Air Conditioning via Solar Panels

[Rule303]

Originally Posted by noelex 77

Power cats have lots of room for solar, but at a rough estimation you are using 2,000 Ahrs (@ 12v) a day just for the A/C.

That would seem right, but I guess that it's closer to 15Kw per day, those numbers are peak observed, and I assume cycle during the day and drop in the wee small hours. We have 8 panels that peak around 1650W, and 2200Ah of Lithium, with a 1200AH bank that the AC draws from. Adding a data logger is on the list.....

One Unit will keep her around 24C, but we need both to chill her way down.

[Rule303]

Originally Posted by a64pilot

A small unit running continuously will do a better job of dehumidifying than a larger unit that cycles.
It will also do more cooling with less power if efficiency is the same, reason is when a unit first starts, it consumes energy for quite some time before it actually begins to cool.
Same as with refrigeration, cycling is inefficient.

Using Nolex’s estimation, and I do believe it to be correct, you need 6000 W of panels to deliver 2000 AH at 12 VDC.

Of course it can be done, it’s simply a matter of scaling.
What does the number of required panels cost installed, along with a bank big enough to store the energy, remember that 2000 AH just covers air conditioning, it doesn’t charge anything.

Then compare the costs and disadvantages of all that Solar against a small built in Diesel genset, and I think you’ll see why it’s not often done.

For us, the panels are pretty much just for the AC. We have a 6.5KVA genset, but it annoyed me to be running it while underway, and before the overhaul, we didn't have enough Inverter capacity to run the AC's.

Now we have 2 x 5KW Inverter chargers, one sucking from a 1200AH bank for the AC and one from a 1000AH bank for the rest. A couple of hours of motoring or running the gen will charge them back up regardless of sun state. More importantly, when the generator stops working, which it seems to do whenever is most inconvenient, we can use either of the other two diesel burning money sinks to generate power.

Yep, the costs are higher than just the diesel cost of running the genset, but the redundancy of not needing to run the genset is worth it.

[geoffa]

Outstanding, balanced and reflecting real world technology uptake. BZ

[Checkswrecks]

First off, I'm a proponent of "right tool for the job" and have a number of Li-ion batteries where weight and space are issues. The benefit for sailboats can be huge.

You are talking about a really big system which is probably bigger than most would need. At the same time, a 1200 AH bank ought to be 12 x 100 AH batteries, which probably cost somewhere shy of $1,000 each, so $12,000. (Assuming you are paying something near retail.) Plus a rough $3/W times 6000W for the panels adds $18,000, and that is assuming there is space for that many panels. Plus controllers, inverters, installation, etc.

Again, I'm a big proponent of the technology and costs are coming down fast. But a 6 kW diesel generator is probably $6-8,000, so IMHO the >$23,000+ difference in cost/benefit ratio doesn't seem to be there yet. The tool for this still seems to require burning diesel in a genny or engine.

[noelex 77]

Focusing only on the cost is missing the bigger picture.

Solar is quiet, reliable and requires almost no maintenance. It is very difficult to factor in the value of these attributes. Some would argue the lack of problems associated with a generator is priceless.

However, you need to be aware of the limitations. A/C is power hungry.

[Buzzman]

Those portable battery charges listed on the Macfarlane site are actually made by Christie Engineering. Used to know the owners. They're good ****, but not cheap.

12 VOLT 80 AMP – Christie Engineering

[CookiesnTequila]

This is all great information. But I'm having a bit of a hard time putting it all together and figuring out how much air con we should install. Air con is our last mod before setting off to the tropics next year, but I am getting sizing and recommendations and energy usage estimates that are all over the board!

We have a Catana 50 cat, equiped with (some in the process of installing) 600ah Li @ 24v., 2000w solar, 3600w (continuous) generator to feed a 120ah @ 24v. charger and 5000w inverter to run watermaker, air con and misc AC house loads. We cook with gas snd are not overly heavy energy users.

We want air con prinarily for dehumidification (my wife is allergic to mold, and sneezes uncontrollably if there's any around) and for sleeping on those nights when it's too hot and humid, or raining. Not looking to cool below 28c (82f).

???

