Grid-scale renewable energy is becoming increasingly affordable and competitive, as national policies mandate a switch from fossil fuels. Renewables are becoming the investment of choice for new power generation, making this an exciting growth industry.
Overview
This module introduces specific equipment and operating conditions for three renewable power categories:
- Wind,
- Solar, and
- Geothermal.
And includes the following topics:
- The basic concept of a renewable,
- Power industry terminology,
- Onshore and offshore wind power
- Solar photovoltaic power and
- Concentrated solar power,
- Overcoming operational challenges, and finally the
- Environmental issues.
Introduction to Renewable Power Generation
The key factor driving renewables growth is the United Nations Paris Climate Accord Agreement of 2015.
UN membership set targets to ensure that global temperatures this century do not increase by more than 2°C above pre-industrial levels.
Today the world is not meeting these targets, and the U.S. stopped its participation in 2017. Europe is making the most progress, but more still needs to be done in power generation, home heating and transportation.
Renewable Energy Basic Concepts
Energy is defined as renewable when its source is replaced within a brief space of time — unlike fossil fuels, which take millions of years to develop.
Renewable electricity can be economically generated from a range of sustainable sources such as the wind blowing, the sun shining or underground heat, called geothermal. Hydropower and biomass are important too, but are not part of this module.
Wind and solar are classed as variable sources of energy because they depend on the intermittent availability of wind and sunlight. Geothermal facilities produce steady-state power.
Energy storage is the key to resolving the variability of wind and sun power. To learn more, visit our module on Energy Store Management.
Finally, fossil fuel power plants are typically placed near centres of demand and can receive fuel by rail, ship or pipelines.
In contrast, renewable power plants tend to be near their energy source, which can often be some distance from customers.
According to the BP Annual Review of World Energy, renewables need to grow by 400% to meet the Paris Climate Accords by 2040.
Power Industry Terminology
Before we get started, let’s introduce some of the terminology used to measure power consumption and generation.
The common term is a watt, which indicates how much power a device consumes, over a specific time period.
A watt is relatively small. So, residential and industrial electricity usage is measured in kilowatt-hours, or kWh — the energy used to keep a 1,000-watt appliance running for one hour.
A typical washing machine uses half a kWh per load. A typical household in the US uses about 11, 000 kWh annually.
In Europe, annual household power consumption varies widely: from around 2,000 kWh in Poland and the Baltic countries, to 8,000 – 10,000 kWh in Finland and Sweden.
The next unit up from a kilowatt is a megawatt — equal to 1,000 kilowatts. A megawatt hour, or MWh, typically refers to larger-scale industrial electricity usage.
A standard 500 MW power plant produces 3.5 billion kWh per year, which is enough energy to power 4 million light bulbs all year.
The industry tends to use megawatts and megawatt hours interchangeably.
Gigawatts, or GW, represent one million kilowatt hours.
This term is used to measure the contribution by wind and solar to a country’s total energy demand.
In 2020, Germany had over 29,500 wind turbines providing nearly 56 GW of wind power capacity. This is almost 29% of Germany’s total energy demand.
Onshore Wind Power
Onshore wind farms refer to turbines sited on land that are used to generate electricity. They are located in areas of high wind force, with low property and/or habitat value.
Onshore wind power is popular because:
Low-cost land allows for a large number of wind turbines.
Wind turbine farms can be constructed in months, unlike a natural gas power station, which can take over 10 years with environmental approvals.
Note that a single 2.5 MW wind turbine can produce enough power to meet the yearly needs of over 1,400 households.
One of the world’s largest onshore wind farms is the 20 GW Jiuquan Wind Power Base in China that covers approximately 15,000 acres or 59 km², which is the size of Manhattan.
How Wind Power Works
In a wind farm, large turbine blades are on top of towers that can be 45 meters above the ground.
The tower diameter is large enough to provide stability and this allows maintenance access and equipment to be moved from ground level.
As wind passes over the turbine blades, they slowly turn.
There are usually three blades, shaped like airplane propellers, to maximize the energy transfer.
The box shape at the top of the tower is called the nacelle.
It is on a pivoted bearing to allow the entire assembly to turn as wind direction changes.
Inside the nacelle the turbine blades turn a shaft which is connected to a generator, using a gearbox and brake to control speed.
The generated power is moved to the base of the tower and transformed to the right voltage before being exported to the grid.
Many wind farms now have battery storage to stabilize power output.
Optimal Locations – Wind Power
In Europe, the best wind power locations are:
For onshore – the coastal regions of northwest Europe, especially Ireland, Scotland and Norway.
For offshore – the North Sea, Irish Sea, Baltic Sea and the Western Approaches to the English Channel.
In the U.S. the best wind power locations are:
For onshore wind power – in the Rocky Mountains and Great Plains, often called “Tornado Alley.”
