Thermal Desalination Systems: From Traditionality to Modernity and Development

Abstract

As well known, the basic birthrights of human are the clean air, clean water, healthy food, and green energy. So, clean water is the second important requested need of all living organisms on Earth. To know the importance of water to our human bodies, a deficiency of just 2% in our body’s water supply indicates dehydration. Nowadays, all countries suffer from the problem of freshwater shortage. Despite the importance of clean water for our lives, only 0.01% is available as surface water such as the rivers, lakes, and swamps. These frightening facts have made it a national and humanitarian duty for scientists to research how to overcome the water problem and how to provide alternative sources of safe drinking water using renewable energies. Desalination is the most famous and operative technique used to overcome this problem. In this chapter, the different desalination techniques are reviewed and reported. Also, the solar distillation processes are mentioned with an extended review on the solar distillers. Besides, the application of artificial intelligence in improving the performance of desalination systems is reported. The main conclusions are stated at the end of this chapter.

1. Introduction

Any human being needs the water as the second most important fluid after air to be able to live on the Earth. Actually, around two-thirds of the Earth is covered by water (~ 71%), but more than 7% of this water cannot be used because they are in the form of ocean, ice caps, glaciers, ground, and aquifers [1]. Therefore, the freshwater that is available to use by the people is only around 1% all over the globe. So, the need for desalinated freshwater arises day by day [2]. In addition, the applications of freshwater such as cooking, drinking, and farming make it in a difficult situation. As a result, providing safe drinking water is a major challenge all over the world [3, 4]. The predicted freshwater shortage problem in 2030 is illustrated in Figure 1.

Figure 1.

Estimated global water scarcity in 2030 [5].

The simplest well-known cycle for the water is the hydrological cycle, which had sequentially the steps of water surface evaporation, condensation, cloud creation, runoff, and rain. So, the water controls the ecosystem of any society [6, 7]. This is because the potable water shortage is a problem of both the remote and urban communities [8].

The thermal desalination processes follow the same principles of the natural hydrological cycle, but they use huge energy amounts. Distillation is accomplished by introducing saltwater into the process, which produces two output streams: one of freshwater and the other of brine water. Distillation of seawater produces freshwater [9, 10, 11, 12, 13].

Distillation becomes a key source of freshwater for most of the world’s regions. Because of the ample water, the distillation procedure is mostly considered in coastal places. The most significant feature of this procedure is that it is completely safe for everyone involved—in other words, it has no negative consequences for the environment [14]. According to a survey conducted in the preceding decade, roughly 75 million people around the world rely on distillation for their everyday requirements. Distillation is the only source of freshwater for many countries. Saudi Arabia, the United States, the United Arab Emirates, Spain, and Kuwait are the top five countries in terms of desalination plant capacity, with 17.4, 16.2, 14.7, 6.4, and 5.8%, respectively [15]. The capacity of distillation processes for various countries is depicted in Figure 2; for (a) the globe, (b) USA, and (c) Middle East countries in 2002.

Figure 2.

Capacity of distillation processes for (a) the globe, (b) USA, an (c) Middle East countries in 2002 [16].

Till 2015, the total capacity of desalinated freshwater is 86.55 million cubic meter a day, which were obtained from around 18,000 plants for desalination around the globe. The Middle East and North Africa account for almost 44% of the above total capacity. Over the last 20 years, 80% of the energy utilized for freshwater producing has been lowered because of contemporary advancements in distillation technologies [16].

2. Desalination techniques

Desalination of brackish/saline water can be done in a variety of ways. Among the distillation methods, multistage flash desalination (MSFD), multi-effect distillation (MED), and reverse osmosis (RO) are considered the most commercial and economic viable technologies for distillation. These techniques are the most effective, and they will continue in the future [17]. Desalination can be done in several ways, as listed below.

2.1 Multistage flash distillation (MSFD)

MSFD is based on flashing of evaporation. Evaporation of saltwater occurs as a result of a decrease in pressure rather than a rise in temperature. Broadly speaking, regenerative heating is used to achieve a maximal production and best MSFD’s economics. The seawater flashing in the flashing chamber regenerates and transfers its thermal energy to the saltwater passing via the flashing action. Because this is a regenerative heating system, it must be completed in stages. Then, it would be better if we increased the incoming saline water temperature [18]. The condensation heat (released when condensating the water vapor) is responsible for the progressive raise in saline water temperature. The heat input, heat recovery, and heat rejection are the most important factors for MSFD unit [19].

