Bigger is not always better. In the world of power generation, large grids that carry electricity from powerful turbines may not always be the best choice.
This brings us to a relatively new development in power distribution—the microgrid transmitting power generated by a small power source to nearby customers. A microgrid is all about proximity. It is an integrated energy distribution system that delivers power from distributed energy sources (renewable energy, fuel cells, battery storage, flywheel energy, combined heat and power systems, internal combustion engines, or microturbines). They operate concurrently and in parallel with the main power grid when the main grid is fully functional. However, when the main power grid goes down, microgrids can continue to operate as controlled islands of electrical power to their local customer base.
A microgrid is defined as an integrated energy system consisting of interconnected distributed generation sources, along with energy storage devices and controllable loads located at or near the end-use customers at the distribution level. The portfolio of these small-scale generation and storage technologies (which are generally known as distributed energy resources) includes renewable energy technologies, internal combustion engines, microturbines, fuel cells, battery storage, and flywheel energy storage. Such systems can operate in parallel with the grid during normal conditions or in a controlled islanded mode during emergency situations. The deployment of a microgrid is expected to increase system robustness, resilience, and security; deliver higher power security to critical loads; allow renewable integration; and enable inclusion of emerging technologies. A connected web of microgrids fully integrated with the main power grid can improve the robustness of the systems and ensure that key organizations and facilities (hospitals, fire and police, telecommunications, etc.) remain functional even during a blackout.
What makes microturbines a good power source for microgrids? As mentioned above, there are multiple energy sources that can provide power to a microgrid. They all have one thing in common—appropriate scale of power output suitable to the relatively small size of the microgrid’s demand load. In general, integrating any distributed energy source with a microgrid offers several technical, operational, environmental, and economic benefits. It also requires the overcoming of several technical challenges such as potential power quality problems, safety issues especially during instances of islanding operation, and integration of highly variable renewable power source with the main grid’s steady state power output, etc.
Microturbines have emerged as the favored power source for microgrids for several reasons. They can start up quickly and at a predictable power output (unlike renewable sources). Their operation can be regulated so as to provide peak load shaving, precise voltage regulation, and load-following capabilities to adjust power output as demand for electricity fluctuates throughout the day. This last consideration is of serious importance because even within the narrow confines of a microgrid service area, the demand for electrical power will vary during a normal work day. Within a microgrid, microturbines can be easily disconnected and reconnected to the main grid with little or no disruption in service. Microturbines can provide instantaneous power and adjust that power to accommodate ever-changing needs. In other words, microturbines offer both reliability and flexibility.
The microturbine market is segmented by power rating, application, end-user, and geographical region. Power rating segments include 12 KW–50 KW, 51 KW–250 KW, and above 250 KW. Applications are primarily combined heat and power (CHP) and the use of microturbines as standby power systems. End-users are divided into residential, commercial, and industrial markets. Worldwide, the microturbine market is divided into continental regions (North America, Latin America, Europe, Asia Pacific, Middle East, and Africa). North America leads in microturbine applications with more than 40% of the global market (2016). This is in response to increased regional production of biogas along with generous government support and incentives combined with strict air quality control regulations.
Turbine Types and Operation—From Chemical Energy to Electrical Energy
Begin with the latent chemical energy stored in solid fuel (coal or biomass), liquid fuel (gasoline, diesel, or biodiesel), or gaseous fuel (natural gas, propane, or biogas from anaerobic digesters). Some turbines are heated by waste heat collected and utilized by combined heat and power systems (CHP). Each contains a certain amount of potential energy stored in their chemical bonds. This energy is typically represented in terms of British Thermal Units (BTUs) per pound, the amount of energy needed to raise one pound of water from 60°F to 61°F at sea level. Igniting the fuel in a controlled manner releases large amounts of heat. This heat is applied to a boiler tank full of water, which then turns the water into steam. A gallon of water weighs 8.33 pounds, so 8.33 BTUs are required to raise a gallon of water 1°F. Heating water from a cooled 60°F to the temperature of steam, 212°F, requires a temperature increase of 152°F. Therefore, the amount of energy required to turn a gallon of water into steam is 1,266 BTUs (assuming 100% efficiency).