For offshore – US wind power is best along the Great Lakes and the Atlantic and Pacific coasts.
Interestingly, nearly 40% of the US population lives within 100 miles of the seashore.
Let’s look at offshore wind power in a little more detail… And there are two types.
Offshore Wind Advantages
Offshore turbines enjoy an unlimited resource of higher wind speeds and so are more productive than those on land.
The offshore wind market is growing faster than onshore.
Offshore turbines are larger, taller, and can collect more energy than their onshore counterparts.
One of the world’s largest offshore wind turbines is General Electric’s Haliade-X, a 12 MW turbine which has a 260-meter tower with 107-meter turbine blades longer that the wingspan of an A380 Airbus.
Offshore turbines tend to be placed far out at sea, allowing the installation of many wind turbines per square mile.
A case in point is the Walney Extension Offshore Wind Farm located in the Irish Sea, which is 15 km from shore.
The type of offshore wind farm design depends on whether it is in shallow or deep water.
Offshore Bottom-Founded Wind Power
Bottom-founded or fixed-to-seabed wind farms are installed in shallow water, like the southern North Sea in Europe or off America’s New England coast and the Gulf of Mexico.
- Wind turbines, rooted to the seabed by low-cost monopiles, are restricted to waters less than 50 metres deep. [a]
- Fixed, steel-jacketed structures can be used to a water depth in the range of 60 meters. [b]
- Fixed designs currently far outnumber floating facilities, but that is changing rapidly.
Offshore Floating Wind Power
Offshore floating wind power is best suited to parts of the world with deep water near coastlines, such as California, Hawaii and Scotland.
In deeper water, three types of offshore oil and gas industry designs have been demonstrated in various wind projects in Europe.
First discussed is a Tension Leg Platform, or TLP. They are tied to the seabed with tensioned tethers that eliminate most of the vertical motion — at one time a favourite design system for Shell.
Next discussed is a SPAR, the acronym for Single Point Anchor Reservoir. A SPAR is a gigantic cylindrical tube that floats in the water and is long and heavy enough to provide stability. Mooring lines hold a SPAR in position.The Hywind wind farm in Scotland, commissioned in 2017, uses a SPAR in a 220m water depth.
Semi-submersibles, or Semis, are mobile platforms that have widely spread columns to provide stability and pontoons for buoyancy.
They are anchored, and in the oil and gas industry have been used in over 1,500m water depths.
A key issue in deep water design is how the power is collected and moved the long distances to shore.
Offshore wind farms use subsea transmission cables, either connected directly to the grid or to large energy storage battery farms.
Most have remote instrumentation and sensors that monitor the operating conditions in real time.
In addition, drones are now being used to conduct visual inspections.
Maintenance workers are sent at regular intervals to make required repairs — quite a challenge when the tower is over 200m high!
Let’s now move on to solar power often called photovoltaic, or PV, power.
Solar PV Power
Photovoltaic is the science of converting light into electricity.
The word comes from the Greek word phōs meaning light and the word volt, after the Italian research physicist Alessandro Volta (1745-1827).
Grid-scale solar photovoltaic farms are found both:
· Onshore, and
· Floating on reservoirs and lakes.
Onshore solar farms can be as large as 400,000 m².
A typical 72,000 MWh per year solar farm can meet the annual power needs of approximately 19,000 European homes.
How Solar Power Works
To generate power, a solar panel is made up of a collection of photovoltaic, or PV, cells.
They absorb sunlight and produce direct current, or DC, energy.
Individual cells are consolidated into what is called an array, which can be controlled to automatically follow the sun during the day. [a]
PV-cell DC current is then changed into alternating current, AC, by an inverter before it is exported to the grid. [b]
Many large solar facilities can have backup generators or batteries to stabilize the power level sent to the grid. [c]
Photovoltaic solar farms use a variety of panel technologies, with the best designs reaching an efficiency of about 15%. This is only cost-effective because solar energy is free, and the land is usually very low-cost.
Panel cleaning is essential to ensuring optimal energy output. Most solar panels are tilted, so rainfall clears much of the debris and dust away.
But in desert regions, an increasing number of solar farms are using robots to clear the debris. In water-short, high-dust regions, operators are developing panels that deter dust accumulation, thus reducing the need for scheduled cleanings.
Optimal Locations – Solar Farms
In Europe, the best solar locations are southern Spain and France, Italy, Greece and Cyprus.
The best US locations for solar are the sunny southwestern states of New Mexico, Arizona and California.
Before we leave solar power, let’s spend some time on Concentrated Solar Power, abbreviated as CSP.
How Concentrated Solar Power Works
CSP is a utility-scale system able to generate electricity by using curved mirrors, called heliostats, to concentrate sunlight onto a receiver containing water, synthetic oil or salt. [a]
The receiver heat is converted to steam, with a condenser and preheater, similar to equipment in a stationary power plant. [b]
A turbine generator then produces electricity for the grid. [c]
The advantage of CSP is that daytime heat can be stored as hot water, with steady power throughout cloudy days and some power available for a few hours after sunset.
