CHEMICAL ENGINEERING TRANSACTIONS
VOL. 81, 2020
A publication of
The Italian Association
of Chemical Engineering
Online at www.cetjournal.it
Guest Editors: Petar S. Varbanov, Qiuwang Wang, Min Zeng, Panos Seferlis, Ting Ma, Jiří J. Klemeš
Copyright © 2020, AIDIC Servizi S.r.l.
ISBN 978-88-95608-79-2; ISSN 2283-9216
Modelling Investigation of Optimum Operating Conditions for
Circulating Water Waste Heat Recovery
Li Yanga, Yunfeng Renb, Xiang Zhangb, Zhihua Wangc, Jinshi Wangd,
Xuebin Wangd, Zhouming Hanga,*
aThe College of Electrical Engineering, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018,
China
bZhejiang Pyneo technology Limited Company, Hangzhou 310000, Zhejiang, China
cState Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310012, China
dState Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
hzm0335@126.com
This study is dedicated to present a reliable numerical methodology based on Aspen Plus simulation to assess
the performance of energy-saving for circulating cooling water waste heat and water resource recovery system.
Heat-pump is used in the system to assist or replace cooling towers to recover the waste heat of low-grade
industrial circulating cooling water and convert the waste heat into high grade heat, such as hot water at 90 °C
or low-pressure saturated steam, to meet the specific cooling and heat requirements of industrial process. The
design schemes are simulated using Aspen Plus V10.0 to explore the effects of critical operating parameters,
such as working medium temperature and pressure (water, steam, refrigerant), energy efficiency, economic and
reliability analysis. The operating conditions of each design scheme are adjusted and optimized to achieve the
best energy efficiency and economy. Results proves that it is feasible to use absorption heat pump to recover
the waste heat of circulating cooling water and water resource, which provides theoretical guide and reference
for the utilization of low-grade waste heat using heat pump.
1. Introduction
As the world's energy resources become increasingly depleted and low energy efficiency, the use of waste heat
is becoming increasingly important and has increasing potential. At present, the available waste heat resources
include steam turbine exhaust waste heat, flue gas waste heat, circulating cooling water waste heat and so on,
of which the amount of circulating cooling water is the largest, and the water loss due to evaporation in cooling
tower accounts for more than 50% of the total water consumption. Even with the water harvesting and water-
saving devices, the water loss due to evaporation accounts for 1.2 ~ 1.6 %. With the shortage of water resources,
the increase of water price and sewage charge, it is very necessary to research on energy-saving and water-
saving technology for waste heat and water resource recovery of circulating cooling water in various industries.
Studies have shown that heat pump can enhance the utilization of low-grade thermal energy (Li et al., 2019)
and upgrade the low-grade heat. Imran et al. (2016) pointed out that power production from low grade heat and
waste heat does not only mitigate environmental impact but also improve energy efficiency and reduce energy
cost. Wang et al. (2020) studied the economy of heat pump system which recovering waste heat and water from
high-humidity flue gas after the wet flue gas desulfurization scrubber. Zhang et al. (2020) offers an effective way
to use the wastewater source heat pump recover the large amounts of waste heat which contains in urban
wastewater.
At present, the energy-saving technology of heat pump is developing rapidly, and some progress has been
made from experimental and theoretical research to pilot-scale process. However, there are not many cases of
systematic economic analysis of heat pump technology by the combination of off-design test, theoretical
research and industrial practice. Aspen Plus can be used to complete the whole process simulation of the
system, and to design and optimize the key operating parameters, which is the basis of the system to industrial
application. In present study, lithium bromide absorption heat pump is used to assist or replace the cooling tower
DOI: 10.3303/CET2081210
Paper Received: 31/03/2020; Revised: 29/06/2020; Accepted: 03/07/2020
Please cite this article as: Yang L., Ren Y., Zhang X., Wang Z., Wang J., Wang X., Hang Z., 2020, Modelling Investigation of Optimum
Operating Conditions for Circulating Water Waste Heat Recovery, Chemical Engineering Transactions, 81, 1255-1260
DOI:10.3303/CET2081210
1255
in a power plant to recover the waste heat of circulating water to improve the heat efficiency and heating capacity
of the system, the energy-saving performance of the system is evaluated by numerical method based on Aspen
plus simulation and economic analysis. The result can provide theoretical basis and reference for the practical
application of low-grade waste heat using heat pump technology.
