Energy Resources and Sustainability
1. Renewable Energy Technologies
Learning outcomes
- I can describe how renewable energy technologies convert natural energy sources into useful energy.
- I can compare solar, wind, hydroelectric, geothermal, and tidal energy systems.
- I can explain the energy transfers involved in renewable energy generation.
- I can evaluate the advantages and limitations of renewable energy technologies.
- I can assess the suitability of renewable energy technologies for different environments.
Renewable Energy Technologies
Renewable energy comes from natural sources that are replenished on human timescales.
Major renewable energy resources include:
- sunlight
- wind
- flowing or falling water
- geothermal energy
- tides and ocean movement
Renewable energy technologies do not create energy. Instead, they transfer energy from naturally occurring stores or processes into useful forms, most commonly electrical energy.
The principle of conservation of energy still applies:
energy input = useful energy output + energy dissipated
Why Renewable Energy Matters
Modern societies require large quantities of energy for:
- electricity generation
- transportation
- heating and cooling
- manufacturing
- communication
- agriculture
Historically, much of this energy has come from fossil fuels such as coal, oil, and natural gas.
Renewable technologies provide alternatives that can reduce dependence on finite fuels and reduce greenhouse-gas emissions associated with energy production.
However, every renewable technology also has limitations and environmental impacts.
The best technology depends strongly on location, climate, geography, energy demand, cost, and available infrastructure.
Solar Energy
The Sun is the original energy source for many processes on Earth.
Solar energy can be converted directly into electricity using photovoltaic (PV) cells.
A photovoltaic cell is a semiconductor device that produces an electric current when it absorbs suitable electromagnetic radiation.
The simplified energy transfer is:
radiation from Sun → electrical energy
The electricity can then be:
- used immediately
- supplied to an electrical grid
- stored in batteries
- used to power equipment
Solar Photovoltaic Cells
Solar panels contain many photovoltaic cells connected together.
When sunlight reaches the cells, some of the incoming electromagnetic energy is converted into electrical energy.
Not all the incoming solar energy becomes electricity.
Some is:
- reflected
- absorbed as thermal energy
- lost through electrical resistance
Therefore:
solar panels are not 100% efficient
Factors Affecting Solar Power
The electrical output of a solar installation depends on factors including:
- intensity of sunlight
- panel area
- panel efficiency
- angle of panels
- orientation
- cloud cover
- shading
- temperature
- time of day
- season
- latitude
Solar power is therefore highly dependent on environmental conditions.
Worked Example: Solar Panel
A solar installation receives:
8000 W
of solar power.
Its efficiency is:
20%
Useful electrical power:
Puseful = efficiency × Pinput
Puseful = 0.20 × 8000
Puseful = 1600 W
The panels produce approximately 1.6 kW under these conditions.
Advantages of Solar Energy
Solar photovoltaic systems:
- require no fuel during operation
- produce no direct combustion emissions during operation
- can be installed at many scales
- have relatively few moving parts
- can be installed on rooftops
- can provide electricity in remote locations
Solar installations range from a few panels on a house to enormous solar farms.
Limitations of Solar Energy
Solar electricity production varies with:
- day and night
- weather
- seasons
- location
Large installations can also require substantial land area.
Manufacturing panels requires:
- raw materials
- energy
- industrial processing
Energy storage or backup generation may also be necessary when electricity is required during periods of low sunlight.
Solar Energy and Storage
Solar generation does not always match electricity demand.
For example:
Solar production may be high at midday but electricity may also be needed after sunset.
Excess electricity can sometimes be stored using:
- batteries
- pumped-storage hydroelectric systems
- thermal storage
- hydrogen production
Stored energy can then be used later.
Wind Energy
Wind turbines convert the kinetic energy of moving air into electrical energy.
The energy pathway is:
kinetic energy of wind → rotational kinetic energy of turbine → electrical energy
Moving air pushes against turbine blades.
The blades rotate a shaft connected, directly or indirectly, to a generator.
The generator converts mechanical energy into electrical energy.
Wind Turbine Components
Important components may include:
Blades — capture energy from moving air.
