DC-ification - any case stories, hints, tips?

[paulca]

I am going to install an inverter, however I want to keep it much more incidental than permanently running.  Maybe in a few years time with a few more upgrades I can.

I was sitting this morning with the panel producing 301W and the mains grid usage sitting at 425W.  I want to move some of the 425W to the panel.  Sure an inverter makes that easy, especially if it's got mains backup.  But it's wasteful when almost all of that power is being used by devices running on DC via power bricks.

I did a quick catalog of devices accounting towards that 425W.

12V 24hrs - Network switches, routers, Wifi etc.
12V 12hrs - USB hubs/chargers
19V 12hrs - Monitors
19V 8hrs - Work laptop

That lot, when running adds up to over 200W in just my office where the DC power is available.

Today it's 12V and pulling more than about 3A loses a volt.  In about 4 weeks that will go up to 24V which will increase the continuous load capability up to about 100W.  Storage capacity is another factor though.  This iteration it will be about 2.6kWh. 

Thoughts.  The monitors run off of basic laptop bricks, 120W and 150W 19V bricks.  They are in use for 8-14 hours a day.  Same for the work laptop.  They don't pull 150W.  I did have the laptop running on a USB charger at one point from solar and it was pulling 18W with the screen closed.  Unfortunately with screen open it isn't interested in that power source, says it's too weak and disconnects it entirely.  That's fine. 

My thoughts are:
Take the raw 24VDC solar feed and supply a single junction box.  Within said junction box are a set of buck convertors of suitable size to step the 24V down to either 19V or 12V and route those out using standard 2.5mm DC barrel jacks or laptop jacks for the monitors.

For when there is a dull week and I run out of storage I can use a MeanWell DC supply off the mains and a manual switch with the help of a super cap to switch from "offgrid 24V" to "shore power 24V" ideally without reseting everything connected.  Automating that switch over could go on the back burner.  If I forget to switch it off and the batteries LVC that's my fault.

[paulca]

Given the voltage drops, it might be better to distrib the 24V around the room and buck it down to 19V nearer the device?

[fourtytwo42]

Three thoughts :-

1/ Efficiency gain is only one conversion (let's call it the inverter) as the buck simply replaces another converter, LV buck may be less efficient than HV half bridge!

2/ Convenience, when everything is grid powered it rolls over seamlessly to/from battery/pv depending on sun & charge.

3/ Economics, one inverter is probably cheaper than many buck converters meaning payback time is less

[paulca]

Three thoughts :-

1/ Efficiency gain is only one conversion (let's call it the inverter) as the buck simply replaces another converter, LV buck may be less efficient than HV half bridge!

2/ Convenience, when everything is grid powered it rolls over seamlessly to/from battery/pv depending on sun & charge.

3/ Economics, one inverter is probably cheaper than many buck converters meaning payback time is less

1.  DC->AC->DC, could be more efficient that direct DC->DC.   I would probably expect that to be the case for higher wattage systems.  The distribution should be considered and dropping 1 or 2V from 240V is negligible.  Dropping 1 or 2 voltages at 24VDC is rather irritating.

2.  Convenience yes.  A single invertor feed and a bunch of DC adapters plugged into it like it was (and is) a normal power extension is a lot easier.    I'm not concerned, yet, with grid connection.  I have other plans for that, but that needs to be done BTB and via the right channels, needs budgetted, financed etc. etc. etc.   That system will be entirely "normal".  The system we are discussing is my "shed" system.  Which ultimately I want to power my garage, my lab and my office.

3. 19V 10A buck convertors, even in automotive potted form are £10 max.  £20 if you want a marata branded one.  A cheap, sensibly speced cheap inverter for 1kW is £300.  Which depending on your luck you might regret.  YMMV like most cheap products.  EDIT:  Going lower on teh inverter highlights it's pointless-ness for low wattages.  Nobody goes really low.  500W is about the lowest.  If I want 1kW peak load to match the 40A maxima I "choose", then to be save I but the 2k or 3k cheap version.

Given it's a low input/output system.  I'm concerned the quiescent power to run the invertor will end up being a very non-negligible portion of the power budget.  If I'm only pulling an average of 50W over 24 hours and the invertor is consuming 20W on top of that, it makes it a bit wasteful.  In that circumstance, running the lower end 24/7 stuff off the DC+Buck and leave the higher power things like the monitors to the mains invertor.

[paulca]

A humbling measurement I made.

My system will power 22W 24hours and be at 30% charge state.  It 'can' recharge fully in one day, but only if it's sunny.  If it's overcast it will take a week to replace that.

Using the solar system to make a dent that way could be a bit boring, compared with short bursts with the invertor running the whole office of it.

EDIT:  Note.  A lot of these smaller DC devices will probably run absolutely fine on the direct feed from a 4S lifepo4.  Maybe just a capping on the 14.4V max.  No buck required.... until I go to 24V   Which I need for the inverter.

[paulca]

On the other end of the spectrum.

Just powering the garage.  That would include the washing machine and the dryer.

A 3k invertor would do one at a time or maybe even both.  It might consume 1kWh for the wash and 2kWh for the dry, would make an interesting "shed" experiment on a really sunny june day.

[paulca]

The plans around the nice quiet, low voltage DC supply and trying to preserve it has taken a tumble.

Introducing more loads onto the 12VDC circuit I already have, has made it well ... not quiet.

It might seem odd, you'll say I'm nuts, but I'm considering adding a local battery power rail which can charge from the 240V inverted solar and then disconnect and power a silent 12V rail.

That takes care of extra conductors for 240VAC + 12/24VDC and the voltage drop on the later. 

It will change my model of working with the solar.  I don't want to run the inverter 24/7 so I need to schedule the load (until there is enough storage and generation that I don't care). 

The locally (to the office/lab) LiFePO4 pack providing what my current 12V silent system provides sounds like a "fun" option.