Another look at Solar Powered refrigeration on sailboats

[nilespf]

My experience,
I decided to design my own reefer control system replacing the existing fridge and freezer temp sensors/thermostats. I replaced each with RTD type sensors and used 3 temp controllers. One for the fridge, one for the freezer and a third for what I call a set back controller. I continued using the BD35 and evap coil.from Adler Barbour. The intent was to reduce reefer power consumption and control box temperatures (and of course design/play and keep my self entertained with yet again another project). The results have been mixed:
Temp sensor attachment/placement was critical. The best D.C. obtained is 48% Essentially, the system bleeds BTU's 1/2 as fast as the compressor can add them. Haven't been able to make any better.

Compressor speed is based upon load. Load is detected only by temp differential between detected and programed. The setback is a brute force turning off of the reefer during nite hours by raising the freezer temperature set point.
I have 500W of solar AND a battery bank. 500W was selected based upon an estimated best case solar day of illumination less what the boat uses in that same 24 hr. period. I also put in a factor for the shading effects and how the bypass diodes are configured in the Renogy panels. Series, parallel and series/parallel combinations along with effects of one or two solar MPP controllers to control the configurations were compiled.

I cannot seem to reliably put back what the boat removes in a 24hr period and I wouldn't have a chance of starting the BD35 w/o a battery bank on a less than optimum solar day. Sometimes the solar controllers will trip off line from the compressor surge (if batteries are disconnected)
Has anyone out there measured their system duty cycle getting much higher numbers?


pn

[sailcrazy]

following

[Richard Kollmann]

nilespf, Sometimes the solar controllers will trip off line from the compressor surge (if batteries are disconnected)
Has anyone out there measured their system duty cycle getting much higher numbers?

The standard control module for BD35s’ P terminal with no wire connected to it stops compressor when voltage drops below 10.5 volts. The Danfoss module for solar power use is 101N0400 with no P terminal can handle a voltage range from 10 to 45 vdc. Danfoss recommends panel wattage of 120 watts as long as compressor starting power consumption is under 120 watts. To me this means compressor should start and run at minimum speed of 2000 Rpm and draw less than 7.1 amps.

At 2000 rpm compressor will only draw an average of 3 amps (45 watts) with the correct size standard evaporator. If you want cooling at night Plastic ice packs are recommended. If you want redundancy power when solar is not enough on some days an OFF and ON power switch can be installed. If you want more refrigerator capacity when alternator is running compressor can be turned on and its speed can be increased use a $40 battery combiner voltage switch.

[nilespf]

Richard, my compressor after it reaches steady state and the box has reached set temperature of 20 F it will be pulling about 4A. I think to achieve any serious headway towards a reduction in daily Ah usage will require a change to my duty cycle. In my mind I cannot change D.C. without adding foam (assuming of course refrigerate levels and condensing circuit is OK) I have increased CFM across the condenser which made about 2% improvement to the D.C. If 50% is OK/typical/normal than short of rebuilding that corner of the galley I'm not going to be able to get anything else out of the system.


pn

[a64pilot]

The best way to reduce power consumption is through insulation.

I however went a different route, rather than chasing increased efficiency, because I believe it’s very quickly diminishing returns once you begin that chase, I went out instead to increase power production.
So I have a kilowatt or Solar, and if I could have fit more without excessive work or expense, I’d have more.

However I believe the AEO module has done more to decrease power consumption than anything else that I have done.

That and scrapping the spill over concept. It works, but doesn’t work well in my opinion, I was having to keep the freezer excessively cold, but then go a couple of days without adding anything to the fridge and things would freeze there. Fridge temp control was passive and not very stable at all, put in a case of beer and it would warm up way more than I was comfortable with for at least a day.

So I taped over the spillover holes and drilled another hole higher up in the divider and installed a small computer fan, I connected this fan to a cheap digital thermostat and set 34f as a turn off point and 37f as a turn on point for the fan.
My fridge now maintains a constant 36f and I can raise my freezer temp to 15f, where before I was having to keep it to low single digits.

[nilespf]

a64, after your system reaches the set point of the stats and enters sustainment what is the duty cycle of your sytem i.e the compressor not the spill fan.


pn

[RaymondR]

Originally Posted by Richard Kollmann

nilespf,


If you want cooling at night Plastic ice packs are recommended.

Which is effectively adding a eutectic cold storage component to your refrigeration enclosure??

[seandepagnier]

I'm constantly hearing about refrigeration on sailboats and its complications and power consumption. As someone who has never used refrigeration, I can't understand what use they have or what problem they would solve. If I had one I would remove it to simplify the boat.


