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ENERGY HARVESTING AND POWER MANAGEMENT IN NANO-SATELLITE
Thomas John1, Ankur Dev1, Aditya Shanker1
Co-Author: Kshitij Shashank1
1: Manipal Institute of Technology, Manipal University (www.manipal.edu)
Abstract
This paper focuses on building an efficient Power
System for a nanosatellite. The proposed idea is to harvest
solar energy by means of solar panels mounted on the
satellite surface. This energy is then optimized using a
Maximum Power Point Tracking system that runs on
board and the output voltage is regulated so as to allow the
charging of the batteries present. The loads on the satellite
are operational on two voltages (4.2 V or 3.3 V), hence
two separate buses are created so as to provide a constant
supply line to the components. The battery in itself is a
lithium ion battery and hence a Constant Current Constant
Voltage algorithm exists prior to it so as to ensure the
proper charging of the battery. There is also a protection
circuitry that acts as a failsafe measure in case the battery
is overcharged, in such cases the circuitry cuts of supply
to the battery. The loads are also exclusively protected
from latch ups that may occur in space by means of a
protection circuitry prior to it.
I. Introduction
Parikshit is a nanosatellite that weighs
approximately 2Kgs. It has body mounted solar panels on
4 of its 6 sides. The two sides are neglected because the
antisunward facing side (-x) never receives any
illumination and the other side (i.e. the nadir side) contains
the payload (a thermal imaging camera).
consists of Solar Panels connected in a hybrid pattern
(mixture of Series and parallel) followed by a battery
charge regulator that implements the MPPT functionality.
The MPPT ensures that at all points of operations,
maximum power is obtained from the solar panels. This is
done by varying the effective load on the panels so as to
fix the voltage and current at a value that ensures the
maximum output. For storage there are three Li-ion
batteries connected in parallel along with equal number of
battery protection ICs. Two separate buses are created
after proper conditioning of the available power. A 4.2V
and 3.3V as required by the different components in the
satellite. The 3.3V bus is obtained from the 4.2V bus itself
by using a DC-DC buck convertor. A major constraint in
a nano-satellite is managing the available power, within
the satellite during all phases of its operation while in the
orbit. This is taken care of by a highly efficient Power
Management Algorithm.
There is a Kill Switch which is electrically connected
between the main power bus and the loads. Its main
purpose is to connect the bus to the loads once the satellite
has been deployed out of the deployer. The satellite has
two kill switches in parallel to ensure that if one fails, the
other is present to complete the connection between the
main power bus and the loads.
Figure 1: Satellite axis orientation
Solar energy acts as the base source of accessible
power in space, and it is on this power that the continuity
of the entire satellite is dependent. The Electric Power
System (EPS) conditions this power, stores and distributes
it to the other subsystems. The foundation of the EPS
Figure 2: System Engineering Level diagram for entire
satellite
SOLAR PANELS
-Y SIDE
SOLAR PANELS
-Z, +Z SIDE
Bypass
Diodes
Bypass
Diodes
Blocking
Diodes
Blocking
Diodes
Blocking
Diodes
Battery Charge
RegulatorSPV1040
Battery Charge
RegulatorSPV1040
Battery Charge
RegulatorSPV1040
SOLAR PANELS
-X SIDE
Battery
Protection
In eclipse phase, the solar panels tend to act as current
sinks and thereby may cause a small dark current to flow
out of the batteries and dissipate across the panels. This
unwanted current may cause damage to the panels and the
individual cells. In order to avoid this, we use blocking
diodes between the battery and the solar panel. These
diodes are placed with the p side facing the panels and the
n side facing the battery. During dark current flow, the
diodes become reverse biased, effectively open circuiting
them. Thus, no current is allowed to flow. We are using a
SL23-61N8 Schottky Diode for its low forward voltage
drop and high efficiency.
Kill
Switch
4.2V
BATTERY BOX
3 Li-ion cells in Parallel
Bypass
Diodes
4.2V
OCPC
Buck
Converter
Buck
Converter
LOAD
@4.2V
3.3V
of the shaded cell, due to the extra current being produced
by the fully illuminated cells. Thus, as a result, the entire
excess current is dissipated across the shaded cell. This
effect is known as hotspot heating. Thus, in order to
prevent this, we connect bypass diodes across groups of
solar cells. When the cells are fully illuminated, the bypass
diodes are reverse biased with respect to the solar cells,
thus open circuiting them. However, during hotspot
heating, the reverse bias of the shaded cell drives the diode
to forward bias, thus short circuiting it. This provides a
path for this current to flow, eliminating hot spot heating.
