How to control climate in greenhouses with an earth air heat exchanger

This article is a follow up on my article about 14 Strategies to Keep Your Fruiting Chamber Cool in Hot Climates. In it, I talked about the use of earth to air heat exchanger to bring down the temperature inside your greenhouse during hot seasons and heating during colder seasons.

Due to the questions which came upon this topic, I want to provide more details about this system like, for example, how it works and what design parameter someone should consider.

Before going deeper into this topic, I want to stress out that there are many names for this kind of cooling/heating system. You will find names like earth-to-air heat exchanger with abbreviations like ETAHE, EAT, EAHX, or EATH.

Others call them earth tubes, earth pipes, or earth batteries, and you will also find the name geothermal cooling/heating for this kind of system.

I try to stick mainly to the name earth air heat exchanger (EAHE) as it is self-explanatory.

With that said, let’s jump right into this fascinating topic.

How do earth tubes work?

If you want to understand how the earth air heat exchanger is working, we have to take a look at the temperature profile of the earth (Fig. 1).

As shown in figure 1, the temperature of the earth at a depth of 1.5 to 2 m (4.9 to 6.5 ft) remains relatively constant throughout the year[1]. This temperature is called the earth’s undisturbed temperature (EUT). The EUT remains higher than the ambient air temperature in winter and lower than the ambient air temperature in summer[2].

Figure 1: Shallow Ground Temperature Variation with Season[3]

Principle of an earth-to-air heat exchanger

These temperature differences are used by placing pipes down into the earth and pumping air or liquid through them (Fig. 2). As the air flows through the tube (s), it exchanges due to the temperature gradient heat with the soil and, therefore, cools itself down or heats up.

When it comes to the design of an EAHE, the diameter of the pipe, pipe length, and the number of tubes are the leading parameters to be determined. With an increase in the length of pipe, the pressure drops, and thermal performance increase.

A longer pipe of smaller diameter buried at a greater depth and having lower airflow velocity results in an increase in the performance of the EAHE system[4].

Figure 2: Pictorial representation of the EAHE system[5]

Things to do before installing an EAHE system?

If you want to install such a system, you should first think and doing the following things.

Soil probing to figure out the soil composition.
Calculating the needed heating/cooling capacity.
Calculating the size of the EAHE.
Getting the necessary permits if needed.
Reading this article gives you a better understanding of the topic, and it helps me with the Google algorithm. A win-win situation.
Defining a money budget and a time budget.
Site selection.

How the thermal conductivity of different soils impacts your EAHE

Before you build an EAHE, you should definitely check the soil composition of your site on which you are considering putting it in.

This is since the higher the conductivity of the soil, the better the heat transfer between the tube and the ground. And, therefore, the better the cooling/heating capacity.

Table 1 gives you an overview of different soil conductivities. How these values translate in performance can be seen in figure 3.

Table 1: Soil properties values[6]

Figure 3: Impact of the soil type on the Performance of an ETAHE[7]

As shown in figure 3, a soil conductivity of 0.35 W/mK gives you roughly 1000 W. If the soil conductivity increases to 1.2 W/mK, the Power can be doubled to 2000 W.

How much cooling/heating can I expect?

But what does this actually mean in cooling/heating performance? An experimental study made in 2019 found that at an airflow rate of 2 m/s, a temperature drop of around 5 to 6 °C or 9 to 10.8 °F could be achieved.

Figure 4: Comparison between simulated temperature and experimental temperature at 2 m/s airflow velocity[8]

These values depend not only on the soil but as mentioned earlier on the design of the overall earth air heat exchanger.

What is the difference between an open-loop and a closed-loop earth-to-air heat exchanger?

There are two different types of EAHE systems. The first shown in figure 5 and the second one is shown in figure 6. In the first version, the heat exchange is between the outside and the inside of the greenhouse (open-loop).

In contrast, in the second version, the air is circulated throughout the system (closed-loop).

Figure 5: Schematic of open-loop EAHE[9]

Figure 6: Schematic of closed-loop EAHE[10]

What type of EAHE does exist?

But that is not the only difference when it comes to the design of an EAHE. How you layout the pipes and how you place them into the ground has a high impact on the system.

