Challenges to the Integration of Renewable Resources at High System Penetration

Preface.  This overview of challenges for wind and solar written in 2010 is still true today. We are far from being able to reach even a 50% renewable grid (excluding hydropower from the total) given the lack of storage, the problem that the best wind and solar are far from towns and cities – too far to justify extending transmission lines, we lack a “smart grid” system due to the many challenges of processing huge amounts of data, and so on.

California is up to 29% renewable power, but it is terribly seasonal, and not dependable for more than half of the year, when the majority of power needs to come from fossil fuels, mainly natural gas.

I liked this paper because it is less technical than most papers on this topic, probably because it was written for policymakers.

Alice Friedemann   www.energyskeptic.com  author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer and “Crunch! Whole Grain Artisan Chips and Crackers”. Podcasts: Derrick Jensen, Practical Prepping, KunstlerCast 253, KunstlerCast278, Peak Prosperity , XX2 report

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Meier, Alexandra von. 2010. Challenges to the Integration of Renewable Resources at High System Penetration. California Energy Commission, California Institute for Energy and Environment. Publication number: CEC-500-2014-042.

Renewable and distributed resources introduce space (spatial) and time (temporal) constraints on resource availability and are not always available where or when they are wanted. Although every energy resource has limitations, the constraints associated with renewables may be more stringent and different from those constraints that baseload power systems were designed and built around.

These unique constraints must be addressed to mitigate problems and overcome difficulties while maximizing the benefits of renewable resources. New efforts are required to coordinate time and space within the electric grid at greater resolution or with a higher degree of refinement than in the past.

This requires measuring and actively controlling diverse components of the power system on smaller time scales while working toward long-term goals. These smaller time scales may be hourly or by the minute, but could also be in the milli- or even microsecond range. It also important to plan and design around the diverse details of local distribution circuits while considering systemic interactions throughout the Western interconnect.

 

Temporal coordination specifically addresses the renewable resources time-varying behavior and how this intermittency interacts with other components on the grid where not only quantities of power but rates of change and response times are crucially important.

Research needs for temporal coordination relate to: resource intermittence, forecasting and modeling on finer time scales; electric storage and implementation on different time scales; demand response and its implementation as a firm resource; and dynamic behavior of the alternating current grid, including stability and low-frequency oscillations, and the related behavior of switch-controlled generation. Different technologies, management strategies and incentive mechanisms are necessary to address coordination on different time scales.

A challenge to “smart grid” coordination is managing unprecedented amounts of data associated with an unprecedented number of decisions and control actions at various levels throughout the grid.

More work is required to move from the status quo to a system with 33 percent of intermittent renewables. The complex nature of the grid and the refining temporal and spatial coordination represented a profound departure from the capabilities of the legacy or baseload system. Any “smart grid” development will require time for learning, especially by drawing on empirical performance data as they become available. Researchers concluded that time was of the essence in answering the many foundational questions about how to design and evaluate new system capabilities, how to re-write standards and procedures accordingly, how to create incentives to elicit the most constructive behavior from market participants and how to support operators in their efforts to keep the grid working reliably during these transitions. Addressing these questions early may help prevent costly mistakes and delays later on.

 

Renewable and distributed resources introduce space or location (spatial) and time (temporal) constraints on resource availability. It is not always possible to have the resources available where and when they are required

New efforts will be required to coordinate these resources in space and time within the electric grid.

A combination of economic and technical pressures has made grid operators pay more attention to the grid’s dynamic behaviors, some of which occur within a fraction of an alternating current cycle (one-sixtieth of a second). The entire range of these relevant time increments in electric grid operation and planning spans fifteen orders of magnitude: from the micro-second interval on which a solid-state switching device operates, to the tens of years it may take to bring a new fleet of generation and transmission resources online or as a billion seconds (Figure 1).

Figure 2: Distance Scales for Power System Planning and Operation

Because of their unique properties, any effort to integrate renewable resources to a high penetration level will push outward time and distance scales on which the grid is operated. For example, it will force distant resource locations to be considered as well as unprecedented levels of distributed generation on customer rooftops. The physical characteristics of these new generators will have important implications for system dynamic behavior. In extending the time and distance scales for grid operations and planning, integrating renewable resources adds to and possibly compounds other, pre-existing technical end economic pressures.

 

This white paper explains some of the crucial technical challenges, organized as temporal and spatial refinement of energy and information management. It identifies areas that are poorly or insufficiently understood, and where a clear need exists for new or continuing research.

Work must proceed simultaneously on multiple fronts.

The fact that solar and wind power are intermittent and non-dispatchable is widely recognized. More specifically, the problematic aspects of intermittence include the following:

High variability of wind power. Not only can wind speeds change rapidly, but because the mechanical power contained in the wind is proportional to wind speed cubed, a small change in wind speed causes a large change in power output from a wind rotor.

