Renewables

Renewable Energy Study Guide

The goal of renewable energy:

• Almost all renewable energy sources share the same goal which is to reduce the amount of CO₂ emissions to net zero. Meaning we are not producing any excess CO₂ compared to the rate of CO₂ being removed from the atmosphere from processes such as photosynthesis.

• The main drawback to many renewables, however, is that many of them are not at a current capability that we can use them solely as our source of energy.

  • For Example: Solar and Wind have intermittent power supply due to factors like location, weather, etc. so we cannot rely on them solely. 

Types of Renewable Energy

How some renewables provide us double

• Energy sources such as Biomass, Geothermal, and Solar have the benefit that not only can we use them as a way to generate electricity, they can also be used as a source of heat for homes, buildings, etc. 

• This ability to combine heat and power generation is known as cogeneration

  • For example, Geothermal is used in Iceland to provide the steam for turbines and also pumped around buildings for their heat too!

The challenge of dependence on fossil fuels

• We know that using fossil fuels has long lasting effects on the environment from the production of multiple sources of pollution

• The problem is, to move away from fossil fuels would be costly and renewable energy is not fully capable of being our power source for all electricity. 

• That is why we are looking at 2 renewable energy sources as more conventional alternatives as we use them around the world and they have been used for many decades if not millenia!

Renewable Conventional Alternatives: Bioenergy and Hydropower

Bioenergy - The energy of biomass

Overview

• Bioenergy is energy derived from biomass.

  • Biomass consists of organic material derived from living or recently living organisms, and it contains chemical energy that originated with sunlight and photosynthesis

• In principle, bioenergy is renewable and releases no net carbon dioxide into the atmosphere as it assumes any carbon released had been absorbed in the production of the biomass. 

Sources of Biomass

• Most of our sources of bioenergy come from plant matter such as:

  • Wood: From trees, it is an easy material to burn and can be used for multiple purposes.

  • Charcoal: not the same as coal! This is a man made source of energy as we slow burn wood over a long period of time to form the charcoal.

  • Crop Residue: leftover material from fields after harvest 

• Combustible animal waste can be used as well to provide the energy needed

Dependence on Fuelwood

• In rural regions of developing countries, people heavily depend on fuelwood to burn in their homes for heating, cooking, and lighting.

• This further increases issues like deforestation which only increases soil erosion and desertification of these regions. 

Bioenergy in Developed Countries - The Biofuels

• In developed countries, we have been exploring different sources of bioenergy to produce biopower, generating heat and electricity.

• This has led to the creation of many liquid biofuels that we are looking as a valuable alternative to the 

Ethanol

• Ethanol is an alcohol produced by fermenting biomass, generally from carbohydrate - rich crops (such as corn), in a process similar to brewing beer, wine, etc. 

• Ethanol is seen as a potential replacement for gasoline in the US as we have the largest dependence on Oil of all nations right now.

Pros and Cons of Ethanol

Pros:

• It is easy to source the biomass needed for the production of ethanol

• These fuels have shown promise as alternatives to gasoline

• Ethanol can be blended into fuels, for example E - 85 fuel is a mixture of 85% ethanol and 15% gasoline that certain model cars can use for their energy 

• Other plants besides corn can be used for ethanol like sugar cane, sorghum, etc. as long as they are high in carbon!

Cons:

• Efficiency concerns exist as the EROI is not that strong to justify 

• For the US, one of the major concerns about generating and using ethanol as a fuel source, is that we primarily source the biomass for ethanol from corn.

• These concerns relate around the sustainability of growing corn for the purpose of ethanol production including:

  • Intensifying pesticide use, fertilizer use, and freshwater depletion which are already facing concerns with today just for the production of corn as a food source. 

  • The amount of land necessary for replacing gasoline with ethanol would expand the acreage for corn fields by more than 4 times. 

    • This also assumes that all of this corn will go into the production of ethanol

Biodiesel

Overview

• Biodiesel is a fuel produced from vegetable oil, used cooking grease, or animal fat to be used as a potential alternative to diesel fuels further reducing our dependence on petroleum (oil)

Pros and Cons

Pros:

• Less vehicle emissions.

• Nearly identical fuel economy.

• Cost effective.

• Nontoxic and biodegradable.

• Can be sourced from food waste.

Cons:

• EROI is usually (<1-8):1, so its return of energy is lower compared to our fossil fuels today.

