Microturbines—gas- or liquid-fuel-fired turbine-generator units with an electrical output between 30 and 500 kilowatts—are being used increasingly for 24/7 onsite power generation. And many areas of the world will very likely see a major increase in microturbine-based onsite power generation over the next five to 10 years.

A cutaway of a Capstone microturbine. The company's 30 and 60-kilowatt units have just one moving part—a shaft that turns at 96,000 rpm.

Since making their commercial debut a mere five years ago, microturbines have been installed with considerable success in office and apartment buildings, hotels and motels, supermarkets, schools and colleges, office and industrial parks, small industries, and numerous other facilities both in the US and abroad. They provide not only electricity, but the thermal energy to provide for all heating and cooling needs.

The reasons for the growth in microturbine installations lie in the intrinsic advantages of this technology, including:

• Low to moderate initial capital cost
• Fuel flexibility, allowing them to burn either gaseous (natural gas, propane, biogases, oil-field flared gas) or liquid fuels (diesel, kerosene)
• Heat released from burning the fuel not only generating electricity, but also providing all heating and cooling needs for a facility through cogeneration, combined heat and power (CHP), and combined cooling, heat and power (CCHP)
• Extremely low air emissions for NOx, CO, and SOx
• The ability for a facility to continue operating even during a regional power brownout or blackout, offering greater energy reliability

The Capstone Turbine Interview

To get the latest information on the proper application, installation, operation, and maintenance of microturbines, DISTRIBUTED ENERGY contacted the world's pioneer and lead player in microturbines, Capstone Turbine Corporation of Chatsworth, CA (www.capstone.com).

In two sessions we interviewed Keith Field, Capstone's director of corporate communications. We quickly discovered that Field has a masterful command of the subject. Prior to joining Capstone five years ago, he had held marketing and public relations positions with firms in the consumer electronics and health fields.

DE: Perhaps a good place to start is with the basics. What is a microturbine?

Field: Think of it as a small jet engine integrated with an electric generator. The engine itself is about the size of a beer keg. The most popular models have just one moving part—a shaft with a turbine wheel on one end, a permanent magnet generator on the other end, and an air compressor wheel in the middle. This assembly rotates at up to 96,000 rpm. At that speed, traditional oil-lubricated bearings are severely challenged. Accordingly, the most popular microturbine engines use air bearings to float the shaft.

Not only is the turbine turning at a high rpm, so is the generator. The generator in turn produces a high-frequency electrical output, which is then converted by a power-electronics unit to grid-compatible 400- to 480-volt-alternating-current, 10- to 60-hertz, 3-phase power.

DE: My understanding is that the gas turbines that are used in distributed energy situations can vary in size from 30 kilowatts all the way up to 20 megawatts. Correct?

Field: The smallest-capacity gas-powered turbine commercially available is the Capstone 30-kilowatt C30. The only upper limit for onsite distributed generation would be the needs of the facility. There are some on-site gas-turbine installations that produce well in excess of 20 megawatts.

DE: I also understand that gas-turbine systems between 30 kilowatts and 500 kilowatts are often referred to as microturbines; and those from 500 kilowatts up to 20 megawatts are simply called industrial turbines. Correct?

Field: There's no particular set limit for the microturbine term. Currently, the highest-capacity microturbine on the market is an Ingersoll-Rand model rated at 250 kilowatts. DTE has been working for a few years now on developing a 400-kilowatt model that they call a "miniturbine." That seems to be as good a nomenclature cross point as any.

DE: How long have microturbines been commercially available? When did they first come onto the market and why? Is their use growing and, if so, why?

Field: Capstone Turbine was the first to introduce a commercially available microturbine—in December 1998. It sold its first three 30-kilowatt microturbines that month, and has since shipped over 3,000 more worldwide.

The dawn of these microturbines has made small-scale, daily, or continuous distributed generation [DG] possible. Such compact units could supplement the power and heating and cooling needs of a facility more efficiently, more cost-effectively, and with fewer emissions than their traditional electric utility grid or the onsite boiler approaches.

Before then, continuous-rated truck-engine gensets (e.g., an onsite generator driven by a diesel engine) weren't commonly used in the few-hundred-kilowatts-or-less size range due to problems with carcinogenic soot and other emissions, noise, and ever-present oil leakage and other hazmat problems.

The lengthy economic recession that predated and then followed September 11, 2001, hit capital-equipment manufacturers hard. Businesses tended to invest in equipment that was core to their business—e.g., a faster widget stamper—rather than DG equipment that would reduce external costs. That is, they channeled scarce investment dollars into equipment that would make money—rather than into equipment that would save money. Those few companies that did cross that DG barrier were averse to doing so with less-proven equipment like microturbines.

That's the bad news. The good news is that we at Capstone Turbine are now experiencing a huge turnaround in orders. Our CEO, John Tucker, has said that our microturbine sales revenue will see at least a 100% rise in our current fiscal year. And we are likely to beat that goal.

DE: Why is this turnaround occurring in the growth of microturbines? Anything to do with soaring energy costs in California and with power blackouts in other regions of the country?

