Introduction
In this section we will look at the other ways of generating energy sustainably; hydro power; wave power; tidal energy. In each case the energy carrier is water. We will not look in great technical detail at how energy is extracted but instead focus on the potential of each source and problems extracting the energy. New generation technologies are also considered.
Water
Everyone must have heard at some time how the unique properties of water make life on earth possible. Water (H2O) is an inorganic, transparent, tasteless, odourless, and nearly colourless chemical substance, which is the main constituent of earth's hydrosphere and the fluids of all known living organisms (acting as a solvent). It is vital for all known forms of life, even though it provides no calories or organic nutrients. Pure water has its highest density 1000 kg/m3 at temperature 4°C (39.2°F). The energy from water can be harnessed to be useful in a variety of different ways. As water moves through some body, such as a river, its potential and kinetic energy vary. Additionally, if the area through which the water is moving changes size the pressure can also change. Therefore, the kinetic and potential energy can be harnessed and transformed into a type of useable energy, such as electricity.
‘Water vapour is a greenhouse gas; only a matter of time till it’s declared a pollutant’
Water vapour is the largest contributor to the earth’s greenhouse effect. On average, it probably accounts for about 60% of the warming effect.
However, water vapour does not control the earth’s temperature, but is instead controlled by the temperature. This is because the temperature of the surrounding atmosphere limits the maximum amount of water vapour the atmosphere can contain. If a volume of air contains its maximum amount of water vapour and the temperature is decreased, some of the water vapour will condense to form liquid water.
This is why clouds form as warm air containing water vapour rises and cools at higher altitudes where the water condenses to the tiny droplets that make up clouds.
Water is also important in the fundamental reaction that sustains all life. During photosynthesis, plants take in carbon dioxide (CO2) and water (H2O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxyen and the carbon dioxide into glucose.
But what is so special about water? Simple technical details are stated in the caption under Fig. 1, but the key point is that water is a liquid at the ambient temperature of the planet, when nearly all other substances are liquid or gaseous. Without a neutral liquid medium in which to operate, it would be impossible for the organic molecules that make up living cells to react and interact.
Water is involved everywhere as we look at energy generation and energy issues, be it the operation of a steam turbine for generating electricity, electrolysis to make hydrogen, a central heating system to move heat around the home, the massive contribution of water vapour to the greenhouse effect, or even fusion to produce clean energy in the future.
Nuclear fusion is the process that powers the sun. Two protons are forced into such close proximity that the nuclear force is engaged and binds them together to form helium, releasing a huge amount of energy. Fusion has already been achieved in the lab but not in a controlled productive way. Fusion is considered by many scientists to be the most likely way energy will be sustainably generated. Water, the raw material, is plentiful, but a very high temperature is required to overcome the long-distance repulsive force between protons that normally prevent them coming close together. The temperature must be raised to millions of degrees, but no conventional vessel can be used to contain a material at this temperature. A magnetic vessel must be constructed to confine the plasma, but it is extremely difficult to produce a magnetic barrier that is completely secure.
Figure 1: Liquid water consists of polar molecules made up of one oxygen atom and two hydrogen atoms bound through hydrogen bonding: This is the continuous splitting and recombination of the molecule:
H2O <—> H+ + OH-
It is this unique bonding that gives water its unusual chemical properties which complement the activity of inorganic compounds that are the basis of all cell activity.
[source] used under fair dealing.
Activity 1
- When water solidifies, it becomes less dense. What would be the consequence if instead ice were denser than liquid water?
- What is heavy water?
- What is a catalyst and how does a catalyst work?
- How much energy is needed to reduce the temperature of 1 m3 of water from 10oC to 0oC? When water then freezes, does it absorb or release energy, and how much? The specific heat capacity (SHC) of water is 4.2 kJ kg-1 K-1 and the latent heat of fusion is 334 kJ kg-1. This calculation is relevant to ground source heat pumps.
Solution
a.
Water becomes less dense when it freezes. This means that ice is lighter than water and floats, insulating the water underneath and slowing down the freezing process. If ice were heavier, the ice would drop to the bottom leading to a greater degree of ice formation in winter. You might rightly feel that such ‘conditional subjunctives’ are pointless speculations, but as far as global warming is concerned to understand it is interesting to note a big difference in the amount of sunlight reflected by water compared to ice.
b.
