A home energy management system created by Fraunhofer incorporates electric vehicles (EVs) into the household energy network and creates charging itineraries to allow owners of home photovoltaic (PV) systems to charge their EVs.

This functionality is already in use at a collection of row houses built to the “Passive House” standard in the German city of Fellbach. The group of homes was upgraded as part of the “Fellbach ZeroPlus” project to include electromobility enhancements as well as a comprehensive energy management system.

The rooftop PV systems provide more power than the inhabitants consume over the long term, allowing surplus power to be used to charge a household electric vehicle.

To efficiently incorporate electromobility enhancements into the daily routines of the households, Noeren Dominik Noeren, a scientist at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, and his team designed a 22-kilowatt fast-charging station as well as a home energy management system (HEMS) for five of the seven homes. The Java-based HEMS software runs on small computers known as embedded systems and collects data from electricity meters around the house, including those for the photovoltaic system, the electric vehicle, the heat pump and general household power. The system displays power flows and informs homeowners about their current electric power consumption.

Homeowners can determine how much power is coming from the public grid or the household solar system, and see where it is going—to the heat pump, household appliances or the electric vehicle. HEMS forecasts solar intensity over the next 20 hours and provides users with information on how much solar power is available.

An adaptive algorithm also computes anticipated household power loads for each quarter hour. Using this data, homeowners may determine how much PV power is available for the electric vehicle.

“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,” says Noeren.

During two years of field testing, an Android application was created using homeowner feedback. The HEMS app provides a visualization of all processes and electricity flows in real time and gives solar intensity forecast readouts in graphical and numerical form. An adaptive algorithm works to optimize the use of the power generated by each household. Through the app, users can control the charging station as well as view the battery charge level and EV charging times.

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 best for recharging the vehicle using the greatest possible proportion of household-produced solar energy.

The HEMS system helps consumers use data on driving times, solar intensity forecasts and current household energy consumption to synchronize electric vehicle charging times with rooftop energy production. This not only helps homeowners lower costs, but enhances the goal of low-CO₂ homes and personal mobility.