I don't eat meat unless I catch a fish, and I sun dry that. I don't like cold drinks and I am perfectly happy to never drink beer. I know for a fact that vegetables can last longer than an ocean passage in the tropics, and they taste terrible once refrigerated.


So, could someone explain to me what I could gain by installing a refrigerator, what am I missing, and why these things are popular and what you put in them.

[Richard Kollmann]

Originally Posted by RaymondR

Which is effectively adding a eutectic cold storage component to your refrigeration enclosure??

I was suggesting to nilesph’s post ways to maybe supplement energy storage as his Adler Barbour evaporator that would not store enough energy power only available by day time solar panels. Very few ice box conversion refrigerators have or would boat owner want eutectic evaporator plates. In the Danfoss papers I got the solar power recommendations I also got the Plastic Ice Packs recommendation.. Many boaters even supplement their eutectic plates with frozen water ice in plastic bottles. I have a three door two drawer Samsung refrigerator with metal energy storing plates that act as heat sink flywheels when doors are opened and closed.

[Boatwright]

We installed an Isotherm unit 3 years ago and are very happy with the performance. After researching the Smart Energy control we decided the benefit would be minimal.

Use the basic thermostat. Choose the freezer compartment style evaporator. Install an inside the box fan - we used a quality ball bearing computer type 12v unit that runs when the compressor is on. This requires a simple relay at the compressor control terminals. Tweak the thermostat setting to keep the freezer frozen solid and the rest of the box at 35-40 deg F. Once you find the correct setting, leave it alone.

The internal fan is the key. By circulating the air inside a well insulated box (5" of closed cell foam with a tight gasketed top loading lid) you will achieve the "storing the cold in the contents" effect without the expense and complexity of the Smart Control.

We also installed a simple 12v hour meter to measure performance. 20-30 AH / day to cool a 6 cu ft box. Power supplied by a 200 watt solar setup that yields 60 - 80 AH/day (80AH x 12v = 960WH). We have all LED lighting, very simple electronics, and a 400 AH battery bank. We RARELY have to run the engine for charging - only after a 3 or 4 day period of cloudy weather. Our boat is on a mooring when not in use - so no help from shore power. We use the batteries very conservatively - never below 75%. This is in the northern Great Lakes area. Summer highs in the 80 -90 deg range.

Pay attention to the posts about how to calculate power requirements (volts, amps, watts, watt hours, etc.)

[a64pilot]

Originally Posted by nilespf

a64, after your system reaches the set point of the stats and enters sustainment what is the duty cycle of your sytem i.e the compressor not the spill fan.


pn

I don’t really know, and it’s my opinion that duty cycle isn’t as relevant with an AEO module, reason is the AEO will control the compressors speed to reach the desired cycle time and therefore its duty cycle. if it can do so of course. You have to have a system capable of achieving set temps, an AEO module may not necessarily increase cooling, it may only if the resistor that was chosen has the compressor running at less than full speed. I believe the controlling of compressor speed by resistor value is a feature so that a manufacturer can adjust if you will compressor size to fit their requirement, and it means that one compressor can be manufactured as opposed to having to manufacture three or more.
Sort of how like many outboards are the same motor actually, the New Suzuki for example, the 9.9, 15 and 20 are the same motor.
Without an AEO module your compressor is either on or off, with the AEO module it can run at different speeds and adapt to the required cooling demand.

It’s my understand that from an efficiency perspective a 100% or near 100% duty cycle is preferred, you size a compressor to be just barely big enough to do the job, that is most efficient.
However if you size for that, how are you going to cool down that watermelon or that case of beer you just tossed in? It will take days, and that isn’t desirable.

The heat pumps on my house handled that in an interesting way, they each had two compressors, a big one that could do a lot of cooling, and a small one that was very efficient due to its size, normally you only ran on the small one of course, the big one kicked in if there was need of excess cooling or heating.
Enabling the variable speed feature of the AB compressor with an AEO module I believe does the same thing, allows the compressor to operate at a slower, more efficient state when possible, and yet speed up when extra cooling is required.

[missourisailor]

Originally Posted by nilespf

a64, after your system reaches the set point of the stats and enters sustainment what is the duty cycle of your sytem i.e the compressor not the spill fan.


pn

I also have a Cool Blue Holding plate system using a BD35 compressor with just a resistor to maintain full speed.

The ice box is approx. 4 cubic feet with some added insulation to the inside. Total about 3" of blue foam. Fridge is cooled by spill over.

Once the system reaches equilibrium, with a set point of -10 degrees (holding plate temp, compartment temp about 0 degrees), I have a 50% duty cycle.
5 amp draw times 12 hours, calculates to about 60Ahr.

