Dagli scienziati italiani un prototipo per trasformare i rifiuti in risorse.

 A DUE ANNI DALLA XXI GIORNATA MONDIALE DELL’AMBIENTE, ACCADEMIA NAZIONALE DEI LINCEI.

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ECCO COSA CI HA PROMESSO LA SCIENZA.

QUANTO NON REALIZZATO NON E’ COLPA DEI NOSTRI SCIENZIATI, MA DELLA  CIECA, IGNORANTE E CORROTTA BUROCRAZIA CHE CI CIRCONDA.

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 QUALE RICERCA PER L’ENERGIA DEL FUTURO

CONVEGNO INTERNAZIONALE CON IL PREMIO NOBEL CARLO RUBBIA.

5 e 6 novembre all’Accademia dei Lincei con la Fondazione ENI Enrico Mattei

Le più recenti ricerche a livello mondiale per produzione, uso e distribuzione delle fonti energetiche sia fossili sia non convenzionali, sono state presentate all’Accademia dei Lincei nei giorni 5 e 6 novembre 2013, dai più eminenti scienziati internazionali tra cui Carlo Rubbia, Michael Graetzel e Daniel Nocera, nel convegno “La sfida dei terawatt: Quale ricerca per l’energia del futuro?”, organizzato con il supporto della Fondazione ENI Enrico Mattei.

-La crescente domanda di energia, che è fattore fondamentale per la qualità della vita e per lo sviluppo economico, ha portato i consumi ai livelli di migliaia di miliardi di watt, una quantità che si misura in “terawatt” e ben indica le prospettive della domanda globale di energia.

–La necessità di superare l’impiego degli idrocarburi con nuove fonti richiede lo sviluppo di tecnologie capaci di armonizzare sicurezza economica ed energetica e la ricerca sta svolgendo un ruolo determinante per assicurare un futuro alle richieste degli abitanti della Terra.

Recupero energetico da rifiuti mediante celle a combustibile microbiche.

 Rosa Anna NASTRO, Giacomo FALCUCCI, Elio JANNELLI, Mariagiovanna MINUTILLO, Vincenzo PASQUALE, Stefano DUMONTET (Università di Napoli ‘Parthenope’) – Michele PAVONE, Maria TOSCANESI, Marco TRIFUOGGI (Università di Napoli ‘Federico II’).

Più sotto: development of nuclear energy in a sustainable frame (Rolando Calabrese Università di Padova).

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Proposta di enunciato finale derivato dal Convegno Terawatt a Roma.

 

Nella attuale produzione dell’energia su scala mondiale

-le fonti fossili, in particolare gli idrocarburi, rivestono tuttora un ruolo egemone per cui il loro paventato repentino esaurimento  porterebbe ad una grave crisi economica. In realtà la loro produzione è soggetta ad un rilancio in seguito al miglioramento dei processi di trivellazione con conseguente  scoperta dello shale gas,  e alle concrete prospettive di utilizzare le immense risorse di gas naturale intrappolato  nei sedimenti marini ai margini della piattaforma continentale delle regioni artiche.

-Questi eventi,  pur essendo controversi per i danni che possono arrecare sull’ambiente, stanno però influenzando il quadro geopolitico e geo-economico mondiale,  imponendo a tutti i paesi una  attenta formulazione delle proprie strategie ed uno sforzo in settori di ricerca innovativi. 

Per quanto riguarda

-l’Italia, resta aperta, e risulta quanto mai urgente,  la formulazione di un piano energetico capace di operare scelte che abbiano rilevanza strategica e facciano leva sui nostri  punti di forza culturali e tecnologici. In particolare tenendo adeguato conto del ricupero  delle attività produttive la cui crisi sta  impedendo la crescita economica  severamente penalizzata dall’elevato costo dell’energia.

-In questo quadro risulta anche importante l’intensificazione della produzione nazionale di idrocarburi nel pieno rispetto del territorio, obiettivo non irraggiungibile se si adottano   le tecnologie moderne.

Resta comunque  aperta la sfida di avvicendare l’impiego dei carburanti fossili con fonti alternative, possibilmente rinnovabili, con un bilancio nullo dell’anidride carbonica prodotta dalla combustione.

–Particolare attenzione deve essere ovviamente posta all’energia proveniente dal sole, anche se  il suo sfruttamento su una  scala confrontabile con i consumi primari di energia è strettamente vincolato alla creazione di adeguate infrastrutture, obiettivo che non  appare perseguibile in tempi brevi:   la  periodicità della produzione di energia solare richiede infatti ampie ristrutturazioni delle reti legate alle difficoltà che tuttora si incontrano nell’immagazzinare l’energia elettrica. E non va sottaciuto il fatto che  il tentativo di accelerare l’utilizzo delle fonti periodiche, quale la solare, e stocastiche,

-quale l’eolica, in sostituzione delle  tradizionali termoelettrica e nucleare, sta creando a livello europeo un’enorme richiesta finanziaria, dell’ordine di mezzo trilione di euro, per la costruzione di infrastrutture (How to lose half a trillion euros, Economist, october 12TH, 2013).

