Renewable Integration Benefits

Executive Summary

To one extent or another, every region will have a significant amount of renewable energy (RE) fueled generation capacity. Consider, for example, that a growing number of states in the United States have what is called a “renewables portfolio standard” (RPS) which requires that a specific portion of all generation be fueled by RE. To find out more about the RPS in specific U.S. states, please click here.

As use of RE generation increases, there is a commensurate need for electric resources that can address the unique characteristics of renewable energy fueled generation – primarily the variability of its output. Energy storage is well-positioned to play a significant role in that regard in the modern electric grid. Three key challenges that storage can address include accommodating RE generation’s:

  1. Output variability,
  2. The temporal (time-related) mismatch between generation and demand,
  3. Uncertainty regarding weather forecasts, and
  4. Undesirable electrical effects on the electrical grid caused by some RE generation.

Discussion

Storage is expected to play a significant role in the successful integration of RE electricity generation into the electric grid of the future (renewables integration).

Most importantly,storage is expected to enable effective and reliable integration of RE generation whose output is variable and somewhat uncertain.
The challenge of variability is primarily related to wind and solar, although future ocean wave and tidal generation could also pose challenges related to variability. Examples of wind generation variability are shown graphically in Figure 1 and solar (photovoltaic) generation variability is shown graphically in Figure 2.



RE generation’s output variability can be categorized as a) short-duration or b) long-duration. Short duration variability – lasting a few seconds to many minutes – is caused by wind speed variability, sometimes involving significant moment-to-moment variations, and rapid fluctuations of solar energy due to clouds. Long term variability occurs from year-to-year, season-to-season, day-to-day and, most importantly, throughout each day. Storage can be used to address both short-duration and long-duration variability.

Energy Storage to Address Short-Duration Variability

With respect to short-duration variations: Wind generation output power is proportional to the cube of the windspeed, meaning that even modest windspeed fluctuations may result in significant variation of power output from wind generation. For example, if wind speed doubles, the power output from the wind turbine will be eight times greater (23). Similarly, solar generation output can vary quite rapidly as clouds pass overhead. Those rapid variations of output – called ramping – must be accommodated by other resources on the grid. 

Not surprisingly, there is growing need for what is known as “ramping resources” to offset the rapid output fluctuations from solar and wind generation. To perform ramping, storage output fluctuates in such a way that it cancels out the RE generation’s variability. For example, if wind generation output drops due to lower wind speed or solar power output diminishes due to numerous passing clouds then storage output is increased by the amount needed to compensate for the rapid reduction of output from RE generation. Similarly, if wind RE generation output increases rapidly, storage output is reduced commensurately. An example of ramping is shown graphically in Figure 3.

Many types of storage are especially well-suited to providing this ramping service – unlike most non-renewable (i.e., fossil-fueled) generation. This is because for most types of generation maintenance, wear, fuel use and air emissions are lowest when power output is constant.If storage is used in lieu of generation for ramping, then the benefit is related to a combination of:

  1. Reduced need and cost for generation equipment to provide ramping service, and
  2. Reduced variation of generation output.

Energy Storage to Address Long-Duration Variability

Storage is also well-suited to address intra-day and possibly day-to-day variability. Figure 4 shows how wind and solar generation output vary throughout a typical day, relative to the grid system demand.

Note that a significant portion of wind generation output occurs at night, when the energy has relatively low value. And, in some cases there is so much “baseload” generation – whose output cannot be varied – plus wind generation that system operators must either shut down (“curtail”) wind generation or pay other utilities, businesses or residential end-users to take the excess energy – sometimes referred to as “negative pricing.” 

With storage that “off-peak” energy from wind generation can be stored and used during the day, reducing:

  1. The need for generation capacity (equipment), and
  2. Operation during peak demand periods (thereby reducing air emissions, fuel use, maintenance and wear related costs).

Storage could do the same with energy output from RE generation that does have a constant output, especially biomass and geothermal fueled generation. 

