ULTRA-COMPACT COOLING SYSTEMS BASED ON PHASE CHANGE MATERIAL HEAT RESERVOIRS

Abstract

A cooling system includes a first stage heat reservoir arranged to absorb heat from a heat source. Heat is transferred from the first stage heat reservoir to a second stage heat reservoir. The first stage heat reservoir includes a material with a heat capacity lower than that of the second stage heat reservoir but with a thermal conductivity higher than that of the second stage heat reservoir. Heat transfer structure increases heat transfer rate from the first stage heat reservoir to the second stage heat reservoir.

Description

FIELD OF THE INVENTION

The present invention relates generally to cooling systems, such as for electronics and the like, and particularly to a cooling system that employs phase change materials.

BACKGROUND OF THE INVENTION

Cooling of power electronics components and diode-lasers has traditionally been performed using systems that transfer the heat to the environment (e.g., thermo-electric coolers, chillers, heat pipes, air-blown heat-exchangers, radiators). Many platforms (missiles, pods, satellites) do not readily allow for the elimination of heat generated within the system because of size, weight, electrical power, operating scenario limitations or other factors. For such applications, the concept of heat reservoirs to store the heat close to its source until completion of a mission can impart significant advantages in terms of simplicity, weight, and low power requirements. While heat reservoirs can operate on the basis of raising the temperature of a material with moderate heat capacity, the use of phase change materials (PCMs) has significant benefit in terms of much higher specific heat capacity and much lower increases in temperature.

However, a disadvantage of PCMs is their low thermal conductivity compared to standard heat sink metals.

SUMMARY OF THE INVENTION

The present invention seeks to provide a novel cooling system that employs phase change materials, as is described more in detail hereinbelow.

The present invention overcomes the low thermal conductivity of PCMs through the use of multi-stage heat reservoirs: a first stage is optimized to contain heat generated during a single heat pulse of finite duration, and a second stage uses a high-capacity heat-reservoir optimized to absorb heat between pulses. The invention may also use advanced heat transfer structures, such as densely packed, thin, diamond-coated copper fins extending throughout the PCM.

The operational challenge in adopting PCM based passive cooling systems is adapting heat generation to a periodically pulsed format and adapting the source to operate effectively despite a temperature that varies between the phase change temperature (equal to the initial system temperature) and the maximum allowable source temperature. Design simulations show that duty factors of many tens of percent can be achieved and that maximum temperatures can be maintained to below typical laser diode maximum temperatures.

Power electronics components and diode lasers generate heat from concentrated areas. Heat densities can reach 1 kW/cm². This heat load must be removed from the area of the heat source and then dealt with. Traditional high capacity cooling systems remove the heat from the system and dump it into the environment. Examples are Freon based chillers, thermo-electric coolers, and forced convection heat-exchangers. Some lower capacity cooling systems utilize heat pipes to transport the heat from the source to the point where it is expelled to the environment.

Another approach would be to store the heat within the system. This has been done in the past by incorporating large amounts of heat spreader metal in the mechanical design. In systems containing laser diodes the material most often used is aluminum [C_p=0.9 J/(gm·K), K=165 W/(m·K)] since it is used anyway for the base to which all of the optics are mounted. In power supplies, copper [C_p=0.38 J/(gm·K), K=385 W/(m·K)] might be the material of choice because of its high electrical conductivity. If heat transfer rate were not a factor, then considerable weight could be saved by switching to some higher heat capacity material. Water [C_p=4.2 J/(gm·K), K=0.58 W/(m K)] is but one example. If the heat reservoir is allowed to increase in temperature by 10° C. in order to store heat energy, then 100 gm of water can hold 4.2 KJ. Phase change materials (PCMs) are much more effective absorbers of heat. Hexadecane, for example, melts at 18° C. (C_p=2.1 J/(gm·K), C_latent=230 J/gm, K=0.15 W/(m·K) and 100 gm will absorb 23 KJ of thermal energy at the melting point and an additional 2.1 KJ if the temperature shifts by 1° C. around the melting point.

Thermal conductivity strongly affects the peak temperature during heating. Thus, placing the heat source on a simple water or PCM cell will not work. The present invention proposes some solutions, such as: a) use a material with lower heat capacity but reasonably high thermal conductivity, b) design a heat reservoir containing fins or other heat transfer structure to speed up the heat flow, or c) use a two-stage heat reservoir where the first stage has high power handling capacity but low integral energy storage capacity and a second stage optimized for high energy storage capacity. One embodiment of the present invention involves the first option in designing the first stage, and the second and third options in designing the second stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

FIG. 1 is a simplified illustration of a cooling system, in accordance with a non-limiting embodiment of the invention;

FIG. 2 is a simplified illustration of time profiles for the heating pulse and for the temperature of the heat source under different cooling scenarios, for the cooling system of FIG. 1;

FIGS. 3A and 3B are illustrations of long and short-term temperature variations, respectively, in a single-stage aluminum heat reservoir, in accordance with a non-limiting embodiment of the invention;

FIG. 4 is a simplified illustration of a two stage heat reservoir with the first stage as in FIGS. 3A-3B but with an added water-based 2^nd stage, in accordance with a non-limiting embodiment of the invention;

FIGS. 5A and 5B are simplified illustrations of using two different PCM materials: hexadecane (FIG. 5A) and gallium (FIG. 5B), in accordance with another non-limiting embodiment of the invention; and

FIG. 6 is a simplified illustration of two-stage heat-reservoir cool-down times as a function of the number of fin cooling surfaces, in accordance with a non-limiting embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to FIG. 1, which illustrates a cooling system, in accordance with a non-limiting embodiment of the invention.

