Investigating the Prospects of Using Novel Thermal Power Pump

Cycle Coupled with Reverse Osmosis System for Water Desalination

Abhijit Date+1

, S.V. Ghaisas2, Ashwin Date

1 and Aliakbar Akbarzadeh

1

1 School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Australia

2 School of Energy Studies, University of Pune, Pune, India

Abstract. This paper presents theoretical and experimental study of new thermal power pump cycle for

water desalination. The operation, thermodynamic cycle and design of the proposed pump-cycle-operated

reverse osmosis system are explained with the aid of system schematics and thermodynamic process

diagrams. Theoretical performance of the thermal power pump cycle alone and in combination with a reverse

osmosis system is presented. The advantages of the proposed thermal power pump cycle in relation to

conventional power cycles are discussed. The proposed system is predicted to consume between 29MJ and

250MJ of thermal energy at approximately 80C in order to produce 1m³ of fresh water from 2m³ of feed

water with salt concentration between 5,000g/m³ and 45,000g/m³.

Keywords: desalination, reverse osmosis, thermal energy, water pump, low temperature heat.

1. Introduction

With the ever increasing human population the demand for fresh water is also increasing. Similarly

energy is in short supply in many regions of the world and approximately 80% of the world’s primary energy

comes from fossil fuels which are becoming scarce and are the main contributors to greenhouse gas. In

recent years most of the developing countries have experienced rapid increase in primary energy demand [1].

To support the growing fresh water needs, desalination technology has been extensively used in high demand

and high drought regions. Worldwide more than 60 million m³ of desalinated water is produced every day

and most of the desalination processes are very energy intensive and rely on energy from fossil fuel [2, 3].

This has motivated development of sustainable desalination systems powered by renewable energy.

Researchers around the world are developing new desalination systems to utilise renewable energy.

In addition, some industrial processes generate waste heat at low temperatures (below 100°C) which

cannot be converted efficiently to useful work and therefore most of it is discarded by industry. This has

become an environmental and economic problem both because of the thermal pollution it causes and because

of ever increasing fuel prices. For example, the diesel engines on-board of most ships have an efficiency of

approximately 50% and about 50% of the fuel energy is lost in the form of heat through exhaust gas and

engine coolant [4]. On sea ships fresh water is not readily available, while saline sea water is abundantly

available; therefore most sea ships have on-board desalination plants. Ideally the waste heat from the engines

could power a desalination process.

The most commonly used phase change processes are Multi-Effect Distillation (MED), Multi-Stage

Flash (MSF) and solar still (ST) and the main single phase process is the Reverse Osmosis (RO) desalination

process, MED, MSF and RO are the main technologies used globally in the production of desalinated water

[2]- [6]. Desalination is an energy intensive process and the energy consumption, per m³ of desalinated water

produced, varies with the process. Table 1 shows the range of specific energy consumptions for various

processes [5].

The RO system uses high pressure water pumps to force the saline water through a semipermeable

membrane, which allows water molecules to pass while retaining the salt molecules on the pressurised side

Corresponding author. Tel: +61 399250612; fax: +61 399256108.

E-mail address: abhijit.date@rmit.edu.au.

2014 3rd International Conference on Environment Energy and Biotechnology IPCBEE vol.70 (2014) © (2014) IACSIT Press, Singapore

DOI: 10.7763/IPCBEE. 2014. V70. 26

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of the membrane. The most energy intensive component of the RO process is the high pressure water pump.

Generally, electrically driven high pressure water pumps have been used in RO systems.

Table 1: Performance range of solar desalination plants

Process /

Technology Feed water type

Specific energy consumption

MJ/m³ (kWh/m³)

Main energy

type

Recovery

ratio %

MSF Brackish water, Seawater 291 – 518 (81 – 144) Thermal 0.6 – 6

MED Brackish water, Seawater 180 – 698 (50 – 194) Thermal 6 – 38

Conventional RO Brackish water, Seawater 4.8 – 68 (1.2 – 19) Electrical 10 – 51

In 2013 Date and Akbarzadeh [7] proposed an innovative thermal water pump cycle that can utilise low

temperature thermal energy for pumping water. This paper investigates the prospects of using this thermal

water pump for supplying saline water at high pressure to RO desalination process that would operate at heat

sources temperatures below 100°C. This paper describes the working principle of the new high pressure

thermal water pump with assistance of pump schematics and thermodynamic curves (P-v and P-h). Analysis

of this high pressure thermal water pump connected to a RO membrane is carried out in order to predict its

thermal performance.