Field: In California, the energy crisis came to a head in the winter of 2000–2001. At that time, there were somber predictions that there would be rolling blackouts in the summer of 2001. But that never happened. And that fact coupled with a recession had an adverse effect on microturbine sales.

Instead of blackouts, what did emerge in California was an energy-cost crisis—power rates up 50% in some areas. This situation, coupled with an improving economy, has over the past several months made business managers more willing to invest in distributed energy. With these microturbines, a company would be looking at an investment payback period of two to four years—given current gas and electricity rates.

DE: What sort of future do you see for onsite power development over the next five years?

Field: Coming out of the recession beginning in early 2004, there has been much higher interest in distributed energy than in the recession-plagued last two or three years.

Many facilities managers are looking for a way to cut operating costs. Many of them had long since adopted measures to cut energy consumption—energy-saving lighting and appliances, energy conservation, etc. Few managers, though, have yet to explore the great energy savings to be realized from installing onsite power systems that provide both electricity and heat. Yet some are beginning to look at the onsite option.

DE: Where are microturbines used? The most common applications?

Field: Here are the most common microturbine applications:

• Office buildings: The microturbine is designed to operate 24/7 to meet all heating and cooling needs of the building and some of the electrical needs.
• Educational facilities: college campuses, high schools, etc. In some cases, the microturbine installation provides for most of the heating and cooling needs of the site. In other cases, its thermal output is used only to heat campus swimming pools or to preheat water for campus steam boilers. A cluster of microturbines would usually not be powerful enough to heat a large campus.
• Hotels and motels: At a hotel, the peak time is usually morning. In such peak periods, much water would be heated in the building's traditional boilers. During afternoons and evenings, on the other hand, when demand is far below morning peaks, the traditional boilers would be turned down and the microturbine installation would be providing all heating needs.

Ingersoll-Rand's MT250 synchronous microturbine has an integrated waste-heat recovery system. The 250-kilowatt unit is designed for indoor use.

DE: Is there a trend toward using microturbines for onsite power generation?

Field: Yes. There is a very definitely a trend toward installing microturbine systems onsite—not only for generating electric power, but also for meeting site heating and cooling needs. Such microturbine configurations are called combined heat and power, or combined cooling, heat,and power (cogeneration) systems.

The core idea is this: When burning a fuel in a microturbine unit, don't just use the resulting heated gases to spin a turbine and generate electricity. There is still a huge amount of thermal energy in the turbine exhaust. Don't waste that valuable energy to the atmosphere—which is what they do in most central power plants (because there is no use for the heat in remote areas). Instead, use a heat exchanger to capture much of that thermal energy and use it to meet all the heating and cooling needs of the site.

When a microturbine unit is arranged in CHP or CCHP mode, heat from the turbine stack is captured and used to meet some or all of the heating and cooling needs of the facility. This makes for much more efficient fuel use. Instead of just using 35% of the thermal energy released during fuel combustion (as with a traditional central power plant), with CHP and CCHP one would be using 65% or more of the fuel's thermal energy. This realization is a major reason the federal Department of Energy has been strongly encouraging the advance of onsite power generation with CHP and CCHP.

Yet, there are a few applications where installing microturbines strictly for generating electricity makes sense—e.g., in oil fields or at landfill sites, where free flare gases are used as turbine fuels. In such installations, there is usually little heat demand at the site; so it makes little sense to capture the waste heat and use it. On the other hand, if all the electricity generated can't be used onsite, it could be readily exported by connecting to the utility power grid.

It is important to realize that microturbines have only been on the scene now for five years. The very fact that they are now available in itself constitutes a powerful force pushing for more onsite power generation.

DE: What type of fuel do you usually burn in these compact gas-turbine units?

Field: At the present time, the Capstone 60-kilowatt microturbine is compatible only with natural gas. But the older 30-kilowatt model is very versatile, being able to burn several gaseous or liquid fuels—natural gas, propane, biogases, diesel, and kerosene.

We are working on making our 60-kilowatt unit more fuel-versatile. In fact, Capstone will be exhibiting a biogas-compatible version at some industry trade shows in fall 2004.

DE: Is fuel versatility really all that important? As long as a microturbine can burn natural gas, shouldn't that be sufficient?

Field: Certainly in North America, where the majority of microturbines are installed, natural gas is the most common fuel source. But fuel versatility enables greater market penetration. If you have a model that can use currently flared waste gases, that opens up the possibility of using microturbines at landfills, sewage plants, oilfields, and livestock facilities. And in all but one of those cases, the gas is renewable.

And if you have a model that will run on kerosene, that would open up the Japanese market, where kerosene is currently the most common and best economic choice.

DE: Could fuel versatility be important for the more mainstream onsite power applications as well—apartment complexes, office parks, etc.? Wouldn't that give them more options and more leverage in dealing with fuel suppliers?

Field: Some facility managers are showing much interest in equipment that can handle more than one fuel. Now, as I mentioned, the Capstone 30-kilowatt unit is fuel-flexible; it can handle several different fuels. But there is one type of injector for gaseous fuels, another type for liquid fuels. But switching that unit from gas to liquid or liquid to gas could be costly.

