Mechanical holding plate refrigeration systems?

[chromisdesigns]

We are in the planning stages for a 4x4 camper. Everybody seems to use 12v refrigeration these days, or 12v/propane, in some cases. Does anyone use mechanical, holding-plate systems driven by a compressor belted to the vehicle engine?

We lived aboard a sailboat for 8 years, and had nothing but great experiences with our holding plate fridge/freezer. An hour a day running the diesel at just above an idle not only pumped down the fridge (which would stay cold for a few days once the plates were frozen, if needed) but also gave us about 150 amp-hours a day of electric to use from our 440 AH battery bank, which was plenty since we didn't have a high-amp 12v fridge to worry about. It took about a quart of fuel per day, max, on days when we wouldn't otherwise run the engine.

It seems like one of these systems would be ideal for an expedition vehicle, especially since you could build the box into whatever space you wanted.

----------edited later: such as system would have to have a fan-cooled condenser, like a truck refrigeration unit, rather than the water-cooled versions used in boats. But even without an engine-drive compressor, an air-cooled 12v compessor with holding plates should use about 1/2 the AH draw of constant-run evaporator units.

[dwh]

Not sure, but I seem to recall Captain Cuthbert mentioned cold plates somewhere in regards to his truck Shachagra. I do recall he used a small fridge and a drop-in freezer - sailboat style. Not surprising considering his degree in naval architecture and all of the sailboat style details he used in his truck.

The issue with the engine driven compressor is that of complexity. Quite a few overland campervans use diesel fired hydronic units to both heat the cabin and hot water and also pre-heat the main engine. These are tied into the engine's cooling system. This also allows the main engine to heat the system.

With a cooling compressor driven from the engine, you now have to add the compressor - or 2nd compressor if there is one there already for the a/c - and tie that into the camper cabin.

More complexity equal more potential points of failure.

With the advent of the Danfoss variable speed DC compressor, many 12v (and 24v) refrigerators (and a/c systems) are quite efficient electrically, so there isn't much benefit gained from extra complexity.

Cold plate technology is however becoming more popular I think. Novakool has been doing it for years, and I think they have a fridge designed for off-grid solar which is a standard Danfoss compressor fridge with cold plates added, and which detects when the battery bank reaches full charge and then uses excess solar wattage to power up the cold plates. This makes the already efficient intermittent compressor cooling run at an even lower duty cycle by storing "extra" solar power in the cold plates.

[cwsqbm]

This reminds me of what I read on kimberly's site about "eutectic refrigerators", designed so that it doesn't need to draw power as frequently.

I agree there's no point in an engine driven compressor - the loss of first converting it to dc is trival since the land vehicle's engine produces vastly more power than what's needed for a refrigerator.

[chromisdesigns]

This reminds me of what I read on kimberly's site about "eutectic refrigerators", designed so that it doesn't need to draw power as frequently.

I agree there's no point in an engine driven compressor - the loss of first converting it to dc is trival since the land vehicle's engine produces vastly more power than what's needed for a refrigerator.

On a boat, the advantage of direct drive compressors is the ability to pump down the system very fast. The compressor on our boat probably drew about 5 hp freezing 3 big plates, and it would do that in an hour max from room temp. By comparison, even a large alternator can't be used efficiently for 12v refrigeration, unless you have a huge battery bank to absorb the power, because the 12v units can't use it that fast.

Without a water cooled condensor, though, the difference between the two is probably a lot less. I had a 3/4 HP beer-box compressor plumbed into the holding plates on a separate tubing circut for use when we had shore power available, and it took several hours to freeze the plates, as it was air-cooled and less capacity than the engine unit, as well. Probably similar to the best 12v units available.

I guess with a large battery bank and several solar panels, 12v becomes reasonable. But I think holding plates would still be a good idea, even with the Danfoss cooling unit.

[dwh]

By comparison, even a large alternator can't be used efficiently for 12v refrigeration, unless you have a huge battery bank to absorb the power...

...OR you run the engine/alternator several hours a day - for instance while driving here and there in the truck.

