ENSP101Chapter 20 – Sustainable Energy20.1 Conservation- Much of the energy we consume is wasted- Our ways of using energy are so inefficient that most potential energy in fuel is lost as waste heat, becoming a form of environmental pollution  Of the energy we do extract from primary resources, however, much is used for frankly trivial purposes- Conservation techniques are simple and cost effective:o Compact fluorescent bulbs produce 4x as much light as an incandescent bulbof the same wattage, and last up to 10x as longo Light-emitting diodes (LEDs) also are even more efficient, consuming 90% less energy and lasting hundreds of times as long as ordinary light bulbso New national standards  All new washing machines have to use 35% less water than older models; air conditioners also are required to be about 20% more efficient than previous modelso One of the most direct and immediate ways to conserve = turn off applianceso Industrial energy savings = more efficient electric motors and pumps, new sensors and control devices, advanced heat-recovery systems- Energy efficiency - a measure of energy produced compared to energy consumed- Green buildings can cut energy costs by ½- Automobiles and light trucks account for 40% of the US oil consumption and produce 1/5 of its carbon dioxide emissionso Environmentally responsible: Walking, biking, “smart cars”o Hybrid gasoline electric engines offer the best fuel economy and lowest emissions of any currently available vehicleso Plug-in hybrids – vehicles with hybrid gasoline-electric engines adapted with a larger battery array (enough to propel the vehicle for 50 km or so on the batteries alone) and a plug-in to recharge the batteries from a standard electric outlet)o Hydrogen fuel-cell-powered vehicles  Using hydrogen gas for fuel, these vehicles would produce water as their only waste product- Cogeneration – the simultaneous production of both electricity and steam or hot water in the same planto By producing 2 kinds of useful energy in the same facility, the net energy yield from the primary fuel is increased from 30-35% to 80-90%20.2 Tapping Solar Energy - If we could devise cost-effective ways to use this vast power source, we would neveragain have to burn fossil fuels- Passive heat absorption – using natural materials or absorptive structures with no moving parts to simply gather and hold heato Olden days  Built thick-walled stone and adobe dwellingso Modern adaptation  Glass-walled “sun-space” or greenhouse on the south side of a building- Active solar system – a mechanical system that actively collects, concentrates, and stores solar energyo Active collectors can be located adjacent to or on top of buildings rather thanbeing built into the structure- How can solar energy be stored for times when it’s needed?o In a climate where sunless days are rare and seasonal variations are minimal, a small insulated water tank is a good solar energy storage system  For areas where clouds block the sun for days at a time or where energy must be stored for winter use, a large, insulated bin containing a heat-storing mass, such as stone, water, or clay, provides good solar energy storage20.3 High-temperature solar energy - There are 2 ways to use mirrors to collect solar energy to generate high temperature:o One technique uses long curved mirrors focused on a central tube, containing a heat-absorbing fluid. Fluid flowing through the tubes reaches much higher temperatures than possible in a basic flat panel collectoro Another high-temperature system uses thousands of smaller mirrors arranged in concentric rings around a tall central tower. The mirrors, driven by electric motors, track the sun and focus its light on a heat absorber at the top of the “power tower” where molten salt is heated to temperatures as high as 500C which then drives a steam-turbine electric generator- Simple solar cookers can save energy- Energy policies in some states include measures to encourage conservation and alternative energy sources. Among these are:o (1) “Distributional surcharges” in which a small per kWh charge is levied on all utility customers to help renewable energy finance research and developmento (2) “Renewable portfolio” standards to require power suppliers to obtain a minimum percentage of their energy from sustainable sourceso (3) Green pricing that allows utilities to profit from conservation programs and charge premium prices fro energy from renewable sourceso E.g. Revolving Loan Fund- Some nonutility companies are investing in sustainable energyo E.g. BP- Photovoltaic cells capture solar energy and convert it directly to electrical current by separating electrons from their parent atoms and accelerating them across a one-way electrostatic barrier formed by the junction between 2 different types of semiconductor material- During the last 25 years the efficiency of energy capture by photovoltaic cells has increased from < 1% of incident light to > 15% under field conditions and > 75% in the lab- Photovoltaic prices are now dropping about 7% per year- Amorphous silicon collectors – photovoltaic cells that collect solar energy and convert it to electricity using non-crystalline (randomly arranged) thin films of silicono Silicon collectors already are providing power to places where conventional power is unavailable, such as lighthouses, mountaintop microwave repeater stations, villages on remote islands, and ranches in the Australian outback- Storage is a problem for photovoltaic generation as well as other sources of electric power- An exciting potential of plug-in hybrids is that they could serve as an enormous, distributed battery array. You’d recharge your battery at night when power plants have excess generation capacityo Another strategy is to store energy in a form that can be turned back into electricity when needed. Pumped-hydro storage involves pumping water to an elevated reservoir at times when excess electricity is availableo Or we could use surplus electricity for electrolytic decomposition of water toH2 and O220.4 Fuel Cells- Fuel cells are devices that use ongoing electrochemical reactions to produce an electric currento They are very similar to batteries except that rather than recharging them with an electrical current, you add more fuel for the chemical reactiono Te fuel cell provides direct-current electricity as long as it is supplied with hydrogen and oxygen For most