Telecom tower microgrid · 60 kW BESS

The site and the problem

A telecom tower site in Ladakh at 3,800 m altitude operated in an off-grid regime: the nearest DISCOM connection was 8 km away, making grid extension economically infeasible. The site relied on a 40 kVA diesel generator for base load (15 kW continuous for radio transmission) and a 30 kW solar array for daytime top-up. During summer (May-August), diesel burn was minimal. During winter (October-March), solar generation collapsed to 30-40% of summer output due to cloud cover, ice on panels, and low sun angle. A 60 kVA DG set ran 16-18 hours per day during December-February, consuming 300-350 L of diesel monthly.

The core challenge was logistics. Diesel delivery to a 3,800 m site in Spiti Valley meant 2-day truck hauls from Shimla, costing ₹95-110/L. Pilferage was endemic (local herders sometimes siphoned fuel during winter road blockages). The tower operator also faced altitude-related DG inefficiency: air density is 60% of sea level, degrading combustion and requiring larger air intakes that froze in winter.

Sizing the system

We modelled 18 months of solar irradiance and diesel burn data. Summer solar (400-500 Wh/m2/day) could run the 15 kW base load for 26-28 hours, implying a 30 kW solar array was undersized for winter autonomy goals. A 60 kW BESS (IZ-60K-3P) with 120 kWh usable capacity was sized to bridge the winter valley. The hypothesis: 80 kWp PV (a 2.7x increase in summer capacity) generates 120-140 kWh on a clear winter day. The BESS absorbs this midday overage and releases it at 15 kW over 8 hours (night operations). This reduces DG burn to 8-10 hours daily in winter, versus the previous 16-18 hours.

Capex was ₹38 lakh for the BESS + ₹65 lakh for additional PV panels. Payback hinged on diesel displacement: at ₹100/L, saving 250 L/month (winter 5-month season) meant ₹1.25 crore fuel savings over 10 years, yielding a 3.8-year payback. Altitude cooling was a bonus: 3,800 m ambient averages -8 to 18°C year-round, meaning the LFP battery operates in its optimal thermal window (10-35°C) for 7-8 months per year.

Engineering details

The IZ-60K-3P is a modular cabinet housing LiFePO4 cells in 48 V, 100 Ah strings. The system was built as a three-phase islanded microgrid: the 80 kWp solar array connects to a string inverter (Fronius Primo 10), which charges the BESS via AC coupling. The DG remains hard-wired as a backup, with an automatic transfer switch (ATS) configured to start the DG only if battery SOC drops below 15% over 30 consecutive minutes (avoiding false triggers from brief cloud cover).

The site presented two extreme climate challenges. Summer ambient swings from -5°C at dawn to 28°C at noon. Winter nights routinely hit -15°C. The BESS cabinet included a 2 kW immersion heater (thermostat controlled) to maintain cell temperature above 0°C during winter nights, preventing lithium plating and low-temperature voltage sag. Cooling was passive: the cabinet's large surface area dissipates heat to the thin 3,800 m air, needing no active compressor.

Core technical specifications for the altitude deployment:

• LFP chemistry, 6,000-cycle life (rated to -15 to 50°C ambient, though optimum is -5 to 35°C)
• Immersion heater (2 kW, thermostat at -2°C cutoff) for winter cold-soak protection
• 60 kW three-phase AC output at 415 V, 50 Hz, island-mode capable
• Modbus gateway to Fronius inverter for real-time solar telemetry
• DG ATS logic: start DG only if SOC <15% sustained for 30 minutes, preventing flicker from cloud cover
• Passive cooling; no active thermal management needed at altitude

What changed after commissioning

Winter DG runtime fell 82%, from 16-18 hours/day to 3-4 hours/day. Monthly diesel consumption during the 5-month winter season (October-March) dropped from 300-350 L to 40-60 L, a 85% reduction. Annual diesel cost fell from ₹4.2 lakh to ₹0.8 lakh. Site autonomy improved: the tower operator could now predict DG-free operation during November and January (when solar is weaker but not absent), and only run DG during December-February peaks.

Solar fraction reached 86% (calculated as annual renewable generation divided by total load). This meant 86% of the year's 15 kW base load was powered by PV + BESS, with DG filling only the deepest winter gap. The site also eliminated diesel pilferage risk (no stored fuel means no theft). Maintenance burden dropped: the DG ran only 500-600 hours annually instead of 5,000 hours, extending overhaul intervals from 3 years to 5+ years.

Lessons we carried into the next deployment

Remote altitude sites operate under unique constraints. Subsequent remote microgrid builds incorporated three key learnings.

• Altitude-driven efficiency loss is real and scales non-linearly. A 40 kVA DG at sea level produces 40 kW. At 3,800 m, derate is roughly 35%, yielding only 26 kW effective output. We now model altitude-specific power curves (available from engine manufacturers) rather than nameplate ratings. This changes BESS sizing dramatically.
• Cold-soak battery management is non-negotiable in Himalayan sites. LFP cells below 0°C see rapid voltage sag and plating risk. Our 2 kW immersion heater costs ₹0.8 lakh but prevents catastrophic cell failure. Subsequent designs budget 2% of BESS capex for low-temp conditioning.
• DG ATS logic must ignore cloud cover flicker. Our first iteration triggered DG start on every 5-minute cloud shadow. The 30-minute sustained-low-SOC rule eliminated nuisance starts. For altitude sites, we now default to 30-45 minute windows, not 5 minutes.