L’impiego delle biomasse, nella forma tradizionale sino ad ora utilizzata, deve essere perseguito con oculatezza per non compromettere la filiera agricola, favorendo quindi il passaggio ai biocarburanti di seconda generazione basati sull’impiego di raccolti dedicati  verso le masse cellulosiche. Particolare attenzione deve essere però volta  alle

–nuove interessanti  prospettive derivanti dalla “Synthetic Biology” (biologia sintetica)il cui successo potrebbe rivoluzionare  la  produzione di molti prodotti chimici con   importanti implicazioni per la produzione di biocarburanti di terza generazione. 

Inoltre, si deve esplorare l’ottenimento di  energia carbon free mediante la fissione nucleare sviluppando tecnologie basate su reattori di piccole dimensioni e quindi più sicuri,  rivolti a particolari applicazioni quali la desalinizzazione dell’acqua marina, il riscaldamento e il trasporto.

Senza dimenticare che Fukushima è ancora pericolosamente funzionante e produce sempre più acque radioattive che si riversano nel Pacifico e nessuno è in  grado di trovare una soluzione !

In conclusione, risulta inderogabile la formulazione di un piano nazionale dell’energia.

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Recupero energetico da rifiuti mediante celle a combustibile microbiche.

Dai rifiuti solidi urbani un’energia amica dell’ambiente.

-“Il nostro studio conferma che le celle microbiche costituiscono una promettente tecnologia per la produzione di energia elettrica da fonti rinnovabili e per il contemporaneo trattamento dei rifiuti organici”.

E’ stato realizzato un prototipo di cella a combustibile microbica (MFC), che utilizza come combustibile la frazione organica dei rifiuti solidi urbani (FORSU) provenienti da un impianto di trattamento della provincia di Napoli. Scopo dello studio è di valutare il possibile utilizzo della FORSU per la produzione di energia elettrica. Il prototipo realizzato, a singola camera, in cui l’anodo e il catodo sono costituiti da piastre di grafite, ha funzionato per un periodo di 30 giorni durante i quali sono stati monitorati la temperatura, il pH, la densità di corrente e la potenza  prodotte dalla cella. I valori di corrente e potenza sono stati rapportati all’unità di superficie anodica (m²) e di massa di rifiuto trattata (Kg). I dati ottenuti hanno dimostrato una potenza massima di 10 mW m^-2 Kg^-1 ed una densità di corrente di 129 mA m^-2 Kg^-1 con una diminuzione della BOD₅ del rifiuto del 98%.

-Altro obiettivo dello studio è stato quello di caratterizzare il biogas che si forma da tale processo ed eventualmente individuare i possibili utilizzi. I risultati preliminari hanno mostrato il possibile utilizzo della frazione organica dei rifiuti come combustibile per le MFC.

 STUDI PER LA SOSTENIBILITA’ AMBIENTALE.

Introduzione

La sostenibilità ambientale è diventata un problema di portata globale, apparentemente a causa di due fattori:

–aumento della consapevolezza che le risorse di energia non rinnovabili sono limitate e, allo stesso tempo, che la biosfera ha una capacità limitata di assorbire i rifiuti prodotti dalla varie attività umane (Ulgiati e Brown, 2002). Attraverso la digestione anaerobica della Frazione Organica dei Rifiuti Solidi Urbani (FORSU) è già possibile accoppiare il trattamento dei rifiuti alla produzione di CH₄ e quindi di energia.

–In questo caso, il rifiuto stesso diventa risorsa (Karagiannidis, 2009). Particolare interesse suscita la capacità di alcune specie batteriche di trasferire – in assenza di ossigeno – gli elettroni derivanti dai processi ossidativi della sostanza organica a elettrodi polarizzati.

-Questo ha suggerito la possibilità di utilizzare i microrganismi per la produzione diretta di energia elettrica mediante pile a combustibile definite “microbiche” (Logan et al, 2006; Du et al., 2007). Questa nuova tipologia di “cella a combustibile microbico” o MFC (Microbial Fuel Cell)

–rende possibile, in linea di principio, la conversione dell’energia chimica contenuta in qualsiasi forma di biomassa biodegradabile (Pant et al, 2010), , in energia elettrica rinnovabile ad elevato rendimento, operando a bassa temperatura, in sistemi di semplice realizzazione e dal costo relativamente contenuto. Inoltre, tali dispositivi consentono di abbinare la produzione diretta di elettricità con il trattamento del rifiuto stesso a temperatura ambiente (Du et al., 2009).

