When power discussions turn to distributed energy resources (DERs), renewable energy technologies such as wind and solar usually come to mind. However, engines and small turbines have served in the role of distributed power sources for a much longer period. Originally, they were used primarily for backup supply and emergency power, and also in applications where facility operators realized they could economically produce hot water or steam for manufacturing or domestic requirements and obtain power at the same time. Continuing advances in turbine and engine technology are making these “old-time” DER staples an attractive option for remote power supply in a growing number of applications.
Engine and small turbine power systems offer all the electrical advantages of renewable DERs and they are not intermittent. They can eliminate transmission power losses, provide stand-alone onsite power in remote areas, and can reduce voltage dips, transients, surges, and frequency variations when correctly designed. They can also be designed to supply standby power, peak shaving, and lower cost energy supply in certain applications. Unlike wind and solar sources, engines and turbines can readily be configured to supply thermal energy in addition to electricity for higher overall efficiency.
From a utility perspective, reciprocating engines and turbines can be used to optimize the performance of existing grid assets in the event of an emergency or in anticipation of infrequent issues. Such equipment can also address grid issues on a short-term basis while permitting line upgrades. Further, given their shorter lead times, these distributed resources allow a just-in-time commitment of capital for permanent upgrades as opposed to the historical approach of oversizing grid systems in the near term in order to meet a 20-year working horizon.
Turning to the technology choices, reciprocating engines have served the power industry since the beginning. They are attractive because of the broad range of fuels they can use and a nearly infinite choice in available sizes. Reciprocating engines are manufactured with electrical capacities ranging from several kilowatts to 85 MW. They are responsive to power requirement changes over a broader range than turbines and they perform better at low power levels. Engines also may be less expensive than turbines for many applications. The capital cost of reciprocating engines ranges from US$300 to US$800/kW, according to the National Institute of Building Sciences.
Small turbines, particularly those in the size range of 25 to 500 kW, known as microturbines, are the new kids on the block, relatively speaking. Like the smaller size range of reciprocating engines, modern electronics are increasing possible applications, increasing autonomy, reducing interconnection costs, extending operation intervals between maintenance requirements, and improving economics. Microturbines have a very small number of moving parts, are low in weight, compact in size and purportedly exhibit the lowest emission rates of any noncatalyzed fossil fuel combustion system. Recuperated microturbines have a heat exchanger that recovers heat from an exhaust stream and can lead to overall efficiencies above 80% in combined heat and power (CHP) applications.
Unrecovered microturbine capital costs range from US$700 to US$1100/kW according to the National Institute of Building Sciences. Digital processor controls and the use of output inverters simplify grid connection and allow modifications that equip microturbines to serve as hybrid UPS systems. However, the use of the microturbine option usually makes the most economic sense when a CHP or combined cooling, heat, and power (CCHP) application exists.
While advancements in digital technology are creating more and more opportunities for advanced turbine, engine, and other DERs, the integration of such equipment into the grid to maximize customer and grid system benefits is still a learning experience. A recent article in T&D World magazine (July 2019), contributed by Justin Orkney from Tucson Electric Power (TEP), describes a research project sponsored by TEP and EPRI to determine how disparate distributed resources perform when aggregated on the grid to meet flexible loads and function dynamically. While this study is still underway, the findings not only indicate considerable potential, but also limited off-the-shelf solutions, and they highlight the critical importance of selecting a distributed energy resource management system (DERMS) capable of coordinating the available assets. Some of the issues developers face sound reminiscent of ground we’ve plowed with IEC 61850. If true, there is a lot of valuable experience we can draw upon to advance DER integration going forward.