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"Daylight drive" direct PV solar for equipment

 
Posts: 6
Location: Appalachia
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Just wanted to share this with everyone, it's an interesting idea that could reduce the complexity of a solar setup if you can live with the tradeoffs. Their well pump and DC-powered workshop only operate during the day, while lights and such are on a smaller NiFe bank. They even adapted cordless tools to run off the solar DC feed. https://www.notechmagazine.com/2019/07/living-energy-farm.html (Living Energy Farm, via No Tech Magazine)

I don't have solar yet, but I've been looking at systems, and this approach looks pretty attractive. (I am allergic to both unnecessary complexity and expensive maintenance.)

The PDF overview gives a more in-depth look at things.

Any thoughts from people who have or have seen similar setups?
 
gardener & bricolagier
Posts: 2611
Location: SW Missouri
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Cool! I haven't seen a setup like that,but it's interesting to me, I'm into low tech/low maintenance/low finances solar and wind generating. Thanks for the links! Welcome to Permies, an interesting first post!
:)
 
Brett Coburn
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Thanks :) I now see that there are some other posts here about direct PV (like the one right below mine, how did I miss that) but I do think this sort of "hybrid" system where you have some direct PV and some battery-fed PV seems more rare. Of course I am fairly ignorant on solar compared to people who actually know what they are doing. I just like how this system puts extra power to use during the day rather than building a massive (expensive!) battery bank to store everything.
 
gardener
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Location: SoCal USA
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This is certainly an appealing setup, I've been purchasing my solar system parts and aiming for as small of a system as reasonable, and the inverter power draw is an annoying part. Even in standby there's a phantom load from the inverter itself, and of course you'll lose about 10% of your power in the DC to AC conversion, so a 100w AC load will take 110w of DC. As the inverter capacity goes up (to handle those power hungry devices), so does the cost, and man the charger/inverter model prices are steep! So focusing more power use to DC to limit those AC needs can become cost effective.

Does anyone have experience with DC lighting, such as good sites for buying the lamps/fixtures which use DC bulbs? I have a good supply of AC-powered LED light bulbs I bought when our utility was promoting them, I think they were like $2 each. While I have AC lamps, in a few years I'll be installing fixtures in the cabin and if DC-powered options are affordable, I can do that and run a DC fuse box and skip the inverter.

DC powered fridges are easy enough to find, although pricey compared to the $25 used AC chest freezer I have connected to a $25 thermostat controller which sets the temp to 35F instead of 0F, making it use just 240wh per day from my testing in a very warm garage. With inverter perhaps 300 watts of battery charge per day. Spending $1000+ for a DC fridge for similar power use and I get freezer space and can also power it directly without the inverter, so turning off the inverter when not needed could save that phantom draw.


It all ends up resulting in less batteries needed. While the Edison nickel iron batteries are nice (especially their lifespan), the cost is still really high relative to FLA. Some sample math: 1kwh of estimated power use per day, 5 days of no charging during a winter storm would mean at least 10kwh of storage using 50% DoD for flooded lead acid batteries. NiFe Edison batteries can handle 80% DoD, so total storage needed for that 5 day window would be (5000/.8) 6.25kwh. For a 12v system, that's about 520AH and IronEdison sells 500AH worth of storage for $5468 plus tax/shipping (how much shipping for 800+ pounds of battery?).

It's rated for 11,000 cycles at 80% DoD, but since this will only happen a few days of the year during that winter storm, the cycles are probably much higher, say 15,000-20,000. Being 40-50+ years for that many daily cycles, it's essentially a once-in-a-lifetime purchase for a middle-aged purchaser.

Compared that to say a pair of Rolls Surrette 6 CS 25PS FLA batteries, similar usable capacity and about 5000 cycles for the averaged out DoD, runs $2200 plus tax (bought locally, no shipping) for almost 14 years of use for that many cycles. As a final comparison point, I picked up a pair of 1 year old FLA 225AH 6v batteries, $150 for the pair including tax and no shipping. Probably 1500 cycles left under my calculated typical use. To get the appropriate total watt/hours would cost $600 total for 4 years of use.

So overall the costs are pretty similar per year of use for FLA, if you don't abuse the battery DoD. The NiFe are less toxic and a little less maintenance, but cost the most up front and per year. Here's a point never mentioned about total cost of ownership that will always place the Edison batteries in last place- assuming you invested the *price difference* and earned interest, for example a conservative 4% return long term, the Rolls batteries are about $3400 less up front; that $3400 is worth almost $5900 after 14 years of 4% compounding interest (when you'd buy a new pair of batteries), so all future battery purchases (costing $2200 for another 14 years of batteries) are free using the interest earned, and you still have that initial $3400 of savings which is actually growing. And 4% is considered a conservative long term withdrawal rate to preserve initial capital.

So the take away is that if you have the money up front to buy the Edison batteries, and total cost is a primary driver, they are not worth buying. You can instead purchase flooded lead acid batteries, invest the rest of the money, and effectively never pay for the replacement FLA batteries again because they will all be purchased using just the interest on that 4% investment return. You essentially have "lost" that same chunk of money in that you don't get to spend it on other stuff, but at any point you can take that cash out and use it for something else, like a totally new energy storage technology, versus the Edison battery purchase is a sunk cost unless you resell the batteries. Sort of a moot point to me, I'm not trying to bash them or say it's a bad choice. But when I see sellers always pushing how the long-term cost is always less, that seems deceptive. It's the same with lithium claims of costly less long-term, it's not true when you include the opportunity cost of investing the money. Of course we could argue the chance that society collapses and those investments are lost, anything's possible. The reality is, unless you abuse FLA or hit 50% DoD on a regular basis, they are still the most cost effective option over the long term.
 