Heavy water (D2O), also called deuterium oxide, water composed of deuterium, the hydrogen isotope with a mass double that of ordinary hydrogen, and oxygen. (Ordinary water has a composition represented by H2O.) Thus, heavy water has a molecular weight of about 20 (the sum of twice the atomic weight of deuterium, which is 2, plus the atomic weight of oxygen, which is 16), whereas ordinary water has a molecular weight of about 18 (twice the atomic weight of ordinary hydrogen, which is 1, plus oxygen, which is 16).
Deuterium is different than the hydrogen that usually occurs in water (protium), since each atom of deuterium contains a proton and a neutron, while more commonly occurring hydrogen contains only a proton.
Heavy water does occur naturally, however in much smaller quantities than regular water. Approximately, one water molecule for every twenty million water molecules is heavy water. Since deuterium is a stable isotope, heavy water is not radioactive.
c.
Complex molecules are formed by combining atoms or simpler molecules. The process only works if the component parts are correctly aligned with respect to one another, like for example jigsaw pieces. A catalyst is a material with a surface that attracts the interacting parts and aligns them in such a way they can combine if energy is supplied. The slider of a zipper is a good analogy for a molecular catalyst. It effects a complex link and is unchanged in the process. Catalysts are not consumed in the reaction though they may become contaminated and need replacing.
d.
The energy E required to raise the temperature of a mass (m) of a material with specific heat capacity (SHC) by (ΔT) degrees Celsius is:
E = m x SHC x ΔT
1 m3 of water weighs 1,000 kg. The specific heat capacity of water is 4,200 J kg-1 oC-1, and the temperature rise is –10 oC.
The energy is 1,000 x 4,200 x -10 J = -42 MJ.
The negative sign means that energy has to be removed not added.
When water freezes, energy is released.
When 1 m3 of water freezes at 0 oC, 334 kJ kg-1 x 1,000 kg = 334 MJ of energy is released.
When water evaporates, energy is absorbed and it is by this process energy leaves the surface of the oceans and is transported into the upper atmosphere when water later condenses at high altitude.
Hydroelectricity
Water will flow from a higher level to a lower level under the influence of gravity. In doing so, the original potential energy is converted to kinetic energy. Fig. 2 shows a cube of water of volume (V) suspended at height (h) above ground level. The total potential energy is:
PE = density x volume x gravitational acceleration x height
= ρVgh,
where the density of water is 1,000 kg m-3 and the gravitational acceleration is 9.81 ms-2. If the mean height of a 1 m3 volume cube of water were 10 m, the stored energy would be 1,000 x 1 x 9.81 x 10 J = 98.1 kJ.
Referring to Fig. 3, a hydroelectric scheme converts gravitational potential energy to electrical energy. Water flowing down its natural course is constrained by a barrier causing the water level to rise, causing the stored energy to increase both by virtue of the increasing mass of water and the rising head of water (the height of the body of water above the turbine position). When electricity needs to be generated, water is allowed to flow through a turbine (Fig. 4). Generation efficiency can approach 90%, more than double that of a thermal power station.
Figure 2: The height of the water above the generator should be as great as possible. E.g. Water from an uphill reservoir is piped down to a turbine 600 m below. 1 m3 of water will therefore generate 1,000 x 1 x 9.81 x 600 / 3,600,000 x 0.9 = 1.47 Units Units of electricity. Large hydro schemes can be very effective and are completely sustainable.
© UHI, Owen Inger – Jones, LCC
Hydroelectricity takes on an role of energy buffer in an integrated system that includes intermittent renewable energy sources such as wind power, because it can respond very rapidly (less than 30 seconds) and compensate for any sudden drop in wind speed and ensure continuity of supply.
The difficulty is that there are very few locations suitable for a hydroelectric scheme, even in a mountainous country with heavy rainfall such as Scotland. The topography must be such that water from a significant catchment area is directed through a narrow channel that can be effectively blocked using a barrier. Alternatively a major river can be dammed, but this will often flood a significant area of fertile land.
Demand for electricity is not constant. Demand goes up and down during the day, and overnight there is a reduction for electricity in homes, businesses, and other facilities. Hydroelectric plants are more efficient at providing for peak power demands during short periods than are fossil-fuel and nuclear power plants, and one way of doing that is by using pumped storage, which reuses the same water more than once.
Pumped storage (Fig. 5) is a method of keeping water in reserve for peak period power demands by pumping water that has already flowed through the turbines back up to the reservoir above the power plant at a time when customer demand for energy is low, such as during the middle of the night. The water is then allowed to flow back through the turbine-generators at times when demand is high and a heavy load is placed on the system.
