[QUOTE=fb_bf;10791]It sounds like you have the solar cells, and your estimating that you can generate 20Kwh of electricity on a good day. I think most people use lead acid batteries for solar storage. With LiFePo, you’ll need some form of battery management system to keep from overcharging them on good solar days, and not drawing them down below 2.5 volts on poor solar days when you run your device A/C?? .You will have to find out what voltage your inverter from the solar panels will put out, and how to get that to charger your cells. Then you’ll need to get the DC voltage back to AC to run your 3 ton device. I can’t help with all of that. For LiFePo cells, you could use 38 160AH Thundersky cells. That gives you 19.6 Kwh of capacity. You’ll have to add more information for me to try to help farther.

Frank Home[/QUOTE]

Ok, I did a LOT of extra work on this yesterday. Here are the results.

Anyone see anything wrong with this?

Objective: To power a 3 ton air conditioner from solar panels and a large battery bank.

Boat Description: The boat, in question, is a catamaran with a 45’ LWL and a 25’ beam. The deckhouse roof is approximately an 20’ x 20’ square, all available to solar panels. The design goals for the boat were:

*High Performance Cruising (similar to Gunboat)

*Comfort for Charter Guests (water, hot showers, HVAC)

*Light Weight

*Reliability

*Self sufficiency away from the dock (it will never dock, except momentarily for fuel)

*Outboard engines have been chosen to keep the boat light and to raise them, allowing the boat to slip through the water more easily, reducing drag.

*Lack of metal below the waterline also means no zincs to replace

Usage of Boat: Boat is a term charter vessel, taking people on vacation for a week at a time. Charters run from November to March in the Bahamas and from June to September in the Northeast, USA.

Electrical Design Strategy: I am using the design strategy of using the power hog (HVAC) to size the system, then adding the small items (anchor lights, etc…) later on. This is a good strategy for an initial pass designing an electrical system. It is not necessary, at this point, with the HVAC load size, to add up every tiny little item. That will come later.

HVAC System: Based on another thread and a lot of thinking on my own, I have decided to go with a 3-4 ton split, reverse cycle air conditioning system, like you would see in a home. Outside is an air cooled condenser. Inside are 3 evaporators for different zones. They are connected by refrigerant lines. SEER Rating should be about 23 SEER or as high as possible. For the sake of the math, let’s assume a 3 ton, or 36,000 BTU system. (I apologize about the imperial units).

36,000 BTU’s of AC/Heat divided by 23 SEER = 1,565 Watts of Power

This means the air conditioner will use approximately 1.5 kW each hour. Since kVa is not often used in the USA, and I do want to stick to proper units, we now have to find out how many kWh I will use in an average day of air conditioning.

Does anyone have any thoughts on that? My technique is to overestimate and imagine the worst heat wave, though that is usually not the case on a boat. Heat may be needed more often.

I will just assume a 10 hour heat or air conditioning run time per day. Does that make sense? If not, we can adjust the amount after figuring out the battery and solar needed to keep this system going.

So, in summary, the HVAC unit will use 15kWh of power each day.

The HVAC unit will also never exceed 1.5KW of power demand at any given instant, except on startup, which really doesn’t matter, so long as the inverter can handle the momentary rush. The battery bank can take a quick starting rush like this.

HVAC on Boats - Real World Usage: I find that in season, I do not typically need heat or air conditioning often. Sometimes if you are in a very hot, windless anchorage in a city, you need to run an air conditioner in the evening, as the sea breeze slows down and you are going to bed. Often, it is not necessary to run it through the night and fans work just fine. Heat, if you are sailing in season, is much the same. You sometimes need to turn it on in the morning to take the chill off. A few hours into daylight, with ports and hatches closed, the boat heats up like the inside of a car. If it gets too hot, you open up the ports and hatches and allow the sea breeze through.

15kWh or 30kWh of Power? If the solar panels of my proposed system were broken, I would need 15kWh available from the battery bank to run the HVAC for 10 hours. However, there is always some light and some power coming from the panels, even on the cloudiest day. So, in reality, you would never need the full 15kWh of power each day. So how do I size this system? Do I just say, “ok, I need the 15kWh each day. This is my buffer?”

Solar Panels Assuming I need 15kWh per day to run the air conditioner, let’s go through the solar panel sizing.

Fact: 5 hours of sunlight average, per day is what they use to calculate the number of panels you need.

Our load here is 15,000 Watt Hours.

15,000 watt hours divided by 5 hours is 3000 watts.

3000 watts is the amount of power the solar array needs to produce per hour to give me 15kWh of power each day in use.

Actual power output random “Astronergy” 240 watt panel, at 28.8VDC, is 8.13 amps x 28.8 VDC = 234 watts, each panel. 28.8VDC is chosen as the charging current because 14.4 is the charging current for a 12V system. Doubling that, you would use 28.8 to charge a 24VDC system.

To make 3000 watts of power when the panel puts out 234 watts of power, it will require 13 panels. Let’s round up to 14 panels so it looks nice on deck.

So, 14 panels will supply a 3 ton, 36,000 Btu air conditioner with enough power to run 10 hours per day.

Price of Panels These example panels go for $304 each. $304 x 14 panels is - $4,256. Solar Array Cost - $4,256

Weight of Panels They are heavy. 44lbs each. I will have to see if I can find lighter ones. The total weight of the solar panels is 616lbs.

Batteries LiFePO4 batteries come in small, 3 volt cells. They have 2000-3000 cycles in them, if put to an 80% discharge. That’s a lot of days of air conditioning. To get 15kWh out of a set of batteries per day, running at 24VDC, you would need to have a 625AH bank. But, the batteries can only be discharged to 80%, so we need to add another 20% of AH’s on there to make up for this. 625*20% = 125AH. Total required AH @24VDC is 750AH to run the 3 ton, 36,000 BTU air conditioner for 10 hours.

LiFePO4 batteries I found store energy at a density of 131Wh per kg. Taking the 80% discharge level into account, if I need to store 15000Wh this means I need 136kg of LiFePO4 batteries to make a 15kWh storage bank. Or… 299lbs of batteries.

LiFePO4 batteries I found also store energy at a volume of 247 wH per liter. This means I need a battery bank that is 73 liters in volume, or 20 US gallons in volume. This equates to 4620 cubic inches of volume, which, taken as a cube, would a a cube that is 2.6 cubic feet, weighing 300lbs. OR… ABOUT THE SAME SIZE AS AN ENGINE. Interesting.

Cost of batteries: The cost of LiFePO4 batteries is $375 per kWh. I need 18 kWh, so the total cost of the bank would be $6,750.

Solar Benefits

*Total Cost - $11,000

*Total Weight - 1,000 lbs

*Runs FOREVER without buying fuel or doing any maintenance - no additional cost