Assessment of the Potential for Balancing Wind Power Supply with ...

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ScienceDirect Energy Procedia 72 (2015) 250 – 255

International Scientific Conference “Environmental and Climate Technologies – CONECT 2014”

Assessment of the potential for balancing wind power supply with biogas plants in Latvia Ilze Dzene*, Francesco Romagnoli Riga Technical University, Institute of Energy Systems and Environment, Azenes iela 12/1, Riga, LV 1048, Latvia

Abstract The aim of this study is to assess the potential of biogas plants in Latvia to balance power supply from wind power plants. Several balancing options are discussed in the paper, including demand driven biogas production with biogas storage, flexible biogas production, and the use of biogas in Power-to-Gas systems. Theoretical surplus wind energy accumulation potential using operational biogas plants in Latvia has been calculated. Results show that 83.75 MW of surplus wind power capacity in Latvia could be theoretically balanced by using dynamic biogas upgrading in biogas CHP plants in Latvia. That would allow balancing 3.6 % of surplus wind electricity in a 100 % wind energy scenario. © by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2015 2015The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment Keywords: biogas; wind; balance power; Power-to-Gas; electricity storage

1. Introduction Biogas is a well-known, sustainable and versatile energy carrier. In order to contribute to the European energy and climate targets, member states of the European Union have committed to increase significantly the share of renewable energy sources (RES) in the final energy consumption. In Latvia the target is to increase the share of RES from 34.9 % in 2005 up to 40 % in 2020. Within these ambitious targets, biogas plays an important role because it is a versatile fuel and can be used in various ways – for heat and electricity generation, and for application in the

* Corresponding author. Tel.: +371 29629982 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 Riga Technical University, Institute of Energy Systems and Environment doi:10.1016/j.egypro.2015.06.036

Ilze Dzene and Francesco Romagnoli / Energy Procedia 72 (2015) 250 – 255

transport sector [1, 2]. In this paper another aspect of biogas versatility is highlighted. Contribution of biogas for balancing fluctuating sources of power generation is discussed. Currently in Europe biogas is mostly used for combined heat and power (CHP) generation and covers the base load of the energy demand [2]. To a lesser extent biogas is upgraded to biomethane, injected into the natural gas grids or compressed and used as transport fuel. During the last years another potential use of biogas is gaining much attention. The role of biogas for balancing fluctuating power generation sources has been investigated by various research groups. Two research trends are further described: (i) demand-driven biogas production and (ii) dynamic biogas upgrading for Power-to-Gas (PtG) systems. 1.1. Demand-driven biogas production Demand-driven biogas production means generating electricity from biogas flexible on demand. A demanddriven biogas production is essential for balancing power generation [3] – both in the short and long term. Demanddriven biogas production can generally be achieved through biogas storage or flexible biogas production concepts. Hahn H., Krautkremer B. et al. [3] have reviewed several concepts for a demand-driven biogas supply. The summary of the concepts is given in Figure 1.

Fig.1. Overview of the concepts for demand-driven biogas supply.

Within these concepts it is required that biogas availability should be guaranteed at all times when it is necessary to generate balancing power and to cover peak loads. On the other hand, during energy surpluses it is necessary to reduce biogas production or store it on-site of the plant (short-term storage) or in the form of biomethane in the natural gas grid (long-term storage). Concepts of flexible biogas production focus on influencing the anaerobic digestion process to adapt biogas production to biogas demand [3]. This is done by changing the substrate composition and feeding rate or by changing substantially the configuration of a biogas plant, e.g., by separation of hydrolysis, acidogenesis and methanisation steps of the anaerobic digestion process. From the cost analysis perspective, biogas storing concepts are favourable for supplying biogas for flexible power generation 8 hours per day, while flexible biogas production concepts are suitable for longer periods without biogas demand – up to 72 hours [4]. Optimizing biogas plants with storage capacity and additional CHP units has also been analysed by Hochloff and Braun [5] and Østergaard [6]. 1.2. Biogas in Power-to-Gas systems Besides flexible power generation energy storage systems is another option for balancing weather dependent renewable electricity production. Jentsch et al. [7] compared Power-to-Gas (PtG), Power-to-Heat (PtH) and shortterm storage systems regarding their flexibility for balancing electricity surpluses in an 85 % renewable energy scenario for Germany. Authors of the above mentioned study concluded that the PtG option has the highest flexibility and integrates most of the electricity surpluses. However, from the costs perspective, the availability and

