NREL study reveals counterintuitive relationship between flexibility options and downsizing power systems with high solar penetration


The increasing penetration of variable renewable energy (VRE) into power systems is expected to increase reduction – the reduction in renewable energy supplied due to oversupply or lack of system flexibility. But while reduction can be the new normal in the evolving network, and can even be managed in a way that makes the network more flexible, it is important to find an optimal level of it to get the most out of VRE resources.

The NREL Study on Reducing High Solar Energy Futures provides insight into the importance of operating thermal generators at average PV penetration levels (25% to 40%) and the potential need to revise the rules eligibility of operating reserves and compensation structures as PV penetrations continue to increase. Image by Cynthia Shahan, CleanTechnica

While it is generally accepted that a lack of flexibility in the system increases reduction, it has not been well understood how individual flexibility options affect reduction, especially in high VRE futures. Analysts at the National Renewable Energy Laboratory (NREL) have completed the first in-depth study of how different approaches to system flexibility could impact reducing VRE (primarily solar) penetration levels. Flexibility options include battery storage, thermal generator flexibility, transmission, and allowing ERV and storage to provide operating reserves, among others.

Results – published in a Joule item – reveal two key findings that collectively constitute a “reduction paradox” that emerges as the system evolves towards higher levels of solar penetration. First, the flexibility of thermal generators has the greatest impact on reducing ERVs in the solar photovoltaic (PV) penetration averages, but not in the low or high ranges. Second, when ERV and storage are allowed to provide operating reserves, system-wide operating costs and abatement levels decrease, which in turn reduces the economic incentive to operate. PV to supply these reserves with reduced energy in a wholesale market environment.

Modeling approach

NREL’s analysis used a system that is roughly based on the Los Angeles Department of Water and Power’s (LADWP) production and transmission system, taking advantage of the data sets developed for the Los Angeles Study on 100% Renewable Energy (LA100). NREL used capacity expansion modeling to establish future scenarios of building the least expensive power systems with increasing levels of penetration of ERV resources. Next, the analysts used a utility-grade production cost modeling database to optimize the least expensive production and transmission resources to assess the detailed operation of each future construction scenario.

This is the first time that this complete suite of models has been used for a realistic and highly resolved system in high future PV to identify the operational factors that contribute the most to reduction and the potential value of PV to provide reserves. operating with reduced energy.

Paradox 1: The flexibility of the heat generator has the greatest impact on reduction only at mid-PV penetration levels

The flexibility of the thermal power plant – ramp-up rate, minimum production levels, and minimum rise and fall times – allows the power system to meet grid fluctuations as needed and to maintain a balance between supply and demand. request.

In the study, NREL made the counterintuitive observation that the flexibility of thermal power plants has a much greater impact on reducing ERV in a “transition zone” at average PV levels (about 25% at 40% in the study system) only at lower or higher levels. Of the various aspects of thermal generator flexibility, minimum production levels have the greatest impact on reduction in this area.

However, when PV penetrations are low (around 20%), there is not enough ERV for thermal flexibility changes to have a significant impact on the system. When PV penetrations are higher (around 45%), there is not enough incentive to use the remaining heat capacity to adjust operations and produce significant abatement effects.

“We also call the transition zone the ‘Goldilocks zone’ where it’s just the right combination of PV and thermal generators to result in thermal flexibility impacts on reduction,” said Bethany Frew, NREL senior energy analyst and principal investigator. of the study.

This aspect of the reduction paradox reveals the importance of solar evolution and interaction with the rest of the system, especially with regard to the flexibility of thermal power plants in power systems that are moving from fleets to thermal dominance. predominantly VRE. He also suggests that a phased approach might be needed to support the ongoing transformation of the power system.

Paradox 2: Using ERV and storage for operating reserves means lower operating costs and reductions, but reduced revenues

Limited and stored renewable energy is increasingly seen as a potential source of operating reserves, or the capacity available to the system operator in a short period of time to meet demand during events such as load forecast errors or scheduled outages.

As modeled in the study, simulated high PV penetration scenarios in which VRE and storage resources are not allowed to provide operating reserves result in significant increases in reduction and operating costs – indicating the aggregate value of enabling these resources to provide operating reserves.

However, this value does not necessarily translate into increased revenue potential in a wholesale market environment – which is the second aspect of the reduction paradox.

“Allowing ERV and storage to provide operating reserves results in low prices, which reduces the incentives for PV to provide operating reserves with reduced energy,” said Frew. “This aspect of the solar reduction paradox reveals the importance of proper value alignment and grid system compensation.”

Storage built for capacity and energy transfer services often has spare capacity for reserves, especially during periods of breeding. Because storage has a cost close to zero, it results in lower overall operating reserve prices, especially during PV reduction. Allowing storage to provide operating reserves also reduces the amount and hours of reduction, limiting the periods in which PV could use the reduced energy to provide operating reserves.

Add VRE on top of storage and the prices drop even more. Overall, there is little incentive for photovoltaics to provide operating reserves with reduced energy. It cannot compensate for the drop in income due to greater levels of reduction and lower energy prices with high levels of ERV.

“We have found that PV provides value to the system without sufficient opportunity for monetary compensation,” said Frew. “Market designers may need to revise operating reserve eligibility rules and compensation structures as PV penetrations increase.”

Future work

Overall, the study highlights the highly nuanced nature of flexibility and its role in the solar reduction paradox, and indicates that storage and thermal generators are important factors in a system’s flexibility needs. (and reduction levels) with high solar penetration levels.

Future work could explore other sensitivity factors with additional storage penetration levels and various system configurations. In addition, a comprehensive cost-benefit analysis of thermal generator flexibility upgrades could assess the overall cost competitiveness with storage and other flexibility options as power systems evolve towards greater penetration of thermal generators. ERV.

Learn more about NRELs energy analysis research.

Article courtesy of NREL.

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