Standing the Test of Time Variations

Abstract

In his News Focus story “Renewables test IQ of the grid” (10 April, p. [172][1]), D. Charles describes the widely held belief that large-scale use of renewables will become viable with a smart electric grid and short-term energy storage devices (such as pumped storage and batteries). However, the problems of integrating renewables with the electricity grid in a low-carbon world are much larger. Renewables are a time-varying source of energy that does not automatically match the demand-side time variations. Electricity demand varies daily, over a 3-day weather-related cycle, weekly, and seasonally. The seasonal variation in electric demand in much of the country is greater than a factor of 2, with seasonal mismatches between renewable production and demand. In a low-carbon world, the major energy options (nuclear, fossil fuels with carbon-dioxide sequestration, and renewables) have high capital costs and low operating costs. These high–capital-cost systems for electricity generation operate most economically when they are at full load all the time. For renewables, this is the maximum output given solar or wind conditions. To optimize their operation and to meet the storage needs of renewables requires systems that can store large amounts of energy and can do so on a daily to seasonal basis. The only existing such technologies are large hydroelectric dams—but there is not enough water storage capacity to meet the need. There is limited ongoing work on several new technologies. For example, in nuclear-hydrogen peak electricity systems, the nuclear reactor operates at steady state on a continuous basis ([ 1 ][2]). At times of low electricity demand, steam and electricity from the reactor are converted to hydrogen and oxygen by a solid-oxide high-temperature electrolyzer. At times of peak electricity demand, the reactor sends electricity to the grid and the electrolyzer is operated in reverse on hydrogen and oxygen from storage as a high-temperature fuel cell to produce electricity. Large-scale seasonal hydrogen storage is economical with use of current underground natural gas storage technologies. Eliminating greenhouse gas emissions from electricity production and large-scale use of renewables is not possible until we develop multiple, seasonal, energy storage systems with fast response capability to address mismatches between electricity production and demand. ![Figure][3] Hydroelectric dam. Dams such as this one can store large amounts of energy on a flexible schedule, but they have insufficient water storage capacity globally to be our sole renewable energy source. CREDIT: JUPITERIMAGES 1. [↵][4] 1. C. W. Forsberg, 2. M. S. Kazimi , 4th OECD-NEA Information Exchange Meeting on the Nuclear Production of Hydrogen, 14 to 16 April 2009, Oakbrook, IL; . [1]: /lookup/doi/10.1126/science.324.5924.172 [2]: #ref-1 [3]: pending:yes [4]: #xref-ref-1-1 View reference 1 in text