"Renewables": some resources support a healthy grid, other challenge it.
The first part of this series discussed some of the shortcomings of the renewable/nonrenewable dichotomy. Renewable generation resources are not necessarily sustainable or environmentally sound and non-renewable options can be clean and highly sustainable. For example, you will find many ardent environmentalist groups strongly opposed to “renewable” biomass generation. Similarly, more and more environmentalists are dropping their objections to “nonrenewable” nuclear power. For those who are concerned with the health of the planet as well as those who want to use the earth for human flourishing the renewable/nonrenewable dichotomy is losing relevance. Referring generally to “renewable” and “nonrenewable” resources or structuring policy to favor renewable does more harm than good as we face the complicate challenges ahead in maintain an adequate electric power supply in an environmentally responsible manner.
This posting examines the impacts of various generation alternatives s on the power system and the electric grid. Renewable resources do not have a general impact on the grid; impacts vary by resource type. The various renewable resources alternatives available today differ greatly in how they impact the grid and should not be clustered. Hydro resources with storage for example, work well to support the electric grid. In fact, it may be the best resource available considering the varied needs of the major grids. Demanding loads that stress the system are often best located near hydro resources. Other “renewable” resources to a greater or lesser extent may present challenges to the operation of the grid and grid reliability. In assessing the challenges of changing resources, reports that a particular grid is operating with 80% renewables may be impressive or virtually meaningless. Of course, a grid can function well depending on 80% hydro resources, or 78% hydro and 2% wind and solar. That’s very different and much less challenging than operating a grid with a penetration level of 40% wind and solar. Let’ look at some of the important characteristics of generation resources and how they differ among resource types.
Load Following/Scheduling/Dispatch/Resource Availability
Generation alternatives, especially some renewables, differ greatly in how and when they supply power. Starting out, the first question, “is the provided energy dependable?” The chart below is the daily output taken from A solar array It shows the energy produced on a typical sunny day with clouds rolling by. You can see the unpredictability introduced by random clouds superimposed upon the predictable daily curve made by the suns path. A bigger footprint or aggregating more farms would tend to smooth the output and increase predictability. But even when greatly smoothed, the energy output is following the sun not the system load.
Click on the image to enlarge.
Electric generation in the form of alternating power has to match load on an instantaneous basis. It’s important that the total generation supply be able to match the load as it ramps up or down. Resources that are dependable and can ramp up and ramp down to follow changes in load are very valuable allowing system operators to depend on them. Plants with the best dispatchability include hydro storage facilities, natural gas plants and batteries/storage. In the middle are plants which provide dependable baseload but are more limited in how well they follow load. These plants rang from coal, combined cycle gas plants, biomass and geothermal to nuclear. Wind and solar generally complicate load following as they are not dependable or able to follow load without attached storage. Run of river hydro is rare and to the extent it is unpredictable it can create difficulties. Speaking generally of the dispatchability of “renewable” resources makes little sense.
One manifestation of load following problems is known as the “duck curve”. In 2014 I warned of the emerging duck curve here and discussed in depth issues related to dispatchability for various generation resources. Since then, as expected, it has worsened considerably. The chart shows that as solar kicks in and fades out, the remaining resources have to ramp up and ramp down at a very steep rate that greatly challenges system operation.
Click on the image to enlarge.
While the daily impacts of solar resources ramping in and out present challenges, at least solar is somewhat predictable. Wind in some locations at sometimes is predictable to a degree but in other locations and at other times it can be highly variable. Sometimes wind and solar track load changes and sometimes they work against the variance in load.
This variability presents problems beyond load following. Generating resources that can’t be counted on need back up resources. If resources might become available, dependable generation must back up those resources. As noted above hydro and batteries can work well to balance energy and it can be easy to have them on standby for that purpose. Other resources however mut be committed and running on line at lower generation levels or depending on the timeframe, in standby mode. Also coal and gas generation have start up times which may delay their availability as well as minimum cool down periods during which they cannot be operated.
I am not trying to go into detail about the intricacies of scheduling and dispatch here, but rather trying to illustrate that generation availability can create challenges that are highly dependent upon the available resource mix and the specific characteristic of the individual resources. All individual renewable and non-renewable resources have their own unique characteristics. When we wonder as to how the grid might responds to renewable, we need to know which renewables are under consideration.
