Running an Air Conditioner From The Sun - The Holy Grail

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

  1. CatBuilder

    CatBuilder Previous Member

    Good input, Phil. Thank you.

    When deciding between standard marine units and the Fuji or Sanyo type split air systems, I was left with one big, glaring issue:

    The split residential (air cooled condenser) systems are rated at 23+ SEER.

    Obviously, the SEER rating is the most important part of this entire system. Do you have any information on the SEER ratings of marine units? I have never see them published.

    What are your thoughts there?

    PS: My engine bays are completely, 100% empty. No engines in them, just equipment, so no problem.
  2. mydauphin
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    mydauphin Senior Member

    The big best panels need high end mppt to work period because of higher voltages anyway. Trust me I have being looking for 96v cheap controllers that bring voltage down to 24v for the batteries, and cant find them. Like the outback 80 mppt. This one is about $600 for the size I need, and it is most efficient. If you can find cheaper ones let me know.
  3. CatBuilder

    CatBuilder Previous Member

    We agree on that, Mydauphin. MPPT takes the over voltage and uses it. Up to 30% more power from MPPT over standard controllers. Despite Phil's excellent air conditioning advice, I will go MPPT.

    I am wondering if Phil knows of any marine air conditioning units (raw water cooled) that are 23 SEER or higher.
  4. philSweet
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    philSweet Senior Member

    No, compactness and peak amp draw are usually more important. And that rating is just that- a rating, and it does not necessarily reveal the most efficient way to be comfortable. There have have been several attemps to get a recognised comfort rating, but it is dependant on climate. Most ACs rated over about 15 won't pull enough humidity out of the air in the tropics at sea level. With some, you will never feel comfortable so you keep lowering the temp but it does no good and you end up using more energy. I've gone round and round with building inspectors in Florida over this.
  5. CatBuilder

    CatBuilder Previous Member

    Hmmm.... Air conditioning can be tricky.

    I sized a unit in my rv that works in 106 degrees, 98% humidity in Florida. What i did was assume an always on compressor for energy consumption and sized it just a little small, so it ran compressor on most of the time, for removing humidity.

    Can't I do the same here? The btu value (done by both Daiquiri and i) came out to just over 40,000 btu. If i run 36,000 btu, wouldn't that mean a compressor on most of the time pulling humidity better?

    Also, with less cycling, the starting current isn't as big of a deal, right? My rv ac cycles about 3x an hour, usually.

    Daiquiri, what do you think of these topics? Aside from Daiquiri, Rasorinc's son, a pro Hvac installer, also suggested this setup. So, I'm pretty confused as to why it would not work.

    Also, are the split residential systems really set up so inefficiently as to run the large compressor at 100% even when you are using only 1 of the 3 evaporators? I would think since you have 1/3 the available flow through the refrigerant lines at that point, it would take less energy, no? I mean you can't pump the whole flow through just one evaporator, so what happens in that case? Compressor slows down?

    One thing that does make a lot of sense to me is many small air conditioners though.
  6. PAR
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    PAR Yacht Designer/Builder

    If you want to kill a compressor quickly, don't let it cycle (continuous operation). Sizing a unit for any application isn't especially difficult. A common mistake is to over size it, thinking you'll save money, because the unit doesn't cycle as much. The bottom line is heat transference. A 5 ton will do it faster then a 4, but will also pull more electricity in the process, considering the volume of air that must be pushed. If you don't want condensation or other issues, then size it properly.

    I just sized up a PV system off that site. $20K installed, so amortization is still several years, though admittedly much lower then the last time I looked it up.
  7. CatBuilder

    CatBuilder Previous Member

    Now wait a minute. My understanding of electric alternating current motors is that they undergo the most stress when turning on and off. The LRA's go right into heat and mechanical stress on the motor. A motor that is constantly running, as opposed to turning on and off lasts longer. That's my understanding. What am I missing?

    Fwiw, the air conditioner in my rv has run for 2 summers may to Sept without being turned off. It is necessary to not have short cycling in order to remove humidity, right?

    Calling Rasorinc or Daiquiri... Need a little input from the pros here. :) I'm getting confused by these air conditioning related topics that, to some degree, seem counter to what we worked out in the Hvac thread.
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    FAST FRED Senior Member

    It is necessary to not have short cycling in order to remove humidity, right?

    A large unit will create the temperature required in a short time , so there is little time for humidity to be removed.

    Copeland and other high end compressor units will have more than one speed , great for efficiency.

