OFF GRID POWER; various thoughts on...

[Dave Sparks]

For the distances being discussed, say a maximum of 500 feet from top to bottom, there is very little difference between AC and DC transmission losses.   It is mainly a question of volts and amps.  AC does have the advantage of lower cost disconnects and fusing compared to DC.   It could be worth a look at it. 

A detailed look at what the actual AC and DC loads in the house and the cabins needs to be done before any real planning on one system vs another, IMO.

Totally agree and even longer distances than 500 feet. Much better to keep all the electronics and batteries close to the house and run DC the distance. Run the DC at 300 to 400DC mppt and live the good life!  I do this as much as possible for my 85 offgrid homes.

[old_guy]

First, thank you for the replies.  The internet is full of information, but these few paragraphs here have given me more insight than many hours of online searching and reading.

We will need 240 at the barn and possibly at the house.  Occasional tools usage at the barn, and possible well usage near the house.  Well is actually TBD.  A local well company said wells in that area are generally about 500' and $10,000.  Our nearest neighbor collects water into a cistern from 2 year-round springs, and said he has found at least one spring on our hill which may be a better option than the well.  I will verify necessity for a well.

I have done some power estimates using our home as the example, and will grab actual info from our next bills.  We are terribly wasteful in our current (pun intended) power usage, with AC, 2 refrigerators, 1 large freezer, incandescent bulbs everywhere, several TVs always waiting for input from a remote, etc.  Our youngest children have been (almost) gone for several months, and we already see that our electricity needs will shrink drastically.  One of the refrigerators will go, and the freezer can shrink by 1/2.  The AC will morph into mini-splits, and whatever TV we keep will probably get turned OFF when not in use.  We will not have natural gas in Tennessee, however, so may use electricity to replace some of our current gas usage. 

We will not have a grid connection.  We are about 3500 feet from the pavement and nearest power line.  Our nearest neighbor was told it would be over $20,000 for them to get power brought in, and they are less than half that distance.  $30,000 - $40,000 for us to run power lines through a forest full of line-breaking limbs seems a poor investment.

The cabins ( 3 @ 16' x 24' ) will each have lights, a small fridge, and a microwave.  Probably also a TV for DVDs and video games.  Propane for cooking, heat and water heating.  I expect no AC.  Water will be piped in from uphill, with pressure regulated at the cabins.

As I get better usage numbers I can better size the system, but am hoping to determine how to do it, then later figure out how big.  FWIW, the property is at about 36.45 degrees north latitude.  Again, thanks for the feedback.

[old_guy]

Totally agree and even longer distances than 500 feet. Much better to keep all the electronics and batteries close to the house and run DC the distance. Run the DC at 300 to 400DC mppt and live the good life!

I had to ponder this for a bit, but think I get it.  The high voltage DC reduces voltage drop because there is less current than at a lower voltage.  Any voltage drop from transmission losses will be between the panels and the house (batteries, electronics).  The controller will convert this DC (at whatever voltage) to the appropriate voltage for charging the batteries, and the inverter can then provide AC at full voltage without the line drop.  If converted to AC at the panels, the AC voltage at the house could drop considerably.

Transmission losses still exist, but do not affect the AC voltage at the house.

A possible downside would be more expensive high-voltage DC switches and breakers?

Thank you.

[Dave Sparks]

Well, voltage drop is voltage drop, AC or DC is for our purposes the same. It does take DC rated switches and other components.
But yes the high voltage will mean less current and so there will be less loss or drop. And the best part is # 8 or so wire for a 5,000 watt solar system.

You are right the new roof solar converts the DC at the panel and makes it simple for residential as there is not a DC run. I dislike them because in hot climates electronics on a roof is just against everything I know.

The cost difference is small for DC rated components, especially if you are my client....Drop me a line or go to my webpage

Shameless plug but actually a very good deal for anyone who can do the basic handyman skills of installing solar. All the other things I help with are on my webpage!

[MountainDon]

FWIW, I believe you can trust Dave and what he says.

[old_guy]

Well, voltage drop is voltage drop, AC or DC

Thank you for the reply, Dave.  And yes, Don, Dave seems to really know his stuff.  I am glad I asked my questions during one of Dave's "very active" phases.   : - )

It seems to me, though, that having the long run be DC between the panels and the controller and batteries would be better regarding the drop.  The voltage drop will happen there, but the controller will then correct it to the proper charging voltage for the batteries, and full converted AC voltage will be available at the house.

