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A generative hidden Markov model of the clear-sky index
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics, Byggteknik.ORCID iD: 0000-0001-6292-0695
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics, Byggteknik.ORCID iD: 0000-0003-0051-4098
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics, Byggteknik.ORCID iD: 0000-0003-4887-9547
2019 (English)In: Journal of Renewable and Sustainable Energy, E-ISSN 1941-7012, Vol. 11, no 4, article id 043703Article in journal (Refereed) Published
Abstract [en]

Clear-sky index (CSI) generative models are of paramount importance in, e.g., studying the integration of solar power in the electricity grid. Several models have recently been proposed with methodologies that are related to hidden Markov models (HMMs). In this paper, we formally employ HMMs, with Gaussian distributions, to generate CSI time-series. The authors propose two different methodologies. The first is a completely data-driven approach, where an HMM with Gaussian observation distributions is proposed. In the second, the means of these Gaussian observation distributions were predefined based on the fraction of time of bright sunshine from the site. Finally, the authors also propose a novel method to improve the autocorrelation function (ACF) of HMMs in general. The two methods were tested on two datasets representing two different climate regions. The performance of the two methodologies varied between the two datasets and among the compared performance metrics. Moreover, both the proposed methods underperformed in reproducing the ACF as compared to state-of-the-art models. However, the method proposed to improve the ACF was able to reduce the mean absolute error (MAE) of the ACF by up to 19%. In summary, the proposed models were able to achieve a Kolmogorov-Smirnov test score as low as 0.042 and MAE of the ACF as low as 0.012. These results are comparable with the state-of-the-art models. Moreover, the proposed models were fast to train. HMMs are shown to be viable CSI generative models. The code for the model and the simulations performed in this paper can be found in the GitHub repository: HMM-CSI-generativeModel.

Place, publisher, year, edition, pages
2019. Vol. 11, no 4, article id 043703
Keywords [en]
Markov processes, Photovoltaics, Machine learning, Solar energy, Statistical models, Solar irradiance
National Category
Energy Systems Other Environmental Engineering Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-389945DOI: 10.1063/1.5110785ISI: 000482886400012OAI: oai:DiVA.org:uu-389945DiVA, id: diva2:1339916
Funder
StandUpSwedish Energy AgencyAvailable from: 2019-08-01 Created: 2019-08-01 Last updated: 2024-01-17Bibliographically approved
In thesis
1. Modeling and Forecasting of Electric Vehicle Charging, Solar Power Production, and Residential Load: Perspectives into the Future Urban and Rural Energy Systems
Open this publication in new window or tab >>Modeling and Forecasting of Electric Vehicle Charging, Solar Power Production, and Residential Load: Perspectives into the Future Urban and Rural Energy Systems
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The urban and rural energy systems are undergoing modernization. This modernization is motivated by the need to increase sustainability in both systems. Some characteristics of this modernization include electrification of industries, transports, and heating and cooling loads. Additionally, there has been an increase in building-applied photovoltaic (PV) systems, and in the flexibility of customer loads. This thesis aims to progress the knowledge regarding the electric power production and consumption in the future urban and rural energy systems. In total, three models were developed and applied to case studies: a spatial electric vehicle (EV) charging model, a residential load forecasting model, and a clear-sky index (CSI) generative model. The results of the EV spatial model showed that there is an aggregation effect for the charging of the EVs. If all EVs charge opportunistically upon arrival using 3.7 kW, at most 19% of the EVs in a large area will charge simultaneously. Delaying the charging to after 22:00 will result in a significant increase in the simultaneity factor — to 59%. Two forecasting models were compared for the residential load. Both models achieved a root mean square error (RMSE) smaller than 4%. One model had a slightly sharper forecast than the other model — by 2.6% — and a variable prediction interval (PI) which decreased at night. As regards the spatiotemporal matching between PV power production and EV charging in rural and urban areas, the results showed that there were no correlations between the building type in each part of the city and the temporal matching. Both residential and workplace areas had similar temporal matching. This is because of the orientations of the roofs in the cities and the sizes of the parking lots. Considering the impacts of EV charging on the distribution grid of a Swedish municipality (Herrljunga), it is shown that 3.7 kW chargers will result in at most a 1% decrease in the voltage of the grid. No under-voltages were witnessed. In conclusion, the urban and rural energy systems can withstand the penetration of PV and EVs in the nearby coming years. Extreme scenarios might, however, require increasing the flexibility or performing upgrades to the systems.  

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. p. 100
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1952
Keywords
Electric vehicles, Residential load, Photovoltaic, Distribution grid, Grid integration, Self-consumption, Elbilar, hushållsel, Solel, Lokalnät, Regionnät, Elkraftsystem, Egenanvändning
National Category
Energy Systems Energy Engineering
Research subject
Engineering Science with specialization in Civil Engineering and Built Environment
Identifiers
urn:nbn:se:uu:diva-416754 (URN)978-91-513-0985-9 (ISBN)
Public defence
2020-09-22, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2020-08-27 Created: 2020-08-03 Last updated: 2020-09-02

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