Solar powered RV AC energy budget and feasibility?

[MattiFin]

OK, lets assume small RV/stealth camper van in southern Europe (Fiat Ducato, Dodge Promaster for mericans)

"Stealth" camper van so no windows in living area or small windows to shadows side. Insulated wall between cabin and living quarters.
30 mm insulation with PIR foam(lambda 0,022) or similar, shell is approx 38 square meters, well insulated with no major thermal bridges.

We get 28 watts per 1 degree celcius, winter use in -25C weather would need 1.25kW to heat.
Hot day in southern Europe might be +35 cel or 95F but large part of the body shell is going to heat up in sunshine.
Assuming fairly light colored van the sunny side surface temp might rise to +55 cel?
Almost half of the shell is at +55 cel and rest is at air temperature 35 cel.
Average outer shell temperateure would be +45 and if we want +25 indoors it would give 20cel temperature difference --> 560W cooling needed?

 

[MattiFin]

Part2 (had to take kids to Sauna..)

Modern variable speed split AC units are pretty efficient. A+++ energy efficiency Daikin Ururu specifies 110W input on lowest compressor speed and 600W cooling capacity.

OK, the Daikin unit is really expensive. Cost-effective cheapo like Samsung DC09RQ.SSJ specifies 170W input for 890W cooling https://cms.intermetal.bg/storage/uploads/2019/10/28/5db6a64c9a7d1--PCSDC.pdf

Ducato size car roof fits about 1kWp worth of panels (and if you really push it you could mount 3x450W panels on it)

It looks like 1kWp panels would produce about 5-6kWh in southern Europe on hottest/sunniest summer days.
For example this https://pvoutput.org/list.jsp?df=20200701&dt=20200731&id=27116&sid=24811&t=m&gs=0&v=0

5kWh out of solar panels would let you run the Daikin Ururu on lowest setting for non-stop and even leave about 2kWh for other uses.

 

[Thafella]

Thanks for running the numbers. I’ve been thinking about a similar problem but have been a bit too lazy to formalize it.

In general I think you’re about right, it’s more feasible than people think. I’ve got a few assorted points to make about some of your assumptions. It may seem like nitpicking but you’ve made some generous assumptions that may end up compounding to be significant.

You say no major thermal bridges but you’d be surprised how major a minor thermal bridge can be, it’s impact isn’t proportional to it’s size but it’s conductivity. So a good extra 30-100% of your heat loss can be through any you inevitably end up with when you get to building.

Similarly you’re going to open doors to enter and exit occasionally, and when you do you’re going to trade a lot of the air with the outside world. So budget some constant cooling and heating for just bringing new air and objects up to temperature. It can take a long time to actually reach equilibrium and until then you’re going to be in a space out of your target temp so expect to run at a higher power only to start over again every time a door opens.

I’d love to have someone clarify this part for me: I couldn’t verify any of the units you mentioned from your links (pdf didn’t load) but a COP of 6 is indeed possible with new units from what I understand. Though those efficiency ratings are measured based on pretty mild conditions (EER is 35C outside for 27C inside and SEER is outside ranging from 18C to 40C). The efficiency drops off pretty fast outside those ranges from what I understand. (Apparently a SEER 13 unit breaks even with a resistive heating element at -23C)

In the summer this is probably fine, you can get all your fresh/heat sink air from somewhere cool like under the vehicle. In winter you’re in trouble, your efficiency gets worse as your sunlight dwindles, and in your example of a -25C winter as you note the equilibrium work load is pretty extreme at 1.25kW.

You could just install a cheap Chinese diesel heater to cover the short fall in winter. They’re $200 on eBay and you’ll get around 10kW/L of heating from diesel. It will be a draw of 8-40W/h so you’ll be using some power too but you’ll get 1.5-5kW/h of heating out of it.

Also don’t forget to include 100W of heating per adult at rest inside from their metabolism That helps the winter case tho hurts the summer case. Not to mention any cooking etc in summer.

Of courses the only problem now is you have $2000 of solar which is only just covering your air conditioning needs for half the year, and you may have been able to survive by just parking under a tree for all but a few of those weeks

I’m in a bigger vehicle and the math gets a tiny bit better as you get more roof space as it tends to scale at a better ratio to the insulation surface area of your shell. Though you could always install a simple solar panel tilting system to get more solar, but that isn’t very stealth.

Having said all that I still plan to install a mini split at some point in my build, but I’m already building the solar for power needs regardless and I expect it to only represent a small percentage of my generation or as a luxury to burn excess power generation on and I’ll still be using diesel all winter I expect.

 

[MattiFin]

Thafella:
You are correct about the thermal bridges. I think this would be as much insulation project as solar project.
Van-based campers are usually pretty horrible in this regard. I was actually thinking of vacuum insulated panels for some of the problem areas like panels on the end of the bed to create bit more space for bed. Side door is one problem area that needs some "inventions" like 3" thick insulate blanket. That would also help with the huge door opening and limiting air exchange.

Larger "real" integrated RV would be much better start with the insulation and lot easier shape but the size is a serious compromise on tiny curvy roads.

Winter numbers: Should have been more clear but that was only given as a some sort of sanity check or reference. RV heating with winter sun would be possible only in few most southern locations in Europe. Diesel heating otherwise. 1kW solar wouldn't even power diesel heater where I'm currently living!