• It is still new, so we don’t have the infrastructure in place right now to produce it at such amounts that we could use it to replace our dependence on fossil fuels.

Novel Biofuels

Algae and Cellulosic Ethanol

• Many species of algae are now being cultivated to produce biodiesel, ethanol, or even jet fuel.

• Algae can be grown at a large scale in outdoor ponds, closed tanks, or in closed transparent tubes called photobioreactors.

Cellulosic Ethanol Production - ways to not use food sources as ethanol sources.

• Researchers are refining techniques to produce cellulosic ethanol by using enzymes to produce ethanol from cellulose.

  • Cellulose is the main sugar found in all plant material and has no food value as we are unable to digest this as a source of energy for our own bodies

• The largest benefit to using cellulose is that it can be produced rapidly from native plants such as switchgrass which are not used for food and grow faster than other plants we source right now. 

Carbon Emissions of Bioenergy

• In principle, energy from biomass is carbon-neutral, releasing no net carbon into the atmosphere.

  • Burning biomass releases carbon dioxide that was recently pulled from the air by photosynthesis.

• This only holds true if forests, which sequester carbon dioxide, are not cut and used as a source of biomass. 

  • Which is a problem, especially in developing countries who use fuel wood as a cheap source for subsistence over electricity generation means for many places biomass is not net 0 at this point.

Hydropower - The energy of moving water

Overview and Use

• Hydroelectric power, or hydropower, uses the kinetic energy of flowing river water to turn turbines and generate electricity.

• The amount of electricity generated by hydropower plants is dependent on the volume of water and the distance that it falls.

Approaches to Hydropower

• Hydropower uses three approaches:

  1. Storage Technique (the most used approach!):

     • involves storing water by first building a barrier across a rivet to form a reservoir

     • This reservoir water is then released through a series of turbines within the dam and releasing water through a series of turbines in a dam.

  2. The run-of-river technique:

     • Diverts a portion of the river’s flow through a pipe or channel, passing it through a powerhouse, and returning it to the river. 

     • This is much less disruptive of the river’s natural flow, but does not generate as much power.

  3. Pumped storage:

     • Hydropower works by transporting water from a lower reservoir to a higher reservoir during times when demand for power is weak and prices are low.

     • When demand is high again, the water is allowed to flow back downhill through a turbine.

Advantages and Environmental Impacts of Hydropower

• Hydropower is and renewable and does not produce air pollutions, yet it has some impacts

• Hydropower has three advantages over fossil fuels:

1. It is a completely renewable source of energy.

2. Hydropower is efficient, with an EROI ratio of more than 80:1, higher than any other modern energy source.

3. No carbon dioxide or other pollutants are emitted.

• Hydropower does have negative impacts on streams:

  • Natural riparian habitat is flooded upstream of the dam, and deprived of water downstream of the dam.

  • Suppression of flooding prevents river floodplains from receiving nutrient-rich sediment from the river.

Ecological Effects

• Dams cause thermal pollution if the river downstream is kept artificially shallow and it warms rapidly in the sun.

• Thermal shocks can occur if cold water is suddenly released from the dam’s reservoir.

• The passage of fish and other organisms through the river is blocked.

The New Renewable Energy Sources

Definition and Characteristics

• Conventional alternatives to fossil fuels, such as hydropower and biomass, are renewable but they can be depleted with overuse and they exert some undesirable environmental impacts.

• Energy from the sun, wind, geothermal heat, and ocean water are 'new renewables' because:

  • They are just beginning to be used on a wide scale.

  • They are harnessed using technologies still in a rapid phase of development.

  • They will likely play much larger roles in the future.

Advantages of New Renewables

• The use of new renewables can provides many benefits:

  • Reduction in air pollution such as SO_X (sulphur oxides), NO_X (Nitrous Oxides), Hg (Mercury), Pb (Lead), etc. 

  • Reduction in greenhouse gas emissions, like CO₂ (carbon dioxide) and CH₄ (methane), that drive global climate change.

  • Diversification of an economy’s energy mix, which allows it to become more resilient to changes in conditions

Supportive Policies

• One of the major issues with the use of renewables is they can have a high upfront cost,(especially solar), so we use policies in governments to help encourage the use of these new renewables

• Policies that support renewable energy may include:

  • Feed-in tariffs, such as those used in Germany.

  • Goals or mandates set by governments that a minimum percentage of power comes from renewable sources.

  • Government investment in research and development.

  • Lending programs to renewable energy start-up businesses.

