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TECHNICAL
REPORT
ELECTRIC
POINTS: “WHAT IS A SOLAR BOAT?”
In
the seventh of a series of short articles on technical aspects of
electric boating, EBA Technical Officer Paul Lynn discusses
solar-powered boats and proposes a new measure of performance, the
Solar Boat Index (SBI).
An
eye-catching new craft, furnished with solar photovoltaic (PV)
modules, arrives at an electric boat rally. Onlookers naturally
assume she is “solar-powered”. But what exactly does this
mean, and how might we quantify the performance of a boat that
uses sunlight for propulsion?
There
is no easy answer. The viability of solar propulsion depends on
the amount of PV carried, the motor power needed to achieve
cruising speed, the intended pattern of use and – of course –
the amount of local sunshine. In this article I should like to
examine these factors and propose a quantitative measure, the Solar
Boat Index (SBI), to
indicate the comparative performance of a wide variety of craft.
The
nub of my proposal is that any boat which, on average, derives 1
hour per day, or 7 hours per week, of summer cruising from its PV
modules deserves the accolade “solarpowered”. Most users
confine their boating to weekends, and 7 hours of weekend cruising
is widely regarded as a reasonable amount for an electric leisure
craft, whether solar or not. And I shall show that the criterion
is met, in English summer conditions, provided the boat’s peak
PV power is at least equal to one third the power required by its
propulsion motor at normal cruising speed. This, in turn, will
lead to a definition of the Solar
Boat Index.
To
illustrate the argument, let’s consider a boat having 0.5kW peak
of PV modules, mounted horizontally, and a propulsion motor that
requires 1.5kW to drive the boat at normal cruising speed.
According to the above proposal, this boat will just meet the
criterion of being “solar-powered”. How much energy do the
modules produce? Using arguments similar to those in EBA
Information Sheet No.2 on Solar Photovoltaics, in the summer
months May to August in England, 0.5kW of PV may be expected to
deliver an average of about 1.5 kWh per day of usable stored
energy to the boat’s batteries. And since the motor needs 1.5 kW
at normal cruising speed, this solar input will clearly allow an
average 1 hour cruising per day, or 7 hours at the weekend,
assuming adequate battery storage.
But
what is “normal” speed? I should like to suggest 5 mph (8 kph),
the speed limit on the river Thames, for boats based on rivers and
lakes; and 4 mph (6.4 kph), the speed limit on the English canals,
for barges and other canal boats. Most people would probably agree
that these are realistic, and enjoyable, speeds at which to
travel.
I
now propose the following Solar
Boat Index (SBI): SBI = 3 (Ppv)/(Pm)
Where Ppv is the peak power in watts supplied by the PV panels,
and Pm is the motor input power required to give a cruising speed
of 5 mph (6.4 kph) on rivers and lakes, or 4 mph (6.4 kph) on
canals. I have included the factor 3 in the definition so that a
boat giving an average 7 hours of weekend cruising in England,
which is just “solar-powered”, achieves an SBI of unity (1.0).
This is easy to explain, discuss, and remember.
A
further advantage is that a boat’s SBI value equals the expected
number of cruising hours per day provided by its PV modules. For
example, a boat with SBI = 2 gives an average 2 hours per day of
solar cruising at “normal” speed. Let’s take some more
examples of solar boats, this time real-life ones. My own
catamaran Solar Flair,
which completed the first
solar voyage along the non-tidal Thames in 2003, has four 75W PV
panels, giving Ppv = 0.3kW, and needs about 0.55kW of motor power
to travel at 5 mph (8kph) in calm water. Her SBI is therefore
equal to 3 x 0.3/0.55 = 1.64 (which, I note with relief, is
comfortably above unity). The catamaran Collinda,
in which Malcolm Moss made
the first solar voyage across the English Channel in 1997, has a
more impressive SBI of 2.8 making her well and truly “solar-powered”.
And Cedric Lynch’s canoe, often seen running at unlikely speeds
in weak sunlight, manages an SBI of 4.3. This one will be hard to
beat!
We
should also consider the local climate. Data for England should
also apply reasonably well to countries such as Germany, Holland,
and the northern half of France. A boat based in the south of
France might get an additional 30% from its PV panels in summer,
and one based in Spain or Australia perhaps 50% more. Such
variations, as well as different patterns of use in different
climates, should be taken into account when interpreting SBI
values.
Finally,
a few words of caution. An SBI value can only be an approximate
indicator of performance because of the uncertainties surrounding
the solar climate and a boat’s pattern of use. I therefore
recommend that SBI’s should only be quoted to 2 significant
figure accuracy. And we should avoid saying that a boat based in
England and having an SBI of, say, 0.80 is definitely “not
solar-powered”, or that one with an SBI of 1.2 “definitely is”.
A little flexibility is required. With such provisos I launch the
SBI upon uncharted, but hopefully unruffled, waters.
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