Thermal Science & Engineering Vol.27 No.1 (2019)

Heat Pumps for Efficient Low Grade Heat Uses: From Concept to Application
R.Z. Wang*, Z.Y. Xu, B. Hu, S. Du, Q.W. Pan, L. Jiang and L.W. Wang
Abstract
Utilization of the widely available low grade heat including the solar thermal energy, waste heat and even the ambient
heat offers great opportunities for environmentally friendly energy harvesting. Despite the efforts on developing new
low grade heat conversion technologies, heat pump is still one of the most effective ways to utilize low grade heat in thecurrent stage. Regarding the heat pump for efficient low grade heat uses, this paper unrolls a comprehensive view fromthe thermodynamic cycle, working medium, as well as heat and mass transfer enhancement, to the low grade heatutilization systems. Different heat pumps including vapor compression heat pump, absorption heat pump, andadsorption heat pump are included. The aim of this paper is to share our experience in the evolution from heat pumpconcept to low grade heat utilization. It is also helpful for researchers who try to build a bridge between thermal scienceand thermal engineering.
Key Words: Heat pump, Waste heat recovery, Thermodynamic cycle, Heat and mass transfer
commonly used HPs including the vapor compression,
absorption, and adsorption HPs will be introduced. The
complete technology evolution of HPs could also
provide a guideline for the future development of otherlow grade heat utilizing approaches.

1 Introduction
Low grade heat typically refers to the heat withtemperature lower than 150 oC. Except for the widely
available low grade heat from solar collector andgeothermal well, large amount of industrial waste heatalso offers great opportunities for low grade heatutilization. Among the waste heat with different forms
and temperature levels, nearly half of it belongs to thelow grade heat. Significant energy savings and emission
reduction could be achieved if the low grade heat could
be recovered. However, the low grade heat is difficult tobe used due to its poor ability for power conversion.
Even the low temperature heat-to-power conversion
systems including the organic Rankine cycle and Kalina
cycle are not efficient especially for the driving
temperature below 100 oC. In this case, various new
approaches have been proposed aiming at the low grade
heat recovery including the thermoelectric system,
pyroelectric system, thermogalvanic system, etc.
However, these systems are either in the concept level or
too expensive for large scale applications. Heat pumps(HPs) converting the low grade heat into hightemperature heat output or low temperature cooling
output are still the most effective way to utilize the low
grade heat.
The researches of HPs cover the thermodynamic
concept innovation, working pair selection, heat andmass transfer enhancement and its integration into waste
heat recovery systems. In this paper, we will give acomprehensive view of HPs on all these aspects aspresented in Fig. 1, which illustrates how to develop
practical systems from the original concepts. The mostLow grade heatutilization

Thermodynamic
concept

Working medium Cycle construction
Practical system

Heat & masstransfer

Cycle optimization

Fig. 1. Development of low grade heat utilizing
technology: from concept to application.
Before specifying HP technology, the discussion starts
from the ideal HP cycles for low grade heat utilization asshown in Fig. 2. The ideal HP cycle with double heatsources shown in Fig. 2(a) is the reverse Carnot cycle.
By consuming work input, it could capture the low grade
heat with temperature of Tr and deliver heat output at ahigher temperature of Th. The ideal HP cycle with triple
heat sources shown in Fig. 2(b) is the cascading of the
Carnot cycle between high temperature heat source (Th)and medium temperature heat source (To), and thereverse Carnot cycle between medium temperature heatsource (To) and low temperature heat source (Tr). Workoutput from the Carnot cycle will be delivered to thereverse Carnot cycle. In the ideal three-heat-source HP

Manuscript accepted: November 17th, 2018; Editor: Katsunori Hanamura
1 Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China-1-

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Thermal Science & Engineering Vol.27 No.1 (2019)an example of HP cycle with double heat sources, andsorption (absorption and adsorption) HPs will be
introduced as examples of HP cycles with triple heatsources.

cycle, the low grade heat could act as either the hightemperature heat source or low temperature heat source,
depending on the temperature level of low grade heatand user demands. Other ideal HP cycles with triple heatsources could also be built by different combinations
between Carnot cycle and reverse Carnot cycle, aiming
for different kinds of low grade heat utilization.

2 Vapor compression heat pump
By consuming electricity input, vapor compression
HPs capture the low temperature heat in the evaporator
and output the high temperature heat from the condenser.
The output temperature generally ranges from 55 to
120 °C, depending on the temperature of low grade heat
input. Vapor compression HP has drawn a lot of
attentions in the last decades due to its decreasing
investment and increasing efficiency. Currently, thevapor compression HP still needs to be improved on two
aspects. (1) Applications of vapor compression HP for
low grade heat utilization mainly emphasize on theindustrial waste heat recovery, and high temperature
vapor compression HP is favorable since heat output at
higher temperatures could meet more user demands.
New working fluids and the corresponding compressors,heat exchange technologies for high temperature
operation should be developed. (2) Another importanttask for vapor compression HP is the alternativerefrigerants for environmental protection. The commonlyused refrigerants like R134a will be phased out in thenear future. Alternative working fluid with low Global
Warming Potential (GWP) or even natural fluid and thecorresponding compressors, heat exchange technologies
should be developed. Considering the aforementioned
situation, we will discuss more about the working
medium in this section.

Fig. 2. Ideal cycle of HPs: (a) HP cycle with double
heat sources, (b) HP cycle with triple heat sources.
With such ideal cycles of HPs, we can easily find theimportance of heat and mass transfer enhancement byconsidering different temperature differences between
the heat source and the process in the ideal cycle. Fig. 3
shows the performance degradation of HP cycle withtriple heat sources. The ideal refrigeration cycle underheat source temperatures of 80 oC/30 oC/10 oC and heattransfer temperature differences of 0 oC/0 oC/0 oC could
yield a refrigeration COP of 2.0. However, if the heattransfer temperature differences are increased to 10 oC/10o
C/5 oC which is common in real systems, the ideal COP
will be decreased to 0.69. If the heat and mass transfer
could be enhanced to minimize the temperature
difference in a reasonable and economical way, the total
efficiency would increase a lot.

