EPSc 116: Resources of the Earth Lecture 15 on Ch. 6: Renewable Energy Focal Points What are the sources of renewable energy? How do we extract the energy from them and convert it to our use? What are the specific advantages of using each type of renewable energy? What are the specific challenges to upscaling the production of this energy, i.e., for use of it and reliance on it at a national scale? Efficiency of energy conversion, i.e., % made available Cost of energy production Intermittence of energy production History of (lack of) success
Sources of Renewable Energy Wind Power Solar Photovoltaic Solar Thermal Geothermal energy Ocean Energy (wave, thermal, tidal) Hydroelectric Power Biomass (burning, anaerobic digestion to "fuel") Nuclear fusion
Energy flow to Earth s Surface SOURCES 1) Solar (SW) 2) Tidal 3) Heat flow from below Text, Fig. 6.1
Solar Energy Solar Thermal Low-quality solar, e.g., to heat water passively to less than 100 C High-quality solar, i.e., concentrate sun s rays to produce high temperatures About 25% efficiency of conversion Text, Fig. 6.23 Solar Photovoltaic P-N junction solar cells are moving toward 33% efficiency; routinely 10-25% Some novel cells 44% Polymer-based cells may reach 10% Text, Fig. 6.22 Light interacts with a semiconductor to cause electrons to flow within it, thereby producing electricity Use of photovoltaic cells = solar cells; array
Satellite Solar Power Station Schematic only Large arrays of photovoltaic cells orbiting the earth capture the sun s energy, convert it to electrical energy, and beam it back to the earth via a microwave beam. Text, Fig. 6.24
Wind Energy Wind = movement of air caused by (unequal) heating by the sun Windmills (individual or in wind farms) convert movement of air to operation of a turbine, which produces electricity. Various designs of windmills to account for absolute wind speed (high vs. low), (variable) direction of wind, intermittence of wind. Theoretical limit to conversion efficiency is 59%. Usually < 50%. Images from http://www.darvill.clara.net/altenerg/wind.htm
Hydroelectric Power Energy of sun and gravity Electricity generated from the energy captured from the flow of water. Usually produced by large dams (capture water in a reservoir) Mechanical energy directly drives turbines 80-90% efficiency of energy conversion; only a 2-step process Compare to 35-40% for coal-fired and 30% for nuclear power plants Pumped water storage also possible, as at Taum Sauk in Missouri Text, Fig. 6.28 Principal countries producing electricity from hydroelectric dams Text, Fig. 6.26
Wave Power Wind, which blows due to solar heating, causes waves to form Waves vs. wind of same velocity: Waves have 800x more energy, because of the much greater density of water (a lot more mass is moving in water) Tremendous amounts of wave energy are used up along coasts erosion Multiple designs (not very successful) to capture wave energy: Flexible air bags mounted along spine of reinforced concrete Floating rafts or tubes that transmit mechanical energy Problems of corrosion and storms
Ocean Power Usually refers to Ocean Thermal Energy Conversion (OTEC) Sun warms surface of the water (by energy absorption), which leads to a gradient (difference) in temperature between surface and depth Creates basis for a heat engine, using the (only) 20 C difference in temp. Only about 2-3% efficiency of energy conversion, BUT there is a huge amount of total energy. (Small %)x(huge #) = BIG # Text Fig. 6.32a Typical temperature variation with depth in equatorial ocean Heat engine via thermal gradient Text Fig. 6.32b
Tidal Energy Not derived from sun, but rather from gravitational attraction of Moon and Earth Cycle of the tides causes fluctuation in height of water level Rise and fall can drive a water-powered turbine to generate electricity Best sites are where tidal fluctuation > 10 m (30 ft) Bay of Fundy Nova Scotia Bristol Channel, UK Patagonian coast of Argentina Murmansk coast, Barents Sea Turbine operates during both ebb and inflow of tide Rance Estuary, French coast
Geothermal Energy Comes from below (geothermal gradient) due to original heat plus heat from decay of naturally radioactive elements Geothermal gradient is 15-75 C/kilometer (i.e., 25-120 F/mile) Vast amount of heat available (but dispersed) at depth Usually select an area of very high heat flow near surface: volcanoes, spas, geysers, hot springs, hot dry rocks. Not truly renewable. Can directly tap either steam or superheated water that can be allowed to flash into steam as it rises and depressurizes. Also can pump cold water down into hot dry rock ; let water rise to surface and drive turbines to generate electricity. Wikipedia
Generating Electricity from Geothermal Energy Text Fig. 6.38 Iceland** Italy New Zealand Text Fig. 6.37a Steam from geothermal well drives turbines to produce electricity. Only ~1% efficiency in recovery of energy Pump cold water down into hot, dry rocks. Retrieve it.
