Another look at Solar Powered refrigeration on sailboats

[a64pilot]

Never seen a pure Solar fridge on a boat, don’t see any reason to do so.
I’m aware that there are controllers made to run from panels direct, but don’t understand the use case.

I think we have sort of made the point here a few times that a cold plate and a battery bank are sort of very similar things, both can store energy when there is an excess of it so that a fridge can get by the times when energy generated is less than the demand.

I think Richards point is that if properly designed, there is no significant difference in efficiency between a cold plate and a thin walled evaporator/
The difference I think may be that to be efficient a this walled evaporator has to be rather large, and most are not?

[a64pilot]

Originally Posted by RaymondR

It's far cheaper to store cold in the form of a frozen eutectic fluid with an unlimited lifetime and no ongoing maintenance costs than it is to store electricity which requires expensive batteries which require regular replacement.

Maybe, I say maybe cause think of how much space cold plates occupy.
I think just as an example that between my two cold plates, they contain about 4 gl ea. As a guess of antifreeze/water mix, that is a lot of lost space.
I have if anything too big a box, so it’s not a concern for me, but if I had a smallish box to begin with, it would be.
Then there is the fact that most don’t want to admit to, that is that most boats have to occasionally at least augment the Solar by running something, and that excess power is best stored in a battery bank, cause then it’s usable for other things besides just refrigeration.

[belizesailor]

Given that the typical cruising boat has a house battery bank...what problem is it you are trying to solve with direct solar powered refrigeration?

[a64pilot]

Originally Posted by RaymondR

This looks like a rerun of the thread last year where Ozepete's company set up a test jig with both a constant cycling refrigeration system and a eutectic in the same box and then ran comparative tests to find that the eutectic system consumed far less electrical power than the constant cycling, cold plate system.

Pete was very honest in using his own systems for both tests, however Pete did admit that his evaporator was undersized which meant that due to its small surface area that it had to be run much colder to absorb the same amount of heat that the larger surface area cold plate did.
The efficiency difference was due to the compressor having to run at lower COP to get to the colder temp.

You control excessive cycling in a system by several ways, one is to widen the temps, turn on and off temps are opened up a little. Of course if you want excessive cycling you have the temps be one degree apart, and you will get what you want.
Then secondly it seems a lot of tests are run with an empty box, meaning no thermal mass to even temp swings out, but who cruises with an empty box?

Unfortunately to my knowledge there is no testing standard for boat refrigeration, and while I’m not very smart, I know enough to tailor the test to get the desired results.
I’m not saying anyone does, just that if I were biased to begin with, I’m smart enough to ensure that I would test under the circumstances that the system I wanted to “win” would shine, even if subconsciously with no ill intent.
You brag about what your kid does well, not what they can’t do. It’s human nature.

You want efficiency? Invest in insulation, nothing trumps insulation efficiency wise.


I have a cold plate system myself, and it works very well, so I’m not defending anything I personally have.
However I know enough to understand that the reason my system works as well as it does is that it’s a simple, well designed system with matched components, that is the secret, not the evaporator type.

[missourisailor]

Well said A64!!!!!

[a64pilot]

I believe also that John, (Cold Eh) has also published some testing that showed a very efficient installation, with an evaporator. He did that I assume with excellent insulation, and a well matched system.

[Typhoon]

Yes pilot

We did many tests , all of the results are on this forum and videos on our website . With a well insulated box and a properly designed system results can be spectacular . Done right such a system can give many years of trouble free service with no daily attention. If the system uses very little energy it does not become a burden to ships power . So set it and forget it .

Regards John
www.coldeh.com

[a64pilot]

The only real advantage I see in a cold plate is in its ability to store or hold over, and the best way I see it implement that would be different temp set points based on voltage, when you have excess Solar, turn the temp down and try to harvest and store as much of that excess you can, then when Solar isn’t available, voltage drops of course and your set point could be higher. You don’t want to freeze your fresh produce though so there are limits, unless we are talking freezer.

I’ve not seen any systems that do this though. I thought I found one, but it didn’t work well at all for a cold plate system.
You could also do this with a timer and in fact a simple timer may be best cause there are issues with using voltage, the issue is of course that voltage will drop when compressor is on and of your only using voltage it’s not going to be stable causing cycling.

