LUNAR ROVER

POWER SYSTEM

In remote systems – especially space systems – the available power to each of the components is at a premium. Without the luxury of the power grid, remote systems are forced to either carry heavy, limited-lifetime batteries or rely upon locally generated power. Because of the mass-dependent expense of launches, the majority of space-based systems utilize solar power to provide electricity. Solar cells produce electricity for the lifetime of a mission and are even more efficient than on Earth due to the lack of an atmosphere. Currently, silicon solar cells dominate the commercial market, providing a cheap, readily available supply. Sunpower produces some of the most efficient, affordable photovoltaic (PV) panels with cell efficiencies near 22%. Silicon cells offer the advantage of low cost, but other technologies are more efficient. For example, gallium-arsenide cells, with a wider, direct bandgap, can offer a substantial efficiency improvement depending on the complexity of the cell design. A leading producer of GaAs PV panels for space applications, Surrey Satellite Technology offers a triple junction GaAs/Ge solar cell with 28% efficiency. Both technologies were considered for the rover power system.

Design Parameters

Incident Light Intensity

I₀ = 1353 W/m²

Incident Angle Factor

cos θ_avg = 0.707

PV Panel Area

A = 0.2016 m²

Table 1. PV System Design Parameters

As the cheaper option, silicon solar cells were considered first. In addition to a lower nominal efficiency, silicon cell performance degrades at higher operating temperatures. After taking the lunar surface temperature into account, the rover’s PV panel would only produce 30 W of power using silicon solar cells, which is not enough to power the onboard systems. Current silicon PV systems cost between $3/W and $4/W, so a conservative estimate of $5/W prices this system at $155.

Silicon PV Panel Characteristics

Nominal Efficiency (25ºC)

η₀ = 22 %

Temperature Loss (100ºC)

5.5%

Panel Wiring Loss

0.5%

Operating Efficiency

η = 16%

Average Output Power

30.9 W

Cost

$155

Table 2. Silicon Cell Parameters

In order to obtain enough power for the various systems onboard the rover, the triple junction GaAs/Ge solar cells were required. These cells provide an additional benefit because GaAs cells are only about half as sensitive to temperature as their silicon counterparts. After working through the calculations, the GaAs/Ge cells will provide over 47 W of power. Unfortunately, the cost of this system is difficult to estimate; however, a rough estimate of $20/W gives a complete panel cost of approximately $1000.

Triple Junction GaAs/Ge PV Panel Characteristics

Nominal Efficiency (25ºC)

η₀ = 28 %

Temperature Loss (100ºC)

3%

Panel Wiring Loss

0.5%

Operating Efficiency

η = 24.5%

Average Output Power

47.2 W

Cost

$1000

Table 3. GaAs/Ge Cell Parameters

By choosing the more efficient GaAs/Ge solar cells, the rover’s electronics and motors can be powered with enough left over to transmit the gathered data back to either the landing station or directly to Earth. The final breakdown of the power system is as follows:

Component

Power (W)

Wheel Motors

18

Transmit Power

15

CPU/Control Electronics

10

Encoder

0.4

Cameras

2

Proximity Sensors

0.8

Wiring Losses

1

Total

47.2