Going Off-Grid.. AC / DC Coupling - What, How, Why - A Plan?

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Using Capacitors for storage..

Postby Tracker » Wed Jan 08, 2014 7:06 am

Warpspeed wrote:My situation is different to many of you, I live in suburban Melbourne, am on grid, wish to reduce my power bills a bit (?) and eventually my total reliance on the power utility. And I don't have a fortune to spend.


no... not so... most of us here have the same issues.. most want to increase their own generation and reduce the grid usage... with the possibility of one day going off grid..

as for your proposal... I like it and it would be a practical commercial proposal for someone.
you are literally using the grid as the battery to supply power when solar is unavailable, and then use whatever power is available from the sun

the problem with the proposal is that you have no means of using surplus solar power.. but that is OK..

you could do something similar using commercially available parts and at safer voltages.. eg .. a 48v capacitor bank and inverter, fed by either a solar charge controller or a grid supplied battery charger..

this latter method would mean that you would be dealing with "approved " components .. ;)
and if something failed, you could buy another, rather than starting from scratch..

PS... something that I do even now, is to use a conventional solar charge controller, fed by a transformer and rectifier and capacitor... it works great and gives the MPP current benefits and all the accurate charging benefits of true charge controller.. not that you would be looking for that as i do use a battery..

were you to go down that track of using a 48v based system, you could then consider grid feed of the surplus solar power.
you could do this by control of voltage levels and using a Latronics PvE1200...
this grid inverter legally works by starting to grid feed at 54v and stops at 52v. and has a very soft control in between, delivering as much or as little grid power as practical..
so... your house inverter would run on 48v.. eg a pretty typical computer UPS.
SOLAR would charge to say 55v.
Grid Rectification would charge to say 50v...

the obvious issue with the concept of rectifying the grid, and then re-inverting, is all the conversion losses.

now.... if you simply set up a solar system and NET feed the grid, then you are really achieving the SAME result... ie using YOUR solar power when it is available, and using the GRID when solar is not available, and then you have the added benefit of a small amount of grid feed... ;)
and a significant benefit would be that it would all be using legal and approved products..

Now, you did mention the critical eroded... budget..

considering that you will need Solar Panels anyway.... it is very easy to buy an inverter such as the PVE1200 for as little as $100... :idea: all just food for thought.. :!:
..
.
Last edited by Tracker on Wed Jan 08, 2014 8:13 am, edited 2 times in total.
Retired Engineer and keen PV experimenter - Always ready to learn and share.
2 x CMS2000 (fan cooled) GCI and SE 170W panels
1.7kW First Solar/Outback Island circuit - Peak Replacement Power
Governments won't save the world :-) They will just TAX it :-(
Tracker
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Re: Going Off-Grid.. AC / DC Coupling - What, How, Why - A P

Postby Warpspeed » Wed Jan 08, 2014 7:59 am

Tracker wrote:
Warpspeed wrote:My situation is different to many of you, I live in suburban Melbourne, am on grid, wish to reduce my power bills a bit (?) and eventually my total reliance on the power utility. And I don't have a fortune to spend.


no... not so... most of us here have the problem with the proposal is that you have no means of using surplus solar power.. but that is OK...


I have been thinking about that, and my initial thoughts are that a large storage battery is not going to be cost effective in my particular situation.
However, I may change my ideas on that at a later time, after some long term data logging and some more thinking.

Batteries would certainly provide night time storage, but the power load at night is minimal, and batteries during winter are not going to contribute much when there is total cloud cover for weeks on end.
The aim here is not complete energy independence from the grid, at least not initially, but gaining as much solar benefit as I can with minimal financial outlay.

The dollar amounts are not huge, it's mainly a problem to gnaw away on in retirement.
And its fun, a technical challenge, and its interesting.

This latter method would mean that you would be dealing with "approved " components ..
and if something failed, you could buy another, rather than starting from scratch..

Thats not a problem here.
I am a retired power electronics engineer.
I used to design commercial electronic products in my former life.
Designing and building (say) a really high power inverter from scratch is as easy as it would be for a chippy to knock up some new shelves at home, or an auto mechanic to change his own fan belt.

The trick is to keep it all really simple, and super efficient, something I am really having fun with.
Doing it all totally myself, means I can build it EXACTLY how I want it to be, at minimal cost using many parts I already have here.
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Re: Going Off-Grid.. AC / DC Coupling - What, How, Why - A P

Postby Tracker » Wed Jan 08, 2014 10:34 am

Warpspeed wrote:I am a retired power electronics engineer.
I used to design commercial electronic products in my former life......

Well then you are very well placed to achieve your goals..

For normal mortals, the reality is that NET FEED will achieve the same outcome..

Without thinking deeply, I think that the only issue with your plan, is that it is not readily complemented with battery, at a later time, whereas the alternative of aiming for 48V as the base voltage, means that ANY amount of storage can be added..
my initial thoughts are that a large storage battery is not going to be cost effective in my particular situation.
However, I may change my ideas on that at a later time, after some long term data logging and some more thinking.

We ALL develop our thinking as time goes on.. Goodness - how my thinking has changed..! :lol:

Clearly, changing usage practices can make a deal of difference.. ie.. generate and use as much as you can, when the sun shines..

Your project will be interesting to follow.. Working at high voltage levels will make it relatively efficient , provided you can keep your fingers away from the HOT end..
do keep us advised..
..
.
Retired Engineer and keen PV experimenter - Always ready to learn and share.
2 x CMS2000 (fan cooled) GCI and SE 170W panels
1.7kW First Solar/Outback Island circuit - Peak Replacement Power
Governments won't save the world :-) They will just TAX it :-(
Tracker
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Re: Going Off-Grid.. AC / DC Coupling - What, How, Why - A P

Postby Warpspeed » Wed Jan 08, 2014 12:11 pm

Tracker wrote:Your project will be interesting to follow.. Working at high voltage levels will make it relatively efficient , provided you can keep your fingers away from the HOT end..
do keep us advised..
..
.

Yes I certainly will.
System low side dc voltage is designed to run between 42v min to 68v max. (basically a 48 volt system).
Initially it will run off raw solar panel voltage directly, but it would also be compatible with batteries later on.

System high side dc bus voltage will be nominally +340v and -340v.

The voltage converter that converts up from 48 volts up to +/- >340 volts has been prototyped and tested, but I need to get a pair of iron cored chokes specially wound for it, which is not going to happen until after the Christmas/New Year holiday break finishes.
Magnetics design is complete, and all set to get wound by a mate that has a transformer manufacturing business, as soon as he resurfaces.

Efficiency is 92% cold and 88% after heat soaking at max full power for a while.
Design output power 400 watts, and no load input power less than 7 watts.
The output dc voltage is very closely regulated, no current limiting is used or really needed.
If the output is overloaded it should just drag down the panel voltage, its not like having a very low impedance battery source, solar panels are more a current source, and that simplifies things.

Planned to be used with just a pair of 24v 200 watt panels to begin with, which I have yet to get.
Its all going to take a while to materialise, but its coming together... slowly.

This could very easily be scaled up to any power level, and a multi kW version would be no more complicated to build than my puny little prototype 400 watter. Just much bigger physically.
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