Types of renewable energy

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What is Renewable Energy?

Definitions vary but generally comprise energy sources which will last indefinitely from a human perspective. (There are no sources of energy which can actually be renewed from nothing – that would violate basic laws of Physics[1] and the concept of "renewable energy" has been criticised for the way its use has affected the way energy policy is debated and conducted[2]). Energy sources considered "renewable" include solar, wind, wave, tide, hydro, geothermal, and biomass. Wind, wave and biomass are derived from solar energy hitting the Earth and providing energy for plants to grow, evaporating water to create rain, and creating winds, which create waves. Geothermal energy comes from decay of radioactive isotopes or Uranium, Thorium and Potassium in the Earths core. Tidal energy is derived from the kinetic energy of the Moon's orbiting of the Earth and the Earth's orbit of the Sun, and has the effect that very slowly the Moon is moving further away from the Earth.

Not all renewable energy is carbon-free, environmentally friendly, small-scale, or safe, and the word is often used politically to mean any non-fossil fuel energy source except nuclear energy. A more useful term would be "sustainable energy" (the term chosen by the late David MacKay for his excellent book Sustainable Energy Without The Hot Air) – energy which we can continue to use as much of as we need, without harmful consequences such as climate change, air pollution, and other environmental degradation, for the forseeable future.

Important factors to consider with any energy source are:

  • Energy Density – how much land is required to produce a given amount of energy.
Energy density tells us what proportion of our land is needed to meet our energy needs from a particular source. For example Britain simply does not have enough land to grow enough biomass to supply all our energy needs even if we wanted to.
  • Capacity Factor (or load factor) – what proportion of its maximum output it produces on average.
Capacity factor is important because when a generator, such as wind or solar, is producing less than its maximum output, then demand for energy has to be met from other sources — which are generally fossil fuels or biomass.
  • Despatchability – can it be turned on when needed?
Despatchability matters because supply of electricity has to be exactly matched to demand at every instant, so sources of energy which can be turned on and off – or up and down – at will are essential to avoiding power cuts.
The overall EROEI of all our energy sources determines how much energy our civilisation has to spare for activities like producing food, housing, transport, education, industry etc.

Various schemes have been proposed for powering regions, countries or even the entire world using only "renewables" – some further restricting choices by excluding hydro or biomass. Most of these plans are non-scientific offerings of anti-nuclear "environmentalist" organisations; those that have been peer-reviewed have not been found favourable by the IPCC's assessments.

Solar energy

The sun's energy can be exploited in passive solar buildings, used for water heating or even cooking, or used to generate electricity via photovoltaic (PV) panels or concentrating solar power (CSP) plants. Solar generator output varies with weather and obviously with day and night, with peak output around midday, declining to zero by evenings. This is usually a poor match for demand, resulting in a "duck curve" where demand rises quickly whilst solar output is declining, resulting in challenges to operators of electricity systems to ramp up output from other sources (usually gas) quickly enough to make up the shortfall.

Some Concentrating solar plants have been built using molten salt to not only transfer heat from the sun to turbines to generate electricity, but to store heat so that the plant can continue generating power past sunset. Whilst PV does generate some useful power under overcast conditions, CSP requires direct sunshine to operate, so is only suitable for areas with very little cloud cover. Both PV and CSP require large areas of panels or mirrors, which have to be kept clean, which can pose challenges in windy, dusty areas.

The intensity of solar radiation is highest at low latitudes (nearest to the equator), and the variation between summer and winter power output is also lower. At higher latitudes (such as in Britain and Germany) the seasonal variation is such that winter output is effectively negligible and other forms of supply are needed. British energy experts at the Department of Energy and Climate Change assessed that solar is effectively useless for the UK energy mix, but it was encouraged with Feed-In Tariffs for political reasons.

Solar panels destroyed by Hurricane Irma: St Thomas, 2017

PV panels contain various toxic elements such as arsenic, cadmium and lead, and are not recycled. They are vulnerable to damage by storms, such as occurred when Hurricane Irma hit St. Thomas in 2017, and hurricane Maria hit Puerto Rico in 2018.