[geoffa]

As we write the numbers are getting better you should be able to source mono crystalline for 50c a watt, Tesla have achieved LiFePo4 for $51kwhr and simple I mean standalone not anyway connected to mains inverters 6kva for $400. The splitst always seem to go 30%30%30% solar storage inverter! The other 10% is control gear etc etc.
My observation is 48v house, 12v start/operational, 240v where you can and keep it below 2.4kw and plan on no more than 2x10amp gpo ccts and you have a safe very cost effective system. This avoids the complications of ever needing a shore or generator.
Modern inverter ac with coops of 5 or so run perfectly fine within this system, the rest is sound demand management and lots of foam insulation. Stops the boat sinking too!

[Rule303]

Originally Posted by CookiesnTequila

This is all great information. But I'm having a bit of a hard time putting it all together and figuring out how much air con we should install. Air con is our last mod before setting off to the tropics next year, but I am getting sizing and recommendations and energy usage estimates that are all over the board!

We have a Catana 50 cat, equiped with (some in the process of installing) 600ah Li @ 24v., 2000w solar, 3600w (continuous) generator to feed a 120ah @ 24v. charger and 5000w inverter to run watermaker, air con and misc AC house loads. We cook with gas snd are not overly heavy energy users.

We want air con prinarily for dehumidification (my wife is allergic to mold, and sneezes uncontrollably if there's any around) and for sleeping on those nights when it's too hot and humid, or raining. Not looking to cool below 28c (82f).

???

When we leave our cat in the marina, I leave one AC unit on, and set to 26C. Just went back and checked pre solar panels, I was being charged for around 12KW per day, that would be running the AC, a Freezer and two fridges, bilge pumps etc. 24/7
We have a very similar setup to yours, except when we are moving we are getting up to 400A@12V from the alternators. You shouldn't have any dramas.

I was sceptical when we brought the boat re the domestic AC split system, but, and I know I will forever regret typing this, they have run for 10 years with only very periodic servicing. Also, you can get them serviced anywhere. The heat pump units are in an externally vented locker centre aft, and the head units are in the salon. We are adding ducting with inline fan to get air to the forward state rooms, but a fan and an open door has sufficed to date.

[john61ct]

Originally Posted by CookiesnTequila

This is all great information. But I'm having a bit of a hard time putting it all together and figuring out how much air con we should install.

Because doing this off batteries is **very** difficult, and so expensive hardly anyone has even tried, much less been successful.

Put in multiple units, each of a different size, as if you will be on shore power only.

The smallest one should be **very** energy efficient and placed to service the smallest tightly sealed best insulated space,

as if you were going to (try to) run that off batteries alone. Of course that factor is very rare and will be very pricey.

Frigomar's inverter-based brushless units are apparently the way to go.

If you really need all the living space serviced, your genset must be very robust and able to handle all the aircon units cycling together.

Anything else will not be certain to work, much less will likely be a roll of the dice.

And of course may require buying a bigger boat, or spending more time in marinas than you'd planned.

[CookiesnTequila]

Originally Posted by john61ct

Because doing this off batteries is **very** difficult, and so expensive hardly anyone has even tried, much less been successful.

Put in multiple units, each of a different size, as if you will be on shore power only.

The smallest one should be **very** energy efficient and placed to service the smallest tightly sealed best insulated space,

as if you were going to (try to) run that off batteries alone. Of course that factor is very rare and will be very pricey.

Frigomar's inverter-based brushless units are apparently the way to go.

If you really need all the living space serviced, your genset must be very robust and able to handle all the aircon units cycling together.

Anything else will not be certain to work, much less will likely be a roll of the dice.

And of course may require buying a bigger boat, or spending more time in marinas than you'd planned.

With respect, I don't think that is true. Not with Li batteries, the way we propose to use it!

Where I'm at now, my best guess at sizing, is to use a Wabasto 20000btu chilled water air con system. That uses a max 8A @ 230v, which is a 70A (@26v) draw on the Li batteries. Add another 5A for air handler and 15 (max) for house loads, and the maximum draw I can see is 90A, which is less than 1/2 the battery's maximum rated continuous discharge rate of 200A.

With 600ah batteries I expect to have at least 300Ah available for the air con load, which equals 4 hours (300/75) of 100% cycle time at maximum cooling.

