Profound changes are occurring across the domains of traditional energy, renewable energy, demand-response and overall energy management. Energy efficiency has emerged as a topic of keen interest across these domains. By Maria Vallet, and Jérôme Brenguier, Schneider Electric
As the energy marketplace transforms itself, developers and facility managers are questioning whether power distribution efficiencies are being affected by a new mix of loads within buildings. Profound changes are occurring across the domains of traditional energy, renewable energy, demand-response and overall energy management. Energy efficiency has emerged as a topic of keen interest across these domains.
As a result, the traditional ways that energy has been produced, stored, and delivered are being re-evaluated. New applications, such as DC connected solar energy and storage, and new technological breakthroughs such as lithium-ion batteries are raising new questions as to how power distribution networks could possibly evolve to ensure high levels of safety and efficiency. This application note presents a researched comparison of both AC and DC power distribution efficiencies in building environments.
Building power distribution is going through a period of evolution. For many decades, AC power served as the primary source for distributed power throughout buildings. In recent years, changes in both the makeup of electrical loads (a growth in DC-based technologies such as batteries) and power generation (the proliferation of renewable energy sources such as solar) have given birth to the idea of creating hybrid and DC power distribution networks within buildings. Such an evolution may help to turn what was once only a dream into a technological and practical reality: the ability of a building to actually produce more energy than it consumes.
AC and DC power electrical distribution systems have co-existed over a long period of time. Whether AC or DC systems are implemented is often determined by application attributes, business needs, and cost considerations. AC distribution is the primary and traditional choice of designers for many applications. Building electrical distribution systems, for instance, are AC power supplied (although DC power distribution for buildings is an option that is often discussed in the literature). Examples of popular DC-oriented installations include telephone system central offices, data centres, uninterruptible power supplies (UPS), electric vehicle (EV) charging, auxiliary power supplies for power plants, emergency lighting for buildings, charging facilities that support marine activities, and photovoltaic power plants to name a few.
Companies like Schneider Electric leverage their expertise based on a proven track record when it comes to the analysis of criteria for evaluating both AC and DC power distribution solutions, for designing efficient architectures, for sizing solutions, for modifying existing power distribution networks and for growing the application value of power products.
This application note analyses one aspect of the building power distribution paradigm: AC and DC building power distribution system efficiency. The two approaches are compared with emphasis on observation of energy source, energy path and load type criteria.
Power Distribution Efficiency Comparison Study
A recent study, endorsed by IEEE, analysed the average efficiency difference when a building’s loads and sources are linked either to a DC distribution bus or to a standard AC distribution system. Average efficiencies of several energy paths were analysed. Building power distribution efficiency performance at different times of the day both during the work week and on weekends was considered. Each case analysed corresponded to a power utilisation path from the source(s) to the load(s). This application note summarises the assumptions and the findings of the study. To access the detailed analysis, see the IEEE document entitled “Efficiency Gap between AC and DC Electrical Power Distribution System”.
The sources considered for the comparison included:
- AC public distributed power supply
- Photovoltaic panels
The loads considered for the comparison fell into one of the following four categories:
- Resistive loads (eg: heaters, incandescent lamps, water heaters, tea kettles or ovens)
- Asynchronous motor loads (eg: fans, pumps, refrigerators or freezers)
- Low power electronics (eg: mobile phone chargers, touch pads chargers, or Light Emitting Diode (LED))
- High power electronics (eg: computers, televisions)
A battery storage system acted as either source or load.
Conditions for fair comparison Equivalent distribution voltages were utilised (230V AC compared to 380V DC) as were equivalent levels of technology for the loads. The same power electronics converter technology was used and equivalent cable characteristics and cable sections, lengths and cable loading were used. Efficiencies were calculated for AC/DC, DC/AC and DC/DC converters interfaces. All possible power paths were taken into account to establish an efficiency average. The power path efficiency was based on an average over at least a weekly duration and took into account the relative weight of each power flow. Efficiency differences were expressed in terms of a final energy bill.
The difference between the average equivalent efficiency of AC and DC distribution power systems was computed for a working weekday and for a non-working two-day weekend. Then a weighted average was projected based on an entire week.
For buildings connected to the AC grid with no DC sources, the level of losses (and thus the efficiency) is equivalent for both AC or DC distribution. In the case of an islanded building with no connection to an AC mains supply, DC distribution results in about 3 percent lower losses.
For a building connected to the AC power supply and equipped with DC storage and sources, losses in a DC distribution scenario vary between 2 percent and 5 percent. In a similar scenario, AC losses vary from 1 percent to 7 percent. The gap between AC and DC efficiency in these cases depends on energy paths and self-consumption and battery use scenarios. Therefore, efficiency advantages in favour of either DC or AC will vary but remain very small.
Building study conclusion DC distribution did not result in any significant efficiency gain, except in cases where a DC power source such as photovoltaic (PV) was available. However, buildings without any access to an AC power source (eg: an islanded building with PV and storage) saw efficiency improvements of less than 3 percent using DC distribution.
Applications accessing AC and DC sources with self-consumption and frequent use of battery improved their efficiency by 0 to 2 percent when a DC distribution bus was deployed. The gap depends on the use cases. This analysis shows that low voltage AC and DC power distribution architectures in buildings demonstrate virtually the same efficiency, suggesting that a move to a DC-based architecture is unwarranted on the basis of efficiency alone.
Other considerations such as safety, cost performance trade-offs, electromagnetic compatibility (EMC), ageing, energy availability and stability should also be factored in when choosing AC or DC for low voltage electrical distribution. When comparing the familiar world of AC power distribution to DC, observers recognise that the transposition of AC to DC cannot be made directly. There are many parameters to consider showing significant differences including the need for specific switching and protection devices, additional capacitor banks, and regulated input currents, which have significant cost and performance consequences.