DEGREE PROJECT IN
NUCLEAR ENERGY ENGINEERING
,
SECOND CYCLE, 30 CREDITS
,
STOCKHOLM SWEDEN 2018
Author
Simon Wakter
Nuclear Energy Engineering KTH Royal Institute of Technology
Location for Project
Stockholm, Sweden
Examiner
Professor Pavel Kudinov Nuclear Energy Engineering KTH Royal Institute of Technology
Supervisor
’I have said that I thought that if we could ever competitively, at a cheap rate, get fresh water from salt water, that it would be in the long-range interests of humanity which would really dwarf any other scientific accomplishments.’
— John F. Kennedy, April 12 1961 [1, 2]
Acknowledgements
This degree project will conclude the author’s education toward a Master of Science degree in Nuclear Energy Engineering carried out at the KTH Royal Institute of Technology in Stockholm, Sweden.
I would like to thank my supervisor, Janne Wallenius at KTH, for supporting the project idea and for advice and help in completing the project.
I also wish to express my heartfelt gratitude to my family and to my friends and girlfriend for providing unconditional support and continuous encouragement throughout my years of study and through the process of researching and writing this thesis. This accomplishment would not have been possible without them.
Thank you
Abstract
Water scarcity is an increasingly serious problem, exacerbated by an increasing world population, increasing water demand per capita and climate change effects. Seawater desalination provides a viable option for reducing the effects of water scarcity but is an energy-intensive technology that sits at the centre of the energy-water nexus. Increasing environmental concern regarding greenhouse gas emissions poses an additional obstacle to increased desalination as a means to mitigate water scarcity.
This degree project in nuclear energy engineering examines the feasibility of powering desalination plants with Small Modular Reactors and whether they could improve the energy efficiency of the desalination process. The report presents the findings of a comprehensive literature review alongside the results of the development of a new concept for desalination and the evaluation of said concept.
The concept proposal consists of a feed water pump mechanically coupled to a steam turbine powered by a Small Modular Reactor. Cogeneration of electricity and water through the proposed concept can provide positive synergy effects and the SMR form factor provides several benefits.
Evaluation of the concept using conservative estimates and calculations suggests a gain-output-ratio (GOR) of 60 kg of product water per kg of steam and a specific energy consumption (SEC) of 3.9 kWh/m3.
The results indicate that the concept has the possibility of reducing desalination energy consumption by around 10 % while reducing the associated greenhouse gas emissions by more than 90 % when compared to energy provided from a future low-emission energy mix.
Keywords
Sammanfattning
Vattenbrist är ett allvarligt globalt problem som förvärras av en ökande världsbefolkning, ökande vattenbehov per capita och klimatförändringar. Avsaltning av havsvatten utgör ett potentiellt alternativ för att minska effekterna av vattenbrist. Men avsaltning är energiintensivt och utgör en knutpunkt mellan förbrukning och tillverkning av elektricitet och vatten. Ökande miljöhänsyn avseende utsläpp av växthusgaser utgör ett ytterligare hinder för utökad installation av avsaltningsanläggningar för att minska den globala vattenbristen.
Detta examensarbete inom området kärnenergiteknik utreder möjligheten att driva avsaltningsanläggningar med små modulära reaktorer (SMR) och huruvida de kan förbättra avsaltningsprocessens energieffektivitet. Rapporten presenterar resultaten från en omfattande litteraturstudie samt resultaten av utvecklingen av ett nytt koncept för avsaltning och utvärdering av konceptet.
Konceptförslaget består av en matarvattenpump som är mekaniskt kopplad till en ångturbin som drivas av en liten modulär reaktor. Produktion av både el och vatten genom det föreslagna konceptet kan ge positiva synergieffekter och användandet av SMR medför flera ytterligare fördelar.
Utvärdering av konceptet med konservativa uppskattningar och beräkningar antyder en Gain-Output-Ratio på 60 kg färskvatten per kg ånga och en specifik energiförbrukning på 3.9 kWh/m3.
Resultaten indikerar att konceptet har möjligheten att minska avsaltningsanläggningens energiförbrukning med omkring 10 % och utsläppen av växthusgaser med mer än 90 % jämfört med energi från en framtida energimix med låga utsläpp.
Nyckelord
Contents
Introduction
1 Background
1
1.1 Source water type and availability . . . 3
1.2 Water composition . . . 4
1.2.1 Measuring and describing water composition . . . 5
1.2.2 The role of sodium chloride in drinking water . . . 6
1.3 Drinking water quality . . . 8
1.3.1 Drinking water quality aspects and guidelines . . . 9
1.3.2 Drinking water quality standards around the world . . . 10
1.3.3 Providing safe drinking water through desalination . . . 12
1.4 Water scarcity . . . 12
1.4.1 Water in conflicts . . . 13
1.5 Water losses through leakage . . . 14
1.6 Desalination processes . . . 15
1.6.1 Multiple-stage flash distillation (MSF) . . . 17
1.6.2 Multiple-effect distillation (MED) . . . 18
1.6.3 Reverse Osmosis (RO) . . . 19
1.6.4 Hybrid desalination processes . . . 21
1.7 The Energy-Water Nexus . . . 21
1.7.1 Water use in energy and electricity production . . . 21
1.7.2 Energy use in desalination . . . 22
1.7.3 Current state of nuclear desalination . . . 23
1.8 Small Modular Reactors, SMR . . . 24
2 Project description
25
2.1 Problem statement . . . 25
2.2 Project method . . . 26
2.2.1 Literature review . . . 26
2.2.2 Concept proposal . . . 26
2.2.3 Evaluation of the proposed concept . . . 27
2.3 Delimitations . . . 27
2.4 Purpose and goal . . . 28
Concept Development and Evaluation
3 Concept development
29
3.1 Economics and energy consumption of desalination . . . 293.2 High-pressure pumps, couplings and drivers . . . 32
3.2.1 High-pressure pumps . . . 33
3.2.2 Pump drivers . . . 34
3.2.3 Driver-pump couplings . . . 35
3.3 Concept description . . . 36
3.4 Evaluation of concept energy efficiency . . . 37
3.4.1 Conventional electric motor drive efficiency . . . 37
3.4.2 Concept steam turbine drive efficiency . . . 38
3.5 Analysis of concept specific energy consumption . . . 38
3.5.1 Required pump power . . . 39
3.5.2 Steam turbine design . . . 40
3.6 Prospects of SMR for desalination . . . 41
3.6.1 Burnup and load following . . . 41
3.6.2 Scalability and modularity . . . 42
3.6.3 Decreased capital cost . . . 42
3.6.4 Smaller size . . . 42
3.6.5 Potential for military applications . . . 43
3.6.6 Possible future advantages . . . 43
3.6.7 Environmental and health impacts . . . 43
Results and Discussion
4 Results
45
4.1 Viability of nuclear desalination . . . 45 4.2 Concept energy consumption . . . 45
5 Discussion
46
5.1 Discussion and comparison of results . . . 46 5.2 Implications of widespread desalination . . . 47
6 Conclusions
48
Appendix
A Thermodynamic minimum specific energy
58
B Energy savings from increased number of RO stages
60
C Steam turbine design
61
D Environmental and health impacts in figures
68
Introduction
1
Background
Securing a reliable supply of energy and freshwater is critical for the sustainable development and prosperity of human society. Besides being needed for development and prosperity, freshwater is also an essential resource for human survival and an adequate supply should be guaranteed to all people. Despite this, freshwater scarcity is a serious problem around the world with an estimated 4.0 billion people experiencing severe water scarcity for at least one month of the year in 2016 [3] and 2.1 billion people lacked access to safe drinking water in 2015 [4]. A total of 68 countries suffer from high to extremely high Baseline water stress [5], measured as the ratio of total annual water withdrawals to total available renewable water supply, see Figure 1.1. In addition to population growth, the increasing demand for freshwater is also driven by improved standards of living, changing consumption patterns, increased water use in the industrial and agricultural sector and climate change [6, 7].
