Biomethane can be produced from biogas by upgrading it to the quality of natural gas. This way it is possible to obtain a renewable substitute for natural gas, which may be injected into the natural gas grid and used in existing technologies without the need for replacement. Grid-supply of biomethane offers several important advantages. It allows substituting fossil for a renewable energy source in the technologies which require gaseous fuels, e.g. natural gas-fired industrial furnaces, gas

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... nes and combined cycle gas turbine cogeneration plants. Biogas, after purification, can be diverted from distributed biogas-fired power plants, which often are relatively inefficient due to the lack of sufficient heat loads, to large district heating systems with efficient cogeneration possibilities. Biomethane supply via a natural gas grid would also facilitate maintenance and development of the natural gas supply infrastructure in future when consumption of natural gas may decline due to an increased share of renewable energy. However, several important questions arise. First, is biomethane a sound choice from the environmental point of view in comparison with natural gas and potential renewable alternatives, e.g. biofuels, which also can be often used in natural gas-fired systems? Second, is biomethane an economically feasible alternative to natural gas? If not, what is the level of support required for production of biomethane to be put in price parity with natural gas? And finally, how should a support policy for a biomethane supply chain be designed to make it sustainable over time? These questions have been addressed in the research performed at the Institute of Energy Systems and Environment of Riga Technical University and the results have been published in several peer-reviewed papers. In principle, the design of a biomethane supply system involves the system innovation from the ecodesign perspective since it allows improving environmental performance of the whole system currently using natural gas. Therefore, by following the ecodesign approach one should employ tools of environmental impact assessment, e.g. life cycle assessment (LCA), for the comparison of design alternatives from the environmental standing. This has been done by applying LCA to the case of brick production industry in Latvia and analysing "cradle-to-gate" environmental impacts per 1 ton of the product when the following fuels are used in the industrial furnace: natural gas, biomethane, and 1 st and 2 nd generation liquid biofuels. "ReCiPe" has been used as the environmental impact assessment method. The results show that the environmental impact could be reduced by circa 50% when biomethane is substituted for natural gas, and that biomethane and the 2 nd generation biofuel have roughly equal environmental impact as the 1 st generation biofuel, being far worse than even natural gas. Other studies based on life cycle inventory data have indicated that the use of biomethane instead of natural gas may reduce greenhouse gas emissions by nearly 80%. The environmental benefits of fuel substitution, i.e. system innovation versus gradual improvements of natural gas-fired equipment efficiencies, can thus be clearly seen. Development of a biomethane supply chain can be started by installation of upgrading facilities to the existing biogas plants and their connection to the natural gas grid. The study of the location of the biogas plants relative to the natural gas transmission pipelines has been done for Latvia using GIS. The results show that the average distance from the biogas plants to the natural gas transmission pipeline was circa 18 km with the shortest distances being less than 1 km. In many places, the connection could be made to the natural gas distribution pipeline instead, thus, reducing the connection distance. Therefore, it can be claimed that in many places the connection of the biogas reactors to the grid may be feasible. The total costs of grid-injected biomethane produced would have to include capital, operation and maintenance costs of biogas production, upgrading and injection into the natural gas Bazbauers 4 grid. The total costs have been calculated for 5 different biogas upgrading technologies and it has been found that the difference between the total biomethane production costs among these technologies was relatively small, i.e. approximately 3%. The total costs can be reduced significantly, i.e. by circa 30%, if biogas producers cooperate and build a single larger biogas production and upgrading facility instead of several of a smaller size. This indicates that the spatial distribution of biogas reactors and possibilities for cooperation among biogas producers to a large extent will determine competitiveness of grid-injected biomethane with natural gas. Currently, financial support is required for production of grid-injected biomethane since the minimum total costs of production achieved at the best scenario with 20 years of economic lifetime are nearly 50 EUR/MWh, exceeding the price of natural gas by approximately 16%. For economic lifetime of 10 years, the difference is about 34%. Therefore, we come to the need for the design of an appropriate support policy for the development of a biomethane supply system. System dynamics modelling is a well-suited tool for the design of a renewable energy support policy since it allows analysing complex dynamic systems with feedbacks, delays and non-linearity. System dynamics has been successfully used at the Institute of Energy Systems and Environment for modelling of power and district heating supply systems, design of energy efficiency and waste management policies, analysis of the transport sector and sustainable energy supply systems. The challenge for energy policy makers lies in setting up the support system which leads to the perceived optimal growth of the renewable energy share and, for that reason, can be sustained over a long period of time. As experience shows, this is not a case in many countries, including Latvia, where support has led to the overshoot of the perceived limit of the financial support which can be carried by national economy, and it was corrected by a complete or nearly complete stall in the support. It is often noted that due to complexity of the energy supply system policy decisions for support of sustainability are difficult, i.e. it is difficult to foresee the dynamics of evolvement of the required support. Therefore, system dynamics has been chosen for creating the model which reveals the most crucial part in the support policy implementation, i.e. the structure of the policy system which leads to successful implementation of the support. Very few studies using system dynamics for the analysis of renewable energy policies have a focus on the structure of the support policy itself but rather look at the consequences in terms of the renewable energy share if the support is implemented or absent. The structure of the support policy for biomethane is built up by 2 parallel flows, i.e. the flow of granted permits to receive support, and the flow of actual investments into biomethane production facilities. Actual biomethane production is compared with the target value, which is based on the perceived limit of financial support at a certain period of time, and the feedback is taken back for adjusting dynamics of permit granting. This structure is similar to biological systems where a marginal increase of distribution of a certain plant in a limited territory decreases over time as space available for new plants shrinks. It is considered in the analysis of dynamics that not all permits will lead to actual investments and some will be cancelled after a certain period of time. This modelling approach allows comparison of several support alternatives, i.e. investment subsidies versus feed-in-tariff or feed-inpremium. Development of specific investments into technologies due to the learning effect and dynamics of a natural gas price are considered as well. Combining life cycle assessment, GIS, technical and economic calculations with system dynamics modelling can be viewed as a method for system innovation leading from sound evaluation of alternatives to the actual policy for implementation of a particular system. This approach can be applied in the design of sustainable energy systems, waste management systems and other similar applications. Microbial fuel cells (MFCs) are an emerging type of biological wastewater treatment units with simultaneous power generation. The present study demonstrates an effective treatment of greywater and generation of electricity in a double-chambered MFC. This MFC was fabricated using costeffective and easily available materials replacing expensive materials like Nafion membranes, graphite electrodes, etc. Experimental results showed a maximum open circuit voltage of 0.64 ± 0.04 V and 114 ± 1.41 mA current during the study period. The results further indicate a maximum power generation of 24.50 mW along with 307.69 mW/m² of power density; 34.62 mA/m² of current density, 1.33 W/m³ of volumetric power density, 0.15 A/m³ of volumetric current density and a power yield of 0.40 mW/kg of COD removal. The chemical oxygen demand (COD) removal efficiency was 77.6%. The use of low-cost and easily available raw materials has brought down the total manufacturing cost of MFCs used in this study to less than USD 4.0. However, the performance of the MFCs used in the current study is comparable with other sophisticated MFCs built with expensive raw materials, as reported in the literature. This costeffective MFC used in the present study might be an effective replacement of expensive MFCs for wastewater treatment at scaled-up levels.