2.1 Working medium of vapor compression HP
Taking the environmental issues and high temperatureapplications into consideration, properties of low GWP
refrigerants for high temperature operation are focused inthis section. Table 1 presents a comparison of selectedrefrigerants for high temperature vapor compression HPs.
Among these refrigerants, R1234yf, R1234ze(E),R1234ze(Z), R1233zd(E) are relatively new refrigerants(Brown et al., 2009; Sánchez et al., 2017), while R152a,
R717, R600 and R744 are conventional refrigerants.
In the current stage, R134a and R245fa are still
popular for high temperature vapor compression HPs.
However, R134a and R245fa have high GWP values, and
HFOs including R1234ze(E), R1234ze(Z), andR1233zd(E) are studied as substitutions.
Natural refrigerants also have low GWP values and are
typically cost effective, but most of them also has theirown drawbacks. (1) R717 (ammonia) demonstrates
excellent thermo-physical properties, but it has the toxic
and flammable issues. (2) R600 (butane) could cover awide range of high temperature applications with low
GWP value and high critical temperature of 154 °C.
However, the flammability limits its application in small
plants. (3) R744 (CO2) has a low critical temperature,

Fig. 3. Efficiency of ideal HP cycle with triple heatsources and its degradation with external heatexchange losses.
Except for the ideal HP cycles, we still need proper
working media to realize these cycles, and theintegration of working medium into HP cycle also brings
next-step researches including the detailed cycle
construction and heat/mass transfer enhancement basedon the property of working media. In the following
sections, the vapor compression HP will be introduced as-2-

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Thermal Science & Engineering Vol.27 No.1 (2019)

which makes its operating pressure much higher thanother refrigerants. (4) R718 (water) is easy to be obtained
and has excellent thermo-physical properties. Therefore,high temperature vapor compression HP with R718

refrigerant is promising. However, there are still a lot of
obstacles to be overcome including its low pressure, highspecific volume, and immiscibility with lubricant oil.

Table 1. Properties of the low GWP refrigerants (Lemmon et al., 2013).
Refrigerants
Formula
GWP100NBP (℃ )Tcrit(°C)
R-152aC2F2H2
124
-24.0
113.3
R-134aCF3CH2F
1430-26.1
101.06R-1234yf
C3F4H24-29.45
94.7
R-1233zd(E)
C3H2F3Cl
1.4
18.26
166.5
R-245fa
CF3CH2CHF2
950
15.3
154.01
R-1234ze(E)C3F4H2
6.0-18.95109.37R-1234ze(Z)C3F4H2
1.4
9.745
150.12
R-600
C4H10
<1
-0.5
152.0
R-744
CO2
1
-57
31.1
R-717
NH3
<1
-33
132.3
R-718
H2O
0.2
99.9
373.95

Pcrit (MPa)
4.52a
4.06
3.38a
3.63
3.65
3.64
3.54
3.81
7.38
11.33
22.06

Safety
A2
A1
A2L
A1
B1
A2L
A2L
A3
A1
B2
A1

expansion valve with reduction of pressure and returns to
the evaporator, thus completing a full cycle. In this cycle,
thermal energy at the evaporation temperature (wasteheat) is upgraded to that at the condensation temperature.
The output heat with higher temperature can be used forvarious industrial heating applications. The steady-state
performance of a vapor compression HP cycle isevaluated by coefficient of performance (COP) definedas COP = Qh/Wt, where Qh is the heat output power and
Wt is the work/electrical input power.

Fig. 4 demonstrates the COP of vapor compression
HPs with different refrigerants and different
condensation temperatures. It is indicated that COP
ranges from 4.234 to 5.747, under the temperature lift of
45 °C. R1234ze(Z) shows the best performance whencondensation temperature is lower than 105 °C. Whencondensation temperature is higher than 105 °C,
R1233zd(E) is the better choice. Considering the similar
COP and working temperature ranges, R134a can be
replaced by R1234ze(E), and R245fa can be replaced by
R1234ze(Z)/R1233zd(E) for lower GWP.

(a)(b)
Fig. 5. Working principle of the vapor compression
HP: (a) schematic diagram, (b) T–s diagram (Hu etal., 2018).
Fig. 4. COP vs. condensation temperature for vapor
compression HP with single stage cycle, temperature
lift (Tc - Te) of 45 °C, superheating of 3 °C, andsub-cooling of 5 °C.

In order to achieve higher efficiency or lager
temperature lift, vapor compression HP could be
improved by different cycle constructions. Fig. 6 shows
several advanced vapor compression HP cycles,
including vapor injection cycle with a flash tank forinternal heat recovery (Fig. 6(a)), the two-stagecompression cycle (Fig. 6(b)), the three-stagecompression cycle (Fig. 6(c)) and cascading compression
cycle (Fig. 6(d)). For vapor injection cycle, heat ofrefrigerant vapor at the outlet of the condenser can berecovered by a flash tank, thus improving the overall
performance. Two-stage and three-stage cycles use twoor three compressors to achieve higher temperature lift.
To achieve higher temperature lift, cascading cycles use
the condensing heat output of one cycle to serve as the

2.2 Cycle construction of vapor compression HP
As shown in Fig. 5(a), the basic construction of vapor
compression HP cycle consists of a compressor, an
expansion valve and two heat exchangers i.e. theevaporator and condenser. The state variation ofrefrigerant on T–s (temperature–entropy) diagram isshown in Fig. 5(b). In the evaporator, liquid refrigerant is
evaporated by absorbing low temperature heat. Then the
superheated vapor is compressed and condensed in thecondenser. The liquid refrigerant then passes through the-3-

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Thermal Science & Engineering Vol.27 No.1 (2019)heat input into evaporator of another cycle, and eachcycle could use different refrigerants to achieve the best
performance. In these cycles, various compressors
including centrifugal compressor, screw compressors,
turbo compressor, roots compressor, reciprocating
compressor and rotary compressor could be used
depending on the temperature lift, output temperature,
capacity and other requirements.

