Capstone Turbine Corporation is aiming to follow in the footsteps of Apple, and make available to the masses what has traditionally been a shared centralized resource. Playing the role of the Macintosh is Capstone's line of microturbines which look like nothing so much as computer towers the size of small cars, right down to the ventilation slots and gray sheet metal cladding. Ranging in capacity from 30-200 kW, these units put electrical generation capacity directly on-site giving their owner partial or complete independence from the electric grid. These systems are capable of using a wide range of fuels, from pipeline natural gas to diesel to biogas. Besides cutting electric bills these units are even more effective when deployed as cogeneration or trigeneration systems to provide space heating and cooling. Though the idea of owning a miniature power plant might sound like something only a factory operator would consider, these clean quite systems are in fact suited to a variety of commercial buildings as well as multi-unit residential facilities. Depending on energy usage and local prices ROI's will vary greatly, but in general don't expect to see 18-24 month payback times.

Like the engine in your car, microturbines rely on the expansion of hot combustion gases to power them. Except, unlike the piston in your Honda's V4, in a microturbine, it is the turbine blades which turn the thermal energy derived by burning fuel into usable mechanical work. The turbine shaft is in turn connected to a generator to produce electricity. Interestingly, the technology for these systems is largely based on work originally done on engine turbochargers, so they do, in fact, have automotive roots. Gas turbines are favored for energy generation and powering airplanes because they are much more compact than steam- turbine systems and piston cylinder engines. This same fact applies regardless of scale. As the name would imply, microturbines are basically scaled down versions of the gas turbine systems seen in large power plants. Whereas standard gas turbines generate anywhere from 500 kW to 250 MW, microturbines generally put out 30 kW - 250 kW. Capstone itself makes units of 30 kW, 65 kW and 200 kW (a 15 kW was designed for sale Europe but has not proven to be economical in the U.S). For comparison 200 kW is equal to 268 HP, enough to make any morning commute exciting or just power 200 homes. These units are small enough and cheap enough that they can be purchased by private building owners and installed on-site. But why would anybody want to buy such a system when they have a perfectly good electrical hook-up already?

Capstone microturbines and other similarly sized generation products (MT250 from FlexEnergy, KMGR line of reciprocating engines from Kraft) are classified under distributed power. Nearly all electricity that we use comes from a vast web power lines spreading across the United States and feeding our nation's ever growing demand for electricity. This network is generally referred to as the electric grid and at its main nodes sit power plants, which supply power to numerous homes and businesses. This model, where one facility supplies power to 1000's of customers, we will refer to as "centralized power." While this distribution scheme has worked for a very long time it does have shortcomings which only become more apparent with every passing year.


The vast majority of energy is derived by burning fossil fuels and then using some sort of power cycle to convert the chemical energy stored in the fuel into usable electrical energy. This conversion process is roughly 42% efficieny in the best cases (natural gas plants). To reach the customer, electricity must then travel over many miles of transmission line which incurs further resistive losses. In the end, customers only receive about ⅓ of the energy that was actually available from the fuel. To increase the efficiency of these plants, engineers sometimes harness the thermal energy contained in the hot exhaust gases. These gases are usually vented up a flue, or smoke stack, and lost to the atmosphere, but if they are run through a heat exchanger they can be used to raised steam. The steam can then be distributed to buildings to use for heating or as process heat in various industrial applications (i.e. food processing). This type of energy generation is referred to as combined heat and power (CHP) or simply cogeneration. It can increase the total efficiency (thermal + electrical) to over 60%. The problem is that the site of steam generation must be fairly close to the site where the steam is being used. In most cases, power plants are located too far from any large steam users to make CHP worthwhile.

By installing a Capstone generation system on-site, it becomes convenient to hook the turbine's exhaust stream into the building's heat load. The C65 - ICHP (integrated heat and power) model contains a water heater which sits right on top of the base-model C65 (65 kW) and increases the system's total efficiency from 29% to as much as 82%. This single package means there is no need for additional floor space to accommodate this extra functionality. Capstone also offers an external heat exchanger for use with its other models. It is unclear why an integrated ICHP option is only offered for the C65, as the MT250 and all Kraft engines come with heat exchangers standard. Note that these are water heaters, the operative word here being "water" not steam. If you're looking to generate steam for process heat or somehow condition the exhaust for use in other applications additional equipment will be required. This can be handled through the Capstone distributors some of whom use the company's microturbines as part of larger more complex installation designs. One example of this is the PureComfort® trigeneration unit built by UTC power. Trigeneration, or CCHP (the extra C is for cooling) incorporates an absorption chiller to utilize waste heat to drive a refrigeration cycle. This way, one system can meet both heating and cooling needs.