Posts: 20
Location: Proebstel, Washington, USDA Zone 6B
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What I found intriguing in that article was the fact that they took a conventional stick framed home, leaned strawbales against it, and then cobbed over that. I would have moisture concerns about that arrangement. It looks like they lost a bit of overhang from their roof. But they lost no interior floorspace. Anyone know if anyone else has given this a try?
 
steward
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Location: Cache Valley, zone 4b, Irrigated, 9" rain in badlands.
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I have an electric assist bicycle. It uses 200 Watts. Hmm. That's the same as the output of a typical solar panel. Therefore, I'm imagining a solar panel resting on a bike trailer for those times when I just have to take a hundred mile bike ride.

 
pollinator
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Mark Brunnr wrote:This is certainly an appealing setup, I've been purchasing my solar system parts and aiming for as small of a system as reasonable, and the inverter power draw is an annoying part. Even in standby there's a phantom load from the inverter itself, and of course you'll lose about 10% of your power in the DC to AC conversion, so a 100w AC load will take 110w of DC. As the inverter capacity goes up (to handle those power hungry devices), so does the cost, and man the charger/inverter model prices are steep! So focusing more power use to DC to limit those AC needs can become cost effective.

Does anyone have experience with DC lighting, such as good sites for buying the lamps/fixtures which use DC bulbs? I have a good supply of AC-powered LED light bulbs I bought when our utility was promoting them, I think they were like $2 each. While I have AC lamps, in a few years I'll be installing fixtures in the cabin and if DC-powered options are affordable, I can do that and run a DC fuse box and skip the inverter.

DC powered fridges are easy enough to find, although pricey compared to the $25 used AC chest freezer I have connected to a $25 thermostat controller which sets the temp to 35F instead of 0F, making it use just 240wh per day from my testing in a very warm garage. With inverter perhaps 300 watts of battery charge per day. Spending $1000+ for a DC fridge for similar power use and I get freezer space and can also power it directly without the inverter, so turning off the inverter when not needed could save that phantom draw.


It all ends up resulting in less batteries needed. While the Edison nickel iron batteries are nice (especially their lifespan), the cost is still really high relative to FLA. Some sample math: 1kwh of estimated power use per day, 5 days of no charging during a winter storm would mean at least 10kwh of storage using 50% DoD for flooded lead acid batteries. NiFe Edison batteries can handle 80% DoD, so total storage needed for that 5 day window would be (5000/.8) 6.25kwh. For a 12v system, that's about 520AH and IronEdison sells 500AH worth of storage for $5468 plus tax/shipping (how much shipping for 800+ pounds of battery?).

It's rated for 11,000 cycles at 80% DoD, but since this will only happen a few days of the year during that winter storm, the cycles are probably much higher, say 15,000-20,000. Being 40-50+ years for that many daily cycles, it's essentially a once-in-a-lifetime purchase for a middle-aged purchaser.

Compared that to say a pair of Rolls Surrette 6 CS 25PS FLA batteries, similar usable capacity and about 5000 cycles for the averaged out DoD, runs $2200 plus tax (bought locally, no shipping) for almost 14 years of use for that many cycles. As a final comparison point, I picked up a pair of 1 year old FLA 225AH 6v batteries, $150 for the pair including tax and no shipping. Probably 1500 cycles left under my calculated typical use. To get the appropriate total watt/hours would cost $600 total for 4 years of use.

So overall the costs are pretty similar per year of use for FLA, if you don't abuse the battery DoD. The NiFe are less toxic and a little less maintenance, but cost the most up front and per year. Here's a point never mentioned about total cost of ownership that will always place the Edison batteries in last place- assuming you invested the *price difference* and earned interest, for example a conservative 4% return long term, the Rolls batteries are about $3400 less up front; that $3400 is worth almost $5900 after 14 years of 4% compounding interest (when you'd buy a new pair of batteries), so all future battery purchases (costing $2200 for another 14 years of batteries) are free using the interest earned, and you still have that initial $3400 of savings which is actually growing. And 4% is considered a conservative long term withdrawal rate to preserve initial capital.

So the take away is that if you have the money up front to buy the Edison batteries, and total cost is a primary driver, they are not worth buying. You can instead purchase flooded lead acid batteries, invest the rest of the money, and effectively never pay for the replacement FLA batteries again because they will all be purchased using just the interest on that 4% investment return. You essentially have "lost" that same chunk of money in that you don't get to spend it on other stuff, but at any point you can take that cash out and use it for something else, like a totally new energy storage technology, versus the Edison battery purchase is a sunk cost unless you resell the batteries. Sort of a moot point to me, I'm not trying to bash them or say it's a bad choice. But when I see sellers always pushing how the long-term cost is always less, that seems deceptive. It's the same with lithium claims of costly less long-term, it's not true when you include the opportunity cost of investing the money. Of course we could argue the chance that society collapses and those investments are lost, anything's possible. The reality is, unless you abuse FLA or hit 50% DoD on a regular basis, they are still the most cost effective option over the long term.



Outside the US the math may change. Everything is more expensive in Canada, especially shipping... those Rolls would be a pretty penny here. OTOH, lack of tariffs meant my chinese LiFePO4 cells were noticeably cheaper here than in the states.

Being in a tinyhouse, some of the other upsides are nice: sealed cells that require no venting and do not offgas, and occupy less space. Keeping lead acid cells outside is not practical with extended periods of freezing weather..

Of course given that they are chinese made, the long term value of my cells is something that only time will tell..
 
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