Synthetic tree: A means to remove CO2 from the air

In 2003, a Dr. Klaus Lackner, a Columbia University physicist, had designed a synthetic tree that could draw carbon dioxide from air and retain the carbon. It was the first step in application of carbon sequestration technology.

However, Dr. Lackner’s synthetic tree did not look like a tree or perform any of the functions of a real tree. The synthetic tree was merely an air capture device to remove carbon from the atmosphere and store it.

Now, in September 2008, Abraham Stroock and Tobias Wheeler, of Cornell university, have created a synthetic tree that simulates the process of transpiration by which trees draw water to its branches and leaves.

What are synthetic trees?

A synthetic tree is a palm-sized microfluidic system that mimics the main features of transpiration. In synthetic trees, evaporation or transduction of water in the vapour phase into negative pressures in the liquid phase, takes place followed by stabilization and flow of liquid water at very high negative pressures.
A real tree can transfer water to great heights, up to 85 meters tall, from the roots to its leaves, through its trunk. The synthetic trees, created by Stroock and Tobias Wheeler, can transfer liquids in a similar way to amazing heights.

The synthetic tree comprises two networks of parallel channels placed next to each other in a thin sheet of hydrogel – a material used to make contact lenses – connected to a main channel, thus replicating a tree’s vascular system.

In an synthetic tree, the tranparent sheet of hydrogel is 1 millimeter thick. There are 80 parallel channels etched into the hydrogel sheet. These parallel sheets are connected to a main channel that enters into a network of microchannels in the leaf or root network. The channels, in a synthetic tree are approximately 100 micrometers wide.

How do synthetic trees work?

Real trees use xylem, a tubular tissue-like substance, to pull water out of the ground and pass it to the leaves.

Because of negative pressure, the water remains in a metastable state, something between a liquid and vapor.

For their synthetic tree, Stroock and Wheeler decided to use hydrogel, or polyhydroxyethyl methacrylate, to replicate the plant membrane. Hydrogel is a porous solid with the mixture of the solid and liquid phase at the molecular level. This makes the pores very tiny – much less than the maximum allowable 10 nanometers to hold the water – so that the negative pressure is high enough to suck the water. If the pores are larger than 10 nanometers, then the pores will fail to hold on to the liquid.

Using hydrogel to replicate xylem capillaries in synthetic trees, scientists can build negative pressures to rival those found in trees.

Benefits of Synthetic trees

1. Synthetic trees can remove large quantities of CO2 from the atmosphere,and could be a partial solution for vehicle and industrial emissions
2. Synthetic trees can be used to move liquids without using mechanical pumps.
3. Synthetic trees can be used in building heat-transfer applications to cool devices like laptop computers, vehicles, and buildings. Heat-transfer applications use vaporization to carry heat, to say, the fan on the edge of a computer. The technology used for synthetic trees can do the same more effectively. In buildings, the technology used in synthetic trees can be used to transfer heat through the building and then pull the water back.
4. Synthetic trees can provide effective methods for soil remediation by pulling contaminated water out.
5. Synthetic trees can used to tap water from semi-dry ground.
6. Synthetic trees can be tweaked to extract purified water from above the water table.

Lackner’s synthetic trees

Unlike Abraham Stroock and Tobias Wheeler, Dr. Lackner’s synthetic trees are artificial trees that can work like real trees to draw out carbondioxide from the air and retain the carbon. Unlike real trees, synthetic trees do not release oxygen, but they successfully purify the air. Synthetic trees do not look like real trees either. Dr Klaus Lackner, a Columbia university physicist, who has designed the synthetic trees has said that synthetic trees look like goal posts with venetian blinds. As of now, synthetic trees exist only on paper, but Dr Lackner says that if synthetic trees are built, each synthetic tree will be able to remove 90,000 tonnes of CO2 in a year, which is the emissions equivalent of 15,000 cars.

How do Lackner’s synthetic trees work?

Synthetic trees are a step in the application of carbon sequestration technology that is considered a vital tool for reducing carbon emissions to the atmosphere. Simply put, carbon sequestration means carbon capture, separation and storage or reuse.

If the synthetic tree prototype can be scaled up, then it can help deal with  vehicles that together represent over 20 percent of global CO2 emissions. Synthetic trees can provide solutions on how best to manage carbon emissions from available resources and infrastructure, and how to dispose of the carbon that has been collected.

Benefits of Synthetic Trees

Synthetic trees can be more effective than establishing renewable energy sources. Synthetic trees would require much less space to operate, then say windmills, which also help rid the atmosphere of CO2.
Synthetic trees are more practical dealing with vehicles that pollute the atmosphere. Instead of trying to collect CO2 emissions from exhausts – storing CO2 emissions in cars are very impractical – synthetic trees would be able to capture the CO2 emissions much better.

The most important benefit of Synthetic trees or other air capture devices – if they can be implemented on a large scale – is that they would enable reductions to take place irrespective of where carbon emissions occur. Synthetic trees can be located at a specific physical location where there is a need to sequester a certain amount of CO2.

Synthetic tree can offset high re-outfit costs of making existing power plants more eco-friendly. Synthetic trees can capture emissions from power lants without adding retrofit costs.
However, there could be a cost issue with implementing synthetic trees. Dr Lackner says that the biggest cost would be in recycling the material that absorbs carbon. If the material is not refreshed, then the it would fill up with carbon dioxide very fast.

The two divergent synthetic prototypes of synthetic trees – Stroock and Tobias Wheeler synthetic tree that mimics a tree’s transpiration system and  Lackner’s synthetic tree that can extract carbon – would prove to have exciting implications, if they are are scaled up and implemented.