Off-Grid system with grid fall back.

[paulca]

Just testing how insane this is.

Take a mains feed, through a AC/DC power supply to produce 24V (regulated after beefy diodes).  Feed this directly into the solar inverter input.   Without solar power at all this should run the inverter, although a complete waste of time doing so.

However if the solar charged batteries are above 24V they will 'over power' the ACDC brick and supply 100% of the power to the inverter.  When the solar batteries deplete below 24V however the ACDC brick will supply power to the inverter.

Surely this can't be that simple?

I wondered about if the solar batteries drop below 24V somehow, such as cell failure.  Surely then the ACDC brick would just charge the batteries to 24V and as long as it's rated to be capable of doing that and running the load, but limited to something sensible this is not an issue?

What other issues does this kind of set up have, especially when compared with the alternative transfer switch solution?

I'm toying with the hobby grade idea of powering a 100W 24/7 365 from solar, but in a way that allows things to start small and slowly scale up modular fashion, eg, more panels, more batteries.

Oh, before there are suggestions of just grid tieing the inverter.  In the UK I simply can't.  To do so I would need to have the whole rig certified for grid tie and pay the cost of the micro-gen meter.  Which would instantly make the whole thing to expensive.  If I simply connect a grid tie inverter there is a potential I would run the clock work meter backwards and... that's considered a crime of tampering with the utility.

[nctnico]

I'm missing the MPPT for the solar panels.

[paulca]

I'm missing the MPPT for the solar panels.

I'm thinking the panels run through a normal MPPT controller and the load side goes to the inverter, as standard.  The ACDC over-ride could connect on the circuit between the MPPT load output and the inverter input or directly to the batteries.  The former would (should?) prevent charging of the batteries and rely on the MPPT controllers diodes, if it has any... however on my small one the battery voltage is exactly the same as the load output voltage so i suspect that is not diode protected.

[f4eru]

Won't really work:

1) A PSU is typically not intended to be used as a charger. it'S current limiting is typically only a protection, not designed to be used continuously
2) Proper Battery management is not an option. This setup would over and undercharge, killing the battery in a few cycles, and thus, converting your nice project into an unwanted moneypit. Get proper battery management !
3) the PSU kicking in as soon as the battery voltage goes under 24V would prevent proper battery discharging to take place when grid power is present.
4) PSUs in the 100W class often can not run properly if overvolted, due to the typical primary control bein supplied with an AUX winding
5) why grid voltage for such a low continuous 100W load ? why not get this load in a 24 or 48V version, and skip the problematic inverter ??

[David Hess]

What you are proposing can be made to work and some solar charge controllers do exactly that with an auxiliary power input which could be provided by an off-line power supply.  Grid-tie inverters which support operation from a backup battery are essentially this.

[Bratster]

If you have not purchased your inverter yet,
 the simplest and most reliable method would be to just buy an inverter that has a grid input and battery charger built in.
 then you can just program it so that if the battery starts getting too low it will switch the loads over to the grid and turn on the battery charger.

Or if it's not that smart, make a circuit that looks at battery voltage/state of charge and connects the grid input through a relay when the battery is get too low.

Since this would just be receiving power from the grid you don't run into all the grid-tie legality issues you were avoiding.



Sent from my Pixel 2 XL using Tapatalk

[paulca]

I'm beginning to think this might turn out to be too big a project to attempt just yet.

Basically I'm moving house and will therefore have considerably more room for my "Hobby Gen" solar.  I'm looking for a project to upscale to from a 50W panel and 40Ah battery.  Running a 24/7 server seemed plausible, but the more I work through the numbers, even doing it hobby grade is getting into numbers which are comparable with full profressional roof installations.  Basic, broad numbers, even with the right inverter are look at £2000.  Yet a 3k roof installation is about £4000+fitting.

A much more reasonable solution is to step it up a bit and run garage and garden lights, plus, maybe workshop power sockets in the garage.  This allows me to easily run DC cables around the garage (which is not plastered) without upsetting the house electrics.  It also does not require 24/7 operation and if the solar is down due to long periods of rubbish weather I will still have the grid mains light and plug sockets without any complex fail over.

Don't want to seem easily defeated but I figure there are a few more sensibly sized steps between where I am and getting to that 100W 24/7 load.

[Ian.M]

There are plenty of makes of smart battery chargers for marine applications that are designed to be permanently connected to one or more battery banks, and when mains power is available, assess the state of charge, if required charge, then subsequently float the battery bank(s).  They are designed to intelligently cope with other charging sources and varying DC loads on the bank(s).

The only adaptation you would need to make (as they prioritise charging from mains) would be to add a SSR in the charger's mains feed, + a circuit to monitor the battery voltage + panel output, so the charger only gets mains if the battery charge is low and there is insufficient solar output.

