• Energy Research

Many industrialised nations recently concentrated their focus on hydrogen as a viable option for the decarbonisation of fossil-intensive sectors, including maritime transportation. A sustainable alternative to the conventional production of hydrogen based on fossil hydrocarbons is water electrolysis powered by renewable energy sources. This paper presents a detailed techno-economic optimisation model for sizing an electrolyser and a hydrogen storage embedded in a multi-domain virtual power plant to produce green hydrogen for seaborne passenger transportation. We base our numerical analysis on three years of historical data from a renewable-dominated 60/10kV substation on the Danish island of Bornholm, and on data for ferries to the mainland of Sweden. Our analysis shows that an electrolyser system serves as a valuable flexibility asset on the electrical demand side, while supporting the thermal management of the district heating system and contributing to meeting the ferries hydrogen demand. With a sized electrolyser of 9.63MW and a hydrogen storage of 1.45t, the hydrogen assets are able to take up a large share of the local excess electricity generation. The waste heat of the electrolyser delivers a significant share of 21.4% of the annual district heating demand. Moreover, the substation can supply 26% of the hydrogen demand of the ferries from local resources. We further examine the sensitivity of the asset sizing towards investment costs, electrolyser efficiency and hydrogen market prices.

Many industrialised nations recently concentrated their focus on hydrogen as a viable option for the decarbonisation of fossil-intensive sectors, including maritime transportation. A sustainable alternative to the conventional production of hydrogen based on fossil hydrocarbons is water electrolysis powered by renewable energy sources. This paper presents a detailed techno-economic optimisation model for sizing an electrolyser and a hydrogen storage embedded in a multi-domain virtual power plant to produce green hydrogen for seaborne passenger transportation. We base our numerical analysis on three years of historical data from a renewable-dominated 60/10kV substation on the Danish island of Bornholm, and on data for ferries to the mainland of Sweden. Our analysis shows that an electrolyser system serves as a valuable flexibility asset on the electrical demand side, while supporting the thermal management of the district heating system and contributing to meeting the ferries hydrogen demand. With a sized electrolyser of 9.63MW and a hydrogen storage of 1.45t, the hydrogen assets are able to take up a large share of the local excess electricity generation. The waste heat of the electrolyser delivers a significant share of 21.4% of the annual district heating demand. Moreover, the substation can supply 26% of the hydrogen demand of the ferries from local resources. We further examine the sensitivity of the asset sizing towards investment costs, electrolyser efficiency and hydrogen market prices.

With growing popularity of grid-connected battery energy storage systems (BESSs), operators require electrical models for optimal utilisation. These models should be provided by suppliers or manufacturers based on testing methods applied to individual cells or modules in specialised laboratories. However, operators are also interested in developing electrical models on their own. This paper explores the feasibility of modelling a grid-connected BESS without dismantling it, using only the data from its energy management system (EMS) and battery management system (BMS). The goal is to characterise a BESS directly on-site, controlling it through the available commands of its power converter system (PCS). The aim is to represent the electrical dynamics of the BESS with an equivalent Thevenin electric circuit composed of open circuit voltage, resistances, and capacitances. The overall usable capacity of the BESS and the efficiency of the PCS are also estimated. The subject of the investigation is a 300kW/652kWh Nickel-Manganese-Cobalt (NMC) Li-ion BESS composed of ten racks, each equipped with a PCS and ten battery modules. The analysis proves the feasibility of modelling the grid-connected BESS via data from EMS and BMS. An equivalent cell model is derived, with the open circuit voltage and internal impedance expressed for the entire state-of-charge range. The total resistance assumes values in the range of 1.580–2.424mΩ, whereas the total capacitance is 609.5–1,580kF. Consequently, the normalised total resistance is 3%–4%, aligned with the expectations from other NMC Li-ion cells. Finally, the energy capacity and PCS efficiency are reported as a function of the power of the PCS. The usable energy capacity per rack is approx. 59.7kWh, which is 91.5% of the rated DC energy capacity, and it is independent of the power at which the BESS is discharged. The PCS efficiency is above 94% when operating at 15% of the PCS rated power or higher, during both rectifier and inverter mode. The obtained efficiency curves differ by approx. 2% from the ones reported in the converters’ data sheet.

