Category Archives: geothermal

How much cooling does my greenhouse need?

When looking at different cooling methods, the first thing you need to know is how much energy (heat!) you need to remove.  You have to know your energy inputs and outputs.  The inputs are primarily the sun or solar radiation, but they can also be air from the outside.  We’ll focus on solar radiation input, as it’s the biggest factor.  Our outputs are our cooling mechanisms.  Probably the most common method is simple air exchange with the outside air.  We’ll add to that list geothermal and water chillers, but we can even include evaporative coolers and traditional air conditioners.  We’ll start out with a very rough way to estimate and go into more accurate forms in subsequent blogs.

Estimating Input using the Solar Constant

The average solar radiation received by the earth is about 1300W/m^2.  After atmospheric absorption, it’s around 1000W/m^2.  That makes things easy.  All you have to do to get your radiation input is to multiply the surface area of your greenhouse by 1000.  My greenhouse is 8.9m^2 so my energy input is 8900W or 8.9kW.

Energy Input = 1000W/m^2 * GH Surface Area(m^2)

Now that we have a rough estimate how much energy we need to remove, we can look at how we can cool that with different methods.

Geothermal Water

This type of cooling requires 4 basic parts: geothermal temperature, setpoint, tubing, and a means of transferring the energy into the water.  For our setpoint, we are using 32C and assuming the geothermal temperature is 15C.

We assume for simplicity that we are able to transfer all the energy input from the sun into the water.  This is really important but we could dedicate an entire article on just this topic.  I’ll briefly mention a few methods of getting the energy into the water and move on.  Methods include: copper/aluminum air-to-water heat exchangers, tubing distributed in the greenhouse flooring (using the surface area of the greenhouse flooring to transfer the heat), direct radiation to a water reservoir.

We also don’t assume any bonus energy transfers from condensation or evaporation.

We are going to use a script I wrote to try to figure out how much tubing we are going to need to bury underground.  We run it like this:

python3 –greenhouse redhouse.json

redhouse.json is my greenhouse and you will want to edit it for yours.  We just need greenhouse_dimensions and optionally “pump_flow_rate” for this script.  We can also change the flow rate with the “–flow_rate” option.

Tubing diameter : Watts per meter tube length
1/2in (0.0127m)  : 0.72W

Using the –tube_diameter argument we can see how much doubling the tubing size helps:

python3 –greenhouse redhouse.json –tube_diameter 0.0254

Tubing diameter : Watts per meter tube length
1in (0.0254m) : 1.44W

I use 1/2″ tubing, so to get enough cooling for 8900W, I’d need 8900/0.72 = 12420 meters of tubing length.  That’s a lot of tubing!  Is it correct?  I am not sure.  My script is open source, so feel free to look at it and check it.  We need to be clear what the script is telling us.  It is trying to transfer all the solar radiation to the soil.  That is a factor of the thermal conductivity of soil, the temperatures of each, and the surface area in contact.  We can play around with our setpoint, the flow rate and the tubing diameter and this will change out outcome.  Most likely, geothermal will be a supplementary method used with other cooling methods.  So we can play around with the energy input as well.

Geothermal Air

Air is going to be similar to water in how we figure out how effective it will be.  We will use our script to figure out cooling potential per meter length of tubing.  We will use the same setpoint of 32C and 15C for the geothermal temperature.  We will use 0.102m for the tubing diameter (4in pipe), and an air-flow rate of 94 liters per second (about 200 cubic feet per minute).  Like our water calculations, we are not assuming any condensation/evaporation effects which are complicated and unreliable.

python3 –greenhouse redhouse.json –tube_diameter 0.102 –flow_rate 94

This gives us a rate of 5.77W/m of tubing.  If we use 0.152m tubing (6 inches), we get 8.59W.

We require 1038 meters of tubing for my greenhouse.  That’s still a lot of tubing, but much less.

Is geothermal air better than water?

It certainly transfers more per meter of tubing because of the increase in surface area.  Water of the same tubing size has identical using this script.  The differences are going to be in the transfer of energy to air.  Air doesn’t absorb much visible light.  So all energy absorption comes from its atoms bumping into things that do absorb the direct solar radiation.  This is a slow process because air has the worst thermal conductivity of almost anything.  The good news is the surface area is huge: effectively the entire surface area of everything in your greenhouse.

Energy transfers to water much more easily (24 times better) and can store 4 times the energy per gram, but performance is going to greatly depend on surface area of radiation to water heat exchange and the water to soil heat exchange (the tubing diameter).