[a64pilot]

I want to try to make something clear.
By stating that I can skew a test in the direction I want it to go, that was not meant by any means to mean Pete, it was an observation that a truly non skewed test is difficult, cause if nothing else we are human and proud of our accomplishments.
I am not trying to show Pete or his test in any kind of disdain, in fact It is an honest test, it’s not often that you find a test that tests apples to apples, that is difficult, but Pete’s test comes close to that.
Also the excessive cycling comment was not meant to be Pete’s test. But another manufacturer publishes test data that shows excessive cycling of a thin plate evaporator, I meant to show that excessive cycling can be controlled if it’s an issue, it’s not necessarily an operating principle of a thin plate evaporator.

Just from my admittedly very limited perspective, from my observation the Danfoss AEO module is a game changer as it has the ability to adapt the compressor speed to seek optimum cycle times, if they are short it will reduce compressor speed, and I believe that just as a general statement, within limits of course, the slower the compressor runs, the more efficient it is.
It also seems to be a whole lot less noisy to my HF radio, so much that I don’t turn the fridge off anymore.

[RaymondR]

My own conversion to eutectic systems using small DC compressors occurred after I had rebuilt my refrigerator using a kit which included a fin and tube evaporator.

The box was prismatic and fairly long and deep but not particularly wide and because of the hull shape had a considerable volume under a ledge on the outboard side. Also, I was fairly fed up with having to unload a lot of stuff at the top to get at that at the bottom.

In consequence I installed plywood walls front and back with horizontal slats to allow plywood sliders into which plastic storage containers would fit. I could slide them sideways to get at the lower ones without removing the upper.

The fin and tube evaporator I fitted behind the lower left hand side of the back plywood panel to allow cooled air to exit from the front and I fitted a fan to the upper right hand side to draw warmed air out of the box and recirculate it through the evaporator. Since I wanted to put things on the bottom of the box without them being in melted frosting from the evaporator I had also put a removable floor about 3" above the bottom of the box.

The system ran well and some months after I had commissioned it I decided to remove the plywood bottom and check how much water was there, there was no drain as it being a steel boat I did not want water in the bilge.

To my surprise there was about 2" of frozen water in the bottom of the box.

Rather than try to break it out and risk holing the fibre glass box I put everything back, turned the compressor off and wired the in-box fan to run continuously. It took about a day to melt the ice and the fridge stayed cold whilst it did so as one would expect.

I realized I had accidentally built myself a eutectic refrigeration system and since I had a very limited battery and solar panel facility at that time decided to exploit the discovery to assist with my power problem.

Rather than get involved with trying to switch the compressor on and off around the eutectic temperature I wound the thermostat way up and fitted a 24 hour timer driving a relay to switch the compressor on at about 0800 hours or so and off at about 1500 hours and put another computer fan under the compressor to assist with cooling.

My battery charging problems had disappeared as the batteries were no longer discharging during the night hours and since I had a hold over capacity the system tolerated overcast days much better and since the panels provided sufficient current to both charge the batteries with the low amps needed at the top of the charge cycle and run the compressor
my limited DC power facility no longer needed engine alternator charging at any time.

I ran the system that way for about 3 years until I decided to rebuild the box and never finished it before selling the boat.

My new (to me) boat has about 700 amp hours of AGM storage which has been depleted because the constant cycling fridge was pulling the batteries down overnight and there were insufficient panels to recharge the batteries during the daylight hours. I have increased the panel capacity to 580 watts from 400 and am installing a 12 V eutectic fridge and 200 amp hours of lithium batteries which I believe will be sufficient without the overnight fridge loading.

I believe I will achieve a far better balance of charge/store/usage with this system at lower cost in the long term.

[Richard Kollmann]

Originally Posted by a64pilot

I want to try to make something clear.
By stating that I can skew a test in the direction I want it to go, that was not meant by any means to mean Pete, it was an observation that a truly non skewed test is difficult, cause if nothing else we are human and proud of our accomplishments.
I am not trying to show Pete or his test in any kind of disdain, in fact It is an honest test, it’s not often that you find a test that tests apples to apples, that is difficult, but Pete’s test comes close to that.
Also the excessive cycling comment was not meant to be Pete’s test. But another manufacturer publishes test data that shows excessive cycling of a thin plate evaporator, I meant to show that excessive cycling can be controlled if it’s an issue, it’s not necessarily an operating principle of a thin plate evaporator.