[Seekonk]

 Running just a few lights is a total waste of solar. Heat water for the best payback.  In a small system, nearly 100% of potentially generated power can be used to heat water.  It will have almost no potential to overheat. Water heaters need about 50W just to maintain temperature.  A tempering valve can be added with much larger panels.  I have a little 500W 60V system at home just for testing.  It heats water and keeps my 12V camp battery charged.  I use a $3.50 PWM to do that with a modified buck converter that keeps the panels at power point.  The water heater is also kept at power point voltage.  An actually useful and cheap solar application. That is if anyone can actually put a few parts together which seems to be a thing of the past.  I have an off grid camp made from all this home brew stuff running on just a car battery.  Wife made me install a dishwasher and that has heated dry.  Don't buy anything except grid tie panels.

[paulca]

I have seen the hot water concept before, but I can't help but think...

If I heat the water with the solar panel and then do not use that hot water it will just cool down again wasting the energy.  So it's fine if I put several hundred watts of heat into the hot tank and then when I have a shower the oil/gas might have to heat the water about 0.5*C less.  However if I am not home for another few hours the water will have cooled by at least that 0.5*C and the net gain is much lower.

[Seekonk]

Yup, that is what your water heater does all day. Question is, do it with free energy or stuff you pay for. If you raise the tank temperature with solar to other heat source will not turn on. Pre heaters of water is also an option. But with the heat loss, temperatures much above ambient add little value.  If you don't do something serious with solar you might as well just buy garden lights from the Dollar Store.  Here is the cheap system that heats water and charges a battery. Once upon a time electronics enthusiasts would jump at the chance to build something simple like this.

[paulca]

So a few points on the water thing.  The house I am buying has a vented heating system with an upstairs hot tank.  There is no way into that tank.  I could try and feed DC into the immersion heater, but if I accidentally switched the immersion on I would instantly vapourise the solar system by back feeding 240V into it.  Okay, so diodes and SSRs could play a part to stop that.

The hot water heater will only be on when I want to do the dishes (or have a bath) as the house has an electric shower.

The heating system (oil fired), to be determined, but I believe it's a single pipe system, so when it's on it will heat the water anyway.  In summer it will be off except a quick 5 minute maintenance burn first thing in the morning.

So I think even if I wire up the immersion heater to take solar DC the amount of heat I could actually add to that tank would simply escape before I could get to use it in the evenings anyway.

My solar set up with be 80% hobby experiment and 20% energy saving.    If I wanted a proper energy saving solution I would take the $$$ hit and get a professional 3k grid tie roof system installed.

[capt bullshot]

If it's for experimenting and gaining of experience purpose, have look at mine:
http://wunderkis.de/pvbat/index.html

It supplies an always on 50W ... 60W distributed all over the house DC load and saves about half of the energy over one year.

[Red Squirrel]

It's crossed my mind to do that for my server room.   Essentially a normal solar setup, but also have rectifiers powered by commercial AC that turn on if the voltage gets too low.

So basically:

solar panels -> MPPT controller -> battery bank -> inverter

Then also:

Commercial AC -> rectifiers -> same battery bank

Rectifiers here meaning AC-DC converter.  They would only power on if battery voltage is too low.   This would only work with lead acid batteries though, if you plan to use lithium then this would be more complicated as they need to be balance charged etc so the setup would be more complicated.  Lead acid batteries can just be floated and you don't have to worry about current, just voltage, and they don't need balancing.  The loads will draw whatever current they need.

[fourtytwo42]

So a few points on the water thing.  The house I am buying has a vented heating system with an upstairs hot tank.  There is no way into that tank.  I could try and feed DC into the immersion heater, but if I accidentally switched the immersion on I would instantly vapourise the solar system by back feeding 240V into it.  Okay, so diodes and SSRs could play a part to stop that.

I had a fairly failsafe solution to that, the existing immersion was plugged into a 13A socket on the wall, so I simply added another socket labelled solar where the heater is now always plugged in. I also have an oil boiler and previously the immersion heater was never used.

So I think even if I wire up the immersion heater to take solar DC the amount of heat I could actually add to that tank would simply escape before I could get to use it in the evenings anyway.

Well I have a standard 250Liter tank and it has about 25mm of sprayed on foam insulation, that losses about 7C from 7pm to 7am. As we do not use masses of hot water the solar system does for us (1Kw panels) all summer without using the boiler unless there are several very cloudy days in succession. Even in winter it saves oil by pre-heating the water, work the boiler does not have to do

Please note standard immersion heater thermostats are NOT suitable for switching DC! In my case I set the existing immersion thermostat to max and then use and electronic stat that signals the converter to shut down (electronically) when the tank has reached ~62C.

