Search

The main aim of the current paper is to create a mathematical model for dual layer shell type recuperation system, which allows reducing the heat losses from the biomass digester and water amount in the biogas without any additional mechanical or chemical components. The idea of this system is to reduce the temperature of the outflowing gas by creating two-layered counter-flow heat exchanger around the walls of biogas digester, thus increasing a thermal resistance and the gas temperature, resulting in a condensation on a colder surface. Complex mathematical model, including surface condensation, is developed for this type of biogas dehumidifier and the parameter study is carried out for a wide range of parameters. The model is reduced to 1D case to make numerical calculations faster. It is shown that latent heat of condensation is very important for the total heat balance and the condensation rate is highly dependent on insulation between layers and outside temperature. Modelling results allow finding optimal geometrical parameters for the known gas flow and predicting the condensation rate for different system setups and seasons.

Purpose. The paper is aimed to present the new approach to the promotion of university students’ entrepreneurship which was elaborated and researched in the Faculty of Engineering Economics and Management of Riga Technical University within the study course „Economics of Entrepreneurship”. Methodology. It is argued that opportunity identification (Baron, 2006), opportunity creation (Sarasvathy, et al., 2003) and opportunity development (Virtanen, 2010) are the prerequisites for the creation of new economic values which makes the heart of entrepreneurship (Drucker, 1993). In order to make the study process more entrepreneurship enhancing, it is necessary to avoid traditional teaching and learning methods such as lectures, literature reviews and examinations (Gibb, 2002). Instead, the working out of opportunity-centered (Rae, 2003) and problem-based (Tan & Frank, 2006) study models, maximally “pushing” students into entrepreneurship through the structuring of learning like an entrepreneurial process (Hjorth & Johannisson, 2007) should become the key elements of contemporary advanced education for entrepreneurship. Therefore the “Opportunity-oriented 7-step-problem-based learning model for developing students’ entrepreneurship” (Oganisjana, 2015) was elaborated and tried out in secondary schools of Latvia and Malaysia within ASEM (Asia-Europe Lifelong Learning Research HUB) from 2011 to 2014. The analysis of the results of the research served as a basis for adapting this model for university students with its further application in practice within the framework of the National Research Program EKOSOC-LV in the autumn semester of 2014. It was qualitative research for discovering the aspects of the new approach to the teaching and learning process and getting students’ feedback in the beginning and in the end of the course. Students filled in Google Drive electronic forms and sent them to the authors. The questions of the reflections concerned what students liked and disliked in the course, what and who helped or disturbed them while working in teams, what and who surprised them, what they considered in the course to be especially important to them and what they would recommend to change, etc. The qualitative content analysis of the texts of the reflections provided the discovery of the factors which promote and hinder the opportunity–oriented problem-based teaching and learning for entrepreneurship in the university. Results. Students evaluated the course very positively, mentioning: 1) the given opportunity to solve real life problems and work with entrepreneurs; 2) development of their creativity, presentation, analytical and social skills, 3) the democratic atmosphere and teachers’ enthusiasm, professionalism and positive attitude towards them. However there was certain criticism related to overload with work, not clear instructions in some cases and the time of the lectures which took place very early in the morning. The theoretical contribution. The paper presents an entrepreneurship promoting study model, which links and unites two stages of lifelong learning: secondary school and university, thus making a universal theoretical basis for solving this topical educational problem. Practical implications “Opportunity-oriented 7-step-problem-based learning model for developing students’ entrepreneurship” can be an appropriate basis for organising studies not only in secondary schools and universities, but as well in adult and kindergarten education. Keywords: development of entrepreneurship, opportunity-oriented learning, problem-based learning. Paper type : Research paper.

ESTCube-1 is a student satellite project lead by the University of Tartu, Estonia, and supported by the European Space Agency (ESA) via Plan for European Cooperating States (PECS). Development of ESTCube-1 has been a collaborative effort with many international partners. The satellite is shown on Figure 1 [1].

The scientific mission of ESTCube-1, launched in May 2013, is to measure the electric solar wind sail (E-sail) force in orbit. The experiment is planned to push forward the development of the E-sail, a propulsion method recently invented at the Finnish Meteorological Institute. The E-sail is based on extracting momentum from the solar wind plasma flow by using long thin electrically charged tethers. ESTCube-1 is equipped with one such tether, together with hardware capable of deploying and charging it. At the orbital altitude of ESTCube-1 (660–680 km) there is no solar wind present. Instead, ESTCube-1 shall observe the interaction between the charged tether and the ionospheric plasma. The ESTCube-1 payload uses a 10-m, partly two-filament E-sail tether and a motorized reel on which it is stored. The tether shall be deployed from a spinning satellite with the help of centrifugal force. An additional mass is added at the tip of the tether to assist with the deployment. During the E-sail experiment the tether shall be charged to 500 V potential. Both positive and negative voltages shall be experimented with. The voltage is provided by a dedicated high-voltage source and delivered to the tether through a slip ring contact. When the negative voltage is applied to the tether, the satellite body is expected to attract the electron flow capable of compensating for the ion flow, which runs to the tether from the surrounding plasma. With the positive voltage applied, onboard cold cathode electron guns are used to remove excess electrons to maintain the positive voltage of the tether. In this paper we present the design and structure of the tether payload of ESTCube-1.

