Is 48v feasible in my case ( limited solar )

[gcap]

I've been thinking about this for a long time but this is my first attempt at pv so please bear with me.

I started out wanting to create a battery bank and inverter to power 2 glycol circulators and associated electronics for when my home lost power rendering my large solar thermal system vulnerable to overheating. I don’t lose power vey often but the possible damage caused to the system makes this a necessity. We get planned outages every so often and I'm forced to rent a generator every time.

Watching hours of videos and reading Will’s book as well as skimming another, I’ve come up with a few conclusions/ideas (please help/correct me)

Learning that 48v systems were the most efficient sold me on this concept, also, the easy peasy all in ones were perfect for my home situation. I then discovered standby or idle inverter power losses and decided against these units in favor of separates but still quite surprised that victron phoenix inverters have larger no load draws at 48v vs. 12 & 24 for any given VA size. Efficiency % do seem to be better though, maybe just less losses from higher voltage.

This got me thinking that I should install PV to offset the losses…, while at it, use pv to run the whole system…, why just run the thermal system, keep going! I realize now that I have issues to deal with:
With limited space and shading considerations, can I even charge a 48v battery?

I’ve used up so much of my perfect, south-facing roof (+/- 19’ x 30’ ) with solar thermal collectors that I don’t have much space left for pv, and what I do have isn’t great location quality. I can place 2 x 250W+ panels horizontally and low on the same roof but some shading will occur which is why I’d ideally not want to have them in series. In hindsight, I should have lowered the thermal collectors that are less prone to solar shading leaving the top part of the roof for future PV.

Are there single panels with enough voltage to charge a 48v battery?

As I grow the system there’s another spot where I can place a panel that will only get sun from 10am on because of shading from my house as well as partial tree shading in winter with the low sun, all clear in the summer. Also considering a semi-fixed 2 panel ground mount array.
Even with these sub-optimal conditions, I would at least like to try PV if only to start as an offset to the inverter and other component losses.

I need the backup and I have to start somewhere.
Am I just better to stick to a 24v battery bank to reduce the headaches?

I don’t mind doing what’s necessary to make it work within my tight space constraints. I ‘d search for high watt, high voltage panels but do realize that my limited space for properly mounted panels is a factor.

Thank you,

 

[MichaelK]

OK, a couple of discussion points here. The central theme here is the the glycol must be continously circulated while absorbing sunlight? Rather than the expense of an AC system, could a DC pump be placed in-line? Assuming the glycol only needs circulation while the sun is shining, that might be self regulating. I could envision a bypass being installed, so you have both the regular AC pump, and also a DC unit.

An MPPT controller needs some voltage overhead to function properly, and without knowing what controller you have, let's say that voltage is 72V. My 250W panels put out 29.5V, so two in series would only reach 59V. During the hottest summer days, the voltage may go down lower. So, just two panels might not be adequate here. For a 24V system though, 50 something volts will be more than adequate.

A second point is that you haven't established how many watts the AC system would be consuming per day, so it's hard to say if 500W would be adequate. To decide if 500W is adequate, we need to know what the total consumption will be, which includes both what the pump needs, and also the inverter powering it. What kind of sun-hours do you have at your location? I normally guestimate 2.5sh in winter and 5.0sh in summer. So, in December, that might not be more than 1200Wh.

 

[gcap]

Hi MichaelK, thank you for chiming in with your experience.

You are correct about the glycol during sunlight hours yet there are electronics that must run continuously. I could deal with these via a timer but I only gave the basic info in my original post so as to not veer off course too early.

There are 3 circulators in my thermal system, the Main one, the Heat dump and seeing as the actual pool pump will probably not go solar I havent used its glycol circulator in my measurements. There’s also another 2 for closed loop in-floor hydronic heat so a total of 5. I hadn’t thought of the DC pumps though, good call, this may be a future upgrade.

