Low asset utilization is an increasing financial challenge for both conventional and renewable assets, which are sometimes curtailed due to over-generation at various times of day. Integrating large-scale thermal energy storage with combustion turbines in a Liquid Salt Combined Cycle can transform thermal generation assets into low-carbon resources that deliver capacity and energy at far lower cost than batteries.
Solar and wind resources are displacing conventional generation, which is increasingly used to follow and firm variable generation. Thermal plants are operating at reduced capacity factors, are called on for load-following, and are being cycled, sometimes requiring two starts per day. Under these conditions, the heat rate increases, emissions limit run-time, and thermally stressed components have dramatically shortened useful lifetimes.
Renewable over-generation and fast ramps up and down, such as depicted in the California Independent System Operator’s (CAISO’s) Duck Curve, create reliability challenges for grid operators. Market prices are increasingly volatile, often introducing negative prices in day-ahead markets, driving baseload retirements. Extended cloudy periods have challenged reliability in Hawaii, while becalmed wind generators have driven price spikes on the Electric Reliability Council of Texas (ERCOT) system.
But as renewable deployment increases, it is not only thermal generators that must adapt. Having achieved its 30% renewable portfolio standard (RPS) goal, CAISO was forced, for example, to curtail almost 32 GWh of renewable power generation on April 21, 2019. By the middle of the next decade, over-generation is expected to be a daily occurrence as shown in Figure 1.
1. Up to 100 GWh of daily storage would be needed to avoid renewable curtailment when California’s renewable portfolio standard exceeds 50%. Source: California Energy Storage Alliance and LS Power analysis of CAISO Open Access Same-time Information System (OASIS) Data (2016), 2030 IRP [Integrated Resource Plan] Proposed Reference System Plan Scenario
The power industry requires energy storage to play a critical role in the low-carbon transition, with North American potential estimated to exceed 100 GW, according to a presentation given by Ilya Chernyakhovskiy, who is a member of the Grid Systems Group in the Strategic Energy Analysis Center at the National Renewable Energy Laboratory (NREL). Chernyakhovskiy spoke in September at the Advanced Clean Energy Summit in Denver. He suggested for capacity and renewable time-shifting with duration of four to 12 hours, the storage system costs will need to be less than $150/kWh to make economic sense.
All Energy Storage Is Not Created Equal
Storage systems are comprised of charging and discharging systems that transform electricity to and from the internal energy of a storage medium. Different approaches are taken to achieve cost reductions.
Some technologies share charging and discharging equipment to reduce cost. These include reversible pump-turbines for pumped storage hydro (PSH) and rectifier-inverter power electronics for electro-chemical batteries. Others, such as compressed air energy storage (CAES), use independent charging and discharging equipment.
Some technologies use cellular storage, while others use bulk storage media (such as water, air, gas, or flowing liquid electrolytes). Bulk storage tends to reduce the system cost at large scale, because adding more storage only requires a larger tank or reservoir, which enables economy of scale. Cellular batteries replicate modules to add more energy, so cost reduction relies on manufacturing economy, with modest scale effects.