EME 810
Solar Resource Assessment and Economics

8.8 Time Horizons of Interest

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Reading Review

  • J. Kleissl (ed), 2013. Solar Energy Forecasting and Resource Assessment. Elsevier Science, Academic Press. Read pp. 171- 177 from Chapter 8: "Overview of Solar-Forecasting Methods and a Metric for Accuracy Evaluation", by Coimbra, Kleissl, and Marquez
As you are reading, consider the following questions. What time horizons are of critical interest for grid management and renewable energy forecasting? What economic flows and energy flows, and meteorological flows are we concerned with for our clients?

Electricity Markets and Grid Management

The first forecasts that we will explore from the reading are related to the modern electrical power grid. You are about to observe us jump from a +20 year time horizon in Lesson 7 (Life Cycle Cost Analysis), right down to spans of days and hours. If you recall from the reading by von Meier, this jump in orders of magnitude is pretty common for analysis related to financial and engineering decisions in energy systems.

Within the technological ecosystem of the grid, one follows the demand for electricity. Recall from Lesson 7 that the term for energy demand (including losses) is called the Load. Supply must be managed to match those dynamic demands. So, why might we forecast for loads in the electricity market? The time horizons of interest to engineers and financial experts working with the grid have been developed within the technical ecosystem of the modern power grid. When managing the grid as a dynamic system, we can think in time spans of seconds to minutes (Intrahour), over a few hours (Intraday), and over the course of a few days (Intraweek).

CAISO Example

Recall that California mainly has an Independent System Operator, with the exception of entities like SMUD (Sacramento Municipal Utility District). Also recall that an ISO uses markets (a zonal or nodal market) to manage the grid. In the last super graphic, which tied together weather scales and grid management scales, we observed "Hour Ahead" and "Day Ahead" markets along the top. But in our reading, we see that the time horizon is a bit different from the simple description:

  • Day Ahead Market: Operational Day begins at 12:00a in the morning.
    • Release load forecast at 5:30a in the morning of the day prior to the operational day. This means a +18.5h lead time from the beginning of the forecast. (5h:30 + 18h and 30 min = 24h)
    • The forecast ends at ~11:59p of the forecast day. The end of the forecast is a +42.5h lead time from the end of the forecast that was initiated at 5:30a the day before. (5h:30 + 42h and 30 min = 48h)
  • Hour Ahead Market: Operational Hour begins at the 0h.
    • Release the hourly load forecast 1h and 45 min prior to the operational hour.
    • Also release an advisory forecast with a +7h window from the beginning of the operational hour.
  • Other Planned Markets: CAISO is considering intrahour forecasts for 5 minute intervals. Midwest ISO already does this.
  • FERC: Notice of Proposed Rulemaking -- opportunity to schedule transmission every 15 min.

Time Horizons of Interest to Meteorologists

By reviewing the reading on Taylor's Hypothesis in Ch. 5 of the SECS text, we see that periods in time and distances can be related: a series of changes in time for a fixed place is due to the passage of an unchanging spatial pattern over that locale. So, in the following list, we can connect space and time (in fact, we do the same for power systems).

Taylor’s hypothesis permits a time series of irradiance observations over fixed locations to be converted into an equivalent translation across space (at the advective or propagation speed to the corresponding spatial pattern). Hence, all time scales are also spatial scales so long as the advective wind speed is much greater than the time scale of the evolving meteorological event being investigated, as is often the case.

-Brownson, SECS (2013)

Just keep this in mind with forecasting, as units of time actually also imply units of distance, and vice versa.

  • Synoptic-scale weather: large-scale meteorology (also termed cyclonic-scale), with common atmospheric phenomena spanning 1000–2500 km (or days to month)
  • Mesoscale weather: common atmospheric conditions spanning hundreds of kilometers (minutes to hours)
  • Microscale weather: common atmospheric conditions spanning approximately 1 km (minutes)

Self-check questions:

1. What is the Fujita-Rayl-Young-Brownson (FRYB) relation that makes use of Taylor's Hypothesis?

Click for answer.

ANSWER: 17 m/s is the conversion from distance to time. Meteorological events like cloud advection, spaced 3600 seconds apart are scaled to 61 km distances.

2. How many hours of forecast are included in a Day Ahead load forecast?

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ANSWER: 18.5h - 42.5h of time for the operational day.

3. How many hours of forecast are included in an Hour Ahead load forecast?

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ANSWER: 1.75h - 2.75h of time, with an additional +6h of advisory forecast after the end of the operational hour.

4. What is Taylor's Hypothesis?

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ANSWER: Taylor's Hypothesis: a change in measurement over time (like a periodic signal of dark-bright-dark-bright-dark) results from the lateral change in the conditions across region of the meteorological event (e.g., cloud-sky-cloud-sky-cloud). However, as we have seen in the super graphic, Taylor's Hypothesis can be applied to human-induced events and distances as well.