1. 2. Simulation
Aspen Plus is a general process simulation software with a huge physical property database (Somers et al.,
2011) and mostly used in the chemical industry field and the field of heat pump and refrigeration (Mansouri et
al., 2015), which have common theoretical bases on the fundamental equation of states, transfer equation and
constitutive equation of matters (De Guido et al., 2015). In this paper, the following design conditions are
simulated using Aspen Plus V10.0 (Figure 1): the total circulating water flow rate of a power plant is 80,000 t/h,
the inlet water temperature of the circulating cooling water is 33 °C, the outlet water temperature is 43 °C, and
16 new cooling towers with 5,000 t/h circulating water treatment capacity per unit are planned.
1.1 Introduction of system
Generally, the circulating cooling water of the power plant goes directly into the cooling tower, cools down and
then returns to the cooling water main pipe. In this design scheme, a branch is connected to the circulating
cooling water main pipe (43 °C, 5,000 t/h), a certain proportion of circulating water is taken into the heat pump
for precooling, and the circulating water after precooling is mixed with the original circulating cooling water (43
°C) and entered the cooling tower together, and finally returned to the circulating water system (33 °C, 4,914.5
t/h, assuming the evaporation water loss is 1.71 %). The design scheme directly used lithium bromide absorption
heat pump technology to assist or replace cooling tower to cool the circulating cooling water to 33 °C or less.
The temperature of wet saturated air at the cooling tower exit decreased and the humidity content decreased
because of the precooling of circulating cooling water, which causing condensing of part water vapor and
allowing the recovery of the condensed water recovering the condensed water.
Figure 1: The cycle model implemented in Aspen Plus
At the same time, a small amount of low-grade steam (0.2 ~ 0.8 MPa) is used to drive the absorption heat pump,
to recover a large amount of low-grade waste heat from circulating cooling water and convert it into higher-grade
heat (high-temperature hot water, ≤ 90 °C, temperature adjustable) or cooling capacity (refrigeration) for users.
The construction cost of cooling towers with corresponding circulating water treatment capacity is saved and
the evaporation water loss is nearly zero, this design scheme not only can decrease the construction and
operation cost of cooling tower whitening, but also decrease the humidity content of the wet saturated air even
further using a small part of high-temperature heat source produced by the heat pump to heat the humid
saturated air, which achieving the goal of water recovery and energy conservation. This design scheme will
have a very broad application prospects in the circulating cooling water treatment of electric power, chemical
industry and other industries.
1.2 Model description
The design schemes are simulated using Aspen Plus V10.0 to explore the effects of critical operating
parameters, such as working medium temperature and pressure (water, steam, refrigerant), energy efficiency,
economic and reliability analysis and so on. This software has an integrated structure and includes process-
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related unit operation modules besides components, properties and state equations (Mansouri et al., 2015).
Every component of heat pump system is designated the corresponding module. The description of the module
symbols used in the models: B3- generator, B5-condenser, B9-absorber, B8-evaporator, 81- pump and B13-
heat exchanger. Typical state point of basic working condition results from Aspen Plus models is shown in Table
1. The operating conditions of each design scheme are adjusted and optimized to achieve the best energy
efficiency and economy, so as to guide and modify the engineering practice.
Table 1: Typical state point of basic working condition results from Aspen Plus models
State point Position Temperature (°C) Pressure (kPa) Flow rate (t/h)
21 Absorber Inlet 55 / 5,500
10 Condenser outlet 80 53 5,500
5 Generator inlet 250 / /
12 Evaporator inlet 43 4.16 5,000
13 Evaporator outlet 32 4.16 5,000
In order to simplify the mathematical model, several assumptions in the process of model building are adopted
as follows (Yang et al., 2013): first, the system is in a steady state; second, the condensation pressure is the
same as the generator pressure; third, the refrigerant water at the condenser outlet and the refrigerant at the
evaporator outlet are in a saturated state; fourth, the pressure drop of the piping and valves in the system, the
power consumption of the circulating pump and the heat loss are ignored.
2. 3. Modelling result and discussion
An off-design analysis model of recovering the waste heat from the circulating cooling water using lithium
bromide absorption heat pump technology is built, and the performance of the system design is simulated under
the design working condition. Then the system performance under off-design condition is studied by changing
the input parameters.
2.1 Effect of heating network water temperature on COP
The heating network temperature of the water supply and the return water for users has a great influence on
the coefficient of performance (COP) of the whole system. Heating network water gets into the absorber of heat
pump, come out from the condenser, and absorbs the heat and then the temperature of heating network water
rises. The outlet heating network water temperature from condenser represents the grade of heat for users. The
simulation conditions of the model are based on the design parameters of the basic working conditions, such
as the data in Table 2. Change one or more of these parameters to design the others accordingly. All simulations
follow this principle and will not be repeated in other section.