The Capacity Factor of solar PV installations vary from almost 30% in Arizona to under 10% in the UK.

The late Professor David MacKay discussed the potential for solar energy in his book "Sustainable Energy Without The Hot Air" (SEWTHA), pages 38-49.

More on Solar energy

Wind energy

Wind is a form of indirect solar energy, produced by convection currents in warmed air. Wind has been used for millenia to produce mechanical power for milling corn and pumping water, but nowadays increasingly big wind turbines are used to produce electricity. The power available from wind depends very strongly on the wind's speed – it is proportional to the cube of wind speed – and wind speeds are faster the higher one gets above the ground, so taller turbines can produce much more power than shorter ones. Wind tends to be more consistent (less intermittent) at higher altitudes so taller turbines also have slightly higher capacity factors. Wind speeds at sea also tend to be higher than on land so offshore wind farms such as those in the North Sea can have capacity factors up to about 45%, although around 30% is more common for wind farms in general.

However offshore wind farms are more expensive to build, and also more dangerous (in common with other maritime occupations, from fishing to oil and gas rig operations). Wind turbines also kill birds and bats. The numbers of birds killed are far fewer than those killed by domestic cats and by collisions with windows, but they tend to kill rarer, sometimes endangered, species such as eagles and other raptors.

The potential for wind energy in the UK is estimated by David MacKay in Sustainable Energy Without The Hot Air pages 32-34, and Professor MacKay explains the physics of Wind Energy in pages 263-268.

More on Wind energy

Hydropower

The water cycle

Hydropower is another form of indirect solar energy, produced by the sun's radiation heating water (on land and in the oceans) making it evaporate and rise into the atmosphere where it condenses into clouds and falls as rain (or snow). Some of it falls in hills and mountains where it has the potential to release energy as it descends to lower altitudes (and eventually back to the sea).

The main ways of exploiting this energy are with water-wheels or turbines in flowing rivers, or by building dams creating artificial lakes or reservoirs which store large amounts of water which can drive turbines as it is released. Dams with turbines driving electrical generators can produce large amounts of electricity with good capacity factor and despatchably: they are turn-on-and-off-able, very quickly.

See also the EIA's article Hydropower explained, and main Hydropower article on this site.

Articles announcing that a country has run for so-many days on renewables alone, such as this and this about Costa Rica, often feature pictures of wind turbines and solar panels when the prime energy source is actually hydro.

Pumped hydro

A hydroelectric dam where water released from the dam is collected in a lower dam (or goes to the sea) can be arranged so that water can be pumped back from the lower reservoir (or sea) to the upper reservoir, where it can generate electricity again later. These sorts of pumped storage schemes act like enormous batteries, effectively storing electricity (although there is a trade-off: only about three-quarters of the electricity put into pumping up the storage comes back as electricity generated). Pumped storage schemes such as that at Dinorwig in North Wales and Diablo Canyon in California have long been used to match the constant output from baseload power stations, especially nuclear, to demands which tend to be lower at night, when the pumped system can use surplus electricity to pump water and then release it for extra generation during peak periods in the the daytime.

Pumped storage may also be able to partner with solar to provide electricity 24*7, in certain parts of the world such as the proposed Valhalla project in Northern Chile.

Safety and environmental

Hydroelectricity generally has a very low carbon footprint. Hydro dams are mostly quite safe, but when they do go wrong they can be very dangerous: the world's worst energy disaster (massively worse than Chernobyl) was the collapse of the Banqaio dam which killed around 200,000 people and made about 12 million homeless). Building dams often floods homes and displaces communities, and has large ecological impacts, especially downstream where patterns of river flow and flooding are disrupted by the dam. Silt carried in the river feeding the dam can also deposit in the dam reservoir, reducing its storage capacity. Climate change is likely to alter patterns of rainfall and is already impacting hydroelectric production in some parts of the world.




Special Report on Renewable Energy Sources and Climate Change Mitigation IPCC Working Group III; 2012

The IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation (SRREN) provides a comprehensive review concerning these sources and technologies, the relevant costs and benefits, and their potential role in a portfolio of mitigation options.
For the first time, an inclusive account of costs and greenhouse gas emissions across various technologies and scenarios confirms the key role of renewable sources, irrespective of any tangible climate change mitigation agreement.