This paper presents the mission analysis, requirements, system design, system level test results, as well as mass andpower budgets of a 1-unit CubeSat ESTCube-1 built to perform the first in-orbit demonstration of electric solar wind sail (E-sail)technology. The E-sail is a propellantless propulsion system concept that uses thin charged electrostatic tethers for turning themomentum flux of a natural plasma stream, such as the solar wind, into spacecraft propulsion. ESTCube-1 will deploy and chargea 10 m long tether and measure changes in the satellite spin rate. These changes result from the Coulomb drag interaction with theionospheric plasma that is moving with respect to the satellite due to the orbital motion of the satellite. The following subsystemshavebeendevelopedtoperformandtosupporttheE-sailexperiment: atetherdeploymentsubsystembasedonapiezoelectricmotor;an attitude determination and control subsystem to provide the centrifugal force for tether deployment, which uses electromagneticcoils to spin up the satellite to one revolution per second with controlled spin axis alignment; an imaging subsystem to verify tetherdeployment, which is based on a 640 × 480 pixel resolution digital image sensor; an electron gun to keep the tether at a highpositive potential; a high voltage source to charge the tether; a command and data handling subsystem; and an electrical powersubsystem with high levels of redundancy and fault tolerance to mitigate the risk of mission failure.

In the last fifty years in medicine, engineering and other sciences widely is used finite element method (FEM), it is cost-effective alternative to the experimental approach for solving wide range of implant-biological tissue issues [1,2]. This method is applied for determination of stress-strain states in different systems. In the present study a stress-strain state of reconstruction system bone-implant was determined in theory by finite element program ANSYS. In order to solve this problem the mechanical characteristics of bone tissue were taken from literature [3], but characteristics of implants were obtained in experimental way in biomechanics laboratory in Riga Technical University. Afterwards obtained results were compared with the experimental ones.

This paper presents the design, implementation, and pre-launch test results of the Command and Data Handling Subsystem (CDHS) for ESTCube-1. ESTCube-1 is a one-unit CubeSat, which will perform an electric solar wind sail experiment. The development process of the CDHS for ESTCube-1 was focused on robustness and fault tolerance. A combination of hot and cold hardware redundancy was implemented. Software, including a custom-written internal communications protocol, was designed to increase the system's fault tolerance further by providing fault detection and fall-back procedures. Tests were carried out to validate the implementation's performance and physical endurance. The final CDHS design is operational within the set requirements. Tests that verify fault tolerance of the system in orbit are suggested.

This paper presents a novel low-power imaging system for nanosatellite proximity operations. A robust independent camera module with on-board image processing, based on the ARM Cortex-M3 microcontroller and fast static random access memory, has been developed and characterized for the requirements of the ESTCube-1 mission. The imaging system, optimized for use in a single unit CubeSat, utilizes commercial off-the-shelf components and standard interfaces for a cost-effective reusable design. The resulting 43.3 mm £ 22 mm £ 44.2 mm (W£ H£ D) aluminium camera module weighs 30 g and consumes on the average of 118 mW of power, with peaks of 280 mW during image capture. Space qualification and stress tests have been performed. A detailed case study for the ESTCube-1 10 m tether deployment monitoring and Earth imaging mission is presented. For this purpose a 4.4 mm telecentric lens, 10 bit 640£ 480 pixel CMOS image sensor, 700 nm infrared cut-off filter and a 25% neutral density filter are used. The resolution of the assembled system is 12.7 mm and 1 km per pixel at distances of 10 m and 700 km, respectively. Custom on-board image evaluation and high dynamic range imaging algorithms for ESTCube-1 have been implemented and tested. Optical calibration of the assembled system has been performed.

This paper presents the design, development, and pre-launch characterization of the ESTCube-1 Attitude Determination and Control System (ADCS). The design driver for the ADCS has been the mission requirement to spin up the satellite to 360 deg¢s i1 with controlled orientation of the spin axis and to acquire the angular velocity and the attitude during the scientific experiment. ESTCube-1 is a one-unit CubeSat launched on 7 May 2013, 2:06 UTC on board the Vega VV02 rocket. Its primary mission is to measure the Coulomb drag force exerted by a natural plasma stream on a charged tether and, therefore, to perform the basic proof of concept measurement and technology demonstration of electric solar wind sail technology. The attitude determination system uses three-axis magnetometers, three-axis gyroscopic sensors, and two-axis Sun sensors, a Sun sensor on each side of the satellite. While commercial off-the-shelf components are used for magnetometers and gyroscopic sensors, Sun sensors are custom- built based on analogue one-dimensional position sensitive detectors. The attitude of the satellite is estimated on board using an Unscented Kalman Filter. An ARM 32-bit processor is used for ADCS calculations. Three electromagnetic coils are used for attitude control. The system is characterized through tests and simulations. Results include mass and power budgets, estimated uncertainties as well as attitude determination and control performance. The system fulfils all mission requirements.