arrivee - Wastewater treatment plants as control component in intelligent distribution networks with renewable energy generation

Background

The increasing need for a compensation of severely fluctuating power generation is a result of the ongoing extension of wind and solar power plants. These renewable energy sources depend on their naturel availability, which can cause energy network problems by fluctuating energy supply. This is caused by the abandonment of nuclear power and the reduction of fossil-based energy production, which covered a large part of the basic load and the commitment to raise the part of renewable energy to 80% by 2050. As a consequence the impact of renewable energy sources will increase. In case of an energy surplus their input cannot be fed into the energy grid and the generators are shut down. In future, that gap between supply and consumption has to be closed by an additional amount of control reserve as well as storage capacities which could be supported by the local water management. The superior function of WWTP is the treatment of (waste-) water which must not be affected in a negative way. Nonetheless the energy potentials of WWTPs have to be taken into account as far as technically and economically feasible.

The core objective of the joint research project “arrivee” is the integration of widely available wastewater treatment plants (WWTP) with anaerobic sludge digestion into an optimized control reserve and storage concept. Therefore the excellent technical conditions of municipal WWTPs, such as combined heat and power (CHP) units as well as gas storage units, will be used and enhanced by using innovative technologies. System services are provided with new developed solutions which are necessary today and in the future to compensate fluctuating energy production (Figure 1). Therefore external influences on the WWTP are analysed and assessed.

Currently WWTP use the produced digestion gas in most instances for self-sufficiency purposes to reduce the use of external energy sources. In the future WWTP could act as a more active participant in the energy grids. This could be realised by a more energy grid related energy production and smart intermediate storage of the gas. Furthermore WWTPs provide the opportunity to transform energy into a chemical long-term storage matter (methane - CH₄) which can be fed into the natural gas grid and use that stored energy to provide system services in a Power-to-Gas-to-Power concept with multiple synergy effects on (waste-)water treatment and the energy sector.

Intermediate results

Within the arrivee project a survey of the national anaerobic digestion plants has been performed to determine the potential to provide flexibility via electricity production from existing CHP units throughout Germany. The basis to that potential are the consideration of so far unused gas, efficiency improvements on the plants, conversion of WWTPs from aerobic into anaerobic sludge digestion (if economic feasibility is given) and the use of spare capacities of existing digestion tanks (operation as “sludge assembly centres”). Results show that the actual electricity production of 1.25 TWh/a could be augmented up to 2.11 to 2.61 TWh/a. This corresponds to nearly 300 MW_el which is equivalent to around 15% of the current demand of negative control energy.

To make full use of this potential of WWTP besides the CHP-units, additional flexibility options are identified (Figure 2). Every WWTP differs in operation, size, polluting load and other local boundary conditions. In this regard, different plant concepts are developed to ensure a wide application area. This includes innovative plant concepts using energy surplus in the energy grid to produce hydrogen (H₂) and oxygen via an electrolyser. H₂ and the digestion gas are used to produce high quality methane (Sabatier-reaction) which can be fed into the natural gas grid. With that plant configuration WWTP work as a Power-to-Gas plant and provide long-term energy storage with the additional benefit of producing electric energy with their CHP-units if needed.

The utilization of WWTP as a storage and control unit in energy grids has to be taken into account in conjunction with the regular operation of the WWTP. Negative influences on the purification processes have to be excluded. To ensure this objective a mathematical model of the pilot WWTP has been created, tested and verified with field tests on the pilot plant. Load-shedding was tested for the aerator and the return sludge pumps. So far, no negative impacts have been observed on outlet values for different stress periods and switch-off durations. The possibility to switch on/off plant components to provide flexibility is realised with a developed tiered control concept. To offer that obtained flexibility on the energy markets WWTPs have to be “pooled” in a virtual power plant because a single plant can’t provide enough power capacity to act on the market individually. These plant pools consist of many small power plants to match the restrictions given by the transmission system operators. To test the impacts of those external interventions measuring and control devices are installed at the pilot WWTP. The flexibility of WWTP can be used for different purposes. Under this aspect today and future markets are determined and rated.

Furthermore an in-depth analysis of significant energy-law relations between the different actors (WWTP, transmission system operators, network operator, and electricity suppliers) was performed. In addition, the current legal and political framework was examined and with regards to the project objectives assessed. With these findings there could no fundamental obstacles or restrictions be identified on providing flexibility with WWTPs. On the contrary, multiple (political) declarations of intent support the objectives of arrivee.

Outlook

Results show that WWTPs in Germany already now could provide 143 MW_el and are capable to yield up to 300 MW_el by optimising the existing infrastructure. Investigations on load-shedding and switch on onside plant components are continued to provide maximum flexibility for the required services for the energy grid. Along with implementing electrolyses and smart plant management energy surpluses and deficits could be used to enhance energy efficiency and/or store energy instead of losing it by shutting down renewable power plants due to overproduction. Field tests and simulations for the pilot WWTP show that load-shedding doesn’t affect the treatment processes significantly under controlled conditions and can be used in a smart control concept. The implementation of PtG-concepts will increase the impact of WWTPs as municipal participant for system services significantly and will show that those plants are capable to operate not only as a consumer, but as a producer of energy on a stable operation of energy grids. Hence WWTPs will become a valuable source of flexible energy production which is needed in the future German energy sector. Furthermore an expert workshop for practitioner, researcher and decision maker will take place on the second quarter of 2016 to develop potential energy market based scenarios. This input will match the requirements from the practical side with the project status and will give an important feedback of the current project status and future development.