Multistage evaporators, with roughly 19–28 stages, are utilized in recent MSFD plants [20]. The MSFD operating temperatures are between 90 and 120°C. The more the operating temperature of unit, the higher the efficiency of the system. Also, the pressure should be controlled under the water saturation temperature.

In the unit (see Figure 3), various devices or accessories such as demisters, decarbonators, and vacuum deaerators are employed for various reasons. Demisters are used to prevent the carryover of saline water into condensed distillate. Removing the dissolved gases in brine is the function of decarbonator and vacuum deaerator [22].

Figure 3.

Illustration of processes of MSFD [21].

2.2 Multiple-effect distillation (MED)

MED method is the earliest of all distillation techniques. Thermodynamically, it is the most constitutional technique among all other distillation methods. The term “effect” of the MED name refers to how many evaporators’ series. Then, the main fundamental of MED is the decrease in ambient pressure through various effects. In this process, no additional heat is required after the first effect. This is due to that MED automatically makes the saline water to be boiled many times through the corresponding pressure.

In this process, the saline water temperature is increased to its boiling value at the first effect through the heat exchanger tubes. The entering saline water is sprayed over the evaporator surface for better vapor generation. The created vapor is condensed on the other side of tubes. On the second hand, the condensation process is utilized on the same time to heat up the saline feedwater. Figure 4 illustrates the desalination with solar energy.

Figure 4.

Desalination by solar energy [2].

MED economy relies on its effects number regarding its chained processes [8]. So, the first effect has some generated vapor from the seawater entering the effect. After that, the remaining water is fed to the next effect. The amid tubes are warmed up using the first-effect created vapor. Also, that vapor is condensated to produce potable water. Besides, the generated vapor heats up the remaining saline water of the next effect. This action is repeated from one effect to another under low pressure and temperature till the end of process with around 4–21 effects [23].

2.3 Vapor compression distillation (VCD)

As it is known, raising the pressure of steam vapor leads to increase its temperature, and hence, the heat released from this vapor is increased. This concept is utilized in the VCD method, where the vapor-increased heat is utilized to evaporate the incoming saline water. So, the main key of VCD is reducing the medium pressure, which leads to decrease the boiling temperature of water [24]. This process can be achieved by either steam jet technique (thermo-compressor technique) or mechanical compressor device (electrically driven technique), and these techniques are utilized to condense vapor content into distillate and use the corresponding latent heat as a heat source for evaporating the incoming saline water.

In the technique of thermo-compressor, a venturi orifice is used in the stream of steam jet to create low pressure. After that, the vapor content is compressed via the steam jet with the help of the venturi orifice device. Therefore, the generated vapor content is condensated over the tubes’ surfaces with releasing heat to vaporize the incoming salt water.

In addition, there is another kind of VCD, which depends on lowering the temperature inside the VCD, and hence, this kind needs only power to run. This technique of VCD is useful for small desalination units due to its simple construction, better efficiency, and process reliability [25]. As a result, VCD is applicable in resorts, industries, and drilling sites.

2.4 Reverse osmosis (RO)

It is known that the water flows naturally from the freshwater direction to saltwater direction. RO depends mainly on a critical parameter called osmotic pressure, which should force the saltwater to flow in the opposite direction of natural flow. As a result, we need an external source of pressure to overcome the osmotic pressure. That external pressure must be more than the osmotic pressure. Hence, the name of this method (reverse osmosis) refers to the process meaning reversing the direction of normal water flow through the membrane. By applying this process, the salts in the saline water are left behind and not allowed to cross the membranes [26]. This process produces potable water (permeate) and brine water (concentrate) as illustrated in Figure 5. Also, Figure 5 illustrates the main components of RO unit such as the pretreatment, membranes, high-pressure pumps, and post-treatment.

Figure 5.

Schematic of RO desalination technique [16].

