5 Common Myths or Misconceptions about Climate Battery Greenhouses
I’ve talked to enough folks about climate batteries to hear quite a number of repeated misconceptions or simply myths (falsehoods, but I needed another M word). Many of these hint at a truth but fall short of a true understanding of what’s really going on.
So why address them here? Misunderstanding how & why climate batteries batteries work is important in the design process. Climate batteries are permanent installations. They’re relatively expensive investments, so it’s critical that they work effectively and generate savings for our customers in order to accelerate a return on investment. A misconception that translates into a poor design means lost opportunity and oftentimes wasted money.
We’re constantly learning about climate batteries through monitoring our own installations, simulating how they work, partnering with other farms, and collaborating with engineering students. I fully expect that our designs will continue to evolve over time as we learn from these sources, but our responses to these 5 items are what we’ve learned so far. With each I’ll state the misconception as I’ve heard it, then attempt to explain why it’s either incorrect, or only true in a qualified way.
Misconception #1 - “The soil is a constant temperature X feet underground”
This misconception is a tricky one because in a qualified way, like many misconceptions, it’s correct. The temperature of the earth does become more constant the deeper you go (until you get to get very deep, then it gets rather hot!). In our Mid Atlantic climate, folks bury their frost-free water hydrants to a 3’ depth to avoid the valve mechanism freezing. It’s not that the soil at that 3’ depth is a constant temperature, it’s that it doesn’t freeze. In colder climates, the hydrant would need to be buried deeper because the 3’ depth does freeze.
The truth is, the temperature of the earth, even down to 20’ below the surface, fluctuates seasonally and varies regionally (see graphic). The temperature of the earth fluctuates less the further removed you are from the surface (see temperature by depth and season graphic to the right). Climate batteries are typically buried within the first 4-8’ below grade and are therefore subject to seasonal temperature changes. The seasonal soil temperatures changes can be used to benefit the climate battery greenhouse grower, though, as temperatures at deeper soil depths peak later in the year, around the same time as the start of the greenhouse heating season.
Additionally, this is an important misconception to understand more deeply because it can mistakenly lead the grower to think that there’s either a limitless source of either heating or cooling. In truth, a climate battery is properly named because it acts like a battery: excess heat from the greenhouse ‘charges’ the battery (the soil) with heat, while using the climate battery for heating ‘discharges’ the battery. If not properly managed, just like a battery storing electrical energy, a climate battery can be ‘drained’ to the point where it’s not storing enough energy to be useful. In our climate, we try not to allow the climate battery soil temperature dip below the low 40s Fahrenheit as the amount of usable heat in the soil is greatly diminished at that point.
Ground temperatures vary by region and approximate the year-round average temperature for a given region.
The temperature of the earth varies by depth and season. This graph applies, I believe, to a study done in Virginia, but the same seasonality in temperature swings would apply in other areas with a different base temperature (in this case, 62F).
Misconception #2 - “Pipe length should be between 25’-35’ ”
This misconception goes along these lines: “the ideal pipe length for heat transfer piping (usually in the form of corrugated perforated drain tile) is 25-35’ long.” Like the ground temperature misconception above, this misconception is also true in a qualified way: the main issue is not necessarily the length of the pipe, but rather the velocity of the air within the pipe.
In smaller climate battery greenhouses with smaller climate battery fans, achieving a pipe length of 25-30’ is not realistic (in some cases, the largest dimension of the greenhouse may actually be shorter than the “recommended” 25’). In larger greenhouses, a pipe run longer than 35’ may be necessary to interface with as much soil as possible.
In both of these instances, ensuring that airflow velocities will be in the optimal range of 13ft/s (4m/s)* while also ensuring that airflow is relatively balanced across all of the tubing is the more critical factor to pay attention to.
“Rather than aim for a specific pipe length, the goal at Atmos is twofold: to balance the airflow and ensure that the resulting airflow falls in the ideal range of 4+ m/s.”
Let’s look at the example of our first climate battery installed at my farm, Threefold Farm. I went by conventional wisdom and made sure that my tubes were all 30’ long, right in the middle of that 25-35’ recommendation. We even installed the tubing on an angle across the width of the greenhouse to ensure that the full 30’ would be possible. However, when examining the airflow characteristics of the tubing setup in study over the years that followed, I found that some tubes were receiving a much greater flow rate than others due to the nature of the tubing setup. Much of the installed tubing was effectively wasted from a heat transfer perspective because it received very little airflow.
In the Atmos climate battery design, we aim to balance airflow across all of the tubing, ensuring that none of it is wasted. Our individual tubing lengths do tend to fall in the range of 25-35’ range, but that was not the design goal. Rather than aim for a specific pipe length, the goal at Atmos is twofold: to balance the airflow and ensure that the resulting airflow across all of the tubing falls in the ideal range of 4+ m/s.
