34 ISE Magazine | www.iise.org/ISEmagazine
Energy-intensive industries, such as aluminum, iron
and steel, cement, glass, ceramic, chemicals, pulp and
paper, oil rening, and food and beverage, consume
a great share of energy consumed in the economy. In
doing so, their thermal processes reject big quanti-
ties of heat energy, and most of this wasted heat is
dumped into the atmosphere without beneficial use. Though
technologies of waste heat recovery have been in place for de-
cades and are used in oil rening and gas processing applica-
tions, the heat reuse process now is being emphasized. The ef-
fort focuses on improving energy efficiency in all industries by
reducing the consumption of prime energies, a main strategy to
combat climate change.
Here we will discuss the potential of waste heat sources re-
covery, different types of these sources, the main uses and ap-
plications of recovered heat, with particular emphasis on the oil
and gas industry and the limitations of recovered waste heat.
The U.S. Department of Energy reports that the American
industrial sector consumes 35% of the total energy consumed
within the country, and estimates that around 20% to 50% of
the energy input into this sector is wasted in exhaust gasses,
cooling water and heat lost from hot equipment surfaces and
heated products. The estimation of waste heat recovery poten-
tial in the European Union is about 300 trillion-watt hours per
year. A German official said this waste could heat space in all of
the buildings in the EU.
That creates the high potential for recovering this energy
that can be reused to reduce fuel consumption without any
addition of greenhouse gas emission. This recovered heat has
many uses inside industrial facilities; for example, the preheat-
ing of combustion air and boilers’ feed water, besides generat-
ing electricity. In addition, the recovered waste heat is a main
source of energy for district heating operators.
Waste heat sources are generally classified into three tem-
perature categories. Low temperature waste heat refers to levels
below 200 degrees Celsius/392 Fahrenheit, which represents
60% of waste heat resources in the U.S. In the EU, one third of
the unused waste heat is of low temperature; another 25% is in
the range of 200-500 C and the rest is above 500 C/932 F. The
energy source of high temperature is of higher quality and can
be recovered more feasibly.
While high-temperature heat has been captured and reused
for decades, industries focus now on the low-temperature en-
ergy sources that contain large quantities of heat. Its quality can
be improved by using heat pumps to raise the temperature to
suit end use.
For example, the grid water temperature of district heating
is 70 degrees C (158 F) for modern facilities; that level of heat-
The potential energy savings
of heat reuse
Recovering wasted heat can cut fuel consumption, greenhouse gas emissions
By Alaa Kafafi
The potential energy savings
of heat reuse
Recovering wasted heat can cut fuel consumption, greenhouse gas emissions
By Alaa Kafafi
November 2020 | ISE Magazine 35
ing can be attained by recovering heat from low temperature
sources and the addition of a heat pump within the delivery
system. In contrast, Lonsdale Energy Corp.s heating plant that
was commissioned in 2003 and provides space heating and
hot water to the North Vancouver area in British Columbia,
Canada, was designed to keep the temperature of the hot water
flowing in the pipe network at 82 degrees C (179 F), which
restricted the use of low-temperature waste heat sources. Thus,
keeping the temperature of the domestic water to the mini-
mum usable limit facilitates the use of low-temperature energy
sources and reduces the rate for consumers.
To understand how a heat pump decreases energy losses, re-
fer to Figure 1 (Page 36). To generate 1 million BTU (British
thermal unit), a heat pump with a coefcient of performance of
4 needs an equivalent 0.25 million BTU of electric energy that
goes to the compressor to create the work needed to compress
the flowing fluid in the heat pump, and 0.75 million BTU of
waste heat source to produce 1 million BTU of useful heat. If
a boiler has an efficiency of 80%, the boiler needs an input of
1.25 million BTU (from fuel burned) to generate heat energy
of 1 million BTU. In addition to the higher efficiency in us-
ing a heat pump, lower fuel consumption and greenhouse gas
emission is achieved.
Besides the uses of heat recovered outside the industrial fa-
cility discussed above, the high temperature energy has many
uses inside the manufacturing plant, such as combustion air,
boiler feedwater and load preheating, steam and power gen-
eration and transfer to liquid or gaseous process streams. The
main equipment used in heat recovery applications are heat ex-
changers. Different types of heat exchangers are used to make
standard modules that suit each end use applications and are
available commercially.
