<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 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>The paper discusses the current developments in the recycling of electrical machines. The main attention is devoted to three types of motors: synchronous reluctance motor, permanent magnet assisted synchronous reluctance motor, and induction motor. Base materials of such electrical machines are also described in the paper. Rare-earth permanent magnets used in electrical machines are review separately. Moreover, the paper considers the features of the disassembly and recycling options.</abstract>

Re-Use and Recycling of Different Electrical Machines

<abstract>
<title style='display:none'>Abstract</title>
At the end of 2019, the EC presented the European Green Deal – a set of policy initiatives with the ultimate goal of making Europe climate-neutral by 2050. This means that all EU member states must reach climate neutrality. Emissions in all sectors shall be substantially reduced to achieve this goal. Most of the energy is produced using fossil fuels, which contributes to climate change, water and air pollution, as well as the depletion of natural resources. Individual and district heating systems, which supply residential buildings with heat and hot water, are responsible for a part of global emissions.
Electricity generation also produces emissions. The EU recognises the mitigation of climate change as one of the main challenges that must be overcome to ensure sustainable development of the European Union, which is why it has defined ambitious goals to promote decarbonisation and is deliberately moving towards the implementation of renewable energy.
This article aims at analysing the opportunity of using renewable energy resources in multi-apartment residential buildings in the context of sustainability.
</abstract>

Installation of Alternative Energy Sources and Equipment in Residential Buildings in Latvia

<abstract>
<title style='display:none'>Abstract</title>
This study explores the recent legislative changes allowing prosumers and consumers in Latvia to form energy communities (EnCs). In spite of legal advancements, the absence of supporting regulations and potential barriers, such as the lack of tailored support schemes and forecasting tools, pose challenges. International experiences highlight additional hurdles, including weak stakeholder dialogue and limited EnC modelling tools. To address these issues, the study proposes a prosumer and consumer EnC business model aligned with Latvia’s legislation, emphasising a peer-to-peer (P2P) trading approach. The model incorporates state aid, electricity trader responsibilities, and reinvestment of EnC profits, eliminating the need for a dedicated trading platform. The study introduces a user-friendly planning tool in MS Excel, evaluating economic viability through case studies and scenarios. Findings emphasise the optimisation of EnC effectiveness by adjusting tariffs, minimising initial investment costs, securing external funding, and managing loan durations. The planning tool serves as a universal instrument, offering insights into individual EnC economic indicators based on their unique characteristics. The research provides valuable guidance to stakeholders and policymakers in Latvia’s evolving energy landscape.
</abstract>

Assessment of Various Factors Affecting Economic Indicators in Prosumer and Consumer Energy Communities: A Case Study of Latvia

<abstract>
<title style='display:none'>Abstract</title>
The paper proposes a method of simultaneous reconstruction of the electrical conductivity and magnetic permeability profiles of planar metal research objects based on the results of single measurements by eddy current probes using surrogate optimization techniques in a reduced compact subspace design and accumulating the full amount of the most important a priori information about the modes of electromagnetic objects. In addition to the information on the response of probe signals to changes in electrophysical parameters, a priori information includes the data on multifrequency sensing and changes in the lift-off between metal research objects and eddy current probes. All the main stages for the implementation of the method of solving the inverse problem are demonstrated, namely, creating a uniform computer quasi-design of the experiment with improved 2D-projections based on LPτ-Sobol’s sequences; creating surrogate models on fully connected deep neural networks; reducing the dimensionality of the full design space using the principal components method of PCA; reconstructing profiles as a result of surrogate optimisation in a compact subspace. Numerical examples of the method are also presented in the paper.
</abstract>

Reconstruction of Electrophysical Parameter Distribution During Eddy Current Measurements of Structural Features of Planar Metal Objects

