<abstract>
<title style='display:none'>Abstract</title>
This review article covers the topics of evaluation and experimental determination of oxygen mass transfer coefficients (<italic>kLa</italic>) for their application in characterising bioreactors and fermentations processes. The article provides a comparison of different experimental approaches for determining <italic>kLa</italic> in bioreactors. Additionally, the influence of bioreactor design and fermentation parameters on <italic>kLa</italic> is discussed. The aim of the article is to provide useful information regarding the approaches for selecting bioreactors and their working regimes to achieve optimal fermentation results.
</abstract>

Oxygen Mass Transfer Coefficient Application in Characterisation of Bioreactors and Fermentation Processes

<abstract><title style='display:none'>Abstract</title>The paper covers the main aspects of designing a low voltage three-phase PWM rectifier for bidirectional AC/DC power flow with unity power factor. A model in Matlab/Simulink environment has been built for a 10kW active rectifier with an LCL filter connected to grid side of the rectifier. The primary goal of the model is to achieve low grid current harmonic content for frequency ranges described in worldwide applicable standards and above. LCL filter parameter design procedure is described in the paper and implemented in the rectifier model to achieve a better power quality with limitations in passive element size. A simple “p-q” theory-based voltage oriented control algorithm is used in the model and described in the present paper. Model performance is characterised by dynamic response, stability and grid parameters during simulation. The simulation results demonstrate that the modelled rectifier system is stable and the grid current harmonic content is low both in the low-and high-frequency ranges.</abstract>

Design of a Three-Phase Bidirectional PWM Rectifier with Simple Control Algorithm

<abstract xml:lang="en"><title style='display:none'>Parallel Path Magnet Motor: Development of the Theoretical Model and Analysis of Experimental Results</title>Analytical and numerical modelling is performed for the linear actuator of a parallel path magnet motor. In the model based on finite-element analysis, the 3D problem is reduced to a 2D problem, which is sufficiently precise in a design aspect and allows modelling the principle of a parallel path motor. The paper also describes a relevant numerical model and gives comparison with experimental results. The numerical model includes all geometrical and physical characteristics of the motor components. The magnetic flux density and magnetic force are simulated using FEMM 4.2 software. An experimental model has also been developed and verified for the core of switchable magnetic flux linear actuator and motor. The results of experiments are compared with those of theoretical/analytical and numerical modelling.</abstract>

Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga, LV-1063, LATVIA

Parallel Path Magnet Motor: Development of the Theoretical Model and Analysis of Experimental Results

<abstract><title style='display:none'>Abstract</title>Single-crystal, microcrystalline and nanocrystalline nickel oxides (NiO) have been studied by Raman spectroscopy. A new band at ~200 cm−1 and TO-LO splitting of the band at 350–650 cm−1 have been found in the spectra of single-crystals NiO(100), NiO(110) and NiO(111). The Raman spectra of microcrystalline (1500 nm) and nanocrystalline (13–100 nm) NiO resemble those of the single crystals. They all contain the two-magnon band at 1500 cm−1, indicating that the oxides remain at room temperature in the antiferromagnetic phase. Besides, a new sharp Raman band has been observed at 500 cm−1 in nanocrystalline NiO. Its temperature dependence suggests the magnetic origin of the band, possibly associated with the one-phonon–one-magnon excitation at the Brillouin zone centre.</abstract>

Magnon and Phonon Excitations in Nanosized NiO

<abstract><title style='display:none'>Abstract</title>X-ray diffraction and x-ray absorption spectroscopy at the Cu K-edge were used to study the atomic structure in copper nitride (Cu3N) thin films. Textured nanocrystalline films are obtained upon dc magnetron sputtering on substrates heated at about 190 °C, whereas amorphous films having strongly disordered structure already in the second coordination shell of copper are deposited in the absence of heating.</abstract>

Study of Copper Nitride Thin Film Structure

<abstract>
<title style='display:none'>Abstract</title>
The paper analyses the constraints related to optical tracking of an HMD with a single commercial binocular stereoscopic optical tracking system and offers an overview of different factors affecting the best active fiducial set-up and marker geometries for reliable tracking with a focus on the rotational accuracy of a marker.
Two IR diode models with different emission characteristics were tested as active fiducials and the results were compared to localization accuracy of passive fiducials. In terms of preferable marker geometry, it was found that the area enclosed by the fiducials should be maximized. Thus, due to geometrical constraints, very small marker geometries may not be stable or feasible entirely. Rotational accuracy was analysed for cases when the marker was not directly facing the tracking device and it was found that rotation about one axis introduced errors to the determined orientation in space related to the other axes as well.
</abstract>

