an experimental and numerical analysis of the performances of a wankel steam expander

Abstract In the last decades, the energy market increased its interest towards the smart grids and electrically isolated systems. These systems utilize small size power generators in which volumetric expanders may be employed for a wide range of operative conditions, because of their robustness and reliability. In this work a study on a volumetric expander based on the Wankel mechanism was carried out. The aim of this study was to develop a lumped parameters numerical model able to predict the brake effective torque and working fluid consumption of the expander. This model was validated by comparison with experimental results obtained using steam as working fluid. This model allowed to trace the trends of mechanical and thermal losses versus rotating speed and inlet pressure. The experimental results encouraged the need for a further development of this expander, and showed the capability of the numerical model to predict the effective performances of the device

cfd optimization of cpc solar collectors

Abstract The use of solar energy for industrial purpose at medium-low temperature is receiving attention. As a matter of fact, this temperature range, usually between 80-200°C, requires low cost devices to convert solar energy into useful heat. In particular, the use of CPCs collectors has been suggested in literature because they can be operated without the use of a tracking system, at least within certain limits. The thermal losses of these devices are often reduced by using an evacuated pipe, but this solution increases the manufacturing costs and reduces the reliability and the optical efficiency of the receiver. A series of alternative methods for the thermal losses reduction has been proposed in this paper, for working temperature up to 200°C. Their effectiveness was evaluated by means of a previously validated CFD model. A cylindrical receiver and a concentration ratio of 2 were taken into account. The results were analyzed in terms of temperature contours and thermal efficiency. In particular, the optical efficiency was focused as a key parameter in the performances of a CPC. As conclusion, it was found that a proper arrangement of the absorber with a baffle may entail an improvement of the thermal efficiency without significantly increasing the complexity of the system

Influence of emitter-receiver number on measurement accuracy in acoustic pyrometry

Acoustic pyrometry is an interesting technique that may find several useful applications in turbomachinery. As the speed of sound is directly related a medium temperature, this measurement technique estimates the temperature of a gas by considering the time of flight of an acoustic wave moving through it. If only an acoustic emitter-receiver couple is used, only the average temperature along the acoustic path can be determined. If multiple emitter-receiver couples laying on the same plane are used, a reconstruction of the temperature map in the section is possible. This estimation is performed by considering that multiple acoustic paths travel across the same sub-portions of the section and, therefore, the temperature of each sub-portion affects the time of flight along several sound paths. Many parameters affect the accuracy of the measurement, and they are related to the physic of the phenomena involved in the measurement, the accuracy of the instrumentation used, the interaction between the acoustic wave and the flow velocity and the hardware set-up. In this study, the impact of some set-up parameters on the accuracy of the measurement was investigated and, in particular, the number of sound emitter-receiver couples and the number of investigation sub-portions in which the section is divided. A reference temperature map has been considered as a benchmark. This study, which is a preliminary investigation on this technique, was useful to assess the capability of this methodology to correctly describe a temperature distribution in an ideal condition. Therefore, it represents a first step in the set-up of an experimental investigation with an acoustic pyrometer.

experimental results of a wankel type expander fuelled by compressed air and saturated steam

Abstract The work presented in this paper deals with the experimental tests which were carried out on a prototype of a rotary volumetric expansion device based on the Wankel mechanism. This expansion device is addressed to small size power plants (in the range 5-50 kW) for distributed micro-generation using various sources of thermal energy, such as sun, biomass and waste heat. The prototype was built using an internal combustion Wankel engine, employing the shaft, the rotor, the bearings, while the statoric case was newly built on the design of the University of Pisa. Firstly, the tests were carried out with the compressed air produced by a compressor, then the prototype was fed with the saturated steam produced by a biomass boiler. In the first case, the exhaust back-pressure was the atmospheric one, in the second case vacuum conditions were employed thanks to a condenser. The inlet pressure was between 4 and 8 bar. The results showed the capability of the prototype to rotate regularly at 3000 rpm, and allowed the validation of numerical models presented in previous papers. Moreover, the expansion device showed the capability of developing the expected power

poly generation capability of a biogas plant with upgrading system

Abstract Biomass, together with other renewable sources, is increasingly used to provide energy to minigrids and distributed generation systems. Particularly, biogas production seems an interesting solution as it can be used to produce electricity, heat and bio-methane (through an upgrading system). In addition, biogas can be relatively easily stored in gasometers to compensate for small request variations. On the other hand, the amounts of heat, electricity and bio-methane produced are strictly dependent one on the others. A poly-generation scenario was considered starting from an existing case study made up of a digester, a 600kWel micro gas turbine and an upgrading system for bio-methane production. An off-design system simulation was carried out to analyze the energy and mass fluxes between plant components as a function of the fraction of the biogas sent to the upgrader. The constraints and relations between heat, electricity and bio-methane production were extensively analyzed. Results show that this system can be a versatile poly-generation unit

