Another look at Solar Powered refrigeration on sailboats

[Richard Kollmann]

Direct solar powered refrigeration on a pleasure sailboat without battery assist is not practical when refrigerated box size is larger than 100 liters (3.5 cu ft), as I understand Danfoss BD35F solar power charts. It is not possible to be precise because of all the variables, through put load in and out of box, ambient temperatures, desired evaporator temperature and R value and condition of insulation. There are exceptions to this without battery opinion where a 101N0400 special control module available from Danfoss allows solar power voltage to vary from 10 to 45 Vdc and still operate compressors. The real key to operation of a refrigerator without battery assist is there a sufficient amount of solar array to just cool or refrigerate a boat’s insulated box.

If your 12 volt boat refrigerator today in warm weather is consuming 50 amp-hrs per day it will require 600 watts of energy from a solar panel array per day to operate refrigeration system alone. Yes, you might find room for solar panels rated at 600 watts on your boat but actual sun day wattage maybe only ½ the advertised panel wattage rate. At 60 watts normal power consumption for a BD35F compressor per hour this compressor will need to run 10 hours per day which is much more than panels rated at 600 watts can deliver.

Information from boaters cruising in Bahamas indicate it requires 80 to 100 watts solar power used per day per cubic foot of box while maintaining only a drink cooler at +40 degree F box temperatures.

There are two basic types of solar refrigeration Direct solar operated storing energy in ice or Battery assisted energy refrigerator systems. Storing energy in batteries generally allows for greater energy storage capacity and better box temperature flexibility in a converted boxes than tanks containing frozen ice liquids. The disadvantage of relying on energy storage in batteries alone for 24/7 refrigeration is its negative effect on battery life. Alternative sources of electrical battery assisted energy from any source will extend battery useful life such as:
• Any device that energizes power to refrigerator compressor when there is a charging current at or above 13.8 volts.
• Grunert prevented large 12 volt compressors from running when alternator was not running by an oil pressure switch.
• Isotherm’s ASU starts compressor when charging voltage is detected.
• My designs called for a low cost voltage combiner and relay to bypass thermostats control when a charging current was present.
• Twelve volt clock timers have been used to over ride thermostats on Technautics holding plate systems.
• Some boaters have reported they reset thermostat when motoring for long periods of time.
The disadvantages of eutectic solution cycling or holding plates are:
• The small 1/10 to 1/12 horse power 12 volt compressors require too many hours running time per day to support a sufficient size eutectic holding plate.
• Eutectic ice cycling single temperature zone plates were popular a few years back by a few boat refrigeration companies. I believe only Techautics is still marketing true welded stainless holding plates in the US for small 12/24 volt compressors.
• The only way to justify the cost and inconvenience of eutectic plates on a sailboat is if there is surplus electrical energy available from other than the engine alternator. Contrary to what some salesman believe the compressor or the refrigeration unit does not create energy.

[44'cruisingcat]

Your units seem confused. Litres, cubic feet, etc. And you seem to say Watts when you mean Watt hours?

To make 600 Watt hours per day, you don't need to have 600 Watts of solar panels.

[StuM]

Sorry, but this doesn't make any sense.

Originally Posted by Richard Kollmann

If your 12 volt boat refrigerator today in warm weather is consuming 50 amp-hrs per day it will require 600 watts of energy from a solar panel array per day to operate refrigeration system alone.

"Watts of energy" is a misunderstanding of units. Watts are a measure of power, not energy. Similarly "watts... per day" is incorrect. Watts are an instantaneous measure of energy flow - they are not time related in any way.


I presume you meant "will require 600Wh per day".

Yes, you might find room for solar panels rated at 600 watts on your boat but actual sun day wattage maybe only ½ the advertised panel wattage rate.

Rated watts and "watt hours per day" (which i guess you meant by "the nonsensical sun day wattage") are entirely different things. A common rule of thumb is to expect 4 to 5 time the "rated wattage" in watt hours for well sited panels, dependent on time of year and latitude.


So 600W of panels can be estimated to provide 2400-3000Wh per day.

