Alkaline Fuel Cells

Alkaline Fuel Cells

We provide "Cooler, Cleaner, Greener™" power and energy solutions that also include "Pollution Free Power™" systems, and all provide "Renewable Energy Credits." We also offer energy-saving technologies that may include; Absorption Chillers, Adsorption Chillers, Automated Demand Response, Cogeneration, Demand Response Programs, Demand Side Management, Energy Master Planning, Engine Driven Chillers, Trigeneration and Energy Conservation Measures.

Cooler, Cleaner, Greener Power & Energy Solutions™ project development services are one of our many specialties. These projects are Kyoto Protocol compliant and generate clean energy and significantly fewer greenhouse gas emissions. Unlike most companies, we are equipment supplier/vendor neutral. This means we help our clients select the best equipment for their specific application. This approach provides our customers with superior performance, decreased operating expenses and increased return on investment.

Cogeneration Technologies, located in Houston, Texas, provides project development services that generate clean energy and significantly reduce greenhouse gas emissions and carbon dioxide emissions. Included in this are our turnkey "ecogeneration™" products and services which includes renewable energy technologies, waste to energy, waste to watts™ and waste heat recovery solutions. Other project development technologies include; Anaerobic Digester, Anaerobic Lagoon, Biogas Recovery, BioMethane, Biomass Gasification, and Landfill Gas To Energy, project development services.

Products and services provided by Cogeneration Technologies include the following power and energy project development services:

  • Project Engineering Feasibility & Economic Analysis Studies
  • Engineering, Procurement and Construction
  • Environmental Engineering & Permitting
  • Project Funding & Financing Options; including Equity Investment, Debt Financing, Lease and Municipal Lease
  • Shared/Guaranteed Savings Program with No Capital Investment from Qualified Clients
  • Project Commissioning
  • 3rd Party Ownership and Project Development
  • Long-term Service Agreements
  • Operations & Maintenance
  • Green Tag (Renewable Energy Credit, Carbon Dioxide Credits, Emission Reduction Credits) Brokerage Services; Application and Permitting

We are Renewable Energy Technologies specialists and develop clean power and energy projects that will generate a "Renewable Energy Credit," Carbon Dioxide Credits and Emission Reduction Credits. Some of our products and services solutions and technologies include; Absorption Chillers, Adsorption Chillers, Automated Demand Response, Biodiesel Refineries, Biofuel Refineries, Biomass Gasification, BioMethane, Canola Biodiesel, Coconut Biodiesel, Cogeneration, Concentrating Solar Power, Demand Response Programs, Demand Side Management, Energy Conservation Measures, Energy Master Planning, Engine Driven Chillers, Geothermal Heat pumps, Ground source Heat pumps, Solar CHP, Solar Cogeneration, Rapeseed Biodiesel, Solar Electric Heat Pumps, Solar Electric Power Systems, Solar Heating and Cooling, Solar Trigeneration, Soy Biodiesel, Trigeneration, and Water source Heat pumps.

Types of Fuel Cells

Fuel cells are classified primarily by the kind of electrolyte they employ. This determines the kind of chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. There are several types of fuel cells currently under development, each with its own advantages, limitations, and potential applications.

What are Molten Carbonate Fuel Cells?

Molten Carbonate Fuel Cells (MCFC) evolved from work in the 1960’s aimed at producing a fuel cell which would operate directly on coal. While direct operation on coal seems less likely today, operation on coal-derived fuel gases or natural gas is viable.

Molten Carbonate Fuel Cell Design and Operation

Molten Carbonate Fuel Cells use a molten carbonate salt mixture as its electrolyte. The composition of the electrolyte varies, but usually consists of lithium carbonate and potassium carbonate. At the operating temperature of about 1200°F (650°C), the salt mixture is liquid and a good ionic conductor. The electrolyte is suspended in a porous, insulating and chemically inert ceramic (LiA102) matrix.

The Molten Carbonate Fuel Cell reactions that occur are:

The anode process involves a reaction between hydrogen and carbonate ions (CO3=) from the electrolyte which produces water and carbon dioxide (CO2) while releasing electrons to the anode. The cathode process combines oxygen and CO2 from the oxidant stream with electrons from the cathode to produce carbonate ions which enter the electrolyte. The need for CO2 in the oxidant stream requires a system for collecting CO2 from the anode exhaust and mixing it with the cathode feed stream.

