Trees are photoautotrophic organisms because they produce sugars with the help of solar energy, using the process of photosynthesis. During photosynthesis, plants also consume water (H2O) and carbon dioxide (CO2), and produce oxygen (O2). The process of photosynthesis (See Figure 1.) mainly occurs in the leaf cell’s chloroplasts and converts solar energy into chemical food for the plant. The green colour of the leaf comes the red and blue light-absorbing pigment located in the chloroplasts, known as chlorophyll. Chloroplasts are found in the leaf mesophyll cells, averaging approximately thirty to forty chloroplasts per cell.

There are two different processes in photosynthesis: light reactions (photophosphorylation) and the Calvin-Benson Cycle. First, light is absorbed by chlorophyll. The light energy then drives a transfer of electrons and hydrogen from water to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate). Water splits during this process, which is why the plant is able to give off O2 as a byproduct. The electron carrier, NADP+ uses solar energy absorbed by the chloroplasts to reduce it to NADPH by adding two electrons and a hydrogen. Adenosine triphosphate (ATP), the versatile energy currency for cells, is also generated during this process.

After NADPH is formed, it goes through the Calvin-Benson Cycle along with ATP. Carbon dioxide is then accepted by a five-carbon simple organic molecule in the chloroplast with the assistance of ATP and NADP+ in the presence of a catalytic enzyme. The formation of organic compounds (sugars and organic acids) in the Calvin-Benson Cycle is known as carbon fixation.

Most trees grow via what is called a fractal pattern. (See Figure 2.) After a certain period of time, they split into two or more new forks. These forks then will in turn split into two or more new forks and so on.  It is affected by sunlight, as branches try to grow towards the light for optimal absorption. The branches are organized in a horizontal distribution manner so that there is not too much shading. Some slightly overlap, but the needles below receive at least twenty percent of the full sunlight it should receive.

Coniferous trees have branches positioned at angles, which are highly efficient for capturing sunlight. The spruce tree and other species and genotypes have shoot, or apical forms. They grow upward with most branches at acute angles. Some tree species would respond to applied gibberellin hormones best in the roots by increasing the upward angle of their lateral branches. Their growth is affected by mechanical factors. This includes holding a branch, flexibility, swaying in the wind and environmental conditions. The tree must be able to function effectively at ambient temperatures throughout the seasons, be able to handle competition for space with other trees and have good access to direct sunlight, reflected light and ambient (natural) light.

However, for every species and many genotypes within that species there will be a wide range of individual examples which are not optimal for maximum capture of photosynthetically active radiation.  Needles of most species not only capture light, but they also reflect some light onto neighboring needles (Figure 3) and back into the atmosphere. Needles also transmit light of lower intensity to needles below, which are shaded. The trait of genetic predisposition of the branch angle is highly heritable. Thus, fine-branched trees within a wide range of genera and species have a high probability of producing seedlings and saplings that also have fine branches. Coarse-branched trees, which have larger diameter branches that are often inclined upward, have a high probability of producing progeny with large, rapidly growing branches. However, if one grows a tree that is genetically predisposed toward fine branches in an open site with no or few surrounding trees, even that finely branched genotype can produce somewhat coarser, lateral branches, which tend to be inclined in an upward angle. This type of phenotype plasticity is known as the genotype X environment interaction.

Most of the naturally occurring genotypes of spruce tree seem to have finely branched genotypes as the norm, likely because they usually grow in relatively dense stands. A coarsely-branched tree phenotype can be thought of as a tree that is more likely to be successful in an environment that requires rapid growth. A tree that can quickly cover a large area of ground with shade allows it to invade open habitats that might usually have grasses or shrubs as the dominant vegetation. Coarsely-branched trees appear to be genetically programmed to apportion varying amounts of their photosynthate to branch wood. As such, they are programmed to retain a large amount of photosynthate in their branch wood as opposed to transporting it to the main stem. These branches are large, and tend to be horizontal next to the stem, but bend toward an upward acute angle as they grow outwards from the stem. The converse is a tree that is genetically programmed to apportion most of its photosynthate to the main stem. Branches are fine, and the branches tend to be inclined downward. A branch angle is the angle (in degrees) of a branch’s outside connection to the main trunk of the tree.