<abstract>
<title style='display:none'>Abstract</title>
Holograms recorded by focused light beams have several advantages over usual plane wave holograms such as reduced size, white light readout possibility and others. Focused polarization grating recording in an azobenzene-based molecular glasy film 5,5,5-triphenylpentyl 4-((4-(2-(4-bis(2-hydroxyethyl)amino)phenyl)-1-cyanovinyl)phenyl)dyazenyl)benzoate (which we shortly denoted as B11) is experimentally studied and compared with such grating recording in other materials. It has been determined that focusing manifests itself differently than in other materials, e.g., as in a-As2S3 and a-As-S-Se chalcogenide films. Thus, it reduces the holographic grating recording efficiency independently of recording and readout beam polarizations. It has also been found that recording efficiency with circularly and orthogonally <italic>L-R</italic> polarized beams is higher than with linearly polarized <italic>p-p</italic> beams. The highest diffraction eficiency of 26% is achieved with <italic>L-R</italic> polarized unfocused beams.
Recording efficiency grating period dependences for unfocused beams at 200 J/cm2 exposure and at 1000 J/cm2 exposure are different with a maxima at 2 μm and 6 μm, respectively. In contrast, recording efficiency grating period dependences for focused beams at 200 J/cm2 exposure and at 1000 J/cm2 exposure are the same with the maximum at 6 μm.
The obtained results are discussed in terms of <italic>trans-cis-trans</italic> photoisomerization followed by mass transfer and their light intensity dependence, light electric field gradient force, the photoinduced light scattering and photoelastic forces.
</abstract>

Focused Polarization Gratings in an Azobenzene-Based Molecular Glassy Film

<abstract>
<title style='display:none'>Abstract</title>
The accumulation of plutonium produced by LWR operations causes public concern related to its radiotoxicity and proliferation issue. Therefore, there is some interest in developing various methods for consuming plutonium through plutonium incineration i.e., by utilising thorium-based nuclear fuel. Thorium-plutonium mixed oxide fuel (Thorium MOX) is an attractive option considered for plutonium incineration. The aim of the study is to investigate the neutronic characteristics of Thorium MOX fuel in VVER-1000 nuclear reactor fuel assemblies. The VVER-1000 Benchmark model was utilised in the present study. A series of calculations were performed with the MCNP6 code and the ENDF/B-VII library. The calculation results show that a proper increase in Pu content could increase <italic>k</italic>inf of ThMOX fuel, so it could have almost the same <italic>k</italic>inf with MOX fuel or increase Pu content when needed. It was also shown that Thorium MOX fuel assembly had more negative Doppler reactivity due to neutron capture by thorium (232Th). The moderator temperature coefficient was found to be more negative than the Doppler coefficient, and it could be concluded that replacing uranium with thorium could reduce the amount of plutonium produced while increasing plutonium consumption.
</abstract>

Neutronic Investigation on Thorium Mox Fuel in Vver-1000 Reactor Assembly

<abstract>
<title style='display:none'>Abstract</title>
The paper demonstrates a spectrum slicing technique applied to PAM-4 modulated signal spectrum at 40 Gbaud baudrate for optical access networks. The spectrum slicing is shown in VPIphotonics simulation environment, where characteristics of key components are measured in the laboratory and loaded into the corresponding elements of the simulation setup. The spectrum of electrical baseband signal is sliced in four slices introducing an additional degree of freedom – higher data rate signals can be transmitted by re-using already-existing lower bandwidth transceiver components in an optical access network. This tackles the electronic bottleneck enabling the on-demand variable bandwidth allocation and extends the life span of opto-electrical components during network upgrades. Furthermore, the use of the proposed spectrum slicing technique has a positive side-effect when it comes to security concerns, as the signal spectrum is divided into slices. All slices are necessary to obtain the reconstructed signal and even if an eavesdropper gets most of the slices it is still not enough. The simulation results show that the proposed spectrum slicing and stitching approach is a viable solution to overcome the problem of limited frequency bandwidth of electrical components.
</abstract>

Extending Transceiver Capacity with a Scalable Spectrum Slicing Technique for Optical Access Networks

<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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Latvian Journal of Physics and Technical Sciences

The Journal of Institute of Physical Energetics

Latvian Journal of Physics and Technical Sciences (Latvijas Fizikas un Tehnisko Zinātņu Žurnāls) publishes experimental and theoretical papers containing results not published...

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Latvian Journal of Physics and Technical Sciences (Latvijas Fizikas un Tehnisko Zinātņu Žurnāls) publishes experimental and theoretical papers...

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