  9. daiquiri
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    daiquiri Engineering and Design

    Ok, I'm back to this topic. Cat, please let me remind you one more time that the calculations in this post: have been done with very huge margin, to demonstrate how much off were your initial estimates. The numbers you see in those tables are relevant to a very warm and sunny day. The fact that these numbers correspond to the calculations done with a method found on one A/C manufacturer's site demonstrates that A/C manufacturers want to sell you big over-sized systems, to pull more money out of your pocket.

    The first questions you have to ask yourself are:
    1. How many extremely warm and sunny days do I have to expect during the intended period of usage of the boat?
    2. What are the the most typical maximum and minimum temperatures and RHs which will be encountered over a 24-hour period in the geographic ares you plan to cruise around?
    3. What is the mean daily solar energy irradiated per unit area in that period of the year and in that geographical area?

    If this was a residential system, then you could design it for the worst case and count on the electric grid to provide you with the necessary power.

    But here you have a different situation. You want to be self-sufficient, so you cannot design the system with a a huge overkill margin, because your energy supply is very limited. Any excessive margin will be reflected on your AC system efficiency and cost, your boat's electrical system design and cost, and on the overall weight. and weight will have an impact on your boat's performance, volume of the necessary technical vanes, and the passenger's comfort. A huge electric (PV and batteries) system is not compatible with the passenger comfort, because they steal the useful interior space and the outside flat surfaces which your guests would surely know how to use better (suntanning, comfortably sitting around on the deck etc.).
    So the imperative in your case is to NOT OVERSIZE.

    A rational approach to the sizing of your electrical system would be:
    1) create a list of the electrical appliances and machinery, an Excel sheet will do the job
    2) write the electrical power requirement for each one of them
    3) divide the 24 hour day in several significant periods where particular electric load patterns can be foreseen. For example:
    - night time (mimimum load, probably just A/C running at minimum)
    - early morning time (heavy kitchen and shower use)
    - late morning time (minimum loads, time to re-charge batteries)
    - lunch time (kitchen loads and heavy A/C use)
    - mid-afternoon (A/C usage and battery charging)
    - evening and dinner (lights, tv, kitchen and A/C at moderate load)
    - late evening (lights, TV, A/C at minimum).​
    4) for each one of this periods make the sum of required electrical loads
    5) for each of these periods make an evaluation of the available power from your PV panels, taking into account the mean sun elevation at that time of the day.
    6) the net daily energy count (ins minus outs) should be zero or slightly above it. In-power comes from PV and batteries. Out-power is the one eaten by the various electric loads. If the net balance is too much higher than zero, your system is over-sized, with all of the previous implications. If it is negative, it is under-sized and will need a backup from the city grid.​

    But the first step is to give an answer to those 3 basic questions above.

    Modern air conditioners use the Variable Refrigerant Volume (VRV) system. It means that the speed of the compressor is adjusted accordingly to the heat demand from evaporators. It makes the VRV system work more efficiently than a classical on/off systems.
    You can find a lots of info about VRV systems in internet, a brief summary might be this one:
    However, I don't think they are yet available for marine use (small boats).

    Regarding the high-SEER conditionars, PhilSweet is correct when he says that a high SEER is not compatible with high humidity. That's because machines with high SEER have been optimized to work with higher temperatures of the cooling coils, meaning that they can condense less humidity form the air.

    If done for efficiency, then an A/C system sizing is a pretty demanding engineering task, and sometimes (like in this case) can also become quite tricky. ;)

  10. hoytedow
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    hoytedow Carbon Based Life Form

  11. CatBuilder

    CatBuilder Previous Member

    Yes, that is what i was saying.

    My next post will have all information Daiquiri has suggested. This was a preliminary look at if it was generally possible to do this. I think it has passed that test. Now down to specifics...
  12. CatBuilder

    CatBuilder Previous Member

    I thought we both came up with 14kw using an extreme example? Mine was in BTUs, yours was in kw, but they agreed when converted between the two.

    In any case, you suggested a 36,000btu unit was the maximum I should get, if I recall. I thought I needed more, but then relented and decided on 36,000btu or 10kw. This seems reasonable to me given the deckhouse is all glass (360 degrees) and there is a galley in operation.

    Please let me know if you disagree with 10kw or 36,000btu.

    I definitely want to size it right and as small as possible so it can run longer and pull more humidity out.