There are many different systems tested and used around the world such as

Single pipes
U-shape pipes
Parallel or grid pipes
Coils
Vertical or horizontal pipes

and many many more. Each one of them has pros and cons concerning effectivity, costs, and environmental impact.

In this article, I will focus on the multi-layer approach as it provides, in my opinion, (almost) the best of all. To show you what I mean, let’s take a look at the numbers of pipes you need and how long they should be.

The result will give us then a good idea about the space we need and, therefore, on the environmental impact.

How many tubes do I need?

First, let’s calculate the number of tubes you need to cool or heat your greenhouse. Do so, we use one of the following formula depending if you use the metric system or imperial system.

This formula is based on a tube length of 25 m[11].

Metric system[12]: #tubes = room volume / 50 + 1 with room volume in m³

Imperial system[13]: #tubes = room volume / 1750 + 1 with room volume in ft³

Example (metric): length = 10 m, width = 3 m, height = 4 m

Room volume = length x width x height = 10 x 3 x 4 = 120 m³

With the formula we will end up with

#tubes = 120 / 50 + 1 = 3.4 tubes => 4 tubes

Which will give us a total tube length of 4 x 25 m = 100 m.

Example (imperial): length = 33 ft, width = 10 ft, height = 13 ft

Room volume = length x width x height = 33 x 10 x 13 = 4290 ft³

With the formula we will end up with

#tubes = 4290 / 1750 + 1 = 3.45 tubes => 4 tubes

Which will give us a total tube length of 4 x 82 ft = 328 ft.

Another estimation can be done by converting the room volume into the linear foot by multiplying the room volume with 10 % as a rule of thumb.

Example (imperial): length = 33 ft, width = 10 ft, height = 13 ft

Room volume = length x width x height = 33 x 10 x 13 = 4290 ft³

Room volume x 10 % = linear foot

4290 ft³ x 10 % = 429 ft

#tubes = 429 ft / 82 ft/tube = 5.2 or 5 tubes

How long does my tubes have to be?

Now that we know the total length of the tubes, what does that actually mean? If we take a look at figure 7, we can see that the longer the tube, the higher the power of the system.

Figure 7: Impact of the tube length on the Performance of an EAHE[14]

This can be translated into a temperature drop, which is shown in figure 8. With our just calculated tube length, we can expect a temperature drop under ideal conditions of roughly 18 °C or 32 °F.

Figure 8: Influence of length on the temperature drop[15]

Mono-Layer EAHE vs. Multi-Layer EAHE

As we now know the total length of our pipes, we can now think of the layout. To reduce the amount of earth we have to move, we calculate the area needed to place the tubes into the ground.

If we use a mono-layer grid (Fig. 9), we will end up with

120 m / 4 pipes x 1.5 m x 3.25 m = 146.25 m³

And if we use a multi-layer grid (Fig. 10), we will end up with

120 m / 4 pipes x 2 m x 1.25 m = 75 m³ or 50 % less area needed.

When it comes to a high cooling/heating load, a multi-layer pipe-layout works best[16] (Fig 10).

Figure 9: Example of a grid pipe-layout for EAHE System[17]

“Multi-layer pipe-layouts reduce land area requirement significantly for the installation of the EAHE system. Compared to the single-layer pipe layout, two-layer pipe layouts and three-layer pipe layouts reduce the required land area, with 50% and 67%, respectively. Multi-layer pipe-layouts is best, where the cost of land is high, and it is easy to excavate a deep trench. To avoid thermal interaction between the two layers, the distance between the two layers should be at least ten times the pipe diameter.[18]”

Figure 10: Example of a double-layer grid layout[19]

When using a grid layout, it is essential to know that the airflow throughout the system is not equal, as shown in Figures 11 and 12. It is, therefore, carefully considered to use a uniform airflow distribution as this will lead to a pressure drop in the system and, hence, higher operating costs.

Figure 11: Schema of the multi-pipe EAHE[20]

Figure 12: Percentage share of flow rate in each pipe in the total flow rate of multi-pipe EAHE: 5 pipes 90°, L = 76d, dmain = d[21]

What size do my tubes have to be?

As we discussed so far, the length of the tubes, we have to take the diameter of the pipes as well. The influence of the diameter on the outlet temperature (Fig. 13) and the temperature drop (Fig. 14) is shown in the next two figures.

