Water desalination and energy

December 10th, 2014, Published in Articles: Energize

 

Water takes a close second to energy as the commodity most under pressure in the future, and the least available. Energy can be produced, but the supply of fresh water is totally dependant on weather and climatic conditions. For this reason desalination of seawater is becoming an increasingly important factor in the fresh water environment.

Desalination is playing an increasing role in the water supply chain, with the larger desalination plants reaching a capacity of up to 800 000 m3 per day. Unfortunately desalination of seawater is an energy intensive process, and any increase in the use of desalination will result in an increase in energy usage.

Research into the development of energy efficient or energy neutral water desalination processes is ongoing. One of the concerns with the energy composition of desalination is that the increased consumption of energy would increase greenhouse gas (GHG) emissions. A more pressing concern is the cost of desalination, as the price of conventional sources of energy is increasing. Carbon free sources such as renewable energy (RE) and nuclear are high on the agenda as possible future solutions. Bulk water desalination using RE must rank at the top of the list, and there are already several plants powered mainly by RE in operation. The usual argument against RE is that it is variable and cannot be stored, but in the case of desalinisation the product can be stored, allowing the plant to run when energy is available and still give a consistent output. Grid-tied RE is also used in some instances. This however does mean running the plant at a capacity determined by the available energy, and would not be suitable for all desalination methods. Other RE solutions such as CSP with storage are becoming available and are being considered.

As an example, consider how much energy and power would be required to produce 10% of the non-agricultural water needs of South Africa from desalination. The total annual water consumption for South Africa is estimated to be as high as 20-billion m3 [1]. Taking half of this for agriculture leaves 10-billion m3 for domestic and industrial consumption. 10% of this amounts to 1-billion m3 per annum. Table 1 shows the energy and power requirements to desalinate this volume of water based on different levels of energy intensity, and different energy sources.

Table 1 shows that at an energy intensity of 5 kWh/m3, 570 MW of continuous power would be required. Using renewable energy would require 2283 MW of solar PV or 1712 MW of wind power.

Table 1: Energy and power requirements to desalinate 10% of South Africa’s water requirements.
Energy requirement kWh/m3
1 2 5 10 20
Energy required (MWh) 1 000 000 2 000 000 5 000 000 10 000 000 20 000 000
Continuous power required (MW) 114,16 228,31 570,78 1141,55 2283,11
Solar PV power required (MW) 456,62 913,24 2283,11 4566,21 9132,42
Wind power required (MW) 342,47 684,93 1712,33 3424,66 6849,32

Existing methods and energy requirement.

There are no major technical obstacles to desalination as a means of providing an unlimited supply of fresh water, but the high energy requirements pose a major challenge. Theoretically, about 0,86 kWh of energy is needed to desalinate 1 m3 of salt water. The present day desalination plants use 5 to 26 times as much as this theoretical minimum depending on the type of process used [2].

Table 2 shows the energy consumption of different types of water desalination. Exact figures will depend on site parameters, such as the salinity of the water, so a range is given [2].

Table 2: Energy requirements for different types of desalination [2].
Method Electrical energy consumption (kWh/m3) Total energy consumption
Kwh/m3 (electrical and thermal)
Reverse osmosis ( RO) 3 to 3,5 3 to 3,5
Multistage flash distillation (MSF) 4 to 6 13,5 to 25,5
Multi-effect desalination (MED) 1,5 to 2,5 6,5 to 11
MED with thermal vapour compression (MED-TVC) 1,5 to 2,5 11 to 28
Mechanical vapour compression (MVC) 7 to 12 7 to 12

Desalination plants use energy for two purposes:

  • Electricity for water circulation and pumping
  • Steam generation, which may use fuel directly or may be waste heat or solar heat

Desalination technologies

There are two technologies types in use:

Membrane technologies

Reverse osmosis (RO) is the main technology used, although research is ongoing into the use of forward osmosis, electrodialysis and nano-technology solutions. RO involves forcing water through a membrane which inhibits the passage of salt molecules, leaving salt-free water on the one side and concentrated brine on the other. Feed water salinity has the most significant impact on power consumption of RO desalinators as the desalination process must overcome osmotic pressure to reverse the flow the normal osmotic flow. Fig. 1 shows the basic structure of a reverse osmosis system.

Fig. 1: Reverse osmosis desalination system (Lascich [3]).

Fig. 1: Reverse osmosis desalination system (Lascich [3]).

