Renewable energy

Renewable energy sources are energy sources which are replenished faster than they are consumed on a human timescale. In other words, while "renewable energy" may not be infinite, they are practically unlimited. Arguably, this is a misnomer, and the name "ambient energy" is more appropriate.

Solar energy
Solar energy is the energy of the Sun shining on the Earth, originating from entropic fusion reactions at the heart of the star. It can be tapped in many different ways, directly and indirectly.

Direct methods usually refer to either solar panels or passive heat absorption.

Indirect methods would include hydroelectric power, wind power and energy crops. With hydroelectric systems, the sun provides the energy to make the water evaporate, so it can begin its travel anew by raining down on a high ground. With wind power, solar radiation is the ultimate energy source that drives the Earth's weather and climate, including the winds harnessed by turbines. Plants also utilize solar energy in order to perform many energy-costly chemical reactions, which later in the food chain are broken down by life for energy.

Direct solar
There are two main ways of directly getting useful energy from sunlight: either directly converting it to electricity with photovoltaic cells, or using it to heat stuff in various ways.

For now, both kinds of methods are limited by the vagaries of a planet's surface, including cloud cover (curse you, water vapour!), varying amounts of sunlight (curse you, axial tilt!), and the day-night cycle (curse you, planetary rotation!). Space-based solar collection systems that don't suffer from these limitations have been proposed, but these are unlikely to be built in the foreseeable future.

Photovoltaic cells, a.k.a. "solar panels", convert solar radiation into electricity through the use of the photovoltaic effect. Although the Earth's surface receives daily about 4.2 kilowatt-hours per square meter, most solar panels are not perfectly efficient (and they can never be for thermodynamics reasons). In reality, most of them struggle to get over 20% efficiency (8%-15% being more common, even for commercial uses). Heat collection cells take many forms. The one encountered most often is used for heating water for domestic use, which might look like Solar Panels, but in reality, at no point is a conversion to electricity taking place. Other forms of heat collection are greenhouse structures, as well as passive heating architectural schemes making use of either controlled glazed surfaces or a material's native thermal capacity. Since such systems only try to convert into heat (so any "heat losses" are actually aiding them), for their specific applications, they are actually far more efficient than solar panels, with conversion rates of over 60%. (The reason for this being that they don't work equally well in the entire spectrum).

Concentration towers are essentially a specialized heat-capturing system where a lot of mirrors aim thermal energy into a central point, essentially utilizing the same mechanism Archimedes did when he (allegedly) burned a Roman fleet. In a solar tower's focus point, there is usually some kind of heat-absorbing liquid, which is heated and used to turn turbines, similar to the way conventional power plants generate electricity. The advantages of that approach is that, while solar panels are expensive objects, each and every one requiring electrical installations and maintenance, mirrors are… just dumb mirrors which you could clean with a hose. In addition, because the important energy conversion takes place en masse at the central focus points, such facilities only have to improve on that one, whereas photovoltaic ones would have to do so over their entire area. Heat storage strategies can also allow such a plant to maintain generation during brief interruptions (i.e., clouds) or even for a significant part of the night.

Experimental installations like this are already in place in the Southwestern desert in the United States, which is in reasonably close proximity to large cities in California and Nevada. Were the U.S. to switch to a centralized solar generation from sunlit states, it would require the overhauling of the current, decades-old energy distribution systems. A first step towards that would be a nationwide direct current backbone.

Project Desertec is a similar scheme involving huge solar arrays in the Sahara desert, which would then transmit low-loss DC power to Europe.

Advantages of direct solar

 * The technology is proven at this point, and continues to improve. It is rapidly becoming cheaper, yet more robust with each design iteration.
 * Relatively very little maintenance is required.
 * Daily variations in power output are mostly predictable.
 * Can be deployed on top of existing buildings, allowing for lots of expansion in the consumer space.
 * It is the technology with the most appeal to the Greens.
 * The scalability is very good. This allows large-scale solar farms to be reasonable, and for solar panels on homes to lighten the load on the grid.
 * Also works pretty well for low-powered devices where a grid connection is impractical.
 * Does not require any additional energy for a "black start", i.e., restarting an electric grid from nothing.
 * Photovoltaic solar power has very low water usage, and thus does not compete with farms or homes when it comes to using water and can be used in drier climates effectively.
 * A rough estimate for the time it takes to create energy equivalent to what it took to create a solar panel is about 1.5-2 years, depending on the model; after that, it's green energy, baby!
 * Solar panels on roofs block much of the sun from directly hitting the house, slightly reducing the amount of electricity needed for AC, Which is one of the largest pieces of energy usage.
 * Many modern solar panels are guaranteed under warranty to output at least 90% of the initial wattage after 25 years.
 * Direct heating from solar energy (it's not turned into electricity) is particularly costly and energy efficient.
 * Solar concentration towers allow for continuous 24-hour power output with the use of thermal energy storage.
 * Works really well for powering communications satellites.
 * Wind turbines scare away Donald Trump.

