Techno-Economic Feasibility of a Grid-Connected Hybrid Renewable Energy System for a School in North-West Indonesia

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link. open access journalBackground: Schools typically have high diurnal fluctuation in electricity demand, with peak loads during daylight hours, which could be adequately met through harnessing solar renewable resources. This study demonstrates the strength of techno-economic assessment in selection and optimization of a grid-connected hybrid renewable energy system (HRES), utilizing local renewable resources to fulfil the daytime electricity demand for a school in northwest Indonesia. Methods: Three different scenarios are developed for optimizing the HRES configurations, comprising of PV panels, Wind turbine, Battery and Inverter. The following optimization parameters are used—one, technological performance of the HRES, in terms of their energy output to fulfil the energy deficit; two, economic performance of the HRES, in terms of their net present cost (NPC) and payback periods. Results: A clear trade-off is noted between the level of complexity of the three HRES, their renewable electricity generation potentials, NPC and payback periods. Scenario II, comprising of Solar PV and Inverter only, is found to be the most feasible and cost-effective HRES, with the optimized configuration of 245 kW PV capacity and 184 kW inverter having the lowest initial capital cost of US$51,686 and a payback time of 4 years to meet the school’s annual electricity load of 114,654 kWh. Its NPC is US$ −138,017 at the 20th year of installation. The negative value in year 20 is achieved through the sale of 40% of the renewable energy back to the grid. Conclusions: Techno-economic assessment can provide useful decision support in designing HRES relying on solar energy to serve predominantly daytime school electricity requirements in tropical countries

On the extension complexity of combinatorial polytopes

In this paper we extend recent results of Fiorini et al. on the extension complexity of the cut polytope and related polyhedra. We first describe a lifting argument to show exponential extension complexity for a number of NP-complete problems including subset-sum and three dimensional matching. We then obtain a relationship between the extension complexity of the cut polytope of a graph and that of its graph minors. Using this we are able to show exponential extension complexity for the cut polytope of a large number of graphs, including those used in quantum information and suspensions of cubic planar graphs.Comment: 15 pages, 3 figures, 2 table

Realistic time-scale fully atomistic simulations of surface nucleation of dislocations in pristine nanopillars

We use our recently proposed accelerated dynamics algorithm (Tiwary and van de Walle, 2011) to calculate temperature and stress dependence of activation free energy for surface nucleation of dislocations in pristine Gold nanopillars under realistic loads. While maintaining fully atomistic resolution, we achieve the fraction of a second time-scale regime. We find that the activation free energy depends significantly and non-linearly on the driving force (stress or strain) and temperature, leading to very high activation entropies. We also perform compression tests on Gold nanopillars for strain-rates varying between 7 orders of magnitudes, reaching as low as 10^3/s. Our calculations bring out the perils of high strain-rate Molecular Dynamics calculations: we find that while the failure mechanism for compression of Gold nanopillars remains the same across the entire strain-rate range, the elastic limit (defined as stress for nucleation of the first dislocation) depends significantly on the strain-rate. We also propose a new methodology that overcomes some of the limits in our original accelerated dynamics scheme (and accelerated dynamics methods in general). We lay out our methods in sufficient details so as to be used for understanding and predicting deformation mechanism under realistic driving forces for various problems