Sizing of a cogeneration unit based on fuel cells and on steam reforming of diluted bioethanol
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Data di Pubblicazione:
2018
Citazione:
Sizing of a cogeneration unit based on fuel cells and on steam reforming of diluted bioethanol / I. Rossetti, A. Tripodi, M. Compagnoni, G. Ramis. ((Intervento presentato al convegno European Hydrogen Energy Conference tenutosi a Malaga nel 2018.
Abstract:
Introduction
The production of electric energy for small-scale applications, using hydrogen as energy vector, can rely nowadays on the already established fuel cell technology coupled to efficient reforming catalysts and processes [1-3], that set renewable energy sources as viable alternatives to the traditional energy production and distribution chain. Bioethanol, among other feedstocks, is very promising to meet the demand of the large market of distributed energy cogeneration. With respect to reforming plants based on pure ethanol, different strategies can rely on less expensive diluted bioethanol. In this case, the system is much more flexible both for the fuel supply and for the possible operation modes, provided that the additional heat input required to vaporize excess water quantities can eventually be retrieved as useful thermal energy.
With this work, both steady state and dynamic simulation were used for a detailed design of a residential size cogeneration system based on a bioethanol-to-hydrogen-to-power technology. The basic layout and preliminary sizing of a water circuit has been also performed, that recovers a substantial fraction of the waste heat released by a bioethanol reforming unit already sized to meet a home-scale power production of 5 kWelectric. The thermal energy is sufficient to cover the steady-state wintertime dispersion for a class ‘F’ two-storey house in the Northern Italy climate with a traditional radiation system, while the more demanding sanitary water (DHW) production can be met by resorting to a micro-accumulation strategy. The flexibility provided by a diluted hydro-alcoholic feed mixture with a separate reformer-fuel cell layout makes it possible to tune the system performance according to the instant house need, and the use of hydrogen as a vector rather than as a source let foresee an easier feedstock supply and management.
Models and methods
The calculation of the reforming apparatus has been performed using Aspen Plus® V8.8 with the PURE32 Databank. The Peng-Robinson and NRTL thermodynamic systems have been employed for the whole flowsheet, while for the simpler steady-state radiators power we adopted the STEAM-TAB plus PENG-ROB packages. The dynamic simulation of the DHW (Hot Water) delivery has been modeled and solved with Matlab® V7.10 (using the ‘ode45’ algorithm to integrate the differential equations). The algorithm used to estimate a model house heat dispersions runs on MS Excel™.
Results and discussion
An increased water content in the feed, i.e. the use of diluted bioethanol, has an overall beneficial effect on the steam reforming system for hydrogen production. On one hand, the heat subtracted to the burned gas downstream the reformer (Fig.1), once the hydrogen production is accomplished, is released anyway at the condenser. Furthermore, the CO purification is easier through waster gas shift and, hence, the methanation step can be downsized. The lower temperatures and flowrates at the burner exit makes the reformer operation much more stable with respect to the gases split fraction, eliminating the instability range which prevents the increase of the FC power at intermediate utilization factors.
A high water dilution, on the other hand, is not compatible with a full FC exploitation of the reformate, needing separation in a condenser. High dilution can give raise to tricky regime-change and instability phenomena at too low values of the utilization factors of the fuel cell, where the hot gases production and utilization are both enhanced to opposite effects. The details of this behavior are anyway dependent on the absolute ethanol quantity employed, since this value fixes the total power available from the system in any form (electrical or thermal), but
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
I. Rossetti, A. Tripodi, M. Compagnoni, G. Ramis
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