Lenan Zhang and his team at MIT have developed a mathematical model for optimizing the performance of a small, economical, highly efficient device that canprovide fresh water for an individual family , using only the sun for its energy input.

Tech Briefs: What got you started on this idea?

Lenan Zhang: Existing large-scale desalination plants need a power grid and water pipes to send the water to the users’ different locations; however, many water-stressed areas are rural, poor, and off-grid. So, we need small-scale technology that can produce potable water for a single person or family. Solar-powered desalination is a portable, affordable, passive way to achieve this.

Lenan Zhang and his team at MIT have developed a mathematical model for optimizing the performance of a small, economical, highly efficient device that can provide fresh water for an individual family, using only the sun for its energy input.

Tech Briefs: What got you started on this idea?

Lenan Zhang: Seawater is a very abundant resource for clean water, but the problem is how to desalinate it. Commercially, there are several well-developed approaches — large desalination plants — but all of them need centralized installations. You need a well-established supporting infrastructure: you need electricity, that means a power grid; and water pipes to send the water to the users’, different locations.

However, many water-stressed areas are also under-developed — they are rural, they are poor, they are off-grid. They don’t have access to fresh clean water and cannot afford the well-established large plants. So, we need technology that is small-scale and can produce potable water for a single person, or family, without electricity or the need for a centralized installation and distribution infrastructure. Solar-powered desalination is a portable, affordable, passive way to achieve this.

Tech Briefs: There are already such things as solar stills that do something like that, aren’t there?

Zhang: A solar still is any device that heats water using solar energy to generate vapor that is then condensed. From the condensation of the vapor, you can get clean water.

Tech Briefs: How is your system different?

Zhang: Many people are interested in passive solar desalination, but the key thing is the efficiency. Previously, people used just one evaporator. The solar energy input induces evaporation because a liquid-to-vapor phase change requires energy, which in this case, comes from the sun. The vapor then condenses back to the liquid phase, as fresh, unsalted, water. This releases energy, called latent heat; but for the typical system, this released energy is dissipated into the environment as waste heat that cannot be reused. For this reason, their efficiency is limited to 100%.

But that energy can be reused to trigger more and more cycles of vapor generation and condensation, which can produce much more desalinated water. That’s why we proposed this Thermally Localized Multi-Stage Desalination Technology (TMSS), which uses a number of stages instead of a single stage in order to reuse the waste heat. We use that energy to trigger the evaporation process in the next stage and the stages after that. By reusing the thermal energy, we boosted the efficiency. In our prototype we got record high thermal efficiency — as high as 385%, which is more than twice the previous record.

Tech Briefs: That’s hard for me to understand. I’m not sure how you can get more than 100% efficiency.

Zhang: The definition of solar vapor evaporation efficiency is the total amount of water vapor you produce times the energy you need to input to convert a kilogram of liquid to vapor. This is known as latent heat of vaporization enthalpy — the amount of energy to produce a given amount of vapor. This energy over the total solar input flux (watts per square meter) is defined as solar thermal conversion efficiency.

Tests on an MIT building rooftop showed that a simple proof-of-concept desalination device could produce clean, drinkable water at a rate equivalent to more than 1.5 gallons per hour for each square meter of solar collecting area. (Image Credit: MIT)

Tech Briefs: Could you give me some idea of how these stages are arranged physically. Are they piled on top of each other like a sandwich?

Zhang: Yes, it’s actually a very simple structure. And, because of the simplicity of this device, we can achieve very low cost. First, we have a very common commercially available solar absorber: a black sheet that can absorb solar energy. That energy is converted to heat. What we call the evaporator is attached to the back side of the solar absorber. The heat converted by the solar absorber transfers to the evaporator, which contains saltwater, and triggers the evaporation process. For the evaporator we also use a very low-cost material — paper towels. The vapor then moves through a tiny air gap to a condenser, where it condenses to a salt-free liquid. For the condenser, we also use a very simple material — an aluminum plate.

