Thermoelectric Heat Exchanger

Heat exchange plays an important role in many industrial applications to mankind and may refer to: heat transfer, an area of engineering concerned with the transfer of thermal energy (heat). Heat exchange is accomplished with the help of a heat exchanger, a device built for heat transfer from one medium to another (in various viz. air to air, air to liquid, liquid to air, liquid to liquid, surface to air, air to surface, liquid to surface and surface to liquid etc.). The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. Heat exchangers are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine, in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air.

Thermoelectric technology offers the highest quality thermoelectric heat exchangers for all of our heating and cooling needs. We can design and manufacture thermoelectric heat exchangers for heating and cooling applications that demand extremely high reliability. The incentive for using thermoelectrics, lies in their compact size, light-weight, high reliability, and near/subambient heating and cooling application. All thermoelectric heat exchangers are similar in concept: thermoelectric modules are sandwiched between two surfaces. When DC current is applied, the thermoelectric modules ‘pump’ heat from one surface to the other. The ‘cold side’ of the heat exchanger is designed to maximise cooling within the customer’s equipment. The ‘hot side’ of the heat exchanger is designed to efficiently move heat into another medium. The total heat rejected from the heat exchanger is the sum of the heat removed from the customer’s equipment plus the power supplied to the modules themselves. Working of different modes of thermoelectric heat exchangers is summarised as:

Liquid to liquid: Liquid on cold side of the thermoelectric modules is cooled while the heat is rejected into another liquid on the other side. Liquid chillers provide an example of liquid to liquid heat exchangers. The chiller circulates temperature-controlled coolant to equipment and rejects the heat into a facility’s ‘house water.’

Liquid to air: In liquid to air heat exchangers, liquid is cooled on one side of the thermoelectric modules, while heat is rejected into a finned heat sink with a fan to rapidly dissipate the heat into air. Liquid to air heat exchangers are commonly used for cooling machine tools, lasers, liquid chillers or any type of equipment with a coolant loop where the heat is rejected into air.

Surface to air: In surface to air cooling (one type of point-of use cooling) thermoelectric modules are located at the point requiring cooling. The heat is rejected into a finned heat sink with a fan to rapidly dissipate the heat into air. Two examples of surface to air heat exchangers are air-cooled cold plates and air-cooled containers. The cold plate could be in contact with electrical components or a container of fluid.

Surface to liquid: In surface to liquid cooling (point-of-use cooling) thermoelectric modules are located at the required cooling point. The heat is rejected into a liquid heat sink cooled by a facility’s ‘house water.’ Direct cooling can be a cost effective and elegant replacement for a remote chiller.

Air to air: A fan circulates cool air from a finned heat sink inside the area to be cooled. The heat is pumped through the metal wall of the enclosure and rejected to the outside air, again via heat sink and fan. Examples: a common AC, an electronics enclosure cooler.

Air to liquid: In this, air is chilled on the cold side of the thermoelectric modules, while the heat is rejected into a fluid. A typical air to liquid heat exchanger is an air conditioner where the heat is rejected into water. Another example is an air dehumidifier.

The thermoelectric integrated heat exchanger could be used in various applications _ where heat exchange against the thermal gradient is required. Specifically, applications requiring compact solutions, long term reliability, and essentially no maintenance will be the best suited for the use of thermoelectric heat exchanger technology. Additionally, innovations in integration, e.g., allowing direct convection heat removal from the hot side, provide system level efficiencies that are currently unattainable with large scale thermoelectric solutions. Fur ther, the technology is easily scalable increasing the flexibility of potential application. The system includes a thermoelectric heat exchanger having a thermoelectric device configured to pump heat. Heat exchangers are provided for transferring heat to and from the thermoelectric device, and for generating a fluid flow across the device. The conditioned fluid may be placed in thermal communication with a variety of objects, such as a vehicle seat, or anywhere localized heating and cooling are desired. Thermal isolation may also be provided in the direction of flow to enhance efficiency.

An Automotive Thermoelectric Generator (ATEG) is a device that converts waste heat in an Internal Combustion (IC) engine into electricity using the Seebeck Effect. A typical ATEG consists of four main elements: A hotside heat exchanger, a cold-side heat exchanger, thermoelectric materials, and a compression assembly system. ATEGs can be classified into two categories depending on their hot-side heat exchanger: exhaust-based and coolant-based. The exhaust-based ATEGs convert the waste heat from the exhaust in an IC engine into electricity. Alternately, coolantbased ATEGs use the engine coolant’s waste heat to generate electricity. In ATEGs, thermoelectric materials are packed between the hot-side and the cold-side heat exchangers. The thermoelectric materials are made up of p-type and n-type semiconductors, while the heat exchangers are metal plates with high thermal conductivity. The temperature difference between the two surfaces of the thermoelectric module(s) generates electricity using the Seebeck Effect. When hot exhaust from the engine passes through an exhaust ATEG, the charge carriers of the semiconductors within the generator diffuse from the hot-side heat exchanger to the coldside exchanger. The build-up of charge carriers results in a net charge, producing an electrostatic potential while the heat transfer drives a current. With exhaust temperatures of 700°C (~1300°F) or more, the temperature difference between exhaust gas on the hot side and coolant on the cold side is several hundred degrees. This temperature difference is capable of generating 500-750 W of electricity.

A subscale thermoelectric heat exchanger designed, fabricated and optimised for performance through testing and simulation. A thermoelectric heat exchanger concept that integrates solid-state coolers to provide active cooling in a compact, modular package. Specifically, direct fluid contact and jetimpingement were used to improve heat transfer at both hot and cold junctions of the thermoelectric. A schematic of the design concept can be seen in the figure (above). This approach resulted in a five-fold increase in the cooling coefficient-of-performance.

Experimentally validated predictions also demonstrated that a 100-kW heat exchanger is both lighter per unit-power than comparable vapour-compression systems. This feasibility study raises the outlook of reducing thermoelectric technology to practice in large heat load applications. Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other, with consumption of electrical energy, depending on the direction of the current. Such an instrument is also called a Peltier device, Peltier heat pump, solid state refrigerator, or thermoelectric cooler (TEC). The Peltier device is a heat pump: when direct current runs through it, heat is moved from one side to the other. Therefore, it can be used either for heating or for cooling (refrigeration), although in practice the main application is cooling. It can also be used as a temperature controller that either heats or cools. This technology is far less commonly applied to refrigeration than vapour-compression refrigeration is.

The main advantages of a Peltier cooler (compared to a vapour-compression refrigerator) are its lack of moving parts or circulating liquid, and its small size and flexible shape (form factor). Its main disadvantage is that it cannot simultaneously have low cost and high power efficiency. Many researchers and companies are trying to develop Peltier coolers that are both cheap and efficient. A Peltier cooler can also be used as a thermoelectric generator. When operated as a cooler, a voltage is applied across the device, and as a result, a difference in temperature will build up between the two sides. When operated as a generator, one side of the device is heated to a temperature greater than the other side, and as a result, a difference in voltage will build up between the two sides (the Seebeck effect). However, a well-designed Peltier cooler will be a mediocre thermoelectric generator and vice-versa, due to different design and packaging requirements.