Harbec Plastics, a rapid-process manufacturer of custom plastic parts for the medical, automotive, and consumer goods industries, had experienced a number of power disruptions at its facility near Rochester, N.Y. This facility uses a sophisticated production process using 30 computer-numerical-controlled (CNC) spindles and numerous computer-aided design (CAD) stations. When the power outages grew in frequency and duration, it was time for action.
Even during a momentary grid outage, the CNC machines at the Harbec plant lose the fine-tuned sequences they normally perform. In the best case, a machine merely turns off, requiring six to eight hours to reprogram and restart. In the worst case, a CNC machine's cutter damages the produced part, resulting in wasted material and days of lost work — now multiply these scenarios by the 30 CNC machines conducting these processes.
In addition, the plant loses at least one hour of production time for each of the more traditional production machines operating in another part of the facility. Corrupted or damaged files on the CAD stations also result in costly losses.
Harbec's president Bob Bechtold wanted a solution that could deliver independence from the grid and provide a continuous AC power source to all facility loads, including the starting and stopping of motors as large as 60 hp. Other must-haves included availability, economic feasibility, and the ability to generate 3-phase, 60 Hz power with low maintenance requirements, high fuel economy, low emissions, scalability, and some level of redundancy. After considering standby gensets, microturbines, large turbines, fuel cells, and photovoltaics, the company chose a microturbine solution.
Microturbines are compact generators of electricity and heat. They operate on the same principle as a jet engine but can use a variety of commercially available fuels, such as natural gas, diesel, and propane. The units, which are approximately the size of a refrigerator (see Photo), have the ability to operate in grid-connected, stand-alone, and dual modes.
Grid-connected mode allows the unit to operate parallel to the grid, providing base loading and peak shaving. Stand-alone mode allows the units to operate completely isolated from the grid. In dual mode, the units can switch between the two modes automatically.
Microturbines consist of a turbo-generator (see Fig. 1, on page 84), digital power electronics, and a fuel system. The turbogenerator includes a mechanical combustor system that turns a single moving part: a shaft with a turbine rotor at one end, a permanent magnet generator at the other, and an air compressor impeller near the center.
The compressor impeller draws air into the inlet, which increases the pressure of the air and injects it into the recuperator. In the recuperator, which is simply an air-to-air heat exchanger, the system exhaust heats the air. Preheating the air increases efficiency by lessening the amount of fuel needed. The preheated air mixes with fuel and combusts. As the combusted mixture expands, it causes the turbine and shaft to rotate, generating electricity.
Harbec's microturbine system is air-cooled, and the single rotating assembly rides on patented air-foil bearings, minimizing component wear and maintenance and eliminating liquid lubricants, coolants, and related subsystems.
The power electronics also are air-cooled and consist of an inverter based on an isolated gate bipolar transistor (IGBT) and microelectronics based on digital signal processing (DSP).
Other attributes of microturbines include the following:
Continuous Operation. Although often used in load-following, peak-shaving, and standby applications, microturbines are designed to run continuously at full load.
Minimal Maintenance. For units operating on high-pressure gaseous fuels, cleaning or changing the intake air filter and fuel filter is recommended after 8,000 hr of continuous operation (or about one year). At 16,000 hr, cleaning or replacing the temperature sensor and igniter (spark plug) is recommended. Most microturbines do not require an overhaul until 40,000 hr of continuous operation.
Low Emissions. Harbec's digital-power-controlled combustion system (at full load) is factory rated at less than 9 ppm of nitrogen oxides and unburned hydrocarbons on gaseous fuels.
Scalable Power Systems. Built-in software supports paralleling of multiple units to increase capacity or add redundancy.
Cogeneration (combined heat and power). Exhaust heat may be used in a variety of applications. The most prominent uses include space heating, drying applications, and air conditioning via absorption chilling. Use of exhaust heat increases total system efficiency to 70% or more.
System In Action
The Harbec installation consists of 24, 30kW microturbines and five cogeneration units. This system provides 480VAC, 3-phase, 60 Hz power (see Fig. 2, on page 84).
The microturbines are connected in parallel to achieve the required total system capacity and provide a level of redundancy. Although paralleling switchgear is never required, the units share current evenly and each may be disconnected for service without interruption to other units.
At the time of this writing, Harbec's system provides primary power isolated from the utility grid. The grid is used only as backup to the microturbine power source.
Currently, transferring from primary microturbine power to utility power is a manual operation, but the system offers a dual-mode controller option that allows automatic transfers from microturbine to utility power and back again.
The five cogeneration units capture exhaust heat to generate 180° F water. In the winter, some of the hot water is routed to a radiant floor heating system built into the concrete slab of Harbec's warehouse area. The remaining hot water is used for forced-air space heating.
In the summer, the heat fires a 200-ton absorption chiller, which generates chilled water. This water is piped to a fan-coil system that cools Harbec's warehouse and production areas. Thus, air conditioning is made available without adding appreciable electrical load to the facility.
The use of the waste heat provided an added economic benefit to Harbec. The company secured a contract to purchase gas at a delivered cost of $6.85/MCF. At this rate, the value of the hot water recovered from the microturbines equates to $0.03/kWh produced. Power is generated for approximately $0.074/kWh (net of the recaptured heat), compared to the grid price of approximately $0.10/kWh.
This differential will pay for the capital cost of the system. More important, Harbec achieves certainty in its power source during a time of growing uncertainty about the cost and reliability of grid-supplied power.
When momentary disruptions result in lost product, downtime, and missed deadlines, microturbines can provide an economically attractive alternative. They are fuel-flexible — capable of switching from natural gas to propane with a few button presses.
Down the road, Harbec could put a backup source of fuel into place if their needs eventually require it. Until then, they have a continuous source of reliable power that produces near-zero emissions, requires little maintenance and offers energy-efficient heat recovery.
Mike Lasky is the power quality business development manager for Capstone Turbine Corp. He was formerly international marketing product manager at Liebert Corp. after serving as domestic marketing product manager at MGE UPS Systems. Lasky has a BSEE from CalPoly Pomona and an MBA from Pepperdine University.