I also left out the ventilation needs, some (hot) fresh air is needed. On the other hand you don't have the solar radiation load on AC for 12 hours or so and RV shell is only at air temperature. Late night air temps also usually drop to tolerable level at least for couple of hours between 01-07 hours.

 

[HaldorEE]

It might be out of your price range, but Mitsubishi makes a 6000 BTU minisplit heat pump with a 33 SEER rating. That only requires 180W to run. It will also work in cold temperature down to -13F (-25C).

This is 120VAC (US) version, but I am sure Mitsubishi makes this in a 230VAC version as well.

6,000 BTU 33.1 SEER Wall Mounted Mitsubishi Mini Split H2i Hyper Heat Single Zone Heat Pump - MZ-FS06NAH

Designed with enhanced efficiency and supreme comfort, the Mitsubishi MSZ-FS06NA / MUZ-FS06NAH single-zone Mini Split is an ideal wall mounted unit to heat and cool a single room of your home or office. With its enhanced sensors and automatic intelligent controls, the MSZ-FS06NA / MUZ-FS06NAH is...

hvacdirect.com

 

[whatismouse]

It might be out of your price range, but Mitsubishi makes a 6000 BTU minisplit heat pump with a 33 SEER rating. That only requires 180W to run. It will also work in cold temperature down to -13F (-25C).

This is 120VAC (US) version, but I am sure Mitsubishi makes this in a 230VAC version as well.

6,000 BTU 33.1 SEER Wall Mounted Mitsubishi Mini Split H2i Hyper Heat Single Zone Heat Pump - MZ-FS06NAH

Designed with enhanced efficiency and supreme comfort, the Mitsubishi MSZ-FS06NA / MUZ-FS06NAH single-zone Mini Split is an ideal wall mounted unit to heat and cool a single room of your home or office. With its enhanced sensors and automatic intelligent controls, the MSZ-FS06NA / MUZ-FS06NAH is...

hvacdirect.com

Where do you see that this is 120VAC? It looks like it is only 230VAC.

Being in the US, I went with a Pioneer 19 SEER unit for my RV. If I were to do it again, I would get a second inverter, upgrade my electical system to accommodate split phase @ 240V, and go for the highest SEER 240V 9000BTU unit I could get my hands on.

Efficiency of my unit is not great below 40*F (4.44*C) ambient. Especially as it starts going into defrost mode at those temperatures, and upon start-up and for longer and longer periods of time as temperature decreases it is drawing 800-1200watts. I end up using my gas furnace at temperatures below 40*F generally, unless I just need heat for an hour or two and have plenty of battery.

 

[MattiFin]

It might be out of your price range, but Mitsubishi makes a 6000 BTU minisplit heat pump with a 33 SEER rating. That only requires 180W to run. It will also work in cold temperature down to -13F (-25C).

This is 120VAC (US) version, but I am sure Mitsubishi makes this in a 230VAC version as well.

6,000 BTU 33.1 SEER Wall Mounted Mitsubishi Mini Split H2i Hyper Heat Single Zone Heat Pump - MZ-FS06NAH

Designed with enhanced efficiency and supreme comfort, the Mitsubishi MSZ-FS06NA / MUZ-FS06NAH single-zone Mini Split is an ideal wall mounted unit to heat and cool a single room of your home or office. With its enhanced sensors and automatic intelligent controls, the MSZ-FS06NA / MUZ-FS06NAH is...

hvacdirect.com

That would look like a good candidate. 6000 BTU unit might be a better match for small van cooling. (power and physical size)
Maybe slightly less efficient than the Daikin unit but lot cheaper.
Unfortunately it seems like finding the smaller 6000 BTU units here in Europe is lot harder especially if you are looking for a energy-efficient models.

COP, EER, SEER, BTU and kilocalories per UK gallon drive me nuts.
EER in US is BTU/Watt
EER in civilized world is Watts/Watt (albeit even in here people on diet often talk about kilocalories)

 

[Luthj]

560W for cooling seems low. Air infiltration into vehicles is significant, and difficult to estimate.

You also need to include any heat generated by the occupants and gear inside the vehicle. Inverter heat could be 50-100W in some cases. A resting person produces ~100W of heat and a fair bit of moisture.

I would say 700-850W for cooling is more realistic in direct sun and 35C ambient. This assumes you have few thermal bridges, so all your exposed metal is covered with at least 1" of foam insulation. But if you insulation isn't great, you can easily find yourself needing 1,250W of cooling. A 800W solar array should be able to keep up with your cooling needs during the day if you are well insulated and get a good COP rated unit. But its not likely to keep up with a fridge and other major power users to keep your battery charged.

Almost everyone I have seen doing extended off grid AC operation needs some type of slide out solar array on a vehicle that size. Typically yielding 1kw nominal of panels.

At -25C in direct sun your heating needs would probably be around 1.2kw, but wind plays a big factor. With no sun at -25C you are probably going to need 1.5kw. Again this assumes no thermal bridging and goo insulation all over.

If you have a fair bit of cabinetry, that can serve to increase insulation value. A big example would be a garage space under a rear bed. You still need to insulate the shell in this space, but it can be allowed to go 15C above/below the living space temp.