GLOBAL TRENDS IN RENEWABLE ENERGY INVESTMENT 2016 UNEP / Bloomberg

Renewable Energy BBC Radio 4 - The Bottom Line

Intermittency, capacity factor, grid integration

Renewables and grid reliability Challenges integrating renewables

ANALYSING TECHNICAL CONSTRAINTS ON RENEWABLE GENERATION TO 2050 A report to the Committee on Climate Change; Mar 2011

Huge Boost For Renewables' Output And Reliability Just From Location Planning Jeff McMahon; Forbes; 22 Apr 2016

California could boost its solar and wind energy output by nearly 50 percent and reduce volatility just by planning where new plants are installed, according to a Stanford economist. Solar and wind developers tend to cluster installations in locations that offer the highest revenues, said Frank Wolak, director of Stanford University’s Program on Energy and Sustainable Development. But these same locations accentuate volatility, raising the peaks when all are producing and deepening the troughs when all shut down.

Europe has adopted clean energy with so much gusto they've created a strange new problem Rebecca Harrington; Business Insider; 30 Mar 2016

Europe gets so much of its energy from renewables that many leading countries are now facing a new problem: updating their energy grids to handle it all.

Managing Flexibility Whilst Decarbonising the GB Electricity System - Executive Summary and Recommendations Energy Research Partnership; Aug 2015

The Energy Research Partnership has undertaken some modelling and analysis of the GB electricity system in the light of the carbon targets set by the Committee on Climate Change. Firstly a brief examination was made of the German and Irish markets with the hope of learning from their advanced penetration of variable renewables. Secondly a new model, BERIC, was written to simultaneously balance the need for energy, reserve, inertia and firm capacity on the system and its findings compared with simpler stacking against the load duration curve. The intention was to assess the need for flexibility on the system but some broader conclusions also emerged: A zero- or very low- carbon system with weather dependent renewables needs companion low carbon technologies to provide firm capacity
The modelling indicates that the 2030 decarbonisation targets of 50 or even 100 g/kWh cannot be hit by relying solely on weather dependent technologies like wind and PV alone. Simple merit order calculations have backed this up and demonstrated why this is the case, even with very significant storage, demand side measures or interconnection in support. There is a need to have a significant amount of zero carbon firm capacity on the system too - to supply dark, windless periods without too much reliance on unabated fossil. This firm capacity could be supplied by a number of technologies such as nuclear, biomass or fossil CCS

The Duck Pond Bonnie Marini; Power Engineering; 22 Mar 2016

The use of renewable power generation continues to grow around the globe. The challenge introduced with renewable generation is that it can be interrupted by timing and weather, and this variance affects the stability of the power produced. The sum of all generation must meet the demand at the very instant the demand is manifested-simply put, most of the electricity around the globe is produced and used in the very same moment. Without a solution for large-scale, cost-effective energy storage, the only way to fill the gaps in fluctuating generation from renewable resources is by partnering dispatchable power generation.
The California duck curve has become synonymous in the industry with the shape of renewable generation. The renewable duck generation rides on top of the rest of the generation portfolio, which is referred to as the duck pond. This paper discusses the changes in demand for this pond of dispatchable generating resources as they react to the presence and growth of the duck.

The Biggest Solar Breakthrough You've Never Heard Of William Pentland; Forbes; 12 Nov 2015

The breakthrough is a business model used to integrate distributed energy technologies into the mainstream electric grid.
For the first time in the history of the modern power grid, a non-utility, non-RTO entity called GridSolar is managing a part of the electric power grid. This would be illegal in any state other than Maine. GridSolar is demonstrating the value distributed generation, demand response and energy efficiency can provide to the power grid and ratepayers. In the process, it is turning the central argument made by utility trade groups against distributed energy on its head. Rather than imposing additional costs on ratepayers, distributed energy can significantly reduce the economic burden borne by ratepayers.

