A microbial fuel cell (MFC) is a promising renewable energy option, which enables the effective and sustainable harvesting of electrical power due to bacterial activity and, at the same time, can also treat wastewater and utilise organic wastes or renewable biomass. However, the practical implementation of MFCs is limited and, therefore, it is important to improve their performance before they can be scaled up. The surface modification of anode material is one way to improve MFC performance by enhancing bacterial cell adhesion, cell viability and extracellular electron transfer. The modification of graphite felt (GF), used as an anode in MFCs, by electrochemical oxidation followed by the treatment with ethylenediamine or p-phenylenediamine in one-step short duration reactions with the aim of introducing amino groups on the surface of GF led to the enhancement of the overall performance characteristics of MFCs. The MFC with the anode from GF modified with p-phenylenediamine provided approx. 32% higher voltage than the control MFC with a bare GF anode, when electric circuits of the investigated MFCs were loaded with resistors of 659 Ω. Its surface power density was higher by approx. 1.75 times than that of the control. Decreasing temperature down to 0 °C resulted in just an approx. 30% reduction in voltage generated by the MFC with the anode from GF modified with p-phenylenediamine.

Medium-scale solar hot water systems with a total solar panel area varying from 60 to 166 m² have been installed in Lithuania since 2002. However, the performance of these systems varies depending on the type of energy users, equipment and design of the systems, as well as their maintenance. The aim of this paper was to analyse operational SHW systems from the perspective of energy production and economic benefit as well as to outline the differences of their actual performance compared to the numerical simulation results. Three different medium-scale solar thermal systems in Lithuania were selected for the analysis varying in both equipment used (flat type solar collectors, evacuated tube collectors) and type of energy user (swimming pool building, domestic hot water heating, district heating). The results of the analysis showed that in the analysed cases the gap between measured and modelled data of heat energy produced by SHW systems was approx. 11%. From the economical perspective, the system with flat type solar collectors used for domestic hot water production was proved to be most efficient. However, calculation of Internal Rate of Return showed that a grant of 35% is required for this project to be fully profitable.

The cold climate countries require high energy consumption for buildings’ heating. According to EU directives and national law, buildings’ energy efficiency is increasing due to higher investment in the sector. Primary energy consumption for space heating still comprises a large part of global energy consumption. It is essential to develop technological solutions and innovations to reduce energy consumption by using newer, smarter, more natural energy generation and accumulation. The soil layer could be used as a natural material for thermal energy accumulation. The soil’s temperature is higher than atmospheric air in the heating season and is lower in the non-heating season. Underground buildings placed in a soil medium could use less thermal energy for buildings’ heating and cooling during its life cycle. The impact of the wind is eliminated in this underground building case. As the soil temperature rises, the difference in temperature of the building’s inside air and the soil decreases. This means that the heat loss into the soil generates the conditions acting against the heat loss. However, heat spreads further and dissipates in the surrounding soil medium. The analysis of this research results showed that the savings in energy could reach 28 percent in the case of the underground building. Heat loss to the soil could be treated as the charge of the soil by thermal energy. The charging by heat and heat dissipation in the soil was researched experimentally. The dependence of the intensity of the charge on time was analysed and presented in this paper also.

A significant portion of energy losses in industrial systems arises from the inefficient use of high-temperature exhaust gases, emphasizing the need for enhanced heat recovery strategies. This study aims to improve energy efficiency by examining the effects of radiation-intensifying inserts on combined radiative and convective heat transfer in flue-gas heated channels. A systematic literature review revealed a research gap in understanding the interaction between these mechanisms in flue-gas heat exchangers. To address this, analytical calculations were conducted for two geometries: a radiation-intensifying plate between parallel plates and the same insert in a circular pipe. The analysis covered a range of gas-flue and wall temperatures (560–1460 K and 303–393 K, respectively), flow velocities, and spectral emissivity values. Key performance metrics included Reynolds and Nusselt numbers to assess flow resistance and heat transfer. Results indicated that flue-gas temperature has the most significant effect on total rate of heat transfer, and the insert significantly enhanced radiative heat transfer by over 60%, increasing flow resistance. A local Nusselt number minimum at a length-to-diameter ratio of approximately 26 suggested transitional flow behavior. These results provide valuable insights for the design of high-temperature heat exchangers, with future work planned to validate the findings experimentally.