    1. I expect few days that require air conditioning. It is for the time when there is a heatwave, mostly or if someone is too uncomfortable going to bed at night and needs to cool it off. So, I estimate 2-6 days per month of usage.

      Winter charters take place with an average high of 80F (25C) and average low of 62F (17C). There are deviations, for sure, such as colder evenings and warmer days. However, if you have spent time aboard a bridgeck catamaran, you would know they are like a greenhouse when closed up (stay very warm down to 50F). If it gets a little hot, you open up and they have a very refreshing wind blowing on you through the deckhouse. Mostly, it is going to bed at night (too hot) and getting up in the morning (too cold) that need to be addressed.

      Summer charters take place in an area with, basically, the exact same numbers as the winter charters, now that I look at them. However, record highs are 100F (37.7C) and record lows are 32F (0C) in the same place during the summer season.

      Relative humidity is always high because we are talking about a boat. I seldom see indoor relative humidity below 60% and it can get much more humid if there is fog or a very wet wind blowing. Sea air, as you know, away from land is very humid by nature.

      It is mostly these large, out of the ordinary swings in temperature I need to protect the guests from, as well as a nice little touch of waking up to a warm boat and going to bed at a nice, cool, comfortable temperature.

      Much of the need for these systems can be taken care of by nature itself. Closing up a boat when it's cold makes it very warm (like a car). Opening up if it is too hot makes a very nice breeze across all areas, except in berths, it is not always good no matter how much airflow you have. So air conditioning would be good in the evening.

      I see it like a small, part time help with the temperature to take a chill off or cool things down so the boat is not warmer inside than the evening air is outside. The only time it will be used for many hours is during a heat wave in the North or summer operation. For that, I expect we may have to plug into a dock each evening to run the air conditioner and keep a nice charge up for the next day's 10 hours of air conditioning underway. If our guests could not take the heat for a very hot week, that is what we would suggest... a dock each night instead of staying at anchor. It seems reasonable if there is an unusual heat wave.

    Well, it is summer with nearly full sunshine overhead no matter what. We are at 42deg - 43deg latitude for the summer season and 24deg latitude for winter season. So, it is as if we are in the south at all times. This means typical solar setup for Florida will work. Why? Days are very long in the summer in the North. Light out at 5AM or 6AM, dark at 9PM for much of the summer. That is a lot of hours of light and the sun is more directly overhead since it is summer.

    For winter, we are close to the equator. The sun is very *very* strong. No issues there.

    All solar array sizing guides I have seen simply assume 5 hours of production at full capacity to make up for sunrise/sunset times when there is little light.

    Agreed completely. This is indeed the strategy. To right size the air conditioning/heating. I should point out that there is no area that is acceptable for solar panels except my cabin top roof which is not part of the deck and not used by people. That area is approximately 7 meters x 7 meters.

    Luckily, huge battery banks are reduced greatly by the LiFePO4 batteries. They are something of a miracle. They would take up a 1 meter cubic space. I do not have anything going in my engine rooms except equipment such as batteries and electrical watermaker, etc... there are no engines inside the boat. So, I have two open "rooms" in the aft where the engines are designed to go that can be used for this purpose. No problem with a battery bank, but I do not want to oversize it either.

    Doing the Excel sheet as well. Will have it posted up here at some point soon.

    However, I have found over the years putting together systems like this, that if you tackle the largest "worst case" electrical loads first, it is a much more simple process.

    Rather than counting LED bulbs for hours, you can just forget about them because they have nothing to do with the system, really. They use the "spare change" from the large loads. However, I will get some more items on the energy budget, such as:

    *Fans (galley exhaust, berth fans)
    *Slow Cooker/ Crock Pot
    *Hair Dryers (guests only)

    Most of these are tiny impulse loads though and don't contribute more than a few AH's to the electric budget.

    I will, however, start making up a sheet for them. (no lighting or anything that insignificant though - it's not necessary at this stage) It's like using "significant figures" when calculating engineering values. They just don't enter into a 15KWH system. :D
  13. mydauphin
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    mydauphin Senior Member

    I am lucky I have lots of real life experience and see bad theory fall on it side all the time.