Tale processo, utilizzando  rifiuti organici, può affiancare la digestione anaerobica delle biomasse  (Pham et al., 2006), contribuendo alla gestione dei rifiuti solidi (come scarti agroalimentari) e dei reflui organici (civili e industriali) (Mohan , 2011). Inoltre, è stata dimostrata la possibilità di produrre biocarburanti ed idrogeno molecolare attraverso l’utilizzo di celle microbiche (Cristiani e Triburzio, 2010). Le celle microbiche costituiscono una fonte di energia rinnovabile (Du et al., 2007), con minimi impatti ambientali. Un’adeguata analisi del ciclo di vita potrà portare ad una reale valutazione e quantificazione degli impatti ambientali connessi con la messa in opera in vasta scala delle celle microbiche (Panta et al., 2011).

Riportiamo i risultati preliminari di uno studio volto all’applicazione della tecnologia delle MFC al trattamento della Frazione Organica dei Rifiuti Solidi Urbani (FORSU) prelevati presso un’azienda di trattamento dei rifiuti della provincia di Napoli. E’ stato inoltre valutato il possibile utilizzo del biogas che si forma dai processi fermentativi del combustibile organico..

Materiali e Metodi

La FORSU è stata prelevata mediante la tecnica della quartatura da un quantitativo di materiale di alcune tonnellate. Un’aliquota del rifiuto (600 g) sospesa in una soluzione diluita di acido acetico è stata posta in incubazione in atmosfera anerobica alla temperatura di 15±2°C per 7 giorni per stabilizzare la flora microbica.

Il  bio-slurry risultante è stato utilizzato per alimentare la cella microbica a sua volta mantenuta alla temperatura di 25°±2°C per 30 giorni.

Per la realizzazione della cella microbica si è utilizzato un contenitore in vetro, in cui l’anodo è immerso nella cella e il catodo presenta una superficie esposta all’aria, entrambi costituiti da piastre di grafite, collegate tra loro mediante un circuito esterno; inoltre sul contenitore sono state create delle uscite per permettere la raccolta del biogas, e l’inserimento delle sonde di misura.

Il biogas generato è stato analizzato mediante tecniche gascromatografiche con rivelatore a spettrometria di massa (GC-MS).

 Durante la fase di esercizio della cella sono stati monitorati i principali parametri chimico fisici, quali temperatura e pH, e la produzione di energia elettrica, mediante l’utilizzo di un multimetro digitale (Agilent Techologies^©). Le principali caratteristiche chimiche del rifiuto sono state determinate secondo metodiche standard (IRSA-CNR, 1984; APAT, IRSA-CNR, 2003). I risultati sono stati paragonati a quelli di uno studio analogo condotto da Mohan e Chandrasekhar (2011) sull’utilizzo degli scarti di una mensa come combustibile in una MFC. La Densità di Potenza (DP) prodotta così come la Densità di Corrente (DC) sono state rapportate alla superficie anodica (espressa in m² ) ed all’unità di massa di rifiuto trattato (in Kg).

Risultati e Discussione                                       

La cella alimentata da FORSU è stata monitorata per un periodo di 30 giorni. In Figura 2 è riportato l’evoluzione della tensione di circuito aperto (OCV) in funzione del tempo. I valori della cella sono messi a confronto con i risultati della cella ideata da Mohan e Chandrasekar, denominata cella SCW (Solid Composite Waste cell).

 Si osserva che la tensione di circuito aperto (OCV) della cella FORSU è caratterizzata da un picco di 260 mV raggiunto dopo i primi 5 giorni, seguito da una diminuzione a 170 mV ed ad un andamento leggermente crescente fino alla fine dell’esperimento. Nella stessa figura sono riportati i dati relativi alla cella SCW la quale, invece, presenta un picco di 400 mV seguito da una rapida diminuzione a130 mV e, poi, a circa 70 mV.

In Figura 3, sono riportate le curve di polarizzazione di entrambe le celle. La cella alimentata da FORSU registra valori massimi di potenza più elevati seppure intensità di corrente più basse. La massima potenza raggiunta è stata di 10 mW per m² di superficie anodica e kg di FORSU trattato, superiore agli 8.8 mW m^-2kg^-1 ottenuti dalla cella  SCW.

La massima Densità di Potenza (DP) raggiunta dalla cella alimentata da FORSU è stata di 3,01 mW/m² (10 mW m^-2 Kg^-1) mentre la massima Densità di Corrente (DC) è stata di 36 mA m^-2 (129 mA m^-2Kg^-1). Mohan and Chandrasekar (2011) riportano per la loro cella alimentata da scarti di mensa una DP massima di 4 mW m^-2 (8.8 mW m^-2Kg^-1). La DC era, in quel caso di 77,31 mA m^-2 (171 mA m^-2Kg).

Il pH, durante l’intero esperimento, si mantiene ad un valore di circa 4.5±0.2 (Fig. 4). Tale valore è spiegabile con il metabolismo, essenzialmente acido, dei batteri coinvolti nella digestione anaerobica del rifiuto.

