Biomass can be converted into a liquid fuel known as biofuel, with the two main types being ethanol and biodiesel. In order to make these, the plant cell walls must be broken down, and then the intermediates are processed biologically (such as using microbial fermentation) or chemically (typically through the use of particular catalysts) in order to form the biofuel. One of the biggest uses of these biofuels is as a substitute for gasoline in transportation. As the plants grown to make biofuels take in CO2, and it is that same CO2 that is released when they are burned, is considered a carbon neutral fuel, meaning it takes in as much carbon as it releases. This makes them a promising possibility to reduce carbon emissions while minimizing changes to infrastructure. However, the energy return on energy investment for a lot of biofuels is low.

Solar energy centers on using energy from the sun for our uses. There are two broad types of it, with the simpler one being passive solar energy. 

Passive solar energy does not use mechanical or electrical equipment, not does it actually collect or store any energy. It uses the natural heat and light given off by the sun. Examples of passive solar energy include solar ovens, skylights which decrease the need for electricity for lighting, darker colored buildings to absorb more heat, or even things that block the sun such as trees that provide shade and cool homes down. Passive solar energy reduces the need for electricity produced by other means.

The other form of solar energy, which is what is typically thought of when people imagine "solar power" is active solar energy. This uses mechanical and/or electrical equipment to actually capture the sun's energy. 

One of the main forms of this is capturing the sun's heat. Solar water heaters work by capturing the sun's heat in water, which is warmed and then transferred to a home, which reduces the need for gas or electricity being used to heat the water. This can actually be done passively as well, but it is far more efficient and can be used in a wider range of climates if used actively. Another example is concentrated solar thermal (CST) power, which uses mirrors or lenses to concentrate sunlight, using the heat to create steam, spin a turbine, and generate electricity. 

Solar farms are larger-scale and have numerous, larger panels. They generate significantly more electricity than rooftop solar does, but they also result in more habitat destruction and therefore have a larger environmental impact.

Solar energy as a whole is a renewable and nondepletable source of energy. However, it has the problem of intermittency - they are limited by the amount of sunlight available in a particular area. Energy generated during the day can be stored for use at night, however, the battery technology we currently have is expensive and not fully efficient, limiting its use. In addition, the semiconductor metals need to be mined, which is associated with its own environmental problems and concerns.

Hydropower, or hydroelectric power, uses the movement of water to generate electricity. It does this in much the same way as fossil fuels work. The movement of water spins a turbine, which turns a shaft in a generator, which generates electricity.

In its simplest form, hydropower involves the creation of such turbines in moving water, such as a river. This is far from the only way that the movement of water can be harnessed to generate electricity, however. The movement of the tides can also be used - tidal energy centers around the placing of turbines so that the natural ebb and flow of the tides spins them.

Flooding becomes more common upstream of the dam, along with significantly increased sedimentation. Habitats downstream lose out on nutrients that were present in the sediment, along with their decreased water levels. In addition, this disturbs the migration of species such as salmon, that swim upstream to their spawning grounds. Fish ladders, also known as fishways, have been being constructed to allow a path for them to follow their normal migration path. Essentially, there are a series of "steps" where fish jump from pool to poll until they reach the reservoir.

Despite the above listed issues, there are a variety of benefits to using hydropower. Once the dam is built, there is no production of air pollutants or greenhouse gases associated with the generation of electricity in this manner.

Within the Earth, radioactive decay is occurring, which gives off a lot of thermal energy, or in other words, heat. Geothermal energy uses this energy from within the Earth.

One of the biggest issues with geothermal energy is that it is not accessible everywhere. The most active locations for geothermal energy are near plate boundaries. Hydrothermal areas are hot, permeable, and contain liquid within them; these areas can have hot water extracted from them. Most areas do not meet all three of these characteristics, and instead have water pumped down, however, this has been linked to an increased risk of small earthquakes.

As the requisite heat is found in very deep areas within the Earth, the cost of drilling associated with the construction of geothermal power plants is very high.  In addition, while there are far fewer emissions into the air with geothermal when compared to fossil fuels, geothermal plants can result in the release of small amounts of CO2, as well as hydrogen sulfide (a toxic gas that inhibits cellular respiration). Hydrogen sulfide can even be released in small amounts by the above-mentioned natural hot springs.