The pretreatment process is important for eliminating the undesirable materials that damage the membrane [27]. It relies on the membrane kind and configuration, properties of feedwater, recovery ratio, and required permeate quantity and quality. According to those parameters, the pretreatment process has different techniques to be applied such as chlorination, coagulation, acid addition, micron cartridge filtration, multimedia filtration, and dechlorination. Moreover, to overcome the osmotic pressure, high-pressure pumping system such as the centrifugal pump is utilized. Additionally, the different membrane configurations such as the spiral wound and hollow fine fiber (HFF) strongly affect the performance of the RO unit [28]. Besides, adjusting the pH and adding H₂S and CO₂ are performed in the post-treatment process [29].

2.5 Freezing

Another type of desalination processes is nominated freezing, which is simple to conduct and operate. This process depends mainly on the fact that the dissolved salts are removed while forming the ice crystals from the saline water. First, we clean and wash the saline water mixture to leave the salt in the left water behind before freezing the whole water. Then, we can get the freshwater via melting the ice crystals. As a result, freezing has the processes of saline water cooling, partial creation of ice, separation of ice from saline water, ice melting (obtaining freshwater), and finally refrigeration and heat rejection [30].

Freezing units have the merits of low power consumed, few corrosions, and eliminated scaling factors. But it has the demerits of handling water and ice mixtures because it is mechanically hard to treat. Unfortunately, this method still needs numerous improvements to be applied on the commercial level. Limited freezing stations exist all over the globe such as that was built in Saudi Arabia [31].

2.6 Solar desalination

Here, the sun energy is the driving force for such solar desalination systems, which actually are alike the natural hydrological cycle. The natural hydrological cycle takes every day in the form of heating the seawater using the solar radiation to produce vapor, and this vapor content is condensated due to the low temperature in the heights. An application on the solar distillation systems is the greenhouse solar distiller [25, 32].

2.7 Potabilization

This process is almost linked to MSFD systems. This is because MSFD produces distillate with small impurities amounts, and then, the potabilization is utilized to eliminate these impurities [33]. The potabilization process can be conducted via two different methods: injecting CO₂ and hydrated lime [34] and carbonated water is passed through limestone bed filters [35]. Potabilization has four main processes: carbonation, liming, chlorination, and aeration. The main functions of liming and carbonation are increasing the hardness, alkalinity, mineral content, and pH of the targeted water. Also, chlorination (performed via using chlorine gas or calcium hypochlorite) aims at avoiding the infected water [36], while aeration aims at replacing the oxygen inside the water to enhance its taste.

3. Desalination economics

The economics of any system determine its success. In desalination stations, the parameters of fixed cost, running costs, station location, maintenance cost, and energy consumption are the controlling factors of the station economics. There are two opposite directions for determining the economics of the desalination systems. First is the improvements in desalination systems, which leads to reduce the cost of the system as a whole. Second is the pollution and contamination of water, which raises the cost of the desalination system.

The economics and technical properties of the desalination method with the targeted quantity of freshwater are the factors based on which we select the distillation technique. The technical properties include the energy-driving source, consumed energy, freshwater specifications, land space of unit, station reliability, operational aspects, plant size, and the maintenance of spare parts, while the economic parameters include the fixed costs of the station, operating costs, interest rate, life cycle of station, and maintenance costs …etc. [37]. The cost of distilled freshwater is described by $/m³. The cost of desalinated water is determined using the following equation.

$$\mathit{Cost}\mathit{of}\mathit{desalinated}\mathit{water}=\frac{\mathit{All}\mathit{spent}\mathit{and}\mathit{estimated}\mathit{costs}\mathit{through}\mathit{station}\mathit{lifetime}}{\mathit{Total}\mathit{produced}\mathit{water}\mathit{quantity}}$$E1

4. Solar distillers

As explained before, solar distillation is one of the desalination techniques. Solar distiller is one of the solar distillation devices’ family. It is a simple in construction, cheap, and easy to maintain device [38]. But it has the demerit of few freshwater production. As a result, the scientists do their best to improve the yield of solar distillers [39, 40, 41, 42, 43, 44, 45]. The solar distiller has the parts of glazing cover, basin, collecting trough, and some instruments as shown in Figures 6 and 7. The basin is fed by saline water to be heated and vaporized inside the distiller. Then, the vapor is condensed on the inner glazing surface. After that, the condensated droplets are taken out using a hose into the calibrated flasks. The surfaces of the solar distiller are painted with black for maximizing the absorbed solar radiations. Also, the device body is insulated by saw dust or fiberglass for avoiding the thermal losses. Moreover, the measuring instruments are utilized to be able to evaluate the solar distiller performance.

