Capture more turbine energy with CHP
Thermal applications are the key
In the evolving world of combined heat and power (CHP), the use of natural gas-fired industrial turbines rated at 1 MWe and higher with heat recovery has become increasingly attractive. These machines are built for long running hours, heavy loads and high fuel efficiency. Technology for extracting high temperature exhaust heat has also improved, so total system efficiencies of 80% and higher are often achievable.
CHP Is On the Rise
CHP applications powered by natural gas are burgeoning in popularity. One reason is the continued attractive price of the fuel. Ed Mardiat is a Principal and Director of Project Development for Burns & McDonnell, an international engineering, architectural and consulting firm. Mardiat has been extensively involved in a wide range of CHP projects.
He notes, “The abundance of and lower price for natural gas have dramatically helped CHP economics. The other related factor is environmental regulations such Industrial Boiler MACT [Maximum Achievable Control Technology] requirement, which is forcing many of the older coal-fired assets to be replaced with natural gas-fired units.”
Energy System Security
Mardiat also points out that there is growing interest in micro grids. “These in combination with on-site CHP systems can improve energy security. Many industrial, institutional and government facilities are exploring these alternatives.”
Turbine CHP can be used for a wide range of beneficial purposes. It makes the most sense to use turbines where the need for heat is large, the preferred thermal output is steam rather than hot water, the run-hours will be long, and the output is generally at the upper end of the turbine power range.
In a recent presentation at a Technology Marketing & Assessment Forum (TMAF) sponsored by the Energy Solutions Center, Chris Lyons from Solar Turbines and Mike Devine from Caterpillar discussed the opportunities for both large engines and combustion turbines for CHP applications. They noted that natural gas-powered CHP of both types is attractive now because of the low and stable price of the fuel, the need to control emissions, the proven nature of both technologies, high reliability and low life-cycle costs.
Choosing Between Turbines and Engines
Turbines are often the choice where the thermal load requirement is high, steam is preferable to hot water, and where the electric requirement is continuous and relatively even. These units produce large quantities of waste heat at temperatures ranging from 700° to 1,000° F – a range where high-energy steam can be efficiently extracted using a heat recovery steam generator (HRSG).
In certain applications, CHP using larger industrial turbines is an excellent fit. According to Lyons, common applications include food processing, dairy plants, breweries, pulp and paper, pharmaceuticals, district heating and cooling, colleges and universities, healthcare facilities, hotels and resorts, and penal institutions. “This is an ideal choice where there are major thermal or chilling loads.”
Take Advantage of Higher Temperatures
Mardiat from Burns & McDonnell echoes this opinion, saying, “Because turbine simple cycle heat rate efficiency is lower than reciprocating engine systems, with exhaust temperatures in the 700 to 1,000 degree F range, industrial, institutional and government facilities that require an abundance of steam for heating or process loads provide the best fit for this prime mover technology.” He points out that for most applications, it is important to make full use of the thermal output. “The highest efficiency and most economic CHP systems use 100% of the waste heat to provide steam or hot water.” This then results in very high total system efficiencies.
Range of Sizes Available
Solar offers turbines ranging in size from the 1.2 MWe Saturn 20 unit to the two-shaft Titan 250 unit rated at 21.7 MWe. In some situations, owners prefer to have multiple units to further increase system reliability, and to allow the selected units to operate near their peak efficiency.
Lyons points out that the smaller turbines do not have efficiencies as high as with larger units, ranging from 25% to 39%. “But in CHP applications, the overall thermal efficiency can range from 70% to 90%.” He suggests that most buyers size the units to match their thermal load requirement. “However the decision is also very dependent on overall electric rates and factors such as the need for plant reliability.”
Longer Service Intervals
Today’s combustion turbines have high reliability and designers have extended required service intervals. For Solar Turbines, Lyons explains, “Most of our turbines are designed for 30,000 hours between overhauls. However some of the smaller units go well beyond 40,000 hours.” He notes, “From time to time there are issues that might require attention to prevent a premature failure.”
Generally, combustion turbines operate at their peak electrical efficiency near full load. When at part load, a greater part of the fuel energy goes to the thermal load and less to the electrical. Lyons emphasizes that for combined heat and power situations, the efficiency at part load is still quite good. In situations where the turbine operates at a low level for an extended period of time, a supplemental heat source to the HRSG may be desirable.