Identifying bottlenecks in charging infrastructure of plug-in hybrid electric vehicles through agent-based traffic simulation

The effect of different charging infrastructure configurations on the electric-driven distance of plug-in hybrid electric vehicles (e-mileage) has been investigated, using an agent-based traffic simulation. Our findings suggest that the same e-mileage can be achieved with fewer charging poles if the poles support charging from several parking slots around them, and the charging cable is switched from one vehicle to the next. We also find that the charging power supported by most Finnish charging stations, 3.7 kW, and the cable switching delay of 1 h seem to be sufficient for effective workplace charging.Peer reviewe

Effectiveness of smart charging of electric vehicles under power limitations

This article investigates charging strategies for plug-in hybrid electric vehicles (PHEV) as part of the energy system. The objective was to increase the combined all-electric mileage (total distance driven using only the traction batteries in each PHEV) when the total charging power at each workplace is subject to severe limitations imposed by the energy system. In order to allocate this power optimally, different input variables, such as state-of-charge, battery size, travel distance, and parking time, were considered. The required vehicle mobility was generated using a novel agent-based model that describes the spatiotemporal movement of individual PHEVs. The results show that, in the case of Helsinki (Finland), smart control strategies could lead to an increase of over 5% in the all-electric mileage compared to a no-control strategy. With a high prediction error, or with a particularly small or large battery, the benefits of smart charging fade off. Smart PHEV charging strategies, when applied to the optimal allocation of limited charging power between the cars of a vehicle fleet, seem counterintuitively to provide only a modest increase in the all-electric mileage. A simple charging strategy based on allocating power to PHEVs equally could thus perform sufficiently well. This finding may be important for the future planning of smart grids as limiting the charging power of larger PHEV fleets will sometimes be necessary as a result of grid restrictions.Peer reviewe

Review of energy system flexibility measures to enable high levels of variable renewable electricity

The paper reviews different approaches, technologies, and strategies to manage large-scale schemes of variable renewable electricity such as solar and wind power. We consider both supply and demand side measures. In addition to presenting energy system flexibility measures, their importance to renewable electricity is discussed. The flexibility measures available range from traditional ones such as grid extension or pumped hydro storage to more advanced strategies such as demand side management and demand side linked approaches, e.g. the use of electric vehicles for storing excess electricity, but also providing grid support services. Advanced batteries may offer new solutions in the future, though the high costs associated with batteries may restrict their use to smaller scale applications. Different “P2Y”-type of strategies, where P stands for surplus renewable power and Y for the energy form or energy service to which this excess in converted to, e.g. thermal energy, hydrogen, gas or mobility are receiving much attention as potential flexibility solutions, making use of the energy system as a whole. To “functionalize” or to assess the value of the various energy system flexibility measures, these need often be put into an electricity/energy market or utility service context. Summarizing, the outlook for managing large amounts of RE power in terms of options available seems to be promising.Peer reviewe

Sähköajoneuvojen latausstrategioista kaupunkiympäristössä

Due to global environmental concerns, plug-in hybrid electric vehicles (PHEVs) are expected to become more prevalent. In order to avoid overloading the power supply system and to use PHEV recharging to reduce power fluctuations, smart charging strategies could be very useful. This thesis introduces a method for allocating available power to the connected PHEVs through a charging function. To obtain insight on the problem of power allocation, the form and parameters of the charging function were modified and the effect on the electric kilometres (objective function to be maximized) was checked. All results were obtained in a simulated environment, using Helsinki as a case. The results indicate that it is usually a good strategy to allocate more charging power for vehicles that have low state of charge (SOC) and low battery capacity. Obtaining an accurate prediction for the amount of time a PHEV will stay at its current location provides more electric kilometres than an accurate prediction for the amount of kilometres the PHEV will travel before reconnecting with the grid. It is possible to safeguard against inaccurate predictions by using a linear combination of the prediction and the observed average. When there is a sufficient correlation between travelled distance and battery capacity, the effectiveness of our strategies diminishes, rendering them almost redundant. The effectiveness of any strategy is linked to the battery capacity. With too low and too high capacities, the strategies are redundant. This thesis also introduces a method for generating nearly arbitrary charging load profiles for night-time home recharging, the weighted random recharging (WRR). It was shown that with traffic electrification of 39 % and under certain simplifying assumptions, the yearly standard deviation of power consumption could be reduced by 6.3 % while reducing the daily standard deviation by 61 % on average.Maailmanlaajuisten ympäristöongelmien uhan vuoksi on odotettavissa, että plug-in hybridisähköajoneuvojen (PHEV) määrä tulee lisääntymään. Välttääksemme sähköjärjestelmämme ylikuormittamisen ja jopa hyötyäksemme sähköajoneuvojen latauskuormasta pienentämällä tehonkulutuksen vaihtelua älykkäät latausstrategiat voivat osoittautua hyödyllisiksi. Tämä diplomityö esittelee latausfunktion, menetelmän tehonjakamiselle sähköverkkoon kytketyille autoille. Yritimme ymmärtää tehon jakamisen ongelmaa kokeilemalla erilaisia muotoja tälle funktiolle ja selvittämällä, miten ajettujen sähkökilometrien määrä, maksimoitavan kohdefunktiomme arvo, muuttui. Kaikki tuloksemme saavutettiin simuloidussa ympäristössä Helsinkiä edustavassa noodiverkostossa. Tuloksemme viittaavat siihen, että on tavallisesti hyvä strategia antaa enemmän lataustehoa niille ajoneuvoille, joiden normalisoitu lataustaso (SOC) ja kapasiteetti ovat matalia. Tarkan ennusteen saaminen sille ajalle, minkä ajoneuvo viettää nykyisessä sijainnissaan tuottaa enemmän sähkökilometrejä kuin tarkka ennuste sille matkalle, minkä ajoneuvo kulkee ennen liittymistään takaisin sähköverkkoon. Huonoja ennusteita vastaan voidaan suojautua käyttämällä ennusteen ja havaitun keskiarvon yhdistelmää. Kun kuljetun matkan ja akkukapasiteetin välillä on riittävä korrelaatio, käyttämiemme strategioiden vaikutus pieneni tehden niistä lähes tarpeettomia. Minkä tahansa strategian vaikutus riippuu akkukapasiteetista. Liian pienillä ja liian suurilla kapasiteeteilla strategiat ovat tarpeettomia. Tämä diplomityö esittelee myös menetelmän lähes mielivaltaisten latauskuormaprofiilien muodostamiselle, painotetun satunnaislatauksen (WRR). Näytämme, että liikenteen sähköistämisprosentin ollessa 39 ja tiettyjen yksinkertaistavien oletusten voimassa ollessa voidaan vuosittaista sähkönkulutuksen keskihajontaa pienentää 6.3 % päivittäisen keskihajonnan pienentyessä keskimäärin 61 %