VRE and IRES: The Good and the Bad of these Terms
Some of you may want to remind me that some technical work does break down renewables into a separate Variable Renewable Energy (VRE) group or Intermittent Renewable Energy Source (IRES) group when discussing issues of reliability. Certainly, speaking specifically of the challenges of integrating large amounts of VRE or IRES into the grid is an improvement over discussing the challenges of generic unspecified renewable resources. Referring to VRES and IRES can be seen as somewhat a step in the right direction consistent with the recommendations here for clearer language, but we can do better.
The term renewable in both IRES and VRE is redundant. There are no significant nonrenewable generating options that are variable or intermittent in nature. (There are limited amounts of run of river hydro in some areas.) However, my major concern with this term is that it does not recognize the larger problem currently inherent with most intermittent renewable resources. In the series Academics and the Grid, I argued that it seems like there is a deliberate effort to hide the real challenges of a net zero transition by focusing almost exclusively on the problems associated with intermittency. These names seem to help promote the misconception that variability or intermittency stand as the major challenge. .
Imagine a factory with a problem that workers are showing up late and mostly all of them are intoxicated. Upper management instead of reacting to the very real concerns around inebriated employees, focuses instead on tardiness. They bring in experts to help ensure employees show up on time and provide innovative scheduling so that gaps can be covered with extended hours once the employees show up. To the extent that the measures address the problems of “tardy” employees and that they are able to cover the hours, the more problems associated with intoxication will show up. Maybe the business fears that if the intoxication problem becomes known they will have to slow down, make major changes, maybe lose some contracts. One can imagine that a business might short term want to hide the problem of “intoxication” so that it does not impact near term goals, but for long term success it must be confronted.
The big problem with wind and solar generating resources seems almost hidden in a similar manner. Many do not want to see the march towards “renewables” slowed. It’s obvious there are problems with increasing “ the level of available renewables, but if most of the talk about the smaller problems and the greater public ignores the larger problems, perhaps this is better for the current “momentum” of available renewable options. The focus on intermittency is distracting relevant actors from the bigger looming problems ahead. Far too many policy makers think that batteries or other solutions to the intermittency problem will enable us to march ahead towards net zero with resources that currently are not up to the task.
The Big Looming Problem Associated with Increasing Wind, Solar and Batteries
The major challenges associated with increased penetration of wind and solar generation are not caused by intermittency, but rather from how the energy is injected into the grid. The electric energy produced by wind and solar is transformed by a power converter using inverters in order to synchronize with the oscillating grid. In terms of reliability, resources that spin in synchronism with the grid as electric energy is produced are much better for the grid than those resources which use inverter-based technology to convert direct for grid injection. Resources that spin with the system are called synchronous resources, while inverter-based generation is called asynchronous generation. Note – Although wind turbines spin, they do so at variable speeds, requiring them to produce direct current power which must be then be converted to AC before entering the grid.
Synchronous resources readily provide inertia and essential reliability services which support the grid. In addition to dispatchability, hydro resources are valuable because they spin in synchronism with the grid. They are typically large and among spinning resources they have great grid supporting characteristics. Good spinning resources also include nuclear, coal, geothermal, natural gas, biomass, geothermal and molten salt thermal solar plants. In terms of supporting grid reliability, wind and solar resources which rely on asynchronous inverter-based technologies are at the other extreme.
It’s important that we distinguish between inverter-based technology and spinning machines because there is a strong potential to improve inverter-based technology. To some extent now, with additional features, power converters can roughly emulate some behaviors of rotating machines and help provide important reliability functions to the grid. There is hope that inverter-based generating resources will be better able to approximate spinning generation in future years. The National Renewable Energy Laboratory (NREL) recognizes that:
Managing the stability of electric power systems is based on decades of experience with large, synchronous generators. Today’s electric power systems have increasing numbers of nontraditional sources, such as wind and solar power, as well as energy storage devices, such as batteries. In addition to the variable nature of some renewable generation, many of these resources are connected to the power system through electronic power inverters.
