Geothermal air vs water – The math

Math is hard… especially in complex systems.  Therefore what I’m about to do may have errors.  There may be holes in my understanding.  Hopefully, if you see an error or hole, you’ll let me know.  Here goes…

Air holds less energy than water.  About 4 times less.  That means that you can sink lots of energy into water without raising its temperature much.  Water can also transfer energy (thermal conductivity) better than air.  These attributes should make water superior to air at heating or cooling, right?  Well, as I have learned, maybe not.

Air contains water.  Up to 2% of air is water vapor.  How many grams of water per cubic meter of air depends on the air’s temperature. The warmer the air is, the more water it can hold.  For example, one cubic meter of air (1.2kg) at 30C can contain 30g of water at 100% saturation or 100% humidity.  At 15C, air only can hold 12g of water (again at 100% humidity).  I’ll be using those temperatures later in my examples, so take note.

Latent heat

When water changes from a liquid state, to a gas state (and vice-versa), it takes a certain amount of energy.  It takes 2257 joules per gram of liquid water to change it from a liquid to gas.  In my understanding, the energy to phase change the water will come from the warmest available source.  In a geothermal cooling situation, the energy is going to come from the air because it’s warmer.  We can calculate, then, how much energy will be taken out of the air as it goes from one temperature to another.  We can estimate a best-case energy transfer air to a 15C geothermal thermal mass.  I compiled the following chart showing the best-case performance of geothermal air per humidity level:


From the chart, we can see that it’s possible to condense 100% of the available air above 50% humidity.  We also see that we move about 18,000 joules of energy from the air during this transfer.  By contrast, taking the same mass of water (1.2kg) from 30C to 15C without any phase changes transfers 75,312 joules.  Even with the latent bonus, more energy is being transferred with water.

The devil is in the details.  How quickly can you transfer that amount of energy using the various mediums and what is the cost are really the questions.  How likely is it that you will be able to achieve a water temperature shift from 30C to 15C?  I guess it depends on the amount of tubing in the ground.  Likewise, we are making assumptions about the air system -that it is basically 0% humidity when it comes out.

Too make the muddy water less clear, a geothermal water system isn’t exactly 100% sensible (no phase changes).  For example, I see water condensing around my heat exchanger.  Water can also condense around the reservoir surface area but I have not noticed it (nor have I looked for it).  This condensing effect might be just as good as a geothermal air system, but I don’t have the proper tools to test it and my temperatures are probably not warm enough to do the testing until next year anyway.  So we’ll have to leave that as an open question: does the latent effects of a water system with heat exchanger equal that of a geothermal air system?


Air is probably better at cooling than it will be at heating.  The main reason is that the amount of moisture that the air can hold at cool temperatures is low.  If you live in an area like mine where the cool months still have high humidity, this effect is reduced even more because there just won’t be the excess capacity in the air to hold any water vapor.  Without water vaporizing or condensing, there will be no latent heating bonus.

I put together a similar chart showing, again, best-case results where the air picks up as much moisture from the soil as possible (achieving 100% humidity every pass).  I’m assuming that air that wicks up moisture gains the energy from the phase change of liquid from the soil to gas.  I started with an air temperature of 0C and a geothermal mass of 10C.


Both tables are available here:

How does that compare to water?  Well, to take 1.2kg of 0C water to 10C takes 51000 joules of energy (ignoring phase change).  That’s enough to warm the air to “7.2C” over 50 times.  At one complete exchange of the loop per minute, it will take about 22 hours for the water to achieve that temperature.


The up front costs of geothermal air are higher than geothermal water.  For 200 meters of corrugated 6 inch tubing, the cost is around $500 dollars.  For the same length of 1/2″ PEX tubing, it will cost less than $200 dollars.  The blower is also more expensive than the pump.  Below I’ve listed prices I found for both systems:geothermal-air2

What about runtime costs?  The pump consumes around 70W.  This dayton blower consumes above 200W at full speed.  The fan on the heat exchanger runs at about 90W.  So even though the water system has more components, the total sum of components uses less power than the air system.


