| 1. |
- Zhang, Yichi, 1995
(författare)
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Applicability of thermal energy storage in future district heating system - Design methodologies and performance evaluations
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- District heating (DH) enables efficient and economical utilization of energy resources to satisfy the heat and hot water demands in buildings and is, thereby, well-established in Northern European countries. To achieve the future renewable energy system, the current DH systems are proved to undergo transitions towards the future DH systems, with major characteristics including renewable-based heat sources, low temperature networks, lower heating demands and smart controls. An important step is the coordination of heating and electricity sectors to achieve synergies and optimal solutions for the overall energy system, which is also known as the smart energy systems. Such goal could be achieved in a cost-effective manner by the flexibilities added from short-term thermal energy storage (TES) technologies. Despite the importance of TES has been demonstrated in previous studies, giving drastic changes compared to the current systems, the practical applicability of TES in the future DH systems remains unknown. The proposed benefits of TES might deviate from expectation considering the future characteristics, such as the low storage temperature levels and short space-heating period. Furthermore, the current studies about the TES applications have mostly focused on specific case studies. The findings are of limited applicability because they cannot be easily generalized and extrapolated to other future conditions. To explore the practical challenges and optimal applications of short-term TES units in the future, a systematic design framework that considers the diverse factors from top-level targets to bottom-level implementations is developed in this study. The top-level theoretical analysis method is developed to identify the load shifting potentials and associated storage capacities for the whole energy system, by comparing and matching energy supply and demand profiles. Compared to current bottom-up detailed system models, the proposed method requires only the energy profiles, which has resulted in much shorter analysis time. The method is further validated by complex system models, and because a good agreement has been achieved, it can be applied in various scenarios to efficiently pre-study the storage potentials. Then, the design of the practical TES capacity is derived from the theoretical result by considering performance indicators during realistic operations, such as power-to-heat conversion efficiency and heat loss efficiency. On bottom-level implementations, four typical short-term TES technologies were investigated including central water tank (CWT), district heating network inertia (DHNI), domestic hot water tank (DHWT), and building thermal mass (BTM). For this purpose, an integrated bottom-level model to simulate the operation dynamics of the district heating systems and to optimize the use of the TES units is developed. Techno-economic analysis and comparisons of TES technologies were performed on a variety of scenarios, which are representatives of the main characteristics of the current middle-temperature district heating system and future low-temperature district heating system. The changes in the source side, transportation networks and end-use building demands are considered. As a result, a performance map of the TES technologies indicating the strong links between the system characteristics and optimal TES applications has been identified. Based on that, the optimal combinations of TES technologies were proposed for a LTDH system. Consequently, combining this with top-level methods, the overall potentials and roles of short-term TES were identified by a systematic design framework.
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| 2. |
- Zhang, Yichi, 1995, et al.
(författare)
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Applicability of thermal energy storage in future low-temperature district heating systems – Case study using multi-scenario analysis
- 2021
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Ingår i: Energy Conversion and Management. - : Elsevier BV. - 0196-8904. ; 244
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Tidskriftsartikel (refereegranskat)abstract
- With the flexibilities added from thermal energy storage (TES) technologies, low temperature district heating (LTDH) system can coordinate the heat and electricity sectors in a cost-effective manner. Such combinations have therefore become an important step to achieve a 100% renewable energy system. Despite the importance of TES has been demonstrated in previous studies, giving drastic changes compared to the current systems, the practical applicability of TES in the LTDH systems remains unknown. Furthermore, the proposed benefits of TES might deviate from the expectations considering the development of future characteristics, such as the low temperature levels and small space-heating demand. This study investigates the performances and benefits of four typical short-term TES technologies, including the use of central water tank (CWT), district heating network inertia, domestic hot water tank (DHWT), and building thermal mass, based on a case LTDH system in Roskilde, Denmark. Techno-economic analysis is conducted on a variety of scenarios, based on future changes in operation of the heat sources to the end-users. An integrated model is also developed to simulate the operation dynamics of the district heating system with regards to optimizing the use of the TES units. This study provides a performance map of the TES technologies in accordance with the transitions from current to future LTDH systems, indicating the relationships between the system characteristics and optimal TES applications. The CWT is found to be most preferable for integrating the variable renewable energy due to its ability to store heat for long periods. In the end-use side, with the improved building performances and reduced space heating demand in the future, there is less potential for the use of building inertia. In contrary, the benefit of the DHWT, which mainly comes from the reduction of bypass loss during the non-space-heating period, is increased in the future. Furthermore, raising the network temperatures for active storage is found to be infeasible under all future LTDH scenarios because this measure significantly influences the heat source efficiency.
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| 3. |
- Zhang, Yichi, 1995, et al.
