Running an Air Conditioner From The Sun - The Holy Grail

Discussion in 'Boat Design' started by CatBuilder, Mar 6, 2012.

  1. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Look at this site
    They have an 18k btu that is : (from their site) The newest addition to our lineup of fine equipment has been specifically designed for Catamarans and other applications that require low vertical dimensions.

    The Tecumseh horizontal compressor made this all possible. 
Measures only 26.5" long x 14.50" wide x 10.5" high!

    The efficiency and noise is the determined by the manufacturer, that is true. These units can barely be heard if standing right next to them. Don't buy a cheap Chinese unit and you will do find.

    Oh the other reason I like multiple small units, I can stick then in place that would have gone to waste anyway. They are easier to hide.
  2. CatBuilder

    CatBuilder Previous Member

    Not every boat had an AC in all my years, but the last 3 did. On those, they were all standard marine units of some BTU value I cannot recall right now.

    I took a look at your link about the power consumption of the seawater cooled condensers. Thank you. That was a good link. Here's what I found...

    The first model in the link you gave is 18,000 BTU. It uses 13.6 amps at 120VAC to product 18,000 BTU for a total of 1,632 watts to make 18,000 BTU of power.

    Comparing apples to apples, you need two of those to produce 36,000 BTU of air conditioning, so you need 3.2kw to produce 36,000 BTU of cooling with the air conditioners in your link.

    A 36,000 BTU, 23 SEER air cooled heat pump, by definition, uses 1.5KW to produce 36,000 BTU.

    So... the air cooled system is roughly twice as efficient as the water cooled system you provided the link to. You are talking old school. I'm not sure you understand what modern air conditioners are like. These P.O.S. ACs they are selling in the marine market are old fashioned wastes of power. You cannot run air conditioning off solar if you have an inefficient air conditioner that uses 3.2kw to produce 36,000 btu of power. In that case, you need a generator.

    I need an efficient air conditioner to do this. Standard marine ones aren't.

    At $6,000 for an efficient air cooled split system, I'd say those aren't cheap either. They charge a premium because they are worth it in the electricity you save.

    Agreed. The megayachts I have worked on had regular marine air conditioning units in the engine room, ran to evaporators in the cabin. They may be switching to smaller units to simplify, which is great for them. They, however are not designing a solar powered air conditioning system. They have generators bigger than the engine a 50' boat - and boy are they loud! :eek: I always had to wear full ear protection working in the engine rooms of megayachts with those things running.

    Question: How is a chiller (chilled water) system with long runs of pipe going to be the "most efficient" as compared to long runs of refrigeration tubing with the split system. That doesn't even make any sense. Thermal losses through copper pipe, if it has chilled water or freon in it, will be roughly identical. Chilled water can't be the most efficient in terms of losses in the tubing. It would be about the same.

    Anyway, I think I'm done arguing these points. Obviously the new air cooled units are twice as efficient as the seawater ones (see math above and your link) and I am not convinced I have to add up every light bulb for my power budget. I used to do it like that, but it's not worth wasting the time.

    Now I do very large loads first, then medium loads and let the other little items just make up a buffer I add. Works every time (3 times recently)- and I don't use docks, so when I make systems they are really tested. I've been off grid since the early 2000's.

    PS: Cheap Chinese unit?? I am looking at a $6,000 air cooled split system AC heat pump with 3 (or 4) zones and a 7 year warranty. I do not by cheap Chinese *anything*.
  3. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Let m know which units you end up buying and how it works out. You experimenting for all of us. If it works for you I will redo my systems. 36,000 btu for 1.5kw just seems like there is something missing.
  4. hoytedow
    Joined: Sep 2009
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    Location: North of Cuba

    hoytedow Carbon Based Life Form

    How will you combat corrosion. As a previous terra firma HVAC customer service rep I know that salt spray greatly shortens the units' lifespans.
  5. CatBuilder

    CatBuilder Previous Member

    Mounting the unit in salt spray would be a very bad move. Of course I would never do that. It will be protected. "Terra Firma HVAC"... for a quick second, I thought that was the name of the company you worked for! :)

    Tell me though... how's the 7 year warranty if there is corrosion?

    I see commercial vessels with these units on them every once in a while. Rasorinc's son installed 6 or 7 of them on commercial boats. I see 3 of them on commercial boats from where I am building my boat right now.

