Off-Grid Microgrid for Remote Health Clinic in Northern Kenya

Case Study: Off-Grid Microgrid for Remote Health Clinic in Northern Kenya

Scenario

A solar-diesel hybrid microgrid is being deployed for a 24/7 health clinic in Marsabit County, Kenya — an arid, off-grid region with unreliable fuel supply and frequent dust storms. The clinic’s critical loads (refrigerated vaccine storage, LED lighting, medical devices) demand stable 230 V AC power. Due to limited space and transport constraints, only compact, high-efficiency inverters can be shipped. The design must support peak AC demand of 42 kVA with unity power factor for medical equipment, but the chosen inverter’s datasheet specifies 94% weighted efficiency and 0.95 power factor capability (though system-level PF is constrained to 0.85 due to legacy AC loads). A 15% derating is applied for dust-induced cooling reduction and infrequent maintenance access.

Given Data

• AC Output Power (p_ac): 42 kVA
• Inverter Efficiency (eff): 94% → 0.94
• Power Factor (pf): 0.85 (conservative system-level value, lower than inverter capability due to mixed load profile)
• Derating Factor (derating): 0.85

Calculation

Using the tool’s formula:

p_dc = (p_ac × 1000 × pf) / (eff × derating)
     = (42 × 1000 × 0.85) / (0.94 × 0.85)
     = (35,700) / (0.799)
     ≈ 44,681 W = 44.68 kW

Note: The pf term appears in both numerator and denominator implicitly cancels only if PF were 1.0 — but here, since pf = 0.85, it remains explicit in the numerator (real AC power) and does not cancel; the denominator reflects combined efficiency and derating losses on real power conversion.

Result and Decision

The required DC input power is 44.68 kW, guiding selection of a single 50-kW hybrid inverter (e.g., Victron Quattro 48/10000) with integrated battery charging, generator control, and robust IP65 enclosure. Its 50-kW DC input rating comfortably covers the 44.68 kW requirement while leaving headroom for short-term surge loads (e.g., centrifuge startup) and 2-year PV degradation (~0.5%/yr). Diesel generator is sized separately as backup only.

Lesson

In off-grid systems, inverter derating must reflect operational reality, not just datasheet specs — dust, infrequent cleaning, and lack of active cooling maintenance justify aggressive derating (here 15%). Never assume inverter PF capability equals system PF; always use the worst-case expected load PF for sizing.