All To Know About Solar-Powered Desalination Systems

Drinkable freshwater is a precious resource. And while we often take it for granted, access to clean water is a huge problem in many parts of the world. Producing freshwater from saltwater is one way to address the water crisis.

However, the desalination process has traditionally been energy-intensive and expensive.

Because the process requires boiling the saltwater and then condensing the steam back into the water. Thus, making it not a viable solution for many areas.

But what if there was a way to power the desalination process using renewable energy?

Fortunately, there is.

Solar desalination is a process that uses solar energy to power the desalination process. This makes it a more affordable and sustainable solution in areas where access to clean water is a challenge.

In this post, we will explore everything you need to know about solar desalination. We will discuss how solar desalination works, the benefits and drawbacks of solar-powered desalination, and the various types of solar desalination systems.

What is solar desalination?

Solar desalination is a process in which solar energy is used to evaporate seawater and then condense the vapor to produce freshwater.

The process relies on heating the saltwater to separate the freshwater from the salt.

However, saltwater may not be brought to a boiling point depending on the type of system being used. Since evaporation happens at almost any temperature, the systems may use a lower temperature to evaporate the water.

Solar desalination mimics the natural water cycle on Earth, which uses the sun's energy to evaporate water and produce rain.

It replicates this process and produces freshwater from saltwater by using solar energy to power the desalination process.

How does solar desalination work?

Although solar desalination is essentially a distillation process, there are several different ways to execute it. All solar desalination systems either rely fully on solar energy or use a hybrid of solar and traditional energy sources.

In order to understand how solar desalination works, we have to first understand the different types of solar desalination systems.

Direct (Passive) Desalination Systems

A direct, or passive unit is often referred to as a “solar still”. These systems are the simplest and most affordable option for solar desalination.

It works by heating the water in a container, which then evaporates. As the steam rises, it hits a cold plate or condenser, where it turns back into freshwater and is collected.

Although direct solar desalination works fine in theory, it's not very practical. Because the process is very slow and only produces a small amount of freshwater.

The quantity of drinking water that can be generated by the system depends on the surface area of the absorber and the intensity of solar radiation.

Depending on the design, these systems can produce 2 to 3 liters (approximately 0.5 to 0.8 gallons) of freshwater per square meter of space.

Direct desalination systems may be okay for small-scale applications or to meet the temporary need for freshwater. But they are not practical for larger-scale or long-term applications.

Since the process solely relies on solar energy, weather conditions, and varying levels of solar irradiation will affect the output and efficiency of the system.

Here is an example design:

Indirect (Active) Desalination Systems

An indirect, or active, solar desalination system brings together solar energy collection (through photovoltaic panels) with an established desalination method like multistage flash distillation, MEE, or reverse osmosis (RO).

An indirect, or active, solar desalination system entails combining photovoltaic technology with an established desalination technique like multistage flash distillation, MEE, or reverse osmosis (RO).

Active systems can be thermal, mechanical, or electric driven. What that means is that the system can use solar energy to heat up the water, or generate electricity to power the desalination process.

Active systems also don't have to rely solely on solar energy, and they can choose solar power as a backup source to traditional energy sources.

Active desalination systems that work with solar thermal energy are the most common type. In this system, solar collectors are used to heat up the water, which is then run through a standard desalination process.

With this system, the solar energy is captured using concentrating (PTC, LFC, TSC, SDC) or non-concentrating (FPCs, HPC, SP) collectors to power the desalination operations.

Another type of indirect solar desalination is using electricity generated by photovoltaic (PV) cells to power ED, which is the only desalination technology that uses electricity directly to produce fresh water.

RO and freezing desalination methods necessitate mechanical energy which can be acquired from solar energy via heat engines (Rankine, Sterling, and Brayton) or PV panels.

Now, let's discuss the desalination techniques that are used by solar-powered desalination systems.

What desalination techniques are implemented?