The only kind of switching you can do is this: if the unit has gas injectors, then you can accommodate these fuels: natural gas, propane, methane (e.g., from landfills); and if it has liquid injectors, you can accommodate either diesel or kerosene.

Capstone's microturbines are packaged in an enclosure not much bigger than a refrigerator.

DE: Who are the main manufacturers of microturbine systems, the major players?

Field: Capstone Turbine is the only USA-headquartered microturbine company. But globally oriented Ingersoll-Rand is making headway with a 70-kilowatt model, which it made commercially available shortly after Honeywell exited the market in 2000.

Honeywell/Allied Signal had marketed a 75-kilowatt microturbine, and GE has been developing a 175-kilowatt microturbine, but their interest seems more R&D; they've said they have no commercialization plans. On the other hand, Capstone now has a 200-kilowatt pre-production model in beta testing and will announce commercial launch plans at the end of 2004.

There were a couple of European companies—Turbec and Bowman—that had been marketing microturbines, but those companies seem no longer active in the business in the US. In Japan, Ebara has had limited success marketing Elliott models, and Toyota is working on fielding a 50-kilowatt model. But both face tough competition from Capstone models—about 500 of which are deployed in Japan—because their products exhibit much higher emissions than Capstone models.

Incidentally, Ingersoll-Rand has also recently begun to market a 250-kilowatt microturbine. Yet, unlike Capstone, they incorporate a gearbox between the microturbine and the electric generator. This gearbox takes the high rpm output of the turbine shaft and gears it down greatly, producing an output shaft (which is coupled to the electric-generator shaft) at much lower rpm. (See the September/October 2004 issue of DISTRIBUTED ENERGY, "Maintaining Gas Turbine Systems: What You Need to Know," www.distributedenergy.com/de_0409_maintaining.html.)

Capstone's approach is simpler, with just one moving part—the rotating microturbine shaft. There are no gearboxes; no pumps for pumping oil lubricant to a gearbox or to oil-lubricated bearings; no radiators for cooling lubricating oils (we use air bearings); no lubricants or other hazardous materials. In short, there is little in our microturbine system that can break down. There are no mechanical subsystems.

DE: Interesting. Could you please expand further, describing more fully your microturbine system—its major components?

Field: OK. Our microturbine engine has only one moving part, basically a shaft. At one end of that shaft is a turbine wheel; at the opposite end of the shaft is a permanent magnet electric generator; and positioned at the midpoint of that shaft is an air impeller wheel (i.e., an air compressor) for drawing in ambient air, compressing it, then pumping it into the combustor. Fuel is then injected into the combustor, where it then mixes with the compressed air. Combustion occurs and the resulting gases expand and rush out through the turbine, spinning it to a very high rpm.

This whole microturbine system is packaged in an enclosure not much bigger than a refrigerator—about 7 feet tall, 2.5 feet wide, and 6.5 feet deep.

DE: Yes. I had heard that these gas microturbines—with outputs ranging anywhere from 30 kilowatts to 250 kilowatts—do turn at a very high rpm. Shaft bearings for such turbines must be a crucial component, are they not?

Field: To be sure! In either the Capstone 30- or 60-kilowatt unit, the turbine shaft rotates at varying speeds up to 96,000 rpm. The entire shaft is supported with our patented air bearing positioned at the shaft's midpoint.

At such high rpm, oil-lubricated bearings are severely challenged. In our opinion oil is simply too viscous and too fragile a fluid for such requirements. With the air-bearing design used in the Capstone unit, however, the rotating shaft, in effect, is being supported by a film of pressurized air.

DE: Where does the air come from that lubricates this turbine-shaft bearing?

Field: Ambient air is first drawn into the microturbine system enclosure, filtered, then passed over the electric generator, which is kept cool by this passing air. Next, the air is drawn into the impeller (or compressor), which compresses the air before pumping it into the combustor.

Now, a part of that compressed-air stream exiting the impeller (compressor) is diverted to the air bearing. The microturbine shaft in effect now rides on a thin film of compressed air—this film being in the thin annular space between the rotating shaft and the stationary bearing housing.

The Capstone Microturbine engine, shown here being assembled, has just one moving part and no gearbox, pumps, or other mechanical subsystems.

DE: Is this air-bearing technology something… never used before in turbine bearings?

Field: No. Air bearings are commonly used in aerospace applications. For instance, they are used in the air compressors used to pressurize the cabins of aircraft and in the onboard turbine-generators that provide aircraft electricity.

Nonetheless, Capstone has developed its own air-bearing system specifically designed for the microturbine engine. Some time ago, Allied Signal (now part of Honeywell) also developed an air bearing for microturbines and patented it; GE has since bought the right to use that technology in its microturbines.

For some time now the federal Department of Energy has been pushing and funding an advanced microturbine program. As part of that R&D program, GE has been working on a 175-kilowatt microturbine generator, and it incorporates air bearings. Capstone's participation in this R&D program has resulted in a 200-kilowatt model, pre-production versions of which are now operating at beta sites. But currently, Capstone has the only commercially available microturbine with air bearings.

DE: The air bearing is certainly a fascinating technology. But is it really any better—does it perform better—than the traditional oil-lubricated bearing?