I know eventually I will put up about 1.2-1.xKw of panels but I believe that if at some point I only have room for a single ( lower than 50V ) panel in a particular spot, it may hinder the others. Would this thinking be correct? if so, the decision is already made to proceed at 24v. I have not decided on a charge controller till I get this step straight.

the system controller and heating controls idle at .408amps and run at .425amps at 120V so 48- 51W
running normally the system consumes 171.5W in summer and 195.5 with the floor circulators in winter
Should a problem arise such as loss of power or circulator failure and certain conditions are met ( usually in summer ), there’s a heat dump that engages and this is where the larger draw peaks 460W and operates at 440W for about a minute every few minutes until the issue is resolved or dealt with.

 

[Rednecktek]

OK, so rough napkin math for worst case scenario here assuming I understand the math right:

Your biggest draw is the 460w dump, and 200w of circulators so a 1Kw inverter should be more than plenty to run that. Doing a 1Kw inverter on 24v is actually kinda rare as finding a 24v that Small is a little difficult. Fortunately with an inverter that small you can easily go 12v which is what many DC pumps like anyways so it opens that up a little

Now, let's say that the power is out for a full day and you need everything to run. With the sun out the dump load is running for say 12 hours in summer when it's nice and hot out and the circ pumps are running 24hrs. Math-wise that's (440w * 12h = 5280Wh) + (200w * 24h = 4800Wh) = 10Kwh worth of battery. That's a LOT, about 400Ah of 24v or 800Ah on 12v.

Again, this is napkin math based on guesstimates, feel free to correct the math to get more realistic numbers.

The nice thing is that a single "24v" panel is going to be more than enough to recharge the battery and 2 in parallel (if they fit) will go a long way towards keeping those batteries filled up each day. Since the dump load will only be active at about the same time as the panels are producing power that means that 2x 400-ish watt panels should not only just about break even on the dump cooler but still make a significant dent in refilling the batteries from the circ pumps running all night.

The system being 24v or even 12v is perfectly fine for the limited panel space you've got available, just try to get the largest panels you can up there as the system is going to be drawing upwards of 600w when the panels are producing so the more margin the better. The biggest issue is just going to be the batteries and where to stick them.

Just to throw the noodle out there, is ground mounting an option? If you've got panels in series the higher voltage makes running longer wires more tolerable.

Just a quick thought anyways.

 

[gcap]

Hey Rednecktek, thanks for the math, I’d done a bunch myself but replacing numbers in someone else’s thought process allowed me to really gain confidence that my project may actually work.

Does this make sense?
Does it look like I’ve forgotten anything?

The nice thing is that a single "24v" panel is going to be more than enough to recharge the battery and 2 in parallel (if they fit) will go a long way towards keeping those batteries filled up each day.
Yes! I’m confident that I will find some that fit within the 40 inches I have left below my thermal collectors. These in parallel along with 1 mounted elsewhere that can be tilted for summer/winter. As you suggest, the last solar panels I‘m hopping to acquire will be a 2 panel in series ground array that will not be ideally placed to mitigate shading but hopefully will still add to the system. I will have to test locations

the system is going to be drawing upwards of 600w when the panels are producing so the more margin the better.
With the 5 x 300-400W panels now, what kind of power headroom could I be generating and what size inverter would you go with to add more small home loads assuming that the system runs normally with and without the need for a dump?

Your biggest draw is the 460w dump, and 200w of circulators
This is another positive as the dump calculations already included the main circulator and it does not need to run after sundown.
The only time the system runs overnight is in the winter when the main circ is off but the floor circs steal from the thermal tanks. the total system draw here is 73Wh. I didn’t add these in initally as they were much lower than anything else.

1Kw inverter on 24v is actually kinda rare / you can easily go 12v which is what many DC pumps like anyways
I have found a 24v with low standby losses but your point is quite valid and probably opens up more choices

power is out for a full day and you need everything to run. With the sun out the dump load is running for say 12 hours in summer when it's nice and hot out and the circ pumps are running 24hrs.
Math-wise that's
(440w * 12h = 5280Wh) + (200w * 24h = 4800Wh) = 10Kwh worth of battery
(440w * 4h = 1760Wh) + (200w * 8h (12h -4h dump)= 1600Wh) = 3.36Kwh worth of battery.
If the dump is required, it activates for 1 minute every 3-4 minutes or so, 20 mins/hr and already inc. the main system circs

just try to get the largest panels you can up there EXACTLY