Monday, May 21, 2012

Load: A Rough Cut Sprinkled with Secret Sauce

If you're a utility manager, you want the load to be as consistent and as close and safely possible to capacity throughout the day and the year. The most picturesque example of this phenomenon comes from the early days of electricty. Back then, electric trolleys fromed the dominant load, so companies built amusement parks at the ends of their lines to suck up nighttime load and also generate some extra traffic and revenue. This "load balancing" brought us Coney Island. By 1901 more than half of trolley companies operated at least one park with rollercoasters or underground tours or other rides.
-Alexis Madrigal, Powering the Dream
Load in 60 seconds
  •  Load profiles are shaped like a wave with a peak during the day, an off-peak period at night (AKA: trough) and shoulder periods connecting peaks and valleys
  • Weekends and holidays generally have lower peaks than adjacent weekdays due to less commercial and industrial activity
  • Hot climates peak in the summer. The hottest days have the highest peaks due to A/C load.
  • Cold climates peak in the winter. The winter peak is driven by heating loads.
  • Load drives the price of electricity up
  • Supply drives the price of electricity down.
Supply vs. Load vs. Price

The graphic to the right is a supply side depiction of several solar penetration scenarios for California. For my purposes nuclear, coal, wind and geothermal power plants can be disregarded because they remain unchanged in all scenarios. The generators to take note of here are the load followed resources. These being imports, combined cycles turbines, hydro, pumped storage and gas turbines.

In the Base Case all the load following plants follow the same basic pattern - they ramp up in the morning, peak during the day and ramp down in the evening. If you look carefully you can see that as the solar penetration increases the peaks progressively flatten and eventually divide into two peaks - a minor morning peak and a major evening peak. We can also see that the load following plants are all shifted from the middle of the load curve to the right of the curve as the solar penetration increases. For reasons unknown this graphic assumes a fixed load curve for each penetration scenario. This flawed assumption limits the usefulness of this graphic.

An improved graphic would incorporate the fact that solar power drives the price of wholesale electricity down. Electricity isn't known to be all that responsive to prices but there are price responsive customers - these customers would undoubtedly shift their loads around to capture cheaper electricity if they could. An improved graphic would show the load curves reaching incrementally higher peaks and lower troughs with increasing solar penetration.

I'm not going to say I've got it all figured out. Things get really unpredictable really fast. Here's a sketch of some possibilities.

Now: Natural gas peaker plants (AKA: Combustion Turbines or CTs) are currently used to meet peak load.
Next: Photoelectric plants easily cover peak load. CTs go the way of the dinosaur.
Now: Prices peak during the day and bottom out at night.
Next: Prices will flatten during the day and drift up marginally at night. The price peaks will occur at the load shoulders.
Now: Pumped hydro reservoirs fill in the middle of the night and run at the peak.
Next: Reservoirs will fill up at night and run in the morning shoulder then fill up in the middle of the day to run during the evening shoulder.
Now: Base load is approximately half of Peak load
Next: Base loads are going to shrink and Peak loads are going to rise.
Now: Demand Side Management (DSM) programs offer cheap rates to customers who move loads from peak load hours to off-peak hours.
Next: We should see DSM programs adjusted so that the cheapest rates occur during the day. This will move flexible loads (irrigation pumps for example) out of the off-peak load hours and into the peak load hours.
Now: EV buffs imagine these cars charging at night.
Next: EVs charging strategies will be designed to avoid the shoulders - day or night should be fine. You'll aim for the weekends if you can.
Now: People think photoelectrics need storage
Next: People recognize that all the displaced natural gas, hydro and imports will be available to provide back up. People also recognize that photoelectrics mathed to load behaves like a firm product

Germany is already seeing some of this stuff happen. For example, they are seeing lower daytime wholesale prices and higher prices at night. They are seeing double cycles of their pumped hydro plants and they are also seeing the disappearance of CTs. Consider that this is all happening at relatively low penetrations of solar (approximately 4 to 5%) and despite the unexpected decommisioning of six nuclear reactors following the Fukushima Daiichi nuclear disaster. Had those plants been up and running this past year we would have seen even more pronounced daytime price depressions due to excess power supply and even more peaker plants decommisioned due to poor economics.

SEE: Power too cheap for new gas turbines in Germany

To be continued...


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