01 September 2003
Highly automated capable competitor
Fuel cell power plants are well suited to distributed generation.
By George Berntsen
Fuel cell distributed generation plants require very reliable process controls, sophisticated automation logic, and powerful remote monitoring capabilities made practical by modern instrumentation and control technologies.
Hunting is an undesirable oscillation of some magnitude that continues after feedback ends. One of the major problems with single ported valves is they are susceptible to valve hunting. Valve hunting can be the alternate overfeeding and starving of refrigerant flow to the evaporator, which manifests in the constant movement one might see in refrigerant gauges. Hunting can be reduced by either relocating the sensing bulb or by purchasing a valve designed for reduced hunting.
Distributed generation is the production of electrical energy at its point of use rather than at large central power stations. It results in increased reliability and the elimination of transmission and distribution costs.
Although traditional power stations will continue to do the heavy lifting for the near term, distributed generation technologies are filling new and growing market niches.
Because fuel cell power plants are quiet and have near zero emissions, they can be located close to the electrical consumer without evoking environmental and "not in my backyard" objections. In addition, the ultrahigh efficiencies of fuel cells ensure their economic viability.
To be cost competitive and user friendly, fuel cell distributed generation plants must be highly automated and capable of unattended operation. Here follows a technical overview of fuel cell power generation and an examination of the role specific instrument and control (I&C) technologies will play in realizing the benefits of this emerging technology.
UTILIZE HYDROCARBON FUELS
At their most basic level, fuel cells electrochemically combine hydrogen and oxygen to form electricity, heat, and water.
NASA successfully applied fuel cell technology to power the Apollo spacecraft and keep its crew warm and well hydrated.
Given the rarity of free hydrogen (H2 ) in nature and the lack of a hydrogen delivery infrastructure, practical commercial applications of fuel cells utilize hydrocarbon fuels such as natural gas.
These fuels are either internally or externally reformed to provide the necessary hydrogen for the fuel cell process, for example:
where CH4 is methane, the major component of natural gas. One can also process other hydrocarbons to produce hydrogen.
The technology this paper examines comes from FuelCell Energy, Inc.'s carbonate electrolyte internally reforming Direct FuelCell power plants. These plants operate at approximately 1200°F, which is a higher temperature than most other fuel cells. This is an optimal temperature, because it avoids the use of precious metal electrodes required by lower temperature fuel cells and the more expensive metals and ceramic materials of higher temperature fuel cells.
The result is lower costs, higher efficiency, and high quality by-product heat energy suitable for cogeneration.
|Single fuel cell|
Electricity flows from the anode, drives the load (e.g. an electric motor) to the cathode.
CAPITAL INTENSIVE TIME HOG
Five related trends point to a growing fuel cell distributed generation market:
1. Deregulation: The Energy Policy Act of 1999 called for open access to electrical consumers. Most deregulation policies separate the generation and transmission/distribution functions of the traditional electric utility business. This opens up formerly captive markets and allows new power producers to enter.
2. Improved generation technology: New generation technologies, particularly fuel cells and microturbines, make small-scale power plants with efficiencies that rival or surpass large central power stations possible. This contributes to cost-effective distributed generation.
3. Digital economy reliability: Digital technology requires highly reliable power. Power interruptions, glitches, spikes, and voltage sags can cause computer outages and data losses resulting in large lost production and data recovery costs. Distributed generation can greatly enhance critical load reliability by maintaining power during loss of the grid. When the grid is available, it serves as a backup to the distributed generator.
4. Capacity constraints: Electrical demand will probably grow due to population growth and continued expansion of information technology. Increasing the existing transmission infrastructure to match this demand will be capital intensive and time consuming. Environmental concerns may also play a restrictive role.
5. Environmental issues: Continuing concern about air pollution will favor the development and deployment of clean energy sources. Fuel cell power plants represent the cleanest and most efficient technology for converting methane-based fuels to electricity. Their emissions are free from NOx, SOx, carbon monoxide, and other pollutants associated with combustion-based power plants.
|One megawatt can power 750–1000 homes in the U.S. depending on what appliances those homes are using at a particular time.|
ABSORBS HIGH PURITY WATER
A fuel cell driven, 300 kilowatt power plant has a single integrated enclosure consisting of three major sections: mechanical balance-of-plant (MBOP), the direct fuel cell module (DFC), and the electrical balance-of-plant (EBOP).