But this is only after the box has stabilized. Starting with a warm box and nothing but air in either compartment, it can take two days to reach a stabilized compartment temp of 0, with a near 100% duty cycle.
To load a warm box with warm food, I have seen it take up to a week to get to equilibrium.
Even loading with frozen food, it still several days.

This is the biggest complaint I have with my system, The long cool down time.
From what I have seen and heard from others, this is the norm..
Even heard the same complaint from folks with a thin plate evaporator.

As has been mentioned previously, most if not all efficiency tests are done with an empty box.
Which is not a real world test!

I would really like to see a “real world”, side by side test done with loaded boxes.

[RaymondR]

Originally Posted by Richard Kollmann

I was suggesting to nilesph’s post ways to maybe supplement energy storage as his Adler Barbour evaporator that would not store enough energy power only available by day time solar panels. Very few ice box conversion refrigerators have or would boat owner want eutectic evaporator plates. In the Danfoss papers I got the solar power recommendations I also got the Plastic Ice Packs recommendation.. Many boaters even supplement their eutectic plates with frozen water ice in plastic bottles. I have a three door two drawer Samsung refrigerator with metal energy storing plates that act as heat sink flywheels when doors are opened and closed.

I couldn't help myself Richard, it was too tempting a gotcha for me to let it go by.

In Europe and north America block of ice were cut from frozen rivers and stored in ice houses for use during the hot summer months.

When I was a small boy living in a country town most homes had an insulated box with a galvanized tray at the top into which the "ice man" placed a block of ice a couple of times a week. The ice was made in the town's ice works. The ice works had a couple of large reciprocating compressors, a big air or water cooled condenser and a lot of 1" steel pipe running backwards and forwards across the bottom of a big tank of brine. Galvanized steel cans were filled with fresh water and immersed in the brine tank for freezing.

Basically the town was operating a giant eutectic system as is anyone (at a smaller scale) who supplements the refrigeration capacity of a small DC system by placing cool bricks or ice in their boats refrigerator. You are just relocating a eutectic fluid which has had practically all the heat of fusion extracted from it by an outside refrigeration system ie. you are exploiting the eutectic phenomenon related to phase change as were the towns people.

When I bought my first two 80 Watt solar panels about 20 years ago they cost $750 each and were the most expensive components in my DC electrical system however two 120 Watt panels of the same physical size can now be purchased for a few hundred dollars. Panels are cheap.

I am looking at having to replace the bank of AGM sealed lead acid batteries which came with the boat and this will cost about $2,400. I have just purchased 100 amp hours of LiPo and BMS for about $2,000 and am looking at another 100 amp hours of a cheaper brand for $1,000. The point is whatever I do with batteries is going to be costly. Batteries are expensive as is their risk of failure.

You can also buy 12V DC refrigeration compressors for a couple of hundred dollars and if I wished to run two or even three or even four or five of them in parallel to exploit the 600 Watts of panels I have on my boat I can do so and only draw 20 odd amps of the 40 or so Watts I now get from the panels. The heat extraction component is cheap and sealed unit compressors extremely reliable and long lived (Larger DC compressors appear to be available but still fairly expensive in comparison to those produced for the mass market portable fridge market)

The eutectic phenomenon has been reliably exploited by mankind to keep or make things cold for hundreds of years.

We can exploit it using modern developments in solar panels and DC, sealed unit refrigeration compressors cooling a eutectic fluid during daylight hours by running the refrigeration system directly from the panels with a small electrical buffer in the form of a battery.

The subject variable speed electronic control unit for the compressors would help to optimize the system.

[nilespf]

Missourisailor, like yours mine takes a while to pull down. On initial startup the compressor is running wide open, once it drops to within 10 deg of the set point the speed is throttled to medium. It continues until it reaches the set point of 20. This process rubber bands until the spill fan controller can drop the fridge to the 35 deg. set point. It takes 5 if not more cycles because the spill fan is bleeding energy from the freezer. It will settle into about a 48-50% duty cycle once everything stabilizes. I had to bury the freezer RTD temp sensor low in the box to ensure it did not feel the freezer top door opening or the spill fan air movement.

If the sensor was too high and in contact with the evap plate it short cycled. The D.C. was still close to 50% but when you graphed it out it looked like a high frequency square wave. Glad to hear yours is 50% I'm inclined to believe it is all to be expected given the type systems we have been dealt.

If anyone has measured Duty cycles please drop them into the forum.

pn

[nilespf]

Hello Boatwright,
Two questions, do you use water against the condenser?

The 30Ah/day means how many hours per day?



If you say 24 my box may be leaking like a sieve



pn