There are several different ways to remove salt and minerals from water. All of them have their own advantages and disadvantages. The most common desalination techniques are:

Reverse osmosis (RO)

Reverse osmosis (RO) is a pressure-driven desalination technique that uses a semipermeable membrane to remove ions, molecules, and larger particles from saline water.

As the water travels through the membrane, the salt and other impurities are left behind, while the freshwater is collected on the other side. RO not only removes particles but also improves water's taste, odor, and appearance.

RO is one of the most common methods of desalination, as it is relatively simple and efficient.

Reverse osmosis (RO) is the dominant indirect desalination technology and is used for more than half of large-desalination operations.

RO membranes are capable of separating more than 98% of the salt in seawater. The process needs a pressure of 10 to 60 bars depending on the salinity of the feed water for desalination.

RO desalination systems typically recover 45–50% of seawater and 90% of brackish water. The main issues with RO are the high energy requirements, fouling & scaling of the membrane.

The required pressurized feed water for the RO process can be provided by a Stirling or Rankine engine heated by solar energy, or by an electric motor powered by solar panels.

Given that solar energy is seasonal, a continual supply of freshwater requires additional measures such as energy storage in the form of batteries, and hybridization with other energy sources like wind, geothermal, or fossil fuels.

Although water pretreatment before the RO membrane can increase the energy expenditure of the process, RO is still more efficient than phase-changing thermal processes.

Multiple stage flash distillation (MSF)

A multiple-stage flash distillation (MSF) system is made up of a series of adjacent vessels, each with an internal heat exchanger and a condensed water collection.

Each of these vessels is known as a stage. Each stage has its own inner pressure, which ranges from high to extremely low.

Different pressures in each vessel result in different boiling points, with a 2–5°C difference from high to low.

Theoretically, operating an MSF plant at brine temperatures as high as feasible boosts the plant's efficiency.

To avoid scale formation and faster corrosion of metal surfaces in contact with seawater, the upper brine temperature should be kept around 120°C.

In a multi-stage flash distillation system, cold saltwater passes from the cold to the hot side, absorbing heat and forming distilled water. As the brine evaporates inside the vessels, it acts as an evaporator.

Although a greater number of stages improves efficiency, it also raises installation costs. Thus, modern large MSF plants are built to have 19–28 steps.

With this system, the preheated saltwater departs the hottest (or first) stage and enters the collector to absorb more heat before entering the first stage vessel.

The first stage vessel is set to a pressure high enough that the entering hot brine exceeds the boiling temperature at that pressure. As a result, a portion of the incoming brine water evaporates suddenly, which is referred to as the “flash.”

The steam produced by this flash reaches the condenser above and condenses into liquid condensate, which falls on a freshwater collection and is removed as freshwater via a regulated valve.

The demister is utilized to capture any water particles that may erupt during the flash and blend with the freshwater.

Multi-effect distillation (MED)

Multiple-effect distillation (MED) is a distillation method that is commonly used for seawater desalination. It is made up of several stages or “effects” in which the feed water is heated and vaporized.

The feedwater is heated by steam in tubes at each stage, commonly by spraying saline water onto them. Some of the water evaporates, and the resulting steam flows into the tubes of the following stage (effect), heating and evaporating more water.

Each stage basically recycles the energy from the preceding one, with decreasing temperatures and pressures following each one.

There are several configurations, such as forward-feed, backward-feed, and so on. In addition, between phases, this steam uses heat to pre-heat the incoming saline water.

The advantages of MED over MSF are that MED requires lower pressures and temperatures, making it more efficient. MED is tolerant to variations in feed water quality, and the number of stages can be easily increased if more capacity is needed.

The main drawbacks of MED include incompatibility with higher temperature heat sources and the difficulty in scaling down to small sizes due to the complexity and the larger number of parts required.

Freezing Desalination 

Desalination by freezing relies on the fact that ice crystals are generated when saline water is cooled to its freezing point and then further heat is removed. It has three distinct stages: ice formation, cleaning, and melting.

