Fuel cell systems are a newer technology still undergoing development. These systems enable the direct conversion of chemical energy of the fuel into electrical energy without intermittent conversion to mechanical energy. In general fuel cells are like batteries, the only difference being chemical energy (generally hydrogen, natural gas, etc) is stored outside the power generating device. Further, fuel cells are not subject to Carnot cycle limitations and therefore see electric efficiencies as high as 80%. Typical electrical efficiencies range from 20-60% (depending on the fuel) and can reach total efficiencies of 80-90% in CHP mode. Since no combustion takes place, fuel cells are clean and quiet.

Fuel cells can be divided into five different categories based on type of electrolyte and operating temperature. Below this list is a more comparative study on different fuel cell types.

• Polymer Electrolyte Membrane Fuel Cell (PEMFC): This type of fuel cells uses solid electrolyte and operates at low temperatures (~80°C). These cells have high power density, quick start up time, and fast dynamic response. PEMFC are seen as potential candidates for auto applications, although residential systems are being developed. Hydrogen or reformate is used as a fuel. Efficiencies are in range of 40-55% with hydrogen as a fuel and 20-30% with other fuels like natural gas, propane, etc. These fuel cells are best suited for 1 kW to 100 kW applications.
• Alkaline Fuel Cells (AFC): This type of fuel cell uses aqueous KOH solution as the electrolyte. AFC have been widely used by NASA in their Gemini and subsequent space shuttle operations. AFCs have high efficiency in range of 50-70% and don't use precious noble metal catalysts. On the downside, these fuel cells require COx free hydrogen and oxygen, which is exceptionally expensive. Consequently, AFCs will find application only in niche markets.
• Phosphoric Acid Fuel Cell (PAFC): This type uses concentrated phosphoric acid as an electrolyte and operates in temperature range of 150-250 oC. Efficiencies are in range of 35-50%. In CHP mode, efficiencies as high as 80% are achievable. Today, more than 200 phosphoric acid fuel cells have been deployed worldwide. These 200 kW units are well proven for reliability and durability, although they are still relatively costly.
• Molten Carbonate Fuel Cell (MCFC): This type uses a carbonate-salt-impregnated ceramic matrix as an electrolyte and operates in temperature range of 650-700ºC. Due to their high temperature operation, these cells are best suited for large stationary applications. Typical electrical efficiency are in range of 40-55% with potential to reach 85% with cogeneration. Many MCFCs are currently undergoing real-world testing at several wastewater treatment facilities and other commercial and industrial settings.
• Solid Oxide Fuel Cell (SOFC): This type uses solid oxygen ion conduction as an electrolyte and operates in temperature range of 800 - 1000ºC. SOFC makes excellent cogeneration devices for industrial applications where high temperature steam is required.

Table: Characteristics of Major Fuel Cell Types (Source: Energy Nexus Group)

 

Manufacturers: FuelCell Energy UTC Power Siemens Plug Power Ballard

Technical information: Catalog of CHP Technologies: Fuel Cells - EEA/U.S. EPA (Dec. 2008) Center for Fuel Cell Research and Applications Fuel Cells - Energy Solutions Center

Gas turbines have been used to generate power for about 70 years. The technology revolutionized airplane propulsion industry in 1940s and is currently the economic and environmentally preferred choice for new power generation plants in the United States. Typically, gas turbines are in the 500 kW to 250 MW range, but a subset, microturbines, cover the small capacities from 25 kW to approximately 500 kW. All gas turbines exhaust high quality heat that can be used in CHP mode to reach overall system efficiency of 70-80%. Gas turbines have been typically used by the utilities for peaking power capacity, although recent trends show that these turbines are increasingly used for baseload power. Gas turbines have been typically used in combination with steam turbines in a combined cycle plant at central power station with efficiencies approaching as high as 60% LHV. However, simple cycle gas turbines have efficiencies approaching 40% LHV.

Gas turbines operate on the principle of Brayton cycle. In this case, air is compressed in a compressor, heated in a fuel combustion chamber, and then expanded in a turbine (see figure below). The excess power produced by the turbine is used to drive the compressor. The power produced by the turbine and consumed by a compressor is proportional to the absolute temperature of the gas passing through it and is limited by turbine materials. In general, use of high temperature and high pressure ratios results in higher efficiency and specific power.

Figure: Components of a Simple Cycle Gas Turbine

Three different kinds of gas turbines are common. These include:

• Frame gas turbines: These are the industrial gas turbines for stationary power generation and are available in size range of 1 - 250 MW. These are less expensive, more rugged, and are suited for continuous base-load operation with longer inspection and maintenance intervals.
• Mini and micro turbines: Mini and micro turbines are the newer generation of smaller turbines. The capacities of mini turbines range from 100 kW to 1000 kW and micro turbines range in capacities from 25 kW to 100 kW. It is not uncommon to ignore the differentiation between mini- and micro- turbines. For the purpose of discussion at this website all turbines smaller in capacity than 1 MW will be referred to as microturbines. These turbines can use natural gas, propane, and gases produced from landfills, sewage treatment facilities, and animal waste processing plants as a primary fuel. The fuel source versatility of microturbines allows their application in remote areas.
• Aeroderivative gas turbines: These are typically used for jet and turboshaft aircraft engines, and are lightweight, thermally efficient, but expensive. For the stationary power generation, these turbines are typically operated at compression ratios of 30:1, therefore requiring high-pressure external gas compressor. These turbines are available in sizes typically less than 50 MW.

