Comparison of AC and DC power supply for residential buildings equipped with solar + energy storage

Swedish scientists compared AC and DC topologies in photovoltaic-powered buildings using battery energy storage. They found that the DC power distribution system combined with the solar + energy storage system can achieve energy savings.

Researchers at Chalmers University of Technology in Sweden compared the energy-saving potential of alternating current (AC) and direct current (DC) power distribution systems in residential buildings equipped with photovoltaic and battery energy storage systems. In particular, they looked at whether a DC system would reduce energy losses.

"Our work not only demonstrates the reduced losses of DC power distribution in a Nordic climate with low average irradiance, but also demonstrates the contribution of photovoltaics and battery energy storage to energy-saving DC power supply," the researchers said. impact and necessity.”

The scientists used a full-year dataset to analyze the daily and seasonal performance of the two topologies, including load usage, photovoltaic generation, load-dependent power electronic converter (PEC) efficiency characteristics, and battery storage data. They considered the AC and DC configurations of a building with 3.6 kW of solar and storage installed at a 45-degree tilt angle facing south. They assume that the building is heated and domestically hot water supplied by a ground source heat pump.

"We carried out separate measurements for the following equipment, including ground source heat pumps, ventilation, water pumps and photovoltaics," the scientists said. They also noted that the annual load demand was 6,354 kWh, while the photovoltaics produced 3,113 kWh. "This research was done on a grid-connected building that requires a bi-directional AC/DC converter to interface with the grid."

This research work considered four different system topologies: AC (230 V AC with load-dependent efficiency), DC1 (380 V DC with load-dependent efficiency), DC2 (380 V DC with fixed converter efficiency), and DC3 (380 and 20 Vdc with load dependent efficiency).

"DC1 and DC2 also add a secondary DC voltage of 20 volts to power smaller loads and lighting through centralized DC/DC converters," the research team said.

They found that bidirectional converters showed significant differences in losses when modeled with fixed and load-dependent efficiency characteristics. They also found that the DC topology can achieve energy savings even without photovoltaic or battery storage.

They state: "The losses of the grid-connected converter (DC2) using the constant efficiency approach are 34% lower than those of the converter (DC1) using the load-dependent efficiency. The efficiency values for the AC and DC1−3 systems are 95.3%, 94.3%, 95.8% and 93.7%."

The team concluded that DC systems are not a favorable option in terms of reducing losses when not equipped with photovoltaic and battery systems.

The researchers conclude: "In a more scientific context, the error of using constant efficiency for both batteries and power electronic converters stands out. Furthermore, the cost savings are most significant when photovoltaic power is fed directly or via a battery energy storage device. I Acknowledging that the use of DC power distribution systems in buildings is a niche application that presents a dilemma in terms of product supply and demand. However, some special occasions may be of interest, such as coupling photovoltaics, batteries and the interior of electric vehicles DC grid and office buildings with good correlation between PV and load demand.”

The researchers present their findings in the paper "Using DC Distribution to Reduce Energy Losses in Residential Buildings Equipped with Solar Photovoltaic and Battery Energy Storage Systems," recently published in the journal Energies.