2. Concept Design

P(k

Pa

)

v (m³/kg)

1

23

43’

P1,2&3

v1 ≈ v4

P3’, 4

v3

v2

s3s4

T2 & 3

T3’ & 4

T (

ºC)

s (kJ/kg K)

1

2 3

43’

Constant

Pressure Line

Fig. 1: Schematic of thermal water pump RO system along with P-v and T-s diagrams

Figure 1 shows the schematic of the thermal water pump connected to the reverse osmosis system. The

feed water can be saline ground water or it could be sea water that needs to be desalinated. Saline water from

ground or sea is pulled through the low pressure feed water suction line (LPF) into the cylinder. The high

pressure feed water delivery line (HPF) supplies the feed water at the required pressure to the RO system. To

be able to achieve higher delivery pressure the design of the piston & cylinder arrangement of the thermal

water pump is modified as shown in Figure 1. In the modified design there are two pistons, larger piston p1

that sees the working fluid and the smaller piston p2 that sees the feed water and both these pistons are linked

with a connecting rod. By reducing the diameter of the feed water piston, higher delivery and suction

pressures can be generated to suit the operating RO pressure that corresponds to the feed water salinity and

recovery rate. The product out of RO shown in Figure 1 is the treated water with reduced salinity, while the

reject is the water with higher salt concentration. The ratio of product divided by the product + reject is the

recovery ratio of the RO system. The reject comes out of the RO system at operating pressure and energy can

be recovered from this reject using pressure recovery devices as suggested by many researchers in past [8, 9].

The present study does not consider the pressure recovery system in the energy balance analysis.

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3. Performance Analysis of TPP Cycle

Figure 2 shows the theoretical and experimental performance of the thermal water pump without

pressure boosting, i.e. with pressure ratio equal to 1. This means the change in volume of feed water per

stroke would be equal to the change in volume of working fluid per stroke. And the delivery pressure of the

feed water would be equal to the pressure of the working fluid during the delivery stroke. Here it is assumed

that the temperature of the working fluid is equal to the temperature of heat source during the delivery stroke

and equal to the temperature of the heat sink during the suction stroke.

Fig. 2: Theoretical and experimental thermal performance of the thermal water pump with pressure ratio equal to 1

Theoretical analysis shows that the maximum delivery pressure without pressure boosting (i.e. with PR =

1) for any working fluid corresponds to the saturation pressure of the working fluid at the temperature of heat

source. So with R245ca as the working fluid the maximum achievable delivery pressure at a heat source

temperature of 100°C would be 828kPa (gauge). This delivery pressure without pressure boost can be

utilised to desalinate low salinity ground water with salt concentration below 5000g/m³. It can be seen from

Figure 2 the theoretical overall thermodynamic efficiency of the TPP cycle peaks at about 80°C and stays

almost constant till 100°C heat source temperature for isopentane and beyond this temperature it decreases.

The majority of work output happens during the constant pressure process from point 2 to point 3 which is

the delivery stroke of TPP cycle. The delivery work has greater influence on the overall efficiency of the

TPP cycle as compared to the suction work as the delivery pressure is very large as compared to the suction

pressure. Experimental were conducted with Isopentane as the working fluid for four different heat source

temperatures as seen from Figure 2. The experimental delivery head was measured to be less than the

theoretical predictions and this can be attributed to the lower working fluid temperature due to thermal

resistance of the heat exchanger and friction in the piston cylinder device. The experimental overall

thermodynamic efficiency was on average 30% lower than the theoretical predictions and this is mainly due

to heat loss from the cylinder during the delivery stroke and the frictional head loss in the device. The

thermal water pump experiments showed that this type of pump works and the preliminary experimental

results show the actual performance is around 30% lower than theoretical. Further experimental examination

and development is required to improve the experimental performance.

