Giorgos Kardaras

The transition to climate-neutral, resilient, and decentralized energy systems is particularly challenging for regions with strong seasonal demand, high renewable potential, and limited grid flexibility. Islands such as Crete exemplify these challenges. This work provides an integrated framework for evaluating both system performance and replication potential of four Modular Renewable Energy Systems (MRES) for Renewable Energy Valleys in Crete under common Renewable Energy Penetration (REP) targets of 90%, 95%, and 99.9%. The framework incorporates output-based levelized cost indicators (LCOE), Net Levelized Electricity Cost (NLEC), Effective Community Electricity Cost (ECEC), capital expenditure (CAPEX), profitability index (PI), payback time (POT), and life-cycle environmental assessment (LCA) relative to the corresponding business-as-usual (BAU) systems. Output-based LCOE ranged from 0.059 to 0.112 €/kWh, NLEC from 0.017 to 0.074 €/kWh, and ECEC from 0.082 to 1.090 €/kWh, highlighting the influence of surplus-electricity revenues on net system economics. The attributional LCA showed substantial global-warming reductions for most configurations, reaching approximately 86% for MRES 3, although trade-offs emerged in mineral resource scarcity and water consumption. Results highlight that replication potential depends on matching each archetype to local resources, demand profiles, infrastructure constraints, and end-use priorities, while 90–95% REP provides a more balanced compromise than 99.9% REP.

Unlike the image of poverty and a society in disorder promoted by the Byzantine sources for the early Slavs, the scrutiny of the written sources and the archaeological material testify that since the second half of the 6th century, there are the first indications for the development of a social hierarchy. Focusing on the Antes, a series of testimonies and finds analyzed in the chapter, along with some approaches on the political and social organization of the early Slavic society, show clearly the steps of the elite formation and leads to the conclusion that the Antes achieved earlier than their contemporaneous Sclavenians the level of a “developed chiefdom” with a rudimentary central power.

The shipping sector is responsible for a considerable share of global CO₂ emissions and is under pressure to reduce emissions and adopt carbon-neutral fuels. Among the proposed alternatives, methanol produced from green hydrogen and biogenic CO₂ represents a promising option. However, the feasibility of its production is significantly influenced by the composition and variability of the bio-CO₂ feedstock, which can negatively impact the complete value chain. To address these challenges, sampling campaigns were carried out at actual bio-CO₂-emitting sites, namely biogas and biomass combustion facilities, to characterize the impurity profiles and determine the appropriate conditioning requirements. A novel membrane gas absorption system with a Diethanolamine solution was deployed directly in the field to capture, as well as purify to a certain extent, the CO₂ stream. The system demonstrated high efficiency in removing most impurities, achieving high CO₂ capture rates and impurity reduction close to 90%. However, residual chlorine species were detected in the CO₂ streams from biogas plants, suggesting the need for additional conditioning to meet the purity specifications required for methanol synthesis. Given that the feedstock composition and upstream process conditions could significantly affect the final output and present considerable variations, the implementation of additional cleaning measures is recommended before synthesis.

While community energy initiatives and pilot projects have demonstrated technical feasibility and economic benefits, their site-specific nature limits transferability to systematic, scalable investment models. This study addresses this gap by proposing a modular framework for Renewable Energy Valleys (REVs), developed from real-world Community Energy Lab (CEL) demonstrations in Crete, Greece, which is an island with pronounced seasonal demand fluctuation, strong renewable potential, and ongoing hydrogen valley initiatives. Four modular business schemes are defined, each representing different sectoral contexts by combining a baseline of 50 residential units with one representative large consumer (hotel, rural households with thermal loads, municipal swimming pool, or hydrogen bus). For each scheme, a mixed-integer linear programming model is applied to optimally size and operate integrated solar PV, wind, battery (BAT) energy storage, and hydrogen systems across three renewable energy penetration (REP) targets: 90%, 95%, and 99.9%. The framework incorporates stochastic demand modeling, sector coupling, and hierarchical dispatch schemes. Results highlight optimal technology configurations that minimize dependency on external sources and curtailment while enhancing reliability and sustainability under Mediterranean conditions. Results demonstrate significant variation in optimal configurations across sectors and targets, with PV capacity ranging from 217 kW to 2840 kW, battery storage from 624 kWh to 2822 kWh, and hydrogen systems scaling from 65.2 kg to 192 kg storage capacity. The modular design of the framework enables replication beyond the specific context of Crete, supporting the scalable development of Renewable Energy Valleys that can adapt to diverse sectoral mixes and regional conditions.

For lignite intense regions such as the case of Western Macedonia (WM), the production and utilization of green hydrogen is one of the most viable ways to achieve near zero emissions in sectors like transport, chemicals, heat and energy production, synthetic fuels, etc. However, the implementation of each technology that is available to a respective sector differs significantly in terms of readiness and the current installation scale of each technology. The goal of this study is the provision of a transition roadmap for a decarbonized future for the WM region through utilizing green hydrogen. The technologies which can take part in this transition are presented, along with the implementation purpose of each technology, and the reasonable extension that each technology could be adopted in the present context. The WM region’s limited capacity for green hydrogen production leads to certain integration scenarios, with regards to the required hydrogen, electrolyzer capacities, and required power, whereas an environmental assessment is also presented for each scenario.