L’analisi del biogas non ha mostrato la presenza di metano, probabilmente dovuto al basso valore di pH che nel processo anerobico inibisce il processo di metanogenesi. I batteri metanogeni sono in genere inibiti a valori di pH inferiori a 5.0. Solo in casi particolari è stata registrata una limitata attività metanogenetica a pH 4.5 (Kima et al., 2004).

 Fig.4: Andamento del pH.

Le analisi del bioslurry il secondo mese di trattamento ha rivelato una diminuzione della BOD₅ del 98% mentre l’analisi qualitativa del biogas ha mostrato la presenza composti di interesse industriale come acidi organici, alcoli e chetoni.:

Conclusioni

I primi risultati hanno mostrato che la frazione organica dei rifiuti solidi urbani (FORSU) si presta come un possibile combustibile per le celle a combustibile microbiche (MFC), costituite da un’unica camera e operanti a temperatura ambiente.

Dal confronto con il lavoro di Mohan e Chandrasekar (2011) è risultato che le densità di potenza ottenute nel corso del nostro esperimento sono più alte, ma le densità di corrente più basse. Una delle possibili spiegazioni è data dall’elevata resistenza interna della cella, ma non è da escludere l’influenza del basso valore di pH. Ulteriori indagini sono in corso  per chiarire questo aspetto.

Ulteriori studi mirati  alla caratterizzazione elettrochimica della cella e all’individuazione dei processi che avvengono all’anodo e al catodo, sono necessari per definire la dinamica dei processi che sono alla base della generazione di corrente in questo tipo di celle..

-Il nostro studio conferma che le celle microbiche costituiscono una promettente tecnologia per la produzione di energia elettrica da fonti rinnovabili e per il contemporaneo trattamento dei rifiuti organici. .

Riferimenti Bibliografici:

APAT, IRSA-CNR.(2003) Metodi analitici per le acque. Manuali e Linee Guida n°29.

Brown M.T., Ulgiati S. (2002) Emergy evaluations and environmental loading of electricity production systems. Journal of Cleaner Production 10,321–334.

Cristiani P., Tribuzio M. (2010). Stato dell’arte  sulle celle a combustibile microbiche, risultati delle prove di laboratorio – Rapporto ASV ambiente e sviluppo sostenibile.

Du Z, Li H, Gu T. (2007). A state of the art review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy, Biotechnology  Advances.25(5):464-82.

Karagiannidis A., and Perkoulidis G.,.(2009) “A multi-criteria ranking of different technologies for the anaerobic digestion for energy recovery of the organic fraction of municipal solid wastes”, Bioresource Technology 100, 2355,2360.
IRSA-CNR. (1984). Metodi analitici per i fanghi. Vol 2 Parametri tecnologici, Quaderno n°64..

In S. Kima; Moon H. Hwanga, NamJ. Janga, Seong H. Hyunb, S.T. Leec (2004) Effect of low pH on the activity of hydrogen utilizing methanogen in bio-hydrogen process.International Journal of Hydrogen Energy 29; 1133 – 1140.

Logan B. E., Melers B., Reneä R., Schroder U., Keller J., Freguia S., Aelterman P., Verstraete W., Rabay K. (2006). “Microbial Fuel Cells: Methodology and Technology”, Environmental Science  Technology 40 (6).

Mohan, S.V. and Chandrasekhar, K.(2011) “Solid phase microbial fuel cell (SMFC) for harnessing bioelectricity from composite food waste fermentation: Influence of electrode assembly and buffering capacity”, Bioresource Technology 102, 7077-7085.

Panta D., Singhb A., Van Bogaerta G., Alvarez Gallegoa Y., Dielsa L, Vanbroekhovena K. (2011). An introduction to the life cycle assessment (LCA) of bioelectrochemical systems (BES) for sustainable energy and product generation: Relevance and key aspects.Renewable and Sustainable Energy Reviews 15, 1305–1313

Pant D, Van Bogaert G, Diels L, Vanbroekhoven K. (2010). A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production, Bioresource Technology 101(6):1533-43. .

Pham, T.H., Rabaey, K., Aelterman, P., Clauwaert, P., De Schamphelaire, L., Boon, N. and Verstraete, W. (2006). “Microbial Fuel Cells in Relation to Conventional Anaerobic Digestion Technology”, Eng. Life Sci. 6 (3), 285-292.

Zhuwei Du, Haoran Li, Tingyue Gu. (2007). A state of the art review on microbial fuel cells: a promising technology  for wastewater treatment and bioenergy).Biotechnology  Advances 25;464-482.

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Development of nuclear energy in a sustainable frame: 

evaluations on the issues of uranium resources and nuclear waste

Rolando CALABRESE (Università di Padova)

FUKUSHIMA IL PERICOLO CONTINUA E NON SI PUO’ DIMENTICARE.

 “Moreover, safety is a crucial factor for the development of nuclear energy as confirmed by the Fukushima−Daiichi accident that raised new questions in the public opinion forcing countries to the decision to phase−out or to abandon any plan to enter nuclear business”.