Further exacerbating the problem of water scarcity is the diminishing availability and quality of remaining available fresh and brackish water sources. This results from the lack of a holistic perspective in policy-making as well as from mismanagement of water resources [8–11]. Short-term and large-scale infrastructure projects, in particular dams and irrigation for supply-side management, have initially mobilised more water resources but ultimately decreased freshwater availability. Fewer rivers reach the ocean and groundwater levels are dropping in important aquifers while industrial and agricultural sectors are polluting several remaining freshwater sources [10, 11].
Solving the problem of water scarcity is a difficult problem with stakeholders from all areas of society. Engineers are tasked with developing technologies that solve the problems of both the prosperous, unsustainable developed world and the needs of the developing world as it rises out of poverty. Meanwhile, water resource management experts advocate a softer and broader approach, emphasising the intricacies of water policy and the interdisciplinary dimensions between, among others, society, economics, ecology and engineering.
But while water resource management experts argue the need of sound water policy with less emphasis on technological solutions, there is an emerging technological solution that could possibly solve the water scarcity crisis – seawater desalination.
Figure 1.2. World total population growth, probabilistic estimation [13].
1.1
Source water type and availability
Table 1
Overview of global water resource distribution [15].
Resource Volume [km3] Percent of total water Percent of fresh water
Glaciers 24064000 1.72 68.7 Aquifers 10530000 0.75 30.1 Ground ice 300000 0.021 0.86 Lakes 176400 0.013 0.26 Soil Moisture 16500 0.0012 0.05 Marshes 11470 0.0008 0.03 Atmospheric water 12900 0.001 0.01 Rivers 2120 0.0002 0.006 Lithosphere 23400000 1.68 Oceans 1338000000 95.81 Total 1396513390 .
1.2
Water composition
Figure 1.3. Illustration of different pathways of solids into seawater [17].
There are numerous pathways for solids into the oceans, see Figure 1.3, and seawater contains almost all known elements and chemicals, albeit in minute concentrations. The main components can be divided into dissolved inorganic chemicals, dissolved organic matter and suspended fine particles including clay, microorganisms, viruses and colloidal matter.
Water molecules have areas of small positive and negative charge due to the asymmetry of oxygen and hydrogen atoms
Figure 1.4. Salt crystal dissolving
in water [17].
The six ions Cl−, Na+, SO2−
4 , Mg2+ Ca2+ and
K+ together make up more than 99 % of dissolved
solids in seawater [15, 17]. An overview of the most common chemical compounds dissolved in typical seawater (with salinity of 35 000 mg/l) can be found in Table 2. The relative proportions of the different salt ions can vary locally but on average remain almost constant, even though the total amount of salt varies between locations and temperatures. This is known as the rule of constant proportion [17].
Table 2
Overview of sea water composition of common dissolved compounds [17].
Chemical compound Chemical Formula Concentration [mg/l] Percentage of total salinity
Chloride Cl− 19.345 55.03 Sodium Na+ 10.752 30.59 Sulfate SO2− 4 2.701 7.68 Magnesium Mg2+ 1.295 3.68 Calcium Ca2+ 0.416 1.18 Potassium K+ 0.390 1.11 Bicarbonate HCO−3 0.145 0.41 Bromide Br− 0.066 0.19 Borate H2BO−3 0.027 0.08 Strontium Sr2+ 0.013 0.04 Fluoride F− 0.001 0.003 Other compounds <0.001 <0.001
1.2.1 Measuring and describing water composition
Water is divided into the three categories fresh, brackish and sea (alternatively salt or saline) water depending on its measured salinity, as described above in subsection 1.1. Salinity together with Total Dissolved Solids (TDS) are the primary ways of describing water composition [18].
measurements. The conductivity value (in µS/cm) of the sample is compared to those of a salinity standard and is called the Practical Salinity Scale, measured in the unit-less practical salinity unit (psu). There are many other methods used to determine salinity, with the most widely adopted being Absolute Salinity. It is based on the Gibbs function and determines salinity as a function of specific enthalpy, temperature and specific entropy [18]. Measurement units for salinity vary too, with 1 ppt = 1 g/kg being the traditional standard. For freshwater sources, mg/l is a common unit.
For continuous monitoring of changes and field measurements, TDS is determined through multiplication of the conductivity with a specific TDS constant for that location and water composition. If the water composition is not known, the TDS must be determined through evaporation and gravimetric analysis but this process is time-consuming [18].
1.2.2 The role of sodium chloride in drinking water
Even though all quality aspects of drinking water should be considered when discussing desalination, see subsubsection 1.3.1, the discussion about desalination often revolves around the dangers and removal of salt. Usually, this refers to a specific type of salt; sodium chloride, which is formed by the cation-anion pair of Na+ and Cl− and more commonly referred to as
table salt. Because of the differing molar masses, sodium chloride salt consists of approximately 40 % sodium and 60 % chloride.
While sodium is essential for many bodily functions such as osmoregulation and neuron function, high sodium intake is also correlated with increased blood pressure and several non-communicable diseases (NCDs), including cancer and cardiovascular diseases. [19, 20]. The increase in blood pressure is a major risk factor for cardiovascular diseases [20, 21].
Regulation of sodium in the human body
High salinity water
Ingesting a small amount of higher salinity water, e.g. seawater, is not directly harmful if the water is clean. It is still dangerous as the ingestion of water with a higher salinity than the urine salinity will result in the addition of more sodium than water to the body, causing a net loss of water. The ability of the kidneys to rid the body of excess sodium is ultimately limited by the maximum possible salinity of the excreted urine. The maximum urine concentration is limited by the highest concentration gradient that the body can generate in the kidney. When the concentration is at its maximum, the only way for the body to increase sodium excretion is to excrete more urine, leading to dehydration. Normal sodium concentration in human urine averages between 1170 mg/l and 4390 mg/l [23] with the maximum recorded concentration being 8625 mg/l (8625mOsm/l) [24, 25]. This is significantly less than the average salinity of seawater of around 35 000 mg/l.
