Technical Report and Model Presentation.
2. Methodology
3. Estimating the Technical Hydropower Potential
4. Estimating the Economically Viable Hydropower Potential
5. Results and Post-Processing
Conclusion
Sources
Water courses suitable for hydropower generation have sustainable and ideally high flow rates as well as steep gradients between intake and powerhouse creating the necessary head. Hydropower facilities require a diversion dam to direct water from a stream into the hydraulic system that conveys the water to a powerhouse. Turbines and generators convert potential energy into electricity before the water returns to the stream.
To locate hydropower opportunities, reliable elevation data, information on land-use and land-cover as well as precipitation data is required. Remote sensing data products can be used to derive a digital elevation model (DEM) of the study area and to classify vegetation structures. Information about precipitation may be obtained from global climate databases, if local precipitation data is not available. Local discharge measurements and high accurate elevation data will increase the accuracy of the outputs.
Several data products with different accuracy levels can be used when applying the hydroMinds model. Even minimum standard data provide a good overview and allow comparing river sections as of their estimated hydropower potential. More accurate data products along with local data and expertise may lead to first capacity appraisals of identified sites.
The grid points represent cells with a square shape with the grid point being located in the center of the cell. All relevant spatial information is joined to the grid points according to its spatial location. The attribute data of all grid points is transferred to a SQL-database serving the computer-based analysis and decision-making tool to estimate the technically and economically viable hydropower potential.
- Digital Terrain Analysis The objective of the Digital Terrain Analysis is to identify catchment boundaries and to model the topographic characteristics of the catchment as well as the resulting stream network.
The GIS-based terrain analysis is subject to the presumption that the direct runoff of any given cell flows downhill in the direction of the greatest slope. To allow all cells of the input DEM-data draining downhill, the elevation model is cleared of errors such as surface depressions, which would act as water sinks. >/p>
To calculate the flow direction for each grid point, the deterministic 8 (D8) algorithm is applied [4]. According to the flow direction of all cells, each grid point is assigned a value corresponding to the number of cumulated cells flowing to it [8]. Cells with no inflow correspond to the pattern of ridges and form catchment boundaries.
To introduce a lower boundary for the calculation of the hydropower potential, a minimum hydraulic head and a minimum area to accumulate runoff water are defined. The minimum size of the hydrologic catchments is set to 4.5 km² which allows a sensible minimum flow accumulation. The data processing routine recognizes only those grid points as “river” that are connected to at least the minimum catchment size.
All “river” grid points of each catchment are joined to form the primary river of the respective catchment. All other grid points which are not defined as water courses are assigned information about elevation, vegetation, soil and rainfall according to their spatial location, and are linked to the river data point they drain into. This allows the calculation of the discharge for every point in the river.
- Hydrological Modeling To minimize skew results of the hydrologic modeling process it is very important to apply only models that are suitable for the study region. Several hydrologic models have been developed and verified for the use in certain regional areas of the world.
For all study areas with a good availability of essential input data, regionally verified hydrologic models can be applied. This allows considering any kind of specific climatic condition of the study area that may have a strong impact on the runoff processes, as for example snow and ice occurrence during winter time in moderate climate zones.
For all areas with limited data availability, a modified version of the US Soil Conservation Service Curve Number (SCS-CN) method for modeling the precipitation-runoff processes is applied to allow first assumptions about the hydropower potential of the stream network. Globally available satellite-based data products were obtained to compensate for any missing yet relevant input data. The original SCS-CN method is an empirical approach based on simplified, experimentally derived relationships. The combination of land-use, land-cover, hydrological soil type and the antecedent moisture condition of a grid cell are reflected in defined curve number values [1], [11], [12].
According to the modified version, the direct runoff of each grid cell is calculated under consideration of variable runoff coefficients depending on the CN value and a 21-day prior rainfall-index as well as of regional climatic conditions [9], [13]. As steep slope conditions reduce the infiltration rate, a linear regression algorithm based on the slope inclination of the grid cell complements the hydrological modeling. Depending on the temporal resolution of the precipitation data, the available mean discharge at each river data point is calculated.