If The AC was converted at the panels then sent the 300-400 feet to the house, the voltage drop would mean that AC house voltage would be lower.

The drop still happens, but the house voltage is good.  If converted at the panels house voltage is reduced.

[Dave Sparks]

Don, I am still really happy about the LG linear fridge compressors you pointed me at.  That was the last load in an offgrid home that had a surge with it off my list.
Not because of the surge load on the inverter but alot of ambiance is lost when a dimmer for lighting has a slight flicker from the fridge cycling.

I have had to run two lighting circuits for a few clients who had this way up on their list.  I did agree with them also!

[glenn kangiser]

I have to put a word in here about used forklift batteries..... Super.

So much better than the old L16's I have been using.  I got one from my brother years ago and rejuvenated it.. bypassed one bad cell and charged it up.  It was a 36volt that fell off a scrap trailer breaking a couple cells.  I found 24volts in line that were good and tapped into them then later had to take one cell out and hook to one of the spare cells that were good.

They are gigantic 2 volt cells with a lot of capacity.  Makes me think one would be ahead to just buy a forklift battery new rather than spend tons of money over the years replacing so many L16's or golf cart batteries.

[Dave Sparks]

Hey Glen,

I have your phone number you left a few weeks ago. Glad you are back in the county!

The problem with forklift batteries is they are too heavy to easily move around. Someone like you no problem.

The other problem they have for Offgrid is they are designed to be deeply cycled and charged for 16 hours or so. They work an 8 hour shift and charge for 16. People do use them offgrid but they really need to know what they are doing or they end up with problems long term.

I pretty much use the 2V 1,100 AH L16 and get 10 years or 5 if the client does not follow the rules.

With the newest battery technology hanging on the wall, like the Samsung Lithium Ion NMC cells there are some nice things ahead if they can make them safe enough for your home.

[MountainDon]

I am hoping to get another couple years out of the golf cart batteries at our cabin, then see what the market has to offer in lithium batteries. Our GC-2's were put into service 7/2009; 81 months ago.  They don't get used every day though; good / bad.  Or maybe just do another set of GC-2's, though I like the apparent ability of the LFP batteries to be stored at partial charge over many months.

[Dave Sparks]

There is alot to like about the Lions but there are some spectacular U-tubes about how they fail. I think I would rather have lightning!
But they can be done right! One of the challenges for someone who sells them is that they become HAZ MAT when on the shelves for more than 6 months.

[MountainDon]

But they're not HAZ MAT after being installed for more than 6 months?   


I haven't looked for any youtube video fails... are they LFP or some other Lion?  I have seen a couple of cylindrical 3.7 volt cell fires; INR or ICR. LFP is supposed to be safer?

[Dave Sparks]

I can't say what I think yet because of an NDA. But, there is no rush to know this yet as none of the biggies are selling them yet.
I am talking the biggies, the Schneider's and the Outbacks.

[MountainDon]

Whenever you can tell whatever, I'll be interested.    I would like to see one or both of those biggies do something in the lithium battery field. Too many small, unknown names saying lithium is the best thing since sliced bread.

[Dave Sparks]

Next year!

[MTScott]

Thank you for the reply, Dave.  And yes, Don, Dave seems to really know his stuff.  I am glad I asked my questions during one of Dave's "very active" phases.   : - )

It seems to me, though, that having the long run be DC between the panels and the controller and batteries would be better regarding the drop.  The voltage drop will happen there, but the controller will then correct it to the proper charging voltage for the batteries, and full converted AC voltage will be available at the house.

If The AC was converted at the panels then sent the 300-400 feet to the house, the voltage drop would mean that AC house voltage would be lower.

The drop still happens, but the house voltage is good.  If converted at the panels house voltage is reduced.

What should be considered is overall "power" (wattage drop), vs only voltage (given a load - there is no current without a load of some sort).

AC is always used to transmit power over long distances because it is easy to step down (decrease voltage, increase current capacity) and up with a transformer.  DC is not so easy (transformers don't work on DC).  Most of the "step down" and "step up" conversions for DC create more loss than the transmission line itself.  You can convert AC into DC fairly simply with a rectifier, but the opposite isn't as simple or efficient.  So once you have DC, you better off leaving it as DC as well as at the same voltage. 

