Bioenergy

Sugarcane plantation to produce ethanol in Brazil

A CHP power station using wood to supply 30,000 households in France

Bioenergy is energy made from biomass, which consists of recently living (but now dead) organisms, mainly plants. Types of biomass commonly used for bioenergy include wood, food crops such as corn, energy crops and waste from forests, yards, or farms. The IPCC (Intergovernmental Panel on Climate Change) defines bioenergy as a renewable form of energy. Bioenergy can either mitigate (i.e. reduce) or increase greenhouse gas emissions. There is also agreement that local environmental impacts can be problematic.

Terminology

See also: Biofuel § Terminology

Biomass plant in Scotland.

Since biomass technically can be used as a fuel directly (e.g. wood logs), some people use the terms biomass and biofuel interchangeably. However, more often than not, the word biomass simply denotes the biological raw material the fuel is made of. On the other hand, the word biofuel is often reserved for liquid or gaseous fuels, used for transportation.

Input materials

Main article: Biomass (energy)

Simple use of biomass fuel (combustion of wood logs for heat).

Wood and wood residues is the largest biomass energy source today. Wood can be used as a fuel directly or processed into pellet fuel or other forms of fuels. Other plants can also be used as fuel, for instance corn, switchgrass, miscanthus and bamboo. The main waste feedstocks are wood waste, agricultural waste, municipal solid waste, and manufacturing waste. Upgrading raw biomass to higher grade fuels can be achieved by different methods, broadly classified as thermal, chemical, or biochemical:

Thermal conversion processes use heat as the dominant mechanism to upgrade biomass into a better and more practical fuel. The basic alternatives are torrefaction, pyrolysis, and gasification, these are separated mainly by the extent to which the chemical reactions involved are allowed to proceed (mainly controlled by the availability of oxygen and conversion temperature).

Many chemical conversions are based on established coal-based processes, such as the Fischer-Tropsch synthesis. Like coal, biomass can be converted into multiple commodity chemicals.

Biochemical processes have developed in nature to break down the molecules of which biomass is composed, and many of these can be harnessed. In most cases, microorganisms are used to perform the conversion. The processes are called anaerobic digestion, fermentation, and composting.

Comparison with other renewable energy types

Further information: Renewable energy § Bioenergy

To calculate land use requirements for different kinds of power production, it is essential to know the relevant surface power production densities. Vaclav Smil estimates that the average lifecycle surface power densities for biomass, wind, hydro and solar power production are 0.30 W/m², 1 W/m², 3 W/m² and 5 W/m², respectively (power in the form of heat for biomass, and electricity for wind, hydro and solar). Lifecycle surface power density includes land used by all supporting infrastructure, manufacturing, mining/harvesting and decommissioning. Van Zalk et al. estimates 0.08 W/m² for biomass, 0.14 W/m² for hydro, 1.84 W/m² for wind, and 6.63 W/m² for solar (median values, with none of the renewable sources exceeding 10 W/m²). Fossil gas has the highest surface density at 482 W/m² while nuclear power at 240 W/m² is the only high-density and low-carbon energy source. The average human power consumption on ice-free land is 0.125 W/m² (heat and electricity combined), although rising to 20 W/m² in urban and industrial areas. Generally, bioenergy expansion fell by 50% in 2020. China and Europe are the only two regions that reported significant expansion in 2020, adding 2 GW and 1.2 GW of bioenergy capacity, respectively.

Plants with low yields have lower surface power density compared to plants with high yields. Additionally, when the plants are only partially utilized, surface density drops even lower. This is the case when producing liquid fuels. For instance, ethanol is often made from sugarcane's sugar content or corn's starch content, while biodiesel is often made from rapeseed and soybean's oil content.

Smil estimates the following densities for liquid fuels:

Wheat fields in the USA.

Ethanol

• Winter wheat (USA) 0.08 W/m²
• Corn 0.26 W/m² (yield 10 t/ha)
• Wheat (Germany) 0.30 W/m²
• Miscanthus x giganteus 0.40 W/m² (yield 15 t/ha)
• Sugarcane 0.50 W/m² (yield 80 t/ha wet)

Jet fuel

• Soybean 0.06 W/m²
• Jathropa (marginal land) 0.20 W/m²
• Palm oil 0.65 W/m²

Biodiesel

• Rapeseed 0.12 W/m² (EU average)
• Rapeseed (adjusted for energy input, the Netherlands) 0.08 W/m²
• Sugar beets (adjusted for energy input, Spain) 0.02 W/m²

Eucalyptus plantation in India.

