There is clear consensus that fossil fuels are the highest source of GHG emissions in the world. Unfortunately, the world is far too energy-dependent to simply stop burning fossil fuels and call it a day. We need alternative sources of energy.
To reduce GHG emissions as fast as possible, countries can reduce fossil fuel consumption by transitioning to low-emission sources of energy, like renewables. The faster we complete this transition, the less GHGs we’ll emit due to energy use. Investing in renewable energy accelerates this transition, as long as the focus remains on replacing fossil fuels and ensuring energy security. Increasing renewable energy production without lowering fossil fuel consumption doesn’t do us any good. Especially since all renewables have life-cycle impacts of their own.
In this section, we’ll see how different renewables supply energy and what drawbacks they might have.
Solar and Wind
Solar panels convert solar energy into electricity, thanks to engineered materials like semi-conductors. In 2019, solar PV [photovoltaic] accounted for roughly 2.6% of electricity production worldwide.
Wind turbines convert the mechanical energy from their rotating turbines into electricity, thanks to a gear system connected to a generator. They’re usually very tall since wind speeds increase with elevation. In 2019, wind turbines accounted for roughly 5.3% of electricity production worldwide.
While these technologies are far more sustainable options than fossil fuels, they also emit GHGs and toxic compounds during production and disposal. Most environmental impacts associated with solar and wind arise from their metal demands, including high demands of rare metals.
Solar panels are made from a wide range of metals [depending on the type of panel] including arsenic, copper, gallium, germanium, indium, and/or tellurium to name a few of the critical ones. Solar panels use a bunch of silicon as well, which may be considered abundant – but is rarely found in its pure form.
Wind turbines are giant metallic structures [mostly composed of zinc and copper] that also depend on a few ‘critical‘ commodities, like dysprosium, neodymium, praseodymium [all used for turbine magnets], aluminum and rare earth elements. For reference, the typical onshore wind plant is made with 9 times more mineral resources than a gas-fired plant.
Unfortunately, both technologies are also intermittent, since sunlight and wind vary throughout the day. That’s why energy storage is essential to provide a continuous energy supply throughout the 24-hour day. This is where batteries come in.
Chemical batteries store energy chemically and release it as electricity. Unfortunately, these types of batteries are also metal-intensive and require several critical materials such as cobalt, graphite, nickel, rare earth metals, and of course lithium [for lithium-based batteries]. Battery storage systems will play an important role as we look to decarbonize our societies, as clearly indicated by their projected increasing metal demand.
The following graph shows the extent to which ‘clean’ energy technologies will increase their specific metal demands from 2020-2040 in the Sustainable Development Scenario [SDS]. For example, the clean-tech sector’s lithium demand is expected to increase 42-fold by 2040 [i.e. clean-tech’s 2040 lithium demand will be 42 times its 2020 demand].
Even if we’re able to extract all the necessary metals to help renewables overtake fossil fuels, there’s still the issue of lifetime. Solar, wind, and batteries use technologies that degrade over time, so they need to be replaced every 2-4 decades, at most [e.g. wind]. Consequently, if we choose to complete the ‘green revolution’ and deplete many metal reserves, we better have a plan for how to manage the second/third/next waves of the revolution.
Solar thermal [or concentrated solar power – CSP] is another type of renewable technology that depends on solar power, but focuses on capturing solar heat – which can then be used to generate electricity. This technology currently isn’t making an impact at the global scale, as it faces many practical challenges of its own [e.g. isolated in deserts but requires water] – in addition to some that solar PV faces. In 2018, solar thermal generated just 13.4 TWh of electricity – compared to 570 TWh for Solar PV.
Hydrogen has gained so much clout over the past few years that some companies and governments decided to switch over to this energy ‘source’. It’s already being used by industries for other applications than electricity generation, and that’s expected to ramp up in the next few decades.
Hydrogen isn’t really an energy source – or at least that depends on how it’s retrieved. If it’s made through hydrolysis, then it acts like a battery. If it’s stripped from natural gas, then it’s basically just a fossil fuel. Once hydrogen has been produced/separated by either method, it can store energy [much like a propane tank] until it’s used to generate electricity via a fuel cell.
Through chemical reactions, a fuel cell provided with hydrogen and oxygen can generate electricity. The beauty of a fuel cell is that their only by-products are clean water and heat. With oxygen abundantly present in air [21%], the fuel cell can simply intake nearby air, use the oxygen, and release the air’s other gases that weren’t needed. Alas, hydrogen is required as well.
