Atmospheric methane removal: Solar chimney and photocatalytic semiconductor technology

Industry

Methane is a highly potent greenhouse gas and its reduction is given ever greater priority in international emission reduction policies (see, for example, https://www.globalmethanepledge.org/). There are several suggested ways to remove atmospheric methane (see Nisbet Jones et al. 2021; and Ming et al. 2022; see in this report also iron salt aerosols and zeolites). One of the main issues with methane removal is that atmospheric methane concentrations are very low. This means that very large volumes of air, and related energy demands, are required, making the use of ventilators like those used for DACCS (see direct air capture) more complicated (Nisbet-Jones et al. 2021).

De Richter et al. (2017) first suggested that it might be possible to combine photocatalytic reactors, which would degrade methane into water vapor and CO2, with a solar updraft tower that uses large volumes of air that are passively moved inside it by incoming solar radiation to power generators. De Richter et al. (2016 & 2017) suggest multiple solar chimney power plants with photocatalytic reactors (SCPP-PCR) could potentially produce renewable energy and process large enough amounts of air to significantly reduce methane levels.

Technological Readiness Level (TRL)

Low 1

Solar chimneys have been in use for a long time in warmer regions of the world. The first ideas to combine the passive system with power generation were proposed in the 70s (Zhou et al, 2010; Kasaeian et al, 2017). Multiple small pilot plants have since been built. The largest of these was already constructed in Spain in the 1980s. Recently, interest in the technology seems to have been growing, with multiple studies exploring different elements of its design and effectiveness (See for example Ming et al, 2021; or Xiong et al, 2023). However, apart from the pilot plants, the technology has not yet been operationalised or commercialized, and questions remain about ultimate feasibility to scale.

The other part of this technology is the photocatalytic methane removal systems. These are already in use in various forms (Wang et al, 2022). With growing interest in methane removal technologies, the development of such photocatalytic technologies is also advancing (Li et al., 2019). However, scaling up is a major issue of concern to all methane mitigation technologies, given the large amounts of air required (Lackner, 2020; Jackson, 2021). Although new studies are exploring crucial issues (see for example Ming et al, 2021 and Huang et al, 2021), studies will have to provide experimental data and show how effective and scalable such a system can be.

Cobo et al's (2023) review assigns photocatalytic methane degradation in general a low technological readiness level of 3 to 4.

Technological Readiness Level (TRL)

A technology with a TRL of 1-3: TRL 1 – Basic; TRL 2 – Concept formulated; TRL 3 – Experimental proof of concept

Scalability

Unknown 0

Scaling up is a major issue of concern to all methane mitigation technologies, given the large amounts of air required (Lackner, 2020; Jackson, 2021). Huang et al (2021) already showed that size of the towers would significantly influence the system’s efficiency. Nisbet-Jones et al (2021) note that the use of solar towers for energy generation seemed to be best feasible through especially large structures, but that it is not sure if this holds true for methane removal too.

Because solar towers depend on solar radiation to function, it is likely that physically scaling across the globe will not see similar efficiency rates, and greatest efficiency will be achievable in parts of the globe with the greatest amount of sun hours.

Timeliness for near-future effects

Unknown 0

Northern + Arctic potential

Low 1

Given this technology’s dependency on sunlight, it is likely the Arctic is not the most promising for such structures. Although variants to a chimney system have been proposed for the Polar regions (see Polar chimneys).

Northern + Arctic potential

No noticeable extra positive effect beyond the global average; technology is unsuited to the Arctic

Global potential

Unknown 0

It is not yet clear how effective such systems would be. Some studies extrapolate very high potential capture rates, with De Richter et al (2017) claiming that 2 out of 3 CH4 molecules would be removed from the airflow, and Huang et al (2021) calculating that a large tower combined with an effective photocatalyst could remove up to 42.5%.

Cost - benefit

Unknown 0

Apart from the construction and maintenance costs of the towers, these devices would not require energy input and could potentially be relatively cheap measures for methane removal.

Environmental risks

Unknown 0

Community impacts

Unknown 0

Apart from methane removal and energy generation, solar towers have in other studies also been explored in their capacity to provide other utilities like water production (Zuo et al, 2020; Wu et al., 2020) or to counter local air pollution (Liu et al, 2021). If such co-benefits could be realized, this could be very beneficial to local communities.

Ease of reversibility

Easy 3

Ease of reversibility

Easily reversible naturally

Risk of termination shock

Low 3

Risk of termination shock

Low or insignificant termination shock or damage

Legality/governance

High 3

This will likely fall under national or regional legislation comparable to the construction of other kinds of power plants.

Legality/governance

Currently legal to deploy, with governance structures in place to facilitate it and/or financial incentives to develop it

Scientific/media attention

Medium 2

Although the importance of methane mitigation is increasingly recognised, this particular measure has not been widely picked up other than in some academic studies. Interest in solar towers seems particularly great amongst specific groups, especially in China, but has not really captured mainstream attention yet.

Scientific/media attention

Some attention within the scientific community, including published research and funding programmes; some media attention; some commercial interest

References

Cobo, S., Negri, V., Valente, A., Reiner, D. M., Hamelin, L., Mac Dowell, N., & Guillén-Gosálbez, G. (2023). Sustainable scale-up of negative emissions technologies and practices: where to focus. Environmental Research Letters, 18(2), 023001. https://doi.org/10.1088/1748-9326/acacb3

Huang, Y., Shao, Y., Bai, Y., Yuan, Q., Ming, T., Davies, P., ... & Li, W. (2021). Feasibility of solar updraft towers as photocatalytic reactors for removal of atmospheric methane–the role of catalysts and rate limiting steps. Frontiers in chemistry, 9, 745347. https://doi.org/10.3389/fchem.2021.745347

Jackson, R. B., Abernethy, S., Canadell, J. G., Cargnello, M., Davis, S. J., Féron, S., ... & Zickfeld, K. (2021). Atmospheric methane removal: a research agenda. Philosophical Transactions of the Royal Society A, 379(2210), 20200454. https://doi.org/10.1098/rsta.2020.0454

Kasaeian, A. B., Molana, S., Rahmani, K., & Wen, D. (2017). A review on solar chimney systems. Renewable and sustainable energy reviews, 67, 954-987. https://doi.org/10.1016/j.rser.2016.09.081

Lackner, K. S. (2020). Practical constraints on atmospheric methane removal. Nature Sustainability, 3(5), 357-357. https://doi.org/10.1038/s41893-020-0496-7

Liu, Y., Ming, T., Peng, C., Wu, Y., Li, W., De Richter, R., & Zhou, N. (2021). Mitigating air pollution strategies based on solar chimneys. Solar Energy, 218, 11-27. https://doi.org/10.1016/j.solener.2021.02.021

Ming, T., Caillol, S., & Liu, W. (2016). Fighting global warming by GHG removal: Destroying CFCs and HCFCs in solar-wind power plant hybrids producing renewable energy with no-intermittency. International Journal of Greenhouse Gas Control, 49, 449-472. https://doi.org/10.1016/j.ijggc.2016.02.027

Ming, T., Davies, P., Liu, W., & Caillol, S. (2017). Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis. Progress in Energy and Combustion Science, 60, 68-96. https://doi.org/10.1016/j.pecs.2017.01.001

Ming, T., Gui, H., Shi, T., Xiong, H., Wu, Y., Shao, Y., ... & de Richter, R. (2021). Solar chimney power plant integrated with a photocatalytic reactor to remove atmospheric methane: A numerical analysis. Solar Energy, 226, 101-111. https://doi.org/10.1016/j.solener.2021.08.024

Ming, T., Li, W., Yuan, Q., Davies, P., De Richter, R., Peng, C., ... & Zhou, N. (2022). Perspectives on removal of atmospheric methane. Advances in Applied Energy, 100085. https://doi.org/10.1016/j.adapen.2022.100085

Nisbet-Jones, P. B., Fernandez, J. M., Fisher, R. E., France, J. L., Lowry, D., Waltham, D. A., ... & Nisbet, E. G. (2022). Is the destruction or removal of atmospheric methane a worthwhile option?. Philosophical Transactions of the Royal Society A, 380(2215), 20210108. https://doi.org/10.1098/rsta.2021.0108

Wang, Q., Xiong, H., & Ming, T. (2022). Methods of Large-Scale Capture and Removal of Atmospheric Greenhouse Gases. Energies, 15(18), 6560. https://doi.org/10.3390/en15186560

Wu, Y., Ming, T., de Richter, R., Höffer, R., & Niemann, H. J. (2020). Large-scale freshwater generation from the humid air using the modified solar chimney. Renewable Energy, 146, 1325-1336. https://doi.org/10.1016/j.renene.2019.07.061

Xiong, H., Ming, T., Wu, Y., Li, W., Mu, L., de Richter, R., ... & Peng, C. (2023). Numerical analysis of a negative emission technology of methane to mitigate climate change. Solar Energy, 255, 416-424. https://doi.org/10.1016/j.solener.2023.02.048

Zhou, X., Wang, F., & Ochieng, R. M. (2010). A review of solar chimney power technology. Renewable and Sustainable Energy Reviews, 14(8), 2315-2338. https://doi.org/10.1016/j.rser.2010.04.018

Zuo, L., Liu, Z., Ding, L., Qu, N., Dai, P., Xu, B., & Yuan, Y. (2020). Performance analysis of a wind supercharging solar chimney power plant combined with thermal plant for power and freshwater generation. Energy Conversion and Management, 204, 112282. https://doi.org/10.1016/j.enconman.2019.112282

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