Phase Change Cooling and Heating: The Heat Pump (2/3)
*Disclaimer, I like to use an evaporation chamber and condensation chamber for my heat pumps purely for the simplicity of how the build and some other benefits involved including the low power usage. I do not say this is the best method of making a heat pump, but it is a very good entry point for anyone looking to build such a system.
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Part 3: math and optimizations
Basics of a Heat Pump:
For the overall basics of the heat pump using phase change devices it can be summed up to the evaporation chamber will take in heat, the evaporated gases will go to a condensation chamber where it will condense and drop off the heat. If that sentence sounds weird to you, then good. Evaporation cools and Condensation Heats up. How this works is actually based on the connected gas network to these two chambers. If you want to cool a gas network, you need to connect it to an evaporation chamber. If you want to heat up a gas network, you want to connect it to a condensation chamber. A simple phase change Heat Pump will look something like :
T - E = C - T
T are tanks or gas networks, E is the evap chamber, C is the condensation chamber. The condensation chamber and evap chamber will feed each other the gas or liquids they need to keep doing the phase changes. The tanks or gas networks connected to the respective chamber will be Cooled or Heated by the chamber as it does the work. The overall movement of thermal energy will flow towards the condensation chamber. This is what a heat pump does, moves thermal energy from a cooler source to a warmer source.
We can review what happens:
- A tank of hot air connected to the evaporation chamber will warm up the evaporation chamber by giving some of its excess thermal energy to the contents in the evaporation chamber.
- With the increased heat, liquids in the evaporation chamber will evaporate, cooling the chamber down.
- The evaporation chamber will push out the excess amount of gas inside it to the gas pipe leading to the condensation chamber until its pressure setting is reached.
- The condensation chamber will take in this excess gas into itself. It now has too much gas for its temperature and pressure and some will condense and heat up the chamber.
- The condensed liquids will leave the condensation chamber. But now the chamber is warmer than its connected gas pipe network. Some of the extra thermal energy will move to that connected gas pipe network to equalize their temperatures.
- The condensed liquids will be drawn back to the evaporation chamber, the cycle repeats back to step 1.
Even if the phase change devices arent used, any phase change heating or cooling setup will have a place where evaporation occurs, a place where condensation occurs, and some method of heat transfer at these two points.
Coolant choices:
While the physical hardware of pipes and devices are easy to set up, what does the majority of the work is the coolant you fill these with. This is probably where the user can be most tripped up what to use, how much to use, and what are the limitations. All these factors must be considered to make an adequate heat pump or even a functional one. Stationpedia lists all the gas card information and all of this factors in what to use. If we disregard the gases in the heat exchange connection (please respect freezing points at least), you must consider the temperature range of the lowest you want to get on the evaporation side to the highest point on the condensation side. Once you see if the desired gas type fits within this range, you now need to do some math to find the range: Latent Heat / Specific Heat. If the temperature swing is greater than this, you would need to do this heating or cooling in 2 steps (or see a later post on optimizations).
In short:
- If the lowest temperature is below the freezing temperature of a gas type, dont use that gas as a coolant.
- If the highest temperature is above the max liquid temperature of a gas type, don't use that gas as a coolant.
- If the selected Coolant's Latent Heat / Specific Heat gives a range lower than the two temperatures, this coolant will never get the work done. Consider splitting the work into two heat pump setups.
Chances are you will have multiple gas choices you can use for moving heat across certain temperature ranges. In that case use your best judgment on what to use based on availability or choose the gas with the higher Latent Heat.
For example: if you are trying to cool a gas down to -10C and the max temperature you expect to have is 40C, both Pollutants and Nitrous Oxide can work as the coolant for the heat pump. Pollutants have a Latent heat of 2kJ and Nitrous Oxide has a latent heat of 4 kJ. Nitrous oxide would be the best coolant to use for moving the thermal energy faster. The only reason we can't use water is because water freezes at lower than 0C, which removes it from the ability to use water as a coolant.