Rotor — rotating assembly containing the blades.
Nacelle — housing containing important machinery.
Generator — converts mechanical energy into electrical energy.
Tower — raises the turbine into stronger, more consistent winds.
Some designs also use gearboxes, while others use direct-drive generators.
Factors Affecting Wind Power
Wind turbine output depends strongly on wind speed.
Other important factors include:
- blade size
- air density
- turbine efficiency
- turbine height
- location
- spacing between turbines
A site with stronger and more consistent winds can generate much more electricity than a poor wind site.
Why Turbines Have Large Blades
The blades sweep through a circular area.
Larger blades intercept moving air across a larger area.
Therefore, large modern turbines can capture energy from a greater volume of moving air.
This is one reason modern wind turbines can be extremely large.
Onshore Wind
Onshore wind farms are constructed on land.
Advantages can include:
- relatively accessible construction
- easier maintenance than offshore installations
- no fuel requirement
- low direct operational emissions
Possible limitations include:
- variable wind
- visual impact
- noise concerns
- land-use conflicts
- impacts on wildlife if poorly located
Offshore Wind
Offshore wind farms are constructed in oceans or large bodies of water.
Offshore winds can often be stronger and more consistent.
Large turbines can also be installed away from densely populated areas.
However, offshore wind farms can be:
- more expensive to construct
- difficult to maintain
- exposed to harsh marine conditions
- dependent on underwater electrical cables
Hydroelectric Energy
Hydroelectric power uses moving water to generate electricity.
Many hydroelectric systems use water stored behind a dam.
Water at a high elevation has gravitational potential energy.
The energy pathway is:
gravitational potential energy → kinetic energy of water → rotational kinetic energy of turbine → electrical energy
As water flows downward, it turns turbines connected to generators.
Hydroelectric Dams
A dam can create a large reservoir.
Water stored high behind the dam has gravitational potential energy:
Eₚ = mgh
where:
- m = mass of water
- g = gravitational field strength
- h = height difference
Greater height can mean more gravitational potential energy is available per kilogram of water.
Worked Example: Hydroelectric Energy
Suppose:
1000 kg
of water falls:
50 m
Using:
g = 9.8 N/kg
Change in gravitational potential energy:
ΔEₚ = mgΔh
ΔEₚ = 1000 × 9.8 × 50
ΔEₚ = 490,000 J
If the system converts:
90%
into electrical energy:
Eelectrical = 0.90 × 490,000
Eelectrical = 441,000 J
Advantages of Hydroelectric Power
Hydroelectric systems can provide:
- large amounts of electricity
- relatively predictable generation
- rapid changes in power output
- long operating lifetimes
- relatively low direct emissions during operation
Reservoir systems can sometimes respond quickly to changes in electricity demand.
Limitations of Hydroelectric Power
Large dams can have major environmental and social effects.
Reservoir construction may:
- flood ecosystems
- alter river flow
- affect fish migration
- change sediment movement
- displace communities
- alter downstream ecosystems
Suitable geographical locations are also limited.
Run-of-River Hydroelectricity
Not all hydroelectric plants require enormous reservoirs.
Run-of-river systems use the natural flow of rivers with less water storage.
These systems may cause less flooding than large reservoir projects.
However, electricity production can vary as river flow changes.
Pumped-Storage Hydroelectricity
Pumped-storage hydroelectricity is primarily an energy-storage system, rather than an original energy source.
When excess electricity is available:
electrical energy → gravitational potential energy
Water is pumped uphill into an upper reservoir.
When electricity is needed:
gravitational potential → kinetic → mechanical → electrical
Water flows downhill through turbines.
This allows energy generated at one time to be used later.
Geothermal Energy
Geothermal energy comes from thermal energy within Earth.
Sources include:
- Earth's internal heat
- radioactive decay within Earth
- hot rocks
- underground hot water
- steam
In suitable regions, underground heat can be used to generate electricity or provide direct heating.