[paulca]

I didn't expect to see those kind of numbers, but thinking about it, 1000W can heat tap water from cold to 60*C in 7 hours. 

That 7*C loss in 12 hours, is that for 62*C down to 57*C?  That would be the worse case for heat loss and works out about 60W.

Do you just match a DC element to the panel or do you use an MPPT controller?  I figure the controller gives you an electronic way to control the heater on/off. 

Does the MPPT controller not need a battery to run properly?  How do you stop the battery emptying into the heater?

[fourtytwo42]

That 7*C loss in 12 hours, is that for 62*C down to 57*C?  That would be the worse case for heat loss and works out about 60W.

Down to 55C but my figures are roughly anyway as the heat loss is a little dependent on overnight air temperature but on the whole never worse than that.

Do you just match a DC element to the panel or do you use an MPPT controller?  I figure the controller gives you an electronic way to control the heater on/off. 

It's a custom buck/boost controller driven by an MPPT algorithm.  What you need depends upon the match between your heater & pv array, in my case the heater is a standard 240V/3Kw so about 19R resistance and the panels MPPV is ~120 so it needs buck below around 750W and boost above it (it's dynamic not fixed), not an easy conundrum to resolve and I do not know if there any commercial solutions available. Ohh yes the thermostat is an input to the control software.

Does the MPPT controller not need a battery to run properly?  How do you stop the battery emptying into the heater?

The MPPT controller is powered directly from the panels themselves, there are no batteries in the system.

The advantage of this system for me is it gives priority to water heating. I have seen commercial solutions using a standard grid-tie with an immersion controller giving priority over export BUT that still means no water heating while your cooking etc. Also that solution involves an expensive grid-tie from the start.

I think what I try to keep in mind with all this solar stuff is how much is it going to cost and whats the payback period. It's easy to get carried away and defeat the object (if that's economic) and this one I worked out on the basis of oil fuel saved, I reckon the payback as about 3 years maybe less (as oil keeps rising).

[paulca]

Well the topic was driven off trying to find somewhere to expand my solar setup to in the new house.  Hot water heating is definitely an option.

It sounds like you Magivere'd it a bit with a custom converter.  I realise there is probably a spectrum between frankenDIY and 100% 'to code', just wondering how close to the later one can get before costs explode or the code says "No!"

Anyway it's food for thought.

My tiny little 50W window system will come with me and the MPPT controller is good for about 200W of panels @24V, I can upgrade it to run the garage lights, security light, garden lights etc.  100% Franken-DIY "Solar shed" style.  I can maybe look at options for the grid fall back there as a hobby thing.

Hot water heating sound like a more serious option with maybe 900W of panels.  (I will only have 50% a long peaked garage roof which is directly south facing).  OF course if I decide to go natural gas combi boiler it might make solar water heating difficult.

[paulca]

I did a bit more digging into these options.  The original fall back option and the PV solar immersion.

The trouble I'm having is finding information on available controllers.  For PV immersion I have seen a bunch of youtube videos and most made custom controllers or folks using grid-tie systems but bleeding off excess power to the immersion through the inverter.

I realise you can just connect a string of DC panels to a suitably matched immersion, but it's sub-optimal and only tuned for peak sunlight.  So MPPT seem to be a must.

I realise MPPT has nothing to do with charging batteries, but it seems the term has become entwined with "MPPT Charge Controller".  This makes finding a non battery based controller next to impossible.  The next most common controller that comes up for my searching is inverter based off-grid systems.

I also realise that an MPPT controller is relatively simple to make a buck/boost converter with (or without) a micro-controller monitoring output power and varying the current to achieve the maximum power point in a feedback loop.

However, while I would no issues with building such a system on my bench and running a few watts through it, I would be less inclined to build one to run 1000W through it and allow it to run when I'm out of the house!

On "Grid fall back", there are apparently off grid inverter based systems that have a mains input which it will fall back to when solar falls off.  However I find it difficult getting any information and have not found a representative model with enough information on how it manages it's batterys or manages without batteries.  Plenty of things that look like the right thing, but not enough info on how it would respond in the real world.

I did consider a basic MPPT charge controller but set it's "Load disconnect" voltage to be quite high so it never actually runs the battery down.  Then attach a "token" battery, say a single 100Ah.  However thinking this through, in low light conditions the 1kW load will pull the battery voltage down, it will disconnect, the battery voltage will spring back up and the load will reconnect.  As this would be using a "canned" charge controller there would be little to no way to alter the hysteresis.   Maybe a second PWM style current limiter watching the MPPT output and trying to match the load to the PV, but already sounds like it's getting too complex and I could just use a custom MPPT only.

I suppose it leaves the option of lots more research, reach out to some people and find links to the open source or DIY controllers people are using. 

