Intelligent Flow Control Supports Clean, Low Cost Energy

Every engineer knows that in machine design, you can’t get something for nothing.

No system is 100 percent efficient, and when that system involves combustion for heat and electricity generation, efficiency is the main driver in delivering low cost, reliable power.

Current best available gas turbine technology is close to 40 percent thermally efficient…. a good number, but that’s a lot of BTU’s of heat energy going up the stack. “Combined cycle” systems recover some of that heat, but as every engineer knows, it’s the “delta T” that constrains energy recovery.

The exhaust of the first combustion stage is much lower than the peak burner temperature and that’s the starting point for the second stage. Just as in the age of steam, where triple expansion represented the limit of energy recovery from coal combustion, more machinery quickly reaches the point of diminishing returns.

Consider, however, the other uses for that waste heat.

Turbine exhaust temperatures can exceed 600C…. with lots of heat capacity in the working fluid to provide process heat for many applications, from building HVAC to manufacturing operations.

Called combined heat and power (CHP), (or more accurately “cogeneration” when the energy is used for other purposes like cooling) CHP systems save fuel and reduce pollution as well as lowering CO2 emissions.

Major urban areas have markets for both products of a cogeneration operation: electricity and heating/cooling energy, and “district heating and cooling” is commonly used worldwide to serve commercial and residential customers. 

As an example, NRG Energy Center Omaha has delivered reliable and affordable steam and chilled water thermal energy services to its customers in Downtown Omaha for 50 years.

Three NRG district energy plants in Omaha deliver steam and chilled water through an extensive pipeline network. Each building is instrumented to monitor usage for billing purposes.

Steam measurement was originally by pressure and temperature measurement through an orifice plate, along with a BTU meter. Chilled water was measured with simple positive displacement flow measurement, temperature monitoring and BTU meters. Circular chart recorders generated paper records of measurements.

Although state-of-the-art when installed, the original metering systems had limitations. The orifice plate system had a limited “rangeability” (turndown ratio) as the system dealt with wide variations in flow, a problem shared with the chilled water positive displacement flowmeters.

And as the original BTU meter vendor ended support for that legacy product, a system upgrade was ideal. NRG replaced the orifice plate devices with Endress+Hauser Prowirl 73F vortex flow meters, Cerabar PMP71 pressure transmitters and TH13 HART Smart temperature transmitters.

Chilled water flow is measured using Promag 53W Magnetic flow meters. NRG rounds out the system with an RMS621 Steam Flow Computer/Energy Manager and RSG30 Paperless Recorder/Multi-channel display.

Results have been excellent, with improved accuracy and reliability, and an additional benefit of lower pumping costs with the vortex and magnetic flow meters. The RSG30 has eliminated the storage and tracking issues of paper charts, and the systems is managed by an Endress+Hauser ReadWin 2000 system for billing.

An added advantage is that the new system’s instrumentation is traceable to NIST standards. The new technology was initially implemented for NRG’s new customers but now existing equipment is being upgraded to take advantage of the lower costs and improved accuracy of the automation.

The monitoring of chilled water flow is handled with Endress+Hauser Promag 53P electromagnetic flowmeters, while steam condensate mass is measured with Promass 40E Coriolis flowmeters.

The video below explains the process of Coriolis flow measurement:

NRG sells heat and power, but they also buy a major input: natural gas.

With significant gas consumption, accurate measurement is a necessity for profitable operation. When NRG’s existing BTU measurement system needed replacement, the utility installed an Endress+Hauser RMM621 Advanced Math Module and RMC621 Natural Gas Flow Computer/Energy Manager.

The new system detected a fault in the gas company’s PD metering, which had created a 0.5 percent measurement error in favor of the gas supplier.

By adding advanced monitoring and process control to an already efficient generation and distribution system, the NRG Omaha Energy Center delivers low-cost heating and cooling with minimal environmental impact.

For more information, visit the Endress+Hauser website, or download Endress+Hauser’s eBook Learn to Measure pH of Ultrapure Water in Power Industry Applications.