Power production.

Production calculation, step by step.

Feasibility studies or projecting of hydropower plants are based on anticipated power demand, power prices and run off. No “equation” is available to get an answer to that problem so a trial and error method must be used. Terms as e.g. “Mean year”, “Median year” or “Duration curve” are only included in Alab to assist users having another angle of attack. “Wet year” or “Dry year” is not defined in Alab.

Analysis of production.

An Alab principle is that both production and run off shall be correctly related in time.  This principle will be broken if production is calculated by use of duration curves, mean year or median year data.

Run off minus water consumption a specific day determines level changes and eventually spill. Units will stop if drawdown level is exceeded independent of operation strategy. 

Type of plants.

Five types of power plant are defined in Alab.
                Run of river
                Buffer plant
                Reservoir plant
                Peak Power Plant
                Irrigation plant (During development)

Run of river is characterized by a small intake pond and level control equipment setting turbine stroke. No operation strategy is necessary, power production are related to run off. Run off greater than turbine capacity will go through spillway. All water will be bypassed if the defined bypass flow is greater than run off. Turbines will start if available flow is greater than minimum flow.

Production overview.
Based on run-off tables from NVE.

Buffer plant has an intake dam with a reasonable capacity. Units will be started at an upper level and will operate with an output according to operation strategy until the low level is reached. In case of spilling units will be opened to maximum power. Residual flow are independent of run off which may result in a negative water balance.

Reservoir plant has reservoirs with great capacity; power production is not directly related to run off. Alab will continuously calculate reservoir levels based on run off curves and operation strategy. Intake dam water level will control transfer of water from reservoirs located above intake dam. Overflow and residual flow is taken into consideration; if intake dam draw down level is reached units will be stopped.

Peak Power Plant has reservoirs both in headrace and tailrace and RPT units may be installed. The natural run off is based on day-mean values; operation strategy may use either day-mean or hour-mean values. System conditions are: Overflow from headrace reservoir will discharge into tailrace reservoir. Bypass flow will leave the system. Units will start and stop independent of operation planner if level limits are exceeded.

Irrigation plant is primarily storing water for irrigation purposes. To utilize the potential for power production turbines are installed. Operating strategy defines irrigation flow, prices for water and energy as day-mean values. Alab will continuously calculate water levels and possible spill based on run off and irrigation outflow. Related to intake water level water will be transferred from above located reservoirs. Turbines will run as long as head and available flow are within operation envelop. Residual flow is taken into consideration.

Future runoff.

Available run off data is normally presented as day-mean values, m³/s. It is not taken into consideration if it rains for only one hour each day. The day represents therefore the resolution of the calculation. An Alab project may store runoff data for a period of 100 years, but a continuous period of maximum 30 years can be used for production calculations. The idea is that the selected historical period is estimated to be representative for the future run off, eventually adjusted by a scale factor. Alab will calculate the “Mean year” or “Median year” run off for this period. If run off data are unavailable a run off curve for a single year may be created manually based on size of areas and precipitation – or on data from a neighbor area.

Three types of single years are available:
                Mean year-
                Median year-
                Manually created curve.
In addition a year curve for bypass flow (residual flow) and flow from a controlled source may be created and combined with the cases above.

Operation strategy.

Anticipated yearly power or irrigation demand for the selected period is the base for the Operation strategy defining power and price. Operating strategy is based on day-mean for plants with yearly cycles, for Peak Power Plants it may be actual with day cycles and hour-mean. Operation strategy shall cover all days/hours in a year. A Day-factor defines the part of the day the defined power will be run. Alab will interpolate between available points to fill the tables.

Plant efficiency.

Alab calculates a Plant efficiency taking into consideration all losses in the system. If more than one unit is installed Alab will optimize the load distribution between the units. The actual combination will be selected according to the operation strategy.

Reports.

For reporting day-mean calculations each year may be divided in 12 periods of different length, for hour-mean the number of periods are 52. Year-mean production and total production are calculated and displayed. Tables which are used for curves may mainly be stored as excel-files.

 Reservoir level is displayed for each day during the period (max. 30 years) as curves.

Time of operation for each turbine combination is reported.

Tolerances.

You should note that the power production calculations are based on much estimation and should be considered as a method to show the importance of the different parameters involved.

 Runoff registrations may have margins of 15 – 20 %, future power consumption, political involvement are difficult to estimate so going to deep into details may be waste of time.