A framework to identify Pareto‐efficient subdaily environmental flow constraints on hydropower reservoirs using a grid‐wide power dispatch model

Abstract

Hydrologic alteration due to hydropeaking reservoir operations is a main concern worldwide. Subdaily environmental flow constraints (ECs) on operations can be promising alternatives for mitigating negative impacts. However, those constraints reduce the flexibility of hydropower plants, potentially with higher costs for the power system. To study the economic and environmental efficiency of ECs, this work proposes a novel framework comprising four steps: (i) assessment of the current subdaily hydrologic alteration; (ii) formulation and implementation of a short-term, grid-wide hydrothermal coordination model; (iii) design of ECs in the form of maximum ramping rates (MRRs) and minimum flows (MIFs) for selected hydropower reservoirs; and (iv) identification of Pareto-efficient solutions in terms of grid-wide costs and the Richard-Baker flashiness index for subdaily hydrologic alteration (SDHA). The framework was applied to Chile's main power grid, assessing 25 EC cases, involving five MIFs and five MRRs. Each case was run for a dry, normal, and wet water year type. Three Pareto-efficient ECs are found, with remarkably small cost increase below 2% and a SDHA improvement between 28% and 90%. While the case involving the highest MIF worsens the flashiness of another basin, the other two have no negative effect on other basins and can be recommended for implementation.

Modeling the Multi-Seasonal Link between the Hydrodynamics of a Reservoir and Its Hydropower Plant Operation

“…This coupling is required because the Rapel hydropower plant is part of Chile's Central Interconnected Power System (SIC), organized as a mandatory pool with audited costs. Thus, the operation of all power plants in central Chile is prescribed by an independent system operator (ISO) following a cost-minimization criterion of the entire power system [5,12]. The following subsections detail this coupling.…”

“…MIP-UC minimizes the operational costs of the power system (composed of thermal, hydro and renewable power plants) while satisfying the energy demand of each node and a set of technical constraints. A detailed formulation of the MIP-UC model can be found in previous studies [12].…”

“…As the system possesses large water reservoirs (besides Rapel), MIP-UC uses an opportunity cost function of stored water (in terms of the stored water at the end of the simulation horizon) [12]. This future cost function is computed by the ISO using a stochastic medium-term optimization tool [27] and can be understood as the displaced future thermal costs.…”

“…The EC scenario is a combination of a minimum instream flow (MIF) and a maximum ramping rate of the turbine outflow (MRR). Based on the findings of [12] (which studied the trade-off between the cost of implementing environmental constraints and their effectiveness in improving sub-daily hydrologic alteration), we chose efficient values for our environmental constraints. This resulted in 20% and 6% of the median monthly inflow as MIF and MRR, respectively.…”

“…In response, several indexes for measuring the hourly fluctuation have been proposed in the last decade [35][36][37]. The Richard-Baker (R-B) index [35] for flow flashiness is one that has found numerous applications in the last five years [5,6,12,38,39]:…”