The steam becomes the working fluid, which is used to turn the blades of a turbine, thus translating heat energy into mechanical energy. This superheated working fluid flows through the interior of the turbine and impacts the vanes connected to the turbine’s shaft, which is aligned with its axis of rotation. The impellors are set at an angle relative to the alignment of the shaft and given a curvature so as to maximize the amount of energy that can be translated from the impact into spinning motion of the shaft. The shaft is connected to a disc-shaped assembly referred to as a rotor. The rotor is encased and encircled by the stationary stator. When the rotor turns, it causes the field of electromagnets to move past the conductors mounted in the stator. The bundled coils of wire in the spinning rotor spin in a magnetic field, creating electrical current and electrical energy. This then causes electricity to flow and a voltage to develop at the turbine’s output terminals. Basic operations aside, turbines can come in a wide variety of sizes and power outputs. These can range from massive turbines operating at large hydroelectric dams such as the 2,080 MW generated by Hoover Dam, to small, portable microturbines with outputs between 25 and 500 kW.
A traditional turbine uses steam as a working fluid—steam heated by an external fuel source. A turbine design variant is the gas generator. This modified turbine design uses the air that has been heated by direct combustion of the fuel as the working fluid that turns the turbine. As such, it operates like a small jet engine. Key components include compressor module, fuel injection ports, and a combustor unit. An air intake sucks air into a compressor unit where it is compressed to much higher pressures and densities and mixed with fuel. This compressed volume of fuel and air is ignited, creating a rapidly expanding mass of high pressure hot air (up to 2,900°F) which passes through the turbine like steam, turning the turbine’s drive shaft. The drive shaft is connected to a separate generator shaft which actually turns the rotor. The connection is made via an interconnecting gear box which allows changes in frequency between the normal 50 and 60 Hz operating ranges. The high operating temperatures require special cooling of the blades with a continuous airflow along their surfaces.
A further refinement of the gas turbine is the combined cycle system. As can be imagined, a gas turbine can generate huge amounts of excess waste heat. The combined cycle design captures this waste heat and puts it to use, thereby increasing the efficiency of a gas turbine by an additional 50%. Instead of allowing the hot expanding gas to escape out the end of the turbine, it is instead captured by a heat exchanger. The heat exchanger takes this otherwise waste energy and uses it to heat water to steam to run a second, smaller turbine to generate additional electricity. The water is continuously recycled, being condensed back into water after turning the secondary turbine rotor, and reheated again in the heat exchanger.
Lastly, there are microturbines. They differ from standard turbines in several ways, not just their physical size or power output. The power output of the turbines ranges from 25 kW to 500 kW. They combine small size and physical footprint (about the size of a refrigerator ranging up to about a minivan) with high efficiency, especially when utilizing natural gas for fuel. As a result, they produce little in the way of air pollution (NOx emissions less than 9 ppm). Their small size allows for their installation in areas with severe size constraints. Even with their small size, they can take advantage of cogeneration with the use of a sheet metal heat exchanger called a recuperator, and thereby achieve an even higher level of efficiency.
The inner workings of a microturbine define its operational function. These can vary from the simple design (single shaft, bearing configuration using either oil or air), and the more complicated (double-shaft, inter-cooled, and reheat systems, split-shaft for machine-drive applications, recuperated CHP or simple un-recuperated single cycle). Microturbines operate at design speeds of 40,000 rpm to 120,000 rpm. The simpler microturbines operate at lower efficiencies (as low as 15%) but are cheaper to own and operate. The more complicated combined cycle microturbines with CHP heat recapture achieve overall fuel efficiencies of 85%, but are far more expensive. They also require unique construction and rely heavily on parts made from heat-resistant ceramics.