Combusting solid biomass is more energy efficient than combusting liquids, as the whole plant is utilized. For instance, corn plantations producing solid biomass for combustion generate more than double the amount of power per square metre compared to corn plantations producing for ethanol, when the yield is the same: 10 t/ha generates 0.60 W/m² and 0.26 W/m² respectively.

Oven dry biomass in general, including wood, miscanthus and Napier grass, have a calorific content of roughly 18 GJ/t. When calculating power production per square metre, every t/ha of dry biomass yield increases a plantation's power production by 0.06 W/m². Consequently, Smil estimates the following:

• Large-scale plantations with pines, acacias, poplars and willows in temperate regions 0.30–0.90 W/m² (yield 5–15 t/ha)
• Large scale plantations with eucalyptus, acacia, leucaena, pinus and dalbergia in tropical and subtropical regions 1.20–1.50 W/m² (yield 20–25 t/ha)

In Brazil, the average yield for eucalyptus is 21 t/ha (1.26 W/m²), but in Africa, India and Southeast Asia, typical eucalyptus yields are below 10 t/ha (0.6 W/m²).

FAO (Food and Agriculture Organization of the United Nations) estimate that forest plantation yields range from 1 to 25 m³ per hectare per year globally, equivalent to 0.02–0.7 W/m² (0.4–12.2 t/ha):

• Pine (Russia) 0.02–0.1 W/m² (0.4–2 t/ha or 1–5 m³)
• Eucalyptus (Argentina, Brazil, Chile and Uruguay) 0.5–0.7 W/m² (7.8–12.2 t/ha or 25 m³)
• Poplar (France, Italy) 0.2–0.5 W/m² (2.7–8.4 t/ha or 25 m³)

Smil estimate that natural temperate mixed forests yield on average 1.5–2 dry tonnes per hectare (2–2,5 m³, equivalent to 0.1 W/m²), ranging from 0.9 m3 in Greece to 6 m³ in France). IPCC provides average net annual biomass growth data for natural forests globally. Net growth varies between 0.1 and 9.3 dry tonnes per hectare per year, with most natural forests producing between 1 and 4 tonnes, and with the global average at 2.3 tonnes. Average net growth for plantation forests varies between 0.4 and 25 tonnes, with most plantations producing between 5 and 15 tonnes, and with the global average at 9.1 tonnes.

As mentioned above, Smil estimates that the world average for wind, hydro and solar power production is 1 W/m², 3 W/m² and 5 W/m² respectively. In order to match these surface power densities, plantation yields must reach 17 t/ha, 50 t/ha and 83 t/ha for wind, hydro and solar respectively. This seems achievable for the tropical plantations mentioned above (yield 20–25 t/ha) and for elephant grasses, e.g. miscanthus (10–40 t/ha), and Napier (15–80 t/ha), but unlikely for forest and many other types of biomass crops. To match the world average for biofuels (0.3 W/m²), plantations need to produce 5 tonnes of dry mass per hectare per year. When instead using the Van Zalk estimates for hydro, wind and solar (0.14, 1.84, and 6.63 W/m² respectively), plantation yields must reach 2 t/ha, 31 t/ha and 111 t/ha in order to compete. Only the first two of those yields seem achievable, however.

Yields need to be adjusted to compensate for the amount of moisture in the biomass (evaporating moisture in order to reach the ignition point is usually wasted energy). The moisture of biomass straw or bales varies with the surrounding air humidity and eventual pre-drying measures, while pellets have a standardized (ISO-defined) moisture content of below 10% (wood pellets) and below 15% (other pellets). Likewise, for wind, hydro and solar, power line transmission losses amounts to roughly 8% globally and should be accounted for. If biomass is to be utilized for electricity production rather than heat production, note that yields have to be roughly tripled in order to compete with wind, hydro and solar, as the current heat to electricity conversion efficiency is only 30–40%. When simply comparing surface power density without regard for cost, this low heat to electricity conversion efficiency effectively pushes at least solar parks out of reach of even the highest yielding biomass plantations, surface power density wise.

