Integrating renewables may call for some combined cycles to start twice daily, increasing emissions

The renewables challenge. Steve Royall, director of fossil, solar, and small-hydro generation for PG&E, and a member of the CCUG Steering Committee, spoke about the demands imposed on conventional generation assets by California’s requirement that one-third of the state’s kilowatt-hours come from renewables in 2020. The load profile projected by the California Independent System Operator (CalISO) for January 2020 (chart) illustrates the challenge well. Note the (1) rapid ramp in conventional generation required in the early morning (8000-MW increase within two hours); (2) the 6300-MW mid-morning decrease as demand levels off and solar generation increases rapidly, and (3) the 13,500-MW ramp in conventional generation required within two hours as the work day comes to an end.

The chart suggests that at least some combined-cycle plants might be forced to start twice daily to satisfy California’s requirements. Wear and tear on equipment is only one concern. One attendee asked: “What about the increase in total annual emissions because of the greater number of starts? Will permits be relaxed?” The reply: Still too early to have all the answers. Group think suggested that “operational flexibility” might be an ancillary service in the future because generators must be compensated for withdrawals from their metal-fatigue bank accounts. More peakers is not necessarily a viable strategy, the 70 attendees concluded. The efficiency of a simple-cycle engine is lower than that of a combined cycle, which contributes to higher emissions.

Royall also shared some details on PG&E’s aggressive solar PV program, put in place to comply with the state’s renewables statute. In both 2011 and 2012, the company commissioned three facilities totaling 50 MW; three more solar plants with a combined capability of 50 MW are scheduled for service by the middle of next year. PG&E plans to install a total of 250 MW of photovoltaic capacity before 2016 and contract for another 250 MW as well. The owned PV infrastructure is expected to cost about $1.5 billion. The price the utility receives its solar power is capped at $246/MWh. The solar generation it purchases competitively may not exceed $246/MWh.

Some facts of interest include the following:

• The stationary PV panels installed by PG&E have a guaranteed life of at least 25 years, and an average annual degradation rate of less than 1%.

• PV panels generate electricity best under bright sunshine when the ambient temperature is cool. The spring and autumn months are best for solar generation.

• Solar PV sites require from six to 10 acres per megawatt.

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