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Hydropower Resources—Overview

Hydropower is a renewable energy source utilizing water moving in the hydrological cycle (water cycle), which is “powered” by radiation energy from the sun. Close to 50% of all solar energy reaching the surface of the earth is used for evaporating water and thereby converted to latent energy in water vapour. An currents bring some of this water vapour in over land, where it condenses into clouds and precipitation. Precipitation falling on laud generates runoff which moves back to the ocean under the influence of gravity. This runoff is the basis for hydropower generation and, since the water cycle is powered by solar energy, water will continue to flow as long as the sun shines, ensuring a renewable energy supply. The distribution of precipitation and elevation on land area determine the runoff volume and tuning and thereby the potential for hydropower generation.

How to compute hydropower generation potential

Hydropower (except hydrokinetic plants) is generated by converting potential energy in water as it moves from high to a lower elevation, into mechanical and electrical energy. The output (Power, P) depends on three factors, flow (Q), head (H) and efficiency (q) and can be computed using equation 1:

P Electrical power output (J/s = W) p Density of water (1000 kg/m3) g Acceleration of gravity (9.81 m/s2)

Q Water flow per unit time (m3/s)

H Elevation drop or Head (m) r| Efficiency in the conversion process

Assuming values for density and acceleration of gravity as given above, the equation can be simplified to give output directly in kW, equation 2:

The amount of energy (E) produced depends on the duration of the flow. If duration At is given in horn s (h) the amount of energy produced (kilowatt hour, kWh) can be computed using equation 3:

Another usefiil equation gives the energy output in kWh per m3 of water as a function of H and q, often called “Energy equivalent of water” (EEKY), equation 4:

In addition to kWh, other commonly-used units of electrical energy are megawatt hour (MWh), Gigawatt hour (GWh) and Terawatt hour (TWh), where:

  • 1 MWh = 1000 kWh (106 Wli)
  • 1 GWh = 1000 MWh (109 Wli)
  • 1 TWh = 1000 GWh (1012 Wli)

For comparison to other renewable or thermal energy sources, the unit Exajoule (EJ) is often used. One EJ (1018 J) equals 277.78 TWh. and one TWh equals 0.0036 EJ. The global hydropower production in 2018 was 4239 TWh. corresponding to 15.26 EJ.

The capacity factor (Cf) of a power plant is the ratio of the total energy output from the power plant over a period of tune (typically one year) to its potential output if it had operated at frill rated (nameplate) capacity over the same period. This is also called plant load factor (plf) or capacity utilization factor (CUF). Typical capacity factors for hydropower plants are in the range of 0.35 to 0.5, world average is 0.42 for hydropower, see Table 1. Due to maintenance or other limitation, sometimes equipment is not available for power generation ел ей if water and demand are there. This is known as “plant availability factor” (PAF).

Definition of hydropower potential

In order to compute how much hydropower that can be generated (the potential) within an area (a catchment, a region, a country, etc.), the usual procedure is to identify all feasible sites with a suitable combination of flow (Q) and Head (H), where hydropower plants can be located. The potential at each site is computed by applying equations (1-3), considering the topography, volume of water and its variability in time. Results are usually given as potential atmual energy generation (GWhyear), for each site. Hydropower potential within the area is then computed by adding up the potential production from all feasible sites, omitting sites with environmental restrictions, high cost or social constraints.

It can be argued that this definition of potential is not precise because the selection of ‘feasible sites’ will depend on technology, economic parameters and social and environmental preferences, and all these can change with time. This has led to the use of other definitions like: ‘Theoretical potential’, ‘Technical potential’ and ‘Economic potential’. Some also argue for the use of definition ‘Sustainable potential'.

Global and regional hydropower potential

The International Journal on Hydropower & Dams World Atlas & Industry Guide (IJHD), provides the most comprehensive inventory of current hydropower installed capacity and atmual generation, and hydropower resource potential, updated every year. The Atlas pror ides three measures of hydropower resource potential, all in terms of atmual generation, but gives no detailed explanation of how the potentials were computed or limits for what is defined as technically or economically feasible. A possible explanation for each, based on other sources, is given in parenthesis:

Gross theoretical potential (Gross potential at all known sites)

Technically feasible potential (Portion of potential with cost low enough to justify site assessment) Economically feasible potential (Potential with cost less or equal to large thermal power plants)

The total global technical potential for hydropower is estimated at 15778 TWh/year (IJHD, 2017), this is nearly four times the current generation. Economically feasible potential was estimated at 9623 T Why ear. more than twice the existing generation in 2018 (4239 TWh/year). The different potential estimates are summarized in Table 3 and visualized in Figure 7.

Table 3. Global hydropower potential (IJHD, 2017).

