Fluid Displacement Thermal Actuators

Actuators that use a hydraulic medium to transfer material phase displacement as a thermally reactive mechanical force

Similar projects worth following
When a material melts or freezes, it changes volume. As the material expands, it displaces a hydraulic fluid that can be used to do work.

Thermal actuators use the change in volume that occurs when a Phase Change Material (PCM) changes phase between solid and liquid. Traditional thermal actuators require a membrane to contain the material in liquid phase. Fluid Displacement Thermal Actuators (FDTAs) use a hydraulic fluid that is insoluble with the phase change material to transfer the PCM volume displacement to hydraulic mechanisms. Basically, as the PCM melts, it expands and creates hydraulic force. 

Thermal actuators are generally inefficient, but extraordinarily reliable. This inefficiency can be overlooked when the thermal actuator is powered by waste or ambient heat. They are often used as a thermal feedback mechanisms, such as the wax thermostat found in many water cooled engines. Environmental control systems can be created where a thermal actuator reacts to the environment, such as a shade device that deploys when ambient temperature rises above the melting temperature of the PCM.

One advantage of FDTAs over traditional thermal actuators is that they essentially have no limit to the amount of force or displacement generated, allowing for thermally reactive mechanisms on practically any scale. FDTAs can be designed with no moving parts, allowing for extraordinarily high reliability.

Final Capstone.pdf

This is the final printout that was submitted for the project back in 2009.

Adobe Portable Document Format - 27.90 MB - 07/16/2018 at 10:34



Some drawings from early 2009 exploring a folding mechanism to deploy a shade.

AutoCAD DXF - 1.77 MB - 07/16/2018 at 09:46


  • Uses & Paths

    Andrew Benson07/16/2018 at 11:55 0 comments

    When I began this project back in late 2008, I was working with Phase Change Materials (PCMs) as latent heat storage systems with architectural applications. while trying to design a latent heat storage system for solar energy, I destroyed my first prototype storage system when the PCM expanded. I had designed the container to have the exact volume of the frozen PCM, and spent a good week scratching my head after the container failure. In retrospect, I am actually kind of surprised that I came up with the idea of creating a hydraulic mechanism over simply enclosing the PCM with a compressible gas; either way, I discovered Wax Thermostats after this revelation. By combining a latent heat storage system with a usable hydraulic cylinder, I was able to come up with an interesting final thesis project.

    My initial designs were for environmental control systems that would allow buildings to both respond to their environment, as well as store thermal energy. I went a bit off the deep end trying to come up with boolean logic thermal actuators, and otherwise trying to find the limits of the technology. After graduation, I attempted to write a patent for the system, but failed to articulate the limits of the system before the "one year after public exhibition" deadline prevented me from patenting the invention. I burnt myself out on the project, and threw my endeavors into digital fabrication and open source entrepreneurial endeavors.

    In the passing years this project has haunted many hours of speculation. I have sketchbooks filled with drawings of skyscrapers that radically change forms with the seasons, walls that prevent conduction when the thermal batteries are full, and rivers that are pumped uphill by daily temperature oscillations. Before these designs can be taken much further, some pretty basic research needs to be done on optimizing the design - hence entering the project in the hackaday prize. I hope that by publishing this project openly on the web, others may explore this simple concept further.

  • Moving Forward

    Andrew Benson07/16/2018 at 11:01 0 comments

    For the next iteration of the design, I decided to machine a basic FDTA out of Aluminum. Previous versions of the device have been made out of glass or acrylic to allow visual inspection of the PCM. Now that I have experience with machining Aluminum, I can explore higher pressures than were possible with glass. I designed this actuator around some scraps I had in my shop, with a usable stroke of approximately 40mm.

  • Intermediate thoughts

    Andrew Benson07/16/2018 at 08:34 0 comments

    I threw together some videos of the project in the years since I graduated:

  • Project Origin

    Andrew Benson07/16/2018 at 07:42 0 comments

    This project was my graduating thesis from the University of Arizona College of Architecture in 2009. When I saw Thermal Actuators show up on Hackaday, I figured I should dust off the project and have another go at it. When I built my first Fluid Displacement Thermal Actuators (FDTAs), I was limited to my college's laser cutter for digital prototyping. I have since built a modest fabrication laboratory, and am excited to revisit this project with nearly a decade of personal development (and frequent contemplation of the project). When I was working on the project, I did extensive research into Thermal Actuators, and to the best of my knowledge this is a unique design. Of all the thermal actuator patents I sifted through, few used a hydraulic medium, and all of those did so with the assistance of a flexible membrane between the Phase Change Material (PCM) and the hydraulic fluid, severely limiting the stroke length.

    Me, demonstrating the self deploying shade mechanism I designed (2009)

View all 4 project logs

  • 1
    Basic Requirements

    To build a Fluid Displacement Thermal Actuator (FDTA), you need three things:

    1. Phase Change Material (PCM)
    The PCM creates the change in volume that powers the device. The PCM can be selected on a number of criteria: melting temperature, repeatable performance, container compatibility (i.e. corrosion), compressibility, displacement percentage, and compatible hydraulic medium being of primary concern. My previous research in PCMs focused on paraffin wax and Polyethylene glycol 600. Paraffin Wax is an affordable, easy to find PCM that has favorable characteristics. It begins melting at 37 °C, expands by more than 10%, and can create functional expansion under high pressure. Polyethylene Glycol is rather expensive, but the melting temperature of the material can be altered by changing the molecular length of the hydrocarbon. Currently I am exploring the use of biobased commercial PCMs. 
    2. Hydraulic Fluid
    The hydraulic fluid transfers the expansion of the PCM into usable work. It is important that the hydraulic fluid be insoluble with the liquid phase of the PCM, and that it stay liquid within the operating temperature of the device. In the past I have used water/antifreeze as the hydraulic fluid with paraffin PCMs, and oil as the hydraulic fluid for Polyethylene Glycol PCMs.
    3. Container
    The container resists the expansion of the PCM and includes the hydraulic mechanism. I have generally used hydraulic cylinders, although other hydraulic mechanisms could be used. The container must be oriented such that the stratification of the liquid PCM and the hydraulic fluid prevents the PCM from solidifying in the hydraulic mechanism. I have found that some amount of back-pressure on the device is also helpful for creating reliable contraction of the device. Design criteria for containers include: operating pressure, displacement volume, hydraulic mechanism, and thermal flux.
    The first FDTA I built was a glass bottle connected to a hydraulic cylinder with tubing. It lifted an arm whenever the ambient temperature rose above 20°C.

View all instructions

Enjoy this project?



Similar Projects

Does this project spark your interest?

Become a member to follow this project and never miss any updates