RO systems comprise the majority of systems in use today. The efficiency of the system can be improved by pre-heating the seawater, and this often done where desalination is combined with power generation.

Evaporation or thermal systems

These systems all use steam to evaporate seawater, which is then condensed and collected. There are two main technologies in use today:

Multistage flash distillation (MSF)

This process uses low pressure and low temperature steam (Fig. 2). The seawater entering the system is preheated by passing through tubes in the evaporation stages. Steam is used to heat the seawater which is then passed into the evaporators where a vacuum exists, causing evaporation. Water condenses on the cold seawater tubes and is collected. The temperature decreases and the vacuum increases, and further evaporation takes place in each stage.

Fig. 2: Multistage flash distillation (Engineering agenda [4]).

Fig. 2: Multistage flash distillation (Engineering agenda [4]).

Multi-effect distillation (MED)

A multi effect desalination unit is an evaporator where sea water is evaporated in one or more evaporation stages at low temperature (< 70°C ). Steam produced in each stage is fed to the next stage where it provides heat for further evaporation (Fig. 3).

Fig. 3: MED desalination system (Hamed [5]).

Fig. 3: MED desalination system (Hamed [5]).

Hybrid units

Hybrid units combine thermal and RO solutions to give a performance which is better than either on its own.

Reduction of energy requirements and costs

For many of the processes, thermal energy constitutes a large portion of the total energy consumed, and the electrical energy required is a small portion of the total. This suggests the use of “waste” heat from industrial processes or power plants as a source of energy for desalination. This approach has been used in several power plants or industrial plants located at coastal installations. Not only is the heat used for desalination at low cost, but the cost of running condensers is reduced, as well as the use of water for condensing.

Power and water generation

Power to water ratios (PWR)

This is an important metric in combined power/desalination plants, and is the ratio of power generated (in MW) to the quantity of desalinated water produced (in million imperial gallons per day (MIGD)). Typical ratios are 10 to 15.

CHP using CCGT or steam turbine plant

Common applications are collocated with closed cycle gas turbines, with the exit steam from the steam stage being used directly or indirectly in the desalination process [6]. Outlet steam from the steam turbine is fed to the water desalination (WD) plant and used in the desalination process. This reduces the need for condensers and provides steam for the desalination process.

Fig. 4: Combined gas turbine desalination plant (Veolia Water Technologies [6]).

Fig. 4: Combined gas turbine desalination plant (Veolia Water Technologies [6]).

Current developments

Existing plant including combined power and water units, use mainly fossil fuels, the price of which is increasing and hence the cost of water desalination is on the upward trend. Two options for future WD plants are being pursued, namely nuclear, either WD or power and WD (P+WD), and CSP WD or CSP P+WD.

Nuclear

Desalination is particularly suited to working together with nuclear plants, which are often located at coastal sites, and use huge amounts of seawater for cooling. A study undertaken amongst ten nuclear operators showed that using heat from nuclear power stations for desalination would be cheaper than other methods [7].

Nuclear power plus water distillation (NP and WD)

Water distillation plants have been used together with nuclear power plants for many years, but mainly producing water for internal usage, although here are several operational plants providing water outside of the plant [7]. Nuclear plants are largely located at the coast, because of the huge amounts of seawater used for cooling. This offers an opportunity to use the large amount of heat which is carried away by seawater to desalinate the same seawater. The development of high temperature reactors will make much more heat available for this purpose, and future installations could operate power generation and desalination in parallel rather than just using waste heat.

There are several pilot NP and WD plants in operation worldwide. One example is the nuclear desalination demonstration plant (NDDP) operated by the Indian Bhaba atomic research centre (BARC) [8], which consists of a 170 MWe PHWR reactors coupled with a hydrid RO/MSF desalination plant (Fig. 5). The unit delivers 5400 m3 of water/day as well as 166 MWe of electricity.

South Korea has developed a small nuclear reactor design for cogeneration of electricity and potable water. The 330 MWt SMART reactor (an integral PWR) has a long design life and needs refuelling only every 3 years. The main concept has the SMART reactor coupled to four MED units, each with thermal-vapour compressor (MED-TVC) and producing total of 40 000 m3/d, with 90 MWe [9].

Marine nuclear plants have been used for water desalination for many years and several types of marine reactors have been adapted for moored barge operation primarily as WD units with a small amount of electricity production. Capacities in the range of 40 000 m3/d have been reported.