For solar, storage could be charged (filled) at night with low-cost electricity from the grid,and the energy could be used duringpeak demand time, -- usually hot summer week day afternoons – when the solar generation is falling off for the day. Perhaps less desirable, but still interesting, is the possibility of time-shifting the energy produced in the morning, before peak demand times (in the afternoon) so that energy is available to serve peak demand as the solar generation is falling off. 

To address longer-duration variability throughout agiven day, storage can be used to provide power when two conditions exists:

  1. RE generation power output is less that its full power output, and
  2. When electricity demand is high.

To do that, the storage discharges to “fill-in” when the RE generation is not producing full power. The effect – of storage used in in concert with variable RE to address daily variability – is what is sometimes called “firming,” meaning that the result is constant power output, especially during times of peak demand.

Energy Storage to Address Renewable Generation Forecasting Uncertainty

One challenge associated with RE generation is that weather-related forecasts are imprecise. So there is always some uncertainty about prevailing weather conditions – wind speed and cloudiness – when RE generation is expected. Importantly, this challenge is being addressed and forecasting will improve. For example, there is a significant interest in improving “hour-ahead” forecasts rather than relying heavily on “day-ahead” forecasts. 

Nonetheless there will always be some uncertainty about the weather. And the results can be somewhat dramatic, especially for wind generation because wind can be somewhat to very different than forecasts would indicate, especially when the forecast is a day-ahead. 

When unexpected shortfalls of RE generation output occur some other resource is needed to make up the difference. Storage could be a valuable and flexible resource for addressing this challenge by providing “back-up” when RE generation is significantly less than expected.

Energy Storage to Address Power Quality Effects From Renewable Generation

Storage can also be used to offset undesirable effects on power quality (PQ) at and near points where problematicRE generation is connected to the grid. Of special note are parts of the distribution system where there is a significant penetration– 30% of peak demand or higher – of distributed photovoltaic (PV) systems. In parts of the distribution system with such high penetration of distributed PV, significant voltage variability may occur, affecting the overall electric service quality.

In some cases, the distributed photovoltaics produce an amount of power that exceeds the local demand. In those situations the excess electricity must be transferred to other sections of the distribution systems, or even to other regions. Unfortunately, that requires electricity “back-flow” (from lower voltage to higher voltage) through equipment that is designed for one way electricity flow (higher voltage to lower voltage). Under some conditions, wind generation can also cause electrical problems.

Storage can be used to address many PQ related challenges from RE generation by absorbing, filtering or otherwise offsetting many power quality anomalies. For example, when the voltage on a local distribution system varies too much, the storage can absorb excess power and even some types of abnormal power, such as current surges. Storage can also provide “real power” or “reactive power” to improve the voltage. Storage can also absorb excess power to reduce undesirable back-flow. Much larger storage could do the same for the transmission system in locations where many large wind turbines are installed.

Conclusions & Observations

Energy storage and variable RE generation have important synergies, making them quite complementary. Energy storage power input and output can be controlled precisely and varied rapidly to offset effects from RE generation short duration variability. Storage can store RE generation output when that output has low value for use later when the energy has higher value (energy time-shift). Storage can also be used to firm power output from variable RE generation during times when electricity demand is high. The resulting benefits include:

  1. Reduced need for conventional generation,
  2. Increased value of the electricity generated by variable RE generation,
  3. Reduced suboptimal operation of conventional generation due to reduced need for ramping, and
  4. Reduced need for other power conditioning equipment to accommodate large wind farms and high penetrations of distributed photovoltaics.

Given the aforementioned synergies and benefits, it is quite reasonable to assume that energy storage is likely to play a significant role in the more sustainable, cleaner, variable and distributed electricity grid future by:

  1. Offsetting negative effects on the grid from RE generation variability,
  2. Increasing the value of variable RE generation output, and
  3. Enabling more variable RE generation deployment.