1
EPSRC Thermal Management of
Industrial Processes
Case Study
Techno-economic Feasibility of Absorption Heat
Pump using Waste-Water as Heating Source for
Desalination
Progress Report
January 2011
Researcher : Dr Hanning Li
Investigators: Professor J Swithenbank,
Professor V Sharifi
SUWIC, Department of Chemical &
Process Engineering
Sheffield University
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Case Study
Techno-economic Feasibility of Absorption Heat Pump using
Waste-Water as Heating Source for Desalination
Executive Summary
The thermal desalination process consumes a large quantity of heat energy for seawater
evaporation. The utilization of low grade heat could significantly reduce the energy
consumption in thermal desalination plants. However, the temperature of the low grade heat
is normally lower than that of sea-water evaporation, i.e. 64-70 °C for MED (multi-effect
desalination) technology. Heat pumping is an effective technology to convert low grade heat
to higher-temperature energy. This report studies the feasibility of utilizing low-grade waste
heat for thermal desalination via a hybrid absorption heat pump system. The proposed heat
pump system consists of an absorber, a generator and vapour compression. Ammonia-water
is used as a working fluid. The generator is used to absorb low-grade heat and the absorber
releases higher-temperature energy for evaporating sea-water. A thermodynamic model is
proposed to simulate the steady-state operation of the proposed system and then evaluate
power consumption and waste-water usage. The size designs of both generator and absorber
are studied for heat pump operation suitable for fresh-water production of 5,000 T/day using
MED technology. As conclusions, both capital and operating costs are mainly dependent on
construction and power consumption for the compression system. Reducing the ratio of
absorber pressure (Pa) to generator pressure (Pg) could reduce the power consumption and
waste-water usage. Profitability will be achieved at operation pressure less than 35 bar in the
absorber. The recommended operation pressure in absorber is less than 25 bar and that will
obtain payback in less than 10 years.
Accordingly, a case study is presented in which hot-water, discharged from the process
industry, can be used for generating 20,000 T/day of fresh-water from sea water. The
proposed process is a sequential 4-stage heat-pump system. Each stage is designed for the
fresh-water production of 5,000 (T/day) and operated at different temperature/pressure
conditions. According to calculation, 83% energy will be saved based on the energy
requirement for water evaporation with individual ranges from 91% to 73%. The waste-
3
water usages are between 25 and 61 T/h for different stages. The cost estimation is up to €9
millions and economic profitability could be achieved from 1-4 years for different stage
configurations..
Acknowledgements:
The authors would like to thank the Engineering and Physical Science Research Council
(EPSRC Thermal Management of Industrial Processes Consortium) for their financial and
technical support for this research work.
4
List of Content
1- Introduction 2- Description of Proposed Process (case study 1)
2-1 Introduction to MED process 2-2 The proposed heat-pump process
3- Models for Heat-Pump Operation 3-1 Thermodynamic model 3-2 Parameter determination 3-3 Sea-water evaporation rate
4- Equipment Design and Evaluation 4-1 Generator 4-2 Absorber 4-3 Compressors
5- Results 5-1 Power consumed by compressor 5-2 Waste-water usage 5-3 Equipment costs
6- Economic profitability 7- Case Study 2 : Results References Nomenclature
5
1. Introduction
Water is an abundant natural resource that covers three quarters of the earth’s surface.
However, only about 3% of all water sources are potable. About 25% of the worlds’
population does not have access to good quality and/or quantity of freshwater and more than
80 countries face severe water shortages. Worldwide drought and desertification are expected
to sharpen the problem. Even countries that at present do not face water shortages may have
to tackle the problem of fresh water scarcity in the near future.
In UK, water companies in the South of England are looking at desalination technology,
mainly due to climate change (i.e. drier summers) and shifting population demographics,
which exacerbate water shortages in certain areas of accelerated population growth. This
promotes the application of desalination technology for meeting future demands. Currently,
there are two seawater desalination projects underway in UK. These are : i) Beckton Project
and ii) Newhaven Project. Thames Water is in charge of running Beckton project. The
Beckton desalination plant started its operation in June 2010 in East London. The Beckton
plant has a capacity of 150 MLD. It transfers desalinated water to Woodford Reservoir where
it is blended with existing supplies. The project cost for this plant was approx. £200 Million.
Both Beckton and Newhaven projects utilize Reverse Osmosis (RO) technology. Figure 1-1
shows the aerial view of Beckton water desalination plant.