At 60 watts normal power consumption for a BD35F compressor per hour this compressor will need to run 10 hours per day which is much more than panels rated at 600 watts can deliver.
.

60 watts power consumption is not "per hour". It is the power consumed at any time the compressor is running. (again "watts per hour" is meaningless)


It tells us nothing about how many Watt hours are required per hour or day.



You need to consider the duty cycle. That will depend on many factors including ambient temperature, insulation efficiency, how may times the fridge is opened, etc. If the compressor runs non stop, 24 hours a day, you have a major problem. Assume that it runs on average for 15 minutes per hour (it shouldn't run much at all in the early hours of the morning), the average energy consumption is 15Wh per hour which is 360Wh per day. Compare that to the expected output of 600w of solar panels.


Added, I am assuming that by "without battery assist", you don't actually mean that but are saying without additional inputs from a battery that is charged by other means. Obviously, a battery of some sort is required to store the excess energy during the day when the solar panels are working so that the energy is available at night.

[Richard Kollmann]

StuM, Sorry I got carried away with solar power without battery assist. You are correct a 600 watt-hr panel will handle my example 50 amp-hr a day system as long as surplus energy can be stored in batteries.

[Uncle Bob]

Originally Posted by Richard Kollmann

Direct solar powered refrigeration on a pleasure sailboat without battery assist is not practical when refrigerated box size is larger than 100 liters (3.5 cu ft), as I understand Danfoss BD35F solar power charts. It is not possible to be precise because of all the variables, through put load in and out of box, ambient temperatures, desired evaporator temperature and R value and condition of insulation. There are exceptions to this without battery opinion where a 101N0400 special control module available from Danfoss allows solar power voltage to vary from 10 to 45 Vdc and still operate compressors. The real key to operation of a refrigerator without battery assist is there a sufficient amount of solar array to just cool or refrigerate a boat’s insulated box.

If your 12 volt boat refrigerator today in warm weather is consuming 50 amp-hrs per day it will require 600 watts of energy from a solar panel array per day to operate refrigeration system alone. Yes, you might find room for solar panels rated at 600 watts on your boat but actual sun day wattage maybe only ½ the advertised panel wattage rate. At 60 watts normal power consumption for a BD35F compressor per hour this compressor will need to run 10 hours per day which is much more than panels rated at 600 watts can deliver.

Information from boaters cruising in Bahamas indicate it requires 80 to 100 watts solar power used per day per cubic foot of box while maintaining only a drink cooler at +40 degree F box temperatures.

There are two basic types of solar refrigeration Direct solar operated storing energy in ice or Battery assisted energy refrigerator systems. Storing energy in batteries generally allows for greater energy storage capacity and better box temperature flexibility in a converted boxes than tanks containing frozen ice liquids. The disadvantage of relying on energy storage in batteries alone for 24/7 refrigeration is its negative effect on battery life. Alternative sources of electrical battery assisted energy from any source will extend battery useful life such as:
• Any device that energizes power to refrigerator compressor when there is a charging current at or above 13.8 volts.
• Grunert prevented large 12 volt compressors from running when alternator was not running by an oil pressure switch.
• Isotherm’s ASU starts compressor when charging voltage is detected.
• My designs called for a low cost voltage combiner and relay to bypass thermostats control when a charging current was present.
• Twelve volt clock timers have been used to over ride thermostats on Technautics holding plate systems.
• Some boaters have reported they reset thermostat when motoring for long periods of time.
The disadvantages of eutectic solution cycling or holding plates are:
• The small 1/10 to 1/12 horse power 12 volt compressors require too many hours running time per day to support a sufficient size eutectic holding plate.
• Eutectic ice cycling single temperature zone plates were popular a few years back by a few boat refrigeration companies. I believe only Techautics is still marketing true welded stainless holding plates in the US for small 12/24 volt compressors.
• The only way to justify the cost and inconvenience of eutectic plates on a sailboat is if there is surplus electrical energy available from other than the engine alternator. Contrary to what some salesman believe the compressor or the refrigeration unit does not create energy.

