Another look at Solar Powered refrigeration on sailboats

[Boatwright]

nilespf:

The compressor draws approx. 7 amps. So at maximum, about 4 hours/day. When the boat is on the mooring, with a cold box, and no opening of the lid it runs as little as 2 hours. 200 watts of solar does the job with no worries. I logged the compressor hour meter the first year for a very good May to Oct. warm months daily average.

We usually launch in May with fully charged batteries, hook up the solar, load the box with sodas, beer, and bag ice for drinks and forget about it. Once the sweet spot was found, I haven't touched the thermostat in years. When we cruise (5 weeks last summer), we start out with the freezer compartment filled with hard-frozen meat and the rest of the box filled with drinks, bag ice, and groceries.

If you are running 24 hours/day I wonder if you have a possible problem with heat loss. Our original box would use 20# of bag ice in a day or so. It was insulated with 2" of broken down urethane foam. We gutted that box and put in 5" of blue foam lined with FRP shower panels from our local Home Depot, rebuilt the lid with a tight gasket, and installed the Isotherm unit ourselves.

Possible problem #2: You have slow Freon leak. With low Freon your compressor can run all day and not move much heat.

Possible problem #3: Your condenser is clogged with dust or has an airflow problem. Ours is ducted to draw cool air from the bilge. Isotherm also has a one-hole water cooled heat exchanger / condenser setup that is said to improve performance in hot climates, although I don't think you would need it in Wlimington.

Hope this helps.

[Richard Kollmann]

Missourisailor, your reported Cool Blue refrigeration with eutectic plate evaporator performance is typical but what was missing were the ambient air and water temperatures.
Small 12/24 volt compressors much smaller then 1/6 horse power lack the ability to remove heat from a warm box and at the same time freeze eutectic ice plates. Blue Water cruising day after day with alternative electrical surplus energy is when you can benefit from energy storing plates. The BD 35 compressor and eutectic plate cooling a six cubic foot box is pushing the envelope too far in a warm climate.

Where boat owner Boatwright is cruising at Latitude 44.76 N his 30 to 40 amp-hrs per day for a 6 cubic ft box is manageable. Boatwright’s power consumption below Latitude 26N could be closer to100 amp-hrs per day.

It is important to remember that there will never two boat refrigerators that perform the same. These ice box conversion installations are all different they will not run the same hours a day or cycle the same.

[missourisailor]

Originally Posted by Richard Kollmann

Missourisailor, your reported Cool Blue refrigeration with eutectic plate evaporator performance is typical but what was missing were the ambient air and water temperatures.
Small 12/24 volt compressors much smaller then 1/6 horse power lack the ability to remove heat from a warm box and at the same time freeze eutectic ice plates. Blue Water cruising day after day with alternative electrical surplus energy is when you can benefit from energy storing plates. The BD 35 compressor and eutectic plate cooling a six cubic foot box is pushing the envelope too far in a warm climate..

Totally agree!
And BTW, water temp is 70~80 F. Air temp 80~90 F.
But remember, Cool Blue system's are air cooled. So water temp is really irrelevant.



If I was to replace the evap/cond unit, I would get one with the largest condenser and no less than a BD50 compressor.
A64's system is ideal, IMO, with the BD90 compressor. As long a a variable speed controller is installed.


Honestly, I think the main advantage of a holding plate system is the TXV.



IMO, If two identical compressor systems were put in identical boxes, one having a holding plate with a TXV and the other a thin plate evap with a TXV, I would be willing to bet that there would be virtually no difference in both performance nor efficiency.

[Richard Kollmann]

Originally Posted by missourisailor

Totally agree!
And BTW, water temp is 70~80 F. Air temp 80~90 F.
But remember, Cool Blue system's are air cooled. So water temp is really irrelevant.



If I was to replace the evap/cond unit, I would get one with the largest condenser and no less than a BD50 compressor.
A64's system is ideal, IMO, with the BD90 compressor. As long a a variable speed controller is installed.


Honestly, I think the main advantage of a holding plate system is the TXV.



IMO, If two identical compressor systems were put in identical boxes, one having a holding plate with a TXV and the other a thin plate evap with a TXV, I would be willing to bet that there would be virtually no difference in both performance nor efficiency.

There are two differences in performance between a TXV and Cap tube refrigerant flow control. The TXV and its liquid storage tank has a wider Btu control range and refrigerant flow volume/pressure is not effected near as much by ambient climate conditions.

When a cap tube is used on a low temperature eutectic plates Latent heat phase transfer is at a fixed slow rate. And with a cap tube evaporator super heat changes with ambient temperature do to fixed amount of refrigerant available.

As you can see I do agree with your IMO on eutectic holding plates. Boats that travel south every winter may be better off with TXV control on standard evaporators like John's Cold Eh Marine system.

[OzePete]

Originally Posted by missourisailor

Totally agree!
And BTW, water temp is 70~80 F. Air temp 80~90 F.
But remember, Cool Blue system's are air cooled. So water temp is really irrelevant.