Related technologies

Bioenergy with carbon capture and storage (BECCS)

Carbon capture and storage technology can be used to capture emissions from bioenergy power plants. This process is known as bioenergy with carbon capture and storage (BECCS) and can result in net carbon dioxide removal from the atmosphere. However, BECCS can also result in net positive emissions depending on how the biomass material is grown, harvested, and transported. Deployment of BECCS at scales described in some climate change mitigation pathways would require converting large amounts of cropland.

This section is an excerpt from Bioenergy with carbon capture and storage.[edit]

Example of BECCS: Diagram of bioenergy power plant with carbon capture and storage.

Bioenergy with carbon capture and storage (BECCS) is the process of extracting bioenergy from biomass and capturing and storing as much as possible of the resulting CO2, thereby moving it from the biogenic carbon pool to the geological carbon pool. BECCS can theoretically be a "negative emissions technology" (NET), although its deployment at the scale considered by many governments and industries can "also pose major economic, technological, and social feasibility challenges; threaten food security and human rights; and risk overstepping multiple planetary boundaries, with potentially irreversible consequences". The carbon in the biomass comes from the greenhouse gas carbon dioxide (CO₂) which is extracted from the atmosphere by the biomass when it grows. Energy ("bioenergy") is extracted in useful forms (electricity, heat, biofuels, etc.) as the biomass is utilized through combustion, fermentation, pyrolysis or other conversion methods.

Some of the carbon in the biomass is converted to CO₂ or biochar which can then be stored by geologic sequestration or land application, respectively, enabling carbon dioxide removal (CDR).

The potential range of negative emissions from BECCS was estimated to be zero to 22 gigatonnes per year. As of 2019, five facilities around the world were actively using BECCS technologies and were capturing approximately 1.5 million tonnes per year of CO₂. Wide deployment of BECCS is constrained by cost and availability of biomass.

Applications

Biomass for heating

Main article: Biomass heating systems

Biomass heating systems generate heat from biomass. The systems fall under the categories of direct combustion, gasification, combined heat and power (CHP), anaerobic digestion, aerobic digestion. The types of biomass heating are fully automated, semi-automated, pellet-fired, and combined heat and power.

Biofuel for transportation

Main article: Biofuel

Sugarcane plantation to produce ethanol in Brazil

Based on the source of biomass, biofuels are classified broadly into two major categories:

First-generation biofuels are made from food sources grown on arable lands, such as sugarcane and corn. Sugars present in this biomass are fermented to produce bioethanol, an alcohol fuel which serves as an additive to gasoline, or in a fuel cell to produce electricity. Bioethanol is made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as corn, sugarcane, or sweet sorghum. Bioethanol is widely used in the United States and in Brazil. Biodiesel is produced from the oils in for instance rapeseed or sugar beets and is the most common biofuel in Europe.

Second-generation biofuels utilize non-food-based biomass sources such as perennial energy crops and agricultural residues/waste. The feedstock used to make the fuels either grow on arable land but are byproducts of the main crop, or they are grown on marginal land. Waste from industry, agriculture, forestry and households can also be used for second-generation biofuels, using e.g. anaerobic digestion to produce biogas, gasification to produce syngas or by direct combustion. Cellulosic biomass, derived from non-food sources, such as trees and grasses, is being developed as a feedstock for ethanol production, and biodiesel can be produced from left-over food products like vegetable oils and animal fats.

Climate and sustainability aspects

This section is an excerpt from Sustainable energy § Bioenergy.[edit]

The climate impact of bioenergy varies considerably depending on where biomass feedstocks come from and how they are grown. For example, burning wood for energy releases carbon dioxide; those emissions can be significantly offset if the trees that were harvested are replaced by new trees in a well-managed forest, as the new trees will absorb carbon dioxide from the air as they grow. However, the establishment and cultivation of bioenergy crops can displace natural ecosystems, degrade soils, and consume water resources and synthetic fertilisers.

Approximately one-third of all wood used for traditional heating and cooking in tropical areas is harvested unsustainably. Bioenergy feedstocks typically require significant amounts of energy to harvest, dry, and transport; the energy usage for these processes may emit greenhouse gases. In some cases, the impacts of land-use change, cultivation, and processing can result in higher overall carbon emissions for bioenergy compared to using fossil fuels.

