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Increased Coal Plant Flexibility Can Improve Renewables Integration

Analysis Insights: April 2015

Flexible Generation

Increasing wind and solar penetration also increases opportunity for savings associated with improved coal flexibility. Using a commercial production cost model, NREL estimated the impact of generator flexibility on the integration of wind and solar generators, at penetrations between 15% and 60%. Increased flexibility was simulated by reducing the minimum generation level of the coal fleet from 60% to 40% of nameplate capacity. Minimizing generation levels can allow plants to stay online during periods of low energy prices, such as at night, and minimize the need for and impacts of cycling.

In the more flexible system, fuel costs and CO2 emissions are lower, as are variable operations and maintenance costs.

Coal plant flexibility can be increased through retrofits and operational practices that enable lower turndowns, faster starts and stops, and faster ramping between load set-points. One North American coal generating station accommodates significant cycling thanks to key hardware modifications and operational changes—e.g. monitoring and managing temperature ramp rates, implementing inspection programs for affected equipment, and continual operator training. These changes have minimized potential damage and minimized the cycling-related cost of maintenance at the plant.

System-wide flexibility can be enhanced by retrofitting just a portion of conventional generators within a power system. NREL investigated the costs and benefits of retrofitting ~25% of coal and natural gas plant fleet capacity for improved operational flexibility. The study showed that retrofits that improve the turndown level of gas-fueled and coal-fueled power plants have a net-benefit to the system, although there may or may not be a benefit at plant level. Additional analysis could help determine the system-level costs and benefits of faster ramp rates and faster and less expensive starts.

Flexible Load – Demand Response Resources

Demand response (DR) is a load management practice of deliberately reducing or adding load to balance the system during periods of peak demand or high market prices. DR resources represent a potentially large and relatively untapped resource for supplying flexibility to the grid systems. NREL is working with other national laboratories to estimate the economic value of D resources—including water heating, space heating and cooling, and water and wastewater pumping—for the Western United States. The value is calculated in terms of reduced system production costs and potential revenue streams. Because some DR resources are more flexible or better correlated with system requirements, revenue from energy or grid services provided can vary significantly.

Flexible Storage

Electricity storage technologies—such as pumped hydro, batteries, flywheels, and compressed air—have had limited deployment in the U.S. power grid, but could potentially allow greater use of variable renewables by matching energy dispatch to periods of high demand, providing flexible load during periods of high RE output, and providing a variety of high-value ancillary services. One of the challenges faced by storage developers is quantifying the value of storage, particularly benefits that may not be fully captured within U.S. electricity markets.

The operational value of storage is expected to increase with increasing penetration of variable wind and solar generation. For storage applications involving time-shifting of generation (i.e., load-leveling), variable generation tends to suppress off-peak prices more than on-peak prices, increasing price differentials for energy storage. Additional operating reserves required with higher levels of variable generation, also increase the potential value of storage when providing these reserves. Significant charging from renewables, and the associated net reduction in carbon emissions, does not occur until VG penetration is in the range of 40%-50%. Other sources of storage value, such as the ability to replace or defer generation, transmission, and distribution investments may also be impacted by the deployment of VG resources.