When opening a normal fridge, cold air will fall out (cold air goes down) and will be replaced by warm air (from your room), leaving the fridge warmer than before. The fridge will start cooling this new air, thus using more energy.
If you take a look at top opening freezers, cold air will pretty much stay inside (cold air goes down). The cheapest way to create a low-energy fridge is simply by turning the cooling down a little. A little rearrangement of shelves might be necessary.