Prof Jerry Murphy outlines an alternative renewable energy approach: to ‘green the gas grid’ using anaerobic digestion, gasification and converting surplus renewable electricity to gas


Author: Prof Jerry D Murphy, Science Foundation Ireland, Marine Renewable Energy Ireland Centre, Environmental Research Institute, School of Engineering, University College Cork

An ongoing task for engineers is to predict the future and plan infrastructure based on these predictions. This is a difficult task and is not an exact science. Questions abound. What will the population of the country be in 2050? How much housing is needed and where is it required? Where will industry be situated? How will we provide energy to industry and to houses?

This is not just about electricity, but renewable thermal energy and renewable transport sources. In terms of final consumption, electricity consumes approximately 20% of energy, while transport and thermal energy consume about 40% each. Table 1 indicates a very simplified analysis of how these different sectors will contribute to the overall renewable energy target of 16% by 2020. It must be stated that plans for renewable electricity are far more advanced than for renewable heat or transport.

Table 1: Simplified analysis of renewable energy targets

Energy vector Percentage of final energy consumption Renewable energy Target Contribution to renewable energy target
Electricity 20% 40% 8%
Thermal Energy 40% 12% 4.8%
Transport energy 40% 10% 4%
Total 100% 16%

Going beyond 2020 and planning for the future, how will renewable energy targets be met? One approach is to electrify all energy, plan for 100% electric vehicles and use electricity for heating. This requires a massive ramp-up in electric capacity, from 20% of final consumption to 100% of final consumption.

A major issue with renewable electricity is the variable intermittent production associated with wind electricity. By 2020, it is expected that we will need to curtail between 7% and 14% of wind electricity (McGarrigle et al, 2013). In summer 2020, it is projected that wind capacity will exceed minimum demand for electricity by over 25% (Ahern et al, 2015). Therefore, the ‘electrify all energy’option must be based on interconnection with the UK and Europe. This is not a cheap option. It also makes the country depend on other countries for energy security.

Natural gas can support variable renewable electricity. It can provide electricity when the wind is not blowing. It is used for thermal energy and provides transport fuel to over 16 million vehicles across the planet. It is a fossil fuel, albeit a cleaner fossil fuel with a lower carbon footprint than coal or oil. Can the gas grid be taken out of the ground, assuming it is a soon-to-be-defunct piece of fossil-fuel infrastructure?

Greening the gas grid

An alternative renewable energy approach is to ‘green the gas grid’. Within Europe, six different gas-grid operators (Denmark, Sweden, Belgium, the Netherlands, France and Switzerland) have signed up to 100% carbon-neutral gas by 2050 under the green gas commitment. This may be effected using three different technologies.

  • Anaerobic digestion

Wet organic biomass may be biologically converted to biogas. This is a well-proven technology. Germany has over 8,000 anaerobic digesters. These digesters typically are sized at 500 kWe to 1 MWe. Feedstocks include for energy crops (maize in Germany). However, from a sustainability perspective, non-food crops (such as grass) and residues (slaughter waste, slurries, agri-food wastes, house hold wastes) are preferable. The potential for this technology is significant. A recent paper by Wall et al (2013) suggests that 1.1% of grassland can allow 10% renewable energy supply in transport (RES-T).

This industry was sized at 170 digesters treating 10,000 tons of grass silage and 40,000 of dairy slurry per annum each. Food waste is an ideal feedstock for anaerobic digestion and is the most urban of feedstocks; it is ideally coupled with provision of renewable gaseous fuel for buses or for industry.

Future third-generation feedstocks include for seaweed. Seaweeds are abundant in waters off these islands. The island of Orkney has a kelp forest of 1 million tons covering 22,000 hectares along 800 km of coastline. For sustainability, it is preferable to leave natural resources as they are and to cultivate seaweed for energy. A model proposed is to align seaweed farms with salmon farms. The farmed seaweed will extract nutrients from the water released by the farmed salmon. Seaweed can ameliorate the negative impacts of salmon farming.

Anaerobic digestion allows for waste treatment, environmental improvement, decentralised renewable energy and employment in rural and coastal communities. Upgrading biogas (removing CO2) and injecting to the natural gas grid allows for distribution of renewable gas to the cities and zones of energy demand.