Pressure Settings
Both the devices need to have their pressure settings set to work right. This step happens after you have selected your coolant. Afterall different gases will have different pressures at different temperatures. Consider Water vs Nitrous Oxide. At 20C, water needs approximately 15kpa of pressure to stay as a liquid, but Nitrous Oxide needs approximately 1015 kPa of pressure to stay as a liquid. You must consult the phase chart in the gas's stationpedia to find these values. These settings are also good for setting a safety limit on your devices. For example if you are using a heat pump with a water coolant, you would want the evaporation chamber to reach 20 celcius, so you set it to 15 kpa. But if you dont want the condensation chamber to get above 100 Celcius, you would set it to 100Kpa so that the water wont make that connected gas line hotter than you want.
In short:
- Look up the pressure settings by consulting the coolant's phase curve on stationpedia.
- Evaporation chambers will cool down to the temperature based on a pressure setting
- Condensation chambers will heat up to the temperature based on a pressure setting
- Evaporation chambers set to almost 0 pressure means it will cool to the coolant's freezing point.
- Condensation Chambers set to 6000 kpa (6 MPa) will mean it will heat up to the coolant's maximum liquid temperature
Heat Dissipation or Heat Generation:
If you use a phase change pair to cool or heat, you must have a way to either dissipate the heat or have enough heat for your heat pump setup to work. Keep in mind that they are moving thermal energy, they are not magically deleting it or creating it. Heat must move from the evaporation side to the condensation side. For an evaporative cooling side the condensation side must not get hotter than the coolant's max liquid temperature. You must keep its connected gas line cooled down. On the reverse for a condensation heating setup, the evaporation side must never get below the coolant's freezing temperature. If you do not dissipate or generate heat for the respective setups, then one side will stop phase changing which will starve out the other side.
Example: On a Vulcan phase change cooling setup using pollutants as a coolant. Pollutants have a max liquid temperature of 152C. This means if you want to use the Vulcan atmosphere to keep the condensation chamber cooled, it must be below 152C or else your cooling setup will stop working. You can get around this by prefilling a tank with cold night air at 127C, this way your pollutant condensation chamber can still keep working to allow your evaporation side to cool.
Example2: On a Europa phase change heating setup using pollutants as a coolant. Pollutants have a freezing temperature of -100C. This means you cannot use Europa's atmosphere to draw heat for your heating setup. But if you attach a tank of gas to the evaporation side, it will slowly cool down to -100C. Once it gets below -100C, your evaporation chamber will stop evaporating and your heating setup will stop working.
How much Coolant to fill?:
How much coolant you use is up to you. Only a certain amount of the coolant will condense or evaporate. Typically I see something like 3 mols worth of latent energy at max in the chambers. That doesnt mean you can just barely fill the chambers and expect it to work. I encourage you to pressurize the liquid pipes between the two devices first to avoid unwanted evaporation that could lead to freezing there. You need to first fill both chambers such that the evaporation chamber is filled pressure wise and then fill the condensation chamber such that condensation is actually occurring. Once this has occurred, my stopping point is usually 3-5L of liquids present in the evaporation chamber. For a heating setup you want to monitor the gas pipe connected between the two chambers so that there is little to no gas present while evaporation and condensation is happening at the two chambers.
Why so little? 2 reasons, Safety and they dont need to be full.
Safety reasons are that you are filling them at a specific state with two temperatures on each side. When they get to their desired temperatures, this will change the pressures at each chamber and will thus have more or less of the coolant in them. If the heat pump gets hotter than normal, you will have more coolant gas present in the gas pipe between the two devices, this can lead to condensation and pipe breaks. At worst this could mean the evaporation side is full and you have evaporation in the liquid pipe network. If the heat pump setup gets colder than normal, you run the risk of outputting freezing liquids or freezing gases.
Wait, One directional?:
Sadly heat pumps are one directional. Heat always moves from the evaporation side to the condensation side. No you cannot use an evaporation side to heat up that area and vice versa you cannot use the condensation side to cool its area. In order to manage temperature at a range, you may need a second setup, in reverse to heat up or cool down a gas network that overcooled or overheated a bit. Usually though our main problem is cooling.