Making Hydrogen Option 1: Hydrolysis
Hydrolysis is the opposite reaction of what happens inside a fuel cell. This time, it’s the electricity and water that are consumed to create oxygen and hydrogen. There’s an apparent obstacle right from the get-go with this method – we need to supply electricity. The yield for this reaction is around 80%, meaning that 20% of input electricity is wasted.
Combining hydrolysis and fuel cell efficiencies, only 40-48% of the electricity input is output from a fuel cell. That’s roughly half as much as ‘normal’ lithium batteries that are 80-95% efficient. In other words, twice as much electricity is required for the same final energy output. Unfortunately, increasing energy consumption has significant drawbacks, especially in regions of dirty electricity mix.
In 2019, hydrolysis accounted for less 0.1% of hydrogen production worldwide – an insignificant share compared to other production processes that consume gas, coal, or oil.
Making Hydrogen Option 2: Steam-Methane Reforming
Since hydrolysis is inefficient, the vast majority of industries adopt steam-methane reforming [SMR] instead. Most natural gases are hydrocarbons, meaning they’re composed of hydrogen and carbon atoms. Gases like methane have 4 hydrogen atoms for each carbon atom [i.e. CH4]. SMR plants strip the hydrogen from the carbon in the natural gas to ‘produce’ hydrogen. The carbon ends up as CO2 and is released into the atmosphere in the absence of pollution control measures.
This type of hydrogen production is far more efficient than hydrolysis, even though it requires high energy input for the reaction to occur – which explains why it accounts for 75% of global hydrogen production.
Alas, between the carbon emissions and SMR’s high energy requirements, this process is far from clean.
Tying It All Back Together
Once the hydrogen is made, there are still a few challenges left. Notably, hydrogen lacks the infrastructure to be easily transported. While it could certainly be moved through pipelines, other transportation methods are often inefficient due to the safety risks associated with driving trucks filled with flammable gas.
Hydrogen research and technology haven’t made many advances that would suggest hydrogen is the fuel of the future. Some interesting industrial applications exist – where hydrogen used in the right setting shows some potential to reduce emissions [e.g. smelting]. However, as long as hydrogen production pollutes our atmosphere – what’s the point?
Geothermal [not to be confused with geo-exchange which relies on thermal energy generated from solar radiation], as the name indicates, consists of using the heat present in the Earth. The heat is extracted by heating a fluid to generate steam and rotate turbines. Alas, we’re not able to dig very deep into the Earth, which explains why geothermal energy production is limited to regions of high geothermal activity, like Iceland.
As a result, geothermal energy cannot be produced in enough locations to take on a significant part of the world’s energy mix. In fact, geothermal accounted for less than 0.5% of electricity production worldwide in 2019.
Hydroelectric dams use a very simple principle to produce energy: gravity. Upstream, a dam retains the water elevation much higher than on the other side of the dam. The higher the difference in elevations, the greater the pressure differential. Passing through the dam, the water rotates turbines to generate electricity. Out of all renewable energy systems [excluding biofuels & waste], it holds the greatest share of the world’s energy mix with 2.5% [in 2018] and electricity mix with 16% [in 2019].
Unfortunately, there are quite a few drawbacks.
First, dams imply that a reservoir must be allowed to accumulate upstream. Ecosystems can be completely destroyed by such installations, as can human-inhabited environments. For example, the world’s largest dam in China forced 1.4 million people to relocate between the 13 cities, 140 towns, and 1,350 villages flooded by the reservoir. The larger the dam, the more energy can be produced. However, this also implies more water accumulation, which results in more damage to the environment.
Second, building dams isn’t a feasible option everywhere. Even if hydroelectric dams can help supply select regions of the world with nearly all of their electricity, not everyone can follow suit.
Third, dams are often at the mercy of precipitation. With increasing global temperatures, droughts are becoming more frequent and intense. That forces regions to rely more heavily on fossil fuels during times of drought – when the dam’s water elevation doesn’t permit electricity production.
Fourth, hydroelectric dams create reservoirs of decaying organic matter. Just upstream of the dam, loads of dead vegetation and animals accumulate on the reservoir floor. The decay process releases GHGs and consumes oxygen. While better reservoir management practices can significantly reduce those emissions, many dams aren’t up to par. This explains why certain regions of the world have cleaner hydropower systems than others. The IPCC evaluated hydropower’s life-cycle emissions and found that the median hydropower plant emits just 24 g of CO2e per kWh of electricity supplied, while the most polluting plants emit 2,200 g of CO2e per kWh.