Sodium intake
The necessary sodium intake for a grown person is less than 0.5 g (or about 1 g of sodium chloride salt) per day. This amount is necessary, among other things, in order for cells to maintain homeostasis [20, 22]. Most dietary guidelines recommend an intake of less than 2 g of sodium or 5 g of salt per day [26]. Actual sodium intake varies considerably across different population groups, from 0.06 g per day among the Yanomamo Indians of Brazil to 22.5 g per day in the Republic of Korea and the Bahamas [27–29]. Many people around the world consume several times the recommended intake, with consumption in some countries averaging around 9 g to 15 g of salt per day [22, 30].
Drinking water is not a significant part of dietary sodium intake in humans. Daily consumption of 2 l of drinking water containing 20 mg sodium would only lead to an intake of 40 mg which is not of consequence except for people on a highly sodium-restricted diet. Because infant kidney function is not fully developed, drinking water with a high level of sodium may exacerbate problems for infants with gastrointestinal infections that already suffer from difficulties retaining fluids [31].
Guidelines for sodium and chloride concentrations in drinking water
250 mg/l are increasingly likely to be detected by taste. The vast majority of countries (77 out of 104) specified this value. Chloride will also make the water more corrosive, which will shorten the life of mains piping and increase leakage. Chloride concentration is therefore regulated to around 100 mg/l in some countries [34, 35].
Table 3
Guideline and regulatory values for sodium and chloride concentration in drinking water specified by different countries [33].
Sodium WHO Guideline value None specified
Number of countries specifying a value 81 out of 104
Maximum value 400 mg/l
Minimum value 100 mg/l
Median value 200 mg/l
Chloride WHO Guideline value None specified
Number of countries specifying a value 100 out of 104
Maximum value 1200 mg/l
Minimum value 20 mg/l
Median value 250 mg/l
1.3
Drinking water quality
The negative effects on human health stemming from contaminants in drinking water are significant. Improving access to and quality of drinking water are cost-efficient and meaningful interventions that provide considerable benefits to health. Therefore, substantial effort has been devoted to establishing standards and guidelines for safe drinking water, both by individual countries as well as by international organisations. This section describes both international and national drinking water quality guidelines. In addition to the guidelines, the United Nations (UN) in 2010 adopted the resolution 64/292 The human right to water and sanitation [36] stating that the UN
Recognizes the right to safe and clean drinking water and sanitation as a human right that is essential for the full enjoyment of life and all human rights. [36] In 2015 these rights were reaffirmed and expanded on through the resolution 70/169 The human
rights to safe drinking water and sanitation [36] and goals relating to these human rights were
formally adopted as goal 6 of the 2030 Sustainable Development Goals through resolution 70/1
1.3.1 Drinking water quality aspects and guidelines
Most guideline values for chemical substances in drinking water are based on a Tolerable Daily Intake (TDI) which is determined from a No Observed Adverse Effect Level (NOAEL) in experimental tests on laboratory animals using a safety factor, most often of 100. The TDI can also be determined using a benchmark-dose (BMD) instead. If there is no basis for exposure to a specific substance the permissible dose from drinking water is generally (in accordance with the World Health Organization) set at 10 % of the TDI. If it is well known that the exposure from drinking water is large in comparison to other sources the permissible contribution to TDI may be set as high as 80 % [38]. This is the case for uranium.
Perhaps the most comprehensive standard is the Guidelines for Drinking-water Quality (GDWQ), published by the World Health Organization (WHO) since 1958 with the latest edition being the Guidelines for drinking-water quality, 4th edition, incorporating the 1st addendum [32]. The guidelines divide the contaminants in drinking water into four categories;
Microbial aspects
The biggest health risk associated with drinking water is infectious disease resulting from the exposure to microbial pathogens. These pathogens include bacteria, viruses and parasites (helminths and protozoa) and the exposure could be through ingestion, inhalation or contact with water droplets. The presence of these pathogens is generally due to contamination with excreta from humans and/or animals. Some contamination pathways include poor choice of source water and insufficient treatment of the water, low water pressure and leaks or contamination during storage.
Chemical aspects
Radiological aspects
The risks resulting from exposure to radionuclides in drinking water are very small compared to the risks from microbial pathogens and chemicals. Drinking water normally contains a number of different naturally occurring radionuclides resulting from the decay chains of thorium and uranium along with potassium-40. These daughter products include radon-222, radium-226, radium-228, uranium-234, uranium-238 and lead-210. Additional artificial radionuclides from human activity may be the result of discharge from nuclear fuel facilities, nuclear power stations or production for medical and industrial purposes. There is no difference in risk assessment between naturally occurring radionuclides and artificial radionuclides but there is a difference in risk management as it is generally easier to control the contamination pathways of human-made radionuclides. The radiation dose from ingesting radioactive substances in drinking water is of little to no consequence, except in exceptional circumstances. Nevertheless, it is recommended that drinking water is screened for radioactive substances.
Acceptability aspects
Water that is unacceptable in terms of odour, taste and appearance may cause consumers to turn to water sources that are not as safe. Discoloration or unpleasant taste could indicate problems but are not reliable indicators of how safe the water is for consumption. Some contaminants affect the acceptability but have no specified guideline, see Guidelines for sodium and chloride
concentrations in drinking water in subsubsection 1.2.2. Two noteworthy examples are sodium
and chloride, both of which have no health-based guidelines, see Table 3. The taste threshold of both depends on temperature and the specific cation-anion pair. For the sodium-chloride pair at room temperature the taste threshold is around 200 mg/l.
1.3.2 Drinking water quality standards around the world
Implementation of water quality standards vary from loose recommendations to strict legislation that dictates standards, monitoring procedures and enforcement of drinking water quality. Some examples from around the world are detailed below:
USA Under the Safe Drinking Water Act (SDWA) of 1974, the US Environmental Protection
Europe In the EU, the European Council and the European Commission issue directives that
the member states must adhere to, but exactly how the directives are implemented is up to each individual country. A country may choose to implement stricter versions of these directives. The European council established minimum requirements on the quality of drinking water through the Council Directive 98/83/EC of 3 November 1998 on the quality
of water intended for human consumption [41] which has since been updated several times,
most recently through the Commission Directive (EU) 2015/1787 of 6 October 2015 [42]. The European Atomic Energy Community (EURATOM) has issued the Council Directive
2013/51/Euratom of 22 October 2013 regarding radioactive substances in drinking water.
Sweden The above European directives have been implemented through the Livsmedelsverkets
föreskrifter (SLVFS 2001:30) om dricksvatten [34].
China The WHO GDWQ were implemented on July 1, 2012. The implementation made China
the first developing country to adopt strict regulation of drinking water quality and the number of regulated items now exceed those of most developed nations. Meeting the standards may prove a challenge with some areas being severely polluted (including some high-risk but non-regulated chemicals) or lacking in water sanitation infrastructure [43].
India The Ministry of Drinking Water and Sanitation (MDWS) first established the Indian
Standard, Drinking Water - Specification (Second Revision) in 1983 which has since
been updated several times. The standard is based on the EU directives 80/778/EEC and 98/83/EC, the US EPA standard EPA 816-F-02-013 and WHO GDWQ 3rd Edition together with the Indian Manual on Water Supply and Treatment, third edition. India is struggling with water quality and supply as they develop infrastructure with the aim of providing every person in rural India with adequate safe water for drinking, cooking and other domestic basic needs on a sustainable basis through the National Rural Drinking Water Programme (NRDWP) [44].
