Another look at Solar Powered refrigeration on sailboats

[hellosailor]

No, you miss the point. Whether you store thermal energy in the thermal mass of a huge lock of stone (or meter-thick masonry walls in a home) or you store thermal energy by tying it up in the phase change of a eutectic plate, you are still only storing thermal energy. One may be more efficient than the other, but as Chairman Mao once said:
"Black cat, white cat, all same. Catch mice."
Most of use would say a black cat is not the same as a white cat. In some cases, they are the same.

You're storing thermal energy, the rest is just details.
As opposed to the flywheel (mechanical/kinetic energy) or the battery (electrochemical energy).

If anyone ever figures out how to efficiently convince water molecules to come apart, rather than using brute force on them, you could just as easily use excess solar power to split the water, then recombine the hydrogen and oxygen to generate more power on the next day. Totally unfeasible now--but only because the physics of "gently convincing" is still beyond us.

[RaymondR]

No matter how big you make the evaporator plates, all other things being equal, the plate evaporators refrigerator is going to cycle more often than the eutectic and consequently have higher power losses.

Varying the speed of the compressor will allow a reduction in the start/stop/stabilization losses because it will match refrigeration effect to heat flow into the box, however that's not going to make best use of the available power from the panels.

Non degrading, infinite lifetime eutectic fluid which stores cold is always going to be more economical than chemical process based battery storage (that might change if someone gets around to inventing supercaps)

The primary value of eutectic has always been it's ability to time shift the active refrigerating part of the cycle. With the old engine driven compressors it reduced it to a relatively short period every one or two days (if you were lucky whilst you were motoring somewhere) With 12 V compressors the shift is into the daylight hours when solar power is readily available.

[Richard Kollmann]

Q Xopa, Has the correct understanding of storing ice whether from water or eutectic solution. During the temperature reduction of a substance before liquid is frozen is defined as Sensible heat change of 1 Btu per pound per degree. When the liquid reaches its freeze point Water 32 dgrees F or Predetermined mixture of Eutectic liquid ice starts forming at rate equal to compressor/condensing units output.

Very little energy is stored in the sensible reduction of heat until the phase change of liquid to ice where 144 btu of energy as latent heat per pound of ice is formed. If the freeze point of eutectic solution is set to zero degrees F the compressor’s Btu output will be cut in half maybe doubling compressor running time for latent heat phase change. Example a 2.5 cu ft refrigerated area with one gallon eutectic zero degree F plate and BD50 compressor will need to run 4.7 hours just to freeze a zero degree F eutectic ice.

Instead of a zero degree plate this is what happens with freezing water instead of zero degree eutectic solution. Whether water or eutectic is used in a holding plate the Latent and Sensible energy stores in making or melting latent ice is almost the same. The addition of Glycols or Brines to water do little to change actual energy stored in phase change to ice. The latent energy stored in 32 degree F of frozen water ice might last 24 hours while the zero degree eutectic ice melts away two or three times faster.

There is nothing magical in eutectic plate evaporators except they store energy when there is surplus energy and the belief they create energy over a standard evaporator is false.There are many good reasons to use eutectic plate evaporators but because they are more energy efficient is questionable without alternative electrical. Boats connected to shore power and take day are week end trips are good can dates for holding plates

RaymondR, The size of any refrigerant evaporator with the exception of energy storing holding plates is determined by the installation application engineer or might even my be the boat owner. You do not want the evaporator too big or too small. In selecting evaporator it should have the Btu capacity to handle the maximum capacity of compressors. Yes compressor cycling prior to the variable speed compressor where one size fits all meant energy consumption was a problem. If your Danfoss variable speed compressor is cycling off and on more than twice per hour the equipment components are not correctly balanced together. It is true these small compressors will require a 10 minute pump down on the first start of the day. Once box and evaporator is stabilized the pump down is only a few seconds with the correct evaporator. If your refrigerator has a eutectic plate the pump down may be extensive.

