June 15, 2008

Microgrids promise improved power quality and reliability

Angela Neville, JD

Industry analysts view microgrid development as a major transition in the way electricity is generated, delivered, and controlled. The deployment of microgrids could shift electricity supply away from today’s highly centralized universal service model toward a more dispersed system.

One element of a dispersed system, according to Chris Marnay of Ernest Orlando Lawrence Berkeley National Laboratory, is the clustering of power generation sources and loads into semiautonomous microgrids. This new delivery model will result in more electricity generation closer to end users, often involving combined heat and power (CHP) for building heating and cooling (see sidebar), increased local integration of renewables, direct control of end-use equipment, and the possible provision of heterogeneous qualities of electrical service to match the requirements of various end uses.

Free fuel powers water reclamation plant

In 2001 the Santa Margarita Water District and the South Coast Air Quality Management District (SCAQMD) funded the installation of two Capstone 30-kW microturbines and one Microgen hot water generator at the Chiquita Water Reclamation Plant in southern California. Two additional 30-kW microturbines were commissioned in 2003. The microturbines are fueled by anaerobic digester gas from the reclamation plant. Waste heat from the microturbines is used to heat the anaerobic digesters (Figure 1).

 

1. Firing digester gas. The Chiquita Water Reclamation Plant in Orange County, California, uses four 30-kW microturbines and one hot water generator fueled by anaerobic digester gas. The turbine exhaust heat is used to heat the digesters. Courtesy: Advanced Power and Energy Program at the University of California, Irvine

The systems are all baseloaded at full electrical power and typically deliver 26 to 30 kW each. Their electrical efficiency is around 20% to 22%. Waste heat from the first two microturbines was sufficient to allow shutting down the two boilers that originally fed hot water to the digesters. Additional heat from the two newer microturbines may be used to dry sludge in order to lower shipping costs and/or heat future anaerobic digesters.

Phase 1 installation costs were $114,020, plus SCAQMD support. Phase 2 installation costs were $160,582. Energy costs savings were estimated at approximately $60,000 per year, yielding an expected payback period of around two years for Phase 1 with the SCAQMD support. For more information about the pilot project, go to www.chpcenterpr.org.

How does a microgrid differ from what we now call distributed generation (DG)? A handful of randomly selected DG professionals who were asked this question at the ELECTRIC POWER Conference in Baltimore in early May couldn’t say—or hadn’t heard the term “microgrid.” That may be a sign of the term’s newness. But in an interview after his presentation in the DG track, Imre Gyuk, director of energy storage research at the Department of Energy, explained: “It’s a continuum.” Whereas a single generator probably shouldn’t be called a microgrid, Hawaii is “the ultimate microgrid.” It depends, he said, on the amount of internal generation supplied and the degree of self-sufficiency provided. And, as you’ll see, the sophistication of the system’s controls also plays a large role in enabling a microgrid to serve all the roles of the traditional grid (and sometimes more).

One key feature of a microgrid is its ability to separate and isolate itself (islanding) from the utility’s distribution system during a grid disturbance, as distributed generation systems typically can today. This is accomplished via intelligent power electronic interfaces and a single, high-speed switch. During a disturbance, the distributed energy resources (DER) and corresponding loads can be separated from the utility’s distribution system, isolating the microgrid’s load from the disturbance (and thereby maintaining high-level service) without harming the integrity of the utility’s system. Intentional islanding of DER and loads has the potential to provide a higher level of reliability to end users than that provided by the distribution system as a whole. Then, when the utility grid returns to normal, the microgrid automatically resynchronizes and reconnects itself to the grid, in an equally seamless fashion.

In the U.S., this new microgrid technology was first proposed by Robert Lasseter, a professor emeritus of electrical and computer engineering at the University of Wisconsin-Madison. Under the Consortium for Electric Reliability Technology Solutions (CERTS), funded by the Department of Energy and the California Energy Commission, Lasseter developed the electronics control technology that enables large businesses to switch back and forth from generating their own power to pulling power from the main grid (see sidebar).

CERTS microgrid R&D program

The CERTS Microgrid Test Bed is located at American Electric Power (AEP) Co.’s Walnut Test Facility in Groveport, Ohio, and is designed to demonstrate the CERTS microgrid concept, which is an advanced approach for enabling the integration of multiple distributed energy resources (DER) into an electric utility’s distribution system/power grid (Figure 5).

 

5. Testing different grid options. The Microgrid Test Bed at American Electric Power’s Ohio test site is used to develop advanced distributed generation technologies. Courtesy: Consortium for Electric Reliability Technology Solutions and American Electric Power

AEP shares a common vision with CERTS of the potential for advanced technologies within the electric power industry and, in particular, the increasing value of DER. The utility has developed a unique research and testing facility for generation, energy storage, and power quality devices.

The CERTS microgrid concept envisions enabling the integration of (in principle) an unlimited quantity of DER (including distributed generation and energy storage) into the electric utility grid. The original concept was driven by two fundamental principles:

• A systems perspective was necessary to enable customers, utilities, and society to capture the full benefits of integrating DER into an energy system.
• The business case for accelerating adoption of these advanced concepts will be driven, primarily, by lowering the first cost and enhancing the value of microgrids.

Each innovation was created specifically to lower the cost and improve the reliability of small-scale distributed generation (DG) systems (typically defined as installed systems with capacities ranging from less than 100 kW to 1,000 kW). The goal was to increase and accelerate realization of the many benefits offered by small-scale DG, such as the ability to supply waste heat where it’s needed or to provide a higher level of power quality to some, but not all, loads within a facility.

From a utility perspective, the microgrid concept is attractive because it recognizes the reality that the nation’s distribution system is extensive, aging, and will change over time.

CERTS says that potential applications of its microgrid include industrial parks, commercial and institutional campuses (locations that require uninterrupted power supplies and high power quality), combined heat and power systems, greenfield communities, and remote applications. In short, wherever economic and DG colocation considerations indicate the need for multiple DG units within a (or among) site, the CERTS microgrid offers the potential for a more reliable, flexible, and lower-cost solution compared to traditional engineering approaches for integrating DG.

For more information about the CERTS Microgrid Test Bed project, go to http://certs.aeptechlab.com.

“What is unique about the CERTS microgrid is that by incorporating peer-to-peer and plug-and-play concepts for each component within the microgrid, it can provide this technically challenging functionality without extensive and expensive custom engineering,” said Lasserter in a statement released by University of Wisconsin. “In addition, the design of the CERTS microgrid also provides the high system reliability and great flexibility in the placement of distributed generation within the microgrid” (Figure 2).

2. Connected, but independent. Thanks to such elements as a static switch and power flow controller, the CERTS microgrid concept should enable high penetration of distributed generation systems without requiring redesign or reengineering of the utility’s distribution system. Source: Lawrence Berkeley National Lab

Currently, research, development, demonstration, and deployment of microgrids are being conducted on at least three continents. For example, in the U.S., American Electric Power and CERTS signed a memorandum of understanding in 2006 to work cooperatively in promoting the microgrid concept. CERTS includes four DOE national labs (Lawrence Berkeley National Laboratory, Sandia National Laboratory, Oak Ridge National Laboratory, and Pacific Northwest National Laboratory); the Power Systems Engineering Research Center; and the Electric Power Group. Additionally, research is under way at several other U.S. institutions and at many others overseas, including the Advanced Power and Energy Program at the University of California- Irvine, GE Global Research, U.T.C., NEDO in Japan, and various groups funded by the European Commission. New programs are also starting up in Singapore and Korea.