This is the basic structure of the first stage. For the second stage, the evaporator is attached to the back side of the condenser. Once the vapor condenses, it releases heat, which can transfer across the condenser to the evaporator. Evaporation will occur at the second stage and will condense on the condenser of the second stage and transfer to the following stage. The following stages are the repeat structure. Each stage is almost the same — evaporator, condenser; evaporator, condenser. The only difference is the first stage, which uses a solar absorber to convert the solar flux to heat.

Tech Briefs: So, there’s just that one solar input at the first stage?

Zhang: Yes, we reuse that solar energy again, and again, and again.

Tech Briefs: How do you collect the fresh water?

Zhang: We apply a hydrophobic coating to the condenser. The liquid condenses onto it and forms small droplets that are fed by gravity into a slot. There is an open port at the end of the slot in each stage. The water flows through each of these ports into a collection container.

A diagram of the desalination system. Sunlight passes through a transparent insulating layer at left, to heat up a black heat-absorbing material, which transfers the heat to a layer of wicking material (shown in blue), where it evaporates and then condenses on a surface (gray) and then drips off to be collected as fresh, potable water. (Image Credit: MIT)

Tech Briefs: Where does the salt go?

Zhang: The salt that has been removed from the water is contained in the evaporator in a highly concentrated form. The seawater at the bottom of the device, on the other hand, has a relatively low concentration of salt. This is during the daytime, but during the nighttime, when there is no sun, no vapor will be produced. At night, since the salt in the evaporator is highly concentrated, it will automatically diffuse back to the seawater. During the daytime, the cycle repeats: salt accumulates as evaporation generates clean water. At night the salt diffuses back again because of the difference of concentrations.

Tech Briefs: How do you feed the water in?

Zhang: It’s passive. The paper towel can automatically suck water in because micro-pores in the towel create capillary pressure. Once you place the device in contact with seawater, the water will be automatically driven by the capillary force. In our experiment, we placed it on top of a saltwater reservoir.

Tech Briefs: How large was the device you used for your test apparatus?

Zhang: 10 x 10 cm.

Tech Briefs: Where do you see this going next?

Zhang: There are several goals we need to pursue. These sorts of multi-phase structures have already been developed for the industry, but they are very large-scale setups. We have transferred this technology to small-scale portable devices. Our key contribution is that we have devised a theory — a model — to understand the fundamental limit of the device. For example, 700 – 800% efficiency, in theory, can be achieved with careful optimization. So, the key contribution of our work is that we propose a practical doable optimization strategy. For example, how large the air gap needs to be, how many stages we should use, and how large the device should be.

We provide a rigorous and practical way to predict and optimize those parameters to achieve better efficiency. In the future, we will pursue higher efficiency, approaching our theoretical limit. Our second goal is to reduce cost. Currently, this 10 x 10 cm setup costs only about $1.50, because we use low-cost materials. However, we use a nylon frame made by 3D printing for our convenience in the experiment. This accounts for more than 70% of the total cost. So that means there is plenty of space to further reduce the total cost. We could replace the frame with easier-to-fabricate material, for example. A third goal is to improve the water collection. In our current design, although we produce lots of clean water, 20% of it is inevitably lost due to the inefficient water collection process. This is a technical and engineering problem. So, I think we can work in these three directions to make this scalable and affordable, and at the same time achieve better performance.

Tech Briefs: How do you plan to scale up the system?

Zhang: Although the per meter square production rate of this system is high, due to the small size of the single system, it can only give you a glass of water if you operate it for one day. To make family-sized amounts, or even a single person’s demands, we need to scale up. Our current strategy, for example, is to use an assembly of 100 of these devices to achieve an area of 1 m2, which will increase the total production by 100 times to create 10-20 liters of clean water per day.

An edited version of this interview appeared in the April Issue of Tech Briefs.