Field: Air bearings can respond virtually instantaneously to changes in load demands and resulting changes in shaft speeds. As the load demand increases from zero kilowatts on up to 30 or 60 kilowatts, the microturbine shaft can surge from 50,000 rpm up to 96,000 rpm almost instantaneously. Oil-lubricated bearings introduce viscous drag and can't be as responsive. And that type of heat and variation stress can take quite a toll on lubricants, even the more exotic ones.

DE: Can we turn now to another topic, namely linkage of the onsite microturbine to the electric utility grid? Is linking all that important? Isn't generating electric power onsite for consumption what's really essential?

Field: No! It's crucially important to be able to link your onsite microturbine to the utility grid. Why so? Because then the power grid can be used to supplement your electric power needs; and to serve as a backup power source when your DG equipment is offline.

It is important to understand that, in most onsite power installations at commercial and industrial sites, the microturbine system is arranged in a combined heat and power (or cogeneration) configuration. That is, the thermal energy (heat) released from the burning of the fuel in the microturbine's combustor will be used not only to spin a turbine-generator to generate electricity, but the heat exiting the turbine will also be captured and used to meet the thermal needs of the site. Not to capture that thermal energy and use it is to waste it to the atmosphere—which would greatly lower the energy efficiency of the microturbine installation, thus greatly reducing its economic attractiveness.

Generally, engineers size the microturbine system for any given commercial or industrial installation to meet the heating needs, the thermal load, at that site. This means that, in many cases, the microturbine system will not be generating enough electric power to meet 100% of the site's electric power needs. Said another way, if the engineer sized for electric needs, then one would usually end up with a larger installation that would waste much heat to the ambient.

Accordingly, many microturbine DE installations have to import electric power from the utility grid. That's the most economical and reliable way to go.

DE: Specifically, how is the microturbine heat used in a typical combined heat and power microturbine installation?

Field: The voluminous hot exhaust gases from the microturbine are used to meet thermal loads, such as heating water for both space heating and hot water in buildings and apartments, heating swimming pools, etc.

CHP is very simple and efficient with microturbines, since all the thermal energy is contained in the exhaust gases. With piston-engine-driven generators, on the other hand, CHP is more complicated: you want to siphon off some of the heat from the engine but not too much, lest the engine run too cold—a delicate balancing act.

One of the coolest uses, literally, of microturbine exhaust heat energy is to drive absorption chillers, devices that use heat energy, instead of electric energy, to chill air and water. Absorption chillers are nothing new; there are millions of them around the world. But most of them are fueled by natural gas—gas burners release the heat energy.

Using a microturbine's "waste" heat to drive such a chiller makes great sense. It creates air conditioning—with almost zero electricity use and zero natural gas use. And it does so during summer months, when peak electric rates are double or triple their off-peak levels.

That's why we're seeing quite a few microturbine-based CCHP installations in place with many more on the way. One of our distributors, United Technologies' UTC Power division, now has a very-high-efficiency double-effect CCHP solution that directly uses exhaust from four or more 60-kilowatt Capstone microturbines to drive their 110-ton, 1.3 COP Carrier absorption chiller.

DE: Could you please explain how the heat is actually captured from the hot turbine exhaust gases?

Field: The hot turbine flue gases (500°F to 600°F) flowing up the stack pass through a finned heat exchanger near the top of the stack—much as moving air flows through the spaces between fins in an automobile radiator. Or it's like holding an automobile radiator (its plane horizontal) above a campfire. Now as the flue gases flow up through the microturbine's heat exchanger, they transfer much of their thermal energy to water circulating through the inside of the heat exchanger.

DE: Very good. Now how do you convey the heat-exchanger hot water to the point where it is needed? Further, in installing a CHP or a CCHP microturbine system, is there usually major plumbing involved?

Field: Usually the microturbine is located right in the building where the heat is to be used. So getting the hot water from the microturbine heat exchanger to the point of use is merely a matter of running pipe from the heat exchanger to the use point—typically less than a few hundred feet. Of course, you then have to install a cold-water return line to the inlet to the heat exchanger.

In installing a CHP or CCHP system, is there much plumbing involved? Usually no, for the simple reason that an existing building already has its plumbing system: it is merely a matter of running pipes from the new microturbine heat exchanger to the existing plumbing.

So, no, there's no major plumbing involved. It is merely a matter of connecting up the microturbine with the existing plumbing—running an outbound and a return pipe from the microturbine's heat exchanger to either a building water loop, a furnace, an absorption chiller, or some other heat-consuming device.

DE: Does the microturbine system usually provide all the electric power needs of a site?

Field: As I mentioned, very often it does not. Usually, the microturbine is sized to meet the heating and/or cooling needs of a facility or specific process. And if that is the design approach, then the microturbine system will produce well under the facility's baseline electric power needs.

For energy-efficiency reasons, the design engineers want to use 100% of both forms of microturbine energy: the heat and the electricity. When the microturbine system is sized like this—just large enough to provide all the heating needs of the facility (anything larger would mean that heat would now have to be wasted to the environment)—then there will not usually be enough electricity being generated to meet baseline electric power needs. And that means that the facility will still have to buy some electric power from the utility grid—but much less.