In our solar panels, bypass diodes come inbuilt.
3.3V
OCPC
LOAD
@3.3V
Figure 3: Block diagram for Electric Power System
A direct connection from the solar panels to the
batteries via a voltage regulator will only give a sub
optimal output as when compared to implementing an
MPPT algorithm .Our MPPT system effectively finds the
point of maximum power point by using a continuous
‘Perturb and Observe’ method. We have used an IC from
ST Microelectronics (SPV1040) in order to implement a
Battery charge regulator and MPPT functionality on our
satellite. The basic IC specifications are:



Minimum Vin (cold start) = 0.7 V
(Soft start) Vin= 0.4V
Vin range= 0 - 5V
II. ENERGY HARVESTING
We have used Improved Triple Junction (ITJ) solar
panels from Spectrolab which have a rated efficiency of
26 % during its Beginning of Life (BOL). The solar panels
are fully functional during the sunlight phase, whereas
during the eclipse phase the entire satellite is driven by the
batteries. The input of the solar panel varies from 0-5 V
and as such requires regulation to a value of 4.2 V in order
to allow charging of the batteries present.
Typical solar cells are made up of several individual
solar cells connected in series. This leads to an optimal
balance between voltage and current generated. However,
the series current of the panel is limited to the current
produced by the least illuminated cell. Thus, if a certain
part of the panel is shaded, this can lead to reverse biasing
Figure 4: Charge of Battery vs. time
DOD Required – (300/5282.301) = 5.678%
Capacity at DOD – 5942.286mAh
Based on the above values obtained, we have set a
maximum DOD value for the battery of 6%. Should the
battery capacity go below 94%, the satellite has a Power
Management Algorithm running onboard that will
immediately switch to a preprogrammed safe mode that
will effectively only supply power to the most essential
components, and thereby give priority to battery charging
till it is above the set limit.
Figure 5: Battery Discharge Graph when Payload data is transmitted during eclipse
period
III. STORAGE AND MANAGEMENT OF ONBOARD POWER
The power management algorithm is used to ensure that
at all times, the satellite has enough power to power a
specific set of components and that the battery is kept
above a certain level of discharge, to ensure maximum
cycle life. The algorithm is designed to perform its
functions by switching between different modes. This
switching will be done on the basis of battery capacity,
which will be reported to the satellite microcontroller by
the DS2784 IC.
The nano-satellite has 3 lithium ion rechargeable cells
in parallel for the purpose of storing energy for usage
during the eclipse phase. Each of these batteries are of
2100mAh and hence constitute a total capacity of
6300mAh. Since it is a Lithium ion battery, we follow a
Constant Current Constant Voltage (CCCV) algorithm so IV. DISTRIBUTION OF POWER
as to ensure a safe and efficient charging curve for the
The batteries and the solar panels are connected to the
battery. The SPV1040 IC which is mentioned above
rest
of
the system via a kill switch that is open during the
carries out the CCCV algorithm and safely charges the
period
that the satellite is in the deployer. It is a
battery.
mechanical pressure based switch and opens immediately
A protection circuit which also acts as a fuel gauge upon release of the satellite into the orbit.
for the battery is connected in parallel to the battery. This
After the kill switch the power lines are divided into
circuit has a programmable overvoltage as well as an
overcurrent threshold, so that it acts as a failsafe should a 3.3 V bus and a 4.2 V bus respectively. The lines are
the battery be subject to overcharging. The circuit also connected to the components based on their input voltage
constantly monitors the health data of the battery. This rating requirements.
includes the voltage, current input, average current input,
Each of the functional component on board the
temperature and the accumulated charge on the battery. satellite is electrically protected from single event latch
These values are saved as part of the housekeeping data ups that may occur in the space environment. This job is
and is downlinked to the ground station every time the done by an overcurrent protection circuit. The
satellite makes a pass over it. The IC used as the fuel overcurrent protection circuit is a high side MOSFET
gauge and protection circuit is the DS2784 from Texas switch designed to protect loads from sudden current
instruments. The battery health data is logged every 1 surges due to a latch up. It can also be used as a
second and hence it provides a recent and accurate controllable switch by the MCU to switch off loads in
measurement of the battery condition and effectively the case of an emergency or in case the PMA demands it.
satellite. For our application, we calculated a minimum The IC used as the overcurrent protection circuit is the
life of 1 year for the satellite and thereby we set a MAX890l by Maxim Integrated.
