Renewable Energy Sources

Lecture Question –What are the renewable energy sources? Make a list, as comprehensive as possible. –What are the environmental impacts of these energy sources? –Renewable Energy Sources Radiant solar energy –Solar heating (passive and active), solar power plants, photovoltaic cells Biomass energy –Direct: combustion of biomass –Indirect: chemical conversion to biofuel Wind energy Hydro energy Geothermal energy –Power plants, direct use, heat pumps Ocean energy –Tidal; salinity-driven

Hydro Energy Advantages –Cheap to operate Long life and lower operating costs than all other power plants –Renewable –High yield Lower energy cost than any other method –Pretty plentiful Some countries depend almost entirely on it –Not intermittent (if reservoir is large enough) –Reservoirs have multiple uses Flood control, drinking water, aquaculture, recreation –Less air pollution than fossil fuel combustion

Hydro Energy Disadvantages: –Human population displacement –More significant breeding ground for disease –Reduces availability of water downstream –Ecosystem impacts Barriers to migrating fish Loss of biodiversity both upstream and downstream Coastal erosion Reduces nutrient flow (dissolved and particulate) –Water pollution problems Low dissolved oxygen (DO) Increased H 2 S toxicity; other DO-related problems Siltation a big problem (also shortens dam life) –Air pollution Actually may be a significant source of GHGs (CH 4, N 2 O, CO 2 ) –Decommissioning is a big problem The Size Issue –Many (most) of the above problems are significantly worse for larger dams –However, small dams have shorter lifetimes, less capacity, and are more intermittent

Wind Energy How it works –Wind turbines directly generate electricity –Quite efficient (not a heat engine) Wind Energy Potential shading gives potential for in- state electrical needs numbers give total potential for total US needs

Wind Energy Advantages –High net energy yield –Renewable and free –Very clean source of energy No pollution (air or water) during operation –Long operating life –Low operating/maintenance costs –Can be quickly built; not too expensive –Now almost competitive with hydro and fossil fuels –Land can be used for other purposes Can combine wind and agricultural farms

Wind Energy Disadvantages –Energy storage issues An intermittent source of energy; need backup (eg stored energy) for low-wind days Or must be connected to the electrical grid –Only practical in areas that are windy enough –Visual pollution –Danger to birds New (slow turning) designs largely eliminate this problem –Low energy density of wind Must use large areas of land

Biomass Energy What is it? –Biomass energy is the use of living and recently dead biological material as an energy source –Ultimately dependent on the capture of solar energy and conversion to a chemical (carbohydrate) fuel –Theoretically it is a carbon neutral and renewable source of energy How it works? –Traditional: forest management, using wood as fuel –Use of biodegradable waste Examples: manure, crop residue, sewage, municipal solid waste –Recent interest in agricultural production of energy crops Should be high yield and low maintenance Examples: corn, sugarcane, switchgrass, hemp, willow, palm oil, rapeseed, and many others Does not have to be a food crop Recent interest in bioengineered (GM) plants as fuel sources –Production of a liquid or gaseous biofuel Biogas due to the breakdown of biomass in the absence of O 2 –Includes capture of landfill methane Bioethanol from fermentation, often from corn. Cellulosic bioethanol is usually from a grass (switchgrass) Biodiesel from rapeseed and other sources

Biomass Energy Carbon neutral –CO 2 ultimately released in energy generation is recently captured and so ideally does not change total atmospheric levels –Carbon leaks can result in a net increase in CO 2 levels –Sequestration in soil can result in a net decrease in CO 2 levels

Biomass Energy Advantages –Versatile –Renewable –No net CO 2 emissions (ideally) –Emits less SO 2 and NO x than fossil fuels Disadvantages –Low energy density/yield In some cases (eg, corn-derived bioethanol) may yield no net energy –Land conversion Biodiversity loss Possible decrease in agricultural food productivity –Usual problems associated with intensive agriculture Nutrient pollution Soil depletion Soil erosion Other water pollution problems

Geothermal Energy How it works –Geothermal power plants Use earths heat to power steam turbines –Geothermal direct use Use hot springs (etc) as heat source –Geothermal heat pumps Advantages –Renewable –Easy to exploit in some cases –CO 2 production less than with fossil fuels –High net energy yield Disadvantages –Not available everywhere –H 2 S pollution –Produces some water pollution (somewhat similar to mining)

