Preparation - Pacific Crossing Notes: A Sailor's Guide to the Coconut Milk Run - page 53

Sources

All data underlying the graphics in this chapter was computed by a Python script nomad.py,
developed by the author. Nomad uses PyEphem [#1] to calculate sun ephemeris data. The
source code of the script is available here [#2] .

The 2012 wind data was taken from NOAA's National Buoy Data Center website [#3] . The
source data lists wind speeds averaged over eight minutes at six-minute intervals. Intermit-
tent gusts are higher than the speeds listed at those intervals, but they will also increase air
turbulence which significantly reduces wind generator efficiency.

Solar panel efficiencies: National Renewable Energy Laboratory [#4] , Fraunhofer Institute
for Solar Energy Systems [#5]

In many publications PSH are erroneously used to denote an energy quantity. You
might find a “PSH of 10 kWh” which is equivalent to saying that something
“weighs 3 feet”. The confusion is due to the fact that peak solar irradiation (the light
power received from the sun) is about 1 kW/sqm at sea level in the mid-latitudes. In
this case, the numerical value of PSH is roughly equal to the number of kWh of
daily irradiation per square meter. This can be misleading since actual peak solar ir-
radiation varies significantly with latitude and the time of year as discussed in this
chapter.

[1]

At mid-summer solar noon in the Chesapeake, the sun will be at an altitude of 76°
rather than directly overhead. However, the solar output at that altitude is about 97%
of that of a sun directly overhead - close enough for the rough calculation in our ex-
ample.

[2]

Going back over our log books, I see “Cloud Cover” entries of “2/8” for most of our
sunny days in the tropics. There are of course also fully overcast days, but we would
size our solar capacity for the typical rather than the worst case.
[#1] http://rhodesmill.org/pyephem/index.html
[#2] http://dropbox.namaniatsea.org/44ddfb978c1cef375252c378f2aa14e0/nomad.py
[#3] http://www.ndbc.noaa.gov/
[#4] http://www.nrel.gov/solar/
[#5] http://phys.org/news/2013-09-world-solar-cell-efficiency.html

[3]