Alexandros Tsimpoukis, Ph.D.

The widespread adoption of renewable energy technologies requires adaptable and replicable energy storage solutions tailored to diverse climatic conditions. This paper presents a comprehensive study on the replication potential of a compact hybrid energy storage solution, called MiniStor across various European sites. The MiniStor system integrates solar photovoltaic-thermal (PVT) and solar thermal collectors, a thermochemical (TCM) reactor using an ammonia cycle, a heat pump with phase change material (PCM) storage, and a battery for electrical energy management in buildings. Its core innovation, the TCM reactor stores heat through reversible ammonia-calcium chloride reactions, providing heating during winter and cooling during summer. To assess its broader applicability, the system’s replication potential was evaluated across various European climates and residential typologies, focusing on single-family homes. Simulation, based on specific meteorological and building data, demonstrated that MiniStor can effectively operate across diverse regions demonstrating significant potential for RES integration across Europe, with renewable energy coverage ranging from 32% to over 88% depending on location for the worst-case scenario. This work contributes to accelerating the large-scale integration of renewable energy storage solutions in the built environment across Europe.

A dynamic model was developed in TRNSYS® software package to simulate the thermal behavior of a single family house in Spain during a year. The model takes into consideration the characteristics of the building (orientation, insulation, thermal zones, etc.), heating and cooling devices, weather conditions and electric grid usage. Additionally, the model incorporates occupancy profiles, lighting, heat producing devices and infiltration to estimate thermal loads and photovoltaic production. So, with the utilization of the model, a characterization of the thermal and photovoltaic systems is accomplished, which can be used for the assessment of potential coupling with individual models of thermal energy storage solutions and comparison of cases with and without storage. As a result, the investigation of load shift strategies together with the exploitation of the photovoltaic energy source, will be feasible if the required input becomes available. According to the model, the annual thermal demands are 1618 kWh for space heating, 2358 kWh for space cooling and 1671 kWh for domestic hot water. The total electricity consumption of the heat pump and all circulating pumps is 1273 kWh, while the photovoltaic array produces 6512 kWh per year.

This paper explores the development of positive energy communities using Eordaia, Greece, as a case study. The approach combines building and district-level energy analysis to achieve nearly zero energy performance through retrofitting, district-level storage systems, and renewable energy technologies. A parametric analysis utilizing RETSCREEN Expert and EnergyPlan software determines the optimal mix of technologies based on technical and financial parameters, with Eordaia, a region in energy transition and part of the RESPONSE Horizon project, illustrating the practical benefits. It includes a neighborhood of 105 mixed-use properties and two municipal buildings where a range of renewable energy sources and energy efficiency measures are applied. Insulation, photovoltaic systems, LED lighting, predictive thermostats, and windows coated with nanotechnology are some of the key interventions considered. The findings show considerable reductions in CO2 emissions and energy use, with payback periods ranging from 8.7 to 9.6 years. This study underscores the value of district-level strategies over individual building retrofits, highlighting cost savings and improved energy performance. These findings offer valuable insights for urban planners and policymakers aiming to transform urban areas into sustainable, positive energy districts, supporting the EU’s 2050 net-zero emissions goals.

The linear oscillatory two-dimensional rarefied gas flow in the zig-zag channel between the shuttle and stator fingers of typical comb elements, due to the harmonic motion of the shuttle finger with arbitrary frequency, is numerically investigated, via the linearized unsteady Shakhov kinetic model equation, subject to purely diffuse boundary conditions. The amplitude and phase distributions of the velocity components, normal and shear stresses, density and temperature are thoroughly investigated in the whole range of the gas rarefaction and oscillation parameters, characterizing the flow. The low, moderate and high flow oscillation regimes are defined by the ratio of the gas rarefaction over the oscillation parameters. It is shown that in the low oscillation regime the flow may be approximated by the corresponding steady-state analysis, while in the high oscillation one by the superposition of the associated oscillatory one-dimensional normal sound propagation and Couette flows. In the former case edge and compressibility effects are considerable and in the latter one the flow is characterized by gas trapping along the shuttle finger. In the moderate oscillation regime the fully oscillatory two-dimensional flow must be analysed. Here, the antiresonance and resonance states of the flow are considered. The ratios of the gas rarefaction over the oscillation parameters, where the average normal and shear stresses acting on the shuttle finger are minimized are computationally specified. Changing the main geometry parameters of the comb finger block, significantly affects the normal and shear stresses acting on the shuttle finger, mainly in the low oscillation regime. The present work may be applied to minimize the damping forces in comb finger blocks, as well as to optimize the design of new generation devices operating in moderate and high frequencies.

