Harvesting diurnal temperature fluctuation with wax

Firstly, there is a general point that should be made about the subject of solar Architecture and solar technologies, it is misleading to think that it is relatively new and driven by recent technical developments and a industrialised society. The evidence of buildings constructed with solar design principles stretches over several thousand years, this demonstrates that historically it has been well understood. If the interested reader needs convincing then one place to start is to read "A Golden Thread"^[4]. The sub-text to every chapter in that book is topical, efforts to demonstrate the feasibility of exploiting the free clean energy from the sun arise from the depletion of the easy and cheap alternatives of the day. It was true for Greece thousands of years ago when they faced severe timber shortages for after deforesting vast areas of their country and it was true in the 1970s during the oilcrisis. Would it not be a good idea for significant investment to be made ahead of the curve?

Butti and Perlin's book makes the point that once a easy and cheap alternative to exploiting solar energy has been found (or fought for) then the pressure is off, unfortunately the technology and knowledge disappears with it. The book chronicles the ironic legacy that many of the necessary technologies are known, if only those so inclinded choose to look.

For clarity it should be explained that this project is not about storing thermal energy in liquid wax in large quantities as an alternative to using a thermally massive material e.g. concrete or stone. Using wax phase-change to store heat energy is a subject of interest for researchers because one of the properties of wax its high Latent heat of fusion. This research project is about using small quantities and exploiting another property of waxes, its high volumetric expansion when wax melts.

The principle is simple, when wax melts it expands and when it freezes it contracts. Most substances do this but what makes wax interesting (and useful) is that it expands a lot, between 7% and 20% by volume. The material properties of waxes during phase-change affect the amount of expansion depending on the type of wax and its purity and how much pressure it is placed under. The phase-change of certain waxes can occur at temperatures that are typical around buildings which means it is a phase-change that can be passive. The 'phase-change' of wax from solid-to-liquid has several distinct properties in common with other substances and together they are often referred to as 'phase-change materials' or PCMs.

The temperature at which the liquid-to-solid transitions takes place when it cools it the wax's melting-point. Wax is capable of withstanding a lot of compression load when it is a liquid and even more when it is a solid, therefore its expansion when melting can be used to do mechanical work. In simple terms, wax as a material can be used to convert thermal energy into mechanical energy. This can be done completely passively, that is without any extra energy being transferred other than that available from the difference between warm daytime conditions and cooler night-time conditions.

Wax is highly incompressible both as a solid and liquid, therefore it can be used to generate a large hydrostatic force. For example, if the wax is contained inside a metal cylinder with a seal and a sliding piston, it can drive the piston just like a hydraulic actuator. In fact, wax has been described as a 'thermo-hydraulic' fluid.

Using wax as the hydraulic substance in mechanical actuators is an established art in many industries, the most relevant to this project is the greenhouse ventilator. The wonderfully simple device is completely self-contained and self-powered while being capable of lifting open a 15kg window, the design for the motor has been granted a Patent^[1] and numerous mechanisms that exploit the principle of a heat-motor have been the subject of further Patents^[2][3].

Unfortunately, the literature on the subject calls these devices by many different names; wax motor, wax actuator, HOP actuator, wax motor etc. It is a device that converts thermal-to-mechanical energy and so might fairly be called a 'heat-motor' simply because that describes what it does when operating.