The rate of installation of new hydroelectric dams is decreasing while removal rates rise, as developing countries are starting to label hydroelectricity as uneconomical. Hydropower isn’t expected to reach its target of 5,722 TWh of electricity produced per year by 2030, although it was already generating 4,333 TWh in 2019. Not fantastic news for the world’s current largest renewable energy source.
Biomass – Energy Crops
The use of energy crops for biomass production can be considered renewable, but certainly not sustainable. Energy crops are trees and plants that are grown for the sole purpose of producing energy. There are numerous applications and types of energy crops – ranging from woody biomass burnt at power plants to corn-based ethanol mixed in gasoline.
Governments around the world are considering using biomass to slightly reduce their dependence on fossil fuels. Unfortunately, that often doesn’t make sense from a carbon emission standpoint. For an equivalent energy output, burning wood can be 2-3 times as carbon emitting as fossil fuels when considering life-cycle impacts.
That being said, it’s true that sustainable agriculture/forest management practices can help make biomass more sustainable than fossil fuels. Unfortunately, setting up these sustainable practices will take lots of time and land. And those are 2 things that are already stressed by the environmental crisis and food production.
We should also note that some of these sustainable logging practices consist of planting ‘compensation forests’ to maintain the total amount of trees on Earth. Alas, this isn’t ideal since it would assume that the vegetation being cut down for energy production has the same value as the vegetation being planted to compensate, which often isn’t true. Vegetation plays many important roles in an ecosystem – there’s more to it than just the carbon cycle. For example, replacing a tree cut down in Brazil with another tree planted in Finland isn’t really a fair deal as far as Brazilian biodiversity is concerned.
To truly help the environment, preserving vegetation while it’s still alive might be a much more efficient way to mitigate climate change and biodiversity loss. However, there is an excellent reason to use biomass and harness its energy. That is if the biomass would instead be wasted. And one of the best ways of converting organic waste into energy is by producing biogas/renewable natural gas.
Biogas and Renewable Natural Gas
Biogas is produced by landfills, industrial manure ponds, and anaerobic digester plants [e.g. for sewage treatment] during the decomposition of organic waste. When captured, biogas can either be flared to reduce GHG emissions, burned to produce energy, or purified into renewable natural gas [RNG]. This RNG can then be injected into the natural gas grid to be burned like ‘normal’ natural gas later on.
The eco-friendly aspect comes from the assumption that biogas and RNG can replace some electricity or natural gas in the grid, thus lowering fossil fuel consumption. This logic only holds if we assume that the amount of energy supplied by the biogas or RNG would have been supplied by other energy sources anyway. That’s a pretty reliable assumption.
But if biogas and RNG emit as much CO2 as natural gas when burned, then what’s the point? To answer that, we have to make one big assumption. We have to assume that all organic waste CO2 emissions are “biogenic” – a fancy term to say that these emissions would have occurred naturally. That makes sense: organic items like vegetables can’t emit more carbon than they’ve absorbed during growth. Organic waste emissions are part of a natural carbon cycle.
Since biogas and RNG are produced during the decomposition of organic waste, the CO2 emitted during their combustion is simply regarded as biogenic – so we don’t count those as emissions. On the other hand, emissions due to fossil fuel combustion are considered “anthropogenic” since they would not have occurred naturally. These types of emissions are counted.
So if biogas and RNG can replace fossil fuels, then biogenic emissions can replace anthropogenic emissions. Considering biogas/RNG is produced from waste biomass, it’s pretty clear that the use of biogas/RNG is eco-friendly.
Biogas and RNG Alternatives
What if organic waste was instead composted? A significant amount of carbon would then be stored in the compost – while the amount of CO2 emitted would be considered biogenic. There doesn’t seem to be much of a difference between these waste-to-energy and composting scenarios. We’ll go over organic waste management in greater depth in later sections.
Evaluating biogas/RNG is tricky since it can either be a fantastic solution in areas equipped with poor waste management systems, or just a decent solution if the organic waste could have instead been composted. Composting also has other benefits. Notably, the fact that it can provide nutrients in a much simpler process, which avoids inefficiencies and additional impacts [e.g. biogas/anaerobic digester plant infrastructure, RNG leaks, etc…].
In the end, biogas and RNG are produced from waste [and it should stay that way – see biomass], which means they’re already limited in terms of how much energy they can provide.
Reducing GHG emissions as fast as possible is a priority. However, focus has to remain on replacing fossil fuels and ensuring energy security. Renewable energy technologies have drawbacks, and understanding each of them is important for planning purposes. Lowering energy demand worldwide is an effective way to minimize these drawbacks and accelerate the phasing out of fossil fuels.