Solar Array Sizing (space-systems/subsystems/solar-array-sizing)
Use when sizing the photovoltaic array of a spacecraft EPS: computing the
required array area in square meters from the orbit-average power demand,
the eclipse fraction, the solar cell efficiency, the packing factor, and
the end-of-life degradation over the mission life, and verifying that the
sized array still meets the bus load at end of life.
Domain quick reference
The array generates only in daylight, so the required daylight power
exceeds the orbit-average demand by the daylight fraction factor:
- Required daylight power:
P_day = P_demand / (1 - f_eclipse) * (1 + margin).
Worked: 500 W demand, eclipse fraction 0.35 -> 500 / 0.65 = 769.23 W;
with a 0.20 sizing margin -> 923.08 W.
- End-of-life degradation factor:
D = (1 - r_annual) ** mission_years
(compound annual loss). Worked: 0.02 per year over 10 years ->
0.98 ** 10 = 0.8171.
- End-of-life specific power:
p_eol = G * eta * PF * D, where G is the
solar irradiance (W/m2), eta the cell efficiency, PF the packing factor.
Worked: 1367 W/m2 * 0.30 * 0.85 * 0.8171 = 284.82 W/m2.
- Required array area:
A = P_day / p_eol. Worked: 923.08 / 284.82 =
3.24 m2.
- Second anchor (no margin): 300 W demand, eclipse fraction 0.40,
eta = 0.28, PF = 0.90, r = 0.03, 5 years -> P_day = 300 / 0.60 = 500 W;
D = 0.97 ** 5 = 0.8587; p_eol = 1367 * 0.28 * 0.90 * 0.8587 =
295.82 W/m2; A = 500 / 295.82 = 1.69 m2.
- Reference irradiance: 1367 W/m2 mean solar constant at 1 AU; an array
off-pointed from the sun by angle theta receives G * cos(theta).
All worked numbers are verified by running scripts/solar_array_sizing_logic.py
(see scripts/test_solar_array_sizing.py, behavior contract below).
Workflow
- Collect the inputs: orbit-average power demand (W), eclipse fraction
(0 to <1), solar irradiance (W/m2), cell efficiency (fraction), packing
factor (fraction), annual degradation rate (fraction per year), mission
years, and the array sizing margin (default 0.20).
- Compute the required daylight power with
daylight_power(power_demand_w, eclipse_fraction, array_margin).
- Compute the end-of-life degradation factor with
degradation_factor(annual_degradation, mission_years) and the
end-of-life specific power with eol_specific_power(...).
- Compute the required array area with
required_array_area(power_demand_w, eclipse_fraction, solar_irradiance, cell_efficiency, packing_factor, annual_degradation, mission_years, array_margin).
- Verify the sizing: feed the computed area back through
array_power_available(...) and power_margin(...); a positive margin
means the array meets the daylight demand at end of life. Confirm the
deterministic checks with the contract test.
Pitfalls
- Confusing this leaf with power-thermal-budget: that leaf sizes the
overall EPS power budget and the battery; this leaf sizes the array
area from cell efficiency, packing factor, and degradation.
- Confusing this leaf with communication-link-budget: RF link power,
EIRP, and path loss are unrelated to photovoltaic generation sizing.
- Confusing this leaf with the adcs leaves: sun-pointing computes the sun
vector geometry, attitude-control-sizing sizes reaction wheels, and
magnetorquer-control sizes coils; none of them size the array area.
- Using begin-of-life specific power for an end-of-life requirement:
forgetting the degradation factor D < 1 oversizes the array.
- Omitting the packing factor: the panel area is larger than the bare
cell area, so A without PF is the cell area, not the panel area.
- Using a linear degradation estimate (1 - r * n) instead of the compound
form (1 - r) ** n: the two diverge for long missions.
- Sizing from orbit-average power without the 1 / (1 - f_eclipse) factor:
the array generates only in daylight and must cover the eclipse too.
- Assuming normal-incidence irradiance at all times: off-pointing by
angle theta reduces the received flux to G * cos(theta).
Behavior contract (gate 3)
The sizing, degradation, and margin logic is exercised by the gate 3
contract test: scripts/test_solar_array_sizing.py against
scripts/solar_array_sizing_logic.py (stdlib unittest, offline). Run:
python3 scripts/test_solar_array_sizing.py
Compliance
- Standards referenced, not reproduced: ECSS standards are freely
downloadable (copyright ESA); summary-only per standards-map.yaml
and brief 06.
- compliance: STANDARDS-REF, gated: false.