(författare)
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Assessment of district heating and cooling systems transition with respect to future changes in demand profiles and renewable energy supplies
- 2022
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Ingår i: Energy Conversion and Management. - : Elsevier BV. - 0196-8904. ; 268
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Tidskriftsartikel (refereegranskat)abstract
- District energy systems are about to shift towards closer temperature configurations, i.e. low-temperature district heating and high-temperature district cooling. Challenges and benefits of these transitions are mostly analyzed from a perspective of current energy demand and supply scenarios while the influence from future changes in these domains remains unknown. Based on a representative residential community in the Nordic district heating context, centralized district heating and cooling (DHC), ultra-low temperature district heating (ULTDH), and bi-directional fifth generation 5GDHC systems were assessed from technical, economical, and environmental aspects. Moreover, the applications of thermal energy storage (TES) and their roles in the future DHC systems were also investigated. The assessment was done by a generalized methodology framework, integrating the future changes, multiple operation scenarios modellings and system design optimizations. Results suggest that in the future low-energy building stock, the increased cooling demand makes the 5GDHC system the most economically attractive choice. In the supply side, with a 50% share of wind power in the future national grid, the electricity prices can make 5GDHC and ULTDHC either cost-saving or more expensive compared to the central DHC system dependent on if nuclear plants are decommissioned or not. Besides, with increasing power production from VRE, the limited application of TES for active shift of electricity demand is found when a system’s heat-to-power ratio is high. The methodology framework can be applied to similar systems to increase the understandings on system transitions.
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| 4. |
- Zhang, Yichi, 1995, et al.
(författare)
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Feasibilities of utilizing thermal inertia of district heating networks to improve system flexibility
- 2022
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311. ; 213
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Tidskriftsartikel (refereegranskat)abstract
- The use of district heating network inertia (DHNI) has been regarded as an efficient and cost-saving method to improve the flexibility of the energy systems. However, the acclaimed benefits in most studies are found in specific cases with traditional middle-temperature systems. The cases where DHNI is not feasible are unclear and the applicability of DHNI in future low-temperature district heating (LTDH) systems requires further investigations. Therefore, this study applied a top-down methodology where the practical storage potentials of DH networks are evaluated based on field investigations of 134 Swedish DH networks and 25 Chinese DH networks with various sizes and demand densities. Empirical relationships between the heat density and storage potentials are established and analyzed. Then, bottom-level analysis from technical and economical aspects are conducted on a variety of application scenarios for DHNI, including different temperature levels, heat sources, control strategies and renewable energy profiles. It is found that in LTDH system, by raising the network temperatures to actively use the DHNI, the heat source efficiency is reduced regardless the size and density of the network and, thereby, making the DHNI infeasible. This implies that the DHNI is only applicable in middle-temperature systems with combined heat and power plant (CHP) as a heat source in the extraction mode. Furthermore, the back-pressure mode is not economically attractive. In summary, the results from a multi-scenario analysis identified limited benefits of the DHNI, implying a proper consideration of its roles in future works.
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| 5. |
- Zhang, Yichi, 1995, et al.
(författare)
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Quantification of overlapping heating and cooling demand for the feasibility assessment of bi-directional systems over Europe
- 2023
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Ingår i: Energy and Buildings. - 0378-7788. ; 294
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Tidskriftsartikel (refereegranskat)abstract
- With warmer climate conditions, the growing need to cool down building environment in Europe calls for efficient solutions to solve the heating and cooling demand altogether. To meet such requirement, the bi-directional fifth generation district heating and cooling (5GDHC) system has been proposed in recent years, while having the capability of integrating the renewable and waste energy sources. The overlapping heating and cooling demand, or referred to as the simultaneous demand, is proved as the key pre-requisite to guide the application of 5GDHC through several case studies. However, how large is this overlapping part in the practical building stock and where are the feasible areas for 5GDHC remain unclear. To close the gap, this study assesses the quantities and geographical distributions of overlapping heating and cooling demand across Europe in EU 27, UK, and EFTA countries. Using GIS-based methods and data sources, the study region is split into 28 million hectare-sized units where the overlapping demand is specifically calculated. Moreover, the possible future changes to the heating and cooling demand alone brought by warmer climate, building renovations, and increased cooling area were investigated. The results from reference condition at year 2016 reveal that less than 0.1% of the building stock has DOC larger than 0.3, which is the threshold for 5GDHC being energy efficient. These potential areas are primarily found in city centres involving cooling demands from commercial and industrial processes. In the future scenarios of year 2050, while a better energy performance of buildings and warmer climate may decrease the heating and increase the cooling demand, the overlapping part is only slightly increased by around 5%. Accordingly, around 2500 ha-sized units from the entire study region are found to meet the DOC limit required by the 5GDHC while having large enough demand density to support district energy systems. The presented works geographically identified the potential areas for 5GDHC and can be extended with regional details to serve as the connection between top-level planning and bottom-level case studies.
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| 6. |
- Zhang, Yichi, 1995, et al.