The potential energy of downhill flowing water of a stream regardless of any physical, technical or economic limitation is defined as the gross theoretical hydropower potential. According to physical and technical reasons hydropower plants aren’t able to fully use the gross theoretical hydropower potential. The technical potential of hydropower describes the energy capacity that is actually useable when technical, infrastructural, ecological and other conditions are taken into consideration [3].
Applying the hydroMinds model, the technical hydropower potential is calculated for each grid point representing a river. Thus, each assessed river point forms a virtual powerhouse location. The virtual intake for the respective virtual project is defined being 1,000 m upstream. This assumption creates a series of virtual hydropower projects along the considered river to ensure compatibility.
The technical hydropower potential for every possible virtual project combination is calculated according to the following equation:
The following technical, physical and ecological influences reducing the gross theoretical hydropower potential are:
- Any friction losses, that occur from water flowing through hydraulic conduits such as the intake, the trash-rack, canals and penstock including valves and other installations, are taken into account by reducing the actual available gross head relative to the flow in the conduits. Thus, the net head is the geodetic elevation difference between virtual intake and virtual powerhouse (hgeo) minus hydraulic losses (hloss) resulting from friction in the water conduit. It is assumed that friction losses are proportional to the penstock length and defined as 0.5 m loss of height per 100 m penstock length.
- The predicted discharge amount used for hydropower calculations is expected to be available statistically at 30% of days per year. To estimate the discharge with an exceedance probability of 30%, a flow duration curve is synthetically generated for every analyzed stream.
- The amount of discharge usable for hydropower in an ecologically sustainable way is set to 75% of the available discharge at any point of time, while 25% of the water remains in the rivers as ecological flow (Qeco) preserving the local aquatic ecosystem.
- The plant efficiency summarizes all energy conversion losses occurring in the process of electricity generation using turbines, generators and related equipment and is set to be η = 0.80.
- The density of water is assumed to be ρ = 1,000 kg/m³.
- The strength of the gravitational field is depending on the location and mainly affected by the parameters latitude, altitude, topography and geology and varies between g = 9.77 and 9.83 m/s².
With extending the penstock of a virtual project a higher elevation difference may be utilized resulting in a higher hydroelectric production capacity, but also increasing investment costs. Thus, a filter routine has been implemented, taking into account economic parameters such as investment costs of a virtual hydropower generation plant, average annual power production, project lifetime expectancy as well as the feed-in tariff for selling electricity, calculating the net value of the virtual project. The virtual projects that have a negative net value are excluded from further analysis. The Internal Rate of Return (IRR)- method is applied to identify the top economically viable hydropower projects.
As all economically related parameters are very sensitive and may lead to skew results of the analysis, the parameters and assumptions need to be modified and determined carefully according to local pricing conditions. For every analysis, government agencies, local experts, manufactures and suppliers are consulted to provide input data. Based on the received information mean values for the calculations of the computer based decision-making tool can be calculated.
For every virtual project combination the net value is calculated applying the following formula and assumptions as used for a study on small tropical islands in the Caribbean:
- Installed Capacity The installed capacity is the capacity corresponding to a discharge with an exceedance probability of 30% minus ecological minimum flow, the head and the overall plant efficiency as used to estimate the technical hydropower potential.
- Capacity Factor The capacity factor is the ratio of the annual hours the virtual hydropower plant is operated at full design capacity in relation to the plant operating at full capacity full time (8,760 hours per year). The capacity factor as used in this analysis is defined as 0.5.
- Feed-in Tariff The feed-in tariff is the amount of money per unit that a generator of electricity is remunerated for feeding-in electricity to the public grid. The feed-in tariff is often used as a policy mechanism designed to promote renewable energies. Feed-in tariffs of US$ 0.10 to US$ 0.20 per kWh are considered. However, in many countries there is no fixed feed-in tariff yet.
