On-Site Gas: Engines and Turbines
When a site needs dense, dispatchable power and a pipeline reaches the property, natural gas combustion is the workhorse. It comes in two main forms, and the choice between them shapes the plant.
Reciprocating gas engines are the modular option. These are large piston engines, close cousins of ship and locomotive engines, sized in blocks of a few megawatts each. Their strengths are directly useful behind the meter. They achieve high electrical efficiency, often in the low-to-mid forties percent on a simple-cycle basis, and they hold that efficiency well at part load, which matters when your load swings. They start in minutes and tolerate frequent cycling. Because they come in modular blocks, you build a plant by stacking units, which gives you redundancy for free: with several engines, the loss of one is a small fraction of capacity, not a blackout. That N-plus-one structure is why engine plants suit sites that treat generation as primary rather than backup.
Gas turbines are the other path. A simple-cycle turbine burns gas to spin a generator directly; efficiency in that mode is lower than a reciprocating engine, often in the thirties. The turbine's advantage shows up at scale and in heat. Aeroderivative turbines, derived from jet engines, start fast and pack large capacity into a small footprint. Larger industrial-frame turbines produce enormous, high-quality exhaust heat. Capture that heat and you change the economics entirely.
That heat recovery is the concept every serious gas buyer must understand: combined heat and power, and combined cycle. In combined heat and power, you use the engine or turbine exhaust to make steam or hot water for a process, a district loop, or space heating. Because you are now getting useful work from fuel that would otherwise go up the stack, total fuel utilization can climb toward the seventies or eighties percent, and your effective cost per unit of energy drops sharply. In combined cycle, the turbine exhaust drives a steam turbine that makes still more electricity, pushing electrical efficiency toward the high fifties or low sixties. The rule is simple: if you have a use for heat, or enough scale to justify a steam bottoming cycle, capturing the exhaust is often the single biggest lever on gas economics.
Against those strengths, price the liabilities honestly. The first is fuel. When gas is the primary source and the plant runs thousands of hours, the fuel bill dominates lifetime cost. You are now exposed to gas price volatility and to firm delivery: a plant that must run needs firm pipeline capacity or on-site fuel storage, and interruptible gas service can leave you dark on the coldest days when both power and gas markets spike together. The second liability is emissions and permitting. Continuous combustion produces carbon dioxide, nitrogen oxides, and other regulated pollutants. Emergency-only generators enjoy light permitting; a continuously running plant faces a far higher bar, often requiring emissions controls such as selective catalytic reduction and oxidation catalysts, and an air permit that can take many months and cap your annual run hours. In areas that already exceed air-quality standards, that permit may be the binding constraint on the whole project.
The third liability is operational. A plant that runs as primary power needs maintenance, spare parts, on-site staff or a service contract, and a controls system that can manage multiple units, respond to load changes, and, if you island, hold frequency and voltage on its own. This is real industrial operation, not a switch you flip.
How do you choose between engines and turbines? Match the machine to the profile. Choose reciprocating engines for modular redundancy, high part-load efficiency, frequent starts, and sites without a large steady heat sink. Choose turbines where you want maximum capacity in a small footprint, where you have a big continuous heat or steam demand that suits combined heat and power, or where combined-cycle scale justifies the extra equipment. Many large behind-the-meter builds combine both, or phase engines first for speed and add heat recovery later.
The strategic takeaway is that on-site gas is the default dense-power option, but its economics live and die on two numbers: whether you can capture the heat, and how many hours the air permit lets you run. Get those right and gas is a genuinely competitive primary source. Get them wrong and you have built an expensive machine that either wastes most of its fuel as heat or sits throttled by its permit.