As the operating temperature increases, the theoretical operating voltage for a fuel cell decreases and with it the maximum theoretical fuel efficiency. On the other hand, increasing the operating temperature increases the rate of the electrochemical reaction and thus the current which can be obtained at a given voltage. The net effect for the Molten Carbonate Fuel Cell is that the real operating voltage is higher than the operating voltage for the Phosphoric Acid Fuel Cell at the same current density.

The higher operating voltage of the Molten Carbonate Fuel Cell means that more power is available at a higher fuel efficiency from a Molten Carbonate Fuel Cell than from a Phosphoric Acid Fuel Cell of the same electrode area. As size and cost scale roughly with electrode area, this suggests that a Molten Carbonate Fuel Cell should be smaller and less expensive than a "comparable" Phosphoric Acid Fuel Cell.

The Molten Carbonate Fuel Cell also produces excess heat at a temperature which is high enough to yield high pressure steam which may be fed to a turbine to generate additional electricity. In combined cycle operation, electrical efficiencies in excess of 60% (HHV) have been suggested for mature Molten Carbonate Fuel Cell systems.

The Molten Carbonate Fuel Cell operates at between 1110°F (600°C) and 1200°F (650°C) which is necessary to achieve sufficient conductivity of the electrolyte. To maintain this operating temperature, a higher volume of air is passed through the cathode for cooling purposes.

As mentioned above, the high operating temperature of the Molten Carbonate Fuel Cell offers the possibility that it could operate directly on gaseous hydrocarbon fuels such as natural gas. The natural gas would be reformed to produce hydrogen within the fuel cell itself.

The need for CO2 in the oxidant stream requires that CO2 from the spent anode gas be collected and mixed with the incoming air stream. Before this can be done, any residual hydrogen in the spent fuel stream must be burned. Future systems may incorporate membrane separators to remove the hydrogen for recirculation back to the fuel stream.

At cell operating temperatures of 1200°F (650°C) noble metal catalysts are not required. The anode is a highly porous sintered nickel powder, alloyed with chromium to prevent agglomeration and creep at operating temperatures. The cathode is a porous nickel oxide material doped with lithium. Significant technology has been developed to provide electrode structures which position the electrolyte with respect to the electrodes and maintain that position while allowing for some electrolyte boil-off during operation. The electrolyte boil-off has an insignificant impact on cell stack life. A more significant factor of life expectancy has to do with corrosion of the cathode.

The Molten Carbonate Fuel Cell operating temperature is about 1200°F (650°C). At this temperature the salt mixture is liquid and is a good conductor. The cell performance is sensitive to operating temperature. A change in cell temperature from 1200°F (650°C) to 1110°F (600°C) results in a drop in cell voltage of almost 15%. The reduction in cell voltage is due to increased ionic and electrical resistance and a reduction in electrode kinetics. Molten Carbonate Fuel Cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. Molten Carbonate Fuel Cells are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. Since they operate at extremely high temperatures of 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Improved efficiency is another reason Molten Carbonate Fuel Cells offer significant cost reductions over Phosphoric Acid Fuel Cells (PAFCs). Molten Carbonate Fuel Cells can reach efficiencies approaching 60 percent, considerably higher than the 37-42 percent efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be as high as 85 percent.

Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, Molten Carbonate Fuel Cells don’t require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high temperatures at which Molten Carbonate Fuel Cells operate, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.

Molten Carbonate Fuel Cells are not prone to carbon monoxide or carbon dioxide "poisoning" —they can even use carbon oxides as fuel—making them more attractive for fueling with gases made from coal. Because they are more resistant to impurities than other fuel cell types, scientists believe that they could even be capable of internal reforming of coal, assuming they can be made resistant to impurities such as sulfur and particulates that result from converting coal, a dirtier fossil fuel source than many others, into hydrogen.

The primary disadvantage of current Molten Carbonate Fuel Cell technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance.

Phosphoric Acid Fuel Cells

Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte—the acid is contained in a Teflon-bonded silicon carbide matrix—and porous carbon electrodes containing a platinum catalyst. The chemical reactions that take place in the cell are shown in the diagram to the right.

The phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially, with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses.

PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells, which are easily "poisoned" by carbon monoxide—carbon monoxide binds to the platinum catalyst at the anode, decreasing the fuel cell’s efficiency. They are 85 percent efficient when used for the co-generation of electricity and heat, but less efficient at generating electricity alone (37 to 42 percent). This is only slightly more efficient than combustion-based power plants, which typically operate at 33 to 35 percent efficiency. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. Like PEM fuel cells, PAFCs require an expensive platinum catalyst, which raises the cost of the fuel cell. A typical phosphoric acid fuel cell costs between $4,000 and $4,500 per kilowatt to operate.