Disadvantages of direct solar

 * Photovoltaic cells degrade over time; they have a non-infinite lifespan and will eventually have to be replaced. This is improving, however, with several manufacturers guaranteeing 90% of original efficiency after 25 years.
 * Photovoltaic cells require the use of toxic chemicals for their manufacture. Depending on the type of cell and the employed process, this can be greatly diminished if not eliminated.
 * Solar cells also require a lot of rare earth metals, and we might not have enough. Wars have been fought and countries conquered over resource rights in the past, and the materials needed for solar are unlikely to be different. Recycling might mitigate this problem, yet we currently don't recycle all of our old electronics.
 * Photovoltaic solar power plants do not have any output at night. However, electricity demand tends to be lowest at night.
 * Unfortunately, in colder climates, the peak demand for electricity doesn't occur around noon, but in the late afternoon when people get home from work, whereas solar energy peaks at noon and may only be producing a minimal amount of energy when it's needed most. However, some warmer climates have peak demand in the early afternoon due to air conditioning needs. This means that any solar-heavy grid will need to be paired with a large amount of storage capacity.
 * Many highly populated parts of the world don't get consistent high levels of sunlight. This is a problem, especially in countries where peak power use is during the winter (which it usually is). This could be remedied in some areas by switching to heat pumps from resistive heating. Even still, this could be resolved by using HVDC lines to transport power over long distances efficiently.
 * Photovoltaic solar outputs 10-25% of its normal energy output when it's cloudy. Solar concentration power doesn't have as big an issue with this, as thermal energy is stored and can keep producing electricity for up to 24 hours after not receiving light.
 * Low power density (a lot of land is needed per unit of power), but higher than wind or biofuels. Though, to be fair, the amount of land needed for 100% solar electricity in the U.S. (assuming 100% efficient storage and transmission) is not even a third of that currently used for corn ethanol.
 * Solar collectors (whether PV or thermal) require the occasional cleaning, though not as much as people tend to think. While they use much less water than, say, coal, that doesn't mean they use no water. Displacing coal with solar power would reduce water use, but building new solar power plants where there are no power plants at all will increase local water usage slightly, which is a problem when the best spots are the areas with the least cloud cover.
 * Although solar energy plants do not emit CO2 when operated, they require a lot more to set up. This is partially due to chemical reasons. Silicon is refined from SiO2, commonly known as Quartz. The other partner in the reaction is carbon anodes, which "burn" up, creating CO2. On the other hand, Silicon can be recycled quite well nowadays, and the amount of obsolete electronics filling the trash every year is a huge, almost entirely untapped source of fresh Silicon. Most Modern solar panels will produce the amount of energy it takes to create them within 1.5 - 2.5 years.
 * Third-party-owned (TPO) residential systems are common in some parts of the United States, and there is no evidence that they add value to the house or save people money in the long run. This is in contrast to homeowner-owned residential systems, which cost a lot upfront but do add value to the house. TPO sales tactics can be high-pressure and rely on fear, uncertainty and doubt. Third-party ownership and decades-long contracts can create real headaches.
 * Solar panels absorb heat better than sand. The whole plan to coat the Sahara with solar panels would probably not end very well. Coating the Sahara with solar panels could lead to unintended warming of the area around them, and thus heat nearby ocean currents. This, of course, means warmer water will make its way to the arctic, which is... y'know... pretty bad.
 * Space-based solar, which could get around most of the problems of solar, requires infrastructure we just don't have and is unproven (so we can't depend on it). It also isn't technically direct solar since it converts sunlight into microwaves, so maybe it shouldn't be mentioned here, but that's not going to stop us. Also, we probably shouldn't mention that any satellite that can "accidentally" be aimed at something other than a solar collector is an orbital doom laser, and in violation of a treaty or two. It's an open question as to whether giving the man-babies in control of society a bunch of orbital death beams to play with would be an improvement over climate change...

Hydroelectric energy
Hydroelectric power, or water power in general, involves taking advantage of water that has been raised due to evaporation by the sun and deposited on high ground. As it then flows back towards the ocean, its potential energy can be tapped by turbines or water wheels, which are usually built into dams to increase the drop.

Many hydroelectric schemes are also designed to store energy, as water is pumped into the dam when energy is cheap at night and then allowed to run out when demand is higher during the day. Although not particularly efficient, it is perhaps the only commonly used method of "storing" significant amounts of electrical energy. Sometimes an artificial upper reservoir is constructed - the facility generates no net power and is dedicated to energy storage.

Advantages of water power

 * On average, it's by far the cheapest source of energy. And "by far", we are talking less than a third of the cost of fossil fuels.
 * It is relatively clean once the equipment has been built and installed
 * The technology is relatively simple and very robust, meaning it doesn't require anywhere close to the maintenance and knowledge base required for, say, a nuclear plant.
 * If you need a flood control dam, it is very cheap and low impact to add electricity generators to it (at least compared with leaving the dam without turbines)
 * Creation of artificial lakes upstream of the dams to be used for recreation, wildlife preserves, or potable water supplies
 * Can slow erosion, such as in the case of Niagara Falls, where the falls are "turned off" when the tourists aren't watching and diverted to the hydro plant
 * Effectively supplies power on demand (actually about the most responsive major power source on any grid). With the reservoir, virtually 100% of electricity produced by other renewables can effectively be used at any hour of the day, up to the total consumed at any given hour e.g., if the inflow to a dam's reservoir is only capable of sustaining 100MW of average production, a town is consuming 200MW, and the wind is producing 180MW, the dam will reduce to 20MW and the reservoir will fill up, and when the wind is producing only 50MW the dam will produce 150MW.
 * In addition to simply producing energy whenever it is needed, by installing pumps that return water upstream, the dam can act like a gigantic battery for the hours when renewables produce more energy than is consumed – in fact; this is the cheapest, most reliable and most efficient large scale energy storage in existence. In the above-mentioned example, if the wind is producing 250MW but the town only needs 200MW, normally, the dam can't go below 0 MW produced, and the wind turbines will have to be shut.  Instead, all 250MW will be used, and even more water will be available for later use.  This is incidentally one of the major uses for artificial lakes in the Alpine countries and Scandinavia (Europe has an interconnected high voltage grid, allowing electricity to be moved from Denmark to Italy without catastrophic transmission losses)
 * Doesn't need significant electricity or power to start, making it useful for black starts.
 * The only renewable proved to be able to take significant market share away from fossil fuels. Technically, fossil fuels took market share away from Hydro, not the other way around, considering the first commercial power plant in the U.S. was a Hydro plant located at Niagra Falls.
 * Old mine shafts can be used as reservoirs for pump-based water power (primarily used for energy storage)

