Off-Grid Clinic Power System in Rural Kenya

Off-Grid Clinic Power System in Rural Kenya

Scenario A solar microgrid was designed for a rural health clinic in Kitui County, Kenya — an arid region with unreliable grid access and frequent power outages. The clinic operates 8 hours/day (6 AM–2 PM) and must maintain refrigeration for vaccines during nights and weekends. Constraints included limited roof space (max 25 m²), budget cap of USD $4,500, and requirement for ≥3 days of autonomy due to seasonal dust storms reducing irradiance.

Given data

• Load Power (p_load): 620 W (LED lighting, vaccine fridge, diagnostic devices, laptop)
• Load Time (t_load): 8 h/day
• Days of Autonomy (days_autonomy): 3
• Battery Voltage (v_battery): 24 V
• Depth of Discharge (dod): 50 % (to extend flooded lead-acid battery life in high ambient temps)
• Average Daily Solar Irradiance (h_sun): 5.2 kWh/m²/day (measured from NASA POWER dataset for Kitui)
• Performance Ratio (pr): 0.78 (accounting for high ambient temps >35°C degrading panel output and soiling from red dust)
• System Losses (l_system): 18 % (due to long DC cable runs and undersized charge controller in initial design iteration)

Calculation

1. Daily Energy Demand (e_daily) = p_load × t_load = 620 W × 8 h = 4,960 Wh
2. Total Energy Requirement (e_total) = e_daily × days_autonomy = 4,960 Wh × 3 = 14,880 Wh
3. Battery Capacity (c_battery) = e_total ÷ (v_battery × (dod/100)) = 14,880 Wh ÷ (24 V × 0.5) = 14,880 ÷ 12 = 1,240 Ah → Selected two 24 V, 600 Ah deep-cycle AGM batteries in parallel (1,200 Ah usable; 3.3% margin shortfall accepted for cost control).
4. Recommended PV Array Size (p_pv) = e_daily ÷ (h_sun × pr × (1 − l_system/100)) = 4,960 Wh ÷ (5.2 kWh/m²/day × 0.78 × 0.82)
   = 4,960 ÷ (5.2 × 0.78 × 0.82) ≈ 4,960 ÷ 3.32 ≈ 1,494 Wp → Rounded up to 1,560 Wp (13 × 120 W monocrystalline panels) to accommodate future load growth and ensure winter margin.

Result and decision A 1.56 kWp rooftop array (13 × 120 W panels, tilted at 15° on east-west split mounting), paired with 2 × 24 V / 600 Ah AGM batteries and a 3 kW MPPT charge controller, was installed. System commissioning confirmed 98% of daily demand met year-round, including during 4-day dust events (autonomy held at 3.1 days). Total cost: $4,420.

Lesson Always derate the performance ratio for high-temperature, high-soiling environments — using the default PR=0.8 in Kitui would have undersized the array by ~6%, risking chronic undercharging and battery sulfation.