The operation of future power systems must be based on the physical properties and control responses of traditional large, synchronous turbine generators as well as inverter-based resources. But there is no established body of experience for operating hybrid power systems with significant inverter-based resources at the scale of today’s large interconnections. To operate such systems, the assumptions that underlie generation design and control must be reexamined and modified—or even redefined—to take account of the challenges and opportunities presented by inverter-based generation.
Most inverter controllers today are grid-following and built on the assumption that system voltage and frequency are regulated by inertial sources. Such control approaches cannot guarantee system stability in low-inertia setting and are unlikely to sustain an inverter-dominated infrastructure. This limitation has inspired an investigation into grid-forming control methods for power electronic inverters, which provide functionalities that are traditionally provided by synchronous machinery.
Certainly, progress can be made. However, the Eastern Interconnection in the United States in the largest, most complicated machine in the history of the world. The challenge of adding significant amounts of asynchronous inverter-based generation increases the complexity tremendously. I have written extensively on the challenges of increasing the levels of penetration from asynchronous generation sources here, here, here, here and here. I believe that running any large complex grid without support from large rotation machines in very close to impossible within planning time frames. I see the challenge of allowing high penetration levels of asynchronous inverter-based resource as tougher than rocket science or brain surgery. Such an achievement would go beyond the moon landing, the sequencing of the human genome, the atomic bomb or a cure for the common cold.
Many entities, including the National Renewables Energy Laboratory, FERC, and NERC, are working hard to improve the functionality of asynchronous inverter based technology. They may well meet their goals to allow better grid support from these resources within the next five to ten years. Better support is possible. But that will still be a long way from eliminating the grids reliance on rotating machines.
Those who are more optimistic about the development of such technologies may end up being right. I hope so. It may be that we may have functional large grids driven by wind, solar and batteries within the lifetimes of some of our readers. But to get there, we have to recognize and acknowledge the difficulties inherent in such a transformation. Those who assume or pretend that there is no problem are either woefully ignorant or ignoring the looming problem in favor of short-term goals. Unfortunately, the challenges of asynchronous inverter based generation will not be adequately addressed of ever fixed if they are hidden.
Conclusion
Generically speaking about the impact of renewables on the grid, or what renewables can accomplish generates more confusion than clarity. Power system engineers know that it is easier to add wind, solar and batteries where there are existing supportive large hydro resources. It is harder to add wind, solar and batteries where the other resources are not that strong. Comparing the penetration levels of renewable generation for different areas with vastly differing amounts of hydro can be misleading, especially when hydro is included in the renewable category. Failing to distinguish between the capabilities of hydro, and wind and solar can lead to unrealistic selections for low hydro areas. Misconceptions on capabilities are especially a concern as existing hydro resources are increasingly challenged.
Large grids certainly can operate reliability without fossil fuel generation. Large grids can operate on 100% “renewable” energy. Reliability is not a function of the source of the energy input to the grid, but rather how that energy is injected into the grid. If solar powers spin a generator in synchronism with the grid (perhaps through the use of molten salt) that better supports reliability. If wind turbines could be made to spin only in synchronism with the grid, that would aid reliability. If the energy from hydro flows were captured with variable dc motors and injected to the grid with inverters, reliability concerns would increase. As the grid changes and we address reliability, is so much easier, direct and honest to speak of synchronous generation and inverter-based generation. The impact of “renewables” on reliability is too vague to be meaningful. Although this is the case, you frequently see references to renewable percentages which do not distinguish among the resources employed. These figures are quoted by those who want to push a perspective and they impact policy makers.
We do not face a challenge in integrating large amounts of renewable generating resources into the grid. We face operational challenges from intermittent resources. We face large reliability challenges from integrating large amounts of asynchronous inverter-based generation into the grid. The better the problem is understood, the better the problem can be addressed. We many disagree on the scope and magnitude of problems caused by intermittency as well as by asynchronous inverter-based generation. But be assured addressing the emerging problems will be go smoother, the more precisely the problems and successes are identified and defined.
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ABOUT THE AUTHOR
Russell Schussler is a registered Professional Engineer with expertise on climate change and energy alternatives, and a frequent contributor to Climate Etc.
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