Distributed generation refers to the small scale, on-site, production of electricity. Currently, hundreds of sites in the US operate some type of distributed generation equipment, including: reciprocating engines, combustion turbines, microturbines, and fuel cells. Benefits of these local electricity production units include reduced transmission losses, improved power reliability and energy efficiency, and peak load reduction. All of these distributed generation systems have benefits, but the focus of this blog will now turn toward microturbines.
Microturbines in particular have a few additional benefits. By eliminating the need for a gear box and pump, microturbines reduce the number of moving parts and therefore improve reliability. They are also low maintenance because they need no liquid coolants or lubricants. The modularly designed microturbines are versatile because they can be operated in grid parallel or as stand alone as well as be used in remote locations. Of all the distributed generation systems, microturbines offer the widest fuel flexibility by being able to run on natural gas, propane, flare gas, gasoline, diesel, and kerosene. Capstone, a major manufacturer, also advertises that their microturbines can be easily modified to run on waste gases from landfills, water treatment facilities, or agricultural and food processing facilities. In addition, many microturbine applications will make use of the high temperature exhaust gases to generate hot water or hot air thus being characterized as a combined heat and power unit.
However, work is still being done to try to improve the overall efficiency of microturbine systems. One area of research has been aimed at incorporating an Organic Rankine Cycle as a bottoming cycle to provide waste heat recovery and generate additional electricity. An Organic Rankine Cycle is similar to a regular Rankine cycle in that a fluid is boiled then passed through a turbine to produce power. The difference is that an Organic Rankine Cycle uses an organic working fluid such as a refrigerant or some other complex hydrocarbon. These fluids are able to boil at temperatures as low as 350 Kelvin, meaning that they can utilize the waste heat from other sources to generate more power.
By simulating a Microturbine coupled Organic Rankine Cycle (ORC) system, I found that the overall electrical efficiency was boosted from 30% to 37% by the addition of the ORC, and the ability to be used in a combined heat and power application can still be utilized. The application of such a system has vast benefits including the reduction of peak demand. As more electricity providers move toward real time pricing and smart meters a distributed generation system could be activated during times of peak demand to reduce the grid load. This allows the microturbine user to avoid the peak prices and would also reduce emissions, grid strain, and transmission losses.