Dedicated nuclear water desalination (NWD)

In addition to combining desalination with nuclear power generation, small nuclear plants dedicated to heat production for desalination are being investigated and developed. This process uses reactors designed specifically for heat generation to desalinate water. An example is the
200 MW nuclear heating reactor (NHR-200) coupled with MED WD plant developed by the Institute of Nuclear Energy Technology (INET) of Tsinghua university, China. The feasibility study on this plant has been completed, and the Chinese government has agreed to build a nuclear seawater desalination plant of this type in the Shandong Peninsula of China. Two different kinds of MED processes, high temperature stacked VTE-MED and low-temperature horizontal tube MED-TVC, have been investigated and compared, and their capacities for freshwater production are 160 000 m³/d and 120 000 m³/d, respectively [10].

Fig. 5: NDDP, a hybrid MSF and RO plant, at Kalpakkam (BARC [8]).

Fig. 5: NDDP, a hybrid MSF and RO plant, at Kalpakkam (BARC [8]).

Concentrated solar power water desalination (CSPWD)

As with nuclear, both combined application of CSP for power generation and WD, and dedicated application for WD are possible. The low temperature steam requirement of thermal WD systems makes the use of fresnel and trough type solar collectors ideal, and thermal storage allows extended operation of the plant.

The main problem with CSP is the large amount of land required. For seawater desalination the plant would have to be located in a coastal area with high solar radiation levels, which rules out most coastal areas, where land is at a premium, but opens up possibilities for arid or semi-arid areas.

CSP P+WD

Studies have been done on the feasibility of combined CSP and WD in the MENA region [11, 12] , and show that this is both feasible and cost effective. This raises the question of whether this could not be implemented on the west coast of South Africa.

Dedicated concentrated solar WD systems

Solar thermal systems under development use either trough or fresnel mirror systems to heat oil which is used to generate low temperature steam which drives a thermal based desalination process [13]. Systems are claimed to be more efficient than solar PV or wind driven RO systems, and have a higher recovery rate. The advantage of these systems over RE is that well established thermal storage can be used allowing extended operation of the plant, and thus reduce the size of the WD plant required.

Future developments

Advancements include such new and emerging technologies as forward osmosis, low temperature distillation, membrane distillation, pressure retarded osmosis, biomimetic and graphene membranes. Hybrid plants (especially those using MED) and reverse osmosis are gaining wider use in the Middle East, which has traditionally been home to facilities using more energy-intensive thermal technologies such as MSF.

References

[1] DWAF: “Strategic overview of the water sector in South Africa”, DWA Directorate: Water services planning and information, http://nepadwatercoe.org/wp-content/uploads/Strategic-Overview-of-the-Water-Sector-in-South-Africa-2013.pdf
[2] Desware: “Energy requirements of desalination processes”, Encyclopedia of desalination and water resources, www.desware.net/desa4.aspx
[3] U Lascich: “Optimising the efficiency of reverse osmosis seawater desalination”, http://urila.tripod.com/Seawater.htm
[4] Engineering agenda: “Multistage flash distillation”, http://engineeringagenda.com/agenda/2014/04/multistage-flash-distillation-msf/
[5] O Hamed: “Evolution of thermal desalination processes”, Saline water desalination research institute (SWDRI).
[6] Veolia Water technologies: “Combination of desalination plant with gas turbine and heat recovery boiler”, www.sidem-desalination.com/en/Process/Cogeneration/GT-and-DP/
[7] G Seneviratne: “Research projects show nuclear desalination economical” Nuclear news April 2007.
[8] PK Tewari and BM Misra: “Technological innovations in deslination”, BARC newsletter, www.barc.gov.in/publications/nl/2001/200109-01.pdf
[9] UIC: “Nuclear desalination” UIC Nuclear Issues Briefing Paper # 74, www.tbc.school.nz/elearning/localsites/uic/nip74.htm
[10] J Haijun: “Nuclear Seawater Desalination Plant Coupled with 200 MW Heating Reactor”, International Symposium on the Peaceful Applications of Nuclear Technology in the GCC Countries, Jeddah 2008.
[11] IRENA: “Water desalination using renewable energy”, IEA-ETSAP and IRENA technology brief I12, March 2012.
[12] J Blanco: et al “Assesment of CSP+D potential in the MENA area”. Solar Paces task VI activity, April 2013, www.solarpaces.org/images/task/SolarPaces_TaskVI_CSPD_Activity_Final_Report.pdf
[13] FX Water: “A modular, solar-thermal water desalination system”, http://waterfx.co/aqua4/

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