(a)3 Absorption heat pumpCompared with vapor compression HPs, absorption
HPs upgrade the heat with the cost of low grade heat
input other than electricity input, which could cover awide temperature range of waste heat. Based on the
different functions, absorption HPs could be classified
into two main types, i.e. heat-amplifying type andtemperature-boosting type.
Heat-amplifying absorption HP (known as the 1st typeHP) is driven by high temperature heat. When the input
low grade heat has temperature of 30–50 °C, the HP isable to deliver heat output of 70–95 °C, and the heating
COP is always higher than 100%.
Temperature-boosting absorption HP (known as the
2nd type HP, or heat transformer) uses medium
temperature heat input to yield high temperature heat
output, while releasing some low temperature heat toambient heat source. With heat input at 70–90 °C, heat
output at 90–150 °C could be realized, and the COP for
this case is always below than 100%.
Researches about absorption HP cover the advanced
thermodynamic cycles, working pairs and enhancementof heat and mass transfer. Since working pairs ofabsorption HP has been widely investigated, it is notnecessary to discuss this too much, and we will discuss
more about the cycle construction and cycle
optimization in this section.

(b)

(c)
(d)
Fig. 6. Advanced vapor compression HP cycles: (a)vapor injection cycle, (b) two-stage compression cycle,(c) three-stage compression cycle, (d) cascading
compression cycle (Hu et al., 2017).

3.1 Working medium of absorption HP
Working medium of the absorption HPs is composedof the refrigerant and the absorbent. Various working
pairs have been investigated for obtaining better
performance, higher temperature lift capability, wider
working temperature range, lower crystallization risk,
and so on. Absorption working pairs can be classified
according to the refrigerants as shown in Table 2. The
first type of working pairs is based on water, which isalways an excellent choice for heat sink with temperature
higher than 0 °C. Salt solution or lye will serve as theabsorbents (such as LiBr). The second type of working
pairs is based on ammonia, which is a natural refrigerant
able to work efficiently below 0 °C. Water, salt or
aqueous solution of slat can be the absorbents. The third
type of working fluids is based on organic refrigerants
including R32, R124, R134a and TFE, the absorbents arealso organic fluids like DMAC, DMEU, NMP and E181.
Among these working pairs, LiBr-water andwater-ammonia are the most popular options.

2.3 Extension to three-heat-source HP
A three-heat-source HP cycle shown in Fig. 2(b) can
be formed by combining the organic Rankine cycle withvapor compression HP cycle for low grade heat driving
heat pumping or refrigeration. Here we take an exampleof turning waste heat into power, e.g. organic Rankine
cycle, for vapor compression air-conditioning. For theheat source temperature, ambient temperature andcooling temperature of 150 oC, 30 oC and 5 oC
respectively, organic Rankine cycle and vapor
compression refrigeration cycle have efficiency of ~15%
and ~4.0, and the overall efficiency of such combinationis ~0.6. Due to that the organic Rankine cycle cannot
work efficiently under low driven temperature, this is notan efficient and flexible combination for low grade heatutilization especially when the heat source has atemperature below 150 oC.
Alternatively, if a heat source with temperature of
150 oC is used to power the double effect absorption
chiller, a cooling COP over 1.2 could be reached. Thus,
to achieve better performance for such three-heat-source
heat pump or refrigeration, sorption HPs including theabsorption HPs and adsorption HPs are better choices
due to their different vapor compression processes.

Table 2. Working pairs for absorption HPs (Xu andWang, 2017a).
Refrigerants
Absorbents

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Water

LiBr, LiCl, CaCl2, LiBr + LiNO3,
LiBr + CH3COOK, NaOH + KOH,

Ammonia
R32, R134a
TFE

H2O, LiNO3, NaSCN, H2O + LiBr
DMAC, DMEU, NMP, DMETEG
E181, NMP, PYR

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Thermal Science & Engineering Vol.27 No.1 (2019)

To fulfill different demands of temperature lift,
various advanced absorption HP cycles for bothheat-amplifying and temperature-boosting have been
proposed (Xu and Wang, 2017a), and some of theexamples are shown as follows. (1) For larger
temperature lift of heat-amplifying or lower driving
temperature, double lift absorption HP cycle can be used
as shown in Fig. 8(a). Theoretically, temperature lift of
50 °C can be achieved under driving temperature of~90–130 °C with COP of 1.3. (2) For better utilization ofheat source with higher temperature, double effectabsorption HP cycle can be used to improve theefficiency as shown in Fig. 8(b). It requires higher
driving temperature than the single effect cycle for thesame temperature lift. Temperature lift of ~30 °C can beachieved with COP of ~2.3. (3) For temperature lift
between single effect cycle and double effect cycle, thevariable effect absorption HP cycle shown in Fig. 8(c)can be used. Its temperature lifting ability is between the
single effect cycle and double effect cycle, and thetheoretical COP varies from 1.7 to 2.3 (Xu et al., 2013).
(4) For higher temperature lift with temperature-boosting,double absorption heat transformer can be used as shownin Fig. 8(d). Temperature lift can be increased by
50–60% in comparison with the single stage
configuration, however, its COP will be decreased to
0.25–0.4.

