Plan: Off grid solar with a Model S battery pack at the heart

[Dave EV]

The low charge current itself isn't the issue. It's the *continued* low charge current once the pack is full that is.

But that's just it - 4.05V isn't "full" and neither is 4.1V. Full is somewhere closer to 4.2V, but for short periods of time can probably take 4.3V or more.

If the goal is to make the cells last as long as possible, don't worry about "floating" the cells - just worry about keeping your "full" voltage as low as possible and keeping the temperature of the cells low.

 

[wk057]

First uninterrupted week 100% off grid. (As of yesterday at ~noon).

Some stats for last Saturday at midnight to midnight this morning (1 week):

AC Power used: 844.3 kWh (~907.5 kWh DC side taking inverter average efficiency into account)
PV Power generated (DC): 971.7 kWh
Estimated PV power wasted due to high battery SoC: ~150kWh.
Estimated battery capacity at midnight this morning: ~135kWh. (Using 5%-90% SoC window, similar to the Model S)
Approximate Model S miles driven this week (two cars): ~700
Overall weather for the week: crappy.
Compromises made: zero.

 

[ModelX]

wk057, I don't understand much of this thread-but still find myself reading it! This seems like an amazing project!

 

[Banahogg]

Estimated PV power wasted due to high battery SoC: ~150kWh.

That looked like a pretty nice hill in the back of one of your pictures - time for some home pumped hydro (and/or home scuba pool)? ;-)

http://www.wolframalpha.com/input/?i=%28150+ft+*+50+ft+*+50+ft%29+*+%28water+density%29+*+9.86+m%2Fs^2+*+50+ft+in+kWh

 

[wk057]

That looked like a pretty nice hill in the back of one of your pictures - time for some home pumped hydro (and/or home scuba pool)? ;-)

http://www.wolframalpha.com/input/?i=%28150+ft+*+50+ft+*+50+ft%29+*+%28water+density%29+*+9.86+m%2Fs^2+*+50+ft+in+kWh

Interesting.

Well, part of the hill is fixed now.

Brought in ~25 dump truck loads of dirt and built up that back corner almost 5'. Can actually reach that last combiner box now. Hehe. Should look pretty nice once I get the rest of the weed-block (black paper stuff under the panels) down, some nice stone (leaning towards Alabama river rocks), fence done, grass fixed, etc.

Edit:

For comparison, here is a pic of the first couple of loads of dirt that were dropped off:

Note that a 6' ladder is still something like 2' below that last combiner box in that picture. Definitely was a lot of dirt...

 

[AudubonB]

Yikes.

While the end result looks great, what those photos reminded me of was the Russian Method Of House Building: put up the house first, then dig the basement and erect the foundation. Howinsamhill did you get all that fill into and around the PV framework? Mostly by muscledraulics, or did you have a teeny skidsteer to squeeze in there?

 

[Sogorman]

@wk, congrats from a small time 7.5Kwh Grid tied generation station. Congrats on your first week of grid indepediance!

 

[wk057]

Yikes.

While the end result looks great, what those photos reminded me of was the Russian Method Of House Building: put up the house first, then dig the basement and erect the foundation. Howinsamhill did you get all that fill into and around the PV framework? Mostly by muscledraulics, or did you have a teeny skidsteer to squeeze in there?

The plan was to have this done after concrete was poured but before the racks went up (this way the concrete was embedded in the original earth so no question about stability). But the landscaping crew fell behind the time table and I wasn't holding up progress for them.

They ended up just bringing 8 guys with shovels, pushed the dirt up against both sides of the array posts with the skid steer, and they went to work building up the dirt underneath and used a plate packer on it. Landscape company owner was probably kicking himself for not sticking with the original schedule and getting it done while it was easy (where the skid steer would have easily just fit through the middle of the posts), but this was planned weeks in advance, and he knew the time line. We even fell behind on the racking system install a bit due to weather and some emergency work the guys I had helping me (local electricians) had to do. So, the landscape crew had plenty of time to get this done. I tried to get them over here several times before the bracing went up and was told it was fine and they'd make it work. So, there it is.

@wk, congrats from a small time 7.5Kwh Grid tied generation station. Congrats on your first week of grid indepediance!

Thanks!

 

[nativewolf]

What to do with all the juice

First uninterrupted week 100% off grid. (As of yesterday at ~noon).