Another Face to Geothermal Energy The Direct Application of Geothermal Energy From: http://climatelab.org/geothermal_energy
Energy from Biofuels and Waste Products Much of the developing world relies on readily available combustibles: renewable and widespread wood, grain stalks, dried animal dung Known as biofuels because they come from recently living organisms Can lead to deforestation erosion, loss of fertile soil, water depletion Closed carbon cycle : grow, burn, release CO 2, absorb CO 2 in new growth Fermentation process called anaerobic digestion (special bacteria) can convert discarded biomass into methane. C.f. coal gasification Pyrolysis (controlled heating) produces liquid fuels from biomass: gasohol Not just from waste products: Brazil s remarkable production of biofuels from sugar (cane, beet), cassava Single-celled algae grown in ponds, harvested, fermented -- methane
Hydrogen for Energy Hydrogen is extremely abundant on earth, but usually bonded with other elements, as in H 2 O (water) and CH 4 (methane = natural gas) Hydrogen burns easily by combining with oxygen and forming water H 2 is abundant, burns to produce much heat, only water as combustion waste Challenges: How to produce large amounts of pure H 2 How to transport and handle H 2 (explosive) Pass electric current thru H 2 O Keep H 2 under pressure, as a cool liquid Hydrogen-powered cars (?): fuel cells Schematic of a fuel cell
Photocatalytic Conversion: CO 2 to Hydrocarbons Roy, S., et al. 2010. Toward solar fuels: Photocatalytic conversion of carbon dioxide to hydrocarbons: ACS Nano, v. 4, 1259-1278.
Nuclear Fusion: Ultimate Energy Source? It s good enough for the sun! Nuclear fusion, in which nuclei of lighter elements combine to form larger nuclei, i.e., heavier elements, with the release of huge amounts of energy Fusion of 2 deuterium ( heavy hydrogen ) atoms produces helium + ENERGY 2 H + 2 H 3 He + n + energy (3.2 mev) 1 1 2 Problems: Lots! Have fused deuterium + tritium to form helium, but explosive & uncontrolled Thermonuclear bomb, AKA a hydrogen bomb Must reach 100 million C to form a plasma. How do you contain this?! Doughnut-shaped magnetic field; inertial confinement (compression) Pay-off: HUGE amount of energy without any radioactive waste
From: Penn State College of Earth and Mineral Sciences Accessed in 3/2014; date of data, unknown
Efficiency of Energy Conversion Accessed in 3/2014; date of data, unknown http://www.mpoweruk.com/energy_efficiency.htm
Some Final Thoughts We want sources of energy that are reliable, available, affordable, & clean. There is no reason to believe that everyone, everywhere should or will use the exact same energy source or even tap similar energy sources in the same way. Particular challenges to green renewable energy sources: How much space will they require to tap necessary amounts of energy? How can we deal with the problems of intermittent access to the energy? How can we balance the use of huge amounts of land for energy retrieval against 1) other needs for that land and 2) the alternative of nuclear energy? Reasons for making these big choices: Desire to retain the environmental integrity of the earth Determination to be more self-reliant rather than dependent on others Depletion of our historical fossil fuels, especially petroleum
Scientific American, Oct. 2013, p. 100