Energia dalle maree, tra vecchie barriere e nuovi impianti

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Energia dalle maree: quali prospettive si aprono, dopo decenni di tentativi, per lo sfruttamento delle masse d’acqua in movimento? Facciamo un salto nel passato, per la precisione una sosta nel lontano 1967, anno della costruzione della prima grande  centrale adibita allo scopo sull’estuario del fiume Rance, in Francia. Una diga e 24 turbine a bulbo realizzate in un’area in cui la marea raggiunge i 13,5 metri di dislivello. Incredibile ma vero, ancora oggi  l’impianto è perfettamente funzionante, con all’attivo 480 GWh di produzione annuale. L’energia viene prodotta dall’affluire e defluire dell’acqua all’interno di un bacino. Il flusso acquista velocità passando attraverso dei tunnel ed innesca il movimento delle turbine a loro volta collegate a generatori.

I diversi impianti possono generare energia mareomotrice sfruttando il movimento di ruote a pale o di turbine azionate dall’espansione dell’aria compressa in cassoni; sollevando un peso contrapposto alla forza di gravità; tramite il riempimento e svuotamento di bacini veicolato da turbine. I principali nodi che ostacolano lo sviluppo di questa forma di energia alternativa e rinnovabile, oltre ai costi elevati ed all’impatto ambientale, risiedono nella difficoltà di soddisfare la richiesta nelle ore di punta. Se l’afflusso d’acqua è insufficiente, infatti, la produzione di elettricità si interrompe. Inoltre è difficile localizzare aree con un dislivello ottimale.

Progetti simili alla prima centrale francese sono stati realizzati anche in altri Paesi del mondo quali il Canada, la Cina, la Russia, non senza sollevare, però, interrogativi assolutamente leciti sull’impatto ambientale di simili strutture. Erigere quelle che sono delle vere e proprie barriere necessarie allo sfruttamento dell’energia delle maree lascia infatti un’impronta pesante sul territorio ospite, impatto che si concretizza principalmente in fattore di disturbo per la fauna ittica e nell’erosione delle coste dovuta ai cambiamenti nei flussi di marea apportati dagli impianti.

E allora come aggirare il problema? Prima di menzionare alcuni degli sforzi che si stanno compiendo nella direzione di progetti ad impatto ambientale ridotto, è bene ricordare che l’energia ricavabile dalle maree non è certo determinante ma nemmeno trascurabile. Lo spostamento delle masse d’acqua generato dalla gravità della Luna e del Sole, riuscirebbe, pensate un po’, a garantirci, con uno sfruttamento ottimale, ben 575.000 GWh all’anno che equivalgono al 2% del fabbisogno energetico mondiale. Per garantire una potenza di 3 kW ad una famiglia, si rivelererebbe necessaria una corrente di 1 m² che si sposta ad  un’andatura di 11 km/h. In Italia il potenziale energetico delle maree è racchiuso nelle correnti dello stretto di Messina, a Ganzirri per la precisione. La produzione annuale ipotizzata è di 538mila kWh, ovvero un decimo di quanto produceva la centrale nucleare di Caorso (dati Focus).

Piuttosto che innalzare barriere, gli impianti di sfruttamento del futuro dovranno trovare il modo di utilizzare direttamente la marea, senza corrodersi a contatto con l’acqua marina ed erigendosi in maniera meno invasiva e più contenuta nelle dimensioni. Al momento si contano diversi progetti volti a scovare una soluzione che migliori la produzione energetica senza per questo necessariamente pesare sull’ambiente. Per citarne solo alcuni il Delta Stream, in Inghilterra, il sistema Free Flow negli States e Solon a Singapore. L’impianto Deltastream consta di unità da 1,2 MW composte ciascuna da tre turbine collegate a triangolo che non richiedono agganciamento al fondale. In Irlanda del Nord merita una menzione il progetto SeaGen, avviato nel 2008. Enel Green Power ha annunciato, di recente, che investirà in un parco sperimentale per testare nuove modalità di sfruttamento dell’energia delle maree. Non nel Mediterraneo, però, perché il moto ondoso è piuttosto scarso.

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60 commenti su “Energia dalle maree, tra vecchie barriere e nuovi impianti”

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