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Renewable energy sources are the key for long-term decarbonization of energy. However, the intermittent nature of renewables does not always meet the energy demand in the electrical grid. Thus, electrical heated Thermal Energy Storage systems (TES) coupled with sCO2 power cycles are investigated at Helmholtz-Zentrum Dresden-Rossendorf as a possible solution to balance this mismatch. In this study, a Printed Circuit Heat Exchanger (PCHE) is considered as candidate for the 1 MW primary heat exchanger, given the mechanical challenge induced by drastic pressure difference between the hot fluid of the TES and the cold fluid of the power cycle. The present work consists of two parts, one elaborates a 1D model in order to optimize the PCHE regarding the pump power required to compensate the pressure loss. It was found that the hot fluid coming from the TES accounts for 80 % of the total pump power after optimization because of its low density and its high mass flow rate. Furthermore, 3D simulations by Computational Fluid Dynamics (CFD) were done and compared to the results from the 1D model to ensure its validity. It was observed that the results from the 1D model and the CFD simulations are consistent, with a slight potential deviation in the calculation of the pressure profile.

Renewable energy sources are the key for long-term decarbonisation of the energy system. However, the intermittent nature of renewables, such as solar energy or wind energy, does not always meet the energy demand in the electrical grid. Considering the fact that both electricity production and consumption vary independently, balancing the grid is a major challenge for the development of an energy system based on renewable energies. Within this framework, Thermal Energy Storage systems (TES) coupled with a power cycle have gained popularity since they can store energy from renewable sources during the periods of high production and release it when necessary. To convert thermal energy into electricity, a power cycle is required. Given the relative high temperature range (600 - 1000 °C), supercritical CO2 (sCO2) is the most promising material as working fluid for the power cycle, from efficiency and safety considerations. Thus, the Primary Heat Exchanger (PHX) must be carefully designed as the fluid pressures in the TES and the power cycle are namely 1 - 10 bar and 200 - 250 bar. The present work consists of two parts, one elaborates a 1D model in order to design the PCHE regarding a certain set of boundary conditions. The model requires heat transfer and pressure loss correlations from the literature to estimate the Nusselt number and friction factor, which strongly depends on the geometry. It was found that the zigzag channel design intensifies both heat transfer and pressure drop phenomena, which is not suitable for the hot fluid from an economic prospective. Furthermore, 3D simulations by Computational Fluid Dynamics (CFD) were done and compared to the results from the 1D model to ensure the validity of the correlations. It was also found that the results match those from the literature, thus validating the 1D model.

Renewable energy sources are the key for long-term decarbonisation of the energy system. However, the intermittent nature of renewables, such as solar energy or wind energy, does not always meet the energy demand in the electrical grid. Considering the fact that both electricity production and consumption vary independently, balancing the grid is a major challenge for the development of an energy system based on renewable energies. Within this framework, Thermal Energy Storage systems (TES) coupled with a power cycle have gained popularity since they can store energy from renewable sources during the periods of high production and release it when necessary. To convert thermal energy into electricity, a power cycle is required. Given the relative high temperature range (600 - 1000 °C), supercritical CO2 (sCO2) is the most promising material as working fluid for the power cycle, from efficiency and safety considerations. Thus, the Primary Heat Exchanger (PHX) must be carefully designed as the fluid pressures in the TES and the power cycle are namely 1 - 10 bar and 200 - 250 bar. A Printed Circuit Heat Exchanger (PCHE) with straight or zigzag channels is numerically studied. A 1D model were developped and Computational Fluid Dynamics (CFD) simulations were conducted to design and to optimize the PCHE.

The necessity of energy independency, political and climatic concerns is driving the global attention towards sustainable energy sources. The European commission targets carbon-neutrality with net zero greenhouse gas emission by 2050 [1], which results in a decarbonization of the energy sector by renewable energy sources. Nevertheless, a significant challenge arises from the fluctuating power infeed of wind and solar based plants, which may result in grid instability, overstrain and mismatch in supply-demand. Thus, energy storage systems (ESS) are required to decouple electricity generation from the demand and to enhance the reliability of the energy system. Furthermore, a successful implementation of such a storage technology on a large scale requires cost-effective cycles for energy storage and reconversion. Therefore, a Power-to-Heat-to-Power system is proposed, based on electrical charged high temperature ceramics and a supercritical CO2 (sCO2) power cycle for discharge. The high temperature sensible thermal energy storage is manufactured from waste material of the aluminum production, which is a low-cost storage solution. As a result, such a thermal storage combines an important contribution to the circular economy with an economic solution for large scale, location-independent energy storage. CO2 becomes supercritical when temperature and pressure are higher than 31 °C and 74 bar. In this phase the viscosity is low and the density as well as the heat capacity are high. Hence, the installed components, in particular the turbomachinery of the sCO2 power cycle are highly compact, which results in low investment costs and low thermal losses. Furthermore, high temperatures up to 600 °C can be utilized to generate electricity at high thermal efficiency. An inhouse code was used to study various heat transfer fluids, materials and designs for the thermal storage vessel during the charge and discharge cycle. Furthermore, the heat exchanger transferring the heat from t

As a part of my graduation internship in engineering and master’s degree, I work in the Helmholtz Zentrum Dresden-Rosseendorf institute for an internship of 6-months. I am a part of a team consisting of my supervisor, Dr. Sebastian Unger and a PhD student, Mrs. Malini Bangalore. The latter studies a thermal energy storage (TES). The storage would store electricity as heat from renewable energy sources when the supply in electricity is higher than the demand. In case of residual load demand, the stored thermal energy would be released to a power cycle and supply the electrical grid. Therefore, a heat exchanger , connecting the thermal storage system to the power cycle, needs to ben designed and here is where I intervene. Before I came, it was already decided, that the power cycle uses supercritical fluid as working fluid and working fluid in the thermal energy storage circuit, more specifically the hot fluid, is CO2 at atmospheric pressure. It was also decided that the heat exchanger type is a printed circuit heat exchanger (PCHE), that the heat transfer rate is set to 10 MW and the hot fluid inlet temperature is 600 °C. However, to completely determine the geometry of the PCHE, the boundary conditions, i.e. temperature, pressure and mass flow rates in hot and cold channels, need to be determined. To that end, a review of cycles and their modelling is made. It helps to calculate the boundary conditions for the cold side. Then, a cost-optimization analysis of the PCHE leads to determine the boundary conditions of the hot fluid. Indeed, as the hot fluid is CO2 at atmospheric pressure, even the smaller pressure drop has a significant impact on the operation cost of the PCHE. Therefore, a reference case may be determined. Finally, CFD simulations were conducted on the reference case geometry. Furthermore, the behaviour of the PCHE is examined, when the mass flow rate is changed, e.g. because of different system operation. Finally, the CFD simulation results are compared