In Germany, the energetic use of biomass has seen a rapid increase in the last decade (Fig. 2, first four columns). At present, bioenergy contributes significantly to the production of heatFootnote 2 and power and has also seen a growing share in the mobility sector even if this has lessened since 2010. Especially in the biomass-based electricity sector, the increase of new installations in the last 15 years has been a success of the German policy (the German feed-in tariff law, EEG). In the last few years, the average of newly installed production capacity per annum has been between 300–500 MW. Figure 2 represents different scenarios about bioenergy and its shares in the three energy sectors (electricity, heat and mobility).
Currently, there is no universal/official scenario/plan or target for the future share of bioenergy on the different sectors, but discussions on the future role of bioenergy are ongoing. In a study mandated by the German biogas association (Fachverband Biogas), the IZES gGmbH analysed the future contribution of bioenergy to the electricity sector [7].
Before the introduction of the ‘flexibility bonus’ within the German renewable energy law in 2012 (EEG 2012), which remunerates the installation of additional capacity able to provide more flexible modes of operation, most existing bioenergy installations kept their plant capacity at the same level for the whole year. Installations built before 2012 have been designed and optimised to run constantly. With the newly introduced flexibility bonus, construction companies and installation owners started to experiment with different operation modes, using gas reservoirs, variable feeding of the fermenters and etc.
Besides these technical aspects, the question of who should profit from this flexibility has been discussed largely in Germany as most biogas plant owners simply sold their flexible production according to spot market prices. These aspects were discussed in [7]. Theoretically, biogas can be used flexibly as natural gas. Furthermore, from a technical point of view, wood is more flexible than coal regarding the partial loads behaviour. This discussion arouses from the underlying question of the short-, middle- and long-term roles of bioenergy in the energy market. Concerning the electricity system, two basic characteristics play an important role in this discussion: the general flexibility and the possibility of highly efficient provision of electricity and heat. Especially, biogas can offer this adjustable flexibility (unlike VRE such as wind or photovoltaic) because of the inherent storing function of biomass and its multifunctional usability. Therefore in the further discussion, biogas is highlighted.
In order to model and analyse the possible costs of the flexibilisation of biogas (depending on the degree of existing and newly transformed capacities), a proper biogas facility database has been established [7] which includes data of existing plants (2014) and a forecast of possible new installations until 2020.
Furthermore, the study distinguishes between the variants of ‘complete’ and ‘partial flexibilisation’: Partial flexibilisation as well can allow more flexible operation modes with less capital intensive solutions such as gas or heat storages.
Figure 3 shows the possible capacity gains (in GW) for 16 different cases: Four scenarios have been set up with different degrees of flexibilisation, and these four scenarios have been combined with four different modes of operation:
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Scenario 1: 20% of the existing stock and 50% of new installations exceeding 500 kW are made flexible
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Scenario 2: 20% of the existing stock and 75% of new installations exceeding 150 kW are made flexible
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Scenario 3: 50% of the existing stock and 100% of new installations exceeding 150 kW are made flexible
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Scenario 4: all existing and new biogas capacities are made flexible;
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Operating mode 8S/16E: the installation interrupts or stores its production during 8 h and sells during 16 h (e.g. following specific price patterns or for participation in the tertiary reserve market)
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Operating mode 16S/8E: the installation interrupts or stores its production during 16 h and sells during 8 h (e.g. following specific price patterns or for participation in the tertiary reserve market)
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Operating mode 12S/12E: the installation interrupts or stores its production during 12 h and sells during 12 h (e.g. base-load hours vs. peak hours or for participation in the secondary reserve market)
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Operating mode 10S/4E/6S/4E: the installation interrupts or stores its production during 10 h, sells during 4 h, interrupts or stores during 6 h and sells for another 4 h (selling during the daily price peaks in the morning and in the evening and interrupting from 8 p.m. to 6 a.m. and from 10 a.m. to 4 p.m., adapted to the ‘phelix sun peak future’).
