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.