Comparison of Passive and Active Fiducials for Optical Tracking

<abstract><title style='display:none'>Abstract</title>
Sustainability and longevity of existing gas grid exploitation perspective are closely related to two fundamental issues: their ability to adopt to changing gas fuel production and supply landscape in the context of methane-based fuels, mostly, biomethane, and in the context of non-methane-based fuels, mostly, low carbon and green hydrogen. Renewable gases and their ever-growing presence in gas transmission and distribution systems open up a discussion about the necessity to revise and restructure the original – vertically integrated layout of the gas systems, where gas supply is only technically possible from the transmission system towards distribution one, and not vice versa. Development of numerous decentralized biomethane production facilities connected to the gas distribution system causes a necessity to ensure the possibility to pass biomethane surplus of a certain production area into the gas transmission grid, thus avoiding necessity to install biomethane storage capacities locally and granting other regions an opportunity to use said surplus in their gas consumption immediately. The article addresses biomethane production trends and actions taken towards the development of reverse flow gas stations in France – one of biggest biomethane producers in Europe to date, and opportunities and challenges, which this technique might face in smaller and less active renewable gas markets as the one of Latvia.
</abstract>

Injection of Renewables Gases Into the Existing Gas Distribution Grids and Employment of Reverse Gas Flow Technique

<abstract><title style='display:none'>Abstract</title>
Persistent luminescence (PersL), also called long-lasting phosphorescence or simply afterglow, is a luminescence characterised by the emission of radiation from a few seconds to several days after the excitation source has been switched off. Over the past two decades, research on PersL materials, both in fundamental and applied physics, has developed rapidly; however, the explanation for the physical processes that cause afterglow still needs to be clarified. Today, PersL materials are used mainly for luminescent paints, safety signs and decorations. At the same time, research into using such materials in medicine, information storage, anti-counterfeiting technology, etc., is underway.
Currently, information on the long persistent luminescence materials with emission in the blue and green spectral range is widely available. In contrast, the number of publications on the afterglow in the red and near-infrared spectral range is considerably lower.
Within the framework of this research, Mg2SiO4: Mn2+; M3+ (M3+ = B3+; Al3+; Ga3+; In3+) materials were synthesised using solid state reaction synthesis. When excited with X-rays, the materials exhibited a broad Mn2+ PersL band with two maxima at approximately 625 nm and 730 nm. After cessation of irradiation, an afterglow of at least 6 hours could be observed.
The research focuses on the trap properties of the materials. It was concluded that at least three discrete trap levels with activation energies approximately between 0.4–1.6 eV were present in the samples. Additionally, co-doping with Al3+; Ga3+; In3+ ions improved PersL longevity of the Mg2SiO4: Mn2+ material.
</abstract>

Red Persistent Luminescence and Trap Properties of Mg2SiO4: Mn2+, M3+ (M3+ = B3+; Al3+; Ga3+; In3+) Material

<abstract><title style='display:none'>Abstract</title>
Quadrotor unmanned aerial vehicles (UAVs) are small, agile four-rotor systems suitable for various applications, from surveillance to disaster support missions. Hence, achieving high-precision trajectory tracking is crucial for their successful deployment. This research focuses on modelling, parameter identification, and Linear Quadratic Regulator (LQR) control design for quadrotors, aiming to enhance their trajectory following capabilities. The quadrotor dynamics are a sixth degree-of-freedom (6DOF) equation of motion derived from Newton’s second law, encompassing moment of inertia, centre of gravity, weight, and thrust of propeller parameters. Experimental measurements are conducted to accurately determine these parameters, ensuring a realistic representation of the quadrotor system. Subsequently, a linearized model is constructed to provide a suitable plant for control system development. The LQR control design is intended to improve the trajectory tracking performance. This control strategy is validated through simulation and practical experiments, demonstrating its effectiveness in achieving high-precision trajectory following capabilites. The proposed approach demonstrates that LQR control effectively guides the quadrotor to resemble a predefined trajectory, experiencing only 3 % overshoot observed during the initial phase of flight.
</abstract>