There is a maximum capacity shift potential when combining scenario 4 with the operation mode 2 (16S/8P) with 16 GW of capacity shift, slightly followed by the operation mode 4 (10S/4P/6S/4P). From a technical point of view, biogas is thus able to deliver important quantities of capacity shift and therefore contribute to the different needs of the electricity system.
Consequently, the next step in the study has been to ask whose demands can be satisfied with these flexibility potentials as the flexibilisation of the biomass always should preferably be pursued according to the needs of the system transformation.
One priority flexibilisation aim has been identified when analysing the origin of negative prices in the spot market of the EPEXSpot. In order to maintain system security, a minimum power plant capacity must remain in operation in order to deliver instantaneously ancillary services (particularly the primary and secondary reserve). Today, they are delivered by conventional power plants and partly contribute to the formation of negative prices at the day-ahead market of the electricity exchange.
Usually conventional power plant operators market their whole production in advance in the long-term markets if at least they can achieve their marginal costs. Having sold their capacity, they carry out a monetary optimisation in the day-ahead auctions by replacing their own production with renewable energies sold ‘unlimited’ (which means at the lowest price limit). If the quantity of substitutable conventional production exceeds the production of renewable energies, positive prices occur in the power exchange day-ahead trade. In the opposite case, when the production of renewable energies cannot be substituted completely, negative prices occur. Conventional producers are, either due to the supply of balancing energy or due to reasons of microeconomic optimisation of a single power station, resp. their portfolios, willing to pay for electricity to avoid a still more expensive reduction or a complete switching off.
Consequently, current bioenergy power plants should be empowered to replace these conventional must-run capacities by offering and delivering all forms of balancing energy. Accordingly, the legislator should continue to remove tangible obstacles for the use of bioenergy as balancing energy (further shortening of offer periods, approximation of trading dates to the delivery date, further synchronisation of the trading dates of the bulk energy markets and of the balancing energy markets, etc.). In doing so, it should be achieved by appropriate regulations that the bioenergy plants behave less ‘spot market price fixed’.
Due to the spot market price-related shift of the production of electricity from biomass, actually, a substitution of fossil electricity is only partly achieved. As Fig. 4 shows, buffering biogas in low-price periods and selling it in high-price periods creates the necessity to produce more electricity from lignite and even less gas-fired electricity. In the end, the ecological effect is rather negative due to a higher share of coal.
Therefore, it seems adapted to pursue a spot market-based operation of bioenergy plants starting from the time where the production of variable renewable energies contributes to more than half of the electricity production. At this time, we can more frequently expect hours in which real surpluses of VRE occur. With VRE surpluses taking place, switching off bioenergy may prevent VRE from being thrown away and thus generate system-wide and environmental benefits.
Regarding the energy system transformation, it seems necessary to pay more attention to the separation of functions of real ‘peak load power plants’ (especially combined cycle power plants and gas turbines) and the biomass-based CHP plants, whose operation is more linked to the fluctuation and seasonality of the heat demand. Thus, the German legislator should implement measures to check and if necessary, revoke the exceptions concerning minimum percentage of heat recovery of biogas gas plants applied for the direct marketing of their electricity. For the future, it is not desirable that bioenergy plants whose economic calculation is too unilaterally based on incomes resulting from the electricity sector are built. In this context, it should be checked if the minimum proportion of combined heat and power production can be seasonally differentiated if bioenergy plants show a seasonally strong diverging operation. So during the heat period, the heat production level could be considerably higher, whereas in summer months, it could be reduced which would also serve the aims of developing solar thermal and waste heat use combined with thermal storages and heat grids fed by these devices.
It therefore seems that the principal role for biogas plants, besides the provision of highly efficient combined heat and power, should be to provide ancillary grid services as shown in Fig. 5 (i.e. frequency stability, voltage stability and reactive power compensation, delivery of grid losses, re-dispatch, congestion management resp. or black start capacity). Basically, bioenergy plants have the ability to provide these system services.
Developing and marketing these abilities of bioenergy plants seem actually quite important in order to replace the existing must-run capacities by conventional power plants.Footnote 3