http://www.nrel.gov/docs/fy15osti/63038.pdf[font face=Serif][font size=5]USING WIND AND SOLAR TO RELIABLY MEET ELECTRICITY DEMAND[/font]
[font size=4]LEVERAGING RENEWABLE ENERGY TO ACHIEVE LONG-TERM ADEQUACY[/font]
[font size=3]An important aspect of power system planning is ensuring that adequate generation capacity exists to meet electricity demand during all hours of the year. Mechanical failures, planned maintenance, or lack of on-demand generating resources (especially for variable renewable resources) may leave a power system with insufficient capacity to meet demand. Grid planners project future peak demand patterns and estimate the relative contribution of each generator towards achieving a reliable supply of energy. When generating during peak demand periods, variable renewable energy (VRE) such as wind or solar PV provides capacity value to the system. By providing capacity value, VRE can help to defer capital investments in traditional generation and transmission infrastructure.[/font]
[font size=4]CALCULATING CAPACITY VALUE[/font]
[font size=3]Determining the capacity value of VRE is more complex than it is for conventional generation technologies due to the inherent variability of these resources. The simplest way to calculate capacity value of a renewable generator is to examine how well VRE generation aligns with regional demand patterns. This approach considers the output of a generator over a subset of periods during which the system faces a high risk of an outage, such as the 10 to 100 hours of highest net load in a year |1|. The result is expressed as either a capacity value (kW, MW) or as the fraction of the renewable generator’s capacity that adds to system reliability or can offset conventional capacity (see sidebar).
More accurate approaches used by utilities and system planners employ detailed reliability-based metrics to assess capacity value. One widely used statistical approach calculates the effective load-carrying capacity (ELCC) of additional generation. The ELCC of a generator is defined as the amount by which the system’s loads can increase when the generator is added to the system while maintaining the same system reliability. In this case, system reliability can be described using two metrics: loss of load probability (LOLP) and loss of load expectation (LOLE). The LOLP is defined as the probability of a loss-of-load event in which the demand is greater than available generating capacity during a given period |2|. The LOLE is the sum of the LOLPs during a planning period—typically 1 year. LOLE gives the expected number of periods in which a loss-of-load event occurs. Power system planners aim for a certain LOLE target, such as 0.1 days/year or 0.1 events/ year |3|. Figure 2 illustrates the steps used to calculate the full ELCC of an added renewable generator.
Studies have found a large range in capacity values, ranging from 5% to 40% for wind and 5% to 75% or higher for solar PV |2|, |4|, |5|. Once capacity value is known, a monetary value per unit of installed VRE capacity can be calculated using a variety of approaches |1|.
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