Electrical conundrum - mains aircon in motor home

[David]

This first post is a place holder in part, so that I don't forget to ask the question, as I usually do once I get involved in stuff during the day.

The Motor Home has mains powered habitation air conditioning - that is an ELectrolux slug on the roof.

So for it to work you need to be on a site with mains electricity.

There is also the issue of the power surge on start-up compared to the power demand on normal running.

What I would like to be able to do:

(1) Run the A/C whilst driving - that would I assume involve an inverter which could take power through the 12V electrics buffered by the habitation batteries (unless the demand needs a direct connection to the alternator charging circuit instead of via the charge controller which charges the habitation batteries). This also allows starting the engine, firing up A/C and then stopping the engine and letting the A/C run on using the habitation batteries.

(2) Run the A/C when away from mains power - using a small Honda generator which might be able to meet most of the demand apart from start-up. I am envisaging perhaps the generator pushing power into the system whilst the A/C takes power out so that the use of habitation battery charge is slowed. This probably equates to running a UPS (that is, power in to UPS for charging, power always out of UPS for running the device) and using a small generator to keep feeding some power into the UPS during a power cut. The generator may not fully meet the power demand but it slows the rate of discharge.

This does seem to demand a lot of inefficiency, though, potentially with the generator input being converted from 240V to 12V then back again. It would be nicer if the battery 12V power could be used to boost the 240V input from the generator so that most of the power comes directly in at

240V.

Bottom line is that one way may require blending two 240V inputs into a single output, with obvious (I think) requirements to lock the wave forms of the 240V together.

If I can manage ASCII art:

12V -> 240V -> Blender -> A/C 240V -----------^

I think this is probably not realistic because you would have to prevent back flow which is why there are so many issues with combining power inputs connected to the grid. Then again, it is only like combining a solar panel and mains (but that does allow back flow into the grid).

I need to dig out the handbook for the A/C to check all the power demands, but meanwhile does this sound in any way feasible?

I know (2) could be met by just buying a bigger generator, but I bought the little Honda because I could just shoehorn it into the available storage and anything bigger is too tall/wide to fit anywhere usable.

Cheers

Dave R

[philipuk]

Non starter. Aircon requires an immense amount of energy that you are just not going to get out of the 12V system or a battery.

You possibly could add an aircon compressor to the engine if there is space for one and all the other components of course (many engines just substitu te an idler pulley instead of the compressor if the aircon option isn't cho sen) but it would be hugely expensive.

Probably better all round to open the window when driving and sit outside i n a deckchair when camped without leccy hookup.

Philip

[David]

There is already cab aircon which is fine for driving along and can also cool the interior of the MH unless it is really hot - keeps the cab cool but the interior of a MH has a lot more volume than a van cab or a car.

Also does not solve the 240V issue as you would have to run the engine all the time even when you are on a powered site.

Also not good to cool the van when you are out at the beach - which seems to be one of the favourite reported usages.

Cheers

Dave R

[Chris J Dixon]

To get some idea of the overall feasibility, what is the actual power demand from the aircon?

Chris

[David]

The handbook is not entirely clear. **

The unit is an Electrolux Blizzard B1500.

The technical data includes:

Cooling consumption 715 watts Heating capacity 800 watts.

Looking at the manual it seems that the unit is not designed to be used as a heat pump when blowing warm air (as you might expect) but has a small heating element - so from the data it looks to be an 800 watt fan heater (less a small deduction for the power to turn the fan).

The 715 watts may be the maximum start up for the compressor, or may be the average after the compressor has started.

Google isn't helping me much so far (apart from several links about the heater giving of a burning smell then stopping working) but there is one foreign link which related the "1500" in the brand name to 1.5kW. The fuse recommended is a 10 amp slow blow, so good for up to about 2.5kW, which may hint at the expected starting load.

I suppose what I really need is a power meter of some sort to accurately measure the power draw when it is fired up via the mains.

Can anyone recommend a reasonably priced meter which can do this sort of thing?

Amazon offers plug in meters from about £7 upwards - to about £55.

In this case I won't be looking for milliwatt accuracy, but who knows what I might need to check in the future.

Oh, and the generator is rated at 450 watts normal 550 watts maximum at

50Hz. It is badged as a 650W generator as this is the maximum output at 60Hz.

Assuming that the real draw from the A/C is 715W then the generator couldn't run it stand alone, but could supply a reasonable percentage of the power.

Cheers

Dave R

[spuorgelgoog]

That might be for 110 volt? Or it might refer to cooling capacity of 1500 watts, with electrical consumption of 715 watts, which would be a coefficient of performance (COP) of about 2 which sounds reasonable for a small heat pump.

