Novel Strategies to Reduce Variation of Wind and Solar Generation at its Source

Stephen RoseEric 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 Array1 Day

4

Curtailing a wind farm creates a reserve of power

Pow

er [k

W]

Time [sec]

Possible Power

Actual Power

Delta Power

Delta Control + Ramp Up & Down Limits

Power lost to Ramp Up Limit

Power gained by Ramp Down Limit

Ramp Up Limit

Ramp Down Limit

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

Charge/ Maintain Energy

Curtailment

“Flat Power” Output (within deadband)

Wind Power vs. Time

Wind + Gas Power vs. Time

Wind + Gas + Battery Power vs. TimeSodium Sulfur (NaS)

Battery

A Co-located wind/natural gas turbine/energy storage system can deliver “Baseload” power

7

8

The scale of energy storage in these systems is reasonable

Wind Nameplate Capacity (MW)* 0 25 43 67 Delivered Wind Energy (% of total) 0% 12% 19% 30%

Average Cost of Electricity ($/MWh) 62 64 65 67

Contribution of NaS Battery to Average Cost of Electricity (percent) 0% 0% 0.5% 1%

NaS Battery Capacity (MWh) 0 0 10 21

* 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