Novel Strategies to Reduce Variation of Wind and Solar Generation at its Source Stephen Rose Eric Hittinger

Variability has always been an issue for electrical grids Traditional Problem: Dispatchable generation and variable load Generators are scheduled to meet expected demand Ancillary Services deal with the sorter-term variability (regulation) and backup (reserves) Energy storage is exceedingly expensive (except hydro) and generally neither required nor utilized 2

Changes in the structure of the grid challenge the traditional model Generation will cause an increasing fraction of the variability on the grid Transmission constraints are getting tighter due to greater power demands and very little new transmission Energy storage costs are decreasing, making them more attractive Bottom Line: The varieties and sources of variability are changing, thus the traditional model of variability control may need to change as well 3

Wind and solar power are variable and non-dispatachable Source: Jay Apt, CMU 12 MW Wind Farm 15 Days 4.5 MW PV Solar Array 1 Day 4

Curtailing a wind farm creates a reserve of power 5

Reserve power from curtailment costs 20 – 200% more than regular power 2 MW of reserve costs 150% more per MW 10 MW of reserves cost 30% more per MW 6

100 MW Natural Gas Turbine Charge/ Maintain Energy Curtailment “Flat Power” Output (within deadband) Wind Power vs. Time Wind + Gas Power vs. Time Wind + Gas + Battery Power vs. Time Wind Generation Sodium Sulfur (NaS) Battery A Co-located wind/natural gas turbine/energy storage system can deliver “Baseload” power 7

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The scale of energy storage in these systems is reasonable Wind Nameplate Capacity (MW)* Delivered Wind Energy (% of total) 0%12%19%30% Average Cost of Electricity ($/MWh) Contribution of NaS Battery to Average Cost of Electricity (percent) 0% 0.5%1% NaS Battery Capacity (MWh) * System Target Power Output is 100 MW 9

Market and regulation issues are critical for co-located smoothing Current standard for renewable energy is to allow unlimited variability Some grids with higher fractions of variable generation (Denmark, Texas) are starting to limit allowable variability Without market incentives or system protocols to discourage variability, wind and solar generators have no motivation to reduce variability 10

As variable generation increases, market rules will need to change Variable generation will likely have to internalize variability, but the exact implementation is important: Ramp-rate limitation Increased subsidy for reduced variability “Variability tax” Variability reduction market 11

Future Research Questions What time scales for variability are most important/expensive? Are wind/solar integration costs evenly distributed in time/space? Where and when is co-located smoothing a economical? How will different market mechanisms affect the deployment and operation of variable generation? 12