Note: Descriptions are shown in the official language in which they were submitted.
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ENE~GY- OENERATING PLANT, PARTICULARLY
PROP~LLE~-TYPE SIIIP'S Pl~OPULSION PLANT,
INCLUDING A SOL~ GENE~TOR
This invention relates to an energy-generating plant
including a solar generator, the energy-generating plant
supplying energy, in particular to a ship's propulsion plant
having a propeller and the plant having the characteristics
according to the definition of the species of claim 1. In
the preferred case, the propell~r can be a ship's propeller,
which propels the vessel directly, or it may be a turbine
wheel of a pump jet, with which the water is accelerated
and caused to leave the housing in such a manner that the
water jet leaving the housing brings about the propulsion
of the vessel. In the latter case, not only can ~he propulsion
of the vessel be brought about, but the direction of travel
of the vessel can also be determined by changing the direction
of the water jet leaving tlle housing.
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Such propelling systems, to which the invention relates,
are generally known and are also in use.
For the generation oE energy, the use of solar generators,
in which photovoltaic solar cells convert light into electric
energy, i5 also known~ If a photovoltaic solar generator is used
for propulsion particularly of smaller vessels, attention must
be paid to the hiqhest possible efficiency of the components used,
in order to keep the area required for the solar generator and
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the weight of the energy storage system, the motor, and the
propulsion system, as small as possible. In other words, a
propulsion system should be available which overall has a high
efficiency.
Accordingly, it is an object of the invention to provide
a photovoltaic energy arrangement which takes maximum electric
power from the solar generator guaranteeing thus an optimum energy
yield. For this purpose, it is necessary to operate the solar
generator at its maximum power point (MPP). This is to be
ensured with the invention without having the expense of operating
the system at the MPP in an economically unjustifiable manner.
The invention is explained in greater detail below with
reference to the drawings~
It is a key feature of the invention to determine the
maximum power point as a fun~tion of two parameters, namely,
"the strength of the solar irradiation" and the "temperature at
the solar generator", and to set the conditions for this operating
point by means of a computer by adapting the input resistance of
a direct current converter for charging an energy storage device
for supplying the power to the electric motor of, for example,
a ship's propeller. These determining parameters are thereby
taken into consideration and, for practical purposes, there is
an optimum utilization of the photovoltaics for the propulsion
of a vessel.
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In the drawings, by means of which the invention is
described in greater detail:
igure 1 is a schematic diayram of the components of
a photovoltaic boat propulsion system and for
optimum energy management thereof, and
igure 2 indicates the generator characteristics in
relation to the working range of the direct
current converter' in the form used as a
component of the invention.
The propulsion system of a vessel tthe vessel not
being shown) includes a propeller 1, in an individual arrange-
ment, or the propeller 1 as one,of several similar propellers,
is used. The propeller 1 is fixed on, and rotatable with, the
drive sha~t of an electric motor 2. Electric energy is supplied
to the electric motor 2 from a d.c.-to-a.c. converter 3, which
draws electric energy from a number of batteries 4, which serve
to store the energy of the system. The batteries 4 are charged
with electric energy by means of a direct current converter 5,
the electric energy being obtained by means of a solar generator
6 having a plurality of solar cells.
Assigned to this system is a microcomputer 7 and a
data field 9, to which are supplied, as input quantities, the
generator voltage as signal 12, the generator current as signal
13, the batter~ voltage as signal 14, the battery charging current
as signal 15, the measured values of the pilot cells 10, which
serve to measure the solar irradiation as signals 16 and 17,
and the measured value of the temperature sensor 11 at the solar
generator as signal 18. From the processing of these signals in
the microcomputer 7, which contains a data field 9, a control
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signal 19 is obtained, which is supplied to the direct current
converter.
, The input resistance of the direct current converter 5
can be freely varied within a particular operating range. The
output voltage of the converter follows the voltage of the
connected storage battery up to the end of charge. This control
input forms the interface with the overriding microcomputer
system 7. The microcomputer is supplied with information from
different measuring sites in the system.
Due to the nonlinearity of the characteristic of the
solar generator 6, which is, in addition, liable to permanent
changes, also with respect to its shape, it is not possible
to specify a nominal value for the design of a closed control
loop with the direct current converter 5 being the electric
actuator.
In the propulsion system described hereafter, a control
algorithm has to be developed and programmed, which operates
the solar generator 6 at the maximum power point, the MPP
through successive optimizations. In this system, the voltage
and current are measured at the output of the direct current
converter 5 and supplied to the mîcrocomput~r 7. From these
values, the power is calculated, which the direct current
converter 5 delivers to the energy storage system 4. This
power must be maximized in order to obtain optimum power
and hence optimum energy yield of the solar generator 6.
When the system is started up, the value of the control
signal is set by the microcomputer 7 to an initial value, as
is the input resistance of the direct current converter 5. Since
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the position of the valid MPP is not knwon, the values to which
ths control signal and the input resistance of the direct current
converter must be changed, cannot be specified. The value of the
control signal is now increased with the largest step width w
and the voltage and current are measured again and the power is
determined. If the power is greater, the search is in the right
direction and the value o~ the control signal is again increased
by the step width w and the voltage and current are measured again.
