9. Energy resources - Future energy supplies 3: renewable energy resources

Question 1

what are the properties of renewable energy resources?

Answer 1

• intermittency (not available all the time)
• difficult to predict how much energy can be harnessed
• low energy density
• cannot be stored unless they are converted into other forms of energy
• many modern day technologies have been developed to use fossil fuels and not renewable energy
• usually have low environmental impacts
• requires specific natural processes or geological positions
• size of available resource depends on the natural process that produces the energy
• many renewable technologies are not yet fully developed
• the cost of replacing existing equipment with new equipment for renewable energy is very high

Question 2

what are the problems with solar power?

Answer 2

1. intermittency: the availability and intensity of sunlight depends upon daily and seasonal cycles
2. reliability: changes in energy intensity caused by changes in cloud cover cannot be predicted accurately
3. energy density: the low energy density of solar power requires very large areas of solar collectors to harness significant amounts of energy. the angle of incidence also changes in daily and seasonal cycles so there is no single optimum position for a solar panel to harness solar power. this can reduce the energy density further

Question 3

explain the local constraints of solar power

Answer 3

solar power can be used anywhere in the world but it is most viable where light levels are highest such as dry sunny deserts. Systems that concentrate using parabolic reflectors only work where there is no cloud so the rays of light are parallel and reflect onto the absorber. Areas that are very long away from the equator have long summer days which increases the availability of solar power but winter days are short and light levels are low.

Question 4

What are the different ways to harness solar power?

Answer 4

1. Photothermal solar power
2. Passive solar architecture
3. Photovoltaic (PV) solar power
4. Heat pumps

Question 5

What is photothermal solar power?

Answer 5

Photothermal systems absorb sunlight to produce heat, usually to heat water for low-temperature uses such as space heating or domestic hot water. The heat harnessed by the photothermal panels can be retained in a thermal store for later use. This is usually a well insulated tank containing material such as water, sand or concrete. Molten salt can be used if the energy has been concentrated to produce much higher temperatures.

Question 6

What is passive solar architecture?

Answer 6

Buildings can be designed to maximise the absorption of sunlight for heating without the use of active working equipment. Overheating in the summer can be reduced with a fixed solar screen (brise soleil) that deflects the sunlight, adjustable screens or by ventilation.

Question 7

What are heat pumps?

Answer 7

A heat pump uses the change in state of a fluid from liquid to gas to absorb the heat from the environment and releases it within a building when the gas condenses to a liquid. The change in state is caused by changes in pressure using a compressor pump to cause the gas to condense and a pressure relief valve to cause the liquid to boil. The heat source may be the atmosphere or the ground. Heat pumps absorb heat energy from low energy-density sources and produce higher temperatures in the building to be heated. The energy released into the building can be four times as much as the energy to run the heat pump, although less is released if the temperature of the heat source is lower. NB heat pumps can also harness other sources of heat such as geothermal energy.

Question 8

What is photovoltaic (PV) solar power?

Answer 8

When a PV cell absorbs photons of light, electrons are dislodged from atoms in the upper layer of the PV cell. These will flow along an electrical conductor from this electrically negative layer to the relatively positive lower layer. The moving electrons provide the electric current to power electrical appliances.

Question 9

What are the different types of PV cells and how efficient are they?

Answer 9

A wide variety of improved PV cells have been developed. Some have lower efficiencies but are cheaper to manufacture, while more expensive ones may have higher efficiencies at converting light to energy.

Multi junction - 46% max efficiency
Single junction gallium arsenide - 29.1% max efficiency
Crystalline silicon - 27.6% max. efficiency
Organic cells - 20.1%
Amorphous silicon - 13.6%

Question 10

What are the environmental impacts of manufacturing solar panels?

Answer 10

Making solar panels requires the extraction and processing of materials such as metals, plastics, paints, and silicon. Making PV solar panels produces toxic wastes such as silicon tetrachloride and small amounts of cadmium. These can be controlled but this adds to the manufacturing costs.

Question 11

What are the environmental impacts of solar power during use?

Answer 11

Solar panels do not require much maintenance, although cleaning requires water which may be scarce in the areas best suited for solar power. Large solar farms can occupy land that would have been used for other purposes, although there are very large areas of urban roof space that could have panels installed without any land use conflicts. In desert areas, they can make land productive although there may be environmental impacts on local flora and fauna.