Richard, as others have noted, some of your post appears to be confused, perhaps revisiting and revision may make it clearer.
As to the portion highlighted, I believe that a well designed and sized eutectic system that is designed to run when power is abundant, in a well insulated box to be far superior to a simple evaporator plate that must run for a period each hour of the day.
As to the portion of the quote highlighted in red, it is fair to say that is correct, however one must acknowledge that in fact nothing creates energy, it is merely converted from one form to another, as in a diesel engine is used to run an alternator, converting the energy stored in the fuel into another form, electricity. The same holds true for wind, solar etc.
We can store this energy as electricity and use it at our leisure for whatever and we can also store it as a frozen bank within the refrigerator, eliminating the need to use the batteries through the night to replenish the refrigeration.
Should the need arise I will replace my refrigeration system with eutectic, based solely on personal observation of others systems.

[AKA-None]

If you don’t want to use batteries then a solar powered refrigeration system will work but I’m not sure you would have the surface area for the heat exchanger.

[RaymondR]

I know from personal experience that BD 35 run constantly between the hours of 0800 and 1530 every day with about 1 1/2 cu ft of eutectic fluid will keep a 6 cu ft refrigerator at the correct temperature overnight with about a days holdover capacity without the compressor running at all. The subject box had 4" of polyurethane insulation.

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.

The only valid way to refute Ozepete's data and subsequent conclusions is to set up your own test jig and run similar tests. This sort of thing is part of the scientific method and is widely accepted as a valid protocol where controversy exists regarding a matter.

Richard, I always read your posts and have referenced a fair bit of the other work you give access to via the net and have found that you are very knowledgeable regarding boat refrigeration matters and offer a great deal of very good and valuable advice however you appear to have a bit of a blind spot regarding euttectisc systems using 12V compressors.

[Uncle Bob]

Originally Posted by RaymondR

I know from personal experience that BD 35 run constantly between the hours of 0800 and 1530 every day with about 1 1/2 cu ft of eutectic fluid will keep a 6 cu ft refrigerator at the correct temperature overnight with about a days holdover capacity without the compressor running at all. The subject box had 4" of polyurethane insulation.

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.

The only valid way to refute Ozepete's data and subsequent conclusions is to set up your own test jig and run similar tests. This sort of thing is part of the scientific method and is widely accepted as a valid protocol where controversy exists regarding a matter.

Richard, I always read your posts and have referenced a fair bit of the other work you give access to via the net and have found that you are very knowledgeable regarding boat refrigeration matters and offer a great deal of very good and valuable advice however you appear to have a bit of a blind spot regarding euttectisc systems using 12V compressors.

And their manufacturers.

[StuM]

Originally Posted by Richard Kollmann

StuM, Sorry I got carried away with solar power without battery assist. You are correct a 600 watt-hr panel will handle my example 50 amp-hr a day system as long as surplus energy can be stored in batteries.

That's 600W, not watt-hr. You really do need to get your units sorted out. Try here.


http://www.cruisersforum.com/forums/...ml#post1933764

[valhalla360]

I didn't look up the models but are we talking about cold plate systems or standard fridges?

Standard fridges are pretty much not viable no matter what size...depending on latitude and season...somewhere around 4pm, the solar stops putting out any significant power and doesn't start up again until around 9am the next day. That's 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.

[Richard Kollmann]

StuM, hopefully this will help reduce confusion about the limits of mobile boat refrigeration when operated solely from solar panels without battery assisted back up. Only when power is drawn from a solar panel is it measured in watt hour or watt minutes.

First lets examine the facts of my example.

I used Danfoss information from {How to get started Danfoss Compressors – BD Solar. }

Following the box size in Liters and power consumption in amp-hrs the Danfoss chart revels a 100 liter size box is about the maximum size for operating refrigeration and storing energy in eutectic containers, without battery backup.

My example was the 100 liter or if you prefer a 3.5 cubic foot box with BD35 134a compressor operating at 2500 Rpm averaging a amperage of 5 amps per hour. Five amps per hour is the same as 60 watts hours consumed per hour. Designing conceptually for a 50% compressor running cycle in a warm climate compressor could run as much as 10 hours per day or much less if box temperature was maintained for only a drink cooler.