Figure 5: Typical Pumped Hydro Storage [source]
Activity 2
- A hydro scheme has a head of 250 m, a catchment area of 1000 km2, an installed capacity of 90 MW, and a load factor of 0.67. Assume the turbine and generator together are 85% efficient. Calculate the energy is produced per year, energy produced per cubic metre, annual volume of water, and the annual rainfall?
- Discuss the advantages and disadvantages of hydroelectric power?
Solution
a.
The hydro scheme has an installed capacity of 90 MW and a loading of 0.67, hence
Energy produced per year = 90x106 x 0.67 x 24 x 3600 x 365 = 1,902,620.8 GJ
1 m3 of water released from the dam will make a quantity of electrical energy equal to mgh times the efficiency:
Energy produced by 1 m3 of water = 0.85 x 1,000 x 9.81 x 250 = 2.085 MJ
The volume of water flowing in one year is the energy produced in one year divided by the energy from 1 m3 of flow:
Annual volume of water = 1,902,620.8x109 / 2.085x106 = 912,527,961.6 m3
If the catchment area is 1000 km2, this implies that the annual rainfall was at least:
Annual rainfall = 912,527,961.6 / 1,000,000,000 m = 0.91 m (approx.)
b.
Advantages
- Clean and renewable
- Using water to generate electricity doesn’t release harmful pollutants into the air or water. While there are some environmental considerations that come with building large hydropower facilities like dams and reservoirs, once operational, hydropower plants themselves don’t require burning any fossil fuels.
- Facilities don’t use up water as they operate, making hydropower a completely renewable electricity source. As the water cycle naturally runs, hydropower will always be a viable way to generate electricity.
- Hydropower pairs well with other renewables
- The majority of hydroelectric plants are storage or pumped storage facilities that store large amounts of water in reservoirs, and will almost always have stored water to pull from to generate power. Hydropower’s reliance on stored water in reservoirs means that it is generally a reliable source of power in the sense that hydropower plants can be a stable source of supporting energy for more intermittent energy sources like wind and solar. Wind power and solar energy rely on the natural availability of wind and sunlight; just like an energy storage system, at times of low wind or at night when the sun isn’t shining, hydropower provides electricity when solar and wind can’t, making them more economical and practical sources of electricity.
- Certain hydroelectric systems meet peak demand
- Both storage hydropower and pumped storage hydropower facilities have the ability to generate electricity on-demand (by releasing dammed water through turbines), making many hydroelectric plants dispatchable resources.
Disadvantages
- Adversely affect surrounding environments
- Environmental impacts involved with building hydroelectric plants. Storage hydropower or pumped storage hydropower systems interrupt the natural flow of a river system. This leads to disrupted animal migration paths, issues with water quality, and human or wildlife displacement.
- Building facilities are expensive up-front
- Many hydropower plants are large infrastructure projects that involve building a dam, a reservoir, and power-generating turbines. requiring a significant monetary investment. While a large hydropower facility can often provide low-cost electricity for 50 to 100 years after being built, the upfront construction costs can be large. This, combined with the fact that suitable places for reservoirs are becoming rarer over time means that large-scale hydropower plant construction costs may continue to rise.
- Facilities rely on local hydrology
- Hydropower is a reliable energy source, but it is still ultimately controlled by weather and precipitation trends. The amount of water available for hydropower systems can vary, thus electricity production at a hydroelectric facility can also vary.
Wave Power
Ocean waves build up through the effect of wind on the surface of the water over enormous distances (the fetch). When winds blow across a body of water, they transfer some of the energy to the water. The rougher the water gets the more surface area there is between the water and the air. This improves the transfer of energy to the water from the wind. Therefore, the waves get larger the more wind there is. Large stretches of ocean and strong winds combine to make large waves with lots of energy.
The energy these waves have can be broken down into two key components: Potential energy which comes from the elevation of the water surface and Kinetic energy which is due to the movement of the water particles. Throughout the creation and movement of waves, the amount of each type of energy a wave has stays the same when averaged over the wavelength. However, the type of energy it holds will fluctuate between kinetic and potential.
There is the illusion that the water flows; in fact water molecules travel in circles relaying energy and momentum to the next molecules along the path of the wave; waves are an energy transport phenomenon (Fig. 6).