Condensing Air Conditioner

These are the easiest to estimate for because they usually come with a rating in BTU/hr.  1W = 3.412BTU/hr, so we can do a simple conversion to get the required size of air conditioner.  8900 x 3.412 = 30367 BTU/hr.  These are expensive up front and expensive also to run.

Water chiller

Another expensive option is a water chiller.  There are two varieties air cooled and water cooled.  If you have need of both heating and cooling, the water cooled option is a good one.  Either option is a good addition to a geothermal  water system.  It can also be used to cool LEDs if those are water cooled.  These are usually rated in “tons”.  Each ton is equivalent to 12,000 BTU/hr or about 3500W per ton.  For my greenhouse without geothermal, I’d need a 3 ton.

Water cooled chillers require a second water loop to transfer the heat.  It can be transferred to a pool heater system, a water cooling tower, or even used to heat water for your home.

I want to look more into both advanced estimation solar input and these different cooling methods including more in subsequent blogs, so stay tuned.

Preemptive Greenhouse Cooling with Machine Learning

In order to improve cooling, I added a 1/2 HP water chiller to my system.  When used with the geothermal, it cools pretty well.  This is evident by the amount of condensation I can see on the pipes and the HVAC water reservoir:


and the heat exchanger:

The chiller, however, is only a bit over 400W consumption.  This means at 100% efficiency, I can remove 400W of energy.  That’s not good enough.  Using pysolar I estimate I need to remove an average of 1764W of solar energy.  Combining the chiller with geothermal, however, I have so far been able to maintain sub-40C temperatures with 22% shadecloth (1375W average solar input) at up to 30C outside.  However, near the end of the day, the reservoir sensors claims the water temperature is 30C.  I think this is higher than it actually is (sensor may be polluted by outside air due to its location), but it’s warm enough that condensation stops.  Condensation represents a nice boost in cooling because it takes more energy to phase change water gas to liquid and that energy will be pulled from the ambient air.

How much energy can my reservoir absorb?

Doing the math, I can estimate how much my 151L reservoir can absorb at different starting temperatures.  If I can start my reservoir at 9C, it can absorb around 245W of energy throughout the day (15 hours until it reaches 30C).

This is more than enough even without any bonuses from condensation.  So how do I get my reservoir that cool at the beginning of the day?  One established method is to pre-cool the reservoir during the night.

Pre-cooling and saving energy

It would be relatively easy to pre-cool the reservoir at night.  I just keep the geothermal pump and HVAC pumps going at night until I reach the desired temperature.  However, if I want to maximize efficiency, I need to understand the next day’s needs.  I need to preemptively decide whether or not pre-cooling is necessary.  If it’s going to rain the following day, and solar radiation drops below 400-500W/m^2, I don’t need to pre-cool.  How do I understand my future needs so that I can be efficient in how I pre-cool?

Weather forecast is inadequate

I can easily get tomorrows estimated maximum temperature.  However, this is not adequate.  In a greenhouse, it may be 15C outside, but if there’s high solar radiation, it will easily be 30C or higher inside the greenhouse.   I cannot use predicted outside temperatures alone in my preemptive decision.  If I knew the forecast for solar radiation, I could base my preemption on that.  However, the Weather Underground (WU) forecast API, the API I’m currently using, doesn’t have solar radiation estimates/predictions.

Machine learning to predict greenhouse temperature

I can get tomorrows and historic atmospheric conditions from the WU API.  This will give me a rough idea how much solar radiation I’ll have.  I also have a couple years worth of greenhouse temperature data.  Using Tensorflow, I should be able to train a neural network linear regression model using historic weather data and data for my greenhouse and use that model to predict future temperatures using weather forecasts.

To start out with, I used 2 months worth of data.  The data retreival process was the slowest part.  With a free WU API account, I can only get 10 API calls per minute.  Once complete, I have 3700+ data points to work with.  I can expand this to more data as needed (depending on accuracy requirements).

Machine learning prediction results

Looks like my prediction results are within about 4-5C.  I selected 3 recent temperature points and associated conditions over the past week to test:

Outside Temperature: 15.1C
Condition: “Overcast”
Prediction: 18.5C
Actual: 17.9C

Outside Temperature: 17.7C
Condition: “PartlyCloudy”
Prediction: 21C
Actual: 25.8C

Outside Temperature: 27.2C
Condition: “Clear”
Prediction: 32.5C
Actual: 36.6C

This level of accuracy might be fine.  If it predicts anything over 23 (which is my typical setpoint), I’ll know I need to preemptively cool during the night.