Use of farmland for growing biomass can result in less land being available for growing food. In the United States, around 10% of motor gasoline has been replaced by corn-based ethanol, which requires a significant proportion of the harvest. In Malaysia and Indonesia, clearing forests to produce palm oil for biodiesel has led to serious social and environmental effects, as these forests are critical carbon sinks and habitats for diverse species. Since photosynthesis captures only a small fraction of the energy in sunlight, producing a given amount of bioenergy requires a large amount of land compared to other renewable energy sources.

Carbon neutrality for forest biomass

Further information: Biomass (energy) § Climate impacts

GHG emissions from wood pellet production and transport (Hanssen et al. 2017).

IEA defines carbon neutrality as "any CO₂ released into the atmosphere from human activity is balanced by an equivalent amount being removed. Becoming carbon negative requires a company, sector or country to remove more CO₂ from the atmosphere than it emits." The actual carbon intensity of biomass varies with production techniques and transportation lengths.

Oven dry wood emits slightly less CO₂ per unit of heat produced compared to oven dry coal. However, many biomass combustion facilities are relatively small and inefficient compared to the typically much larger coal plants. Further, raw biomass can have higher moisture content compared to some common coal types. When this is the case, more of the wood's inherent energy must be spent solely on evaporating moisture, compared to the drier coal, which means that the amount of CO₂ emitted per unit of produced heat will be higher.

How much extra CO₂ is released depends on local factors. Estimates range from 10% to 31%. IPCC argues that focusing on gross emissions misses the point, and that the net effect of emissions and absorption taken together: "Estimating gross emissions only, creates a distorted representation of human impacts on the land sector carbon cycle. While forest harvest for timber and fuelwood and land-use change (deforestation) contribute to gross emissions, to quantify impacts on the atmosphere, it is necessary to estimate net emissions, that is, the balance of gross emissions and gross removals of carbon from the atmosphere through forest regrowth."

IEA Bioenergy provide a similar argument: "It is incorrect to determine the climate change effect of using biomass for energy by comparing GHG emissions at the point of combustion." They also argue that "the misplaced focus on emissions at the point of combustion blurs the distinction between fossil and biogenic carbon, and it prevents proper evaluation of how displacement of fossil fuels with biomass affects the development of atmospheric GHG concentrations." IEA Bioenergy conclude that the additional CO₂ from biomass "is irrelevant if the biomass is derived from sustainably managed forests."

Forest protection

Plantation forest in Hawaii.

An old forest will eventually stop absorbing net CO₂ because CO₂ emissions from dead trees cancel out the remaining living trees' CO₂ absorption. The old forest (or forest stands) are also vulnerable for natural disturbances that produces CO₂. The IPCC writes: "When vegetation matures or when vegetation and soil carbon reservoirs reach saturation, the annual removal of CO₂ from the atmosphere declines towards zero, while carbon stocks can be maintained (high confidence). However, accumulated carbon in vegetation and soils is at risk from future loss (or sink reversal) triggered by disturbances such as flood, drought, fire, or pest outbreaks, or future poor management (high confidence)." Summing up, IPCC writes that "landscapes with older forests have accumulated more carbon but their sink strength is diminishing, while landscapes with younger forests contain less carbon but they are removing CO₂ from the atmosphere at a much higher rate."

The "competition" between locked-away and unlocked forest carbon might be won by the unlocked carbon: "In the long term, using sustainably produced forest biomass as a substitute for carbon-intensive products and fossil fuels provides greater permanent reductions in atmospheric CO₂ than preservation does."

Carbon payback time

Further information: Biomass (energy) § Short-term vs long-term climate benefits

Researchers agree that in the short term, emissions might rise compared to a no-bioenergy scenario. IPCC for instance states that forest carbon emission avoidance strategies always give a short-term mitigation benefit, but argue that the long-term benefits from sustainable forestry activities are larger. Similarly, addressing the issue of climate consequences for modern bioenergy in general, IPCC states: "Life-cycle GHG emissions of modern bioenergy alternatives are usually lower than those for fossil fuels." Consequently, most of IPCC's GHG mitigation pathways include substantial deployment of bioenergy technologies. Limited or no bioenergy pathways lead to increased climate change or shifting bioenergy's mitigation load to other sectors. In addition, mitigation costs increase. The National Association of University Forest Resources Programs agrees, and argues that a timeframe of 100 years is recommended in order to produce a realistic assessment of cumulative emissions.

Carbon neutrality for energy crops

Further information: Energy crop § Carbon neutrality

Miscanthus x giganteus energy crop, Germany.

Many first generation biomass projects are carbon positive. The IPCC states that indirect land use change effects are highly uncertain. Some projects have higher total GHG emissions than some fossil based alternatives.

During plant growth, CO₂ is absorbed by the plants. While regular forest stands have carbon rotation times spanning many decades, short rotation forestry (SRF) stands have a rotation time of 8–20 years, and short rotation coppicing (SRC) stands 2–4 years. Perennial grasses like miscanthus or napier grass have a rotation time of 4–12 months. In addition to absorbing CO₂ in its above-ground tissue, biomass crops also sequester carbon below ground, in roots and soil. Typically, perennial crops sequester more carbon than annual crops because the root buildup is allowed to continue undisturbed over many years. Also, perennial crops avoid the yearly tillage procedures (plowing, digging) associated with growing annual crops. Tilling helps the soil microbe populations to decompose the available carbon, producing CO₂.

Soil organic carbon has been observed to be greater below switchgrass crops than under cultivated cropland, especially at depths below 30 cm (12 in).

Environmental impacts

Main article: Biomass (energy) § Environmental impact

Bioenergy can either mitigate (i.e. reduce) or increase greenhouse gas emissions. There is also agreement that local environmental impacts can be problematic. For example, increased biomass demand can create significant social and environmental pressure in the locations where the biomass is produced. The impact is primarily related to the low surface power density of biomass. The low surface power density has the effect that much larger land areas are needed in order to produce the same amount of energy, compared to for instance fossil fuels.

Long-distance transport of biomass have been criticised as wasteful and unsustainable, and there have been protests against forest biomass export in Sweden and Canada.

Future trends

Almost all available sawmill residue is already being utilized for pellet production, so there is no room for expansion. For the bioenergy sector to significantly expand in the future, more of the harvested pulpwood must go to pellet mills. However, the harvest of pulpwood (tree thinnings) removes the possibility for these trees to grow old and therefore maximize their carbon holding capacity. Compared to pulpwood, sawmill residues have lower net emissions: "Some types of biomass feedstock can be carbon-neutral, at least over a period of a few years, including in particular sawmill residues. These are wastes from other forest operations that imply no additional harvesting, and if otherwise burnt as waste or left to rot would release carbon to the atmosphere in any case."

See also

• Biochar
• Biogas
• Biomass heating system
• Biomass to liquid
• Biorefinery
• Pellet fuel
• European Biomass Association