Wind turbines are typically found either in wind farms (groups of turbines in flat, open areas) or offshore wind (essentially wind farms in oceans or lakes). They are often clustered together to make servicing, repairing, and building transmission lines easier. Offshore wind requires longer transmission lines, and a significantly more complicated construction process, resulting in greater expenses. However, there are faster winds, resulting in the generation of more electricity.

Wind turbines do not release air pollutants or greenhouse gases while generating electricity, they are a non-depletable source of energy, and they can share land with other things (such as agriculture), resulting in a lack of habitat destruction. 

Like with solar power, wind turbines suffer from intermittency as it isn't always windy. Extra energy can be stored, but we can't store much with our current technology and grid capabilities, so wind power works best as a supplement to other sources of energy. In addition, wind turbines can kill flying species, particularly birds and bats, although research has shown that painting a single blade black greatly reduces the number of animal deaths. Wind farms are often considered an eyesore, which results in people not wanting them built near their homes or neighborhoods. 

Steam reforming involves the burning of methane, and using steam to separate out the hydrogen gas. This releases CO2 and requires the use of methane, a fossil fuel. Unfortunately, as this is the cheapest way to produce hydrogen gas, it is by far the most used.

Electrolysis involves the use of an electric current with water, splitting water into oxygen gas and hydrogen gas. This requires electricity, but does not release any CO2 and is a more sustainable method. Recently, a highly efficient (nearly 100%) method for using electrolysis to create hydrogen gas from seawater was developed, using cheap and plentiful catalysts.

Along with switching to more renewable sources of energy, it is also helpful to conserve energy and simply use less of it. This can be done either at smaller scales, making changes around the home, or larger scale, where it occurs community-wide.

Small-Scale Energy Conservation

• HVAC Changes - Adjust the thermostat at home to use less energy. Set the heat to a lower temperature in the winter, set the AC to a higher temperature in the summer, and just try to use each of them with reduced frequency.

• Water Conservation - You might be wondering how conserving water conserves energy. It takes energy to transport the water and use it at home. In addition, it requires energy to treat and filter the water for municipal use. There are a variety of strategies to reduce water use at home: native lawns (planting native plants eliminates the need for water once they are established while also increasing local biodiversity; this is a part of conservation landscaping), using rain barrels for water plants or washing cars, and low flow/more efficient appliances (such as showers, toilets, and dishwashers) can get the same job done while using less water.

• Energy-Efficient Appliances - There are a variety of appliances that are made that are more efficient and can get the same job done without using as much energy. In addition, many appliances have "eco" or "green" settings to work in a more efficient mode.

Large-Scale Energy Conservation

• Fuel Economy - The government can set standards regulating the minimum fuel efficiency standards for vehicles. In the US, CAFE (Corporate Average Fuel Economy) sets this standard. Using hybrid or electric cars, rather than ICE (internal combustion engine) vehicles, reaches this same effect, as they can reach farther distances with the same amount of energy input. Subsidies and tax breaks can be put in place to encourage their use, and some states are beginning to adopt policies mandating that all new cars after a certain year be BEV (battery electric vehicles). 

• Public Transportation - If more people use public transportation, then there are fewer vehicles being used in total. Carpooling, as well as greatly increased use of public transportation, helps to reduce the amount of energy being used for travel.

• Green Building Design - Buildings can be designed in a way to reduce the amount of energy needed. Recycled materials can reduce the energy used to produce house materials. Passive solar can be used for heating or to reduce the need for using electricity for light. Deciduous trees can be used to provide shade in the summer, but allow sunlight through in the winter when there are no leaves. Green roofs and walls absorb the sun's heat and cool buildings.

• Smart Grid - Some electricity companies use a variable price model, in which they charge a higher price in peak demand to discourage use. They will also charge lower prices when individuals use less energy, which incentivizes a lower overall use. In addition, smart grid supports the use of more varied sources of energy, such as allowing surplus energy made by rooftop solar to be directed back to the grid for use in other homes, and the overall incorporation of other renewable energy sources.