Just from my admittedly very limited perspective, from my observation the Danfoss AEO module is a game changer as it has the ability to adapt the compressor speed to seek optimum cycle times, if they are short it will reduce compressor speed, and I believe that just as a general statement, within limits of course, the slower the compressor runs, the more efficient it is.
It also seems to be a whole lot less noisy to my HF radio, so much that I don’t turn the fridge off anymore.

Eutectic energy storing evaporator tanks offer one advantage and that is to store surplus energy if it is available. Unless a boat has a propane or diesel flame heat fired refrigeration the source of energy for refrigerator is produced by a hydrocarbons fuel turning engine’s electrical alternator or a large Btu engine driven refrigerant compressor both can normally produce surplus energy. Engine alternator wattage output is far greater in most cases than a small refrigerant compressor can convert input wattage energy into Btu energy to be stored in a eutectic ice plate. The belief that energy supplied by alternator can be creatively increased within the refrigeration system by any of the system components is false. It is true a compressor can be more efficient in converting the conversion of electrical energy to Btu energy by operating Danfoss BD compressor at a slower speed. Danfoss and others developed the variable speed compressor to assist manufacturers of refrigeration units to better marry compressors with system components for standard day temperature applications. Danfoss went one step further by developing an Adaptive Energy Optimization (AEO) module that is ideal for boat refrigeration compressor performance in varying ambient climate temperatures.

Because of the demand for 12 volt electrical power on most sailboats with or without refrigeration the only efficient place to store available energy when engine is running is in batteries.

Where does surplus energy come from to freeze a eutectic holding plate definitely not from a tiny 60 to 80 watts per hour 12/24 volt compressor especially if compressor is running at its best coefficient of performance (COP). With the limited amount of Btu energy from these small compressors a day even with eutectic plates the larger percent of refrigeration energy comes from what is stored in house battery bank. A 70 to 100 amp-hr alternator is producing a lot of energy but only a small number of those amp-hrs are going to be consumed by refrigerator from the engine alternator per, engine running hour. This small compressor will make only two pounds of eutectic ice per hour. How many hours a day do you run the engine alternator? Twelve pounds of eutectic ice produced from six hours of this small compressor running will produce a nice four cu ft drink cooler in a 90 degree F ambient temperature.

On most sailboats 12 volt refrigeration systems demand a boost to the boats direct current power grid either by larger battery capacity or alternative energy from Wind, Solar, or Water generator. If a boat does not have a source of alternative electrical energy it is a mistake to waste available box space and dollars on single temperature zone eutectic plates.

[RaymondR]

Originally Posted by Richard Kollmann

Eutectic energy storing evaporator tanks offer one advantage and that is to store surplus energy if it is available. Unless a boat has a propane or diesel flame heat fired refrigeration the source of energy for refrigerator is produced by a hydrocarbons fuel turning engine’s electrical alternator or a large Btu engine driven refrigerant compressor both can normally produce surplus energy. Engine alternator wattage output is far greater in most cases than a small refrigerant compressor can convert input wattage energy into Btu energy to be stored in a eutectic ice plate. The belief that energy supplied by alternator can be creatively increased within the refrigeration system by any of the system components is false. It is true a compressor can be more efficient in converting the conversion of electrical energy to Btu energy by operating Danfoss BD compressor at a slower speed. Danfoss and others developed the variable speed compressor to assist manufacturers of refrigeration units to better marry compressors with system components for standard day temperature applications. Danfoss went one step further by developing an Adaptive Energy Optimization (AEO) module that is ideal for boat refrigeration compressor performance in varying ambient climate temperatures.

Because of the demand for 12 volt electrical power on most sailboats with or without refrigeration the only efficient place to store available energy when engine is running is in batteries.

