't work, which seemed crazy. I can now see why this is the case, and what folks should do (get a transfer switch and generator) to be able to power their home with their solar panels.

tlrobinson2• 8 years ago

Pacific Gas & Electric (which powers much of CA) recently announced that they may pre-emptively shut off power to certain parts of Silicon Valley out of concern for potential fires

Huh, where can I read about this?

rconti• 8 years ago

Even just going to pge.com i get an interstitial about it, but no detail.

A couple links:

https://yubanet.com/regional/pge-shutting-off-power-during-h...

https://www.mercurynews.com/2018/06/12/pge-could-shut-off-po...

mcpherrinm• 8 years ago

TLDR: Power lines are a cause of wildfires, and PG&E may shut down power on some transmission lines in times of extreme fire danger.

https://www.sfchronicle.com/business/article/For-PG-E-cuttin...

https://sacramento.cbslocal.com/2018/06/25/pge-cutting-power...

gnicholas2• 8 years ago

I read about it in The Almanac [1]. Also, note that the map that shows higher and lower risk zones doesn't actually indicate what locations may be affected by outages. They only show where the affected lines are, but those apparently feed homes/buildings that appear to be in the clear.

1: https://www.almanacnews.com/news/2018/07/25/pge-may-cut-powe...

EGreg• 8 years ago

Can you please elaborate about the transfer switch and generator - how to use them?

What about just using batteries?

TheSpiceIsLife13• 8 years ago

Typical grid-tied solar system would still require a transfer switch, either auto or manual, they're called break-before-make transfer switches because they disconnect the mains supply before switching over to the battery + inverter system or generator. These transfer switches are mechanically interlocked so it's not possible to have both supplies connected simultaneously as this would generally be release-the-magic-smoke catastrophic.

Typical auto transfer switches are fast enough that computer equipment etc won't notice.

Edited to add: if you want truly uninterruptible power then you should do what data centres do: everything runs off the UPS all the time (well, all the computers, maybe not all the cooling system), the only thing that changes is the supply: either mains, batteries, solar, or generator. Or some combination of batteries + the other three.

bacon_waffle3• 8 years ago

Unfortunately, adding a generator and a transfer switch will probably not enable you to get power from the solar panels. You'll also need an inverter that's designed to operate off grid; the anti-islanding feature on a normal grid tie inverter will prevent it from providing power when the grid is gone.

There are a few ways that anti-islanding systems can work, but it's reasonable to imagine that the inverter is measuring the impedance between its output and a nominal AC source: the power grid. If that impedance is too high - and it will be unless you've got a rather large generator - then the inverter will not source power.

Johnny555• 8 years ago

During 2017, I saw a lot of news articles talking about how the Evil Power Companies were being Meanie McMean by not letting people with solar panels use them when the grid was down. The implication (in many news articles) was that these powerless people with solar panels could use them to power their home while the grid was down, if only the evil power company didn't require that solar not work if the grid was down.

I don't remember seeing any articles that blame the power company for home solar not working off-grid... I do remember seeing articles saying that most home solar installations won't work if the power grid is down, but nothing that implied that the power companies were behind it.

0xCMP• 8 years ago

In Florida it was popular around hurricane season after we lost power.

I'm glad I finally understand why because all that I'd seen reported was the solar systems installed must be turned off. I'd always assumed all these systems (many of which are meant to be used in a storm + lower electricity costs) had a transfer switch.

PJDK• 8 years ago

Here's one example https://boingboing.net/2017/09/18/rep-ray-rodrigues.html

tlrobinson2• 8 years ago

To get any sort of stable battery-free power, you have to run the panels well, well below peak power (30-50% of peak is as high as you can really run), and even then, you have a horrifically unstable system. If the array power briefly drops below demand (perhaps an airplane has flown over), you shut down the entire output for a while

It doesn't seem like a battery or capacitor to handle blips like an airplane flying overhead would need to be very large?

derefr3• 8 years ago

No, but using a small battery means using a battery that get heavily charged/discharged by such events, and therefore has a short lifespan (less than a year, probably, if you used the kind of battery that's in a UPS.) The reason the battery banks for solar are so large isn't just to capture the entire daytime output for night-time use, but also so that they can have a lifespan closer to that of the solar array itself.

foota2• 8 years ago

That's interesting! Is it like an SSD where you plan for a certain amount of cells to fail or does it reduce load on the cells so they last long individually?

grecy• 8 years ago

heavily charged/discharged by such events, and therefore has a short lifespan (less than a year, probably, if you used the kind of battery that's in a UPS.)

But surely such events (i.e. grid power outages) are pretty rare. In Australia it's rare for the power to be out more than once a year.

PinguTS3• 8 years ago

That you could do with a super capacitor, but not with a battery.

I know of some railway train setup, which is deployed in Saudi Arabia by Siemens, that uses a combination of battery and super caps to power the train. While the train starts it takes the sparking current from the super caps. While riding it will take the energy slowly from the batteries. While deceleration it will recuperate and will charge up the super caps, because they can be charged easily with a high current. So the super caps are used as a high available energy buffer with very fast charge and discharge capabilities. While the battery is only used during the times of less energy consumption. Also at the stations the super caps are super charged very quickly. With that concept, they can run the train the whole day with only 40% discharge of the battery. The battery is actually way smaller then in a Tesla.