As far as transmission loss goes it is about voltage vs. current.  Higher voltage and less current is always better (given the same total wattage).  It can be explained mathematically with ohms law, and there are plenty of sources out there if you're curious about the equations. 

The bottom line - If you increase the voltage, and therefore drop the current you end up with less overall resistance and therefor power loss (because calculating resistance is based on the relationship of voltage and current).  Since it is easier to step down AC voltage, you're better off leaving it as AC for as far as you need until you convert it to DC.  Once power makes it to it's destination as AC, you can step the voltage down with a transformer, then convert it to DC via a rectifier (if DC is your end goal).

I'll qualify this by saying I'm not exactly sure what we're doing with the panels, but this is what I know about line transmission.

[Dave Sparks]

What should be considered is overall "power" (wattage drop), vs only voltage (given a load - there is no current without a load of some sort).

AC is always used to transmit power over long distances because it is easy to step down (decrease voltage, increase current capacity) and up with a transformer.  DC is not so easy (transformers don't work on DC).  Most of the "step down" and "step up" conversions for DC create more loss than the transmission line itself.  You can convert AC into DC fairly simply with a rectifier, but the opposite isn't as simple or efficient.  So once you have DC, you better off leaving it as DC as well as at the same voltage. 

As far as transmission loss goes it is about voltage vs. current.  Higher voltage and less current is always better (given the same total wattage).  It can be explained mathematically with ohms law, and there are plenty of sources out there if you're curious about the equations. 

What makes you think that we do not consider overall power offgrid? What makes you think we are stepping up DC offgrid?
what makes you think that AC is always used over long distance? What is long distance to you?
What makes you think transmission loss is only about voltage and current?
You make good points but the word always is wrong because there are"always" other strategies that you may not be considering.

The bottom line - If you increase the voltage, and therefore drop the current you end up with less overall resistance and therefor power loss (because calculating resistance is based on the relationship of voltage and current).  Since it is easier to step down AC voltage, you're better off leaving it as AC for as far as you need until you convert it to DC.  Once power makes it to it's destination as AC, you can step the voltage down with a transformer, then convert it to DC via a rectifier (if DC is your end goal).

I'll qualify this by saying I'm not exactly sure what we're doing with the panels, but this is what I know about line transmission. I understand that you do not understand the solar panels   --Dave

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[MTScott]

What makes you think that we do not consider overall power offgrid? What makes you think we are stepping up DC offgrid?
what makes you think that AC is always used over long distance? What is long distance to you?
What makes you think transmission loss is only about voltage and current?
You make good points but the word always is off because there are"always" other strategies that you may not be considering.
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Eh, some of my posts sound better in my head before I type them.  Always was strong - a poor choice I'll admit.  Maybe "usually" is a better term.  That is usually what you see with high power lines.  That's also why I put my last statement in there - I'm not exactly sure what you guys are working with, so it may not apply.   

I was really just relaying some theory.  No I don't believe that transmission loss is "only" about voltage and current, but if ALL all other things are equal (same wire, distance, load, environment, etc..), modifying the voltage and current during transmission can change the overall power loss.  If power loss over the a long distance "is" a concern, then efficiency could be gained if it is possible to transmit at a higher voltage, and the same will be true for DC or AC.  As a basic example, ff a guy started with 120V AC, but wanted 12V DC in the end, I would expect that transmitting at 120V would be more efficient rather than converting to 12V first, then transmitting.  That is all I was saying, and it may not apply here.    How long is long enough to make it worth doing?  That I'm not sure on, but it probably could be calculated.

DC does have its advantages over AC in transmission as well, but modifying DC voltage has losses.  If a guy is starting with AC at the source, then he could maybe take advantage of long distance transmission efficiency by adding transformers.  ..maybe.  I'm sure there are diminishing returns for both cost, and efficiency.

So, you're absolutely right, there is no "always" better solution.  I was just throwing more ideas in the hat.

[MountainDon]

With a PV system we have DC first at the panels. At the other end we more than likely want to be using 120 VAC, not DC, or at least not much DC.

When I ran the numbers for our system, with a 325 foot distance between the PV and the cabin, it did not save me anything on wire to run AC the longer distance. Our plan was to use about 650 watts of panels. That was 2009; at today's prices I would do more. Three panels in series meant about 90+ volts and 8 amps of DC traversing 325 feet. At the cabin the uses included a few kitchen appliances with large but short duration current flows. Microwave of almost 10 amps, toaster, coffee maker, etc. As well some lights, a stereo, a TV.

