PSU 2008/09

power supply unit discussions to go here

[ftp://ftp.star.le.ac.uk/db68/presentations/ ftp server presentations folder

OK, being sensible now.

The PSU team are:

Lucy Benson ku.ca.el|43bml#ku.ca.el|43bml
Rich Wrigley ku.ca.el|12wlr#ku.ca.el|12wlr
Matt Cro ku.ca.el|1cwjm#ku.ca.el|1cwjm
Tamsyn Evans ku.ca.el|4eat#ku.ca.el|4eat
Roisin Boadle ku.ca.el|24bar#ku.ca.el|24bar

PSU

PSU Introduction

Our PSU is going to consist of:

Part Description
Solar Panels primary source of power for satellite
MPPT, Max Power Point Tracking is an electronic DC to DC converter to optimise match between solar panels and battery/DC bus
Battery charge/Discharge control ensures battery doesn't overcharge or discharge through solar panels
Battery back up power supply, the solar panels will charge the battery
Kill Switch turn off cubesat while in P-Pod for launch
DC-bus to provide subsystems with power
Priority control so that we can turn off inessential systems that use too much power
CPU, Attitude control etc these are our subsystems that need power provided to them

nb. The MPPT is likely to be very useful. It is needed because the solar panels and the battery do not operate at their maximum efficiency at the same voltage. The MPPT solves this problem HOWEVER it draws power itself so we shall need to consider that in our budget calculations.

Solar Cells

These are going to be our source of power, but will compete for space with the nanometeoroid detector so we need to think hard about how to get the best efficiency from them with the minimum area.

Type Symbol Efficiency %
Silicon Si ~16
Gallium Arsenide GaAs ~18
Triple Junction Gallium Arsenide 3GaAs ~24

3GaAs are basically 3 GaAs cells stacked on top of each other to trap more solar energy, they also give a higher output voltage (~2.1V) than the Si cells (~0.6V).

3GaAs do cost more than Si cells but are going to be worth it particularly since we cannot now coat the entire cubesat in cells.

Solar cells are affected by radiation and dust and over a lifetime of several years a reduction of 20% can be expected. Our cubesat is going to have alifetime of ~1year so hopefully we shouldn't see the effects of this too badly.

Also, we have no idea what kind of orbit we're going to get. Some cubesats have been put in helious-synchronous orbit so they get 100% day, but we need to plan for a less advantageous situation. Assume 2/3 of the orbit is in daylight.

Solar Cell Power
Manufacturer
SPECTROLAB

Technology
GaInP2/GaAs/Ge

Type
TASC

Junctions
Triple

String Length
2

No. of Strings
13

BOL Panel Voc at Min Temp
5.88V

BOL Panel Vmpp at Min Temp
5.35V

BOL Panel Vmpp at Max Temp
3.92V

BOL Panel Vmpp at 28C
4.27V

BOL Panel Power at Min Temp
2.41W

BOL Panel Power at Max Temp
1.92W

BOL Panel Power at 28C
2.15W

There will be some further analysis of this later (particularly in view of the current comms design for antenna).

Emissivity/Absorptivity

Absorptivity=0.92
Emissivity=0.85
source: data sheet for spectrolab ITJ cells which Clyde Space say have the same properties as they are made from the same materials

The cells have virtually no mass, so there is no heat capacity. Also, they are so thin that the front of the cell will be almost exactly the same temperature as the rear of the cell. Clyde Space bond the cells to the panel substrate with thermally conductive adhesive (The properties of this will be sent later). The panel substrate is a PCB with a copper layer front and back in order to take the heat away from the cells.

Getting Solar Cells

There are restrictions on buying technology from USA, usually you need to get a permit which can take several months. Not a massive problem if we plan far enough ahead, but many European cubesats are looking for European manufacturers. Clyde Space (www.clyde-space.com) are our best bet, plus they're a British company. They make complete solar arrays which are expensive (£10,000) but they also do separate solar panels.

Weight

This is an estimate from www.spectrolab.com an American company popular for cubesats.

Their solar panels weigh 84mg/cm2

so if we assume that we have 4 sides available for panels (this will probably change)

Total weight= Area of side x no of sides x weight per cm2
=10 x 8 x 4 x 84mg = 26.88g

Batteries

There are different types of battery available for use. Traditionally, NiCad and NiH2 are the most common but Li-Ion are becoming more and more used.

Type Advantages Disadvantages
NiCad Heritage low output voltage ~1.2V, low energy density ~40Wh/kg, memory effects
NiH2 Heritage, no memory effects, higher energy density than NiCad ~60Wh/kg low output voltage ~1.2V
Li-Ion highest energy density ~ 100Wh/kg, 3x the voltage of NiCad and NiH2 ~3.7V, no memory effects, flat discharge profile (ie constant voltage level), wider operating temps, very light ~40g per cell can suffer from overcharging if several are connected together then it explodes

Now in addition to these there are Lithium Polymer batteries which Clyde Space are using in their complete power pack. The ones they're using have an output voltage of 5V-8.4V. They sell these as well as higher capacity ones. We're looking at 55g for the battery though.

Voltage/current monitoring

Another team producing a CubeSat 2 years ago used a DS2762 protection circuit to help protect the battery and circuits from short circuiting etc. To test the protection circuit board, the following steps must be used:

  • Wire DS2762K evaluation board to battery.
  • Connect DS2762K to computer using supplied USB adapter and phone cord.
  • Verify software identifies circuit. Software interface should show values for voltage, current flow, battery capacity and temperature.
  • Place resistors on the breadboard and wire to the circuit.

Such devices can be purchased at:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3950

PSU To-do list

Battery

  • Identify battery- type, cost, company, dimensions
  • Charging/discharging currents, number of cycles etc
  • Test battery

Solar Cells

  • Identify- type, company, cost, dimensions
  • Efficiency- Lucy has a computer model on this
  • Protection of solar cells (against back emf)
  • magnetorquers are available (type, company cost, dimensions, mass, are these worthwhile ADCS?? etc) are these required

PCB

  • identify components-block diagram
  • design MPPT and test -RICH
  • Protection of the circuits- kill switches, diodes etc
  • measuring voltage and current
  • priority control systems- OBDH to design, then integration with the system (PSU-OBDH collaboration)
  • look into the discharge bypass- where should it go?

Useful Links

Clyde Space
www.clyde-space.com

Online data sheet for Clyde Space power system
http://www.clyde-space.com/CubeSat%20Power%20Datasheet.pdf

Complete report for Clyde Space power system
There is also a more complete report about the Clyde Space power system, which I've emailed everyone in the group. If anyone else would like a copy, email me on ku.ca.el|24bar#ku.ca.el|24bar and I'll send it you. I'm not sure how to put files on here.

News article about Clyde Space
http://www.spacedaily.com/reports/Clyde_Space_Gets_SMART_with_Off_the_Sh
elf_CubeSat_Power_Management_Solution.html

Spectrolab
www.spectrolab.com

The Physics of solar cells
Once again a file, emailed to all group members. Email rab42 if you want a copy.

Power Supply report for DTU's cubesat
Another file!

EMCORE- manufacturers of solar panels
www.emcore.com

Azur Space- manufacturers of solar panels
http://www.rwespace.de/

**Maskin Konstruktion - In german but has technical drawings, might be useful
http://www.lcto.dk/downloads.htm

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