Utility is how useful a thing is.  If it has more than one uses, the better.  Are there multiple uses for the air we can take advantage of?  Yes.  We can use the cool air from the geothermal system to cool lighting such as LEDs but we need extra fans.  Ambient air cooling will probably not be  adequate.  Channeling the air might also be difficult.

Water will have much more utility.  Water is going to be better for spot cooling things and for moving energy around.  The tubing is much smaller and cheaper and pumps are less expensive.  You can use the same pump for the entire system where with air, multiple fans are requires for spot applications.  The costs add up.  We can easily use a water system to spot cool CO2 generators, lights, and virtually anything else we need to without any additional moving parts or electrical components (generally just tubes and adapters).


Geothermal with air as a medium might be better than water at cooling if water has no latent effects.  If water does have latent effects and if latent effects are equal, water is probably better due to it’s capacity to store energy.

The latent effects of air for cooling are diminished in dry environments when air is below 50% humidity.  Humidity in high desert states like Utah may not get over 30% during summer days.

For heating, the endothermic bonus for air is not great and since it cannot be used as thermal mass, water could be better.

Temperature stability is going to be better with water because the required amount of energy to change the water’s temperature is greater.

Utility is also better for water because it can be used to spot cool virtually anything without adding additional costs relative to trying to spot cool with air.

Optimizing the greenhouse cooling system

My current cooling solution is composed of the following:

  • 15″ x 12″ intake water-cooled heat exchanger
  • 12″ exhaust fan
  • 8″ water-cooled heat exchanger with 240mm fan x2
  • Geothermal loop 5-6′ deep
  • Soil bed and aisle loop 3-4″ deep

Since the beginning of spring, I’ve been optimizing the cooling system.  Now that it’s summer, I can test it’s capabilities.  Here are some of the principles I’m trying out.

Variable setpoint

In my area day/night temperatures can swing violently.  7C at night, and 31C during the day isn’t uncommon.  It will become more consistent as summer progresses, but for now, I want a system that will change the setpoint based on the fact that nights are colder.  I want the system to store more heat during the day so it can keep things warmer at night.  So I have a bit of machine learning built into the system that takes into consideration the night time temperature and allows for a higher setpoint during the day to store more heat.  I have to keep the setpoint within livable conditions for my plants.  Right now, I’ve got 9C as my lowest setpoint, and 34C as my highest and the “ideal” setpoint at 24C.  For every night the temperature drops below 9C, I raise the running setpoint by one degree.  In the winter, that will probably mean that the temperature could reach 34C and maintain there for a while.  In the summer, if the nights are warm, the setpoint will decrease as long as it’s above the 24C.

With this type of “learning”, I shouldn’t have to ever manually set my setpoint.

Oh, and one more thing.  If the setpoint is kinda high -too high for humans to comfortably work in the greenhouse, I have a rule where if the human lights (a white LED strip) are on, change the setpoint to something that a human can tolerate.  For me, that’s 23C.

CO2 On-demand

The rule for this is simple.  If the temperature is below the variable setpoint and it’s daytime, burn some hydrocarbons and make some CO2.  If it’s night, only turn on if the temperature drops below the minimum threshold (9C).  Plants only need CO2 when there’s light, so trying to maintain a certain CO2 level at night will become expensive.

Exhaust only when needed

I have my exhaust set to turn on when the temperature exceeds the maximum threshold of 34C.  Why?  Well, if there’s a certain CO2 level, and it’s 25C (vs the 24C setpoint) we don’t want to exhaust the CO2.  That’d be a waste!

We will, however, exhaust the CO2 if we get a gas alarm from the harmful gas sensor.

Variable speed intake

My intake has 6 200CFM 120mm fans controlled by a photon which can PWM a MOSFET to control how much voltage the fans get.  Using a PID algorithm and the variable setpoint, On the photon, I get 8bits of control resolution (256 steps) so I’m able to suck in just the amount of air needed to cool.  Almost all of the cooling work is being done by this and the other heat exchanger.