Figure 1-1 Aerial view of Beckton, east London including the Thames Water desalination plant and sewage works. Photograph: S Downward
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Traditionally MSF technology (Multiple Stage Flash) has been the most popular technology
used in desalination industry. However, it is now losing its share in the market to RO
(Reverse Osmosis) and MED (Multi-effect Distillation) systems, due to the improvement of
membrane technologies and also the cost advantages of MED. Main advantages of Multiple
Effect Distillation system (MED) are: i) low investment and running costs and ii) high
modularity (ETAP, 2006). Relatively high amount of energy is required to run a desalination
plant. Usage of low-grade heat could be used as a heating source for running desalination
plants . MED technology is generally used in small (100,000 gal/day) and medium size
(100,000-500,000 gal/day) desalination plants.
MED operation requires steam at 180 °C as the heat source (i.e. high energy consumption
process). Heat pumping is a technology that could convert low-grade heat, i.e. 50 °C, to a
suitable heat source for MED operation. There are four main types of heat pump systems.
These are: i) thermal vapour compression system, ii) mechanical vapour compression system,
iii) absorption vapour compression system, and iv) adsorption vapour compression system
(Ettouney HM, 1999). Performance of the above 4 systems was investigated by Al-Juwayhel
et al. (1997). It has been shown that the coupling of an absorption heat pump (AHP) to a
MED unit is one of the best ways to make thermal desalination technology more competitive
when compared to the reverse osmosis process (Diego-César Alarcón-Padilla et al.2007).
Working fluids used in the absorption heat pumps are LiBr-water, ammonia-water and
hydrocarbon. Hydrocarbon as working fluid is mostly used with careful design due to its
flammability. Bromide-water absorption system has been studied by several researchers
(Mandani et al., 2000; Diego-César et al., 2007, 2010). This system can use low grade heat or
solar heat as the heating source of the heat-pump evaporator. However, such system requires
a high temperature steam as heating source to rise working-fluid temperature. This report
studies the feasibility of utilizing low-grade waste heat for thermal desalination via a hybrid
absorption heat pump system. The proposed heat pump system consists of an absorber, a
generator and vapour compression. Ammonia-water is used as a working fluid. The
generator is used to absorb low-grade heat and the absorber releases higher-temperature
energy for evaporating sea-water
Ammonia–water absorption systems constitute an old technology that has been in use since
the middles of the nineteenth century (Stephan, 1983). From the beginning, its development
has been linked to the evolution of the energy prices and mainly to the expansion of the
compression refrigeration systems. As the energy prices increased and the compression
7
systems expanded, the applications for absorption system decreased. Ammonia-water
absorption pump is usually used in the cooling system. The application for desalination
requires an increase in the temperature of low-grade heat source to that of MED operation.
The possibility of using the ammonia-water absorption heat pump in order to increase
temperatures has been studied by several researchers. Slesarenko (2001) demonstrated that
thermodynamically ammonia-water heat pump could be used as a heating at low temperature.
Mineaa et al. (2006) explored the use of ammonia-water based compression heat pump for
district heating system. In his study, hot water (at 55 °C) was produced using industrial waste
water (at 36 °C) as a heating source. Tarique and Siddiqui (1999) studied the performance of
ammonia systems. Their results indicated that the performance of ammonia-water system is
better than that of pure ammonia.
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2. Case study 1 : Description of Proposed Process
2.1 Introduction to MED process
Figure 2-1 schematic diagram of a multi effect distillation process(MED)
Multi-effect distillation (MED) systems have been used for sea-water evaporation and
subsequent production of freshwater. The MED process has been designed for utilization of
thermal heat as a heat source in the form of hot water in the range of 0.8–3 bar, which is
supplied to the first effect of the desalination unit with lower top brine temperatures in the
range of 64–70°C(Henry, 2005). Figure 2-1 shows a schematic diagram of MED system. In
first effect, heat source provides the steam that is generated from a boiler. If operation
temperature in first effect is designed at 70°C as saturated temperature, the operation boiling
temperature of MED should be higher than 70°C, depending on salt concentration in water.
For example, the actual boiling temperature of salt concentration of 3.8% will rise 1.55°C
(Supersystems Inc., 1995). To effectively force the heat transfer from steam to sea water, the
temperature of steam has at least 3 °C higher than the boiling temperature of sea water. In
practical, 4°C (or higher) difference between them is desired for MED operation
(Supersystems Inc., 1995). As the result, the steam is condensed to liquid and releases the
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latent heat; salt water is evaporated into steam as receiving the heat. In the second effect, the
steam from first effect provides heating source to the second effect. Subsequently, n stage
heating source is provided by the steam from n-1 stage. In general, the thermal performance,
operational and capital cost are directly proportional to the number of effects in the MED
system. The increased number of effects reduces energy consumption and increases capital
costs. The practical range of MED effects is 4 to 21 effects (Khawajia et al., 2008).