This CHP and CCHP approach can dramatically reduce per-kilowatt-hour costs and has an even greater impact on costs during hours and seasons of high demand. Utility "demand" charges can range anywhere from a few dollars to $20 or more per kilowatt. Accordingly, if a building has a 500-kilowatt peak demand, that could add up to a whopping $10,000 charge for a single month.

Yet, if the manager of that building were to put in, say, four C60s, that monthly demand charge could be cut in half. And if the facility were a 24/7 business like a hotel, it would slash its kilowatt-hours for the month by over 150,000—a savings of more than $10,000 in the higher-rate states.

For this reason, having the capability to interconnect the microturbine system with the electric-utility grid is most important.

One other point: Depending upon the local electricity rate structure, it may make sense to power down the microturbine system at certain hours of the day. For example, it may be prudent to completely turn off the microturbine system in the evenings and nights, when the utility power rate may be lower than what it would cost to generate electricity with the microturbine.

Accordingly, being able to connect a microturbine system to the utility power grid is vitally important. Of course, in some applications, it would be impossible to connect to a grid—e.g., on an offshore oil-drilling platform or an on-land oil field in a remote area. But if there is a grid available, then it makes sense to connect to it.

DE: How then do you connect the onsite power unit to the electric-utility grid? What sort of equipment is involved here?

Field: The biggest issue in connecting an onsite power unit to the utility grid is this: What happens if a vehicle or a storm knocks down a power line? A utility repair crew will be sent out. But is there an onsite generator somewhere that is still sending power into that downed power line? If so, the repair persons face a risk of electrocution.

So, if a facility manager wants to install an onsite power unit, then connect it to the utility grid, the local power company will need assurances that it will not be sending power into a line after it has been downed.

Now the Capstone microturbine unit has built into it a component called the power electronics unit (or inverter). Its main purpose is to take the electrical output from the high-rpm generator and convert to the proper voltage and frequency. The proper voltage and frequency is whatever is on the utility power line coming into the facility. This power electronics package senses what is on that utility power line and puts out an output that matches it. In other words, the inverter makes the same kind of electricity that is being supplied by the utility grid.

The microturbine power electronics unit can also sense when there is a problem on that utility power line. If there is a problem, it will automatically disconnect the microturbine generator from the utility grid, thereby protecting utility repair crews from danger.

DE: Are the traditional reciprocating-engine gensets also connected to the utility grid in a similar way?

Field: With a traditional reciprocating-engine generator, an engineer desiring to connect it to the utility grid would have to install considerable electrical/electronic hardware (which includes a protective relay) between the generator and the grid. This is to ensure that the onsite generator limits its impact on grid voltage variability, and to prevent it from exporting power to a downed power line.

Now such electrical/electronic interface equipment is expensive, a significant part of the cost of smaller gensets. In fact, this extra cost has often been a deal killer; it is the main reason that, until recently, there has been very little in the way of onsite 24/7 power generation (as contrasted with emergency backup power) in the 30- to 250-kilowatt range.

Let's be clear: Distributed energy—onsite power generation—is not new. It has been done for decades, but mainly in large industrial facilities such as paper mills and steel mills. Since those facilities use massive amounts of electricity and heat, it made sense for them to generate power onsite.

But what is new is the application of distributed energy to smaller energy-using sites—office and apartment buildings, schools and colleges, hotels and motels, supermarkets. And the dawn of the microturbine—including its inexpensive means of interconnecting with the utility grid—is a major reason for this trend toward distributed energy at smaller sites.

DE: Is that it then concerning connectivity to utility power grids? Is there any variation in requirements as one moves from interfacing with one electric utility to another?

Field: You are hinting at an important issue. Indeed, the toughest problem in achieving interconnectivity with the power grid stems from the fact that there are over 3,000 electric utilities in the US, most with their own rules and regulations for connecting generating equipment to the grid. Many of these regs were written assuming that an electric generator powered by a reciprocating-engine genset would be the onsite power source.

To address this interconnect issue, some electric utilities have been moving to standardize the interconnection rules. The California Energy Commission (CEC), for example, has worked with all major utilities there to develop a statewide set of rules and regulations (Rule 21) concerning distributed-generation interconnection.

This standardized set of rules helps all sides—the utility grid operators, the DG equipment manufacturers, and the building owners putting in DG. All players now know what is expected and required. Capstone's 30- and 60-kilowatt systems were the first to be state-certified to meet the requirement, and several other DG equipment offerings have since been certified to comply with California's Rule 21 interconnection standard.

There is a similar set of rules for DG interconnection in New York, and there is a standard enacted by UL [Underwriters Laboratories]—its 1741 rule—that is somewhat accepted. But, to date, there is no all-encompassing DG interconnection rule.

DE: Turning away now from issues of where to apply and how to install microturbines, can we now talk about what happens once the microturbine has been properly installed? Specifically, what maintenance is required on the Capstone 30- and 60-kilowatt units?

Field: After the first 4,000 hours (6 months) of turbine-system operation, we recommend an inspection of air and fuel filters (natural gas passes through a filter) to make sure the environment they're in isn't unusually dusty. There is no oil to change or oil filters to inspect, for there are no lubricants used. Otherwise, minor service intervals are at 8,000 hours (11 months if operated continuously). The maintenance service is simply to clean or to replace all filters.