Where does surplus energy come from to freeze a eutectic holding plate definitely not from a tiny 60 to 80 watts per hour 12/24 volt compressor especially if compressor is running at its best coefficient of performance (COP). With the limited amount of Btu energy from these small compressors a day even with eutectic plates the larger percent of refrigeration energy comes from what is stored in house battery bank. A 70 to 100 amp-hr alternator is producing a lot of energy but only a small number of those amp-hrs are going to be consumed by refrigerator from the engine alternator per, engine running hour. This small compressor will make only two pounds of eutectic ice per hour. How many hours a day do you run the engine alternator? Twelve pounds of eutectic ice produced from six hours of this small compressor running will produce a nice four cu ft drink cooler in a 90 degree F ambient temperature.

On most sailboats 12 volt refrigeration systems demand a boost to the boats direct current power grid either by larger battery capacity or alternative energy from Wind, Solar, or Water generator. If a boat does not have a source of alternative electrical energy it is a mistake to waste available box space and dollars on single temperature zone eutectic plates.

I tend to the opinion that with the complexity of interacting circumstances present in modeling a boat refrigeration system the only practical comparison regarding the overall efficiency of eutectic versus continuous cycling systems is compressor run times.

On one hand we have a cycling system which switches on and off to average say 20 minutes per hour (being mindful of the inefficiencies propagated whilst the system establishes a stable condensation/evaporation regime) versus one which runs continuously during that same hour.

If we say that 15 minutes of the continuous run time is spent counteracting heat incursion into the refrigerator (not an error, we don't have the stabilization times to consider) then 45 minutes of the hour with the eutectic system is actually spent removing heat from the eutectic fluid.

Then, if the heat incursion rate is constant we have a stored "cold" time period allowance of three times the system running time of the continuously cycled system the eutectic will carry through the time period when there is no solar power available.

However, I also tend to the opinion that we need bigger capacity compressors to more effectively use the power from or solar panels on eutectic systems.

In any circumstances, the eutectic is going to prove far more economically and materials usage efficient since eutectic fluid does not "wear out" no matter how many times it is cycled.

[Richard Kollmann]

MY comments to RaymondR opinions

I tend to the opinion that with the complexity of interacting circumstances present in modeling a boat refrigeration system the only practical comparison regarding the overall efficiency of eutectic versus continuous cycling systems is compressor run times.

Answer: True, Rapid compressor and evaporator cycling is a thing of the past now that equilibriums between compressor speed and system’s components will improve total system performance. Optimization with the AEO electric control module or manually set speed runs compressor longer improving compressor’s volumetric efficiency and SCOP.

On one hand we have a cycling system which switches on and off to average say 20 minutes per hour (being mindful of the inefficiencies propagated whilst the system establishes a stable condensation/evaporation regime) versus one which runs continuously during that same hour.

Answer: True, Boaters have reported using timers or manually switching compressor OFF and ON with eutectic evaporators whe solar, wind or water generator power is available. Again the main problem is unless eutectic plate is large enough to except the heat absorbing energy when alternative energy is not available a negative ice condition in plate will occur.

If we say that 15 minutes of the continuous run time is spent counteracting heat incursion into the refrigerator (not an error, we don't have the stabilization times to consider) then 45 minutes of the hour with the eutectic system is actually spent removing heat from the eutectic fluid.

Answer: The mechanical engineering papers can provide box infiltration figures based on insulation, box dimensions and ambient temperatures but not the variables involved in a cruising boat. I have used in my concept designs 600 Btu for each cubic ft of refrigerator and 1200 Btu for each cu ft of freezer. Why are my box heat loads so high is because they are based on worst case conditions 90 degrees ambient air and 90 degree F seawater temperature. If you want to adjust my figures to your planed cruising areas subtract 2 percent per for each degree of cooler temperature. Another reason my heat loads are higher than the salesman’s I add affect of box throughput as ambient temperatures increase.

Then, if the heat incursion rate is constant we have a stored "cold" time period allowance of three times the system running time of the continuously cycled system the eutectic will carry through the time period when there is no solar power available

Answer: Constant cycling is just an old tired tail from the past when trying to justify and sell eutectic energy storage plates.

However, I also tend to the opinion that we need bigger capacity compressors to more effectively use the power from or solar panels on eutectic systems.