The concept of sustainable development addresses the capability to meet present needs without compromising the ability of future generations to meet their own needs. Sustainability is therefore a fundamental approach in the definition of future energy systems to accomplish economical, social, and environmental criteria.

In this frame, nuclear energy is characterized, aiming at tackling human-induced climate changes, by low greenhouse gas (GHG) emissions while facing drawbacks such as the shortage of natural uranium resources, the steady increase of spent nuclear fuel stockpiles as well as proliferation risks. The demand for electricity generation is growing and the definition of an energy portfolio capable of meeting needs in a sustainable manner is more and more urgent given the narrow timeframe left for a transition to a low carbon electricity generation.

In this study, the sustainability of the electricity generation sector was evaluated and the share of competing energy sources calculated for three scenarios by means of the Analytic Network Process (ANP). This evaluation was based on a set of indicators defined according to the findings published in the open literature. Afterwards, aiming at studying the impact of uranium consumption and nuclear waste stockpiles, according to the projections of electricity demand, the material flows were calculated in the hypothesis of an open fuel cycle. The future perspectives of nuclear energy trust on the development of innovative power plant designs where fast reactors play a major role and a transition to a closed fuel cycle is planned. Preliminary investigations are presented to identify the share of nuclear energy according to the improvements in the sustainability criteria achieved through the foreseen transition to innovative nuclear power plants.

1. Introduction

The concept of sustainable development addresses the capability of meeting present needs without compromising the ability of future generations to meet their own needs. Energy is fundamental to improve living standards and to support societal development. The use of fossil energy sources, accounting for about 80% of the global primary energy needs, leads to phenomena of increasing concern such as: urban air pollution, regional acidification, human−induced climate change.

Nuclear energy is capable of tackling GHG emissions at competitive costs. If, on the one hand, this latter aspect is assumed as a fundamental driver for its development, on the other hand nuclear energy faces drawbacks such as the management of radioactive waste and their proliferation risks. Moreover, safety is a crucial factor for the development of nuclear energy as confirmed by the Fukushima−Daiichi accident that raised new questions in the public opinion forcing countries to the decision to phase−out or to abandon any plan to enter nuclear business.

Nevertheless, main reasons promoting the so-called nuclear renaissance seen in the last decade are still valid. This statement is consistent with the projections that, even though to a smaller extent, confirm an increase in the deployment of nuclear energy. This contribution presents a study on the role of nuclear energy in the electricity sector where other energy sources are competing in the frame of a sustainable development. The method used for the analysis is the ANP whereas the list of indicators was defined according to the open literature. Based on these results and the projections of the electricity demand, the uranium resources consumption and the burden of nuclear waste stockpiles were calculated in an open fuel cycle option. In the concluding part, preliminary conclusions concerning the improvements of selected indicators achieved through the transition to a closed fuel cycle and the deployment of next−generation fast reactors were drawn. The evaluations of sustainability were carried out by means of the Super Decisions code while the calculations of material flows were carried out by means of the Dynamic Energy System-Atomic Energy code (DESAE).

2. The Analytic Network Approach

The Analytic Network Process is a theory that extends the Analytic Hierarchy Process to cases of dependence and feedbacks through the use of the concept of supermatrix. The ANP provides a framework to include clusters of elements connected in any desired way to investigate the process and deriving the adopted priorities from the distribution of influence among elements and among clusters. This method permits to describe the dependence among components as well as feedbacks, outer and inner dependence respectively. A supermatrix, W, is a complete system matrix of components, {C_a, C_b, C_c,…, C_n}, and their linkages, Wij. The supermatrix represents the impact of all model elements relative to the complete set of elements. The actual elements that make up the columns (Wij) of the supermatrix are the eigenvector solutions within the components. The final priority weights are calculated by multiplying the supermatrix by itself until the columns stabilize (limiting matrix). The analysis presented in the paper was performed by means of the Super Decisions 2.2 code.

3. The DESAE code and the calculation of material flows

As previously mentioned, material flows calculations were performed by means of the DESAE code, a software developed at the Kurchatov Institute (Moscow) to support the activities of the IAEA International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO). The DESAE code allows predicting financial and materials resources needed for a sustainable nuclear energy policy at country, regional and global level. The analysis is performed on user-defined deployment scenarios where reactors, fuel cycle facilities and energy demand projections are properly defined. It allows studying both open and closed fuel cycles taking into account uranium and thorium recycling. The code does not perform burn-up or core management calculations so that fresh, equilibrium and spent core isotopic composition are required. The code deals with 18 nuclides, with one additional variable accounting for the remaining fission products. The layout of a single nuclear energy system may be composed of seven different nuclear power plants and four recycling plants in case of closed cycle options. In this regard, reprocessing losses are not taken into account.