If (this is where I lack knowledge) we can get enough cooling and dehumidification from 20000btu at 50% cycle time, then we can go through the night on batteries (8hrs). And even if not, the generator will come on automatically and put about 110a into the batteries, which is more than enough to run the air con.

Under normal sunny conditions, our solar should easily generate 300ah. Subject to the sbove, our total daily consumption when running air con should be about 540ah. So that means we have to make up about 240ah from motoring (100a alternators, so 2.5 hours) or generator (120a charger, so sbout 2hrs). Even if we had to produce the full 540ah, that is 4.5 hrs of generator run time out of every 24hr period.

Am I missing anything?

Big question here that I still have: is 20000btu enough!

[CatNewBee]

I guess, it is a question where you are sailing and what your comfort temperature is.

I can run 12000 BTU (one of four units) on solar all day long if I do not use other systems, my battery would be able to run it all night long too, but than at some point I would need to re-charge back to the starting point SOC (battery full) the next evening, this will be unlikely the case on solar alone with 1650Wp to do this inifinitely.

We need the power for other tasks anyway. Two units in parallel will drain the battery slowly already during the day.

[john61ct]

Originally Posted by CookiesnTequila

With respect, I don't think that is true. Not with Li batteries, the way we propose to use it!
…
Am I missing anything?

No, 600AH of a properly installed LFP system is exactly the kind of "very difficult & expensive" electrickery I was talking about.

Just be aware actual AH/hr draw will vary a lot with conditions.

> Big question here that I still have: is 20000btu enough?

Besides cubic volume area,

how well insulated / sealed up the living space will be is critical.

After those, varying ambient temps, as opposed to your wife's comfort zone temps will answer both questions.

Hence my idea of a smaller unit dedicated to a smaller better insulated space. . .

With one big system, you need to design for a worst case scenario that maybe only applies for a very small number of days per year.

[a64pilot]

Solar radiation, as in the Sun is the huge deal breaker with air conditioning, meaning if your trying to cool the boat at 4 o’clock in the afternoon 20k btu isn’t going to be enough, but if your just really trying to reduce humidity at night so you can sleep, it’s likely going to be plenty.

My 38’ mono hull has two AC’s a 16 k btu and a 6 k btu, both Webasto’s.
In the Summer in North Fl or Ga, during the day in a Marina, it takes every bit of both to keep the interior cool, they both run continuously and don’t cycle in the full heat of the afternoon.
I assume my mono has both less interior volume, certainly fewer windows and likely less exposed to the Sun hull.

Some of the numbers I’m seeing are a little optimistic, like 400 amps at 12 VDC from the alternators, I guess maybe if you have some real special alternators, but probably not.
Solar is of course another variable, weather affects output and on my boat so does boat orientation. To make the most during these Winter months here in the Northern Hemisphere, my stern has to be facing South to East, facing North really cuts output.
Maybe due to my shading issues, I have some serious shading issues due to size constraints and equipment I have, it’s a well less than perfect install.
For our uses, it seems Solar is an excellent augmenter, but I’m still tell to a generator, Solar significantly reduces how much and how often I need to run it though, and having an AGM bank as opposed to Lithium doesn’t help either

[valhalla360]

Originally Posted by Rule303

Just so happens that I have a Hobart registered 54' Power Cat that now lives in Thailand. We run two domestic split system 240v Reverse cycle A/C units pretty much 24/7. On the Inverter panel, they show 1200W draw during the day, 1600W for a few hours when the sun comes directly in the back, and around 800W at night.

42C outside, 20C inside.

Something is missing from the explanation.

We had a much smaller 34' cat with a 12k BTU air/con and it struggled to keep up in 42C temps. On a sunny day, it might keep it 25-28C inside running continuously..

It pulled about 1200w when running. I'm struggling with how a 12k BTU unit would possibly keep up on a 54' cat (which is probably 4 times the volume to cool). Even if the split units are drastically more efficient, I'm not struggling with how this would work.

Was it 1200w per unit and you have multiple units running?
Does the boat have unusually good insulation?

PS: You mention your system generating 15KW but...for the day. Are you confusing KW-Hours with KW? 15KW of panels would be massive even for a 54' cat. 15KW-Hours would require about 3KW of panels.