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ScienceDirect Energy Procedia 73 (2015) 87 – 93

9th International Renewable Energy Storage Conference, IRES 2015

Wastewater treatment plants as system service provider for renewable energy storage and control energy in virtual power plants – A potential analysis Michael Schäfera*, Oliver Gretzschela, Theo G. Schmitta,Henning Knerrb b

a Institute of Urban Water Management, University of Kaiserslautern, Paul-Ehrlich-Str. 14, D-67663 Kaiserslautern, Germany Tectraa – Institute for Innovative Waste Water Technologies, University of Kaiserslautern, Paul-Ehrlich-Str. 14, D-67663 Kaiserslautern, Germany

Abstract In future, an additional potential of control reserve as well as storage capacities will be necessary to compensate fluctuating energy generators such as wind and solar power plants. 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 those plants such as combined heat and power (CHP) units as well as gas storage units will be used. By means of analyzing energy consumption and production processes of existing WWTPs the potential to provide ancillary services for energy grids is investigated. Using a mathematical model of an existing WWTPs the effects of external interventions for the supply of ancillary services under different conditions are tested. Therefore, different processes will be analyzed in detail. The integration of WWTPs into a virtual power plant (VPP) is mandatory because a single plant can’t provide enough capacity to participate on energy markets. The initial results present a theoretical potential of control reserve of WWTPs in Germany focusing on a CHP/gas unit of a magnitude of 300 MW el. This results show a substantial potential of WWTPs to provide ancillary services, by reshaping the existing infrastructure in a sustainable, ecological and economic way. This may contribute significantly to a stable operation of energy grids and to further integration of renewable energy sources in the frame of energy system transition. ©2015 2015The TheAuthors. Authors. Published by Elsevier © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy Keywords:waste water treatment plants; renewable energy; energy storage;power-to-gas;

* Corresponding author. Tel.: +49-631-205-4643; fax:+49-631-205-3905. E-mail address: [email protected]

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy doi:10.1016/j.egypro.2015.07.566

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Michael Schäfer et al. / Energy Procedia 73 (2015) 87 – 93

1. Introduction The increasing need for a compensation of severely fluctuating power generation is a result of the ongoing extension of wind and solar power plants. This is promoted by the new energy policy enacted by the German federal government. In this regard the remaining nuclear power plants together with capacities from fossil fuel leaded power generation, that previously covered a large part of the basic load, will diminish. As a consequence the impact of renewable energy sources will increase. These energy sources depend on their naturel availability, which can cause network problems by fluctuating energy supply. 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 has to be closed by an additional amount of control reserve as well as storage capacities. The core objective of the joint research project “arrivee”, founded by the German Federal Ministry of Education and Research, is the integration of widely available wastewater treatment plants (WWTPs) with anaerobic sludge digestion into an optimized control reserve and storage concept. Therefore the excellent technical conditions of those plants, such as combined heat and power plants (CHP) as well as gas storage units will be used to take part as a storage facility to store or generate electrical energy if needed. This paper focuses on first results of the potential analysis within the scope of “arrivee” but also describes the project approach.

2. Approach of arrivee By means of analyzing the energy consumption and production processes of existing WWTPs the potential to provide ancillary services for energy grids is investigated. Using a mathematical model of an existing pilot WWTP, the effects of external interventions for the supply of ancillary services under different conditions are tested. Therefore, different processes inside the WWTP will be analyzed in detail. These processes are complemented by new and innovative plant elements in order to optimize participation on providing control reserve (Figure 1). In case of energy surplus that energy is used in a first step to split water into oxygen (O 2) and hydrogen (H2) using an electrolyzer. Oxygen can be used manifold, such as for processes inside the treatment plant (e.g. aeration tanks) or advanced applications (e.g. ozone generation for ozonation). Hydrogen is used for methane (CH 4) conditioning, which can be realized chemically through Sabatier process [1] or biologically inside the digestion tank [2]. Furthermore it can be stored into the natural gas grid at a ratio up to 2 – 5 vol% depending on the local gasgrid[1].

Figure 1: Modules of WWTPs to provide control reserve.


Besides hydrogen, carbon dioxide (CO2) is required for the methanation process. CO2 can be obtained from internal sources, such as emissions from aeration tanks or the CHP system. The most easily accessible source of CO 2 is the raw biogas produced with the anaerobic sludge digestion process, which consists of around 65 % CH4 and 35 % CO2. The purity of the raw biogas from the digestion tanks increases from about 65 % to more than 95 % CH4. Based on this purity, the produced synthetic natural gas (SNG) is nearly 100 % compatible with the existing regular natural gas grid [2] and the grid can be used as a nearly infinite gas storage (Figure 2).

Figure 2: Possible use of power-to-gas technology on WWTPs incl. methane conditioning (adopted from [4]).

Furthermore the increased CH4 proportion leads to a higher methane energy content which increases electricity generation. In case of energy surplus, CHP units are shut down and the generated gas is stored. In addition, energy consuming sequences inside the WWTP, such as sludge treatment are started as well. If the condition inside the power grid changes and there is an electricity shortage, non-vital plant components are shut down and the CHP-unit(s) are powered up. To improve the impact of this positive control energy redundant CHP units and existing emergency generators could be used in future. In this case the natural gas grid is not used as a storage option, but rather as a source to provide gas for energy production [3] (Figure 3). To make those services available, WWTPs will be integrated into a virtual power plant (VPP) to counterbalance fluctuations (Figure 4).

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Figure 3: Counterbalance of fluctuating energy (adopted [5]).