    It's not like they aren't used. They just aren't used on small boats. It will be a challenge to fit it in place and protect it, but should be worth it if I can go solar.

    Thanks to this post, I'll probably open up the case and vasoline all the electrical connections. Thanks for the reminder, Hoyt.
  6. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    Warranty covers manufacturing defects, not exposure to harsh conditions. Some had five and some had ten year warranty, depending on which deal they went with. If unit wasn't maintained at least 1 time within every 12 months it would void the warranty, being considered neglect on the part of the customer. The salt spray to which I was referring was from it being blown onto beach-side communities usually along the Atlantic Coast of South Florida.
  7. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Cat, Mydauphin is correct here that it will be difficult to implement air-cooled condenser in your boat.

    You have to consider that the air-cooled units need a correct airflow through the condenser, in order to have them working at the design SEER. And it's a lots of air for a 36,000 BTU/h unit - around 1200 cfm (2000 cu.m/h). It will require a ductwork with a cross-section of 350x350 mm (14"x14"), one duct for the inflow and one for the outflow. You will need to have the same size opening on your hull walls, which is not a nice thing to see on a modern-looking catamaran like the one you are building.
    If you fail to create a good and smooth ducting with as little pressure loss as possible, you will see your SEER drop down by at least some 10%. You will also see it drop down due to the fact that you have to spend additional electric power for running the condenser's blower against the pressure losses in the ducts.

    On a hot summer day (35°C), the hull-mounted external grills will spit out air at 50°C, so you'll have to be careful not to let that air get back towards the living areas.

    By the way, did I mention that I believe that 36.000 BTU/h is too much for your boat? :p

    Did you check these A/C units from Veco: ? Two of those could be enough for your boat imo - one 16,000 BTU/h (or even 12,000 BTU/h) unit for the upper deck and one 10,000 BTU/h unit for the cabins. They are water-cooled, hence would require two through-hull seacocks (one for each unit).

    A 12,000 BTU/h and a 10,000 BTU/h would require 1.6 kW peak electric power. But, assuming that only the bigger one (in the deck salon) will work during a daytime, it means that the A/C alone would require 940 W peak power (12,000 BTU/h unit). It is then possible to construct a graph of power consumption for the air conditioning during a summer day, starting from 8 a.m., up until 8 p.m. It will look approximately like this:

    Power required.gif

    It will actually be more bell-shaped, but not very dissimilar from this linearized shape I've used for convenience of calculation. The cooling loads will follow pretty closely (very little phase delay) the variation of solar radiation and outside temperatures during the day, because your boat's walls have low thermal inertia and because of huge plexiglass windows all around the cabin. So it will be rather difficult to apply the strategy of "peak shaving" with the energy coming from batteries, because "peak shaving" requires periods where PV panels produce more energy than is necessary for cabin cooling, so the excess energy in those moments can be used for re-charging the batteries. In your case, solar energy available at any time will have to be used almost entirely for cooling the ambients heated by that same solar energy, leaving nothing for battery re-charging. That's because the solar radiation curve will have a similar shape as the previously seen curve of cooling loads. So you will need a certain area of PV panels dedicated to A/C powering, and an additional PV area for all the rest of needs, including the battery bank re-charging.

    The amount of PV panels for the A/C system can be calculated as follows.
    The total daily electric energy required to power the AC is in the last column, and is around 5.6 kWh. But PV panels have a mean efficiency of around 10-12% (considering occasional shadings, dirt, age etc), and the inverter will have an efficiency around 90-95%. Let's put them all together into one single efficiency of 10%. So, a solar radiation of 56 kWh is necessary in order to get 5.6 kWh of electric power.

    Now, take a look at this map here:

    (taken from

    It shows that the average daily insolation in the US, in latitudes around 40° and in July, is around 5 kWh/sq.m .
    Considering that the map shows the insulation of a flat plate oriented towards south and tilted up by (latitude) degrees, it has to be corrected for a horizontal-plate case (which is the situation you have on your boat's roof). So we have to multiply the values shown in the map by cos(40°) = 0.77. The average daily insolation then becomes 3.85 kWh/sq.m.

