3 8 Almacenamiento Paneles Fotovoltaicos
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Transcript of 3 8 Almacenamiento Paneles Fotovoltaicos
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The power system
Solar arrays
Batteries
Solar array regulators
Distribution system
Secondary power system
4
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The power system
Function: to provide electrical power to all the units on board
Energy is taken from the sun by the solar arrays
Then it is processed and distributed to the units
Exceeding energy is stored in batteries for the eclipse periods
A secondary conversion might be needed at unit level
Overview
The power system is autonomous
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The power system
DC/DCCL
CL
CL
CL
CL
Solar Array
Battery
SAR
AOCS
Comms
DHS
Motors
FPGA
Main Power Bus
Distribution Lines Payloads
DC/DC
DC/DC
DC/DC
DC/DC
A typical configuration could be:
Primary power system
Distribution system
Secondary power system
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The power system Solar Arrays
In the majority of satellites the power is generated by solar cells.
There are several types:
Silicon
GaAs
Multi-junction
Nowadays, the most used solar cell is the triple junction GaAs cell.
GaInP
GaInAs
Ge
The conversion efficiency is currently 28%
The near future target is 30%
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The power system Solar Arrays
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The power system Solar Arrays
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The power system Solar Arrays
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The power system Solar Arrays
The I-V curve of a solar cell is as follows:
It can be defined with three points: Open circuit (OC), Short-circuit (SC) and Maximum Power Point (MPP)
Depending on the operating point, it can behave as a:
Current source
Voltage source
Cur
rent
[A]
Power curve
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The power system Solar Arrays
The I-V curve changes with temperature
0 1 2 30
0.5
1
1.5
Voltage [V]
Cur
rent
[A]
@28 degC@88 degC
IMPP
The power available is higher at lower temperatures
The Voc changes significantly. This is very important for the power converter connected to the SA.
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The power system Solar Arrays
It can be mathematically modelled as follows:
RS = cells series resistance I = generated currentRSH = cells shunt resistance V = cell operation voltageIL = photovoltaic current across junction k = Boltzmanns constant (1.38 E-23 J/K)Io = reverse saturation current of ideal diode T = absolute temperatureI = generated current q = electronic charge (1.6E-19 coulombs)V = cell operation voltage A = curve fitting factor between 1 and 2ID = Io * exp[q*V / (A*k*T)] -1 (combined from 2 terms with A=1 for diffusion current and
A=2 for recombination current)
0 1SV I Rq SA k T
LSH
V I RI I I e
R
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The power system Solar Arrays
It can also be modelled using the following four parameters:
VOC ISC VMPP IMPP
ln 1
1 1mp cell mp
sc mp oc
I v V
I V Vmpcell sc
sc
Ii I e
I
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The power system Solar Arrays
Thermal model
Temperature is a key parameter to determine the I-V curve and the available power.
The temperature dependence is assumed to be linear.
The equilibrium point can be calculated as follows:
Tfront
Tback
Pabsorbed Pemitted_front
Pemitted_back
Pelectrical_outRth Cth
_ _
_
1frontabs em f em b out
panel
front back th em b
dTP P P P
dt C
T T R P
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The power system Solar Arrays
Degradation
Radiation has provokes degradation on the solar cell performance
After 15 years in GEO, the performance will be approximately 88%compared to BOL.
The effects on the voltage and the current parameters are different
UV also degrades the performance of the cover glass
Micrometeorites impacts obviously degrade the performance as well
A fleck of paint left this crater on the surface of Space Shuttle Challenger's front window on STS-7.
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The power system Solar Arrays
How are the solar cells arranged to form a solar array?
The basic idea is to combine a number of them in series [n] to form a string
And then to connect a number of strings in parallel [m] so that:
The voltage level and current capability can cope with the demand foreseen
1
2
n
2 m
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The power system Solar Arrays
To improve reliability, protection diodes are added:
A diode is added in series with each string
And another one in parallel with each cell
Otherwise, a short in one string will short the array To avoid that the less
illuminated cell biases the current of the string
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Section 1
SADM
Section 2
Section k
Satellite body
The power system Solar Arrays
The strings are arranged in sections
SADM: solar array drive mechanism
The SADM has a limited number of lines that can go through
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The power system Solar Arrays
Solar cell parasitics
The most important parasitic is the cell capacitance.
It is non linear and depends on the voltage and the temperature
In triple junction cells it is in the order of 1uF
Note that if the cell was switching, there will be relatively high losses on the switching device