Answer” True, Twelve volt 800 Btu compressors have trouble producing 300 Btu per hour to freeze eutectic solution with a freeze point of zero degrees F. One half horse power 110 volt AC condensing units can deliver 3400 Btu and Engine driven compressors 6,000 Btu to 12.000 btu

In any circumstances, the eutectic is going to prove far more economically and materials usage efficient since eutectic fluid does not "wear out" no matter how many times it is cycled.

Answer” This statement is far from true “Eutectic plates are far more economically” is False.. It is true that aluminum evaporator plates are not as durable as a welded 16 gauge stainless eutectic plate. It is true that True eutectic plates last forever but no one is selling eutectic plates for pleasure boat refrigeration with True eutectic plates they are now all Glycol and water plates. It is a known fact that glycol eutectic chemistry will separate over time from water and Glycol eutectic has a fast sliding freeze point raising plate temperature. The trend now is to use eutectic plates as cycling compressor plates instead of holdover holding plates to compensate for the fact they can not equal the temperature control of conventional evaporators.

[hellosailor]

"around 17 hr per day where the fridge is warming up with no ability to take heat out of the compartment...this is why you need to include a battery."
Not really. Batteries store energy as electrochemical storage. Eutectic plates are just a cheat for thermal mass, if you took a 500-pound block of igneous rock (granite? basalt?) and chilled it down to ten below zero, you'd still have a damn cold rock in the morning. Or, you could take the solar panel output and--as is being done experimentally in many places by many companies--you can store that energy by spinning up a dense flywheel, typically sealed in a vacuum with magnetic "zero friction" bearings.
You can store energy many ways, and with eutectic plates you're still going to have the problem that if tomorrow is a gray day, those plates now need to store enough energy to carry the cooler through perhaps 36 hours without solar power. With batteries you've got lots of slack in your numbers and easy reconfiguration. With thermal plates and the need to fix the design parameters pretty much one and forever before you start...I wouldn't think the eutectic approach would be much better than that big block of icy stone.
Bottom line is that BTUs (or the equivalent, any unit of thermal energy) are what have to be considered, and all the other numbers, including watts, can be converted to them.
My grandparents grew up before refrigeration was the norm. The ice man brought around block ice, and if you couldn't afford it or didn't want to...it was only four generations ago that even landlubbers didn't have refrigeration or ice boxes. There are still plenty of cookbooks that go into how to deal with that, and keep your pantry well stocked.

[Q Xopa]

Originally Posted by hellosailor

"around 17 hr per day where the fridge is warming up with no ability to take heat out of the compartment...this is why you need to include a battery."
Not really. Batteries store energy as electrochemical storage. Eutectic plates are just a cheat for thermal mass, if you took a 500-pound block of igneous rock (granite? basalt?) and chilled it down to ten below zero, you'd still have a damn cold rock in the morning. Or, you could take the solar panel output and--as is being done experimentally in many places by many companies--you can store that energy by spinning up a dense flywheel, typically sealed in a vacuum with magnetic "zero friction" bearings.
You can store energy many ways, and with eutectic plates you're still going to have the problem that if tomorrow is a gray day, those plates now need to store enough energy to carry the cooler through perhaps 36 hours without solar power. With batteries you've got lots of slack in your numbers and easy reconfiguration. With thermal plates and the need to fix the design parameters pretty much one and forever before you start...I wouldn't think the eutectic approach would be much better than that big block of icy stone.
Bottom line is that BTUs (or the equivalent, any unit of thermal energy) are what have to be considered, and all the other numbers, including watts, can be converted to them.
My grandparents grew up before refrigeration was the norm. The ice man brought around block ice, and if you couldn't afford it or didn't want to...it was only four generations ago that even landlubbers didn't have refrigeration or ice boxes. There are still plenty of cookbooks that go into how to deal with that, and keep your pantry well stocked.

Re block of cold stone vs Eutectic plate. Eutectic plate works the same as the old block of ice from the ice man. It is a phase change, ie from solid to liquid, Latent heat.

So you can get more cooling effect from your Eutectic plate or block of ice than you can from a similar size block of cold stone with no phase change.