Improving Operational Flexibility of Integrated Energy Systems Through Operating Conditions Optimization of CHP Units

As a type of energy system with bright application prospects,the integrated energy system(IES)is environmentally friendly and can improve overall energy efficiency.Tight coupling between heat and electricity outputs of combined heat and power(CHP)units limits IES operational flexibility significantly.To resolve this problem,in this paper,we integrate operating mode optimization of the natural gas combined cycle CHP unit(NGCC-CHP)into dispatch of the IES to improve flexibility of the IES.First,we analyze operational modes of the CHP units from the perspectives of thermal processes and physical mechanisms,including the adjustable extraction mode,backpressure mode,and switching mode.Next,we propose an explicit mathematical model for full-mode operation of the CHP units,in which the heat-electricity feasible region,switching constraints,and switching costs are all formulated in detail.Finally,a novel economic dispatch model is proposed for a heat and electricity IES,which uses the full-mode operation of CHP units to improve operational flexibility.The Fortuny-Amat transformation is used to convert the economic dispatch model into a mixed-integer quadratic programming model,which can then be solved using commercial solvers.Case studies demonstrate the proposed method can reduce operational costs and obviously promotes wind power utilization.

Power system planning with high renewable energy penetration considering demand response

Electric system planning with high variable renewable energy(VRE)penetration levels has attracted great attention world-wide.Electricity production of VRE highly depends on the weather conditions and thus involves large variability,uncertainty,and low-capacity credit.This gives rise to significant challenges for power system planning.Currently,many solutions are proposed to address the issue of operational flexibility inadequacy,including flexibility retrofit of thermal units,inter-regional transmission,electricity energy storage,and demand response(DR).Evidently,the performance and the cost of various solutions are different.It is relevant to explore the optimal portfolio to satisfy the flexibility requirement for a renewable dominated system and the role of each flexibility source.In this study,the value of diverse DR flexibilities was examined and a stochastic investment planning model considering DR is proposed.Two types of DRs,namely interrupted DR and transferred DR,were modeled.Chronological load and renewable generation curves with 8760 hours within a whole year were reduced to 4 weekly scenarios to accelerate the optimization.Clustered unit commitment constraints for accommodating variability of renewables were incorporated.Case studies based on IEEE RTS-96 system are reported to demonstrate the effectiveness of the proposed method and the DR potential to avoid energy storage investment.

Increasing Coal-Fired Power Plant Operational Flexibility by Integrating Solar Thermal Energy and Compressed Air Energy Storage System

This paper proposed a novel integrated system with solar energy,thermal energy storage(TES),coal-fired power plant(CFPP),and compressed air energy storage(CAES)system to improve the operational flexibility of the CFPP.A portion of the solar energy is adopted for preheating the boiler’s feedwater,and another portion is stored in the TES for the CAES discharging process.Condensate water from the CFPP condenser is used for cooling compressed air during the CAES charging process.The thermodynamic performance of the integrated system under different load conditions is studied.The system operations in a typical day are simulated with EBSILON software.The system enables daily coal saving of 9.88 t and reduces CO_(2)emission by 27.95 t compared with the original CFPP at 100%load.Under partial load conditions,the system enables maximum coal saving of 10.29 t and maximum CO_(2)emission reduction of 29.11 t at 75%load.The system has maximum peak shaving depth of 9.42%under 40%load condition.The potential of the system participating ancillary service is also discussed.It is found that the integration of solar thermal system and CAES system can bring significant ancillary service revenue to a conventional CFPP.

Economic Dispatch Considering Hourly Capacity Allocation with a Variable Renewable and Hydro-Based Generation Portfolio

The objective of this paper is to assess an economic dispatch considering a power system portfolio, which includes predominant amount of hydro power and increasing quantities of intermittent renewables in relation to the total electric capacity. With growing importance of intermittent wind and solar generation taking part into power systems worldwide, there is need for greater chronological resolution to estimate the flexibility of the power system to offer firm capacity. In this way, a linear optimization model operating hourly is developed to calculate the minimum power system cost, while stablishing the capacity allocation to meet the projected load throughout one-year simulation, as an estimation of how the hourly economic dispatch impacts the scheduling of generators belonging to a power system with this portfolio composition. A central focus is how to operate the available hydro capacity to back up intermittent renewables, evaluating the physical hydro operating constraints, monthly energy balance and maximum power availability. A case study was simulated based on the Brazil’s power system configuration, showing that existing hydro capacity provide hourly flexibility to back-up intermittent renewables, potentially saving 1.2 Billion R$, about 3.6% of total system cost referred to 2019. It is worthwhile to realize that the developed methodology can be employed to other power systems with similar capacity portfolio structure for the purpose of calculating its optimum allocation for a specified region and target year.