This concept can be applied also locally. The problem: these super capacitors are way expansive so you need to design them to your specific requirements.

sramam• 8 years ago

Slightly beside the point you are making - would there be a problem if the train makes an unscheduled stop between stations for any reason?

Dylan168075• 8 years ago

You can do it with a battery. An 18650 that can supply a hundred watts is $5 or less. Even limited to half its discharge depth for longevity, it can take you through "blips" two minutes long.

I wouldn't want to ride a battery pack like that all day and every day, but it should be fine for emergencies.

namibj6• 8 years ago

Also, while not necessary trivial, it is not hard to throttle consumption of devices, and if it's with two circuits, one for devices that care about voltage, and one for devices that actually brown out. The latter is easy to scale by just reducing the voltage far enough that it doesn't sink too much power, and devices like e.g. desktop or server computers are theoretically easy to rapidly throttle, e.g. by reducing cpu clock to a few hundred megahertz, halting HDD operations or even cutting their power (most are made to handle hot-swap well). A large, modern CPU with 140 W TDP can cut usage within 10 ms to 10W, but dram can't scale that way due to how it works, or rather, this scaling is not supported in mainstream hardware.

If you use a small supercapacitor, which commonly has a minimum discharge time of about 2~20 seconds (similar to Lithium-ion pouch cells "LiPo", which are available with different "C" ratings, and negatively correlated gravimetric/volumetric energy density and maximum average discharge power (i.e., optimizing discharge current vs time to maximize the ratio of total power extracted divided by time until empty). You need more metal in the electrodes and the plate/foil that aggregates the current of the individual electrodes, leaving less space/weight for the parts that actually store energy.), you can use much more advanced power reduction techniques for electronic devices, as you have time for something approaching a proper shutdown. Compare e.g. the time your Laptop takes until it is off from the moment you close the lid.

Oh, and yes, these capacitors are cheap. One about 1kg / 1 liter size for about 100$ can provide 20kW average power, for 2 seconds. The main difference is that they don't care (much) if you cycle them at 100% depth of charge, apart from internal losses heating it up and thereby causing damage.

bacon_waffle3• 8 years ago

There are all sorts of issues with using a regular grid-tied inverter in an off-grid situation; there are regulatory issues besides the anti-islanding (which is a complicated topic all on its own), and technical challenges too. A lot of the technical challenges go back to the inverter effectively being a thing that bolts on to the existing house wiring; I think that if they could be more of a pass-through thing (like a UPS) then the picture would be a lot simpler.

An off-grid capable inverter needs to handle several state transitions smoothly, and some of those are hard. For instance, starting up can be a lot harder when there's no mains voltage. An inverter will have a maximum output current, which is generally not too far from the maximum power output of the inverter divided by minimum mains voltage. If you have a load that presents as a constant resistance, like a water heater, then as the inverter's output voltage goes from 0 to 240V (120V for areas with weenie power) the current will smoothly go from nothing up to whatever the load normally draws, along with the voltage, and there's no problem. But! A lot of loads aren't resistive - things like computers, battery chargers, and inverter-driven motors (as newer appliances often have) will tend to look more like constant-power loads. So, once they decide it's OK to turn on, if the voltage happens to be half of nominal at that point, they will try to pull twice their nominal current. If that causes the total draw to exceed what the inverter can provide, it stops and maybe tries again. It's practically impossible to measure what the load characteristic of something like a house, from the perspective of an inverter, so the best you can do is something like guess-and-check, and that's not a great solution.

Then, how to handle reconnection with the grid; you either need to re-synchronise before reconnecting, or ensure that the intrepid homeowner first shuts off the house before turning the switch to reconnect.

Switches that can handle that job may need to be specified per-installation, and there are all sorts of regulations around how they work and where they can go. They are not cheap, as a rule. Some regions have the solar generation stuff on a separate export power meter from the import one. This means that the switch needs to be on he supply side of both meters, and that could be regulatorily complicated.

What about multiple inverters? These things all need to play nice with each other when solo, or installed with others. Perhaps you didn't design those, and they might try to start up too, then fire up their anti islanding which should promptly trip out again.

Remember this all has to work with variable input power available, and it needs to be packaged up so that installation is straightforward, and operation is basically hands-off. Oh, and inexpensive too!

ghubbard• 8 years ago

Great article, lots of technical details. A simple answer to the 'Why' can be found in the middle under the 'Off Grid Without Batteries' headline.

If you have a typical grid tied system (microinverters or normal string inverters, so easily 95+% of installed rooftop solar), the system is technically incapable of running off grid (without additional hardware). There's no waveform to sync with,

raverbashing3• 8 years ago

"there's no waveform to sync with" is technically correct and a very poor excuse

A residential no-break has no waveform to sync with as well.

Something capable of syncing to the grid and then more or less keeping pace even if the main grid goes down should cost very little today. (And when the grid goes back it shouldn't have drifted too much unless something ridiculously big happened)

TheSpiceIsLife13• 8 years ago

When the grid goes back up the system would check the grid frequency and phase, and slowly match that waveform over a few minutes by making light changes.

When the two are aligned within a good-enough tolerance the system will switch back to it's regular state of mains + solar ( + batteries + wind + generator + etc / whatever).

These are all solved problems with commercial off the shelf components.

zaarn19• 8 years ago

The grid isn't a fixed frequency. While on average the grid doesn't change phase, the reality is that the grid may be out of phase by several seconds most of the time.