The MBOP contains the piping, valves, heat exchangers, and other equipment necessary to prepare the fuel gas—typically natural gas—for supply to the fuel cell module.
The natural gas first undergoes processing to remove the sulfur, which serves as an odorant. The fuel gas then heats several hundred degrees using the exhaust heat from the fuel cell module.
As the fuel gas heats, it also absorbs high purity water. This takes place to prevent coking and to supply the water needed in the reforming reaction. The heated, humidified fuel then flows to the fuel cell module.
The fuel gas entering the fuel cell module receives superheat, and then it internally reforms to yield hydrogen. This fuel preparation process is much simpler than external reforming fuel cells. Fresh air supplies the oxidant for the fuel cell reaction.
These are the carbonate fuel cell reactions.
The carbonate ion electrolyte is
The fuel cell products are heat, water, and direct current (DC) electricity. The use of a hydrocarbon fuel instead of pure hydrogen results in carbon dioxide as an additional emission.
The high temperature (approximately 1200°F) fuel cell module exhaust gas heats the fuel gas in the MBOP. Following this onboard heat recovery, the exhaust gas temperature is high enough (approximately 800°F) to produce high-pressure steam for cogeneration applications.
The DC electrical power output from the fuel cell module applies to the power conditioning unit portion of the electrical balance-of-plant.
The power-conditioning unit (PCU) draws DC from the DFC and converts it to synchronized three-phase alternating current power. During normal operation, the fuel cell power plant synchronizes to the utility grid.
Loads distribute so that the fuel cell power plant carries the customer critical loads and some of the customer's noncritical load. If the utility grid is lost, the fuel cell power plant tie breaker opens and the PCU output reduces to match the customer critical load.
In addition to the PCU, the EBOP includes breakers, motors, transformers, variable speed drives, a protective relay, a programmable logic controller (PLC), and the unit's human-machine interface (HMI).
|Fuel cell power plant|
INSTRUMENTATION AND CONTROLS
To achieve economic viability, small, distributed generation power plants need to operate primarily unattended. The operators who infrequently interface with the power plants will not be fuel cell specialists.
These considerations require that fuel cell power plants incorporate tight and dependable process controls so that they can maintain high operating efficiency without the occasional manual adjustment. Automation logic has to simplify operation and minimize requisite operator knowledge. Reliable protection logic is necessary to detect hazardous plant conditions and automatically place the plant in a safe state without human intervention.
Finally, it is essential to incorporate extensive remote monitoring, control, and diagnostic capabilities so off-site specialists can interface with the plant when necessary.
Not long ago, it would have been impractical and cost prohibitive to meet these demanding requirements. Today, however, people are practically and cost effectively deploying modern I&C technologies to help realize the benefits of fuel cell distributed generation.
Field devices: Smart pressure and flow transmitters provide longer stability than previous analog devices. This permits longer calibration intervals and contributes to the minimal maintenance goals of the unattended fuel cell power plant.
In this megawatt-class power plant, Foundation fieldbus field devices are at work. Digital communication down to the field level yields a wealth of device information not previously available and enables remote diagnostics of plant sensors and actuators.
On/off valve controllers record the number of valve strokes and retain baseline opening and closing times as well as the most recent valve stroke times. Regulating valve controllers keep track of total valve travel distance over time as well as the number of changes in direction.
This information could allow engineers to remotely identify valve hunting. Tuning changes can then take place as necessary to minimize valve wear without requiring an on-site visit.
Process control logic: The fuel cell power plant requires discrete and regulatory control of various fuel preparation processes as well as coordination of fuel and power. To avoid relying on occasional manual operator adjustments to maintain high plant efficiencies, sophisticated trim controls handle a wide range of operating conditions.
Control loop techniques such as cascaded, feedforward, and constraint control proportional, integral, derivative (PID) serve, and more advanced predictive modeling techniques are under investigation.