4. Analysis of the indicators used in the evaluations of sustainability

Climate change concern has become one of the most challenging issues for the new century. The level of CO₂ is moving steadily towards the limit of 450 ppm a value leading to an increase of the average global temperature by 2 °C. New trade−offs are necessary especially in the developing countries, to account for the demographic increase, energy resources scarcity and improving of living standards.

The total primary energy supply (TPES) moved from 6107 Mtoe in 1973 to 12717 in 2010; the electricity generation raised, in the same period, from 6115 TWh to 21431 TWh showing a nearly four−fold increase. In 2010, the production of electricity from fossil fuel was 67.4% with a slight reduction from the value of 75.1% in 1973. New electricity generating plants are foreseen to be installed during the coming two decades with a projected total capacity of the same order of that installed in the past century.

 The share of nuclear electricity increased from 3.3% to 12.9% as well as natural gas share moved from the 12.1% in 1973 to 22.2% in 2010. The concept of sustainability addresses, besides environmental, other dimensions, social and economic. The planning of a sustainable energy mix needs therefore a deeper analysis through a multi−parametric approach. For this purpose, a review of the indicators used in the evaluation of sustainability published in the open literature was carried out.

The review confirmed the need for several indicators in the three dimensions of the sustainability (economic, social, environmental), therefore, in these studies, the analysis was performed through multi−criteria approaches such as the Analytic Hierarchy Process (AHP), the Multi Criteria Decision Analysis (MCDA), the Analytic Network Process (ANP), this latter applied in presented study. The vision of each stakeholder confirmed to be of great importance in defining the ranking of energy sources, nevertheless, nuclear energy proved to be, in comparison with renewables, a sustainable energy source as well. The importance of public opinion was also highlighted as in the case of Japan after the Fukushima accident.

5. Sustainability indicators and modelling

The study presents an assessment of a sustainable energy mix, where nine alternatives are given, based on twelve indicators grouped in economic (3), environmental (7) and social (3). The indicators selected for this study were:

Total cost.

Loss of expected life.

CO_2.

SO₂.

Efficiency.

Availability.

Fuel cost.

Reserves-to-Production (R/P) ratio.

External cost.

Land use.

Water use.

Heated water.

The values used in this study were defined in agreement with the data published in the open literature. The energy system was modelled through a top−level network (no sub−networks). The model consists of four clusters (Alternatives, Economic, Environmental, Social) connected by bidirectional links. The cluster Alternatives contains nine nodes for the description of the different energy sources taken into account. The other clusters contain the nodes representing the corresponding criteria selected for the analysis. An inner dependence was introduced between the nodes of the Alternatives cluster to take into account the status of the electricity sector assuming that the share of competitors has an effect on the development of each energy source.

The values of criteria were applied through pairwise comparisons of elements when the energy sources acted as children and the indicator as parent. In the opposite direction, when the energy source was the parent and the criteria the children, the inverse of ranking achieved by the alternative for each criteria was applied. No inter−relationship were created between the assumed criteria. A preliminary analysis was performed aiming at reproducing the status of the electricity sector assigning, in the comparison, nearly all the priority to the Alternatives cluster. Assuming that a share of 25% is given to the Alternatives cluster, three scenarios were analyzed. The scenario Economic assigned 50% of priority to the Economic cluster and 12.5% to the Environmental and Social clusters. The Environmental and Social scenarios were defined swapping properly these values.

Based on the projections of the electricity demand at 2030 and the share of nuclear energy as in the previous evaluations, the material flows of an open fuel cycle were calculated aiming at studying the needs for uranium resources and the burden of nuclear waste stockpiles. In the concluding part of the study, the improvements expected in the indicators of sustainability for a closed fuel cycle option were discussed.

6. Conclusions

Economic, social, environmental criteria are fundamental for the evaluation of the sustainability of an energy mix. Multi−parametric approach is therefore mandatory for the investigations of new trade−offs between growing energy demand, improving of living standards and environment.

The study focused on the electricity generation sector presenting sustainability evaluations based on twelve criteria by means of the Analytic Network Process. Consistent predictions were obtained in reproducing the status of the sector highlighting, in the results of scenarios, the need for a transition to low carbon technologies. In this frame, nuclear energy proved to be, in agreement with the conclusions published in the open literature, a sustainable source of energy. Based on these results, assuming the use of current technology in an open fuel cycle, an evaluation of natural uranium needs and nuclear waste burden was presented. In the concluding part, the perspectives of the new generation nuclear plants were discussed investigating the change in the sustainability indicators and the related share of nuclear energy.