For a eutectic plate to match performance of correct Btu evaporators they need latent mass energy storage. Today’s so called hold over plates do not use true efficient mass control eutectic solutions. They all seem to use glycol and water where only fifty percent of the ice mass melts at preset temperature point requiring over sized plates. In other a four cubic ft freezer must have two, two gallon eutectic plates.

[a64pilot]

I know of only one Eutectic system that actually allows the fluid to phase change, thereby actually using the fluids huge heat holding capacity.

In truth I’m at a little bit of a loss trying to understand the function of a cold plate that never goes through a phase change, yes it works, but how, why? Without the phase change, it may as well be that rock that was discussed earlier?

[Richard Kollmann]

Originally Posted by a64pilot

I know of only one Eutectic system that actually allows the fluid to phase change, thereby actually using the fluids huge heat holding capacity.

In truth I’m at a little bit of a loss trying to understand the function of a cold plate that never goes through a phase change, yes it works, but how, why? Without the phase change, it may as well be that rock that was discussed earlier?

Do you believe that 25 pounds of zero degree F eutectic solution ice stores more energy than 25 pounds of 32 degree F plain water ice?

Do you believe a frozen eutectic solution plate to be as energy efficient as the correct sized aluminum thin plate evaporator?

[RaymondR]

We can theorize and infer until the cows come home but in the real world what was established by Ozepete's companies tests was that the eutectic system cycled approximately once per day whereas the plate evaporator cycled multiple time for an energy use of approximately twice that of the eutectic system.

Whilst it might be claimed that part of the huge energy use difference was because the plate evaporator was undersized any increase in it's size could be readily countered by increasing the size of the eutectic tank thereby increasing the amount of stored energy.

The only valid way to negate the results of the Ozepete tests is to redo the tests by setting up another test bed and re-run the tests which I suspect would be futile because the results would be the same.

Using a variable speed compressor might allow the effects of the stop/start losses to be negated but my experience with DC motors is that the power curve is parabaloid in shape with the most efficient motor running speed about half the free running speed. Consequently if the compressor is designed with the motor to run at it's most efficient speed at the rated capacity speed of the compressor slowing it down is going to put the amps draw through the roof and it may prove that intermittent cycling is far superior to variable speed operation from an energy usage viewpoint.

[a64pilot]

Originally Posted by Richard Kollmann

Do you believe that 25 pounds of zero degree F eutectic solution ice stores more energy than 25 pounds of 32 degree F plain water ice?



Do you believe a frozen eutectic solution plate to be as energy efficient as the correct sized aluminum thin plate evaporator?

Just going off memory, plain ole water as energy storage is tough to beat, but I may be thinking heat required to change to steam.
But you through in the 25 degree difference in temp so I truly don’t know.

I believe that a properly designed thin plate evaporator system can be as energy efficient as a eutectic system.
One catch is who manufacturers an eutectic system? I have cold plates but do not think it’s actually a eutectic system as it’s anti freeze and water mix. A proper eutectic goes through phase change all at once where antifreeze and water does not?
Then how many actually phase change?
Pete’s is the only one I know of that does, but of course there are many, many that I have not seen.

My belief is that proper design trumps parts used, by that I mean that just because you have a cold plate does not mean your more efficient.
Then of course you need to have a bigger box to have the same usable storage space with cold plates, so don’t forget that this bigger box will lose more heat, just due to its size, assuming insulation is identical.
Then in truth my brain wants to think that storing heat in eutectic solution to then transfer to the box is actually less efficient than going directly to the box as you have a medium in between.

Now, as I have said, I have a dual cold plate system myself, and I’m very happy with it, but my box is 14 cuft, so I don’t mind losing some space, I almost have too much space, as I have almost the volume of two 55 gl drums in my box. I think my box is 105 gls. or about 400 liters for you metric types.
If I had a much smaller box, then I wouldn’t want to lose any space to a cold plate.

My only point is that a cold plate system is not necessarily better, just cause it has a cold plate, there is a whole lot more to it than that.


If a cold plate system were just by itself that much more efficient, you would see more of them I think, but other than boat refrigeration, how many do you see?

