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
. 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
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
• 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
• 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.