Phase Change Cooling and Heating Basics
So this post and a few others will be aimed as more of a background or compiled information for information to glean or use when you are planning to make a phase change heat pump. This will cover how we see different effects and what can make evaporative cooling or condensation Heating work for your advantage. I realize not everyone has taken courses in thermodynamics or are AC technicians so I hope to break the information down to something you can use. Most of this information is what I have learned but if I made errors or need corrections, feel free to tell me. I would rather get correct information out than giving false or misleading information that could hinder someone when they attempt to make these systems work or seek to heavily optimize them to the N'th degree.
Nav Edits:
Basics:
First of all, we need to cover the gas card information the game displays to us. Each gas card has several key pieces of information which is absolutely critical for why we would choose a gas or why not. Every gas has Specific Heat, Latent Heat, Mols per Liter (only in the liquid card), Freezing Temperature, Phase Change Curve graph, and Max condensation Temperature and Pressure.
- Specific Heat: This tells exactly how much thermal energy in Joules (J) it takes to make 1 mol of the gas heat up or cool down exactly 1 C or 1 Kelvin. Ex: Water has a specific heat of 72 J whereas Nitrous Oxide has a specific heat of 37.2 J. This means if you add 1000 J of energy to 1 mol of each gas, Water would only go up 1000 / 72 = 13.8 degrees but Nos would go up 1000/37.2 = 26.9 degrees.
- Latent Heat: This is how much Thermal Energy is generated or consumed when a phase change is happening and is measured in kiloJoules per mol that does the phasechanging . Condensation generates or adds this energy to the gases whereas Evaporation needs to remove this much energy from the gases. The greater this number, the greater this gas's capability to heat or cool with phase changes.
- Freezing point: The temperature the gas freezes, you must avoid this when handling your gases. This is 1 factor in deciding if a gas will be a good coolant option.
- Max liquid Temperature and Pressure: The absolute highest temperature this gas can exist as a liquid and the pressure (usually 6 MPa). This information is also 1 factor in deciding if a gas will be a good coolant option. Rule of thumb, if a gas can't condense at a temperature you need it to be, it wont work for phase change heating or cooling.
- Phase Change Graph: This stationpedia graph is where you must consult to see at what point a gas will condense based on temperature and pressure. This is also information you need when setting your phase change devices to reach a target temperature based on the pressure setting.
One piece of information you must derive for yourself is how high of a temperature change you can have with phase change heating or cooling for a specific gas. To get this number , you do Latent Heat / Specific Heat. This tells you the absolute max temperature change you can have and it will have an affect if you are pushing a phase change heat pump to the limits with a wide temperature swing.
Phase Change Devices:
Typically you will work with Evaporation or Condensation chambers unless you build your own homebrew system. They sport a heat exchange port and depending on the chamber it will be a liquid in/gas out or a gas in/liquid out. It is easy for us to make "condensation chambers" because we can always push a gas into a pipe network with pumps and let the gas condense. Evaporation chambers have some features that help make them hard to make. Evaporation chambers will suck in ONLY Liquids from their liquid input. They will also only do so until they are filled with 20L of liquid. This can allow you to make sure only a very specific liquid is brought into the chamber to evaporate, even if the liquid pipe network has all sorts of pressurant gas types. These devices also only cost 50W of power to keep them running, this is extremely cheap in terms of power. Most pumps start at least 100W of power so even making a home made condensation chamber you are already sacrificing extra power for hopefully better gains.
Coefficient of Performance:
One thing to mention is that in order to tell how well a heating or cooling system works is to derive the Coefficient of Performance. This is a simple division of Heating or Cooling power / Power used to do that. Wall and pipe heaters spend 1000W of power to add 1000J of energy, their COP is 1. If an atmospherics AC can spend 360W of power to provide 5,000 J of cooling, that COP is 13.89. If you use a Phase Change pair system for cooling and spend 100W of power to provide 4000J of cooling, its COP is 40. This measure is helpful to gauge how impactful the system is in terms of cooling or heating. When you make modifications you can see if the extra power draw is worth it. Let's say you spend 100W more to add an extra 1000J of cooling. For each example above, the wall heater would jump to 1.8, the AC would drop to 13.04, and the phase change setup would massively drop to 25. If you want to optimize your performance, you would need to carefully consider how much extra power you are using and note how much extra performance you gained.
Isolated Coolant Loop or not:
If you isolate the coolant and not allow any temperature changes or pressure changes to occur to them you are essentially removing any potentially unwanted factors from occurring. This can be as simple as using insulated piping or running the pipes in such a way that nothing unwanted will occur to your coolant as it "flows" for condensation or evaporation to occur. Usually you want a loop so that your coolant gas will not run out. You can just have a single evaporation chamber being fed by a tank of pollutants to cool something. This can work in the short term if you cant condense the output gas but you will eventually exhaust the tank of pollutants. Of course that also means your COP will increase at the cost of losing Coolant. On a hot planet like Venus, this could be a possible solution until you can figure a way to cool the hot pollutants to condense back. I highly recommend you have a way to condense your coolant to return back to your evaporation chamber to make sure you can always cool. But this leads to the fact that the excess heat needs to be removed from them.