  • Tax credits and rebates to individuals and businesses who produce or buy renewable energy.

Solar Energy

Overview of Solar Energy

• Solar energy is energy from the sun.

• The Earth receives enough solar energy each day to power human consumption for 25 years.

• Finding cost-effective ways of efficiency collecting and converting this energy has been a major challenge.

Methods of Collecting Solar Energy

Passive:

• Involves designing buildings to maximize absorption of sunlight in winter while keeping the interior cool in the heat of summer such as:

  • Planting vegetation as a temperature buffer.

  • Using thermal mass materials that absorb, store, and release heat slowly to build the floor, walls, roof, etc.

• It does not involve the use of moving, focusing, or storing energy from the sun!

Active:

• Active solar energy collection makes use of devices to focus, move, or store solar energy such as:

  • Photovoltaic Cells: Use the energy of sunlight to generate electricity by causing electrons to move across silicon plates.

  • Concentrated solar power (CSP): intensifies solar energy by gathering it from a wide area and focusing it on a single point

    • The parabolic trough approach uses curved mirrors to focus sunlight onto synthetic oil in pipes, which is then used to heat water into steam, which then drives turbines.

  • Solar cooker: portable ovens that use reflectors to focus sunlight and cook food.
    
    

Pros and Cons of Solar Energy

Current Trends in Solar Energy

• Solar energy is expanding:

  • Due to lack of government investment in past decades, solar energy contributes just 0.56% of the U.S. energy supply.

  • This is changing, however, as PV technology is the fastest growing power source today, having recently doubled every two years.

  • China is currently the world leader in solar technology, subsidizing its production so much that it is driving U.S. manufacturers out of business.

Wind Power

Overview of Wind Power

• Wind power is generated from the movement of air caused by differentially heated air masses.

• Wind turbines convert the mechanical energy of wind into electrical energy.

• Wind turns the blades of a rotor, which rotates machinery inside of a nacelle.

Wind Turbine Design and Functionality

• Engineers design turbines to yaw, or rotate back and forth in response to changes in wind direction, ensuring that the motor faces into the wind at all times.

• Wind speed highly influences electricity generation: A doubling of wind velocity increases power output by eight times.

Wind Farms and Production

• Turbines are often erected in groups; this development is called a wind farm.

• Wind power capacity is doubling every three years.

• China, the United States, and Germany have the greatest rates of production.

Benefits and Limitations of Wind Power

• Wind power has many benefits:

  • A 1-megawatt wind turbine prevents the release of:

    • 1500 tons of carbon dioxide

    • 6.5 tons of sulfur dioxide

    • 3.2 tons of nitrogen oxides

    • 60 pounds of mercury

  • Wind turbines produce 20 times more energy than they consume, an EROI value better than most other energy sources.

  • Wind power can also be easily scaled up by adding more turbines to an area of land.

• Wind power has limitations:

  • Like solar energy, wind is an intermittent resource that we cannot directly control.

  • The geography of the landscape determines how much wind power is available.

Challenges in Wind Power Implementation

• In the United States, mountainous areas and the Great Plains have the greatest potential.

• Most of the U.S. population lives along the coasts, however, so continent-wide transmission networks will be needed.

• When wind farms are proposed near population centers, residents often oppose them.

  • Local people object to wind farms for aesthetic reasons, creating a not-in-my-backyard (NIMBY) syndrome.

• Wind turbines also pose a threat to birds and bats, which are killed if they fly into the rotating blades.

Geothermal Energy

Definition and Sources

• Geothermal energy is thermal energy that arises from beneath Earth’s surface.

• Geothermal energy manifests itself at the surface in these ways only in certain areas.

Harnessing Geothermal Energy

• We harness geothermal energy for heating and electricity.

• Wells can be drilled toward heated groundwater, which can then be used to provide direct heating of buildings.

• Geothermal power plants use heated underground water and steam to generate electricity.

Case Study: Iceland

• At Iceland’s Nesjavellir power station, geothermal steam is used to heat water from a lake, which is sent to the capital city for residents to use or to generate electricity.

Benefits and Limitations

• Geothermally heated water can release dissolved gases, including carbon dioxide, methane, ammonia, and hydrogen sulfide, but at far smaller levels than fossil fuel combustion.

• Geothermal energy is renewable, but can be depleted if plants use heated water faster than it can be recharged.

• The Earth’s crust shifts naturally over time, so areas that produce hot groundwater may not always do so.