The oil and gas industry has abundant sources of waste heat.
The research and development of heat recovery methods go
hand-in-hand with other methods of enhancing energy ef-
ficiency and curbing of greenhouse gases emissions. It’s esti-
mated that the market of the waste heat recovery application
within the oil and gas industry will reach $15 billion in 2024.
We will present two hands-on applications of heat recovery
within the industry. The first is the use of an absorption chiller
that uses waste heat to produce cheap cooling in gas processing
plants, rather than using a vapor compression chiller that con-
sumes a large quantity of electricity. The second application is
organic Rankine cycle (ORC) that generates electricity from
low temperature heat sources.
Use of absorption chillers
In a natural gas processing plant, the removal of natural gas
36 ISE Magazine | www.iise.org/ISEmagazine
The potential energy savings of heat reuse
liquids (NGL) ethane, propane, butane
and pentanes from the natural gas stream
is a main process prior to transportation
to consumers. It leaves only methane in
the natural gas stream as a requirement
to what is called “pipeline quality.” The
separated natural gas liquids have a high
monetary value when sold separately and
used as energy sources, raw material for
petrochemical products and in oil en-
hanced recovery – a process that injects
water or gas into depleted oil reservoirs
to increase pressure and extract more oil.
Cooling the natural gas stream is a
requirement for the separation of NGL.
This is conventionally done using com-
pression chillers that produce the re-
quired cooling through a compressor that
exerts mechanical work on the refriger-
ant (Freon); the cooling cycle of com-
pression chillers is akin to the air condi-
tioning cooling process. The compressor
consumes a large quantity of electricity,
making this process expensive.
To save energy consumption in the
cooling process, absorption chillers have
been used. Waste heat recovered from
a gas turbine exhaust is used to operate
these chillers instead of the compressor
used in compression chillers. Since there
is no compressor in the cooling cycle of
the absorption chiller, only a pump to
circulate the refrigerant, electricity con-
sumption is one-tenth that of the com-
pression chiller.
There are two kinds of absorption
chillers: one uses water and lithium bro-
mide (H²O-LiBr), the other water and
ammonia. For the former, water is used as
a refrigerant and lithium bromide as the
absorbent; ammonia is the refrigerant and
water is the absorbent for the latter. The
working principles of absorption chiller
are simple: The evaporator allows the re-
frigerant to evaporate, due to the pressure
difference between the evaporator and
condenser, and to be absorbed by the ab-
sorbent, a process that extracts heat from
the chilled water loop. The combined
fluids then go to the generator, which is
heated by the waste heat stream, driving
the refrigerant back out of the absorbent.
Heat pump cuts energy loss
Using a fossil-fuel fired boiler to generate 1 million BTU of thermal energy or a heat pump
with a source of waste heat can produce the same output.
Data centers a viable source of reusable heat
One major source of wasted heat occurs in data centers, where 98% of energy is lost in
the form of low-grade heat. As a result, some companies are looking at ways to reuse a
portion of this energy by transforming the wasted heat into usable energy.
Some examples cited in a recent Data Center Knowledge article include IBM in
Switzerland reusing such heat to warm a swimming pool; Finnish data centers by
Yandex and Academica using heat to warm 500 to 1,000 residents’ homes; and
Amazon reusing its data center heat for a Seattle biosphere project.
The two main challenges are converting low temperature heat to useful energy, then
finding a way to transport it effectively. For the former, highly efficiently heat pumps have
been used to take waste heat that originally measures between 80 and 95 degrees F
(28 C) and increase it to 130-160 F. To transport it, waste heat can be transferred to a
liquid medium and used in industrial process or district heating needs.
Capturing waste heat for such use, however, is not easy or inexpensive, and involves
insulated ducting or plumbing. As a result, much of the heat reuse from data centers
thus far involve venting it to a nearby greenhouse or other building in its pure form.
Yet waste heat can also serve as a source of income to help defray such expenses.
