Renewable Energy Choices - a basic lowdown
by: Paul Kando
As we have observed earlier, the economy is a collection of institutions and processes that, in quest of an enjoyable life for all, turn low-entropy energy and matter into useful objects, which eventually become valueless, high entropy waste. Productivity is ultimately limited by the net energy left after the energy required to obtain it has been expended - the ratio of energy expended to energy yield or Energy Return on Energy Invested (EROEI). A sustainable economy demands careful energy choices. Harmful effects, water and rare metals use are all critical issues. And, regardless of energy source, maximum energy efficiency must come first. Energy not used will always cost less in terms of both environmental impact and money.
photo credit: SF State University
Global energy demand, currently 14 terawatts (TW), will probably double by 2050. (see table of binary prefixes). The EROEI of oil -- 35:1 (1990), 18:1 (2005), 12:1 (2007), 5:1 (shale oil), 3:1 (tar sand oil) -- compares less and less favorably with renewable alternatives that convert natural energy flows directly into electricity, avoiding the environmental harm and inherently inefficient thermal energy conversion of nuclear and fossil-fueled power generation. We can harness the Sun's electromagnetic radiation as heat or electricity. Passive solar and water heating are familiar applications. Concentrating solar power (CSP), with an estimated global potential of 249 TW, requires 4 to 8.4 acres of land per megawatt (MW). Its water needs are technology dependent. Desert ecosystem interference is possible.
Photovoltaics (PV) could meet 40% of 2030 world energy demand using 0.29% of the globe's land area. Water needs are minimal. For crystalline cells, silicon and silver are limiting materials; as are tellurium, indium, and germanium for thin film cells. PV's cadmium emissions are minimal compared with those associated with fossil fuels. PV has a global potential of 340 TW. The EROEI ranges between 10:1 and 30:1.
Wind, with an estimated global potential of 40 to 85 TW, and over 238 GW installed capacity worldwide, is the most successful renewable electricity source after hydroelectricity. Wind could provide half the projected 2030 world energy demand on 1.17% of global land area. Water needs are minimal. A limiting material need is neodymium for permanent magnet generators. Average EROEI: 18:1.
The EROEI of hydropower ranges between 11:1 and 267:1, depending on size, location and technology. Estimated global potential is 1.6 TW. Land and water needs are significant. A limiting material is neodymium. Diversion, pollution, evaporation, river ecosystem damage, flooded lands and methane emissions are potential negatives.
The estimated global potential of wave power is 500 gigawatts (GW); of tidal 20 GW. Water needs are minimal but interference with shipping lanes, archaeological sites, pipeline infrastructure, and nature conservation could be problems. Neodymium is a limiting material requirement. Sedimentation, biodiversity loss, interference with migratory bird, fish and mammal populations may occur. However, artificial reefs are a net positive. The EROEI is 15:1 for wave power and 6:1 for tidal.
Biomass has an estimated global potential of 3.7 TW. Land and water needs are significant, but there are no limiting material requirements. Deforestation, biodiversity loss, pesticide and fertilizer use, and land degradation are environmental negatives. Average EROEI values: 9:1 for Brazilian sugarcane ethanol, 3:1 for soy biodiesel, 9:1 for palm oil biodiesel and net negative for U.S. corn ethanol.
Geothermal electricity has an estimated global potential of 2.6 TW. Land requirement: 1 to 8 acres per MW. Dry steam plants use geothermal steam to turn turbines. Flash steam plants pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines. In binary cycle power plants, which work with fluid temperatures as low as 135�F, geothermal water heats a secondary fluid with a much lower boiling point, causing it to flash-vaporize. This vapor drives the turbines. Enhanced Geothermal Systems (EGS) don't require natural hydrothermal resources. Water travels through fractures in dry hot rock, capturing heat, which is retrieved through a second borehole as very hot water. The heat is converted to electricity either by a steam turbine or a binary power plant. Unlike in hydraulic fracturing used in natural gas recovery, the water is reused repeatedly in a closed loop operation.
Enhanced Geothermal Systems (EGS) don't require natural hydrothermal resources. Water travels through fractures in dry hot rock, capturing heat, which is retrieved through a second borehole as very hot water. The heat is converted to electricity either by a steam turbine or a binary power plant. Unlike in hydraulic fracturing used in natural gas recovery, the water is reused repeatedly in a closed loop operation.
Like fossil fuel plants producing power 24 hours a day, hydrothermal and EGS technologies can provide base-load electricity. EGS is feasible anywhere in the world, depending on the economic limits of drilling depth. Water needs vary from 0.2 liters per kWh for binary systems to 1.2 to 2.73 liters per kWh for enhanced geothermal systems (EGS). Flash systems require little water, but are subject to 10.2 liters per kWh geofluid evaporation. There are no limiting material requirements, but environmental impacts may include deforestation, interference with sensitive ecosystems and seismic activity associated with EGS technologies. The EROEI for geothermal electricity ranges from 13:1 to 2:1; for geothermal heat pumps from 5:1 to 3:1.
Obstacles to wisely choosing alternative energy resources are not technical but political. Where public policies encourage local action in planning, source-selection, implementation and day to day energy management, renewable energy systems have already been proven more than competitive. Where changing to renewable alternatives is left up to the conventional energy industry, the opposite seems to be true. Distributed and decentralized, renewable energy fits well into a nature-harmonious economic model but not the traditional economic paradigm built around centralized power generation and control of the energy supply.