Disadvantages of water power

 * Flooding of landscapes, causing a disruption of ecosystems.
 * Displacement of residents and burial underwater of important cultural landmarks. This a serious enough issue in the developed world, but in a country where the wounds of inter-ethnic violence are still fresh, this can become a huge issue.
 * Limited by geography. In most developed countries, all the good spots are already taken.
 * The flooded vegetation and soil decompose anaerobically to methane, a far worse greenhouse gas than carbon dioxide, which causes further global warming. Of course, cutting down trees before flooding and similar measures can somewhat reduce that, but it is rarely done.
 * The failure of a dam can be disastrous. Large dam failures rank among the most deadly industrial accidents in history, with the Banqiao dam failures of 1975 killing around 100,000 in China. Dams are also targets for military or terrorist activity, though, in fairness, the larger dams are thick enough that you'd need explosives on the equivalent of tactical nukes to do more than scratch the paint.
 * Hydroelectric dams pose barriers to migrating fish. This can be remedied to some extent by constructing fish ladders.
 * As any geoscientist worth their salt will tell you: Artificial lakes tend to silt up. Depending on the amount of sediment the river brings from upstream, this can be an alarmingly rapid process (as is the case with Lake Nasser on the Nile) or hardly perceptible on human timescales. Eventually, the river "wants to" fill up all artificial lakes to the level of the dam.
 * Large, mostly stagnant bodies of water can be breeding grounds for mosquitoes and other bugs. Mosquitoes are vectors for some of the deadliest diseases known to humanity, including Malaria, Dengue (for which neither cure nor vaccine exists), Yellow fever or West Nile. Also, Zika, now that it spread to important countries the media started paying attention to that.
 * The sheer weight of the water in a reservoir may have an effect on earthquakes. Though this field needs more study, although the dams do cause the Earth to rotate slower. Barely.
 * As mentioned earlier, hydroelectric power refills slower in the winter months, as there's less precipitation and what's there is usually in the form of snow/ice that won't fill up the reservoir until it melts. At the same time, the winter months often have a higher demand for electricity.  It's not too much of a problem if managed properly, but the turbines can't run at max capacity indefinitely, so another power source is needed.  Wind, which produces more electricity in winter but needs storage, would pair well with hydro, except that the hilly terrain that's ideal for hydro tends to have very limited locations for wind.

Wind energy
This is one of the more developed forms of renewable energy, and major projects are underway worldwide. Although only 5% of the world's power comes from the wind, some European countries produce a lot more. The heaviest user is Denmark at 48% in 2019, followed by Irelands's 36.3% in 2020. In general, annual wind power growth in Europe is a surprising 32%. However, it should be noted that wind-heavy countries have among the highest electricity prices in the EU. In recent years, China has also gotten into the game and (as with other things, like high-speed rail or photovoltaics) come from behind to overtake the rest of the world and sweep up the floor with them. In 2019, the total installed wind capacity of China was 236,402 Megawatts, more than double that of runner-up U.S.A. at 105466 MW and nearly quadruple that of third-placed Germany at 61,406 MW. In 2019 alone, China added 26,155 new Megawatts of wind energy capacity, which not only equals slightly over 43% of the worldwide added capacity of that year, but is also more than a third of the whole world's wind output capacity.

Advantages of wind power

 * Once the infrastructure is in place, it is very clean.
 * The technology is well-established and proven.
 * The land between the wind turbines can be used for farming or pastures.
 * Can realistically be built on water, reducing land usage and, with it, a number of the below-listed disadvantages.
 * ...which is especially useful as wind is much higher over the water (no hills to block it), and the majority of people live near the coasts.
 * Has been shown to produce more energy in the winter, when consumption is higher in most temperate countries.
 * On the most basic level, only abundant construction materials and "low tech" are needed - this is especially advantageous for developing countries which lack complex infrastructure and logistics. Unlike oil and potentially nuclear or solar, there won't be too many geopolitical headaches over wind power.

Disadvantages of wind power

 * Output that is difficult to accurately predict due to the power in the wind varying as the cube of the wind speed; many turbines are required over a large area to compensate for this (even with many turbines over a large area, the total power will sometimes drop near zero)
 * Less intuitively, wind turbines must be shut down and their blades feathered if there is too much wind, since this can damage their gearboxes or, in extreme cases, cause them to catch fire.
 * Due to the relatively unpredictable output, other sources of energy are needed to back up wind power. In the obvious case, you need extra power to come online when the wind is not high enough to meet demand; but in the less obvious case, when more electric power is available than is demanded, you need extra power to go offline.  Coal and nuclear plants are not flexible enough for this, so in practice, wind power displaces hydroelectric and natural gas.  Furthermore, dams generally are expected to provide or withhold water for many conflicting needs, such as flood control, irrigation, and salmon migration, as well as baseload power production, and balancing wind production is another conflicting need.  This also makes wind power unsuitable for baseload power.  In theory, this could be mitigated with better energy storage systems, but on a scale capable of powering a large region, only pumped-storage would be cost-effective, which by their nature are limited to hilly or mountainous areas that generally aren't ideal for large-scale wind farms.
 * Depending on where the wind farm is built and site management practices, land use change might release more CO2 than the wind farm will save, even ignoring the carbon cost of construction (peat bogs are an especially bad location).
 * Low power density: 2 W/m2, meaning large amounts of land are required. This can be mitigated somewhat with VAWT (vertical axis wind turbine) in addition to HAWT (horizontal axis wind turbine), though these take up more space and are less efficient due to being in the slower, low-to-the-ground winds.  At least they are easier to repair, since you don't need a crane to access the gearbox.  Of course, if you are filling your fields with VAWTs, this makes it a bit difficult to use the land for grazing or crops, so VAWTs may be limited to wastelands.
 * Land between the wind turbines is less desirable for human settlement due to the risk of turbine blades breaking away, ice throw, and noise - however, it is rather desirable for the farmers owning the land and prevents them from selling it for development, leading to yet more urban sprawl and squeezing out even more farmers, so in many ways, this isn't a bug, it's a feature.
 * Wind turbine blades have been known to be a cause of avian and chiropteran mortality, though to what extent is still unknown. Chances are, however, it's still less than from coal or what would happen if significant climate change occurred... Furthermore, any significantly tall structure will, on occasion, kill birds that fly against the windows. Most reports in the media of bird fatalities from wind turbines come from the Altamont Pass Wind Farm in California, which happens to be located in the middle of a flight path and uses turbine designs which are now long obsolete for practical reasons as well as being deadlier to birds than more modern designs. Essentially, the latter is the wind power equivalent of Chernobyl. Let's also note that those anti-wind advocates who appear so concerned with the well-being of birds are oddly silent on the issue of the domestic housecat - cats kill billions of birds a year compared to the hundreds of thousands (at most) estimated by wind turbines. Meanwhile, air pollution—to which fossil fuels are a large contributor—contributes to one in six premature human deaths.
 * Wind turbines are generally designed to last about 20-25 years, generally lasting even longer, but afterwards, well, they aren't easily scrapped. We've been recycling metals since the Bronze Age, but the composites and special materials, such as fiberglass and carbon fiber, used for the blades are not so easy to recycle. This means that those blades will find their way into a landfill, and they aren't exactly biodegradable. There is now at least one company recycling the fiberglass into boards and injection-mold-able pellets. Though, given that much of the current recyclable materials are, well, not recycled, the blades will likely still pile up.
 * It's worth mentioning the carbon involved in manufacturing/building. A single wind turbine requires a little more than 200 tons of steel and 700 tons or so of concrete, with some requiring more or less depending on the design.  Typically it's 150/450 tons of steel/concrete per MW capacity, and given that steel produces 1.85 tons of carbon per ton of steel and concrete .65 tons per ton, that's around 550 tons of carbon per MW, without including the mining and transportation.  Coal produces a little less than 3x its weight when burnt, so every 1MW of windmills is the equivalent of burning 200 tons of coal.  Said coal would only produce about 400MWh of electricity, whereas the 1MW wind turbine will produce an average of 6MWh per day, so it takes about 2-3 months to break-even carbonwise.  This is far less dirty than coal but it's not completely green.

