A fridge, and in an increasing number of cases even a freezer, is an item of domestic equipment that can transform life on board, but they can also represent the biggest single drain of battery power. Therefore care must be taken to avoid premature failure of an expensive battery bank. (See also: How can I save battery power with my refrigeration system?)
The first step that can be taken to reduce the power needed to run a fridge is improving the insulation. In many cases this is woefully inadequate – ideally a marine fridge will be encased in four-inches of insulation. Smaller units – those below around 100 litres – may be able to get away with less than this. However a freezer should ideally have some six inches of insulation.
If yours has less than this, improving the insulation may be easier than you think. Many units have a big air gap between the refrigerated box and the surrounding joinery. This allows for a very cost effective solution of simply gluing the type of extruded polystyrene insulation you can buy from builders’ merchants around the outside of the box. More awkwardly shaped gaps could also be filled with expanding foam.
If these options are not available to you, the best alternative may be to construct a new box that’s designed to make best use of the available space, to both optimise the size of the unit and maximise insulation. The number of boat builders, even among the quality brands, that, in the past at least, have failed to use the space on offer effectively is legion. This means there’s every chance that the cost and effort of fitting a new unit will be repaid with a efficient system that maximises the available space.
The siting of the compressor can also cause unnecessary power consumption, especially if it’s tucked away in a corner with next to no ventilation and no means of allowing heat to escape effectively. If your installation suffers from this problem the best solution is to move the compressor to a location with a good airflow. Most can be mounted at least 1.5m from the fridge, and some types allow for a significantly larger distance than this.
An effective alternative is to fit ducting and a fan to suck warm air away from the unit – these are available to buy for popular units. Systems that employ water cooling add an additional level of efficiency that may well pay for their additional costs, especially for larger units or installations in which it’s impossible to locate the compressor with a good air flow.
Front-opening vs top loading
Traditionally marine fridges were mostly of a top-opening style that prevented cold air escaping downwards when the lid is opened. However, there’s a growing trend towards front opening fridges that offer greater convenience, but are less efficient. That may not be a problem for boats that spend each evening in a marina with a shore power connection that can be used to maintain battery charge, especially if the door seals are maintained in good condition and only infrequent access is needed to the fridge. However, the extra power requirements can pose a problem for vessels that like to spend time in more remote places, or that make longer passages.
Drawer type units offer many of the advantages of a front-opening model, but are more efficient in that cold air can be retained in the drawers when they are opened. These units also have the benefit of it being easier to wedge items in place in a big sea.
The power used in a decent installation will be significantly reduced if items placed in the fridge are already cold – it takes much more energy to reduce the temperature of a can of beer to 5 degrees Celsius than it does to maintain it at that temperature for 24 hours. Furthermore, if you add a frozen five litre water bottle, this will help to keep the temperature down for several days with a minimum of power needed.
With sensible use, a well-planned refrigeration system with good insulation may draw as little as an average of around one amp hour averaged over a 24-hour period. However, badly set up system can see this number double or treble, even on a modest sized fridge and larger yachts that have fridges of several hundred litres in size may see even greater losses.
Once a boat has a number of electrical systems that draw a high current it’s invariably worth investing in a battery monitor that will calculate the amount of charge put into the batteries and the charge drawn out of them (see also: 8 ways to make boat batteries last longer). This will both help to ensure that you have adequate provision for replacing all the charge that is drawn out of the batteries, as well as indicating when they are down to 50 per cent of charge – the point at which they must be recharged to maximise battery life.
If you’re not able to plug into shore power each day it’s likely to be worth installing a solar panel to help give the batteries a hand. Even in the Mediterranean summer, with its high summer temperatures, an 80W panel can be ample to supply all the power requirements of a well insulated fridge. In many cases the cost of the panel will be offset by longer battery life.