Solar Tracking

Prototyping Passive Sun-Tracking Systems with Thermally Active Plastics

Passive Solar Tracking is an exploration the challenges and benefits of using thermally active materials to actuate a sun-tracking surface. Orienting a surface perpendicular to the sun throughout the day has potential benefits for both solar energy generation and daylight management. The reliance on inexpensive passive materials rather than a mechanical actuator can reduce complexity and operational energy. In this project, several prototypes were constructed that use a combination of thermally active plastics to rotate a surface to face the sun throughout the day. 

Architects will always have to contend with the challenges of solar energy, either by providing protection from it, or by harvesting it for energy and daylight. While static solutions are effective for the north and south facades, there is no optimal solution for the east and west. Photovoltaics are becoming more efficient and widely used every year, but the limiting factor is the timing imbalance between mid-day peak energy production and morning and evening peak demand.
The solution to both the daylighting problem and the energy problem is a dynamic system that rotates to face the sun at any hour of the day. Several dynamic solutions exist, but the benefits are partially offset by the technical complexity, material cost, or operating cost of the system.
The following prototypes are dynamic sun-tracking systems that rely only on thermally active materials to operate. This eliminates the need for an external power supply, and makes solar energy both the object and actuator of the system. They were developed as part of the University of Texas at Arlington Digital Architecture Research Consortium, and later as part of the Lake Flato Research and Development Program.

Benefits of Passive Solar Tracking

The US electrical grid is an aging system that is becoming difficult to maintain. A transition to on-site solar power would make the grid far more resilient, efficient, and less vulnerable to outages. Solar power is becoming less expensive and more efficient every year. By rotating a photovoltaic panel to track the sun perfectly throughout the day, it can increase the total power generation by 25 to 33%. This is a very significant margin of improvement considering that PV efficiency only increases by a few percentage points every few years. While tracking is highly beneficial, a high degree of precision is not. A perfectly aligned systems collects 100% of the potential power, 15 degrees of misalignment still gathers 97%, and a full 30 degrees of misalignment still gets 90% of potential power. A solar tracker does not need to be perfect, but needs to rotate just enough to capture most of the benefits of alignment.

Existing Passive Systems

There are already passive sun tracking systems that exist today. Many of them use liquid freon or another highly responsive, volatile liquid as the thermally active component. The liquid condenses in certain areas when exposed to sunlight, causing a shift in the center of gravity that tilts the array towards the sun throughout the day. The problem is that these systems can be about 25-30% more expensive than static systems and the benefits of tracking are outweighed by their added cost. Volatile liquids are also a potentially hazardous and expensive actuator.

Even if solar power becomes extremely inexpensive in the future, and maximizing solar power through tracking is not necessary, architects will always require dynamic solutions for improving the qualities of interior spaces by controlling glare, solar heat gain, and natural light. Some systems are being developed as an architectural shading application such as these prototypes by DOSU Studio Architecture.

These leverage the reactivity of thermal bimetal, a composite of two metals that bend when heated, and fold over to obstruct the window during times of high solar heat gain. The drawback of this is that a completely transparent glass surface becomes opaque at certain times and loses all visual transparency. A binary compromise between transparency and opacity is materially inefficient and not ideal for controlling something as dynamic as heat and daylight.

To address this challenge, while working at the Digital Architecture Research Consortium at UT Arlington, we developed a bimetal facade system that remains diagonally aligned and allows more solar heat gain during cold temperatures, and then folds over to block solar heat gain during warmer temperatures, while always maintaining visual transparency perpendicular to the window.

Bimetal Prototypes

PROTOTYPE 1.0

The first prototypes were built with thermally active bimetal that bends when exposed to direct sunlight. The bimetal is composed of two layers of metal that expand at different rates when heated. Fixed to a 3D printed joint, the metal fins remain unbent and parallel at room temperature, and curl into a more horizontal position when heated. On a south-facing facade, this allows for more solar heat gain from low-angle morning or winter sun when temperatures are typically colder, while preventing solar heat gain from high-angle mid-day or summer sun when temperatures are typically warmer. The fins bend on the axis perpendicular to the glass to maintain exterior views. This avoids the drawback of other bimetal shutter systems in which views are sacrificed when the system is active.

PROTOTYPE 2.0
Later we transitioned from using a bimetal surface as the active component to using a bimetal coil as the active component. These are coils of bimetal that rotate when heated, while the center of the coil remains fixed. These were used to rotate a flat aluminum surface, which improved the rotational ability of the system. It was rather difficult to calibrate the rotation of the coil to the rotation of the aluminum fins, and nearly impossible to keep all the aluminum fins aligned.

By making the coil the only active component, this allowed for greater freedom in the fabrication of the CNC-cut aluminum fin. These fins were lifted and lowered by a wire suspended from the coil at the top of each column of fins. This allowed about 70 degrees of rotation, an improvement from the previous prototype.