The formulation of the half-range moment method (HRMM), well defined in steady rarefied gas flows, is extended to linear oscillatory rarefied gas flows, driven by oscillating boundaries. The oscillatory Stokes (also known as Stokes second problem) and the oscillatory Couette flows, as representative ones for harmonically oscillating half-space and finite-medium flow setups respectively, are solved. The moment equations are derived from the linearized time-dependent BGK kinetic equation, operating accordingly over the positive and negative halves of the molecular velocity space. Moreover, the boundary conditions of the “positive” and “negative” moment equations are accordingly constructed from the half-range moments of the boundary conditions of the outgoing distribution function, assuming purely diffuse reflection. The oscillatory Stokes flow is characterized by the oscillation parameter, while the oscillatory Couette flow by the oscillation and rarefaction parameters. HRMM results for the amplitude and phase of the velocity and shear stress in a wide range of the flow parameters are presented and compared with corresponding results, obtained by the discrete velocity method (DVM). In the oscillatory Stokes flow the so-called penetration depth is also computed. When the oscillation frequency is lower than the collision frequency excellent agreement is observed, while when it is about the same or larger some differences are present. Overall, it is demonstrated that the HRMM can be applied to linear oscillatory rarefied gas flows, providing accurate results in a very wide range of the involved flow parameters. Since the computational effort is negligible, it is worthwhile to consider the efficient implementation of the HRMM to stationary and transient multidimensional rarefied gas flows.

The nonlinear oscillatory fully developed rarefied gas flow between parallel plates due to an external harmonic force is investigated by the Direct Simulation Monte Carlo (DSMC) method in terms of ...

The pulsatile pressure driven fully developed flow of a rarefied gas through an orthogonal duct is investigated, based on the time-dependent linear Bhatnagar, Gross, and Krook equation, by decom- posing the flow into its steady and oscillatory parts. The investigation is focused on the oscillatory part, which is characterized by the gas rarefaction and oscillation parameters, the duct aspect ratio, and the accommodation coefficient. As the oscillation frequency is increased, the amplitude of all macroscopic quantities is decreased, while their phase angle lag is increased reaching the limit- ing value of $\pi$ /2. As the gas becomes more rarefied, higher frequencies are needed to trigger this behavior. At small and moderate frequencies, there is a critical degree of gas rarefaction, where a maximum flow rate is obtained. As the duct aspect ratio is decreased and tends to zero, the flow rate and mean wall shear stress amplitudes are increased, while their phase angle lags are slightly affected. The accommodation coefficient has a significant effect on the amplitude and a very weak one on the phase angle of the macroscopic quantities. The computation of the inertia and viscous forces clarifies when the flow consists of only one oscillating viscous region or of two regions, namely, the inviscid piston flow in the core and the oscillating Stokes layer at the wall with the velocity overshooting. Finally, the time average oscillatory pumping power is increased as the oscil- lation frequency is reduced and its maximum value is one half of the corresponding steady one.

In this paper, data from 12 meteorological stations located throughout the greater metropolitan area of Thessaloniki - Greece are used for the calculation of location specific monthly Heating (HDD) and Cooling (CDD) Degree Days utilizing hourly records of the last three years. The HDD are calculated for base temperatures of 15 and 18°C and the CDD for base temperatures of 22 and 24°C by compacting average hourly data. The results show that the HDD average value of the various locations examined in Thessaloniki during the examined period (2010-2013), as compared to the corresponding value for the city center, is increased from 19{\%} up to 48{\%} (depending on the base temperature). The difference in the average value of CDD for the specified time period is more pronounced, as differences range from -10{\%} to -40{\%} compared to the corresponding value for the city center.