Life-cycle Coordination Of Large-scale Hydroelectric-photovoltaic Hybrid Energy Systems

Exploitation and utilization of renewable energy sources(RESs)is an important measure to guarantee future energy security and to address global climate change.However,because the RESs(e.g.,solar and wind)are intermittent,fluctuant,and stochastic,they are confronted with great difficulty in accommodation,resulting in serious curtailment problems.To cope with this issue,one promising way is to use flexible hydropower to complement those intermittent power sources and operate them as a hybrid energy system(HES).As the HES has higher dimensions,more uncertain inputs and more competitive scheduling objectives,it becomes extremely challenging to manage the HES safely and efficiently.In this context,how to enhance the synergy among different energy sources within a HES needs in-depth study.Based on China’s Longyangxia hydro–photovoltaic(PV)HES,we systematically study its planning and operation problems from four time-scales: real-time,short-term,medium-long-term,and life cycle.The main work is as follows:(1)A robust unit commitment model considering the uncertainty in forecasting PV power is presented for the economic dispatch problem of the hydro–PV HES.This model seeks robust solutions(i.e.,hydro unit status)under multiple PV scenarios to minimize the hydro plant’s water consumption when external load demands are imposed onto the hybrid system.A two-layer nested optimization framework is proposed to solve the model in a hierarchical structure.In the outer layer,evolutionary algorithms,combined with a twodimensional encoding strategy,optimize the unit’s on/off status that can meet the load demand under all PV scenarios.In the inner layer,load dispatch schemes for the given on/off status are determined by dynamic programming.Results show that the proposed methodologies could provide robust and effective generating schemes within a relatively short period.The generating scheme not only improves water use efficiency but coordinates PV power generation.(2)A bi-level programming model coupled with an economic dispatch module is established for the day-ahead generation scheduling problem of the HES.The upper model maximizes the hybrid’s energy production by optimizing its delivered power output while satisfying specified load characteristic(e.g.,bimodal pattern).The lower model minimizes the hydropower plant’s water consumption under the given delivered power output by optimizing the hydro unit status and load dispatch strategies,.To solve the model effectively,the original problem is first decomposed into three related sub-problems(i.e.,determining the hybrid’s delivered power output,determining the hydro unit status,and determining the load dispatch strategies).Then a heuristic algorithm,a cuckoo search algorithm,and dynamic programming are respectively used to solve these sub-problems in a hierarchical structure.Results show that the proposed methodologies can provide satisfactory generation schedules that simultaneously meet load and water demands within an acceptable time.(3)For the long-term complementary operation problem of the HES,a method coupled with a PV-energy loss function is proposed to optimize the HES’s operating rule curves.First,we analyze possible situations of energy curtailment in the joint operation of hydropower and PV power.A short-term simulation model is then established to estimate the amount of curtailed PV energy arising from specified long-term hydropower decisions,in which the relationships are represented as PV energy-loss functions.By incorporating the PV-energy loss function,a multi-objective optimization model for the long-term operation is constructed.Finally,a multi-objective cuckoo search algorithm is used to identify the optimal type and parameters of the operating rule curves.Results show that: the PV-energy loss function exhibits an S shape;maximizing the hybrid’s energy production and energy supply reliability are non-competitive objectives;and the optimal operating rules can achieve good balance between hydropower generation,PV integration,and downstream water supply in the long run.(4)An integrated sizing method based on a cost-benefit analysis and a long-short nested dispatching model is proposed to determine the optimal PV size within the HES.This method aims to maximize the net revenue of the PV plant over its life-span,while considering power demands of the wider electricity grid and the impact of complementary operation on the water system.To capture the hybrid’s operational performance,historical average sampling,sliding window sampling and Bootstrap sampling methods are adopted.Finally,the optimal PV size and the complementary dispatching functions are determined synchronously.Results show that: the optimal PV size is most sensitive to the feed-in tariff;the derived dispatching functions could effectively guide the long-term complementary operation of the hybrid system;and the proposed sizing method is valid and effective,which could coordinate the capacity planning and operations management of the PV plant.