Power-to-X

Seasonal storage through Power-to-X —the key to a robust energy grid?

Just a few years ago, both the media and politicians also started talking about Power-to-X (PtX), and we are developing a common understanding that Denmark may benefit from venturing into PtX.

Henrik Lund Frandsen. Fotograf Mikal Schlosser — Professor Henrik Lund Frandsen. Photo: Mikal Schlosser.

Monday 09 October 2023

Column by professor Henrik Lund Frandsen published in Energy Supply October 2023.

The reason is that we need to balance an energy grid with fluctuating energy sources (wind and solar), which means that we sometimes have inexpensive power available that we may as well derive value from, while the transport sector in particular will have difficulty meeting the climate goals without green fuels from PtX.

I believe we may even be able to use PtX to solve another challenge: ensuring that we have a competitive, robust, and sufficient energy storage solution. I’m not referring to a hydrogen storage, which is an inefficient and expensive solution for converting electricity into hydrogen and back again, but a new solution; a biomass battery, which will be explained in detail below.

The need for storage

When we phase out fossil fuels, we will in Denmark need a terawatt-hour-sized energy storage solution to get through the winter. The capacity of terawatt hours (TWh) equals millions of car batteries, so it’s not something we can solve using standard batteries. Such capacities can only be supplied by underground cavern gas storage — for example, Denmark’s natural gas storage capacity is ~12 TWh.

In addition to seasonal storage, a robust energy storage concept will also facilitate faster integration of renewable fluctuating energy (solar and wind), as greater seasonal variations can be covered by such a concept as well. One side effect of this would be cheaper energy, but also independence from foreign energy supply, which means we would not have to pay enormous sums for hydropower from Norway, for example.

Finally, a robust energy storage solution will also be a security necessity, if a large part of our energy supply come under attack—for example, offshore wind turbines connected to the grid by a vulnerable cable. Fuel self-sufficiency also makes sense from a security point of view.

Biomass shortage

Before I start describing possible long-term storage solutions, we first have to apprehend an obstacle that we must take into account when piecing the puzzle together.

Biomass is already a maligned energy source, as quite a lot of it—wood in particular—is imported from neighbouring countries in the name of the green transition. However, Denmark has quite large amounts of ‘residual biomass’ available (approx. 200 PJ = 55 TWh annually [ref: Gylling 2012, Graudal 2013]). This may not all be readily accessible, but it is nevertheless available.

Most people interested in PtX probably know that the circular carbon [SK1][HF2]in biomass will become a scarce resource worldwide, as it is vital for producing various fuels, chemicals, plastics via PtX, but also has many other applications—in particular in construction. So we should use biomass wisely, and we should do this already. The alternative is direct carbon capture from the air, where CO2 only makes up 0.04%, which obviously is an expensive and energy-intensive proposition.

The biomass battery

A new solution for storing energy is what I call the ‘biomass battery’, illustrated in the figure. When the wind blows and the sun shines, we can use the abundant green power to produce PtX products. When we lack energy in the system, we use the ‘biomass battery’ by using our available residual biomass in combined heat and power (CHP) plants to generate power (and heat).

But we have to collect the green CO2 from these and store it for when the wind blows again, so we can use the CO2 for PtX products. This approach requires a gas storage solution, but CO2 is cheaper and more risk-free to handle than, for example, hydrogen. The CO2 may initially be obtained with carbon capture and temporarily stored in a cavern.

Figure of biomass battery by Henrik Lund Frandsen — Figure of biomass battery

The collection could be done in more ways than the slightly expensive carbon capture; by oxy-fire using oxygen from the electrolysis, or with an interesting oxy- CO2 firing concept, which I have presented in the figure. Here, oxygen will be diluted with CO2 in relation to transport and storage. Here, too, the majority of the flue gas will be CO2 and energy loss in the capture is avoided. This will make the solution safer and, unlike pure oxy-fire, it can be used in existing installations. In the slightly longer term, efficient high-temperature fuel cells could likely be used with economic benefits [ref: Jacobsen 2023].