Question 12

What are the features of multi-junction photovoltaic cells?

Answer 12

Multiple layers made of different materials, each of which absorbs different wavelengths of light. This means that a greater amount of the light available is absorbed and converted into electricity.

Question 13

What are the features of anti-reflective surfaces?

Answer 13

PV cells with smooth surfaces reflect about 30% of the light that hits them. Having a grooved or textured surface reflects light into the cells rather than away from them. Some designs mimic the structure of corneas of moth eyes which are very efficient at absorbing light.

Question 14

What are the features of concentrating solar power (CSP) with thermal storage

Answer 14

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Question 15

What are the features of photovoltaic/thermal hybrid systems (PVT systems)?

Answer 15

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Question 16

What are the features of transparent PV cells?

Answer 16

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Question 17

What are the features of heliostats?

Answer 17

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Question 18

What are the features of self-cleaning panels?

Answer 18

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Question 19

What is hydroelectric power (HEP)?

Answer 19

The gravitational potential energy of rainwater that lands on upland areas can be harnessed as kinetic energy as it flows downhill. The kinetic energy of flowing water has been used for thousands of years to provide mechanical power and, more recently, to generate electricity. Water wheels have been replaced by turbines which are more efficient.

Question 20

What does practical exploitation of HEP depend on? (2)

Answer 20

• requires a large enough volume of water available

- suitable topography that creates high water pressures or high flow velocities

Question 21

What does the ideal site for harnessing HEP include?

Answer 21

• large water catchment area
• high total rainfall, evenly distributed throughout the year
• low water turbidity
• impermeable bedrock beneath the reservoir
• low seismic activity
• suitable topography - narrow exit to a large basin
• no serious land-use conflicts
• close to consumers, or electricity grid, to reduce transport costs

Question 22

What are the environmental impacts of reservoir creation for HEP?

Answer 22

• Dam construction requires large amounts of material such as rock, sand, gravel and cement. Extraction, processing and transport of the materials requires energy.
• access to roads may also be required
• the reservoir created by the dam will flood land which can cause the loss of wildlife habitats, farmland, and homes.
• the reservoir provides an area of static water in which suspends soils and dead organic matter carried by the river sink. Decomposition of DOM can produce anaerobic conditions and the release of methane gas.
• the reservoir can also have a positive impact on wildlife by providing a new habitat for aquatic wildlife.

Question 23

What are the environmental impacts of HEP on the river?

Answer 23

• sedimentation in the reservoir can reduce turbidity downstream and the replenishment of nutrients onto the flood plain around the river.
• the natural flow fluctuations of the river are replaced by the water flow that is controlled by the power station operators. This may involve a constant flow rate to produce the maximum power output. The environmental impact of changes in flow rate depend upon the difference between the natural and new flow regimes.

Question 24

How are HEP stations used to meet peak demands for electricity?

Answer 24

The reservoir stores surplus water when the flow rate of the river upstream is high. The reservoir is used to maintain water flow through the power station when river flow is lower. So, as a result, the water flow rate through the dam can can vary greatly.

Question 25

What are the environmental impacts of altering the natural seasonal flow of the river to a constant flow? (3)

Answer 25

• Dry season sandbanks used by nesting birds or turtles are lost
• The loss of periods of high flow allows sediments to build up so gravel fish spawning sites are lost
• The loss of high river levels downstream stops the seasonal flooding of the surrounding land which may be essential for plants, breeding fish, and other aquatic species.

Question 26

What are the environmental impacts of sudden flow increases from a regime of constant flow? (3)

Answer 26

• Increased turbidity caused by high flow rates makes it difficult for fish-eating birds to see their food
• Reduced light penetration makes photosynthesis by aquatic plants more difficult
• Species that cannot resist high flow rates may be washed away

Question 27

What are the environmental impacts of HEP on aquatic species? (2 examples)

Answer 27

The dam and reservoir divide the river into two sections which prevents the movement of aquatic species:

• Fish such as salmon and sturgeon cannot move upstream to spawn
• Aquatic animals that live in flowing water, such as platypus, may not be able to pass the dam so populations become isolated.

Question 28

What are 2 new HEP technologies?

Answer 28

1. Low-head turbines - some new developments allow increased use of low-head locations where the water drops a short distance
2. Large HEP schemes - these are being constructed in many countries with expanding industrial economies, for example, China, Brazil)

Question 29

What are the pros and cons of water wheels?

