Waste Heat Recovery

What is Waste Heat Recovery?

Many processes, especially in industrial applications, produce large amounts of excess heat – i.e., heat beyond what can be efficiently used in the process. Waste Heat Recovery methods attempt to extract some of the energy as work that otherwise would be wasted.

Typical methods of recovering heat in industrial applications include direct heat recovery to the process itself, recuperators, regenerators, and waste heat boilers. In many applications – especially those with low-temperature waste heat streams, such as automotive applications – the economic benefits of waste heat recovery do not justify the cost of the recovery systems. Innovative, affordable methods that are highly efficient, applicable to low-temperature streams, and/or suitable for use with corrosive or “dirty” wastes could expand the number of viable applications of waste heat recovery, as well as improve the performance of existing applications. Our focus is on the development of innovative Waste Heat Recovery processes and techniques that are (1) more efficient than conventional methods, yet still cost-effective; and (2) applicable to waste streams from which heat cannot be recovered easily with conventional methods.

Turning to cooling, air conditioning systems consume approximately 10% of the energy used in U.S. buildings and are key contributors to peak demand. Consequently, improving the energy efficiency of air conditioning systems would substantially reduce overall energy consumption and enhance grid reliability. For example, compressors require cooling to dissipate the heat produced during compression and could benefit from improved surface heat transfer – innovative designs could increase the available heat-transfer area or materials enhancement could increase the heat flux between the hot and cool sides of a heat exchanger. Similarly, a reduction in the requirement for condenser cooling could provide significant energy savings if more-efficient, cost-effective technologies were developed.

This is where we believe waste heat recovery integrated with our Solar Trigeneration energy systems represents a unique opportunity for commercial and industrial clients. Industrial Waste Heat Recovery - Waste Heat Recovery from exit gases can significantly increase the energy efficiency of industrial processes. Energy can be recovered from flue and stack gases, vent gases, and combustion gases at a variety of temperatures at large-scale industrial plants (chemical plants, petroleum refineries, biorefineries, pulp and paper mills, etc.).

The Market and Potential for Waste Heat Recovery technologies and solutions

There are more than 500,000 smokestacks in the U.S. that are “wasting” heat, an untapped resource that can be converted to energy with Waste Heat Recovery technologies.

About 10% of these 500,000 smokestacks represent about 75% of the available wasted heat which has a stack gas exit temperature above 500 degrees F. which could generate approximately 50,000 megawatts of electricity annually and an annual market of over $75 billion in gross revenues before tax incentives and greenhouse gas emissions credits.

Waste Heat Recovery technologies represent the least cost solution which provides the greatest return on investment, than any other possible green energy technology or “carbon free energy” opportunity!

What is a Bottoming Cycle?

A Bottoming Cycle in power and energy generation uses the primary energy source applied to a useful heating process such as melting iron or scrap metal. The heat from this process is recovered to generate electricity.

The typical Bottoming Cycle directs waste heat from a process to a waste heat recovery boiler which converts the thermal (heat) energy to steam which provides mechanical energy to power one or more steam turbines, which is connected by a common shaft to a synchronous generator, which generates electricity and is used onsite or sold to the electric grid.

What is the Brayton Cycle?

Gas turbines operate on the principal of the Brayton Cycle, which is defined as a constant pressure cycle, with four basic operations which it accomplishes simultaneously and continuously for an uninterrupted flow of power.

Background Information and History of Rudolph Diesel and Sadi Carnot

Rudolph Diesel was educated at the predecessor school to the Technical University of Munich, Germany. In 1878, he was introduced to the work of Sadi Carnot, who theorized that an engine could achieve much higher efficiency than the steam engines of the day. Carnot envisioned a cycle in which a gas is compressed, heated, allowed to expand, and then cooled. After the gas is cooled, the cycle begins anew. Mechanical energy is used to compress the gas and thermal energy to heat it. In turn, expansion of the gas yields mechanical energy, and its cooling yields thermal energy. The net result is conversion of thermal energy to mechanical energy.