If the power becomes less, the search direction is wrong and the
value of the control signal is decreased by the step width w.
The voltage and current are measured once more and the calculated
power is compared ~ith the specified value.
Due to the large step width w the characteristic of the
solar generator is quickly cove~ed. Moreover, the procedure ensures
that local maxima, if any, are skipped and the area of the absolute
maximum is detected.
If now the operating point has overstepped the MPP, the
direction of the search is reversed and the MPP is traversed in
the opposite direction. The operating point thus oscillates about
the actual MPP. Because of the large step width, the operating
point is still relatively far distant from the actual MPP. If now,
as described above, the MPP is traversed on~e in both directions,
then the step width w is halved and the optimization is continued.
This halving of the step width is continued up to the smallest
possible step width. In this way, the operating point is brought
as close as possible to the MPP.
At the end of a search process, that is, when the
step width is a minimum the searching process is interrupted
as long as the power yield remains constant~
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The search process remains switched off until the power ~ ,'
yield changes by a value to be established, that is, until the
position o~ the MPP has changed. In order to attain an accurate
setting of the new MPP as quickly as possible when there have
~een small changes, the searching process i~ continued with the
smallest step width, only after a predetermined number o~ ~arch
steps in the same direction the step width i5 doubled, at most
to the largest step width w. In order to accelerate the optimiza-
tion process ~n the further operation, the MPP values ~oun~ are
filed in a data ~ield. For the identification of the MPP, on the
one hand, and the value of the control signal, on the other, the
values of the solar irradiation and the generator temperature
are required being the determining parameters for the position
of the M~P. -;-
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The solar irradiation is ~etermined with the help o~ r,-
at least one so-called pilot cell 10. Such a pilot cell i~ an - ~;-
accurately calibrated re~er~nce cell, which i9 operated in ,-
circuit. The short~circuit current is a dlrect measure of the
magnitude o~ the solar irradiat~on, which can actually be
processed by a photovoltaic solar generator With respect ;;-
to the solar radiation the pilot cell is installed in the ¦ -~
ame position as the solar generator in it~ i~mediate vlcinty
as regard~ the solar irrad~ation, so that ~he same irradiation ~
condition~ can be assumed. For reasons of redundancy, it is -;
advisable to u~e at least two pilot ce~ls 10.
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The generator ~empera~ure iq measured with two temperatu
sensors 11, whlch arei~co~p~rq~within the rear of the solar ¦
generator 6 at varlous plac~s. -~
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These values are also supplied to the microcomputer 7,
so tha~ the measured irradiation and temperature of the solar
generator can be stated for each operating point.
Each MPP found, an MPP being regarded as found when it
was possible to terminate the search process, is filed in the
data field. In doing so, the value of the control signal for
said MPP is filed in the field element, and the index values
for the determination of the field element are the valueSof
the solar irradiation and the generator temperature.
If now the search process is started, or started anew,
the microcomputer searches in the created data field for an MPP
value which is valid for the instantaneous temperature and solar
irradiation, i.e. the control slgnal value. If it can find such
a valid MPP, the value of the control signal is set immediately
and the generator is thus operated at the MPP, the searching
process can be omitted. I~ there is no valid MPP value, a search
for the MPP must be conducted in the manner described aboveO
In order to prevent a wrong value being writ~en into
the MPP data field due to the shading or partial shading of the
solar generator or the pilot cells, a plausibility test is carried
out before each entry. On the basis of the family of characteristics
of a solar generator and with the short circuit current of the
pilot cells and the generator temperatures as input quantities,
the anticipated output at the MPP can be calculated with an accuracy
of about + 10%. This is too inaccurate for the MPP control, but
entirely adequate for a plausibility test.
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Before each entry of a supposed MPP into the data field,
the actual generator output, calculated from the generator voltage
and the generator current, is compared with the anticipated output
by means of the previously mentioned calculation. If the actual ~
output deviates by not more than about 10% from the calculated
one, the value i9 plausible and is entered in the data field;
if not, the search process is continued.
This plausibility test is also carried out before the
value of a control signal is set by means of the tabulated values.
This prevents an incorrect operating point being set because o~
shading of the pilot cell or of the solar generator~ -
It is evident that the energy, obtained with the help of
the plant described and stored in the batteries 4, can serve not
only for the propulsion of a propeller 1 serving for the propulsion
of a ship, although being derived in particular from the object of
the invention. The invention.can also find application in conjunc-
tion with a land-based plant. In such a case, the d.c.-to-a.c.
converter 3 is not coupled to the electric motor 2 of the propeller
1, but the power produced is fed, for example, into an electrical
grid instead of to the batteries 4. For example, a consumer, who
is not connected to the electrical grid, is supplied with
electricity, the installation operating rationally, because it -
always wor~s at the maximum power point with respect to the
practical requirements.
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