Some stats for last Saturday at midnight to midnight this morning (1 week):

AC Power used: 844.3 kWh (~907.5 kWh DC side taking inverter average efficiency into account)
PV Power generated (DC): 971.7 kWh
Estimated PV power wasted due to high battery SoC: ~150kWh.
Estimated battery capacity at midnight this morning: ~135kWh. (Using 5%-90% SoC window, similar to the Model S)
Approximate Model S miles driven this week (two cars): ~700
Overall weather for the week: crappy.
Compromises made: zero.

So you have too much juice and a slope? The only logical outcome is a for you to create a winter sports playground, turn the pool into a kids skating area and build a sled run complete with snow gun and artificially cooled mats, you could then put a large hockey rink down at the bottom of the hill. Problem solved, juice has a home and the slope and pool put to good use in the winter.

 

[jbabb]

First uninterrupted week 100% off grid. (As of yesterday at ~noon).

Some stats for last Saturday at midnight to midnight this morning (1 week):

AC Power used: 844.3 kWh (~907.5 kWh DC side taking inverter average efficiency into account)
PV Power generated (DC): 971.7 kWh
Estimated PV power wasted due to high battery SoC: ~150kWh.
Estimated battery capacity at midnight this morning: ~135kWh. (Using 5%-90% SoC window, similar to the Model S)
Approximate Model S miles driven this week (two cars): ~700
Overall weather for the week: crappy.
Compromises made: zero.

Nice stats!
What is the water temperature of the pool? I think it needs to be a little warmer to avoid wasting solar power.

 

[nativewolf]

Nice stats!
What is the water temperature of the pool? I think it needs to be a little warmer to avoid wasting solar power.

Warmer???? Cooler is what it needs to be, to soak up all that power he needs skating, an ice rink would do just fine.

Or, a high powered searchlight beaming up into the sky with the days stats burning into everyone's sub conscious. They'll look up and see that he's produced 160 kWh today and they'll writhe with jealousy.

 

[wk057]

Yeah, I've been trying to wrap my heard around some good loads to waste excess power... but there aren't really any that can absorb the amount of power that I have in excess on days like this past Sunday, yesterday, and today.

Today is probably going to be my most wasteful day yet. My wife is out until ~5-6PM, I have work to do at home, and it's barely in the 70s outside so little need for HVAC. Combine that with a beautiful sunny summer day and my batteries being over 80% already at sun rise and it's a recipe for much wasted power.

Tomorrow things should be less wasteful as her car tops off in the morning, but still probably not going to be enough.

The only way I can figure to get rid of that much power would be into the grid, and I'm not setup for that currently (and I didn't plan to any time soon). It also takes months to get an interconnect here for renewables, so not even worth it right now since by the time it gets done it'll be winter and I'll need most of my power. Honestly, I almost still think it may not be worth the wear on my inverters to pump back into the grid at the super low prices they're willing to pay ($0.03 to $0.04/kWh). Even if I had 100 kWh/day excess most of the summer we're talking about like $300-$400/yr in earnings. To do the interconnect will cost me ~$300+ in fees, plus I need to install an external grid disconnect by my meter (probably $1000+). So while it might be nice to not waste the incoming power... honestly, it's pretty much not worth it to me to help out the grid. Maybe if the interconnect wasn't such a PITA and expensive I would do so, but probably no help there.

I'm considering coming up with some way to store energy across seasons, or even across weeks, but nothing that really makes a lot of practical sense just yet.

Storing power as potential energy by say, pumping water up 30' and capturing power as gravity reclaims it later, is impractical. If I did my napkin math right, ~30,000 gallons of water pumped 30' up, with 100% efficiency, only has ~3 kWh of energy coming back down. Not very useful.

Some way to store excess power as heat might make sense with awesome insulation. But from what I've determined, even with super insulation thermal storage is only really good for a couple of weeks at best, so probably impractical, although I kind of want to make up a small water tank to test this. Like a 5 gallon bucket filled with hot water (~150F), cap it off, wrap it in mylar reflective film (reflect IR), then some aerogel insulation, then repeat that layering a couple of times.... then let it sit for a week in my electrical room that's at 64F and then check the temperature again.

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Warmer???? Cooler is what it needs to be, to soak up all that power he needs skating, an ice rink would do just fine.

Or, a high powered searchlight beaming up into the sky with the days stats burning into everyone's sub conscious. They'll look up and see that he's produced 160 kWh today and they'll writhe with jealousy.

hah

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Nice stats!
What is the water temperature of the pool? I think it needs to be a little warmer to avoid wasting solar power.

Pool temp is pretty high already just from normal solar exposure.

