8.2 Power and Energy Potential Study

8.2 Power and Energy Potential Study

Introduction to Power and Energy Potential

The core of hydropower planning lies in accurately quantifying a river's Power (instantaneous capacity) and Energy (total work done over time) potential. This study transforms raw hydrological data into actionable engineering and economic parameters. It determines the optimal installed capacity of a plant, guides the selection of the most suitable plant type, and forms the basis for financial viability assessments and Power Purchase Agreements (PPAs). This section details the methodologies for calculating potential, selecting capacity, classifying plant types, understanding their components, and the critical role of reservoir regulation.

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1. Power and Energy Potentials

1.1 Fundamental Concepts and Equations

1. Hydraulic Power (Theoretical Power):

   • The raw power available in the flowing water before accounting for any losses.

   • Formula: $P_{theoretical} = \rho g Q H$ Where,

     • $P$ = Power (Watts)

     • $\rho$ = Density of water ($\approx 1000 \ \text{kg/m}^3$)

     • $g$ = Acceleration due to gravity ($\approx 9.81 \ \text{m/s}^2$)

     • $Q$ = Flow rate ($\text{m}^3/\text{s}$)

     • $H$ = Net head (m) (Gross head minus hydraulic losses)

2. Firm (Primary) Power & Energy:

   • The power and corresponding energy guaranteed to be available 100% of the time (or a very high percentage, e.g., 95%) based on historical flow records.

   • Derived from the minimum reliable flow (often the flow exceeded 95% of the time, $Q_{95}$).

   • Significance: Forms the "must-run" or baseload portion of a PPA; is critical for grid stability.

3. Secondary (Seasonal) Power & Energy:

   • The additional power available during high-flow seasons (monsoon) beyond the firm power.

   • Significance: Increases the plant's annual energy output and economic value but is not guaranteed year-round.

4. Peak Power:

   • The maximum power a plant can deliver for short durations, often utilizing storage.

1.2 Key Hydrological Tools for Assessment

1. Flow Duration Curve (FDC):

   • A plot showing the percentage of time (probability) that a given flow rate is equaled or exceeded in a historical period.

   • X-axis: Percentage of time flow is equalled or exceeded ($\%$).

   • Y-axis: Flow rate ($Q$ in $\text{m}^3/\text{s}$).

   • Primary Use:

     • To read $Q_{95}$, $Q_{50}$ (median flow), $Q_{20}$, etc.

     • To calculate Firm Energy (area under FDC to the right of $Q_{95}$) and Secondary Energy (area between $Q_{95}$ and plant design flow, $Q_d$).

2. Mass (Cumulative) Curve:

   • A plot of cumulative volume of water (or flow) against time.

   • Primary Use: For reservoir sizing and regulation studies. The slope of the line represents the inflow rate.

3. Sequential Streamflow Data Analysis:

   • Analyzing long-term daily or monthly flow sequences to simulate plant operation and energy generation over dry and wet years.

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2. Methods of Fixing Installed Capacity

Installed Capacity ($P_{inst}$) is the maximum net electrical output a plant's generators can produce under specific conditions. Its selection is an economic optimization, not just a hydrological one.

2.1 Key Parameters

• Design Flow ($Q_d$): The flow rate for which the turbine is designed to operate at maximum efficiency. It is a key decision variable.

• Plant Load Factor (PLF): Ratio of actual energy generated in a period to the maximum possible energy if the plant ran at full capacity all the time. $PLF = \frac{\text{Actual Annual Energy (kWh)}}{P_{inst} (\text{kW}) \times 8760 \ (\text{hours})}$

• Capacity Factor (CF): Often used synonymously with PLF. It reflects the utilization of the plant.

2.2 Selection Methods

1. Based on Flow Duration Curve (FDC Area Method):

   • Plot the FDC. Calculate the energy corresponding to different candidate design flows.

   • The relationship is: $Energy \propto \int_{0}^{Q_d} H \cdot Q \cdot dt$

   • Choose the $Q_d$ (and corresponding $P_{inst} = \eta \rho g Q_d H$) that maximizes the net economic benefit (value of energy minus cost of turbine/plant). Typically, $Q_d$ is selected between $Q_{30}$ and $Q_{60}$ for run-of-river projects.

2. Load Factor Method:

   • A target PLF is chosen based on the type of plant and market needs (e.g., baseload plants target high PLF >60%, peaking plants accept lower PLF ~20-40%).