When the grid comes back from a blackout, chances are that it browned out beforehand so your sync is to a low frequency and coming back it'll be high frequency because the grid wants to compensate for loads jumping back on.

Additionally the components to generate your own waveform are cheap, yes, but not that cheap, adding them to the microinverter would increase cost quite a bit.

And you'd still need a transfer switch because if you happen to be 180 degrees out of phase, which CAN HAPPEN then your panel will behave like a dead short at double grid voltage. The current flow will definitely exceed the maximum tolerances and the magic smoke goes out.

You will absolutely need a transfer switch just so you don't fry all your devices the moment the grid comes back. Even then, syncing to the grid is a rather delicate maneuver since the grid will be constantly changing phase and it'll be simpler to shut down all inverters, connect back and have it all run back up on the grid itself.

avianlyric2• 8 years ago

It might be worth reading the rest of the article. Which points at that the big reason why you can run off grid is because solar panels without a battery store are effectively incapable of usefully powering variable loads without:

Only running at a fraction of their max available output allow enough headroom for high peak startup draw from loads, and headroom for clouds and planes passing overhead.

Frequently cutting in and out as the load regularly exceeds available supply.

Keeping the frequency sync is the easiest problem to solve, the article even covers what would happen if you tried to use a little generator to produce a sync signal.

jccalhoun• 8 years ago

I agree that there are a lot of good technical details. However, I wish the article had started off by saying, "It's Not Power Companies Being Evilly Evil - It's Homeowners Being Cheap" instead of waiting more than half way through it.

CaliforniaKarl• 8 years ago

I've read the detailed reports for outages in the West and the North-East, and they touch on alot of the same concepts that were in this (awesome) article.

Does anyone know, what's a good resource for learning more about power generation, grid regulation, etc.? The relationship between frequency/voltage and available vs. demanded current, frequency sync, grid maintenance, etc.…

throwawaycanada• 8 years ago

If you have a reasonable electrical background I would suggest this textbook:

Electrical Machines, Drives and Power Systems by Theordore Wildi

ggm3• 8 years ago

We found that even a well made Honda petrol genset was highly unstable feeding volts into a UPS for a machineroom. They are good at lights. They are good at a cooker. They suck at anything which expects regulated, non-noisy volts.

Lots of homebrew 'you can plug this magick cable I made' methods to supply volts across the fence to neighbours houses when the power goes down are really bad: behind the meter is just as lethal as in front of the meter, if you get it wrong.

TL;DR if you need to read a website like this for clue how to avoid losing power, you are putting your life at risk if you don't follow code, and don't know what you're doing.

driverdan6• 8 years ago

Inverter generators like the popular Hondas are not good for surges. If you run them at a steady draw they'll give you nice, clean power. If you your demand fluctuates significantly they will stall or cut out and the voltage during or after the stall may be dirty.

You have to read the specs carefully to make sure they are a good match for your use.

ggm3• 8 years ago

Fridge. Surge load. Washing machine. Huge torque surge load. Airconditioning. That's basically a fridge?

So.. "home" solar setup.. home.. like fridges, aircon and washing machines...

Do you see where this is going?

modeless• 8 years ago

So much complexity to deal with synchronizing AC frequency. I wonder if DC wouldn't be a better choice for a modern power grid?

sooper• 8 years ago

The costs to refit would be enormous.

I think there are some issues on longer lines with things like inductance, cable size requirements, etc as well.

rollulus• 8 years ago

High-voltage DC [1] is being used to transport power over longer distances because it can be cheaper and there's less power loss.

Like a sibling commented here, the costs would be enormous to convert an AC grid into a DC grid. AFAIK are there two reasons that the grid is AC: generators produce AC, and AC is so easy to transform up and down. That transforming is essential since you want to transport energy at high voltages [2] but you don't want 380kV in someone's house.

[1] https://en.wikipedia.org/wiki/High-voltage_direct_current [2]: https://en.wikipedia.org/wiki/Electric_power_transmission#Ad...

avianlyric2• 8 years ago

Your right that AC is super easy to transform. But it’s quite easy to build generators that produce DC.

They’re frequently called dynamos and you find them on old bikes to power the lights.

With modern electronics doing DC-DC transforming is pretty easy (because modern electronics is basically magic). And the reason why high voltage DC transmission is a thing, is because you can get way more power down the same wire with DC compared to AC.

Many high voltage DC lines used to be high voltage AC that was converted to increase the transmission capacity without building more cables.

gmueckl2• 8 years ago

But, apart form installation costs, a low voltage DC power network inside homes might make sense (e.g. 12V or 24V). There's a lot of modern electronics that doesn't depend on the regular powerline voltage anymore. Unifying their power supplies into a central unit should have some benefits.

Max_aaa• 8 years ago

AC also allows for smaller switches and relays, as you are guaranteed to hit 0 V within a phase, so you can disconnect loads without giant sparks/arcs.

taneq3• 8 years ago

Backfeeding the power grid, according to some lineworkers I've talked to, is really not a big concern for two reasons. First, lineworkers assume lines are live until proven otherwise. And, second, no residential system is going to successfully backfeed a large dead section of grid. [...] it's really not that big a concern from a technical/safety perspective.

This is wrong and dangerous. Suddenly energising a section of grid which has been isolated from all known power sources could easily kill someone working on it. Yes, lineworkers will isolate and test lines before touching them. No, this won't protect them because your inverter could start outputting after they've tested. Yes, your inverter will probably overload and trip out before putting too much voltage into your local section of the grid, depending on where it's isolated. No, it's not OK to bet a random stranger's life on it.