At 20,000 hours (or after 2.3 years of continuous operation), we recommend cleaning and possible replacement of the fuel injectors. There are three injectors inside the combustor, the chamber wherein fuel is injected into incoming, compressed air. The oxygen in the air reacts with the fuel, releases much heat and light and greatly expanding the volume of the gases, which then rush through the turbine section at high speed.

Also at 20,000 hours, we recommend replacing the thermocouple and the igniter (i.e., the "sparkplug") inside the combustor. And we recommend servicing the battery, which is needed to start the unit if operating as a standalone unit (i.e., a "black-start" system that can start itself in remote no-grid areas or as a backup generator when the grid blacks out).

At 40,000 hours (about 4.5 years), we recommend a factory engine overhaul. We simply remove the customer's existing gas-turbine engine and replace it with a rebuilt turbine. The system was designed so that this is a rather simple process that can be done in just a few hours. We then take the old turbine back to the factory to do any work that might be needed for the next 40,000 hours of operation.

DE: To get you to expand a bit more on needed maintenance, let's assume that a given facility has installed an onsite microturbine system. What would be that owner's most important maintenance task?

Field: For the owner, there is no maintenance—ever. The microturbine operates automatically and "transparently" to the facility. Any service ever needed is done by a Capstone factory-trained technician. The system's built-in modem or network connection allows us or our distributors to check and diagnose virtually all operation hours and parameters, real-time and historically. At the appropriate intervals, a factory-trained authorized service technician comes to the facility to do the service. The system is then back online in a couple hours—and sometimes in as little as 20 minutes.

DE: Why is checking the air filter all that important?

Field: Just as in a car engine, a clogged air filter will impair the microturbine's operation. Turbines draw in a lot more air than the traditional truck-engine gensets so often used onsite for emergency backup power. Accordingly, air flow is important.

Over time, massive amounts of air will have passed through the microturbine's compressor, combustor, and turbine. If there are abrasive or corrosive particles in that air, they could cause significant damage to microturbine components over time.

The idea is to prevent such potential problems from ever emerging by (1) installing a quality air filter to screen out harmful particles, and (2) ensuring that the air filter is either cleaned or replaced periodically. Servicing the air filter in a timely manner is probably the single most important maintenance measure.

Ingersoll-Rand microturbines generate power at the ACME Landfill in Martinez, CA.

DE: How often does one of these air filters need to be serviced?

Field: We recommend that a service technician check a filter after the first 4,000 hours of operation or less. This takes just a minute and can be done while the microturbine is running. That initial inspection will give the technician a good idea of local ambient-air conditions and of how often the microturbine filter will need to be checked, cleaned, or replaced in the future.

As a general guide, filters should be cleaned or replaced after every 8,000 hours of microturbine operation. In dusty conditions like those at landfills, where microturbines are fueled by flare gas, air filters may have to be serviced more often.

DE: Besides servicing the air filter, what is the next most important maintenance task that must be done?

Field: The fuel injectors should be inspected at 20,000 hours (about 2.3 years if run continuously). In many cases where clean natural gas is the fuel, there will be no significant deposits or wear on the injectors.

DE: Could you please expand a bit more on these fuel injectors—how many, their function, what can go wrong with them, what fuels they can accommodate, etc.?

Field: There are three injectors inside the combustor. Their purpose is the same as the injectors in an automobile engine: namely, to control the rate of fuel flow into the combustion chamber. An injector is cylindrical in shape with the diameter of a quarter.

What can go wrong? An injector can get clogged with dust or other contaminants in the natural gas piped in from the utility—if there is not an appropriate gas line filter.

There is one type of injector for our gas-fueled microturbines (natural gas, propane, landfill gas); and another type of injector for our liquid-fueled (kerosene, diesel) models. Currently, it would not be economically practicable to convert a microturbine from gas to liquid fuel, or vice versa: it is too costly to switch injectors.

As far as adjustments to be made on the injectors for controlling the fuel rate, all that is controlled by the microturbine's computer; it is not something set by a service technician.

DE: In sum, the microturbine's combustor needs some servicing after 20,000 hours of operation—check its fuel injectors. Is there anything else inside the combustor that needs checking?

Field: Yes. The igniter—in effect, the sparkplug—also needs to be inspected at the same time (20,000 hours). If it is too worn or corroded, [it] could affect the quality of combustion.

The igniter looks like a sparkplug—only it's longer. Unlike car engines that have to spark thousands of times per minute, turbines use a continuous combustion process. It's kind of like your gas stove. You only have to ignite the gas once, then the flame keeps going, igniting all new incoming fuel as it reaches the burner. Likewise, the continuous combustion process of a turbine has to be lit only at startup. Incidentally, the power for that spark is drawn from the grid connection or, if it is a standalone unit, from the microturbine's battery pack.

What can go wrong over time with an igniter? It can wear down just like an automobile sparkplug. And, as with an auto sparkplug, carbon deposits can build up on it. It can also corrode. The gap does not need to be adjusted.