Geothermal Electricity Generation
A simplified geothermal electricity pathway is:
thermal energy within Earth → thermal/kinetic energy of fluid → mechanical energy of turbine → electrical energy
Hot underground water or steam can be brought toward the surface.
Steam, or another working fluid heated by the geothermal source, can turn a turbine.
The turbine drives a generator.
Geothermal Heating
Geothermal energy does not always need to generate electricity.
Hot underground water can sometimes be used directly for:
- building heating
- industrial processes
- greenhouses
- district heating
Avoiding unnecessary energy conversions can sometimes improve overall efficiency.
Advantages of Geothermal Energy
Geothermal systems can provide:
- relatively steady output
- electricity day and night
- low direct greenhouse-gas emissions compared with fossil-fuel combustion
- relatively small land requirements
- direct heating
Unlike solar and wind power, geothermal output is not normally dependent on daily weather.
Limitations of Geothermal Energy
High-quality geothermal resources are geographically limited.
Challenges can include:
- expensive drilling
- mineral-rich fluids
- corrosion
- release of some underground gases
- changes to underground water systems
- induced seismic activity in some projects
Location is therefore extremely important.
Tidal Energy
Tides are produced primarily by gravitational interactions involving Earth, the Moon, and the Sun.
Tidal energy technologies capture energy from predictable movement of ocean water.
Two important approaches are:
- tidal-stream turbines
- tidal barrages or lagoons
Tidal-Stream Turbines
Tidal-stream turbines operate somewhat like underwater wind turbines.
Moving water turns turbine blades.
Energy pathway:
kinetic energy of moving water → rotational kinetic energy → electrical energy
Because water is much denser than air, moving water can exert substantial forces on turbine blades.
Tidal Barrages
A tidal barrage is a structure built across a tidal estuary or bay.
Water levels change as tides rise and fall.
Water flowing through turbines can generate electricity.
The system uses differences in water level and tidal movement.
Advantages of Tidal Energy
An important advantage of tidal energy is:
predictability
Tidal cycles can be forecast accurately far into the future.
Other potential advantages include:
- renewable resource
- no fuel combustion during operation
- high energy availability at suitable sites
Limitations of Tidal Energy
Tidal projects require very specific locations.
Possible challenges include:
- high construction costs
- difficult underwater maintenance
- corrosion
- marine growth on equipment
- effects on marine ecosystems
- effects on sediment movement
- interference with navigation or fisheries
Comparing Renewable Technologies
| Technology | Main Energy Source | Major Advantage | Major Limitation |
|---|---|---|---|
| Solar PV | Solar radiation | Can be installed at many scales | Variable sunlight |
| Wind | Moving air | Strong output at good sites | Variable wind |
| Hydroelectric | Elevated/moving water | Controllable in reservoir systems | Major ecosystem changes possible |
| Geothermal | Earth's internal thermal energy | Steady output | Geographically limited |
| Tidal | Ocean tides | Highly predictable | Few suitable locations |
Intermittent Energy Sources
An intermittent or variable energy source does not provide constant power whenever demanded.
Solar power varies with:
- daylight
- clouds
- seasons
Wind power varies with:
- wind speed
- weather systems
This creates a challenge because electricity supply and demand must be balanced.
Dispatchable Energy
A dispatchable power source can adjust its electricity production when needed.
Reservoir hydroelectric plants can often increase or decrease output quickly.
Some geothermal plants can also provide relatively steady generation.
This can help support electrical grids containing large amounts of variable solar and wind generation.
Energy Storage
Energy storage helps match renewable generation with demand.
Possible storage technologies include:
- batteries
- pumped-storage hydroelectricity
- thermal storage
- compressed-air storage
- hydrogen
Energy storage involves energy conversions, and each conversion has some losses.
Therefore:
energy recovered < energy originally stored
for real systems.
Worked Example: Battery Storage
A battery system receives:
1000 kWh
of electrical energy.
Later it returns:
850 kWh
to the grid.
Round-trip efficiency:
efficiency = useful output / input × 100
efficiency = 850/1000 × 100
efficiency = 85%
Energy dissipated:
1000 − 850 = 150 kWh
Choosing the Right Technology
There is no single renewable technology that is best everywhere.