Answer 29

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Question 30

What are Kaplan turbines?

Answer 30

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Question 31

What are helical turbines?

Answer 31

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Question 32

What are micro-hydro schemes?

Answer 32

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Question 33

What are the 2 main types of wind turbines?

Answer 33

Most aerogenerators have blades or vanes that rotate around an existing in either the vertical or horizontal plane:

• vertical axis wind turbine (VAWT)
• horizontal axis wind turbine (HAWT)

Question 34

How has the use of wind turbines changed?

Answer 34

Early uses of wind power harnessed kinetic energy of the winds to drive machinery such as grain mills and water pumps. Modern wind turbines generate electricity.

Question 35

What are the advantages of HAWTs? (2)

Answer 35

• technology is more advanced and well established than VAWTs
• higher efficiency than VAWTs

Question 36

What are the disadvantages of HAWTs? (3)

Answer 36

• stress cracking can occur at the base of the blades, caused by cyclical gravitational forces
• taller towers are needed to keep the blades above the ground
• the weight of the generator requires a stronger tower than for VAWTs where the generator can be at ground level

Question 37

What are the advantages of VAWTs? (4)

Answer 37

• turbines are driven by wind from any direction so they can be used where winds are turbulent such as cities
• turbines do not need to be turned to face into the wind so there is no need for a motor
• VAWTs are quieter than HAWTs
• operate at lower wind velocities

Question 38

What are the disadvantages of VAWTs? (2)

Answer 38

No very large VAWTs have been built

- lower efficiency as the blades are not absorbing wind energy throughout their rotation

Question 39

What are the land use conflicts of wind power farms?

Answer 39

• ecological impacts
• land requirement
• telecommunication interference (aerogenerators can interfere with radio and radar systems)
• public opposition

Question 40

How can the problem of ecological impacts of wind power be mitigated?

Answer 40

Turbines may be located away from bird migration routes, high bat populations, or sensitive habitats such as bogs where turbine foundations or access tracks may affect the hydrology of the area.

Question 41

How can the problem of conflicting land requirements involving wind power be mitigated?

Answer 41

The turbines in a windfarm require a large area of land over which the aerogenerators are dispersed. To minimise the ‘wind shadow’ effect aerogenerators are normally spaced with an interval of three to five times the diameter of the blades. The land between the aerogenerators can still be used for purposes such as agriculture, so the actual amount of land that is ‘lost’ is much smaller than the overall area of the windfarm.

Question 42

How can public opposition to wind farms be mitigated?

Answer 42

Some people object to wind farms so they are often located away from scenic and urban areas.

Question 43

What are the locational constraints of wind power?

Answer 43

1. Wind velocity - The reliability and strength of winds are affected by the latitude, with the windiest areas being in temperate and polar regions. The topography of an area will also affect wind velocity as the friction and turbulence of surface features can slow the wind down.
2. Isolation - Small-scale use of wind power can provide electricity for isolated uses such as small rural communities. Large-scale wind farms may be located near the exiting electricity grid to avoid the cost of laying new cables to join the electricity grid.
3. Land use conflicts

Question 44

What are the environmental impacts of wind power? (>)

Answer 44

1. Manufacture and installation (production and transport of materials)
2. Noise (more aerodynamic designs are quieter)
3. Habitat damage (area needed for foundations of aerogenerators and for access paths and transformers)
4. Bird strikes (rotating blades may kill birds)
5. Bat deaths (the changes in air pressure caused by the passing blades can kill bats)

Question 45

Name 5 new wind power technologies

Answer 45

1. Blade-tip fins
2. Nacelle brushes
3. Direct drive turbines
4. Helical VAWT blades
5. Wind-assisted ships

Question 46

What are the benefits of blade-tip fins?

Answer 46

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Question 47

What are the benefits of 2?

Answer 47

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Question 48

What are the benefits of 3?

Answer 48

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Question 49

What are the benefits of 4?

Answer 49

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Question 50

What are the benefits of 5?

Answer 50

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Question 51

Where is the kinetic energy of waves greatest?

Answer 51

• where mean wind velocities are high and winds are consistent in strength and direction to allow the wave height to increase
• where there is a long ‘fetch’ - distance of open water over which waves can build up