--- title: "Solar PV versus hydro and their combination" subject: "Power Plant Design" chapter: 1 key: "semester_8/power_plant_design:solar-pv-hydro-combination" source: https://www.bctengineer.com/q/semester_8/power_plant_design/solar-pv-hydro-combination site: BCT Engineering --- # Solar PV versus hydro and their combination Power Plant Design · Chapter 1: Energy Sources and Electric Power Generation - Asked in: 2075 Bhadra, 7+3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/solar-pv-hydro-combination - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-1 ## Question Compare the operational characteristics of solar photovoltaic power plant with hydropower plant on the basis of reliability, grid connection, efficiency, operating range, maintenance perspectives and so on. Also, discuss the beauty of combination of such two kinds of power project for an electric system. ## Short answer PV has 15–20% capacity factor, inverter-based grid connection with no inertia, 15–22% efficiency, non-dispatchable output, and very low maintenance. Hydro has 50–65% capacity factor, synchronous connection, 85–92% efficiency, 0–100% dispatchability, and higher maintenance. Combining them uses the reservoir as a battery, with solar in dry season and hydro for evening peaks and frequency support. ## Answer A **solar PV plant** converts sunlight directly to DC electricity, delivered to the grid or loads through inverters. A **hydropower plant** converts the energy of water through turbines and synchronous generators. #### Comparison of operational characteristics | Basis | Solar PV plant | Hydropower plant | |---|---|---| | Reliability | Daytime only; cloud dependent; capacity factor about 15–20% | Day and night; RoR seasonal; storage firm; capacity factor about 50–65% | | Grid connection | Inverter based; no inertia; anti-islanding, harmonics, voltage rise issues | Synchronous; inertia, reactive support, frequency control, black-start | | Efficiency | Module about 15–22% | About 85–92% | | Operating range | Not dispatchable; output set by irradiance; curtail only | 0–100% dispatchable via governor; fast ramp | | Maintenance | Very low; module cleaning, inverter replacement every 10–15 years | Turbine, gates, civil works; sediment erosion; skilled staff | | Construction time | Months | Years | | Capital cost | Low, modular | High | | Life | About 25 years | 50+ years | | Location | Near load, rooftops, flat land | Fixed by river and head | #### Beauty of combining solar PV and hydro 1. **Seasonal complement** – in Nepal, river flows are low in the dry season (winter, spring), when skies are clear and solar output is high; in the monsoon, hydro is abundant. 2. **Daily complement** – solar supplies daytime energy; the hydro reservoir saves water during the day and releases it for the **evening peak**. The reservoir acts as a large, cheap "battery". 3. **Smoothing variability** – fast hydro governors balance sudden changes in solar output due to clouds, keeping frequency stable. 4. **Grid strength** – synchronous hydro machines provide inertia and voltage support that inverter-based PV lacks. 5. **Shared infrastructure** – floating PV on hydro reservoirs uses the same substation and transmission line; less land needed and less evaporation. 6. **Better economics** – higher utilisation of transmission lines and lower need for battery storage. ```text Power | solar | /^^^^\ hydro raised | / \ for evening peak |-----/--------\-----------/^^^\---- | hydro (held back by day) \__ +---------------------------------> hour 0 6 12 18 24 ``` Such a **hybrid** system gives reliable, clean and low-cost supply. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a solar PV plant need more care for grid connection than a hydropower plant? A. It is inverter based and gives no inertia, with harmonics and voltage rise issues B. It uses synchronous generators that are hard to synchronise C. It must be built far from load centres D. It produces AC at a variable frequency Answer: A. PV is connected through inverters, so it has no rotating inertia and can cause harmonics and voltage rise, while hydro uses synchronous machines that give inertia and reactive support. (Key point: Grid connection difference) 2. Which statement about operating range is correct? A. PV is dispatchable from 0-100% by a governor B. Hydro is dispatchable 0-100% through the governor, while PV output is set by irradiance and can only be curtailed C. Both plants can only be curtailed D. Hydro output is fixed by sunshine Answer: B. Hydro governors give fast, controllable ramping; PV is not dispatchable and output follows irradiance. (Key point: Operating range) 3. How does a hydro reservoir complement solar PV over the day? A. It stores sunlight as heat B. It stops generation at night to save the grid C. It holds back water during the day and releases it for the evening peak, acting like a cheap battery D. It runs at full load during midday to match PV Answer: C. Solar supplies daytime energy while hydro saves water and releases it for the evening peak. (Key point: Daily complement) 4. Why do solar and hydro complement each other seasonally in Nepal? A. Both peak in the monsoon B. River flow is low in the dry season when skies are clear and solar output is high C. Solar is strongest in the monsoon when rivers are low D. Hydro is strongest in winter Answer: B. Dry-season low river flow coincides with clear skies and high solar, while monsoon gives abundant hydro. 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