A hybrid energy system, based on renewable energy sources and with hydrogen storage, can become an alternative for stand-alone electricity and heat supply. The objective of this work is to evaluate the feasibility of a completely renewable supply of power and heat for an isolated household, and a comparison to reference and alternative energy supply scenarios. In this paper, an energy system using fossil and renewable energy sources is compared to a system using only renewable energy sources (solar and wind) with hydrogen-based energy storage technologies. A reference household in Slovenia's coastal region was used for modelling and numerical simulation. Simulations and optimal energy system identification were conducted by considering the geographical location and availability of energy sources, load dynamics, and components' technical and economical characteristics. A household with electricity consumption of 11 kWh/day, hourly peak power demand of 3.8 kW and 660 L of oil-equivalent yearly heat demand was considered as the stand-alone load. The results show that 100% renewable electricity and heat supply of a reference household is technically feasible and is more cost-effective, compared to systems utilising fossil heat.
COBISS.SI-ID: 13637147
Efficiently combining proton exchange membrane fuel cell (PEMFC) stack with methanol steam reformer (MSR) into a small portable system is still quite a topical issue. Using methanol as a fuel in PEMFC stack includes a series of chemical processes where each proceeds at a unique temperature. In a combined MSR-PEMFC-stack system with integrated auxiliary fuel processors (vaporizer, catalytic combustor, etc.) the processes are both endothermic and exothermic hence their proper thermal integration can help raising the system efficiency. A concept of such fully integrated and compact system is proposed in this study. Three separate systems are designed based on different PEMFC stacks and MSR. Low-temperature (LT) and conventional high-temperature (cHT) PEMFC stack characteristics are based on available data from suppliers. Also, a novel high-temperature (nHT) PEMFC stack is proposed because its operating temperature coincides with that of MSR. A comparative study of modelled systems is performed using a mass and energy balances zero-dimensional model, which is interdependently coupled to a physical model based on finite element method (FEM). The results indicate that a system with nHT PEMFC stack is feasible and has the potential to reach higher system efficiencies than systems with LT or cHT PEMFC stacks.
COBISS.SI-ID: 13635611
The work presented here characterises the influences of different waste-derived fuels on the combustion process in a microturbine. The two most common types of solid waste are converted into liquids by liquefaction in polyhydric alcohols and by pyrolysis to produce the novel fuels LW (liquefied wood) and TPO (tire pyrolysis oil). Baseline results were obtained with diesel fuel and with raw polyhydric alcohols otherwise used in wood liquefaction process. The original contribution of this analysis is the in-depth comparison of the combustion and emission phenomena of LW and TPO under different operating conditions. The results revealed significantly higher CO (carbon monoxide) and THC (total hydrocarbons) emissions for LW due to its reduced atomisation ability and increased spray penetration in line with the physical and chemical properties of the fuel. The combustion properties of TPO resembled those of diesel fuel in terms of CO and THC. NOx (nitrous oxides) emissions reflected the elemental composition of the fuels. In addition, pronounced soot formation is observed when utilizing TPO, whereas for LW, the opacity measurements are surprisingly low. These results indicate that TPO can be exploited directly in gas turbines, whereas LW requires adaptations in the fuel supply system and a narrower range of operating conditions.
COBISS.SI-ID: 13791003
Innovative lignocellulosic biofuel, obtained through solvolysis of spruce wood in multifunctional alcohols was tested in an experimental turbine engine with different primary air temperatures. Variation of primary air temperature was used to emulate two types of microgas turbine generators - fully recuperated and simple cycle setups resulting in different temperatures of combustion chamber intake air. Results indicate that different temperatures, velocities, and flow conditions in primary zone of combustion chamber strongly influence on droplet penetration depth and rate of mixture formation as well as emissions formation. For the innovative lignocellulosic biofuel emission trends of CO and THC were found to be significantly higher in simple cycle mode, whereas no influence of operation mode was observed for baseline diesel fuel. NOx emissions of innovative biofuel generally increased in regenerative cycle mode and also became sensitive on turbine inlet temperature, with similar trends being observed with diesel fuel. In the case of innovative fuel, deposits of char and polymerized fuel were observed on the combustor walls after operation in simple cycle mode, whereas in regenerative cycle mode, only small amounts of ash deposits were found on hot path surfaces, indicating beneficial influence of high primary air temperatures on combustion efficiency of innovative fuel.
COBISS.SI-ID: 13804059
Ultrasonic horn transducers are frequently used in applications of acoustic cavitation in liquids, for instance, for cell disruption or sonochemical reactions. They are operated typically in the frequency range up to about 50 kHz and have tip diameters from some mm to several cm. It has been observed that if the horn tip is sufficiently small and driven at high amplitude, cavitation is very strong, and the tip can be covered entirely by the gas/vapor phase for longer time intervals. A peculiar dynamics of the attached cavity can emerge with expansion and collapse at a self-generated frequency in the subharmonic range, i.e., below the acoustic driving frequency. Here, we present a systematic study of the cavitation dynamics in water at a 20 kHz horn tip of 3 mm diameter. The system was investigated by high-speed imaging with simultaneous recording of the acoustic emissions. Measurements were performed under variation of acoustic power, air saturation, viscosity, surface tension, and temperature of the liquid. Our findings show that the liquid properties play no significant role in the dynamics of the attached cavitation at the small ultrasonic horn. Also the variation of the experimental geometry, within a certain range, did not change the dynamics. We believe that the main two reasons for the peculiar dynamics of cavitation on a small ultrasonic horn are the higher energy density on a small tip and the inability of the big tip to wash away the gaseous bubbles. In the spirit to distinguish the observed phenomenon with other cavitation dynamics at ultrasonic transducer surfaces, we suggest to term the observed phenomenon of attached cavities partly covering the full horn tip as acoustic supercavitation.
COBISS.SI-ID: 13368347