The upward (Figure 4.) and downward bending of branches seems to be governed by the interaction of several plant growth hormones, namely the gibberellin class of growth hormones and auxin (indole-3-acetic acid). Gibberellins can cause individual cells to expand and increase cell division in the apical and cambial meristems of trees, which govern shoot elongation and branch diameter growth. Auxins can cause cells to expand, and appear to act together with gibberellins, and may induce, or trigger gibberellin synthesis. If cells in the tip meristem, sub-apical meristem, and the cambial meristem grow more than the cells on the upper side of the branch, the branch grows at an upward angle.

Conversely branches where the bottom-side cells do not grow more than cells on the topside, since they cannot successfully resist the pull of gravity tend to grow downward. Horizontal branches lie somewhere in between, having more cell growth on the bottom than on the top to counter gravity. Downward growth versus upward growth seems to be governed by a balance of the two hormones, gibberellins versus auxins. Speculatively, an imbalance in auxin or gibberellin levels may mean that the tree will allocate less photosynthate to the branch wood. If so, the branch angle would likely be less acute and the branch may grow downward due to this differential lower side reduced cell growth versus upper side greater growth.

In terms of photosynthetic efficiency in the form of carbon fixation per plant, a large rapidly growing tree with coarse, large branches which are horizontal or tend to be somewhat upwardly angled, would seem to be most efficient genotype. Finer branches will tend to be angled downward, thus, coarser branches would be more productive in photosynthetic processing. Acute angles may seem to be most efficient in capturing light within in a stand of trees, but downwardly angled branches can perform the same task just as efficiently.

The efficiency of acute angles is moderated by the genetically determined ability of needles to continue to photosynthesize under ever-diminishing levels of mutual shading by other needles above them. In an open habitat, the taller the main stem and the larger the large lateral branches, the more efficient it is because it has the greatest exposure to more solar energy. Additionally, one should keep in mind that a pioneering tree that has thick needles or leaves may be highly efficient because the majority of radiant solar energy is captured, regardless of branch angle. Therefore, trees have what appears to be a highly efficient design for collecting sunlight. Based on the evidence provided above, it can be concluded that a tree’s branch angles can be studied to determine the optimal angles of solar collectors for creating a solar tree.

Photovoltaic panels are powered by light photons, a form of energy that travels at an extremely fast speed. (See Figure 5.) When a photon strikes an atom, it interacts with the electrons and is absorbed. The added energy can drive one of the atom’s outer electrons off. The sun’s surface is approximately 6000 degrees Celsius, and can provide a maximum of one thousand watts per square meter of energy. Each photon that reaches the earth has all of the energy it possessed eight and a half minutes ago when it left the sun.

Solar panels are made of silicon, a semiconductor material that absorbs the sunlight’s energy. This energy hits electrons in the material, pushing them loose to cause movement within the atoms. Silicon atoms have fourteen electrons each, arranged in three different shells. The inner two shells are full of electrons, whereas the outer shell has only four electrons. A silicon atom attempts to fill the last four electron spaces by sharing electrons with its neighbouring silicon atoms, forming the crystalline structure of the panel. In the cells, impurities in the silicon such as phosphorous are added to fill up the electron spaces. This allows for one positive proton and electron to remain, as phosphorous has five electrons to fill up four spaces. Because of this, bonds with other atoms are weaker, and allows for easier separation of electrons. Such silicon is known as the Negative-type silicon, which is known as the most commonly effective conductor for photovoltaics.

Positive-type silicon is made using boron atoms, which have three electrons each. Since there are extra electron spaces, the positive charge can be carried at this location. Both of these types of silicon are then combined together to meet at a junction point where extra electrons fill spaces. This generates an electric field, forming a barrier to make it more difficult for electrons to cross from negative to positive. The field acts as a diode that creates an external current path for an electrical current to pass through. This causes disruptions in electrical neutrality, and generates voltage throughout the panel.

The panels have been designed to be impervious to ultraviolet and thermal degradation, resistant to weathering, have minimal expansion and contraction, and transmit at least 86% of the energy absorbed. They also provide good aesthetic value and require a minimum amount of maintenance procedures once every twelve days or so depending on location and latitude. (A typical photovoltaic panel is shown in Figure 7.)

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