The WHO has produced a comprehensive overview of the drinking water quality standards of 104 countries with a total population of 6.5 billion, approximately 90 % of the world population at the time, called A global overview of national regulations and standards for drinking-water
1.3.3 Providing safe drinking water through desalination
In order to make water fit for human consumption, all of the water quality aspects must be improved by removing the contaminants in the source water until it meets guideline levels. This includes organic and inorganic chemicals as well as microbial pathogens. Desalination is a group of methods designed to remove all types of contaminants, including suspended solids, from the source water. Different desalination technologies have different advantages depending on the salinity of the source water and the desired end product. This is covered in subsection 1.6. Salinity here refers to all salts, e.g. magnesium and sulphate salts, and not just sodium chloride. Both distillation and membrane type desalination methods are capable of removing all contaminants, with the possible exception of organobromides and boron for membrane processes.
1.4
Water scarcity
When an individual lacks access to safe water at an affordable price to satisfy the needs for drinking, washing and livelihood, that individual is referred to as water insecure. When the same lack of access applies to a large group of people for an extended period of time, the area is called water scarce.
A number of different indicators have been proposed in order to quantify and define water scarcity, e.g. the Baseline water stress mentioned previously. Falkenmark, Lundqvist, and Widstrand [45] proposed the Falkenmark indicator, or ”water stress index”, with 1700 m3 of
renewable water resources per capita per year as the threshold, based on household, agricultural, industrial, energy sector and environmental requirements. It is noteworthy that water demand for food production (estimated at 1200 m3 per person per year) is roughly 70 times higher than
the basic per capita household water use of 18.2 m3 per year [46]. Countries that are unable to
sustainably supply this amount are said to experience water stress. A country is experiencing
water scarcity when supply falls below 1000 m3 and absolute scarcity when supply falls below
500 m3.
1.4.1 Water in conflicts
Water is also a tool and a catalyst for conflict. Conflicts involving water are plentiful throughout history and there are several present-day examples [47, 48]. When water supply becomes more uncertain it is likely that disputes will become more common [49, 50]. The Pacific Institute currently lists 551 water conflicts throughout history in their Water Conflict Chronology database [51], see Figure 1.5. Unintentional or accidental adverse impacts arising from water management decisions are not included in the database, which does include threats of violence or acts of violence involving water, using the following definitions:
Trigger: Water as a trigger or root cause of conflict, where there is a dispute over the control of
water or water systems or where economic or physical access to water, or scarcity of water, triggers violence.
Weapon: Water as a weapon of conflict, where water resources, or water systems themselves,
are used as a tool or weapon in a violent conflict.
Casualty: Water resources or water systems as a casualty of conflict, where water resources, or
water systems, are intentional or incidental casualties or targets of violence.
Defence administrations around the world have started paying more serious attention to climate change and water scarcity. One example is the U.S. Department of Defense, stating that:
DoD recognizes the reality of climate change and the significant risk it poses to U.S.
interests globally. [52]
with the addition that
Case studies indicate that in addition to exacerbating existing risks from other factors (e.g., social, economic, and political fault lines), climate-induced stress can generate new vulnerabilities (e.g., water scarcity) and thus contribute to instability and conflict
even in situations not previously considered at risk. [52]
It should certainly be mentioned that although disputes and conflicts involving water continue to exist, most interactions are cooperative and for every one dispute or conflict there are more than two cooperative interactions [48]. Water is generally considered too cheap a commodity (and too difficult to successfully store and deliver) to go to war over [49], an idea best conveyed by a quote from the previous Major-General of the Israeli Defence Force:
Why go to war over water? For the price of 1 week’s fighting, you could build five desalination plants. No loss of life, no international pressure, and a reliable supply you don’t have to defend in hostile territory — Avraham Tamir [53, chapter 2] Even while international war over water is unlikely, local water conflicts and social unrest may well result from unresolved water scarcity [54]. Especially as for the first time in human history, freshwater scarcity has reached a level where it will potentially limit food production [8].
1.5
Water losses through leakage
Drinking water is distributed to consumers through distribution systems. These water mains networks are made from concrete, steel and plastic piping and span enormous distances. The mains network in Europe alone spans 4 225 527 km, enough to cover the distance to the moon and back more than five times [55].
(a) NRW in percent of total water produced [62] (b) NRW in m3/(km d) [63]
Figure 1.6. Global map of Non Revenue Water
extraordinary differences between countries, see Figure 1.6. The percentage of NRW of total water produced varies from a low of 3.75 % in Singapore [57] to a high of 89.13 % in Kiribati [58] and water lost per kilometre varies from a low of 0.48 m3/(km d) in Niue [59]to a high of
426.18 m3/(km d) in Cameroon [60]. These statistics should be considered mostly as an indication
of trends and not be interpreted as exact measurements. Many utilities have not reported any statistics and some have even reported large negative losses (e.g. in Bosnia and Herzegovina [61]), severely skewing the average. In Europe the mean average NRW is 23 % and 2171 m3/(km d)
[55].
Water losses through leakage have been modelled experimentally as a function of the average pressure in the pipe, the length of the pipe and two leakage parameters that are calibrated for different pipe characteristics [64]. In this model, the leakage is directly proportional to the length of pipe.
1.6
Desalination processes
Desalination refers to the wide range of processes designed to remove salts from water by using energy and saline feedwater as input to output freshwater and brine, see Figure 1.7. All of the processes require differing degrees of pretreatment and posttreament of the water. They can be divided into groups based on phase change of the feedwater and type of energy used, see Table 4.
Figure 1.7. Schematic and definition of the desalination process.
change of the feedwater. The energy required for this phase change is called the latent heat (or latent enthalpy) of vaporisation. This increased energy consumption contributes to a higher cost since energy cost is a considerable fraction of the final product cost [65].
Table 4
Overview of different types of desalination.
Type Desalination process Type of energy used Phase change Multiple-Effect Distillation (MED) Thermal
Multi-Stage Flash distillation (MSF) Thermal Geothermal desalination Thermal Solar humidification-dehumidification (HDH) Thermal Multiple Effect Humidification (MEH) Thermal
Seawater Greenhouse Thermal
Vapour Compression (VC) Mechanical/Thermal Freezing desalination Electrical
No phase change Electrodyalisis Reversal (EDR) Electrical Forward Osmosis (FO) Electrical
Ion exchange Electrical
Before 2000, the majority of desalination installations used MSF [65] but it has since been overtaken by RO, see Figure 1.8, which now provides about 60% of installed capacity.[66]. Each of the three technologies; MSF, MED and RO are described in more detail in the following subsections.
Figure 1.8. Cumulative installed desalination capacity from 1970 to 2011 [66].
1.6.1 Multiple-stage flash distillation (MSF)
Figure 1.9. Overview of the MSF distillation process [67].
great simplicity. It requires comparatively minimal pretreatment, is suitable for large capacity installations and can handle very dirty and high salinity water while still producing high quality water with Total Dissolved Solids (TDS) below 25 ppm [65]. The major weaknesses of this process are high capital cost, high energy consumption and high ratio of feedwater to product water flow compared to RO. The Gain Output Ratio (GOR) of MSF is generally better than that of MED [65]. Gain output ratio is a measure of kilogram (or mass) of distilled water produced per kilogram of steam consumed.
