With wind, solar and other clean energy sources gaining popularity worldwide, engineers are seeking ways to make renewable energy systems more affordable and to integrate them with existing ac power grids. Much research focuses on distributed power systems and the concept of microgrids, in which multiple electrical generation sources, energy storage, and loads connect as a single point on the grid.
Researchers from the Center for Power Electronics Systems (CPES) are tackling the issue from what they call a nanogrid perspective. Less extensive than a microgrid, a nanogrid can be as small as the energy management system for an entire building, explains ECE professor Paolo Mattavelli. A nanogrid includes the generating source, in-house distribution, and energy storage functions and can be extended to multiple buildings.
Mattavelli is working with professors Fred Lee, Dushan Boroyevich, and Khai Ngo, and research engineer Igor Cvetkovic and a team of graduate students on the effort. We are focusing on energy conservation for buildings, because residential and commercial buildings represent 68 percent of U.S. electricity consumption and almost 40 percent of the countrys carbon dioxide emissions, he explains. We are applying power management to reduce electrical energy use and to make renewable energy more cost effective.
The team is investigating smart ac nanogrids for todays technology and dc nanogrids for the smart, sustainable homes and buildings of tomorrow. The ac nanogrids incorporate smart appliances, lighting and heating/ventilation/air conditioning (HVAC) with on-site power generation and an Energy Control Center (ECC). The ECC can communicate with the power system operator for energy trading purposes, while also acting as a data acquisition unit. The ECC can collect and record the power flow data not only from and towards the grid, but also from all the converters and smart appliances in the home.
CPES researchers have developed and demonstrated unidirectional and bidirectional inverters that operate with an ECC to isolate a house from the utility grid, work in stand-alone mode, and synchronize and reconnect the house to the grid without load power interruptions. The next step, Mattavelli says, is to integrate energy storage and renewable energy sources with cost-effective solutions
Power management via ac nanogrids can achieve significant energy savings, he says. However, if we change the architecture from a single ac system and go straight from dc renewable energy sources and storage elements to dc loads, we can significantly reduce power losses and cost, he adds.
A dc nanogrid would start with fewer power converters, higher overall system efficiency, and easier interface of renewable energy sources to a dc system. There would be no frequency stability and reactive power issues, and less conduction loss, he says. Moreover, the consumer electronics, electronic ballasts, LED lighting, and variable-speed motor drives can be more conveniently powered by dc.
CPES researchers envision a dc nanogrid with two dc voltage levels: a high-voltage (380 V) dc bus powering HVAC, kitchen loads, and other major home appliances, and a multitude of low-voltage (48 V, 24 V, etc.) dc buses powering small tabletop appliances, computer and entertainment systems, and LED lighting.
There are many challenges for implementing the ac and dc nanogrids, the main one being fault current interruption in higher voltage dc systems. Communication, control and optimization also present challenges, Mattavelli says. This is a very complex system. We want to control the overall system at the same time we are controlling at the component level. How do we optimize each part How do we implement a cost-effective communication that enable system energy optimization
Power management with ac and dc nanogrids is not an entirely new concept, Mattavelli says. Similar systems have already been applied in the transportation area, with the all-electric ship and the more-electric airplane. Experience with such standalone systems gives power electronics researchers confidence in their success.
Many of the same concepts are also used in low-power applications, he says. A notebook computer, for example, has different load converters, source converters and even a single mother board. A dc nanogrid could potentially have even a higher level of power management as a notebook of course at a different power level.
Ultimately, Mattavelli says, the sustainable home will have the net-zero energy consumption from the utility grid and zero-emissions. This is our goal, he says.