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Cogeneration March 1, 2019 - How should cogeneration blocks be treated from the perspective of reporting capacities, output, efficiency and reliability data?

GADS OS and the system on which it was based, NERC GADS, did not contemplate reliability and capacity for more than electric energy generation. That is, thermal energy/steam as an output in terms of capacity and availability was not contemplated, nor reported, nor tracked by NERC GADS.

NERC GADS generally assumes that ALL generators basically are stand-alone utility-owned or merchant units built and operated for the sole purpose to deliver to the bulk electric system (BES) power for resale to third-party, wholesale or retail customers.

Combined heat and power (CHP) machines most often are built to serve an end-user thermal load with any residual electricity sold as a by-product to the grid. The tight integration of CHP to the end user’s steam/thermal requirements makes it inseparable from the load.

Our GADS OS software currently addresses cogeneration plants and CHP plants within the limitations of NERC GADS reporting requirements.

In 2019, GADS OS is being modified to begin accepting thermal production data and events; however, this data will not be reported to NERC or any of the ISOs. The data collected will allow the comparison of CHP plants with differing host steam requirements using a consistent methodology.

Goal – A consistent method must be defined for handling data for cogeneration facilities. Specifically, it must take into account the Net Dependable Capacity, Net Maximum Capacity, Efficiency, etc., down to a major component basis, and address the concept of redundant capacity, and/or utilization variations from major plant components with rebalance or reallocation of steam generated and utilized.

Cogen plant Details - In most cases, the cogeneration and CHP plants vary widely from a configuration perspective, but consist of a minimum of two or three basic elements. The basic elements might include a primary boiler, combustion turbine generator, a steam turbine generator, a back-up boiler, and at least one steam flow to a host, whether central district heating, or an industrial consumer.

We will use BLR, HRSG, CTG, STG, BLRbu, HRSGf and STM to discuss these elements.

  • BLR indicates a primary boiler involved in the regular conversion of fuel energy release to thermal energy in the form of steam or hot water.
  • HRSG is a heat recovery steam generator consuming only waste heat to produce steam or hot water.
  • BLRbu (back-up boiler) provides steam or hot water only when other primary components are not available, and is considered extra steam production capacity.
  • HRSGf (fired HRSG) represents the fired fuel - duct burners or supplemental burners - to augment the steam or hot water produced solely from waste heat and is also considered to be extra steam production capacity.

The elements on which a Net Dependable Capacity and Net Maximum Capacity are calculated includes the CTG (MWs), the STG (MWs) and the STM flow (MWs and/or MMBtu). To establish the Net Dependable Capacity, the values used are to be those which represent the normal allocation of output energy, based on an assumed thermal/electric output, which is typical, and is defined by the GADS OS user in the software.

To summarize this point, BLR, BLRbu, HRSGf, and CTG are fuel conversion elements. STG is a steam to electric element. HRSG is typically tied directly to a CTG, but not always.

The code enhancements result in the three following points:

  • To establish the Net Dependable Capacity at the components, use what is the standard delivery operation or, said another way, thermal/electrical balance of delivered energy based on the users physical steam system energy balance. That is, what is the typical mode or mix of thermal and electrical energy delivered from the cogen block.
  • To establish the Net Maximum Capacity, using the sum of the design capacity of each of the components from the user’s block equipment.
  • To establish the Net Available Capacity for each cogeneration process steam extraction using the Steam Pressure, Temperature, and Flow Rate for the available steam flow reduction due to each event. Where applicable, the energy transfer capacity reduction at each energy level leaving the power block, as a result of the event.

The maximum capability available/utilizable will be calculated from the maximum steam production from all sources, before any utilization of steam, either through thermally delivered steam or hot water, or before conversion to electrical energy through a steam turbine generator.