CONSTRUCTION AND MODIFICATION OF FLEXIBLE FEATURE-BASED MODELS

A new approach is proposed to generate flexible featrure based models (FFBM), which can be modified dynamically. BRep/CSFG/FRG hybrid scheme is used to describe FFBM, in which BRep explicitly defines the model, CSFG (Constructive solid feature geometry) tree records the feature based modelling procedure and FRG (Feature relation graph) reflects different knids of relationship among features. Topological operators with local retrievability are designed to implement feature addition, which is traced by topological operation list (TOL) in detail. As a result, FFBM can be modified directly in the system database. Related features' chain reactions and variable topologies are supported in design modification, after which the product information adhering on features will not be lost. Further, a feature can be modified as rapidly as it was added.

New Hybrid CHP System Integrating Solar Energy and Exhaust Heat Thermochemical Synergistic Conversion with Dual-Source Energy Storage

For the efficient use of solar and fuels and to improve the supply-demand matching performance in combined heat and power(CHP)systems,this paper proposes a hybrid solar/methanol energy system integrating solar/exhaust thermochemical and thermal energy storage.The proposed system includes parabolic trough solar collectors(PTSC),a thermochemical reactor,an internal combustion engine(ICE),and hybrid storage of thermal and chemical energy,which uses solar energy and methanol fuel as input and outputs power and heat.With methanol thermochemical decomposition reaction,mid-and-low temperature solar heat and exhaust heat are upgraded to chemical energy for efficient power generation.The thermal energy storage(TES)stores surplus thermal energy,acting as a backup source to produce heat without emitting CO_(2).Due to the energy storage,time-varying solar energy can be used steadily and efficiently;considerable supply-demand mismatches can be avoided,and the operational flexibility is improved.Under the design condition,the overall energy efficiency,exergy efficiency,and net solar-to-electric efficiency achieve 72.09%,37.65%,and 24.63%,respectively.The fuel saving rate(FSR)and the CO_(2) emission reduction(ER_(CO_(2)))achieve 32.97%and 25.33%,respectively.The research findings provide a promising approach for the efficient and flexible use of solar energy and fuels for combined heat and power.

Guest editorial:special section on energy storage systems and operational flexibility in power systems

The energy storage system(ESS) is becoming an important component in power systems to mitigate the adverse impact of intermittent renewable energy resources and improve power grid reliability and efficiency.However,storage devices driven by different technologies can have specific grid impacts.This special section is dedicated to reflecting the

Synergetic process between wind power and sewage sludge pyrolysis in a novel renewable-energy microgrid system

To relieve the stress of sewage sludge(SS)disposal and effectively increase the use of renewable energy,a novel renewable-energy microgrid system(REMS)was developed,specifically designed to integrate a wind power plant(WPP)with energy storage and the SS pyrolysis process for heat and power generation.Based on a lab-scale pyrolysis experiment and 7-day numerical analysis,we studied the energy-recycling potential of SS and simulated the operational behaviours of REMS.According to the results,the calorific values of the pyrolytic gaseous and liquid products were better than those of the raw material,at 16.19 and 33.53 MJ/kg,respectively.The proposed REMS performed well in power supply and energy utilization with a design performance index of 99.23 when the WPP capacity was 200 MWe and the initial wind-energy curtailment rate was 30%.It indicates that by converting SS into flammable gas,condensable liquid and carbon-rich solid residue,curtailed wind energy could be saved and the synergy between wind power and the SS pyrolysis process enabled the proposed microgrid system to effectively utilize renewable energy and provide reliable on-demand power service.The REMS installed with a 155-MWe WPP achieved the optimal design in system performance,environmental benefit and construction cost under the initial wind-curtailment rate of 34.12%.The design scheme makes REMS capable of satisfying the 15.10-GWh power demand of end users and the 1700-t/day SS disposal need,and the curtailed wind energy could be reduced to zero.