Figure 6.

Single-basin distiller [46].

Figure 7.

Solar still with simple basin [40].

5. Methods of improving the solar distiller performance

One of the biggest problems of the twenty-first century is the global freshwater scarcity, which has numerous side effects on the mankind [47, 48]. The widespread of the water problem on a global level has made more impacts on the lives of people who live far from urbanization and remote areas, and more and more on the lives of the poor who do not have the costs of using high technology to produce water. As a result, the science developed various high and low water desalination technologies to be used at the level of industrial and commercial production and at the level of individuals and families [49]. The water desalination technologies can be categorized by membranes or thermal processes. Nevertheless, the high technologies demand building complex and large central installations, which causes them to be infeasible for developing regions such as distributed, poor, and remote areas. In addition, the rural, arid, and remote areas need desalination methods with no or minimum maintenance, supervising, and operating requirements [41, 45]. Consequently, the solar-powered desalination units such as solar distillers meet all these conditions, which make them as an efficient candidate to provide drinkable water in these regions [42, 43].

Nevertheless, the solar distillers have low output productivity (1.5–2.5 L/m² day) and low thermal efficiency (≈ 30%), which are the main bottlenecks of this distillation technique [50, 51, 52]. As a result, numerous investigations focused on improving the performance of solar distillers. Consequently, the solar stills such as stepped type [53, 54, 55] (the absorber of the basin takes the steps shape), disk type [56] (the main effective absorber is a rotating disk), vertical type [57] (the distiller horizontal width is very small compared to its vertical height), tubular type [58, 59, 60] (the outer shape of the distiller is tubular/cylindrical), drum type [61, 62] (the absorber is a rotating drum inside the basin still), PV/T active type [63, 64] (distiller powered by PV panels), finned type [65] (the absorber of the distiller is a collection of fins), trays type [66, 67, 68, 69] (the main effective absorber has trays to enlarge the surface area), inclined type [70] (the absorber is inclined), wick type [10, 71, 72] (the wick material is spread over the absorber of distiller), corrugated type [73, 74] (the absorber of the distiller is a collection of corrugated shapes), spherical type [75] (the distiller takes the shape of sphere), double-effect type [76, 77, 78] (the distiller has two stages of water basins), multistage type [79] (the distiller has multistages of water basins), inverted absorber type [80] (the distiller has inverted absorber inside it), hemispherical type [81] (the distiller takes the shape of sphere), convex type [82, 83] (the absorber has a convex shape), and pyramid type [84, 85, 86] (the distiller takes the form of pyramid) are found in the literature. In addition, numerous modifications were performed to improve the distiller performance. These modifications included the use of condensation unit [87, 88, 89], nanofluids [90, 91], heat exchanger [92], floating aluminum sheet [93], desiccant [94], solar ponds [95], glass cooling [44], volcanic rocks [96], wick materials [71], rotating parts [97, 98, 99], coated absorbers [100, 101], phase change materials (PCM) [102, 103, 104], fins [105], half barrel and corrugated absorbers [106], solar collectors [107], sun-tracking systems [108], multiple-effect basins [109], reflectors [110], vapor extraction [111, 112, 113], and reusing latent heat of evaporation [114].

6. Solar distiller types

The solar distillers can be classified into single-effect and multi-effect distillers (according to the number of effects of distiller) with a subcategory of active and passive distillers (according to the vaporization heat source) inside every classification [2]. In the passive distillers, the vaporization occurs directly without external sources of heat, while using external heat sources such as collectors and concentrators are used in the active solar distillers.

6.1 Single-effect distillers

This is the traditional solar distiller (or conventional distiller) without any modifications [115]. Also, it is used as a reference for the other modified distillers’ performances. Here, in this type of distillers, there is just one glass cover over the basin water. Also, the thermal losses in this type of distiller are large due to the single glass cover, which hence decreases its performance. Therefore, the efficacy of this distiller is low around 30–40% [2]. As a result, numerous experimental and empirical investigations were performed to augment the distiller performance. This type of distillers (single-effect solar distillers) is categorized into active and passive distillers.