Owain

[David]

Thanks - another bit of the data says that it is 230 volts at 50Hz.

Cheers

Dave R

[Bob Minchin]

Whatever scheme you use you need the VA requirements of the compressor NOT the watts to get an inverter or genny to power it. You also need to be able to source the Starting VA of the compressor which will be far higher than the running figure by several times.

One way to estimate pk current is to look at the rating of the fuse and knock off a little.

You will soon see the requirements are far too high for a toy genny or a battery.

Basically this is a non starter. Forget it.

[Dave Plowman (News)]

Vehicle AC is invariably driven direct from the engine - the compressor via a belt drive, etc. You'd need a massive alternator to provide anywhere near enough current. And a massive inverter.

[The Natural Philosopher]

Is not possible.

When I took an RV across the Mojave desert, it had aircon that was powered by a little donkey engine/genny at the back that chewed petrol. Only used it in anger once, near Lake mead. There was just enough humidity to make the 40C+ temps unbearable.

It was noisy too.

[Capitol]

Agreed. A big alternator will give you 2.4 Kw, nowhere near enough to start a locked compressor. Try sourcing a refigerated container heat pump and see what it takes.

[Fredxxx]

As soon as we know the spec, it might be possible to assist.

For a narrow boat I installed a 3kW inverter (with surge capability),

150A alternator and 4 100Ah batteries. It can be done though challenging. I doubt you'll have the space to fit a second alternator.

Again without knowing the rating of the air con, and the size of your portable gen, we're in the dark.

You cannot "blend" 2 AC voltages.

[harry]

t not going to get out of the 12V system or a battery.

ce for one and all the other components of course (many engines just substi tute an idler pulley instead of the compressor if the aircon option isn't c hosen) but it would be hugely expensive.

in a deckchair when camped without leccy hookup.

+1

Automotive AC runs directly off the engine such is the power required.

Such AC can be retro-fitted but not cheap.

[Dave Plowman (News)]

Snag is many motors if rated at nominally under 13 amps take a great deal more when starting. Not a problem for a normal mains supply - but an inverter or even generator may not like this.

[David]

Can nobody see my post of 12:29 today? This contains the details, but I keep getting "as soon as we know the specs" responses much later in the day.

With regards to blending two AC voltages, how does solar from the roof blend into the UK grid, then? Thought it was pushed out of an inverter and synced with mains - depending on how much you are drawing at the time you should get all inverter (plus some pushed back up the wires) or part mains and part solar. Or all mains if it is night time!

Or am I missing something about how solar power is delivered from the rooftop?

Cheers

Dave R

[Fredxxx]

My apologies.

That sort of power is doable; 4 a 100Ah batteries at 12V at 750W, taking into inverter efficiencies and battery characteristics might give you 2 or 3 hours at most. Your biggest problem is how to charge them back up! I believe a generator would be a better solution.

You are converting the DC power from the PVs into AC using an inverter. They are purpose built. You cannot just join 2 AC sources, it either requires some form of inverter or "manual" synchronisation of a generator to a source. I doubt the inverter will survive as it is most likely a modified sine wave type.

[harry]

Solar power is DC. It is linked to the mains by means of a grid tie inverter. The mains AC is used to "create" synchronised AC from the DC.

Synchronisng two separate AC supplies is quite another (more difficult) matter.

[harry]

[David]

However, in the scenario I am exploring I was looking at blending a source of 240V AC with the output of an inverter driven by 12V batteries.

So in this case a "grid tie inverter" might do the trick. That is, blending the output from an inverter and the output from 240V mains. However I think that this is likely to be far too expensive (from memory of solar installation quotes) for the application I was considering.

I know that the power demand from the A/C is likely to rule it out in my specific case but I am now exploring general principles. For instance if you want to run a microwave or hair dryer which is just over the power output of your genny.

It does raise the possibility of a very limited solar system (couple of panels on the roof of a motor home) being able to boost the output of some of the more flaky (especially continental) on site power supplies on sunny days.

Cheers

Dave R

[David]

Some obvious confusion (see my response to Harry).

When I am talking about blending two AC sources I am talking about having an external 240V supply to the motor home (generator or mains) and an internal one from an inverter.

Which seems to be much the same as having solar power and mains joined together, which also involves an inverter.

In the motor home I have 2 * 110Ah leisure batteries so (assuming the information from the handbook is correct) I might be able to handle the running load but possibly not the starting load. Which was more or less where I came in (I think).

Bottom line is the question "Can you boost the 240V supply to a motor home by using a grid tied inverter to feed in battery and/or solar power from on board sources?"

The answer may be "Yes, but too expensive.".

Cheers

Dave R