2.2 The proposed heat-pump process
The proposed process is a hybrid absorption heat pump that replaces steam boiler as heating
source of MED in desalination process. Figure 2-2 shows a schematic diagram of the heat
pump system coupled with installation with MED first stage. In the absorber of the heat
pump system, the heat-sink of heat pump (75 °C) becomes the heat-source of first effect in
MED (70°C) for sea-water evaporation.
Figure 2-2 schematic diagram of hybrid absorption and compression heat pump system applied for desalination.
The heat pump system has various components and its state points are shown in Figure 2-2.
The superheated ammonia vapour, leaving the generator (State 1), is compressed into state 2.
In the absorber, the absorption of ammonia vapour into poor solution (state 8) releases the
heat to the first effect of MED while the poor solution becomes rich solution (state 3). The
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rich solution, after heat exchanger and expansion valve, is throttled to the generator (state 5).
In the generator, the rich solution, once heated by external heat, is separated into ammonia
vapour (state 1) and poor solution (state 6). The poor solution, leaving the generator at state
point 6, is pumped via the heat exchanger to the absorber (state 8). The cold solution from
the generator and hot solution from the absorber exchange heat through the heat exchanger.
This completes the cycle. The low grade waste heat (state 9) provides the heat source to the
regenerator.
The power consumption, the waste-water usage and the capital cost for a fresh-water
production of 5,000 T/h (i.e. a medium-size desalination process) were calculated in this
study..
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3. Models for the Heat-Pump Operation
A thermodynamic model was selected and used to simulate the steady-state operation of the
heat pump (Figure 2-2). The assumptions made in the model are as follows:
• Steady state conditions of all unit operations, including compressor, generator, absorber bed and heat exchanger.
• Model parameters, such as the fluid density, enthalpy at each state point are assumed constant.
• The amount of ammonia generated in the generator is equal to that of ammonia absorbed by absorber.
• No heat losses to the surroundings.
Based on the above assumptions, the mass and thermal balances for each unit operation were
conducted in order to evaluate parameters such as power consumption, waste-water flow rate
and poor/rich solution flow rates. The power consumption is considered as the main
operational cost.
3.1 Thermodynamic model
In the heat pump cycle, the mass, concentration and energy balances for the absorber are as
follows (see Figure 2-2 for state points):
m3 = m2 + m8 (3-1)
m3 x3 = m2y2+ m8x8 (3-2)
Qa + m3 h3 = m2 h2+ m8h8 (3-3)
x3 and x8, mass fractions of ammonia in rich and poor solutions, are calculated according to
(Salavera, 2005):
logPNH3 = f (T, X) (3-4)
y, mass fraction of ammonia in the vapour, is calculated by ( Pt - P wat)/ Pt. Pt is operation
pressure. P wat is vapour pressure of water, calculated by Antoine equation:
TC
BA
+−=)(Plog wat10 (3-5)
The parameters A, B, and C are selected according to temperatures below 100oC.
In the heat pump system, the mass flow rates and concentrations of poor solution and rich
solution are assumed to be the same. Also, mass flow rate of ammonia is assumed to be the
same as well.
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Rich solution: m3 = m4 = m5 (3-6)
Poor solution: m6 = m7 = m8 (3-7)
Rich solution: x3 = x4 = x5 (3-8)
Poor solution: x8 = x7 = x6 (3-9)
Ammonia vapour: m1 y1= m2 y2 (3-10)
In the generator, the external energy is calculated according to the following energy balance:
Qg = m1 h1+ m6h6 - m5 h5 = m2 h1+ m8h6 – m3 h5 (3-11)
The compression work, WC, is calculated by the following equation (Cacciola et al., 1990):
−
••== 105.2Th-h
235.
1
2112
P
PWC (kJ/kg) (3-12)
In the heat exchanger, the thermal transfer rate, QH, is calculated using the following energy
balance:
QH = m3(h3 - h4) = m6(h7 - h8) (3-13)
The pump work, WP, is calculated by:
100P
h-h a
67 ×−
=L
g
P
PW
ρ(kJ/kg) (3-14)
Pa and Pg are pressures of absorber and generator. ρL is poor solution density.
