5122 Natural Gas Fueling

Natural gas (NG) is the fuel of choice for small business and domestic MTs but requires compression from essentially ambient pipeline pressures to levels exceeding MT compressor delivery pressure. The compressor outlet pressure is nominally three to four atmospheres. Adiabatic efficiencies approaching 40% have been recently measured (Rodgers, 1989) and required approximately 6% of the engine output power. Positive displacement, rather than dynamic, compressors more efficiently handle the very small, low specific gravity flow of natural gas. The selection of MT cycle pressure ratio requires consideration of the MT gas supply equipment.

Gas injection at the engine compressor inlet, in combination with burning a dilute air-gas mixture, is being researched. At the time of the writing of this book, at least three small companies are experimenting with this alternative to the gas compressor dilemma. Note that the air-gas mixture results in a 5% lighter gas and will slightly reduce overall compressor pressure ratio. Durability and safety aspects are prime concerns exceeding those of achieving efficient gas compression. Additional alternatives to NG compression include some form of lower pressure ratio (2.0) cycle, or the concept of discrete gas injection at the compressor inlet, and possible migration to a rich primary zone for stable combustion (Rodgers, 1991) plus the inverted Brayton cycle with atmospheric combustion. A conceptual inverted MT is shown in Figure 5.2.

Intercooler

FIGURE 5.2

Inverted Brayton cycle schematic diagram.

Intercooler

FIGURE 5.2

Inverted Brayton cycle schematic diagram.

During the late 1970s, AiResearch built and tested components of a 35 kW semi-closed, inverted Brayton cycle gas turbine (Friedman, 1977). The unit was designed for 27% thermal efficiency (ETATH) running at 90 krpm and 815°C. The inverted cycle was selected to allow for natural gas injection and combustion at atmospheric pressure and lower speed turbomachinery components.

Pitfalls that the inverted cycle presents include (1) both recuperator and intercooler effectiveness need to exceed 90%, (2) larger overall system size, (3) elevated compressor inlet temperature, (4) subatmospheric inboard seal leakage, and (5) 70% increase in package weight and cost. 'Emissions might be reduced with partial compressor exit flow recirculation.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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