The paper discusses the principle of power amplification, as it can be found in many systems powered by PV panels and buffered by batteries. Under different categories of application examples, one is given by compacting containers for plastic or other waste materials. The main goal of such systems is to supply high-power intermittent systems from low power sources. The studied example discusses an energy harvesting system using a low power solar PV collection system dedicated to energize a specific application, sequentially operated at high power. The transformation of the power level is achieved using intermediary storage, where the charging sequence is characterized by a very low power level for longer time, followed by a shorter discharge sequence of the storage means with a much higher instantaneous power. The performance of the PV harvesting system is discussed from the point of view of its energy efficiency. Several solutions are discussed, and finally, a new 2-stage harvesting system is introduced. The requirement of a multistage amplification system is related to the power amplification ratio itself. The design method for the system relies on the concept of the so-called “Modified Ragone Representation”, MRR, that is shortly introduced in the paper. A prototype realization of the two-stage system is also presented.

The continuous quest for improving the performance of heat exchangers, together with evermore stringent volume and weight constraints, especially in enclosed applications (engines, electronic devices), stimulates the search for compact, high-performance units. One of the shapes that emerged from a vast body of research is the disc-shaped heat exchanger, in which the fluid to be heated/cooled flows through radial -often bifurcated- channels inside of a metallic disc. The disc in turn exchanges heat with the heat/cold source (the environment or another body). Several studies have been devoted to the identification of an “optimal shape” of the channels: most of them are based on prime principles, though numerical simulations abound as well. The present paper demonstrates that -for all engineering purposes- there is only one correct design procedure for such a heat exchanger, and that this procedure depends solely on the technical specifications (exchanged thermal power, materials, surface quality): the design in fact reduces to a zero-degree of freedom problem! The argument is described in detail, and it is shown that a proper application of the constraints completely identifies the shape, size and similarity indices of both the disc and the internal channels. Goal of this study is not that of “inventing” a novel heat exchanger design procedure, but that of demonstrating that -in this as in many similar cases-a straight forward application of prime principles and of diligent engineering rules may generate “optimal” designs. Of course, the resulting configurations may be a posteriori tested as to their performance, their irreversibility rates, their compliance with one or the other “techno-economical optimization methods”, but it is important to realize that they enjoy a sort of “embedded” optimality.

The paper develops generalizing entropic approaches of irreversible closed cycles. The mathematical models of the irreversible engines (basic, with internal regeneration of the heat, cogeneration units) and of the refrigeration cycles were applied to four possible operating irreversible trigeneration cycles. The models involve the reference entropy, the number of internal irreversibility, the thermal conductance inventory, the proper temperatures of external heat reservoirs unifying the first law of thermodynamics and the linear heat transfer law, the mean log temperature differences, and four possible operational constraints, i.e., constant heat input, constant power, constant energy efficiency and constant reference entropy. The reference entropy is always the entropy variation rate of the working fluid during the reversible heat input process. The number of internal irreversibility allows the evaluation of the heat output via the ratio of overall internal irreversible entropy generation and the reference entropy. The operational constraints allow the replacement of the reference entropy function of the finite physical dimensions parameters, i.e., mean log temperature differences, thermal conductance inventory, and the proper external heat reservoir temperatures. The paper presents initially the number of internal irreversibility and the energy efficiency equations for engine and refrigeration cycles. At the limit, i.e., endoreversibility, we can re-obtain the endoreversible energy efficiency equation. The second part develops the influences between the imposed operational constraint and the finite physical dimensions parameters for the basic irreversible cycle. The third part is applying the mathematical models to four possible standalone trigeneration cycles. It was assumed that there are the required consumers of the all useful heat delivered by the trigeneration system. The design of trigeneration system must know the ratio of refrigeration rate to power, e.g., engine shaft power or useful power delivered directly to power consumers. The final discussions and conclusions emphasize the novelties and the complexity of interconnected irreversible trigeneration systems design/optimization.

Preheated Schizochytrium sp. raw MAO was evaluated as a fuel in a single-cylinder four-stroke diesel engine to produce a comparative study of MAO and DO critical parameters. In particular, brake power, brake specific fuel consumption, brake thermal efficiency, in-cylinder pressure, exhaust gas temperature, NO_x and CO emission were investigated. Additionally, an engine durability test for longevity was undertaken over a 30 h period, using raw MAO as the fuel. The study demonstrated that the preheated MAO could be successfully used in a Diesel engine smoothly. The use of MAO reduced the engine brake power by 26% and increased brake specific fuel consumption by 20%. The most significant finding from this research study is that there was a significant reduction in NO_x and CO emission by 42% and 60% when using raw MAO, respectively. Therefore, these findings demonstrate that algae oil is a highly credible fuel for use in diesel engines and offers a promising solution to diesel engine emissions.

The decarbonisation of the energy sector is probably one of the main worldwide challenges of the future. Global changes urge a radical transformation and improvement of the energy-producing systems to meet the decarbonisation targets and a reduction of greenhouse gas emissions. The hydrocarbon industry also contributes to this transition path. In a mature stage of oil and gas fields, the production of hydrocarbons is associated with formation waters. The volume of produced water increases with the maturity of the assets and the conversion into geothermal wells could be an alternative to the mining closure. In the described transition scenario, the geothermal energy seems very promising because of its wide range of applications depending onthe temperature of extracted fluids. This flexibility enables to propose projects inspired to a circular economy vision with the integration in the territory and social acceptance. In Italy, since 1985, 7246 well has been drilled for hydrocarbonof which 898 are located on-shorewith a productive or potentially productive operational status. The paper presents a preliminary investigation on oil and gas fields located on shore in Italian territory based on the available informationon temperature distribution at different depth. Then, taking into account the local energy demand, existing infrastructure, and land use of the territory, a conversion strategy for the producing wells has been proposed for three case studies.