* Numerical simulation of soil heat exchanger-storage systems for greenhouses, Gauthier, 1997 https://www.sciencedirect.com/science/article/abs/pii/S0038092X97000224
Misconception #3 - “Air speed must be slow to allow time for heat transfer”
This misconception goes like this: “if you move air too fast, it won’t have enough time to transfer heat before it exits the battery.” However, both simulations, the whitepaper cited above, and results from our farm, Threefold Farm, show that there is a positive correlation between the speed of the air in the piping and the rate of heat transfer. In the past 2 years, armed with this information, we swapped out older fans for higher powered fans. The result has been an increase in the ability to create a higher temperature differential between the inside of our greenhouse and the outside air temperature, meaning we’re transferring more heat.
Why is this? One thought is that the higher velocity (speed) air creates more turbulence within the piping, leading to increased contact with the walls of the piping, leading to greater heat transfer. However, this is one of the least studied parts of climate battery design and we would like to have a better grasp on it. But the thought that the air speed needs to be relatively slow within the piping does not seem to be true.
We at Atmos are very interested in learning more about underground heat transfer physics and hope to do our own simulations in the future to understand and visualize how heat transfers into the climate battery and moves through it. In fact, a team of engineering students is studying this exact topic for the 2022/2023 school year. Stay tuned for result!
And if you’re a engineering type who would like to help out with those types of simulations or have access to multiphysics software, please reach out! We’d love to understand this and further refine our designs with the information.
Misconception #4 - “A climate battery is sufficient to provide cooling during the summer months”
This misconception is once again tricky, because it’s once again true in a qualified way. A climate battery does allow you to cool the structure by using the ground as a sort of heat sink. On our farm, we are able to almost fully close our greenhouses through the winter months and even parts of the shoulder seasons of fall and early when the sun is low, not very strong, and when the climate battery itself is rather cool and makes for a good heat sink.
However, as the ground begins to warm in the spring due to the climate battery running, there are two problems with climate battery cooling that begin to present themselves:
First, as the climate battery warms there are diminishing returns to be had by pumping warm air underground. All other things being equal, pumping 80 degree air into 40 degree ground will have a greater cooling effect than pumping 80 degree air into 60 degree ground. As the ground continues to warm, especially through the spring months, the ability to effectively cool your greenhouse diminishes and other means of ridding the greenhouse of heat will be necessary: roll-up sides, vents, exhaust fans, shade cloth, etc.
Second, the sun continues to gain strength as you approach the summer months. If you view the climate battery as a radiator and the sun as an engine, that engine will continue to put off greater and greater amounts of heat as the season moves into summer, while the size of the radiator stays the same. The amount of heat generated by the sun during the summer is far more than the climate battery can effectively handle, so at some point you’ll simply need to exhaust much of the heat rather than attempt to store it.
In order to design a climate battery for cooling in most of the United States, the size of the heat sink (climate battery) would need to be massive. A massive installation likely wouldn’t be cost-effective, so counting on the climate battery for cooling in the summer simply isn’t practical for most climates.
Misconception #5 - “A climate battery lets you tap into an unlimited heating source — the earth”
The reasoning for this misconception goes something like this: “since the ground at a certain depth is constant, if you are able to tap into it, you have access to that constant earth temperature and therefore an ‘unlimited’ amount of heat.” So, for instance, in our area of Pennsylvania where the prevailing ground temperature is around 50F, the thought is that you have access to an unlimited supply of 50 degree ground and therefore the air you extract from the climate battery should always be around that temperature.
Both the starting premise (that the ground at a climate battery depths is a constant temperature) and the conclusion (that it never ‘runs out’ or is unlimited) for this one are incorrect. The premise is incorrect and is explained above under Misconception #1. The conclusion is also incorrect because it operates off a faulty view of how a climate battery works.
Since a climate battery works by actively managing the temperature of a block of soil underneath the greenhouse, it will fluctuate in temperature with the seasons and with usage. It truly functions like a battery, meaning we can ‘charge’ it so that it’s warmer than the surrounding soil, and we can ‘discharge’ it if we’re not careful, even to the point where the temperature can actually be colder than the surrounding soil.
Imagine going into the winter months with climate battery soil that’s in the 40s. Since the winter months are cold, we may be tempted to run the climate battery to keep the greenhouse as warm as possible at night, effectively ‘discharging’ the battery and chilling the soil as a result. Since winter days are the weakest in terms of available sun to ‘recharge’ the battery, we can, over the course of a really cold week, over-chill the battery to the point where it’s colder than the surrounding soil. If given enough time without running, the surrounding soil that is warmer in this instance may help to warm our climate battery, but this process is slow because the rate at which heat moves through soil is relatively slow.
Final Thoughts
We wrote this article because we care deeply about climate battery design and unfortunately have seen far too much poor information and poor examples of climate battery design on the Internet. Remember, it’s expensive to dig in the ground, and expensive to buy a bunch of piping. Our hope is that by dispelling some of the misconceptions here we’ll help you to think more critically about climate battery design (including our own designs!).
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