Alkaline Fuel Cells

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C (212°F and 482°F). However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C (74°F to 158°F)

AFCs’ high performance is due to the rate at which chemical reactions take place in the cell. They have also demonstrated efficiencies near 60 percent in space applications.

The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO2 ). In fact, even the small amount of CO2 in the air can affect this cell’s operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell’s lifetime (the amount of time before it must be replaced), further adding to cost.

Cost is less of a factor for remote locations such as space or under the sea. However, to effectively compete in most mainstream commercial markets, these fuel cells will have to become more cost-effective. AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. To be economically viable in large-scale utility applications, these fuel cells need to reach operating times exceeding 40,000 hours, something that has not yet been achieved due to material durability issues. This is possibly the most significant obstacle in commercializing this fuel cell technology.

Direct Hydrogen Fuel Cells

Direct Hydrogen Fuel Cells are the fuel cell of choice for an increasingly greater number of automobile manufacturers. A Direct Hydrogen Fuel Cell uses hydrogen directly from the fuel tank which is supplied to the direct hydrogen fuel cell, where it combines with oxygen from an air compressor. An electrode that is coated with a catalyst splits the hydrogen into electrons and protons. The movement of the electrons generates electricity, which is then sent to the traction inverter module. The traction inverter module converts the electricity for use by the motor/transaxle, which then transforms the electric energy into mechanical energy that turns the wheels. The only emissions coming from a Direct Hydrogen Fuel Cell are water vapor and heat.

A Direct Methanol Fuel Cell vehicle operates similarly to a Direct Hydrogen Fuel Cell but a Direct Methanol Fuel Cell also has a reformer to extract the needed hydrogen from the methanol.

One of the significant challenges facing car manufacturers with introduction of a mass-produced fuel cell vehicle is providing hydrogen to the car’s fuel cell stack module. At present, there are two alternatives for supplying hydrogen to the fuel cell:

  • Direct Hydrogen Fuel Cell cars that store hydrogen onboard the vehicle; or
  • Direct Methanol Fuel Cell cars that extract hydrogen from liquid fuels, such as gasoline or methanol.

Direct Hydrogen Fuel Cells provide the greatest environmental benefit and the simplest fuel cell system design, but requires the development of a hydrogen economy which includes the hydrogen infrastructure, where drivers with cars using the Direct Hydrogen Fuel Cells can re-fuel their cars with hydrogen.

Direct Methanol Fuel Cells

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode.

Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cells since methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure since it is a liquid, like gasoline.

Direct methanol fuel cell technology is relatively new compared to that of fuel cells powered by pure hydrogen, and DMFC research and development are roughly 3-4 years behind that for other fuel cell types.

Polymer Electrolyte Membrane (PEM) Fuel Cells

Polymer electrolyte membrane (PEM) fuel cells—also called proton exchange membrane fuel cells—deliver high power density and offer the advantages of low weight and volume, compared to other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. They need only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like some fuel cells. They are typically fueled with pure hydrogen supplied from storage tanks or onboard reformers.

Polymer electrolyte membrane fuel cells operate at relatively low temperatures, around 80°C (176°F). Low temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, it requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen’s electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO.

PEM fuel cells are used primarily for transportation applications and some stationary applications. Due to their fast startup time, low sensitivity to orientation, and favorable power-to-weight ratio, PEM fuel cells are particularly suitable for use in passenger vehicles, such as cars and buses.

A significant barrier to using these fuel cells in vehicles is hydrogen storage. Most fuel cell vehicles (FCVs) powered by pure hydrogen must store the hydrogen onboard as a compressed gas in pressurized tanks. Due to the low energy density of hydrogen, it is difficult to store enough hydrogen onboard to allow vehicles to travel the same distance as gasoline-powered vehicles before refueling, typically 300-400 miles. Higher-density liquid fuels such as methanol, ethanol, natural gas, liquefied petroleum gas, and gasoline can be used for fuel, but the vehicles must have an onboard fuel processor to reform the methanol to hydrogen. This increases costs and maintenance requirements. The reformer also releases carbon dioxide (a greenhouse gas), though less than that emitted from current gasoline-powered engines.

Molten Carbonate Fuel Cells

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. Since they operate at extremely high temperatures of 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel cells can reach efficiencies approaching 60 percent, considerably higher than the 37-42 percent efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be as high as 85 percent.

Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs don’t require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high temperatures at which MCFCs operate, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.