DE: In view of the fact it is being subjected to high-temperature flame, is there anything else inside the combustor that needs attention?

Field: No. We cannot really discuss anything further about the combustor without getting into proprietary information.

DE: OK. I understand. Let's turn then to the microturbine itself. Does that need to be inspected after so many hours? Do the turbine blades or vanes ever get damaged and need replacement?

Field: Under proper operating conditions, no. We have seen examples where dirt in liquid fuel has damaged the turbine. This was in an OEM [original equipment manufacturer] application of our microturbine engine to hybrid electric buses. In those cases, there was no fuel filter installed in the fuel line to remove dirt. In stationary applications, there has been no such issue since such microturbine systems provide for a fuel filter in the fuel line.

DE: And what about the main air bearing supporting the microturbine shaft? Does that have to be inspected at some point? And what sort of problems can it develop?

Field: No, it is not an item for periodic maintenance inspection. But the bearing is inspected during the 40,000-hour factory overhaul. There has never been an issue with our air bearings—unless the system was operated without any air filter in place. But in the 6.5 million hours of accumulated operation of Capstone microturbines in the field, the Capstone air bearings have worked perfectly.

DE: What other important maintenance task must be performed on a microturbine system?

Field: For our standalone units, we recommend servicing the battery at 20,000 hours of operation. If the battery is not operating at capacity or isn't holding a charge, it should be serviced or replaced.

DE: Can we just back up here for a moment? In a typical microturbine system, what is the purpose of a battery, its main functions?

Field: It provides the electric energy to start up the microturbine system. That electric energy is used by standalone, grid-independent models: (1) to start cranking the turbine so that the compressor will be able to begin drawing in outside air; (2) to energize the igniter.

DE: Before starting the system, do you have to first purge the microturbine of potentially explosive gases?

Field: Unlike with the larger industrial turbines, purging the microturbine system of potentially explosive gases before startup is not a significant issue with small microturbines.

Why not? A key reason is that a microturbine system will spin down briefly after a shutdown command or after the grid feed is lost in a blackout. This purges the system of gases and cools the system down gradually.

So, yes, the system is purged of dangerous gases at the time it is shut down. But the purging of a microturbine system is not the big deal it is with large gas turbines. The reason: a microturbine system is small, the combustor itself, for example, being less than a cubic foot in volume, so there's exceedingly little uncombusted fuel to be an issue. That tiny amount may cause a "poof" sound as it escapes the combustion chamber during spin-down, but that's about it.

DE: I'm not completely clear. How would the microturbine system be able to purge itself of potentially explosive gases if it were in shutdown mode?

Field: The air rushing through the system continues during the spin-down momentum, but the injectors aren't putting any new fuel in. This continuing fresh air will (1) push out or purge any lingering gas or fuel vapors from the system, and (2) help to cool the turbine system down.

DE: How does the battery crank the turbine shaft—the actual mechanics here?

Field: In startup mode, the generator is run in reverse so that it behaves as an electric motor or a starter motor to start spinning the shaft, so the compressor now starts drawing in air so that combustion can proceed.

DE: What sort of battery is used in a typical microturbine system? And what specific maintenance tasks need to be performed when?

Field: It's an array of several sealed lead-acid batteries, 13 amps. The array is charged either by the utility grid, if grid-connected, or by the microturbine itself. If the system is not grid connected and is not called on to operate for an extended period of time, the system can be set to periodically start itself to maintain proper battery charge.

DE: Turning away now from strictly hardware questions to the softer issues, is it important for the owner/operator of a microturbine onsite power system to have a person on staff for maintaining and servicing it?

Field: No, there is no need to have an on-staff maintenance person, for this 30- or 60-kilowatt microturbine system requires very little maintenance. Even if the owner is operating it 24/7, the microturbine system only requires simple maintenance once per year. Nonetheless, those tasks must be performed only by factory-trained service technicians, either one of our own or one with a Capstone distributor who sells, installs, commissions, and services the product.

DE: What sort of warranty is available on these microturbine units?

Field: Our 30-kilowatt units come with a one-year warranty. This covers all parts and labor except for filters. Concerning our 60-kilowatt unit, the warranty covers all parts and labor for one year; and all parts for an additional two years, except for maintenance parts like filters or an igniter. That's the factory warranty. Our distributors who do the actual servicing of the equipment often offer extended, all-inclusive service contracts.

DE: Is it OK for the owner to do his own maintenance on the microturbine unit?

Field: No. The installing distributor has the responsibility for proper maintenance of these units.

The maintenance of these microturbine systems is relatively minor and inexpensive. It is not like the heavy maintenance required of the much bigger (1- to 20-megawatt) industrial gas turbines. Such maintenance on these large turbines can mean days of downtime a year, many more parts, and much more costly service contracts.

And ditto for the continuous-duty reciprocating-engine gensets that need major overhauls at least a couple times a year and lots of minor servicing in between.

DE: Could you give us some ballpark cost? What would be a typical price range for a 30- and for a 60-kilowatt microturbine? And what would be the price of a typical one-year service agreement?

Field: The equipment price (not including installation cost) of a 30-kilowatt microturbine would be in the $30,000–$35,000 range; and of a 60-kilowatt unit, in the $50,000–$60,000 range.