In reality, it is the nature of all crystals to reject contaminants from their structures as they grow.

Unlike distillation, the freezing method makes use of the phase transition of water from liquid to solid. It is necessary to separate the ice crystals from the brine, clean the ice crystals to remove the clinging salts on the crystals' surface, then melt the ice to obtain fresh water.

The freezing technique has a lot of advantages over other desalination methods, particularly the distillation method. It requires less energy to transfer, requires almost no pretreatment, and has few corrosion and metallurgical issues.

There are a variety of methods for freezing saltwater. In order to acquire freshwater from a saline source, three distinct phases must be completed: ice creation due to the removal of heat from the saline water, isolation of ice from the brine, and melting of the ice.

As a result, practically all freezing procedures involve functional components because they use comparable mechanisms for creating ice and separating it from the brine.

All freezing desalination processes have four essential components: a freezer, a washer, a melter, and a heat removal system.

The freezer is made up of a vessel in which ice crystals and vapor are generated at the same time.

The apparatus used to extract heat from the brine in order to form ice crystals that can be easily transferred, removed, separated, washed, and melted varies between freezing methods. The heat removed during the freezing process is normally transferred to the melter and used to melt the ice.

There are two types of freezing processes: indirect freezing and direct freezing. Indirect freezing is when the heat is transferred from the brine to a heat exchanger, which then transfers it to the ice-forming medium.

Direct freezing, on the other hand, is when the brine and ice-former are in direct contact with each other.

Because of its many advantages, such as low energy requirements, immunity to fouling and corrosion or scaling, and essentially no pre-treatment, the freezing process has grown significantly over the last 50 years.

All that said, desalination by freezing is still being researched, and more improvements need to be made before it can become a more widespread and commercially successful technology.

The benefits of using solar energy for desalination

There are many benefits to using solar energy for desalination. Here are a few of the most notable ones:

Using Renewable Energy

Solar energy is a renewable resource, so it is a more sustainable option than using fossil fuels to power the desalination process.

Even the traditional desalination process, which relies on electricity from fossil fuels, can be supplemented by solar energy. This reduces the amount of greenhouse gas emissions released into the atmosphere and helps to combat climate change.

Reduced Energy Costs

Since solar energy is free once you have installed the necessary equipment, it can significantly reduce the cost of desalinating water.

As just mentioned, the traditional desalination plants that implement solar power can reduce their overall energy costs by supplementing with solar power.

May qualify for government subsidies

Many governments offer incentives and subsidies to help offset the cost of installing solar energy systems.

So if you are considering desalination as a way to provide your community with clean water, it is worth checking to see if there are any available subsidies or grants that could help reduce the cost.

The drawbacks of solar desalination

While there are many benefits to using solar energy for desalination, there are also some drawbacks. Here are a few of the most notable ones:

Higher upfront cost

The initial cost of installing a solar desalination system can be expensive. However, this cost is often offset by the lower cost of operating the system over time.

Especially, if you take into account the subsidies and incentives available from local and national governments.

Limited water production

Solar desalination systems are not as efficient at producing water as traditional desalination plants. This means that they can only be used in areas where there is a high demand for water and low availability of freshwater resources.

Requires a sunny location

Solar desalination only works in locations that receive a lot of sunlight. This limits the locations where solar desalination can be used. Therefore, solar desalination is not a viable option for projects in areas with little sun exposure.

Solar energy is intermittent

The sun only shines during the day, so solar desalination can only operate during daylight hours.

This means that it cannot continue to produce water at night or on cloudy days. In order to get around this, solar desalination systems must be paired with storage tanks that can hold the water until it is needed.

Despite the drawbacks, solar desalination is a promising technology with the potential to provide clean water for communities around the world.

If you are considering implementing solar desalination, be sure to do your research and consult with experts to determine if it is the right solution for your community.