Microalgae based technology for biofuels production and CO2 capture: the role of mathematical modeling and genetic engineering

Alessandro CONCAS, Roberto CUSANO, Massimiliano ORSINI (Center for Advanced Studies, Pula-Ca) – Cristina COSTELLI, Giacomo CAO (Università di Cagliari) – Andrea ANGIUS (IRGB-CNR, Monserrato)

World economy is strictly linked to the availability of fossil fuels, which nowadays meet the world’s growing energy demand. However, the intensive exploitation of fossil fuels as main source of energy is currently recognized to be unsustainable due to the depletion of available resources as well as to their contribution to environmental pollution and greenhouse gases emissions (Ahmad et al., 2011). Moreover, the geographic distribution of oil reserves around the world can lead to economic dependence issues which can result in geopolitical instabilities in several countries. For these reasons the production of sustainable and renewable sources of energy, such as biofuels, is recognized to be critical to fulfill a sustainable economy and face global climate changes (Concas et al., 2010). Among the several feedstock’s that are potentially useful for producing biofuels on the large scale, microalgae are today considered to be one of the most promising since, unlike first generation biomasses, their cultivation can be accomplished in non-agricultural lands without competition with the food market. Moreover, when compared with first generation feedstocks, microalgae are characterized by higher bio-oil productivity and their cultivation might be coupled with the direct CO₂ capture from point sources of emission (cf. Cao and Concas, 2008, 2010). However, the existing microalgae-based technology for CO₂ sequestration and biofuels production is still not widespread since it is affected by economic and technical constraints that might limit the development of industrial scale production systems. Therefore, in view of industrial scaling-up, the current technology should be optimized in terms of oil productivities and CO₂ uptake capability. To this aim, mathematical models and process engineering techniques might be suitably exploited to identify the operating conditions of photobioreactors (i.e. light supply, mass transfer, growth media composition etc.) that maximize growth rate, lipid accumulation and CO₂ fixation as well as the economic viability of the technique. As an example, the possibility to exploit mathematical models to properly design the growth medium, thus avoiding the use of excess concentrations of nutrients and thus improving the economic sustainability of the process, is addressed in what follows. In fact, one of the most impacting cost item of the microalgal technology is related to the need of a continuous replenishment of macronutrients (mainly CO₂, nitrogen and phosphorus) during algal cultivation (Jiang et al. 2011). Since large scale cultivation of microalgae implies the consumption of huge amounts of such macronutrients, the economic feasibility of the entire process could be seriously affected by the erroneous evaluation of their depletion kinetics. Thus, in view of industrial scaling-up, the effect of nutrients concentration in the medium on biomass composition and productivity should be quantitative evaluated. Since nutrients concentration and supplies are among the most controllable factors in microalgae cultivation, at least the main macronutrients (i.e. nitrogen and phosphorus) uptake rates need to be quantitatively evaluated for the microalgae strains candidate to industrial exploitation. Along these lines, Concas et al. (2013) investigated the growth kinetics of a relatively unknown strain of microalgae, namely Nannochloris eucaryotum, in batch photobioreactors with the aim of determining useful kinetic parameters which might be used to suitably design the growth media and photobioreactors. The maximum growth rate, the half saturation constants and yields coefficients for nitrate and phosphate uptake were determined by suitably fitting the experimental data. The reliability of the obtained parameter values was successfully tested by suitably predicting experimental data obtained under several different operating conditions (cf. Figure 1).

Figure 1 Comparison between model predictions and experimental data in terms biomass concentration of N. eucaryotum cultured under different nitrogen and phosphorus concentrations in the growth media, adapted from Concas et al. (2013). 

Another important challenge in the field of biofuels producing through microalgae is related to the potential exploitation of flue gases as carbon source. In fact, in this case, cultivation of microalgae might be coupled with the direct bio-capture of CO₂ emitted by industrial activities that use fossil fuels for energy generation. However, the effects resulting from such operating mode, i.e. low pH values and high dissolved concentration of CO₂, which might severely affect microalgae growth, should be investigated and simulated in order to properly control the main process parameter during the industrial scale operation of the photobioreactors. Along these lines, Concas et al. (2012) proposed a novel mathematical model of the growth of Chlorella Vulgaris in semi-batch photobioreactors fed with pure CO₂ (100% v/v), thus simulating suitable gaseous streams that might be obtained as result of the CO₂ capture technologies adopted. Specifically the proposed model was capable to simulate temporal evolution of cells, light density and macronutrients concentration within the growth medium as well as carbon dioxide and oxygen concentration in liquid and gas phase. Moreover, by taking advantage of comprehensive kinetics and considering the ion speciation phenomena taking place in the growth medium, the model was able to quantitatively describe the dynamics of pH evolution and its effect on microalgae growth. Such aspects, often neglected by mathematical models available in the literature, are indeed critical in order to develop suitable control strategies for the optimization of photobioreactor’s operation when using flue gas as carbon source for microalgae growth. As shown in Figure 2, the model results obtained by Concas et al. (2012) were successfully compared with the experimental data, thus confirming the capability of the proposed model to quantitatively describe the culture behavior within semi-batch photobioreactors both in terms of biomass concentration and pH evolution during the cultivation.

Figure 2. Comparison between model predictions and experimental data in terms of biomass concentration (a) and pH (b) as a function of time when culturing C. Vulgaris under 100%v/v CO₂ in semi-continuous photobioreactors, adapted from Concas et al. (2012)

Moreover as it can be observed from Figure 2, the experimental results obtained by Concas et al. (2012) confirm that C. Vulgaris strain was capable to adapt to high levels of CO₂, probably by changing the carbon uptake mechanisms. Therefore, while the experimental results have shown that such microalgal strain might be potentially exploited for the bio-capture of CO₂ from concentrated industrial flue gases, the mathematical model developed allows one to identify the operating conditions that maximize the CO₂ capture and the simultaneous biomass productivity in semi-continuous photobioreactors.

A further challenge in the field of biofuels production and CO₂ sequestration through microalgae, is the identification of the geometry and the operating mode of photobioreactors that allows to optimize the distribution of photon flux, the CO₂ capture, the removal of photosynthetic oxygen and the nutrients utilization. To this aim, horizontal or helical tubular systems, as well as combinations of vertical flat panels and bubble columns appear to be the reactor configurations whose scale up may be relatively straightforward (Sierra et al., 2008). In particular, the helical configuration named BIOCOIL is characterized by very simple and relatively inexpensive design, easy to assemble and operate. However, only few mathematical models, aimed to the simulation, the optimization and the control of such photobioreactor typology have been reported in the literature. Only recently, Concas et al. (2010) proposed a comprehensive model which was able to quantitatively describe the growth of Spirulina Platensis in a re-circulating helical photobioreactor, i.e. the BIOCOIL shown in Figure 3-a.

 Figura  SEQ Figura \* ARABIC 1  Errore. Il segnalibro non è definito.

Figure 3. Schematic representation of the BIOCOIL photobioreactor (a) and biomass concentration of S. Platensis in the final section of the tube (log scale) as predicted by the proposed model and comparison with literature data, adapted from Concas et al., 2010.

The model accounted for “mass structured” population balances which permitted to properly simulate cell growth, replication and the biomass distribution within the tubular-helical photobioreactor. Specifically, a novel mass-dependent growth kinetics was proposed. The latter one took into account cell size, light intensity, nutrients concentration and inhibitory effects of dissolved oxygen. Model results and literature experimental data (Travieso et al., 2001) in terms of dry biomass content and its distribution within the photobioreactor tube were successfully compared as shown in Figure 3-b, thus demonstrating the validity of the proposed model as well as its predictive capability. Therefore, the proposed model might be potentially useful for the optimization of design and process parameters of the BIOCOIL.

While the mathematical models can be helpful to identify the operating conditions that optimize the microalgae cultivation in suitable photobioreactors, specific genetic engineering tools could be exploited to manipulate genome of existing strains with the aim of increasing their intrinsic photosynthetic efficiency (Melis, 2009) and/or regulate their metabolism (Radakovits et al., 2010) in order to achieve an abundant accumulation of lipids coupled with an high biomass productivity or an high capability of  CO₂ uptake. Along these lines, an intense experimental activity is being carried out in our research group aimed to the identification of specific genes involved in the bio-synthesis of fatty acids and carbon dioxide uptake mechanisms of 5 strains belonging to the phylum of green algae, i.e.. Chlorella sorokiniana (SAG 111-8k), Picochlorum eukaryotum (SAG 55.87), Monodopsis subterranea (SAG 848.1), Scenedesmus obliquus (SAG 276-1) and Tetraspora sp. Specifically, the activity consists of the extraction of the genomic DNA from the above mentioned 5 strains grown under typical operating conditions, and the “de novo” sequencing of such strains by a Next Generation Sequencing approach using the Illumina’s sequencing by synthesis (SBS) technology (Imelfort M and Edwards D, 2009). This strategy returns enough high quality data to ensure a coverage of about 80-100X, over two libraries with different insert size, for each strain. This approach will allow us to apply a bioinformatics workflow to optimize the assembly of both organelles and nuclear genomes (Mascher et al,  2013; Naito et al, 2013). RNA-Seq analysis on Whole Transcriptomes is currently being performed on the above strains. Subsequently, the latter ones will be grown under specific operating conditions, i.e. for example low nitrogen or high iron concentration in the growth medium, which are capable to induce the corresponding bio-synthesis. Afterwards, the lipid-enriched strains will undergo the same transcriptomic characterization which was previously performed on the corresponding ones grown under typical operating conditions. By comparing the corresponding trancriptome profiles of such strains, the genes over or under expressed, when fatty acids synthesis is promoted, will be recognized as well as the key regulatory genes controlling the biosynthetic pathway. Such results will permit to implement suitable genetic engineering strategies to transfect the genes involved in lipid synthesis within the microalgal cells. If successful, such activity will permit to create microalgal strains characterized by an intrinsic high capacity to produce fatty acids and thus to make the current microalgae-based technology for producing biofuels economically viable.

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