1.6.2 Multiple-effect distillation (MED)
Figure 1.10. Overview of the MED distillation process [67].
Although slightly more complex than MSF, the MED process is still a simple process with high robustness. Due to lower brine temperature, MED has a lower energy consumption and is less prone to scaling, requiring little if any pretreatment of seawater. It tolerates large variations in feedwater quality and produces high quality product water with TDS below 25 ppm. Compared to MSF the capital cost of MED is lower and although the overall GOR is also slightly lower, the GOR at a given steam temperature is much higher for MED than for MSF. The most important weakness is the low capacity per unit and the difficulties with scaling installations to fit different needs.
1.6.3 Reverse Osmosis (RO)
Figure 1.11. Osmosis in a U-shaped tube [68].
Osmosis is the movement of a solvent across a semi-permeable membrane to a region with a solution of higher solute concentration. Osmotic pressure in turn is a measure of the tendency of the solution to take in more solvent, see Figure 1.11. Osmotic equilibrium is reached if a pressure is applied to the solution such that the flow through the membrane
Figure 1.12. Overview of the RO distillation process [67].
In a typical RO desalination plant the seawater is pressurised to between 50 and 80 bar. The water exiting the membranes still carries with it significant energy which is partially recovered through an Energy Recovery Device (ERD), and may provide around 25 % energy savings. In an RO application it is typically mechanically coupled to a booster pump to increase the pressure of the feed water before the main pump. More common ERDs today are different types of pressure exchangers which transfer energy hydraulically from the brine concentrate to the feed water. Pressure exchangers cause some minimal mixing of brine concentrate and feedwater, thus slightly increasing the salinity of the feedwater, but have a very high efficiency with modern devices usually having an efficiency of over 95 % [69].
Heavy pretreatment of feedwater is required in the RO process to avoid fouling and scaling of the RO membranes. Pretreatment steps include a screening process, addition of coagulant (as a clarifying agent through flocculation), dissolved air flotation, hard coal and sand filtration and ultra-fine filtration. The feedwater is then pumped through RO membranes at a pressure of between 50-80 bar. The RO membranes are arranged in modules and in many plants the water goes through two separate RO passes. Posttreatment of the permeate includes removal of boron, remineralisation and disinfection.
1.6.4 Hybrid desalination processes
Different configurations of desalination processes are often used together in hybrid systems to leverage the strengths of each system and improve efficiency and reduce cost of product water. Most hybrid systems use a combination of RO with either MSF or MED pretreatment. There are two main advantages of this approach. Firstly, product water from RO has a relatively high salinity content and can be diluted with distilled water from MED or MSF to increase flexibility of water quality to meet the needs of various customers (from distilled to irrigation quality). Hybrid processes also have greater flexibility in terms of feedwater quality. Secondly, increased feedwater temperature improves feedwater flux through RO membranes by 2% to 3% per 1◦C, although this has the disadvantage of increasing salinity of the product water by roughly 1.25% [70]. Most membranes have an upper temperature limit of 45◦C.
1.7
The Energy-Water Nexus
Producing and delivering water requires large amounts of energy. Water in turn is used in every part from resource extraction, to processing, extraction and generation of energy. This inseparable link between energy and water is referred to as the energy-water nexus.
1.7.1 Water use in energy and electricity production
Water is used at all stages throughout the energy production process with most of the water used as working fluid for electricity generation in thermoelectric power plants. In Europe, thermoelectric generation was responsible for 21% of total water withdrawn in 2014 [71]. In the year 2000, thermoelectric power production was responsible for 39 percent of fresh water withdrawals in the U.S., a number that has been stable since 1985 [72]. Together with agricultural irrigation withdrawals that are roughly equal, these two sectors now account for the vast majority of fresh water withdrawals [73]. The withdrawals for agriculture irrigation make up an even larger percentage in developing nations, reaching over 90% of total fresh water withdrawals in the least developed nations [73]. The connection to agriculture for food production is referred to as the energy-water-food nexus. Non-food agriculture water withdrawals are also significant.
1.7.2 Energy use in desalination
Desalination of water is an energy-intensive process. The energy required in the desalination processes is commonly referred to as Specific Energy Consumption (SEC) which is the amount of energy consumed to produce one unit of product water. In SI units this is presented in kWh/m3.
The SEC can be broken down into a thermal and an electrical component. Further, the SEC depends heavily on local conditions but the average SEC of each desalination method displays a clear trend, see Table 5, with RO consuming considerably less energy than other desalination methods. The data presented in Table 5 is from 2010 but newer data confirms the trend, with a reported SEC for large scale thermal plants between 5 and 16 kWh/m3 compared to a reported
SEC of between 2 and 4 kWh/m3 for large scale RO plants [69].
While the table presents typical unit sizes, it is in no way indicative of the smallest or largest installations. Desalination installations using RO range from small installations producing around 100 m3/d to large installations, with the Sorek desalination plant in Israel currently being
the largest RO facility, producing 624 000 m3/d [74]. The largest desalination plant in the world
at the moment is the Ras Al Khair plant in Saudi Arabia which uses a hybrid MSF-RO process to produce 1 000 036 m3/d [75].
Table 5
Overview of specific energy consumption of desalination processes (data from 2010, [76]).
MSF MED RO
Typical unit size [m3/d] 50 000 - 70 000 5000 - 15 000 24 000
Electrical SEC [kW h/m3] 4 - 6 1.5 - 2.5 3 - 5.5
Thermal SEC [kJ/kg] 190 - 390 230 - 390
-Gain Output Ratio 12.2 - 6 10 - 6
-Electrical equivalent1SEC [kW h/m3] 9.5 - 19.5 5 - 8.5
-Total equivalent SEC [kW h/m3] 13.5 - 25.5 6.5 - 11 3 - 5.5
1The electrical energy which is not generated as a result of the extraction of heating steam.
A large part of the total energy consumed in the desalination process may be accounted for by the the work done to overcome different losses, e.g. pressure losses in membrane modules. In order to improve the sustainability and affordability of desalinated water, these losses should be minimised. There is however a fundamental thermodynamic minimum energy consumption associated with desalinating water. This is the minimum energy required to separate salt and water in a theoretically ideal process and can be used to define (see Appendix A for definition) a Carnot-type efficiency of any desalination process
ηE= Ethermodynamic,min
Ethermodynamic,min+ W
(1) where
ηE= energy efficiency.
W = thermodynamic work [J] (or kWh/m3when describing SEC for RO).
The thermodynamic work is the work done in order to overcome losses to provide the necessary energy to separate the salt-water mixture into salt and water. In a theoretically ideal process with no losses, there would be no extra work required in addition to the thermodynamic minimum energy and the process would have a perfect efficiency of 1 (or 100 %).