3.2 Parameter determination
Power consumption and waste-water usages are determined according to the following
equations. Firstly, a variable X (x/(1-x)) is introduced. Equations (3-1) and (3-3) are
rewritten into:
mw (1+X3 ) – mw (1+ X8) = mw(X3 – X8) = m2 (3-15)
Qa + mw (1+X3 ) h3 = m2 h2+ mw (1+ X8) h8 (3-16)
Here, Mw is water circulation rate. Combining equations (3-15) and (3-16) into:
Qa + mw (1+X3 ) h3 = mw(X3 – X8) h2+ mw (1+ X8) h8 (3-17)
The water circulation rate is then calculated using the following equation:
Qa/mw = (X3 – X8) h2+ (1+ X8) h8 – (1+X3) h3 (3-17a)
The flow rates of rich-solution and poor-solution are calculated by:
m3 = mw (1+ X3); m8 = mw (1+ X8); (3-18)
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In the generator, the external energy, Qg, is calculated by (3-11). Waste-water flow rate is
then calculated by
m9 = Qg/Cp(T9 – T10)
Cp is specific heat capacity of waste-water. T9 and T10 are inlet and outlet temperatures of
waste-water in the generator. The temperature difference between T9 and T10 is assumed to
be 10 °C. The powers for compression and pump are calculated by:
QC = m1WC/η (3-19)
QP = m8WP/η (3-20)
3.3 Sea-water evaporation rate
The amount of fresh-water evaporated from the sea-water is determined according to the
following mass balances, equations (21) and 2(2).
Global Mass balance: mf = md+mb (3-21) Salt balance: mfxf=mbxb (3-22)
mf, md and mb are mass flow rates of sea-water inlet, fresh water and sea-water outlet. xf and
xb are mass fractions of salt in inlet and outlet solutions of MED first effect.
The energy for water evaporation is estimated according to energy balance as shown below:
Qo= md∆Hwater (3-23)
∆Hwater is the latent heat of water.
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4. Equipment Design and Evaluation
Absorption heat pump system for desalination consists of i) ammonia generator and ii)
absorber.
Generator
In an ammonia generator, ammonia is released from the water under the external heat. The
process can be operated in a shell-tube heat exchanger. Figure 4-1 shows a schematic
diagram of the generator. The waste-water flows inside the tube and ammonia-water solution
flows on the outside of tube. The waste-water heats the ammonia-water solution through the
wall of tube. The system design consists of two parts, tube size (heat transfer area) and shell
size (tower height and diameter).
Figure 4-1 schematic diagram of the proposed evaporator
In the design of heat transfer area, the heat transfer coefficient is determined according to
tube-side and shell-side values. On the tube side, the Reynolds number (Re) is higher than
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104. The Nusselt number is then calculated based on the following equation (i.e. turbulent
flow in the tube):
Nu=αwdi/λ=0.023 Re0.8 Pr0.3 (4-1)
Here αw is the heat transfer coefficient inside the tube, di is the inner diameter and λ is the
thermal conductivity of waste-water
On the shell side, ammonia-water liquid is heated on the tube. The heat-transfer coefficient
(αg) is determined according to measurement by Arima et al. (2003).
The thermal resistance across a thin stainless tube wall is considered to be negligible because
of its high thermal conductivity. The overall heat transfer coefficient is then calculated by
1/α=1/αw + 1/αg (4-2)
The heat transfer area, A, is calculated by:
)( gwaste TT
QA
−=
α (4-3)
Twaste is average temperature of inlet and outlet waste-water. Tg is the bed-temperature of the
generator. Q is the external heat provided by the waste-water. The cost of heat exchanger
inside the evaporator is then estimated by the cost equation for heat exchanger (Holmberg,
2007):
Costeq=660A0.7 (4-4)
The heat exchanger is designed using multiple pipes with vertical arrangement inside the
shell. The pipes of length L and diameter di construct a bundle of heat exchanger with the
total number of:
2)4/( idL
An
π= (4-5)
Pipes are arranged in triangle space arrangement. The inner diameter of the shell can be
calculated by (Yao, 2001):
)5.12)11.1(70 idnD ×+−•= (4-6)
In the generator, the liquid section is mainly designed for the heat transfer purposes. Above
that, vapour is placed for operation, which is estimated by:
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spaceV
VDH =
4
π (4-7)
Here, V is vapour rising flow rate(m3/s) and Vc is the space velocity of vapour. The height
of the vapour section in the generator is then estimated:
2
4
DV
VH
space
vapourπ
= (4-8)
The total height of evaporator is obtained as:
Htotal= Htube + HVapour+Lend (4-9)
Lend is the length for the end effect of heat transfer section. In general, a minimum height of
1.8 m of vapour section is required to avoid foam effect.
The size of the generator can then be evaluated by using the values for diameter (eqn. 4-6)
and height (eqn. 4-9). The volume and weight of the material which is required to construct
the generator can be evaluated based on the size of the generator (Wt =ρπDHδ, δ: wall
thickness; ρ: material density). The shell cost of the generator is then evaluated based on the
material price, i.e. $ 7/kg-steel.
The cost of the generator is calculated by adding the shell and heat-exchanger costs together.
Table 4-1 presents the estimated size and cost of the ammonia generator with a fresh-water
capacity of 5,000 T/day.