I also think the term "huge battery bank" needs to be defined. Would 200ah@12v be considered huge? That would certainly be enough to store a goodly amount of power for later use and not an unreasonable size bank even for something as small as an SUV.

[chromisdesigns]

...OR you run the engine/alternator several hours a day - for instance while driving here and there in the truck.

I also think the term "huge battery bank" needs to be defined. Would 200ah@12v be considered huge? That would certainly be enough to store a goodly amount of power for later use and not an unreasonable size bank even for something as small as an SUV.

For sure -- if you are driving every day, or every couple of days for several hours, no problem.

Re battery bank capacity, we had 440 AH in two house banks, plus another 220 AH for starting the diesel (3 pairs of Trojan 220 AH 6 volt batts). I do not consider that "huge", but it was adequate. It probably would not have been if we had electric refrigeration. Here's why:

For good deep cycle battery life and fast charging, you can only use about 60% of the rated capacity -- you don't want to drain them below 20%, and you can't bulk charge at high amp rates past about 80%. We alternated house banks every day, so we had a daily "effective capacity" of about 130 AH, give or take. We could bulk-charge that back into a battery set in an hour with a 160 amp alternator on the diesel. With a large fridge and freezer, we would have used at least half of that just for refrigeration, and more in the tropics. This was back in the '80s, before LEDs and compact fluorescents, solar was expensive, and wind generators didn't give too much power. With the mechanical refrigeration, we never lacked for power -- we could run the stereo, boat lighting and instruments, computer, fans, pumps, pretty much whatever we wanted, on about a quart of diesel a day.

Some of our friends had 12v fridges, and they had often problems either keeping their food cold or using too much current, or both. If we had gone that way, I would have wanted at least twice as much house capacity.

If we were a little careful about lights, etc. we could go several days at a time in the tropics, and close to a week in colder areas, before we had to recharge the house banks or pump down the fridge and freezer. Which came in very handy a couple times when the diesel was down!

[chromisdesigns]

Not sure, but I seem to recall Captain Cuthbert mentioned cold plates somewhere in regards to his truck Shachagra. I do recall he used a small fridge and a drop-in freezer - sailboat style. Not surprising considering his degree in naval architecture and all of the sailboat style details he used in his truck.

Now, he had what I would properly term a "huge" battery bank! 2100 AH!!!! And something like 24 KW of solar panels?

[dwh]

Um. I think you missed a decimal point there. 2.1kw I think his array was. Or maybe 2.4kw.


As you say, that was then, this is now. Solar is cheap and LEDs can make an enormous difference. Small wind generators are, or course, still useless.

There is also the question of total load on the electrical system. Are you going to have radios in your truck? Radar? Bilge pumps? Will you use the radios in the truck as much as you did in the boat?

Were your friends with 12v fridges using fridges with variable speed compressors? Another question is the size of the cold food storage. Are you planning to have the same capacity that you (and/or your friends) had in your boat? You'll probably have less, no? And if you are building it yourself and using a built-in refrigerator, adding "way too much" insulation around the fridge can make an enormous difference. As can be seen here, the 4.3 cu' Novakool draws 480wh/day, but by adding 2" insulation that can be cut in half:



http://www.backwoodssolar.com/catalog/refrigerators.htm#NOVAKOOL R-3800

With a 200ah@12v (2400wh) battery bank, that would represent only a 10% draw down of the bank.

With AGM batteries, you could calculate your daily budget based on a 50% Depth of Discharge as opposed to the 60% you used for the Trojan FLAs and depending on what sort of truck you'll have, you could easily have 400w of solar on the roof, which should be just about right to replenish a daily 50% DoD on a 2400wh battery bank.


You are operating in a different regime here. If you could get by with a daily energy budget of 130ah in a (wild guess here) 40' boat, then you can almost certainly get by with much less in a 20' or 25' overland truck; especially using modern energy saving devices.


Another thing worth mentioning, is that normal voltage regulated alternators are pretty poor battery chargers. Replacing the voltage regulator in your truck with something like the Sterling Power Products 3-stage voltage regulator would make a big difference (temperature compensated as well):

http://sterling-power-usa.com/advancedregulators.aspx