Figure 13: Influence of pipe diameter on the air outlet temperature[22]

As both figures are indicating, the larger the diameter, the temperature in the system is more unstable, and the temperature drop decreases.

Figure 14: Influence of the diameter on the temperature drop[23]

What spacing do I need for my EAHE pipes?

If we use a multi-layer grid, we have to consider the spacing between each pipe as well as the spacing between each layer. It was found that the space between two pipes should be 1 m or 3.3 ft (pipe centers) at an airflow rate of 100 m³/h[24].

As shown in figure 15, the higher the airflow rate, the deeper the tubes have to be buried.

The distance between each layer should be around 1.5 m or 4.9 ft[25].

Figure 15: Percentage of decrease in the heat power of a heat exchanger depending on the space between tubes at different air flow rates: (1) 100 m3/h; (2) 407 m³/h; (3) 1500 m³/h. Tube length is 23 m diameter 0.23 m; depth of burial 4 m[26].

How deep do I have to burry my tubes?

Next on our list is the depth of our tubes. As mentioned in the beginning, the temperature of the soil in depth between 5 and 10 meters stays primarily stable throughout the year (Fig. 16).

During the wintertime, the ground temperature increases with the depth, while during the summertime, the temperature cools down.

Figure 16: Temperature changes with depth in Ottawa, Canada[27]

It was found that the optimum depth for a multi-layer EAHE system is 1.5 m or 4.9 ft for the first layer and 2 m or 6.5 ft for the second layer[28].

Remark: The tubes should be placed with a slope of 1 to 3 % into the ground[29].

How much space does an EAHE need?

As we now have all parameters to calculate the area, we need I come back to the calculation, which I did in the Mono-layer vs. Multi-layer section.

Here we calculated a volume of 146.25 m³ for the mono-layer and a volume of 75 m³ for the multi-layer system.

If we take a look at the formula we used, we have to explain the 3.25 m more in detail.

120 m / 4 pipes x 1.5 m x 3.25 m = 146.25 m³

This 3.25 m consists of the distance between the pipes (here 1 m) and the diameter of the pipe (here 0.25 m) itself. As we have 4 pipes, which are 1 m apart, we will end up with a total of 3 m. To this value, we have now to add the pipe diameter as the distance of the pipes is the center distance to get the total distance.

Space is calculated by the length of the pipes multiplied by the distance.

Area = pipe length x total distance

Area = 30 m x 3.25 m = 97.5 m²

How much airflow do I need?

One to the last parameters we have to keep an eye on is the airflow velocity. As in the next figures, 17 and 18 are shown, the higher the airflow velocity, the lower the temperature drop or the higher the heat gain.

For steel, the best result was achieved at an airflow velocity of 2 m/s (Fig. 17).

Figure 17: Comparison of the cooling performance for different airflow velocities[30]

Figure 18: Comparison of the heating performance for different airflow velocities[31]

The best airflow velocity is according to figure 19 between 2 and 3 m/s.

Figure 19: Influence of the air velocity on the temperature drop[32]

What is the best tube material?

Researchers have found that the overall impact of the pipe material is small. In one study, steel pipes and PVC pipes were compared (Fig. 20 and 21). It was found that the difference for cooling was 2 °C or 3.6 °F and for heating was 0.2 °C or 0.9 °F.

Figure 20: Comparison of the cooling performance for different materials at an airflow of 2 m/s[33]

Figure 21: Comparison of the heating performance for different materials at an airflow of 2 m/s[34]

A similar result was found when Zinc pipes and PVC pipes were compared (Fig. 22).

Figure 22: Comparison between Zinc pipe and PVC pipe[35]

It can, therefore, be concluded that the costs and the durability of the pipe material are the main drivers.

Remark: Don’t use concrete as it is porous and, therefore, let radon enter the system. More about radon can be found in the chapter disadvantages of EAHE.

What do I need to control the EAHE?

To run your EAHE, you need first sensors that are measuring all the values you need to know, such as the temperature, humidity, airflow rate, differential, and pressure.

To run your EAHE efficient, you need these sensors twice. One set off sensors will be placed in the inlet pipe, and the others set off in the outlet pipe.