(a) (b)
Figure 2: Effect of water parameter of heating network (a) and circulating cooling water temperature (b) on COP
Modelling result indicated that increasing the heating network water supply temperature can promote the energy
efficiency of the whole system. The temperature of circulating water will drop greatly when the heating network
water supply temperature is increased to 95 °C or more, which directly affect the stable operation of heat pump
system. The maximum heating network water supply temperature in this work is 90 °C. To further increase the
temperature of hot water for specific applications, or even to produce saturated steam, the hot water can be
7,000
6,500
6,000
5,500
5,000
4,500
4,000
3,500
3,000
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heated to about 90 °C by heat pump, and then further heated to a higher temperature by a peak heater (Che
et al., 2014), or directly using the second type of absorption heat pump.
The effect of temperature and flow rate of heating network return water on COP is shown in Fig. 2a. Results
show that the system COP changes little with the increase of the heating network return water temperature
when keeping the heat release of the absorber and condenser constant and adjusting the heating network water
flow rate. With a constant heat production by the heat pump system and heating network water supply
temperature, the system COP is not affected by the temperature and flow rate of the heating network return
water, and the system is the most stable at this time with the best off-design performance.
2.2 Effect of circulating cooling water temperature on COP
For the circulating cooling water of power plant, the larger the temperature drop of circulating cooling water, the
higher the energy efficiency. However, there is no linear relationship between cooling water temperature drop
and energy efficiency for certain system, such as heat pump system, considering the economy and COP of the
whole system. In this section of model, the outlet temperature of circulating cooling water increased with the
decrease of the temperature drop of circulating cooling water in the evaporator, which will affect the pressure of
the evaporator and the absorber when the inlet temperature of circulating cooling water and other parameter
are constant. Results are shown in Figure 2b. The COP of heat pump system is increased with the increase of
the outlet temperature of the circulating cooling water. The reason is that the temperature drop decreases with
the increase of the outlet temperature of the circulating cooling water, causing the increasing of the circulating
cooling water quantity and the decreasing of driving steam consumption, which leading to the increasing of
COP. This is consistent with the conclusion of that the performance coefficient of heat pump, unit power and
the exergy efficiency increased with the increase of the outlet temperature of circulating cooling water or the
decrease of the temperature drop of circulating cooling water in evaporator make (Zhang and Chen, 2013). It is
necessary to adjust the temperature of circulating cooling water in order to improve the performance and
economy of the whole system.
2.3 Effect of driving steam parameter on COP
The COP of heat pump and the energy efficient of the whole system all will be affected by the driving steam
parameters, including steam temperature, pressure and flow rate. Driving steam consumption is also a key
parameter to measure the economy of the whole heat pump system. In this model section, the effects of driving
steam temperature, flow rate and generating temperature in the generator on the system COP are simulated
under the basic working conditions. The condensing pressure and evaporating pressure remain unchanged,
and the temperature and flow rate of circulating cooling water remain constant. Results are shown in Figure 3.
(a) (b)
Figure 3: Effect of driving steam parameter (a) and generating temperature (b) on COP
As shown in Figure 3a, the COP of the system changes little with the increase of driving steam temperature and
the decrease of the steam consumption, when keeping the heat produced by the heat pump system as a certain
amount, which means that under the condition that the heat produced by the heat pump system constant, the
system COP is not affected by the temperature and flow rate of the driving steam, and the system is stable at
this time with the best off-design performance.
Considering that the temperature and pressure of the driving steam directly affect the generating temperature
of the generator, the effect of the generating temperature in the generator on the system COP is then simulated
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and analyzed, assuming that the heating network water temperature rises from 55 °C to 80 °C, the flow rate is
constant, the condensing pressure and evaporating pressure remain constant and the temperature and flow
rate of circulating cooling water remains unchanged. Results are shown in Figure 3b, the system COP is
unchanged with the increase of driving steam temperature while the COP increases with the increase of
generator temperature, when keeping the heat produced by the heat pump system as a certain amount. The
COP of the system is about 2.224 when the generator temperature reaches the maximum of 154 °C in this work.
The results indicated that the waste heat recovery model can raise the thermal economy of the system, however,
the improvement of the energy-saving performance of the model depends on the development of the heat pump
technology.
3. 4. Economic analysis and comparison
The economic analysis and comparison are conducted for a single cooling tower of a power plant circulating
cooling water treatment capacity of 5,000 t/h, circulating cooling water temperature cooling from 43 °C to 33 °C,
assuming that the total water loss of evaporation, floating and Sewage is 1.71 %. 4 technical solutions are
compared: cooling tower with hot air mixture, 10 % of circulating water into the heat pump, 20 % of circulating
water into the heat pump, 100 % of circulating water into heat pump.