Other control functions such as set point and compensation algorithms and function generators are also used. Modern digital controllers make all this possible at low cost and in a very small physical footprint compared to vintage control systems. Computer and telecommunications advances also permit online, remote changes to control strategies when necessary or desired for performance improvement.
Sequential logic: Local distributed generation fuel cell operators are general facilities personnel and not dedicated fuel cell technologists. Interaction with the fuel cell power plant is limited to initiating plant heat up, starting power generation, changing the desired power level, taking the plant off load, and commencing plant cool down.
To minimize prerequisite knowledge, these activities are single control-button functions. This is the result of a significant amount of automation and sequential logic made practical by modern digital controllers.
Protection and alarm logic: To support unattended plant operation, protection logic must place the plant in a safe condition for all credible events without relying on operator intervention.
The protection logic philosophy is to respond to worsening plant conditions in a graded manner to preclude further degradation and minimize lost generation. The protective actions include ramp holds, power ramp backs, total load shed, and complete plant shutdown.
Warning alarms use more conservative set points than protective functions to notify operations personnel early about potential problems. Notification proceeds via a pager utility and phone modem.
Mode-dependent alarm logic ensures that during normal operation no alarms are present. A first out function identifies the initiating condition for any plant trip to assist in plant recovery and restart.
Meeting the protection and alarm logic demands of fuel cell distributed generation plants would be impractical with vintage relay logic and annunciator systems. Modern digital controllers, however, do the job inexpensively and with a small footprint.
|Fuel cell power plant|
HUMAN FACES THE MACHINE
The HMI for this particular power plant is a flat-panel PC using the Windows 2000 operating system and running GE Cimplicity HMI software. The following functions are vital to supporting unattended operation and would not be feasible or even possible without recent advances in computer and telecommunications technology.
Local HMI: Operator monitoring and control functions can be accomplished locally at the power plant via a flat-panel touch screen. There are two levels of HMI access: basic operator and specialist.
Basic operator functionality is the HMI default mode. Controls are limited to push buttons to initiate plant heat up, start power generation, change the desired power level, take the plant off load, and commence plant cool down. There are alarm list and high-level plant parameter displays.
The specialist level HMI is password accessible. Detailed loop controls and process parameters appear on piping and instrumentation diagram mimic screens. Plant setup screens provide the ability to modify key plant constants and options. Alarm and trip set points are also adjustable.
Remote control and monitoring: To provide off-site fuel cell specialists access to plant information and controls, the HMI computer has remote access software installed and a dedicated phone modem. For additional security, local personnel can disconnect and reconnect the phone modem when specifically requested by off-site personnel.
In addition to using the HMI software, authorized off-site personnel can perform data historian archiving tasks and view PLC code execution. If necessary, remotely fabricated PLC program changes can download to the PLC without interrupting plant operation.
Web viewer: The HMI computer contains a Web server to allow read-only, password protected access to HMI screens using a standard web browser such as Internet Explorer or Netscape Navigator. Web Server access can be limited to a customer's local-area network or connected to the Internet to allow off-site viewing.
Pager: All plant alarms assign to higher level categories, each of which will send out a pager text message via a dedicated phone modem. Pager recipients can receive some or all alarms. When a pager alarm arrives operation and/or service personnel can either log in remotely or go to the local HMI computer to diagnose the situation.
DCS interface: Industrial users will often have an existing plant distributed control system (DCS) that they need to interface with the fuel cell distributed generation plant. Because these systems come from various manufacturers and are of different vintages, Modbus serial communication is most often the fuel cell plant-to-DCS communication link.
As the automation industry continues to develop and deploy various open protocols, it is expected that more powerful and easy-to-use third party communication links will be available for customer DCS interfacing.
Fuel cell distributed generation offers ultraclean, highly reliable electrical energy to new and growing market niches. State-of-the-art instrumentation and control technology has contributed to the realization of fuel cell technology benefits. IT
Behind the byline
George Berntsen has a Bachelor of Science degree in Technology—Instrumentation & Electronics and over twenty years of commercial instrumentation and control experience in the fuel cell and nuclear power industries. He is the manager of instrument and controls engineering at FuelCell Energy, Inc. He is also an ISA member. Write him at email@example.com.
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