maximum depth of discharge point (DOD) for the battery,
so as to ensure that the cells last for the speculated time V. TEST RESULTS
period. The calculations done are as follows
V.1 SPV1040:
Assumed Capacity of Single Battery – 2100mAh
The SPV1040 device is a low power, low voltage,
monolithic step-up converter with an input voltage range
from 0.3 V to 5.5 V, and is capable of maximizing the
Number of Complete Charge Discharge Cycles at full energy generated by even a single solar cell (or fuel cell),
discharge – 300
where low input voltage handling capability is extremely
Number of Complete Charge Discharge Cycles required important. By using the embedded MPPT algorithm
(Perturb and Observe), even under varying environmental
for completion of operating time – 5282.301
Total Capacity of 3 batteries – 6300mAh
conditions (such as irradiation, temperature) the SPV1040
Precision measurements of voltage, temperature, and
offers maximum efficiency in terms of power harvested current, along with cell characteristics and application
from the cells and transferred to the output.
parameters are used to estimate capacity. The available
capacity registers report a conservative estimate of the
The device employs an input voltage regulation loop, amount of charge that can be removed given the current
which fixes the charging battery voltage via a resistor temperature and discharge rate.
divider. The maximum output current is set with a current
sense resistor according to charging current requirements.
Figure 6: SPV1040 circuit
Figure 8: DS2784 circuit
Figure 7: Output Voltage vs. Input voltage graph for SPV1040. The
satellite load is taken to be 40.2Kohm
V.2 DS2784:
The DS2784 operates from 2.5V to 4.6V for
integration in battery packs using a single lithium-ion
(Li+) or Li+ polymer cell. Available capacity is reported
in mAh and as a percentage. Safe operation is ensured
with the included Li+ protection function and SHA-1based challenge-response authentication.
Figure 9: Battery charge graph while using DS2784 as the battery
protection IC
X-axis= Time
Y-axis= Output Voltage
Graph description
The above shown is a Voltage vs. Time graph based
on a simulation test run on the LTC 3533. The IC is
expected to step down the voltage to a value of 3.3V so as
to provide the required input for a few specified loads.
Comments: We can see that after a time period 1 ms, the
output voltage attains a constant value of 3.3V.
Inference: The LTC 3533 steps down the 4.2 V to a 3.3V
value.
Figure 10: DS2784 real-time data log while charging
V.3 Buck Converter
The function of the buck converter is to take the
regulated 4.2V input from the battery and step it down to
3.3V, to run the various loads that operate at that voltage
level. We use two bucks in parallel to increase the
redundancy of the system, as well as reduce the current
handled by each converter. In case of a buck failure, a
single buck alone can handle the input, as the IC has a high
current rating of 2A, which is more than sufficient for the
satellites bus.
We have chosen the LTC3533 by Linear
Technologies due to the following characteristics:



Figure 12: Circuit diagram of LTC 3533
VI. CONCLUSION
Ideal range of input and output of the IC Input =
1.8 to 5.5V; Output = 1.8 to 5.25V
We have successfully studied and tested the various
High efficiency of 96% (93% at 4.2V)
phases involved in building the power system of a
Sizeable continuous current rating of 2A at
nanosatellite. Upon implementation, it has proven to be
Vin> 3V
very efficient and is viable methodology for nanosatellites to adopt. We observed that an efficient power
management algorithm is highly essential for a nanosatellite to perform all satellite housekeeping tasks and
payload tasks. This is important because in a nano-satellite
the available power is usually quite low. The Battery
Charge Regulator along with the Maximum Power Point
Tracking Algorithm ensures that maximum power is
generated from the solar panels, and the batteries are
charged at a constant voltage of 4.2V with minimum
ripples.
Figure 11: Simulation for Buck Converter (LTC 3533)
The graphs obtained in Figure. 4 and Figure. 5 show
that the Power system is highly reliable. The battery
charge never goes below 96% Depth of Discharge, in the
14 orbit cycle per day. The payload data can be
downlinked to the ground stations on earth even during
eclipse phase of the orbits without any crisis of power in
the satellite.
REFERENCES
[1] Moacyr A. G. de Brito, Leonardo P. Sampaio,
Luigi G. Junior, Carlos A. Canesin ”Evaluation
of MPPT Techniques for Photovoltaic
Applications” Industrial Electronics (ISIE),
2011 IEEE International Symposium on 27-30
June 2011.
[2] Thanh Tu Vo, Weixiang Shen, Ajay Kapoor
“Experimental Comparison of Charging Algorithms
for a Lithium-ion Battery” IPEC, 2012 Conference
on Power & Energy on 12-14 Dec. 2012
[3] ST Microelectronics “High efficiency solar battery
charger with embedded MPPT” SPV 1040
datasheet 08-Oct-2010 [Revised 21-Mar-2013]