3.2 Cycle construction of absorption HP
Absorption HPs may use different cycles such as
single effect cycle, double effect cycle, double lift cycle
and variable effect cycle according to the heat source andheat sink temperatures (Xu and Wang, 2016). Absorption
HP (heat-amplifying type) requires high temperature heat
input in generation process and delivers heat output inabsorption and condensation process as shown in
Fig. 7(a) (Gen, Con, Abs and Eva represents generation,condensation, absorption and evaporation, respectively).
The evaporation process is usually heated by ambient or
waste heat. Compared with the heat input, the heat
output has a lower temperature and larger capacity,which results in COP (total heat output/high temperature
heat input) higher than 100%. This type of absorption HP
is usually employed for heating supply and domestic hotwater supply. Typical single effect LiBr-water absorption
HP has temperature lift of ~50 °C with theoretical COP
about 1.7 (Xu and Wang, 2017a). A boiler based onburning of oil or natural gas can act as the heat source,
but waste heat from industrial processes or power plant
is a better choice.
Temperature-boosting type absorption HP is usuallyreferred as absorption heat transformer as shown in
Fig. 7(b) . It requires heat input in generation andevaporation processes, and supplies heat output inabsorption process. Condensation process is usuallycooled by ambient heat sink. Compared with heat input,
heat output has a higher temperature but smaller quantity
resulting in COP (high temperature heat output/total heat
input) lower than 100%. As absorption heat transformer
is able to produce heat output of 100 °C or higher, it has
more potential applications in industrial processes.
Typically, the heat input temperature is about 60–90 °C
and heat output temperature is about 100–130 °C. A
common temperature lift of 70–100 °C can be achieved
compared with ambient temperature, and COP of 0.4–0.5
could be achieved (Xu and Wang, 2017a).

HPG

C

Input

Output
Input
HPA

LPG

OutputAmbient

Output
E

LPA

(a)

(b)
Output
HPE

HPA

LPE

LPA

C

G

Input

Output
Con

Gen

Input

SHX
OutputAmbient
Eva

Ambient

Abs

(c)
(d)
Fig. 8. Advanced absorption HP cycles: (a) double lift
cycle, (b) double effect cycle, (c) variable effect cycle,(d) double absorption heat transformer (Xu andWang, 2017a).

(a)
Input

OutputEva

Abs

Ambient
ConInput

3.3 Cycle optimization with internal heat recovery
Compared to the LiBr-water absorption HPs,
water-ammonia absorption HPs have advantages such as
the applications at the evaporation temperature below
0 °C and easier maintenance because of the positive
system pressure. Moreover, it is superior for heat output
with temperature glide due to its large concentration

Input
Gen

(b)
Fig. 7. Single effect absorption HP cycles: (a)heat-amplifying type, (b) temperature-boosting type.

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Thermal Science & Engineering Vol.27 No.1 (2019)

glide (corresponding to the saturation temperature) in theabsorption process. However, the system configuration ismore complicated due to the rectification process, andthe efficiency strongly depends on the system
configuration. In this case, cycle optimization is essential,
and it can be achieved through internal heat recovery.
The authors analyzed the optimal internal heatrecovery strategies of heat-amplifying water-ammonia
heat pumps by pinch technology. According to different
operating conditions, three strategies of internal heatrecovery, named as Split cycle 1, Split cycle 2, and GAX
cycle, are obtained as shown in Fig. 9 (Du et al., 2014,2015). In the figure, CGCC, HSC, and CSC respectively
represent the distillation column grand composite curve,
the hot stream curve and the cold stream curve, whichshows the correlation between the heat load and thetemperature. Curves 1 to 6 represent the weak solution(low percentage of ammonia) from the generator,
absorption process, generation process, rectification
process, condensation process and the strong solution
from the absorber, respectively. The overlapped portion
between the curves is the internal heat recovery process,and the projected length on the horizontal axis indicates
its heat load. It can be found that absorption heat has alarger temperature glide than the condensation heat. As aresult, the hot water could be heated to a higher
temperature. By applying such cycle optimization, theoptimized absorption cycle including Split cycle 1, Split
cycle 2 and GAX cycle could be derived under different
conditions, and COP enhancement of 20–90% could beachieved (Du et al., 2014).

(c)Fig. 9. Three strategies for the internal heat recovery
of water-ammonia absorption HPs: (a) split cycle 1,(b) split cycle 2, (c) GAX cycle (Du et al., 2014).
4 Adsorption heat pump
The adsorption HP shares the same thermodynamic
concept with the absorption HP due to the similarity
between absorption process and adsorption process.
Sometimes, absorption systems and adsorption systems
are collectively termed as sorption systems. The
difference is that adsorption process utilizes solid-gas
working pairs, which makes the cycle construction andheat/mass transfer totally different. The use of solid-gas
working pair also makes the adsorption free of pumping
process, which is favorable for small scale application.
Besides, a lot adsorption working pairs are available,
which offers the possibility for lower driven temperature
utilization.
Different from the liquid working medium used invapor compression HPs and absorption HPs, solid porous
medium is used as adsorbent in adsorption system, whichmakes the heat and mass transfer enhancement more
important. In this case, we will discuss more about theheat and mass transfer enhancement in this section.
4.1 Working medium of adsorption HP
The working pair of adsorption HP consists of therefrigerant/adsorbate and the adsorbent. Water, methanol
and ammonia are the most commonly used refrigerants(Wang et al., 2014b), which are all environmental-benign
substances with large latent heat of vaporization.
Methanol and ammonia have low freezing point which
are suitable for subzero evaporation conditions. For
typical working temperatures, water and methanol needvacuum environment while ammonia works under highpressure. Methanol and ammonia are combustible andtoxic, so they are not suitable for applications where highsafety requirement is required. Methanol also takes a riskof decomposition, so its application is generally limited
below 120 °C.
Various adsorption working pairs have been proposed
based on water, ammonia and methanol, which could beclassified into two types based on the adsorption

(a)

(b)

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Thermal Science & Engineering Vol.27 No.1 (2019)mechanism, i.e. physical adsorption and chemisorption.
Due to the need of simultaneous heat and mass transfer,

porous medium is typically used as the adsorbent.
Table 3 shows several commonly used working pairs.