    And here comes the final part: you need 56 kWh of solar radiation and you can get 3.85 kWh/sq.m . It means that you need at least 56/3.85 = 14.5 sq.m of solar panels, to cover just the daily electric requirements of just one single A/C unit like the one in the above link (12,000 BTU/h).

    But during the day you also have to re-charge your batteries and to feed other electric loads on board. I will leave you the task of calculating the necessary PV panel area to get all the rest of your electric systems running. I have shown you the method and a practical example, now it's time for me to close (for a moment) this endless book. :)

    Last edited: Mar 10, 2012
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    FAST FRED Senior Member

    The more I read , the more I think this problem would be easiest to solve with a smooth 3 cylinder diesel with a 300A 24V bus alternator and a bunch of AGM and a 4000W inverter, or two..

    The unit would operate 24/7 , the loading would be good as the engine speed would be matched to the load required.

    The AGM would be recharged at night when house loads are minimum , and the AGM could boost the output for large Air Cond start loads .

    A small DC motor on the main shaft could eat "extra " power as required.

    The usual truck reefer engines have huge oversized oil pans , so the unit would not need service during a 1 or 2 week charter..

    The savings over the solar acres being proposed both in initial cost , service life and problems that could ruin the charter are huge. Even if two gensets were required for reliability or peak loads , it should still be cheaper.

  9. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Back again, to continue the previous work. Hey Cat, don't get discouraged - the job is still not over. ;)

    I need you to give me the list of other electric loads in your boat, so I can construct a graph of the hourly power use during a day.
    This is your list if items from the previous page, I have found some typical power and energy requirements and have added them:
    - Freezer: 100 W
    - Refrigerator: 15 W
    - Fans (galley exhaust, berth fans)
    - Watermaker: 0.02 kWh/liter
    - Slow Cooker/ Crock Pot: 1200 W - 0.6 kWh per dinner
    - Microwave: 0.1 kWh per meal - 0.5 kWh per dinner
    - Hair Dryers (guests only): 0.1 kWh each usage

    Won't you have a TV and a PC on board? A washing machine for your guests?
    So, possible additional loads you might want to consider:
    - an LCD TV 120-150 W
    - laptop PC 50 W
    - radio receiver and sound system 100-150W
    - sat-receiver 80 W
    - DVD player 30 W
    - coffe-maker 0.01 kWh per cup of coffee
    - toaster 0.04 kWh each toasting cycle
    - washing machine 0.5 kWh each washing cycle
    - icemaker: 3.2 kW

    I am keeping navigation systems out of this because I assume that you will feed them from a separate battery bank and a genset, will you? You cannot remain without navigational safety systems because of several cloudy days in a row with no wind and because someone wanted to stay cool down in a cabin.

    Please check out the above list, add other stuff you think should be there, cancel what you think shouldn't and let's make a comprehensive list of electrc loads on board. Then a more detailed graph of hourly power consumption could be made.

  10. CatBuilder

    CatBuilder Previous Member

    Thank you very much guys. I am happy for the participation in the thread and general momentum.

    I am not fully discouraged yet, but Daiquiri did hit on something that is a big hot button for me: weight.

    I had a long discussion with the wife about this topic this morning. We both prefer not to have generators because we don't like them - don't like having to run it, don't like having to listen to it, don't like putting fuel in it.

    One solution to the problem is this:

    1) Set up the solar system sized for the house loads without air conditioning or heat, but leave room for a large expansion of the system. (ie: put solar panels far forward or aft on the deckhouse, leave room for 3x the batteries in the engine room, use scalable charge controllers, etc...)

    2) Launch the boat and use it - we plan to take 6 months off between the grueling work/build we are in now and the beginning of the charters.

    3) During the time off, I can monitor exact power production and usage, then plan an expansion of the system to run air conditioning systems (if possible) or not.

    This way, instead of pure theory, which we are going on here (who knows what figures we are using are accurate or not?), we will have actual data to go on.

    If the solar system is up to the task and I have enough real estate (and weight availability), I will install the solar and air conditioners. If the solar system is not up to standards for running the extra load of an air conditioner from time to time, I will install a gasoline generator, since this is the propulsion fuel I have aboard.

    That is the best plan I could come up with... choice by experimentation, since I am not confident in the assumptions and values we are developing regarding solar power.

    So... while I am very happy for all of the input and the time taken here by those who have contributed (thank you), I will make this decision via experimental data instead of theory.