Table 4-1 Estimation of ammonia generator dimensions (Pa=20 bar; Pg=5 bar)
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4.1 Absorber
In ammonia absorber, the absorption of ammonia vapours into poor solution releases the heat
that evaporates sea-water. The process can be implemented by a falling film-type heat
exchanger with internal tubes placed vertically, as shown in Figure 4-2. The falling film of
the poor solution slides down the inside of the tubes (Figure 4-3). The compressed ammonia
vapour fills in the tubes and is absorbed into the falling film coupled with generating heat.
The generated heat in the film provides the heating source to sea water through the wall of
tube.
Figure 4-2 Multi-tube absorber with poor solution falling film
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Figure 4-3 Ammonia vapour absorption on the wall and heat transfer to sea-water
The size of absorber is mainly determined by the configuration of the tubes. The diameter
and number of tubes are selected so as to calculate solution flow rate wetted on the tube (Gwet
= m/(πdin), m: solution flow rate, di: tube diameter. n: number of tubes). Reynolds number
(Re = 4Gwet/µL ) of the solution is calculated to find the flow regime. The falling film
thickness is then estimated by
3/1
2
4.2
g
G��
L
Lwet
ρ
µδ (4-9)
The ammonia vapour velocity is calculated according to vapour flow rate and cross–section
area of the vapour flow. The vapour velocity is then examined to be less than the vapour
velocity at flooding point, which is calculated by the correlation proposed by Hughmark
(1980).
Once the diameter and number of tubes are determined, the height of tubes is calculated
according to heat transfer area (H=A/(πdin), A: heat transfer area). The heat transfer area is
calculated according to A= Q/( α(Ta-Tb)). Ta is the temperature of absorber. Tb is the bed-
temperature of the sea-water. Thermal transfer rate, Q, is calculated based on energy
required by water evaporation rate. The overall heat transfer coefficient, α, is generally
dependent on thermal resistances of solution-falling film, tube wall and sea-water outside the
tube. The thermal resistance across a thin stainless tube wall is considered to be negligible
because of its high thermal conductivity. The falling film thickness, δ (from equation 4-9), is
small (in the order of 2×10-4). Thus, the overall heat transfer coefficient does not strongly
depend on the wall or the falling film thermal properties. The overall heat transfer coefficient,
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α, is mainly depended on the property of sea-water outside of the tube, which is calculated
according to sea-water evaporation (Al-Ansari, 2001). The overall heat transfer coefficient is
in the order of 2.8 kW/m2°C.
The space for water vapour is estimated according to section 4.1. The safety design factor of
20% is applied to the total height of tube.
The inner diameter of sea-water evaporator (shell diameter) is calculated based on the
triangle space arrangement between tubes (Yao, 2001):
)5.12)11.1(70 idnD ×+−•= (4-10)
The cost of ammonia absorption tower is then estimated based on the column size, and the
number and size of the tubes, as shown in Table 4-2.
Table 4-2 sizing and cost estimation of absorption bed
4.2 Compressors
The cost of compressors is calculated based on FACT (First Approximation Costing
Technique, School of Chemical Engineering, Cornell University):
Less than 5 atm, $ 100/HP 5 – 15 atm, $ 1250/HP Larger than 15 atm: $ 1500/HP
The price includes compressor and driver.
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5. Calculation Results
The main calculation results for power consumption, waste-water usage and the cost analysis
are presented in this section. The calculation results are based on the production of 5000
T/day of fresh water in a MED with 10 effects.
5.1 Power Consumed by Compressor
Tg=30 C, Pg=5 bar
0
1000
2000
3000
4000
5000
6000
7000
8000
15 20 25 30 35 40
Pressure (bar)
Po
wer
(kW
)
Figure 5-1 Power comsumption by compressor at generator pressure 5 bar. Tg: 30 °C;
fresh-water capacity: 5,000 T/day
In the absorption heat pump system, the generator produces ammonia vapour and compressor
increases the pressure to a value that is specified by the absorption process. The power
consumption is mainly dependent on the vapour compression. Two variables affect the
consumption of compression power. These are: generator pressure(Pg) and absorptor
pressure(Pa). As expected, an increase in Pg or a decrease in Pa reduces the compression
power, as shown in Figures 5-1 and 5-2. Figure 5-1 shows that the compression power
increases with the absorber pressure. Figure 5-2 shows that the compression power is
decreased when generator pressure is increased. In general, it can be seen that the
compression power is dependent on the ratio of Pa(absorber pressure) to Pg (generator
pressure). An increase in the ratio of Pa/Pg requires a high compression-power. Lowering the
ratio of Pa/Pg could result in saving energy!
21
Figure 5-3 shows that the compression power is not significantly affected by the generator
temperature. A slight decrease in the power can be attributed to a slight increase of vapour
enthalpy (in generator) that will slightly reduce the power consumption by compressor.
