Solar Desalination Technology

By closely combining the solar desalination system with conventional modern desalination technology, drawing on its advanced manufacturing process and achievements in enhanced heat and mass transfer, and complementing the advantages of solar energy itself, a more ideal effect can be achieved.

1. Solar desalination technology

Traditional desalination technology requires high investment and consumes too much energy, with the energy mainly coming from fossil fuels such as oil and coal, making it difficult to promote desalination technology.

Data studies have shown that a seawater purification system with a daily output of 1,000 cubic meters of fresh water consumes 10,000 tons of oil per year. For areas lacking fossil fuel resources, especially some remote areas with low population density and no large-scale connection to the power grid, it is difficult to build traditional seawater desalination equipment. Therefore, using ubiquitous solar energy to desalinate seawater and brackish water is not the best choice.

The solar desalination system is actually a combination of solar energy utilization devices and traditional desalination devices, using solar energy instead of traditional energy to supply the energy required by the desalination device.

Some of the combinations are shown in Table 1

Figure 1 Schematic diagram of solar desalination

Figure 2 Trough solar concentrating thermal system

The trough solar thermal system has the characteristics of large scale, long life and low cost, and is currently the most mature large-scale solar thermal utilization technology. There are three main ways to produce steam for trough solar desalination: flash evaporation, direct evaporation and indirect evaporation.

1.1 Application of multi-stage flash evaporation

The direct evaporation method may have operational stability problems. Flow instability can lead to flow loss in the affected pipe section and even cause overheating of the collector tube and permanent damage to the selective absorption coating.

For the indirect evaporation method, the main disadvantage of the system is that most heat transfer fluids have special properties such as being difficult to prepare, flammable, and easily decomposed.

flash evaporation system can effectively avoid the above defects, and has the advantages of simple structure, stable operation, high efficiency and low construction cost. Therefore, the flash evaporation system is suitable as a research and development object.

Figure 3 Principle of solar flash evaporation

1.2 Characteristics of multi-stage flash evaporation

It has the advantages of high reliability, good anti-scaling performance and easy large-scale development.

At present, 60% of the global seawater desalination output is obtained by the multi-stage flash evaporation method. At the same time, multi-stage flash evaporation is also the seawater desalination method with the largest single-unit capacity (up to 100,000t/day), which is suitable for large and ultra-large desalination plants.

1.3 Working principle and process of multi-stage flash evaporation

The principle of the multi-stage flash evaporation process is as follows: the raw seawater is heated to a certain temperature and then introduced into the flash chamber. Since the pressure in the flash chamber is controlled to be lower than the saturated vapor pressure corresponding to the temperature of the hot brine, the hot brine becomes superheated water after entering the flash chamber and is rapidly partially vaporized, thereby reducing the temperature of the hot brine itself. The generated steam is condensed to become the required fresh water.

Multi-stage flash evaporation is based on this principle, whereby hot brine flows through a number of flash chambers with gradually decreasing pressures, evaporating and cooling step by step. At the same time, the brine is gradually concentrated until its temperature approaches (but is higher than) the natural seawater temperature.

The process flow of the multi-stage flash evaporation system is shown in Figure 3 above. The main equipment includes brine heater, heat recovery section of the multi-stage flash evaporation device, heat exhaust section, seawater pretreatment device, vacuum system of the non-condensable gas exhaust device, brine circulation pump and inlet and outlet water pumps, etc.

2. Basic process parameters

(1) Circulating brine flow rate

The characteristic of multi-stage flash evaporation is that it relies on the circulating brine to continuously cool down through multiple stages, releasing its own sensible heat, thereby vaporizing part of the water in the superheated brine to achieve the purpose of producing fresh water and concentrated brine.

Therefore, from the perspective of heat balance, the sensible heat released by each stage of the circulating brine is equal to the latent heat required for the generated fresh water. Therefore, for the entire multi-stage flash evaporation system, the following relationship exists: RS(t0-tn) = DL

Where, R is the circulating brine flow rate (kg/h);

S-average specific heat of salt water (kcal/kg·℃);

t0: first stage inlet temperature of circulating brine (℃);

tn－the final outlet temperature of circulating brine (℃);

D-total freshwater production at each level (kg/h);

L-average latent heat of vaporization of fresh water (kcal/kg).

The above formula can be used to obtain the circulating brine flow rate under certain fresh water production requirements.

Salt balance FCf = BCb

Water balance F(1-Cf) = D+ B(1-Cb)

In the formula,

Cf is the mass concentration of salt in raw water (kg/kg);

Cb-mass concentration of salt in discharged brine (kg/kg)

Substituting the concentration ratio α=Cb /Cf into the above two equations, we can get:

It can be seen that under the condition that the fresh water output is known, the flow rate F of supplementary raw water and the flow rate B of discharged brine are mainly determined by the concentration ratio of the system.

The concentration ratio refers to the ratio of the final brine concentration (total dissolved solids TDS) of the flash evaporation device to the replenishment seawater concentration (TDS). It is generally limited to scale prevention safety based on specific water quality conditions, and the salt discharge concentration generally cannot approach 70,000 mg/L .

For seawater desalination, there is no "single" best solution for the choice of technology. Instead, it should be based on the characteristics of each project and the actual conditions, including scale, energy costs, raw water quality, climatic conditions, and technical and safety requirements.

Generally speaking, reverse osmosis is suitable for independently established seawater desalination plants, but if there is a thermal power plant (latest nuclear power plant), thermal distillation technology is more economical and reliable.

The multi-stage flash evaporation method is not only used for seawater desalination, but has also been widely used in boiler water supply in thermal power plants and petrochemical plants, the treatment and recovery of industrial wastewater and mine brackish water, as well as the recovery of waste alkali liquor in the printing and dyeing industry and the papermaking industry.