References

Sources

• Pishvaee, Mir Saman; Mohseni, Shayan; Bairamzadeh, Samira (2021). "An overview of biomass feedstocks for biofuel production". Biomass to Biofuel Supply Chain Design and Planning Under Uncertainty. Elsevier. doi:10.1016/b978-0-12-820640-9.00001-5. ISBN 978-0-12-820640-9. S2CID 230567249.
• Akhtar, Ali; Krepl, Vladimir; Ivanova, Tatiana (2018-07-05). "A Combined Overview of Combustion, Pyrolysis, and Gasification of Biomass". Energy & Fuels. 32 (7). American Chemical Society (ACS): 7294–7318. doi:10.1021/acs.energyfuels.8b01678. ISSN 0887-0624. S2CID 105089787.
• Liu, Guangjian; Larson, Eric D.; Williams, Robert H.; Kreutz, Thomas G.; Guo, Xiangbo (2011-01-20). "Making Fischer−Tropsch Fuels and Electricity from Coal and Biomass: Performance and Cost Analysis". Energy & Fuels. 25 (1). American Chemical Society (ACS): 415–437. doi:10.1021/ef101184e. ISSN 0887-0624.
• van den Broek, Richard (1996). "Biomass combustion for power generation". Biomass and Bioenergy. 11 (4): 271–281. doi:10.1016/0961-9534(96)00033-5.
• Flores, Rilner A.; Urquiaga, Segundo; Alves, Bruno J. R.; Collier, Leonardo S.; Boddey, Robert M. (October 2012). "Yield and quality of elephant grass biomass produced in the cerrados region for bioenergy". Engenharia Agrícola. 32 (5): 831–839. doi:10.1590/s0100-69162012000500003.
• Ghose, Mrinal K. (2011). Speight, James (ed.). The Biofuels Handbook. Cambridge: Royal Society of Chemistry, The Ingram Publisher Services distributor. ISBN 978-1-84973-026-6. OCLC 798795266.
• Schwarz, H. (January 1993). "Miscanthus sinensis 'giganteus' production on several sites in Austria". Biomass and Bioenergy. 5 (6): 413–419. doi:10.1016/0961-9534(93)90036-4.
• Smil, Vaclav (2008). Energy in Nature and Society: General Energetics of Complex Systems. MIT Press. ISBN 978-0-262-69356-1.
• Smil, Vaclav (2015). Power density: a key to understanding energy sources and uses. Cambridge, Massachusetts: The MIT Press. ISBN 978-0-262-02914-8. OCLC 897401827.
• National Academies of Sciences, Engineering, and Medicine (2019). Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. doi:10.17226/25259. ISBN 978-0-309-48452-7. PMID 31120708. S2CID 134196575. Archived from the original on 25 May 2020.
• IPCC (2019a). "Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems. Chapter 4. Land Degradation" (PDF).
• IPCC (2019d). "Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems. Chapter 6. Interlinkages between desertification, land degradation, food security and GHG fluxes: synergies, trade-offs and integrated response options" (PDF).
• Chatham House (2017). "Woody Biomass for Power and Heat. Impacts on the Global Climate" (PDF).
• IEA Bioenergy (2019). "The use of forest biomass for climate change mitigation: response to statements of EASAC" (PDF).
• The Manomet Center for Conservation Sciences (2010). "Biomass Sustainability and Carbon Policy Study".
• Reid Miner (2010). "Impact of the global forest industry on atmospheric greenhouse gases" (PDF).
• IPCC (2019b). "Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems. Summary for Policymakers".
• NAUFRP (2014). "Letter of transmittal".
• IPCC (2007). "Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Chapter 9. Forestry" (PDF).
• IPCC (2019f). "2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. Volume 4. Agriculture, Forestry and Other Land Use. Chapter 4. Forest Land" (PDF).
• Hanssen, Steef V.; Duden, Anna S.; Junginger, Martin; Dale, Virginia H.; van der Hilst, Floor (2017-01-17). "Wood pellets, what else? Greenhouse gas parity times of European electricity from wood pellets produced in the south-eastern United States using different softwood feedstocks". GCB Bioenergy. 9 (9). Wiley: 1406–1422. doi:10.1111/gcbb.12426. ISSN 1757-1693.
• Fajardy, Mathilde; Köberle, Alexandre; Mac Dowell, Niall; Fantuzzi, Andrea (2019). "BECCS deployment: a reality check" (PDF). Grantham Institute Imperial College London.
• Climate Central (2015-10-23). "Pulp Fiction, The Series". Retrieved 2022-02-12.
• IFL Science (2016-03-14). "British Power Stations Are Burning Wood From US Forests – To Meet Renewables Target".
• Forest Defenders Alliance (2021). "Standing up for forests and against the Swedish forestry model: A letter to EC policymakers".
• STAND.earth (2021-03-23). "Risk Map: Primary forest and threatened caribou habitat overlap with preliminary estimated wood pellet haul zones for Pinnacle/Drax in British Columbia".
• Tester, Jefferson (2012). Sustainable Energy: Choosing Among Options. MIT Press. ISBN 978-0-262-01747-3. OCLC 892554374.
• Smil, Vaclav (2017a). Energy Transitions: Global and National Perspectives. Praeger Publishing. ISBN 978-1-4408-5324-1. OCLC 955778608.
• IPCC (2014). Edenhofer, O.; Pichs-Madruga, R.; Sokona, Y.; Farahani, E.; et al. (eds.). Climate Change 2014: Mitigation of Climate Change: Working Group III contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. ISBN 978-1-107-05821-7. OCLC 892580682. Archived from the original on 26 January 2017.
• World Health Organization (2016). Burning Opportunity: Clean Household Energy for Health, Sustainable Development, and Wellbeing of Women and Children (PDF). ISBN 978-92-4-156523-3. Archived (PDF) from the original on 13 June 2021.
• IEA (2020). "Going carbon negative: What are the technology options?".