Tg=30 C, Pa=25 bar
0
1000
2000
3000
4000
5000
6000
7000
3 4 5 6 7 8
Pressure (bar)
Po
we
r (k
W)
Figure 5-2 Power comsumption by compressor at absorber pressure 25 bar; Tg: 30 °C;
fresh-water capacity: 5,000 T/day
22
Pg=5 bar, Pa=25 bar
0
1000
2000
3000
4000
5000
6000
7000
0 10 20 30 40 50 60
Temperature (C)
Po
we
r (k
W)
Figure 5-3 Power comsumption by compressor affected by generator temperature. Pa:
35 bar; Pg: 5 bar; fresh-water capacity: 5,000 T/day
5.2 Waste-water usage
This section presents the results obtained for different generator bed-temperatures when using
waste water as a heating source.
23
Tg=30 C, Pa=25 bar
0
50
100
150
200
250
300
3 4 5 6 7 8
Pressure (bar)
Flo
w-r
ate
(kg/s)
Figure 5-4 Waste-water usage affected by generator pressure. Ta: 25 bar; Tg: 30 °C;
fresh-water capacity: 5,000 T/day
Tg=30 C, Pg=5 bar
0
50
100
150
200
250
300
350
400
10 15 20 25 30 35 40
Pressure (bar)
wate
-wate
r Flo
w-r
ate
(kg/s)
Figure 5-5 Waste-water usage affected by absorber pressure. Pg: 5 bar. Tg: 30 °C;
fresh-water capacity: 5,000 T/day
As shown in Figures 5-4 and 5-5, the waste-water flow-rate is decreased with generator
pressure (Pg) and increased with absorber pressure (Pa). As a result, the waste-water usage is
generally increased with the ratio of absorption pressure to generator pressure (Pa/Pg) due to
the higher energy requirement for releasing ammonia at the higher ratio (Pa/Pg).
24
Figure 5-6 shows the effect of generator temperature on waste-water usage. With an
increased bed-temperature, generator will release more ammonia from the solution. To
produce the high rate of ammonia vapour, high rate of external heat is required, which could
lead to an increase in waste-water usage. As the temperature increases from 40 °C to 50 °C,
however, the waste-water flow rate is not significantly increased. This can be demonstrated
by circulation rates of rich and poor solutions, as shown in Figure 5-7. When the bed-
temperatures of generator are increased from 20 to 40 °C, the difference between rich
solution and poor solution is increased with the temperature, representing an increase in
ammonia releases. Between 40 °C to 50 °C, the difference between the two solutions is
insignificant. .
Pg=5 bar, Pa=25 bar
0
50
100
150
200
250
300
0 10 20 30 40 50 60
Temperature (C)
wa
te-w
ate
r F
low
-ra
te (
kg
/s)
Figure 5-6 Waste-water usage affected by generator temperatur. Pg: 5 bar. Pa: 25 bar;
fresh-water capacity: 5,000 T/day
25
Pg=5 bar, Pa=25 bar
0
20
40
60
80
100
120
0 10 20 30 40 50 60
Temperature (C)
solu
tio
n F
low
-ra
te (
kg
/s)
rich-solution
poor-s olution
Figure 5-7 rich and poor solution circulation rates at various generator-temperatur. Pg: 5
bar; Pa: 25 bar; fresh-water capacity: 5,000 T/day
5.3 Equipment costs
The main components of the proposed heat pump system include absorber, generator, and
compressor. The methods to evaluate the cost of the equipment can be found in section 4.
This section presents the results for various operational conditions. In general, the costs of
compressors and drivers are very high, which is accounted for more than 90% of total cost.
Figure 5.8 shows the estimated prices of compressors and other equipment for the heat pump
system used for production of 5,000 T/day of fresh-water from sea-water. The equipment
cost for this proposed heat pump is estimated to be more than 5 million euros at the high Pa.
Low Pa (particularly at lower Pa/Pg ) significantly reduces the capital cost.
26
Tg=30 C, Pg=5 bar
0
1000
2000
3000
4000
5000
6000
7000
10 15 20 25 30 35 40
Pressure (bar)
eq
uip
me
nt
cost
(k
eu
ro)
compress or
other equipment
Figure 5-8 Equipment costs at various absorber pressures. Pg=5 bar; Tg =30 °C
Figure 5-9 shows the costs of the absorber and the generator. The generator cost increases
with absorber’s pressure, due to an increase in waste-water flow rate, as shown in Figure 5-5.
The cost of absorber is the same at various heat pump operational conditions.
