Energy Savings Using a Direct-Current Distribution Network in a PV and Battery Equipped Residential Building

Licentiate thesis, 2020

Energy from solar photovoltaic (PV) are generated as direct current (DC) and almost all of today’s electrical loads in residential buildings, household appliances and HVAC system (Heating Ventilation and Air-conditioning) are operated on DC. For a conventional alternating current (AC) distribution system this requires the need for multiple conversion steps before the final user-stage. By switching the distribution system to DC, conversion steps between AC to DC can be avoided and in that way losses are reduced. Including a battery storage–the system’s losses can be reduced further and the generated PV energy is even better utilised.

This thesis investigates and quantifies the energy savings when using a direct current distribution topology in a residential building together with distributed energy generation from solar photovoltaic and a battery storage. Measured load and PV generation data for a single-family house situated in Borås, Sweden is used as a case study for the analysis. Detailed and dynamic models–based on laboratory measurements of the power electronic converters and the battery–are also used to more accurately reflect the system’s dynamic performance.

In this study a dynamic representation of the battery’s losses is presented which is based on laboratory measurements of the resistance and current dependency for a single lithium-ion cell based on Lithium iron phosphate (LFP). A comparative study is made with two other, commonly used, loss representations and evaluated with regards to the complete system’s performance, using the PV and load data from the single-family house. Results show that a detailed battery representation is important for a correct loss prediction when modelling the interaction between loads, PV and the battery.

Four DC system topologies are also modelled and compared to an equivalent AC topology using the experimental findings from the power electronic converters and the battery measurements. Results from the quasi-dynamic modelling show that the annual energy savings potential from the suggested DC topologies ranges between 1.9–5.6%. The DC topologies also increase the PV utilisation by up to 10 percentage points, by reducing the associated losses from the inverter and the battery conversion. Results also show that the grid-tied converter is the main loss contributor and when a constant grid-tied efficiency is used, the energy savings are overestimated.