Follow-up Report: Penikese Island School Page: 3 of 6
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2 MassPUP Project 1, -nitoring
Since replacing the batteries, the refrigerator has operated without a problem
until the unusually dark December. One of the immediately noticeable effects
of replacing the batteries is that the freezer was able to refreeze partially
thawed food. Th food restocking procedure involves purchase of frozen food
at 9am, loading on the boat for delivery to the island and placing in the
freezer at 3pm. Before the batteries were replaced. the partially thawed food
was never fully refrozen.
Suggested limits of the original design
The system design was reviewed. The contractor assumed a duty cycle of four
hours per day for the refrigerator and freezer compressors. Th e compressors
draw five amps each, yielding a daily requirement of 40 amp-hours of charge
or 480 watt-hours of energy. Twenty watt-hours per day or 1.7 amp-hours was
added for the two-way radio. Using the technique and default values
presented in the Photovoltaic System Design Practices Manual, the system
should provide adequate power and three to four days of storage for the
However, a conversation with Becky Chambers at Sunfrost revealed that the
system is seriously underdesigned. The assumed load is far too low. The
sixteen cubic foot refrigerator-freezer consumes 45 amp-hours per day when
the ambient temperature is 70*F and the refrigerator maintains temperature
and has no additional load. Consumption increases to 65 amp-hours per day
when the ambient temperature is 90*F.
The no-load values are higher than the design load for the system. Mr.
Akens' belief that the system is inadequate for the load is a true belief.
Adding 20% to the energy usage to account for refrigerator usage and
redesigning the system based on the new design load revealed the following:
December is the worst month, even considering a 904F ambient
temperature in August.
Using the UMass Lowell loss-of-load-probability-based (LOLP) design
method, 11 35 Watt modules and 5 Absolyte 12-5000 batteries (378 a-
h), giving 5.6 days of storage, are required for an LOLP of 0.005.
Loosely translated, this LOLP means there will be about one day of
energy shortfall in every 200 days.
Using the Photovoltaic System Design Practices Manual, 12 modules are
required, and 5 Absolyte batteries (383 a-h) for five days of storage.
Mr. Akens took a step in the right direction by installing 400 amp hours of gel
Here’s what’s next.
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Berg, B. Follow-up Report: Penikese Island School, report, December 1, 1992; United States. (digital.library.unt.edu/ark:/67531/metadc666469/m1/3/: accessed January 22, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.