My example of using a 600 rated watt solar panel was to call the attention to the compressors hourly running power consumption 60 watt hours per hour for 10 hours removed from a solar panel, a total of 600 watt hours spread over much less than 24 hours. A 3.5 cubic ft refrigerator’s eutectic plate for this size box should contain 1 ¾ gallon of eutectic solution. Or 3 and ¼ gallons if box is used as a freezer.

After spending twenty five years designing over 75 different size and shapes of custom eutectic plates and a couple hundred standard holding plates I would like to think I know what works and what does not. Type and eutectic freeze point plus volume of frozen solution determines temperature holdover. Thermo conductive of plates design will determine box temperature efficiency. Eutectic plates containing Glycol mixed with water are life limited do to glycol separation from water with the heaver glycol sinking to bottom of plate.

In conclusion no matter how much solar or wind power you have make sure the house battery bank can assist absence of sufficient sunshine or wind energy.

[billgewater]

It is always interesting to read up on any developments with refrigeration. It certainly is a case where simple principles apply.

But underlying the efficiency of any system is ultimately related to the heat ingress from the outside.
Obviously lifting the lid or opening the door is not helpful ...
Also the temperature differential between the desired internal temperature and the ambient outside temperature play a great role. Not sure what to do about that. (Live in Alaska? Drink cool beer but have no freezer?)

What does intrigue me a bit is that we know heat energy can be transferred mainly by conduction, convection and radiation (maybe some other methods as well such as physical vibration or chemical princesses or even transfer along the pressure and discharge lines or the condensing (then freezing) of water vapour that enters when the lid is opened - a problem in high humidity environments)

With the current best insulating materials and the sheer practicalities of building an insulated enclosure what proportional role do each of these heat transfer methods play? You would assume that convection is probably limited to heat movement inside the container and that insulation is a function of the material (thermal conductivity) and thickness. But how much heat enters via radiation? White paint may seem like a good idea but maybe white in the optical part of the spectrum does not necessarily mean it is highly reflective in the Infra red part of the spectrum.
Have there been any developments/preferences here?
Also, have there been any advances in vacuum walled containers? What is the relative advantage, if any with vacuum panels? Has anyone tried making their own?
Carbon dioxide (an example) has a lower thermal conductivity than air so maybe panels containing some gas may be viable from a construction viewpoint, although convection would then become an issue to be dealt with.
I understand the Carnot cycle and other issues affect the overall energy transfer efficiency of any such system, but the issues I refer to (I suspect) are the villains in the piece when it comes to holdover at the freezer/refrigeration box.
The commercial boxes found in all the RV shops match a market place that meets the users conditions (mainly in RV vehicles or caravans). But boats are different. We don't like running motors just for freezing and for many of us we require greater capacity. Plus we often want to "build in" our systems rather than bolt them to the floor of the SUV.
I have seen earlier discussions on the forum but have there been any recent developments?

[LoudMusic]

Solar basically requires batteries in order to be useful. Why are you trying to run a refrigerator without batteries? A couple consecutive cloudy days and your food is all garbage.

[valhalla360]

Originally Posted by billgewater

The commercial boxes found in all the RV shops match a market place that meets the users conditions (mainly in RV vehicles or caravans). But boats are different. We don't like running motors just for freezing and for many of us we require greater capacity. Plus we often want to "build in" our systems rather than bolt them to the floor of the SUV.

RV's don't typically run the motor to keep the fridge running. If away from shore power, they run on propane.

Not much difference fitting an RV fridge into a boat cabinet...in fact our prior cat had a small RV fridge. The challenge is proper venting and keeping level an ammonia absorption fridge. On a cat it's doable. On a mono, much more challenging. (this assumes you don't have a propane free boat but that's a different discussion)

[RaymondR]

Originally Posted by LoudMusic

Solar basically requires batteries in order to be useful. Why are you trying to run a refrigerator without batteries? A couple consecutive cloudy days and your food is all garbage.

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.