3 8 Almacenamiento Paneles Fotovoltaicos

The power system

Solar arrays

Batteries

Solar array regulators

Distribution system

Secondary power system

4

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

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

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%


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


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.


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


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


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


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.


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


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

Section 1

SADM

Section 2

Section k

Satellite body


The strings are arranged in sections

SADM: solar array drive mechanism

The SADM has a limited number of lines that can go through


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

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3 8 Almacenamiento Paneles Fotovoltaicos