Renewable Energy: Combined Heat And Power With Fuel Cells
When deciding how to best meet the power and thermal energy needs of their organizations, facility managers (fms) consider a range of priorities, including cost, safety, reliability, and environmental stewardship. In recent years a new option has become available in the form of clean and highly efficient on-site heat and power generation from fuel cell power plant systems. Stationary fuel cell power plants can address these priorities economically.
Fuel cells have been in development for many years, and fms may be aware of these as something used in the space program, or under development for vehicles, requiring pure hydrogen as fuel. But systems that use clean natural gas or renewable biogas fuel are now commercially available and deployed in stationary power and heat applications around the world.
What makes fuel cells distinct as a power source is they extract energy from fuel without burning it. Instead, the fuel is used in an “electrochemical” reaction, similar to the reactions that consume zinc or cadmium in common batteries. The difference is that when the battery reactants are consumed they are discarded, or the battery is recharged. Fuel cell reactants are continuously fed to the fuel cell, which makes power as long as fuel and air are supplied. The easiest fuel to react electrochemically is hydrogen. Today, fuel cell power plants are able to extract hydrogen from natural gas or biogas—making them viable wherever natural gas or biogas is available.
The electrochemical conversion process has several advantages over combustion. Since there is no flame, many of the harmful pollutants created by burning fuel are not created. Also, fuel cells create electricity directly from the fuel, at higher efficiencies than mechanical systems that require multiple energy conversion steps. And the absence of rotating mechanical components makes fuel cells inherently quiet.
Fuel cell technologies are characterized by the electrolyte used to separate the fuel and air electrodes, and several types are commercially available for on-site combined heat and power (CHP).
One of these, the carbonate fuel cell, offers distinct advantages. First, they operate at about 1000°F, and the high temperature promotes high electrical efficiency; when used for CHP, carbonate fuel cells can approach 90% efficiency. Furthermore, for CHP, the heat can be fed back into on-site heat and hot water systems, absorption chilling systems for facility cooling, or be sold off to neighboring facilities. And finally, fuel cells also provide fuel flexibility; the process of extracting hydrogen from natural gas or biogas (called “reforming”) occurs directly in the fuel cell stack. Carbonate fuel cells can use either as fuel almost interchangeably.
The photo below shows a typical on-site CHP application of a carbonate fuel cell power plant. Rated at 1.4 megawatts, this plant operates at California State University’s East Bay campus near San Francisco. The fuel stack module contains four fuel cell stacks, each with about 400 cells. Direct current power from the stack module is sent to the power conversion system, which produces alternating current power at the correct grid voltage and frequency. Mechanical equipment (air blowers and heat exchangers) is on a skid behind power conversion equipment. The system exhaust is directed to a waste heat recovery system, which produces hot water from heat left over from the fuel cell.
In the 10 years since commercial carbonate fuel cell power plants first became available, the cost has reduced more than 60%. Currently more than 80 of these plants are installed at over 50 sites around the world. The largest single facility is a 59 MW system being installed in South Korea with full power production anticipated by the end of 2013 or early 2014.
Leo is vice president, application engineering and advanced technology development, at FuelCell Energy, headquartered in Danbury, CT. Working at the company (then Energy Research Corporation) since1978, he holds a BS in Chemical Engineering from Rensselaer Polytechnic Institute.