[Richard Kollmann]

I ask two direct questions about melting energy of ice stored in a metal tank. These questions have no bearing on how ice is made or the area around tank where ambient heat is drawn from or where the energy to form ice comes from.

1. Do you believe that 25 pounds of zero degree F eutectic solution ice stores more energy than 25 pounds of 32 degree F plain water ice?

2. Do you believe a frozen eutectic solution plate to be as energy efficient as the correct sized aluminum thin plate evaporator?


I expected the answers to question number one to be one of the following:
a. Btu Energy stored is almost the same
b. Ice from water will store more Btu
c. Ice from eutectic solution will stores more Btu
d. I do not know for sure.

Question two is more complicated than the first question as it relates Sensible and Latent Btu heat differences between eutectic Glycol and waters stability in melting heat absorption and eutectic plate’s inside evaporator configuration.

A third question has come up relating to COP or volume-metric efficiency of hermetically sealed 12 volt variable compressors speed. Do you believe these compressors are more energy efficient operating at slower speeds as long as there is a balance of energy that satisfies the daily heat load objective.

[Q Xopa]

Originally Posted by a64pilot

I know of only one Eutectic system that actually allows the fluid to phase change, thereby actually using the fluids huge heat holding capacity.

In truth I’m at a little bit of a loss trying to understand the function of a cold plate that never goes through a phase change, yes it works, but how, why? Without the phase change, it may as well be that rock that was discussed earlier?

Yes true. You need a correct compostion of the eutectic solution, ie correct ratio of glycol or salt to water to get the phase change to match the temperature set point. This could be messed with to set this up correctly. Higher sslt or glycol ratio for lower fridge temperature set point.

As you say if not it is the same as the rock/ block of i e/ water jug etc. Still a flywheel, thermal inertia effect but to a lesser extent.

So there is no efficiency gain per se, its only a question of storing and matching power supply to load demands timings.

Its like a bank account having a cash buffer. Living hand to mouth or sock some away in times of plenty for leaner times. If you have reserves you can buy your power when its cheap.

[a64pilot]

Originally Posted by Richard Kollmann

A third question has come up relating to COP or volume-metric efficiency of hermetically sealed 12 volt variable compressors speed. Do you believe these compressors are more energy efficient operating at slower speeds as long as there is a balance of energy that satisfies the daily heat load objective.

Yes.
Then again if slowing it down optimizes cycle on / off times, then you get a doubling of efficiency increase.
I think the new AEO module is really something, I’m impressed with it.

[RaymondR]

Danfoss have had a motor control which can be run directly from solar panels by varying the compressor speed for the power available for about fifteen or more years now.

I vaguely recall reading a review on them when they first came out and think they were targeting a perceived market for drug refrigeration in third world clinics in remote places where battery systems were a problem. If my recollection is sound then the best way to use them in these systems would be with eutectic storage.

[Richard Kollmann]

RaymondR, I am not familiar with the companies manufacturing and selling eutectic plate systems in Australia for small twelve volt compressors. Here in the US most companies that did produce small ice box conversion compressor systems with eutectic plates stopped marketing them except for Technautics. How efficient a eutectic plate is depends on its design and how fast you try to freeze them. Eutectic plates unlike compressors are more then likely going to produce negative COP. Technautic’s Cool Blue machine with a eutectic plate and at minimum compressor speed has the lowest daily energy used, has set the design standard in small box conversions.

Solar power in conjunction with a small variable speed DC compressors are believed by Danfoss to be able up to an ambient climate temperature of +32 degree C to power refrigeration without batteries. This means cooling a 200 liter refrigerator or a 100 liter Freezer from solar alone is possible. There are a number of companies producing the small solar units generally in quantizes of 50 for distribution by other. I was hired 20 years ago as a consultant for an Ohio company to assist in the development and testing to gain certification for their first order. If you are interested in variable speed solar Danfoss has two solar papers with all information needed to get started. Also check out Global Who web site at www.who.org. Performance Specifications.