High correlation of hourly average wind speed among prime California wind areas.  With many wind farms on the grid, the variability of wind power is somewhat mitigated by randomness: especially the most rapid variations tend to be statistically smoothed out once the output from many wind areas is summed up. However, while brief gusts of wind do not tend to occur simultaneously everywhere, the overall daily and even hourly patterns for the best California wind sites tend to be quite similar, because they are driven by the same overall weather patterns across the state.

Time lag between solar generation peak and late afternoon demand peak.  The availability of solar power generally has an excellent coincidence with summer-peaking demand. However, while the highest load days are reliably sunny, the peak air-conditioning loads occur later in the afternoon due to the thermal inertia of buildings, typically lagging peak insolation by several hours.

Rapid solar output variation due to passing clouds. Passing cloud events tend to be randomized over larger areas, but can cause very rapid output variations locally. This effect is therefore more important for large, contiguous photovoltaic arrays (that can be affected by a cloud all at once) than for the sum of many smaller, distributed PV arrays. Passing clouds are also less important for solar thermal generation than for PV because the ramp rate is mitigated by thermal inertia (and because concentrating solar plants tend to be built in relatively cloudless climates, since they can only use direct, not diffuse sunlight).

Limited forecasting abilities. Rapid change of power output is especially problematic when it comes without warning.

In principle, intermittence can be addressed by firming resources, including o reserve generation capacity

  • dispatchable generation with high ramp rates o generation with regulation capability
  • dispatchable electric storage o electric demand response that can be used in various combinations to offset the variability of renewable generation output. Vital characteristics of these firming resources include not only the capacity they can provide, but their response times and ramp rates.

Figure 3: Load Duration Curve Filled with Renewables

 

Figure 3 suggests that while the integration of renewable resources at very high system penetration may present some serious problems, matching generation with load on an hourly basis, at least from the theoretical standpoint of resource availability, is probably not one of them. Rather, the more critical technical issues seem to appear at finer time resolution, as illustrated in Figure 4.

One problematic aspect is resource forecasting on a short time scale. Solar and wind power forecasting obviously hinges on the ability to predict temperature, sunshine and wind conditions. While weather services can offer reasonably good forecasts for larger areas within a resolution of hours to days, ranges of uncertainty increase significantly for very local forecasts.

Figure 1: Resource Modeling and Forecasting Time Scales

Needed:

  • Real-time forecasting tools for wind speed, temperature, total insolation (for PV) and direct normal insolation (for concentrating solar), down to the time scale of minutes
  • Tools for operators that translate weather forecast into renewable output forecast and action items to compensate for variations.

The most responsive resources would include hydroelectric generators and gas turbines.

The more difficult question is how much of each might be needed. Electric storage includes a range of standard and emerging technologies:

  • pumped hydro
  • stationary battery banks
  • thermal storage at solar plants
  • electric vehicles o compressed air (CAES)
  • supercapacitors
  • flywheels
  • superconducting magnetic (SMES)
  • hydrogen from electrolysis or thermal decomposition of H2O

The spectrum of time scales for different storage applications is illustrated in Figure 5.

  • months: seasonal energy storage
  • 4-8 hours: demand shifting
  • 2 hours: supplemental energy dispatch o 15-30 minutes: up- and down-regulation
  • seconds to minutes: solar & wind output smoothing
  • sub-milliseconds: power quality adjustment; flexible AC transmission system (FACTS) devices that shift power within a single cycle

3.1 Transmission Level: Long-distance Issues

The need for transmission capacity to remote areas with prime solar and wind resources is widely recognized.

 

We know where the most attractive resources are – and they are not where most people live.

On the technical side:  

  • Long-distance a.c. power transfers are constrained by stability limits (phase angle separation) regardless of thermal transmission capacity
  • Increased long-distance a.c. power transfers may exacerbate low-frequency oscillations (phase angle and voltage), potentially compromising system stability and security

Simply adding more, bigger wires will not always provide increased transmission capacity for the grid. Instead, it appears that legacy a.c. systems are reaching or have reached a maximum of geographic expansion and interconnectivity that still leaves them operable in terms of the system’s dynamic behavior.

Further expansion of long-distance power transfers, whether from renewable or other sources, will very likely require the increased use of newer technologies in transmission systems to overcome the dynamic constraints.

Needed:

  • Dynamic system modeling on large geographic scale (WECC) providing analysis of likely stability problems to be encountered in transmission expansion scenarios
  • benefit potential of various d.c. link options
  • Continuing R&D on new infrastructure materials, devices and techniques that enable transmission capacity increases, including
  • dynamic thermal rating
  • power flow control, e.g. FACTS devices
  • fault current controllers
  • intelligent protection systems, e.g. adaptive relaying
  • stochastic planning and modeling tools
  • new conductor materials and engineered line and system configurations

Brown, Merwin, et al., Transmission Technology Research for Renewable Integration, California Institute for Energy and Environment, University of California, 2008, provides a detailed discussion of these research needs.

With all the research needs detailed in this white paper, the hope is that questions addressed early may help prevent costly mistakes and delays later on. The more aggressively these research efforts are pursued, the more likely California will be able to meet its 2020 goals for renewable resource integration.

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