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Renewable energy (RE) sources like wind and solar power have been introduced to power systems to make the energy market more sustainable and environmentally friendly. For this reason, the supply demand mismatch in power systems is more severe than before, in terms of both frequency and magnitude. It is therefore necessary to reinforce the capability to make supply and demand adjustments in power grids. Here, we clarified the dynamic characteristics of biogas plants with combined heat and power (CHP) systems, that represent controllable RE power sources, and evaluated the possibility of using these systems as a new resource for supply-demand adjustments in the power grid. Additionally, the properties of the exhaust gas from the investigated CHP engines were measured as an environmental evaluation. We considered real-world biogas CHP systems operating in both Germany and Japan. Based on the results, biogas CHP systems have the potential to contribute to a long-term power equivalent adjustment such as tertiary control reserve (TCR) in Germany. However, these systems required several minutes between starting and stopping. Furthermore, increased emissions of methane and formaldehyde were measured when the CHP systems were starting up. We found that an operation with as few starts and stops as possible is therefore desirable.

In this study, based on 55 years’ worth of high-resolution simulated wave data using numerical modeling off the southern coasts of China, intra-annual and decadal variations of the wave climate and wave energy were evaluated. The results show that it is important to consider a sufficiently long time period for wave energy assessment to take into account the changing climate. The high-resolution wave dataset enabled the quantitative analysis in both nearshore and offshore, and the quantitative analysis was performed in two phases: First, using two different approaches. i.e., “Climate-dependent Sustainability Index” and “Wave Exploitability Index”, the wave power and its short and long-term changes were considered to prioritize the candidate stations for further assessment. Then, a modified “Multi-Criteria Approach” consisting of both sea state and Wave Energy Converters (WECs) was applied to determine the most suitable combination of WEC and location in the domain, which is Wave Dragon in the eastern parts of the domain with the energy production of around 92, 000 MWh for a single device. The results provide the quantitative analysis for different scenarios of development plans in the study area on the selection of appropriate location and technology.

This is the accepted version of the article published in Renewable Energy, v. 68, pp 403-413, 2014.

In standard operating conditions, a floating offshore wind turbine (FOWT) suffers from the combined action of nonlinear waves and wind. It is important to explore the effect of higher-order hydrodynamic loads induced by the nonlinear waves on its motion response. In this work, a fully coupled aero-hydrodynamic analysis method for FOWT is developed. The hydrodynamic part is based on the nonlinear potential flow theory and the perturbation approach, using a higher-order boundary element method in the time domain. Blade element momentum theory is applied to evaluate the aerodynamic load on the turbine rotor. The mooring system is modelled by the catenary theory. The developed method is verified against the published data of the NREL's 5-MW baseline wind turbine supported by the DeepCwind semi-submersible platform. The verifications are conducted stepwise in the following ways, including hydrodynamic wave excitation load, free-decay response, the load-displacement relationship of the mooring line, the steady-state response of the wind turbine, and fully coupled aero-hydrodynamic interaction. It is found that the nonlinear effect mainly influences the surge motion. The coupled configuration has little effect on the linear wave force and platform displacement but notably impacts the nonlinear wave force and displacement. The FOWT motion is found to be critical in changing the wave field, the maximum wave elevation position around the platform, and the nonlinear wave force acting on the floating platform. The results illustrate that the traditional indirect time-domain method using the Quadratic Transfer Function (QTF) based on assumed response cannot appropriately evaluate the nonlinear wave force of a FOWT in coupled motions. The results also reveal that nonlinear wave hydrodynamics is necessary to understand the motion response of a fully coupled FOWT.

Since 2010 the Dutch photovoltaic (PV) market has been growing fast, with around doubling of installed capacity in 2011 and 2012. Four quarterly inventories have been made in 2012 for modules, inverters, and systems that are presently available for purchase in the Netherlands. We have found that the average selling price of modules, inverters, and systems decreased with 44.3, 14, and 7.3e10.2%, respectively: average selling prices are 1.26 V/Wp, 0.41 V/Wp, and 1.46 V/Wp for modules, inverters, and systems on tilted roofs, respectively, at the end of 2012. Average installation costs amount to 0.43 V/Wp. Using an energy yield of 900 kWh/kWp, 25 years system lifetime, 6% discount rate, and 1% operation and maintenance (O&M) cost, a levelized cost of electricity (LCOE) is calculated for a 2.5 kWp system to be 0.194 V/kWh for a system price of 1.98 V/Wp (including installation). Grid parity conditions are apparent, with electricity retail prices of around 0.23 V/kWh.