A64pilot, If refrigeration is in a 70 degree F controlled environment a standard control module with correct speed resistor is OK. If box is smaller than four cu ft standard control module with no speed resistor is usually the best. For boxes four cu ft and larger and changing ambient temperatures the AEO electronic module is a good investment when purchasing a new Danfoss designed compressor system. Manufacturers like Adler Barbour pre select compressor speed by compressor matching to evaporator capacity. Unfortunately all installations of systems are not the same so without the AEO module the boat operator may need to correct a condition of compressor not maintaining desired box temperature or compressor cycles too often.

[a64pilot]

It should be noted that at least my system had the resistor value (1700) to cause the compressor to run at its highest speed whenever it was on.
If I understand the thermal expansion valve would then modulate refrigerant flow as needed.
It’s a good, simple system and since the compressor is at full speed it can remove as much heat as is possible, but it’s always at high speed, if you control speed by a resistor, it’s a fixed speed, it can’t vary.
I believe it’s done so that a manufacturer can specifically tailor the compressor cooling to their specific requirements and can make one size compressor work in a more flexible way.

However as you stated, the Cool Blue system sets the gold standard for energy efficiency and did so decades ago, but with the compressor at low speed.
It’s very efficient run at low speed, but of course if I toss in a case of beer or a bunch of meat to freeze, it can’t increase cooling to return to set temp as quick as it can if it’s running at full speed.

As I understand it, this is where the AEO module is a game changer, after a period of time if higher speeds are not needed to keep cycle times from being excessively long, it will turn down the speed of the compressor, saving power.
But if I toss in warm stuff that needs a lot of heat removed, it will speed up the compressor and remove that heat, then turn the speed back down because the excess cooling isn’t needed.

Now it does this by monitoring cycle time, it doesn’t know you threw in that case of beer, so it’s not an instantaneous response, but it does adapt.
I have a pretty sensitive DC amp meter and watch it, after a day or so and everything reaches equilibrium, I swear my BD80 compressor and 14 cuft box doesn’t pull any more power than my Engel.
Which if you take its size into account, really isn’t all that efficient a freezer.
Neither compressor is running now, but it does seem to actually measure down to the .1 amp. Right now it’s showing my house load with no refrigeration, IPad / phone charger, NMEA 2000 network and sailing instruments and my Garmin inreach and a Bluetooth GPS.

[Richard Kollmann]

a62Pilot, I did not qualify my statement about the Cool Blue machine correctly. The Cool Blue with a BD35 compressor running at fixed speed 2,000 Rpm installed in four or less cubic ft box, set a, if you prefer, a low energy Gold standard for any other company to equal.

The only way to know for sure what rpm the DB80 compressor is running at is to have a mA meter in series with the thermostat wire, 2 mA would produce 4400 rpm.
When the BD80 was new there were concerns that it was short lived because it was ratted as a Low Back Pressure (LBP) only compressor limiting refrigerant flow to less than 800 Btu. Also BD80 compressor failures occurred on systems where oversized evaporator tubing coils were used.

There are two forces that control and limit refrigerant flow from a system with a TXV super heat inside evaporator and compressor Rpm. With the AEO module at a warm evaporator compressor start up the TXV orifice is wide open for maximum flow. At the first BD80 compressor start with a large warm eutectic plate the AEO starts compressor at 3250 rpm and then spends 48 minutes slowly ramping compressor speed up to max 4400 rpm or less rpm if thermostat/ plate temperature is satisfied and stops compressor in less than 48 minutes. From this point on compressor cycle times will determine the optimum performance of the system.

Having the 1700 ohm resister in thermostat wire may provide high refrigerant flow for several minutes but in theory an overall increase daily energy consumption cab be expected.

[a64pilot]

Well, I hope mine lives a long life.
I have always been astonished at how long sealed compressors last.

Our last house fridge was still going strong at 20 yrs. I only replaced it because the door was beginning to rust and you could see wear on the handles.
When our Son nicknamed it “Rusty” the Wife said it had to go, didn’t matter if we were selling the house and going cruising
If you assume a 50% duty cycle, then that compressor was approaching 90,000 hours of operation.
I mean really what mechanical device do you know of that will run for that long?