Boatworks

The time has come when the prospect of cold drinks and long-term food storage has you thinking about upgrading your icebox to DC-powered refrigeration. Duncan Kent has been there and done that, and has some advice

Everything you need to know before upgrading or installing a fridge onboard

Fresh food must be kept at a refrigerated temperature of 40 degrees F to stay edible for a week, whereas a freezer needs to be able to retain a constant 0 degrees F. The successful operation of any refrigerator is almost entirely dependent on the efficiency of the insulation surrounding the icebox. Insulation that is too thin or of the wrong material can double its operating time and hence its power consumption.

If you are planning a new installation and calculating the physical dimensions of the box, you must allow for the thickness of insulation required to construct a thermally efficient unit. If you already have an unrefrigerated icebox that you plan to convert, check the thickness and type of insulation first, as it will almost always require upgrading. The minimum insulation thicknesses recommended for the Northern Hemisphere are: ( fridge/ freezer) 2in/ 3in ( sides), 3in/ 4in (bottom) and 1in/2in (lid). For tropical cli-

mates, add another 1-2in all round.

Proprietary insulated boxes, as stocked by numerous specialist agents and chandlers, will often fit your available space with a few minor modifications. However, many do not meet the recommended insulation standards, as most are designed for use in trucks where power consumption is not an issue, so you’ll often need to fix additional insulation around the outside of the box.

You can also make your own liner by constructing a mold, laminating it in fiberglass and then coating the inside with gelcoat or paint. It’s not a difficult project and can be a lot of fun. The WEST System website is a great resource if you want to try this.

An especially easy route may be to use one of the refrigerator boxes made by companies like Isotherm, where the fridge box and compressor are combined in a single unit. They are available in a number of shapes and sizes, and are increasingly used by production boatbuilders.

COOLING UNITS

Your choice of cooler will make the difference between a “vaguely cool” and “properly cold” fridge. The two main methods of cooling are thermo-electric (aka: Peltier-effect, as found in many budget portable coolers) and the more effective compressor-driven units.

Peltier-effect fridges work by fanning air over a solid-state thermo-couple. They are compact and inexpensive, but won’t cool food sufficiently in hot climates and can only be used with iceboxes up to 1.25ft³ capacity. They are fine for short periods if you load them with prechilled food and/or throw in a block of ice. However, they will consume a considerable amount of power if the ambient temperature is high and food at room-temperature is loaded into them, making them less than perfect in many situations.

A proper marine fridge requires a compressor-driven cooling unit, as with a domestic fridge, where an inert gas is cycled through an evaporator plate inside the fridge, collecting heat that is then released via a condenser outside the fridge. (Once the heat is dispersed, the coolant is pumped back to the evaporator, after which the process is repeated.) Condensers are commonly air-cooled, so in a closed environment such as a boat it is vital to ensure it is adequately ventilated or it can lose up to 50 percent of its efficiency.

Boats in southern climes or travelling to the tropics may also want to consider fitting a water-cooled fridge. These most commonly utilize a hull-mounted, sintered-bronze condensing heat exchanger (aka: keel plate), which can be at least 30 percent more efficient than air-cooling. Alternatively, the Isotherm SP system comes with a special galley sink drain skin fitting with an integral cooling matrix for the fridge.

These days the components of a compressor-driven cooling system can be bought in kit form. They come with pre-gassed pipes sporting sealed end-valves, allowing them to be used as soon as the pipes are connected to the various other bits.

When you’re planning your system it is also important to know that the greater the surface area of the evaporator plate, the more heat it can absorb and the quicker it will cool the box down, so chose the largest size that will fit the box. The plate can also be bent during manufacture, which might allow you to cover more than one side.

SAVING ENERGY

Fridge compressors driven from a 12-volv DC supply usually draw

between 3.5 and 5.0 amps while operating (usually around 20 minutes every hour in moderate climates), so a 48W/12V fridge can consume around 32 amp hours (Ah) of battery capacity (or 40Ah including battery inefficiencies) over a 24-hour period. Most new models incorporate a low-battery cutout switch, which will disconnect the fridge should the battery voltage drop below 11.5 volts. Many can also switch autimatically between 110V AC and 12/24V DC to help preserve your batteries when you’re hooked up to shore power.

Though these are not so common on sailing boats anymore, you can also install an engine-driven compressor, powered by a belt from the crankshaft, to bring the fridge temperature down rapidly. Although these ideally require the engine to be run for one or two hours a day, the addition of a holding (eutectic) plate (such as the CoolBlue system by Technautics), or a thermo-electric cooler will allow it to retain the cold for the long periods in between. This can be especially useful at night, when a noisy compressor would otherwise keep you awake.

Holding plates work by using a fluid that freezes at a temperature above that of water. The coolant runs through a matrix within the plate, freezing the fluid and turning it into a “cold reservoir,” much like a regular freezer block.

The downside with engine-driven compressors is that they’re not as controllable as a purely electric fridge and the temperature in the box tends to vary from very cold to cool between engine boosts. However, most new electric fridge compressors, such as the Isotherm ASU and SX coolers, also incorporate a “Smart Energy” voltage sensor that automatically switches the compressor pump to full speed whenecer it detects the engine running or the alternator charging. When the engine is switched off it then drops back down to half speed, which in turns halves the power consumption. This system works exceptionally well when combined with a holding plate.

Of course, cold air sinks, so top-opening, rather than front-opening fridges don’t lose nearly as much cold air when opened—the exception being the very latest “drawer style” front-opening fridges (Dometic and Vitrifrigo for instance), where the food is stored in a sealed drawer, as in a domestic freezer, to stop the cold air spilling out every time the door is opened.

A fridge also needs a drain, but a simple open hole at the bottom will allow the cold air to quickly seep out. To prevent this, drains should have a pump, tap, plug or “U” bend filled with water.

ALTERNATIVES

Like many “marine” items, DC fridges are expensive when compared to their domestic AC counterparts. In fact, some opt to install the latter on board using a DC-AC inverter, despite the inherent inefficiencies of such a method. Domestic fridges, however, are not designed to operate when heeled over and can run inefficiently or even fail completely if powered up when sailing. If you only daysail and switch your fridge on once you’re settled at anchor or in a marina, you might just get away with it, but you’d have to restrict the number of times you open it during the day—particularly if it is front-opening with no drawers.

Motorhome and RV fridges are cheaper and can often run on AC, DC or sometimes even propane. But once again, these don’t like working on an incline and using them on propane would require an external flue. Also, their insulation is usually inadequate.

Portable iceboxes, either thermo-electric or compressordriven, are popular too and usually come with both 110V AC and 12V DC leads. Ideal for weekends or short trips, they can be filled with cold food from home, kept cool on the journey via your car’s 12V auxiliary power socket and then plugged into the boat’s AC/DC system when aboard. They do, however, consume a lot of power (5 to 8 amp[s) when on DC continuously, especially models without a thermostat. s