Radiant Solar Energy How it works –Solar power plants Steam produced to turn turbine –Solar heating Active and passive systems –Photovoltaic cells Solar batteries use special semiconductors Advantages –Renewable and free –High energy yield –A very clean source of energy No air/water pollution during operation –Low operating costs Will pay for themselves over time Disadvantages –Intermittent source Energy storage issues –Low energy density Requires pretty much land

The Hydrogen Economy Lecture Questions –What is the hydrogen economy? –Explain how the hydrogen economy could potentially serve as the basis for a renewable energy system that emits little or no air pollution Definition –The Hydrogen Economy is a hypothetical large-scale system in which elemental hydrogen (H 2 ) is the primary form of energy storage Fuel cells would be the primary method of conversion of hydrogen to electrical energy. –Efficient and clean; scalable In particular, hydrogen (usually) plays a central role in transportation. Potential Advantages –Clean, renewable –Potentially more reliable (using distributed generation) BUT many roadblocks including potential showstoppers –Poses great technological challenges for efficient hydrogen production, storage, and transport

Components of the Hydrogen Economy Infrastructure needs –Production –Storage –Delivery –End use

Hydrogen as a Transportation Fuel

Hydrogen Production Fossil Fuels –Steam Reforming of Natural Gas Combination of methane and steam produces hydrogen gas –Carbon monoxide is also produced –The water gas shift reaction can produce further hydrogen from the carbon monoxide. Carbon dioxide is produced too. Most economical; main current method –Carbon sequestration one method to reduce CO 2 emission –Partial Oxidation (POX) of Hydrocarbons HC partially oxidized to produce hydrogen and carbon monoxide –Coal Gasification Gasified at high temps, then processed Can also be used to get hydrogen from biomass

Hydrogen Production Electrolysis –Efficiencies 70-85% –Produces highest purity of hydrogen –Currently, the electricity consumed is usually worth more than the hydrogen produced Experimental methods –Biological hydrogen production –Direct photolysis –Thermolysis

Renewable Solar Paths to Hydrogen

Hydrogen Storage Large-Scale Stationary Storage –Underground in depleted oil/gas fields, aquifers, caverns Intermediate- and Small-Scale Stationary/Mobile Storage –The focus of most current research –As a liquid Advantage: higher energy density, cheaper transport Disadvantage: economic/energy cost of liquefaction is significant –As a compressed gas Probably best short-term method, particularly with advanced materials to decrease weight Advantages –Rapid charging/discharging –Lower costs than liquid storage Disadvantages: –Low energy density, Probably still acceptable for motor vehicles –Safety (esp public perception) –Metal hydrides Hydrogen is absorbed under pressure, released when heated Less filling pressure needed Low energy density, long recharge time, expensive Experimental Methods –Improved hydrides; carbon nanotubes; many other materials (eg conversion to ammonia)

Chemical Batteries Lecture Questions –What are batteries and how do they work? –Whats the difference between a regular and a rechargeable battery? Why are rechargeable batteries sometimes referred to as storage batteries? –Chemical batteries are based on reduction-oxidation (redox) reactions, which are reactions where electron transfer occurs. The oxidation half-reaction occurs at the anode Electrons flow from the anode, through an external circuit, and to the cathode, where the reduction half-reaction occurs –Chemical batteries are a very efficient method of generating electricity Efficiencies of 80% are regularly achieved Significantly (2-3 times) more efficient than heat engines –Rechargeable batteries can be hooked up to an external source of electricity to regenerate the redox reactants through electrolysis. Rechargeable batteries thus provide a means of storing electricity in chemical form. They are thus sometimes called storage batteries.

The Alkaline Battery

The Lead Storage Battery

Fuel Cells Lecture Questions –What is a fuel cell and how does it work? –What is distributed generation? –A fuel cell is basically a battery in which the reactants are continually supplied to the electrodes, and the products are continually removed. Much more efficient (2-3 times) than heat engines at generating electricity Most common type of fuel cells based on hydrogen (there are others) –Fuel cells are scaleable Large ones can power homes or neighborhoods Small ones can be used in appliances Distributed generation is a decentralized power system consisting of hydrogen generators and fuel cells

A Hydrogen Fuel Cell

Hydrogen Fuel Cells: Scalable

Polymer Electrolyte Fuel Cell

Alkaline Fuel Cell (AFC)