Land-use efficiency of energy crops

15/11/2024

For an energy transition, not only wind turbines, solar modules and electricity grids are needed, but also land on which these technologies can be operated. This is why, for example, the Windenergieflächenbedarfsgesetz (German Wind Energy Area Requirements Act) obliges the federal states to make between 0.5 and 2.2% of their land available for wind power by 2032. This may not seem like much at first glance, but in times of high lease prices for agricultural land, increasing pressure on land due to settlement and traffic areas and also increasing utilisation restrictions for nature conservation and environmental protection purposes, there is great potential for conflict.

Against this backdrop, the cultivation of energy crops such as maize or rapeseed is subject to persistent criticism. Although residual and waste materials are increasingly being used to produce biofuels and biomass electricity, energy crops continue to play a dominant role in the bioenergy sector [1,2]. These plants have a reputation for being particularly land-inefficient, i.e. requiring a lot of land per unit of energy produced. Studies repeatedly emphasise that photovoltaic systems or wind power could generate many times more electricity per hectare.

Physically, this argument is correct. However, it ignores important technological, economic and political aspects of the energy transition. The possible advantages of bioenergy are overlooked and potential disadvantages are distorted. In the following, five criteria from the above-mentioned dimensions are presented, which should be part of a reliable comparison of energy crops with other renewable energy options:

• Technological flexibility of use: energy crops offer the advantage that they can be used to produce fuels or other combustibles in addition to electricity. Biofuels can be used for almost all energy requirements, even in sectors that are difficult to electrify, such as aviation and maritime shipping. The direct use of electricity has its technical limits here, as long-haul aircraft, for example, would require such large and heavy batteries according to the current state of the art that they would hardly be able to transport any more freight or passengers. By converting electricity into liquid fuels (PtX), it could also be utilised for air and sea transport, but this step comes at the cost of high efficiency losses. As a result, electricity-based solutions in these areas of application require significantly more space than direct utilisation of electricity. A reliable comparison of the land utilisation efficiency of PV electricity and bioenergy would therefore have to be made with the land requirements of electricity-based fuels, at least with regard to biofuels.

• Temporal flexibility of use: Energy crops not only facilitate the provision of energy in different fields of use, but also at different times. This added value is particularly evident in times of so-called Dunkelflauten (dark doldrums), when there is neither sun nor wind available. The decreasing supply of renewable electricity then tends to lead to rising prices. These price increases show that not every kilowatt hour produced is of equal value, but can have a lower or higher value depending on when it is provided. Biogas plants can increase their production in times of severe electricity shortages, whereby the electricity generated is then of higher value than that from wind or solar plants. Conversely, they can produce more electricity in times of peak demand, i.e. when there is a particularly high demand for electricity. In both cases, they can help to cushion sharp price rises and make the energy transition less expensive. This is another reason why equating wind or solar power with electricity from biomass falls short. An appropriate comparison between these energy options requires wind and solar power to be combined with electricity storage systems in order to ensure comparable temporal flexibility of the electricity supply. The additional costs incurred must be included in the comparison.

• Costs for grid infrastructure and stability: A comparison of PV electricity with biogas plants should also include costs for the electricity grid infrastructure. It will have to be further expanded or converted if PV is used to multiply electricity generation on the current areas for energy crops. Even if the exact amount of PV electricity per hectare were generated as is currently provided from biomass, additional grid capacities would probably be required. In contrast to electricity generation from PV, the flexible generation of electricity from biomass makes it possible to utilise idle grid capacities at times when PV and wind power plants are feeding in little electricity. In addition to rising electricity transport costs in the event of further expansion of volatile renewable energies, the costs for measures to ensure the stability of electricity grids, such as the provision of balancing power, may also increase. However, the fact that biomass electricity enables cost advantages in the form of lower grid expansion and balancing power requirements presupposes that biogas plants actually generate electricity in line with demand (flexibly), which is currently only partially the case. This makes it even more difficult to compare the costs of the two options. In any case, a purely area-based comparison between biomass electricity and other renewable electricity generators can lead to significantly higher costs for the energy transition if infrastructure costs are not taken into account. If a cost-optimised portfolio of renewable energy options is to be compiled, the so-called systemic LCOE, which takes such additional costs into account, must therefore be used (LCOE system, see e.g. [3]) instead of pure LCOE.
   
• Food security: Biofuels from agricultural biomass such as maize and rapeseed in particular have been criticised for reducing the amount of land available for the production of food and feed. In fact, the high energy yields of photovoltaics per hectare initially suggest that this energy option is more likely than bioenergy to be able to minimise the impact of the energy transition on food security. However, it is often overlooked that the cultivation of energy crops for the production of fuels usually goes hand in hand with the production of animal feed or that vegetable oil is a by-product of the animal feed industry. Areas with energy crops are therefore not completely “lost” to the food sector. Rather, there is a dual utilisation for energy and animal feed purposes. If energy crops are replaced by PV systems, other areas must therefore at least partially be used for the cultivation of animal feed. In the worst-case scenario, this leads to an increase in feed imports from regions with low environmental and social standards compared to the status quo. Even if this is not the case and the land intensity of the energy transition can be reduced overall with PV modules, the advantage for food security due to the co-production of animal feed is likely to be less than the pure ratio of the energy yields of the two energy options suggests. It should also be taken into account that the potential positive effects of energy crops on food security are foregone if land is converted. These include the stabilisation of agricultural prices, additional incentives for agricultural investments or the possibility of converting land for food production at short notice in times of crisis [4] (see next point).
   
