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Okjoek

DC Solar power systems?

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Posted · Original PosterOP

I was watching a recent Techquickie vid...

and it got me thinking. Most solar power systems I've seen involve an inverter that changes the DC coming from the charge controller into AC for home appliances. I was wondering though, if the point of AC is just for transferring electricity over long distance, why would you need it for a solar power kit for a home system? Couldn't you just find appliances capable of running off DC so you're not losing efficiency by turning it into AC and then back again?

 

Like I'm sure there's already DC compatible lighting systems, small electronics chargers like phone chargers, USB appliances and stuff, but what about some bigger fish like perhaps...

laptop chargers?

A small refrigerator?

A water pump?

A router-modem?

 

 


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9 minutes ago, Okjoek said:

I was watching a recent Techquickie vid...

and it got me thinking. Most solar power systems I've seen involve an inverter that changes the DC coming from the charge controller into AC for home appliances. I was wondering though, if the point of AC is just for transferring electricity over long distance, why would you need it for a solar power kit for a home system? Couldn't you just find appliances capable of running off DC so you're not losing efficiency by turning it into AC and then back again?

 

Like I'm sure there's already DC compatible lighting systems, small electronics chargers like phone chargers, USB appliances and stuff, but what about some bigger fish like perhaps...

laptop chargers?

A small refrigerator?

A water pump?

 

 

Electrical energy is distributed as alternating current because AC voltage may be increased or decreased with a transformer. So it would be hard to turn 110V source to 12V or 9V charging output for other devices or even appliances.(ex. you'd need much BIGGER power bricks) Not to forget that it'll output much more heat. I think engineers thought of that a long time ago. :) 


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14 minutes ago, Okjoek said:

I was wondering though, if the point of AC is just for transferring electricity over long distance, why would you need it for a solar power kit for a home system? Couldn't you just find appliances capable of running off DC so you're not losing efficiency by turning it into AC and then back again?

Yes, you can find appliances that run off of DC, but they are very expensive compared to the AC-stuff and you do, actually, have to keep efficiency in mind, as well: the lower your voltage, the higher the current you need to push through the wires for the same power, and that means bigger losses in the wiring, unless you go for very thick and expensive wiring. Just think about e.g. some appliance that consumes 2000W -- with 230V AC, you need about 8.7A of current, but with a 12V DC - system, you'd need a whopping 167A of current, meaning REALLY THICK WIRES.

 

For all intents and purposes, it's both simpler and cheaper to use an inverter and go with AC.


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7 minutes ago, phoon said:

So it would be hard to turn 110V source to 12V or 9V charging output for other devices or even appliances.

It's actually not that difficult, it's just basic SMPS-design.


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3 minutes ago, WereCatf said:

It's actually not that difficult, it's just basic SMPS-design.

You're right, but it costs more to produce and SMPS designs are outdated, not that efficient and can fault more than newer technologies.


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Posted · Original PosterOP

@phoon

@WereCatf

Ooooh. Okay. 

 

Reason I was wondering is because it's so hard to figure out what kind of solar system one would need and adding the need for an inverter throws another variable to be unsure about what's good enough into the system.

 

Like I want to plan systems designed for people using improvised shelter, migrants, homeless, anarchists, minimalists and settlers around mostly essentials like...

 

Lighting

 

Food preservation

 

Running water

 

Communications, internet if possible.

 

 


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35 minutes ago, Okjoek said:

@phoon

@WereCatf

Ooooh. Okay. 

 

Reason I was wondering is because it's so hard to figure out what kind of solar system one would need and adding the need for an inverter throws another variable to be unsure about what's good enough into the system.

 

Like I want to plan systems designed for people using improvised shelter, migrants, homeless, anarchists, minimalists and settlers around mostly essentials like...

 

Lighting

 

Food preservation

 

Running water

 

Communications, internet if possible.

 

 

Yeah but as I said our current technologies are much more efficient and safe to use as they are. I get where you're coming from. Good topic though


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1 hour ago, Okjoek said:

Like I want to plan systems designed for people using improvised shelter, migrants, homeless, anarchists, minimalists and settlers around mostly essentials like...

 

Lighting

 

Food preservation

 

Running water

 

Communications, internet if possible.

Lighting is easy enough with all the new LED lights, a lot of them have a separate power supply, so just chuck that and wire 12V straight in.

 

Fridges are a lot harder to find, but there are some options out there meant for RVs. They are really small tho. Same goes for stoves, stick with gas/wood for them.

 

Water is another easy one, a lot of 12/24V pumps available, at least from a minimalist approach. I'm not seeing much over 7.5GPM, where a typical house is going to be 15-30GPM.