1.7.3 Current state of nuclear desalination
There are an estimated 20 000 desalination plants operating in the world, producing more than 87 million m3of fresh water every day, making up only around one percent of world drinking water.
However, over the next five years an estimated 5.7 million m3/d of new production capacity
will be added with capacity projected to double by 2030 [16]. When desalination plants use heat or electricity from commercial nuclear power plants it is referred to as nuclear desalination. Currently, there are less than 20 nuclear desalination plants (excluding marine reactors that provide nuclear desalination), representing less than 0.1 % of global desalination capacity [78].
Despite these discouraging numbers, nuclear power and nuclear desalination are very mature technologies. There is over 17 000 reactor-years of experience operating commercial nuclear reactors in more than 30 countries and around 200 reactor-years of experience operating different types of nuclear reactors for commercial nuclear desalination [12]. Additionally, there is 13 000 reactor-years of experience operating some 180 nuclear reactors for marine propulsion and on-board desalination on 140 ships (most being submarines) [79].
reactor BN-350 near Aktau on the Caspian Sea peninsula in Kazakhstan provided a capacity of 135 MW of electricity and 120 000 m3/d of drinking water for 27 years before it was shut
down in 1999. That experience is unique because Aktau was a fast neutron reactor and the production capacity was orders of magnitude higher than most other desalination facilities, including conventional desalination plants [12]. There are also operational nuclear desalination facilities in many other locations, for example in Pakistan at the Karachi Nuclear Power Plant (KANUPP-1), in Russia at several locations and in India at Kalpakkam and Kudankalam. In addition to this, many countries are either currently constructing or planning to construct nuclear desalination plants. These countries include Algeria, Argentina, China, Egypt, France, India, Indonesia, Japan, Kuwait, Republic of Korea, Libya, Pakistan, Russia, Saudi Arabia, South Africa and the United States [80]. Ongoing research and development efforts include off-the-shelf floating nuclear desalination units from China and Russia [81, 82] and Jordan has been planning the construction of four reactors for nuclear desalination. The first reactor will be constructed earlier along the coast and the remaining three when the Red-Dead Project (a project to replenish the falling water level in the Dead Sea) is up and running by 2025 [83].
1.8
Small Modular Reactors, SMR
The acronym SMR is confusingly used to both refer to Small and Medium Sized Reactors [84] and Small Modular Reactors [85]. The IAEA defines ”small” as reactors with an output under 300 MWe and ”medium” as up to around 700 MWe. In this paper the acronym SMR will refer to Small Modular Reactors, meaning reactors assembled on site from parts manufactured in serial production in factories. This paper also keeps the IAEA definition of small reactors having an output less than 300 MWe. This includes a subcategory of very small reactors, vSMR, with an output below 15 MWe proposed for remote communities and other off-grid applications.
1.8.1 Overview of SMR designs
2
Project description
This section outlines the underlying motivation and describes the problem. The research method and and the proposed solution are presented along with delimitations as well as the purpose and goal of the thesis work.
2.1
Problem statement
The world is experiencing a significant lack of access to safe drinking water. Desalination of seawater provides a viable option of mitigating water scarcity but requires large amounts of energy. At the same time the world is faced with an ever increasing demand for energy and the pressure of reducing the associated carbon footprint of that energy.
Large scale nuclear power plants provide affordable energy with almost no emissions. Lately however, large scale nuclear projects in the western world have been plagued by long construction times, exceeded budgets and increased capital costs. Meanwhile, a new category of reactors called Small Modular Reactors is experiencing a renaissance moment with innovation at an all time high. The smaller reactors promise short construction times, low cost and the ability to tailor installations to a wide range of energy requirements.
2.2 Project method
The overall method consists of an extensive literature review, the development of a novel concept solution and the evaluation of the proposed solution.
2.2.1 Literature review
A comprehensive literature review was first conducted in order to assess the fundamental aspects of the problem as well as to obtain detailed information regarding underlying theory and operation of the many systems involved.
The first objective was to investigate whether desalination is a feasible and practical solution to reduce water scarcity. The research identifies water scarcity as a complex problem that will require addressing the fundamental causes but also indicates that desalination is a worthwhile endeavour.
The second objective was to obtain information about desalination technology and processes. It was determined that Reverse Osmosis is the most commonly used process and also makes up the majority of new installations. Reverse Osmosis also has potential for further cost reduction and improvements of energy efficiency.
The third objective was to acquire detailed information and performance data for the drive coupling and powering of feed water pumps for RO desalination plants. The gathered information suggests that there is a potential for a direct mechanical drive coupling between steam turbine and feed water pump to offer meaningful reductions of energy consumption.
Lastly, possible additional advantages and disadvantages of utilising Small Modular Reactors to power the steam turbines were investigated. These are not easily quantifiable but suggest that there are several tangible advantages that support the feasibility of providing SMR powered desalination. In addition to this there are also concerns that, while not critical, will need to be addressed.
2.2.2 Concept proposal
to electrical energy in the generator. This electrical energy is then transmitted to the electric motor which converts it into mechanical work by rotating a feed water pump. All of the energy conversion processes are thermodynamically imperfect and associated with an efficiency less than 100 %. This results in a loss of energy in every step of the energy transformation from beginning to end.
The underlying assumption of this report is that it should be possible to reduce the energy losses by providing a more direct form of energy transfer from the producer to the final consumer. The literature review revealed that direct mechanical drive of feed water pumps from a steam turbine is commonplace in many industries, for example coal power plants. All power plants consume some fraction of their gross power output for various auxiliary purposes and internal electricity demand. For a medium size power plant this internal electricity demand may amount to 10 % of gross power output [86] with half (5 % of total) due to feed water pump electricity consumption [87]. For this reason, steam turbine mechanical drive is used to power the feed water pumps, reducing internal electricity demand and improving overall energy efficiency. No example of steam turbine mechanical drive or other direct mechanical drive of feed water pumps for desalination was found during the literature review.
This report examines the feasibility of applying the concept of mechanically coupled steam driven feed water pumps to desalination processes and powering the steam turbine using Small Modular Reactors.
2.2.3 Evaluation of the proposed concept
The proposed concept should be evaluated and compared to existing desalination processes. This requires the computation of performance parameters and efficiency of the proposed concept. The resulting performance parameters should be consistent with those used in the desalination field to facilitate the comparison between different processes.
Environmental and health effects should also be evaluated.
2.3
Delimitations
Desalination and nuclear power are broad scientific subjects and therefore the author has introduced several delimitations of the scope of the study. These are
Hybrid Desalination processes Hybrid processes show great potential for improvements, but
Small Modular Reactors (SMR) In this paper the acronym SMR will refer to Small Modular
Reactors, meaning reactors assembled on site from parts manufactured in serial production in factories. This paper also keeps the IAEA definition of small reactors having an output less than 300 MWe. This also includes a subcategory of very small reactors, vSMR, with output under 15 MWe proposed for remote communities and other off-grid applications.
Environmental impact of desalination Desalination produces a waste stream consisting of
brine with concentrated levels of dissolved solids (e.g. salts and heavy metals) and an elevated temperature. The direct discharge of this waste stream into the environment carries with it a potential for negative environmental impact. There are research efforts dedicated to mitigating the negative effects on the environment and to the extraction of materials such as uranium from the waste stream. However, this is not within the scope of this thesis.
