For years, advocates of cleaner power have offered up renewable energy as the solution to our environmental problems. With growing concern about climate change and greenhouse gas emissions like carbon dioxide, renewables are finally receiving serious attention abroad and in the United States.
With that attention has come pushback from those parties that have a lot invested in the status quo, especially those associated with coal. Originally they said renewables don't work, but renewable technology has become proven. Then they said renewable technology was not reliable, yet it has become very reliable. Then they said renewables could not be done on a large utility scale, and now the large wind farms have proven that wrong. Then they said there were not enough renewables to meet our energy needs, but studies show that there are ample sun, wind and water resources to meet our energy needs many times over, even without energy conservation, which itself will dramatically moderate our growing energy demands.
And now the naysayers have come up with a new reason: renewable can't meet our energy needs because "renewables cannot provide baseload power."
"Baseload" is a term that utilities use to describe their large centralized power plants, usually fueled with coal or nuclear fuel. Traditionally, these plants have been the lowest cost to operate, so they are used first, and usually run for long periods, making up the "base" of utilities' generation and handling the typical "load," with more expensive generators being used to meet the rest of demand on those days when it is higher than usual.
But this traditional way of operating a utility system is about to be turned upside down. Environmental and human health costs will likely make these "baseload" plants more expensive than other options, and as their costs are rising, the costs of renewable resources are coming down.
As we plan for the future, we must overcome the myth that the electric utility system cannot be operated without these "baseload" plants because renewable resources are too variable since the wind does not blow all the time and the sun does not shine at night.
While it is true that some renewable resources have different operating characteristics than current utility plants, some of those differences are positive, while others will require different operating procedures. The problem is not the variability or reliability of the renewable resources, but rather the desire by utilities to not change the way they operate their systems.
To understand this, we must first understand how an electric utility operates its system. Because such large volumes of Alternating Current (AC) electricity are being consumed all the time, it is very difficult and expensive to store this power in the volumes necessary to meet customer demand. So, instead, a utility performs a constant balancing act of making or purchasing just the correct amount of power at any moment to meet customer demand and correct for line losses. If the utility makes too much or too little for an extended period, the system voltage will rise or fall to the point that protective relays in the electric system will open up and the result will be a blackout.
Maintaining this exact balance between supply and demand, constantly 24/7, is the job of the dispatchers. They deal with customers adding or dropping loads, as well as generators coming on and off line. Customer load, which must be matched with generation, varies according to time of day (less power is used at night), time of year (more power is used in the summer and winter for heating and cooling) and by weather (very hot and cold periods require more power). Dispatchers do not look at the use of individual customers (except for very large industrial customers), but instead can fairly accurately predict how much power will be needed in the aggregate, hours or days ahead, using the season, time of day and forecast weather and temperatures.
The concept of "baseload" comes from the method that planners and dispatchers have used in the past to meet demand. Since a certain minimum amount of power will be needed no matter the time of day or year, dispatchers have used their lowest cost large units to meet this "base" demand, then added more expensive-to-operate generators to meet the additional demand during peak times of the day. But this is simply the way that the system has been operated in the past, not a requirement. The only real requirement is that the correct amount of power be provided as that power is demanded and needed by customers.
While these large centralized plants have in the past been used by dispatchers to meet around-the-clock demand, they also have their own set of problems for system operators. Their huge size causes major difficulties if they have a problem and a 500 megawatt plant suddenly trips off line. Dispatchers must scramble using a mix of spinning reserve, quick starting peaking units and borrowed power from neighboring utilities to quickly make up this deficit. To protect against the possibility of big generators tripping off-line, utilities have had to invest a lot of money in extra generating capacity, called a reserve margin (in the range of 20 percent more than their projected highest peak load), spinning reserve (capacity running, but not loaded, for quick emergencies) and interconnections with other utilities. All of these are parts of the ratepayer cost of using large "baseload" plants.
It is also difficult to replace one large unit for another. If a large coal generator trips off line with mechanical problems, getting a replacement unit on line can take a while. It can take 24 hours to start-up, synchronize and load a large coal-fired unit from a cold start. Even from a warm start, the process can still take 6 hours, so large coal units are not started and stopped as load varies. Instead, the units can be backed-down during lower use periods, and operated in a lower output mode, which is also less efficient.
Nuclear power plants add even more operational difficulties. For the most part, these units cannot safely be backed-down during low demand periods, so coal units also on the system must be backed-down even further making them even less efficient. This can cause some real operational problems during minimal demand periods (like spring and fall with no heating or cooling load, late at nights, on a weekend when industry is shutdown). If the nuclear plants can't be backed-down, and the coal units are backed down as far as they can go, the electric system can be threatened by too much power.
One way to deal with the problem of nuclear power plants' inability to safely reduce output during low use times was the development of pumped-storage hydro. In these plants, power is used during off- peak times when there is more power than needed, to pump water uphill to a storage reservoir, then the water is later run back downhill during peak times to generate electricity. While these plants are expensive, they have offered assistance in dealing with large fixed output nuclear plants. There are over 23,000 megawatts of pumped-storage hydro capacity in the U.S. today, located mainly where utilities are also using nuclear power. So utilities have developed expensive but effective ways to deal with the problems inherent in the use of large centralized coal and nuclear plants, and the ratepayers have paid these costs.
