Literature Review of Industrial Waste Heat Recovery with Many Parallels to Automotive Waste Heat Recovery

1. Credit: Department of Energy
Industrial Waste Heat Recovery
Alfred Piggott
4/20/2012
MEEM 4220 – Internal Combustion Engines

2. 1.0 Introduction ....................................................................................................................................... 3
Table of Contents
2.0 Waste Heat Grades .......................................................................................................................... 3
3.0 Systems for Waste heat Recovery ............................................................................................. 5
3.1 Heat Exchangers .......................................................................................................................... 5
3.2 Load Preheating .......................................................................................................................... 5
3.3 Low Grade Recovery .................................................................................................................. 5
3.3.1 Deep Economizers.............................................................................................................. 6
3.3.2 Indirect Contact Condensation Recovery.................................................................. 6
3.3.3 Direct Contact Condensation Recovery ..................................................................... 6
3.3.4 Transport Membrane Condenser ................................................................................. 6
3.3.5 Heat Pumps ........................................................................................................................... 6
3.3.6 Closed compression cycles ............................................................................................. 6
3.3.7 Open Cycle Vapor Recompression ............................................................................... 6
3.3.8 Absorption Heat Pumps ................................................................................................... 6
3.4 Power Generation ....................................................................................................................... 7
3.4.1 Generating Power via Mechanical work .................................................................... 7
3.4.2 Direct Electrical Conversion Systems ......................................................................... 7
4.0 Conclusion .......................................................................................................................................... 8
5.0 Bibliography ...................................................................................................................................... 9
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3. 1.0 Introduction
As fuel prices rise, supplies decrease, and concerns about environmental impact intensify, we
start looking for new and better energy sources. A pollution-free source of energy that is
often overlooked is waste heat. Waste heat is a byproduct of converting energy from one
form to another. As governed by the second law of thermodynamics, no process of energy
conversion is 100% efficient. Typical fossil fuel energy conversion processes include
converting coal or natural gas to electricity or gasoline to vehicle power.
In 2008 world energy consumption was roughly 505 quadrillion BTU (505 X 1015 BTU) (1).
Conversion of fossil fuels to usable energy accounts for roughly 84% (1) of the world energy
consumption. The efficiency of these fossil fuel conversion processes tends to be around 28-
43% (2). This means 57-72% or 217-288 quadrillion BTU is turned to heat and not part of
the usable output. This equates to roughly 4-5 times more energy going to waste heat than all
the renewable energy (Wind, Solar, Hydropower, Biomass, Geothermal) usage which was
about 50 quadrillion BTU in 2008 (1).
2.0 Waste Heat Grades
The second law of thermodynamics also governs the amount of waste heat that can be
recovered. The higher the temperature of the waste heat, the greater the proportion that can
be recovered. This can be seen with the equation for Carnot efficiency (equation 1). TH is the
temperature of the waste heat and TL is the temperature of the environment, for example the
ambient air temperature or the temperature of a lake or river where a portion of heat not
recovered will be “dumped”.
𝑇𝐿
𝜂 𝐶𝑎𝑟𝑛𝑜𝑡 = 1 −
𝑇𝐻
Equation 1: Carnot Efficiency
Waste heat temperatures are generally classified into three categories (3). These categories
were chosen based on typical industrial waste heat temperatures and the commercially
available equipment to recover the waste heat.
High-Grade 1100 ≤ TH ≤ 3000◦F (590-1650◦C)
Medium-Grade 400 ≤ TH ≤ 1100◦F (205-590◦C)
Low-Grade 80 ≤ TH ≤ 400◦F (27-205◦C)
Table 1 shows various sources of high-grade waste heat. Although high-grade waste heat can
be recovered at a higher efficiency than the lower grades, the cost to do so will be higher due
to special materials and equipment design needed to withstand the higher temperatures.
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4. Table 1: Sources of High Grade Waste Heat [Source (3)]
Table 2 shows sources of medium grade waste heat. This is a temperature range that can still
be economical (3) without the higher cost associated with high-grade recovery conversion
equipment.
