[en] Highlights: • PV + storage can reduce CO₂ emissions while lowering cost of abatement. • Storage optimal power rating seems to be lower than 25% of PV capacity. • Storage with low energy-to-power ratio is cost effective with lower PV shares. • PV + storage systems would achieve breakeven point with expected costs projections. • Optimal configurations can reduce carbon emissions in the DEC/DEP system up to ~57%. This study explores the performance of the Duke Energy Carolinas/Progress (DEC/DEP) electric power system under one hundred forty-one configurations combining different capacities of utility-scale photovoltaics (PV) and battery energy storage (lithium-ion batteries or BES). The different configurations include PV installations capable of providing 5–25% of the systems energy and batteries with varying duration (energy-to-power ratio) of 2, 4, and 6 h. A production cost model comprised of a day-ahead unit commitment and a real-time economic dispatch simulates the optimal operation of all the generation resources necessary to supply hourly demand and reserve requirements during the year 2016. The model represents in detail the generation fleet of the system, including 221 nuclear, natural gas, coal and hydro power generators with a combined installed capacity of 37.8 GW. Results indicate that: 1) adding BES to a power system that includes PV further reduces carbon dioxide emissions while also lowering the cost of carbon abatement. 2) The optimal power rating of a BES system that supports PV seems to be lower than 25% of the capacity of the PV. 3) BES of short duration (2-h) are more cost-effective (i.e., result in a lower cost of abatement) when the level of PV penetration is low (lower than ~12.5%), while BES of longer duration (6-h) are more cost-effective when there are larger shares of PV. 4) The installation of optimal configurations of PV + BES to reduce carbon emissions in the DEC/DEP system by ~14–57% would increase the levelized cost of electricity (LCOE) ~8–65%. 5) If projections of declining costs for the next decade materialize, the installation of up to 15 GW of PV + 1.88 GW / 3.76 GWh of BES would reduce the LCOE while achieving up to 33% reduction in carbon emissions.

[en] Highlights: • Study assesses renewables and storage synergy in China’s coal dominated system. • Renewables increase flexibility needs, reduce coal units’ generation and efficiency. • Storage improves coal units’ performance by reducing start-ups and partial loading. • Energy storage alone reduces system’s coal use, costs (2.8%), CO₂ emissions (1%). • Paired with renewables storage reduces system’s costs (8.1%) and emissions (6.5%). Variable renewable energy (VRE) and energy storage systems (ESS) are essential pillars of any strategy to decarbonize power systems. However, there are still questions about the effects of their interaction in systems where coal’s electricity generation share is large. Some studies have shown that in the absence of significant VRE capacity ESS can increase CO₂ emissions. This paper shows that contrary to this intuition, ESS reduces operational costs and emissions even without higher penetration of VRE in power systems with large shares of coal. It also shows that when combined with VRE, ESS delivers higher benefits. These findings are based on the examination of China Southern Power Grid under seven VRE and ESS penetration scenarios. Results show that at the 2018 penetration levels, ESS alone reduced operational costs by 2.8% and CO₂ emissions by 1% and that by being paired with VRE, these reductions increased to 8.1% and 6.5%, respectively. The results clarify the synergy between ESS and VRE and explain the underlying mechanism. While VRE lowers coal units’ economic efficiency and environmental performance (measured in RMB/MWh and kg CO₂/MWh), ESS offsets this effect by increasing large coal units’ power generation and improving their efficiency. ESS reduces coal consumption and CO₂ emissions by substituting power generation from low-efficiency coal units with electricity from high-efficiency units and allowing them to operate at levels closer to full capacity and avoid start-ups.

[en] Highlights: • A power system with hydro, wind, and PV is partitioned into local/global clusters. • Clusters are formed according to the regulating ability and proximity. • A multi-objective function is employed to maximize power and minimize fluctuation. • Local and global complementary hydropower sequentially smooth output fluctuation. • The discharge fluctuation is lower for global complementary hydropower than for local. -- Abstract: Hydropower can be used to smooth out the output fluctuations from power systems including wind and solar sources. This paper proposes a double-layer model for coordinating the operations of cascaded hydropower and neighboring wind and photovoltaic (PV) facilities. In a local complementary model, hydropower with less regulating ability (local complementary hydropower, LCH) is dispatched to alleviate short-term fluctuations (i.e., hourly variations), whereas in the global one, hydropower with greater regulating capacity (global complementary hydropower, GCH) is operated to alleviate long-term fluctuations (i.e., intraday peak-valley differences). The results of a case study of a hydro-wind-PV cluster project in southwestern China show that most (90%) short-term fluctuations can be smoothed by LCH with a minimum impact on the capacity factor (−0.02). In contrast, alleviation of long-term fluctuations can be achieved with GCH, but this causes a large capacity factor reduction (−0.25). The discharge fluctuation of GCH is notably lower (70–75%) than that of LCH. In this system (which includes 5200 MW wind and 1400 MW PV), the output fluctuations can be eliminated if hydropower capacity reaches 2200 MW. These findings demonstrate that clustering of hydropower cascades with wind and solar generation facilities is a promising avenue for the decarbonization of electricity systems.

[en] Highlights: • Reservoir refilling procedure is imposed on multienergy integration. • Target water level is meet by uniformly adjusting hydropower output. • A multiobjective function is employed to maximize power and minimize fluctuation. • Optimal integration is in refilling stage when output is less than maximum capacity. • Integration should be discouraged in filled stage for ecological and economic issues. -- Abstract: Hydropower facilities are an ideal solution to complement the intermittent production of energy from wind and solar photovoltaic facilities in electric power systems. However, adding this task to the multiple diverse duties of a reservoir (e.g., flood mitigation, water supply, and power generation) poses a challenge related to pursuing multiple and sometimes conflicting objectives. This study proposes an approach for integrating hydro, wind, and photovoltaic power during a reservoir’s refill period. Specifically, this approach simultaneously minimizes the fluctuation in the combined power output of these three resources and maximizes their combined power generation while adhering to the target reservoir’s water levels. The proposed approach uses a multiobjective optimization model that prescribes a day-ahead optimal hourly operation for a hydropower facility in terms of spilled water, water stored in the reservoir, and water used for power generation, while meeting a daily target to refill the reservoir. The prescribed scheduling is then used as the input into a model that simulates the actual operations of the power system. This study focuses on a hydro-wind-photovoltaic system located in southwestern China, where the peak power generating capacity of the hydropower facility is ten percent larger than the combined installed capacity of the wind and solar power. The results show that by using the proposed model, the hydropower facility effectively smooths the fluctuations in the combined power output caused by variable wind and photovoltaic power and concurrently meets the reservoir replenishing targets under dry, moderate, or wet hydrologic scenarios. Furthermore, the trade-offs between power generation maximization and power fluctuation reduction were found to depend on two conditions: whether the reservoir is full, and whether the turbine is generating electricity at its maximum capacity. The hydro-wind-photovoltaic integration is more cost-effective when the reservoir is not full and the turbines are not generating electricity at their maximum capacity. When the reservoir is full, hydropower still has the ability to balance the wind and photovoltaic power without curtailment but tends to result in water spillage (22–402 m³/s) and reductions in electricity generation (0.1–11.4 GWh per day). The proposed method for scheduling operations allows hydropower facilities to complement wind and photovoltaic power output, while meeting the target water levels during the refill period.