Links to Manufacturers ALM Turbine Alzeta Corporation Bowman Power Systems Capstone Turbine Corporation Elliot Energy Systems General Electric Power Systems Honeywell International Ingersoll Rand Pratt & Whitney Power Systems Siemens Westinghouse Power Generation Solar Turbines Teledyne Technologies Turbec

More Technical Information Catalog of CHP Technologies: Gas Turbines - EEA/U.S. EPA (Dec. 2008) Gas Turbines - Energy Solutions Center

Reciprocating or piston-driven engines have been developed for over 100 years. The technology has improved dramatically over the past three decades due to increased environmental and economic pressures, and the need for power density improvements. These engines offer a low capital-cost CHP system that is easy to start up, has proven reliability, offers good load-following characteristics, and has excellent heat recovery potential. There are two basic types - spark ignition and compression ignition. The former uses natural gas as the preferred fuel, although other fuels like propane or gasoline can be used. The electric efficiency of natural gas fired engines range from 28-40% LHV based on size of the engine. The waste heat can be recovered in CHP mode to achieve an overall efficiency of 70-80%. The compression ignition type of engine operates on diesel or heavy fuel and typically has efficiencies in the range of 30-50% HHV.

The spark ignition engine operates on the principle of Otto cycle while the compression ignition engine operates on Diesel cycle. For the most part, these two cycles are identical except for the method of igniting the fuel. These engines use a cylindrical combustion chamber in which a close fitting piston travels the length of the cylinder. Complete cycle consists of intake stroke, compression stroke, power stroke, and exhaust stroke. The piston is connected to the crankshaft that transforms the linear motion to the rotary motion.

Reciprocating engines tend to have higher emissions than other technologies. In some locations, especially where local air quality standards are high, emissions permits could limit engine use, although the use of exhaust catalysts and better combustion design and control has significantly reduced emissions for current technology.

Steam turbines have been used to generate power for about 100 years, and today, they generate most of the electricity produced in the United States. Typically, steam turbines are in the 50-500 MW range, and they are widely used in CHP applications. Conventional steam turbines have multiple turbines to enhance system efficiency. The installation also requires a boiler to generate steam, fuel handling systems, and steam handling systems. The separation of boiler from the power generating equipment enables operation with wide range of fuels. Steam turbines are well suited to medium- and large-scale industrial and institutional applications where inexpensive fuels, such as coal, biomass, various solid wastes and byproducts (e.g., wood chips. etc.), refinery residual oil, and refinery off gases are available. The electrical generating efficiency of standard steam turbine power plant varies from high of 40% HHV for large, electric utility plants designed for highest practical annual capacity factor, to under 10% HHV for small, simple plants which make electricity as a byproduct of delivering steam to processes or district heating systems.

Steam turbine operates on the principle of Rankine Cycle (see figure below). Water is first pumped to medium to high pressure. It is then heated to the boiling temperature corresponding to pressure, boiled, and most frequently superheated. The turbine expands the pressurized steam to lower pressure and the steam is exhausted either to a condenser at vacuum conditions or into an intermediate temperature steam distribution system that delivers the steam to the industrial or commercial application.

 

Figure: Components of a Boiler/Steam Turbine

Three different types of steam turbines are common. These include:

• Condensing Steam Turbines: These turbines exhaust directly to the condensers that maintain vacuum conditions at the discharge of the turbine. Typical use is at central power generation plants where maximum power and electrical generation efficiency is desired from steam supply and boiler fuel.
• Back-pressure turbines: These turbines are a non-condensing type, and exhaust the entire flow of steam to the industrial process or facility steam at conditions close to process heat requirements. The discharge pressure depends on the type of CHP application. Typical use is district heating systems and industrial processes.
• Extraction turbines: These turbines extract a portion of the steam at some intermediate pressure before condensing the remaining steam. The regulated extraction permits more steam to flow through the turbine to generate additional electricity during periods of low thermal demands.

Manufacturers: Skinner Power Systems Turbosteam

Technical Information: Catalog of CHP Technologies: Steam Turbines - EEA/U.S. EPA (Dec. 2008) Steam Turbines - Energy Solutions Center

Stirling engines have the potential to be much more efficient than reciprocating engines. A Stirling engine uses the Stirling cycle, which is unlike the cycles used in internal combustion engines. The gases used inside a Stirling engine never leave the engine, and this allows great fuel flexibility such as most variations of fossil, biomass fuel and waste gases (see animation). The engine operates by converting differences in temperatures into electrical output. An external heat source is typically used to create the temperature differential, but theoretically a Stirling engine could run on chemical processes that produce cold instead of heat, such as the sublimation of dry ice or the boiling of liquid nitrogen.

In addition, these engines are extremely quiet, because no combustion takes place inside the cylinders. As a result, specialized applications like submarines or auxiliary power generators for yachts, where quiet operation is required, are some of the initial applications for Stirling engines. Recently this technology has been used for residential applications. Several utility companies are demonstrating the technology for residential applications. Electric efficiencies of more than 40 % and system efficiencies of more than 95 % have been achieved. Stirling micro-CHP packages are targeted to cost $2,500/kW (thermal electric) compared to $1,200/kW or less for combustion turbine and IC CHP packages. Service intervals of between 3,500 and 5,000 hours (equivalent to over one year's economic operation) are expected with a product lifetime of more than six years' continuous operation. Stirling engines are pre-commercial, but are expected to contribute to residential and small business needs.