    Thank you for the posts... we can continue and I would be happy to, but in practice, I would prefer to put in a smaller solar system, test it in the real world, then scale it up to take the HVAC loads.
  11. CatBuilder

    CatBuilder Previous Member

    I know this is thread drift and I really don't want an engine running all the time, but Fred, can you expand on this idea a little bit?

    What model of motor would be appropriate and how do I attach a DC alternator to the flywheel side? This would be the right thing to do, not using alternators on the front of the engine. Can you suggest a way to attach high power alternators to the flywheel so I don't have to run a diesel at less than rated power?

    Would probably still use LiFePO4 batteries no matter what. Lead batteries can't really compete on any level, so we can just assume a battery bank in general here.

    Would much rather use all available HP from the shaft.

    The usual truck reefer engines have huge oversized oil pans , so the unit would not need service during a 1 or 2 week charter..

    After fuel and maintenance, generators are more expensive than solar, always. I look at things 10 years into the future, not just next year.

    Any generator type setup would need to be efficient (all power from the flywheel) and good on fuel.

    Let's take a look...

    The most fuel efficient, 5KW diesel generator I can find uses 3/4gal per hour. That's $3 an hour to run it before you even look at maintenance and up front costs. It's a low end 10HP Yanmar powered air cooled diesel generator:

    $4000 for cheap 5kw generator
    24/7 running is $504 per week, or $2000/mo.
    A year of that is $24,000 per year, plus an oil change every 2 weeks ($10 oil change), plus the initial cost is $28,250 the first year and $24,250 the second year, unless it breaks before then.

    That's with the 24/7 model.

    Assuming I'm only running it an average of 6 hours per day, that takes the above prices and cuts them into 1/3. So, $9,400 the first year and $8000 every year after.

    5 years out, using it 6 hours per day
    $40,000 in generator costs, best case scenario

    You can buy a lot of solar panels for $40,000. :eek:

    With the current price of diesel and the current price of solar panels, I don't see how you can generate power with a generator for less than solar.

    If there is a more efficient way to run a generator, I'm interested in hearing it though.
  12. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    I have a 3cyl Deutz, with two 180 amp alternators, Charges batteries, heats the water and I hope to have run Freezer/AC on it soon. It is very quite and runs at basically one speed. If you run it 2 hours a day, it solves all your problem. It only weights about 400 lbs.

    It is the way it has been done of sailboats forever.
  13. CatBuilder

    CatBuilder Previous Member

    I'm not so sure that setup solves all my problems. Are you forgetting about running the 3.2KW air conditioners from the link you provided? I still need to do that too. But... I'm curious about the alternators.

    So you are putting 360A amps into your battery bank for two hours, producing 720AH of power in 2 hours?

    What brand of batteries do you have? What size bank is it?
  14. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Basically I have 6 1000 amp Gel batteries. But I charge one with solar, and other alternators to charge other. Reason for two, is backup and I can charge only one or both. Most of the time I am not using this, I have AC chargers presently for both banks and using shore power for AC, But planning system that can sit at anchor for extended periods. Probably replace batteries with 6 volt traction batteries later. I run diesel just to keep it in standyby. I have 1kw, 1.5kw and 3kw inverters. So no need for generators, but then I don't have guests sleeping on my boat.

  15. pdwiley
    Joined: Jun 2008
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    pdwiley Senior Member

    We've sort of been over this in the diesel-electric thread.

    Unless you build/buy a custom alternator designed to run at a given engine speed, so you can bolt it to the flywheel, the best setup is a short stub shaft on the flywheel and a toothed belt drive with step-up ratio as applicable to a generator head on a layshaft. Support with outboard bearings as required by the load.

    You need to start from the engine and what RPM it is going to run at and work from there. The alternator may well have an influence on the engine; I'm simply not up on 24V alternators but toothed belt sprockets are straight industrial equipment & easily available in whatever ratios required. The reason I say toothed belt is the can transmit far more HP than V belts for the same belt tension (and therefore side load on bearings). My 5HP lathe has a 30mm wide toothed belt and it transmits the full 5 HP, has done for years.

    None of the engineering side is hard or particularly bulky, I could easily make the entire setup in my shop. In fact I may as I have an old small Yanmar marine diesel in search of a role in life.

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