∫ Microturbines – A Primer

You’ve probably seen turbines on airplane jet engines before. They’re responsible for getting you up in the air and keeping you there. If you’ve seen a windmill, then you’ve also seen a turbine.

Turbines are responsible for almost all electricity on the planet. Solar photovoltaic panels are one of the only sources of electricity that do not involve turbines. In addition to wind energy, turbines transform coal, gas, oil, hydro, tidal, and nuclear energy into the electricity that powers your and your neighbors’ homes. Turbines are a major part of the modern energy landscape.

But you probably have not seen a microturbine before. You might soon, and this post is designed to help make sense of them. The last fifteen years has seen the commercialization of microturbines, an electricity generating workhorse designed to power only a single building. Microturbines are installed on site, where the power is needed, rather than at a centrally generating power plant that distributes the energy long distances over power lines. (Microturbines are also feasible in cars, and some versions are small enough for homes though their efficiencies remain low).

The physics of turbines are complex, and there are many varieties of turbines, of which microturbines for commercial buildings and small industrial plants are just one. The short version is that microturbines are powered by the combustion of compressed natural gas or other fuels. This combustion produces high temperature and pressure gases that flow into turbine blades, spinning them. As the blades spin, they turn a rotor and generate electricity.

Microturbines are more expensive than traditional generators like reciprocating engines (more on cost below), but they have several advantages. Most noteworthy are lower emissions and higher temperature waste heat. High temperature waste heat is easy to turn into useful energy that can power an absorption chiller or provide heating to a building. This heat capture process, more commonly done at a centralized power plant, is known by many names including cogeneration or combined heat and power. When used for electricity generation alone, however, microturbines only have efficiencies of 25-35%, well below modern combined cycle natural gas turbine plants which achieve efficiencies of 60% (though central plants lose some energy in transmission).

 

Costs

When you include capital costs as well as the ongoing cost of fuel and maintenance, microturbines produce electricity for anywhere between $0.09-$0.14/kWh. The cost of the turbines themselves is between $1.20-$1.60/Watt, and installation, commissioning, permitting and development runs an additional ~$2/Watt. This depends heavily on how large a system is being installed.

As solar panel costs have dropped dramatically in the past few years, it’s worth comparing solar costs to microturbine costs. A solar array of comparable size to a typical microturbine project, say 1 MW, would cost on the order of $4/Watt installed, only 10-15% more than a microturbine. Solar panels themselves actually cost less than microturbines, at only $1/Watt.

The drawback for solar is therefore not one of cost. This is an important point in and of itself.

The real issues are space requirements and intermittency. One MW of microturbines needs a footprint of a 500 square feet for a concrete pad. One MW of solar needs roughly 100,000 square feet. Additionally, microturbines can be turned on and off at will (as long as there is fuel available) whereas solar energy production fluctuates with cloud cover and time of day.

Emissions

When used for electricity generation alone, the carbon dioxide emissions of microturbines burning natural gas will be worse or better than those of the existing utility grid depending upon location. At an efficiency of 31%, microturbines produce approximately 1.29 lbs CO2/kWh. California’s electricity grid (ignoring imported power) is much cleaner, with emissions of 0.49 lbs CO2/kWh, though some states are much higher. Fuel cells, a technological competitor to microturbines, generate electricity more efficiently; when burning natural gas, they have emissions ~0.75-1.00 lbs CO2/kWh and can be as high as 50% efficient.

Microturbine emissions for non-CO2 sources are actually better than the grid’s. According to the EPA, Capstone microturbines’ NOx emission are 0.00015 lbs/kWh and their CO emissions are 0.00010 lbs/kWh. Emissions from the grid (even in relatively clean California) are 0.00062 NOx lbs/kWh.

 

Reliability

Since they became commercially available, microturbines have gained in popularity and have therefore been subject to review. The Environmental Protection Agency’s Environmental Technology Verification Program (sponsored by the Federal Department of Organization’s Unnecessarily Cumbersome Nomenclature Initiative) measured and signed off on microturbine output frequency, voltage, power factor, harmonic distortions, and other factors. The data is a good starting point though limited in the number of cases reviewed. Sample reports can be reviewed here. At Fort Drum, the US Army conducted its own review of microturbines and found they did not degrade electrical power to their facility. A University of Colorado study found that microturbines efficiencies matched those predicted by the manufacturer.

Several issues have cropped up regarding microturbines, though most problem areas involve auxiliary equipment not related to the reliability of the turbines themselves:

  • Noise. High-pitched squeals, albeit at a low volume. Exhaust silencers may be installed.
  • Air filters. Need to be kept clear and may not last as long as suggested.
  • Fuel compressors. The US Army could not maintain the 55 psi gas they needed.
  • Heat exchangers. Problems reported in the heat exchangers used to transfer waste heat.
  • Commissioning issues. Bearings, diodes, electrodes, software sequencing, and other pieces of equipment can cause issues during set up (as with any new equipment installation).

 

Efficiencies

Microturbine efficiency varies with outdoor air temperature. According to EPA, at 80°F outdoor air temperature, the microturbines are about 3% less efficient than at 50°F outdoor air temperature. Using government weather data from a Typical Meteorological Year, one can determine the average temperatures for all 8,760 hours in the year when the turbines will be operating.

Load also impacts efficiency. If a building does not demand much energy and the microturbines need to turn down, the turbine efficiency will drop. Low load efficiencies are 10% or more below those of full load efficiencies. NOx and CO emissions per kWh increase as the load drops.

There is also an output penalty for microturbines: a turbine rated at 200 kW will only produce between 160-190 kW of actual power due to parasitic loads like running the natural gas compressor and other inefficiencies. The output efficacy decreases at higher temperatures.

 

Manufacturers

In the United States, a single company, Capstone, controls approximately 80% of the domestic market for microturbines. Over the past many years they have, Leviathan-like, swallowed up a host of competitors. Flex Energy is one of Capstone’s only main competitors, having acquired the microturbine technologies of Ingersoll-Rand.

The two products are similar. Each manufacturer makes a range of slightly different sized turbines form the other (e.g. Capstone’s 200 kW unit vs. Flex’s 250 kW unit). Flex’s turbines cost a bit less than Capstone’s but are also less efficient than Capstone’s at higher outside air temperatures.

 

Conclusions

Microturbines are an interesting if imperfect technology with a lot of potential, especially in locations that can make use of its high temperature waste heat. It will be exciting to watch how microturbine manufacturers and the makers of competing technologies innovate to bring about ever cleaner, cheaper, and more reliable energy solutions.

 

One Response to ∫ Microturbines – A Primer

  1. I find this article of much interest. One thing to look into further would be second law efficiency or exergy efficiency of these turbines. Exergy is defined as the potential to do “useful work”. Another interesting thing to look into would be how small these turbines can be, to the point where the heat loss due to the size is greater than the power that can be generated.

    Reply

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