How and Why Microturbines Are Used
From asking what are microturbines, we turn now to how and why they are used. They are uniquely suited to act as standalone or emergency power sources, and as part of a distributed energy system. Their ability to quickly start up and deliver a consistent electrical load makes them ideal as emergency backup power sources. Conversely, they can also start up when peak power demand exceeds normal grid power outputs, providing peak shaving at minimal cost and no disruption to the grid. Microturbines also adapt easily to a CHP system, using waste heat to heat the working fluid that drives the microturbines. The result is a highly efficient low-cost power system, almost as if the entire power grid was equipped with the secondary turbines found in individual combined cycle systems. Individual use for customers on the edge of a power grid or completely isolated from it is another obvious application of microgrids. Furthermore, their use can improve both the durability and the performance of general power grids of only marginal quality.
Microturbines, being smaller and less complicated, are cheaper and easier to maintain. This is of special importance in isolated applications where fully staffed maintenance crews may not be available. Their small size and light weight allow for easy installation into superstructures and onto foundations that would not normally allow for auxiliary power systems. Being small and lightweight also reduces vibration and noise from operations. Microturbines can improve the appearance
of a facility by being small enough to be hidden and can do away with intrusive overhead power lines. They are by definition “flexi fuel,” able to use everything from methane and propane to biogas and waste heat. However, microturbines tend to be less fuel efficient (large turbines can take advantage of scale to generate more kWh per unit of fuel) and can have their performance degraded by higher ambient temperatures.
But the bottom-line remains the bottom-line. How does the cost of operating a microturbine compare to large-scale units? First, look at capital costs. These are the startup costs associated with hardware and installation, instrumentation and controls, software and communications, staffing and training—and can run from $700 to $1,100 per kW. Labor costs can add another $200 to $500 per kW. Customizing the microturbine system with add-ons such as CHP systems can increase the costs further by $75 to $350 per kW—making a total range of almost $1,000 to $2,000 per kW. General operations and maintenance costs run between almost $50 to $150 per kW annually.
Proper Integration of Microturbines and Microgrids
The point of powering a microgrid with a microturbine is to provide an appropriate-scale power source that enhances the microgrid’s inherent advantages in resiliency and flexibility. As they operate independently of the main power grid for a relatively small number of customers, microgrids don’t need massive, urban-scale power sources. Microgrids have also opened the way for wider acceptance of nontraditional energy sources such as renewable solar and wind, as well as energy storing devices such as batteries or fly wheels.
Microgrids are intended to be robust and resilient, surviving natural disasters such as hurricanes, tornadoes, and even earthquakes—stepping in to provide power to critical locations should the main grid go down. Their small size and simple configuration makes them less vulnerable to the forces of nature so they can continue to service populations and critical facilities such as hospitals, police stations, sewer treatment, and stormwater flood control pump systems. Though originally intended for emergency situations, microgrids have found expanded uses in servicing commercial, industrial, educational, and business operations. Given these advantages, both state governments and the Department of Energy have created grant funds and programs to encourage the spread of microgrids including advanced designs, engineering applications, and infrastructure development.
Flexibility is the characteristic of microgrids that has made possible the widespread adoption of distributed energy, renewable energy, and alternative fuels. Variable sources of renewable energy can directly sell their peak power output back to the grid via the connection points with the microgrid. During nighttime or when the wind is not blowing, the microgrid can acquire energy from the main power grid to make up the demand shortfall. In this way, a renewable energy
source can be integrated into the main power grid without heavy reliance on auxiliary energy storage systems. For nonrenewable sources, the same flexibility applies with the local microturbines providing inexpensive peak power when needed without having to pay peak rates to the main power grid. This can provide a significant, if indirect, source of cost savings to the microgrid system.
