Grass and slurry will play a key role in decarbonising Ireland’s heat sector, according to the Sustainable Energy Authority of Ireland (SEAI).

The SEAI recently published the National Heat Study, which details how the heat sector will transition to net-zero by 2050. The challenge is great.

Heat-related carbon dioxide emissions in Ireland are rising, representing 38% of energy-related emissions in 2020.

These largely come from the fossil fuel combustion in homes, businesses and industry, and indirectly from the use of electricity.

Under Government plans, Ireland aims to reduce emissions by 51% from 2018 levels by 2030.

Silage is Ireland's untapped energy crop.

The study identifies several key technologies, including heat pumps, district heating and carbon capture and storage, which will need to be rolled out in order to achieve this.

The report also details the role that anaerobic digestion (AD) will have in decarbonising the heat sector. Biomethane, produced from AD plants, can act as a direct renewable replacement for natural gas. Despite this, the sector remains chronically underdeveloped in Ireland due to lack of financial support. According to the SEAI, the sector needs to develop rapidly in order to meet our decarbonisation target.

Scale

The SEAI examined two scenarios on how an AD industry would develop. Under the first scenario, the dairy herd increases by 10% by 2030, but the suckler herd reduces by 23%, giving an overall reduction in the national herd of 0.5%.

The SEAI estimated that an AD industry, supplied with a mixture of grass silage produced from these farms along with slurry could deliver 2.7 Terawatt hours (TWh) of biomethane by 2030. This is equivalent to 5% of Ireland’s current gas demand. The biomethane can be injected directly into the gas grid or transported directly to a customer.

Under the second scenario, the dairy herd increases by 10% by 2030, but the suckler herd reduces by 45% (about 440,000 head), giving an overall reduction in the national herd of 8.4%.

As a result, more land would be available for silage production and would deliver an extra 1.4 TWh of biomethane. Once combined with another 1TWh produced from food waste, total biomethane production could reach 5.1 TWh by 2030. This is 10% of current gas demand.

For context, the minimum size for a farm-based AD plant, upgrading biomethane for injection into the gas grid, would be around 20 Gigawatt hours (GWh) fed with grass silage mixed with slurry on a 50:50 ratio. Therefore, according to the SEAI, there is potential for between 135-255 farm-based AD plants in Ireland by 2030, depending on how much the suckler herd will reduce by.

Slurry

Slurry has a high water content, which means that the volume of biogas produced per tonne is relatively low. This low energy density means that it is not cost effective or energy efficient to transport it any further than 5km to 10km from an AD plant.

However, slurry will be instrumental in meeting the biomethane sustainability criteria set out in the European Renewable Energy Directive (REDII). Currently, this means that biomethane must reduce emissions by 70% compared to natural gas.

Modern AD plants require a lot of slurry, around 1,000 dairy cows per 2GWh according to the SEAI – however the report outlines that Ireland has an ample supply of slurry to meet both scenarios.

Grass silage

While silage has a higher energy density than slurry, it is still expensive and inefficient to transport long distances, meaning it must be sourced close to the plant.

Interest in producing grass silage for AD has been steadily increasing. However, it is vital that any cultivation of grass silage for energy is done sustainably and does not cause conflict with food and fodder production in line with national policy, the report outlines.

The report also suggests that the inclusion of clover in grass swards and the optimisation of soil fertility would sustainably increase grass production on farms which could be used to supply AD plants.

This, along with the grass produced from land taken out of beef production, would free up between 160,000ha to 200,000ha for grass production. It is assumed that cattle slurry would be available at no cost, and the price for silage has been estimated to be between €47/t to €49/t.

Tillage

The report advises that tillage farmers could grow an energy crops for AD as a break crop in their tillage rotation, thus avoiding a further significant reduction in tillage area.

For example, maize and grass could be grown for a year in between cereal crops, the SEAI has said.