Region

Gross theoretical potential

Technically feasible

Economically feasible

TWh/year

%

TWh/year

%

TWh/year

%

North America

7601

18

1891

12

1045

11

South America

7848

19

2859

18

1728

18

Europe

3136

7

1195

8

852

9

Africa

4423

11

1647

10

1124

12

Asia

18248

44

8000

51

4786

50

Australasia Oceania

658

2

186

1

89

1

World

41914

100

15778

100

9624

100

Different estimates of hydropower potential in six regions (IJHD, 2017)

Figure 7. Different estimates of hydropower potential in six regions (IJHD, 2017).

Cost Issues

Cost of hydropower is site specific and can, therefore, vary considerably from one project to another. The main cost components are:

  • (1) Upfront investment (capital cost)
  • (2) Operation and maintenance (O&M) cost
  • (3) Decommissioning cost

The Levelized Cost Of Energy (LCOE) includes all these cost elements for the entire lifetime of the project, and is usually given in units of US с/kWh or US S/MWh. The LCOE for hydropower depends on these cost components, but also on several others:

  • (4) Capacity factor
  • (5) Lifetime of project
  • (6) Cost of capital (discountrate).

Capital cost includes the cost of civil structures (dams, tunnels, powerhouse, etc.), electromechanical equipment (turbine, gates, governor, generator, transformer, control systems, etc.), access roads, powerlines, cost of planning and cost of mitigation measures (mitigation, resettlement, fish ladders, etc.). The cost of civil structures is usually the dominating share in large projects (60-70%), but for small projects, the electro-mechanical cost can be 50% or more. Typical capital cost for hydropower varies from < 1000 $/kW up to 3000 S/kW capacity for large hydro and from 1500 to 6000 $/kW or even higher for small hydro (IPCC, 2011; IRENA, 2015; NREL, 2012; IHA. 2018). However, there are also many examples of projects with costs as low as 500 $/kW, for especially good sites.

Typical lifetime for hydropower varies from 40 to 80 years or more (IPCC, 2011). Electro-mechanical equipment usually has a shorter lifetime and civil structures a longer lifetime (50-100%). During an average life span of 80 years, it will be necessary to replace much of the electro-mechanical equipment at least once; the cost of this is included in the typical 2.5% per year assumed O&M cost.

This can be compared to LCOE for other renewable energy sources (RES), see Figure 8 which is based on data from a large number of recent projects, compiled and published by The International

Average global levelized cost (USD/kWh) of electricity from utility-scale renewable power generation technologies

Figure 8. Average global levelized cost (USD/kWh) of electricity from utility-scale renewable power generation technologies,

2010 and 2017. Based on data from (IRENA, 2019).

Renewable Energy Agency (IRENA, 2015; IRENA, 2017). Tlie figure shows the cost trend for the seven most important renewable technologies during recent years (2010-2016). Hydropower is, on average, still the technology with the lowest average cost, followed by onshore wind, geothermal and bioenergy. A closer look into IRENAs cost database reveals that small hydro generally has a higher cost than large hydro, and that there are large regional differences. Regions like Europe and North America, where a larger share of available resources have already been developed, show a higher cost than regions with large untapped potential, like Asia and South America.

The following is a summary of comments from the IRENA report on power cost of renewables (IRENA, 2015):

“Hydropower produces some of the lowest-cost electricity of any generation technology. The LCOE of large-scale hydro projects at excellent sites can be as low as USD 0.02/kWh, while average costs are around USD 0.05/kWh where untapped economic resources remain. Small-scale hydropower can also be veiy economic, although typically it has higher costs and is sometimes more suitable as an option for electrification that can provide low-cost electricity to remote communities or for the local grid.

There is a clear cost dichotomy for hydropower between regions with remaining economic resources to exploit and those where most of the economic resources have been exploited already. Asia, Africa and South America all experience LCOEsfor hydropower projects of on average USD 0.04 to USD 0.05/kWh. In contrast, in regions which have exploited their most economic resources, weighted average LCOE ranges are around USD 0.09 to USD 0.10/kWh (e.g., in Europe, Eurasia, North America and Oceania). In addition to the higher costs, these regions ate also constrained in the amount of economic capacity that still remains to be added.” (IRENA, 2015).

An overview of typical cost components in a large (500 MW) hydropower plant is given in Figure 9. Civil cost components (dam, tunnels, powerhouse, etc.) will typically add up to over 50% of the total cost, while electro-mechanical components are typically < 20%. For small hydro and projects without dams and long tunnels, the share of electro-mechanical cost is usually larger, up to 50% or more.

Capital cost breakdown for a 500 MW hydropower plant. Based on data from (NREL, 2012)

Figure 9. Capital cost breakdown for a 500 MW hydropower plant. Based on data from (NREL, 2012).

 
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