Plug-in-sähköajoneuvolaivueiden lataaminen kaupunkiympäristössä

This thesis studies the performance and charging of electric vehicle fleets in an urban environment,using three research questions that focused on: 1) allocation of limited total charging power; 2) effect of different charging infrastructure parameters on the total distance driven in electric-only mode, and; 3) effect of cold and warm ambient temperature on the performance and charging of the fleet, respectively. An agent-based computer simulation was employed, with parameters tuned such that the resulting vehicle travel patterns would resemble the observed behaviour of conventional cars in the city of Helsinki, Finland. Two different simulators were used: in the first one, the vehicles travel in a node network according to a stochastic trip-generation algorithm. In the second one, the vehicles' status changes according to the results of a Finnish travel survey. Two different vehicle battery models were also used: a simple linear "kilowatt-hour counter" and a more advanced battery model with temperature dependency. It was found that smart charging power allocation can improve the total distance driven in electric-only mode compared to a "dumb" equal allocation strategy, but the gain is heavily dependent on the battery capacity. If no predictions about the future are made, the gain is small (1%), but with full knowledge on future travel patterns, it increases to over 5%. In general, more power should be allocated to vehicles that depart earlier and travel longer distances before their next charging session. Among plug-in hybrid electric vehicles, those with small battery capacity gain the most benefit from improved charging infrastructure, in terms of total distance driven in electric-only mode. Battery capacity holds the highest potential out of all parameters tested. The second and third most important infrastructure parameters are the number of parking slots around a single charging station and the number of these charging stations. A charging station should be placed in a central location with several parking slots around it, to allow sequential charging of multiple vehicles by switching the charging cable from one vehicle to the next. Low battery temperature has a negative impact on fully electric vehicle charging. This manifests as slightly reduced median state of charge (3–6%-units) for the vehicle fleet and significantly lower median charging rate (15% in terms of self-weighted mean charging power). Battery heating can be used to achieve higher state of charge, as well as increased charging rate for certain vehicles. Deviation from the close-to-optimal +20°C temperature for the cabin and the battery results in reduced efficiency (km/kWh) and eventually reduced number of planned trips that can be realized. Cabin preconditioning and active battery thermal management improve the median efficiency of the fleet around 8-9% at −10°C and +40°C.Tässä väitöskirjassa tutkitaan sähköautolaivueiden toimintakykyä ja lataamista kaupunkiympäristössä käyttämällä kolmea tutkimuskysymystä, jotka keskittyvät: 1) rajoitetun kokonaislataustehon jakamiseen usealle lataavalle autolle; 2) erilaisten latausinfrastruktuuriparametrien vaikutusta kokonaismatkaan, joka ajetaan täysin sähköllä; ja 3) kylmän ja lämpimän ympäristön lämpötilan vaikutusta autolaivueen toimintakykyyn ja lataamiseen. Tutkimusmenetelmänä käytettiin agenttipohjaista tietokonesimulaatiota, joka säädettiin siten, että simulaation tuloksena syntyneet ajotiedot muistuttaisivat tavallisten polttomoottori-autojen käyttäytymistä Helsingissä. Väitöskirjassa käytettiin kahta erilaista simulaattoria: ensimmäisessä autot liikkuvat noodiverkostossa stokastisen matkageneraattorin ohjaamana ja toisessa autot muuttavat tilaansa suomalaisen henkilöliikennetutkimuksen tuloksiin perustuen. Lisäksi käytettiin kahta erilaista akkumallia: yksinkertaista "kilowattituntilaskuria" ja edistyneempää lämpötilariippuvaista akkumallia. Havaittiin, että lataustehon älykäs jakaminen voi nostaa täysin sähköllä ajettua kokonaismatkaa verrattuna yksinkertaiseen lataustehon tasajakoon, mutta tämä nousu on vahvasti riippuvainen laivueen akkukapasiteetista. Jos mitään ennusteita ei käytetä, kasvu on pieni (1%), mutta täydellä tietämyksellä tulevasta ajosta kasvu on yli 5%. Yleisesti ottaen, lataustehon jakamisessa tulisi suosia autoja, jotka ovat poistumassa aikaisemmin latauspaikalta ja jotka kulkevat pidemmän matkan ennen seuraavaa lataussessiota. Plug-in-hybridisähköautoista ne, joilla on pieni akkukapasiteetti, hyötyvät eniten latausverkoston kehittymisestä, kun mittarina käytetään täysin sähköllä ajettua kokonaismatkaa. Akku-kapasiteetilla on suurin vaikutus kaikista kokeilluista parametreista. Toiseksi ja kolmanneksi tärkeimmät parametrit ovat yhden lataustolpan ympärillä olevien pysäköintiruutujen määrä ja lataustolppien määrä. Lataustolppa tulisi sijoittaa keskeiselle paikalle siten, että sitä voidaan hyödyntää useasta parkkiruudusta vaihtamalla latauskaapelia autosta toiseen. Matala akuston lämpötila huonontaa täyssähköautojen latauskykyä. Tämä näkyy hieman alentuneena mediaanilataustasona (3–6%-yksikköä) ja huomattavasti alentuneena mediaani-latausnopeutena (15% itsepainotetussa keskilataustehossa). Akuston lämmitystä voidaan käyttää kohottamaan lataustasoa ja nostamaan latausnopeutta tiettyjen autojen kohdalla. Kun auton sisätilan ja akuston lämpötila poikkeaa +20°C:sta, autojen tehokkuus (km/kWh) laskee ja poikkeaman kasvaessa joitakin suunniteltuja matkoja joudutaan lopulta perumaan. Sisätilan esi-ilmastointia ja aktiivista akuston lämmönsäätelyä käyttämällä voidaan mediaanitehokkuutta nostaa noin 8–9% lämpötiloissa −10°C ja +40°C

