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Ahern E.P.,University College Cork | Ahern E.P.,Marine Renewable Energy Ireland nter | Deane P.,University College Cork | Persson T.,Energiforsk Swedish Energy Research Center | And 3 more authors.
Renewable Energy | Year: 2015

This paper examines the potential role for Power to Gas (P2G) as applied to an island energy system with high levels of renewable electricity penetration. P2G systems require both a supply of green electricity and a source of CO2. Cheap electricity is essential for a financially sustainable P2G system. Using a PLEXOS model it was determined that deploying 50MWe of P2G capacity on the 2030 Irish electrical grid may reduce absolute levels of curtailed wind by 5% compared to the base case. CO2 capture is expensive. The cheapest method of sourcing CO2 for a P2G system is to employ a methanation process whereby biogas from anaerobic digestion is mixed with hydrogen from surplus electricity. Anaerobic digestion in Ireland has a potential to produce biomethane to a level of 10.2% of energy in transport (19.2PJ/a). The potential CO2 resource from anaerobic digestion could allow for a further 8.9% of energy in transport (16.6PJ/a) from P2G production. An optimal model is proposed including for co-location of a biogas system with a P2G system. The model includes for demand-driven biogas concepts allowing electrical grid balancing and the supply of gaseous transportation fuel. Biofuel obligation certificates allows for a financially viable industry. © 2015 Elsevier Ltd.

Jacob A.,University College Cork | Jacob A.,Marine Renewable Energy Ireland nter | Xia A.,University College Cork | Xia A.,Marine Renewable Energy Ireland nter | And 2 more authors.
Applied Energy | Year: 2015

There are significant resources of coal on the planet. It is likely that a lot of this coal will be combusted. A 1GWe coal power plant operating at 35% electrical efficiency and a capacity factor of 75% produces 6.77 million tonnes of CO2 per annum. A closed cultivation system with a carbon capture efficiency of 80% allows production of 2.69Mt of micro-algal (volatile solids), in a foot print of 19,200ha for a tubular photo-bioreactor (PBR) and 34,000ha for a Flat Plate PBR. An open system (raceway pond) at a carbon capture efficiency of 50% produces 1.68Mt of micro-algal (volatile solids) and requires a footprint of 52,303ha. Employing a three stage sequential process (combining dark fermentation, photo fermentation and anaerobic digestion) to produce bio-hydrogen and bio-methane from the micro-algae could potentially generate 35% of the primary energy in the coal in the form of renewable gaseous fuel if a closed system of cultivation is used. This is sufficient to fuel 600,000 cars per annum. In the cultivation of micro-algae, pumping and circulation is a considerable parasitic energy demand. The ratio of energy output (gaseous biofuel) to energy input (pumping and circulation) is less than 1 for all the three cultivation systems assessed, ranging from 0.71 for raceway ponds to 0.05 for a tubular PBR. If coal powered electricity is the source of this parasitic energy then a tubular PBR system produces more CO2 than the CO2 captured by the micro-algae. © 2015 Elsevier Ltd.

Xia A.,University College Cork | Xia A.,Marine Renewable Energy Ireland nter | Jacob A.,University College Cork | Jacob A.,Marine Renewable Energy Ireland nter | And 6 more authors.
Bioresource Technology | Year: 2015

Fermentative hydrogen from seaweed is a potential biofuel of the future. Mannitol, which is a typical carbohydrate component of seaweed, was used as a substrate for hydrogen fermentation. The theoretical specific hydrogen yield (SHY) of mannitol was calculated as 5mol H2/mol mannitol (615.4mL H2/g mannitol) for acetic acid pathway, 3mol H2/mol mannitol (369.2mL H2/g mannitol) for butyric acid pathway and 1mol H2/mol mannitol (123.1mL H2/g mannitol) for lactic acid and ethanol pathways. An optimal SHY of 1.82mol H2/mol mannitol (224.2mL H2/g mannitol) was obtained by heat pre-treated anaerobic digestion sludge under an initial pH of 8.0, NH4Cl concentration of 25mM, NaCl concentration of 50mM and mannitol concentration of 10g/L. The overall energy conversion efficiency achieved was 96.1%. The energy was contained in the end products, hydrogen (17.2%), butyric acid (38.3%) and ethanol (34.2%). © 2015 Elsevier Ltd.

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