Like pearls embedded in the overall web of the main power grid, each individual microgrid makes up an “island” of power supply. However, this can cause safety issues to those repair crews working to restore the main power grid back to full function and certain safety features are built into the microgrid where it connects to the main grid. These are referred to as a point of common coupling (PCC). The PCC’s primary job is to regulate voltage at the connection to ensure that the voltage from the microturbine is in sync with the voltage from the main power grid. If the grid goes down, the second job of the PCC is to flip a circuit breaker that isolates the grid. While the microgrid continues to provide service to its customers, the repair team that is working to restore the main power grid can work safely in isolation from the potential harm from the current in the microgrid.
The goal of expanded use of microgrids is to provide a more resilient power grid. This goal was stated in Executive Order 13653, “Preparing the United States for the Impacts of Climate Change,” and the goal of “building stronger and safer communities and infrastructure” in accordance with “The President’s Climate Action Plan.” The goal of a more resilient power grid is embodied in the Department of Energy’s Smart Grid R&D Program managed by the DOE Office of Electricity Delivery and Energy Reliability. The program has the twin objectives of: “modernizing the electric distribution grid through the adaptation and integration of advanced technologies (information, communications, and automation) and new operational paradigms (microgrids and transactive controls); and supporting the increasing demand for renewable energy integration and grid reliability and resiliency at state and local levels.”
Microgrids are the key to achieving these objects. The DOE sees microgrids as critical in the development of enhanced system design for residency, improved preparedness and mitigation measures, the design of segmented and agile distributed systems, and the determination of restoration priorities.
Towards these ends, the DOE has awarded $8 million to seven microgrid projects to assist communities to be better prepared for extreme weather events. The DOE’s Pacific Northwest National Laboratory and Washington State University have partnered in the development of microgrids as a resiliency resource by developing power generation assets within the microgrid and by serving critical loads outside the boundaries of microgrids. By extension, these microgrid developments will be designed to service auxiliary systems (water pumps and condensate pumps, e.g.) as black start resources necessary to assist in bringing the main power grid itself back online when it fails.
Parallel research continues in the development and enhancement of microturbine technology along with continued expansion of the microturbine market. Market projections estimate microturbine sales to reach $125 million by 2025, up from $60 million in 2016—more than doubling in less than a decade. According to “Microturbines Market – Global Industry Analysis, Size, Share, Growth, Trends, and Forecast, 2017–2025” (Transparency Market Research, 2018): “Aging power infrastructure and poor grid connectivity in several parts of the world are estimated to increase the need for onsite power generation around the globe. Moreover, the rise in energy prices, increasing oil insecurity, imminent stringent regulations, and the rise in concerns regarding energy consumption and emissions are expected to boost demand for on-site power generation and therefore, the microturbines market.” So, in the race for developing newer and more resilient power sources, it’s time to bet on the little guy.
A worldwide leader in microturbine systems and associated green technology, Capstone Turbine is a manufacturer with over 100 patents. The company provides a complete line of green energy microturbines. These microturbines can utilize a variety of gaseous or liquid fuels (natural gas, associated gas, LPG/propane, flare gas, landfill gas, digester gas, diesel, aviation fuel, and kerosene), though Capstone’s microturbines can be engineered to operate on biogas from organic waste sources, landfills, wastewater treatment facilities, food processing facilities, and agricultural waste process by an anaerobic digester. This makes them ideal candidates for installation at landfills and digester facilities, allowing the owner to actually utilize the gas being produced instead of just flaring it away. Power outputs range from 30 kW to 30 MW, starting with Capstone’s C30 microturbine. The C30 is rated at 30 kW and operates with a 26% efficiency (90% efficiency with CHP modifications), producing three-phase voltage (400 to 480 V) at a frequency of 50 or 60 Hz. Its small compact size (measuring only 30 inches by 60 inches by 70 inches, weighing 891 to 1,271 pounds) allows for easy installation even in confined spaces and limited areas.