Shore Power Independence- Load Analysis
About a week ago a discussion on the practicality of operating a household size refrigerator solely with the power from a set of solar panels was started. After several dozen posts the conclusion was that “yes” it’s probably possible but “no” it’s probably not practical. A day or so ago we found a commercial refrigerator manufacturer of eutectic plate refrigerators who makes fridges that might be able to operate on the power from solar cells. The price of these fridges and whether they would actually work has not been determined. I think the forum discussion has gone as far as it practically can so now it’s up to those who are interested in solar powered refrigerators to buy one and see whether it really works.
Good luck guys. Let’s hope it doesn’t cost too much.
Over the past five years, I and a number of the members of the Murray Chris Craft Constellation forum (the Connie Forum) on the Boat US Manufacturer’s Forum site have been kicking around the concept of Shore Power Independence. Shore Power Independence means making your Connie capable of operating for long periods (days or weeks) without any connection to shore based power sources. Operational, in this case, means both providing a reasonable living environment on board as well as actually cruising. The conclusion of the forum was again that while the concept was possible it was not practical. And so far no one has attempted to actually make their Connie Shore Power Independent.
In order to determine whether shore power independence is practical the first requirements are to make an accurate determination of how much power a Connie uses when away from the dock and whether the generator and/or other power sources can provide it.
None of us had much experience in long term offshore operation but we were able to obtain some very good information from The Voyager’s Handbook; a 570 page tome on ocean crossing and round-the-world-circumnavigations. Sailors who do that obviously need boats equipped for operation for long periods far from shore power cords. The Handbook is a treasure trove of information on offshore operation.
On page 232 we found Table 9-6 which describes the electrical needs of “Highlife” a 52’, eight year old, cutter rigged ketch. The table indicated that the daily DC demands were 115 Ah for House Loads, 75Ah for Activities, and 312 AH for AC loads supplied to the battery bank via an inverter. The peak load is 502 Ah. But not all of these loads occur at the same time so the average load from these three sources is about 301 Ah.
In addition one must add in the power for the refrigerator/freezer which running 24/7 at 12 amps burns about 280 Ah per day. It is the largest power consumer on the boat. The total and average for the day is now 581 Ah.
There are also the inverter loads which include the washer/dryer, hair dryer, toaster, coffee maker, microwave, vacuum cleaner and TV/DVD. They add up to 312 Ah but average out to about 187 Ah. The total and average for the day is now 768 Ah which at 12 volts is 9.22 kWh. This completes the compilation of DC loads.
But there are also AC loads powered by the generator and delivered directly to the loads at 120 volts. They include battery charging at 5 kWh, AC watermaker at 3.4 kWh and a small amount of air conditioning at 5.4 kWh (about enough to cool down the master stateroom so that you can sleep at night). The AC load total is 15.3 kWh.
The total, DC plus AC, load is 9.22 +15.3 = 24.5 kWh which requires 1.2 hours of generator operation at full load, burns about 1.9 gallons of fuel and costs $7.68 at $4.00/gallon.
This is an enormous load and on page 235 the Handbook gives a table which states that the maximum realistic output for solar cells on a 52’ boat is about 100 Ah which at 12 volts is 1.2 kWh. This in turn is not even 5% of the boat’s requirements. There is no chance that solar cells or wind generators are going to generate 24.5 kWh. The only practical source is a diesel generator. The owners of Highlife ran their generator 20 hours a day.
So what can I do? The answer is not much. I could replace the 27 year old refrigerator with a new more efficient one which would reduce the refrigerator and battery charging loads to about one third of their previous levels. But that saves only about 5 kWh (about 1/5 of the total load) which would reduce the daily fuel costs by only $1.53. The old fridge/new fridge swap out would probably cost about $2,000 so at 30 days per year (estimated average use in the Shore Power Independent mode) it would take about 43 years to recover the cost. Not practical!
Sorry this analysis didn’t come out with a more positive answer that indicated solar cells or wind generators would work but that’s the way the cookie crumbles. At least it may have allowed you to avoid a lengthy excursion down a blind alley. In my next post, I’ll try to find a practical way to accomplish Shore Power Independence.
Pete37
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