TheSpiceIsLife13• 8 years ago

I like to always highlight the difference between a technically correct statement and a public health and safety policy statement.

A thing may be technically correct, but for the health and safety of the public at large we need to adhere to a consistent policy with what we say in the public sphere.

Mains voltage is dangerous and kills kills kills. Never connect anything to the mains power supply unless you are licensed to do so, or employ someone who is licensed to do so. Always assume a circuit is live unless you-personally have checked and locked out the breaker with your own personal lockout device that only you have the key to.

maxerickson4• 8 years ago

You are supposed to have a license to plug a TV into the wall?

I'm being silly, but you are going on about how the wording of statements matters.

alkonaut4• 8 years ago

When I work on electrical wires at home, I test if they are hot after flipping what I believe is the correct fuse. If I’m right I know it won’t become hot while I work on it because I control the switches and fuses.

If I worked on wires where I was not in control of incoming power, then I can not be sure the wire isn’t hot just because I measured once. This is for example the case when working with stuff with big capacitors.

So my point is: can line workers really assume a wire isn’t hot just because they tested it? They don’t control both ends, since as you pointed out, anyone could have a) a power source or b) a faulty connection, whether it’s allowed or not. So as a line worker I’d basically just not touch anything even in supposedly dead grid sections.

crististm• 8 years ago

When they work on train power lines, what they do is physically short them to the tracks.

zaarn19• 8 years ago

I recommend shorting to ground. Even if it's your own fuse. Shorting means that if you picked the wrong fuse you notice. And when your friend comes by and flips the fuse back on you don't get a spark shower (something like this has happened to me, don't underestimate people willing to flip fuses for no reason)

Cerium• 8 years ago

You should test if they are hot before you flip the breaker, then test again after. If you only test after you are not protected against faulty test equipment or improper wiring.

VectorLock2• 8 years ago

I'm so paranoid I lockout-tagout my home breaker panel.

lb1lf• 8 years ago

I guess the point is that if your residential solar setup attempts to backfeed a dead grid, it will effectively see a short circuit which it, try as it might, will be unable to generate any voltage across, trip and your house is as dead as the rest of the grid.

For this reason, I'd be most surprised if there's a single backfeed system in existence which doesn't isolate your house from the grid once the grid goes down.

Edit: Finally got around to reading the blog post to the end and found that the author makes this exact point in the paragraph following the one quoted in the comment above.

cesarb2• 8 years ago

Yes, lineworkers will isolate and test lines before touching them. No, this won't protect them because your inverter could start outputting after they've tested.

The lineworkers I've seen that aren't working with live wires use a special tool to ground the wires (it's a set of three clamps connected by a thick wire to a grounding rod). Once the wire's grounded, it's going to be hard to put any voltage on it.

tzs2• 8 years ago

Here's a photo just after they did that while repairing a break near my house a few months ago: https://i.imgur.com/6TckAa8.jpg

The obviously missing third top line is the one that broke. It simply snapped around the middle of the span between that pole and the pole that is outside the photo on the right. They went up in the cherry picker, cut the dangling line, and then put on those clamps.

They then covered the lower arms of the pole with orange sheets: https://i.imgur.com/zZ3DFSd.jpg

Does anyone happen to know what the orange sheets are for? They did not use them anywhere else where they grounded the wires.

sandworm101• 8 years ago

It can also do strange things to outage reporting equipment that assumes power to be flowing a paticular way.

amluto• 8 years ago

I would also expect that a larger residential inverter could successfully backfeed the load side of a single distribution transformer (i.e. just a few houses). If the line workers are repairing a distribution transformer or its associated hardware, this could be a big deal.

I don’t know if the workers make a habit of physically disconnecting all of the houses that are connected.

sathackr5• 8 years ago

I'm not a lineman but am a generally curious person, which has caused me to watch and analyze their work when I see it.

Any time I see them working directly on high-voltage lines, they connect a device that looks like jumper cables in-between the phase(s) and ground. They also put bright orange flags on this device so that it is easily visible to anyone nearby. It seems to be a great practice and seems to make things nearly fool-proof because this effectively shorts out any power that may occur on those lines.

I don't think I've ever seen a lineman work on a power line in a manner that would cause him to be electrocuted if surprise power were to occur on the line. I'd bet it's in policies and procedures somewhere, possibly even in OSHA requirements.

The requirement of not backfeeding is another layer in this multi-layer protection scheme.

If a system is successfully backfeeding the grid, something is badly wrong. If a linemen is then electrocuted by this backfeed, something is almost impossibly wrong.

planteen• 8 years ago

I agree with your comment about back feeding and shorting lines that are supposed to be de-energized, but linemen also sometimes have to work on hot lines. They have all kinds of neat tools and techniques for doing this. Seeing a lineman with a cheater stick (fiberglass pole) or insulated gloves working on a live line is pretty neat.

MertsA2• 8 years ago

It's not as foolproof as it may seem. If linemen are out working it's because something has changed the normal flow of power. If they're fixing downed power lines then all bets are off as to what the circuit diagram has effectively become. They might ground out the spur going to the break but that still leaves everything after the break ungrounded. Take for example, Joe homeowner bought a big 5kW generator and he wants to keep his fridge running. It's easy enough for someone to "install" a generator themselves without calling an electrician and just make a suicide cord to hook it up during a power outage. If the break is just upstream of the generator, or the neutral broke instead of the live line, you can have a situation where the generator is still producing a lethal voltage potential between the live line and ground yet it wouldn't be low enough resistance to short out the generator.