Engineers must consider local conditions.
Sunny Environments
Regions with:
- strong sunlight
- large numbers of sunny days
- suitable open space or rooftops
may be well suited to solar power.
Desert regions may receive excellent solar radiation.
However, engineers must also consider:
- dust
- high temperatures
- water availability for cleaning
- distance from electricity users
Windy Environments
Good wind-power locations often include:
- exposed coastlines
- offshore areas
- open plains
- mountain passes
- elevated regions
Engineers measure wind conditions over long periods before constructing large wind farms.
Mountainous and Wet Regions
Hydroelectric power is particularly suitable where there is:
- reliable rainfall
- large rivers
- significant elevation differences
- appropriate valleys for reservoirs
A flat, dry region would usually have much less hydroelectric potential.
Geologically Active Regions
Geothermal electricity can be particularly suitable in areas with accessible underground heat.
Examples can include regions associated with:
- volcanic activity
- tectonic boundaries
- hot underground rock
In these locations, high-temperature geothermal resources may occur relatively close to the surface.
Coastal Environments
Tidal power requires suitable coastal conditions.
Important factors include:
- tidal range
- tidal current speed
- seabed conditions
- marine ecosystems
- navigation
- distance from electricity infrastructure
A coastline alone does not automatically make tidal power practical.
Renewable Energy and Land Use
Different technologies require different amounts and types of space.
Solar farms may cover large land areas.
Wind farms require turbines to be separated, although some land between turbines can still be used for:
- agriculture
- grazing
- other activities
Hydroelectric reservoirs may flood large areas.
Geothermal plants can have relatively compact surface footprints.
Therefore, land use is an important engineering consideration.
Environmental Impacts
"Renewable" does not mean "no environmental impact."
Renewable technologies require:
- materials
- mining
- manufacturing
- transportation
- construction
- maintenance
- eventual recycling or disposal
They can also affect ecosystems.
A complete evaluation should consider the technology's entire life cycle.
Life-Cycle Analysis
A life-cycle assessment may consider:
raw material extraction → manufacturing → transportation → construction → operation → maintenance → decommissioning → recycling/disposal
This provides a more meaningful environmental comparison than looking only at the power station during operation.
Capacity and Capacity Factor
A power station's rated power is the maximum power it is designed to produce under specified conditions.
However, a renewable generator may not operate at rated power continuously.
The capacity factor compares its actual energy production over time with the energy it would have produced if it operated at rated power continuously.
For example, a solar installation rated at:
100 MW
does not produce:
100 MW × 24 hours
every day because sunlight is unavailable at night and varies during daylight hours.
This distinction is important when comparing energy technologies.
Power and Energy
Remember that power and energy are different.
Power:
P = E/t
Therefore:
E = Pt
Power tells us the rate of energy transfer.
Energy tells us the total amount transferred.
Worked Example: Wind Farm Energy
A wind farm produces an average power of:
50 MW
for:
4 hours
Energy:
E = Pt
Using megawatt-hours:
E = 50 × 4
E = 200 MWh
The wind farm generates:
200 MWh
during that period.
Worked Example: Solar Farm
A solar farm produces an average electrical power of:
30 MW
for:
6 hours
Energy generated:
E = Pt
E = 30 × 6
E = 180 MWh
Worked Example: Hydroelectric Plant
A hydroelectric station produces:
500 MW
for:
3 hours
Energy produced:
E = Pt
E = 500 × 3
E = 1500 MWh
or:
1.5 GWh
Renewable Energy and Electrical Grids
Modern electricity grids can combine several energy technologies.
For example:
solar + wind + hydroelectric + geothermal + storage
Different sources can complement one another.
Solar may produce strongly during sunny daytime periods.
Wind may generate at other times.
Hydroelectricity may respond to changing demand.
Batteries may store excess electricity and release it later.
This is sometimes called an energy mix.
Why Energy Mixes Are Useful
Depending entirely on one energy resource can create vulnerabilities.
For example:
A solar-dominated system may produce little electricity at night.
