DC Building Network and Storage for BIPV Integration

Abstract

Electricity loads in buildings are increasingly supplied by distributed renewable generation sources such as photovoltaic (PV) modules or fuel cells. Often these systems are combined with an electricity storage to raise the level of self-consumption in buildings and to reduce the load on the electric grid. PV production and battery storage are both inherently based on direct current (DC) power. At the same time, an increasing number of energy-efficient devices that are part of the building service systems – such as LED’s, electronic equipment, or variable speed motors for efficient operation of ventilation units and heat pumps – use DC internally. Consequently, direct coupling of PV generation with electricity storage and DC loads in a local DC network has the potential to avoid power conversion losses arising in conventional alternating current (AC) based systems. In addition to the more efficient use of local generation and storage, an independent building micro-grid can provide system reliability in case of power outage. In this work, we perform a first evaluation of potential efficiency gains of low-voltage DC versus AC distribution in residential buildings with PV generation and battery storage. We take into account high-resolution electric load and PV generation profiles for a net-zero energy building. We find that relative to a conventional AC system, a DC network improves energy efficiency by approximately 2% without storage and 4% with storage. Moreover, we analyse the sensitivity of the results to variations in PV system size and battery characteristics. We show that the benefit of the DC system becomes more significant with increasing local generation and storage and that energy efficiency improvement and building self-sufficiency are correlated. The theoretical analysis provides guidance for a lab-scale hardware implementation of a small AC/DC hybrid system, which is currently in preparation.