51 Single Shaft Gas Microturbines

The classic open Brayton cycle described in Chapter 3 is also the basis of gas-fired microturbine (MT) engines. The reader is referred to that chapter for cycle basics.

5.1.1 Overview

Several single-shaft MTs have been developed recently by Capstone, Elliott, and Honeywell with ratings between about 20-150 kW. Some published specifications for these four MTs are listed in Table 5.1. A number of MT flow-path configurations are depicted in Figure 5.1, the most compact of which is the wrap-around recuperator with an annular combustor, as used in the Capstone MT. Configuration choice is dependent upon the application. For

TABLE 5.1

Example Microturbine Specifications

N

Power

Efficiency

Manufacturer

krpm

kWe

% (LHV basis)

Recuperated

Capstone

30

96

28

Yes

Elliott

116

45

17

No

Honeywell

75

75

30

Yes

FIGURE 5.1

Various microturbine flowpath designs.

FIGURE 5.1

Various microturbine flowpath designs.

example, if both simple and recuperated cycle MT variants are to be marketed, it could be advantageous to use the add-on recuperator type, with possible provision for recuperator bypass and coupling with fuel cells. Bypassing also enables more heat to be available for cogeneration purposes.

Optimum MT rotational speeds at typical power ratings are between 60 to 100 krpm with compressor and turbine tip diameters of the order of a few inches, similar to small turbochargers. The state-of-the-art of small turbo-charger turbomachinery has markedly improved in the last decade with the introduction of advanced computational fluid dynamics (CFD) design methodology and the routine use of composite materials and ceramic bearings.

The major aerodynamic difference between the small gas turbine and the turbocharger is the turbine design. The hurdles to a viable MT are not turbo-machinery technology as much as other factors, such as:

• Cost concerns — overall $/kWe and recuperator costs

• Natural gas injection methods and their safety

• Shaft dynamics and bearing design

• Recuperator reliability, effectiveness, and cost

Cost concerns depend heavily on the generator power conditioning and control systems to be discussed later.

A major design feature of small gas turbines is the use of radial flow compressors and expanders. Use of radial flow compressors makes it possible to achieve a high pressure ratio, typically 3 or 4:1 in a single stage compared with about six stages in an axial flow compressor. For higher pressure ratios, more than one radial stage would be used, introducing considerable complexity. Thus, most gas turbines below 100 kW output and all machines in the 100 kW class use single-stage radial compressors. Most of them also use radial flow expanders or turbines.

The inherent characteristics of radial flow compressors lead to a preference for low pressure ratios that can be reached in a single stage. This, in turn, leads to a relatively higher exhaust temperature and a lower efficiency. To produce an acceptable efficiency, the heat in the turbine exhaust must be partially recovered and used to preheat the turbine air supply before it enters the combustor, using an air-to-air heat exchanger called a recuperator or regenerator. The effect is a savings in the fuel requirement and an increase in efficiency by a factor of nearly two.

Simplicity and cost exert major influences in the design of small gas turbines. Other things being equal, the usual economic scaling laws apply, so for small gas turbines to compete with large machines in terms of cost per kW of output, cost must first be pared by using low-cost materials and production methods and reducing the number of components.

The electrical connection to the grid or to a stand-alone load requires control of speed and voltage. MTs usually employ permanent magnet variablespeed alternators generating very high frequency alternating current (AC) which must be first rectified and then converted to AC to match the required supply frequency.

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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