Table 3. Comparison of common working pairs for adsorption HP.
Working pair
Adsorption heat (kJ/kg)
Driving temperature
Condensation temperature
Evaporation temperature

Silica gelwater2800< 100 °C
30–45 °C
> 0 °C

Zeolite-water
3300–4200> 150 °Cwide range> 0 °C

Activated carbonmethanol
1800–2000
80–120 °C
20–50 °C
Could be < 0 °C

Activated carbonammonia
1700
80–200 °C
20–50 °C
Could be < 0 °C

CaCl2-ammonia
2200–2300
100–150 °C
20–50 °C
Could be < 0 °C

chemisorption HP. Actually, reaction of CaCl2 andammonia has three stages, in which CaCl2ꞏ2NH3,
CaCl2ꞏ4NH3 and CaCl2ꞏ8NH3 are the products of every
stage. Adsorption heat of CaCl2-ammonia is about
2200–2300 kJ/kg while adsorption quantity (amount ofrefrigerant per unit adsorbent) of CaCl2-ammonia canreach 1.22 kg/kg, which is much higher than that of thephysical adsorption working pairs. CaCl2-ammonia
working pair requires high driving temperature andmedium level condensation temperature. The general
working conditions of CaCl2-ammonia adsorption chiller
has driving temperature, condensation temperature, andevaporation temperature of 100–150 °C, 20–50 °C and-20–10 °C, respectively. The main disadvantages ofmetal halides-ammonia working pair are the swelling
and agglomeration phenomena during repeatedadsorption and desorption processes, which causes theheat and mass transfer degradation. To solve these
problems composite sorbents of metal halides and porous
matrix (such as activated carbon and expanded graphite)
are developed and will be discussed later.
The above examples of working conditions are
typically for refrigeration or air conditioning purposes,
which shares the same principle for heat pumping. The
differences between condensation and evaporation
temperatures in refrigeration cases are the corresponding
temperature lift for the adsorption HP.

Silica gel-water is the most common working pair foradsorption HP. Its adsorption heat is ~2800 kJ/kg anddesorption temperature can be as low as 50 °C. Working
temperature of silica gel should be lower than 120 °C,
otherwise silica gel would lose the adsorption ability.
Typically, a silica gel-water adsorption chiller could
work efficiently under driving temperature, condensation
temperature, and evaporation temperature of 80 °C,
30–45 °C, and 5–10 °C, respectively.
Since zeolite-water is stable at high desorption
temperature, it can be used to utilize heat above 200 °C.
Its adsorption heat is 3300–4200 kJ/kg, which is higher
than that of silica gel-water pair, resulting in a longer
cycling period. Zeolite-water working pair can operatewith similar performance in a large range of
condensation temperature since its adsorption isotherm
curves are quite insensitive to condensation pressure.
The typical working conditions of a zeolite-water
adsorption chiller has
driving temperature,condensation temperature, and evaporation temperatureof 180 °C, 30–80 °C, and 5–10 °C, respectively.
Adsorption heat of activated carbon-methanol is
1800–2000 kJ/kg, which is lower than that of theprevious working pairs. Activated carbon-methanol canbe driven by heat source at below 100 °C, which makes
it suitable for low temperature heat recovery. The typicalworking conditions of activated carbon-methanol
adsorption
chillerhave
driving
temperature,condensation temperature, and evaporation temperatureof 80–120°C, 20–50 °C and -10–10°C, respectively.
Activated carbon - ammonia is another commonlyused working pair. Its adsorption heat is ~1700 kJ/kg.
Due to the higher operation pressure of activatedcarbon–ammonia pair, cycling period could be reduced
due to the improved mass transfer. Activated carbon ammonia has a wide range of driving temperature from80 to 200 °C. However, limited by the high pressure riskof ammonia system, condensation temperature ofactivated carbon - ammonia system should not be higher
than 50°C. The typical working conditions of activatedcarbon-ammonia adsorption refrigeration has driving
temperature, condensation temperature, and evaporation
temperature of 80–200°C, 20–50°C and -20–10°C,
respectively.
Unlike the above physical adsorbents, metal halides
belong to the chemical adsorbent when they work withammonia. CaCl2 is the most commonly used sorbent for

4.2 Cycle construction of adsorption HP
The common configuration of adsorption HP is a
two-bed system with heat recovery, mass recovery or
both of them (Wang, 2001). Fig. 10 shows the schematicof an adsorption HP system with heat and mass recovery.
Alternating adsorption/desorption of two adsorbers cancontinuously produce cooling or heating. After theadsorption/desorption modes, the system successively
proceeds mass recovery and heat recovery. In the mass
recovery, mass recovery valve (a bypass valve) is opened,making the pressure balanced between two adsorbers. As
a result, adsorption and desorption processes are further
promoted and the cycle adsorption quantity is enlarged,
leading to an improved system performance. Heatrecovery is achieved through circulating thermal fluid
between the two adsorbers, thus sensible heat and evenpart of the adsorption heat could be recovered. With suchheat recovery process, heat input is reduced and better
performance could be achieved.
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Thermal Science & Engineering Vol.27 No.1 (2019)

Meanwhile, HTS desorbs ammonia with the heating by
the intermediate temperature heat source Tin. Various
metal halide-ammonia pairs could be adopted by theresorption HP, thus meeting different requirements oftemperature lift. Theoretically, MnCl2-NaBr-ammonia
working pair could upgrade the heat from 124 to 152 °C
with COP of 0.55, MnCl2-CaCl2-ammonia working paircould upgrade the heat from 87 to 152 °C with COP of
0.46, and NiCl2-SrBr2-ammonia working pair could
upgrade the heat from 130 to 259 °C with COP of 0.43
(Li et al., 2013). Research on resorption temperature lift
HP is still at theoretical stage, though the refrigerationand energy storage prototypes have been established andinvestigated in detail (Bao et al., 2014).