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cost of CO2 for the methanation process in PtG systems is crucial. Biogas that normally contains up to 50 % of CO2 is potentially a good source for methanation in PtG systems. Jürgensen et al. have discussed ‘dynamic biogas upgrading’ that refers to a process which can be switched between two process states: (i) during hours of electricity demand the biogas is used in an internal combustion engine for CHP generation and (ii) as soon as there is no demand for electricity production, the process is switched to the upgrading route to produce upgraded biogas [8]. Dynamic biogas upgrading differs from conventional static biogas upgrading in two possible process states: (i) operation of a biogas plant in a CHP mode when there is a demand for electricity and (ii) operation in PtG mode in periods of excess wind energy. In the latter case, surplus electricity is used for production of hydrogen. Hydrogen is further used for conversion of the biogas CO2 content to synthetic natural gas (SNG) [8] – so called methanation. Methanation of biogas can be done in two ways: (i) methanation of biogas without CO2 separation and (ii) only using CO2 that is separated during the biogas upgrading for the methanation [9]. Two types of methanation are possible – chemical methanation using catalysts in the Sabatier process and biological methanation through the methanogenesis of anaerobic digestion process. M. Reuter has summarized pros and cons of chemical and biological methanation [10]: Chemical methanation is relatively easy to control and easy to scale to the size of the system. On the other hand it requires rare and expensive raw materials as catalysts, catalyser is sensitive to the incoming gas quality – therefore the gases has to have a high grade of purity, the process requires high temperature (>250 °C), high pressure and the system itself has low flexibility. Chemical methanation is mostly applicable for large-scale industrial processes. The biological methanation process does not require high temperatures, when used in biogas plants – have low pressures, the purity of gases is not essential, the system is flexible and can be used with small scale digesters in remote locations. The challenge of the biological methanation is the fact that the process is microbiological and thus difficult to control. Biological methanation is more appropriate for biogas plants. In chemical methanation much research is concentrated to types of catalysts and their materials [11, 12]. In biological methanation the research is focused on in-situ and ex-situ biogas upgrading methods and challenges related to the control of microbiological process (pH, gas-liquid mass transfer rate, microbial community characterisation) [13, 14]. 2. Wind power potential and biogas in Latvia The installed wind power capacity in 2013 in Latvia was 61.8 MW. Zoss et al. [15] have analyzed possibilities to integrate several wind electricity accumulation alternatives in the Baltic States. Multi-criteria analysis was used to compare the application of four alternative technologies: pumped hydro storage, compressed air storage, SNG and hydrogen based energy storage systems for balancing wind electricity in Latvia. Authors of the mentioned study [15] calculated that, in order to cover the total electricity deficiency in a 100 % wind energy scenario in Latvia, a wind power capacity of around 2.3 GW is needed. This would result in a surplus electricity storage demand equal to 889 GWh/year. The calculation in the study was based on actual electricity generation in Latvia in 2013. By the end of 2013 there were 52 operational biogas plants in Latvia, all of which use biogas in CHP generation. The total installed electrical capacity of biogas CHP plants in Latvia in 2013 were 54.42 MW. The average electrical capacity of a biogas plant in Latvia is around 1 MW [1]. In 2013 biogas plants in Latvia generated 275.2 GWh of electricity. 3. Methodology of research The methodology of research is based on calculations performed by Jürgensen et al. [8]. This methodology is applicable for biogas plants with CHP generation. Equations for calculating surplus wind electricity storage potential are based on the stochiometric relationships of the Sabatier process. According to [8], the expressions which could be used to obtain the energy consumed by the electrolysis process and the overall energy storage potential of the dynamic upgrading system based on the electrical power output of existing CHP plants were based on the following assumptions:

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x x x x

Electricity consumption in the conversion and storage processes (except electrolysis) is negligible; Produced biogas consists mostly of CH4 and CO2. Other gases are assumed negligible; A 100 % conversion of CO2 to CH4 during the Sabatier process is assumed; All biogas CHP plants in Latvia in 2013 were considered as potential dynamic upgrading plants.

