Limitations of Existing High Heat Flux Technologies
Heat transfer systems using water based pumped liquid or heat pipe technologies are well suited to the zero to 100C temperature range. They afford good throughput and can achieve very high flux levels, but they have limitations outside of this temperature range.
Pumped liquid systems have limitations related to their operating and storage temperatures, as well as system maintenance. Maintenance can be minimal on small cooling systems, but larger systems often require sophisticated control of the cooling fluid condition. Industrial sized plants using glycol-water mixtures may include deioniser systems which require periodic attention. The addition of deoxygenation plant and chemical inhibitors may be used to minimise corrosion. High temperature cooling plant, used on concentrated solar collection schemes for example, have different maintenance issues. Organic fluids operating at above 350C produce breakdown products which need to be removed from these systems. Vapour pressure of fluids at the higher temperatures can also be an issue. A number of fluids can operate at 350C to 400C, but they generally have high viscosities at lower temperatures and may require trace heating to remain in the liquid state.
Consideration may also need to be given to: technical constraints, surface and/or axial flux issues, operating temperature range, absolute power/size limits, parasitic losses, source to sink distance, operational environment, material compatibility, pressure, acceleration, safety conditions, failsafe on over pressure or temperature, cost, end of life decommissioning, and maintenance.
Considerable research and development has been undertaken to extend the operating envelope and use of the existing heat transfer technologies, including two phase systems such as heat pipes (in all forms), the use of supercritical fluids such as CO2, inorganic salt mixtures and molten alkali metals.
As power densities increase and more difficult operating environments are encountered, alternative heat transfer technologies or approaches to cooling become attractive, if not essential.
The FASTT Technology
The FASTT technology offers a new and simple alternative solution to heat transfer which enables heat transfer operating envelopes to be expanded. It is particularly suited to high heat flux and wide operating temperature range applications and allows integration of the heat transfer medium with the output sink in a single device. High heat flux at cryogenic temperatures is significant relative to other technologies.
The core of the technology comprises solid foils passing slowly through a slotted thermally conductive base or bases. Heat is absorbed as the foils pass into a conductive zone and rejected in a convective transfer zone. The technology does not use liquid or vapour as its working medium.
Designs can be tailored to the application and may include both rotary and linear forms. In a rotary form, the foils rotate slowly through one or more multiple-slotted heat input baseplates as shown in the animation below.
In a linear form, as shown below, foils may reciprocate or run as a continuous loop through the baseplate(s).
The size, number and arrangement of the foils can vary according to the application, operating temperature range and heat flux required. The foils can be polymeric, metallic, ceramic or composite. The slotted thermally conductive baseplate(s) can also be varied in number, material composition and form (including the number of fins/slots) according to the design specifications. Multiple stacked foils increase the surface area over which heat transfer and dissipation to air can be achieved.
The gap between the foils and the slots determines the thermal resistance from the foil and the slot. This thermal resistance is critical to the operation of the device, both in terms of minimizing the temperature drop and in determining the time constant for the absorption of thermal energy into the foil. Effectively, these thermal resistances place constraints on the foil speed on the power throughput.
Theoretical analyses based on a rotary form predict that heating and cooling flux levels of over 400 [Wcm-2] may be possible with slot to foil temperature differences of 20C. Flux levels much higher than this should be achievable with temperature differences above 20C. Full use of the temperature range capability of the foil material may allow flux levels well above 2000 [Wcm-2] ( = 20MWm-2).
The predicted fall in heat flux performance as the temperature goes below zero Celsius is significantly less than for typical pumped fluid and heat pipe systems, with an anticipated loss in performance of less than 10% down to temperatures of 100K (-173C). This feature makes the technology particularly attractive for a number of applications including the cooling of infrared sensors. An added advantage of using solid materials is the potential to use only a single heat transfer material for the range from below 100K to above 500K, rather than a cascade of devices currently used with existing technologies for some applications.
The use of solid material, in the form of a foil, allows low loss movement of the heat transfer medium through the slot and to the convective output section. There is no liquid turbulence or viscosity effects to be considered; the gas in the gap region is very low viscosity compared to any liquid. This feature results in very low parasitic losses for the drive mechanism and as a result the added mass of the drive can be small.
Engineering design analyses of devices using the FASTT technology may adopt a similar approach to that used for specifying normal liquid cooling devices. The substitution of an “equivalent” heat transfer coefficient allows for immediate assessment of the foil/slot parameters using standard fin formulae. The “equivalent” heat transfer coefficient being simply h.t.c. = (thermal conductivity of gas in the gap)/(gap between foil and slot). Rapid assessment of the technology is thus possible for specific applications.