If I was to replace the evap/cond unit, I would get one with the largest condenser and no less than a BD50 compressor.
A64's system is ideal, IMO, with the BD90 compressor. As long a a variable speed controller is installed.


Honestly, I think the main advantage of a holding plate system is the TXV.



IMO, If two identical compressor systems were put in identical boxes, one having a holding plate with a TXV and the other a thin plate evap with a TXV, I would be willing to bet that there would be virtually no difference in both performance nor efficiency.

You would loose your money!
Cheers OzePete

[missourisailor]

Originally Posted by OzePete

You would loose your money!
Cheers OzePete

OzePete,
With nothing but the upmost respect for you and your product, I believe your test was flawed.


Why, because you had both evap's in the same box, at the same time.


When the Holding plate system was running, the thin plate system posed very little thermal mass for the holding plate system. Thus for all practical purposes, the box was empty.


However, when the thin plate system was running, the holding plate system was still there, thus causing a very large thermal mass. Thus the box was not empty.
Hence causing the thin plate system to not only have to cool the box but also cool the holding plate. Causing it to seem to be less efficient.


This is based on the tests you posted here a year or so ago.
If you have done further testing since then, I am not aware of it.

[OzePete]

Originally Posted by Richard Kollmann

There are two differences in performance between a TXV and Cap tube refrigerant flow control. The TXV and its liquid storage tank has a wider Btu control range and refrigerant flow volume/pressure is not effected near as much by ambient climate conditions.

When a cap tube is used on a low temperature eutectic plates Latent heat phase transfer is at a fixed slow rate. And with a cap tube evaporator super heat changes with ambient temperature do to fixed amount of refrigerant available.

As you can see I do agree with your IMO on eutectic holding plates. Boats that travel south every winter may be better off with TXV control on standard evaporators like John's Cold Eh Marine system.

I totally disagree with the above. The TX valve quoted is most likely a Danfoss T2 or TE2 using a OX orifice as these are the smallest they have available. They are Danfoss rated with a nominal 1/3 ton at MBP and 1/8 ton at LBP, far far too big for the BD35 specialy at low RPM used on a cold plate. It is akin to fitting a four barrel Holley carburetor to a motor scooter!!

This is why flood back (super-heat Hunting) was reported at some earlier post and described as a compressor cooling feature!!! lol

TX valves, especially oversized ones, have no place on theses micro or similar systems, as 99.?% (est) of the worlds makers seem to understand.

Cheers OzePete

[OzePete]

Originally Posted by missourisailor

OzePete,
With nothing but the upmost respect for you and your product, I believe your test was flawed.


Why, because you had both evap's in the same box, at the same time.


When the Holding plate system was running, the thin plate system posed very little thermal mass for the holding plate system. Thus for all practical purposes, the box was empty.


However, when the thin plate system was running, the holding plate system was still there, thus causing a very large thermal mass. Thus the box was not empty.
Hence causing the thin plate system to not only have to cool the box but also cool the holding plate. Causing it to seem to be less efficient.


This is based on the tests you posted here a year or so ago.
If you have done further testing since then, I am not aware of it.

Hi Missourisailor, appreciate your comment and can follow your thought processes here but you have it the wrong way around!! The thin plate has the BENEFIT of a small amount of Specific heat (Not Latent heat) holdover from the eutectic plate, while the eutectic plate has virtually no assist from the thin plate.

May I suggest you read the article again and focus on understanding COP and why each type produces different values. The second issue the item raises is inefficient start ups, much easier to understand.

Would be pleased to discuss anything that you may need help understanding

Cheers OzePete

[Richard Kollmann]

I totally disagree with the above. The TX valve quoted is most likely a Danfoss T2 or TE2 using a OX orifice as these are the smallest they have available. They are Danfoss rated with a nominal 1/3 ton at MBP and 1/8 ton at LBP, far far too big for the BD35 specialy at low RPM used on a cold plate. It is akin to fitting a four barrel Holley carburetor to a motor scooter!!

This is why flood back (super-heat Hunting) was reported at some earlier post and described as a compressor cooling feature!!! lol

TX valves, especially oversized ones, have no place on theses micro or similar systems, as 99.?% (est) of the worlds makers seem to understand.

Cheers OzePete

Interesting opinion. But I have even run three TXV T2 Danfoss valves supplying refrigerant to three holding plates at the same time with one single BD2.5 compressor in the hybrid system in my boat.

[missourisailor]

Originally Posted by OzePete

Hi Missourisailor, appreciate your comment and can follow your thought processes here but you have it the wrong way around!! The thin plate has the BENEFIT of a small amount of Specific heat (Not Latent heat) holdover from the eutectic plate, while the eutectic plate has virtually no assist from the thin plate.

May I suggest you read the article again and focus on understanding COP and why each type produces different values. The second issue the item raises is inefficient start ups, much easier to understand.

Would be pleased to discuss anything that you may need help understanding

Cheers OzePete

OzePete
Please clarify something for me.