1---2name: solar-array-sizing3description: Use when sizing a spacecraft EPS solar array or photovoltaic panel: array area, power demand, eclipse fraction, cell efficiency, packing factor, degradation, end of life, solar irradiance. Compute spacecraft solar-array sizing: determine the required photovoltaic array area in square meters from the orbit-average power demand, the eclipse fraction, the solar cell efficiency, the packing factor, and the end-of-life degradation over the mission life, then verify the array margin at end of life. Trigger: solar array sizing, array area, cell efficiency, degradation, power demand, eclipse fraction, photovoltaic.4license: Apache-2.05---67# Solar Array Sizing (space-systems/subsystems/solar-array-sizing)89Use when sizing the photovoltaic array of a spacecraft EPS: computing the10required array area in square meters from the orbit-average power demand,11the eclipse fraction, the solar cell efficiency, the packing factor, and12the end-of-life degradation over the mission life, and verifying that the13sized array still meets the bus load at end of life.1415## Domain quick reference1617The array generates only in daylight, so the required daylight power18exceeds the orbit-average demand by the daylight fraction factor:1920- Required daylight power: `P_day = P_demand / (1 - f_eclipse) * (1 + margin)`.21 Worked: 500 W demand, eclipse fraction 0.35 -> 500 / 0.65 = 769.23 W;22 with a 0.20 sizing margin -> 923.08 W.23- End-of-life degradation factor: `D = (1 - r_annual) ** mission_years`24 (compound annual loss). Worked: 0.02 per year over 10 years ->25 0.98 ** 10 = 0.8171.26- End-of-life specific power: `p_eol = G * eta * PF * D`, where G is the27 solar irradiance (W/m2), eta the cell efficiency, PF the packing factor.28 Worked: 1367 W/m2 * 0.30 * 0.85 * 0.8171 = 284.82 W/m2.29- Required array area: `A = P_day / p_eol`. Worked: 923.08 / 284.82 =30 3.24 m2.31- Second anchor (no margin): 300 W demand, eclipse fraction 0.40,32 eta = 0.28, PF = 0.90, r = 0.03, 5 years -> P_day = 300 / 0.60 = 500 W;33 D = 0.97 ** 5 = 0.8587; p_eol = 1367 * 0.28 * 0.90 * 0.8587 =34 295.82 W/m2; A = 500 / 295.82 = 1.69 m2.35- Reference irradiance: 1367 W/m2 mean solar constant at 1 AU; an array36 off-pointed from the sun by angle theta receives G * cos(theta).3738All worked numbers are verified by running scripts/solar_array_sizing_logic.py39(see scripts/test_solar_array_sizing.py, behavior contract below).4041## Workflow42431. Collect the inputs: orbit-average power demand (W), eclipse fraction44 (0 to <1), solar irradiance (W/m2), cell efficiency (fraction), packing45 factor (fraction), annual degradation rate (fraction per year), mission46 years, and the array sizing margin (default 0.20).472. Compute the required daylight power with48 `daylight_power(power_demand_w, eclipse_fraction, array_margin)`.493. Compute the end-of-life degradation factor with50 `degradation_factor(annual_degradation, mission_years)` and the51 end-of-life specific power with `eol_specific_power(...)`.524. Compute the required array area with53 `required_array_area(power_demand_w, eclipse_fraction, solar_irradiance,54 cell_efficiency, packing_factor, annual_degradation, mission_years,55 array_margin)`.565. Verify the sizing: feed the computed area back through57 `array_power_available(...)` and `power_margin(...)`; a positive margin58 means the array meets the daylight demand at end of life. Confirm the59 deterministic checks with the contract test.6061## Pitfalls6263- Confusing this leaf with power-thermal-budget: that leaf sizes the64 overall EPS power budget and the battery; this leaf sizes the array65 area from cell efficiency, packing factor, and degradation.66- Confusing this leaf with communication-link-budget: RF link power,67 EIRP, and path loss are unrelated to photovoltaic generation sizing.68- Confusing this leaf with the adcs leaves: sun-pointing computes the sun69 vector geometry, attitude-control-sizing sizes reaction wheels, and70 magnetorquer-control sizes coils; none of them size the array area.71- Using begin-of-life specific power for an end-of-life requirement:72 forgetting the degradation factor D < 1 oversizes the array.73- Omitting the packing factor: the panel area is larger than the bare74 cell area, so A without PF is the cell area, not the panel area.75- Using a linear degradation estimate (1 - r * n) instead of the compound76 form (1 - r) ** n: the two diverge for long missions.77- Sizing from orbit-average power without the 1 / (1 - f_eclipse) factor:78 the array generates only in daylight and must cover the eclipse too.79- Assuming normal-incidence irradiance at all times: off-pointing by80 angle theta reduces the received flux to G * cos(theta).8182## Behavior contract (gate 3)8384The sizing, degradation, and margin logic is exercised by the gate 385contract test: scripts/test_solar_array_sizing.py against86scripts/solar_array_sizing_logic.py (stdlib unittest, offline). Run:87python3 scripts/test_solar_array_sizing.py8889## Compliance9091- Standards referenced, not reproduced: ECSS standards are freely92 downloadable (copyright ESA); summary-only per standards-map.yaml93 and brief 06.94- compliance: STANDARDS-REF, gated: false.
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