6.1.1 Active solar distiller

The word “active” means that the solar distiller is integrated with somewhat external source of heat such as the solar concentrators and collectors [2]. Then, the active distillers include the following items:

1. Regenerative distillers

2. Air bubbled distillers

3. Waste heat recovery distillers

4. Distiller with heat exchangers

5. Distiller integrated with concentrators

6. Distiller incorporated with hybrid units

7. Distiller incorporated with heaters

Figure 8 shows the active distiller integrated with evacuated collector.

Figure 8.

Active distiller integrated with evacuated collector [116].

6.1.2 Passive solar distiller

Here, the heat source for basin water vaporization is only from inside of distiller [2]. Then, the passive solar distillers include the following kinds.

1. Basin distiller (conventional distiller).

2. Wick distiller.

3. Weir-type distiller.

4. Spherical distiller.

5. Tubular distiller.

6. Pyramid distiller.

7. Diffusion distiller.

8. Greenhouse combination distiller.

9. Stepped distiller.

For example, Figure 9 obtains a passive distiller incorporated with condenser, and Figure 10 reveals a passive distiller incorporated with internal and external mirrors.

Figure 9.

Passive distiller integrated to condenser [117].

Figure 10.

Passive distiller integrated with mirrors: Schematic and photo diagrams [118].

6.2 Multi-effect distillers

Multi-effect solar distillers are different in design from the single-basin distillers. Also, the modified design of multi-effect distillers helped enhancing the performance of the distillers. Here, in these types of distillers, the condensation latent heat is utilized as a recovery source of heat to obtain more vaporization through the effect of the distillers, and hence, the freshwater production is augmented [119]. Now, multi-effect solar distillers are categorized into two main sections: active and passive distillers as the following.

6.2.1 Active distiller

It is the same concept of the single-effect distiller. The word “active” means that the solar distiller is integrated with somewhat external source of heat such as the solar concentrators and collectors. Various distiller kinds can be found in the literature under the category of active distiller-based multi-effect distillers.

1. Multistage evacuated active distiller.

2. Multi-basin inverted absorber active distiller.

3. Waste heat recovery active distiller.

4. Multi-effect condensation–evaporation desalination unit.

5. Distiller integrated with collectors such as flat plate and tube collector.

A multi-effect active distiller with collector is illustrated in Figure 11. Also, Figure 12 shows a condensation-evaporation active distiller.

Figure 11.

Double-effect single-slope active still: a coupled with solar collector in thermosiphon mode and b coupled with solar collector in forced circulation mode [120].

Figure 12.

Condensation–evaporation active distiller [121].

6.2.2 Passive distiller

Here, the passive multi-effect distillers include the following designs of distillers.

1. Wick distillers.

2. Conventional distillers.

3. Weir-type distillers.

4. Diffusion distillers.

Figure 13 illustrates a passive wick distiller, and Figure 14 shows a double-basin double-effect passive distiller.

Figure 13.

Wick-passive distiller [122].

Figure 14.

Double-effect double-basin passive distiller [46].

7. ANN as a prediction method for the performance of desalination systems

Artificial intelligence (AI) is quickly progressing every day. Computers can easily perform the difficult tasks for the humans to do. AI is first contrived in 1950. AI passed with steps and stages; some failed and others succeeded. Deep learning with powerful computers is the main keys of the success of AI. With time, AI can solve numerous industrial problems including the image processing, recognition of speech, language processing, optimization, prediction, robotic automation, categorization, self-driving cars …etc. AI is of numerous branches, and we will focus, in this chapter, on the neural networks (NNs). NNs help the computers to teach themselves from observing the data to predict the system performances. Preparing data for neural network processing is typically the most difficult and time-consuming task you will encounter when working with neural networks. In addition to the enormous volume of data that could easily reach millions and even billions of records, the main difficulty is in preparing the data in the correct format for the task in question.

7.1 Application of ANN on desalination systems

Contemporary experience indicates that artificial neural networks (ANN’s) may be specifically appropriate to offer tools to assist desalination plant operators in day-to-day operations. Prediction of the productivity of the desalination units is an essential consequence to evaluate its potentiality to provide potable water with involving in conducting more experiments. AI-based methods are stated as robust tools to obtain the correlation between the process parameters and responses [123, 124, 125, 126, 127]. This chapter suggests that ANNs are particularly appropriate as the basis for the development of tools to aid in the various phases of operating a desalination plant. In solar energy applications, different kinds of ANNs predicted and optimized the performance of collectors, cells, distillers, etc. [128]. The most application of ANNs is solving the issues of designing and procedures in electric power systems. Due to the similarities between power plants and desalination plants, the methods of ANNs can be used to predict the performance of desalination plants [129]. The merits of applying ANN included that it handles large amounts of datasets, can discover complex nonlinear relationships among all parameters (dependent and independent), and can find all interactions among all tested parameters.