Improved Market Mechanism for Energy Storage Based on Flexible State of Energy

In this paper,a new operational mode is proposed for energy storage,in which an improved semi-centralized mechanism is proposed for energy storage to participate in the day-ahead energy market.The new operational mode,i.e.,the flexible state-of-energy(SOE)mode,is proposed based on the previous fixed SOE mode,under which the final SOE of energy storage at the end of the last period of the scheduling horizon is not limited to a predefined value.Accordingly,the value of the SOE is introduced to quantify the deviation cost of the final SOE from the predefined value.Under the proposed market mechanism,energy storage submits to the system operator the unit charging and discharging costs and the value of the SOE.The system operator dispatches the charging and discharging power of energy storage according to the data submitted and system operations.A comparative analysis is conducted in the case studies,and results demonstrate that the proposed mechanism is efficient in further realizing the flexibility potential of energy storage and reducing the total cost of the power system.

Flexible Operation Mode of Coal-fired Power Unit Coupling with Heat Storage of Extracted Reheat Steam

In order to provide more grid space for the renewable energy power,the traditional coal-fired power unit should be operated flexibility,especially achieved the deep peak shaving capacity.In this paper,a new scheme using the reheat steam extraction is proposed to further reduce the load far below 50%rated power.Two flexible operation modes of increasing power output mode and reducing fuel mode are proposed in heat discharging process.A 600 MW coal-fired power unit with 50%rated power is chosen as the research model.The results show that the power output is decreased from 300.03 MW to 210.07 MW when the extracted reheat steam flow rate is 270.70 t·h^(-1),which increases the deep peak shaving capacity by 15%rated power.The deep peak shaving time and the thermal efficiency are 7.63 h·d^(-1)and 36.91%respectively for the increasing power output mode,and they are 7.24 h·d^(-1)and 36.58%respectively for the reducing fuel mode.The increasing power output mode has the advantages of higher deep peak shaving time and the thermal efficiency,which is recommended as the preferred scheme for the flexible operation of the coal-fired power unit.

District heating system operation in power systems with high share of wind power

The integration of continuously varying and not easily predictable wind power generation is affecting the stability of the power system and leads to increasing demand for balancing services.In this study,a short-term operation model of a district heating system is proposed to optimally schedule the production of both heat and power in a system with high wind power penetration.The application of the model in a case study system shows the increased flexibility offered by the coordination of power generation,consumption and heat storage units which are available in district heating systems.

A four-dimensional interaction-based appraisal approach towards the performance enhancement of a vehicular waste heat recovery system

The non-linear multifactorial impacts on fuel-saving potential constrain the practical performance of the vehicular waste heat recovery system(WHRS). This study proposed a four-dimensional interaction-based appraisal approach to interpreting these impacts for enhancing WHRS's in-vehicle performance. The interaction incorporates a heat exchanger, configuration, engine,and vehicle. The proposed approach comprises two successive steps, emphasizing evaluation under the rated(Step 1) and off-design(Step 2) heat source conditions. A case study of waste heat recovery from a passenger vehicle was conducted to evaluate the in-vehicle performance of a novel co-split system and two single-split ones(with/without a regenerator) through this approach. The novel system theoretically modifies vehicular performance but remains ambiguous concerning real-world behaviour, which is assessed and verified by the proposed approach. Two key factors determining vehicular performance were identified by Step 1, namely, net power output and engine backpressure. As the co-split system modified both factors, its fuel-saving potential could be increased by up to 20.3% compared with single-split systems. Also, the limiting factor for off-design performance was pinpointed by Step 2, namely, the mismatch between the heat source and working fluid, which led to the solution, i.e., the synergistic split regulation of the working fluid and heat source. An up to 8.8% improvement in net power output was achieved by the co-split system at off-design heat sources compared with fixed split ratios. Consequently, the approach enables holistic performance improvement of the vehicular WHRS under design/off-design heat source conditions.

The hybrid MPC-MINLP algorithm for optimal operation of coal-fired power plants with solvent based post-combustion CO_(2) capture

This paper presents an algorithm that combines model predictive control(MPC)with MINLP optimization and demonstrates its application for coal-fired power plants retrofitted with solvent based post-combustion CO_(2) capture(PCC)plant.The objective function of the optimization algorithm works at a primary level to maximize plant economic revenue while considering an optimal carbon capture profile.At a secondary level,the MPC algorithm is used to control the performance of the PCC plant.Two techno-economic scenarios based on fixed(capture rate is constant)and flexible(capture rate is variable)operation modes are developed using actual electricity prices(2011)with fixed carbon prices($AUD 5,25,50/tonne-CO_(2))for 24 h periods.Results show that fixed operation mode can bring about a ratio of net operating revenue deficit at an average of 6%against the superior flexible operation mode.