It still leaves me with questions on load prioritisation (if I want to run more than just an immersion) and disconnection.  Are SSRs suitable for 1kW load at, say 48VDC?

[fourtytwo42]

The way I started was to choose a panel size that would give me the maximum watts per pound and sufficiently common I would likely be able to buy more later as I only wanted a toe in the water to begin with. Then just compare different panels output when loaded with your heater, some combinations mean your heater can absorb perhaps 80% of the available power without a boost stage meaning you can start with just a pwm at the beginning to prevent the heater overloading the panel/s at low light.

When I say a pwm that would serve ok if it were co-locate with the heater but if not the EMI from long cables becomes overwhelming and the low pass filter of a buck converter is required as well.

As for safety all my stuff is outdoors in fire-resistant steel cases and there are well labelled breakers for use by anyone discovering magic smoke. You do have to be careful with DC but provided everything is conservatively rated you should have no trouble. You will need a micro for control and most come with ADC's & PWM's built in, dont forget you will have to multiply (to derive power) frequently in the MPPT loop. I happen to use a PIC16F written in assembly but there are many options.

SSR's are often AC only (based on triacs) although I have seen DC ones but low power.

Have you moved in yet, do you know if you have a standard immersion ? Once you know its resistance you can start looking at panel combinations

[Seekonk]

MPPT is often a general label when people just want to use higher voltage panels into a battery. As panels are current sources, placing a fixed resistance on them for heating can send them into a death spiral and produce very little usable power.  I use a water heating system that operates at  fixed power point voltage and is in parallel with a MPPT charge controller.  This works fine for heating using excess power not needed for battery charging.  Under a KW is quite easy to build.  I have two tanks and my 60V system can produce up to 400W on each heating element.  This power is only limited by by the heaters resistance.  Considering power adds up for most of the day, several hundred watts is all that is needed unless you have monster water use.  Here is the control board I use derived from a $6 "300W inverter board" minus the capacitor bank.  The heat sinks do not get even mildly warm. You can hardly call that elaborate electronics. Cheap and effective.  I even use the water heaters mechanical thermostat and switch DC.  OH MY GOD! No arcing problems at all because the contacts never see more than a 30V differential on systems up to 100V.

[paulca]

I rewatched Julian Illet's videos on his Arduino MPPT and for the initial parts, where he isn't regulating the battery charge voltage on the output it's pretty much exactly what I need and he presents probably the simplest version.  A few things spring up though.  Inductor sizing.  This seems to require either a large and expensive inductor or high frequency switching, expensive mosfets and probably something faster than an Arduino.  Then there is the high side mosfet switching issue - requiring a voltage above Vcc to slam them on, usually needing a charge pump or boost and mosfet driver.

Looking at panels, it seems that 275W/300W grid tie mono panels are probably the best given price, performance and availability.  These are typically 30V nominal 40V OC.  Three in series for ~900W would produce a rather frightening 90-120V DC, more than enough to kill.  However their ISC is around 11A which makes cabling cheaper with less loss.  4-5mm² cables.  120V, 20A mosfets are available. The inductor will still be expensive. 

A quick online calculator suggests for 120V 20A 31KHz buck a 1mH which is around £20 on ebay and £45 on mouser.  That said output ripple is not that much of a concern and I can drop the inductor to 500uH to accept 10% ripple.  If I can double the frequency to 60kHz that comes down again to a 250uH.  Would be interested in someone with experience of this to advise of course.

As fourtytwo42 suggests, if I look at a reasonable voltage, close to panel MPP voltage, on the output and match an element to that then the output buck can more or less pin 100% duty cycle when the panel is maxed out in sun and only switch it when it's shading to prevent the element pulling the panel down.

Given I will have a few repairs to do based on the home buyer report on the house... and buying furniture, this project looks like it will be a while.  However I can replace my 50W with a 100W and use the 50W and a power resistor in a bucket as a hobby bench test rig in the mean time.

[Seekonk]

Just answer one thing. Are you heating water or charging a battery?

[paulca]

Just answer one thing. Are you heating water or charging a battery?

Both   Although probably not in the same system.  As you can expect from this early stage, things evolve.

Short term I can use my existing MPPT charge controller to power the garage/garden etc. and be a hobby play thing. 

Longer term a larger 600-1000kW system primarily to heat water.

[fourtytwo42]

Longer term a larger 600-1000kW system primarily to heat water.

I am sorry but I had to laugh at the idea of a megawatt water heater, how many Litres a second is that raised by 40C OMG communal showers for the entire village 

On a more serious note, learn to wind your own inductors in time, ETD cores and bobbins are very simple to use but avoid the horrible BLOCK make with square poles as wire much prefers to be bent around a circular pole. Do you know Farnel, they have a good selection available, also can you use Spice ?

Your exactly right with experiment small and grow