(a)

(a)
(b)
Fig. 10. Schematic of a heat and mass recovery
adsorption HP system: (a) thermodynamic cycle, (b)typical adsorption chiller design (Wang et al., 2014a).
Silica gel-water adsorption system with heat and mass
recovery is one of the most developed adsorption
systems which has been commercialized. Studies showedthat silica gel-water HP can achieve COP over 0.5 (Panet al., 2016) for cooling and COP of 1.3 (Núñez et al.,2006) for heating under 80–95 °C driving temperature,
35-45°C intermediate temperature and 10–20 °C low
temperature. Applications of silica gel-water HP mainly
include solar cooling system and combined cooling,heating and power (CCHP) system (Luo et al., 2007;
Núñez et al., 2006; Zhai et al., 2007).
Another type of adsorption HP for temperature lift is
the resorption HP. Resorption HP requires two or more
sorption beds which are filled with different chemicalsorbents (commonly metal halide-ammonia pairs).
Sorption beds can act as the functions of condenser andevaporator, respectively. Schematic of a resorption HP isshown in Fig. 11. At high pressure level of PH, lowtemperature adsorbent (LTS, S/G1) is regenerated by theintermediate temperature heat source (Tin) and releases
ammonia. At the same time, high temperature adsorbent(HTS, S/G2) adsorbs ammonia and releases the heat at
high temperature level of Tout (a temperature lift ∆T isthus created). After cooled down to a low pressure levelof PL, LTS adsorbs ammonia and releases adsorption
heat to the ambient at low temperature level of Tad.

(b)

Fig. 11. Schematic of a resorption HP system: (a)Clapeyron diagram, (b) schematic diagram (Li et al.,2013). S/G-solid/gas reaction, LTS-low temperaturesorbent, and HTS-high temperature sorbent.
4.3 Heat and mass transfer enhancement ofadsorption HP
As illustrated in Fig. 3, the heat and mass transfer
enhancement is necessary to increase the efficiency of
HP considering the external irreversible loss. Comparedto the vapor compression HP and absorption HP, the heatand mass transfer enhancement in adsorption HP is moredifficult since solid porous working medium is used. In
the adsorption HP, there are two major issues including
the stable and efficient adsorption (Wang and Wang,
1999), and the coupled heat and mass transfer
enhancement. Compound adsorbent and consolidated
adsorbent have been developed to address these twoissues.

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Thermal Science & Engineering Vol.27 No.1 (2019)(1) Compound adsorbent. As discussed before, theadsorption working pair is classified into physicaladsorption and chemisorption working pairs based on theadsorption mechanism. For physical adsorption, theadsorption process is stable but the adsorption quantity is
low which will increase the plant size. For instance,
activated carbon-methanol and silica gel-water haveadsorption quantity of 0.2–0.4 kg/kg and 0.15–0.25
kg/kg respectively. On the contrary, the chemisorption
typically has a high adsorption quantity, but theadsorption quantity degrades fast under long term
operation. For instance, CaCl2-ammonia working pair
has adsorption quantity of ~1.2 kg/kg which is 3–6 times
higher than that of the previously mentioned physicaladsorption working pair. However, the adsorption
quantity decreases ~30% after 10 times ofadsorption-desorption, and the capacity of adsorption HP
with such working pair degrades over 80% in half a year
(Wang et al., 2005a). The reason for adsorption quantity
decrease is the permeability degradation due tosolidification of chemical adsorbent.
To overcome the aforementioned problems of physicaladsorption and chemisorption, we proposed the use ofcompound adsorbent which integrates the chemical
adsorbent into the pores of physical adsorbent as shownin Fig. 12. The pores of physical adsorbent offer small
amount of adsorption, and more importantly, they offer
stable mass transfer paths for the efficient chemisorption.
Taking the pure CaCl2 adsorbent as an example, itspermeability decreases from 2.34 × 10-9 m2 to 1.56 ×
10-16 m2 after repeated adsorption of ammonia. When it isintegrated with activated carbon and forms the activatedcarbon/CaCl2 compound adsorbent, the permeability
decreases from 4.60 × 10-11 m2 to 2.77 × 10-13 m2 after
repeated adsorption of ammonia. The permeability after
adsorption is enhanced by 3 orders of magnitude, which
ensures stable and efficient adsorption (Wang et al.,2004).

(2) Consolidated adsorbent. In the adsorption
process, there is a conflict between the heat transfer
enhancement and mass transfer enhancement. Pores
inside the porous adsorbent offer the paths for masstransfer while the solid part offers the paths for heattransfer. When the pore size of the adsorbent is large, themass transfer is enhanced but the heat transfer isdepressed, and vice versa. In order to enhance the heatand mass transfer simultaneously, we developed theconsolidated adsorbent (Wang et al., 2005b) in which theheat transfer of the solid part is significantly enhanced.
In this case, high porosity is acceptable for both heat andmass transfer enhancement.
Fig. 13 shows the consolidated adsorbent made of
CaCl2 and expanded graphite treated with sulfuric acid
(ENG-TSA). The expanded graphite offers both highthermal conductivity and sufficient mass transfer paths
for absorption. The thermal conductivity of CaCl2
consolidated with ENG-TSA is increased up to 88.1
W/(mK), which is 400 times higher than the granular
CaCl2 (Jiang et al., 2014).

(a)

(b)
Fig. 13. Consolidated adsorbent for simultaneous heatand mass transfer enhancement: (a) schematic ofconsolidated adsorbent, (b) consolidated adsorbent of
ENG-TSA (Jiang et al., 2014).

5 Heat pump for low grade heat uses(a)

In this section, the application of HPs for theutilization of typical low grade heat including industrial
waste heat, transportation waste heat and solar heat is
introduced to show how effective of these HP systems
could be in the utilization of low grade heat.
(b)
Fig. 12. Compound adsorbent for stable and efficientadsorption: (a) schematic of the compound adsorbent,(b) activated carbon/CaCl2 adsorbent compound
adsorbent (Wang et al., 2004).