Towards High-Precision Quadrotor Trajectory Following Capabilities: Modelling, Parameter Estimation, and LQR Control

<abstract><title style='display:none'>Abstract</title>
During the pandemic and periods of martial law, educational institutions in Ukraine implemented various forms of organising the educational process, combining in-person and remote classes. As a result, there has been a decrease in the level of use of building premises. The analysis of the characteristics of energy consumption in these conditions requires additional attention. In this study, a dynamic energy model of the building was created using the educational building of Igor Sikorsky Kyiv Polytechnic Institute as an example. Energy consumption for heating needs was determined for normal operating conditions and quarantine conditions in Ukraine (only a specific part of the premises is being operated, ensuring that the standard temperature value is maintained while people are present). Based on the results of the study, the features of the energy consumption of the building during quarantine/martial law restrictions, subject to the partial use of the building, were analysed, and the main disadvantages of such a mode were identified. For the educational building of the university, the total energy consumption for heating needs depends on the number of rooms that are actively used, and the modes of regulation of heating and ventilation; the consumption can decrease depending on the chosen scenario by 61 %, 56 %, and 34 % in quarantine mode. The findings indicate that the efficiency of regulation modes can decrease by more than four times when compared to normal mode due to internal heat exchange with unoccupied rooms.
</abstract>

Energy Performance of Higher Education Institutions Buildings Operating During Quarantine Restrictions and/or Martial Law in Ukraine

<abstract><title style='display:none'>Abstract</title>
Transition metal dichalcogenides (TMDs), specifically those involving V and Ti, possess fascinating material properties, making them interesting candidates for scientific studies. The existing growth methods of these materials are typically limited by scalability – either low yield or high cost. Here, we propose an alternative 2-step method valid for scalable production. In the first step, precursor films of Ti / V are deposited using magnetron sputtering, followed by the second step of selenization of these samples using elemental Se in a vacuum-sealed quartz ampoule for conversion to their respective diselenide material. Synthesized films are char-acterised using scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and X-ray photoelectron (XPS). The method demonstrated here can be used to increase the active surface area of TiSe2 and VSe2 for their potential catalytic and HER applications using nanostructured substrates, while also providing an opportunity for scalable synthesis of films that can be extended to synthesize other TMDs as well.
</abstract>

Towards Scalable Synthesis of TiSe2 and VSe2 Thin Films

<abstract><title style='display:none'>Abstract</title>
This interdisciplinary research is focused on constructing and demonstrating enhanced-reach polarization optical time-domain reflectometer (POTDR) for monitoring single-mode fibre optical communication lines. An optical signal state of polarization measurements enables several new possibilities for real-time monitoring solutions in fibre optics. However, there is no commercial equipment available to determine the location in fibre optical cable where the light polarization state changes. The authors present a monitoring technique of an optical signal state of polarization based on reflected signal time-amplitude analysis with improved operation parameters, namely, timing resolution (2 to 3 ps RMS) and amplitude measurement resolution of nanosecond scale pulses (8 to 10 bits) for monitoring of fibre optical communication lines. Additionally, the demonstrated POTDR provides a considerable optical fibre line measurement reach of up to 40 km.
</abstract>

Enhanced-Reach Polarization Optical Time-Domain Reflectometry Technique for Fibre Optical Infrastructure Monitoring

<abstract><title style='display:none'>Abstract</title>Biomethane is one of the most promising renewable gases (hereafter – RG) – a flexible and easily storable fuel, and, when used along with the natural gas in any mixing proportion, no adjustments on equipment designed to use natural gas are required. In regions where natural gas grids already exist, there is a system suitable for distribution of the biomethane as well. Moreover, improving energy efficiency and sustainability of the gas infrastructure, it can be used as total substitute for natural gas. Since it has the same chemical properties as natural gas, with methane content level greater than 96 %, biomethane is suitable both for heat and electricity generation, and the use in transport.Biomethane is injected into the natural gas networks of many Member States of the European Union (hereafter – the EU) on a regular basis for more than a decade, with the Netherlands, Germany, Austria, Sweden and France being among pioneers in this field. In most early cases, permission to inject biomethane into the natural gas grids came as part of a policy to decarbonize the road transport sector and was granted on a case-by-case basis. The intention to legally frame and standardise the EU’s biomethane injection into the natural gas networks came much later and was fulfilled in the second half of the present decade.This paper addresses the biomethane injection into the natural gas grids in some EU countries, highlights a few crucial aspects in this process, including but not limited to trends in standardisation and legal framework, injection conditions and pressure levels, as well as centralised biogas feedstock collection points and the biomethane injection facilities. In a wider context, the paper deals with the role of biomethane in the EU energy transition and further use of the existing natural gas networks.</abstract>