Fake disadvantages of wind power
Most "anti-wind power" groups, however, tend to focus on the following, which aren't particularly good reasons against wind power, but they're easy to understand.
 * Cause climate change by taking energy out of the jetstream (brought forward by the Alternative für Deutschland, a German fascist party)
 * Visual impact; wind turbines have to be built in highly exposed locations, while conventional power plants can be placed anywhere with decent road access, including being integrated into existing industrial developments. This one comes closest to being a legitimate concern since it's very rare that even the most perfunctory efforts are made to make the wind turbines look like they actually belong where they are; they are generally painted high-gloss white and look like they just escaped from an airport. However, beauty lies in the eye of the beholder, and in some areas, wind turbines have become part of the character and charm of the landscape, just as windmills have in centuries past.
 * Wind Turbine Syndrome.
 * Wind turbines rising up to crush humanity

Plant-based energy (biofuels)
Plant-based energy consists of allowing plants to fix carbon in sugars and cellulose via sunlight, and then to either produce liquid fuels (such as alcohol) from them or burn them directly.

Advantages of plant energy

 * It's (arguably) carbon neutral in the long run. Although, please note the first point under 'disadvantages'.
 * It can produce convenient, high-energy-density liquid fuel, which is very useful in cars and planes.
 * Can use waste products that would otherwise have to be thrown out.
 * As an available alternative, it provides a price cap at which gasoline prices will struggle to rise above. In the U.S., this is a little over $4/gallon.

Disadvantages of plant energy

 * While it is often argued that biomass is carbon neutral in the long term, this claim is highly disputed, especially regarding the combustion of wood obtained from forests. Processing and burning wood for energy is less efficient than coal and gas. This means more carbon is burned, and subsequently, more CO2 is released for every unit of energy produced. For this reason, the argument for the carbon neutral status of biomass relies on 'carbon-counting'; CO2 emitted from combustion will (eventually) be cancelled out. The trees grow back, taking up CO2 from the atmosphere in the process, thereby paying back the carbon 'debt'. This argument ignores several factors:
 * (1) Ancient forests are active carbon sinks. Even old trees continue to absorb CO2 from the atmosphere. In fact, an old tree is able to fix as much carbon within a year as is contained in an entire mid-sized tree that took years to grow. The loss of such carbon sinks increases the carbon debt of biomass energy.
 * (2) Trees also promote the health and stability of the soil. About 80% of the organic carbon contained in the terrestrial biosphere, over three times the amount of atmospheric carbon, is sequestered in the soil. The annual flux of carbon between the atmosphere and soil is about ten times greater than the annual increase in atmospheric CO2. The increase in deforestation to meet the demand for biomass could further perturb this balance, thereby adding more CO2 to the atmosphere in addition to the amount that is directly produced by combustion.
 * (3) The forests may not grow back, or at least to an extent needed to pay back the carbon debt. Deforestation disturbs the environment. The soil is more prone to desiccation and erosion. Normally, the decay of dead leaves and wood returns nutrients back to the soil, which maintains the forest's health. Dryer soils and reduced nutrient availability could stunt the growth of new trees. Furthermore, often a distinctly different set of flora returns in place of the old forest. Sometimes this is done deliberately. Fast-growing pines are often artificially planted after hardwood forests are cut down, but the amount of CO2 these artificial forests will absorb per unit area is less than the original.
 * (4) Depending on the forest type, it takes somewhere between 44 and 104 years for forests to grow back and repay the carbon debt. While better than the millions of years it takes for fossil fuels to pay back their carbon debt, it's not fast enough for wood burning to be considered "carbon neutral" within time frames that are meaningful for addressing climate change, even as a temporary source of energy. Furthermore, during the decades it takes for trees to grow back, in the meantime, the CO2 remains in the atmosphere contributing to global warming. Even if the exact amount of CO2 is returned into the biosphere after many decades, the earth's climate may have already shifted into a different state which won't be restored for hundreds or thousands of years. And an unstable climate may reduce the efficacy of tree growth to repay the carbon debt of biomass.
 * (5) At very large scales (i.e., amounts needed to reduce fossil fuel combustion by a significant margin), wood burning will never pay back their carbon debt. In order for biomass to become carbon neutral in real-time, the carbon flux must reach equilibrium, i.e., trees around the world must absorb the same amount of CO2 that is emitted from the combustion of wood. Trees simply don't grow fast enough to match the rate that we would burn trees at quantities needed to appreciably displace fossil fuels. In order to meet just 3% of global energy demand (~12% electricity) with wood biomass, the demand for harvested wood would double.
 * Another common argument from the biomass industry is that only "wood residues" from logging are used, which would decay and produce the same amount of CO2 anyway. However, this completely omits the effect of time. There is a difference between releasing large amounts of CO2 instantly via combustion compared to the buffer of slow decay over many years, a rate that can actually be balanced by new plant growth. Secondly, not all of the CO2 released from decaying wood ends up in the atmosphere. The long time required for decay allows some of the carbon to be incorporated as organic matter in the soil and/or consumed by other organisms. With the absence of decaying wood, microorganisms would instead consume the organic matter that has been in the soil for many years, thereby releasing carbon that would've been kept sequestered otherwise. More importantly, what the logging industry considers "residue" is very arbitrary. It's not just sawdust. It includes any part of the tree that is deemed not suitable for lumber: bark, thinning, branches, and tree tops, even the trunk itself if a bit diseased, too misshapen, or has too many knots. Basically, a very large portion of the total wood that can be harvested from a forest. Furthermore, it's just not realistic that biomass energy can be met just by the waste that is produced by the timber industry. What is actually happening is that the large demand for fuel pellets (in addition to the demand for lumber) is observably increasing the practice of forest clearcutting. Whether a tree (or tree part) ends up as pellets or lumber is decided AFTER cutting, not before. And large amounts of wood that is good enough for lumbar end up as pellets because a single tree can be judged as "wood residue" if it's deemed low-grade for lumbar and more profitable as biomass. Without the large demand for wood fuel pellets, fewer trees would be cut down.


