Air conditioning your boat from the Sun.

Air conditioning your boat from the Sun

Simon Boyde, Storm Force Marine Ltd

When I first wrote an article on this nine years ago the inescapable conclusion was that generators were absolutely going to be necessary if you wanted to aircondition a cabin on your boat. Technology has moved on now:

Winter time is coming up and who needs air conditioning? Mmm – on that cruise to the Philippines at Easter, or even the summer races to Macau and Mirs Bay you might change your mind and of course, the time to get major work like this done on a boat is over winter when you are less likely to be using it.

Now, on a big motor boat we have great things called engine rooms where we can put the necessary generators etc so its all nice and quiet at night. On a sailing boat we rarely have that luxury so we have to put up with noisy nights.

So why not run it from the batteries?

Indeed why not: inverters and inverter chargers from Victron Energy are well proven and very reliable, typically outlasting generators even.

Variable speed compressor airconditioners from Frigomar pull tiny loads in comparison with their competition and they have no starting surge to speak of as of course you start the compressor at low speed. Couple this with an efficient pump (Frigomar recommend the 90W VMD40 pump for this unit) and we end up with a very very low power requirement indeed. After all, if we are looking to cool one cabin down, most of the effort in cooling this cabin occurs in the first hour when the compressor might run at high speed in order to pump the existing heat away. Subsequently the airconditioner will automatically drop down to a lower speed (meaning that it runs at a much lower power) as all it has to do now is maintain the air temperature.

To take an example of the power required:

the Frigomar variable speed compressor 597NT self contained airconditioner draws between 326W (in ECO mode, 4800btu) and 680W (max output 10,000btu).

The VMD40 pump needs 90W

For the first hour we run the airconditioner at maximum power (10,000btu). For next nine hours of our ten hour cabin cooling time it runs at a low setting of 4800btu.

For all ten hours the pump runs at 90W continuous

So over ten hours we draw 1×680 + 9×326 + 10×90 Wh of energy which adds up to 4514Wh. Our maximum draw is 770W so a Victron Multi Plus 2000W inverter charger can run this with ease.

Normally we think of Amp-Hours in terms of battery capacity on a boat. At a nominal 12V 4514Wh is 376AH.

So if we use straight forward AGM deep cycle batteries, which typically we can draw down to 50% capacity and fully recharge around 500 times (this 50% is the deep cycle in the description – normal car batteries or so called “marine” batteries are NOT deep cycle) we would therefore need 376/50% = 752AH or 7 standard 110AH batteries.

So we can have a cool quiet night, but you have to be able to put the power back into your batteries. Easy enough on shore power, just a big marine grade battery charger (and I mean big – you should be charging your batteries at between 10-25% of rated capacity). The stated Victron Multiplus has a built in 80A charger which is sufficient for this. When not on shore power though you are going to have to get the power from somewhere else.

The Open 60s and the round the world boats normally have high grade flexible solar panels which are designed to be able to walk on. Solbian, the Italian company started by Giovanni Soldini, has been building them specifically for marine use for some years. These are a lightweight panel, fully sealed, designed to be walked on and installed in the marine environment. Giovanni has done the Vendee Globe round the world race using these a few times so I think we can call them well tested (!).

Backed with a Victron MPPT solar controller which enables you to chain up several of these in series and we can get some serious power. They can generate around 140W per square meter so if we put 4 of these big panels on the boat we can chuck 560W into the boat in hard direct sunlight. Just enough in fact to recharge our batteries enough for another ten hour run with the airconditioner. Of course it is unlikely that we really could run them on average at peak efficiency so perhaps 6 panels would be a better choice. Or four and some engine run time and the alternator can put some energy back in.

So there we have it: Yes you can aircondition a cabin on a boat from solar power alone.

It is interesting that when I first wrote on this subject back in 2006 we were adding wind generators and diesel generators to the mix. With the new solar technology from Solbian and the ultra low power and airconditioner technology from Frigomar, teamed with the high efficiency inverter chargers and solar controllers from Victron available now we can dispense with the generator and the noisy wind generator. Which means the cost has come down by around around 60%.


Simon Boyde

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