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[[Category:RefrigerationSystems]]
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= Energy Efficient Fridge/Freezers for Passage Making =
== Summary ==
For serious passagemaking a cold-plate fridge and freezer are the best energy-efficient option. A 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.
== Introduction ==
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 [http://www.dow.com/ Dow chemical] Blueboard (STYROFOAM) or the [http://www.owenscorning.com/ Owens Corning] InsulPINK Foam Insulation Board, plus radiant barrier insulation (RBI).
Polystyrene is water repellent, meaning it does not absorb water, which gives better performance in a moist environment. For the tropics, [http://www.glacierbay.com/ Glacier Bay] recommends 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 F), while freezers are kept at -6.6 C (20 F). Keep the freezer as small as possible.
== 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 (VDC) when running. If it cycles 30 minutes per hour, it will require 39 Amp Hours amp hours (AH) per day (7/2*24). A typical cold-plate system, drawing 35 Ampsamps 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 DC system or engine) when it is running.
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== Options ==
Many production trawlers are fitted with residential [http://www.subzero.com/ Sub-Zero] fridge/freezers. These are constant cycling high-end fridges requiring 110 VAC. 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 AC 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 [http://www.frigoboat.com/ Frigoboat] [http://www.novakool.com/ Nova*Kool] or [http://www.tundra.cc/frige.html Tundra]; or a cold-plate system such as the [http://www.glacierbay.com/ Glacier Bay] Micro HPS 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.
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== References ==
[[Category:FridgeFreezers]]