Mixing solar panels

[phemens]

Hope I don't confused anyone too much, I get muddled with all the numbers, I'm by no means proficient in this, so looking for advice.

I currently have 2 solar arrays. One is 2x325w 72 cell panels in series feeding a Victron 100/50 MPPT. Not planning on touching that one. The other array is a single 250w 60 cell panel feeding a separate Victron 100/30 MPPT. Current total wattage is 900w.

I have room on the roof for 1 more large panel that I was planning to wire in series with the single 250w panel. Ideally I would have preferred to avoid replacing the existing MPPT, but the max wattage for the 100/30 is 440w @ 12v. The unit is able to handle wattage above maximum, but it would be wasted (manual states "If more PV power is connected, the controller will limit input power.").

So here's the question - if I stick with the 250w panel and add another one in series, then am I correct in assuming that I should stick as closely to the rated amperage of the installed panel (8.24A) with any new one (i.e. no point in going to a 300w panel)?

The end objective is to add enough solar to reliably power a residential fridge and get off of propane. I have 600 A/H of LifePO4 (500 A/H usable). The primary consideration is not price (but is a secondary one), it is reliability of the system as we plan to leave the trailer at a remote location for extended periods and I don't necessarily want to worry about propane running out when we're away.

Thanks for your input.

[ajriding]

I quickly scanned n didnt see the answer I am thinking.

A residential fridge is hugely inefficient. If you have a few bucks, and plan to do RVing in the future also, then reconsider all this.

You have plenty of solar for a vehicle compressor fridge, by this I mean the Danfoss-style compressor unit (Engle, ARB, Dometic, Iceco etc). These are made to run off batteries/solar, and the compressor is advanced and uses very little power compared to a house fridge of similar size.
Using a house fridge will suck the batteries low over-night and this will shorten the battery life, and cost more money down the road.

Save the cost of extra panels ($300??) and put it into a unit with the Danfoss compressor. I think someone mentioned a retro-fit for a compressor to fit into some propane RV fridges, this is good as the typical Dometic compressor is a chest style, but there are options for front opening also.

The nice compressor models with two compartments (can be freezer and fridge) are expensive, but no need to point out the benefits anymore given the current social disruptions out there.

I have a big, two door chest, with two GC batteries, and lose very little battery charge overnight. Batteries are topped off before I get out of bed from 200 watts solar.

The compressor might run 7-10 mins per hour and pull 3-5 amps max, I think around 3-4 is typical. A residential fridge would run through an inverter with its own power loss and draw much more power.
Residential fridges are not made for being driven around a lot, though they can hold up it seems, but the vehicle fridges are used and abused by off-road Jeeps all the time with no issues.

[pianotuna]

phemens wrote:
No space constraints for sure. If not tracking then arrays that I can easily access to keep clean. I have a whole education on home solar ahead of me 😉

I would start with a floor plan that satisfies your partner. Next a passive solar design. The fewer mechanical systems the better.

The eco house in Regina had back up heating consisting of a natural gas fired 40 gallon water heater. It used evacuated tubes for space heating, and the windows had a wide gap where Styrofoam peanuts were blown in at night--and then sucked out in the morning. There were no north facing windows. They did use an air to air heat exchange system.

There were two water heaters "in series" the first one only had the pilot running to preheat water.

Will you go 100% electric? A ground heat pump system would be a nice but expensive feature.

I have a triple mini split heat pump in my condo. I've never needed the heating part, but it is unbelievably efficient. My current consumption is 4.3 kwh per day. I suspect most of that is the fridge. I do have a dishwasher, microwave, and induction cook top with a resistance convection oven. My space heating and water heating costs are hidden in the condo fees.

The very first geothermal house in Saskatchewan is near the Manitoba border--and the owners are piano tuning clients of mine. It was monitored for more than a few years by the National Science Counsel from Ottawa. At that time two wells were bored. One for drawing hot water and one for injecting the water back down. Current technology uses a field system that is buried at two layers about ten feet below the surface. I have one client who has the newer system. It is NOT cheap. The mechanical components take up a fair size room.

None of these installations included solar electric generation--but the more modern heat pump folks plan to do so.