The following calculation results are based on the following principles: 5,000 t/h circulating water treatment
capacity of a single cooling tower, the annual operating time is 7,200 h, the water price is 0.1416 USD/t, the
electricity price is 70.7 USD/MWh, and the driving heat source and recovering heat/cooling capacity are
calculated on the basis of GJ and the price is 4.25 USD/GJ.
Table 2: Comparison of water saving performance of heat pump for recycling waste heat of circulating water
Items Unit Solution 1 Solution 2 Solution 3 Solution 4
Water saving t/h 9.8 23 36 85.5
Water saving rate % 13.87% 27 42 100 (theoretical)
Annual water saving t/y 70,560 165,600 259,200 615,600
According to Table 2, for a single cooling tower of the same scale (the circulating water treatment capacity is
5,000 t/h), the water saving rate of a single cooling tower is 42 % and the annual water saving is 259,200 t/y
when 20 % of circulating cooling water directly get into heat pump. Combined with a small amount of driving
heat source (0.2 ~ 0.8 MPa low-grade saturated steam), 764,600 GJ of heat/cooling capacity can be produced
every year for users. When the cooling tower is completely replaced by heat pump system, which means all the
circulating cooling water into the heat pump, the water saving rate can reach 100 % theoretically. Results show
that the waste heat recovery model can greatly reduce the evaporation loss of circulating cooling water and
save water resources. The technical economy of several system design schemes is compared for a single
cooling tower of 5,000 t/h circulating water treatment capacity.
Table 3 Comparison of investment and operating costs of system (unit: MUSD)
Items Solution 1 Solution 3 Solution 4
Cost of cooling tower
65.14(including fans and
pumps)
65.14(including fans and
pumps)
0
Cost of water saving measures 42.48 21.24 0
Cost of heat pump 0 113.28(1×30 MW) 155.76 (3×50 MW)
Cost of civil construction and
installation
51.68 95.86 224.29
Total investment costs 180.82 335.45 785.03
Electricity consumption cost 11.89 0.92 4.59
water replenishment cost 7.50 5.10 0
Cost of driving heat 0 165.25 856.40
Total annual operating costs 19.40 171.34 860.93
annual recovery costs 0 324.83 1624.15
According to Table 3, the investment cost of the cooling tower with water saving scheme is lower than other
schemes, however, the annual operating cost is 19.4 MUSD without any recovery cost and all the waste heat
of circulating cooling water is wasted, which has no return on investment. In addition, a lot of running costs every
year will be needed in this scheme.
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According to the economic comparison results and assuming that all of the recovered energy can be used
efficiently, the investment cost of system scheme of cooling tower + heat pump is 335.45 MUSD, the annual
operating cost is 171.34 MUSD, the annual recovery cost is 324.83 MUSD and the payback period is about 2
years. In practice, however, the recovered energy cannot be used completely due to the technical constraints
and practical application conditions. If 40 % of the recovered energy can be used efficiently, the investment
payback period for the system scheme of cooling tower and heat pump would be approximately 4 years and the
payback period for the system scheme of heat pump is approximately 3 years, which far less than the service
life of the equipment, which shows that it is feasible to use absorption heat pump to recover the waste heat of
circulating cooling water and water resource.
4. 5. Conclusion
This paper studied the system design scheme of circulating cooling water waste heat and water resource
recovery model. The absorption heat pump is effectively connected with the circulating cooling water. Using
Lithium Bromide absorption heat pump to assist/replace the cooling tower to recover a large amount of waste
heat in circulating cooling water and convert into a higher grade of heat output. The energy-saving performance
of the circulating cooling water waste heat and water resource recovery system are evaluated by a numerical
method based on Aspen plus simulation, the effect of heating network temperature of the water supply and the
return water for users, the temperature and flow rate of circulating cooling water, the temperature and pressure
of the driving steam and the generating temperature of the generator on COP were analyzed. At last, the
economic analysis and comparison are conducted. Results prove that it is feasible to use the absorption heat
pump to recover the waste heat of circulating cooling water and water resource, which provides a theoretical
basis and reference for the utilization of low-grade waste heat using heat pump.
Acknowledgements
This work is supported by the Program of Introducing Talents of Discipline to Universities [B08026]. And the
authors would like to thank the financial support of Key Laboratory for Technology in Rural Water Management
of Zhejiang Province, Zhejiang University of Water Resources and Electric Power.
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