Bio-additives for CI engines from one-pot alcoholysis reaction of lignocellulosic biomass: An experimental activity

In the recent years the progressive decrease in fossil petroleum resources and gradual deprivation of the environment have attracted increasing interest towards the use of biomass as renewable carbon source for the production of chemicals and transportation fuels. In particular, lignocellulosic biomass represents an abundant and inexpensive renewable resource with high carbon sequestration ability and non-polluting. In this paper, the valorisation of mixtures made of n-butanol (n-BuOH), butyl levulinate (BL) and dibutyl ether (DBE), in different percentages, as additive fuel for compression ignition (CI) internal combustion engine (ICE) was studied. These mixtures can be directly obtained from the catalytic alcoholysis reaction of the cellulosic fraction of raw and pre-treated lignocellulosic biomasses. Moreover, the possibility to recycle and reutilize the excess alcohol (n-Butanol), during the catalytic alcoholysis reaction, has been considered since it represents an opportunity to reduce the overall costs of the process. Therefore, a blend constituted only by BL and DBE has been also tested. The model mixtures were prepared by using commercial reactants, characterized by compositions analogous to those of the reaction mixtures. These model mixtures were tested as blend with Diesel fuel in a CI-ICE with the measurement of pollutant emission and performance. Results have been compared with those obtained fuelling the engine with a commercial Diesel fuel. As a whole, tests results have evidenced the potentiality of these novel blending mixtures to reduce the emissions of particulate without any significant increase in the other pollutants and negligible changes in engine power and efficiency

Design of the traction battery for a Formula SAE racing car

This paper describes the design of the traction battery for the new electric Formula SAE vehicle of the University of Pisa. A model based design methodology extended to the mechanical, electrical and thermal domains was applied to find the best trade-off between the battery weight and the maximum power available at the wheel. The designed battery configuration was validated by means of electrical and thermal simulations

An Experimental Investigation on the Effect of Exhaust Gas Recirculation in a Small-Scale Fixed Bed Biomass Boiler

Exhaust gas recirculation is a technique that allows for controlling the combustion chamber temperature and reducing the NOx and particle matter emissions. Moreover, it helps to mitigate soot formation and ash agglomeration in combustion systems. The present study investigated the effect of exhaust gas recirculation on combustion temperatures of a 140 kW underfed stoker biomass boiler. To this purpose, a wide range of operating conditions were used, collecting data regarding flue gas and fixed bed temperatures. It turned out that the recirculating ratio has a significant effect on the temperatures in the primary combustion zone, affecting the thermal gradient and the main thermal zones of the biomass combusting bed. The obtained results can be useful for lumped parameter modeling, or CFD validation purposes

Woodchip size effect on combustion temperatures and volatiles in a small-scale fixed bed biomass boiler

Biomass combustion performance is greatly affected by the particle size distribution, which influences heat and mass transport phenomena. The present work investigates the effect of woodchip size distribution on combustion in a 140 kW underfeed stoker boiler. Three different fuel sizes were prepared, and their combustion performance was measured by monitoring temperatures inside and above the fire pit and the gas composition above the fuel bed. The gas composition was then correlated to the particle mean diameter. Although minor effects could be detected in the temperature and composition of the flue gases, a more uniform spatial distribution of volatiles was observed when employing bigger woodchips. The present results can improve the understanding of the impact of fuel size on the performance of woodchip-fired boilers and can be valuably used for numerical model validation

Characterisation of glue behaviour under thermal and mechanical stress conditions

New low-cost measuring devices require that the box housing and electronics have the cost aligned with the sensing system. Nowadays, metallic clips and/or glue are commonly used to fix the electronics to the box, thus providing the same motion of the structure to the sensing element. However, these systems may undergo daily or seasonal thermal cycles, and the combined effect of thermal and mechanical stress can determine significant uncertainties in the measurand evaluation. To study these effects, we prepared some parallel plates capacitors by using glue as a dielectric material. We used different types of fixing and sample assembly to separate the effects of glue softening on the capacitor active area and plates distance. Therefore, we assessed the sample modification by measuring the capacitance variation during controlled temperature cycles. We explored possible non-linear behaviour of the capacitance vs. temperature, and possible effects of thermal cycles on the glue geometry. Further work is still needed to properly assess the nature of this phenomenon and to study the effect of mechanical stress on the sample's capacitance