The heat exchanger in the ICHP comes in both copper and stainless steel versions. Due to copper's superior thermal conductivity, it delivers much higher overall efficiency (82%) than the stainless steel (62%). However, in cases where the water being heated has corrosive elements using a copper core is not an option and stainless steel must be used. The overall efficiency of a Capstone unit will generally be higher than that of the MT250 (71%) but lower than those of the Kraft engines (86%-90%). Microturbine technology is still maturing and efficiencies, especially at smaller capacities, have not yet caught up with those found in reciprocating engines.

Generation efficiency aside, the grid itself has severe problems. Like much of this country's critical infrastructure the electrical grid is outdated and deteriorating. This results in both inefficient distribution as mentioned before and high equipment failure rates. In our increasingly wired, fast paced world losing power for more than a few minutes is a big deal. So while aging electrical conduits may seem like someone else's headache, the issue actually hits closer to home than you think. But what better way to avoid power loss altogether than to have a power plant in your basement? Capstone microturbines can be installed in one of two ways, either grid parallel or stand alone. In grid parallel, the microturbines are connected only to the electrical grid, where they are used for load shaving. Since an electrical meter measures the net energy flow into a facility, the output of a turbine "shaves" off some of this total load and reduces the utility bill. Unfortunately, since the turbine runs through the grid, when the power goes out so does the turbine. The other option is "dual connect mode" wherein the generator is attached to both the grid and the building itself. During normal operation it will operate the same as it would in grid parallel, but when the power goes out the system goes into stand alone mode. Because the unit doesn't carry the entire load of the building to begin with, it can't keep all systems functional during an outage. However, certain critical systems can be identified beforehand and kept operational so that business doesn't come to a grinding halt. The dual mode controller facilitates this switch in less than 10 seconds and since the microturbines are already running there is no need to wait for them to start up.


However, traditional power plants are not simply limited in the way in which they can deliver electricity but also in the fuel sources they can use. Certain types of facilities such as oil rigs, landfills, and waste water treatment plants produce a mixture of combustible gases (primarily natural gas) that can be used as fuel. Unfortunately, there is no way to economically transport this gas to a central generation facility and therefore it goes to waste. Wouldn't it be nice if you could bring the power plant right to the energy source? While most applications will use a natural gas line connection, Capstone's microturbines can actually utilize these marginal fuel sources. Depending on the fuel (wellhead gas, biogas) and the expected operating environment modifications are made to the the base models to optimize their performance. With biogas the major concern is its corrosive content, particularly hydrogen sulfide, which can destroy turbine systems not designed to withstand such harsh conditions. The CR (renewable) series of microturbines are designed for just such conditions so they can generate electricity from landfill and digester gas just as if they were running off regular pipeline gas.

As mentioned before, one alternative to the Capstone systems and to microturbines in general are reciprocating CHP systems, such as those manufactured by Kraft Energy Systems. These units operate on the same internal combustion technology which give cars their motive power . There are any number of differences between Capstone microturbines and Kraft reciprocating CHP units. Besides efficiency, the most obvious of these is the size of the lower capacity systems. While a C65-ICHP (65 kW) has a footprint of just over 18 ft2 and weighs 3000 lbs. the Kraft KMGR-55-4SH produces 55 kW and yet weighs 6500 lbs. with a footprint of 67 ft2 nearly 4 times as large. On the other hand, the 150 kW Kraft model (KMGR-150-4SH) is only 50% heavier than the 55 kW with nearly identical dimensions. Except for the two largest models, all Kraft generator housings have the same 150" x 65" footprint though the weights do vary. The Capstone turbines scale more closely with size and therefore their 200 kW C200 has a 12% larger footprint than the KMGR-150-4SH while weighing about 1 ton less.

The other big factor when dealing with major pieces of equipment such as generators is maintenance. In microturbines, air and fuel filters need to be changed yearly under continuous operating conditions and occasional inspections of the temperature sensors and combustor components are required. However, the major O&M cost is the replacement of the turbine core every 40,000 hours or so. Thankfully, due to Capstone's modular design, this overhaul takes only a few hours and then the unit is back up and running. In comparison, the Kraft engines require far more regular maintenance. Though the servicing costs may actually be cheaper than for microturbines, the down-times are much longer. Oil changes are required every 1000 hours while minor overhauls and major overhauls are required every 17,000 and 34,000 hours respectively. These overhauls involve the replacement of numerous parts such as pistons and bearings and so take days rather hours. Capstone's turbines also use the company's patented air bearings which provide superior lubrication (i.e. lower frictional losses) and eliminate the need for oil changes. They are the only company in the industry (microturbine or reciprocating) to employ this technology.

For those who want to know about the cost involved in installing a generator, be warned that estimates are impossible to generalize. The complexity and duration of any installation really depends more on the application than the choice of generator. Virtually all distributed power systems are designed to be plug-and-play, though in this case the plugging is done with flatbeds and forklifts. For systems of identical capacity and similar ancillary features there should be little difference in the cost of installing a microturbine vs. a reciprocating engine. What really makes a difference are things such as where the unit is placed, how much piping and ductwork will be required to accommodate the new system, and how lazy your contractor is. Remember that the labor and project management costs associated with installing a 1 MW plant are not very much different from those involved in a 100 kW installation. Therefore, the smaller the installed capacity the more the project will cost in terms of $/kW.

