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2	In Other News . . . Renewable energy battles continue in Florida
3	18 Overview: Energy Storage Energy is stored to use it at a different time than when it was generated The process of converting the energy to storable form means that some energy is lost due to inefficiency Additional energy is lost when the energy is released or recovered due to a second inefficiency Ideally, storage is avoided to have a more efficient process Time-of-day metering is likely in the future as metering becomes electronic and inexpensive (like a thermostat) Shifting the energy from usage peaks to low-use times helps the utility, and customers would be rewarded by lower charges
4	18.0 About This Presentation 18.1 General 18.2 History 18.3 Flywheels 18.4 Ultracapacitors 18.5 Pumped Hydro 18.6 Compressed Gas Storage; H₂ 18.7 Superconductors 18.8 Ice Storage 18.9 Financial Storage 18.10 Renewable Energy Funding 18.11 Issues and Trends 18.0 Conclusion
5	18.1 Energy Storage Renewable energy is often intermittent (like wind and sun), and storage allows use at a convenient time Compressed air, flywheels, weight-shifting (pumped water storage) are developing technologies Batteries are traditional for small systems and electric vehicles; grid storage is a financial alternative Energy may be stored financially as credits in the electrical “grid” “Net metering” provides the same cost as sale dollars to the supplier; 37 states’ law; new law needed in Florida
6	18.2 Battery History Alessandro Volta made primary batteries of dissimilar metals (silver, zinc, and a salt water wet paper between them) about 1800 (try touching a dime and a nickel in contact to your tongue) They were “piled” up, and became known as a voltaic pile (from whence came the atomic pile) Johann Ritter developed a rechargeable (secondary) cell about 1802, but there was no generator to recharge them yet George Leclanche’ “wet” cells used carbon rods and zinc He made a wet paste that could be sealed into the cell, thus making a convenient portable energy source; no spilling In 1860, the secondary or rechargeable battery was further developed by Raymond Gaston Planté (lead sheets & acid) A lead paste on the plates provided more active surface area and allowed longer discharge life in 1881 (Faure) Germans made the gel-cell with a sealed case in 1960
7	18.2 Electrochemical Batteries Batteries (groups; from artillery guns) of cells are used separately or in a case containing several cells; a 12V car battery has six 2V cells inside the case Large batteries are often use separate 2V cells placed next to each other in a rectangle Various cell chemistries are used Lead-acid; Nickel-cadmium; Lithium Nickel-metal hydride Zinc-air Best suited to storage periods of 1 second to 60 days Self-discharge and sulphation occur with time Desulphator circuits can reduce sulfates for longer life
8	18.2 Flow Batteries Flow batteries use pumped electrolytes that move outside of the battery case Polysulfide Bromide (PSB), Vanadium Redox (VRB), Zinc Bromine (ZnBr), and Hydrogen Bromine (H-Br) batteries are examples A “filling station” could exchange spent electrolyte for new “charged” electrolyte The power and energy ratings are thus independent since the power is from the battery electrodes while the electrolyte may be replaced periodically
9	18.3 Flywheels Flywheels store energy as angular momentum Best suited to storage periods of 1 second to 10 minutes High temperature superconducting bearings reduce bearing friction to 2% of speed drop per day Ball bearings are so inexpensive that the performance gains of magnetic bearings are irrelevant The flywheel case is designed with a shield to contain a failed rotor and its pieces if it shatters and blows up Batteries are much cheaper than flywheel systems Test buses used flywheels that were spun up by electricity at bus stops; no wires along streets
10	18.3 Flywheels & Trains This trackside flywheel system provides stabilization of voltages on the track system by being both motor and generator Similar types are used to stabilize renewable energy outputs Buses have been operated that use flywheels charged by electricity at the bus stops, thus avoiding the cost of overhead trolley wires
11	18.4 Ultracapacitors Ultracapacitors are very high capacitance units Best suited to storage periods of 0.1 second to 10 seconds Stored energy is 0.5 C V² Capacitances now reach 2.7 kF (kilofarad) Carbon electrode surface areas 1000m² to 2000m² per gram provide high capacitance Electrolytes are sulfuric acid or potassium hydroxide
12	18.5 Hydro Pumped Storage Special turbines can run either to spin an alternator or to act as a pump This reversibility allows excess electrical energy to be used to pump water to a higher storage reservoir to be used as an energy source later Since 2.31 ft of elevation has a bottom pressure of one pound per square inch (psi), a head height of 200 ft is equivalent to 86 psi Japan built a 30MW seawater pumped hydro system at Yanbaru in 1999 Worldwide, pumped hydro is about 90GW, ~3% of total storage, the most widespread high-energy storage technique
13	18.5 San Luis, California
14	18.5.1 Hydro Examples
15	18.6 Compressed Air Pumped Storage "The world's first compressed air energy storage plant was in Germany," Lee Davis (plant manager for the Compressed Air Energy Storage (CAES) Power Plant in McIntosh, Alabama). "The Alabama CAES plant was the first in the United States when it opened in 1991.“ Electrical motors compress air to 1078 psi within underground salt caverns (100 MW); heat is lost in the cavern On release, natural gas is burned to heat the air again, which then passes through a turbine, spinning an alternator (326 MWe) The Norton Energy Company plans a similar site using an abandoned limestone mine 35 miles south of Cleveland, Ohio
16	18.6.1 Compressed Air Energy Storage
17	18.6.2 Compressed H2 and NG Storage Hydrogen is normally stored in 8-inch tubes and tanks H² pressures range from 2000 to 10,000 psi Nickel-metal hydride is a solid pellet or powder storage CNG or compressed natural gas is stored at 3000 psi