Efficient Method of preemptive cooling

I have up to three sources of cooling at night.  The air using the HVAC heat exchanger and fans, the geothermal system, and finally the chiller.  Understanding when to use which will help me cool most efficiently.  My algorithm is roughly this:

  • Use HVAC heat exchange fans and all pumps until reservoir is within 1C of the air temperature
  • Disengage the HVAC fans (180W savings)
  • Continue using geothermal pump until water is within 1C of geothermal battery temperature (I estimate at 12C)
  • Disengage the geothermal pump (60W savings)
  • Continue using chiller pump until water reaches 4C.

Measuring success

The entire system is just about in place.  There’s a few bugs to work out with using the machine learning model and I still need to separate the pumps onto different power supplies.  I have, however, been doing well using only the forecast max temperature to decide preemption and I believe it’s been working.  To determine this objectively, I should be able to look at non-preemptive cooling minimum and maximum deltas between the indoor and outdoor temperatures.  The smaller the difference should mean better cooling.  I started preemptively cooling using the temperature method on 6-6, 2017.  I started preemptively cooling using machine learning on 6-18.  Let’s pull min/max data from may, and compare it to data between 6-6 and 6-15 (a few days before any changes for good measure).  The results look promising:

5-6 to 6-5:

  • min delta: -3.8
  • max delta: 30.0
  • mean delta: 4.0

After 6-6:

  • min delta: -1.0
  • max delta: 15.6
  • mean delta: 3.1

The max of 30 may be anomalous (ie, during a power out), but doing a mean should weed those out.  After implementing preemptive cooling, I’m getting almost 1C better temperatures.  Once I have the machine learning and other improvements made, I’ll check again and see if I see additional improvements.

UPDATE (7/18/2017):

Math about how much energy a 151kg could absorb over a day was incorrect.  Fixed math to cover 15hr period instead of merely 1hr.

Also fixed greenhouse estimated solar input.  New estimate computed with pysolar.

Winter 2016-2017 learnings

What I learned (so you won’t have to)

My goal for this project is to grow tomatoes (and possibly other warm-weather crops) all year round in my greenhouse. I learned several things from my winter experience this year.  I thought I’d record them here to benefit anyone else out there trying to grow tomatoes year-round in uncooperative environments.

For the record.  I “shut-down” the greenhouse in mid-December.  This year (La Nina year), was below average cold for my area.  There just wasn’t enough produce to justify the propane heating cost which was turning out to be double what I expected (9 kg of fuel per month).  Further, there was no future produce either as flowers were not setting fruit.  Reasons listed below.

Light is essential

No duh, right?  This winter, I did not have supplemental lighting.  As a result, growth was slow, the plans were weak, and fruit didn’t set.  A lot of this has to do with improper temperatures, but I believe light was a major factor this year.

Geothermal is unhelpful for winter tomatoes

The concept behind geothermal in the winter is that you can store energy during the day, and recover it at night.  In addition, since the geothermal “battery” recharges from the surrounding soil, there’s a temperature floor which helps keep air temperatures at night above freezing.

The problem with the theory is when there is little to no energy input geothermal temperatures are too low for tomatoes.  Without the sun, which is typical for winters in my area, there’s no “free energy” to store during the day.  To supplement the lack of solar energy, I have a propane water heater as a backup heating source.  This energy isn’t “free” light sunlight and I don’t need to store it in the ground.  In fact, it’s inefficient to store it in the ground during the day because just like the geothermal battery “recharges” from the surrounding soil, the surrounding soil also “recharges” from the geothermal battery when the battery has a higher temperature.  The energy losses to the surrounding soil resulted in lower than acceptable air and soil bed temperatures and wasted propane.  The lower temperatures with the light problems led to slow growth and a lack of fruit production.

When I turned the greenhouse back on, I added a bypass manifold that separated the geothermal from the bed and aisle lines.  Now I can provide radiant heating directly from the propane water heater without any losses to the earth.

The result was very positive.  The water reservoir heated up enough that it was warm to the touch.  Soil temperatures went from just 11C to above 16C (March 6th).

Don’t shut down your greenhouse

When I shut down, temperatures were allowed to drop extremely low.  It was cold enough to crack some PVC pipes, and burst copper tubes in every single heat exchanger in the greenhouse including the propane water heater.  The damage was close to $500…  (all in the name of saving $20 worth of propane).  Instead of shutting down, I should have just lowered the set-point to around 3C.

