Grid of the Future: Quantification of Benefits from Flexible Energy Resources in Scenarios With Extra-High Penetration of Renewable Energy

Executive Summary
The main objective of this study is to quantify the entitlement for system benefits attainable by pervasive application of flexible energy resources in scenarios with extra-high penetration of renewable energy.

The quantified benefits include savings in thermal energy and reduction of CO2 emissions. Both are primarily a result of displacement of conventional thermal generation by renewable energy production, but there are secondary improvements that arise from lowering operating reserves, removing transmission constraints, and by partially removing energy-delivery losses due to energy production by distributed solar.

The flexible energy resources in the context of this study include energy storage and adjustable loads. The flexibility of both was constrained to a time horizon of one day. In case of energy storage this means that the state of charge is restored to the starting value at the end of each day, while for load this means that the daily energy consumed is maintained constant.

The extra-high penetration of renewable energy in the context of this study means the level of penetration resulting in significant number of hours where instantaneous power output from renewable resources added to the power output from baseload nuclear fleet surpasses the instantaneous power consumption by the load.

At extra-high levels of penetration, three major technical barriers stand in the way of renewable energy deployment.

1. The renewable energy resource, particularly wind, is not correlated with the load, which leads to concentration of renewable generation sites at significant distances from load centers and limits their power output to the available transmission system capacity connecting them to the load.

2. Because their fuel is free, renewable sources of energy are typically held at maximum available output. Consequently, they are not counted-on to provide up-reserves to stabilize the power-system frequency in case of unplanned outages of generation sources. This limits their total penetration to a level where non-renewable generation is able to provide sufficient up-reserves for the entire power system.

3. At extra-high levels of penetration, instantaneous renewable power output may surpass the system load, which makes curtailment inevitable unless there is flexibility available on the load side. The flexibility can be provided by energy storage or by scheduling system load to correlate its consumption with the availability of renewable energy. This study evaluates the benefits associated with removing, or partially removing, these barriers to help prioritize development of technologies for addressing the barriers based on their impact or, alternatively, to set the upper-bound for cost of technologies based on their value to the power system.

The findings are as follows. Increasing renewable penetration from 30 to 50% at the footprint of the United States, reduces energy consumed by thermal generation fleet by ~4 quads (quadrillion BTU) and CO2 emissions by ~340 Mtons. At 50% penetration, renewable energy curtailment due to the three constraints are significant. Removing the transmission-imposed limits is the necessary first step. Today’s transmission infrastructure is simply not sized to evacuate this scale of energy from regions where the renewable resources are optimal. With the transmission limits completely removed, the curtailments of renewable energy due to the other two constraints were still significant. Only 79% of available renewable energy was actually utilized, the remaining 21% was curtailed. Assuming that the role of providing upregulation can be covered by flexible energy resources and removing the corresponding part of renewable curtailments resulted in additional thermal energy savings of ~2.8 quads andreduction of CO2 emissions by ~250 Mtons. Finally, load-scheduling to achieve better correlationwith available renewable energy brought the cumulative reduction of consumed thermalenergy to between 3.2 and 3.5 quads and reduction of CO2 emissions to between 290 and 315Mtons.

The study also briefly reviewed the gaps between the existing and novel technologies needed to achieve the estimated benefits. It was found that a new control architecture is needed to overcome frequency stability limit by managing large number of dynamically responding energy resources. Advantageously, however, since the dynamic load response can be provided relative to inherently available signal of system frequency, the deployment of dynamically responsive energy resources is not contingent on demanding communications. The benefits of load scheduling are most dominantly affected by accurate renewable energy forecasting and require system operators to adjust unit-commitment to yield savings in cost of dispatch. Although implementing these adjustments and the corresponding market mechanisms and settlements are not trivial, they too are not contingent on demanding communications.

Published with ARPA-E's Network Optimized Distributed Energy Systems (NODES) Funding Opportunity Announcement.