[phemens]

No space constraints for sure. If not tracking then arrays that I can easily access to keep clean. I have a whole education on home solar ahead of me 😉

[pianotuna]

phemens wrote:
Plan for the house is to have the solar array on a tracking mount to follow the sun. Boss has dictated that the house will be build to consider view first, practicality 2nd 😉

Trackers are not much use unless there are space constraints. They use a fair bit of energy and don't do well in winds. They are also expensive.

Far better to use that money for more panels set at the ideal winter angle.

[Yosemite_Sam1]

phemens wrote:

BFL13 wrote:
Having your own land like that lets you park so you can tilt the panels South. The stick house can be built with the roof slanting the right way too. At our seasonal off grid, I beat that by using a ground based solar set that I could aim, so it didn't matter which way the trailer was parked.

You want the fridge on the North side, the panels tilted South, and the awning on the sunny side. Of course your RV will have the fridge on the wrong side!

Plan for the house is to have the solar array on a tracking mount to follow the sun. Boss has dictated that the house will be build to consider view first, practicality 2nd 😉

And remember, the boss is always right.

A lot of times you are but remember rule no. 1 above.

[phemens]

BFL13 wrote:
Having your own land like that lets you park so you can tilt the panels South. The stick house can be built with the roof slanting the right way too. At our seasonal off grid, I beat that by using a ground based solar set that I could aim, so it didn't matter which way the trailer was parked.

You want the fridge on the North side, the panels tilted South, and the awning on the sunny side. Of course your RV will have the fridge on the wrong side!

Plan for the house is to have the solar array on a tracking mount to follow the sun. Boss has dictated that the house will be build to consider view first, practicality 2nd 😉

[phemens]

Yosemite Sam1 wrote:

Lwiddis wrote:
IMO you have plenty of solar panels. 1100 watts+. Solar panels don’t “power” fridges. Panels and controllers charge batteries. With adequate batteries you can power a residential fridge.

Indeed, do you have enough battery?

Explains why those off grid home with Tesla solar panels as sole energy source. have the fridge-sized. lithium batteries.

Short answer is yes - I have 6x100 A/H LifePO4, which gives me 525 usable A/H (one of the batteries is rated as 'true 100 A/H', or really 125).

[phemens]

pianotuna wrote:

phemens wrote:
Pianotuna, why do you suggest the compressor retrofit? One of the motivations to move to residential is that I can get 40% more fridge space in the same real estate. I imagine the retrofit would end up costing roughly the same?

40% more fridge space would be useful--but likely that means doing wood work. In my case it would mean loosing two storage drawers and that sort of space is always at a premium. Hence retro fit for me.

Just make sure that what ever fridge you purchase has a Danfoss compressor, preferably one that can run on 12 volts. That way you eliminate those pesky inverter losses.

In my condo, I'm using 4.3 kwh per day for all electrical needs. That is a bit of a swindle as I don't directly pay for space heating nor water heating (costs are hidden in the condo fees). That number included 48 hours of air conditioning (triple mini split with just one evaporator running). This time of year, I could manage on your 900 watts of panels.

For your "off grid" home, I'd seriously consider SiO2 lead acid batteries as they can be used and charged down to -40. They are about 1/2 the price of Li. 2800 cycles to 50% or 1500 cycles to 80% discharge. I'd size the bank based on 5 days of no sun to 80% discharge. The only disadvantages of SiO2 appears to be weight and footprint. They are about 10% heavier than a lead acid and a similar size--but in a fixed install that should not matter, and being to store them in an unheated shed, external to living quarters, appeals to me. They are sold in Canadian dollars.

We're probably 5 years away from building the house, so hopefully we'll benefit from some advances in battery technology (but I won't hold my breath).
For the switching of the Norcold to electric, mine is a 2 way, and I won't be there to do it on a daily basis, so that wouldn't work for me. Based on the average draw of a 12CF residential energy star fridge, it would draw down a max of 80-90 A/H per day, which in worst case with zero solar would give me 3-4 days on batteries alone, which would be pretty acceptable for my use case.