Table 2: Sources of Medium Grade Waste Heat [Source (3)]
Table 3 shows sources of low-grade heat. Due to low efficiency at these temperatures, it is
typically not economical to extract work from these sources. Some applications include
preheating process gases, liquids, solids, or space heating.
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5. Table 3: Source of Lowe Grade Waste Heat [Source (3)]
3.0 Systems for Waste heat Recovery
3.1 Heat Exchangers
In medium to high temperature heat recovery systems, heat exchangers use heat from
combustion exhaust gases to preheat pre-combustion incoming air. This reduces the amount
of heat taken from combustion to heat the air and thus more combustion heat is available to
run the intended process. There are many types of heat exchangers used, these include
recuperators, regenerators, heat wheels, passive air preheaters, heat pipes, waste heat boilers
and finned tube heat exchangers / economizers. Each of these has advantages and
disadvantages for a given application.
3.2 Load Preheating
Load preheating refers to the preheating solid materials entering a plant with the waste heat
from the plant process. An example of solid preheating is using the waste heat from a braze
furnace to preheat the parts that will be brazed. This reduces the load on the furnaces and
thus reduced energy consumption.
3.3 Low Grade Recovery
As in high and medium grade waste heat recovery, low-grade waste heat recovery also uses
heat exchangers to accomplish the task. Low-grade recovery has a different set of challenges
than medium and high-grade waste heat recovery. The main challenges are corrosion, large
heat transfer surfaces, and finding a use for recovered heat. Corrosion becomes a challenge
because these heat exchangers cool the gases to a low enough temperature that vapors
condense. These combustion vapors are highly corrosive. Another challenge for low-grade
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6. recovery is the size of the heat exchanger. The laws of heat transfer require a larger surface
area for heat transfer if the difference in temperature of the hot side and cold side is smaller.
3.3.1 Deep Economizers
Deep economizers are corrosion resistant heat exchangers designed to cool exhaust gases to
low-grade 150-160ºF. The heat recovered from the exhaust gas can then be used for another
process.
3.3.2 Indirect Contact Condensation Recovery
Indirect contact condensation recovery units are corrosion resistant shell and tube heat
exchangers that can cool gases enough (100-110ºF) to completely condense vapor which in
turn increases their efficiency.
3.3.3 Direct Contact Condensation Recovery
Direct contact condensation recovery heat exchangers mix process “waste” steam with
cooling fluid that is used to heat or preheat an external system. The direct contact of steam
with the cooling fluid makes this process more efficient than an indirect contact heat
exchanger.
3.3.4 Transport Membrane Condenser
A transport membrane condenser uses capillary action to condense combustion gas vapor
and recover latent heat for use in another process.
3.3.5 Heat Pumps
Heat pumps can increase the temperature of low-grade waste heat for usage in a process that
requires a higher temperature. In certain cases this can be done economically depending on
the temperature rise needed and the cost of fuel and electricity.
3.3.6 Closed compression cycles
The closed compression cycle is essentially a heat pump. This cycle removes heat from one
fluid loop where cooling is needed and adds that heat to another fluid loop where heating is
needed.
3.3.7 Open Cycle Vapor Recompression
These systems use either mechanical or thermal compression to increase the pressure and
thus the temperature of a waste side vapor. This allows the heat to be used in processes
where a higher temperature is needed.
3.3.8 Absorption Heat Pumps
The operation of an absorption heat pump is similar to the closed cycle compression system
but instead of using mechanical compression it uses chemical means driven by heat.
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7. 3.4 Power Generation
Power generation from waste heat typically involves using waste heat to generate mechanical
energy, which subsequently drives an electrical generator. Prevailing technologies for
accomplishing this are the steam Rankine cycle, organic Rankine cycle and the Kalina cycle.
Other types of power generation that currently have not been demonstrated for large-scale
industrial use are thermoelectric, piezoelectric, thermionic and thermal voltaic power
generation. These types convert heat directly to electrical energy.