Only if you have both information, you know if your system is running the way it should.

In addition to that, you should monitor the local weather conditions by installing a meteorological station.

You should log all these data.

Now that all the sensors are in place, it is time to install one or two switches. If you need one or two depends on which climate zone you are living in.

The first switch is for the cooling mode and the second one for the heating mode. For each of them, you have to set two setpoints, which will then turn the EAHE on or off depending on the values.

For example:
Your optimal temperature is 20 °C or 68 °F, but you don’t want to exceed 25 °C or 77 °F. Then these two values are your set points. The latter one turns the EAHE on if the temperature inside your greenhouse exceeds 25 °C or 77 °F.

The former one shuts the EAHE off if the temperature goes below 20 °C or 68 °F.

In doing so, your EAHE doesn’t run 24/7, which will save you money.

What blower do I need?

Now that we have talked about the dimensions of the pipes, airflow, and so on, it is time to think about the blower which you need to get the system to work.

Example:
room volume 120 m3 or 4,237.80 ft3
airflow 2 m/s or 394 ft/min
ACH 8 1/h
CFM 960 m3/h or 565 ft3/min
Diameter 103 mm or 4 in

Before I explain how I calculated all these values, let me talk first about the parameter “ACH.”

ACH stands for air change per hour.

As a mushroom grower, you are familiar with this value as certain mushroom species need different ACH values (Tab. 2).

Mushroom species	ACH range	Temp. range [°C]
Enoki	2 – 4	10 – 16
Shimeji	2 – 4	13 – 18
Shiitake	4 – 8	21 – 27
King Oyster	4 – 5	15 – 21
Yellow Oyster	4 – 8	21 – 29
Pink Oyster	5 – 8	20 – 30

Table 2: ACH and temperature values for different mushroom species [42]

To reduce the heat inside your greenhouse you need to reach a certain ACH value for a given cooling load and solar radiation.

Studies showed[43] that the minimum ACH value is 4 with a recommended ACH value of 8.

To calculate all values, you need the following formulas: Imperial units

Example:

Answer: The blower should provide a minimum of 565 ft3/min or 960 m³/h.

Answer: The pipe should have a diameter of 8 inches or 200 mm.

With these values we can expect according to figure 14 under ideal condition a temperature drop of around 13 °C or 23 °F.

How much does an earth-to-air heat exchanger cost to setup?

After we have investigated all the necessary parameters, we can now go and talk about the cost to set up and run the system.

To do so, we have to know

the material costs (pipes, blowers, sensors, …).
the cost to excavate and refill the area
the labor costs and
the maintenance costs.

In a study down in 2013, the author compared a mono-layer and a multi-layer system according to their costs[36].

The mono-layer, a space of 125 m², was needed. The costs for the materials were at 2,350 US$ for a pipe length of 500 m (4.70 US$/m), and the excavation costs 7,921.88 US$ (42.25 US$/m³).

The spaces needed for the multi-layer was 125 m². The material cost double due to the twice length to 4,700 US$ while the costs for excavation increased to 10,562.50 US$.

It can, therefore, be concluded that the installation costs of a multi-layer grid are roughly 50 % higher than for a mono-layer grid.

On the other hand, if we compare the energy saving with an EATH (Fig. 23), we can then calculate the payback time (Fig. 24). In the investigation done in 2005 is could be found that the payback time is around 7.5 years.

Figure 23: Energy saving for EATH[37]

Figure 24: Payback time for EAHE[38]

Disadvantages of EAHE

So far, we have only addressed mainly the positive side of an EAHE. But besides the already mentioned environmental impacts, there are other disadvantages which have to be taken into consideration.

Mold

The problem with the EAHE systems is that, due to the humid nature of the climate in the rainy season, condensation takes place within the pipes and damages the system[39].

Installing a swamp pump at the lowest point of the system to get rid of excess water.

Adjusting the airflow velocity.

Radon accumulation

Besides mold or mildew, another problem arises due to the accumulation of radon in closed-loop systems. This is especially true if perforated or porous pipes are used.

Radon is a radioactive element that is colorless, odorless, and tasteless noble gas[40]. The concentration throughout the earth varies from region to region and depends on the soil composition (Fig. 25).