Ewod-controlled dual liquid prism for adaptive beam steering

Solar energy accounts for the greatest source of renewable energy on the earth. The use of photovoltaic (PV) and concentrating photovoltaic (CPV) technology has been the most promising method of harvesting solar energy. These CPV systems often require bulky motor-driven tracking devices to steer the sun’s beams onto solar cells. The cost of providing and maintaining these tracking systems is the primary inhibitor for widespread application. The purpose of this work is to overcome the need for mechanical solar trackers through the use of an electrowetting-driven solar tracking (EWST) system. The electrowetting-driven solar tracking system consists of an array of novel electrowetting-assisted dual liquid prisms, which are filled with immiscible fluids that have large differences in refractive indices. The naturally formed meniscus between the fluids can function as a dynamic optical prism. Via the fullrange modulation of the liquid prisms, incident sunlight can be adaptively tracked, steered, and focused onto concentrated photovoltaic cells through a fixed optical condenser (Fresnel lens). Furthermore, unlike the conventional, cumbersome motor-driven tracking systems used today, the liquid prism system introduced in this work would be suitable for rooftop application. The results of this work reveal that the EWST system has the potential to generate ~70% more green energy at 50% of the conventional PV capital cost. NOMENCLATURE θ contact angle ɛ dielectric permittivity γ surface tension d dielectric thickness θy contact angle with no voltage applied c unit area capacitance of the dielectric R reflectance n refractive index INTRODUCTION Solar radiation is the largest source of renewable energy on earth; however, harvesting this wealth of energy on a commercial scale has proved difficult, owing to the fact that conventional methods of transforming solar radiation into electricity are highly inefficient. Conventionally, solar energy is harvested through the use of solar panels with photovoltaic (PV) or concentrating photovoltaic (CPV) solar cells (Luque and Andreev, 2007; Neufeld et al., 2008). However, these PV and CPV cells have run into the same problem: solar cells generate the most energy when the incident solar radiation is hitting the cell at a perpendicular angle. Once the incident radiation begins to hit the cells at an oblique angle, the efficiency drops sharply (Roth et al., 2004). Consequently, people began to devise various methods to track the sun throughout the day. By using methods such as mechanical trackers, people have been able to nearly double the energy output and increase efficiency by nearly 40% compared to stationary PV or CPV cells (Huang et al., 2011). Today, the most common method of harvesting solar energy is through the use of concentrating photovoltaic (CPV) systems. Current CPV cells are significantly more efficient than traditional silicon-based photovoltaic cells (~40% compared to ~20%). However, the widespread use of these CPV cells has been slowed drastically due to the high cost of the multijunction III-V cells. To reduce this cost, people have been developing optical condensers to focus solar radiation onto a smaller CPV cell to harvest the concentrated radiation. The vast majority of concentrators require a system to adaptively track the sun as it moves across the sky, therefore ensuring maximum energy harvesting throughout the entire day. One of the most widely used solar tracking methods is through the use of mechanical, motor-driven dual-axis tracking systems. The use of mechanical trackers incurs additional complexity due to the increase in mechanical moving parts. In particular, bulky passive heat sinks or active cooling blankets are needed in order to ensure the CPV cells do not overheat and breakdown (Mousazadeh et al., 2009). The addition of all of these mechanical parts makes this type of system cost ineffective since it is very power-hungry. In addition, there is a staggering cost to maintain these systems. Lastly, the large, bulky and heavy mechanical system is not suitable for rooftop application. To avoid the use of mechanical moving parts, Pender (2005) patented his motion-free tracking system, which is made up of a beam deflector and a fixed optical condenser that centers the radiation onto a small area. The beam deflectors are made of a pair of adjustable prism arrays made of liquid crystals. By applying varying degrees of intensity and distribution of an electric field, he was able to modulate the refractive index of the liquid crystal. By changing the refractive index of the liquid crystal, the system is able to keep the direction of the deflected beam fixed while the incident beam shifts. Currie et al. (2008) put forward a different approach to harvesting solar radiation. Currie developed organic solar concentrators that are planar waveguides with light absorbed by a thin-film organic coating on a substrate surface. This system gives rise to a near 10-fold increase of power output due to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics