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Boiled Dry: Phase changes, heat pumps and refrigeration
Fri, 06/10/2022 - 2:15pm
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Before embarking on a closer examination of refrigeration, it is prudent to begin with a somewhat mundane and innately familiar scenario: boiling water on a stove. To begin this process, one typically fills a pot with roughly room-temperature water drawn from a tap. This pot is then placed on a heating element of some description, perhaps a highly efficient induction stove or cooktop. The heat energy gained by the cookware is then transferred to its contents through thermal convection and conduction, thus increasing the temperature of the water within.
The amount of heat energy required for this operation is simply the fluid’s specific heat capacity multiplied by both the temperature range over which it is being heated and the quantity of water. In our case, this difference in temperature, or delta T is equal to the boiling point minus the ambient temperature of the water. Assuming our heating device operates at a constant power, the time it takes to raise liquid water from one well-defined temperature to another is directly proportional to the specific heat capacity of that liquid. While water’s specific heat capacity does vary somewhat with temperature, it can be taken to be constant for the sake of simplicity. The specific heat capacity of liquid water is quite high relative to other everyday substances. For example, the specific (mass) heat capacity of dry, room temperature air at sea level and constant pressure is over four times less than liquid water under the same conditions.
This is where our story usually ends; soon after the water has reached its boiling point, that portion of the culinary operation is complete and the next one can begin. However, if one allows the process to reach its exciting denouement, a critical and perhaps slightly counterintuitive observation can be made: Boiling off a certain measure of water takes much longer than heating that same amount of water from ambient temperature to its boiling point. In fact, for typical environmental conditions at sea level, the energy required to vaporize some quantity of water is approximately six times that needed to heat the same water from room temperature to its boiling point. While there will be some significant variance, depending on environmental conditions and the starting temperature of the water, this fact can easily be verified at home.
To codify the observations made above: A vast amount of energy is required to cause liquid water to evaporate into a gaseous state; conversely, that quantity of energy is released when the same gas condenses into a liquid. The numeric value assigned to this quantity of energy is known as the heat of vaporization of the substance.
Understanding that there is an enormous amount of energy involved in the phase change betwixt liquids and gases, we are now equipped to begin to understand the refrigeration cycle. From refrigerators to mini-split heat pumps, this thermodynamic cycle is as ubiquitous as it is vital in the modern world. On the cold side of a refrigeration system, the working fluid’s pressure is rapidly dropped by means of a valve or orifice, causing the liquid to rapidly expand and evaporate, thus absorbing energy from the surroundings while traveling through a heat exchanger, known as an evaporator. After a trip through the compressor, on the hot side, high pressure working fluid condenses from a gas to a liquid, releasing energy to the surroundings. This process occurs in a heat exchanger called a condenser. The repeated execution of this cycle allows the transfer of heat from an area of low temperature to one of high temperature.
While water makes for a natural example to understand the energy available from and required for phase changes, as we are all familiar with its properties, commercially viable refrigeration products use other working fluids. Water is unsuited to this application since its evaporation and condensation occur at temperatures far from and above those which are needed for heating and cooling in typical ambient conditions. A diverse selection of working fluids, known as refrigerants, is in common use today, but there are several shared properties that make them well suited to this task.
First and perhaps most obviously, a refrigerant must have a high heat of vaporization; this allows a significant transfer of energy during each repetition of the refrigeration cycle, thus increasing performance. Second, the refrigerant must have a boiling point below the target temperature, though this factor can be adjusted somewhat by changing the working pressure of the system. For practical mechanical reasons, working fluids requiring very high pressures should be rejected. An ideal refrigerant should also be cheap
to produce, non-toxic, non-flammable and environmentally friendly, should it be accidentally released.
Early refrigerants were toxic and highly damaging to the Earth’s ozone layer, meaning their release or improper disposal caused significant, measurable environmental harm. Modern refrigerants in use today have come a long way in this regard and have significantly reduced, or, in some cases, completely eliminated the potential for environmental damage upon their undue release.
The Solar Initiative offers subsidies to support the adoption of heat pump technologies for residential applications on Block Island. To learn more about our programs, please visit our website www. thesolarinitiativebi.com or email Wade Ortel at wade@thesolarinitiativebi.com