A heat source (such as an electronics or laser component/system) gives off heat, such as a heating pulse. A first stage heat reservoir absorbs the heat from heat source during the heating pulse. The heat is then transferred to a second stage heat reservoir. The dual-stage heat reservoir cooling system is located within a thermally isolated system.

FIG. 2 illustrates time profiles for the heating pulse and for the temperature of the heat source under different cooling scenarios.

The upper part of the graph shows a burst of N heating pulses. The lower part of the graph shows the temperature increase based on “sensible” heat reservoirs in which thermal energy results in a temperature increase, or based on a 1st stage sensible heat reservoir and a 2nd stage PCM heat reservoir. (Sensible heat transfer causes change of temperature of the system while the given state [solid, liquid or gas] remains unchanged.) T0 is the initial temperature, T1 is the “spike” temperature at the end of the heating pulse, T_limit is the maximum allowable temperature, and T_PC is the phase change temperature.

Before making a judgement on the superiority of PCM based heat reservoirs, simulation results should be compared. Three-dimensional time-dependent simulations were performed using the finite element program ANSYS. Samples are shown in FIGS. 3A-5B.

FIGS. 3A and 3B illustrate long and short-term temperature variations, respectively, in a single-stage Al heat reservoir. Equilibrium is reached within 25 sec after the cessation of the 15 sec long heating pulse. The single stage Al heat reservoir is optimally sized to minimize T1 at minimum weight.

FIG. 4 shows a two stage heat reservoir with the first stage as in FIG. 3 but with an added water-based 2^nd stage. FIG. 4 shows T_max and T_min vs. time for 27 mm Al base plus 79 mm water reservoir with 7 fins. Heating pulses are every 1800 sec and T0=20° C. While the water substantially reduced T2, the equilibrium time was increased (despite the addition of seven heat transfer fins).

FIGS. 5A and 5B show two different PCM materials: hexadecane (FIG. 5A) and gallium (FIG. 5B). The heat reservoirs were sized to hold the same amount of energy (7.6 KJ in four pulses). Note the difference in time scales. FIGS. 5A and 5B show T_max and T_min vs. time for 27 mm Al base plus PCM reservoirs with 7 fins. T_PC for hexadecane and gallium are 16 and 30° C. respectively. The minimum temperature rises during the phase transition zone because of the latent heat model were patched into ANSYS. Other melting points can be found for both classes of materials (paraffins and low melting point metals). Hexadecane is even slower than water. Gallium approaches the response time of aluminum.

Table 1 summarizes the weights of two-stage heat reservoirs sized to store 7.6 KJ of heat. The Al—Al heat reservoir includes a block of aluminum placed after the 27 mm block optimal for use with 15 sec pulses. The weight advantage of using PCMs is clear. Their main draw-back is slow equilibration time.

	Type of two-stage heat reservoir	Weight-gm
	Sensible-heat	Al—Al	8560
		Al-Water	960
	PCM	Al-	120
		hexadecane
		Al-metal	390
		alloy

The present invention surprisingly can reduce the heat transfer times to the PCM. The problem is the low thermal conductivity of the PCM compared to that of the 1^st stage. Heat transfer can be increased by reducing the distance that the heat must travel through the PCM, and by increasing the surface area of the heat transport structure. One way of achieving this is to increase the number of fins. This was simulated by reducing the fin thickness as the number of fins increased. This kept constant the amount of PCM in the reservoir. In order to insure that longitudinal heat flow does not limit heat transfer to the PCM, the inventors simulated the use of a diamond-copper-diamond sandwich with K=800 W/(m·K) of the type developed at Civan, Israel, for laser-diode sub-mount/heat-spreader applications. Results for hexadecane are shown in FIG. 6. A dramatic reduction is possible. Equilibrium time was reduced by 35×. At these heat transfer rates, one starts to be limited by the transport through the first stage. The cooling system can be operated in the medium to high duty-factor regime, especially if complete thermal equilibrium is not required. Even faster equilibrium times were simulated with gallium and the high thermal conductivity fins.

FIG. 6 illustrates two-stage heat-reservoir cool-down times as a function of the number of fin cooling surfaces. (The outer two fins have only one cooling surface.). Cool-down times are measured from the start of the heating pulse. The solid line is consistent with complete thermal equilibrium. The dotted lines are consistent with T2−T_PCM=1 or 2° C. The cooling system can be operated in this mode. At 65 cooling surfaces, response time starts to be limited by heat transfer time through the 1^st stage reservoir.

Fins can be produced with an overall fin thickness of approximately 150 μm for the 100 surfaces case. The copper foil may be 50 μm and the nano-diamond coatings on both sides may be 50 μm each.

In conclusion, extremely light-weight cooling systems can be developed on the basis of multi-stage heat-reservoirs that contain phase change materials as the final storage medium. Breakthrough enhancement in recovery time comes about when applying heat fins (such as diamond-copper-diamond fins) to the PCM based reservoirs.

Claims

1. A cooling system comprising:

a first stage heat reservoir arranged to absorb heat from a heat source;

a second stage heat reservoir to which heat is transferred from said first stage heat reservoir, said first stage heat reservoir comprising a material with a heat capacity lower than that of said second stage heat reservoir but with a thermal conductivity higher than that of said second stage heat reservoir; and

heat transfer structure that increases heat transfer rate from said first stage heat reservoir to said second stage heat reservoir.

2. The cooling system according to claim 1, wherein said first stage heat reservoir comprises a sensible heat reservoir and said second stage heat reservoir comprises a phase change material (PCM) heat reservoir.

3. The cooling system according to claim 1, wherein said heat transfer structure comprises cooling fins.

4. The cooling system according to claim 3, wherein said cooling fins comprise a diamond and copper sandwich structure.

5. The cooling system according to claim 1, wherein said PCM comprises gallium.

6. The cooling system according to claim 1, wherein said PCM comprises hexadecane.