That is correct. All other variables being equal it all boils down to efficiency of the compressor
and driving motor
which is the definition of COP. The hermetically sealed Danfoss has the higher COP. Hermetically sealed compressors are rated as a single
unit. The Blitzer spec sheet does not specify the drive method so that COP of 1 is for the compressor
only. Efficiencies lost
in the drive method, belt or direct, and the motor
must be taken into account.
How much diesel
either system requires depends on individual preferences and how you use the engine
. It takes a certain amount of fuel
just to turn the engine
. If it is run just for cooling
(either by charging
batteries or producing power to freeze a cold plate) diesel consumption
will be high per BTU removed. If on the other hand you are also charging
the batteries, making water
, motor sailing, etc. the amount of diesel attributed to cooling
the box will be much lower per BTU.
If you try to freeze the holding plate by battery
alone the total efficiency drops even more. That mean old Dr. Peukert keeps reminding us that the faster you draw power from a battery
the lower the capacity. That means you have to burn even more diesel to recharge the battery. Assume for a moment that both systems are equally efficient and use 80 Ah a day. The hermetic system draws 4 amps for 20 hours and the 1/2 HP holding plate system draws 35 amps for 2.3 hours. Now say the house bank is a pair of lead acid 8Ds with a 20 hour capacity of 460 Ah. That means it will deliver 23 amps (5% of capacity) for 20 hours. (Peukert's Constant for high quality wet cells averages about 1.2) The hermetic system is drawing at a rate of .89% of capacity while the holding plate system is drawing 7.6%. At a 7% rate that 460 Ah capacity becomes more like 420Ah while at less than 1% it is more than 800 Ah. This is for illustration only as nobody will run a battery stone cold dead and have only the refrigerator
drawing power but it does show that capacity goes down as the rate of draw increases which means you have to put more power back into the bank. Here is a spreadsheet so you can play with your own configuration: http://www.smartgauge.co.uk/calcs/peukert.xls
So far we have just talked about the compressor but the evaporator has 3 other advantages over the holding plate. First, with a solar
power source it is fairly self maintaining while you are not aboard. Second, the box temperature remains more even throughout the day. Third, the evaporator is slightly more efficient. Ice is a fairly good insulator. As ice builds up around the cold plate refrigerant tubes the compressor has to work harder to freeze the remaining solution. A properly defrosted evaporator does not face that challenge.