Technology Could Invert the DG World

Issue 3 and Volume 111.

Steve Blankinship, Section Editor

A new inverter technology will soon enable combined heat and power (CHP) installations as small as 100 kW to perform as fully flexible, grid-paralleled backup power providers. The new flexibility and compatibility could have profound implications for distributed generation (DG) by expanding the capabilities of on-site engines, microturbines and fuel cells and even increase the flexibility of solar and wind capacity.

With the support of the California Energy Commission, Sempra Utilities and the U.S. Department of Energy (DOE), TECOGEN, Inc. has developed an inverter-based CHP module using the engine and controls from its current 75 kW magnetic induction genset. The inverter, coupled with a permanent magnet generator, allows variable speed operation for a higher baseload setting, a power boost feature for demand response and greater efficiency than was previously possible when operating at partial loads.

Since 1981, TECOGEN has been producing modular 60 to 75 kW cogeneration units. The gensets consist of 7.4 liter GM V-8 engines modified more than a decade ago (in conjunction with the Gas Technologies Institute) to run on natural gas, coupled to small induction magnetic DC generators. The modular systems can provide supplemental power and thermal energy for space and water heating and for absorption chillers. More than 1,000 are deployed throughout the Northeast, the mid-Atlantic and in California at small public, commercial and industrial facilities including hotels, schools and hospitals.

But engines and microturbines can’t change output quickly enough to match instantaneous load changes and negotiate seamless power transfer in the event of grid failure. To do so requires adding large battery systems and supervisory control equipment to maintain a constant, steady balance of supply and demand. The new inverter technology applied to a fast responding enging changes that.

Robert Panora, president of TECOGEN, says the new inverter-based device allows the engine and generator to push the electricity through a converter that turns the power into DC and then back to AC at 60 hertz. That done, the engine can run at any speed – down to 1,000 rpm for instance – then add voltage and frequency and convert it back to 60 hertz. “You could run the unit at high speed during the day and then at night, when you need less power, turn down the engine speed and it maintains high efficiency throughout the rpm range,” he said. Conversely, it lets a high speed engine shave peak by “sprinting” at 2,500 or 3,000 rpm during periods when electricity is expensive or when time-of-day rates are in effect.

Click here to enlarge image

The California Energy Commission (CEC) and Sempra supported development of the 100 kW inverter-based CHP machine. The inverter control that allows multiple machines to run isolated off the same circuit then transition to parallel mode, came from the University of Wisconsin through its participation in the Consortium for Electric Reliability Technology Solutions microgrid project funded by CEC and DOE.

TECOGEN was asked to supply prototypes of its inverter-based Premium Power Modeule as a platform for the project. Three units are undergoing full-scale testing at AEP’s Dolan Testing Laboratory in Ohio, where they are running in a mircogrid mode subjected to complex patterns of power failure and load variations. The new inversion technology lets engines run at variable speeds and in parallel with the grid without any supervisory control apparatus. Adding complex paralleling equipment has been something HVAC engineers have not generally wanted to get involved with. Now, they can install multiple units in a hospital or hotel. If the lights go out, all the engine sets will run together on the same circuit.

“This makes it plug and play,” says Panora. “You just have to connect four wires.” An engineer who wants to install this in a facility just has to make sure that whatever he or she connects to the gensets doesn’t exceed the units’ installed capacity and that the utility is isolated. Everything is paralleled together and everything matches power without any supervisory control. “That’s a breakthrough,” he says.

In the past, getting distributed generation permitted for grid interconnection has been difficult, especially for small projects. But recent adoption of DG interconnection standards UL 1547 and 1741 have made interconnection easier. The new inversion technology will allow the deployment of one or several 750 kW gensets on-site that can be certified for grid parallel applications.

Click here to enlarge image

With the ability to confer certification on such small projects, Panora believes a developer could send an application to a utility and-at least theoretically-be good to go in 10 days. “That puts small engines like we use into the fast lane with the solar guys and the fuel cell guys,” he says.

Testing at the AEP Dolan facility indicates the product is now ready to go to market. Six units (totaling 600 kW) have been ordered by a technical school in upstate New York. The school campus will use the system to heat water in the winter and may use the heat to run an absorption chiller in the summer. An additional unit is being installed at a tennis club in California.

The technology might prove an especially good fit for small commercial and industrial facilities that want to take part in demand response (DR) programs, which are growing rapidly in the Northeast and California. “CHP is beginning to be recognized for its value in easing capacity and distribution constraints through DR programs such as those offered in parts of New York,” says Panora. In Consoliated Edison’s service territory, CHP owners can receive up to $100/kW annually. With the 25 percent peak power “sprint” feature of Tecogen’s 100 kW Premium Power Module, the payment would rise by an additional $2,500/year, for a total of $12,500.