Desalination plant configuration The exact configuration of a desalination plant has
implications for the environmental impact, energy consumption and total cost. This involves decisions regarding seawater intake, brine discharge and the pretreatment system. While this is an important topic, it is not within the scope of this thesis.
Brackish water desalination Because seawater is so abundant and around half of the world’s
population lives within 100 km of the coast, only seawater desalination will be considered within the scope of this thesis. Mentions of desalination or RO therefore refer to seawater desalination and SeaWater Reverse Osmosis (SWRO).
Optimisation The optimisation of plant configuration, steam turbine drive configuration and
steam turbine design as well as any other optimisation is not considered within the scope of this project.
2.4
Purpose and goal
The purpose of this report is to present the findings of the literature review as well as the development and evaluation of the proposed concept.
Concept Development and Evaluation
3
Concept development
3.1
Economics and energy consumption of desalination
Reducing the energy consumption is important for several reasons. Primarily, it reduces the carbon footprint and overall energy demand. Secondly, energy makes up a significant part of the total water cost for desalination, see Figure 3.1, and therefore reducing the energy consumption also means reducing the resulting product water cost [65, 69, 88].
Figure 3.1. Breakdown of annual operating costs of seawater Reverse Osmosis [88].
However, energy consumption in desalination processes is already approaching the theoretical thermodynamic minimum energy (introduced in subsubsection 1.7.2) with new designs for optimised desalination plants having a reported energy consumption of around 2 kWh/m3. A
breakdown of the SEC for such an optimised desalination plant is presented in Figure 3.2. Energy is required to pressurise the feed water and to overcome friction and other losses. Almost half of the total specific energy consumption of 2.12 kWh/m3 consists of the thermodynamic
minimum energy. Thus, a reduction of energy consumption of seawater is theoretically limited to
50 % relative to the most energy-efficient RO designs of today. It should be noted that most installed RO desalination plants operate with a considerably higher energy consumption of between 3 kWh/m3 and 5.5 kWh/m3, see Table 5 in subsubsection 1.7.2.
While Figure 3.2 provides a useful perspective on the impact of the thermodynamic minimum energy on potential reductions in energy consumption, it does not adequately represent the energy consumption of different components. Pump losses are accounted for in Esystem but the total
Figure 3.2. Breakdown of energy consumption of the SWRO process [89]. Total specific energy consumption is 2.12 kWh/m3.
The actual energy use of the high pressure pumps makes up around 80 % of total operating and maintenance energy consumption for a large seawater RO desalination plant using a partial second stage process, see Figure 3.3. Since an investment in a more expensive but more energy efficient pump will quickly pay for itself through energy savings, pump manufacturers have had a big incentive to develop more energy efficient pumps. Large pumps today may have an efficiency of 90 %, with little room for improvement [69, 90, 91].
Figure 3.3. Breakdown of energy consumption in a large SWRO desalination plant using a partial
second stage [92].
most analyses, this restriction is set to the RO system itself, see the shaded box in Figure 3.4. This means that losses in the coupling between power plant and desalination plant are not considered. This is despite the fact that many desalination plants, especially larger ones, are co-located with power plants, referred to as an Independent Power Producer (IPP).
Figure 3.4. Schematic overview of the RO process. ηi is the energy efficiency of the previous stage [69].
To conclude, energy consumption makes up about 58 % of the operating costs of RO desalination of seawater. High pressure pumps for the first RO pass alone can make up around 80 % of the energy consumption. But pumps are already very effective with little room for improvement.
3.2 High-pressure pumps, couplings and drivers
An RO desalination plant is a very complex system. The main components are the pump driver, the pump, the RO system and the energy recovery device, see Figure 3.5. The energy consumption depends on product flow, operating pressure, recovery ratio and the individual component efficiency.
Figure 3.5 shows the various different drivers that can be chosen to power the high pressure pumps. Depending on the driver and pump requirements, a suitable coupling device will be chosen. Choice of energy recovery device will also influence the choice of pump driver and coupling as the output of the energy recovery device can be either hydraulic work or shaft work. Diesel engines and gas turbines can also be powered by bio-fuels but the scalability and sustainability using current production methods is debatable.
Figure 3.5. Schematic of the main components of the RO desalination process.
The different components and the performance in different stages is highly interdependent on the rest of the desalination process. The selection of components and design of a plant are also influenced by the local conditions and water demand. Electricity price or existing availability of steam will influence the selection of driver and determine the coupling device and energy recovery device. Driver, coupling and pump efficiency also depend heavily on size and will influence the decision making process.
3.2.1 High-pressure pumps
High pressure pumps for desalination are typically centrifugal pumps. The overall efficiency
ηpump,tot is the product of the volumetric efficiency ηvol, the hydraulic efficiency ηhyd and the
mechanical efficiency ηmech;
ηpump,tot= ηvolηhydηmech (2)
and typically varies from around 50 % for small pumps to around 90 % for large pumps [93]. Driver and coupling do not significantly affect the overall efficiency of the pump itself but will influence the size, which in turn determines efficiency. Traditionally, smaller size high pressure pumps have been used for individual RO trains but there is a trend towards large centrifugal pumps serving multiple RO trains. The total efficiency (including electric motor, meaning ratio of electrical power input to mechanical power output) of pumps dedicated to single RO trains is around 80 % to 83 % but changing the configuration so that each pump serves two RO trains can increase efficiency to 85 %. This approach has been greatly optimised through the three-centre RO system design that uses a dedicated pumping centre, membrane centre and energy recovery centre, see Figure 3.6. Total pump efficiency for such an installation at the Ashkelon plant in Israel has shown a practical efficiency of 90 %, currently exceeding the guaranteed long term efficiency of 88 % [91]. For small RO installations with a fresh water production capacity of 950 m3/(km d) (250 000 US gallons per day) or less there is a trend toward using multiple-piston
positive displacement high pressure pumps that can reach a total efficiency between 94 % and 97 %. These units often incorporate the high pressure pump and energy recovery device into a single unit so the comparison to centrifugal pump efficiency is not perfect.
3.2.2 Pump drivers
A driver is needed in order to power the pump. Variations in cost, availability and reliability of the energy supply to the driver are important factors in the choice of driver.
Electric motors
Electric motors are simple and reliable drivers for pumps. The efficiency of an electric motor is the ratio of mechanical power output to electrical power input and primarily depends on the size of the motor. Small motors may have an efficiency of around 60 % while large industrial motors have an efficiency of around 96 % [90, 94]. Large motors are almost exclusively alternating current (AC) motors and therefore inherently constant speed drives by design. This means that the efficiency varies greatly with load and decreases as the load deviates from the rated load. Pump motors need to be oversized initially to compensate for reduced efficiency of the pump over time as it is worn from use.