With growing popularity of grid-connected battery energy storage systems (BESSs), operators require electrical models for optimal utilisation. These models should be provided by suppliers or manufacturers based on testing methods applied to individual cells or modules in specialised laboratories. However, operators are also interested in developing electrical models on their own. This paper explores the feasibility of modelling a grid-connected BESS without dismantling it, using only the data from its energy management system (EMS) and battery management system (BMS). The goal is to characterise a BESS directly on-site, controlling it through the available commands of its power converter system (PCS). The aim is to represent the electrical dynamics of the BESS with an equivalent Thevenin electric circuit composed of open circuit voltage, resistances, and capacitances. The overall usable capacity of the BESS and the efficiency of the PCS are also estimated. The subject of the investigation is a 300kW/652kWh Nickel-Manganese-Cobalt (NMC) Li-ion BESS composed of ten racks, each equipped with a PCS and ten battery modules. The analysis proves the feasibility of modelling the grid-connected BESS via data from EMS and BMS. An equivalent cell model is derived, with the open circuit voltage and internal impedance expressed for the entire state-of-charge range. The total resistance assumes values in the range of 1.580–2.424mΩ, whereas the total capacitance is 609.5–1,580kF. Consequently, the normalised total resistance is 3%–4%, aligned with the expectations from other NMC Li-ion cells. Finally, the energy capacity and PCS efficiency are reported as a function of the power of the PCS. The usable energy capacity per rack is approx. 59.7kWh, which is 91.5% of the rated DC energy capacity, and it is independent of the power at which the BESS is discharged. The PCS efficiency is above 94% when operating at 15% of the PCS rated power or higher, during both rectifier and inverter mode. The obtained efficiency curves differ by approx. 2% from the ones reported in the converters’ data sheet.

Reconfigurable batteries can change their cell topology in real time, which enables them to adapt their voltage during operation. This unique capability makes interfacing power converters redundant in applications where batteries are directly coupled with other DC components or systems. The present paper characterizes a 104 kWh prototype of a reconfigurable battery for high power applications, and derives equations for calculating the battery efficiency for the complete operating area. The battery can adapt its voltage from 0 V up to 1200 V, and reaches power values of 240 kW for charging, and 280 kW for discharging. The results are presented in efficiency maps, showing the dependency on voltage, power, and state of charge. Moreover, the efficiency characteristic is compared to a conventional battery with fixed cell topology and DC-DC converter. The reconfigurable battery can operate at a wider voltage range and achieves better efficiency up to an average power of 44.6 kW during charging, and 46.7 kW during discharging. Conversely, the conventional system performs better above these thresholds. Finally, the presented model can be used to optimize the design of reconfigurable battery strings, and to accurately size such systems for specific applications and purposes.

Reconfigurable batteries can change their cell topology in real time, which enables them to adapt their voltage during operation. This unique capability makes interfacing power converters redundant in applications where batteries are directly coupled with other DC components or systems. The present paper characterizes a 104 kWh prototype of a reconfigurable battery for high power applications, and derives equations for calculating the battery efficiency for the complete operating area. The battery can adapt its voltage from 0 V up to 1200 V, and reaches power values of 240 kW for charging, and 280 kW for discharging. The results are presented in efficiency maps, showing the dependency on voltage, power, and state of charge. Moreover, the efficiency characteristic is compared to a conventional battery with fixed cell topology and DC-DC converter. The reconfigurable battery can operate at a wider voltage range and achieves better efficiency up to an average power of 44.6 kW during charging, and 46.7 kW during discharging. Conversely, the conventional system performs better above these thresholds. Finally, the presented model can be used to optimize the design of reconfigurable battery strings, and to accurately size such systems for specific applications and purposes.

Self-sufficiency is an important metric for various energy concepts, as it reflects what share of the local consumption is covered by local generation. However, the equation commonly used in literature cannot be applied to systems with an energy storage that actively exchanges energy with the grid. With more and more systems incorporating storage units it is therefore necessary to re-think the mathematical definition of self-sufficiency. The present paper addresses this issue by proposing an alternative equation that captures distinctive factors introduced by storage units: (i) Energy exported to the grid can originate from previously imported energy, (ii) initial and final storage energy in a given observation period can be different, (iii) the usage of storage systems entails energy losses. It is demonstrated that neglecting these factors leads to an over- or underestimation of self-sufficiency with values even reaching below 0 % or above 100 %. In contrast, the self-sufficiency calculated by the proposed definition considers the above mentioned factors and always stays within the defined range.

Self-sufficiency is an important metric for various energy concepts, as it reflects what share of the local consumption is covered by local generation. However, the equation commonly used in literature cannot be applied to systems with an energy storage that actively exchanges energy with the grid. With more and more systems incorporating storage units it is therefore necessary to re-think the mathematical definition of self-sufficiency. The present paper addresses this issue by proposing an alternative equation that captures distinctive factors introduced by storage units: (i) Energy exported to the grid can originate from previously imported energy, (ii) initial and final storage energy in a given observation period can be different, (iii) the usage of storage systems entails energy losses. It is demonstrated that neglecting these factors leads to an over- or underestimation of self-sufficiency with values even reaching below 0 % or above 100 %. In contrast, the self-sufficiency calculated by the proposed definition considers the above mentioned factors and always stays within the defined range.