It's unreasonable to assume that a lineman can identify which sections of a downed line are isolated before they get to work repairing the power lines. There are circumstances where the line might even be isolated without being physically disconnected because a fuse blew downstream of where the line was grounded before the lines actually came down. Fuses will have a visual indication that they are blown but it's just a pin that sticks out the bottom and it's easy to miss when you think the only problem is a downed power line. There's an army of fools who know just enough to be dangerous and it's easy to see this on YouTube. Here's a great example of what linemen have to worry about when repairing downed lines.

https://www.youtube.com/watch?v=KiByz1E4IBU

Wouldn't you be worried working with downed lines not knowing if that guy just so happened to live across the street?

WalterBright6• 8 years ago

I'm curious why a large capacitor or inductor cannot be used to provide momentary power for things like electric motor starts, and to smooth out events like a plane briefly occluding the sun.

I built an audio amplifier in college that had a couple of huge capacitors on the power supply (about the size of a can of soda). It would keep the amp running a few seconds after I shut off the power.

PinguTS3• 8 years ago

Capacitors are for DC. You but them in your amplifier after the supply, when you have DC.

Electric motors, especially the simple ones found in compressors for cooling, are running from AC. You can't put a capacitor into the AC path. You can only convert it to DC, charge the capacitor and then convert it back to AC. Where we are back to the original problem.

Dylan168075• 8 years ago

The real problem is having insufficient power to provide AC, not the need to use slightly better logic in the inverters.

WalterBright6• 8 years ago

Capacitors are for DC.

The output of solar panels is DC, too. The capacitors would be placed in the DC side.

mindslight5• 8 years ago

Fundamentally, the size involved. Your amplifier's capacitors were extremely large for the power the amplifier was using, as their purpose was to reduce the ripple voltage from the power supply.

One of those "1 Farad" car stereo caps (not the classiest example) can store 72 joules (watt-seconds). So to handle a 1kW excursion for 5 seconds, would take over 70 of them. Possible, yes. But probably not worthwhile.

The frustrating thing about this article is that it keeps conflating fundamental physical problems (like needing to store energy to deal with peaks), with the limitations of currently-productized circuit topologies. It's not hard to imagine an inverter that would do something similar to MPPT without needing to pull/dump the maximum power possible, would manage a small bit of local storage (a combination of caps and abused batteries), and could intelligently load shed (with "kind" brownouts) on multiple outlets.

It's also not hard to imagine an inverter (brushless motor) air conditioner that would be a lot kinder of a load, and could even operate at variable power.

But of course with the current state of homebuilding, these are bespoke ideas better filed as "off the grid". Which is the entire larger point - an "off the grid" system fundamentally costs more in materials and also presently application engineering time, than the on-grid systems being sold to homeowners.

(Although I do have to wonder about adding a piece to the current puzzle that would generate/manage a local grid frequency, and shed excess power using off-the-shelf electric heaters.)

WalterBright6• 8 years ago

over 70

This doesn't sound that bad to me. Size isn't much of an issue, houses have room for them. Prices for them seem all over the place:

https://www.alibaba.com/showroom/1-farad-capacitor.html https://www.amazon.com/dp/B00381UKD0/ref=psdc_2230642011_t2_...

The idea with a capacitor is they shouldn't degrade like batteries do.

The same with an inductor, which is just a coil of wire.

thinkythought3• 8 years ago

Inverter drive home AC systems that are reasonably priced do exist, you can get a mini split for under $1000 now with variable drive compressor and fans. I was shocked last time i looked that up actually, as i had similar thoughts a few years ago for this application and barely anything was on the market.

There's also inverter drive microwaves, and fridges even. My friend's new condo has an inverter drive fridge

parimm• 8 years ago

The solution to this is to use Hybrid inverters+batteries with a auto disconnect on the mains.

alkonaut4• 8 years ago

My cheap and simple 15-minute UPS device has waveform generation (?) and a battery. Surely with solar you often have to have huge batteries?

In hot climates you have some power use when the sun is up (cooling, pool heating,) but where I live I’d need my power most when the sun is absent in night and winter (power use is probably 80% heating). Any amount of solar power would be nearly useless if only available when the sun is shining.

mcny• 8 years ago

In hot climates you have some power use when the sun is up (cooling, pool heating,) but where I live I’d need my power most when the sun is absent in night and winter (power use is probably 80% heating). Any amount of solar power would be nearly useless if only available when the sun is shining.

Not a subject matter expert by any means but depending on how much sun you get, there are probably better options than solar for you (solar might not be the right choice for you).

alkonaut4• 8 years ago

Solar is increasingly popular around here though (latitudes 60-65) which I only assumed also meant that storage was also handled with batteries. Seasonal storage is probably not, but at least generation during the day and use during the night must be.

I learned from the article that lower temps helps efficiency, so perhaps our climate with short days and subzero temps for 5-7 months is still viable because of higher efficiency?

SigmundA2• 8 years ago

My dad has a 7kw system with the SMA inverter with emergency power outlet for 1.5kw. Works ok, as said the main issue without batteries to buffer is stability.

