We don't use the refrigeration system on board Hello World. It's a water-cooled AC system that pulls down 60 to 70 amps which positively kicks the stuffing out of our batteries. We don't like to run our engine for anything other than propulsion. Diesel engines don't like to be run without a load placed on them. In British Columbia and even going down the Pacific coast, traveling sans refrigeration was fine. Our water tanks were cold enough to drink out of and not having cold beer around means I lost most of my beer belly. We don't need refrigeration!
And then came Mexico. Actually, Cabo San Lucas. The water was 80 degrees which means the water in our tanks were also 80 degrees. An ice cold Coca-Cola was heaven in a bottle. We were getting dehydrated because we weren't drinking enough. And we started to actually catch some fish. That meant whatever we caught, we had to eat right away. Some friends gave us a sierra that we actually had to throw overboard because we couldn't eat it in time.
So we're getting a new refrigeration system. Yeah, I know, we tell everyone we're hard as coffin nails but the truth is we're actually as soft and squishy as Dom DeLuise's underbelly. We've come to accept that life is just better with ice in your gin and tonics and cold milk in your cereal.
Our RequirementsWe've cruised for awhile without refrigeration and learned lots of tricks to provision without stocking perishables. We don't need a lot of space for refrigeration. We want to keep drinks cold (really cold), keep fish we catch longer than 4 hours, and have access to fresh food we would otherwise not be able to buy.
Our main requirements for this system:
- We want to be energy neutral for days on end. I do not want to be in the business of running our engine in order to keep our fridge running.
- We want to keep our fridge cold. Very cold. 35° would just fine with me.
- We want to make and store ice.
- We want to cycle lots of drinks in and out of the fridge.
- We want this system to work in hot, tropical places.
Sizing the SystemOur current fridge/freezer box is a highly under-insulated 11 cubic feet. That's alot of volume to cool and even more so when there's not much insulation around it. So we're going to pull out that box and replace it with a smaller box of around 4.7 cubic feet. Then we can fill in that entire remaining void with a two-part polyurethane foam insulation which should give us a minimum of R30 insulation around the box.
Next, we have to calculate how many BTU's (British Thermal Units) our system needs to pull out of the box. Fortunately, we found kollman-marine.com, a website run by Richard Kollman who specializes in boat refrigeration. His simplified formula for a well-insulated refrigeration heat load for a boat in tropical waters suggests 600 BTUs per cubic foot per day plus 1,000 BTUs per crew member per day.
600 * 4.7 + 1000 * 2 = 4,820 BTU's per day = ~200 BTU's per hour
What Kind of SystemThere are several ways to skin the frosty-cold-cerveza cat. Holding plates are basically large stainless steel boxes filled with a eutectic solution (for the purpose of this blog post, a eutectic solution is a solution with a lower freezing point than plain water). These plates have coils running through them filled with a refrigerant. The holding plates serve as a battery but instead of electricity, they store negative BTU's. It's like having an ice block that can refreeze itself. Holding plate systems are designed to run just a few hours a day to freeze the plates. But they generally require bigger more power-thirsty compressors. They work well for systems that have large amounts of energy available for a couple hours a day. Folks running an AC genset or their diesel engine a couple hours a day would benefit from holding plate systems.
The other option is to install evaporator plates in the cold box. Evaporator plates cool the refrigerated space they are in directly. Systems with evaporator plates cycle on and off frequently throughout the day as the temperature of the cold box surpasses the threshold of the thermostat. They usually draw a lot less power than the holding plate systems but run off and on throughout the day. They are better suited to boats with smaller, well-insulated cold boxes, large battery banks and/or passive energy like solar and wind generators.
A 12V air-cooled system based on the venerable Danfoss BD50 compressor can pull 330 to 500 BTUs per hour depending on speed the compressor is running and the temperature of the cold plate. That exceeds our requirement of 200 BTU's per hour by a comfortable margin of error. Which means it would be running around 40% to 60% of the time. That should pull 50 to 60 amp-hours per day.
Which brings us to...
Electrical RequirementsIn the year and a half that we've owned this boat, I can think of one hour that we've run our engine strictly to charge the batteries. We don't like to do it. It's hard on diesels to run without a load on them and not terribly efficient to run a 46 horsepower engine to put a few hundred watts back into batteries.
We have 240 watts of solar panels on board that keep us in power pretty much as long as we want to sit in an anchorage. We can even run the watermaker for a few hours every couple days and our solar panels keep replacing what we use. We're anticipating refrigeration to add a worst case scenario of 100 amp/hours per day but more likely somewhere around 60 amp/hours. Our current solar array won't keep up with that. We can either add a wind generator or add one or two more 130W solar panels. Or both.
We don't have the cake for both but what a luxury that would be, hey? We're leaning towards replacing the solar panel regulator we have with a MPPT regulator which should wring some more amps out of our current array and support up to two more 130 watt panels.
Our SolutionSo what's all this lead up to? Here's the direction we're heading right now:
- Have someone vacuum out the refrigerant out of our current system. Once the refrigerant is out of it, we should be able to pull it out ourselves.
- Pull the galley counter off - hopefully without destroying it! - and yank the existing 11 cubic foot cold box out.
- Replace the existing cold box with a smaller 4.7 cubic foot cold box. Insulate the remaining void with a combination of polyurethane boards and two-part polyurethane spray foam.
- Install two evaporator plates into the new cold box.
- Run the two evaporator plates from a small 12V air-cooled system, probably based on a Danfoss BD50 compressor.
Next up? Now we have to choose which refrigeration system we're going to go with.
(8 super bonus round points awarded to anyone who identifies the source of the post title - without googling it, McKenzie...)