• Heat Pump
A s's vapor-compression refrigeration cycle 1) condenser 2) expansion valve 3) Evaporator 4) Compressor
• Heat Pipe
Characteristic o Heat pipes contain no mechanical moving parts and typically require no maintenance, though non-condensable gases that diffuse through the pipe's walls, resulting from breakdown of the working fluid or as impurities extant in the material, may eventually reduce the pipe's effectiveness at transferring heat. o The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat. o The wide temperature range over which they are effective, because the boiling point of working fluids depends on the absolute pressure inside the pipe.
• Heat Pump
Characteristic o A heat pump uses a small amount of external power to accomplish the work of transferring energy from the heat source to the heat sink. o The term coefficient of performance (COP) is used to describe the ratio of useful heat movement per work input. o The COP for heat pumps range from 3.2 to 4.5 for air source heat pumps to 4.2 to 5.2 for ground source heat pumps, whereas an electrical resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce only one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move three or four joules of heat from a cooler place to a warmer place. Note that an air source heat pump is more efficient in hotter climates than cooler ones, so when the weather is much warmer the unit will perform with a higher COP (as it has a smaller temperature gap to bridge). When there is a wide temperature differential between the hot and cold reservoirs, the COP is lower (worse). In extreme cold weather the COP will go down to 1.0.
• Heat Pump
Heat sources o Air-source, Water-source, Ground-source Applications o HVAC
In HVAC applications, a heat pump is typically a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow (thermal energy movement) may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building.
o Waste heat can be forced to heat incoming fluids. o For example, air pre-heater (APH)
• Heat Pipe
Working Principle o A heat pipe is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to effectively transfer heat. o At the hot interface of a heat pipe a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid – releasing the latent heat. The liquid then returns to the hot interface through either capillary action, centrifugal force, or gravity, and the cycle repeats.
• Heat Pipe
• Heat Pipe
Heat pipe materials and working fluids
The most commonly used envelope (and wick)/fluid pairs include: o Copper envelope with water working fluid for electronics cooling. This is by far the most common type of heat pipe. o Copper or steel envelope with refrigerant R134a working fluid for energy recovery in HVAC systems. o Aluminum envelope with ammonia working fluid for Spacecraft Thermal Control. o Superalloy envelope with alkali metal (cesium, potassium, sodium) working fluid for high temperature heat pipes, most commonly used for calibrating primary temperature measurement devices.
Low-grade Heat Recovery and Utilization
• Low-grade Heat
o o o o In industrial processes, almost 60% of thermal energy transforms into low temperature (<~300degC) heat. Low temperature heat contains very little capacity to do work, so the heat is qualified as lowgrade heat (or waste heat). Industrial processes, such as oil refining, steel making or glass making are major sources of waste heat. Limitations to the use of waste heat arise primarily from the engineering cost/efficiency challenges in effectively exploiting small temperature differences to generate other forms of energy. A large portion of the waste heat is discharged directly, which leads to energy waste and environmental pollution.