INTRODUCTION

When regarding ecosystems, they can be considered as possessing a superstructure that is analogous to a body exhibiting division of labor. When a part of a body is damaged an artificial prosthetic device can be used to either aid in recovery or to replace the part out right if needs to be. Could there be a way of synthetically simulating the vital, environmental functions of certain components of terrestrial ecosystems, which could be used to replace those components on either a temporary or permanent basis? The components that play the biggest role in terrestrial ecosystems are producers like vegetation, which fix energy. Deforestation claims vast swathes of the Amazon rain forest. In doing so it interferes with the carbon cycle, reducing the ability for one of the world’s largest carbon sinks to remove CO₂ from the atmosphere (Detwiler & Hall, 1988), in addition to interfering with global oxygen production and soil cohesion (Barber, 1995). Devices that function as artificial or synthetic vegetation could be created, to counter this.

History of the idea

So far only one researcher, K. Lackner, has made a serious proposal for a device that performs a function of vegetation. His design calls for a large, supported, pivoting ‘venetian blind’ like array of slats coated in a carbon absorbent material like saturated calcium hydroxide. Provided a large example of such a device, with a capture area of about 3,000m² was maintained properly and its absorbent surfaces were replaced very regularly, Lackner estimates that approximately 90,000 tons of carbon could be removed from the air annually. The main drawback of this proposal is the cost of replacing the absorbent material. Additionally disposal of the carbon once it has been sequestered is another issue (BBC News, 2003). However Lackner and colleagues have illustrated the theoretical feasibility of removing carbon directly from the air (Lackner et al., 1999), which makes large scale aerial Carbon scrubbing an attractive solution.

Design

Theoretically ‘synthetic vegetation’ could be constructed in a variety of configurations, but from an environmental impact point of view the best configurations would be attempts to simulate the structure of living vegetation, at least heuristically. A synthetic ‘tree’ for example could be created with a central trunk constructed from some kind of reinforced plastic or carbon fiber, and a ‘canopy’ of solar cells, providing an input of electrical energy at the top, arranged as a disk perforated with holes to simulate shade patterns on the ground, in order to encourage surface secondary growth. Buttresses could connect the solar arrays to the main trunk and provide conduits for wiring. Beneath the canopy would be arrays of tubes that could collect air for the purging of its CO₂ content. The devices should have roots that act as anchors, allow the exchange materials with the soil, like water (which could be obtained from an environmental source) and to maintain soil matrix cohesion. As there are many active processes that need to be coordinated, the devices would require a sensor network, a central processing unit and a battery for energy storage, which could be located within the base of the trunk.

Sequestering CO₂

The carbon sequestering mechanism would be the most problematic from an engineering standpoint. As it would not be possible to replicate the Calvin-Benson cycle in a machine, some alternative means of capturing and disposing of carbon would be required The object is to create a completely self-reliant device, so traditional modes of sequestering carbon from the atmosphere by using such scrubs as calcium hydroxide, for example would be unsustainable, as this requires regular replacement and the devices would need to be routinely purged of precipitates. The ideal scenario would be for the devices to be able to obtain a carbon store from the environment and to generate a stored carbon product that could be disposed of back into the environment - one that preferably has some ecological role. The ideal carbon store in this case is water. Even though, it doesn’t hold considerable amounts of CO₂ (0.145g/100 ml) it can easily be obtained by a device from the soil, air, precipitation or even a nearby body of water like a lake, river or artificial reservoir. When CO₂ dissolves in Water it produces Carbonic acid according to:

H₂O (l) + CO₂ (g) -> H₂CO₃ (aq)

This carbonic acid can be pumped deep into the soil by the device (simulating yet another function of living vegetation), where it performs several important ecological functions including toxic cation leeching (Johnson et al., 1977) and bedrock erosion, which is of critical importance in the process of new soil formation. The carbon taken from the atmosphere is then not only put to good use, but it is locked in the lithosphere, where it can remain for some time. An engineering challenge would be to design the feature of the synthetic vegetation that could perform the above. One solution would be to design the collection arrays as pneumatic pumps that draw air in from the atmosphere and then through a column of water located in the trunk of the device, which could then be released back into the atmosphere with a lower carbon content. A network of pH sensors could allow the device to regulate the amount of Carbonic acid in the column by having it pumped down a ‘deep root’ into the bedrock. New water can be obtained from the environment, which can be pumped into the column to replace the lost volume.