With this solution with affordable [SK1]modifications (in relation to hydrogen storage, electrolysis, and fuel cells), it will thus be possible to use the existing infrastructure in the form of the natural gas networks and CHP plants, which is an obvious advantage for minimizing investment costs and accelerating the green transition. The CHP plants even have the right capacity to balance the grid.

The CCS pool is a unwise detour

That we may be able to completely avoid carbon capture at CHP plants makes the government’s CCS pool even more questionable. As a point of departure, the solution by no means supports the ‘wind turbine adventure 2.0’ and that Denmark could become a leader in the development of PtX solutions. The circular carbon will now be sent into the ground and not converted into the green fuels that are in high demand by the transport sector. The transport sector already sees the green transition as a business opportunity.

In fact, with the CCS pool, a technology ‘lock-in’ is made on Denmark’s most important resource and the opportunity to be at the forefront of the PtX race (the biomass). Denmark will find it difficult to compete with really cheap power worldwide. But cheap power does not always colocate with biomass availability, so here we can gain an advantage. Now, the green carbon will just go underground and not do any good...

I would go so far as to say that the CCS pool also hampers the future green transition of Denmark, as this solution is not durable and only makes our society more expensive.

‘Recycling’ plastic—as energy storage

In addition to biomass, we should initially use waste as a supplement to other renewable energy. We must assume that waste will become ‘green’ in the long term. If waste constitutes a valuable energy storage, you could also question the sorting of the garbage into more or less valuable fractions. Plastic has a questionable recycling value, and with the biomass battery concept, ‘recycling’ through combustion and carbon collection may be a more cost-effective solution. We should look into that.

Decommission CHP plants?

The biomass battery makes a lot of sense, as we already have a large part of the existing infrastructure available, i.e. our CHP plants. So we should debate whether we should close down our CHP plants before we get that far. Local Government Denmark (KL[SK2][HF3]) has already formulated a plan for the closure of CHP plants across Denmark.

The plants can be used in the biomass battery concept to provide the necessary storage capacity, but also give our energy grid the necessary robustness.

Thus, CHP plants will hereby not become obstacles to the green transition, but rather a prerequisite for it. The fact that we will be able to balance our energy grid with biomass cheaply is actually vital for the further implementation of other renewable energy sources.

The biomass battery deserves further investigation

In short; the biomass battery is an inexpensive solution, where we recycle our existing infrastructure and avoid the losses incurred by first producing an expensive storage fuel (e.g. hydrogen). It is a solution that we can implement quite fast.

However, the idea is new and in order to succeed, several technical aspects need to be investigated—such as the flexibility of CHP plants, the storage stability of biomass, the possibilities of carbon and oxygen storages, and generally the interaction with other green technologies.

However, if our politicians are serious about the green transition, it seems obvious to investigate these relatively few factors. In my view, there is no doubt that if the biomass battery can be introduced as storage capacity, it offers a huge step forward in ensuring a stable future energy supply. [SK1]Antager, at der menes ”økonomisk overkommelige” [SK2]Antager, at det er Kommunernes Landsforening, der refereres til? [HF3]ja

References:
Gylling, M.; Jørgensen, U.; Bentsen, N.S.; Felby, C. & Johannsen, V.K., (2012). ”+10 mio. tons planen”. Udgivet af Aarhus Universitet og Københavns Universitet.

Graudal, L., U. B. Nielsen, E. Schou, B. J. Thorsen, J. K. Hansen, N. S. Bentsen & V. K. Johannsen (2013). Perspektiver for skovenes bidrag til grøn omstilling mod en biobaseret økonomi: Muligheder for bæredygtig udvidelse af dansk produceret vedmasse 2010-2100. København, IGN/Skov & Landskab, IFRO/Skov & Land- skab.

Emilie Jacobsen, Sofie M. Skov, Alessandro Singlitico, Henrik L. Frandsen, (2023) Techno-economic analysis of green aviation fuel production using an integrated electrolyzer and a “biomass-battery” storage system, International Journal of Hydrogen Energy

Contact

Henrik Lund Frandsen Professor Department of Energy Conversion and Storage Mobile: +45 93511618 hlfr@dtu.dk