5.1 Application of vapor compression HP
Waste heat from many industries can be recovered andupgraded into useful heat by vapor compression HP,
which has great potential in energy saving andenvironment protection. Table 4 summarizes several-9-

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Thermal Science & Engineering Vol.27 No.1 (2019)

temperature to low temperature. The widely usedrefrigerants at present and corresponding low GWP
refrigerants in the future are analyzed. As naturalrefrigerants, R744 and R717 are popular in common
heating applications. R1234ze (Z) and R1233zd (E) arecompetitive refrigerants for R245fa and R718 which arequite attractive in high heating temperature applications
due to their outstanding thermodynamic properties. Heretwo cases of vapor compression HP conducted by ourteam are introduced in detail, including a waste heatrecovery system with HP and a high temperature HP withnatural refrigerants.

cases of vapor compression HP with different
refrigerants. For low temperature heat supply (< 70 °C),
R134a, R410a, and ammonia are commonly used. For
medium temperature heat supply (~90 °C), refrigerantsof CO2 and R245fa are suitable, and the heat output canbe used for industrial manufacturing processes ofslaughter house preheating and dyeing liquid heating.
For high temperature heat supply (> 120 °C), R718 could
be used, and again the heat is used for manufacturing
processes, such as furnace drying and process heating.
The better choices of refrigerants and cycles for waste
heat recovery are summarized in Fig. 14. The industrial
waste heat sources are listed in the range from highTable 4. Application of industrial vapor compression HPs (Centre, 2014; Chamoun et al., 2012; Wu et al., 2016).
Country/
Heat resource
Refrigerant/
OutputApplications
Year
Capacity
temperature
Hohenems, Austria/ Waste heat, 20 °C
R134a/
55 °C
Cleaning and heating
2012
257kW
Futtsu, Japan/
Waste gas, 20 °C
R410A/
50 °C
Heating for
2010
180kW
greenhouse
Halifax, UK/
Condensation waste heat, 30 °C NH3/
70 °C
Chocolate factory
2010
600kW
China/
Dyeing waste water, 60 °C
R245fa/95 °C
Dyeing liquid
2016
200kW
heating
Switzerland/
Waste water, 30 °C
CO2/
90 °C
Slaughter house
2010
800kW
preheating
Japan/
Water of drying furnaces, 60 °C R134a/
130 °C
Furnaces drying
2012
627kW
France/Industrial waste heat, 80 °C
R718 (H2O)/
120 °C
Process heating
2014
340kW

Fig. 14. Refrigerant and cycle construction choices of vapor compression HPs for waste heat recovery.
Fig. 15 shows a waste heat recovery system with large
scale centrifugal vapor compression HP built in theAngang Iron and Steel Plant, located in Liaoning, China.
Permanent-magnet synchronous variable-frequency
centrifugal compressor was used, and the total heating
capacity was 9100 kW. Heat output was used for central
heating. As shown in Fig. 15 (a), a serial connection of

external water loop was used to reduce the external
irreversible loss and to increase the efficiency. The test
results showed that COP of 6.13 was achieved underwaste hot water inlet/outlet temperatures of 32.1/24.1 ℃
and hot water supply temperature of 62.8 ℃. The results
showed such serial connected HP system with gradualwater heating is efficient for water heating. The waste- 10 -C 2019 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.27 No.1 (2019)heat recovery system with vapor compression HP could
cover the heating demand of 180,000 m2 (calculated
from heating load of 50 W/m2). The system has beenworking for one heating season till now, consuming 2.43
million kWh of electricity, and the cost was 1.46 million
RMB. Considering that the local heating supply price per
year is 22 RMB/m2, the heating supply of 180,000 m2
per year would cost 3.96 million RMB. The annual net
profit obtained from the waste heat recovery system wasabout 2.5 million RMB. At the same time, by suppling
hot thermal energy of 45,000 GJ from the waste heat,more than 3500 tons of standard coal were saved.

Fig. 16. The water vapor compression HP prototype
for high temperature application.
5.2 Application of absorption HP
Absorption HP can be directly driven by low grade
heat which makes it popular for solar heat utilization(Sun et al., 2015; Wang et al., 2016; Xu and Wang,
2017b; Xu et al., 2015). Fig. 17 shows one of ourresearches about the solar/gas driven single/double effectabsorption HP system to fulfill the heating, cooling andhot water demand in a hotel (Sun et al., 2015). Solardriven single effect mode works when the solar collector
provides enough heat input. In the rest time, gas driven
double effect mode works to ensure the continuous
operation of this system. The single effect mode anddouble effect mode have cooling capacities of 320 kW
and 1280 kW, respectively. Year round operation showedthat the integration of solar energy into this absorption
HP system saved 49.7% of the natural gas consumption.

(a)

Solar
collector(b)Fig. 15. Industrial waste heat recovery by large scalecentrifugal vapor compression HPs: (a) schematic of
the serial connected vapor compression HP system,(b) centrifugal vapor compression HP plants.
Fig. 16 shows our recently developed water vapor
compression HP system developed for high temperature
waste heat recovery. The twin-screw compressor was
designed with three different injection ports ensuring
water injection during the whole compression process.
The internal compression ratio and the rotation speed ofthis compressor was 4.5 and 5000 rpm. The rated heating
capacity of plate condenser and falling film evaporator
were 300 kW and 240 kW, respectively. Experiments
were conducted under steady evaporation temperature of
87 ℃ and increasing condensation temperatures from119.6 to 126.5 ℃. Results showed that the power
consumption ranged from 51.3 to 56.9 kW, the heating
capacity ranged from 226.1 to 230.5 kW and the COP
decreased from 4.40 to 4.03. Compared with the vapor
compression HP system using R245fa, water vapor HP
system has similar performance and better environmental
benefit.