Tg=30 C, Pg=5 bar
0
50
100
150
200
250
300
10 15 20 25 30 35 40
Pressure (bar)
eq
uip
me
nt
cost
(k
eu
ro)
generator (G)
absorber (A)
Figure 5-9 Equipment costs at various absorber pressures. Pg=5 bar; Tg =30 °C
27
In general, power consumption (operational costs) is increased when the pressure ratio (Pa/Pg).
is increased. The power consumption by compression is not significantly affected by the
generator bed-temperature. Waste-water usage also increases with pressure ratio (Pa/Pg).
The effect of generator-temperature on waste-water usage is dependent on the rate of
ammonia generation. The total cost is significantly increased with compression pressure. The
cost of generator is generally increased with waste-water flow rate. The cost of absorber is
dependent on the capacity of sea-water which has been evaporated.
28
6. Economic profitability
The profitability is evaluated in terms of time, cash and percentage return on investment.
Payback time is sometimes taken as the time from commencement of the project to recover
the initial capital investment.
When measuring profitability, the change of money over time must be accounted for. Net
present value (NPV) is a measure of the net cash benefit generated by the project. This report
utilizes NPV to evaluate the profitability of the proposed process:
Capital
enancema CC
−+
−= ∑
=
=
kt
0tt
intt
i)(1
C t)NPV(projec (6-1)
Here, CCapital and Cmaintenance are capital and maintenance costs as discussed above. i is the
interest rate. t is an individual year. k is total number of years. Ct is cash benefit in t year.
Cmaintenance=0.05 CCapital (6-2)
Ct= (HCsave-Q•HCoperation) • τop (6-3)
τop (8400h) is total operating hours in t year. Price of operation, HCoperation, is the cost of
regular operation. In the proposed system, the main cost is the electricity cost (to run the
compressor and pump). The electricity price is selected according to Table 6-1 (DECC,
2010). If Q (kJ/s) is power consumption in the system, the electricity cost should be Q•
HCoperation.
Table 6-1 Coal and electricity prices in 2009, UK(DECC, 2010).
HCsave is defined as an hourly price. Generally, MED desalination system utilizes 180 °C
steam as the heating source. The steam is generated by coal combustion. Thus, the
replacement of absorption heat pump for steam generation can save coal and carbon emission
costs. Coal consumption rate Qcoal (kg/h) for steam generation (kJ/h) is estimated according
29
to coal heating value (i.e. ~ 20 MJ/kg of bitumium) and coal combustion efficiency (80-90%).
Coal price HCcoal is selected according to Table 6-1. If carbon content in the coal is about
70%, carbon release rate QCarbon (kg/h) can be estimated by 70% of coal consumption. The
price of carbon release HCCarbon is currently estimated as $50/t. It is usually predicted that the
carbon release cost will be increased in future. If the boiler is replaced by a heat pump, the
savings in coal price and carbon release prices is estimated by
HCsave = Qcoal HCcoal + Qcarbon HCcarbon. (6-4)
-8000
-6000
-4000
-2000
0
2000
4000
6000
0 1 2 3 4 5 6 7 8 9 10 11
Operation duration (year)
NP
V (
k€
)
20 bar 25 bar
30 bar 35 bar
Figure 6-1 Variation of NPV with operation years at various Pa. (Pg: 5 bar; Tg: 30 °C; fresh-water production: 5000 (t/day)
Figure 6-1 shows the effects of the operational pressures on NPV. The lower absorption
pressure could reduce the payback duration, because of lower power consumption by
compressors. At high pressure operation of 35 bar, a negative NPV is predicted, meaning that
no profitability is achieved. Figure 6-2 shows the effect of generator temperatures on NPV.
The higher temperature will reduce the payback duration.
30
-5000
-4000
-3000
-2000
-1000
0
1000
2000
3000
0 1 2 3 4 5 6 7 8 9 10 11
Operation duration (year)
NP
V (
k€
)20 C
50 C
Figure 6-2 Variation of NPV with operation years at temperatures. (Pg: 5 bar, Pa: 25 bar; fresh-water production: 5000 (t/day)
It should be noted that in this study the profitability calculations were based on the coal and
electricity prices in 2009.
31
7. Case Study 2
In this case study, the following assumptions were made:
1 - Assume energy source (e.g process industry) can provide 100MW of waste heat (60%
of it is available as hot water at 90°C).
2 - The mass flow rate of 90 °C hot water is estimated to be 737 (T/h).
The aim of the case study was to investigate the feasibility of using the above hot water
for MED desalination. In the case study, the hot water is used as the heating source for
the absorption heat-pump, as shown in Figure 7-1. The hot water at 90 °C is used for
heat-pump operation at 70°C. The outlet temperature of hot water from the heat pump is
80 °C that can be reused for another absorption heat pump system. As a result, the 90 °C
hot water will be used for total of 4 stages. 5 °C temperature difference is also assumed
between stages. Table 7-1 show the temperature distributions for the proposed 4 stage
system.. Each heat pump system is the heating source for individual MED desalination
process with fresh-water capacity of 5 kT/day.