The energy transported by a wave in deep water given its main characteristics: its height, between crest and trough, and its period: the time necessary for the wave to propagate along the distance between two crests. Given the velocity potential (that can be obtained through linear wave theory) one can derive the kinetic and potential energy of a wave per meter of crest and unit of surface. The waves energy is therefore;
E = (ρgA2) / 2
Where ρ is the density of water g the gravitational acceleration and A is the amplitude of the wave (half its height H).
Figure 6: Wave movement [source] used under fair dealing.
Figure 7: The pitch, heave and surge responses of a floating object to incident waves
© UHI, Owen Inger – Jones, LCC
To capture energy from sea waves it’s necessary to intercept the waves with a structure that will react in an appropriate manner to the forces applied to it by the waves. A body in the sea subject to waves can respond to six types of movement. These are sway, roll and yaw, not generally harnessed in wave energy conversion technology. Three more, harnessed to varying degrees in most wave energy converters, are: (Fig.7)
- Pitch; waves cause the device to rotate about its axis
- Heave; waves cause the device to rise and fall vertically
- Surge; waves cause the device to move horizontally backwards and forwards
Wave energy devices are classified by;
Operation; (Fig. 8)
- Heaving
- Pitching
- Oscillating
- Surge
Figure 8: Schematic representation of various types of wave energy converter classified by mode of operation [source] (public domain)
Location; (Fig. 9)
- Fixed to the seabed, generally in shallow water (eg TAPCHAN)
- Fethered in intermediate depths (eg Oyster)
- Floating offshore in deep water (eg AWS-III).
Figure 9: Classification of wave energy converters according to location [source] (public domain)
Size and Orientation; (Fig. 10)
- Terminators
- Attenuators
- Point absorbers.
Figure 10: Classification of wave energy converters according to size and orientation [source] (public domain)
Activity 3
- What are the environmental impacts of wave energy technology?
- Discuss the grid integration of wave energy?
Solution
a.
Wave energy converters should be among the most environmentally benign of energy technologies for the following reasons:
- Minimal potential for chemical pollution. At most, they may contain some lubricating or hydraulic oil, which will be carefully sealed from the environment.
- Little visual impact, except where shore-mounted.
- Noise generation is low – generally lower than the noise of crashing waves
- Present a small (though not insignificant) hazard to shipping.
- No difficulties to migrating fish.
- Floating schemes are incapable of extracting more than a small fraction of the energy of storms so will not significantly influence the coastal environment. Concrete structures will need to be removed at the end of their operating life.
b.
When the electrical outputs of several wave energy units are added together, the total output will be generally smoother than for a single unit. If we extend this to an array of several hundred floating devices, then the summed output will be smoother still. In addition, any fluctuations in output will be less important if the electricity is to be delivered to large national systems like those of the UK, where in most locations the grid is ‘strong’ enough to absorb contributions from a fluctuating source.
Although there are short-term fluctuations on second or minute scales, the wave resource also varies on a day-to-day and season-by-season basis. Seasonal variation shown in figure below demonstrates that wave power output reaches a maximum in the bad weather of winter when the electrical demand is greatest.
Figure 11: Seasonal availability of wave energy and electrical demand in the UK
[source] used under fair dealing.
Tidal Energy
Oceans are noticeably influenced by the gravitational effect of the sun and moon on the earth. The moon has the strongest influence, which is itself strongest on the side facing it and weakest on the opposite side of the earth.
This, combined with the weaker effect of the sun, creates a gravitational field with a differential forcing that causes sea levels to rise and fall around the coastlines of the world. Rising and falling sea levels create a tidal cycle of around 12.5 hours, which is dictated by the orbital period of the moon about the earth and the earths own daily rotation. The tidal range describes the variation in the sea levels as they go through a full tidal cycle. The maximum range is known as a spring tide, which coincides with a full or new moon when the sun and moon are working together to create a strong gravitational pull on the earth. Conversely, the minimum range is known as a neap tide and occurs when the sun and moon are working against each other.
This is a highly predictable process and the energy gained by the water can potentially be extracted as another form of marine renewable energy. The sea rises by about 1.0 m in mid-ocean twice per day, much less energy than is associated with ocean waves but the amplitude increases significantly near land when the water becomes constrained by topological features. Another difference between tidal and wave motion is that the water actually moves giving rise to surface and underwater tidal currents or flows.
Figure 12: Potential tidal power per unit area around the UK coast [source] (public domain)
There are a number of ways in which tidal power can be harnessed;
Tidal barrage (Fig. 13) power systems take advantage of differences between high tides and low tides by using a “barrage,” or type of dam, to block receding water during ebb periods. At low tide, water behind the barrage is released, and the water passes through a turbine that generates electricity.
