Page 73 - Sustainable On-Site CHP Systems Design, Construction, and Operations

52 CHP B a s i c s

engines, safety controls are also implemented in case of runaway (or loss of speed
control). Also, as with engine generators, a plant control system is required to monitor
CHP plant equipment and systems; for example, oil pressures and temperatures, turbine
inlet temperature, air inlet and exhaust temperatures, gas pressures, equipment status,
and to meter key plant parameters as identified in Chap. 17. Additional supplemental
systems (such as CTIC systems) also require controls as their specific operating
parameters have consequences on the overall performance of the CTG and the CHP
plant as a whole.

Equipment Life, Operation, and Maintenance
Combustion turbines offer life spans of more than 20 years, when well maintained
and regularly serviced, and offer large amounts of high-quality (high temperature
and pressure) thermal output. Care should be taken to select a combustion turbine
or turbines that closely match the baseline electrical load of the building or facility,
since the efficiency of these machines at part load can be substantially below that of
the full load performance. Combustion turbines are typically designed to operate
between 30,000 to 50,000 hours between overhauls. Requirements for preventative
and predictive maintenance methods are similar to those with reciprocating engines,
even if the actual maintenance is different. In order to limit downtime, many manu-
facturers have rebuilt replacement turbines that can be used to replace a turbine
scheduled for overhaul.

Microturbines
Microturbines are very small combustion turbines, which feature an internal heat
recovery heat exchanger called a recuperator, as previously described. In a microtur-
bine, the inlet air is compressed in a radial compressor and then preheated in the
recuperator using heat from the turbine exhaust. Heated air from the recuperator is
mixed with fuel and ignited in the combustor, and hot combustion gas is then
expanded in the expansion and power turbines. The expansion turbine drives the
compressor and in single-shaft design drives the generator as well. Two-shaft turbine
designs use the turbine’s exhaust to power a second turbine (the power turbine) that
drives the generator. The power turbine exhaust is then used in the recuperator to
preheat the air from the compressor.
Microturbines can be designed to operate on a myriad of fuels, including natural
gas, propane, landfill gas, digester gas, sour gases, and liquid fuels such as biodiesel,
gasoline, kerosene, and diesel fuel/heating oil, for example. Operating fuel pressures
for microturbines may require onboard fuel compressors that are offered as options by
most manufacturers.
Microturbines are ideally suited for distributed generation applications due to their
flexibility in connection methods, ability to provide stable and extremely reliable power,
and low emissions. Types of applications include
• Peak shaving and base load power (grid parallel)
• CHP
• Stand-alone power
• Backup/standby power
• Primary power with grid as backup