Integrated energy systems – unleashing the flexibility between heat and power

Abstract

The transition of the whole energy system from the one relying on fossil fuels to the one extensively using renewable energy, requires integration of all available energy sectors into a single integrated energy system (IES) in a holistic way. Optimal deployment of available coupling technologies, e.g. combined heat and power (CHP) units and power-to-heat (P2H) units, can significantly contribute to the operation of an IES. Such an IES can achieve synergy between different energy sectors and obtain a sustainable, cost-effective, flexible and reliable energy system. However, the transition to the high integration of renewable energy sources into the existing system, as well as development of low-temperature district heating (LTDH) causes generation-load imbalance and high energy losses related problems for both the electricity and heating sectors. These problems can negatively influence reliable, secure and profitable system operation. In this context, flexibility at generation and demand sides can be exploited to address the challenges mentioned above. In parallel with the evident development of coupling technologies, such as CHP units, as well as P2H units, the flexibility provided for heating and electricity sectors can be achieved through optimal operation and control of the coupling units locating at both generation and demand sides of the electricity and heating sectors. From the operation perspective, in the past very limited attention has been paid to approaches linking mechanisms for flexibility provision from one hand and their application in an IES constructed of heating and electricity sectors, from another hand. Therefore, new approaches are needed to 1) schedule the operation of generation-side CHP and P2H units to optimally provide flexibility for integrated heating and electricity sectors and 2) assess the techno-economic performance of demand-side P2H units to provide flexibility for the district heating (DH) sector through the design of new control algorithms for controlling heat load. Hence, this Thesis is focused on flexibility provision for DH and electricity sectors during short-term operation, considering availability and involvement of the following coupling technologies: (1) generation-side large-scale extraction steam turbine CHP unit and backpressure steam turbine CHP unit with bypass operation, (2) generation-side heat accumulators (HAs) and heat pumps (HPs) connected to 1) as a CHP plant, (3) demand-side P2H units particularly electric heat boosters (EHBs) in residential buildings and (4) developed information and communication technology (ICT) infrastructure that enables real-time monitoring and data sharing during the control of (3). Acknowledging these scopes and technologies, the Thesis is proposing three novel and innovative frameworks. The first two frameworks are for optimal scheduling the flexibility provision during the generation-side CHP plant operation. The third framework is for assessing the flexibility provision during demand-side P2H units control. Generation-side CHP plants are characterised by co-generation of heat and power to the IES. Their flexibility delivered to heating and electricity sectors during heat and power dispatch is connected. Moreover, the heat market mechanism requires a day-ahead heat and power dispatch to be planned before the actual real-time operation. Therefore, the real-time flexibility provision to provide balancing service to the electricity sector is limited by the day-ahead plan. In this Thesis, a novel two-stage operation framework is applied to a CHP plant to optimise both heat costs in day-ahead operation and real-time wind power balancing during the flexibility provision. Different technologies and operation modes of the CHP plant are taken into account to compare the performance with the above-mentioned operation framework observed from seasonal variations. Simulation results showed that both connections with HA unit and bypass operation can improve the CHP unit flexibility in different seasons. In view of the challenges during the day-ahead and real-time operation mentioned above, a novel mutlitimescale coordinated operation framework is proposed to optimise the flexibility provision of a CHP plant-wind farm portfolio in both day-ahead heat and real-time balancing markets. Considering multiple uncertainties of heat load, wind power generation, day-ahead, up-regulation and down-regulation electricity prices, here stochastic optimisation technique is used for solving the two-stage operation problems. Simulation results showed that the proposed framework can contribute to enhanced flexibility to reduce operation costs and to improve the operation reliability of the IES in question. In addition to the research related to generation-side coupling, the demand-side coupling is also considered by examining the operation of P2H units. A novel techno-economic assessment framework is developed to evaluate performance of flexibility provision by P2H units under the LTDH scheme, where the supply temperature of the DH networks is no more than 60 degrees. Within the framework, technical aspects of combined heat and power flow and economic aspects of levelized cost of EHBs are analysed from the perspective of the two control algorithms used for EHBs. Simulation results and sensitivity analysis showed that the fuel shift control can significantly contribute to improved flexibility by reducing DH peak load, energy cost of the IES and levelized cost of EHBs. However, it was concluded that the energy losses on electricity networks were slightly compensated. This Thesis is investigating the broader topic of IESs, in the context of flexibility provision for both electricity and heating sectors through optimal operation of coupling technologies of generation-side CHP and P2H units, as well as control of demand-side P2H units. It was concluded that the proposed novel operation and assessment frameworks for flexibility provision are feasible and can be utilised for real-time operation purposes. These frameworks directly contribute to a more sustainable, cost-effective, flexible and reliable operation of an IES constructed of heat and power sub-system.