How cogeneration systems reduce emissions

The CHP process, also known as combined heat and power or cogeneration, is one way businesses and other large facilities can control costs for heating, electricity and cooling, because it extracts multiple forms of usable energy from a single fuel source. The unit produces electric power, which can be used to power some or all of the electricity needed by a facility while a heat exchanger is used to capture heat from the water jacket in the cooling system. Additional heat can be captured from the engine’s exhaust system. The CHP system can also be used for cooling. The heat from the reciprocating CHP system is especially suited for powering an absorption chiller.

Numerous factors allow CHP systems to reduce emissions. Central to CHP’s ability to reduce emissions is the fact that CHP systems are highly efficient, requiring less fuel inputs to achieve the same energy outputs as other systems. CHP systems are able to reduce emissions, even when using fuels such as natural gas. Emissions can be reduced even further when CHP systems are used to enable intermittent renewables such as wind and solar, CHP systems utilize renewable or lower-carbon fuels, or emissions that do occur are reduced through carbon capture or other technologies.

?CHP systems require less fuel inputs for the same energy outputs, reducing all types of emissions, including greenhouse gases such as carbon, criteria pollutants, and hazardous air pollutants.

When heat from CHP is recovered and utilized, it typically displaces steam or hot water from a boiler, which is usually fueled by natural gas. Since the boiler is no longer fueled separately, emissions from the on-site boiler are reduced, lowering the net emissions impact of CHP when compared to separate heat from an onsite boiler and power generated from the grid.

Since CHP systems are typically located close to where the electric and thermal energy will be used, savings are also achieved from reduced line losses, electricity that is typically lost during transmission and distribution from a central power plant to the end user. Within the five major power grids in the United States, average transmission and distribution losses vary from 4,23% to 5,35%, with a national average of 4,48%, and may be even higher when the grid is strained and temperatures are high. By avoiding these transmission and distribution losses associated with conventional electricity supply, CHP further reduces fuel use, helps to avoid the need for new transmission and distribution infrastructure, and eases grid congestion when demand for electricity is high.

Examples demonstrate that, compared to conventional electricity generation, CHP systems can use 32% less fuel and have 50% less annual carbon emissions. Savings for individual systems will depend on a variety of factors, including the fuel use and emissions if heat and power were provided separately, the fuel use and emissions of the CHP system, and the grid emissions where the CHP system is located.

The capacity factor of an electric generation unit describes how often the generation unit operates during a specific period of time, using the ratio of the actual output to the maximum possible output during that period. A generation unit may not run all the time due to maintenance or refueling or due to the intermittent nature of the resource, as is the case with renewable resources such as solar and wind. For example, utility-scale solar photovoltaic facilities had a capacity factor of 24.3% in 2019, while wind facilities had a capacity factor of 34.3%.

CHP systems typically operate at a high capacity, meaning that they often run nearly continuously close to the level of their maximum output.

CHP is an existing technology that can reduce emissions in a cost-effective way. CHP systems are commercially available and immediately deployable, providing a direct path to lower emissions through increased energy efficiency. Well-sited and properly designed CHP systems can be a least cost resource, compared to other baseload resource options that are available.

CHP can also help to reduce emissions overall by providing a consistent source of power that allows more intermittent renewables to be added to the electric grid. CHP serves as an enabling technology to further integrate renewable energy into the fuel mix at the community level. As noted above, CHP’s high capacity factor can help to enable renewable resources such as wind and solar by providing a consistent source of power to smooth out the peaks and valleys of intermittent generation. As a marginal grid resource, electricity generated from CHP does not supplant renewable resources, but rather complements them by providing a consistent source of power.

Historically, CHP units have run on traditional fuels, and many today use natural gas. The first wave of CHP technologies that relied on fossil fuels. However, CHP units can be fueled by renewable and lower-carbon fuels such as biogas, renewable natural gas (RNG) or biomethane, and hydrogen. Use of these lower-carbon fuels can allow CHP systems to reduce emissions even further.

Fuels such as biogas and RNG are already being used in CHP systems, and additional existing systems could run on these fuels, providing a near-term solution for further emissions reductions. Generation equipment currently running on traditional fuels may be able to transition to lower-carbon fuels. In addition, using existing gas pipeline infrastructure can be a low-cost option for delivering these new fuels, and current research is examining how this existing infrastructure can accommodate the distribution of hydrogen gas.

Hydrogen fuel for CHP systems is not yet widespread but could see more extensive use in the future as technologies develop to produce hydrogen fuel and transport it as necessary.

Where a CHP system’s use of fuels still results in carbon emissions, carbon capture and storage (CCS) can help to reduce system emissions. Carbon capture and utilization (CCU) can also be deployed to make use of the carbon that is captured as an input into other products or processes.

CHP can play a significant role in decarbonizing the electricity, buildings, and industrial sectors.

Jose Machado

Source: Energetus, Combines Heat and Power Alliance, MTU solutions, Rolls Royce, Kawasaki Heavy Industries