I have a Magnum 3000w Hybrid inverter in my RV with 320w of solar with a MPPT solar controller. The solar controller feeds the batteries and the inverter draws from batteries or charges them with UPS grade change over. Magnum hybrids can do interesting things like if the sun is out and I am running the air conditioner off generator or shore power it will pull batteries down to float voltage and invert excess power to lower draw, I usually get about 1 amp reduction with 320w of panels. You can also dial in your shore amps and if a load needs more the inverter will step in and provide the overage from the batteries. Nice if you driveway surfing and only have a 15 amp outlet but want to run the microwave or whatever. The RV is basically a giant UPS with generator.

mikestew3• 8 years ago

Having just finished a 450W solar system on our RV, along with new big-ass 200ah battery, I was shocked to find out that house systems have no battery backing. Because I have had to explain to everyone that asks “can it run...?” that you can run anything you want if the battery can handle it, the solar is just a battery charger and doesn’t directly run anything.

But as one who is considering residential solar, I guess there’s more research to do as home solar apparently doesn’t work that way at all (though it probably will when I do it).

Side note: thanks for the inverter mention. That’s next on the list, and the Magnum sounds to be worth a look.

rbritton2• 8 years ago

I went the Victron route on my setup, primarily because they make whole systems that can talk with each other. I had originally been looking at the Magnum, though.

jefftk• 8 years ago

The only real way to get off grid power without batteries is to go with an inverter that has an emergency outlet. Some of the SMA inverters support this (they call it Secure Power Supply) - you feed the whole rooftop array into them, and they can, if the sun is shining, provide 1.5kW or so to a dedicated outlet - assuming there's enough solar power. So, from an 8-10kW array, on a sunny day, you can get 1.5kW by operating well below the peak power point. If the array can't keep up with current demand (a cloud goes over), the outlet shuts down. It's better than nothing, but this is just about the only way you can get battery-free off grid power. To get any sort of stable battery-free power, you have to run the panels well, well below peak power (30-50% of peak is as high as you can really run), and even then, you have a horrifically unstable system. If the array power briefly drops below demand (perhaps an airplane has flown over), you shut down the entire output for a while. Hopefully your devices can handle intermittent power like this. If the array can source 1300W at the moment and a compressor tries to draw 1301W while starting, you collapse the array voltage and shut down the outlet. That's really hard on compressors (and everything else attached to the outlet).

It's not ideal, but this is so much better than what most installs are built with (nothing).

test6554• 8 years ago

I thought there were shutoff devices that could detect a grid outage and instantly disconnect your home from the grid allowing your inverter to power your house until grid power is restored. I assumed something like this would be $10K but well worth it, as basically the whole point of battery-backed solar is energy independence.

imglorp• 8 years ago

Look for "transfer switch".

ianburrell• 8 years ago

From the article, the problem isn't the transfer switch but that the microinverters used on most systems don't work without the grid. If the grid goes down or is disconnected, the microinverters stop providing power.

It is possible to get inverter that will work independently, and batteries to buffer the load, but the off-grid system is a lot more expensive than normal rooftop solar system.

gwbas1c• 8 years ago

That article is TLDR for the target audience.

The end part that basically says, 'solar panels need batteries and a more expensive inverter to run without the grid" needs to be at the beginning of the article.'

The details of power curves then come for those who want more details about why.

foxhop• 8 years ago

It's also not safe for line workers who think power is off to a main and get zapped by a small fleet of home solar in The neighborhood.

tzs2• 8 years ago

I've been looking into backup solutions to keep my fridge and well pump [1] going during outages. Anyone tried something like the following?

The obvious approach is a propane generator. Propane rather than gasoline because we only get a small number of outages a year, and most are fairly short, but there is occasionally one that is 10+ hours. I'd want to store enough fuel for at least 10 hours of generator use, but most years would only actually use a fraction of that, and so I'd have to be constantly rotating the fuel (probably into my car, as the only other gas powered thing I have, my lawn mower, only uses about a gallon a year) to keep it fresh enough to still work for the generator. Propane does not go bad, so storing 10 hours worth of propane for years is no problem.

But both of the things I want to power (fridge and pump) are very intermittent loads. I don't want the generator running all the time during an outage, wasting fuel, just to be ready when the fridge or pump needs to actually run. Even idling the generator will be consuming fuel. I could leave the generator off except when I need to actually have the fridge or well run, but that would be quite annoying as I'd have to go outside to start and stop the generator--and power failures are usually when it is quite unpleasant outside.

So what I've been thinking of is having a dedicated battery and inverter for the fridge, and a dedicated battery and inverter for the well pump.

When power fails, I can start watching the fridge temperature (it has a battery powered wireless temperature monitor, so I can see the temperature without opening the door). When it warms up to the high end of its normal variation, I can unplug it from the wall outlet, plug it into the inverter, and start the inverter running off the battery. When the fridge cools back down, turn off the inverter. Repeat throughout the outage.

Similar for well pump. When I run out of water, go run the pump from the inverter and battery until the tank is full.

I'd still have the generator, but now it would be used to recharge batteries, not power any house circuits or devices. Over time, I'd probably add batteries, eventually reaching a point where I have enough that I can get through most outages without needing the generator.

(Actually, I'd probably do the well directly from the generator instead of inverter and battery. When I go out to turn on the generator to recharge batteries, I'd also plug the well pump into the generator to refill the well).