Zarei and Behyad [130] introduced an ANN to test the effective parameters of the greenhouse on water productivity such as width, length, height of evaporator, and roof transparency. The method obtained an acceptable agreement with the experimental results. Tayyebi and Alishiri [131] used a nonlinear inverse model control strategy based on neural network for predicting the performance of MSF desalination plant. The proposed NNs consisted of three layers identified from input–output data and trained with a descent gradient algorithm. The set point tracking performance of the proposed method was studied when the disturbance is present in MSF unit. Three controllers were provided for checking the saline temperature, last stage level, and salinity. The results obtained that a neural network inverse model control strategy is robust and highly promising to be implemented in such nonlinear systems. An ANN model based on field data was built to investigate the vacuum membrane distillation (VMD) performance at various factors [132]. The introduced model could accurately predict the unseen data of VMD. The correlation coefficient of the overall agreement between ANN predictions and experimental data was found to be more than 0.994. Aish et al. [133] utilized ANN to forecast reverse osmosis performance in Gaza Strip through predicting the next week values of total dissolved solids (TDS) and permeate flow rate of the product water. Multilayer perceptron (MLP) and radial basis function (RBF) neural networks were trained and developed with reference to feedwater parameters including the pressure, pH, and conductivity to predict permeate next week values of flow rate. MLP and RBF neural networks were used for predicting the next week TDS concentrations. Both networks are trained and developed with reference to product water quality variables including the water temperature, pH, conductivity, and pressure. The prediction results showed that both types of neural networks are highly satisfactory for predicting TDS level in the product water quality and satisfactory for predicting permeate flow rate.

Santos et al. [134] simulated the distiller yield based on local weather data such as the solar irradiance, air temperature, glass temperature, and air speed and direction by ANNs. Essa et al. [51] introduced an enhanced ANNs incorporated to Harris Hawks optimizer to anticipate the distiller yield. The optimizer was utilized to get the optimal structure and network factors. Nevertheless, traditional artificial neural networks suffer from trapping in local optima problem during learning process. To overcome this problem, RVFL is proposed as it determines the output weights without involving in time-consuming learning process. Besides, RVFL has a direct connection among the input and output nodes, which is of a substantial impact on the network performance. This aids in avoiding the overfitting issue occurring in other ANNs. Using these merits, the conventional RVFL networks has some restrictions like that there are various conformations can be utilized, and this can affect the quality of water productivity final prediction. Then, Ensemble Random Vector Functional Link Networks (EnsRVFL) [135], which relies on RVFL as a basic modeling, were utilized to eliminate these restrictions of RVFL. A random vector functional link (RVFL) network integrated with artificial ecosystem-based optimization (AEO) algorithm was introduced to predict the seawater greenhouse (SWGH) performance [52]. Power consumption and yield of SWGH were predicted via RVFL-AEO modeling. Besides, the statistical analyses with various statistical tools were conducted to find out the neural network efficacy. The statistical tools revealed a complete match between the field and modeling data. The RVFL-AEO performance was compared with that of conventional RVFL. RVFL-AEO obtained an improved performance as compared by RVFL, which indicates the role of AEO in obtaining the optimal RVFL factors that improved the model accuracy.

8. Conclusions

Based on the above explained sections, the following points can be concluded. The most economic and efficient method to get freshwater is desalination. Demisters are the main component of MSFD technique to prevent mixing the distillate by brine. The thermal performance of MED is better than that of MSFD. For example, GOR is 10 and 15 for MSFD and MED, respectively. RO is considered as the most commercial, economic, and efficient techniques of distillation. The economics and technical properties of the desalination method with the targeted quantity of freshwater are the factors based on which we select the distillation technique. Solar distillers are the simplest and easiest way to get potable water, but they suffer from the few yields. The solar distillers can be classified into single-effect and multi-effect distillers (according to the number of effects of distiller) with a subcategory of active and passive distillers (according to the vaporization heat source) inside every classification.