   • $P_{inst}$ is then back-calculated from the estimated annual energy (from FDC) and the target PLF. $P_{inst} = \frac{\text{Estimated Annual Energy (kWh)}}{PLF \times 8760}$

3. Peak Power Demand Matching:

   • For plants designed specifically to meet peak demand in a grid.

   • $P_{inst}$ is set based on the system's peak load requirement and the plant's ability to ramp up quickly (a feature of storage plants).

4. Optimization Software Models:

   • Use software (e.g., HEC-ResSim, MODSIM) to simulate operation over many years.

   • The model iteratively tests different $P_{inst}$ and $Q_d$ values to maximize a financial objective function (NPV or IRR), considering variable tariff rates, PPA structures, and construction costs.

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3. Types of Hydropower Plants on Various Bases

Hydropower plants can be classified based on several criteria:

3.1 Based on Water Flow Regulation (Most Common Basis)

1. Run-of-River (RoR) Plant:

   • Definition: Has little or no storage capacity. Power generation directly depends on the instantaneous river flow.

   • Characteristics:

     • Requires a diversion weir/barrage (not a high dam).

     • Includes a desilting chamber to remove sediment.

     • High PLF during monsoon, low PLF in dry season.

     • Lower environmental impact (minimal inundation).

     • Common in Nepal (e.g., most projects < 100 MW).

2. Storage (Reservoir) Plant:

   • Definition: Features a large dam creating a reservoir to store water, allowing decoupling of inflow from power generation.

   • Characteristics:

     • Can regulate flow for year-round power, irrigation, flood control.

     • Can operate as a peaking plant.

     • Higher capital cost, significant social and environmental impact due to inundation.

     • Example: Kulekhani I & II (Nepal).

3. Pumped Storage Plant (PSP):

   • Definition: A type of storage plant that pumps water from a lower reservoir to an upper reservoir during off-peak hours (using cheap surplus power) and generates power during peak hours.

   • Characteristics:

     • Acts as a giant battery for the grid.

     • Net consumer of energy but provides valuable peaking power and grid stability.

     • High round-trip efficiency (~70-80%).

3.2 Based on Plant Capacity

• Large Hydro: > 100 MW (e.g., Upper Tamakoshi: 456 MW).

• Medium Hydro: 25 - 100 MW.

• Small Hydro: 1 - 25 MW (common private sector domain in Nepal).

• Mini Hydro: 100 kW - 1 MW.

• Micro Hydro: 5 - 100 kW (off-grid rural electrification).

• Pico Hydro: < 5 kW.

3.3 Based on Hydraulic Head

• High Head: > 100 m. Uses Pelton or Turgo turbines. Shorter water conveyance systems.

• Medium Head: 30 - 100 m. Uses Francis turbines.

• Low Head: < 30 m. Uses Kaplan or Bulb turbines. Requires large flow rates.

3.4 Based on Location of Powerhouse

• Surface Powerhouse: Conventional, on the surface.

• Underground Powerhouse: Excavated inside a mountain. Advantages: better stability, protection from avalanches/rockfall, saves surface land. Used in steep terrains (common in Himalayan projects).

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4. Components of Different Types of Hydropower Projects

4.1 Common Components for All Types

1. Civil Structures:

   • Weir/Barrage/Dam: Diverts or stores water.

   • Intake Structure: Entry point to the water conveyance system, with trash racks.

   • Desilting Basin (for RoR): Settles out abrasive sediments to protect turbines.

   • Headrace System: Conveys water from intake to powerhouse (can be a canal, tunnel, or pipeline).

   • Surge Chamber/Shaft: A standpipe or tank that absorbs pressure surges (water hammer) during sudden valve/turbine closure.

   • Penstock: The final, pressurized pipeline that delivers water to the turbine.

   • Powerhouse Building: Houses turbines, generators, and control equipment.

   • Tailrace: Channel returning water to the river.

2. Electro-Mechanical Equipment:

   • Turbine: Converts hydraulic energy to mechanical energy (Pelton, Francis, Kaplan).

   • Generator: Converts mechanical energy to electrical energy.

   • Governor: Controls turbine speed/power output.

   • Transformer: Steps up voltage for transmission.

   • Switchyard: For switching, protection, and connection to the grid.