Review of energy system flexibility measures to enable high levels of variable renewable electricity

The paper reviews different approaches, technologies, and strategies to manage large-scale schemes of variable renewable electricity such as solar and wind power. We consider both supply and demand side measures. In addition to presenting energy system flexibility measures, their importance to renewable electricity is discussed. The flexibility measures available range from traditional ones such as grid extension or pumped hydro storage to more advanced strategies such as demand side management and demand side linked approaches, e.g. the use of electric vehicles for storing excess electricity, but also providing grid support services. Advanced batteries may offer new solutions in the future, though the high costs associated with batteries may restrict their use to smaller scale applications. Different “P2Y”-type of strategies, where P stands for surplus renewable power and Y for the energy form or energy service to which this excess in converted to, e.g. thermal energy, hydrogen, gas or mobility are receiving much attention as potential flexibility solutions, making use of the energy system as a whole. To “functionalize” or to assess the value of the various energy system flexibility measures, these need often be put into an electricity/energy market or utility service context. Summarizing, the outlook for managing large amounts of RE power in terms of options available seems to be promising.Peer reviewe

Flexibility of electric vehicles and space heating in net zero energy houses

With the increasing penetration of distributed renewable energy generation and dynamic electricity pricing schemes, applications for residential demand side management are becoming more appealing. In this work, we present an optimal control model for studying the economic and grid interaction benefits of smart charging of electric vehicles (EV), vehicle-to-grid, and space heating load control for residential houses with on-site photovoltaics (PV). A case study is conducted on 1–10 net zero energy houses with detailed empirical data, resulting in 8–33% yearly electricity cost savings per household with various electric vehicle and space heating system combinations. The self-consumption of PV is also significantly increased. Additional benefits through increasing the number of cooperating households are minor and saturate already at around 3–5 households. Permitting electricity transfer between the houses and EV charging stations at workplaces increases self-sufficiency significantly, but it provides limited economic benefit. The additional cost savings from vehicle-to-grid compared to smart charging are minor due to increased battery degradation, despite a significant self-sufficiency increase. If the optimization is conducted without taking the battery degradation cost into account, the added monetary value of vehicle-to-grid can even be negative due to the unmanaged degradation. Neglecting battery degradation completely leads to overestimation of the vehicle-to-grid cost benefit.Peer reviewe