A sizable data center (1.2 megawatt) that sells its waste heat could bring in as much as
$350,000 per year. And there are the environmental benefits. A data center of that size
could save nearly 6,000 metro tons of CO2 per year through such recycling.
CyrusOne recently partnered with the municipality of Haarlem, The Netherlands,
and a local business park to research capturing waste heat from its Amsterdam data
center to help heat 15,000 homes. The process involves using heat pumps and
refrigerant gas to capture and transfer the heat, allowing the municipality to cut back
on petroleum fuel use.
“It is crucial that we build data centers that work in a way that is compatible with
a sustainable future,” said Matt Pullen, CyrusOne’s executive vice president and
managing director of Europe.
Sources: www.datacenterknowledge.com, www.datacenterdynamics.com
November 2020 | ISE Magazine 37
The refrigerant then goes to the condenser to be cooled back
down into a liquid, while the absorbent is pumped back to the
absorber. The cooled refrigerant is released through an expan-
sion valve into the evaporator, and the cycle repeats (see Figure
In addition to saving energy costs, absorption chillers are
environment friendly, noise- free and can be easily integrated
within the natural gas processing plants that usually accom-
modate gas turbines for electricity generation, or as mechanical
The Society of Petroleum Engineers presented an analysis of
a gas turbine waste heat-powered double-effect H²O-LiBr ab-
sorption chiller in an integrated natural gas plant. It was found
that waste heat recuperated from the turbine exhaust gases
could be used to provide enhanced process cooling capacity to
the natural gas plant through absorption cooling. The results
suggest that a double-effect lithium bromide absorption chiller
utilizing 34.6 megawatts of gas turbine exhaust heat could pro-
vide 45 MW of cooling at 5 degrees C (41 F). This could save
approximately 9 MW of electric energy required by a typical
compression chiller while providing an equivalent amount of
Organic Rankine cycle
The Rankine cycle of conventional electricity generation en-
tails using heat to generate steam that drives a turbine to gen-
erate power (see Figure 3 on Page 38). Optimum efficiency
can be realized with exhaust streams of a temperature around
650 to 700 F. Low-temperature waste heat streams dont pro-
vide enough energy to superheat the steam that causes con-
densation and the erosion of turbine blades. In addition, there
is the need for a large heat transfer surface, making the heat
recovery units bulkier. The ORC has enabled the utilization of
low-temperature heat streams in power generation. The main
idea of this method is us-
ing organic fluids of less
than boiling temperatures
and higher vapor pressure
than water, such as silicon
oil, propane, haloalkanes
(e.g., Freons), isopentane
and isobutane. The choice
of the working fluid de-
pends on the temperature
of the waste heat stream
and the cost of generating
electricity in dollars per
kilowatt hour.
On the other hand, the
efciency of the ORC is
anywhere from a single
digit to 20%; that is lower
than the traditional Rankine cycle in the range of 30%-40%.
But theoretically, a heat engines’ performance is based on Car-
not efficiency that equals 1- (T sink/T source), where T sink
is the temperature of the fluid in the condenser and T source is
the fluid in the evaporator.
As an example: The efciency of a heat engine operates at T
source of 150 C and T sink of 25 C equals to 1- (298/423) X
100 = 30%. We changed the degrees of Celsius to Kelvin (add
273 to each number). This efficiency is theoretical, in practice;
controlling T source and T sink to the above limits needs a
large heat transfer surface that goes beyond the limitation of
space. However, ORC is still feasible despite its lower efficiency
in cases where waste heat cant be used in other heating pro-
cesses within the facility or sold to a nearby district heating op-
erator. An offshore oil production platform is a typical example
that suits the use of ORC.
The space of offshore platforms is limited and, of course,
expensive. These platforms usually accommodate turbines; in-
stalling a waste heat recovery unit behind the turbine is a good
means to put the heat of the exhaust stream to good use. In
tackling the space limitation, I used to work at engineering,
procurement and construction companies with the designers of
waste heat recovery units (WHRU) during the early stages of
offshore platforms’ front end engineering design to customize
WHRUs to suit the available space. Designers of these units,
such as AMEC Foster Wheeler, GE and Siemens, succeeded in
designing units that occupied 50% of the space needed for the
commercial modules that do the same job.