 

Communications is really easy, just about everything is already 12V or 5V, so worst case scenario you could just put some resistors in to drop to 5V. Even a cheap buck converter could be used to boost to 24/48V for POE.

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As it was mentioned above me, AC is good for long distance transfer (under 300km, after that the skin effect makes HV DC transmission more efficient) and it is waaay easier to step up and down for distribution purposes. Now about your question about DC appliances, most devices can actually run on DC with no modification. The exception being AC transformers, big induction motors, and capacitive droppers for LED lighting. Everything else uses a rectifier right at the input to turn the power back into DC, so it doesn't care if you feed it 100-250VDC. 

 

The lab I work at at ASU is working on a project with HVDC. One of our first projects was testing how SMPSs would hold up on DC. So, I spent a month loading up the supplies to their rated current while feeding them 240VDC. They performed as well if not better than a similar load test at 240VAC.


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2 hours ago, Okjoek said:

-SNIP-

Most of that is very possible since a lot of those systems you are describing has been made for boats and RV's. In most situation you will want to standardize the system on a single voltage either 24V or 48V, the higher voltage will be better than 12V for larger load appliances. One of the main things for electronics is many will require stable regulated  power from the batteries as they will fluctuate when charging and discharging. 

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I've looked into this with the intention of building an off-grid cabin.  Basically, I was going to wire the house for 12V and 110V.

Lights are a no-brainer, go DC.  LED lights sufficient for illumination are easy to find in automotive bayonet-style packaging and "cut what you need" strips, all standardized on 12 volts.  With DC, you need no additional circuitry, and you don't get flicker.  All told, very efficient.  Even if you have an inverter, I suggest running the lights on DC since you would then have lights with the inverter turned off or failed.

 

Electronics--I just looked around.  My Wi-Fi/Ethernet switch, Roku 3, computer monitors, and a handful of other appliances run on 12 volts via wall warts.  Replace their wall warts with simple passive cables and you can run them directly from batteries.  In practice, you'll just never know what voltages or amperages consumer electronics want, so for user friendliness I suggest an inverter for this.
That said, you mention homeless/refugee/emergency use...well, you could boil down any possible need for consumer electronics into one smart phone, and it doesn't take much to power one of those.  I could even see a small laptop-sized device being run from a man-portable solar system.  I would choose something that charges via USB-C here, for simplicity's sake.

 

Running water.  12 volt DC pumps are widely available for RV use; they tend to be suited to pumping water from holding tanks to faucets.  Larger tasks like drawing water from wells is pretty much exclusively done via AC grid power.  I know of a lot of portable or mobile devices for purifying water that require no electricity at all.  I'm thinking things like the Lifesaver Bottle.  For homesteading, you're probably either carrying in water or running a well pump from AC.

 

Refrigeration is a whole other ballgame.  I know of at least three different ways off-grid refrigeration is accomplished. 

  1. Your typical household fridge (along with the heat pump or an automotive air conditioner) uses a motor-driven compressor, condenser, and evaporator to use the fact that boiling a liquid absorbs heat from the surroundings.  There are a few of these available designed for portable or RV use that can run their motors on 110VAC or 12VDC.  It's designed to run on DC from an RV's main engine while driving, and then plug into ground power at a campsite.  They could theoretically be powered from solar, but you'd need lots of panels and batteries to do it continuously.
  2. RV fridges are overwhelmingly absorption fridges.  I've tried to wrap my mind around how they work, but the best I can do is "ammonia is dissolved in water, physics happens, things get cold, then you have this useless ammonia water.  Fortunately you can get the ammonia back out of the water with some heat, then you can cool them down separately and use them again."  These usually have two or three sources of heat:  Propane for "boondocking" or off-grid use, 110VAC for when plugged into shore power, and optionally 12VDC for use while driving* or conceivably on solar.  If doing the off-grid thing, I could see using a 12V mode off solar under the "batteries are charged, no other loads on system, panels would be turned off otherwise" to save some gas.  It would also be possible to power these from sunlight directly, though I don't know of any off-the-shelf units that do that.
    *For those units that can operate while in motion; many require being pretty close to perfectly level to operate.
  3. Some gizmos use solid-state cooling via the peltier process.  It's sort of like an anti-backwards thermocouple.  Put electricity into this thing, and one side gets hot and the other side gets cold.  They're simple, compact and lightweight, but they aren't very efficient and can usually cool to 20 degrees C or so below ambient.  Sort of a "better than nothing" solution, not much else.  In an emergency context, I see this sort of thing being more useful and necessary for keeping insulin cool than preserving food; I would suggest shelf-stable or canned food over fresh.

 

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