Balancing Europe’s wind-power output through spatial deployment informed by weather regimes Christian M. Grams, Remo Beerli, Stefan Pfenninger, Iain Staffell, Heini Wernli; Nature Climate Change letters; 17 July 2017

As wind and solar power provide a growing share of Europe’s electricity1, understanding and accommodating their variability on multiple timescales remains a critical problem. On weekly timescales, variability is related to long-lasting weather conditions, called weather regimes2, 3, 4, 5, which can cause lulls with a loss of wind power across neighbouring countries6. Here we show that weather regimes provide a meteorological explanation for multi-day fluctuations in Europe’s wind power and can help guide new deployment pathways that minimize this variability. Mean generation during different regimes currently ranges from 22 GW to 44 GW and is expected to triple by 2030 with current planning strategies. However, balancing future wind capacity across regions with contrasting inter-regime behaviour—specifically deploying in the Balkans instead of the North Sea—would almost eliminate these output variations, maintain mean generation, and increase fleet-wide minimum output. Solar photovoltaics could balance low-wind regimes locally, but only by expanding current capacity tenfold. New deployment strategies based on an understanding of continent-scale wind patterns and pan-European collaboration could enable a high share of wind energy whilst minimizing the negative impacts of output variability.

Weatherwatch: can we keep the lights on when the wind fails to blow? Kate Ravilious The Guardian; 9 Jan 2018

Rapid growth in both solar and wind (the UK now has more offshore wind power capacity than any other country in the world) has enabled the UK to achieve these impressive statistics, but will the rise in renewables also make UK power more vulnerable to the whims of British weather?
Researchers working on the European Climatic Energy Mixes project have been investigating future risk by assessing how the UK would fare with a repeat of the unusually cold winter of 2009-10. From mid-December 2009 a southward-displaced jet stream allowed cold air to pour in from eastern Europe, bringing widespread snow and plunging temperatures. The mean UK temperature for the entire winter was just 1.5C, the lowest since 1978-79 when it was 1.2C . As a result power demand surged, with electricity consumption between 10 and 20% above average on a number of occasions.
Back in 2010 renewables provided less than 10% of UK electricity. But similar weather now might create a strain. “The low wind conditions in a repeat of winter 2009-10 would lead to a substantial reduction in wind power production over the season, which could lead to increased risks to electricity supply availability when combined with an increased demand due to low temperatures,” writes meteorologist Emma Suckling from the University of Reading. Winters like this might be getting rarer, but we still need a contingency plan when the wind fails to blow.
China

China proposes $50+ trillion Global UHV grid connecting all power generation including massive wind farm at the North Pole by 2050 Next Big Future; 31 Mar 2016

China is proposing a $50+ trillion global energy grid. Global Energy Interconnection (GEI), a vision of a world power grid, was outlined by the State Grid Corporation of China ("State Grid"). It would be based upon a global network of Ultra High Voltage power lines connecting global power generation including massive wind farm at the North Pole and solar power from equatorial areas to energy users around the world.

China’s Vision of a Global Grid Zhenya Liu; The Energy Times; 22 Mar 2016

This is the first of a two-part series, excerpted and edited, from a speech by Zhenya Liu, chairman of the State Grid Corp. of China, delivered recently at IHS CERAWeek in Houston.

Storage

Marine - Tide and Wave

Will Tidal and Wave Energy Ever Live Up to Their Potential? SOPHIA SCHWEITZER; Yale Environment 360; 15 Oct 2015

As solar and wind power grow, another renewable energy source with vast potential — the power of tides and waves — continues to lag far behind. But progress is now being made as governments and the private sector step up efforts to bring marine energy into the mainstream.

Tidal Energy / Tide Power *

Wave

Wave David MacKay; SEWTHA

Carnegie wave energy

CETO wave energy technology that converts ocean swell into zero-emission renewable power and desalinated freshwater.
Uses seafloor-tethered buoys. CETO 6 ~1MW will contain hydraulic - electrical generators, earlier ones seem to have had hydraulic to shore, electrical conversion onshore(?)