The economics of ORC heat recovery on offshore platforms
and other oil and gas applications are not widely documented,
but an article published in Distributed Energy highlighted the
feasibility of this method. It concluded: “An example of a re-
cent successful installation is in Bavaria, Germany, where a ce-
ment plant installed an ORC to recover waste heat from its
Absorption chiller process
The working principles for both a compression chiller (mechanical vapor compression) and an absorption
chiller (thermal vapor compression).
38 ISE Magazine | www.iise.org/ISEmagazine
The potential energy savings of heat reuse
clinker cooler, whose exhaust gas is at about 930 F (500 C).
The ORC provided 12% of the plant’s electricity requirements
and reduced CO2 emissions by approximately 7,000 tons.
Limitations of heat reuse methods
While the energy sources are abundant and the technologies
are available commercially through standard modules that suit
each end-use application, many factors limit the full use of
these unused energy sources.
Cost is the first challenge, especially when the heat stream –
exhaust gas for example – includes agents that cause corrosion
or fouling on the heat exchanger tubes. That suggests the use
of expensive alloys, and when the area of heat transfer is large,
the cost skyrockets. In addition, the operation of heat exchang-
ers entails high maintenance expenses that includes inspection;
plugging leaking tubes that reduces the equipment efficiency;
retubing the heat exchanger, which may involve replacing the
whole tube bundle; cleaning the drums and nozzles; and re-
testing the refurbished heat exchanger. All that adds expen-
sive operational costs to the price of the heat exchanger or the
industrial module needed for such application, which makes
the utilization of such unused heat source unfeasible. Advance
in filtration technologies to make the heat stream clean and
scrubbing the corrosive and fouling substances will help scale
up waste heat recovery.
The temperature of the waste heat
source, as discussed, affects the feasi-
bility of recovery. High temperature
sources are more feasible to recover,
while low temperature heat sources
need a heat pump to improve their
quality. But the quantity of heat recov-
ered is limited to keeping the tempera-
ture of the heat stream above the dew
point (at the temperature below it, the
water vapor condenses and causes cor-
rosion to the heat exchanger surfaces).
In addition, Carnot efficiency denes
the max efficiency from a heat source
to produce power; this efficiency equals
1- (T sink/T source). This denes the
feasibility of the waste heat source to
generate electric power. This means the
temperature of the source must be high
and of the sink low to achieve commer-
cial efficiency, which needs large area of
heat transfer surface and higher costs for
heat exchangers.
Waste heat stream composition also
limits the recovery process. For ex-
ample, flue gases from combustion have
temperatures above 1,000 C and con-
tain acid gases, CO² and H²S that accelerate corrosion on the
heat exchanger surfaces, besides thermal cyclic stresses due to
the cyclic heating and cooling on the exchanger. In this case,
standard materials as carbon steels are not suitable for the heat
exchangers, but expensive alloys are a must.
Finally, the heat in liquid and gas streams can easily be trans-
ported by piping systems in the case of recovering waste heat in
solid materials such as iron ingots, steel and aluminum castings.
The transportability of heat is difficult, and thus, not common.
In summary, the recovery of waste heat promotes energy
efciency in the industrial sector that drives economic com-
petitiveness and curbs greenhouse gas emissions. However,
the recovery process is only feasible and possible for part of the
waste heat sources due to cost, thermodynamic restrictions and
the properties of the waste heat streams.
Alaa Kafafi is a quality management and operational effectiveness con-
sultant and lives in Vancouver, Canada. He has a bachelors degree
in mechanical engineering and a masters in fluid mechanics and has
led quality management efforts with local and international oil and gas
companies in the Middle East (among them British Petroleum) and en-
gineering companies in Calgary, Canada. He was director of quality &
HSE with Toyo Engineering. A Six Sigma black belt, Kafafi practiced
project management and engineering of pipelines, as well as contributed
to training on lean Six Sigma. Contact him at kafaa@gmail.com.
Rankine cycle
Conventional electricity generation that entails using heat to generate steam that drives a
turbine to generate power.