 * It competes with food production and has led to significant price increases and food shortages. To counter this, there are attempts to produce biofuel from crops on marginal soil or from the ocean, where it would not interfere with farming. But these are likely to be much more expensive than using wood or grains.
 * Some systems, such as making ethanol from corn, are very inefficient and might even have an energy return below 1% and may release more CO2 than just burning petroleum directly. To meet the energy demands of the U.S., an area 5 times the size of the land area of this planet would be needed. Currently, the U.S. dedicates 66,000 square kilometers for just corn ethanol alone, twice that needed to power the U.S. with just solar, yet it only provides 4% of transportation fuel. However, the fact that Iowa, a state that grows substantial amounts of corn, has the first caucus in the presidential nomination process has, of course, nothing to do with that.
 * Often, plant matter is harvested without proper concern for replenishment - which is quite ironic, considering the word "Nachhaltigkeit" (German for sustainability) was first used in an 18th-century treatise on forest management, i.e., renewable plant-based biofuels in modern speak.
 * In some cases, tropical rainforests are being cut down to grow biofuels.
 * Where wood is burnt directly for heat, it can present a fire risk.
 * Traditional stoves pose significant risks through smoke, carbon monoxide and other pollutants. While more efficient stoves are rather cheap, they are often inaccessible to the poorest of the poor, who mostly depend on biomass for heating and cooking.
 * Relatively low power density compared to most fossil fuels.
 * Heavy fertilizer use to grow crops may increase nitrogen-associated pollution (algal blooms, NOx emissions etc.). Nitrous oxides are even more climatically relevant per molecule than methane.
 * Speaking of fertilizers, the fertilizers themselves are a less-spoken-of part of the problem of global warming. Synthetic fertilizers use the Bosch-Haber process to create ammonia from methane, which has the net effect of taking carbon out of the ground and pumped into plant biomass before being broken down one way or another into carbon dioxide. This would mean that anything relying on such fertilizers is most definitely not carbon neutral.

Fake disadvantages

 * The anti-GMO crowd likes to fear-monger about "genetically modified" corn being used for biofuels
 * While mono-cultures (vast expanses with only one crop on them) do pose valid problems and concerns, they are not necessarily associated with biofuels. In fact, biofuels can also be extracted from biologically diverse forests or even lawns.

Waste-based energy
Human civilization is constantly producing waste of one type or another, providing a renewable resource for many things, including energy. Much like plant-based energy, waste-based energy relies on producing a combustible material. With waste, the composition of said material may be adjusted via sorting mechanisms to optimise the energy density of the fuel. Prominent examples of this are biogas from bacterial degradation of waste and Energy-from-waste incineration. This is part of the energy recovery option in the waste hierarchy and thus is most sustainably benefited from where the waste cannot be prevented or recycled.

Biogas
Biogas, a fuel consisting mostly of methane, is typically produced during anaerobic digestion (microbial conversion in the absence of oxygen) of waste that is high in organic content in waste processing facilities or less controlled anaerobic degradation of such waste in landfills. While landfills produce biogas we can use, they are also a source for releasing methane into the environment under the degradation of organic waste, which is dangerously combustible and constitutes a severe greenhouse gas. Generally, more controlled facilities are preferred for the management of organic waste.

Advantages of biogas

 * Convenient, high energy density fuel. Very useful in transportation as internal combustion engines can easily be adapted for gas as well as fluid fuels
 * A way to recycle some types of waste and thus get more value out of our products
 * Burns relatively cleanly and with a relatively small carbon footprint
 * Methane is a worse greenhouse gas than CO2, and burning up any that would've otherwise leaked out into the atmosphere is a massive improvement for the environment
 * Waste is a pretty reliable resource
 * Sludge from wastewater can also be anaerobically digested
 * The digestate product of anaerobic digestion is useful as fertilizer in soil

Disadvantages of biogas

 * Requires a certain amount of organic content in waste (at least on the level of municipal solid waste), and sometimes it is not feasible to produce
 * In the case of solid organic waste, composting is usually cheaper and deals much more efficiently with lignocellulosic material (like wood)
 * In the case of solid organic waste, it is difficult to separate plastic contamination, which reduces the quality of the digestate product

EfW incineration
Energy-from-waste incineration is the most common method of direct energy recovery from waste and involves directly burning the waste. It is used—or should be used—to recover some value from different kinds of waste where other resource recovery methods have already had their share. In the past, this method was used to produce by-products irresponsibly and puff them into the surrounding area, like dust or heavy metals; these also included extremely hazardous substances like dioxins and furans. With current engineering standards and properly managed by-products, however, this is no longer a significant problem.