PROTOTYPE 3.0
In this iteration, the wire is wound continuously from a disk attached to the center of the coil, to a disk attached to the center of the aluminum fin. With a higher gear ratio, the fins rotate more actively and consistently. The aluminum fins were stamped in a wooden mold in such a way that they could rotate to a more vertical position without colliding with the vertical support. These ridges along the length of the fin also provide rigidity, allowing them to be made from a thinner gauge of aluminum.

PROTOTYPE 4.0
This prototype eliminates all wire or vertical support to decrease points of failure and increase visual transparency. One end of a central axis is adhered to the glass itself, with a bimetal coil on the other face of glass that directly rotates the fin when heated. This prototype is simpler and does not have the complexity of each fin being connected to the others, however, the fins are not likely to be perfectly aligned.

Heat Engine Typologies

Following these initial prototypes, the goal was to achieve true solar tracking; not just open/close compromises, or vertical/horizontal compromises, but true 2-axis rotation that responds directly to the position of the sun. At the start of the research term, I explored a few new options to make this system truly responsive, reliable, and affordable.

Simple ambient heat engines have been understood and utilized for centuries, but never considered as viable sources of energy generation because of their low energy-to-work potential. Rather than being dismissed as novelties, or inefficient power generators, heat engines can be put to work calibrating and increasing the efficiency of photovoltaic systems.

The first heat engine typology I considered was heat-responsive volatile liquids that vaporize or condense when heated, which then cause an imbalance and rotate the system. This is the same concept used in the classic drinking bird heat engine toy. The lower end of the glass container is warmer than the birds head above, so the methylene chloride inside expands upward, tilts the bird’s head into the water which cools the head and causes the cycle to continue. The volatile liquid needs to operate within a vacuum sealed tube, which is challenging to fabricate and maintain. Also, methylene chloride is not an ideal chemical to be handling and using in a prototype. There are no good chemical alternatives that are both as heat-responsive as methylene chloride and less hazardous.

In the next series of prototypes, I explored industrial rubber bands as actuators. Rubber bands contract when they are heated, so if a balanced wheel supported by rubber bands is asymmetrical heated, the heated bands contract, shifting the center of gravity of a wheel and slowly spins it forward. This requires quite a lot of direct heat to make a very small shift, so it has to be perfectly balanced, or it tends to favor one side, rotate in that direction, and not rotate back.

Thermally Active Plastic Composite

The final option was to use a thermally active actuator similar to a bending bimetal surface. I explored a few options in creating an inexpensive and more responsive bi-material alternative. A combination of two thermally expansive and non-thermally expansive plastics will behave as a bi-material that bends when heated. Polyethylene has a very high thermal expansion rate, making it ideal for the thermally active half of the plastic composite. Acrylic packing tape was ideal for the non-thermally active half because it works as an adhesive to the polyethylene and is transparent, exposing the black polyethylene and increasing its reactivity to sunlight.

PLASTIC PROTOTYPE I
Early tests showed that this bi-plastic was very responsive to heat and sunlight. An advantage of making the bi-plastic manually is that it can be adhered flat at room temperature to make it bend when heated, or adhered on a radius at room temperature so it begins in a bent position and then opens flat when heated.

This prototype is a single square of biplastic that rotates on a central axis. It has pre-bent edges, which become unbent on the side exposed to sunlight. This slightly shifts the center of gravity to that side, and causes the whole surface to rotate and face perpendicularly to the sun. The drawback of this simple prototype is that if there are a few of these next to each other in an array, one surface shades the one adjacent to it, preventing it from receiving sun and activating it.

PLASTIC PROTOTYPE II
The next series used the biplastic only as an actuator in the center of a paper panel that pushes a central rudder. This works by placing two biplastic strips opposite of each other along the frame. When the biplastic strip on the sunlit side is heated, it bends towards the center, pushing the rudder of a balanced paper panel and causes it to rotate towards the sunlit side.

PLASTIC PROTOTYPE III
The final prototype worked on the same principle as the previous one, with two bi-plastic strips pushing a central rudder to rotate a paper panel. To keep the angle of the panels consistent, and to ease fabrication of identical panels, the frame of the panel is made from a laser-cut wooden piece that holds the paper surface and rotates on piano wire on the central axis. The paper panels are tilted upward so that the center of gravity of the entire panel is also the center of rotation on the music wire. This is essential to create a balanced system that does not require a lot of force to be rotated. The upward tilt of the panels and the gap between them was determined through computer simulation to maximize the surface area of the panels while still allowing sunlight to reach the thermally active plastic.

Conclusions

In terms of affordability, the cost of the active plastic components is negligible compared to the metal frame it sits on. Bi-plastics offer a truly affordable option for fabricating thermally active components. There were some fabrication challenges with the first prototypes that relied on hand-made origami-like paper surfaces. It is too time consuming to make more than a few paper surfaces by hand. On later prototypes I used a laser-cut wooden frame on which flat, rectangular paper surfaces and rudders were attached. This was a far faster and more reliable way to mass-produce surfaces.

Such a lightweight system is at the mercy of the wind unless enclosed behind glass, and the long-term durability and performance of the bi-plastic is still unknown. There are some trade-offs to be considered between the durability and predictability of the bimetal compared to the affordability and reactivity of the biplastic.