I could start with one inverter and a battery or two, without a generator. This would probably get me through a large fraction of the outages. Then over time I could add more batteries, more inverters, and the generator to extend the number of things I can use during an outage and how long an outage I can get through, in a nice budget friendly way.

[1] I probably don't actually need the well pump--as soon as an outage starts I fill a couple water bottles to have drinking water, and fill the bath tub to have water for flushing the toilet, and that will almost always be good enough. Why immediately fill things instead of just leaving the water in the well tank and hot water heater until I actually need it? Because I have no idea how much water I have. The outage could have happened right after the pump finished filling the tank, or it could be just before the pump was going to start and fill a nearly empty tank. Filling the water bottles and bathtub right away lets me know what I've got.

stevewilhelm• 8 years ago

Install a Tesla Powerwall. Problem solved. https://www.tesla.com/powerwall

randyrand• 8 years ago

instead of shutting off when the current is exceeded- why can’t you just use a current limiting device and cap it?

modern bench top power supplies can limit current

irrational• 8 years ago

What I would like to know is, if there was an emergency situation and the grid was going to be down for an extended period of time (I'm thinking 3+ weeks, months, years) would there be a way of altering the system so that the home could have electricity, at least during daylight hours?

sounds• 8 years ago

The article points out that the most cost-effective solution is to get an internal combustion generator.

driverdan6• 8 years ago

As this article touches on the solar panels are the cheap part of a solar setup. If you want to run off grid the costs skyrocket.

I live in an off grid bus. My whole electrical system cost in the US$9-10000 range with only 810W of solar. The other components cost a lot more:

* ~$900 for 3x 270W Renogy solar panels

* $1500 for 3.6kWh of LiFePO4 batteries (12 100A 3.6V cells)

* $1600 for a Victron 3kW inverter / charger

* $2000 for a Honda EU3000is generator to run my AC

The rest was the Victron solar controller, color control panel (that runs Linux!), breakers, relays, transfer switch, fuses, cable, and all the other stuff you need to hook it together.

This isn't at house scale either. The cost would go up significantly to support a house scale system.

agumonkey2• 8 years ago

ICE bus ?

driverdan6• 8 years ago

ICE?

driverdan6• 8 years ago

There are no practical electric buses. The bus itself is on the Chevy C4500 chassis with a diesel engine.

VectorLock2• 8 years ago

The solar controller, control panel and miscellaneous cost you $3-4k?

rbritton2• 8 years ago

Having just done similar at a smaller scale, that's plausible. A solar charge controller for that capacity is $600-1000 on its own, and the Victron control panel mentioned is north of $500.

driverdan6• 8 years ago

The controller was more than $200, the control panel is around $550.

I didn't itemize the rest because I don't have the time to list it all. The roof rack and hardware cost around $1000. There are a lot of individual items that cost around $100, (eg starter battery isolator relay, breaker panel + breakers). Heavy gauge cables are necessary and expensive. All the small things add up.

ssdd• 8 years ago

Initial cost might be high but this cost would be slowly return as you save on electricity bills. Also, all this equipment is not perishable and you can still re-sell at good price after your retirement or sell them along with house. Its no different than investing in house upgrades which adds to the house value.

tjohns2• 8 years ago

Both the solar cells and the batteries have a finite lifespan, and will eventually need to be replaced.

Especially the batteries.

nine_k2• 8 years ago

That Honda EU3000is is an 3 kW ICE-based generator, that is, it's not a part of "solar" setup proper.

OTOH not having a combustion-based backup is likely not very wise if you live off-grid. I wonder how often do you need to run it? That is, what part of your energy budget is covered by the solar energy?

driverdan6• 8 years ago

What does ICE mean?

The generator is part of the whole power system. In most implementations solar is not reliable enough on its own. You need a backup. In my case that's the generator and the bus' alternator.

Right now I'm running the generator all day to power my AC. I'm not running it most nights. It could be cycled on and off but I haven't setup an auto start system for it yet.

rdl3• 8 years ago

Maybe this applies in normal markets, but in Puerto Rico the grid is both shit-tier reliability and horribly expensive. Rooftop solar and lithium batteries in an off-grid format are both far more reliable and far cheaper than the grid, assuming a 5% cost of capital. I might throw in some LP or diesel generation capacity too, but solar plus battery is an easy choice.

nine_k2• 8 years ago

Are lithium batteries the best choice here? They have superior energy density, but IIRC there are choices that are heavier but cheaper (per kWh installed), like ready-made car acid-lead batteries, or industrial Ni-Cd batteries.

What do people actually use?

vorpalhex• 8 years ago

Not sure for Solar, but most HAMs using field radio setups still use deep cycle marine batteries as they withstand abuse and are much cheaper.

rdl3• 8 years ago

Tesla PowerWall is only an Apple-level premium (and not a Vertu-level premium) for what it is. I would probably just go with 1-4 of those for residential unless getting something more capable. I don’t own a house yet, though.

My ideal situation would be a microgrid with 10-20 homes (in a compound/neighborhood or condo building) and some commercial/office, which pushes all the electronics costs down to something more reasonable.

