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Energy Efficient Fridge/Freezers for Passage Making


For serious passagemaking a cold-plate fridge and freezer are the best energy-efficient option. AAmpere (amp), SI unit of electrical current general guideline for fridges is 400-600 Btu (British Thermal Units) of cooling per cubic foot of interior fridge space. For freezers, it is 900-1200 Btu per cubic foot.


On weekend cruises, energy efficiency might not be the chief criteria for a cooling box. But for passage making, energy efficiency should be the sole criteria. The most energy efficient refrigeration system is the one requiring the least amount of total energy per day to keep your fridge or freezer at the required temperature.

Obviously insulation is a key factor. The more and better the insulation, the less energy required to keep a unit cold. A good design will have four to six inches of extruded polystyrene, such as the Dow chemical Blueboard (STYROFOAM) or the Owens Corning InsulPINK Foam Insulation Board, plus radiant barrier insulation (RBIRadiant barrier insulation).

Polystyrene is water repellent, meaning it does not absorb water, which gives better performance in a moist environment. For the tropics, consider R20 in a refrigerator and R30 in a freezer, which corresponds to four to six inches of foam. RBI has not been tested in a fridge/freezer, but it is inexpensive to add in a custom build so worth the chance as an experimental outermost layer.

Separating the fridge and freezer units is another good idea. You can optimise each unit without compromising or spilling air from both when you only need access to one. Fridges are usually kept at 4.4 degrees C (40 FFarad, SI unit of capacitance, also Freeboard), while freezers are kept at -6.6 C (20 F). Keep the freezer as small as possible.

Top opening is best for a small chest freezer. Cold air is denser and sinks. You can put larger and less frequently used items in the bottom and use wire baskets on top for smaller stuff. It'sSecond easy to remove a wire basket to get at the food in the bottom.

Front opening is more convenient for a refrigerator. It is easier to reach in and remove day-to-day items from a shelf.


The Type of Refrigeration System

The type of refrigeration system and its power source are the remaining factors. Cooling systems can use everything from acoustics to heat pumps, but constant-cycling and cold-plate systems are best on boats.

Constant Cycling

Constant-cycling systems are like a home refrigerator. A refrigerant gas is compressed and circulated through the unit. Compression removes heat. The compressed gas expands in the unit, absorbing heat. This works only while the compressor is running, so the unit cycles on and off frequently. This keeps the temperature fairly constant. One unit can be used for both fridge and freezer.

The compressor gives off heat, and must be air- or water-cooled. Air-cooling adds heat inside the boat, either in the accommodation or the engine room. In the accommodation, in the summer it will be working against the air conditioning, causing a double energy load. In the engine room, it will be struggling against other heat sources, and increasing the ventilation requirements. Water-cooling, using a keel cooler, is the best way to go.

Cold Plate

A cold-plate system works like an old-fashioned icebox. Small metal tanks, called cold or holding plates, contain a solution that freezes at subzero temperatures. The compressor runs just long enough to freeze the plates. The plates will keep the unit cold for several hours or days, in what is called the holdover cycle. Cold-plates for fridges should run at -3.3 C (26 F) while freezers should run at -15 C (5 F), so separate units are required.

The solution in the plates is either brine, or some other antifreeze, or a eutectic salt. Eutectic solutions don't thaw out gradually. They maintain a constant frozen temperature until they thaw out instantly in a phase change. A phase change is what happens on the highway when traffic suddenly goes from 120 kphKilometres per hour to a crawl for no obvious reason. A system with eutectic salts will have a more constant temperature than one with a brine solution, which increases gradually in temperature as it thaws.


Getting the Size Right

Cooling Capacity

Like heating and air conditioning, you will find no easy answers when trying to calculate refrigeration requirements. Theory and practice based on experience are often apart. As a result, everyone tends to over estimate, which of course increases the energy needed. Conversely, under estimating leads to inadequate performance and frustration. Approaches differ. Some experts use guidelines, and then add a fudge factor; others use heat transfer calculations, and then add a fudge factor. Sigh, maybe we should just use fudge factors.

For fridges, a general guideline is to provide 400-600 Btu of cooling per cubic foot of interior fridge space. For freezers, it is 900-1200 Btu per cubic foot. If you're buying a commercial unit, hopefully the manufacturer will have right-sized the interior volume, insulation and compressor efficiency.

Energy Consumption

When you have determined the cooling capacity and compressor size, the next step is to consider the energy requirements and source. A typical constant-cycling system, using a popular Danfoss compressor, will draw around 7 amps (A) at 12 volts direct current (VDCVolts direct current) when running. If it cycles 30 minutes per hour, it will require 39 amp hours (AHAmpere-hours (A*H)) per day (7/2*24). A typical cold-plate system, drawing 35 amps in the cool-down phase, and cycling for two straight hours per day, will require just 3 AH/day (35*2/24). Overall, a cold-plate system is far more energy efficient, but requires a heftier energy source (larger DCDirect current system or engine) when it is running.



Many production trawlers are fitted with residential fridge/freezers. These are constant cycling high-end fridges requiring 110 VACVolt alternating current. Their key feature is dual refrigeration. They have separate compressors for the fridge and freezer sections. This maintains ideal conditions in each compartment. Frozen foods need very cold, dry air. Refrigerated foods need warmer and moister air. Because each compartment has a separate door, air does not circulate between fridge and freezer, making it harder for foods to pick up unwanted flavours. Sub-Zero fridges are front opening, which means cold air spills out every time you open a door. They also look great. Many other manufacturers offer models with similar features.

Putting aside all other considerations, these types all require 110 VAC. If you spend most of your time hooked up to shore power, or have a large yacht with an ACAlternating current generator running constantly, this is not an issue. But for a medium-sized Passagemaker, you either must have a large-enough battery bank and inverter, or have an AC generator running constantly. Thus, fridge/freezer combinations designed for AC are not a good fit.

A better fit is a marine constant-cycle system such as Frigoboat Nova*Kool; or a cold-plate system running on DC or directly off the engine. Because the constant-cycle system cycles every 10 or 15 minutes (go listen to your home fridge), it is not a good candidate for a direct drive off the engine. You would have to keep the engine running 24 hours per day. It is a good fit to power with a smaller DC system, since the maximum current draw of around 7 Amps does not require a large battery bank.

A cold-plate system is a good candidate for a direct drive off the engine, or large DC battery system. Because it will cycle only once a day for a few hours, you can time it to coincide with a daily engine run to charge the batteries. You can also run it directly off the battery bank.

Another advantage of DC over AC refrigeration is that a well designed system, e.ggram., with trickle-charge backup, can keep your food fresh unattended for periods [1][2][3][4]. An AC system would require remote starting an engine.

If you have a propane stove, you might want to consider a propane fridge such as the Norcold.

The next generation of fridges to watch for may use magnetic, thermoelectric (used in space shuttles) or thermoacoustic cooling.


Web Links

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  1. Refrigeration For Pleasureboats by Nigel Calder, International Marine/Seven Seas, Blue Ridge Summit, PA 17294
  2. Do It Yourself...Boat Refrigeration by Richard Kollmann, 2430 Sugarloaf Lane, Fort Lauderdale, FL 33312
  3. Modern Refrigeration And Air Conditioning, Althouse, Turnquist and Bracciano, The Goodheart Wilcox Co., Inc., Homewood, IL 60430
  4. The Cold Facts, Cruising World, June 1995