  • Gasification

Woody-type biomass may be converted to methane via a thermal conversion process. Gallagher and Murphy (2013) outlined a strategy to meet 12% renewable energy supply in thermal energy. They modeled 11 number 50MWth gasifiers situated on the gas grid producing biomethane. In total, some 75,000 ha of willow were required, spread over 11 sites (approximately 7,000 ha of willow per gasifier site).

  • Power to gas

The third route to green gas is the conversion of surplus renewable electricity to gas. When it is considered that curtailment will be required for 40% renewable electricity production in 2020, it may be appreciated that managing and storing electricity beyond 2020 and on to 2050 (with higher portions of wind electricity) will be a very significant task.

There are a number of options such as pumped hydroelectric schemes, but these tend to be large civil engineering pieces of infrastructure with significant cost, significant lead-in times and potential for disquiet (and planning issues) in the locality for which they are proposed. The end route is electricity produced and available at a different time, when demand for electricity is higher. Power to gas allows storage, but also changes the energy vector from electricity to gas.

The first step in power to gas is electrolysis, a well-understood process of converting electricity to hydrogen. The Hydrogen Economy is not in place; the molecular weight of hydrogen coupled with the low volumetric energy density is such that distributing hydrogen and storing hydrogen is very expensive. A second step allows conversion of hydrogen to methane can be seen in Equation 1: 4H2 + CO2 = CH4 + 2H20.

Power to methane requires cheap electricity and a cheap source of CO2. Capture of CO2 is more expensive if the CO2 is dilute. CO2 is not readily available in concentrated form. The exhaust from power plants is dominated by nitrogen from air used in the combustion process. A paper by Ahern et al (2015) suggests that the cheapest source of CO2 is associated with biogas facilities. To inject renewable gas into the gas grid, CO2 must be removed from the biogas. Biogas consists of about 50% CO2. Upgrading of biogas (removal of CO2) is relatively expensive (up to 30% of the capital investment of a biomethane system).

A biological power to methane system can remove the need for a traditional upgrading system, saving money. Hydrogen (from surplus electricity) may be mixed with biogas in a small anaerobic chamber where methanogenic archae convert the two gases to methane (as described in Equation 1). Typically, hydrogen is added at four times the volume of CO2. The overall effect is that the methane output of the facility is increased by almost double as the CO2 in the biogas is converted to CH4 to add to the existing CH4 in the biogas. The biomethane may now be gas grid injected.

The energy plan

Renewable energy is not all about electricity. Bioenergy is not all about combustion of wood chips. Renewable energy is complex and should be treated as complex. We should not look for silver bullets, but allow for integration of innovative concepts with well-understood concepts. An example is coupling power to gas with a biogas industry; this has significant potential to balance and facilitate high levels of variable renewable electricity.

Renewable gas is an industry that can provide renewable heat to existing houses and facilities connected to the gas grid with minimal investment on the gas user. Large industry including for global multi-nationals are the largest energy consumer in the country; these companies tend to have carbon targets and see a huge benefit in renewable gas for tri-generation (electricity, heating and cooling).

Natural gas is used in buses and heavy commercial vehicles across the world; renewable gas is probably the easiest path to second- (residues) and third- (algae) generation renewable biofuel. The gas grid is a substantial piece of energy infrastructure, which will be an integral component of the Smart Energy Grid.


Ahern, E; Deane, P; Persson, T; O’Gallachoir, B; Murphy, JD (2015). ‘A perspective on the potential role of renewable gas in a smart energy island system.’ Renewable Energy

Wall, D; O’Kiely, P; Murphy, JD (2013). ‘The potential for biomethane from grass and slurry to satisfy renewable energy targets.’ Bioresource Technology (149) 425-431.

Gallagher, C; Murphy, JD (2013). ‘What is the realistic potential for biomethane produced through gasification of indigenous willow or imported wood chip to meet renewable energy heat targets?’ Applied Energy (108) 158-167.

McGarrigle, EV; Deane, JP; Leahy, PG. (2013). ‘How much wind energy will be curtailed on the 2020 Irish power system?’ Renew Energy (55) 544-53. O'RiordanEleccarbon,energy,gas,Ireland,UCC
  Author: Prof Jerry D Murphy, Science Foundation Ireland, Marine Renewable Energy Ireland Centre, Environmental Research Institute, School of Engineering, University College Cork An ongoing task for engineers is to predict the future and plan infrastructure based on these predictions. This is a difficult task and is not an exact science....