Today there are some 153 CSP plants in operation in 23 countries, according to the National Renewable Energy Laboratory of the US Department of Energy.
How Geothermal Works
Unlike solar and wind, geothermal power provides what is called baseload power; that is, steady power throughout the day, seasons and year.
With geothermal power, naturally heated, pressurized water reaching 182°C is pumped from a 4.5 km-deep well. [a]
As water flows upward and the pressure drops, some of the hot water turns into steam. At the surface, the steam is separated from the water [b]
and then used to power a turbine linked to a generator to produce electricity. [c]
Condensed steam is re-injected back into the underground reservoir, thus making this a sustainable resource. [d]
The world’s largest geothermal power plant complex is the 725 MW Geysers Geothermal Complex located north of San Francisco, which supplies 6,000 GW to the grid each year.
To learn more about steam, see our Thermal Power Plant Fundamentals module.
Optimal Locations – Geothermal
The most active geothermal resources are found along major tectonic plate boundaries, where most volcanoes are situated.
One is called the Ring of Fire, which encircles the Pacific Ocean. So, you will find geothermal plants in parts of Chile, California, Alaska, Japan and Indonesia.
In fact, Iceland uses geothermal energy for everything, including:
- Heat and power for people’s homes,
- Greenhouses to produce fresh food,
- Power for internet server farms, and
- Industrial applications, like aluminium production.
Let’s now discuss the main operational challenge for operators of wind and solar power installations:
To match fluctuating output with routine changes in demand. This can be tackled with remote, digital plant computer systems and equipment to stabilize power output.
Remote Power Systems
Traditional fossil fuel and geothermal plants are managed onsite.
Wind and solar power are different.
They rely on remote sensors that automatically and continuously communicate with a control centre, which can be hundreds of miles away.
Renewable power control centres monitor a range of variables that measure efficiency, including:
- Equipment condition,
- Weather forecasts,
- Wind speeds,
- Solar temperatures, and
- Dust conditions.
Once the data is processed and translated into meaningful displays, control centre operators can make real-time operational decisions to match prevailing environmental conditions with market demand.
Similar to other industries, renewable power operators are implementing cybersecurity measures to protect their assets from attack.
Stabilizing Power Output
In traditional coal and gas power generation, it is relatively easy to ramp up generation when more power is required; but with wind and solar, power fluctuates according to the unpredictable strength of the wind or the amount of cloud cover.
To overcome the variability or intermittency of solar and wind power, most industrial-scale renewable power projects are being integrated with energy storage.
An excellent example is the 315 MW Hornsdale Wind Farm in Australia, which consists of 99 wind turbines with a generation capacity of 315 MW.
Alongside, it has the 129MW Hornsdale Power Reserve — currently the largest commercial lithium-ion battery farm in the world.
The Australian Renewable Integration Study of 2020 forecasts that in the coming decade many of the country’s aging coal power plants will be replaced by increasingly cost-effective renewables and storage.
As a result, Australia’s cumulative wind and solar capacity could meet at least 75% of the country’s power needs by 2025.
For more detail on how all this works, see our module on Energy Storage Management.
Now that we have covered the basics of three forms of renewables, let’s wrap up with the environmental issues relevant to developing each type of future renewable generation capacity.
Environmental Issues
Solar Power – The development of grid-scale solar farms takes up extensive plots of land.
Also, some materials used to manufacture solar cells contain hazardous chemicals, which can pose a problem during decommissioning.
Wind Power – People do not like the visual impact of wind turbines on the landscape; others complain of noise and headaches, which is attributed to some propeller designs.
Environmentalists have found that that turbines located along major migration routes are killing wild birds. Turbine blades are huge and currently cannot be recycled.
Geothermal – Water-cooled geothermal power plants dispose of their heated water into local rivers, lakes and streams, potentially killing aquatic life.
Summary
You should by now understand some facts about renewables:
Renewables must grow by 400% by 2040 in order to meet Paris Climate Accords.
A MW is sufficient to supply the average power requirement for around 2,000 homes for one hour.
The best onshore location for wind in Europe is the coastal regions of the continent, and in the US is the “Tornado Alley” region.
Offshore turbines are more productive than those on land.
Solar farms can be located on land or even floating on a lake.
In Europe, the best location for solar projects is the Mediterranean region and in the US the best location is the Southwest.
Unlike wind and solar power which is variable, geothermal provides constant power throughout the year.
To overcome the variability or intermittency of solar and wind power, renewable power projects are being integrated with energy storage.
Sophisticated remote control systems match the variable output of a solar or wind farm with fluctuating market demand.
Each type of renewable power has its own unique environmental considerations.