(författare)
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Roadmaps for heating and cooling system transitions seen through uncertainty and sensitivity analysis
- 2023
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Ingår i: Energy Conversion and Management. - 0196-8904. ; 292
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Tidskriftsartikel (refereegranskat)abstract
- Future district heating systems should enable efficient and economic energy supply, which can be achieved by lowering the system temperatures and boosting it at demand-side. Current solutions include the ultra-low-temperature district heating (ULTDH) and fifth generation district heating and cooling (5GDHC) systems. The transition towards these systems is subject to multiple future uncertainties such as the energy price, investment cost, and demand changes, which were missing in previous works. To investigate the effects of these uncertainties on conclusions brought by established design roadmaps for future DHCs, a five-step framework, which combines the energy system optimization with stochastic simulations, uncertainty analysis and sensitivity assessment, is developed in this study. The framework is applied on a hypothetical 0.25 km2 square district with varying uncertain parameters. Based on stochastic cases, the index named cost-saving probability (CSP) is utilized to reflect the potential of being economic attractive when comparing the energy systems. For the transition towards the ULTDHC, 5GDHC, and individual systems, the most sensitive factors for the CSP are the area demand density, overlapping heating and cooling demand, and linear demand density, respectively. The investment in thermal energy storage (TES) becomes important only when the integration of a larger share of renewable energy is targeted. A roadmap summarizing the promoting and hindering factors for the system transition is provided, pointing out the future focus area for DHC design. The results from the sensitivity analysis also revealed the limited role of TES in integrating variable renewable energy in high-efficiency DHC systems.
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| 7. |
- Zhang, Yichi, 1995, et al.
(författare)
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Techno-economic assessment of thermal energy storage technologies for demand-side management in low-temperature individual heating systems
- 2021
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Ingår i: Energy. - : Elsevier BV. - 0360-5442. ; 236
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Tidskriftsartikel (refereegranskat)abstract
- The combined use of thermal energy storage (TES) technologies and heat pumps in building energy systems has been approved to achieve demand-side management. Although there is an increasing number of case studies about the TES applications, crosswise techno-economic evaluations of different technologies are rare, especially for applications in individual heating systems where the storage temperature range is less than 20 K. Hence, in this study, three TES options; water tank (WT), phase change material tank, and building thermal mass (BTM) are simulated and compared. A systematic analysis approach was proposed to assure impartial comparisons of the energy performance and the life-cycle costs (LCC). Special focus was paid on practical issues such as restricted charging power for different TES technologies. It was found that the majority of LCC savings arises from the peak load reduction. The study also shows that BTM is the most cost-effective TES technology while the WT is the least attractive option, due to larger heat loss and lower storage density. Moreover, less discharged energy and cost savings were found in well-insulated buildings due to the restricted discharging power. Still, there could be more incentives for household TES technologies if additional prices or policies are implemented.
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| 8. |
- Zhang, Yichi, 1995
(författare)
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Transition pathways for future district heating and cooling systems with thermal energy storage
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Buildings’ heating and cooling account for more than 20% of the final energy use within the European countries and are dominated by non-renewable resources. Future district energy systems should enable efficient, fossil-free, and economical energy supply at operating temperatures that end users can directly utilize. This can be achieved by lowering the system temperatures and boosting them on the demand side to increase the overall system efficiency. Ultralow-temperature district heating (ULTDH) and bidirectional fifth-generation district heating and cooling (5GDHC) systems are the solutions. However, the transition of district heating and cooling (DHC) systems from current high-temperature configurations to the future solutions is subject to several uncertainties and challenges, such as energy prices, investment costs, thermal energy storage (TES) distribution, and demand profiles. The variations in these uncertainties were not considered in previous studies. Most of the earlier studies only discussed current perspectives, leaving the future applicability of the DHC system unknown. Hence, a generalized methodological framework combining energy system optimization with stochastic simulations, uncertainty analysis, and sensitivity assessment is developed in this study to investigate the effects of these uncertainties. Based on a variety of stochastic cases, the index named cost-saving probability (CSP) is utilized to reflect the potential of being economic attractive when comparing the energy systems. The preferred future conditions for different DHC systems are summarized in the roadmaps via proposed key performance indicators (KPIs), indicating a future promising area for DHC design. Meanwhile, the applications and roles of TES in future DHC systems were investigated. Furthermore, combined with the geographical information system-based methodologies and data sources, the proposed KPIs for the entire European building stock were calculated at the hectare level to identify the potential areas of 5GDHC. The results reveal considerable differences between the systems as different design and operation objectives on least cost and imported electricity are set. The most sensitive factors of the CSP are area demand density, overlapping heating and cooling demand, and linear demand density for the transition to ULTDHC, 5GDHC, and individual systems, respectively. The roadmap also shows the hindering factors for different transitions, as well as the impact of the objective on imported electricity. Besides, the sensitivity analysis results reveal TES’s limited role in integrating variable renewable energy (RE) in high-efficiency DHC systems. In addition, less than 0.1% of the current European building stock has sufficient overlapping heating and cooling demands to efficiently implement 5GDHC. These potential areas are primarily found in city centres involving cooling demands from commercial and industrial processes. While a better energy performance of buildings and warmer climate in the future may decrease the heating and increase the cooling demand, the overlapping part is only slightly increased by around 4%, leading to limited additional application potentials of 5GDHC.
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