Steam turbines
Steam turbines provide a large amount of power at variable speed with high efficiency and reliability. Multi-stage condensing turbines are typically used in applications such as electricity generation, especially with steam that is nearly saturated as is the case for typical nuclear power plants. The steam is reheated between turbine stages to improve efficiency and reduce damage to turbine blades from water droplets in low quality steam. The overall efficiency of a steam turbine is the product of the internal thermodynamic efficiency and the mechanical efficiency. This should not be confused with electrical generating efficiency which is commonly quoted for steam turbines but refers to the ratio of net electric power generated to total fuel input into the cycle. The internal thermodynamic efficiency of a condensing multi-stage steam turbine is typically around 65 % for small turbines up to over 90 % for large turbines used by industries and utilities [95]. Mechanical efficiency is typically around 95 % [96] to 98 % [97]. The resulting overall efficiency of a large steam turbine is around 85 %.
Diesel engines and gas turbines
Diesel engines are very efficient combustion engines that can run on a variety of fuels and produce high torque but run at low speeds. High maintenance costs have prevented the wide-spread use of diesel engines as drivers for pumps.
Both gas turbines and diesel engines can be run on bio-fuels and gas turbines fuelled by fossil fuels are used today in large scale desalination plants. Nevertheless, it is likely that fossil fuel use will see increasing costs in the future, e.g. in the form of a carbon tax or regulations relating to carbon capture and environmental concerns.
3.2.3 Driver-pump couplings
All systems require a mechanical coupling between the driver and the pump. The coupling transfers energy to the pump and enables speed matching between the driver and the pump. The coupling can also be designed to allow for variable-speed operation to improve driver efficiency under variable pump flow and operating pressure, especially in the case of constant speed drivers such as electric motors.
A constant speed coupling operates at a fixed gear ratio to overcome speed differences between the driver and the pump. For example, diesel engines require speed matching as the engine may operate around 400 rpm with the pump operating around 3600 rpm. The constant speed coupling can be either a direct mechanical coupling or a belt transmission coupling. Belt transmissions are very common in industrial applications of electric motors as they allow for easier disassembling and flexible positioning of the driver in relation to the pump. Direct mechanical couplings operate with a high efficiency of over 99 %. Belt drives exist in many variations with efficiency varying from between 90 % and 98 % for V-belt drives up to between 97 % and 99 % for synchronous belts and chain drives [98].
Steam and gas turbines can be designed to operate at similar speeds of the pump, removing the need for a speed matching coupling. Instead, the driver and pump may be directly mechanically coupled.
Variable loads can also be managed with a fluid coupling. They are commonly used in automobile transmissions. Fluid couplings can be divided into four different categories; hydrokinetic, hydrodynamic, hydroviscous and hydrostatic. The efficiency of fluid couplings is affected by two types of losses, circulation losses and slip losses. Circulation losses include internal friction losses and the power required to drive the oil pump for cooling of the fluid coupling. The slip losses can be calculated by the slip efficiency
ηslip= noutput
ninput
(3) where
noutput= output shaft speed [rpm]. ninput= input shaft speed [rpm].
At maximum output speed (usually 98 % of input speed) the overall efficiency is around 96.5 % with 1.5 % from circulation losses and 2 % from slip losses. At very low speeds compared to maximum designed operating speed, the circulation losses can be neglected and the total efficiency can be approximated as only the slip efficiency.
3.3
Concept description
Desalination of seawater is an energy-intensive process. The energy consumption of SWRO desalination has been reducing dramatically and is fast approaching the thermodynamical limit. Still, potential areas for reduction of energy consumption do exist just outside the scope of most existing research.
Through the application of a direct mechanical coupling from a steam turbine to a high pressure pump it should be possible to further reduce energy consumption. Powering the steam turbine with an SMR could provide significant benefits over existing solutions. This is expanded on below, see subsection 3.6.
3.4 Evaluation of concept energy efficiency
High pressure pumps for desalination are currently powered by electric motors. Large desalination plants are normally constructed with an IPP at the location. This IPP typically uses gas turbines and steam turbines to produce electricity and heat. The heat is used for pretreatment of the feed water and sometimes for the MSF process if the desalination plant uses a hybrid process.
This section will provide an estimate of the comparative overall efficiency of energy transfers for a system with an electric motor driver and one with a steam turbine driver. Both plants have the same product water output and the analysis is performed from after the turbine to just before the high pressure pump. This can also be stated as, the analysis is being performed under the assumption that the turbine and pumps of both systems have the same efficiency and power output. Electricity for electric motors can also be supplied from the grid. This is also associated with losses and inefficiency but these are not a direct burden on the desalination plant but rather grid-operator or society-level considerations.
3.4.1 Conventional electric motor drive efficiency
For the electric motor drive, a generator will first generate electricity from the turbine power output. The electricity is transmitted over a (local) grid and through transformers to the electric motor. Either a VFD or fluid coupling can be used to manage variable load conditions. Both the VFD and fluid coupling perform the same task, with similar efficiency of 97 % at full load.
The overall efficiency of the electric motor drive can be estimated as
ηoverall, electric= ηgeneratorηtransmissionηmotorηcoupling (4)
where
ηgenerator= generator efficiency, estimated as 99 %. ηtransmission= transmission efficiency, estimated as 99 %. ηmotor= electric motor efficiency, estimated as 96 %.
ηcoupling= coupling efficiency, here either VFD or fluid coupling efficiency and estimated as
97 %.
3.4.2 Concept steam turbine drive efficiency
The conceptual steam turbine drive simply consists of a direct, fixed speed mechanical coupling between the turbine and high pressure pump. The overall efficiency of the energy transfer is therefore estimated as equal to the coupling efficiency,
ηoverall, ST= ηcoupling (5)
where both the estimated coupling efficiency and the resulting overall efficiency is 99 %.
3.5
Analysis of concept specific energy consumption
The fresh water output of the desalination concept was set to 200 000 m3/d. This corresponds
to a utility scale RO desalination plant and with an average domestic water demand per person of 0.4 m3/d it would supply 500 000 people with fresh water. Household water demand varies
greatly, from 0.135 m3/d in India to 0.575 m3/d in the US. In Europe the corresponding number
varies from 0.15 m3/d to 0.4 m3/d [99].
To analyse the specific energy consumption of the desalination concept the necessary pump power is first determined. Working backwards through the energy transmission the rest of the parameters can then be computed.
The selected power source for the desalination concept is a scaled up version of the 3 MW Sealer reactor from the Swedish company Leadcold (Blykalla) [100]. Information on the scaled up version has not been published at this time, but preliminary performance values are presented in Table 6.
Table 6
Preliminary values of steam turbine parameters for a scaled up∼ 55 MW
Property Value
Nominal SG power 140 MW
Steam pressure at SG outlet 150 bar Steam temperature at SG outlet 530 Steam mass flow rate 76 kg/s
Generator output 58 MW
Overall cycle conversion efficiency >40 %
Water properties were chosen to be the same as those used by Shrivastava, Rosenberg, and Peery [101] in their calculations, see Appendix A. The osmotic pressure increases gradually as more product water permeates the membranes, thus leaving behind a higher salinity concentrate and subsequently the osmotic pressure at the desired recovery is higher than the osmotic pressure of the feed water, Πrecovery > Πfeed, see Figure A.1. Appendix B provides a brief explanation of
