• Energy Research

In this work, artificial neural networks (ANNs) were applied to describe the performance of a micro gas turbine (MGT). In particular, they were used (i) to complete performance diagrams for unavailable experimental data; (ii) to assess the influence of ambient parameters on performance; and (iii) to analyze and predict emissions of pollutants in the exhausts. The experimental data used to feed the ANNs were acquired from a manufacturer’s test bed. Though large, the data set did not cover the whole working range of the turbine; ANNs and an artificial neural fuzzy interference system (ANFIS) were therefore applied to fill information gaps. The results of this investigation were also used for sensitivity analysis of the machine’s behavior in different ambient conditions. ANNs can effectively evaluate both MGT performance and emissions in real installations in any climate, the worst R2 in the validation set being 0.9962.

This work suggests an interpretation to the quantitatively higher formation of NOx in a compression ignition (CI) engine when fueled with pure biodiesel (B100). A comparative study about the use of rapeseed oil methyl ester (RME) and diesel fuel mixtures on injection timing, in-chamber pressure, heat release rate, and NOx emissions were carried out using a diesel engine equipped with a pump-line-nozzle injection system. Such engines are still widely adopted mainly in agriculture, as the fleet of agricultural machinery is particularly old (often over 20 years) and the use of biofuels can reduce the environmental footprint of the sector. This work aims to supply some general explanations and figures useful to interpret the phenomena occurring within the fuel line and in the combustion process when using biodiesel, as well as in engines with different construction characteristics and fueling systems. Given the contradictory results available in the literature, the so-called “biodiesel NOx effect” cannot be explained solely by the different physical properties of biodiesel (in particular, a higher bulk modulus). Experimental results show that, with the same pump settings, the start of injection with the RME is slightly advanced while the injection pressure values remain almost the same. With the RME, the pressure in the injection line increases faster due to its greater bulk modulus but the pressure rise starts from a lower residual pressure. The start of combustion takes place earlier, the heat release during the premixed phase is steeper, and a higher peak is reached. The NOx emissions with the RME are at least 9% higher when compared to mineral diesel fuel. The greater amount of the RME injected per cycle compensates for its minor lower heating value, and the brake torque at full load is similar to the two analyzed fuels. Finally, a variation of the pump line timing is evaluated in order to assess the effect of the delay and the advance of the injection on the performance of the engine and on the emissions. A viable and simple solution in the variation of the injection strategy is suggested to counterbalance the biodiesel NOx effect.

Abstract In this work a small-scale non-regenerative Organic Rankine Cycle test rig has been developed to experimentally assess the performance of the scroll compressor SANDEN TRS090 converted into an expander. The performance of the expander has been evaluated using R245fa as working fluid in terms of overall efficiency and filling factor under variable flow rates, expansion ratios, rotational speeds and superheating temperatures. A peak isentropic efficiency of 45% has been obtained for an expansion ratio of 1.9–2.1 whilst the rotational speed has a minor effect. The highest values of expander overall efficiency have been achieved for a filling factor ranging from 2.5 to 3 which correspond to rotational speeds higher than 1000 rpm. During the tests the expander has achieved a mechanical power output of about 650 W for an expansion ratio in the range 2–2.2 whilst the peak values of the mechanical efficiency of the plant were >3% (self-consumption of the pump and auxiliaries excluded) for an expansion ratio of 2.2–2.5. Eventually, it was proved that the machine has no advantages in terms of overall efficiency when the fluid is superheated but the whole ORC system has a higher performance with a superheating grade in the range 10–30 K.

Abstract As the demand for energy rises fossil fuel reserves are depleted daily, increasing the interest in alternative fuels. Biodiesel is one of the best candidates in this class and its use is expected to expand rapidly throughout the world. Numerous researchers have been investigating how biodiesel affects combustion, pollutant formation and exhaust aftertreatment. There is general agreement that its combustion characteristics are similar to those of standard diesel fuel, except for a shorter ignition delay, a higher ignition temperature, and greater ignition pressure and peak heat release. Engine power output is similar with both fuels. As regards emissions, reductions in particulate matter (PM) and carbon monoxide (CO) and increases in nitrogen oxides (NOx) are described with most biodiesel blends. The latter is referred to as the ‘biodiesel NOx effect’. The vast majority of researchers who explored the effect of biodiesel did so in mechanical injection engines. They found that the primary mechanism by which biodiesel increases NOx emissions is by an inadvertent advance in the start of injection timing, caused by a higher modulus and viscosity. However, more recent studies show that NOx emissions also increase in biodiesel-fuelled common rail engines, and that in some cases they actually decrease in engines with mechanically controlled fuel injection systems. This cannot be explained solely by differences in compressibility and remains an open question. The present study provides a contribution to the discussion in this field by describing a new method to evaluate the injection advance in engines with mechanically controlled pumps. The experimental data show that the advances in the start of injection timing, using biodiesel rather than mineral diesel, are smaller than those calculated with standard methods and may even not occur at all, depending on injection system design. In addition, they demonstrate that, contrary to common belief, injection pressure does not always increase when using biodiesel. These data may help explain why some researchers have found similar or even reduced NOx emission also with mechanical injection systems.

