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Power from water
- Kinetic energy in flowing water can be used to generate electricity from hydroelectricity, wave or tidal energy sources. In this section, we shed some light on each of the three types of hydro sources.
Hydroelectric power
(a) Introduction
- Hydroelectric power generation is a well-established and mature technology producing, in 2015, 16.6% of the world’s total electricity. China is currently the world’s largest producer of electricity from hydropower and the potential for the growth of hydropower around the world are, 71% Europe, 75% North America, 79% South America, 95% Africa, 95% Middle East and 82% Asia Pacific.
- According to the “🔗 Brighter Africa” report by McKinsey&Company in 2015, it was estimated that there is a potential of 350 GW of hydropower capacity that can be exploited in Sub-Saharan Africa with the with the Democratic Republic of the Congo (DRC) accounting for 50% particularly through the dams currently existing and proposed for construction (such as the massive Grand Inga Dam) at the Inga Falls.
(b) Generation methods
- Three main generation methods are used:
- 1- Dammed water (conventional method). This is the most widely used method which relies upon storing water behind a dam and using the potential energy to drive a water turbine and a generator. The penstock shown in 📷 Fig. 10.1 is a large pipe delivering water from the reservoir to the turbine. As of 2016, the 22.5 GW Three Gorges Dam in China is the largest hydroelectric power station in the world.
- 2- Pumped storage. This produces electricity to meet demands at high peak periods. At times of low demand, excess generating capacity is used to pump water into a high-level reservoir. During periods of high demand, water is released back into a low-level reservoir to generate electricity to meet that surge in demand. It is not considered as an energy source, but instead as means of energy storage (net consumers of electricity). In fact, it is the most commercially important means of large-scale grid energy storage. The 3 GW Bath County Station in the USA is currently the world’s largest.
- 3- Run-of-the-river. This method uses kinetic energy from undammed flowing water, so that only the water coming from upstream is available for generation at that moment in time.
(c) Power generation in dammed hydroelectricity
- The power extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head.
- The potential energy E in the stored water can be given by
- E = Mgh \qquad \qquad (10.1)
- Where M is the mass of stored water (kg), g is the acceleration due to gravity (9.81 m/s2) and h is the head of water (m). Power is the rate at which energy is delivered, hence, the water mass M in (10.1) can be replaced by (ρQ) where ρ is water density (1000 kg/m3) and Q is the volume flow rate (m3/s)
- P = \rho Qgh \qquad \qquad(10.2)
- The potential energy in the stored water is not all converted to kinetic energy, and hence useful electricity. Taking into account conversion efficiency η, the actual power generated is
- P = \eta \rho Qgh \qquad \qquad (10.3)
(d) Numerical example on power generation
- A pumped storage hydroelectric power plant stores a water capacity which can be released for generating electricity during peak demands of up to 3 hours. If the surface if the reservoir is 400m above the 1 GW turbine and the conversion efficiency is 85%, calculate the volume of water required to be stored for the turbines to operate at rated power.
- Q = \frac{P}{\eta \rho gh} = \frac{1 \times 10^9}{0.85 \times 1000 \times 9.81 \times 400} = 300m^3/s
- Vol = 300 \times 3 \times 60 \times 60 = 3.24 \times 10^6m^3
- What is the excess amount of energy required by the pumps if the pumps fill the reservoir in 6 hours and pump the water an additional effective head of 50m? The pumps efficiency is 87%.
- Pump flow rate
- Q = \frac{Vol}{filling time} = \frac{3.25 \times 10^6}{6 \times 60 \times 60} = 150m^3/s
- Power consumed by pumps
- P = Q \rho gh = 150 \times 1000 \times 9.81 \times 450 = 0.662 GW
- Power input to pumps
- P = \frac{0.662}{\eta} = \frac{0.662}{0.87} = 0.76GW
- Excess energy consumed by pumps
- \left (0.76 \times 6 \right ) - \left ( 1 \times 3\right ) = 1.56 GWh
(e) Advantages and disadvantages of hydroelectricity
- Advantages:
- No fuel cost
- Green source of energy, no nuclear waste, no emissions
- Very low labour cost as mostly automated
- Power plants have long economic lives
- Dams can control floods
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- Disadvantages:
- Mostly associated with giant complex infrastructure, hence is quite expensive civil engineering project.
- Can have significant environmental impact:
- Altering existing water flows, which can cause loss of flow in dry months or drowning in wet months.
- Effect on soils
- Effect on water life
- Social and political consequences can result from governmental decisions to build huge dams.
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- This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
Author: Tennessee Valley Authority; SVG version by Tomia -
Tidal power
(a) Clip to watch
- Watch the following short clip about generation of power from tides and answer the questions that follow:
(b) Quiz
(c) Comparison of tidal and wind turbines
- Tidal turbines work in a similar way to wind turbines but are totally submerged under water. The kinetic energy from the water stream is converted to electrical power output, hence the same equation applies as in wind turbines (equation 9.2)
- P = \frac{1}{2} \rho Av^3 \qquad \qquad (10.4)
- where ρ is the density of water (1000kg/m3) instead of density of air (1.201 kg/m3 at 20°C). This means:
- For the same stream velocity and turbine swept area, the power that can be produced by the tidal turbine is more than 800 times that produced by wind turbine
- OR
- For the same power generated, the swept area (hence the size) of the tidal turbine is much smaller than the wind turbine.
- Tidal turbines are totally submerged under water, therefore unlike wind turbines; tidal turbines are likely to present both low visual impact and low levels of audible noise. Output from them is also much more predictable which makes electrical grid management an easier task compared to other sources of energy such as wind.