Advantages of EfW incineration

 * Incineration close to where waste is generated/collected
 * No long-term liabilities
 * EfW now has a track record in many countries
 * Produces biologically sterile ash with a tenth of the volume and a third of the weight of the original waste
 * Emissions are controlled
 * A way to reclaim value from some types of waste
 * Bottom ash can be reused as aggregate in construction
 * BPEO (Best Practicable Environmental Option) for some hazardous wastes
 * Waste is a pretty reliable resource

Disadvantages of EfW incineration

 * Air pollution close to the source of waste (similar to fossil fuels)
 * Extremely high costs and long payback periods
 * Needs long-term waste disposal contracts
 * Needs high calorific value wastes
 * Needs constant emissions monitoring against dioxins and furans
 * Forces redesign of commercial products; if consumer products are toxic when burnt, e.g. styrofoam packing peanuts, they would need to be phased out.
 * Production of ash residues requiring disposal
 * Building new plants has high political costs due to NIMBY concerns

Fake disadvantages of EfW incineration
Groups that are anti-incineration tend to be of the "act first, think later" variety, like Greenpeace or Friends of the Earth, and some of their arguments reflect that:
 * Generates carbon dioxide. While technically true, it's displacing other energy sources that also produce carbon dioxide, and more importantly, biological waste in landfills break down into methane, a gas which is far, far worse than carbon dioxide.  As far as the carbon cycle goes, converting plants into various products which are eventually burnt for energy is carbon neutral.
 * Resources are being lost by incinerating waste. This argument is ignorant of the fact that EfW incineration is so low on the common waste priority pyramid that only the waste from which nothing more can be derived using other methods is supposed to go through it. This also ignores the fact that resources are being recovered by converting waste into energy.
 * Incineration is incompatible with recycling. Again, it is more complementary to recycling than incompatible.
 * It unsustainably produces toxic substances that we have to live with. As mentioned above, times have moved on, and nowadays, problematic by-products are prevented, minimised or controlled by modern engineering standards.

Geothermal energy
Geothermal technologies tap the temperature difference between the surface of the earth and shallow or deep underground regions. Alternatively, they may make use of high-temperature hot springs in geologically active areas. The primary source of the temperature difference is the decay of radioactive elements, so in some sense, it is a form of nuclear power.

A ground source heat pump is often confused with geothermal energy but is not actually geothermal, but a way of making heat pumps more efficient than air source heat pumps since most of the heat is actually from the sun, or the heat pump has been operated in the other direction in the other season (in summer pump heat from the house into the ground, then in winter pump heat from the ground into the house).

There are various types of heat pumps, according to the different loops of pipes used; for instance, closed-loop systems gather energy directly from the earth. Several configurations exist, such as horizontal or vertical heat exchangers. The drilling direction can be radial or directional, and they can be installed around a pond or a lake. On the other hand, open-loop systems require more sophisticated resources to exploit, such as aquifers or groundwater.

Also, it is possible to tap the much higher temperatures hundreds or thousands of meters below the earth's surface via boreholes.

Advantages of geothermal energy

 * Very high energy densities may be naturally available
 * In practical terms, the environmental impact is low
 * Minimal land usage
 * Some volcanic areas are already densely populated due to rich volcanic soils. Thus this form of energy is often available close to its users (e.g. Japan, Indonesia, Hawaii, etc.)
 * Inexpensive compared to fossil fuels if conditions are right
 * Relatively less than expensive on certain islands (i.e., Azores, Montserrat, Dominica) in place of imported fossil fuels
 * As we discover better ways to dig deep underground, more and more spots become economically viable

Disadvantages of geothermal energy

 * Drilling can cause earthquakes
 * Very few good known resources, with only Iceland currently using geothermal for a significant amount of power (and even then, hydro dominates there). There may be more resources out there, but lava pools, even relatively close to the surface, are difficult to detect.
 * If energy extraction is too fast, it is no longer renewable
 * Deep geothermal is effectively fracking for power and is likely to be subject to the same objections if ever adopted on any large scale.
 * The use of geothermal energy may require living close to volcanic and seismically active areas near hot spots and plate boundaries, though many people already do that and have been for centuries, as volcanic soil is very rich in nutrients.
 * Smells bad due to underground sulfides and can release toxic chemicals.

Tidal and wave energy
This relatively new technology is designed to obtain energy from tidal movements or from waves. Although the concept seems simple, few commercial installations are in place. The opened in Brittany, France, at the mouth of the Rance river in 1966, but the significant environmental impact seems to have discouraged later schemes. The world's largest tidal power project is now the opened in South Korea in 2011, using a barrage built for flood control, land reclamation, and other purposes.

If efficient systems could be built, they could provide significant, predictable power (at least from tides).

There are essentially two ways to "trap" tidal energy – to build generation plants that are run by large amounts of water flowing in and out of large estuaries, and to build open ocean devices that somehow tap the energy by letting the tides (and perhaps waves, too) force a floating object up and down relative to an anchored one. The is an example of the latter technology.

Note that while tides are driven by the moon's gravity (and to a lesser extent by the sun's gravity), waves are driven primarily by the wind. This means wave power is a kind of wind power and is thus, ultimately, a form of solar power.

Advantages of tidal and wave energy

 * Whatever we build will provide energy until the moon "runs down".
 * Power generation systems can be built into, or merged with, flood control systems that protect large cities located on major estuaries, such as London and the Thames; this generates the energy close to a large need for it
 * Larger waves cause more erosion, and by harvesting wave energy, erosion can slow down.

Disadvantages of tidal and wave energy

 * They would need to be shown not to interfere with fish stocks or fisheries.
 * Tidal barrages could have environmental impacts, including loss of tidal habitats and possibly buildup of harmful contaminants due to less flushing action, as seems to have happened at Sihwa Lake.
 * The local energy output will wax and wane from maximum to virtually zero four times a day, and this time slowly changes from day to day. This can, however, be mitigated by dammed mill pond-type systems. In certain bodies of water like the North Sea, this can be mitigated simply through an integrated grid to bring electricity generated at a high rate to where it is currently being generated at a low rate (high water in Rotterdam, Netherlands, is hours away from high water in Sylt, Germany, for example).