A wind-dominated system may experience periods of low wind.
Combining technologies can improve:
- reliability
- flexibility
- energy security
Storage and electricity transmission between regions can further improve reliability.
Engineering Trade-Offs
Engineers rarely choose technologies using only one criterion.
They may consider:
- efficiency
- power output
- reliability
- predictability
- construction cost
- operating cost
- environmental impact
- land use
- energy storage
- grid connections
- maintenance
- local climate
- geography
- resource availability
A technology that works extremely well in one region may perform poorly in another.
Example: Choosing Energy for an Island
Imagine an island with:
- strong sunshine
- strong coastal winds
- large tidal currents
- limited land
- no large rivers
Possible choices could include:
Solar: useful, but land availability may limit large installations.
Wind: potentially excellent, particularly offshore.
Hydroelectric: probably unsuitable because large rivers are absent.
Geothermal: depends on local geology.
Tidal: potentially useful if currents and seabed conditions are appropriate.
The best system might combine several technologies rather than rely on only one.
Example: Mountainous Region
Consider a mountainous region with:
- high rainfall
- deep valleys
- large rivers
- moderate sunlight
- weak winds
Hydroelectric power could be particularly suitable because both water flow and elevation differences are available.
Solar could still contribute, but wind might be less attractive if measurements show consistently weak winds.
Technology selection must be based on evidence about the location.
Common Mistakes
Saying Renewable Energy Means Unlimited Energy
Renewable resources are replenished naturally, but the rate at which useful energy can be captured is limited.
Saying Renewable Technologies Produce Energy
They convert or transfer energy from existing natural sources.
Energy is not created.
Saying Renewable Means Zero Environmental Impact
Renewable technologies still require materials, construction, land, maintenance, and eventual disposal or recycling.
Confusing Power and Energy
Power is the rate of energy transfer.
Energy is the total amount transferred.
Assuming Solar Panels Work Only in Hot Places
Solar photovoltaic panels require light, not high temperature.
Sunny conditions are more important than hot conditions.
Assuming Wind Turbines Need Constant Maximum Wind
Wind turbines operate across a range of wind speeds, although their output changes.
Extremely strong winds can also require turbines to shut down for protection.
Assuming Hydroelectricity Has No Environmental Effects
Dams can significantly alter rivers and ecosystems.
Assuming Geothermal Energy Works Everywhere
High-quality geothermal resources depend strongly on geology.
Assuming Tidal and Wind Energy Are Equally Predictable
Wind conditions can change considerably.
Tides are highly predictable because their timing is governed largely by astronomical cycles.
Comparing Systems Only by Rated Power
A 100 MW solar farm and a 100 MW geothermal plant will not necessarily generate the same amount of energy over a year.
Actual generation depends on operating conditions and capacity factor.
Check Your Understanding
- Define renewable energy.
- Name five major renewable energy technologies.
- Describe the energy transfer in a photovoltaic solar panel.
- Give four factors that affect solar electricity generation.
- A solar installation receives 5000 W of solar power and converts 18% into electricity. Calculate its electrical output.
- Describe the energy transfers in a wind turbine.
- Why are modern wind turbine blades very large?
- Compare one advantage of onshore wind with one advantage of offshore wind.
- Describe the energy transfers in a hydroelectric power station.
- Calculate the gravitational potential energy lost by 2000 kg of water falling 40 m. Use g = 9.8 N/kg.
- Explain why reservoir hydroelectricity can help respond to changing electricity demand.
- Explain how pumped-storage hydroelectricity stores energy.
- What is the original energy source in a geothermal system?
- Describe the energy transfers in a geothermal power station.
- Give two advantages of geothermal energy.
- Give two limitations of geothermal energy.
- Explain how a tidal-stream turbine generates electricity.
- What is a major advantage of tidal energy compared with wind energy?
- Give two possible environmental impacts of tidal power.
- Explain why solar and wind energy are described as variable energy sources.
- Give three examples of energy-storage technologies.
- A battery receives 600 kWh and later returns 510 kWh. Calculate its round-trip efficiency.
