By Andy Ungerman, PE, senior mechanical engineer, Stanley Consultants
In their search for reliable, flexible, low-cost sources of power, investor-owned utilities, municipal utilities and rural cooperatives have varying pressures and selection factors but many of them choose the same technology solution: reciprocating engines.
Overall power trends affecting their choices in the U.S. are readily visible: Decommissioning of small and medium coal-fired units, natural gas-fired power generation for peaking and base-load capacity, rapid growth of renewables; growth of distributed generation and mixing of power generation technologies within a utility.
The versatile reciprocating engine is increasingly being selected as the solution to these challenges, from one small engine to a bank of larger engines. While the engines can operate on fuel oil or other liquid fuels, they primarily burn natural gas.
Individual producer needs vary: A tale of a few cities
The city of Alexandria, Louisiana’s contract with the local utility had ended. How could the city upgrade its aging fleet, be self-sufficient and stabilize rates for 20 to 30 years while joining the Midcontinent Independent System Operator? City officials also wanted a diverse, sustainable fuel source.
The city’s prior fuel mix for generation was 80 percent coal, 17 percent gas and 3 percent hydroelectric. As part of the EPC team selected for the project, Stanley Consultants acted as engineer-of-record for the balance of plant for the installation of the seven, 9 MW Wartsila reciprocating engines that satisfied Alexandria’s requirements.
The engines can be started up in about five minutes, which means that sudden demands from the grid could be accommodated. All or one of the engines can be used at any time, providing more reliability and flexibility. The engines even fit in a tight space adjacent to the city’s existing plant in the middle of town and meet the city’s stringent noise requirements. After installation, the fuel mix is 47 percent coal, 50 percent gas and 3 percent hydro.
Tallahassee Florida: Build this plant fast
Tallahassee needed to add power generation quickly, and reciprocating engines fit the bill.
Power production manager Triveni Singh said the multiple small units fits the city’s load profile and will complement the planned 20 MW solar facility. Efficiency, low maintenance and high reliability in addition to the quick-start capability were also keys, Singh said, as well as lower C02 emissions and the ability to be located at distributed sites.
Palmer, Alaska: Extreme temperature changes, earthquake concerns
Matanuska Electric Association is Alaska’s oldest existing and second-largest electric cooperative. MEA’s service area includes more than 4,300 miles of power lines in Southcentral Alaska. MEA had purchased power from another Alaska cooperative but wanted to be more self-sufficient and generate their own. Stanley Consultants served as owner’s engineer for the project.
The needs? Reliable, affordable energy and the ability to operate in temperatures as low as 40 degrees below zero Fahrenheit. In addition, in case the supply of natural gas was interrupted, the plant could switch to fuel oil stored onsite. The cooperative would also be asked to produce as much as 145 MW during the winter and as little as 50 MW during the summer. The installation of 10 new Wartsila 17.1 MW engines met MEA’s needs because of their ability to add grid reliability and fast response time in case of load following. The plant is 30 percent more efficient than its former power supply. MEA is producing electricity for its members at a lower cost and generating additional revenue from selling excess capacity to other utilities.
The benefits of the reciprocating engine
Reciprocating engines provide a proven, reliable technology, but recent upgrades in efficiency and reliability are making them the go-to choice for the addition of small to medium-sized power generation.
For larger applications, reciprocating engines may not be a fit. The overall plant footprint is smaller for a combustion turbine, and the heat rate, or efficiency looks better, especially in combined cycle mode. In larger plants, capital costs can be lower with combustion turbines as well.
According to the U.S. Department of Energy, “Gas-fired reciprocating engines are the fastest-selling, least expensive distributed generation technology in the world today.” The technology does not fit every situation, but has found its way into the power generation ecosystem.