Tank

HX
Absorption HP

User

Cooling towerUser

Gas heater

(a)

(b)
Fig. 17. Application of absorption HP for solar heatutilization: (a) schematic of the solar driving
absorption heating and cooling system, (b)single/double effect absorption HP (Sun et al., 2015).
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Thermal Science & Engineering Vol.27 No.1 (2019)

Except for the direct utilization of solar thermal power,
absorption HP has another advantage which makes itsuitable for waste heat recovery, i.e. large scale
application (Xu et al., 2018). Both the size and cost perunit heating capacity of absorption HP will decrease with
the increase of capacity. Thus absorption HP can be both
efficient and cost-saving for recovering the large amountof waste heat from power plant or industrial processes.
Fig. 18 shows a waste heat recovery system for power
plant built in Lanzhou, China (Xu et al., 2018). Fig.
18(a) shows the schematic of the waste heat recovery
system with LiBr-water absorption HP. The system
consists of 6 serial absorption HP plants and each plant
has a rated heating capacity of 48.46 MW. Condensation
heat of power plant is recovered the absorption HPs.
Heat output of the absorption HPs is used for district
heating. Test results showed that the absorption HPs
achieved heating capacity of 63.57 MW and COP of 1.77
under steam pressure of 0.24 MPa, waste heattemperature of 34.63 oC and hot water outlet temperatureof 81.34 oC. For the year round operation, this system
recovered 1.55 PJ waste heat, and reduced 6.45 × 108 kg
of water assumption and 1.49 × 108 kg CO2 emission.

anti-swaying, anti-corrosion and small bulk, etc. The
prototype, as shown in Fig.19, produces cooling capacityof 33.8 kW and the thermal COP reaches 0.53 under thetest conditions that the temperatures of the cooling water,
secondary refrigerant and exhaust inlet are 26.1 oC, -15.2oC and 567 oC, respectively. This system can be installed
on a small fishing-boat with a diesel engine of 200 kW,
and meet the needs of harvesting preservation. Such asystem could ensure the freezing demand without taking
ice cubes which is promising for small fishing boats.

Fig. 19. Ammonia-water absorption freezer driven bydiesel engine exhaust (Du et al., 2017).
5.3 Application of adsorption Refrigeration
Different from the absorption HP, adsorption HP isadvantageous for small scale, air-cooled and low driven
temperature conditions. Besides, it has much betteranti-vibration performance than absorption HP due to thesolid adsorbent used in the system. In this case, it issuitable for the waste heat recovery of diesel engine intruck or boat. Fig. 20 shows an adsorption freezing
system for refrigerated truck developed by our team(Gao et al., 2016). It is driven by the heat from theexhaust gas of diesel engine, and the system provides
cooling output to maintain the low temperature inside thechamber. The schematic of this adsorption freezing
system is shown in Fig. 20 (b), where MnCl2/CaCl2-NH3
two-stage adsorption cycle is adopted. Experimental
results showed that this system could obtain 1.32 kW
cooling output at a temperature of -10 oC when driven byexhaust gas at 270 oC, and this is enough to keep the lowtemperature in the chamber. The most recent updated
system could even reach more than 2.5 kW refrigeration
power.

(a)(b)
Fig. 18. Application of absorption HPs for industrial
waste heat recovery: (a) schematic of the waste heatrecovery system, (b) absorption HP plants (Xu et al.,2018).
Based on cycle optimization, as stated in section 3.3, apractical water-ammonia absorption prototype isdeveloped for freezing applications, which is driven by
the exhaust of diesel engine on a fishing-boat (Du et al.,2017). According to the operating conditions of marine
application, the split cycle 1, one of the three optimal
strategies for internal heat recovery, is applied and it ismodified in the light of the practical system construction.
Moreover, the prototype is built with the design of(a)- 12 -C 2019 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.27 No.1 (2019)researches in these three levels. This paper also showsthat plenty of chances on novel working principle,
working
medium,cycle
construction,systemoptimization and heat/mass transfer enhancement areavailable in the future development of thermal
engineering systems. As new technologies in thermalscience including HPs are still being developed by many
researchers, we hope such an overview of HP technology
development could provide researchers how technology
is developed from concept to application, and help those
who want to bridge thermal science and thermal
engineering.
A more general view for waste heat utilization could
be shown as Fig. 21, where centralized heat could betransferred into power via Ranking cycle, organic
Ranking cycle, or Kalina cycle depending on its
temperature level and capacity. It could be used togenerate cooling by absorption or adsorption
refrigeration. It could be used directly, or upgraded byheat pump (vapor compression, absorption, adsorption,or chemical reaction). These centralized heat is turned
into network electricity/heating/cooling to its customers(industrial, commercial, or residential). For those small
capacity heat sources, distributed energy system network
could be established by heat transmission via piping or
via heat transportation with thermal storage units. The
distributed localized heat could thus be turned toheating/cooling by various thermal driven systems like
sorption heat pump.
The unique solution would be the unified centralized
and distributed energy systems to form a smart energy
network. An integrated network use of low grade heat
could have great potentials for efficient energy utilizationin the near future.

(b)
Fig. 20. Application of adsorption HP for diesel
engine waste heat recovery: (a) schematic of the waste
heat recovery system, (b) schematic of the adsorption
HP (Gao et al., 2016).
6 Conclusions and Perspectives
In this paper, we have shown the development of low
grade heat utilizing HP system from the thermodynamic
concept to the real application. (1) In the thermodynamic
concept level, ideal HP cycles with double heat sources
and triple heat sources offer the theoretical frameworkfor HP system. (2) In the next step, the technical details
including working medium, cycle construction, cycle
optimization and heat/mass transfer enhancement forvapor compression HP with dual heat sources andabsorption/adsorption HPs with triple heat sources are
introduced. Due to their different features, working
medium, cycle construction/optimization and heat/masstransfer enhancement are emphasized for different HPs.
(3) At last, the applications of different HPs for industrial
waste heat recovery, transportation waste heat recovery
and solar heat utilization are introduced.
The relatively complete view of HPs technology
evolution has been shown through the description ofFig. 21. Network utilization of low grade heat via various energy conversion methods and the integration ofcentralized and distributed energy systems.

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Thermal Science & Engineering Vol.27 No.1 (2019)

Acknowledgement
The authors acknowledge the support of their heat pumpresearches from National Key Research andDevelopment Program (Grant No. 2016YFB0601200).
The support from the Foundation for Innovative
Research Groups of the National Natural Science
Foundation of China (Grant No. 51521004) is also
appreciated.

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