- Explain why solar power might be suitable for a sunny desert region.
- Explain why hydroelectricity may be suitable for a wet mountainous region.
- Why might geothermal electricity be particularly suitable in some tectonically active regions?
- Explain why life-cycle analysis is important when evaluating renewable technologies.
- A wind farm generates an average of 40 MW for 5 hours. Calculate the energy generated in MWh.
- Explain the difference between rated power and actual energy generation.
- Why might an electrical grid use several different renewable technologies instead of only one?
- A coastal region has strong sunshine, powerful tidal currents, weak winds, and no large rivers. Evaluate which renewable technologies would be most suitable.
Key Terms
Renewable energy — Energy obtained from natural sources that are replenished on human timescales.
Photovoltaic cell — Semiconductor device that converts solar radiation directly into electrical energy.
Solar panel — Collection of photovoltaic cells used to generate electricity from sunlight.
Wind turbine — Device that converts kinetic energy from moving air into electrical energy.
Rotor — Rotating part of a wind turbine containing its blades.
Generator — Device that converts mechanical energy into electrical energy.
Hydroelectricity — Electricity generated using energy associated with moving or elevated water.
Reservoir — Stored body of water, often held behind a dam.
Pumped storage — Energy-storage system that pumps water uphill and later releases it through turbines.
Geothermal energy — Thermal energy obtained from within Earth.
Tidal energy — Energy obtained from the periodic movement of ocean water caused primarily by gravitational interactions.
Tidal-stream turbine — Underwater turbine that extracts energy from moving tidal currents.
Tidal barrage — Structure across a tidal region that uses water movement and differences in water level to generate electricity.
Variable energy source — Energy source whose available output changes with environmental conditions.
Dispatchable generation — Generation whose output can be controlled or adjusted to help meet demand.
Energy storage — System that stores energy so that it can be used at another time.
Rated power — Maximum power output a device or installation is designed to provide under specified conditions.
Capacity factor — Ratio of actual energy production over a period to the energy that would have been produced by continuous operation at rated power.
Energy mix — Combination of different energy sources used to supply energy.
Life-cycle analysis — Evaluation of impacts throughout extraction, manufacturing, operation, and end-of-life stages.
Key Takeaways
- Renewable energy comes from naturally replenished resources.
- Renewable technologies convert and transfer energy; they do not create it.
- Solar photovoltaic cells convert solar radiation directly into electrical energy.
- Wind turbines convert the kinetic energy of moving air into mechanical and then electrical energy.
- Hydroelectric plants convert gravitational potential and kinetic energy of water into electrical energy.
- Geothermal plants use thermal energy from within Earth.
- Tidal systems capture energy associated with predictable movement of ocean water.
- Every energy conversion has losses, so no real renewable system is 100% efficient.
- Solar and wind generation vary with environmental conditions.
- Tidal energy is variable but highly predictable.
- Reservoir hydroelectricity can often provide controllable generation.
- Geothermal power can provide relatively steady output where suitable geological resources exist.
- Energy storage can help balance differences between electricity generation and demand.
- Batteries and pumped-storage hydroelectricity are important examples of energy storage.
- P = E/t connects energy generation with power.
- High rated power does not necessarily mean high energy generation over time.
- Capacity factor helps describe how much a generator actually produces compared with continuous operation at rated power.
- Renewable technologies still have environmental impacts.
- Life-cycle analysis provides a more complete assessment than considering operation alone.
- Solar power is particularly attractive in regions with strong solar resources.
- Wind power requires suitable wind conditions.
- Hydroelectricity requires appropriate water resources and geography.
- Geothermal energy depends strongly on local geology.
- Tidal energy requires suitable coastal conditions.
- The most appropriate renewable technology depends on climate, geography, geology, cost, environmental effects, infrastructure, and energy demand.
- Combining different technologies can create a more reliable energy system.
The central engineering question is therefore not simply:
"Which renewable technology is best?"
It is:
"Which combination of technologies provides the required energy most reliably, efficiently, economically, and sustainably in this particular environment?"