• Flexible land utilisation: Contrary to what is often portrayed, there are no specific “bioenergy areas”. Rather, farmers decide whether, when and which areas they use to grow energy crops or other crops depending on the market situation. In some cases, it is only decided at the time of harvest whether the crops will be used for energy production or sold to other markets. Energy crops are part of such flexible utilisation of land and agricultural raw materials. However, the installation of PV modules on arable land previously used for energy crops can lead to a loss of this flexibility. The installation of PV modules means a long-term investment that rules out alternative use of the land for many years. Due to the associated competition with food policy objectives, the Erneuerbare-Energien-Gesetz (Renewable Energy Sources Act) stipulates that PV-only systems are only eligible for funding on land that is already sealed or otherwise not used for agricultural purposes. The occasionally suggested idea that “bioenergy areas” could be converted into solar parks would not only lead to a loss of adaptability of agriculture to changing framework conditions, but is also complicated by political barriers. However, it is conceivable to replace energy crops with PV modules on other, already sealed surfaces. However, in the course of the strong expansion of photovoltaics in Germany and the increasing land utilisation requirements due to settlements and traffic, it may become increasingly challenging to find suitable areas for additional expansion beyond the currently planned level.

The discussion about the competition for land use between bioenergy and photovoltaics (PV) was recently given new impetus by a study by the University of Hanover [5]. The study once again emphasised the higher energy yields of PV. Compared to earlier studies, it represents an important step forward, as parts of the system costs of PV were taken into account (see above). In particular, the costs of storage systems were included, which are required in order to be able to use PV electricity just as flexibly (in line with demand) as biomass electricity. Even under these assumptions, PV proved to be a significantly more advantageous (cost-efficient) energy option: according to this, only one seventh of the arable land currently used for energy crops in Germany would be sufficient to achieve the same – flexibly deployable – energy yield using PV systems.

Despite the study’s significantly improved design, it is difficult to interpret it as clear evidence in favour of PV over energy crops. For example, the study takes into account energy system costs in the form of storage, but not the additional costs for expanding the grid infrastructure, which would be necessary with the extensive expansion of PV described. The long-term scenarios of the Bundesministerium für Wirtschaft und Klimaschutz (Federal Ministry for Economic Affairs and Climate Action), in which the system costs of volatile renewable energies are mapped more comprehensively, do not yet show any cost advantages of scenarios with a particularly strong expansion of photovoltaics [6].

In addition, the potential advantages of bioenergy in terms of food security and flexible use of land mentioned above can hardly be reflected in energy system modelling of this kind. However, the energy transition should not only be optimised in terms of costs, but should also enable ecological and social synergies in order to ensure its acceptance. Furthermore, it should be questioned whether a discussion about the competition between photovoltaics and energy crops is expedient if a combination of both technologies in the form of agrivoltaics is possible and offers promising potential [7]. To what extent and in what form the cultivation of energy crops makes sense therefore requires further discussion, taking systematic account of the effects outlined above.

Sources

[1] Schröder, J. et al. (2023): Monitoring erneuerbarer Energien im Verkehr, DBFZ-Report 44, https://www.dbfz.de/pressemediathek/publikationsreihen-des-dbfz/dbfz-reports/dbfz-report-nr-44.

[2] FNR (2024): Anbau und Verwendung nachwachsender Rohstoffe in Deutschland, Statistik Stand 2024, https://www.fnr.de/fileadmin/Statistik/Statistikbericht_der_FNR_2024_web.pdf.

[3] Hirth, L. et al. (2016): Why wind is not coal: on the economics of electricity generation, in: Energy Journal 37-3, https://doi.org/10.5547/01956574.37.3.lhir.

[4] Bureau, J.-C.; Swinnen, J. F. M. (2018): EU policies and global food security, Global Food Security 16, 106-115, https://doi.org/10.1016/j.gfs.2017.12.001.

[5] Schlemminger, M. et al. (2024): Land competition and its impact on decarbonized energy systems: A case study for Germany, Energy Strategy Reviews 55, 101502, https://doi.org/10.1016/j.esr.2024.101502.

[6] https://langfristszenarien.de/enertile-explorer-de/index.php.

[7] Walston, L. J. et al. (2022): Opportunities for agrivoltaic systems to achieve synergistic food-energy-environmental needs and address sustainability goals, Front. Sustain. Food Syst. 6, https://doi.org/10.3389/fsufs.2022.932018.