Piezoelectric generators
Piezoelectric materials generate voltage when deformed (the opposite is also true - when voltage is applied, they deform). This can allow them to act as potential "free" and renewable power sources in certain applications. For example, there are proposals for piezoelectric floors in dance clubs or piezoelectric sections in roads to power nearby street lights regardless of the grid's status. Usually, the amounts of electricity generated that way/cost ratio are pretty underwhelming. However, things are better when it comes to small-scale applications. For example, piezoelectric fibers woven in someone's clothing could allow them some day to trickle charge their gadgets via their daily activity.

Thermoelectric generators
Thermoelectric generators use materials which generate voltage from heat differences using the (the opposite is also true - applying a voltage will generate a heat differential, known as the Peltier effect). Because of their low efficiencies, they are rarely encountered in anything but small-scale uses, such as temperature sensors and Soviet kerosene-lamp-powered radios. A notable exception is radioisotope thermoelectric generators, the likes of which have been used in deep space exploration probes, making them independent from the fading sunlight in the far reaches of the Solar System. As the name suggests, in RTGs, the heat is provided by the decay of radioactive materials, which automatically makes them a non-renewable energy source. Some forms of radioisotope generators were also once used as batteries for pacemakers, as it is essential that a pacemaker battery be replaced as seldom as possible; however, real and imagined concerns about radiation, as well as the advancements in chemical batteries, have rendered this application very rare.

Earth batteries
Earth batteries create voltage by placing two rods of dissimilar metal into the soil. Functionally, they are no different than sticking two electrodes in a potato, and like any other galvanic battery, they are not eternal, although due to the changing soil conditions, they might end up "recharging" themselves. The voltages generated this way are usually too small for any practical use. A notable exception was their use as an energy source for signal amplifiers in early telegraph installations. Larger installations might also tap into Earth's natural telluric currents.

Energy storage/transmission
While not electricity generation itself, an extremely closely related topic is that of energy storage and transmission. The largest issues with using solar, tidal or wind power as the backbone of an energy grid are
 * The energy supplies are not on-demand energy, and
 * The locations are limited

How would you be able to store solar power in order to provide electricity at night, or wind power from when the wind blew too hard to the times it isn't blowing at all? Note that these issues are not unique to renewable energy, just more pertinent as wind, solar and tidal power will produce what they produce when they produce it, regardless of how much we need or when we need it.

Pumped Energy
By far the most common form of large-scale energy storage, electricity is used to raise the elevation of a large amount of water, which gives the water more potential energy. When electricity is needed again, the potential energy is converted back into electricity. Practically, this involves pumping water up a hill and then using the water as hydropower when needed.

Advantages of Pumped Energy

 * Once built, very little maintenance is required
 * The most efficient and cheapest form of large-scale energy storage
 * It is extremely low-tech; the bulk of construction does not require a complicated supply chain, nor the high pollution and intensive processing of rare materials
 * Many locations, such as abandoned mines, are already pre-dug out, reducing the need for additional construction costs or disturbing the landscape

Disadvantages of Pumped Energy

 * Limited by geography to areas with hills or mountains. Not so much a problem for, say, Switzerland, a country that's basically a series of mountains and valleys, but is simply unavailable for the Midwestern United States, which is so flat and in such long distances that literally 3/4 of all tornadoes worldwide happen here.
 * As with all forms of energy storage, the Laws of Thermodynamics are a bitch, and the generators do not recover all of the energy. About 1/3 of the electricity used in pumping the water is lost.

Electrical batteries
A more conventional type of battery, y'know, it usually zaps you if you lick it. Some common varieties are, Nickel-Cadmium, Lead-Acid, and Lithium-Ion. Like many other types of energy storage, this works by creating an energy difference between two locations with chemical energy, in this case, directly pumping electrons into half of the battery, and when you bridge the gap between them with something conductive (like your tongue), the electrons try to reach equilibrium. Chances are, you have a lithium-ion battery within incineration distance of your genitals in your pocket right now.

Advantages of electrical batteries

 * The energy is already in a form capable of entering the grid without may other complex mechanisms.
 * The energy is stored in a form that is portable, as to provide energy directly where it's needed, e.g., cell phones and Electric cars.
 * The technology behind it is rapidly advancing; the energy density improves year after year.
 * Losses are some of the best in class, with 90%+ of stored energy able to be discharged... at least when the battery is between 20% and 80%).
 * As batteries wear out, they can be repurposed for lower wattage purposes.
 * If batteries reach a point of becoming unusable, the material within is HIGHLY recyclable.
 * Constantly becoming cheaper, to the point where electric cars are expected to reach price parity with ICE cars by 2024, and are expected to continue to fall in price afterwards.

Disadvantages of electrical batteries

 * Lithium-ion batteries, the most common design used today, degrade far too quickly for long-term, large-scale grid power storage. As mentioned above, old car batteries may be used for this purpose, but even so, they still degrade, and even if every single car was electric, the yearly "harvest" of replaced car batteries may be insufficient storage for a country relying upon (non-hydro) renewables alone.  A new generation of cheap/efficient batteries is needed, but they are not yet available at the time of writing.
 * Even ignoring toxicity, those materials aren't particularly abundant, and the sourcing of lithium is rapidly becoming a major political issue. cough cough. There's already talk of mining the ocean floors for the lithium needed, which could have environmental impacts that we don't even realize just yet.
 * Speaking of open-pit mining, one of the more common metals used for batteries is cobalt, which mostly comes from the Democratic Republic of Congo. From excessive pollution to child slavery, cobalt represents just about every possible social ill you could think of.
 * Per watt-hour of storage, batteries are very expensive to create.
 * Batteries are supposed to remain between specific points, kind of with a low and high buffer to extend the life of the battery, essentially handicapping the battery to about 60% capacity if you want the battery to last a while. (Do this for your phone battery too! Try to keep it between 80% and 20%; that 80% will be as much as 100% if you charge it to 100% every night.)
 * Charging takes time, it can be inconvenient to do so, though there has been talk of "battery swap" locations where your battery could be swapped for a full one within a few minutes, and the battery could be charged and stored ready to go.
 * The power inside of the batteries is not greener than the power used to charge them. When you add 100 KWH of electric demand on the grid in order to charge your Tesla, solar and wind farms don't suddenly produce more energy, that extra demand is often filled by increased natural gas and coal consumption.  Also note that this varies by the time of day, as the demand for electricity and the ability to produce both vary.