As for service agreements, those prices are set by our distributors. But they are relatively minor part of operating costs.

DE: Your microturbine power units, sized at 30 kilowatts and 60 kilowatts, are at the lower end of the category of microturbines (30 to 500 kilowatts). Doesn't this limit this equipment to relatively small applications?

Field: Not necessarily, for it is quite common to group together several of these microturbine units and operate them as one—just as is sometimes done with personal computers.

In fact, most of our distributed-energy applications are installations that have two to four microturbine units arrayed together.

The largest number of microturbine units arrayed together is at a landfill site at Sylmar in northern Los Angeles County: a total of 50 Capstone C30s fueled with flare gas from the landfill. Combined, these 50 microturbines generate up to 1.5 megawatts for export into the Los Angeles Department of Water and Power grid.

Incidentally, the Capstone microturbine can readily burn this landfill gas—even though it is only 35% methane (the remainder being carbon dioxide and noxious gases).

DE: But is it cost-effective to go with so many microturbine units? Wouldn't it make better sense to install a much larger gas turbine in such a situation?

Field: Landfill flare gas has only about one-third the energy of natural gas, so it's challenging to use as a fuel. In some large turbine and reciprocating-engine genset installations, they have to "sweeten" the gaseous mix by blending in commercial fuel.

By contrast, the Capstone microturbines can run exclusively on low-grade landfill gas (35% methane), without the expense of adding in costly commercial fuel.

Air emissions are also an issue. Capstone microturbines have extremely low emissions. That's why the LADWP and the South Coast Air Quality Management District teamed up to buy, install, and operate the 50-microturbine array at the Lopez Canyon Landfill.

DE: Interesting. Could you please expand a bit more on the potential for using microturbines to tap the energy wealth in landfills?

Field: The attraction of landfills is that they constitute a source of free gas—and that gas supply is constantly being renewed as biodegradation proceeds in the landfill.

Now landfill biogas can either be vented to the atmosphere, or put to practical use, recycled. If recycling is chosen, then usually there is little or no need for electricity and heat at a landfill site. That means either that a microturbine installation could be built on the site and its electricity and heat exported; or that landfill biogas could be piped to a site in the region that does have a need for electricity and power; and the microturbine installation placed there.

An example of doing this latter is a Capstone microturbine installation in the Chicago area. Biogas from a landfill is piped to a nearby high school. There, the biogas is used to fuel a microturbine installation, which provides both electricity and heating for the school.

DE: You mentioned that the Capstone microturbines have very low air emissions. How low are they?

Field: The Capstone 30- and 60-kilwatt microturbine units are the world's cleanest-burning microturbine engines operating without benefit of any exhaust treatment devices.

The cleanness of the emissions is closely related to Capstone's earlier involvement in the development of hybrid cars. Our engineers originally designed the Capstone microturbine engine for use in hybrid (a mix of batteries and onboard microturbine) vehicles.

The hybrid-vehicle concept was that the car would be powered directly by its batteries; but that the batteries would continuously be charged by an onboard 30-kilowatt (45-horsepower) microturbine fueled by diesel, kerosene, or natural gas or other-type gas. The vehicle designers wanted an engine that would have few moving parts, would be highly energy efficient, and would have air emissions close to zero. And that's how the Capstone microturbine was born.

But it takes a long time for a new-concept car to take hold in the auto industry. Nonetheless, several dozen hybrid buses using the Capstone microturbine have been built.

Several years ago, Capstone began looking for more lucrative markets for its innovative microturbine and concluded that applying this microturbine technology to onsite power generation had great potential.

DE: Fascinating. Quite a story about the evolution of the microturbine! What substances are being released from the microturbine exhaust and in what concentrations? And what is the key to achieving such low air emissions?

Field: Smog-forming NOx emissions are factory-rated at less than 9 parts per million—or 0.47 pounds per megawatt-hour. Carbon monoxide (CO) levels are also very low.

Independent testing reported by the USEPA on a Capstone C60 microturbine operating at a supermarket confirmed the following: NOx emissions were less than 0.15 per megawatt-hour (3 parts per million); and CO emissions, 0.1 per megawatt-hour (3.5 parts per million).

Further, EPA reported SO₂ emissions to be less than 0.02 per megawatt-hour; and CO₂ emissions of this CHP operation to be less than 560 per megawatt-hour.

Now compare those emission figures to those of utility power plants in any region of the US. You'll find that the Capstone microturbines produce power with far less pollutants and greenhouse gas emissions.

In fact, the Capstone microturbine is so clean-burning that the California Air Resources Board, the main air-regulatory body in the state and one of the toughest in the world, no longer requires owners using this microturbine for onsite power generation to monitor its emissions. To qualify for this exemption, a power-unit's emissions need to be less than 0.5 per megawatt-hour of NOx, and less than 6 per megawatt-hour of CO.

Finally, you asked about the key to these low-emission levels. Sorry! We can't tell anyone at this point. That is a closely guarded company trade secret.

Gene Dallaire is a frequent contributor to DISTRIBUTED ENERGY magazine.

DE - November/December 2004