Diesel sought to apply Carnot’s theory to the internal combustion engine. The efficiency of the Carnot cycle increases with the compression ratio—the ratio of gas volume at full expansion to its volume at full compression. Nicklaus Otto invented an internal combustion engine in 1876 that was the predecessor to the modern gasoline engine. Otto’s engine mixed fuel and air before their introduction to the cylinder, and a flame or spark was used to ignite the fuel-air mixture at the appropriate time. However, air gets hotter as it is compressed, and if the compression ratio is too high, the heat of compression will ignite the fuel prematurely. The low compression ratios needed to prevent premature ignition of the fuel-air mixture limited the efficiency of the Otto engine.

Rudolph Diesel wanted to build an engine with the highest possible compression ratio. He introduced fuel only when combustion was desired and allowed the fuel to ignite on its own in the hot compressed air. Diesel’s engine achieved efficiency higher than that of the Otto engine and much higher than that of the steam engine. It also eliminated the trouble-prone electric-spark ignition system. Diesel received a patent in 1893 and demonstrated a workable engine in 1897. Today, diesel engines are classified as “compression-ignition” engines, and Otto engines are classified as “spark-ignition” engines.

What is the Carnot Cycle?

The Carnot Cycle has been described as being the most efficient thermal cycle possible, wherein there is no heat losses, and consisting of four reversible processes, two isothermal and two adiabatic. It has also been described as a cycle of expansion and compression of a reversible heat engine that does work with no loss of heat.

What is Cogeneration?

Cogeneration is the simultaneous production of electricity or power, and thermal energy in the form of heat, hot water, and/or steam, using/burning or combusting the same fuel (such as natural gas) to provide simultaneous heat and power.

Cogeneration power plants are notmally found in one of two basic types of power cycles – either a topping cycle or a bottoming cycle. The topping cycle is the most prevalent as it has the widest industrial application.

What is the Kalina Cycle?

Invented by Alexander Kalina, a Russian engineer, the Kalina Cycle uses a water and ammonia in low temperature Waste Heat Recovery applications, such as geothermal power plants, to increase thermodynamic efficiency and power output.

Problems associated with Kalina Cycle, and why it has never gained significant appeal include;

• GE Power attempted to operate a Kalina Cycle power plant in conjunction with a gas fired power plant but found it neither economical nor competitive.
• Inability to operate efficiently at high temperature or high pressure.
• Nitriding of the alloy steel superheater tubes.
• Complicated distillation columns required to recombine the turbine exhaust into the binary mixture used in the heat exchangers
• Cycle proposals seem to be based on zero frictional pressure drop and zero temperature differences in the main process components, and it is pretty expensive to meet such requirements.
• some of the above information on the Kalina Cycle from www.eng-tips.com with our thanks

What is the Organic Rankine Cycle?

• A Rankine cycle is a closed circuit steam cycle. (Also – see Rankine Cycle).
• An Organic Rankine Cycle uses a heated chemical instead of steam as found in the Rankine Cycle.
• Chemicals used in the Organic Rankine Cycle include freon, butane, propane, ammonia, and the new environmentally-friendly” refrigerants.

Why use a chemical refrigerant?

A refrigerant boils at a temperature below the temperature of frozen ice. Solar heat, for example, of only 150 degrees Fahrenheit from a typical rooftop solar hot water heater, will furiously boil a refrigerant. The resulting high-pressure refrigerant vapor is then piped to an organic Rankine cycle engine.

Why is it called “organic”?

“Organic” is a term used in chemistry to describe a class of chemicals that includes Freon and most of the other common refrigerants.

What is the Rankine Cycle?

The Rankine Cycle is a thermodynamic cycle used to generate electricity in many power stations, and is the real-world approach to the Carnot Cycle. Superheated steam is generated in a boiler, and then expanded in a steam turbine. The steam turbine drives a generator, to convert the work into electricity. The remaining steam is then condensed and recycled as feed-water to the boiler. A disadvantage of using the water-steam mixture is that superheated steam has to be used; otherwise the moisture content after expansion might be too high, which would erode the turbine blades.

What is a Topping Cycle?

The topping cycle is one of the two types of cogeneration cycles, in which the topping cycle utilizes the primary energy source to generate electricity (or mechanical power). The heat that is rejected in the combustion process is recovered in the form of useful thermal energy.

The topping cycle consists of a combustion turbine (which is connected by a common shaft to a synchronous generator, with the turbine exhaust gases directed into a waste heat recovery boiler that converts the exhaust gas heat into steam which drives a steam turbine, extracting steam to the process while driving an electric generator which generates electricity (power) that is used onsite and/or sold to the electric grid.