RankingMember• 8 years ago

What would you say grid uptime (%-wise) is there for you right now?

rdl3• 8 years ago

Depends on where. There are a couple of remote areas with no power still, but generally in San Juan and the areas where most people live it has been good in q2, with a couple of multi-hour outages. Still, at $0.35/KWh in a high sun place solar is obvious.

rm_-rf_slash• 8 years ago

That’s a good point. If a grid is unreliable then an off-grid solar investment would have to factor in the opportunity cost of things like spoiled food, productivity (can’t work remote with no power), amenity offerings (restaurant with A/C versus not) and so on.

dublin3• 8 years ago

Of course. Islands are one of the only places where solar truly makes economic sense - having to ship your fuel in changes the economics equation a LOT...

dublin3• 8 years ago

A huge amount of the info on solar PV power on the net is absolute garbage. (I spent 5 years working on the most advanced solar performance optimization and monitoring system developed to date.) I haven't read all of this article, but so far, it looks dead on (you have no idea how rare that is!) Solar is not the panacea many people wish it was, simply due to the way God built the universe - and there's not a thing we can do about that. (And yes, the panels are now only a small part of the cost of a solar system, especially one capable of operating off-grid...)

rebootthesystem• 8 years ago

Looks like I great article. I'll have to defer reading it. During a quick scan I saw a mention of the SMA technology that allows for dedicated outlets to be powered-up in case of grid failure.

I built a 13 kW ground-mount system feeding a pair of SMA inverters. I have tested this feature by disconnecting from the grid and enabling the outlets (one per inverter). I didn't quite get to the 2,000 W rating SMA claims but got close. Which means that with this size of an array and two inverters I get somewhere between 3kW and 4kW of power to run various devices while the sun is up.

Considering that we might have a couple of power outages a year on average (if that), I felt this was a reasonable investment. Going with batteries is just too expensive and not justifiable at all given the reliability of the grid. One way to think about this is that the grid is your battery. A stretch, I know.

Funny that there's a picture of a gasoline generator towards the end of the article. My guess is that I am likely to invest in a 5 kW to 6 kW generator before I ever add batteries to this system. Again, it's a matter of ROI. Also, I would not go with a gasoline powered generator at all. Gasoline degrades with time and could be a nightmare to maintain the system with sporadic use. I think a propane fueled generator might be a better idea. The fuel does not degrade. So long as you don't have leaks it'll be there ready to go when you need it.

I know way too many people who have been mercilessly duped by these solar companies who come in, hook them on some kind of a lease, install inadequate systems and move on to the next victim. Lack of understanding on the side of consumers has created a situation where solar is equivalent to magic and unscrupulous actors can take advantage of them. That part is sad.

kingkawn• 8 years ago

Forgive my ignorance in advance; is there any viability to a system that pumps water uphill to a storage tank then it flows downhill later to let the current run a generator?

modzu• 8 years ago

you're basically describing a gravity battery

https://en.m.wikipedia.org/wiki/Gravity_battery

taylortbb• 8 years ago

Yes, in fact they're already in use. They're called pumped storage systems, https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

OxO4• 8 years ago

Yes, pumped-storage hydroelectricity is used extensively [0].

[0] https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

iamleppert2• 8 years ago

This is exactly why people need to explore alternative energy options other than solar. The best off-grid setup I’ve seen is a bunch of conductive pipes lay over the ground that heated water from the sun, that in turn was used to generate enough energy to pump water to a storage tank during the day up a large hill. At night the tank was drained and water came back down and the resulting pressure was used to generate power, supply back to the grid. It used minimal components (& simple ones), no batteries, nothing fancy but was enough energy to supply a community of several single family homes in a completely off grid system.

akeck• 8 years ago

Fascinating. By chance, would you have a link or article for further reading?

dragonwriter3• 8 years ago

This is exactly why people need to explore alternative energy options other than solar. The best off-grid setup I’ve seen is a bunch of conductive pipes lay over the ground that heated water from the sun, that in turn was used to generate enough energy to pump water to a storage tank during the day up a large hill. At night the tank was drained and water came back down and the resulting pressure was used to generate power, supply back to the grid.

So, your example of energy options other than solar is solar with hydro storage? Or did you mean “other than photovoltaic” when you said “othet than solar”.

iamleppert2• 8 years ago

Yeah, meant to say conventional PV solar.

HIPisTheAnswer• 8 years ago

Off grid power != AC power.

Duh! Wtf do I want AC for? People think theyre all going to have a stupid fridge which opens from the side to let as much heat in as possible and with a highly inneficient compressor which is tucked behind it where the heat can only escape through it. Or a giant motor to shake their clothes in (electrically heated) water! Ahahah! I swear i keep telling people the KWh is going to cost 150-300 mg of gold (0.10 - 0.20 gold $) and their eyes just glaze over; they think it sounds nuts? If they had off grid power, they might be forced to agree. So the average person will be able to afford a couple or three KWh per month. Two low-powered computers and some lights. We light the whole place with less than 5w. The mere fact that we so often use measures like kilo watts is a good tell tale of the current power glut. The powergrid rate where we are is about 0.0015 gold $ per KWh, or 100 times below sensible prices.

segmondy• 8 years ago

Of course, if you want to run off the grid, the first thing is to minimize your electrical appliance. Get a laptop, the laptop will serve as your TV, computer, radio, etc.

No TV, no Washer/Dryer (you will hand wash), No regular fridge, get a small 12v fridge. No microwave, use stove to heat up food. No A/C, design the space to not get hot, high windows, under the shade, facing away from the sun. Maybe a small fan. The biggest optimization begins with your appliances.

Why a Typical Home Solar Setup Does Not Work with the Grid Down

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