The price, range and charging time of electric vehicles are well-known stumbling blocks when it comes down to a purchasing decision. Also a dense charging network with short distances between stations is not unimportant for customers and the success of electromobility. Considerably less attention is usually paid to the question of proper locations of charging stations. The availability at the workplace and dwelling place have the highest priority for users. Against this background, it is necessary to provide affordable and uncomplicated charging opportunities at these places.
To enhance the attractiveness of electromobility, the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg (Germany) investigates an energy management system which incorporates electric vehicles into the household energy network and creates charging itineraries. One major component of the project are home photovoltaic systems for an even more sustainable household. Rooftop-generated PV energy is not only for household consumption but also to charge personal electric vehicle. Fraunhofer ISE demonstrates in reality that PV power is also suitable for charging personal electronic vehicles. The aim is to test the practicability, the economic operating conditions and to maximize the proportion of household-produced energy. For this purpose a collection of row houses built according to the “Passive House” standard in the German city of Fellbach was upgraded as part of the “Fellbach ZeroPlus” project. The initiative is sponsored by the German Federal Government’s “Electric Mobility Showcase” program.
“The large photovoltaic systems on the rooftops of the houses provide more power than the inhabitants consume over the long term. Surplus power can be fed into the public grid as well as be used for charging the household electric vehicle,” explains Dominik Noeren, a scientist at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg. The researchers desire to incorporate electromobility enhancements into the daily routines of the households efficiently. For this purpose, a 22 kW fast charging station and a home energy management system (HEMS) for five of the seven homes has been designed by Noeren and his team. The Java-based HEMS software runs on small computers known as embedded systems. Data from the various electricity meters in the house, including those for the photovoltaic system, the heat pump, the electric vehicle and general household power are collected by HEMS. The system displays the various power flows. It also informs the homeowners about their current power consumption at any time of the day. “They can see how much power is coming from either the public grid or the household solar system, and they can see where it is going – to the heat pump, household appliances, or the electric vehicle,” says Noeren.
The availability of solar power and a forecast of solar intensity over the next 20 hours is provided by HEMS. An adaptive algorithm also computes anticipated household power loads for each quarter hour. Using this data, it is possible to determine how much PV power can be used at any time to charge the electric vehicles. “Electricity from the PV first goes to the house, and power that is not consumed there is stored in the electric vehicle battery. If there is still any electricity left over after that, it is fed into the public electricity grid,” explains Noeren.
An Android application was created during two years of field testing to use feedback from the homeowners. It visualizes all processes and electricity flows in real time and gives solar intensity forecast readouts in graphical and numerical form. To optimize the use of the power generated by each household an adaptive algorithm is applied. The battery charge level, the charging time of the electric vehicle and the charging station can be managed with the app. “These parameters are necessary in order to intelligently charge the electric vehicle,” says Noeren.
To create an ideal charging itinerary, the system must know the vehicle’s current battery charge level as well as its next planned departure time. The energy management system uses this information together with weather and consumption forecasts to estimate the flows through the household power network. It calculates how much electricity must be topped up, as well as which time periods are ideal for recharging the vehicle using the greatest possible proportion of household-produced solar energy.
“It is more cost effective to consume the self-generated solar electricity than to feed it into the public electricity grid,” says Noeren. According to Fraunhofer ISE it helps homeowners to lower their costs and realizing the ideal of low-CO₂ homes and personal mobility. Maximizing the proportion of household-produced energy consumed helps unburden the public power grid while reducing household feed-in peaks to the grid. Two of the five households in the “Fellbach ZeroPlus” project have been successfully using a car-sharing variant of the system as part of a field test since mid-2014.
Some questions arise when using the described energy management system at home. How much time consumes the system in day-to-day life? The system is an advantage if it supports inhabitants and demand as little attention as possible. Another question is the vulnerability of the system. What about privacy and data protection? The bottom line is that the system has to be affordable and convenient in use. Important steps are done by Fraunhofer ISE.