- Operation and Maintenance Costs (O&M) O&M costs are defined as a percentage of the investment cost of each individual virtual project. This includes the repair of mechanical and electrical equipment like turbine overhaul, reinvestments in auxiliary equipment such as communication and control systems. However, it does not cover the replacement of major electro-mechanical equipment or refurbishment of penstocks, tailraces, etc. The O&M costs are assumed to be 5% of the total investment costs.
- Base Costs of Virtual Project The base costs are assumed to be a fixed amount of US$ 30,000 for every virtual project. These costs cover preliminary studies, designs and all costs that occur in any event when developing a project.
- Costs for electro-mechanical Equipment The costs for the entire electro-mechanical equipment, site access infrastructure, grid connection and the construction of the powerhouse (excluding costs for the penstock) correlates with the installed design capacity of the hydropower plant and is assumed as US$ 3,333 per installed kW.
- Costs for the Penstock The costs of the penstock are dependent on its length, the material used and its diameter. In addition to the material costs there are costs for construction, site preparation as well as shipment and transportation costs.
Corresponding to the design discharge, which is estimated for each river point, different diameters of the penstock are selected based on the rule of limiting the flow velocity in the penstock to 1.5 to 2.5 m/s. It is assumed that the penstock follows the course of the river. Therefore, bends and special penstock elements are required for curves exceeding a certain radius. These extra costs are reflected in 10% higher penstock material costs. It is assumed that all penstocks are made of glass fiber reinforced polymer (GFRP).
The penstock construction costs are based on local wage levels according to skill level and working time of personnel as well as foundation material costs according to local prices for concrete and steel. Each penstock segment is assumed to be installed over ground on reinforced concrete foundations. Its volume varies according to the diameter of the penstock.
According to the penstock diameters, a different amount of segments fit into a 20‘ container. The cost for shipping of a container varies from regional and international destination zones.
It is assumed that every single virtual project will be developed and built in four years. In the first year expenses for the feasibility study, project design and management are incurred which is assumed to be 1/60 of the total project development costs. Costs for civil works and all electro-mechanical equipment are spread almost evenly over the remaining three years. At the end of the fourth year the whole development is finished and all funds disbursed. Full operation time of every project is assumed to be 25 years.
The computer-based decision-making tool identifies the virtual project with the highest IRR of all possible virtual project combinations. This river section is blocked from further screening in order to avoid double selection of the same section when selecting further virtual projects from the remaining river sections according to the next highest IRR value.
The data outputs of the hydroMinds model can be used to produce topographic and thematic maps of the study area using Geographic Information Systems (GIS). Additional diagrams, charts as well as 3D-views and other types of geographic visualization provide a good realistic overview of the study area and the identified locations. The rivers and individual river sections are classified according to their suitability for hydropower, identifying and pointing out the sites with the highest potential.
For a more detailed micro-level assessment of identified locations, the hydroMinds tool, a stand-alone and web-based software, allows modifying all parameters to analyze their impacts on-site. No sophisticated software or high-performance computer systems are required for the post-processing as all calculations are webserver-based and results can be viewed, saved and printed using a web-browser.
With local knowledge and the use of the hydroMinds tool, catchments may be examined individually with customized parameters to enhance the overall accuracy of the outputs. Even without local expertise, the tool allows data exploration to identify the sensitivity of the results to modifications of parameters.
Hydroelectric power opportunities can be identified following the approach of the hydroMinds model using remote sensing and hydrological data. The results are preliminary however, but help concentrating required in-depth studies to pre-identified sites proving the economic viability of the planned hydropower project.
The use of satellite data even allows investigating study areas where local data is not sufficient. Although the accuracy of the recommended remote sensing data products can be considered to be good, local measurements may be required to validate the hydroMinds model.
For hydrological modeling the widely-used SCS-CN method was modified according to the climate of tropical regions and was approved by local measurements, expertise and re-assessing the energy potential of existing hydropower plants in the Caribbean.
To explore the data or to analyze identified locations in more detail, the hydroMinds Tool provides an indication of the estimated range of hydropower plant design capacities according to different input parameters. Both, the hydroMinds model and software-tool can improve the implementation process of new hydropower projects and help establishing a sustainable and climate-friendly energy use.
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