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Combined Heat & Power - What is it?


What is Combined Heat & Power (CHP)?

A CHP system produces both electricity and heat from a single fuel source specifically, from the same prime mover (engine).

CHP systems are highly efficient compared to traditional single cycle power generation systems. Historically, small-scale power generation equipment only made use of the electrical energy produced.


How does CHP work?

In any combustion CHP system, the prime mover will use fuel through a combustion event to generate mechanical energy used to drive an alternator which produces electricity. In legacy systems, the waste heat from the exhaust, various cooling systems & thermal radiation were seldom collected. With an increasing importance (and market) for energy conservation, newer systems collect all of these "waste" sources for use. This is the basis and philosophy of a CHP system.


What technologies are available?

In this article, we will explore the use of combustion gas turbines compared against reciprocating internal combustion engines in CHP systems. There are other technologies; however, they are vastly out numbered by these two equipment types.

Combustion Gas Turbines:

Combustion gas turbines are a type of turbine that can burn several different types of fuel liquid and gaseous, such as natural gas to produce electricity. The combustion gases are used to spin the turbine, which in turn drives a generator to produce electricity. The exhaust gases from the combustion process can be released into the atmosphere or used for heat recovery, making combustion gas turbines a highly efficient candidate as a source of electricity and heat.

The exhaust gases from the turbine combustion process can be used to heat water or to create steam, which can then be used for a myriad of applications. Typically, produced hot water (glycol) is used for HVAC and other industrial processes. This can significantly increase the overall efficiency of the system, making it a highly attractive option for many industries.

Due to the large mass flow of heat from the exhaust of a turbine, they are ideally suited for combined cycle uses.


Reciprocating Internal Combustion Engines (RICE):

Reciprocating internal combustion engines are another type of engine commonly used in CHP systems. These engines use a piston and crankshaft to convert the energy from combustion into mechanical energy, which is used to drive an alternator to produce electricity. RICE engines can burn a variety of fuels, including natural gas, diesel, and other liquid and gaseous sources.

In CHP systems, RICE equipment can also be used to generate both electricity and heat simultaneously. The exhaust gases from the combustion process can be used to heat water in a similar manner to a turbine as described above.


Comparison between Combustion Gas Turbines and RICE Engines in CHP Systems

When comparing combustion gas turbines and RICE engines in CHP systems, there are several factors to consider; including efficiency, emissions, and maintenance costs.


Efficiency

Combustion gas turbines are generally less efficient than RICE engines in single cycle applications. Gas turbines can achieve overall efficiencies of up to 35% (5MW<), while RICE engines typically have efficiencies in the range of 40-45%. For this reason, RICE equipment has been used in power generation in small scale, (5MW<) preferentially. This is changing, as turbine efficiencies have been incrementally improving, as well as the rise in using them for CHP applications where they have emerged as a clear leader.


Emissions

Combustion gas turbines, overwhelmingly, produce fewer emissions than RICE engines. This is particularly in terms of nitrogen oxide (NOx) as well as Sulfur oxide (SOx) emissions. NOx & SOx emissions from combustion gas turbines achieve PPM as low as 7PPM. RICE engines, on the other hand, typically have higher emissions of NOx and other pollutants, making them less environmentally friendly. With the use of SCR and other systems, RICE equipment can achieve low emissions, however, usually at greater upfront expense and maintenance.


Maintenance Costs

RICE engines have significantly higher maintenance costs as compared against combustion gas turbines. This is due to the more complex nature of RICE engines, which require more frequent maintenance and replacement of parts. In contrast, combustion gas turbines can be significantly more reliable and require less maintenance, making them a more cost-effective option in the long run.


Applications

Combustion gas turbines are well suited to large-scale CHP systems, such as those used in power generation or district heating. These systems require high levels of efficiency and reliability, which combustion gas turbines can provide. RICE engines, on the other hand, are better suited to smaller-scale CHP systems, such as those used in hospitals, schools, or small industrial facilities. These systems have lower energy demands and can benefit from the flexibility and lower initial cost of RICE equipment.


Sources:

· U.S. Department of Energy. (2017). "Combined Heat and Power (CHP) Technical Assistance Partnerships (TAPs): CHP Technology Basics." https://www.energy.gov/eere/amo/articles/combined-heat-and-power-chp-technology-basics

· U.S. Department of Energy. (2016). "Combined Heat and Power: A Guide to Developing and Implementing Greenhouse Gas Reduction Programs." https://www.energy.gov/sites/prod/files/2016/10/f33/chp_guide.pdf

· Siemens SGT-50 Gas Turbine, Siemens Energy Website, https://www.siemens-energy.com/global/en/offerings/gas-turbines/sgt-50.html

· Advanced Gas Turbine Systems Research Program, Gas Turbine Efficiency, U.S. Department of Energy Website, https://www.energy.gov/eere/amo/advanced-gas-turbine-systems-research-program

· St. Joseph Energy Center, Indiana, USA, Power Technology Website, https://www.power-technology.com/projects/st-joseph-energy-center/

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