Heat-Pumps...                                       ...or, return to Home Page


Running Hot & Cold...             

Above - Tube de Ranque-Hilsch                                                                              http://en.wikipedia.org/wiki/Vortex_tube

    A heat pump is a machine or device that moves heat from one location (the 'source') to another location (the 'sink'), using work.   

   One common type of heat pump works by exploiting the physical properties of an evaporating and condensing fluid known as the refrigerant.

    When most materials change state, whether it is to melt or freeze, evaporate or condense, then is a certain amount of energy that has to be added to or extracted from the system without there being a change in temperature - this is called Latent Heat.

    The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and pressurized gas is cooled in a heat exchanger called a condenser until it condenses into a high pressure, moderate temperature liquid.  It must therefore render up the Latent Heat that had to be given to it when it was evaporated. This heat can be put to good use, or thrown away to the atmosphere, through a heat exchanger.   

    The condensed refrigerant then passes through a pressure-lowering device like an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. This device then passes the low pressure, barely liquid (saturated vapour) refrigerant to another heat exchanger, the evaporator where the refrigerant evaporates into a gas via heat absorption. The refrigerant then returns to the compressor and the cycle is repeated.

A simple stylized diagram of a heat pump's vapor-compression refrigeration cycle:

 1) condenser 2) expansion valve 3) evaporator 4) compressor. 

    One joule of electrical energy will cause a resistance heater to produce one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place. 

    Sometimes this is inappropriately expressed as an efficiency value greater than 100%, as in the statement, "XYZ brand heat pumps operate at up to 400% efficiency!" This is inaccurate, since the work does not make heat, but instead moves existing heat "upstream"; otherwise, this would be a perpetual-motion machine. 

 The dream...

       ...is not of a Perpetual Motion machine. As Homer Simpson screams at Lisa when her science-project PM pendulum "just keeps going faster and faster."   "In this house we will obey the laws of thermodynamics!" 

    But... Since Heat Pumps can move more energy as heat, by maybe a factor of five, than they consume in energy to move this heat, it would seem logical that if there existed a source of motive power, of perhaps 25% efficiency, then the heat could be pumped using a part of itself, with some left over.

    Sounds like picking yourself up by your bootlaces? It does... But...

    We are still striving to achieve efficiencies of 25% in Diesel engines, which have a very hot source (800°C + ), so it's not tomorrow that we will have efficient engines that run on a temperature difference of 30°...