[BFL13]

time2roll wrote:
Yes match the Imp for series and match Vmp for parallel.
Never going to be perfect but you should still get most of the available power.

Not to be confused with adding all the Voc and adding some more as "margin" for when it gets cold out, so not to exceed the controller's Voc limit, which will fry it immediately, no second chances.

[Yosemite_Sam1]

Lwiddis wrote:
IMO you have plenty of solar panels. 1100 watts+. Solar panels don’t “power” fridges. Panels and controllers charge batteries. With adequate batteries you can power a residential fridge.

Indeed, do you have enough battery?

Explains why those off grid home with Tesla solar panels as sole energy source. have the fridge-sized. lithium batteries.

[time2roll]

Yes match the Imp for series and match Vmp for parallel.
Never going to be perfect but you should still get most of the available power.

[BFL13]

Having your own land like that lets you park so you can tilt the panels South. The stick house can be built with the roof slanting the right way too. At our seasonal off grid, I beat that by using a ground based solar set that I could aim, so it didn't matter which way the trailer was parked.

You want the fridge on the North side, the panels tilted South, and the awning on the sunny side. Of course your RV will have the fridge on the wrong side!

[pianotuna]

phemens wrote:
Pianotuna, why do you suggest the compressor retrofit? One of the motivations to move to residential is that I can get 40% more fridge space in the same real estate. I imagine the retrofit would end up costing roughly the same?

40% more fridge space would be useful--but likely that means doing wood work. In my case it would mean loosing two storage drawers and that sort of space is always at a premium. Hence retro fit for me.

Just make sure that what ever fridge you purchase has a Danfoss compressor, preferably one that can run on 12 volts. That way you eliminate those pesky inverter losses.

In my condo, I'm using 4.3 kwh per day for all electrical needs. That is a bit of a swindle as I don't directly pay for space heating nor water heating (costs are hidden in the condo fees). That number included 48 hours of air conditioning (triple mini split with just one evaporator running). This time of year, I could manage on your 900 watts of panels.

For your "off grid" home, I'd seriously consider SiO2 lead acid batteries as they can be used and charged down to -40. They are about 1/2 the price of Li. 2800 cycles to 50% or 1500 cycles to 80% discharge. I'd size the bank based on 5 days of no sun to 80% discharge. The only disadvantages of SiO2 appears to be weight and footprint. They are about 10% heavier than a lead acid and a similar size--but in a fixed install that should not matter, and being to store them in an unheated shed, external to living quarters, appeals to me. They are sold in Canadian dollars.

[Itinerant1]

phemens wrote:

BFL13 wrote:
It is all a compromise and scenario dependent, but it was no good with a 120v only fridge in our case for off-grid and no generator time for more than a few days. It was too much of a nail-biter if we got enough sun and the weather keeps changing around here.

Takes the fun out of the whole thing if all you do is worry about the fridge and if the weatherman is lying again 😞 . It was so great to get back to having a propane fridge.

OTOH that can't really be run on 120v off grid, taking 34 amps via inverter while it is cycled to on. So it stays on propane while off-grid. Getting more propane is easy in our situation, so that's that.

We toodle around sight- seeing a lot so it is no problem shopping for more food to re-stock instead of having more fridge space.

Everything is a compromise for what suits your own situation of course.

Agreed, I am having the same worry with the propane. I've had a couple of experiences where we came back to the RV and found the fridge off due to propane running out. We've purchased a large tract of land where we'll eventually build a home, but for the time being we'll park the RV there, and it's far enough away (2hr drive) that visits need to be planned. I realize I'm using a sledgehammer to drive a nail (i.e. we could shut off the fridge and restart when back on site, etc), but that's just how my brain is wired! It's also good practice for our future build which will be completely off-grid.

phemens your getting near the same amount of solar and have an additional 100ah of lfp batteries than my setup.

Late spring/ summer/ fall I'll switch my Norcold 821 over to electric (30ah) around 8am and back to propane 6-8pm doing this I extend my 30# tank by 5-7 days and gives you the flexibility of dual power use.

With Lfp charging faster and not going through a long absorb stage allows this to be very convenient with the rv fridges.