The future state of the electricity industry always differs from the scenarios analyzed in large-scale integration studies. Given this fact, studies should focus more on quantifying the relative effect of particular changes in operating policy or technologies than on seeking to precisely quantify the economic or reliability effect of a particular penetration scenario. For example, a conclusion that using fast-ramping storage will reduce ancillary service costs by 10% is likely to be more useful than one that says that the ancillary service costs will be $1.52/MWh for scenario X. This is particularly the case given the fact that new technology and new policies are likely to cause substantial changes in the way that power systems operate in the 10- to 20-year time horizons that are typical in integration studies.
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 Even though production from U. S. hydroelectric plants tripled from 1950 to 1973, demand for electricity grew nearly six-fold in the same period.
 Wind power production of electricity in the United States in 2013 was 18 times that of solar electric power production.
 The RenewElec project (www. RenewElec. org) was supported by the Doris Duke Charitable Foundation, the Richard King Mellon Foundation, the Heinz Endowments, the U. S. Department of Energy, the Electric Power Research Institute, and the U. S. National Science Foundation.
 Pennsylvania has an alternative energy portfolio standard (AEPS), which mandates that a percentage of electricity in the state come from qualified resources. Unlike a renewable portfolio standard, the AEPS allows for the use of resources like waste coal and low- pollution-emissions coal technology.
 Landfill gas, wind, biomass, hydroelectric, geothermal electric, municipal solid waste, hydrokinetic power (i. e., flowing water), anaerobic digestion, small hydroelectric, tidal energy, wave energy, ocean thermal power.
 For a history of the PTC, see www. eia. gov/forecasts/archive/aeo05/issues. html.
 CA, CO, CT, DC, KS, MD, ME, MI, MO, MT, NC, NJ, NM, NY, PA, RI, WA, WI.
 “Firm power” is capacity available 79 to 92% of the time (Katzenstein et al., 2010).
 In “energy arbitrage,” power is purchased when the price is low and sold when the price is high. For example, during the summer, the peak demand and highest price is during the day when air conditioning is heavily used and businesses are in operation. Prices drop in the evening when energy is used less.
 Cycling is when a generation unit is turned off or turned on in response to the need to respond to variability in the grid. Ramping is changing the output of a plant once it is running.
 Frequency regulation is a type of ancillary service that is responsible for the short-term balancing of electric power.
 Capacity factor is the actual energy (in units, for example, of kWh) produced by a generator in a particular time period (for example, a year) divided by the product of the nameplate capacity of that generator (in kW) times the number of hours in the period.
 The amplitude fluctuations are the square root of the power fluctuations, or about
 The amplitude fluctuations are the square root of the power fluctuations, or about a factor of 10 larger for PV than for solar thermal.
 This section is based on Katzenstein and Apt (2009a, 2009b).
 This section is based on Oates and Jaramillo (2013).
 MW (Paish, 2002).
 This section is based on Fertig and colleagues (2014).
 Details on the cost models for CAES and transmission can be found in Fertig and Apt (2011).
 60 70
Opportunity cost ($/MWh)
 MISO changed its name to the Midcontinent Independent Systems Operator, Inc. on April 26, 2013.