Chemical Engineering June 2022 - 31

a higher GWP alternative), so care
must be taken to discuss with potential
suppliers about the availability
of suitable options.
Selection of the final device or
system depends on adherence to
the specifications by the potential
suppliers, along with a tradeoff between
capital and operating costs,
and other technical aspects. Often,
each supplier will quote the standard
system within its catalog that
will cover the service requirements in
the specifications.
It is a good practice to evaluate
different types of heat pumps (for
example, comparing an air-source
heat pump against a ground-source
heat pump), given that higher initial
capital costs may be compensated
by reduced operations costs due to
higher coefficients of performance.
A lifecycle cost analysis (see Ref. 6)
can be used to compare different alternatives
to determine the minimum
total cost over the system lifespan.
An example calculation
An industrial process requires
3,000 kW to raise the temperature
of a stream to 45°C. Two alternatives
are under evaluation: 1) use a
natural-gas-fired heater with 80%
efficiency; or 2) use a heat pump
to transfer heat from a source at
24°C. The proposed heat pump is a
closed one-stage vapor-compression
cycle with an evaporator operating
at 18°C and a condenser operating
at 50°C. When reading the
refrigerant properties at a pressureenthalpy
diagram similar to Figure
1, the following specific enthalpies
are determined:
h1 (evaporator outlet): 1,461 kJ/kg
h2
(compressor discharge):
1,630 kJ/kg
h3 (condenser outlet): 421 kJ/kg
h4 (expansion valve outlet):
421 kJ/kg
The plant manager wishes to compare
the annual energy expense of
both options, assuming that the
plant is required to operate 8,640
hours per year, and that the energy
rates are $4 per million Btu ($0.014/
kWh) of natural gas, and $0.04 per
kilowatt-hour of electricity.
Option 1: Natural-gas-fired heater.
For option 1, the fuel heat consumpW
= Qh/COP = 3,000 kW/7.154 =
419 kW
The total annual energy consumption
is:
419 kW x 8,640 h/yr =
3,620,160 kWh/yr
The total annual energy cost is:
3,620,160 kWh/yr × $0.04 kWh =
$144,806/yr
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Option 2: Heat pump. For option 2,
the electrical power consumption is
the heat load divided by the coefficient
of performance. To obtain the
coefficient of performance, Equation
(3) is rewritten to express the condenser
heat rate and compressor
power as the working fluid flowrate
multiplied by the enthalpy difference
in each equipment, assuming no
losses in piping:
COP = Qh/W = [m × (h2 - h3)]/
[m(h2 - h1)]
Where m is the mass flowrate of the
working fluid.
Since
the
flowrate
through the
compressor is the same as the flowrate
through the condenser, then for
this configuration:
COP = (h2 - h3)]/(h2 - h1) = (1,630
kJ/kg - 421 kJ/kg)/(1,630 kJ/kg -
1,461 kJ/kg)
COP = 1,209 kJ/kg/169 kJ/kg = 7.154
Then, the electrical power consumption
for option 2 is:
tion is the heat load divided by the
heater efficiency:
Fuel heat consumption, option 1 =
3,000 kW/0.80 = 3,750 kW
The total annual energy consumption is:
3,750 kW × 8,640 h/yr =
32,400,000 kWh/yr
The total annual energy cost is:
32,400,000 kWh/yr × $0.014/kWh
= $453,600/yr
This is significantly lower than the
annual cost for option 1.
Concluding remarks
Heat-pump technology is mature,
reliable and safe. It
is an efficient
means to provide heat to a growing
number of uses at increasing
temperatures. Savings in heating
expenses in the long run often outweigh
the initial capital investment
of a heat pump over heaters. New
working fluids are continuously being
developed, first to replace ozonedepleting
refrigerants, and currently
to provide non-toxic, low-GWP, and
ideally non-flammable alternatives to
available refrigerants.
n
Edited by Gerald Ondrey
Acknowledgements
The author would like to thank Mr. Philippe Nellissen
for sharing the figures of his authorship in this article,
and Dr. John R. Abelson for his feedback to improve the
article.
References
1. Secretary of Energy Granholm, Accelerating Cold Climate Heat
Pump Technology and Adoption in the Midwest, January 26,
2022, www.youtube.com/watch?v=z5oeTxVpb2A
2. Nellissen, Philippe " Energy Efficiency in industrial processes
- the role of heat pumps " European Heat Pump Association,
February 6, 2015, Retrieved from: www.ehpa.org/about/news/
article/energy-efficiency-in-industrial-processes-the-role-ofheat-pumps/
3.
International Institute of Refrigeration (2021, January)
Source Heat Pumps for Space Heating and Cooling "
" Air
4. US EPA " Phasing out HCFC Refrigerants to Protect the Ozone
Layer " .
5. Fifth Assessment Report (AR5) of the IPCC (Intergovernmental
Panel on Climate Change).
6. Giardinella, Sebastiano; Baumeister, Zabdyk and Baumeister,
Alberto, Using Lifecycle Cost Analysis for Best Project Value,
Chem. Eng.,December 2020, pp. 32-39.
Author
Sebastiano Giardinella is a visiting
assistant research scientist
and project engineer at the Illinois
Sustainable Technology Center,
Prairie Research Institute, University
of Illinois at Urbana-Champaign
(1 Hazelwood Dr., Champaign,
IL
61820; Phone:
+1-217-953-1424; Email:
sg00@illinois.edu; URL: www.istc.
illinois.edu) where he is involved in energy-storage and
carbon-capture projects. He previously co-founded the
Ecotek group of companies (www.ecotekgrp.com/en),
where he has performed feasibility studies, corporate
management, project management and process engineering
consulting in projects for the chemical and energy
industry. He is a project management professional
(PMP), has a M.Sc. in renewable energy development
from Heriot-Watt University, a Master's degree in project
management from Universidad Latina de Panamá, and
a degree in chemical engineering from Universidad
Simón Bolívar. He has written technical publications for
Chemical Engineering magazine and other international
associations, and holds one patent for an energy storage
system and method.
31
http://www.youtube.com/watch?v=z5oeTxVpb2A http://www.ehpa.org/about/news/article/energy-efficiency-in-industrial-processes-the-role-of-heat-pumps/ http://www.ehpa.org/about/news/article/energy-efficiency-in-industrial-processes-the-role-of-heat-pumps/ https://www.istc.illinois.edu/ https://www.istc.illinois.edu/ http://www.ecotekgrp.com/en http://WWW.CHEMENGONLINE.COM
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