Heat pipes

Heat pipes are often used for cooling system. Heat pipe is defined as a vapor-liquid phase-change device that transfers heat from a hot reservoir to a cold reservoir using capillary forces generated by a wick or porous material and a working fluid (The CRC Handbook of Thermal Engineering). How does heat pipe work?

Heat pipe has three basic components, those are container, working fluid, and capillary structure. When heat pipe receives heat, temperature will increase including working fluid. If working fluid has already saturated, it will change its phase to be vapor. This vapor will through capillary structure and transfer heat to the wall of heat pipe. Then after release heat, working fluid becomes liquid and goes to the heat source again. This phenomenon is repeated again.

Heat pipes has many advantages, such as high heat transfer capacity, precise isothermal control, functional independence of evaporator and condenser, quick thermal response, remote applications, high reliability, small size and light weight. Because of its advantages, heat pipe is often used in cooling system such as electronic cooling system.

Besides electronic cooling system, heat pipe also is used for water heater using solar energy. Because it has very high transfer capacity, heat radiation from sun is received and used to heat water. This hot water is used for residential needs.

The simplest heat pipes that can be made is using pipe (copper) and water as working fluid. To adjust with the requirement, the pressure inside heat pipes should be decreased using vacuum pump. So, boiling temperature of water will be decreased and can be used for below 100 Celsius degree. Other working fluid can be used for low temperature are acetone, ammonia, methanol.

For high temperature, heat should use another working fluid. Based on operating temperature, heat pipes usually use cesium, potassium, sodium, Lithium. Cesium is used in range 300 to 600 Celsius. Potassium is used in range 400 to 1000 Celsius. Sodium is used in range 500 to 1200 Celsius. And Lithium is used in range 900 to 1700 Celsius.

Thermal Resistance in Electronics

Thermal resistance is the ratio between temperature difference and power dissipated. In electronics, Thermal resistance is a great interest for engineer. It is due to every electronic equipment produce heat and need to be cooled. If they cannot be cooled properly, it will be harmed because of overheat problem.

Thermal resistance can be analogized by electrical resistance. Current represent heat flow, voltages represent temperature differences, and Resistor represent thermal resistance. It can be simplified: R_th=∆T/Q is analogized by R=V/I.

There are several kind of thermal resistance, such as conductive thermal resistance, convective thermal resistance, and spreading thermal resistance. Thermal resistance is basically needed to calculate heat transfer from one point/surface to another point or surface.

In electronic, Thermal resistance is the parameter that informs how effective heat dissipated can be transferred to the ambient. Moreover, it can be used to determine heat source temperature, usually chip junction temperature. In other words, heat source temperature can be known if thermal resistance is already known.

Let’s consider a simple electronic device with heat sink. Heat transfer is started from junction to casing and finished at ambient. This construction results several thermal resistances, those are junction to casing thermal resistance, casing to ambient thermal resistance through heat sink. Total thermal resistance of this system is R_total= (T_j-T_amb)/Q= R_JC+R_CA

Power dissipated from electronic device is assumed that all electricity is converted to heat. Therefore heat dissipation is : Q=VxI, where Q represent heat dissipation, V represent Voltage, and I represent Current. For LED heat dissipation, sometimes optical power from LED is considered, so heat dissipation becomes, Q= P_electrical-P_optical = (V*I)-P_optical.

Thermoelectric cooler

Thermoelectric cooler is a device that converts from electricity to temperature difference. This device consists of p-type and n-type materials. When voltage is applied, current flows and result the difference temperature between its surfaces . Hot and cold surfaces can be reversed by reversing the direction of current.

Thermoelectric

Thermoelectric cooler is one of the methods to produce low temperature. Comparing with conventional method (vapor compression), thermoelectric cooler has advantage such as: thermoelectric can be a cooler or heater by change the direction of electrical current, no vibration, quite, reliable, and no refrigerant (environmental friendly)

However, disadvantage of thermoelectric cooler performance is very low. Coefficient of performance of Thermoelectric cooler is only about 0.3 (vapor compression COP : 3-4).

Due to its advantages, thermoelectric cooler can solve the problem that conventional method cannot be applied to it, such as electronics cooling.

The application of thermoelectric has been introduced in many aspects. We can find thermoelectric cooler system at small refrigerator or even thermoelectric air conditioner. In electronics, thermoelectric cooler also is one of the solutions to maintain chip temperature below allowable maximum temperature. Thermoelectric can be attached to the chip and cool the chip directly.

Performance of thermoelectric will decrease if heat dissipation from hot side is not released properly.  Heat sink can be attached to the hot side of thermoelectric cooler to support heat transfer from hot side to the ambient. Moreover, thermoelectric cooler can produce very low temperature by making cascade thermoelectric cooler. It means hot side thermoelectric cooler is attached by another thermoelectric cooler.

Heat Sink in Electronics cooling

Electronics technology has been developing rapidly since the first transistor was invented. Semiconductor technology is the most significant part for electronics technology development. At the first time one transistor with very small size could replace cathode tube. Now, thousand or even million transistors can be operated in one single chip. However, electronics have maximum temperature to be operated properly. A survey showed that the most cause of electronic failure is temperature. Thus electronics cooling cannot be separated in electronics technology.

Many types of cooling system for electronics have been introduced. The most common electronics cooling system is heat sink. Heat sink system can be divided into two parts, forced convection and natural convection.

Natural convection is convection heat transfer without any force applied to heat sink. This convection occurs because of buoyancy force naturally. Fluid (e.g air) has lower density if its temperature is high, this causes air moves up.  In electronics, heat dissipated from chip causes increasing temperature surrounding then air density becomes low, thus air moves up. Since natural convection only uses buoyancy force, usually natural convection heat sink is attached vertically.

Forced convection need additional force to move air flows on heat sink. Fan is usually used to support air flow on heat sink. Heat transfer rate at this heat sink is bigger than natural convection. Many electronic equipments use this type.

Heat sink is attached to heat source to enhance heat transfer rate. When heat dissipated from electronic device cannot be overcome by heat sink, it needs additional system or even different cooling system. Heat pipes or thermoelectric cooler may be an option for additional system in heat sink.