The primary reserve explained, and how it compares with the secondary reserve

The primary reserve explained, and how it compares with the secondary reserve

From the grid’s point of view, they do the same job: bring the frequency back to 50 Hz. They do not do it at the same speed, nor for the same pay.

The primary reserve (FCR) is the flexibility mechanism that demands the fastest response. The battery tracks grid frequency and corrects deviations from 50 Hz on its own within 30 seconds. The secondary reserve (aFRR) follows a setpoint from RTE (the French transmission system operator) and has 5 minutes to do the same.

The prices RTE pays for each kW made available to the primary reserve are fairly close to the downward component of the secondary reserve. Over the last twelve months, a kW held available to RTE earned, for the primary reserve and then the downward secondary reserve respectively:

• At its highest: 2.7 cents per hour from 12:00 to 16:00, versus 3.2 cents at 11:00.
• At its lowest: 0.9 cents from 00:00 to 04:00, versus 0.6 cents at 07:00.
• On average over the day: 1.6 cents, versus 1.4 cents.

It should be noted that the downward direction does not capture the full value of the secondary reserve: capacity held available to RTE in the upward direction reaches 6.7 cents until 20:00. These figures are revenue potentials. Behind the meter, they also have to be weighed against the site’s own needs.

One access condition is the same for both: the minimum bid is 1 MW. That means pooling around twenty 50 kW sites, or hundreds of residential batteries.

This barrier is the reason Nexen GridTech exists: we aggregate and control battery installations under 1 MW to open up access to these mechanisms.

Do you develop, distribute or install residential or commercial batteries? Get in touch to explore what the primary reserve could bring to your projects.