4.2 Specific Components by Plant Type

1. Run-of-River (RoR):

   • Key Feature: Desilting Basin is critical.

   • Headrace: Often a long tunnel to achieve head, especially in mountainous regions.

   • Usually lacks a large dam and reservoir.

2. Storage Plant:

   • Key Feature: Large Dam (earth-fill, rock-fill, concrete) and Reservoir.

   • Spillway: Essential safety structure to release excess floodwater.

   • Bottom Outlet: For reservoir flushing and low-level releases.

3. Pumped Storage Plant (PSP):

   • Key Features: Upper and Lower Reservoirs, Reversible Pump-Turbine units (can act as both turbine and pump), and Motor-Generator units.

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5. Reservoirs and Their Regulation

5.1 Purpose and Functions of a Reservoir

1. Primary: Flow Regulation – To store water in wet periods and release it in dry periods for power generation.

2. Secondary:

   • Flood Control: Provides attenuation of flood peaks.

   • Irrigation & Water Supply: Provides regulated water releases.

   • Recreation & Tourism: Creates lakes for boating, fishing.

   • Sediment Trapping: (Often negative) Reduces downstream sediment load but leads to reservoir sedimentation.

5.2 Key Terms and Levels

• Full Reservoir Level (FRL) / Maximum Water Level (MWL): The highest level to which the reservoir water surface is allowed to rise under normal operating conditions.

• Minimum Drawdown Level (MDDL) / Minimum Operating Level (MOL): The lowest level to which the reservoir is drawn down under normal operation for power generation. Below this, turbine efficiency drops or intake cavitation occurs.

• Dead Storage Level (DSL): The level below which water cannot be evacuated by the normal outlet (power intake). The volume between bed and DSL is dead storage, reserved for sediment accumulation.

• Live/Active Storage: The volume between MDDL and FRL, used for operational regulation.

• Gross Storage: Total volume up to FRL.

• Useful/Live Storage: Volume between DSL and FRL (Gross - Dead Storage).

5.3 Reservoir Regulation (Operation Policy)

The set of rules governing how water is stored and released.

1. Rule Curve (Guide Curve):

   • A pre-defined graph or table showing the target reservoir level for each day or week of the year.

   • Purpose: To balance competing objectives (power generation, flood control, irrigation) throughout the annual hydrologic cycle.

   • Components:

     • Upper Rule Curve: Dictates when to start spilling water to create flood storage space before the monsoon.

     • Lower Rule Curve: Ensures enough water is conserved to meet dry-season demand.

2. Types of Regulation:

   • Seasonal (Annual) Regulation: Large reservoirs store monsoon flows for use over the entire dry season. Example: Kulekhani.

   • Short-Term (Diurnal/Weekly) Regulation: Storage is used to shift generation within a day or week to match peak electricity demand (peak shaving). Water is stored at night (low demand) and released during the day (high demand).

   • Long-Term (Over-year) Regulation: Very large reservoirs can store water from wet years for use in subsequent dry years.

3. Mass Curve (Rippl Diagram) Method for Sizing Storage:

   • Objective: To determine the reservoir capacity required to meet a specified demand pattern from a given inflow sequence.

   • Procedure:

     1. Plot cumulative inflow vs. time (inflow mass curve).

     2. Plot cumulative demand (draft) vs. time (demand line).

     3. The maximum vertical departure of the inflow curve below the demand line represents the required storage capacity to meet that demand without failure.

5.4 Reservoir Sedimentation

• Problem: Inflowing sediments settle in the reservoir, reducing live storage capacity over time ("siltation").

• Impact: Reduces project life and energy generation.

• Mitigation Strategies:

  • Watershed Management: Reduce erosion in the catchment.

  • Sediment Routing/Flushing: Design outlets to pass sediment-laden floods through the reservoir.

  • Mechanical Dredging: Expensive but sometimes necessary.

• Trap Efficiency: The percentage of incoming sediment trapped by the reservoir (high for large, deep reservoirs; lower for run-of-river schemes).

Conclusion: A comprehensive power and energy potential study is the engineering and economic blueprint of a hydropower project. It moves from fundamental hydraulic calculations through the strategic selection of plant capacity and type, to the detailed design of components and the sophisticated operation rules for reservoirs. Mastering these interconnected elements is essential for developing projects that are not only technically sound and financially viable but also optimized for their role within the broader energy system and water resource management framework.