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We designed our farm to take as much advantage as possible from passive heating and cooling and have seen the benefits in reduced electric use and cost. But there’s one often-overlooked element to consider.
Thermal mass has been used for a long time as a way to regulate temperatures. Think about stone cellars or adobe structures. Essentially the idea is to naturally mitigate temperature swings by transferring heat and the energy from light into building materials.
It’s why my farm benefits from being built as a shell inside of a concrete structure.
How it works?
When light hits a material, some of it is converted to heat. That heat slowly conducts (conduction) from the surface to the center of a mass, allowing the entire volume to heat up. When air temperature drops, like in a greenhouse at night or a warehouse farm with all the lights turned off, the mass releases (radiant heating) this heat back out into the environment.
The graph above shows the effects of thermal mass on temperature. Basically, as thermal mass increases, temperature fluctuates less.
Also note that this is different from insulation, which I discuss a little in this article about building my farm and will write a more in-depth analysis of soon.
If you were using thermal mass in a passively heated and cooled space like a home or greenhouse, during the summer, the goal would be to keep the greenhouse cool. Even if shaded or out of direct light, that mass will still absorb some heat from the hot air in the greenhouse.
So why is this important for water?
Well, in most warehouse farms, there isn’t a lot of thermal mass, either by design or being actively used. But there is often a lot of water. And water is one of the best materials to use for thermal mass. Therefore, it is often playing an unintentional role in the heating and cooling of the farm.
Consider this air’s heat capacity per volume (measured in Btu/cubic foot/degrees F) is .018. Water is all the way up at 62 – higher than rock/stone at 25 and concrete at 25-32.
So, when thinking about temperature control for your space, make sure you think about how much water will be there! This is obviously a bigger consideration for DWC systems like ours than NFT channels, but often, as a result of its high heat capacity per volume, your water temperature will be higher than air temperature. That’s why people use things like chillers and it’s important (in addition to stopping algae growth) to keep reservoirs covered.