CETO commercial scale unit

The CETO 6 design builds on the experience gained in all previous CETO generations and incorporates some important improvements.
The diameter of the buoyant actuator has the most significant influence on power output and has been increased to approximately 20m from the 7m diameter 80kW unit successfully tested at the Garden Island site in 2011 (pictured below) and the most recent 11m diameter, 240kW units tested in 2015 at the same site (pictured below).
Apart from being larger, CETO 6 will also incorporate the power generation offshore, inside the buoy rather than onshore as with the current CETO 5 generation being deployed for the Perth Wave Energy Project. Locating the power generation within the buoy removes the need to attach pumps, accumulators and other hydraulic components to the seabed, avoiding the requirement for offshore heavy lift vessel capacity and reducing the offshore installation and maintenance time and cost.The demonstration of CETO incorporating subsea generation and transmission of electrical power will allow Carnegie to take advantage of deeper, more distant to shore wave resources and significantly increases the size of the commercial market for CETO and allow greater responsiveness in the CETO control system.
The Perth Wave Energy Project involved the design, construction and operation of three 240kW CETO 5 units which produced and sold both power and water to the Australian Department of Defence who operate Australia’s largest naval base, HMAS Stirling, on Garden Island and operated for over 14,000 cumulative hours across four seasons.
Work began in 2013 on the next generation CETO 6 design which has a targeted capacity of 1MW. The CETO 6 generation will again be demonstrated first at Carnegie’s Garden Island site in Western Australia ahead of international installations.

EcoWavePower

Eco Wave Power

talks about 1MW

Gibraltar

Gibraltar wave power project surfs up possibilities across Europe Nnamdi Anyadike; Power Technology; 4 Oct 2016

Eco Wave Power’s (EWP) energy project in Gibraltar - the first such grid-connected plant and the only wave energy plant in Europe operating multiple units under commercial power purchase agreement (PPA) terms
in 2014, EWP signed a PPA with Gibraltar for delivery of a 5MW ocean power plant. Phased construction of the Gibraltar plant, located at the Ammunition Jetty, began last year and it is already exporting electricity into the power grid. The system is currently composed of eight ocean energy converter units that supply 100kW, but when completed, with the help of an EU grant, the array will produce 5MW. It is then expected to meet 15% of Gibraltar’s electricity demand. Although currently still in the design phase, the additional units will be much larger than the existing ones.

biological

Biomass / Biofuels *

compost

7 Steps to Build a Compost Water Heater For Hot Water Abundance KATRINA SPADE; Walden Labs; 5 AUG 2015

algae

Urban algae farm eats highway pollution and turns it into organic fuel

Geothermal

New Zealand

Te Ahi O Maui geothermal ready to drill Gisborne Herald NZ; 28 Apr 2016

GISBORNE-based Eastland Group expects to encounter temperatures three times higher than the hottest surface temperature ever recorded on Earth when it drills into the Kawerau geothermal reservoir next month. Following years of planning, the $100m Te Ahi O Maui geothermal project to build a 20mW geothermal power plant 2.3km east of Kawerau is now ready to enter its first production well-drilling phase on land owned by the A8D Ahu Whenua Maori Trust. Te Ahi O Maui project panager Ben Gibson said site works were under way to prepare the well pads and a well-drilling rig would be transported on site later this month. A production well will start on May 10. The first stage of drilling, known as ‘‘spudding’’, will culminate in a 12cm-wide hole into the Kawerau geothermal reservoir. “Extensive field monitoring and computer-based modelling has shown we can expect the drilling equipment to pass through layers of varying substrates and pockets of incredibly hot geothermal steam and fluid, which could be between 200-350 degrees Celsius. “It’s this high-temperature fluid and steam that will ultimately fuel the geothermal power plant.