Capstone designates both high-pressure (HP) and low pressure (LP) options for its turbines. The only difference between these systems is that the LP models have a fuel booster for use with low pressure natural gas feeds. The booster increases the inlet pressure of the turbine up to the level it needs to operate. Since this requires additional energy the LP models will have slightly lower efficiencies than their HP counterparts. The MT250 has a slightly more efficient booster than the Capstone units (3% loss vs. 5-7%). However, if you are like most and plan on fueling your microturbine from a utility gas line the LP/HP consideration is essentially moot since these lines are qualify as high-pressure supplies.

Capstone systems also come with remote monitoring and diagnostic capability. Just like in medicine the key to treating any ailment is catching it early. The ability to perform simple maintenance measures now and prevent costly shutdowns later is key.

One question that must be considered up front is whether you are buying a generation system for its ability to reduce your utility bill, generate cash, or primarily as a back-up system. If you are buying a unit as a back-up system then don't buy a microturbine. In fact, unless you have several hundred thousand dollars just burning a hole in your pocket don't buy any of the systems mentioned here. Instead, call up an industrial supply company and order a stand-by diesel generator. They'll provide the same capacity at 1/10th the price. If this math doesn't work for you consider that Capstone units with stand alone capability cost 30-40% than their grid-connect counterparts. That's before you account for the $10,000's in additional utility mandated equipment. Back-up power is intended to be a side benefit of these units not their primary function.

Those for whom on-site generation actually make sense can be divided into two categories. The first category is those who can run these units without heat recovery and generate savings based only on the discrepancy between electric and gas prices. This is nearly always going to mean that the gas is free, as in wastewater treatment facilities where biogas is a natural byproduct. There are only a few places in the country where gas is cheap enough and electricity expensive enough to make self-generation without CHP profitable. The second category is those customers who have both electrical and heating loads that can use the output of a CHP system. Though generator do have variable output, such systems, particularly microturbines, do not operate as efficiently below their rated capacity. In order to be cost effective they must run at their rated capacity at least 6500-7000 hours a year. Furthermore, since savings as well as incentives are derived by maximizing overall efficiency, a minimum 70%-80% waste heat utilization rate is required. This, of course, is in addition to using all of the generated electricity. The general procedure for sizing a generator is to match is to the your building's electrical load first. Then go through an make sure that you have a use for most or all of the waste heat the unit produces. The issue the arises of varying load requirements over a 24 hour period. While electricity may be sold back to the grid at night when a building's electrical load drops, the same can't be done with waste heat. Therefore, there needs to be a round-the-clock thermal load (heating or cooling). Manufacturing facilities with process heat might fall into this category. There are also a few types of commercial facilities which make very good candidates. Data centers, hospitals, hotels, and even schools. Though thankfully only open during the day, many schools have heated pools which provide a nighttime load.

So how do you choose between generation options? Well consider that the C65-ICHP will take over 6 years to pay for itself. Even when available incentives are taken into account the payback time is remains over 4 years. Compare this to the Kraft's 80 kW engine which has a payback time of 3 years with incentives. At 250 KW this number drops to just over 1 year, a far more attractive prospect then waiting half a unit's rated lifetime to get into the black. It becomes clear quite quickly that for most applications where consumers are looking to save money on their energy bills, reciprocating engines with their lower capital cost and higher efficiencies are the best option. However, because of their more compact size the C65 and the C30 may be able to be installed in indoor spaces where the Kraft and Flex Energy units cannot not fit, say on a mid-level utility floor. Also, if you are looking to place your unit indoors, in a space where noise is a consideration (e.g. hotel) microturbines have a major advantage over reciprocating engines. When placed outdoors the Kraft is acoustically insulated in its container, but when used indoors there is no container to muffle the roar. So unless bleary-eyed guests is your idea of a satisfied customer base, a Capstone microturbine is going to be a better bet. Finally, if you intend on running the generator 24/7/365 and can tolerate extremely little down time than you want a Capstone rather than a Kraft. Yearly maintenance times for a reciprocating engine are measured in days, microturbine down times are measured in hours. Furthermore, Kraft engines will also experience brief but numerous service interruptions to change the oil every 6 weeks.

For cases where energy savings are paramount reciprocating CHP generators such as those from Kraft Energy Systems will be the most cost effective choice. Capstone microturbines are both more expensive and less efficient resulting in considerably longer payback times. On the other hand, in applications where noise, indoor space, and generator availability are major concerns Capstone microturbines are going to be the better choice.

References:

1. http://www.capstoneturbine.com/

2. http://www.kraftenergysystems.com/

3. http://www.flexenergy.com/

4. Catalog of CHP Technologies. U.S. Environmental protection Agency Combined Heat and Power Partnership (Dec. 2008)