18	18.6.3 Liquid Air Energy Storage Mitsubishi Heavy Industries is developing LASE (Liquid Air Storage Energy) The system makes liquid air at nights and weekends for vaporization and electricity generation The turbine is based upon a rocket motor pump This load-shifting provides the economic incentive to use the system Could also be done with liquid nitrogen storage
19	18.7 Superconductors Since a superconductor has essentially zero resistance, a current once started will flow “forever” At a later time, energy could be extracted from the superconductor Since the superconductors must be kept far below usual air temperature (~20K to 80K), energy must be used to compress the gas and make it liquefy Newer superconductors are being investigated to find ones with a higher critical temperature near room temperature
20	18.7.1 Superconductor Example
21	18.8 Ice Thermal Energy Storage Air conditioning systems have a high afternoon load to offset the sun heating of the building and the higher outside temperature Freezing ice during the night provides a latent heat absorber at lower energy prices, assuming demand charges or time-of-use rates are imposed During the day, the ice is melted as the refrigerant is condensed as it passes through pipes in the ice The overall process thus provides air conditioning at a lower cost Bayside High School in Palm Bay FL uses this method
22	18.9 Financial Storage Storage of energy as a credit from the utility company can be the most efficient method No batteries are required with grid intertie, but might be used to provide backup power In net metering states, a single electrical energy meter is used Energy flow moves the meter higher for purchased energy and lower for energy sold from the local site The utility company can avoid meter-reading costs by reading the meter once a year Since the values are only in accounting books, there is no energy loss (likely used by the neighbors) However, ~16 states have yet to regulate the charges, and some utilities may pay $0.023/kWh but charge $0.07 or higher The nonnet-metering system should be designed to reduce the bill to nearly zero but never sell energy into the utility system
23	18.10 Renewable Energy Funding President Clinton served from 1992 through 2000 During 1992-1999, the Dept. of Energy Renewable Energy budget varied from $388M to $488M, reaching its low of $363M in 1997 The 1999 DOE RE budget shows these top areas: Electric Energy Systems $38M Geothermal $33M Hydrogen Research $24M Hydropower $4M Solar Energy was separated out at $112M to $87M in 1997 to $ 116M in 1999 The major budget item in 1999 was biofuels $89M, followed by PV at $79M Budget at 4/2007 at ~$307M vs. ~$200M
24	18.11 Issues and Trends Energy storage provides energy at a different time than when it was generated (time-shifting) Conventional storage systems such as batteries and pumped hydro continue to dominate due to cost Short-term storage or energy-smoothing devices like flywheels and ultracapacitors work well in the 10-second time range Unneeded generators are often kept in “spinning reserve”, motoring without load to act as generators if additional power is required (air and bearing losses) This also stores reactive power (v.a.r.s or vars) Energy storage will smooth peaks and valleys of availability, but load shifting by the users is more useful
25	18 Conclusion: Energy Storage Energy storage is to be avoided due to the losses, but may be economic when load time-shifting is possible Energy must be stored in vehicles since they cannot obtain sufficient power from wind or sun on the vehicle Special student SunRayce PV cars are fragile and light, and cannot be used in normal highway traffic without a significant death rate Protected by team cars travelling with them Newer technologies may increase energy storage density at a lower cost; both are needed for a viable product
26	Olin Engineering Complex 4.7 kW Solar PV Roof Array
27	References: Books Boyle, Godfrey. Renewable Energy, Second Edition. Oxford: Oxford University Press, 2004, ISBN 0-19-26178-4. (my preferred text) Brower, Michael. Cool Energy. Cambridge MA: The MIT Press, 1992. 0-262-02349-0, TJ807.9.U6B76, 333.79’4’0973. Duffie, John and William A. Beckman. Solar Engineering of Thermal Processes. NY: John Wiley & Sons, Inc., 920 pp., 1991 Gipe, Paul. Wind Energy for Home & Business. White River Junction, VT: Chelsea Green Pub. Co., 1993. 0-930031-64-4, TJ820.G57, 621.4’5 Patel, Mukund R. Wind and Solar Power Systems. Boca Raton: CRC Press, 1999, 351 pp. ISBN 0-8493-1605-7, TK1541.P38 1999, 621.31’2136 Sørensen, Bent. Renewable Energy, Second Edition. San Diego: Academic Press, 2000, 911 pp. ISBN 0-12-656152-4. Texter, [MIT]
28	References: Websites, etc. http://www.mhi.co.jp/tech/htm/8353t/e835305t.htm liquid air energy storage http://unisci.com/stories/20013/0802016.htm on compressed air storage http://www.aip.org/isns/reports/2001/025.html on compressed air storage http://www.sandia.gov/media/NewsRel/NR2001/norton.htm on compressed air storage http://www.eere.energy.gov/der/compressed_air.html http://www.hepi.com/basics/history.htm batteries http://www.et.anl.gov/sections/te/research/flywheel.html flywheels http://www.aspes.ch/faq.html http://www.netl.doe.gov/publications/proceedings/01/hybrids/Hybrid%20Workshop%20Group%203%20Breakout%20NREL.pdf http://www.netl.doe.gov/publications/proceedings/01/hybrids/ http://www.electricitystorage.org/sitemap.htm http://www.uptenergy.com/en/traction/casestudy2.htm on electric Chinese bus http://www.acfnewsource.org/science/energy_mine.html ______________________________________________________________________________ www.dieoff.org. Site devoted to the decline of energy and effects upon population www.ferc.gov/ Federal Energy Regulatory Commission www.google.com/search?q=%22renewable+energy+course%22 solstice.crest.org/ dataweb.usbr.gov/html/powerplant_selection.html