East Africa

How Kenya is harnessing geothermal energy to power its growing economy Amy Yee; Independent; 4 Mar 2018

Tapping into heat energy from the East African Rift has helped increase electrical access in Kenya. But making this widely available can be a struggle, and developers face environmental challenges with this seemingly green source of power

Scotland

Heat energy beneath Glasgow British Geological Survey (BGS)

The BGS is working with Glasgow City Council to look into the use of heat energy from the ground to help to warm Glasgow's homes and communities.
Our studies are helping to identify which parts of the city would offer the best prospects of supplying this kind of energy; looking at the potential heat within minewaters, superficial deposits and bedrock aquifers beneath Glasgow.
This new source of energy could help Glasgow to meet government targets to ensure 11 per cent of heat demand comes from renewable sources by 2020.
It will also contribute to Glasgow's ambition, under the Sustainable Glasgow partnership, to become one of Europe's most sustainable cities within the next ten years. BGS expertise can help you find out more about the heat energy beneath Glasgow.

USA

The Future of Geothermal Energy Idoho National Lab; Nov 2006

Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century
Recent national focus on the value of increasing our supply of indigenous, renewable energy underscores the need for reevaluating all alternatives, particularly those that are large and well distributed nationally. This analysis will help determine how we can enlarge and diversify the portfolio of options we should be vigorously pursuing. One such option that is often ignored is geothermal energy, produced from both conventional hydrothermal and Enhanced (or engineered) Geothermal Systems (EGS). An 18-member assessment panel was assembled in September 2005 to evaluate the technical and economic feasibility of EGS becoming a major supplier of primary energy for U.S. base-load generation capacity by 2050. This report documents the work of the panel at three separate levels of detail. The first is a Synopsis, which provides a brief overview of the scope, motivation, approach, major findings, and recommendations of the panel. At the second level, an Executive Summary reviews each component of the study, providing major results and findings. The third level provides full documentation in eight chapters, with each detailing the scope, approach, and results of the analysis and modeling conducted in each area.

MIT-led panel backs 'heat mining' as key U.S. energy source MIT News; 22 Jan 2007

A comprehensive new MIT-led study of the potential for geothermal energy within the United States has found that mining the huge amounts of heat that reside as stored thermal energy in the Earth's hard rock crust could supply a substantial portion of the electricity the United States will need in the future, probably at competitive prices and with minimal environmental impact.
An 18-member panel led by MIT prepared the 400-plus page study, titled "The Future of Geothermal Energy" (PDF, 14.1 MB). Sponsored by the U.S. Department of Energy, it is the first study in some 30 years to take a new look at geothermal, an energy resource that has been largely ignored.
The goal of the study was to assess the feasibility, potential environmental impacts and economic viability of using enhanced geothermal system (EGS) technology to greatly increase the fraction of the U.S. geothermal resource that could be recovered commercially.
Although geothermal energy is produced commercially today and the United States is the world's biggest producer, existing U.S. plants have focused on the high-grade geothermal systems primarily located in isolated regions of the west. This new study takes a more ambitious look at this resource and evaluates its potential for much larger-scale deployment.

Footnotes and References

  1. The Laws of Thermodynamics, which are as well-proven as Gravity, tell us that we cannot get energy from nothing, and that valuable forms of energy (such as electricity) inevitably degenerate into less useful forms (such as low-grade heat) - see for example this explanation at The Khan Academy
  2. "Abandoning the concept of renewable energy", Atte Harjanne, Janne M. Korhonen, Energy Policy, 29 Dec 2018 (The full article can be downloaded from this coauthor download link)
    Abstract
    Renewable energy is a widely used term that describes certain types of energy production. In politics, business and academia, renewable energy is often framed as the key solution to the global climate challenge. We, however, argue that the concept of renewable energy is problematic and should be abandoned in favor of more unambiguous conceptualization.
    Building on the theoretical literature on framing and based on document analysis, case examples and statistical data, we discuss how renewable energy is framed and has come to be a central energy policy concept and analyze how its use has affected the way energy policy is debated and conducted. We demonstrate the key problems the concept of renewable energy has in terms of sustainability, incoherence, policy impacts, bait-and-switch tactics and generally misleading nature. After analyzing these issues, we discuss alternative conceptualizations and present our model of categorizing energy production according to carbon content and combustion.
    The paper does not intend to criticize or promote any specific form of energy production, but instead discusses the role of institutional conceptualization in energy policy.