Molten carbonate fuel cells are not prone to carbon monoxide or carbon dioxide "poisoning" —they can even use carbon oxides as fuel—making them more attractive for fueling with gases made from coal. Because they are more resistant to impurities than other fuel cell types, scientists believe that they could even be capable of internal reforming of coal, assuming they can be made resistant to impurities such as sulfur and particulates that result from converting coal, a dirtier fossil fuel source than many others, into hydrogen.

The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance.

Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50-60 percent efficient at converting fuel to electricity. In applications designed to capture and utilize the system’s waste heat (co-generation), overall fuel use efficiencies could top 80-85 percent.

Solid oxide fuel cells operate at very high temperatures—around 1,000°C (1,830°F). High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.

SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more sulfur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This allows SOFCs to use gases made from coal.

High-temperature operation has disadvantages. It results in a slow start up and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation and small portable applications. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.

Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 800°C that have fewer durability problems and cost less. Lower-temperature SOFCs produce less electrical power, however, and stack materials that will function in this lower temperature range have not been identified.

What are Regenerative Fuel Cells?

Regenerative fuel cells produce electricity from hydrogen and oxygen and generate heat and water as byproducts, just like other fuel cells. However, regenerative fuel cell systems can also use electricity from solar power or some other source to divide the excess water into oxygen and hydrogen fuel—this process is called "electrolysis." This is a comparatively young fuel cell technology being developed by NASA and others.

What is an Absorption Chiller?

Absorption chillers use heat instead of mechanical energy to provide cooling. A thermal compressor consists of an absorber, a generator, a pump, and a throttling device, and replaces the mechanical vapor compressor.

In the chiller, refrigerant vapor from the evaporator is absorbed by a solution mixture in the absorber. This solution is then pumped to the generator. There the refrigerant re-vaporizes using a waste steam heat source. The refrigerant-depleted solution then returns to the absorber via a throttling device. The two most common refrigerant/ absorbent mixtures used in absorption chillers are water/lithium bromide and ammonia/water.

Compared with mechanical chillers, absorption chillers have a low coefficient of performance (COP = chiller load/heat input). However, absorption chillers can substantially reduce operating costs because they are powered by low-grade waste heat. Vapor compression chillers, by contrast, must be motor- or engine-driven.

Low-pressure, steam-driven absorption chillers are available in capacities ranging from 100 to 1,500 tons. Absorption chillers come in two commercially available designs: single-effect and double-effect. Single-effect machines provide a thermal COP of 0.7 and require about 18 pounds of 15-pound-per-square-inch-gauge (psig) steam per ton-hour of cooling. Double-effect machines are about 40% more efficient, but require a higher grade of thermal input, using about 10 pounds of 100- to 150-psig steam per ton-hour.

A single-effect absorption machine means all condensing heat cools and condenses in the condenser. From there it is released to the cooling water. A double-effect machine adopts a higher heat efficiency of condensation and divides the generator into a high-temperature and a low-temperature generator.

Is It Right for You?

Absorption cooling may be worth considering if your site requires cooling, and if at least one of the following applies:

  • You have a combined heat and power CHP) unit and cannot use all of the available heat, or if you are considering a new CHP plant
  • Waste heat is available
  • A low-cost source of fuels is available
  • Your boiler efficiency is low due to a poor load factor
  • Your site has an electrical load limit that will be expensive to upgrade
  • Your site needs more cooling, but has an electrical load limitation that is expensive to overcome, and you have an adequate supply of heat.

In short, absorption cooling may fit when a source of free or low-cost heat is available, or if objections exist to using conventional refrigeration. Essentially, the low-cost heat source displaces higher-cost electricity in a conventional chiller.

In Practice

In a plant where low-pressure steam is currently being vented to the atmosphere, a mechanical chiller with a COP of 4.0 is used 4,000 hours a year to produce an average 300 tons of refrigeration. The plant’s cost of electricity is $0.05 a kilowatt-hour.

An absorption unit requiring 5,400 lbs/hr of 15-psig steam could replace the mechanical chiller, providing annual electrical cost savings of:

  • Annual Savings = 300 tons x (12,000 Btu/ton / 4.0) x 4,000 hrs/yr x $0.05/kWh x kWh/3,413 Btu = $52,740

Actions You Can Take

Determine the cost-effectiveness of displacing a portion of your cooling load with a waste steam absorption chiller by taking the following steps:

  • Conduct a plant survey to identify sources and availability of waste steam
  • Determine cooling load requirements and the cost of meeting those requirements with existing mechanical chillers or new installations
  • Obtain installed cost quotes for a waste steam absorption chiller
  • Conduct a life cycle cost analysis to determine if the waste steam absorption chiller meets your company’s cost-effectiveness criteria.