    First a seawater cooled AC is more efficient power wise than a air cooled. True 5000 btu is 5000 btu whichever you use, but all things consider water is a better at removing heat than is air. It is also easier to get ride of. It is important to get good marine units with stainless components. They are also lighter and simpler, just not cheaper

    On AC sizing and cycling, smaller units are better as long as efficiency is maintained. Small cheap units are not as efficient and more expensive models. Multiple units also give you some redundancy if one fails at least other units work. It also gives people ability to set their own temperature and compressors start in a staggered manner and put less strain on electrical from one big one starting. On a cool day all ACs work great. On a hot day they run non-stop, cycling only happens if unit have built in usage timer that makes them cycle. Remember this is a boat, the sun is brutal, and people have a tendency to leave door open longer than they would in a house. Also a lot of new designs have lots of glass, the real enemy of AC. Boats are not houses, even the big ones

    On electrical system design...
    First you start with critical systems, and make sure these are well fed and taken cared of redundantly. This means dual sets of dual batteries and alternators to drive, starters, Bilge pumps, fans, instrumentation, navigation and emergency lighting. These are all typically DC loads, and you want enough power to last a day and multiple starts on these. Hint, you better have at least 6 2000 gpm bilge pumps, three per hull, in three separate compartments.

    Then work on the lower power comfort luxury system. TVs, DVD, Radios, amplifiers, computers, pretty bathroom lighting, salon lighting. most of these, you will need a few hours of juice for on the inverter when your on anchor unless you want to turn on generator. Typically, the more cabin and guess the bigger the loads. Figure about 1000 watt per cabin.

    Then AC is typically an even bigger load. Figure about 1-2kw per cabin. Salon can take 5-10kw to cool.

    The point is this you start your energy budget from necessary to less necessary. You don't start with AC. When you look at that way, on a boat with guests, you are running AC on a generator. Work on your energy budget carefully otherwise you are dreaming.
  14. CatBuilder

    CatBuilder Previous Member

    I am very lucky. I'm over 20 years into consecutively owning boats and I'm on my 6th one. I haven't lived on land or at a dock (except winter) since the early 2000's until I moved back ashore to work on this boat I'm building. I've also built 2 RVs. Systems are my forte. This is what I know.

    It doesn't mean squat if you can run a TV for an evening. It's the large loads that matter, not the small ones. You first size the system for the largest loads to see if it's possible to do what you want to do. If it can, you add the medium stuff. Now, you just add on the "pocket change" for the lights and stuff. Adding up every light bulb is a waste of time. I always just put a buffer in for that incidental stuff. I always have way more power than I need and enjoy having some reserve available. I don't cycle my batteries even to 50% on any boats.

    With that out of the way, I do mostly agree... but you aren't being specific enough.

    You are saying, as a blanket statement, that seawater cooled ACs are more efficient than air cooled. No, they are not, if they are not designed to be efficient. A 23 SEER, 36,000 BTU air conditioner uses about 1.5kw each hour. How many killowatts does 36,000 BTU of seawater cooled air conditioners use? If you can answer that as less than 1.5kw, I will believe you.

    It would seem that not a single marine air conditioner company lists their efficiency. What are they hiding?

    I am with you on the small units though. It would be best if I could do that, but then again, what does it matter, if you read Daiquiri's post?

    If a compressor adjusts its output (and power consumption) to match whichever evaporator is turned on (one of them, two of them or all three), why are you hung up on small marine AC units? What is the difference between having 3 evaporators on a split system and 3 small AC units? I think the only difference is that it is more efficient to have the larger unit with 3 independently controlled evaporators. Are you familiar with split system residential AC units? Have a Google.

    I am not sure you understand the changes that have happened in the world of air conditioning. You are talking old school.

  15. mydauphin
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    mydauphin Senior Member

    Every place you have a line high pressure and low pressure lines you have loss of heat or cool. The longer the runs the worst the problem. It is simple thermodynamic, water removes heat better than air. And then since you always removing the hot water and bringing in cold the, the system is far more efficient than air. Also seldom is water warmer than the air. If you have a split or any air cooled system you have to get rid of the hot air somewhere. What kind of AC did you have in your boats for 20 years, wall units? Call someone in the marine air business and they are going to tell you to go with something like below. There are reason for it. They are not cheap and just you have a salt water pump going all the time. There is a reason why people don't put home AC units in a boat.

    Take a look at these
    Oh an your wrong about something something else, most big yachts had big split units. Compressors in Engine room and blowers in living areas. They are switching to smaller self-contained units when they do retrofits. They can fit a 10k btu in the space of the old blower/Evaporator. Oh, 36,000 btu of cooling takes a lot of KW or amps or horses - or whatever your going to use to push it along.

    A chiller system is the most efficient of all of these if you don't mind cold water being pumped around your boat.
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