Carbon acquisition calculation

A simple calculation can be used to establish roughly the amounts of carbon that an example of robotic vegetation, scaled to the proportions of a large (400 feet) tree and based on a ‘water scrub’ with access to an external body of water, could remove from the atmosphere. The input of water should be set at 200 Gallons or 758,000 ml, which is approximately double the amount of water passing through a large (400 feet tall, 4 feet diameter) tree on a hot day (Williams and Ley, 1994), multiplied by 365, which equals 276,670,000 ml a year. The amount of water used for the electrolytic production of oxygen and variations in amount throughout the year are to be ignored for the purposes of this calculation. ii) The solubility of carbon dioxide in water is then taken which is 0.145g/100ml, we get the following equation is obtained:

(276,670,000 ml/yr/ 100 ml) * 0.145 g = 401,171.5 g/yr

As there are approximately 900,000 grams in a metric ton, a large tree sized device could realistically remove approximately half a ton a year from the atmosphere. An average healthy, large tree removes approximately 48 pounds of carbon a year, or 21,792 g/yr (Cost et al., 1990). This variety of robotic tree could remove roughly 40 times more carbon than an organic tree of equivalent size. Having the air/water mixing take place under pressure, where Henry’s Law would increase the amount of CO₂ that dissolves, could increase efficiency even more. As synthetic vegetation could be constructed in a variety of shapes and sizes, water throughput and carbon acquisition capacity would vary based on the scale of the device.

Feasibility analysis

Synthetic vegetation presents a potentially attractive alternative to conventional methods of restoration like re-vegetation, as often after deforestation, topsoil becomes loose and is displaced by the wind as dust. As has already been mentioned, vegetation plays a significant role in maintaining soil cohesion. After a process like deforestation, there is potentially only a narrow window of opportunity in which re-vegetation can take place, before the soil degrades to the point where it cannot support vegetable life. If that window is missed then re-vegetation becomes problematic (Barber, 1995). However a machine is used to simulate vegetation-environmental material exchanges, then the presence of degraded soil is no longer a potential problem. One of the functions of synthetic vegetation would be to make use of carbonic acid to generate new soil through bed rock erosion, so in effect the devices could be used to forcibly regenerate a degraded soil, making it fit for secondary growth. Another aforementioned use for the carbonic acid produced would be in detoxifying soil polluted with metal cations through leeching, the presence of which may hinder efforts at re-vegetation. Synthetic vegetation would only be produced with a view to wide scale utilization; as such there are potential economic factors to consider like do the aforementioned benefits of its utilization outweigh the costs of its production. To address this it is necessary to consider the material inputs that would go into the production of these devices. Probably the most costly component is the photovoltaic array, which would provide an input of energy for the devices. Currently Solar technology costs approximately US $5.23 per Watt peak (as of this writing) (solarbuzz.com, 2005). The exact watt-hour usage of the devices would be difficult to estimate, and would vary from device type to device type, but even if current production costs for photovoltaic cells make manufacturing on a very large scale unfeasible, within the foreseeable future, various technologies like thin film silicon deposition will allow for efficient solar cells to be produced using just 1% of the Silicon inputs compared to existing technologies, dramatically cutting the production and materials costs (Hamakawa, 2004).

Figure

Such advances could permit the wide scale production of the power source for the device at reasonable market prices. Other components such as CPUs, storage batteries, body material, pneumatic and hydraulic pumps, all of which are found in domestic and other appliances, are already produced at ‘throw away’ market prices. ‘small’ to ‘medium’ sized examples of robotic vegetation (from 1 to 20 feet high) could be produced en masse, (with thin film silicon cells) depending on demand, whereas much larger examples would conceivably be produced at higher costs in smaller quantities for more specialized purposes.

CONCLUSION

One potential issue is water. The World is becoming more water-limited, and as such it may be harder to justify the potentially large throughput required by these devices. Additionally restoration through conventional means let alone unconventional ones is seldom an issue in areas where there is ready access to water. Even in water-rich areas, these devices could still play a crucial role in restoration, The very presence of a device that could be scaled to interact with its environment on the level of larger examples of vegetation, like trees, is an advantage. Damaged land may only be restored completely through succession, which takes time. Using synthetic ‘trees’ for example, those stages of succession can be bypassed, and the tempo of restoration accelerated. Another factor is the cycling of single devices. One device could be ‘uprooted’ and made to service a multiplicity of locations in a large area undergoing restoration, perhaps on a rotating basis, which could negate the need for large-scale strategic re-vegetation requiring extensive human effort. Perhaps the greatest potential role for these devices may be environmental engineering in areas where water is not an abundant resource, like in regions that have undergone or are undergoing desertification. In these regions, artificial bodies of water could be constructed as either openair reservoirs or as underground cisterns. This centralized water source, which would be replenished periodically, could ‘feed’ synthetic vegetation devices arranged in an extensive network around the main water source, which could then put that water into the soil and promote secondary growth. These secondary growth ‘patches’ would undergo successive radiation away from their synthetic vegetation ‘hubs’, until they ‘linked’ up with other spreading patches of secondary growth, resulting in large scale re-vegetation.

ACKNOWLEDGEMENTS

The author would like to thank K. Lackner and J. K. said for their contributions to developing this idea.

REFERENCES