Rinnovabili, in Alaska il Sealife Center usa l’acqua del mare per il riscaldamento

di Michele Costanzo 42

Nuove frontiere per le energie rinnovabili. L'Alaska SeaLife Center abbandona le fonti tradizionali e gestisce il riscaldamento utilizzano l'acqua di mare come vettore dello scambio termico.

alaska rinnovabili
Al mare ed ai suoi complessi fenomeni cinetici, chimici e termici si è sempre guardato con straordinario interesse nell’ambito delle energie rinnovabili. Sono molteplici i principi tecnici che permettono l’impiego dell’energia marina utilizzando i natali movimenti delle acque o sfruttandone le proprietà chimiche e fisiche. Una innovativa soluzione che permette di utilizzare le acque marine come vettore termico per il riscaldamento degli ambienti è stata presentata dall’Alaska SeaLife Center (ASLC), grande acquario che sorge nella città di Seward in Alaska e che come istituto di ricerca porta avanti anche numerosi progetti scientifici sull’ambiente marino.

Nuove rinnovabili, il calore arriva dai mari

Il Sealife Center ha annunciato pochi giorni fa di aver spostato il 98% del proprio fabbisogno energetico per il riscaldamento dai combustibili fossili tradizionali all’acqua di mare. Quest’ultima è infatti utilizzata come vettore per la produzione di calore attraverso un processo tecnicamente complesso ma anche molto promettente. I risultati ottenuti arrivano da un percorso di ricerca e di sviluppo durato sette anni e basato sui principi della così detta energia talassotermica. Nel progetto infatti si sfrutta la capacità delle acque marine di accumulare e trattenere il calore per lunghi periodi rilasciandolo molto lentamente. Per effetto di questa proprietà nella stagione fredda le acque del mare si trovano ad una temperatura decisamente superiore rispetto a quella dell’aria e possono quindi essere utilizzate per produrre calore da utilizzare ad esempio per il riscaldamento degli edifici. Chiaramente affinché questo schema sia di utilità pratica è necessario poter ‘concentrare’ il calore disponibile fino a ottenere risultati termodinamici paragonabili a quelli ottenibili con i combustibili tradizionali o l’energia elettrica. Ed è proprio in questa direzione che si è mosso il progetto dell’Alaska SeaLife Center realizzato con la collaborazione di YourCleanEnergy.

Ma come funziona più concretamente la nuova via alle energie rinnovabili impiegata dall’ASLC? I due elementi chiave del progetto sono da un lato le acque marine di Resurrection Bay e dall’altro un innovativo scambiatore di calore che utilizza anidride carbonica (CO2). Il processo può essere sintetizzato in quattro passaggi:

  • Il calore presente nell’acqua marina viene utilizzato per riscaldare una miscela di glicole e acqua.
  • Il glicole riscaldato viene quindi inviato a quattro pompe di calore operanti in parallelo dove viene a contatto con un refrigerante composto al 99,9% di CO2.
  • Il calore scambiato tra i due fluidi è sufficiente a portare all’evaporazione la CO2 che sotto forma di gas viene quindi compressa per innalzarne ulteriormente la temperatura.
  • L’anidride carbonica compressa e riscaldata viene infine utilizzata per riscaldare acqua non salata che viene poi inviata alle stanze dell’edificio per regolarne la temperatura.
  • Alla fine del percorso l’acqua salta viene restituita al mare ad una temperatura inferiore a quella iniziale.

Nella prima fase del progetto nelle pompe di calore era stato utilizzato un refrigerante artificiale. Nelle successive fasi di sviluppo del progetto è stato poi implementato l’uso della CO2 nelle pompe di calore andando a ridurre ulteriormente ogni possibile impatto ambientale del sistema. Anche il contributo alla produzione del calore necessario all’edificio è progressivamente aumentato nel tempo. Se inizialmente il nuovo sistema copriva solo parzialmente il fabbisogno della struttura oggi come detto tale copertura è pressoché completa.

Notevoli sono anche i vantaggi in termini economici. L’Alaska SeaLife Center stima che il ricorso alle acque marine per il riscaldamento consentirà un risparmio attorno ai 15 dollari al mese rispetto ai sistemi tradizionali. La sostituzione delle caldaie a combustibile fossile e dei boiler elettrici con un sistema a pompa di calore permetterà inoltre di ridurre le emissioni annue di oltre 550 tonnellate.

Difficile è invece valutare al momento le possibilità di impiego su più ampia scala di questa soluzione. I costi di un sistema di riscaldamento a pompa di calore che utilizzi l’acqua marina sono sicuramente elevati e di conseguenza il modello è replicabile solo per strutture di grandi dimensioni vicine alla costa. D’altro canto nuove evoluzioni potrebbero consentire in futuro l’applicazione del modello anche su scala più piccola ed a costi iniziali più bassi.

Il mare e l’energia talassotermica

Il sistema di riscaldamento realizzato all’ASLC rientra -almeno in parte- tra le modalità di impiego dell’energia talassotermica che deriva dal calore accumulato nelle acque marine ed oceaniche. Classificata a pieno titolo tra le rinnovabili, questa fonte di energia è stata già utilizzata sfruttando i gradienti termici tra le acque superficiali (generalmente più calde) e quelle profonde (più fredde). Anche in questo caso si utilizza un ciclo chiuso in cui le acque a temperatura maggiore sono utilizzate per far evaporare un fluido che poi concentrato in pressione fa muovere una turbina e genera energia. Il gas riscaldato viene quindi condensato e riportato ad una temperatura più bassa sfruttando le acque più fredde per poi riprendere il ciclo.

Energie rinnovabile dal mare possono essere prodotte anche sfruttando la proprietà chimiche dall’acqua (come il gradiente salino) ma allo stato attuale i sistemi più collaudati utilizzano i moti delle acque; rientrano i questa categoria tutti i progetti che sfruttano le maree, le correnti marine e le onde per produrre energia.

Photo | Thinkstock

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