 

[Banahogg]

Storing power as potential energy by say, pumping water up 30' and capturing power as gravity reclaims it later, is impractical. If I did my napkin math right, ~30,000 gallons of water pumped 30' up, with 100% efficiency, only has ~3 kWh of energy coming back down. Not very useful.

Yeah, that was the Wolfram Alpha link I posted. For an interesting amount of storage, you really need something that's basically two extra-deep olympic size pools and at least tens of feet height (ideally more).

Given that you're just throwing away the energy right now, even something low efficiency like hydrogen conversion could be interesting, but that's another level of complicated (though there are actually some plausible kW-level fuel cells purchasable)

One of my favorite posts on this topic: Got Storage? How Hard Can it Be? | Do the Math

 

[jaguar36]

You could start mining Bitcoins with the excess power, dunno what the going rate for that is these days.

To store reasonable amounts of energy in water, you need something like Niagara Falls where they save some of the water that would be going over at night for daytime energy production.

 

[TheTalkingMule]

This is where I think we end up in fairly short order and why I think fuel cells are more viable than the Musk follower consensus. Distributed solar is going to be so incredibly cheap that we'll have a massive overabundance of energy and will need something more flexible than batteries with unlimited capacity. All we're doing is splitting water and then putting it back together again, that has to be the best avenue. It just needs major technological advances in H storage an mild improvements in overall efficiency.

That solar-hydrogen house in the woods in Jersey is the model IMO. It'll just be much more streamlined and prettier.

 

[nativewolf]

Yeah, I've been trying to wrap my heard around some good loads to waste excess power... but there aren't really any that can absorb the amount of power that I have in excess on days like this past Sunday, yesterday, and today.

Today is probably going to be my most wasteful day yet. My wife is out until ~5-6PM, I have work to do at home, and it's barely in the 70s outside so little need for HVAC. Combine that with a beautiful sunny summer day and my batteries being over 80% already at sun rise and it's a recipe for much wasted power.

Tomorrow things should be less wasteful as her car tops off in the morning, but still probably not going to be enough.

The only way I can figure to get rid of that much power would be into the grid, and I'm not setup for that currently (and I didn't plan to any time soon). It also takes months to get an interconnect here for renewables, so not even worth it right now since by the time it gets done it'll be winter and I'll need most of my power. Honestly, I almost still think it may not be worth the wear on my inverters to pump back into the grid at the super low prices they're willing to pay ($0.03 to $0.04/kWh). Even if I had 100 kWh/day excess most of the summer we're talking about like $300-$400/yr in earnings. To do the interconnect will cost me ~$300+ in fees, plus I need to install an external grid disconnect by my meter (probably $1000+). So while it might be nice to not waste the incoming power... honestly, it's pretty much not worth it to me to help out the grid. Maybe if the interconnect wasn't such a PITA and expensive I would do so, but probably no help there.

I'm considering coming up with some way to store energy across seasons, or even across weeks, but nothing that really makes a lot of practical sense just yet.

Storing power as potential energy by say, pumping water up 30' and capturing power as gravity reclaims it later, is impractical. If I did my napkin math right, ~30,000 gallons of water pumped 30' up, with 100% efficiency, only has ~3 kWh of energy coming back down. Not very useful.

Some way to store excess power as heat might make sense with awesome insulation. But from what I've determined, even with super insulation thermal storage is only really good for a couple of weeks at best, so probably impractical, although I kind of want to make up a small water tank to test this. Like a 5 gallon bucket filled with hot water (~150F), cap it off, wrap it in mylar reflective film (reflect IR), then some aerogel insulation, then repeat that layering a couple of times.... then let it sit for a week in my electrical room that's at 64F and then check the temperature again.

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hah

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Pool temp is pretty high already just from normal solar exposure.

Well now lets get creative (the OP obviously has never spent a summer bucolic Hickory NC), you could heat it some more...and have the Guinness Book of World Records cooking pot, need to boil 10,000 hot dogs, no problem. Need to cook a small elephant, no problem.

Really you just need to abandon the idea that it is a swimming pool and consider it a large container of water that should be boiling or frozen.

Personally, I am banking on the spotlight. I could actually see you pulling that off.

 

[beeeerock]

Yeah, I've been trying to wrap my heard around some good loads to waste excess power... but there aren't really any that can absorb the amount of power that I have in excess on days like this past Sunday, yesterday, and today.