Figure 13: Tidal Barrage inlet and outlet [source] used under fair dealing.
The incoming flood tide is allowed to pass though sluices with the generators idling, to trap the water behind the barrage at high tide. This head of water then drives the turbines to generate power on the ebb tide. Flood tide generation is simply the opposite but in either case, given that there are two tides a day, power is produced in two bursts every day. Two-way operation is possible, using both ebb and flow, although power output is not doubled. This is because the tidal basin would normally empty and fill through its entire natural outlet, and when constrained through sluices the full cycle would not be complete before the tide turned.
Tidal stream (Fig. 14) power systems take advantage of ocean currents to drive turbines, particularly in areas around islands or coasts where these currents are fast. They can be installed as tidal fences (where turbines are stretched across a channel) or as tidal turbines, which resemble underwater wind turbines.
Figure 14: Tidal Stream Devices [source] used under fair dealing.
The flow can be used to generate energy by installing underwater turbines (in the same way that wind turbines draw energy from air currents). Since energy output varies with the density of the medium, (kg/m3) and the cube of the velocity, (m3/s), a 10 mph (about 8.6 knots in nautical terms) ocean tidal current would have an energy output equal or greater than a 90 mph wind speed for the same size of turbine system. Therefore, even small increases in velocity can lead to substantial changes in the amount of available power and therefore, smaller faster rotating tidal turbine generators can be used in a ocean based tidal stream system.
Tidal stream generation is a non-barrage tidal scheme, unlike tidal fence energy which uses a physical barrier to extract the energy. Tidal stream systems extract the kinetic energy (energy in motion) from moving water generated by the tides without altering the environment thereby making it a hydrokinetic energy system.
Activity 4
- Calculating tide heights is complex and laborious. An alternative and crude method is The Rule of Twelfths. Explain how it works?
- How much energy could be extracted with a tidal flow speed of 5 m s-1, assuming one hundred 30% efficient turbines with 3 m blades, spaced 10 m apart? Would there be any environmental consequences?
Solution
a.
The height of the tide is measured from an imaginary point known as chart datum. Chart datum is where all the heights and depths are taken from a nautical chart. Using the high and low tide data, we can work out how much water will be over a certain point at high and low tide. During the rest of the day however, it is a little more difficult. Many assume the tide flows from high to low at the same speed, and then low to high at the same speed throughout; however this is not the case. Luckily though, the tides move in quite a predictable way, and although it is not a constant flow, there is a predictable variation in that flow.
Imagining the tidal flow in graph form, it could be illustrated as a bell curve (inverted depending on tidal phase); in fact the speed of the tide is similar to that of a swinging pendulum; in the sense that as the pendulum swings towards the top it slows until it comes to a stop (this would be high tide), then as it drops it also accelerates towards the centre where the pendulum is moving at its fastest (mid tide), then once again it reaches the opposite top and slows to a stop (low tide) before reversing the process.
- So starting at low tide: (imperial units for illustration)
- The first hour, 1 twelfth of the tide would rise 0.5 ft.
- The second hour, 2 twelfths of the total will come in, therefore 1 ft.
- The third hour, the tide is starting to move quickly, and so 3 twelfths will come on, therefore 1.5 ft.
- At this point we are now passing mid tide.
- The fourth hour again is 3 twelfths, so another 1.5ft.
- The fifth hour the tide slows down again, 2 twelfths, therefore 1ft.
- The sixth and final hour, 1 twelfth would come in, and therefore 0.5ft.
The tide would then drop following the same pattern. This is a way of putting a numerical value on the interim period between high and low tide, where the tide is running particularly fast.
Rule of Twelfths diagram. Wikimedia (CC BY-SA 4.0).
b.
Using the equation, E = 1/2 ρ V3
We can work out the energy flowing through 1 m2 area perpendicular to the flow in the same way as with wind, except the density of water is 1,000 kg m-3 compared with 1.22 kg m-3 for air:
Tidal flow energy s -1 m-2 = 0.5 x 1,000 x 53 W m-2 = 62.5 kW m-2
This should be multiplied by the area swept out by the blades, the efficiency and the number of turbines:
Total tidal flow energy s -1 = 62.5 x 100 x 0.3 x 3.14 x 32 kW = 53 MW
This is potentially a huge amount of energy from 1 km length across the stream. Compared to a wind turbine, the propellers are moving very slowly and are unlikely to harm fish or marine mammals.
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