A relationship between available excess electricity supply to the biogas outputs and rated power production from biogas plants is described [8] using Equation 1: Psur =

B F CO PCHP B F CH K CHP K Sab

(1)

2

4

where: Psur – surplus wind derived power, MW ȤCO2B – CO2 content of biogas, % ȤCH4B – CH4 content of biogas, % PCHP – average power output of biogas CHP plants, MW ȘCHP – efficiency of the CHP conversion ȘSab – efficiency of biogas upgrading 4. Results Biogas production from 52 operating biogas plants in Latvia was determined based on the actual electricity production reported to the Ministry of Economics of Latvia in 2013. The annual capacity factor (energy output divided by the theoretical maximum output) for biogas plants in Latvia in 2013 was only 56 %. Therefore, for further calculations, the average power output of 30 MW is used instead of 54 MW installed electrical capacity mentioned before. Following methodology of [8] with an assumption of the biogas output CH4 content of 60 %, CO2 content of 40 % and degrees of efficiency for the CHP conversion and biogas upgrading units of 0.4 and 0.6 respectively, using Eq.1 the surplus wind derived power which could be potentially used by dynamic biogas upgrading is 83.75 MW. A comparison of the calculation results to biogas and wind capacities in 2013 is given in Figure 2. 84

Calculated theoretical balancing capacity of biogas plants

2300

Expected wind capacity in 100% wind scenario Adjusted biogas power output in Latvia, 2013

30

Installed biogas electrical capacity in Latvia, 2013

54

Installed wind capacity in Latvia, 2013

62 0

500

1000

1500

2000

MW

Fig. 2. Biogas plant balancing capacity compared to installed and expected biogas and wind capacities in Latvia.

Compared to capacities in Latvia, it is more than installed wind power capacity in 2013. However, if assuming the 100 % wind scenario discussed in [15], then with biogas plants in dynamic upgrading mode it would be possible to balance only 3.6 % of wind power in Latvia.

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5. Discussions Within this study the maximum theoretical potential for accumulation of surplus wind energy with currently operating biogas plants in Latvia was calculated. Future potential estimations should be based on technical and economical considerations. Other than technical limitations, the spatial distribution of biogas plants and wind power plants should be considered, as well as the distance to the natural gas grids. Spatial analysis and cost analysis were not a part of this study. In further research the geographical location and installed capacity of each single biogas plant should be considered as the main factors for the assessment of economic feasibility of dynamic biogas upgrading. As mentioned in [8], one of the biggest barriers for this concept is the high investment cost, especially for the electrolyser, compared to the low full-load hours of surplus electricity production. To decrease the investment and operational costs, biological methanation could be considered as an option for further research, especially when related to biogas plants. More aspects for further research include the evaluation of requirements from lower network levels, the capacity and proximity of the natural gas network, as well as local advantages to create synergies for using surplus heat of the PtG process [7]. Furthermore, the results of this study can be used to calculate indicators describing the performance of the energy system. For example, electricity production that either has to be exported or needs to be avoided is an indicator of an inability of the system to utilise the production – and thus an indicator of inflexibility of the system. On the other hand, if the system needs to import electricity or produce electricity in condensing mode power plant (as assessed in [15]), this is another indication of the inflexibility of the system [6]. 6. Conclusions Biogas is a versatile source of energy that contributes to renewable heat, electricity and transport applications. However, during the last years another aspect of biogas has been on the agenda of several scientific groups – namely the role of biogas in balancing fluctuating sources of power generation. In this study the potential of biogas plants in Latvia to balance power supply from wind power plants has been assessed. Calculation results show that 83.75 MW of surplus wind power capacity in Latvia could be theoretically balanced by using dynamic biogas upgrading in the biogas CHP plants in Latvia. In a 100 % wind energy scenario for Latvia that would mean providing balancing for 3.6 % of surplus wind electricity.

Acknowledgment This work has been supported by the European Social Fund project “Involvement of Human Resources for Development of Integrated Renewable Energy Resources Energy Production System” (project No. 2013/0014/1DP/1.1.1.2.0/13/APIA/ VIAA/026).

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