Absorption Chiller Refrigeration Cycle

The basic cooling cycle is the same for the absorption and electric chillers. Both systems use a low-temperature liquid refrigerant that absorbs heat from the water to be cooled and converts to a vapor phase (in the evaporator section). The refrigerant vapors are then compressed to a higher pressure (by a compressor or a generator), converted back into a liquid by rejecting heat to the external surroundings (in the condenser section), and then expanded to a low- pressure mixture of liquid and vapor (in the expander section) that goes back to the evaporator section and the cycle is repeated.

The basic difference between the electric chillers and absorption chillers is that an electric chiller uses an electric motor for operating a compressor used for raising the pressure of refrigerant vapors and an absorption chiller uses heat for compressing refrigerant vapors to a high-pressure. The rejected heat from the power-generation equipment (e.g. turbines, microturbines, and engines) may be used with an absorption chiller to provide the cooling in a CHP system.

The basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent. The most commonly fluids are water as the refrigerant and lithium bromide as the absorbent. These fluids are separated and recombined in the absorption cycle. In the absorption cycle the low-pressure refrigerant vapor is absorbed into the absorbent releasing a large amount of heat. The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller. Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases. The added heat causes the refrigerant to desorb from the absorbent and vaporize. The vapors flow to a condenser, where heat is rejected and condense to a high-pressure liquid. The liquid is then throttled though an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling. The remaining liquid absorbent, in the generator passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapors returning from the evaporator so the cycle can be repeated.

Absorption chillers are used to generate cold water (44°F) that is circulated to air handlers in the distribution system for air conditioning.

"Indirect-fired" absorption chillers use steam, hot water or hot gases steam from a boiler, turbine or engine generator, or fuel cell as their primary power input. Theses chillers can be well suited for integration into a CHP system for buildings by utilizing the rejected heat from the electric generation process, thereby providing high operating efficiencies through use of otherwise wasted energy.


"Direct-fired" systems contain natural gas burners; rejected heat from these chillers can be used to regenerate desiccant dehumidifiers or provide hot water.

Commercially absorption chillers can be single-effect or multiple-effect. The above schematic refers to a single-effect absorption chiller. Multiple-effect absorption chillers are more efficient and discussed below.

Multiple-Effect Absorption Chillers

In a single-effect absorption chiller, the heat released during the chemical process of absorbing refrigerant vapor into the liquid stream, rich in absorbent, is rejected to the environment. In a multiple-effect absorption chiller, some of this energy is used as the driving force to generate more refrigerant vapor. The more vapor generated per unit of heat or fuel input, the greater the cooling capacity and the higher the overall operating efficiency.

A double-effect chiller uses two generators paired with a single condenser, absorber, and evaporator. It requires a higher temperature heat input to operate and therefore they are limited in the type of electrical generation equipment they can be paired with when used in a CHP System.

Triple-effect chillers can achieve even higher efficiencies than the double-effect chillers. These chillers require still higher elevated operating temperatures that can limit choices in materials and refrigerant/absorbent pairs. Triple-effect chillers are under development by manufacturers working in cooperation with the U.S. Department of Energy.

How Does an Engine Driven Chiller Work?

Packaged natural gas engine-driven water chillers and direct expansion (DX) units are now available. Commercially proven custom and packaged engine-driven refrigeration units offer excellent reliability and economic advantages for ice rinks, refrigerated warehouses and other applications. The industry is also focusing on developing small, engine-driven heating and cooling systems suitable for small commercial applications.

Operation: Engine-driven cooling systems employ a conventional vapor compression cycle. Their main components are the compressor, condenser, expansion valve and evaporator.

Advantages: The main difference between a natural gas and conventional electric system is the replacement of the electric motor with a gas engine. This change results in variable-speed operation capability; higher part-load efficiency; efficient high-temperature waste-heat recovery for water heating, process heating, or steam generation; and an overall reduction in operating expenses.

  • Requires no more room than conventional electric chillers
  • Lowest operating cost of any available chiller
  • Depending on electric rates and natural gas rates, an engine driven chiller may operate at up to 1/2 of the cost of direct-fired absorption chillers
  • Like absorption chillers, engine driven chillers reduce on-peak electric demand charges.
  • Depending on your electric and/or natural gas supplier, there may be rebates available for purchasing a new absorption chiller or engine driven chiller from your utility supplier. 
  • Environmentally friendly.