The Biomethane Injection into the Natural Gas Networks: The EU’s Gas Synergy Path

<abstract><title style='display:none'>Abstract</title>Comparative analysis is performed for different motors from the standpoint of damage inflicted by them during their lifecycle. Three types of motors have been considered: the synchronous reluctance motor, the permanent magnet assisted synchronous reluctance motor and the induction motor. The assessment of lifecycle has been made in terms of its four stages: manufacturing, distribution, use and end of life. The results show that the production costs of synchronous reluctance motor are lower compared to that of permanent magnet assisted motors, but due to their low efficiency they exert the greatest environmental impact. The main conclusion is that the assessment made at the early designing stage for the related environmental impact enables its reduction.</abstract>

Institute of Physical Energetics, 11 Krivu Street, LV-1006, Riga,

Tallinn University of Technology, 5 Ehitajate Tee, 19086, Tallinn,

Aalto University, P.O. Box 11000, FI-00076, Aalto,

Lifecycle Analysis of Different Motors from the Standpoint of Environmental Impact

<abstract><title style='display:none'>Abstract</title>The Latvian natural gas system is interconnected with transmission networks located in Lithuania, Estonia and Russia. Natural gas commercial metering is provided by GMS “Karksi” (Estonia) and by GMS “Kiemenai” (Lithuania). Natural gas is supplied to all larger urban areas in Latvia. Natural gas is supplied to Latvia along the Latvian–Russian pipeline only during the warm period of the year (April–September), and it is accumulated in the underground gas storage facility in Incukalns. During winter, gas from the underground facility is delivered to Latvian customers, as well as transmitted to Estonia and back to Russia. There is also a connection to Lithuania. Out of the gas supply disruption risks that are assessed at different levels, the essential one with a trans-border impact potential consists in the insufficient technical capacity of Incukalns UGS. Given the current technical possibilities, IUGS cannot pass the gas volume required for the Baltic States to compensate the gas supply deficit. The paper performs system recovery analysis after selected critical events. The paper provides a report describing the steps to be followed in order to restore the gas transmission system to normal operation after selected critical events. A very significant region of the power system of Latvia is the central part of Latvia and Riga region, where both of Riga CHPs, as well as Riga HPP, is located. The restoration time of the gas system of Latvia depends on the gravity of the situation and damage in the gas system and may range from several hours to several days.</abstract>

Risk Treatment and System Recovery Analysis of Gas System of Gas and Electricity Network of Latvia

<abstract><title style='display:none'>Abstract</title>As known, many multi-purpose plants can be used in different industries. This research is focused on the possibilities to utilize hemp as feedstock for thermal insulation products. The most advantageous features of hemp insulation are associated with health and environmental safety. The thermal conductivity of commercially available hemp insulation products is comparable with that of other fibrous insulation materials; however, it is possible to develop new products that could be more efficient in terms of cost and due to other important features.</abstract>

Institute of Physical Energetics, 21 Aizkraukles Str., Riga, LV-1006, LATVIA

Agency of Latvia University of Agriculture Research Institute of Agricultural Machinery, 1 Instituta Str., Ulbroka, Stopinu nov., LV-2130, LATVIA

Thermo-Insulation Properties Of Hemp-Based Products

<abstract><title style='display:none'>Abstract</title>In the present research, the main critical points of gas transmission and storage system of Latvia have been determined to ensure secure and reliable gas supply among the Baltic States to fulfil the core objectives of the EU energy policies.Technical data of critical points of the gas transmission and storage system of Latvia have been collected and analysed with the SWOT method and solutions have been provided to increase the reliability of the regional natural gas system.</abstract>

Consistency Analysis and Data Consultation of Gas System of Gas-Electricity Network of Latvia

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