Prototypes have been constructed to explore various aspects of the new technology. These include high flux air-cooled, cascaded section (to increase input to output flux ratio), thermal transport over distance, small as well as medium sized rotary devices and more recently linear forms.
Initial demonstration of the FASTT technology used a target specification based on the US DARPA MACE initiative in terms of power density, overall mass and parasitic input power (fans plus drive). The specification required: a package volume of 64[in3], dissipation capability 1000 W, parasitic input (device loss and fan) < 33W, overall thermal resistance <0.05 [C/W] and overall mass (including fans etc) < 800gm. An early prototype, which was pressure assisted, achieved a performance of 815W, with a core of 750g, and less than 3W for the active component (without fan). The photos below (left to right) show top and side views of the device.
Scalability of the technology to higher powers and using multiple heat sources has also been demonstrated with a rotary form prototype capable of over 1300W dissipation. Further increases in power throughput can readily be achieved by increasing the height of the foil stack.
A linear form prototype, as shown below, comprising of a simple reciprocating pack of polymeric strips moving back and forth through a centrally placed slotted core, achieved 135W.
The Foil and Slot Thermal (FASTT) Technology Benefits
The theoretical analyses and experimental results to-date show that the FASTT technology has significant potential as an alternative high heat flux heat transfer technology, particularly where existing technologies may be limited or have usage issues. The FASTT technology development is considered to be of similar significance to that of heat pipes and micro channel devices. Its benefits include:
- a wide operating temperature range and high thermal flux performance
- a solid transport medium, not liquid or vapour, as its working medium therefore no boiling or freezing issues
- predicted equivalent heat transfer coefficients of greater than 75000 [Wm-2K-1] at room temperature reducing to 45000 [Wm-2K-1] at 77K
- application to either heating or cooling over a temperature range from below 100K to above 1000K
- does not use hazardous or toxic materials (cf heat pipes and pumped coolant technologies operating at >300C or <0C)
- the ability to transfer heat over several metres from source to sink
- scalable from watts to at least tens of kilowatts
- high heat flux at cryogenic temperatures which is significant relative to other technologies
- a scientific basis that is well understood
- good mathematical modelling correlation with physical experiments
- production techniques for the manufacture of the parts that are well established
- the potential to merge the core heat transfer medium with output sink functions in a single device. The core working material acts as the direct to ambient air heat exchanger as well as providing the thermal transport medium. This overcomes a fundamental issue, often omitted by those providing competitive technologies, in that some form of heat exchanger to ambient air is required in addition to their core heat exchange mechanism
- at high flux levels operating at very low differential temperatures without an inherent “pedestal” differential temperature seen in two phase (liquid to vapour) systems. There is an advantage of “straight from cold storage” operation in sealed FASTT devices
- potential to reduce overall system mass in certain applications
- operates in the absence of high internal pressures enabling thinner envelope walls (for sealed versions) and lower thermal resistance.
- unsealed forms of FASTT devices do not require an envelope to contain the working material, no reservoirs or pipework. Sealed systems using helium or hydrogen can provide much higher fluxes than the unsealed versions, and although an envelope is required for these cases they can operate at internal pressures of no more than ambient and thus can be very thin.
- unsealed versions offer a solid, recirculating material that may be readily cooled directly by ambient air blast, with no intervening barrier or additional heat exchanger.
- can be used either for heating or cooling applications over a temperature range from below 100K to above 1000K. The large input to output flux ratio capability can be tailored to provide a concentrated heat source at high flux from either several low flux sources or from a distributed low flux source. Likewise a high flux input can be coupled to several low flux output sinks.
Comparison of Existing High Heat Flux Technologies with the FASTT Technology
Existing technologies used in high heat flux applications include pumped liquid (micro channel) and heat pipes. A diagrammatic comparison of the FASTT technology versus heat pipe and micro channel cooling technologies is given below.
Direct performance comparisons with existing products based on existing technologies are not possible at this stage due to the absence of production devices based on the FASTT technology. Products based on existing technologies have generally been developed/optimised over many years. The FASTT technology will need to be applied to a number of different target markets before any optimisation can be undertaken. When actual product is available using the FASTT technology sensible comparisons can be made. However, the theoretical analyses, supported by prototypes indicates that even without optimisation the technology has considerable potential.
The work undertaken to-date shows that the FASTT technology offers a new and alternative approach to heat transfer and has significant potential for applications in many markets. Like all new technologies, further research and development will be required to identify appropriate applications, develop products using the technology and for it to gain market acceptance.
Despite the general comments regarding the assessment of the FASTT technology with competing existing technologies, there are applications where the current technologies are limited or can not be used, and where the FASTT technology offers significant benefit. It is within these areas that initial commercial efforts are likely to be focused.