3.4.1 Generating Power via Mechanical work
3.4.1.1 Steam Rankine Cycle
The most common system that converts heat to mechanical work is the steam Rankine cycle.
This system is typically used for medium grade waste heat as it becomes less economical for
low-grade heat. Furthermore, if temperatures are too low, superheat will not be achieved and
if superheat is not achieved, condensation and erosion of turbine blades will occur.
3.4.1.2 Organic Rankine Cycle
Organic Rankine cycle is much more suitable for low temperature waste heat recovery. This
suitability comes from the organic working fluid, which has a higher vapor pressure and a
lower boiling point than water. The higher molecular mass of the organic working fluid also
allows for smaller turbine design due to more energy imparted on the turbine blade per unit
area.
3.4.1.3 Kalina Cycle
The Kalina cycle is basically a Rankine cycle that uses a mixture of two non-reacting fluids.
The benefit of using two fluids is better thermal matching to the waste heat source. This
thermal matching allows the Kalina cycle to achieve significant efficiency gains over the one
fluid Rankine cycle.
3.4.2 Direct Electrical Conversion Systems
3.4.2.1 Thermoelectric Generation
Thermoelectric generators (TEG) utilize the Seebeck effect to convert heat directly to
electricity. When two different semiconductors are connected electrically in series and a
temperature differential is applied, a voltage is created across the series. Thermoelectric
materials are suitable for medium and high-grade waste heat recovery. Currently the costs are
high and efficiencies are relatively low compared with the Rankine cycles. Advances in
materials will make thermoelectric power generation more competitive.
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8. 3.4.2.2 Piezoelectric Power Generation
Piezoelectric Power Generation (PEPG) converts mechanical vibrations into electricity.
These vibrations come from oscillating gas expansion processes. PEPG are suitable for low-
grade heat recovery. These devices are currently very low efficiency and high cost.
3.4.2.3 Thermionic Generation
Thermionic generation devices operate on the principle of thermionic emission. Thermionic
emission is produced when a temperature difference across two metal oxide plates separated
in a vacuum causes electrons to flow through the vacuum gap. These devices are suitable for
high and low-grade heat sources.
3.4.2.4 Thermophotovoltaic (TPV) Generator
TPV generators operate by converting radiant heat into electricity. The heat source heats an
emitter, which gives off electromagnetic radiation. This radiation travels through a filter and
on to the Photovoltaic cell that converts the radiation into electricity.
4.0 Conclusion
Waste heat is a large potential source of pollution free energy. The maximum efficiency of
the system used to recover waste heat depends on the temperature of the heat and is
governed by laws of thermodynamics. There are many types of waste heat recovery
equipment, each with its own pros and cons depending on the characteristics of the waste
heat source. In the future, it will be possible to convert heat directly to electricity
economically on a larger scale with solid-state devices.
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9. 5.0 Bibliography
1. "International Energy Outlook 2011." U.S. Energy Information Administration (EIA). Sept.
2011. Web. 7 Mar. 2011. <http://www.eia.gov/forecasts/ieo/pdf/0484(2011).pdf>.
2. "Environmental Footprints and Costs of Coal-Based Integrated Gasification Combined Cycle
and Pulverized Coal Technologies." United States Environmental Protection Agency. Nexant,
Inc., July 2006. Web. 7 Mar. 2012. <2.
http://www.epa.gov/air/caaac/coaltech/2007_01_epaigcc.pdf>.
3. Doty, Steve, and Wayne C. Turner. Energy Management Handbook - Seventh Addition. 7th ed.
Lilburn: Fairmont, 2009. Print.
4. "Waste Heat Recovery: Technology and Opportunities in the U.S. Industry." U.S. Department
of Energy Efficiency and Renewable Energy. BCS Incorporated, Mar. 2008. Web. 10 Mar. 2012.
<http://www1.eere.energy.gov/industry/intensiveprocesses/pdfs/waste_heat_recovery.pdf>.
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