(d) Status of global installed tidal power
- No technology has yet emerged as the clear standard for tidal power stations. A large variety of designs are being experimented with, with some very close to large scale deployment. The world's first large-scale tidal power plant was the 240 MW Rance Tidal Power Station in France, which became operational in 1966. It was the largest tidal power station in terms of output until the 254 MW Sihwa Lake Tidal Power Station opened in South Korea in August 2011.
- Scotland is currently building what is believed to be the world’s largest tidal power plant, in the Pentland Firth, which has a capacity of 400MW. The project is named the MeyGen Tidal Energy Project and is currently partially operational.
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Wave power
(a) Introduction
- Waves are produced by the wind passing over the sea. This is different from tidal power and the power in ocean currents. Historically, although there have been numerous attempts to convert energy from ocean surface waves to other forms of useful energy, however it is still in experimental stage of development and little or no commercial wave energy converters have been deployed to date.
- It is estimated that 2 TW of potential wave power exist worldwide. 📷 Fig. 10.2 shows a map of average global wave energy in kW per meter of wave front. It can been seen that the northern coasts of Scotland and the southern coasts of South Africa provide quite promising locations for exploitation of wave energy.
(b) Power generated from waves
- The power in ocean waves is largely determined by wave height H (m) and periodic time T (s) between wave crests. A practical expression considering the extractable possible power P from waves in kW per meter of wave front is
- P = 0.5H^2T \qquad [kW/m] \qquad \qquad \left(10.5\right)
- Average offshore conditions in the North Atlantic show T=9s and H=3.5m, giving a power density of approximately 55 kW/m.
- As equation (10.5) shows, the long time period and large amplitudes of ocean waves increase power generation. Wave sizes are generally determined by wind speed and depth and topography of the sea floor.
(c) Wave energy converter technologies
- Different technologies for wave energy converters (WEC) have been experimented. These include point absorber buoy structures, overtopping devices, oscillating water columns and Pelamis machines.
- Watch this short clip about the oscillating water column (OWC) wave energy converter and answer the short quiz that follows.
(d) Quiz
(e) Advantages and disadvantages of wave power
- Advantages:
- Low visual impact
- Impact on coast lines is minimal since only a fraction of overall wave energy is extracted
- No chemical/nuclear waste
- Low environmental impact as almost no problems for marine life
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- Disadvantages:
- High capital costs
- Wave energy converters may present some hazards to shipping
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Energy from biomass
How biomass works
- Biomass is the name given to a range of organic materials that can be used to produce energy. Watch the following short clip explaining how biomass works and then carry out the assignment that follows.
Assignment
- After watching the video clip, and in your own words, write a summary (in no more than 400 words) about how energy is obtained from biomass. Topics to address should include (but not limited to):
- Examples of materials used as biomass fuel
- The difference between biomass and fossil fuels
- Is biomass carbon-neutral?
- How biomass needs careful environmental management to be a sustainable source of energy, and how it can be non-sustainable.
- How to compensate for additional carbon used during transportation and manufacturing of biomass fuel.
- Send your summary to the Tutor.
Numerical example
- A biomass power plant uses 10000 tonnes of wood annually releasing 3300 kWh/tonne when burned. However, only 30% of this energy is turned into useful electricity. The plant feeds 20000 households.
- (a) Calculate the power plant annual electricity output
- (b) If the plant has a capacity factor of 65%, what is the minimum generator rating required?
- (c) If the annual household demand is 2500 kWh per household, what proportion of the annual demand does the power plant provide?
Answer:
- (a) Annual electricity generated=
- 0.3 \times 3300 \times 10000 = 9.9 \times 10^6~kWh
- (b) Number of working hours per year =
- 0.65 \times 8760 = 5695~ hrs
- Generator minimum rating
- \frac{9.9 \times 10^6}{5694} = 1739~kW
- (c) Total annual household demand =
- 2500 \times 20000 = 50 \times 10^6~ kWh
- The plant provides
- \frac{9.9 \times 10^6}{50 \times 10^6} = 19.8\%
- of annual demand
Status of global and sub-saharan biomass
- According to a report by the World Energy Council in 2016 “World Energy Resources, Bioenergy” it has been reported that biomass is the largest renewable energy source in use with 14% of the global energy mix (from overall 18% renewables) coming from biomass.
- In sub-Saharan Africa, biomass is a major source of energy. It supplies over 74% of total energy consumption due to the ease of access of biomass feedstock. It has provided much needed energy to rural areas and is the source of livelihood in many countries. In 2017, Kenya has successfully started operating Africa’s first grid-connected biogas-powered electricity plant (The Gorge Farm AD power plant) with 2.8 MW installed capacity. This plant will use 50000 tonnes of organic crop waste each year and will power approximately 8000 households as well produce 35000 tonnes of rich natural fertiliser as a by –product of the biogas process.
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Distributed generation vs centralized generation
- With the increased use of renewables to meet government’s CO2 reduction targets, generation of energy from some renewable sources provided users with the flexibility of generating power locally at demand/domestic locations. This occurs in a decentralized manner compared to the traditional bulk power production in centralized power stations such as coal and nuclear power stations. This has led to a reduced burden on the high voltage transmission network and also contributed to fuel diversity and improved security of supply since availability of different renewable sources varies at different times, hence more likely to cover demand profile.
- Generation using distributed energy resources has led to the emergence of low voltage distribution networks, commonly known as microgrids. These are modern, localized, small-scale grids, unlike the centralized electricity macrogrid, which can disconnect from the centralized grid and operate autonomously. They are installed by the community they serve and usually employ a mixture of different distributed energy resources.
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