Concentrated PV

Research studies at the University of North Texas claim that electrowetting-driven solar trackers can reduce capital costs by 50% and generate 70% more electricity via CPV. PV Insider investigates how this technique can be applied to CPV.

Solar trackers, a key component in conventional CPV systems, are typically heavy, expensive and unreliable mechanical moving parts that require a lot of operation power (as high as 300 W for SolFocus solar trackers). To overcome these challenges, an alternative, electrowetting-based self-tracking technology is under development, promising a potential breakthrough for the CPV sector.

Designed by Dr. Jiangtao Cheng, associate professor at the University of North Texas (UNT), theelectrowetting-controlled optofluidic concentration system is being developed for adaptive solar tracking and agile sunlight steering without mechanical moving parts. In comparison with traditional silicon-based PV cells, the electrowetting-based self-tracking technology was found to generate 70% more green energy with a 50% cost reduction.

How it works

With two immiscible fluids in a transparent cell, the orientation of fluid–fluid interface can be actively controlled via electrowetting, and the naturally-formed meniscus between the two liquids can function as a dynamic optical prism for solar tracking and sunlight steering.

“Electrowetting means we can change the contact angle of an electrolyte on a dielectric surface by applying a voltage between them. I fill in two immiscible fluids in a glass cuvette and I apply electrowetting in this transparent cell. By applying proper voltages on the sidewalls, I can make the two immiscible fluids interface as smooth as possible – a flat interface.”

“The reflective indices of the two fluids are different. The sunlight hitting on the interface will be directed towards a special direction dominated by the applied voltage on the electrowetting module walls,” Dr. Cheng explains.

“By virtue of electrowetting, I can also make the orientation of the fluid-fluid interface follow the sun’s position. This is called single-axis tracking when activating one sidewall or dual-axis tracking when powering on two neighbouring sidewalls. That’s the mechanism for how electrowetting works for solar tracking and sunlight beam steering.”

An integrated optofluidic solar concentrator can be constructed from the electrowetting-based liquid prism tracker in combination with a fixed and static optical condenser (Fresnel lens). Therefore, the liquid prisms can adaptively focus sunlight on a CPV cell sitting on the focus of the Fresnel lens as the sun moves.

Capital cost reduction

And because of its unique design, electrowetting tracking allows the concentrator to adaptively track both the daily and seasonal changes of the sun’s orbit (dual-axis tracking) without bulky and expensive mechanical moving parts.

This approach can potentially reduce capital costs for CPV and increase operational efficiency by eliminating the power consumption of mechanical tracking. Not only that, but the elimination of bulky tracking hardware, compact design and quiet operation would allow extensive residential deployment of concentrated solar power. Normally, CPV systems with bulky sizes are limited to field-level applications as they cannot be used in residential rooftop applications.

“In our system, the optofluidic solar concentrators allow substitution of cost-effective materials such as Fresnel lenses, glass, and optical fluids for the more costly semiconductor PV/CPV cell materials. And of course, this is solar concentration, thus we can focus a very large area of sunlight onto a very small area of CPV solar cells.

“The area reduction ratio can reach 1000:1. So we only need a tiny amount of the CPV solar cells instead of the whole area of PV. That’s the main source of the capital reduction; we only use a small piece of CPV solar cells, instead of a very large PV panel,” Dr. Cheng stresses.

Different approaches: SolFocus and Suntech

Besides the optofluidic solar concentration technique proposed by Dr. Cheng, there are two other main solar energy harvesting approaches. One uses static and flat PV solar panels without solar tracking capability, as in the case of Suntech Power’s PV panels; and the other employs a mechanical tracking system to focus energy on CPV cells, as in the case of SolFocus CPV systems.

However, the energy conversion efficiency of traditional PV systems is typically 13% to 20%, which means the electric power output from a 100ft² PV panel is below 2000W. And while CPV cells using a motor-driven tracker system operate more efficiently under concentrated light and lead to a higher energy conversion efficiency of around 40% to 41%, traditional solar trackers have expensive mechanical moving parts – sometimes more expensive than the solar cells themselves, with a high maintenance cost.

On the other hand, high-efficiency CPV solar cells are usually made with more pristine materials and complex processes, leading to a high fabrication cost. Typically, the cost of CPV solar cells or modules is about 40% to 50% of the system cost.

The optofluidic concentrator developed by Dr. Cheng can focus sunlight onto specially designed CPV solar cells with a high area reduction ratio, thus reducing the number of CPV cells and claiming to lower the overall power cost by 10 times – in comparison to the 300 W power consumption of SolFocus dual-axis motor-driven tracker system.

“Our analysis indicates that the optofluidic solar concentrators can generate around 3,000W on a 100 ft² rooftop”. This is nearly double the 1,887W generated from a PV panel on a rooftop of the same size, while CPV with motorized tracking cannot be installed on a rooftop in the first place.

In terms of pricing, CPV with liquid prism tracking is targeting a price below $100 per 100W of electric power, compared with the $240 associated with a PV solar panel, and $300 for CPV with motorized tracking.

Another advantage is that the optofluidic technology uses low-frequency square-wave AC signals to enhance the reliability of the electrowetting modules. In contrast to DC voltage driving, the square-wave AV voltage avoids applying a high electric field over a long period to an electrode unit, which significantly increases the reliability and service life of optofluidic solar systems.

Moreover, due to the distributed arrangement of CPV cells, the heat dissipation and thermal management is improved, resulting in exceptional performance of the optofluidic system.

Commercialisation on the cards

At present, Dr. Cheng’s group at UNT is the main team carrying out the optofluidic solar concentrator research, which is funded by the UNT start-up fund.

“We already have papers published demonstrating the mechanism and the functionality of electrowetting-based single solar modules; it can track a 15 degrees sun position. We are continuing to improve the performance and solve the reliability issues associated with electrowetting. The next step is to develop a panel concentrator system. I’m looking for some supporting grants so I can carry out the panel level development. Currently I’m looking out for partners from the industry.” Dr. Cheng tells PV Insider.

“In the long run, we expect the optofluidic solar concentrator technique to be on the market in five years, by 2018 or 2019.”

The findings of the electrowetting-based optofluidic solar concentrator research are available on: