Energy and the EnvironmentSecond Edition

Chapter 5:Renewable Energy Sources II:

Alternatives

Copyright © 2006 by John Wiley & Sons, Inc.

Robert A. Ristinen • Jack J. Kraushaar

Introduction

• Aside from the direct use of solar energy, there are also the less direct technologies.

• These include hydroelectricity, wind energy, biomass, ocean thermal gradients, ocean currents, and ocean waves.

• We have also learned how to exploit ocean tides and geothermal gradients.

Hydropower

• Hydroelectricity dates back to the beginning of the beginnings of the electric power industry more than 100 years ago.

• About 7% of the electric power generated in the U.S. comes from our damming of rivers.

• In other countries, the use of water power is much greater (Norway - 99%, Nepal - 95%, Brazil - 93%).

• Water power is a consequence of natural cycle found on Earth.

Hydropower

• The available energy of water stored at a height above a generator is potential energy stored within the Earth’s gravitational field.

• As an element of water of mass, m falls a distance, h, it attains a velocity, v and a kinetic energy equal to 1/2mv2 equal to its original potential energy mgh.

• The acceleration of gravity, g, is 9.8 m/sec2. Thus the potential energy available for conversion to kinetic energy is 9.8 joules per kilogram of water.

• The distance the water falls, h, is referred to as head. Low-head hydroelectricity can be generated with h as small as 10 ft.

Wind Power

• The use of windmills dates back hundreds to thousands of years. Windmills by the tens of thousands were operating in Europe in the 19th century and several million were in operation in the U.S. by the early 1900s.

• The Earth’s winds are a direct consequence of solar energy. The rate of which solar energy is converted into wind energy in the U.S. is about 30 times the rate of our current energy consumption.

• The relationship of power to wind velocity is given by the equation:

P/m2 = 6.1 x 10-4 v3 where m2 indicates square meter of cross section oriented perpendicular to the

wind’s direction.

Wind Power

• Theoretical analysis of windmill designs shows that no more than 59% of the kinetic energy of the wind is recoverable as mechanical energy.

• Modern windmills range in efficiency between 50 – 70% of the theoretical maximum.

• When a windmill is used for generating electricity, one must consider two efficiencies: the efficiency for converting wind power to mechanical power, and the efficiency for converting mechanical power to electric power. The product of the two is the overall efficiency.

• Mechanical to electric efficiency is typically about 90%.

Ocean Thermal Energy Conversion

• Ocean surfaces are heated more dramatically than at depth and more at low latitudes.

• These warm surface waters move away from the equator toward the poles in the high latitudes within ocean circulation patterns called gyres.

Ocean Thermal Energy Conversion

• It is possible to tap the energy in the ocean currents for electrical power by using gigantic underwater turbines anchored to the ocean floor.

• There have also been proposals to extract energy from ocean currents through the use of heat engines that exploit the temperature differences between warmer water at the surface and colder temperatures at depth.

• Regions of interest in the U.S. include Florida, Puerto Rico, Hawaii and other islands of the Pacific.

• Two types of heat engines have been considered for ocean thermal energy conversion (OTEC): closed-cycle systems, and open-cycle systems.

Ocean Thermal Energy Conversion

• It is estimated that a minimum temperature difference of about 17°C is needed for a practical OTEC heat engine.

• With a temperature difference of 20°C, the Carnot efficiency is only about 7%. This small value means that huge amounts of ocean water are required.

• In the 1970s the U.S. Department of Energy financed the design of large floating OTEC power plants. No plants of the proposed scale were ever actually constructed.

Biomass as an Energy Feedback

• Solar energy stored in plants can be released through combustion either from the original biomass or after it has been converted to another form of fuel.

• When plants use solar energy to form vegetable matter, a chemical reaction takes place:

CO2 + 2 H2O + light → CH2O + H2O + O2

(simple carbohydrate)• The photosynthetic reaction leading to the creation of glucose:

6 CO2 + 6 H2O + light → C6H12O6 + 6 O2

• Only about 25% of sunlight incident on Earth has the right wavelength to produce photosynthesis, and 60 – 70% of that will be absorbed by plants.

• In practical terms, wood is the logical biomass to be considered as a fuel.

Biomass: Municipal Solid Waste

• From 1960 to 2001 the generation of municipal solid waste (MSW) grew from 88 million tons per year to 229 million tons per year.

• Much of this is due to population growth but the rate of solid waste production per person also increased from 2.7 lbs. per person per day to 4.4 lbs. per person per day.

• Majority of MSW is paper (36%), food waste (11%), plant waste (12%), wood (6%), glass (6%), plastic (11%), and metals (8%).

• The burning of MSWs can release an average of 4300 Btu/lb.

Biomass-derived Liquid and Gaseous Fuels• Ethanol from corn is a common form of

alcohol, C2H5OH.• In recent years there has been more of an effort to use ethanol as a fuel for transportation.

• The addition of ethanol to gasoline increases the oxygen content of the fuel, leading to more complete combustion and lower CO2 emissions.

• The blend of 10% ethanol to 90% gasoline is known as gasohol and receives a reduction of 5.2 cents per gallon on the current federal excise tax.

• In Brazil, sugar cane ethanol is more commonly used. In the 1980s, nearly 90% of all automobiles were designed to operate on pure ethanol.

• Biomass can also be converted to a gaseous form, methane CH4.

Geothermal Energy

• Geothermal is one of only two non-solar sources of renewable energy (tidal being the other).

• Geothermal makes use of the heat trapped inside of the Earth itself.

• Geologic regions such as volcanoes, geysers, fumaroles and hot springs are good candidate sites for making use of the internal heat of the Earth.

• The sources can be classified into 6 categories: hot water reservoirs, natural steam reservoirs, geopressured reservoirs, normal geothermal gradient, hot dry rock, and molten magma.

• Wairakei Geothermal Power Station

Tidal Energy

• Tidal energy is fundamentally gravitational.

• Vertical tidal range varies from 2 ft. in Florida to 18 ft. in Maine.

• To enhance the efficiency of capturing energy, damlike structures can be built with gates or channels to direct the flow through the turbine locations.

• For the U.S. the most attractive sites for development are in Cook Inlet and Bristol Bay in Alaska and in the Bay of Fundy region in the northeastern U.S.

Wave Energy

• Waves receive their energy from the wind which in turn derives their energy from the sun.

• The U.S. Department of Energy estimates a total power of 2 to 3 million MW in the waves breaking on the world’s coastlines, and an average energy density that can average 40 MW per kilometer of coastline in favorable locations.

• It is too soon to say what the future of wave energy holds, but as fossil fuels become more scarce and expensive, this may become a more viable option.