Renewable energy plants have their own set of issues that must be dealt with by system planners and dispatchers. Renewable plants tend to be much smaller, so the loss of an individual plant or even a group of plants does not cause the problems of a large centralized plant. Also, most renewable plants can be started and synchronized to the grid very quickly, unlike the large fossil and nuclear plants. So the challenges for planners and dispatchers associated with "baseload" generators generally do not occur with renewable generators. Instead, the major problem with renewables is the variable nature of the power. While no current fossil-fuel or nuclear power plant is available to the dispatcher all the time due to forced outages (something breaking) and planned outages (scheduled downtime for maintenance), renewable plants face longer outage periods due to a lack of fuel (sun, wind or water). Yet these periods are different from the unexpected forced outages that the large coal and nuclear plants experience. Just like expected customer loads can be predicted hours or days ahead by dispatchers using weather forecasts, sun, wind and water produced electric output can be predicted in the exact same way with weather forecasts. Also, geographic diversity of a lot of small renewable generators, like wind turbines, means that all the units taken as a whole have a much higher capacity factor than any individual unit alone.
The other complaint from utilities about wind and solar power is that they are not "dispatchable," meaning utility dispatchers cannot turn these units on or off like they can large centralized power plants. Yet because there is no fuel cost associated with renewable plants, using economic dispatch (which uses the plants with the lowest variable costs first), renewable plants would always be dispatched first, so that dispatchability is a non-issue. The real issue for dispatchers is knowing how much power will be produced by renewable plants in aggregate at any time in the near future and integrating this into their calculations of the amount of additional generation that will be needed. Like customer load that can be predicted using weather forecasts and other factors, renewable generation can also be predicted. So in a sense, this renewable generation will act as negative load, or a reduction in the amount of other load demand that the utility must supply at any given time. Today, future load (say the next day) is predicted by dispatchers, and then they determine which generating plants will need to be on line to meet that predicted load. In the future, the procedure will be the same, except the negative load from renewable generators will be subtracted from the positive customer load, and dispatchers will then need to have the generating assets available to meet the net load.
The question then becomes, with a large penetration of renewable generators, can the electric system be operated without the significant "baseload" units utilities rely upon today? The answer is absolutely yes. While utilities have characterized certain parts of the load they serve as "baseload," and have used large and inefficient centralized plants to meet this part of the load, the reality is that each hour of each day there is a certain amount of load that must be met with power generation, and that load can be met with any type of generation the dispatcher chooses. Today, utilities use large centralized coal and nuclear plants to meet a portion of the load, and then use more expensive gas fired generation to meet the balance. The gas fired generation is also used as a quick start substitute when one of these large plants fail and drops off line.
When renewables become a substantial part of the resource mix, these gas-fired units that now back-up the large "baseload" plants can be used to fill in holes when the mix of renewables is not sufficient to meet predicted load demand. Now that natural gas prices have been coming down as new resources have reached the market, and coal-fired generation is becoming more expensive as pollution controls are needed, many utilities are opting to rely more heavily on gas-fired generation, which should further help with the dispatch of utility systems with large penetrations of renewable generators.
There are also some types of dispatchable renewable plants, such as peaking hydro plants, which can be brought on-line in minutes (not hours), and biomass plants which can be ramped up and down like a large centralized plant. In addition, the pumped-storage hydro plants that have been built to deal with the fixed capacity nature of nuclear plants, can also be used to provide additional backup for renewable generation. And there are solar thermal generating plants being built that will have the capacity to store significant heat for use when the sun isn't shining.
Clearly the customer load can be met, hour by hour, primarily with renewables, without today's 'baseload" plants; in fact these plants may get in the way of renewables providing substantial amounts of the needed power in a given hour.
The problem here is not the nature of renewable resources or any technical hurdle, but rather it is getting utility planners and dispatchers to think outside the "baseload" mindset that they have been stuck in for so many years. Instead of thinking horizontally — adding strips of large "baseload" capacity to run for days or weeks or even months then filling in the gaps, instead the dispatcher needs to look vertically ahead — what will be my load minus my negative load from renewables and then how do I fill any gaps.
The need for large, centralized baseload capacity is not some requirement of the electric power system, but rather a desire to continue to do things as utilities have done in the past, the way they know. What is needed is not additional baseload capacity, but simply the willingness of utilities to look at meeting customer load with different resources, and the development of forecasting tools and dispatch methodologies that easily and reliably integrate clean power sources into their systems.
As the internalizing of the health and environmental costs of the "baseload" plants makes their power more and more expensive, and as it becomes ever more increasing difficult to get these dirty plants to operate under cleaner and cleaner requirements (especially in a carbon-constrained world), utility planners and dispatchers will be forced to think differently as theD switch to clean renewable generators happens, whether they like it or not.
David Brown Kinloch, a Louisville engineer, can be reached at firstname.lastname@example.org.