This paper addresses a hybrid renewable system that consists of a micro-Combined Cooling Heat and Power (CCHP) unit and a solar energy conversion device. In addition to a traditional PV system, a High Concentrator Photovoltaic (HCPV) device, the design of which is suitable for building integration application, was also modelled and embedded in the hybrid system. The work identifies the optimal management strategies for the hybrid renewable system in an effort to minimise the primary energy usage, the carbon dioxide emissions and the operational costs for variable electricity prices that result from the day-ahead electricity market. An “ad hoc” model describes the performance of the HCPV module, PV and Internal Combustion Engine, whilst the other units were simulated based on their main characteristic parameters. The developed algorithm was applied to three different building typologies. The results indicate that the best configuration is the hybrid renewable system with PV, which can provide a yearly primary energy reduction of between 20% and 30% compared to separate production. The hybrid renewable system with HCPV becomes competitive with the PV technology when the level of solar radiation is high.

The paper presents part of the results of two studies, the European “Radar” (Raising Awareness on renewable energy Developing Agro-eneRgetic chain models) Project and the “Energy and environmental plan for the consortium of the municipalities of the Esino-Frasassi mountain area”, conducted in an area in central Italy. The area is characterized by huge forestry biomass resources and by substantial amounts of agricultural residues. The work presents a technical-economic study of a cogeneration plant using a solid biomass-fuelled micro turbine as the prime mover. The energy conversion of solid biomass can be achieved with different technologies, e.g. organic Rankine cycles, micro turbines with an external combustion chamber, or Stirling engines. The choice of the conversion system depends mainly on biomass availability and on the level of user demand. Of the conversion technologies mentioned above, the micro turbine is suitable to meet the requirements of the cogeneration plant examined here, which is applied to a low thermal demand public building. The work describes a micro turbine based on a regenerative Brayton cycle endowed with an external combustion chamber. The inlet air, after being compressed, passes through a regenerator and then through an external furnace fuelled by solid biomass, where it is further heated, and finally expands through the turbine. The outlet air of the turbine, before being funnelled through the chimney, passes through the regenerator and subsequently through a dry kiln, thereby reducing the humidity of the solid biomass. The micro turbine studied produces 75 kWe and 300 kWt. The biomass is made up of olive tree prunings. After the technical analysis, an economic study stresses the critical role of incentives systems (herein provided by the Italian legislation) in making the technology appealing to investors in renewable energy solutions. The energy and economic analysis considers different combinations of three different amounts of annual operation hours, of two operating modes (with/without cogeneration) and three purchase prices of the solid biomass. The incentives mechanism considered is the Feed-In Tariff (FiT) granted by the Italian legislation for plants < 1 MWe. The economic analysis highlights some influential factors for solid biomass-fuelled systems: contract with fuel suppliers, biomass price, availability, transportation, storage, and processing, and plant location. In particular, the purchase price of solid biomass is substantially negotiated between the manager of the energy conversion plant and suppliers. The work demonstrates the crucial role of the incentives mechanisms for economic sustainability; the strong influence of biomass price on investment profitability; and the role of cogeneration in further shortening the payback period.

Micro-combined heat and power (CHP) systems are a key resource to meet the EUCO2 reduction agreed in the Kyoto Protocol. In the near future they are likely to spread significantly through applications in the residential and service sectors, since they can provide considerably higher primary energy efficiencies than plants generating electricity and heat separately. A 28 kWe natural gas, automotive-derived internal combustion engine CHP system was modeled with a view to comparing constant and variable speed operation modes. Besides their energy performances, the paper addresses the major factors involved in their economic evaluation and describes a method to assess their economic feasibility. Typical residential and service sector applications were chosen as test cases and the results discussed in terms of energy performances and of profitability. They showed that interesting savings can be obtained with respect to separate generation, and that they are higher for the household application in variable speed operating conditions. In fact the plant’s energy performance is greatly enhanced by the possibility, for any given power, to regulate the engine’s rotational speed. From the economic viewpoint, despite the higher initial cost of the variable speed concept, the system involves a shorter pay-back period and ensures greater profit.

Abstract The pulp and paper industry is an energy-intensive sector which the need for heat and electricity throughout the year makes an ideal user of cogeneration. This paper presents a survey of cogeneration plants installed in the Italian pulp and paper industry from 1986 to 2010 including the technologies installed and the size and the timeline of installations. The work, carried out in cooperation with ASSOCARTA (the trade organization of Italian pulp, paper and board manufacturers), examines 61 cogeneration plants, 14 of them in detail. The analysis involves 673.5 MWe of installed electrical power, accounting for 75.7% of the sector (890 MWe); the average plant size in the sample is 11.1 MWe, the Italian sector average being 18.2 MWe. Gas turbines coupled with heat recovery steam generators are the commonest technology in the low power range, with 35 plants found between 1 and 8 MWe. If combined cycles (commonly installed above 8 MWe) are considered, the cogeneration plants using gas turbines are 55/61. Our data show that from 1986 to 2010 nearly all plants worked with a positive PES (primary energy saving) index, using less primary energy compared to separate production of electrical and thermal energy. Only two plants had a slightly negative PES index, but the price of electricity and natural gas was such that they made a profit anyway.