Sounds like you're about ready to start your own power utility and sell power to the neighbours... :biggrin:

It makes me wonder, though, what is involved in selling 'green power'. I know there are companies up here that you can pay to provide power certified to be green... at a premium. It goes into the grid somewhere and customers pull it out. But I expect it would require more scale. Maybe people need to get together and create a green power co-op to represent a bunch of people as one big(ger) supplier. More people will have the same conundrum as you as time goes on... (which is actually a Good Thing!).

Failing that... call the pool a really, REALLY, big hot tub....

 

[nativewolf]

Sounds like you're about ready to start your own power utility and sell power to the neighbours... :biggrin:

It makes me wonder, though, what is involved in selling 'green power'. I know there are companies up here that you can pay to provide power certified to be green... at a premium. It goes into the grid somewhere and customers pull it out. But I expect it would require more scale. Maybe people need to get together and create a green power co-op to represent a bunch of people as one big(ger) supplier. More people will have the same conundrum as you as time goes on... (which is actually a Good Thing!).

Failing that... call the pool a really, REALLY, big hot tub....

Currently in the US there is not regulatory structure to facilitate micro power producers. Besides, the insurance and paperwork would likely drown those that tried. Now if you could get a large community of solar power users to share a generating base you'd be in un chartered water but storage options start looking more interesting. Perhaps at that level they could buy the local grid and have a tie in to send power out when possible. Over time we'd see power rates from 10am-4pm drop close to 0. We might anyways.

South of Hickory Duke power runs a very profitable power generation facility that pumps water from a lower reservoir up to a higher reservoir and releases it to create hydro power. I think there is more potential for that sort of play along the escarpment on 77. Micro producers could pump at daytime with cheap solar, move a column of water up 300' or so and then let it run down when power prices justify (5-9pm).

 

[dhanson865]

Over time we'd see power rates from 10am-4pm drop close to 0. We might anyways.

My god, if we can get to the point that daytime PV lowers the cost of electricity to 1 cent per kWH or less it'll drop my summer and winter AC/Heating bills by 95%. I'd just pump heat or cool into the house during those hours and then fall back to no heat/ac most of the rest of the day/night. Can you imagine me heating the house to 80F in the afternoon and letting the temp drift down to the 60s F at night in the winter? Even better when I can pump AC in during the part of the day when the sun is beating down maybe getting into the 60s and let it drift back up into the 70s at night. I'd still have to run AC or a dehumidifier outside of the cheap time of day in the summer but in the winter I don't have to humidify for any but the most extreme winter days.

I'd just stay on grid and get a small PV setup enough to cover my base load (size it for spring/fall historical load) and let the super cheap grid power cover my variable load during the cheapest time summer and winter.

I could surely charge my cars free or cheap during a window like that 2 days a week, charge them for free at work the other days (already doing some free charging at work now).

I've got to think an overkill setup like wk057's is a bet that cheap/free grid electricity won't happen any time soon. I just hope a lot of others go forth and install big arrays as well so I might see a drop in pricing (either in PV panels/rest of system or grid price per kWh at some time of day).

 

[beeeerock]

Currently in the US there is not regulatory structure to facilitate micro power producers. Besides, the insurance and paperwork would likely drown those that tried. Now if you could get a large community of solar power users to share a generating base you'd be in un chartered water but storage options start looking more interesting. Perhaps at that level they could buy the local grid and have a tie in to send power out when possible. Over time we'd see power rates from 10am-4pm drop close to 0. We might anyways.

While I don't expect every homeowner with a big yard to take on a project like wk057's any time soon, his happy problem is one that is sure to become more common in the future. There is certainly a move among homeowners to take ownership of at least a portion of their own power requirements. I expect/hope this snowballs.

Much like an open source project... or perhaps 'crowd-sourcing project' is a better description... the grid and electric economy will have to make some significant adjustments to fit in the future world.

South of Hickory Duke power runs a very profitable power generation facility that pumps water from a lower reservoir up to a higher reservoir and releases it to create hydro power. I think there is more potential for that sort of play along the escarpment on 77. Micro producers could pump at daytime with cheap solar, move a column of water up 300' or so and then let it run down when power prices justify (5-9pm).

In a perfect world, wk057 would be able to sell his surplus power to this cause (or equivalent) at a better rate than his grid tie allows. He *should* be able to! Every kWh that comes from a green source offsets a dirty one. Power needs to become more public and less monopolistic. Easier said than done, I admit.

Rather than hijack his thread any more than I have already, I'll leave it at that... :biggrin: I'm envious of your project, wk057, and happily offer 5 Attaboys for your efforts! :smile: