Which new refrigerator system?

[Roy M]

I asked a straightforward question: How many amp hours are consumed in a 24 hour period? I gave only one variable, a seawater temperature (and assumed air temp) of 80 degrees. I'm getting evasion, cloaked in important sounding studies, tables and hyperbole. I think that effectively makes my point. It's okay for you to choose whatever system you wish. You are paying for it, both in cash and in the resources used to recharge your batteries. My point appears simple to me (but then, I'm a simple guy): Build a superinsulated box, top-loading, with a cold plate sized to the volume and temperature needs. Freeze that cold plate as quickly as possible with the most efficient system available, at the moment. Measure the electricity consumed over a 24 hour period. Convert that into how much money it cost to build, how much to maintain, and how much to provide the electrical power to recharge the batteries storing the juice. Then, the only rationalization one needs to make is why one is not doing what is needed to achieve the lowest cost, over the lifetime of use, to keep the ice cubes in supply. Apples to apples, amp hours to amp hours, and, especially, no bull.

And, by the way, one should actually listen to a Bitzer, with direct drive to the 12 volt motor, as well as place one's ear against a bulkhead to register the transmitted vibration, before expressing the comments about noise and vibration. No bull, please.

[Cheechako]

Doesnt running two compressors (one for fridge and one for freezer) up the amp consumption significantly?

[Roy M]

Cheechako, Absolutely! The system in the picture is a demo used by Bob Van Ness to power a reefer, freezer and airconditioner, all from one compressor to demonstrate the effects of maximum loading. His own system, and the one I'm installing, uses a single coldplate in the freezer, with spill-over holes in the top of the box to chill the reefer. They are a series of holes with corks in them. The temperature gauges on the panel keep you aware of how cold each box actually is. You can simply adjust the chill box by adding or removing corks, depending on the external temperatures, filled volume of the boxes, and other variables. Simple, non-technical and elegant.

[Cheechako]

another option is a little SS vent that rotates to adjust the open area in lieu of corks. I think people worry too much about refrig systems. Simple is nice. I've had belt drive, engine drive and of course simple cold machine types. I would definitely go with the Coldmachine type of system. Although, the one advantage of the engine drive is if you are motoring to get into port, you get "free" cooling that day. On that boat I had engine drive water making so running the engine each day kept everything replenished. On the other hand I had to recharge the freon/compressor now and then.

[Gashmore]

The Danfoss AEO module plays a sort of microprocessor game of "Coffee". The compressor starts up at about mid speed and increases the speed 12.5 RPM/minute and records the speed it was at when it turned off. Next time it starts 400 rpm slower and slowly speeds up. This repeats until it finds a starting point speed that does not exceed the previous shutdown time. The target is a run time of about 32 minutes. (400/12.5)

Frigoboat's SSC is a little more sophisticated. It also starts slow but rather than increasing speed it times how long the compressor runs and compares it to an "ideal profile". If it takes longer than 50 minutes it speeds up on the next cycle. If less, it slows down. The real "ideal profile would be 59 minutes 30 seconds but, as I said, the designers were worried that users might think it was stuck on.

I think it is a sign of progress that no less than Nigel Calder, long a proponent of engine driven holding plate systems, now believes that the efficiency of the new generation of 12V hermetic compressor based evaporator systems has improved to the point that they are a better choice for the long range passage making sailboat. (Professional Boatbuilder, October/November 2004)

[Gashmore]

Roy, you gave ONE variable which is not enough to answer your question. To determine the total amp hours used over 24 hours for either system would require, at a minimum, the capacity and freezing temperature of the holding plates, the heat load on the box and desired box temperature.

However, in comparison for the compressors only and conservatively assuming a freezer with a cold plate eutectic temperature of -20C and 134a refrigerant, a Blitzer IY compressor running at 1,450 RPM and will require 370 watts of power to move 370 watts of heat for a COP of 1.0. A Danfoss BD50 under the same conditions at 2,000 RPM has a COP of 1.38. Considerably more efficient albeit slower.

This is not to disparage belt driven holding plate systems. They have their place when surplus energy is available over relatively short periods but in most situations a well designed evaporator/hermetic 12V system will require less amp hours over 24 hours all other factors being equal. Additionally, the installed cost of a hermetic/evaporator system is usually less than half that of a belt drive/holding plate system and maintenance will be considerably less over the long term. Again, all other variable being equal.

To answer Cheechako's question a dual system will use marginally more power than a single system but allows considerably more flexibility and redundancy. The difference will be a small amount of mechanical loss because the heat load is the same.

[CSY Man]

How many amp hours are consumed in a 24 hour period?

At the "warmest" setting on the thermostat, during the Florida winter, the system will run about 3 hours per day, or about 20 smp/hours.

Medium setting with a cold fridge and the freezer right around -5 C, it will run 7 to 8 hours and consume 50 amp/hours per 24.

Highest setting, with hard ice-cream in the freezer and a -18 to -20 C freezer temp, lots of traffic in the fridge, warm beers in, cold ones out, the system could run 18 hours a per day, or 90 amp/hours.

The above system is a Cool Blue fridge/freezer with spill-over ports adjustable vents.

[Cheechako]

A small well insulated, but not super insulated freezer/ ref tended to consume about 60-70 amp hours per day in my experience. This was a hermetic sealed unit with approx a 2-3 cu ft box and small evap freezer. Not one of the new technology units mentioned though. This is a highly empirical estimate based on engine running time with the boat at anchor. I would run the engine 1-1.25 hours per day to keep up with a 125 amp alternator and 3 step regulator. At anchor the fridge was most of the electrical consumption. as I remember it, these are the approx numbers of Alt output on the meter:
85-90 amps for 15-20 mins~ 30 amp hrs
40-50 amps for 40 mins~ 30 amp hrs
25 amps for 15-30 minutes~ 10 amp hours
Again.... this is pretty general...

[zac]

I'm a big fan of Seafrost, just don't get the controller wet!

[indy]

Don't overlook the Frigoboat. I can't give you any specifications other than to say it shows about 1/2 the current draw of my previous Adler Barbour with cold plate system. Of course, it did last about 20 years. You cannot hear the Frigoboat run and keeps ice cream and anything else frozen like a brick. The biggest negative are the zincs that need frequent replacement on the heat exchanger on the outside of the hull - twice a year.

[bobfnbw]

Originally Posted by indy

Don't overlook the Frigoboat. I can't give you any specifications other than to say it shows about 1/2 the current draw of my previous Adler Barbour with cold plate system. Of course, it did last about 20 years. You cannot hear the Frigoboat run and keeps ice cream and anything else frozen like a brick. The biggest negative are the zincs that need frequent replacement on the heat exchanger on the outside of the hull - twice a year.

If I was upgrading at this time, the frigoboat system would be the one for me. I have a recently purchased boat, came with the AB super cold machine, with evaparator plate, air cooled, and while it works, here in Texas, it consumes a lot of amps, and adds to the heat load on board.

The way I see it, a well insulated box with a water cooled unit with evap plates is the way to go. I am not a fan of holding plate systems.

[hellosailor]

"a Blitzer IY compressor running at 1,450 RPM and will require 370 watts of power to move 370 watts of heat"
There's got to be a typo somewhere in there. In order to MOVE 370 watts of heat, surely you need more than 370- watts of power, or else you'd have an output equal to the input--a perpetual motion machine. No?
There has to be SOME power consumed by the "mover", the compressor itself.

[Whimsical]

The power is only used to move heat energy and not make it. Refridgeration is just moving heat and they have a COP, coefficient of performance. With a COP of 2 it will move twice the heat energy than it consumes. Definitely not perpetual motion.

Mike

[Gashmore]

The Coefficient of Efficiency is the heat removed from the box in watts divided by the energy required to move it. If a system has a COP of 1 it removes 370 watts from the box using 370 watts of electricity. The TOTAL heat involved is 740 watts. Resistance in the motor and friction in the compressor turn the 370 watts of electricity into heat which is given off by primarily by the motor and compressor. The 370 watts of heat from the box is given off by the condenser into either the air or the cooling water.

[Roy M]

I guess we are spiraling into quibbles without listening to each other any more. First, I said the Bitzer is DIRECT DRIVE, not belt driven. Second, in order to compare apples to apples, I am assuming that in comparing two systems, one tries to make as many things equal as possible and limit the variables. Hence, I assumed we were comparing two equally sized boxes, equally constructed (and well designed), with the same internal mix of popsicles and beer distributed equitably. You get each unit up to speed, let them cool to the same temperature, then measure the amp-hours to keep them at the desired temperature range for a 24 hour period. Your preference cycles more often at lower per-minute draw than the direct drive Bitzer. The Bitzer furiously, and powerfully, chills a holding plate for a number of minutes, then shuts down and waits until the thermostat directs it to repeat its task. At the end of 24 hours, one uses more current over time (amp-hours) than the other. The one that achieves this result can then be judged as less efficient than the other. The less efficient unit requires longer run time of the diesel engine, or larger solar panels, or more battery capacity than the other. It can then be seen, over time, to be more costly, both in longterm investment and wear and tear. Apples to apples, no bull. But, in the end, we choose our flavor of religion, politics, and sailing craft as personal decisions, not letting externals confuse us in our certainty. You make your choices and live with the consequences (unless you have lots of money or the ability to radically change course in short order).