When you did the test using the thin plate system, was the test started with a cold (frozen) hold over plate?

[a64pilot]

Originally Posted by missourisailor

IMO, If two identical compressor systems were put in identical boxes, one having a holding plate with a TXV and the other a thin plate evap with a TXV, I would be willing to bet that there would be virtually no difference in both performance nor efficiency.

I think your right as long as, and this I believe is key that the thin plate evaporator was sized so that it didn’t have to run at a colder temp than the cold plate does.
If it has less surface area, then it will have to be run colder to absorb the same amount of heat that the larger surface area cold plate does.

By running it colder it runs the compressor at a lower COP and of course will use more energy.

Unless a phase change occurs, a cold plate is in effect the same as an all metal evaporator.
Or said another way, if the fluid in the cold plate is always frozen, then it is just an evaporator, it’s not functioning as an eutectic system.

[a64pilot]

Originally Posted by missourisailor

OzePete
Please clarify something for me.


When you did the test using the thin plate system, was the test started with a cold (frozen) hold over plate?

Pete, I hope you don’t mind me jumping in.

But I would hope the test was run with everything already at a stasis point, meaning already cooled down and stable. Otherwise there would be so many possible variables, I doubt you could make sense of the data.
Obviously a cold plate holds a tremendous amount of energy and therefore uses a tremendous amount to energy to freeze, if it didn’t, then it would be pointless, just something taking up way too much room.
It’s best use is as a flywheel if you will. A device that allows / requires long cycle times to freeze a plate down, then a long period of off time while the liquid absorbs a huge amount of energy to thaw.
It would likely take some work calculating the correct size of plate and how much bigger a compressor you would need, but I’m convinced it’s possible for a true eutectic system to only run the compressor during the day when there is plenty of Solar (on average) and not use any battery power at night (on average)
My belief is that is the true superiority of a cold plate system, the ability to absorb excess power when it’s available and release it when it’s not.
However that pretty much requires a system that actually phase changes to be most effective

It’s a little like the real world test another poster suggested, unless there is an already existing accepted standard test, then it’s likely that any attempt at a real world test would be almost meaningless.
Cause how I use my system varies widely from one day to the next, to say nothing of how I use mine compared to how you use yours.

Oh, and water temp matters even on an air cooled system, cause you can bet your hull is most likely the same temp as the water, not the air temp.

[missourisailor]

A64,
The reason I asked OzePete the question I did is because....

If the cold plate was already at a “frozen” state, and the thin plate still consumed more energy, then it’s a no brainer which system is more efficient.

However, if the cold plate was at room temp, then the thin plate would not only have to cool the air but also the eutectic in the holding plate. Which common sense tells you, it has to work harder.

If my failing memory is correct, this was brought up on the other thread but never resolved.

Also, and I may be remembering this wrong, I think that OzePete’s systems was tested at fridge temps not freezer temps. Which if is the case, there might be phase change (latent heat) happening which skews the results towards the holding plate system.

[OzePete]

Originally Posted by missourisailor

OzePete
Please clarify something for me.


When you did the test using the thin plate system, was the test started with a cold (frozen) hold over plate?

YES! The test was done with each system involved in maintaining cabinet temperature in sequence. Run periods of 10 days totalling two months in all.
As can be seen the cyclic system benefited greatly by 'inheriting thermal mass from the eutectic system. (See how much less the cyclic used on the first run day of each of its periods thanks to the eutectic system.) This was a great advantage to the thin plate system.
Click on chart to expand....

consumption test.png Views: 113 Size: 196.6 KB ID: 185155" style="margin: 2px" />



Please Note:
1> That testing was originally only for our internal research as we manufacture / market BOTH types. Buy whatever system.. your choice!
2> The results surprised us as said at the time, so we decided to make them available here.
3> The thin plate used had for greater surface are than the eutectic plate.
4> Many fail to consider that a eutectic plate is collecting heat 100% of the time regardless of whether the unit is running or not. The thin plate only collects heat while the refrigeration unit is running therefore has to then run considerably colder.
5> It is ONLY a eutectic system if it is engineered / controlled to run until the liquid eutectic mass is frozen (phase changed to a solid) Then remains off collecting heat until the eutectic mass has thawed.

Cheers OzePete Ozefridge | 12 Volt Refrigeration Systems

[RaymondR]

Ozepete,

Having wandered through the long trail of this and the previous thread I had forgotten just how stark the differences were between the constant cycling and eutectic systems were as illustrated by the data from your tests - the COP of the eutectic is twice that of the cyclic in a sufficiently long series of tests as to ensure those differences were not the result of extraneous circumstances.

However a number of issues have been raised regarding the validity of the results, the inadequate sizing of the evaporator plate used on the continuous cycling system possibly being the most significant.

You state in your last posting that Ozefridge produces both eutectic and continuous cycling systems, was the evaporator plate one as is used in the continuous cycling systems your company manufactures and markets.