Below you can see how well the intake system tracks the temperature (right graph) and also see when the exhaust fan kicks on (left graph).

intake and exhaust

Heat exchanger for geothermal

I redesigned my geothermal system a bit.  Instead of using the same reservoir as the irrigation, it now exclusively uses a 50 gallon drum and a small 5w continuous pump.  This pump pumps water through the geothermal system and back 24hrs a day every day.  In addition, I moved the water cooling pump (the pump that pumps through the intake, LEDs, CO2 generator, etc) to use this dedicated drum as well.

This change allows me to add a bit of chlorine to the water to keep algae from clogging my tubes.  It also frees up my irrigation reservoir for liquid-based feeding (more on that in a future post?).

To compare, here are the designs for both the old system and the new system.

Trip's greenhouse water system

New system:

Trip's greenhouse water system (3)

Attached to the geothermal line is my little 8″ x 8″ heat exchanger with 240mm fan.  The fans don’t push a lot, but it’s the quality that counts.  The air coming from there is cooled from water deep within the earth.


I don’t open my door

With this setup, I have never had the need to open my door so far.  This hopefully keeps pests out and with them, disease.


Over the next several weeks, I’ll be trying to test the potential of this system.  Today’s high was 27C.  My high temperature in the greenhouse was 36C right around the time I increased the aggressiveness of the intake PID algorithm and reset the system (see the missing data on the graph above) .  With that aggressive setting, the intake still only hit 210 pulses out of a possible 255.  My next goal is to optimize the PID settings and tune it better.  This will further give me an idea of the systems capabilities.

Augmenting greenhouse with blue and far red LEDs

I’ve done a few posts about the benefits of blue and far red lights on plants.  I’ve used them on my seedlings.  Now that I have no seedlings for the moment, I wanted to try them out in the greenhouse.

But the greenhouse is outside, right?  It already has the sun, right?  Yes and yes.  However, exposing plants to a higher concentration of blue light before sunrise, can begin the process of the plant opening its stomata (pours the plant uses to “breath” out water and breath in CO2) to make it more ready for photosynthesis and ultimately carbon fixing and growth.

Far red light benefits at the end of the day are already covered in a different blog.  The higher concentrations should help -especially because trees and other objects shade the setting sun from my plants.

I’m trying to put growth and fruit production into overdrive.  My max expected yield should be somewhere around 1-2lbs of tomatoes per day.  I’m getting close to 1lb every 2-3 days.  The blue light along with CO2 boosting in the morning should help with growth and production.  I expect to see less suckers on my tomatoes and more growth with the far red light as well.

My red and blue lights are enclosed in flood light housings.  It was pretty easy to set them up this way.  I used a carabiner to hang them from the wire over my second bed.  I then pointed the lights at my tomatoes and ran the power cord to the ubiquity mfi mpower strip that I recently installed.  The cool thing about the mpower strip is that you can control it over wifi -so I can tie it into my automation system, but it also supports simple schedules including location-based sunrise/sunset.  I created a schedule that starts the blue light 1hr before sunrise and turns it off 1hr after sunrise.

The blue light looks really cool at night.  This is what a 50 watt LED can do.


The schedule I set up for the far red LED was 30 mins before sunset, and turn back of 1 hr after sunset.  It isn’t visibly as bright as the blue, but my infrared camera sees the difference.


I’ll try to remember to follow up if I notice a difference.  If I forget, comment below and I’ll either respond with my findings or make a new “results” post.

Radiant aisle heating/cooling in greenhouse

As I mentioned in a video a bit back I wanted to add more water to the geothermal water system.  This will improve the systems ability to absorb and store more energy.  This should improve cooling in the summer and heating in the winter.  I decided to place a 55 gallon drum to the rear of the greenhouse.  Additionally, to maximize the system performance, I wanted to add radiant heating/cooling pipes in the aisle between the beds.  This will increase the surface area greatly.