Figure 7-1 Schematic diagram of 4-stage heat-pump operations used for MED
32
A low pressure ratio of Pa/Pg was used in the calculations so as to reduce the power
consumption and waste water usage. Table 7-2 lists the values for hot-water usage, power
requirement, equipment sizes and cost estimation for the proposed system. As shown in
Table 7-1, the total cost is approx. €.9 million. The cost of compression system is generally
more than 90% of total cost. The absorber costs are the same for all stage because the design
is based on MED rather than heat pump. In first stage, the cost of heat exchanger is zero - no
heat exchanger is needed because the temperature difference between Tg(=70 °C) and
Ta(=75 °C) is small. The waste-water usages are between 25 and 61 T/h for different stages.
A small portion of hot water (737T/h) from process industry will be used for fresh-water
production of 20,000 (T/day).
Table 7-1 hot-water usages, powers and equipment costs (based on fresh water production: 5,000T/Day on each stage, 20,000T/day for total production)
Figure 7-2 shows that NPV varies with the operation years in the separated stages and in the
whole peocess. The carbon-release cost in the calculation is set to $50/t-C. For a total
profitability, the payback will be achieved within 2 years of operation.
33
-6000
-4000
-2000
0
2000
4000
6000
8000
10000
12000
0 1 2 3 4 5 6
Operation duration (year)
NP
V (
k€
)Stage 1Stage 2Stage 3Stage 4
Figure 7-2 Variation of NPV with operation years in separated stages and in whole process.
The energy savings when using a heat pump can be calculated as:
S
pC
Q
QQ += - 1 Energyin Savings
QC and QP are powers consumed by the compressors and liquid pumps (kJ/s). Qs is the
thermal rate of sea-water evaporation(kJ/s), which is calculated according to producing fresh
water of 5 kT/day from sea-water.
Table7-2 shows the results obtained from these calculations. The first stage can save 91% of
energy. The overall average energy saving is 83%.
Table 7-2 Energy consumed by heat pump and energy required for water evaporation in producing fresh water (5kT/day) from sea water
34
8. Reference
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Arima H., Monde M., Mitsutake Y., Heat transfer in pool boiling of ammonia/water mixture, Heat and Mass Transfer 39 (2003) 535–543
Cacciola G., Restuccia G. and Rizzo G., Theoretical Performance of An Absorption Heat Pump using Ammonia-Water-Potassium Hydroxide Solution, Heat Recovery System & CHP 10(3) 177-185 (1990)
DECC (Department of Energy and Climate Change), Quarterly Energy Prices, Sept. 2010
Diego-César Alarcón-Padilla, Lourdes García-Rodríguez, Application of absorption heat pumps to multi-effect distillation: a case study of solar desalination, Desalination 212 (2007) 294–302
Diego C. Alarcón-Padilla, Lourdes García-Rodríguez, Julián Blanco-Gálvez, Design recommendations for a multi-effect distillation plant connected to a double-effect absorption heat pump: A solar desalination case study, Desalination 262 (2010) 11-14
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35
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Nomenclature
A heat-transfer area (m2) Cp specific heat capacity (kJ/kg °C) Cost equipment cost (€) Ct cash benefit (€) di diameter of tube (m) D diameter of reactor (m) G liquid wetted flow rate (kg/s m) h specific enthalpy (kJ/kg) H height of tube or reactor (m) HCcoal Coal price (€/t) or ($/t) HCCarbon Carbon-release price (€/t) or ($/t) HCsave: operation saving (€/hour) HCoperation: operation cost (€/kWh)
L the length of tube (m) m mass flow rate (kg/s) n number of tubes for heat exchanger (-) P pressure (bar) Q the rate of heat transfer (kW) Qcoal Coal consumption rate (kg/h) QCarbon Carbon release rate (kg/h) T Temperature (°C Ts sea-water temperature (°C) Twaste waste-water temperature (°C) u velocity (m/s) V vapour flow rate (m3/s) Vc space velocity (1/ s) W work (kJ/kg) x liquid mass fraction of ammonia in the solution (-) y mass fraction of ammonia in vapour phases (-) Greeks
36
α heat-transfer coefficient (kW/m2 °C) µ dynamic viscosity (Pa s) ρ density (kg/m3) τop operation duration (hours/year) λ conductivity (kW/m °C) δ liquid film thickness; tube wall thickness (m)
Non-dimension number Re Reynolds number
Nu Nusselt number Pr Prandtl number Subscript:
a: absorber
b: bed of MED
C: compression
f: feed of sea water
g: generator
H: heat exchanger
o: output of fresh-water vapour in MED
P: pump
s: sea-water
w: water
waste: waste-water
