off-the-grid solar-based electrical generation system

[NW-Bound]

If I were going off-grid solar, I think I'd try to rig as much of my house to run on DC as possible, to avoid the power loss inherent in the inverter. You wouldn't have long wiring runs, so I think line losses would be minimal.

Lighting would be easy enough using modern LEDs. I think the bigger challenge (and possible benefit) would be from maybe using an RV-style refrigerator/freezer so it could run on normally DC and even propane in a pinch (without the conversion losses of running the generator). A small backup propane furnace and even adsorptive AC (using propane) would complete the "big-ticket" items and help reduce the "emergency" load on a generator if the batteries run low. A DC well pump would be easy in theory, but might be hard to find in the real world.

The only things one can find running on DC are LED lights meant for RVs. How about other devices like computers, TV, stereos, etc... ?

I converted most of the LED lights in my motorhome, using homebrew DC/DC converters hidden inside the fixtures.

I bought a 25' TV for my motorhome, and it turned out to run off an external power supply that converted 115VAC to 28V. So, I replaced it with a DC/DC converter that runs off 12V instead.

For all other devices, I still use 115VAC. I use a 2KW pure-sine-wave inverter that I mounted hidden inside a cabinet. When idling without a load, it draws only 0.5A or 6W, which is surprisingly low. The efficiency is something like 90% under heavy loads.

Ammonia-absorption refrigerators are commonly used in RVs because the convenience of using propane which has a high energy density. These fridges are very inefficient though. When running the heater with electricity instead of burning propane, my smallish 6-cu.ft. fridge draws 350W.

So, it is better to run a residential fridge with an inverter. This is quite doable in a fixed installation, and in fact some RV'ers do this on their large class A which has more roof area for solar panels than my smallish class C. The overnight battery storage to run a fridge is not outrageous.

Many RV'ers who are full-timers and who are into boondocking now use large lithium battery banks. They even have a large enough inverter to be able to run their roof-mounted AC for a few hours with the batteries supplementing the solar panels. A residential installation would have room for a lot more solar panels than can fit on an RV roof.

So, it is all doable, but the cost may be in the $20K or higher for a small home of perhaps 1,000 sq.ft. One would still need a backup generator for those cloudy days.

 

[travelover]

.......

Ammonia-absorption refrigerators are commonly used in RVs because the convenience of using propane which has a high energy density. These fridges are very inefficient though. When running the heater with electricity instead of burning propane, my smallish 6-cu.ft. fridge draws 350W.

So, it is better to run a residential fridge with an inverter. This is quite doable in a fixed installation, and in fact some RV'ers do this on their large class A which has more roof area for solar panels than my smallish class C. The overnight battery storage to run a fridge is not outrageous.
..........

I was quite surprised to learn how inefficient the absorption refrigerators are, as my camper fridge will run 3 weeks on a 20 pound propane tank. However in the 12 volt DC mode, it will only run about 5 hours on the 100 amp hour battery* . Efficient 12 volt compressor refrigerators are available, but cost 5 or 6 times as much as a 110 volt one of similar size.


* taking the battery to a only 50% discharge to avoid damage

 

[NW-Bound]

Propane has much better energy density than any type of battery ever invented. Lemme see...

20 lbs of propane at 50 MJ/kg can deliver 454 MJ. To provide heat equivalent to a 350W heater, it would last 1.3 million seconds, or 360 hours, or 15 straight days. As the fridge does cycle off when cooled down, a 3-week run is not out-of-line.

Yes, 12-VDC compressor fridges are available, and more common on yachts than on RVs. They are pricey, but absorption fridges are not cheap either. My 6 cu.ft. Dometic costs $900 new. A compressor fridge may be about $1,200.

 

[ERD50]

The only things one can find running on DC are LED lights meant for RVs. How about other devices like computers, TV, stereos, etc... ?

I converted most of the LED lights in my motorhome, using homebrew DC/DC converters hidden inside the fixtures. ...

I was curious about stuff that could run on AC or DC seamlessly. I read up on 'universal' motors, which are common for small appliances, but they have relatively low efficiencies, and use brushes/commutators which are a wear and noise (electrical and acoustic) source.

If I were going off-grid solar, I think I'd try to rig as much of my house to run on DC as possible, to avoid the power loss inherent in the inverter. You wouldn't have long wiring runs, so I think line losses would be minimal.

In almost all cases, you will still need to do DC-DC conversion, I'm not sure you've gained much, if anything - I think efficiency is going to be close? DC-DC converters are switching converters, essentially the same as the AC-DC converters used today. And since solar panel output voltage varies with the sun and loading, you will need a DC-DC converter anyhow to provide stable DC, even if the device runs on voltage in the range of what a solar panel produces.

And if you use batteries for their stable voltage, you will still need DC-DC converters for the various devices that will all run on a different DC voltage, and the solar-battery charger unit will effectively be a smart DC-DC converter, not really much different from an inverter.

So while it seems smart on the surface to avoid a conversion to AC, in practice I'm not sure there is much, if any gain.

On an industrial level, it would seem that if a site could have solar panels and some sort of load that could handle swings from solar, and take the variable DC voltage/current directly, you might avoid the conversion losses. But that still requires switchable loads as the load must be matched to the panel under variable sun levels or you lose efficiency. But that would be simple switches, with near zero loss. But what kind of load would that be?

-ERD50

 

[Totoro]

Aren't there also differences in battery chemistry?

Curious myself (longer term) - specifically a bit confused between 'wet' batteries (NiFe and such) and Li-Ion Tesla powerwall types in terms of when to use which ones.

I would be looking at a small system, just a few kwh per day but I'm guessing that doesn't really matter.

 

[ls99]

I bought a 25' TV for my motorhome, and it turned out to run off an external power supply that converted 115VAC to 28V. So, I replaced it with a DC/DC converter that runs off 12V instead.

......

Man, how big is your motorhome to fit a 25 foot TV??

Just kidding about the typo Especially since I make typos a standard part of my postings.

 

[Totoro]

Multiple screens folded up
CES 2016: Samsung reveals world's largest 170-inch SUHD television

 

[ls99]

Multiple screens folded up
CES 2016: Samsung reveals world's largest 170-inch SUHD television

I suspect it costs more than a camper or a MOHO

 

[NW-Bound]

Aren't there also differences in battery chemistry?

Curious myself (longer term) - specifically a bit confused between 'wet' batteries (NiFe and such) and Li-Ion Tesla powerwall types in terms of when to use which ones.

I would be looking at a small system, just a few kwh per day but I'm guessing that doesn't really matter.

I have read that NiFe batteries have extremely long life, and can stand up to a lot of abuse like over-charging and over-discharging. They are not popular and not easy to get. A Web source quotes a price of $2,900 for a 3.6-KWh battery.

Lithium-Iron-Phosphate (LiFePO4) batteries are more common with RV'ers and boaters. They require a BMS (Battery Management System) to protect from overcharging/discharging, which will ruin them. They can stand up to huge charging and discharging currents. A 3.6-KWh battery costs around $1,900.

Lithium batteries do wear out with use. A manufacturer shows a loss of 20% of capacity after 2,000 cycles of deep discharge. Common lead-acid types would not last that long, even when limited to 50% discharge.