3. First results and discussion The initial results of the “arrivee” project present a high theoretical potential of negative control reserve from WWTPs in Germany. The data basis for this study includes data of more than 95 % of German WWTPs, regarding the following aspects: design capacity, specific purification method and performance as well as used sludge treatment processes. For several federal states additional data were available such as gas production, gas usage, and produced electricity. Figure 4 shows the development of gas usage for electricity generation on sewage plants in Germany from 1990 to 2012. Additionally, the different potentials of optimization fields for power generation are added. These methods take into account: x Consideration of unused gas: Only about 80 % of the German WWTPs using anaerobic sludge digestion utilize the produced CH4 for energy generation. This potential focuses on installing a CHP unit on every existing WWTP with anaerobic sludge digestion. x Efficiency improvements: Additional energy production is generated by using more efficient CHP units (e.g. repowering), optimized sludge treatment and sludge digestion techniques and other technical improvements. x Conversion of WWTPs from aerobic into anaerobic sludge digestion: The background of this optimization method is the increasing economic efficiency of anaerobic sludge digestion compared to aerobic sludge digestion of plants with more than 10.000 population equivalents (PE) [6]. Therefore a transformation of those plants is presumed and their additional gas production is added.


x Use of spare capacities of the digestion tanks: In existing digestion tanks, there is a high unused capacity for additional digestion. For this scenario the free capacities are used by additional fermentation substances. These additional materials can be obtained by collection sludge from smaller WWTPs and building up a “sludge assembly center” or using organic matter in form of co-substrates from other sources (e.g. bio waste). The procurement of such materials is highly controversial and competitively with other sectors, such as biogas plants. Therefore this high potential is excluded by the estimated magnitude of providing control energy.

Figure 4: Development of gas usage on WWTP in Germany and calculated potentials for optimized power generation.

Since 1990 the energy production increased by the factor of 1.5 from 0.8 to 1.25 TWh el/a. In history an optimization of power generation was immaterial for WWTPs but new framework conditions promoted by the new energy policy set a change in motion. In the last years, energy optimization became focus of attention and results show that there is a significant potential which is unused so far. Utilizing this potentials power generation could rise up to 2.11 to 2.61 TWhel/a. This corresponds by a factor 2 compared to the actual use. Under these boundary conditions calculations show that WWTPs could provide negative control reserve of a magnitude of 241 - 298 MWel (Table 1). To provide positive control reserve, an even more in-depth and individual analysis is necessary to make those capacities available. Especially load shedding by shutting down plant components is a sensitive issue, both in acceptance by plant operators and technical implementation.

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Michael Schäfer et al. / Energy Procedia 73 (2015) 87 – 93 Table 1: Theoretical potential of providing energy on WWTP with anaerobic sludge digestion.

gas production1) [million m³/a]

energy production2) [TWhel/a]

electrical power3) [MWel]

644.54

1.25

142.7

use of unused gas

86.74

0.20 – 0.25

22.4 – 28.0

efficiency improvements

100.98

0.21 – 0.26

24.0 – 30.0

215.64 – 323.45

0.36 – 0.67

41.0 – 76.8

46.75 – 70.12

0,10 – 0,18

11.1 – 20.8

(266.67 – 488.90)

(0.56 – 1.71)

(63.3 – 145.1)

406.98 – 516.61

2.11 – 2.61

241.1 – 298.2

current condition* optimization by:

conversion of WWTPs to anaerobic sludge digestion use of spare capacity

sludge assembly centres for small WWTPs (use of external fermentation substances)

total potential *[7] 1) specific gas per inhabitant 20 - 30 l/(E*d) 2) energy level methane: 6.5 kWh/m³ 3) electric level of efficiency of the CHP unit: 0.32 – 0.4 [8]

WWTPs can vary greatly between one another and each one needs to be checked in detail for load shedding. It is considered a switch-off for specific plant components, such as sewage/sludge pumping systems, aeration of the grit chamber, aeration tank and sludge treatment processes to reduce electricity demand. To calculate a positive control reserve, it is necessary to figure out a maximum downtime for those components, which don’t affect the wastewater and sludge treatment process in any negative way. Besides powering up existing CHP units, it is possible to provide extra power generation by using free capacities (modulated operation) and redundant CHP units. Also emergency power units could be used. Further investigations and analyses of load shedding are currently in process.

4. Summery and outlook Initial results of the joint research project “arrivee” show a substantial potential of WWTPs to provide ancillary services, by reshaping the existing infrastructure in a sustainable, ecological and economic way. Increasing the energy generation of WWTPs by a factor of nearly 2 demonstrate the high unused potential. The calculated potential of negative control reserve of a magnitude of 300 MW el may contribute significantly to a stable operation of energy grids and a further integration of renewable energy sources in the frame of energy system transition.


Further investigations to show that WWTPs can provide control reserve in a useful way are in progress. Therefore different components of WWTPs will be analyzed and simulated in detail on a pilot plant, such as gas storage capacities, optimized design of the CHP units and shut down conditions for the power consuming processes on WWTPs. The simulation of load-shedding and integration of new plant components is accompanied by the investigation of the policy and legal framework. Next steps focus on the dimensioning of an electrolyzer in the needed scale and to use available resources and infrastructures to utilize H 2 to produce and use additional CH4. All these aspects depend on the energy market situation as well as on the situation in the transmission grid. Both will be examined and considered within the project. The interaction of the grid situation and the resulting need of control reserve on one hand and the options to provide control reserve on the WTTP on the other hand is a central aspect of the project. Acknowledgments The NaWaM/ERWAS joint research project “arrivee” is funded by the German Federal Ministry of Education and Research and is a research consortium of eight cooperation partners. The authors would like to thank the BMBF for the financial support. (www.erwas- arrivee.de)

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