4. Performance Analysis of TPP Coupled with RO System

In this section performance of TPP cycle pump coupled with RO system is analysed for Isopentane as

working fluid. For an ideal condition Figure 3 shows the specific thermal energy consumption (SECt) of a

TPP cycle operated RO system. It can be seen from Figure 3 that at low heat source temperatures the SECt is

very high. This is due to lower saturation pressure of working fluid at those temperatures which are not

enough to operate the RO system. Hence a pressure boosting is required and hence the feed water piston p2

diameter is reduced to achieve the pressure boost. The reduced feed water piston diameter would reduce the

volume of feed water pumped per stroke while the thermal energy input remains the same. So at lower heat

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source temperatures higher SECt is required. It can be seen from Figure 3 that the SECt decreases with

increase in the temperature and reach the minimum value at about 80°C and stays around this minimum

value till 100°C. Beyond 100°C the SECt again starts to increase and continues to increase with higher heat

source temperatures. This is due to the decrease in the overall efficiency of the TPP cycle beyond the heat

source temperature of 100°C for the working fluids used for this study. The reasons for decrease in the

overall efficiency of the TPP cycle are discussed by Date and Akbarzadeh [7].

0

50

100

150

200

250

300

350

400

450

500

550

40 50 60 70 80 90 100 110 120 130 140

Sp

ecif

ic T

her

mal

En

erg

y C

on

sum

pti

on

(M

J/m

³)

Temperature of heat source (C)

5,000 g/m³ 10,000 g/m³ 15,000 g/m³

20,000 g/m³ 25,000 g/m³ 30,000 g/m³

35,000 g/m³ 40,000 g/m³ 45,000 g/m³

Heat sink at constant temperature of 20 C

Fig. 3: Specific Thermal Energy Consumption of TPP coupled RO system with a recovery ratio of 50% and Isopentane

as working fluid. (without energy recovery)

From Figure 3 it can be said that for optimum operation of TPP cycle with Isopentane as working fluid

the heat source temperature should be in the range of 75°C to 100°C. While the cold side temperatures

should be maintained within 20°C to 24°C to optimise the suction stroke. If the cold side temperature is

higher than 27.5°C for isopentane then the absolute pressure during the suction stroke will be higher than the

atmospheric pressure and this will prevent feed water from being sucked into the cylinder from the feed

water source. This is due to the fact that the saturation pressure of isopentane at 27.5°C is 100.21kPa (abs).

Although due to reduced diameter of the feed water piston to boot the delivery pressure, the suction pressure

on the feed water side also get a boost. But if the available suction pressure on the working fluid side is

positive or very small then the boosting will be of no use. It can be seen from Figure 3 that theoretically the

TPP cycle coupled RO system would consume 170MJ to 250MJ (i.e. 48kWht to 70kWht) of thermal energy

at about 80°C to produce 1m³ of desalinated water from 2m³ of feed water with salt concentration in the

range of 30,000 to 45,000 g/m³ (sea water, NaCl only). Further to produce 1m³ of desalinated water from

2m³ of feed water with salt concentration in the range of 5,000 to 25,000 g/m³ (brackish, NaCl only) a

theoretical TPP cycle operated RO system would consume 29MJ to 140MJ of thermal energy at about 80°C

(i.e. 8kWht to 39kWht). Based on the PV or wind driven RO system research conducted around the world

[10-14] it can be said that the proposed TPP couple RO system has promising prospects for as it utilises low

temperature heat as energy source.

5. Conclusion

Theoretical performance shows that with isopentane and R245ca as working fluids and for a hot side

temperature of 40°C and cold side temperature of 20°C the TPP cycle is about 68% as efficient as a

corresponding Carnot cycle. Further with isopentane and R245ca as the working fluids the theoretical overall

efficiency of a TPP cycle thermal water pump peaks at about 80°C hot side temperature for 20°C cold side.

The experimental efficiency of the TPP cycle at around 75C is estimated to be around 4.2%. It is found

from the theoretical analysis that a TPP coupled RO system with isopentane as working fluid would have

close to minimum SECt when operated between 70°C to 100°C hot side and 20°C cold side temperature. The

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theoretical analysis shows that the TPP coupled RO system would consume thermal energy of about 29MJ at

80°C to produce 1m³ of desalinated water from 2 m³ feed water with salt concentration of about 5,000g/m³.

While the same system would consume thermal energy of about 250MJ at 80°C to produce 1m³ of

desalinated water from 2 m³ feed water with salt concentration of about 45,000g/m³. In the present study the

auxiliary electrical energy required to operate solenoid valves A and B and the working fluid pump is not

included in the specific energy consumption as it is considered to be negligible. Theoretically a single stage

TPP coupled RO system without any kind of energy recovery devices would have less SECt as compared to

most conventional multistage MED and MSF distillation systems, at the same time it can be compact. This

shows the TPP cycle operated RO system has got potential to compete with conventional thermal

desalination systems and hence the next step would be experimental validation of the performance of this

system.

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