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 (Fig 1), distinguished from the more modern definition of a microgrid which interfaces in real time with a larger interconnected grid.

1. SCE’s Pebbly Beach Generating Station serving Catalina Island consists of diesel generating sets dating back to the 1920s (gray roof in middle of photo), micro-turbines and battery storage unit (facing blue diesel tanks at far right), and LPG storage tanks (four white tanks at left of oil tanks)

1. SCE’s Pebbly Beach Generating Station serving Catalina Island consists of diesel generating sets dating back to the 1920s (gray roof in middle of photo), micro-turbines and battery storage unit (facing blue diesel tanks at far right), and LPG storage tanks (four white tanks at left of oil tanks)

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 totaling almost 1.5 MW and a 1-MW battery facility (Figs 2, 3), 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.

2. Twenty-three 65-kW micro-turbine/generators (at left in enclosed area at center of photo) are located adjacent to the 1-MW NaS battery and power conditioning system (right).

2. Twenty-three 65-kW micro-turbine/generators (at left in enclosed area at center of photo) are located adjacent to the 1-MW NaS battery and power conditioning system (right).

In test work prior to the full installation, SCE proved that the Capstone Turbine Corp (Chatsworth, Calif) 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 (Pittsburgh).

3. Close-up of battery and PCS. They allow SCE to optimize diesel operation to minimize emissions and more precisely respond to wide fluctuations in demand

3. Close-up of battery and PCS. They allow SCE to optimize diesel operation to minimize emissions and more precisely respond to wide fluctuations in demand

History

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 (Fig 4). 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.

4. Diesel/generators, spanning unit sizes from 1 to 2.8 MW, were subject to ramp restrictions after SCR was added in 2004 for NOx control

4. Diesel/generators, spanning unit sizes from 1 to 2.8 MW, were subject to ramp restrictions after SCR was added in 2004 for NOx control

Calling all units

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, San Juan Capistrano, Calif, 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.”

I’ll take twenty. 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 (see sidebar) 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, Nagoya, Japan, together with a 1-MW Purewave™ power conditioning system (PCS, supplied by S&C Electric Co, Chicago), 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 (sidebar below) 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.

Tech specs on the micro-turbine, storage units

A. C65 micro-turbine has one moving part—the rotor. The design essentially

A. C65 micro-turbine has one moving part–the rotor. The design essentially mimics a single-stage jet engine

Micro-turbine. Major components of the Capstone C65 (Fig A) are a compressor, recuperator (which recovers exhaust heat recycled to the turbine inlet), combustor, turbine, generator, and solid-state power conditioning system. The last converts the high-frequency AC engine output (spinning at close to 100,000 rpm) to standard 3-phase, 60-Hz AC for the grid.

As the illustration shows, the C65 has a single-stage turbine wheel with three air-lubricated bearings and an electronic gearbox. The shaft is the only moving part. Specifications for the low-NOx turbine used in California include the following:

  • Net power output, 65 kW.
  • Net efficiency (LHV), 12,200 Btu/kWh.
  • Net power quoted is available up to 70F ambient temperature. Output drops linearly to below 50 kW between 70F and 120F.
  • NOx emissions are less than 4 ppm at 15% O2 (volume basis).
  • Firing temperature, 1750F.
  • Weight, 2400 lb (grid-connected version).
  • Height, 103 in.
  • Width, 30 in.
  • Depth, 87 in.
  • Recuperator materials of construction, stainless steel.
  • Shaft (turbine, compressor, generator) speed, 96,000 rpm.

Battery. The roots of the NaS battery chemistry date back to the late 1960s when it was developed by Ford Motor Co. The version used in today’s grid-scale systems has been supplied by NGK (although other sodium-based chemistries are available). Essentially, electrons flow between a sulfur cathode and a molten sodium anode through a beta alumina solid electrolyte (Fig B).

B. A single cell the basic element of an NGK Insulator battery, is at left; a 50-kW module comprised of these cells is at right

B. A single cell, the basic element of an NGK Insulator battery, is at left; a 50-kW module comprised of these cells is at right

The general advantages of this chemistry are the high energy density (one of the highest of commercially available systems), low-cost materials, and relatively high cycle efficiency (Fig C). NGK claims approximately 85% DC-to-DC efficiency and 75% AC-to-AC efficiency. Performance is insensitive to ambient temperature in the range of -4F to 104F; 300 charge/discharge cycles per year at rated capacity should be expected, or 4500 over the life of the unit.

C. Individual batteries in the NGK NaS storage system are arranged in modules of 50 kW each (Fig B); modules are slotted into an enclosure with controller boxes at the bottom. Diagram is of a 1-MW, 6-MWh system

C. Individual batteries in the NGK NaS storage system are arranged in modules of 50 kW each (Fig B); modules are slotted into an enclosure with controller boxes at the bottom. Diagram is of a 1-MW, 6-MWh system

However, the sacrifices include operating temperatures typically much higher than competing systems to keep the sodium in a molten state. And as most former high-school chemistry enthusiasts recall, sodium reacts spontaneously and violently when exposed to water so the cell must be protected from any moisture intrusion.

Like other battery chemistries, unwanted side products of the chemical reactions can build up and cause problems with corrosion and deterioration in efficiency. Others have reported that NGK solved the root cause of the catastrophic fire by adding fuses between cells, insulation boards between blocks in the modules, and anti-fire boards above and below the battery modules.

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 (Fig 5). 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.

5. Substation delivers power to the island through three 12-kV circuits

5. Substation delivers power to the island through three 12-kV circuits

The new Ovation system will integrate all the assets and provide full automation and information for grid management. Construction is expected to be completed in April (2016), with four months slotted 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. GRiD

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