It took several weeks to get all the parts and plan the attack.  But once the parts and plans were in place, it only took a couple hours to retrofit the greenhouse with the upgrade.  Here’s step-by-step what I did with images.

The Plan

Here’s an image from about a week earlier showing what we are working with.  The bed on the left is regulated with the geothermal water system.  It has PEX tubing running through the bottom of the bed.  We are going to attach to that tubing and run 4 more lengths up and down the center aisle between the two beds.  At the end of the aisle we will put the 55gal drum.

Before the retrofit
Before the retrofit

Step 1 – remove tiles and dig trenches

This was pretty straight forward.  I made the trenches about 2-3″ deep.  The first trench and last trench hugged the edge of the bed.  In the middle, I made the trench deep and wide enough for two passes.  The reason for this is I needed exactly 4 passes but the aisle just wasn’t quite wide enough for that with 9 inch spacing between the passes.  The aisle is just under 3 feet.  If it was exactly 3 feet, it would have been perfectly spaced.


Step 2 – Lay out PEX

PEX really doesn’t want to be straight.  Putting some dirt over the ends helped me hold it down enough to get it in the trenches.

PEX layout
PEX layout
PEX layout 2
PEX layout 2

Step 3 – Cover PEX

While digging the trenches, I move the dirt to a wheelbarrow.  I moved it back after laying out the PEX.  I also added a layer of sand to help level the aisle.  I used 4 50lb bags of sand from Home Depot.

IMG_20160423_182124651 IMG_20160423_183406541

Step 4 – Clean tiles

The tiles got kinda dirty over the last 9 months.  This is a good opportunity to clean them off.  These tiles are made from recycled rubber tires.  The cleaned easily with a hose.

Cleaning the tiles
Cleaning the tiles

Step 5 – Weed cover and replace tiles

I put down two layers of weed cover.  Mostly because it was already rolled as two layers and the length was perfect for the aisle.  Rather than unfolding the layers, I just laid it out as it was rolled.

IMG_20160423_184311974 IMG_20160423_185308480

Step 6 – Connect barrel to system

The PEX is 1/2 inch.  I bought some braided PVC tubing of the same inner-diameter to match.  I connected them together with several barb couplers.  For the barrel, I drilled holes in the bung-cap and put some 1/2 inch NPT threaded bulkheads.  The bungs for this barrel are not the same as I’m used to.  These drums are of Japanese origin and it took some extra planning to make adapters for them.


To get the pex to fit on the plastic barbs, I had to heat it up to soften it.  The fit still wasn’t great.  Brass barbs fit better.


Step 7 – Fill the barrel

This was slightly tricky.  My reservoir is 20 gallons.  It fills from 1/4 inch tubing from my rain water barrels via a float valve.  The geothermal water system is all 1/2 inch.  Output is greater than input.  So to fill the 55 gallon barrel, I needed to add more water as needed to the reservoir.  I used my garden hose to add water when needed.  I had to fill it a couple times after it got low.


Note: this may look not level… and it is, but not as much as you might think.  First, the left post of the greenhouse settled about 6 inches.  Second, the barrel is under pressure and is bulging a bit making it lean more to the right.  The bulging is concerning.  I fix might include a reducer before the inlet.


After filling it with water and tightening up a few hose clamps, it was finished.  Let’s enjoy some fresh garden strawberries and celebrate the new 20 megawatts I’ve just added to the system.  This brings my total up to 27mW (20 gallon tank is 7mW at 23C).IMG_20160425_114539348

Water cooling grow LEDs

I needed a few more lights for my second greenhouse bed.  I had the bright idea to use water cooling because, well, I already have a water system, why not direct the heat from the LEDs somewhere useful, like the soil bed?

You can take a look at my previous blogs on how I made the LED strips.  This time, instead of using heat sinks, I used water blocks, which were about the same price.