Flywheel storage
Actually a pretty old tech. Spin a disk, and the energy is stored as rotational energy. The energy can be used to generate mechanical or electrical power on demand. It's found on a lot of old early industrial-era machinery to "even out" any erratic power inputs. However, the old stuff only stores energy for a short while, as the wheel will slow down due to friction, air resistance, and even the rotation of the Earth. Hence today, they are typically used for very short-term fluctuations lasting seconds or minutes rather than, for example, overnight storage. Heavy-duty electric storage involves a more high-tech solution; magnets to suspend the flywheel and encase it in vacuum sealed tube.

Advantages of flywheels

 * Capable of being used as the primary power source of vehicles, if we were desperate
 * Already used in some of the fancier race cars as an additional source of power, storing the energy from braking and used to provide a short burst of power greater than what an internal combustion engine alone could
 * The simpler ones are extremely robust, and can outlast most other pieces of machinery
 * The simpler ones also use relatively common and cheap materials; even worthless lead could make up the bulk of the flywheel if you don't need the thing to be portable, all of which are also very simple to recycle, so widespread adoption would not result in new geo-political headaches over scarce resources... for the cheaper flywheels anyway.
 * Capable of converting stored energy into electricity or mechanical power extremely rapidly, with some systems capable of power outputs in the gigawatt range, for those situations where you need a huge amount of energy to be delivered in a tiny amount of time. E.g., experimental fusion reactors, large hadron colliders, roller coasters, aircraft carriers, railguns, high-powered lasers, mass drivers, doomsday devices, etc.

Disadvantages of flywheels

 * The simple ones lose energy rather quickly for anything other than very short-term storage
 * The more complex ones are much more expensive to produce and maintain
 * Existing systems have much lower capacity than alternatives like pumped storage
 * A flywheel can store a LOT of rotational energy. A damaged flywheel can release said energy all at once in the form of a giant disk of shrapnel.  For this reason, a lot of flywheels are buried under dirt or sand, both to reduce the chance of damage and to reduce the damage done if it breaks.  This does make maintenance and repairs a bit more tedious, of course. There have been occasional accidents, such as in an experimental system at Quantum Energy Storage in southern California, which injured four workers in 2015. However, it isn't clear if flywheels are more intrinsically dangerous than other methods of power storage involving large amounts of water, flammable metals (lithium), and so forth.

Compressed Air
This is really two related ways of storing energy. The first method is to compress air, which can later be used to drive a piston. The second method is to temporarily store the energy from air compression, and then recover much of it back as electricity as the air is expanded.

Advantages of Compressed Air

 * Similar to lithium-ion batteries in terms of energy density and cost, but doesn't require toxic chemicals or exotic materials to produce
 * The lack of exotic materials makes old compression units very easy to scrap and recycle
 * Assuming an airtight seal, the compressed air will not lose potential energy over time
 * Can be used as a primary energy source for vehicles, if we find that we simply don't have enough lithium for an all-electric fleet as we get off oil

Disadvantages of Compressed Air

 * Still relatively expensive, as with all forms of portable energy storage.
 * Compressed air does not like to stay compressed, which is an issue if the storage unit is damaged from relatively routine events such as car accidents. The scientific word for high-pressure gasses rapidly expanding is "explosion".

Transmission
The further electricity has to travel from its origin, the larger the transmission losses will be. This has been reduced in recent years by the adoption of "High Voltage Direct Current" (HVDC), which effectively creates electrical "regions" fairly independent of international borders, with electricity being constantly transported between countries. With greater voltage comes greater responsibility lower current, which results in greater capacity and lower transmission losses. While years ago, HVDC required prohibitively expensive equipment; the manufacturing costs have come down as technology moves forward. When a spike in demand (or generation) occurs somewhere, power plants relatively far away from the site can provide the electricity for a lower cost than turning on another power plant would.

There's been recent investment into (unimaginatively named) "Ultra High Voltage Direct Current" (UHVDC). Such an improvement to a grid enables electricity to be "shipped" across entire continents with only minimal losses. As usual, the biggest investment in such an improvement has been occurring in China, with the creation of a three-THOUSAND kilometer-long transmission line with a capacity of 12 gigawatts. For perspective, that's around 25 times the amount of electricity that New York City alone requires, from as far away as Dallas.

The U.S. has seen some recent development too into UHVDC, namely in Oklahoma; being in the windiest part of the only country on Earth to have a tornado season, Oklahoma is capable of producing far more electricity from wind than it could possibly use, so why not export it and make a small fortune? Other regions in the Midwest are likely to follow suit. Improvements to the grid would enable the bulk of the U.S. to be much more closely interconnected, which solves one of the biggest legitimate issues with renewables; how do you produce electricity when the sun isn't shining or the wind isn't blowing? The wind is always blowing somewhere, and the sun still shines on other parts of the world while your part is still dark. A large-scale UHVDC grid would enable wind and solar (as well as some of the more minor forms of renewables) to be combined with various hydro projects in more mountainous regions (and maybe nuclear for some extra heavy lifting), so it's not impossible to have a low-carbon electric grid.

Persuading people to support renewable energy
Many conservatives in the United States are resistant to suggestions that climate change is real. Giving renewable energy a positive spin can work. If [discussing renewable energy with conservatives] you deliver the message of energy freedom, energy choice, competition, national security, innovation, all of a sudden, you will have a receptive audience and they will listen to you. If you lead off with climate change they’re not going to pay a bit of attention to anything else you say. They’ve been brainwashed for decades into believing, oh, we’re not damaging the environment… Many forms of renewable energy are getting steadily cheaper and are getting steadily better at competing with fossil fuels. Optimists hope consumption of coal and oil could stop rising after 2020. Compared with fossil fuels and nuclear energy, renewable energy consumes much less freshwater yet creates substantially more jobs.

It is a rapidly growing and profitable market.