To send the AC power the same distance still required about the size wires as running DC, and keeping voltage drops at maximum amps low enough.. In part this was because I did not want to build a system that required thinking whether or not one electrical device or another could be used. We lived like that in the RV for years and we frequently caused an overload and an inverter shutdown. Running the AC wires the distance meant having the wires large enough to carry 20+ amps the distance. The DC system I envisioned pushed less than 8 amps and that only a peak power.

Not to mention the fact that I like the idea of the batteries and inverter in place at the cabin and not 325 feet away, down a slope that drops 120+ feet in elevation.

[Adam Roby]

...Not to mention the fact that I like the idea of the batteries and inverter in place at the cabin and not 325 feet away...

Unless one of the batteries shorts out and catches on fire.   

[Dave Sparks]

Unless one of the batteries shorts out and catches on fire.   

In my experience with 87 offgrid homes the real danger of something "catching on fire" as you say is when the equipment is far away and less likely to be noticed. Batteries are protected by fuses or breakers and fire is extremely rare and non existent  (so far) for the systems I design. The most likely failure mode is a dead battery or poor battery performance. This happens every 5 to 10 years or so and can go longer if one is lucky and does the maintenance.

[Dave Sparks]

In the Boeing 787 Dreamliner's first year of service, at least four aircraft suffered from electrical system problems stemming from its lithium-ion batteries. Although teething problems are common within the first year of a new aircraft design's life, after a number of incidents including an electrical fire aboard an All Nippon Airways 787, and a similar fire found by maintenance workers on a landed Japan Airlines 787 at Boston's Logan International Airport, the United States Federal Aviation Administration (FAA) ordered a review into the design and manufacture of the Boeing 787 Dreamliner, following five incidents in five days involving the aircraft, mostly involved with problems with the batteries and electrical systems. This was followed with a full grounding of the entire Boeing 787 fleet, the first such grounding since that of the McDonnell Douglas DC-10 in 1979.[1] The plane has had two major battery thermal runaway events in 52,000 flight hours, which was substantially less than the 10 million flight hours predicted by Boeing, neither of which were contained in a safe manner.[2]

The National Transportation Safety Board released a report on December 1, 2014, and assigned blame to several groups:[3]

GS Yuasa of Japan, for battery manufacturing methods that could introduce defects not caught by inspection
Boeing's engineers, who failed to consider and test for worst-case battery failures
The Federal Aviation Administration, who failed to recognize the potential hazard and did not require proper tests as part of its certification process

[Adam Roby]

I was just petting the cat backwards...  for fun. 

[glenn kangiser]

Hey Glen,

I have your phone number you left a few weeks ago. Glad you are back in the county!

The problem with forklift batteries is they are too heavy to easily move around. Someone like you no problem.

The other problem they have for Offgrid is they are designed to be deeply cycled and charged for 16 hours or so. They work an 8 hour shift and charge for 16. People do use them offgrid but they really need to know what they are doing or they end up with problems long term.

I pretty much use the 2V 1,100 AH L16 and get 10 years or 5 if the client does not follow the rules.

With the newest battery technology hanging on the wall, like the Samsung Lithium Ion NMC cells there are some nice things ahead if they can make them safe enough for your home.

Thanks, Dave... I guess I am kinda special.  You are correct, it hasn't moved from the place I set it about 5 years ago, but it did end my problems with the L16's.  Guess I should cut them out of the mix now....Getting to the age where one year or 5 feels about the same... but all in all doing super great. 

So busy lately I have 16 things waiting ahead of every one I get done.... but batteries is not generally one of them now.

[glenn kangiser]

For my panels now, I am generally running around 100vdc to the MPPT controller and letting it handle the stepdown to 24VDC of my 2  4024 Trace inverters piggybacked for 240v.  It got me down to reasonable size wiring without major voltage drop...  I try to get 500 watts or so of similar panels to run from one controller. 2 controllers are bigger.   I have 6 controllers charging the forklift battery.  They are all reasonably happy.  I have somewhere around 5000 watts of panels.

Of course I run by rule of thumb.... not engineered stuff like our pro, Dave does.  I don't generally have any major fires unless my welding lead moves when I'm charging the forklift battery....    Do as Dave says... don't do as I do.  [ouch]