After a long break from lights, I finally got a system put in to water cool several components at the same time from the same pump.  Check it the video explaining that here:

I’ve got everything I want just about hooked up to the water system including a water cooled air intake system and the solar and CO2 generator.  Only thing left is maybe another heat exchanger and maybe another strip of lights.

Initial testing is promising.  After several minutes (long enough for part of the aluminum back to get very hot to the touch), the water blocks and surrounding area remained very cool.

What about cost?  Well, this actually ended up being cheaper than the previous system.  The water blocks where the same cost, but the savings came in the power supply.  I have been using one power supply per 6 LEDs (180W per 6).  Instead of a 350W power supply (that can’t really do two at full power, I’m using a 400W 24V power supply to power 12 LEDs instead of just 6.  This saves me about $50 per 12 LEDs.

Stay tuned, I have something awesome in the works related to LED grow lights.  I think it will take these lights to the next level.

Grow Lights 101: What kind of light matters

If you are following my youtube channel, you’ve probably already seen this.  If not, here it is again.  This is an introduction to grow lights where I cover what are the important aspects of light relevant to plants and compare a couple different types of grow lights (T5 vs LED).

To sum things up, T5’s are cheaper out of the gate.  But LEDs are more bang for the buck in the long run.

I am building LED grow lights for anyone interested.  Head on over to the new store.

10W Far Red LED Grow Floodlight

Far red (740nm) might be very beneficial to tomato production.  Studies have shown that it can help produce longer “hypocotyl” (the seedling shoot that becomes the stem) by just blasting the plant with 12 minutes of light at the end of the day (16hr photoperiod with T5 lights).  Other studies claim that far red can help reduce or eliminate sucker growth.


Total cost: $33


50W Blue grow light build for seeding

According to my research, blue light is primarily used in plants for vegetative growth.  It follows, therefore that blue light is best for seedlings and clones that you want to grow in size quickly.  Is this logic sound?  I’ll experiment, and report back.

This video is a live build video where I make a $54 dollar blue-only grow LED floodlight.  Note that at the time of posting, the floodlight price has already changed on Amazon.  You can typically find these on ebay for a reasonable price.


Total Cost: $54

LED Stair Lights

Stairs are pretty boring, but they don’t have to be.  I convinced my loving wife that she needed stair lights.  I put the project off for over 6 months while I’ve been greenhousing, but now that that project is more completed, I have time to get back to house projects.  Let’s get building.


First, we will use the APA102 lights.  These are individually controllable.  They are the same lights we’ve used in our ambilight project with the minnowboard max.

I came up with a simple light protocol that supports “instructions” rather than just raw pixel data so it’s fast and light.  I’ve published the library here on github (be sure to use it with my forked Adafruit_dotstar library which has the “driver” for the LightProtocol).  I loaded that onto a particle photon, combined with an level shifter, and powered the thing with a Drok DC-DC power supply and a 24V 5A AC-DC adapter.

To install the LED strips on the stairs, I picked up several aluminum channels from and the corresponding “frosted” covers.  To stick the channels on the stairs, I used 3M automotive double-sided tape.  The aluminum can be drilled and screws can be used to mount, but I didn’t do that.  I used a simple dremel to cut the channels where I needed to.

I’m hiding the photon and the power supply in the closet which is adjacent to the stairs.  I cut a small hole in the wall on the closet side and put a 4 wire, 14AWG cable through the wall.  This is low-voltage (5V), so you don’t need an electrician or a expensive permit to install inside the wall… at least in my area.  On the other side, I combined a 2 socket “keystone” faceplate with a couple two wire speaker jacks.  This doesn’t look half bad.


Using the same python library as the minnowboard max ambilight project, and adding a “driver” that can speak our “LightProtocol” that we’ve installed on the particle photon, we are able to to complex effects and themes on the desktop and change the lights over wifi.

I have three effects coded up: “Chase”, “Random Rainbow Transforms”, and “Rainbow”.  Check out the video for how these effects look on our lights.  What other cool effects can we do?