Microgrid balances variable load, NOx emissions by ‘sculpting’ supply

Southern California Edison Co’s (SCE) Pebbly Beach Generating Station serving Catalina Island (22 miles off the coastal area south of Los Angeles) was a “microgrid” decades before the term became a fixture of power-industry lexicon. That is, it is an isolated small grid serving an island community, distinguished from the more modern definition of a microgrid which interfaces in real time with a larger interconnected grid. To keep up on micro-grids and how they interface with traditional large generation assets, subscribe to our new information service, GRiD Today, at no cost.

As the South Coast Air Quality Management District (SCAQMD) progressively tightened regulations, complying with NOx emissions levels, while meeting the extreme variable loads of the island’s largely tourism-based economy, became at odds with operating a group of diesel engine/generator sets dating back to the 1920s.

With the addition of a set of propane-fueled micro-turbine/generators (Fig 1) totaling almost 1.5 MW and a 1-MW battery facility (Fig 2), SCE is now able to “sculpt” supply to meet the twin objectives of an onerous one-hour rolling average NOx constraint across the diesel units and a load demand profile which can fluctuate from a 6-MW peak to under 2 MW as the island’s population shrinks and swells from one to three times the permanent population of approximately 4000 residents.

Pebbly Beach 1, 2a

In test work prior to the full installation, SCE proved that the Capstone Turbine Corp C65 engine favored for deployment could be successfully and reliably adapted to fire liquid propane gas (LPG).

Pebbly Beach is also a microcosm of the changing objectives of today’s powerplants. If reliability has always been jobs one, two, and three, flexibility is quickly become jobs four, five, and six. The micro-turbines and the battery add new dimensions of responsiveness, especially now that the entire facility is being automated with an Ovation™ control system from Emerson Process Management Power and Water Solutions.

SCE added diesel sets as population—and popularity—of the island grew. The current fleet stretches back to the 1950s and consists of six units. The first installed is 1 MW, then came 1.125-, 1.4-, 1.5-, and 1.575-MW units, and finally a 2.8-MW engine/generator installed in 1995. Diesel fuel is stored in two 125,000-gal tanks, liquid propane gas (LPG) in four 80,000-gal tanks.

As a side note, LPG is vaporized and used directly in the micro-turbines; plus, SCE also supplies propane to island customers via an underground distribution system. LPG vaporized for customers is “cut” with air to better match the characteristics of natural gas.

The engines could ramp comfortably between 5% and 100% of their maximum continuous ratings (MCR), allowing a great degree of flexibility for meeting demand. Then selective catalytic reduction (SCR) had to be added in 2004. The SCR units require a minimum exhaust temperature, which constrained output above 80% of MCR. This made it particularly difficult to control the frequency and voltage of an island grid and still meet NOx emissions limits.

Meanwhile, a little serendipity never hurts. SCAQMD had acquired and warehoused C65 micro-turbine units manufactured by Capstone and the agency was looking for places to use them. They suggested that SCE take 10 to offset emissions from the diesels when they had to cycle.

SCE’s Ron Hite, facility manager, said they were willing to consider the offer but the reliability of the unit first had to be proven on LPG.

SCE agreed to take one unit and run it for two years. One concern: LPG burns significantly hotter than natural gas, which could shorten life from higher wear rates. Independent analysis, conducted by Regatta Solutions confirmed at the end of the long-term test program, that the wear rate was no different than when firing natural gas.

“One component in the combustion chamber burned up,” recalled Hite, “but the supplier was able to successfully redesign and replace the part.”

SCE went back to SCAQMD and asked for as many units as the agency could spare. Ten would offset emissions, 20 would significantly reduce NOx emissions.

So, SCE got twenty-three 65-kW units.

Unlike the diesels, they function as on/off devices. The diesels always stay within 80% to 100% of full load and the C65s meet demand or grid fluctuations in 65-kW increments. Think of a bicycle drive chain (well, most modern day bikes). The diesels are the big sprockets (usually two or three) in front, the C65s are the smaller ones in the back (usually six or seven).

“We run the heck out of them,” Hite said, referring to the micro-turbines. The only unusual maintenance required is associated with the seaside location. “They are literally sitting in the salt spray of the ocean,” Hite stated. “Electronic circuit boards for the controls can collect salt,” he said, “and short out and subsequently we learned they have to be coated with a protective gel.” Other than that, it’s a pretty temperate climate so there’s little concern about output deviations resulting from ambient-temperature changes.

There is a peculiar characteristic of these precise little units. They are lightweight and sensitive to even the slightest excursion in grid voltage and frequency. Island demand fluctuates daily from 2 to 6 MW and there’s a 300-kW load from a rock quarry and other industrial customers on the island. Thus, the C65s can easily trip offline and that’s where the battery comes in.

Battery as fine chisel. The sodium-sulfur chemistry (NaS) battery supplied by Japanese firm NGK Insulators Ltd together with a 1-MW Purewave™ power conditioning system (PCS, supplied by S&C Electric Co), was installed in 2011 and offers another dimension for grid management.

A battery can function as load or supply. Kilowatts can be injected into the island grid at even smaller increments than the C65s for shaving peaks. The battery can discharge its full load within milliseconds. At other times, the battery can reverse and take load off the grid. For example, noted Hite, late at night the island’s demand can drop to 2 MW, the ragged end of the largest diesel’s operating range. When this engine drops below 1.95 MW, the urea pump for the SCR will cut out.

The battery allows for almost instantaneous incremental moves to push or pull power in these “dicey” operating moments—especially considering the NOx-emissions rolling average. The same sequence occurs with the other diesels when they drop below 80% of their respective output curves. The battery adds emissions-free kilowatts during such disruptions and allows the engines to always operate within the optimum NOx emission profile, even when charging the battery.

Pebbly Beach did get caught in the downdraft of NGK’s catastrophic fire and recall of all its operating modules in 2011. “We had maybe the fourth or fifth NaS installation in the US,” said Hite. “We were instructed by NGK to shut down the unit and keep it down until NGK could investigate the root cause.”

They fixed the problem, re-engineered the system, and replaced all the modules at no cost. “The system has performed extremely well since,” Hite stressed. Excellent performance is supported by 24/7/365 monitoring and troubleshooting provided remotely by NGK.

One characteristic of the NaS type battery is its high power density and long cycle time. At Pebbly Beach, the battery can deliver 1 MW for six hours, making it 6 MWh capacity.

Better control coming. Today the diesels are completely manually controlled. For that matter, so is the frequency of the grid, consisting of three 12-kV circuits. Historically, SCE has used the time error correction (TEC) technique dating back to the 1920s using a synchronous clock that runs off the island’s AC power. Several times per shift, the clock is re-synchronized when the reading is compared to a far more accurate time reading from another clock tied to satellites.

The new Ovation system will integrate all the assets and provide full automation and information for grid management. Construction is expected to be completed this month; four months has been budgeted for commissioning.

In considering best practices and lessons learned, Hite mentioned that microgrid owner/operators need to carefully consider inertia. Diesels are “big iron” and provide excellent rotating mass for frequency control. The microturbines are not so much, although Hite added that Capstone now sells a 1-MW machine. The other suggestion from Hite is to “thoroughly evaluate storage technologies for your specific application.” Doing so may help avoid the knee-jerk decision to go with lithium ion, the option which lately has been sucking the oxygen out of the grid-scale storage sector.

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