Solar Thermal Photovoltaic Tree: An Amalgamation with Considerable Potential
Consumption of fossil fuels (primarily CO2, CH4, and N2O) for energy production, agriculture, transportation, and industrial applications is accelerating climate change as we speak. Since the effects of climate change aren’t immediately evident, there’s a lack of awareness about it in a huge
portion of the global demographic. An increase in temperature, changes of precipitation patterns, strengthening of natural extremities, rising of sea-level and melting of ice in the arctic region may very well hamper our economy as a civilization. There’s even the probability of large-scale migration of people in severe scenarios. The following figure shows the predicted temperature
increase due to GHG emissions.
At the current rate, we can expect to see an increase of 4 °C in the global temperature by the end of this century, which is quite concerning. The mass expenditure of fossil fuels at the same time contributes to air and water pollution thus affects human health adversely. So, finding an alternative source of energy that is safe and renewable has become one of the sine quibus non of survivability for human beings.
Radiative Sky Cooling & Thermoelectric Generators
Radiative sky cooling is a process where the cooling of a system is possible by promoting radiative heat transfer with cold space in the 8 to 13 μm atmospheric window. It was popular in some parts of ancient India and Iran. The reason behind its reintroduction is the rapid progress in nano-optics and nano-fabrication. We are no strangers to the fact that we receive about 173K Terawatts of power from the sun in just one hour, while the global energy consumption is approximately 16T Watts which means, if we could use the sun’s total hourly power, we wouldn’t need to generate any excess power for about one year. As a result, sustainable alternatives to fossil fuel, e.g., photovoltaics, thermal photovoltaics, and other solar thermal technologies rose over the last century which harvests (Energy harvesting refers to the extraction and conversion of energy from ambient sources) the sun’s energy. But these techniques are depended on daylight and there were no feasible methods of harvesting energy during the night when energy demand peaks until recently a heat engine has been proposed that works on the radiative cooling principle, exploits the ambient surface temperature and the coldness of vacuum. Not long ago a device was implemented using a blackbody aluminum radiator on the cold side of a thermoelectric generator while the hot side was designed to be heated by ambient air. This device was reported to generate 25 mW/m2 of power at night. Due to the ‘Seebeck Effect’, heat is converted to electrical energy in a thermoelectric generator. To clarify the operating principle of a TEG we can refer to the following figure (figure 2). Th is the temperature of the hot junction and Tc of the cold one. When either Th > Tc or vice versa, thermal diffusion takes place and the charge carriers ambulate from one conductor to another, and thus Seebeck effect arises. The generated voltage depends on the thermal and electrical conductivity besides temperature and the whole arrangement can be compared to a thermal battery, where Seebeck voltage is the EMF. The output voltage of a TEG isn’t constant and is low in amount, so a DC-DC converter is required to increase it as mentioned in figure 2(b).
General TEGs have a conversion efficiency of 5–15% but when we are discussing thermal energy harvesting TEGs have an efficiency of about 5–6%. At night, the efficiency dwindles even further when referring to the device with a power output of 25 mW/m2 mentioned earlier which was constructed in 2019. But, earlier this year, L Fan et al., proposed that by, using a multilayered emitter with spectro-angular selectivity, optimization of environmental convection, and figure-of-merit ZT a power output of 2.2 W/m2 is achievable, which is approximately 88 times of the previous device. To use a TEG during the daytime with maximum efficiency, the utilization of solar concentrators, absorbers, and dual-axis tracking systems are a must. This system is known as a ‘Concentrated Solar Thermoelectric Generator’ which essentially concentrates solar power via a lens on an absorber to increase the value of temperature gradient between the two sides of the Thermoelectric generator in order to produce a higher amount of output with reasonable efficiency (reportedly 15.7% — 23.5%). The incident flux after passing through the concentrator is a function of the flux times concentrator ratio. Due to geographical location, the sun’s respect to earth, atmospheric effects, getting constant solar irradiance isn’t possible, so a dual-axis tracker can bolster the maximum efficiency of the system.
Solar Thermal Photovoltaic Trees
Solar trees incorporate multiple PV modules and other solar energy technologies on a single modular structure. The PV modules are generally placed atop branches while inverters, converters, and storage devices are arranged together in the base of the structure. They serve the purpose of generating power as a complement to rooftop solar panels and spreading awareness about solar energy-based technologies. Large enough structures with many PV modules can generate enough power for agricultural, industrial, and domestic usage by storing the power generated from the modules in high capacity storage devices (batteries). Additionally, solar trees are not restricted by the lack of space and are more efficient where there’s not enough space for placing several layers of rooftop/ground arrays.
Using TEGs in tandem with solar cells can increase the net energy harvested per m2 greatly, this surplus energy could be utilized in a plethora of ways. One being, using an amount of energy for the cooling system of PV modules as the temperature has a substantial impact on solar cells and panels. Typically, a 1°C surface temperature increase of a solar panel accounts for 0.5% degradation inefficiency which can be seen from the following figure 4. The main reason behind this phenomenon is the reduction of the bandgap of a semiconductor due to temperature which in turn affects the open-circuit-voltage along with other parameters.
A TEG as an independent power generating source is also viable but then again using the same TEG during night and day at peak efficiency should be the research focus of this field. The replacement of multilayered emitter (nighttime) with the solar absorber and corresponding layers for daytime optimal performance could be done with the aid of robotic linkages, actuators, and microprocessors. In conclusion, a bottom-up design approach should be employed while designing a highly efficient solar tree with state-of-the-art PV panels (e.g., perovskite tandem cells) and day-night dual functioning TEGs. At preliminary stages, the Solar Thermal PV Tree might not be affordable but with the increase of its popularity, taking the significance into consideration that the whole system is sustainable, its price will eventually fall in a reasonable margin which in turn would help us in reducing GHG emissions by a fair amount.
