(525b) The Role and Value of Negative Emissions Technologies in Decarbonising the Energy System

Negative emissions technologies (NETs) are technologies that can directly or indirectly remove CO₂ from the atmosphere. NETs could, purportedly, offset emissions from sectors with more difficult or expensive mitigation solutions such as transport, and offset residual emissions from fossil fuel power plants. The IPCC Global Warming of 1.5°C report highlighted that NETs were critical in limiting global temperature rise to 1.5°C, as desired by the 2015 Paris Agreement. With the exception of afforestation/reforestation, no NET has been proven as both technically- and commercially-viable at scale. Two technologies—Bioenergy with Carbon Capture and Storage (BECCS) and Direct Air Carbon Capture and Storage (DACCS)—have however been demonstrated as technically-feasible means of delivering negative emissions, albeit not at large-scale. Their potential for climate change mitigation has only been assessed within the context of the technologies in isolation, not within the energy system. Using the UK as a case study, this study investigates the potential role of BECCS and DACCS in meeting greenhouse gas (GHG) emissions reduction targets.

The 2008 Climate Change Act mandated the reduction of UK economy-wide GHG emissions by 80% (relative to 1990 levels) by 2050. To achieve this, the power sector must be virtually decarbonised by 2050. To be consistent with the Paris Agreement, to which the UK is committed, however requires deeper decarbonisation of the economy, including the deployment of 50 Mt_CO2/yr of negative emissions by 2050. We integrate BECCS and DACCS into the Electricity Systems Optimisation (ESO) model to determine their potential role in power sector decarbonisation and deeper decarbonisation in line with the Paris accord. The ESO model determines the least-cost power supply capacity expansion subject to a set of constraints that ensure system reliability and operability at hourly intervals.

We observe that complete decarbonisation of the electricity system—defined here as a carbon intensity of less than 10 kg_CO2/MWh—is very costly in this absence of NETs and requires an investment of £310 billion in the period to 2050. Total installed capacity is seen to rise by 210% to 300 GW by mid-century due to the extensive expansion of intermittent renewable energy sources (IRES), and energy storage and interconnection capacity to compensate for their intermittency and provide additional grid flexibility. A ramping of nuclear capacity from 10 to 24 GW is also needed between 2030 and 2050, after existing plants reach the end of their lifetime. In addition, whilst thermal power plants (CCGTs) are not entirely displaced from the power system, their capacity factors are less than 20% after 2025. CCGT operators will therefore face stranded asset issues due to the underutilisation of their power plants.

Once BECCS or DACCS is made available to the system, the investment needed to achieve complete decarbonisation is reduced to £160 billion and £193 billion, respectively. This is because negative emissions allowed for increased operation of cheaper thermal power plants by offsetting their CO₂ emissions. Increased generation from thermal plants displace the costly additional IRES and energy storage capacity that would otherwise be needed. NETs therefore have the potential to ease the cost burden of decarbonisation. In addition, NETs allow unabated fossil plants (fossil fuel powered plants without CCS) that would otherwise have been constrained off the grid to continue operating and generating revenues. Allowing some of this value to accrue to BECCS or DACCS offers a potential route to their commercialisation.

When BECCS is deployed in the system, we observe that new nuclear capacity is not required. Therefore, flexible carbon negative generation to augment gas generation, and offset gas-derived emissions (from CCGT and CCGT-CCS), proves more valuable to the system than base load zero-carbon generation. On initial deployment in 2030, BECCS generation fluctuates with hourly demand, i.e. it is load-following. CCGT power plants exhibit the same behaviour. BECCS is therefore operating to meet some electricity demand while offsetting CO₂ emissions from CCGT plants. In 2050, when decarbonisation is to be achieved, BECCS operates at its maximum capacity factor, i.e. base load. The emissions target therefore dictates the principal service that BECCS is providing (power generation, CO₂ offset or CO₂ removal).

The System Value (SV) metric was used to evaluate the value added to the electricity system by the integration of BECCS and DACCS. The SV quantifies the reduction in total system cost (TSC) for a given deployment of a technology. The TSC is the total capital and operational expenditure invested into the system over the planning horizon. On initial deployment, the system value of BECCS—£124,000/Kw—is three times greater than that of DACCS; therefore BECCS deployment provides three times the reduction in TSC that is provided by DACS deployment. As its economic level of deployment is approached, i.e. the limit beyond which there is no added deployment as added capacity is made available, SV_BECCS falls to approximately £43,000/kW. We find that BECCS provides greater value to the energy system by providing the service of power generation in addition to offsetting the CO₂ emissions from cheaper, unabated fossil plants.