An automated solar desalination system developed by researchers at MIT and Shanghai Jiaotong University in China can provide more than 1.5 gallons of drinking water per hour per square meter of solar capture area. Such systems are able to provide efficient, low-cost water supplies, serving arid coastal areas.
The article in the journal Energy and Environmental Science described the system in detail: it consisted of many flat layers of solar evaporators and condensers, lined up in a vertical row and had a transparent top layer of thermal insulation. "This new approach makes a lot of sense," said Ravi Prasher, director of the collaborative laboratory at Lawrence Berkeley National Laboratory and a professor in charge of mechanical engineering at the University of California at Berkeley, said. "One of the challenges in solar desalination is low efficiency due to significant energy loss during condensation."
The authors hope their design could contribute to alleviating water scarcity in developing countries where electricity is scarce but seawater and sunlight are plentiful.
The key to the effectiveness of this system is of phasing to desalinate water. At each stage, the heat released from the previous stage is mined instead of wasted. In this way, their device can achieve up to 385% efficiency in converting the energy of sunlight into energy to evaporate water.
Essentially a solar-powered multi-layer distillation tank, the device has a set of evaporating and condensing components similar to those used to distill alcohol. It uses flat plates to absorb heat and then transfers the heat to a layer of water so that the water begins to evaporate and condenses on the next plate. Water is collected while the heat from the vapor condensation is transferred to the next layer.
When steam condenses on a surface, it releases heat; In typical condensation systems, that heat is released into the environment but in this multi-layer distillation unit, the heat flow is released to the next evaporation layer, "recycling" the heat and increasing efficiency. "When you condense water, you release energy in the form of heat," said Wang, a work author. "If there are many stages, you can take advantage of that heat."
Typically, many layers increase the efficiency of converting water distillation energy, but each layer also adds cost and size to the system. The team decided to choose a 10-layer system for their original equipment, which was tested on the roof of the MIT building. This system provides pure water that exceeds the city's drinking water standard, at 5.78 liters per square meter (about 1.52 gallons per 11 square feet), more than twice the record of water separation with old equipment, Wang said.
Unlike some desalination systems, this system does not have salt buildup or salt water that needs treatment after distillation of water. Another advantage of the system is that it is made primarily of inexpensive and readily available materials such as black solar panels available on the market, or use paper towels as a guide to bring water into the solar panel. In most other efforts to create passive solar desalination systems, solar-absorbing materials and absorbent materials are specialized and expensive materials, Wang said.
The most expensive component of the prototype is a layer of aerogel - a form of ultra-light, transparent solid material that is used as an insulator, but the team shows other cheaper insulators can be replaced.
Wang emphasized that the group's important contribution is to create a platform to optimize multi-stage passive systems for desalination, which can be applied to a variety of materials and equipment structures, allowing for further optimization. The systems are based on operating scale, conditions and different materials.