How to understand the carbon footprint of clean hydrogen

The use of hydrogen as a clean fuel is not a new idea. Past hydrogen economy deployment efforts have been generally unsuccessful due to insufficient technology maturity, difficult economics, and lack of supportive policy frameworks. Today equipment required to produce, store, and utilize hydrogen is commercially ready and global efforts to produce it at scale are ramping up.

In addition, regulatory frameworks such as the Paris Agreement and more recently the European Green Deal, Inflation Reduction Act (IRA) and formation of the European Hydrogen Bank are quickly making the hydrogen industry a reality. In particular, the IRA is a strong economic enabler providing subsidies of up to $3 per kg H2. Despite significant efforts, the hydrogen industry is yet to address numerous technical, economic and policy challenges. One of these challenges revolves around hydrogen’s carbon intensity and colour nomenclature.

While our societal awareness of fossil fuels and their respective carbon footprints has evolved over last two decades, we tend to forget that renewable energy generation is not entirely zero-carbon. Generally, renewable energy technologies have significantly lower carbon footprints compared to their fossil fuel counterparts, however, in certain cases their carbon emissions over a lifetime can be significant.

The majority of these emissions come from manufacturing processes, which today rely heavily on fossil fuel based electricity or various forms of carbon for processing respective components. For example, the steel used in the majority of these technologies is responsible for roughly 7-9% of global carbon dioxide emissions. Also, polysilicon production relies on coal to reduce silica and produce metallurgical grade silicon with the intention to convert it further into solar panels or other semiconductor uses.

Some other major sources of carbon dioxide emissions in renewable power generation come from transportation of these technologies from place of manufacture to place of deployment. In addition, some renewable energy technologies may require solid foundations for deployment or additional concrete structures, as in hydropower where reservoir-based hydropower schemes require concrete dams, spillways, and power houses.

High geothermal carbon footprints are often associated with subsurface carbon dioxide being vented during geothermal plant operation. According to the World Bank, the average CO2 emissions from geothermal are 122 gCO2per kWh, while certain geothermal plants may initially release as much as 1,200 gCO2 per kWh, however, these emissions are predicted to reduce over time.

The National Renewable Energy Laboratory (NREL) in Colorado performed an evaluation of the life cycle of greenhouse gas emissions from electricity generation and concluded that while emissions from renewable energy sources and nuclear have significantly lower carbon dioxide footprint than fossil fuel-based generation, some renewable energy technologies can reach levels higher than 200 g of CO2e per kWh (Figure 1).

Using these NREL numbers and assuming electrolyzer power consumption in the range of 48-58 kWh/kg H2, the carbon footprint per kg of generated hydrogen has been derived. The calculations do not include emissions from hydrogen transportation, compression, or further conversion into hydrogen derivatives. Emissions from manufacturing of electrolyzer and other relevant equipment and emissions from deployment of electrolyzer facilities have been omitted as well. As such, these numbers might be higher.

While hydrogen production using renewable energy or nuclear has a significantly lower carbon footprint compared to hydrogen using electricity from fossil fuels, in some cases, it is difficult to call renewable hydrogen carbon-free (Figure 2). Biomass presents an interesting case, where carbon dioxide emissions vary from negative 48 up to nearly 76 kg of CO2e per kg H2. This makes produced biomass-based hydrogen either carbon negative or comparable to that of fossil fuels in terms of carbon intensity.

While the purpose of this article is a very high-level evaluation and comparison of carbon emissions for hydrogen produced from various sources, certain readers may question that we use biomass or fossil fuels to firstly produce electricity and subsequently convert this electricity into hydrogen via electrolysis. Likely, gasification or production of hydrogen via steam methane reforming will have a lower footprint than that presented above. For that reason, values for carbon processing footprint for chemical hydrogen generation from a previous study by Rocky Mountain Institute were compared.

There is ongoing debate and development around low-carbon hydrogen certification. Based on carbon footprint, hydrogen is given different colour classification. Carbon intensities differ depending which certification scheme was used. Some countries may have their own certification schemes, while for others it is still work in progress. In addition, international organizations, and certification bodies like TÜV Süd have developed their own certification frameworks.

Given the carbon intensities illustrated in Figure 2, are these certification schemes reasonable? The answer to this question can be found in Figure 3, and it depends. Taking the lowest threshold of 1.0 kg CO2e per kg H2 assumed by Green Hydrogen Organisation, in the extreme minimum case all renewables and nuclear can meet less than 1.0 kg CO2e per kg H2 threshold, however, not a single technology can meet this requirement for the extreme maximum case. The most liberal CO2 emissions threshold per kg of H2 is assumed by the China Hydrogen Alliance. The 4.9 kg CO2e per kg H2 threshold even in the most extreme case can still be met by hydropower, wind, and ocean energy generation.

As we develop hydrogen projects, we need to be mindful that it is not enough to assume that hydrogen produced from renewable energy via water electrolysis will always have to be classified green or low-carbon, and that it is necessary to understand hydrogen’s carbon footprint at each step of the value chain. As more renewables come online, the carbon footprint of renewable energy generation will decrease as more renewable energy will be used to extract and process raw materials for these technologies. The same applies to transportation of equipment and deployment of renewable generation.

Hydrogen certification schemes are still relatively immature and not broadly tested in real applications due to lack of available projects. As hydrogen industry matures, these certifications schemes will evolve to meet producer, customer, and regulators needs.