⚡ ISS Power System
ISS consumes 84-120 kW continuously - equivalent to 40-50 homes. Solar power is the only practical energy source in orbit.
Power Statistics
- Peak Generation: 120 kW
- Average Usage: 84 kW
- Solar Array Area: 2,500 m²
- Number of Arrays: 8 arrays (4 pairs)
☀️ Solar Arrays
Solar Array Wings (SAW)
- Size: 34m length × 12m width per wing
- Total cells: 262,400 silicon cells across 8 wings
- Cell efficiency: ~14-15%
- Output voltage: 160V DC
- Power per wing: up to 32.8 kW (when perpendicular to sun)
Sun Tracking
Arrays continuously rotate to track the sun using Beta Gimbal Assemblies:
- Alpha Gimbal: Continuous rotation along truss axis
- Beta Gimbal: ±180° rotation perpendicular to truss
- Tracking accuracy: ±0.5°
- Rotation rate: 4° per minute
ISS completes 16 orbits daily, experiencing 16 sunrises and sunsets. Arrays constantly adjust to maximize sunlight exposure.
iROSA Upgrade Arrays
New ISS Roll-Out Solar Arrays installed starting 2021:
- Size: 19m × 6m (smaller than original)
- Efficiency: ~30% (double original)
- Installation: Partially cover existing arrays
- Additional power: +20 kW per channel
- Design life: 15 years
- Roll-out design enables compact transport
🔋 Battery System
Batteries store energy during daylight (~45 min) for use during darkness (~45 min).
| Type | Nickel-Hydrogen (Old) | Lithium-Ion (New) |
|---|---|---|
| Total Count | 48 batteries | 24 batteries |
| Capacity Each | 81 Ah | 105 Ah |
| Mass Each | 375 kg | 195 kg |
| Lifespan | 6.5 years | 10 years |
| Efficiency | ~70% | ~90% |
| Total Storage | 48 kWh | 48 kWh |
Battery charge/discharge cycle occurs ~5,800 times annually - 16 times daily.
⚡ Power Distribution
Distribution Architecture
- Primary Power (160V DC): Solar arrays → Sequential Shunt Unit (SSU)
- SSU dissipates excess power as heat
- Manages battery charging current
- Secondary Power (160V DC): Main Bus Switching Unit (MBSU)
- Manages 8 power channels
- Fault detection and automatic switching
- Tertiary Power (120V DC): DC-to-DC Converter Unit (DDCU)
- Converts 160V to 120V
- US segment standard voltage
- User Power: Power Distribution Units in each module
- Converts to 28V, 120V as needed
- Supplies individual equipment
Power Management
- Total wiring: ~13 km
- Power channels: 8 independent channels
- Redundancy: Dual power sources for critical systems
- Protection: Overcurrent, short circuit, overvoltage protection
- Monitoring: Real-time power usage tracking
❄️ Thermal Control System
Space temperature extremes: +121°C (sunlit) to -157°C (shadow). ISS maintains internal 18-27°C.
Heat Sources
- Electrical equipment: ~70 kW (largest source)
- Solar radiation: Absorbed on sunlit surfaces
- Human bodies: ~100W per person
- Experimental equipment: Variable but significant
Active Thermal Control System (ATCS)
External ATCS (Ammonia-based)
- Coolant: Anhydrous ammonia (NH₃)
- Loops: 2 independent loops
- Total ammonia: ~1,000 kg
- Operating temp: -20°C to +20°C
- Flow rate: ~258 L/min
- Radiator area: ~1,200 m²
Heat Exchange Process
- Internal Loop (water-based): Absorbs heat from equipment, non-toxic water coolant, temp 4-60°C
- Heat Exchanger: Transfers heat between water and ammonia loops (physically separated for safety)
- External Loop (ammonia): Ammonia carries heat to radiators, radiates heat to space, cooled ammonia returns
Radiators
- Total area: ~1,200 m² (three tennis courts)
- 14 panels on P1, S1, P3, P4, S3, S4 truss segments
- Panel size: ~1.7m × 10.7m each
- Cooling capacity: up to 70 kW
- Surface: Reflective silver Teflon coating
- Rotatable to control sun exposure
Passive Thermal Control
- Insulation: Multi-Layer Insulation (MLI) blankets
- Surface coatings: Specialized paints (reflective or absorptive)
- Heat pipes: Passive heat transfer devices
- Insulated windows: Quadruple-pane glass minimizes heat loss
🔧 Maintenance and Upgrades
| Component | Design Life | Replacement Method |
|---|---|---|
| Solar Arrays | 15-20 years | EVA or robotic arm |
| Batteries | 10 years | EVA or robotic arm |
| Pump Modules | 10 years | EVA |
| Control Units | 10-15 years | Internal replacement |
| Radiators | 20+ years | Generally no replacement needed |
Next chapter explores daily life aboard the space station.