Genetics is everything

A number of my tomato plants had poor yields, even in the summer time.  Among the poor producers was the San Marzano that I got from The Home Depot.  This was especially disappointing as even my grafted San Marzano produced almost nothing.  I’m not sure why the San Marzanos didn’t produce well in my greenhouse, but this year I’m trying a greenhouse-specific hybrid: The Pozzano.  This is supposed to be the same style as the San Marzano (paste/plum tomato), but really excels in the greenhouse.  We’ll see.  So far the plants I have look promising.

A new start

It took a while to repair and get things running again.  Sometime learning is expensive and I sure learned a lot this year.  Early February, I was able to transplant some new Pozzano plants into the greenhouse.  I have 7 new plants and 5 of the 7 have flowers.  This is going to be year 3 and I’m hoping this will be the year it “just works”.  Summer is coming and I have big plans, but I’ll save that for the next post.

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 all available water has been condensed and the humidity of the air coming out is almost 100%.

Update (2016/10/24): I originally claimed the air coming out would be 0%.  That is incorrect since not all the water vapor will be condensed.

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.

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

Redhouse December Update

The last few months have been trials, full of learnings and many successes.  This is really only my second tomato growing experience and first experience with a greenhouse.  First  let’s look at the successes.


I’ve harvested several pounds of tomatoes so far.  There’s several more pounds to harvest.   On addition to the tomatoes, I’ve harvested lots of parsley as well.

The greenhouse is fully automated.  The irrigation, the lights, the solar, heater the blower… just about every electronic component is controlled by my custom automation software.  I’ve been tweaking the rules for months and its working really well.


Humidity sufficated many flowers in Sept and October.  They developed mold which prevented fruiting.  Adding a dehumidifier to the system solved that and fruit started to set in November.

Electricity has been my latest trial.  It started getting below freezing so I added an electric heater.  The load from the heater started popping my 20A breaker.  On night it popped and temperatures dropped to -2C air and 9C soil.  This caused serious damage to more than half of the 10 tomato plants.  The planta dropped flowers and aome of the youngest fruits.  To solve the issue temporarily I put the heater on a separate breaker with a long extension cord.

Between the humidity and the cold I probably lost 2 months of future harvests.  After December, I may not see another harvest until March.


Avoid higher than 80% humidity.  85% is a critical threshold and mold will start to take over.

Don’t share electric breakers.  I did not foresee the need for a heater.  Even so, future proof yourself by putting your greenhouse on an independent breaker or two.


The door leaks heat.  It needs to be rebuilt.  When I start, it needs to get done *fast* because I can’t leave it unfinished at night.

CO2 generator/water heater.  I found a water cooled CO2 generator.  This is perfect for a geothermal water system because it heats the water AND produces CO2.  I may be able to eliminate the electric heater if this thing works.

Measure electrical cost.  I plan on adding a current sensor to the new breaker.  I’ll be able to see what my electrical costs are and use that in my automation software.  I’ll be able to create “power saving” rules when costs exceed expected production.  The current sensor has been installed, now I need to get it tied into the system.

I have almost 600 Watts of grow lights that still need to be built and installed.  200 watts on bed A (the tomato bed) and 360 watts for bed B (strawberries, onions, garlic, herbs).  This will help with growth especially with the onions and garlic which don’t get any direct light because of how low the sun is in the horizon and how high my fence is.  It will also help produce heat at night as a fallback if the geothermal system or water heater can’t keep up.


It’s been a rough few months, but things are literally looking up as new growth is being observed in many of the damaged plants.  After the door is rebuilt, I believe the heat and even the cooling and humidity problems will be solved for good.  The new wall where the door will go will have vents and fans for intake and exhaust.  I’m excited about the improvements in progress and seeing some real results.

Happy new year everyone and happy 2016 harvesting!

Geothermal Water Reservoir to Soil Energy Transfer

It was cold today.  It’s been cold all week.  The high for today was 7C (about 44F, yes, Tidder, I love you) and the low was 6C.   This caused my soil temperature to drop to 11C (52F).  This is bad.  In addition, it looks like it was very dark most of the day.  My artificial lighting accounted for 85% of the total light energy in my greenhouse (how my light algorithm works).  The sunlight was only able to warm the greenhouse to 15C.

In a conversation on IRC, a friend, wondered if PEX tubing was very good at thermal conduction (ability to transfer heat energy to another substance).  After a bit of searching, I found that PEX has very poor thermal conduction relative to, say, copper.  If PEX is 0.4W/(mK), copper is like 28.  Big difference.  All is not terrible, however, because water is only 0.6W/(mK).

Looking at today’s data, there was a couple degree discrepancy between the reservoir temperature and the soil temperature.  I would have thought it would be almost the same.  Could thermal conductivity explain the difference?

I conducted an experiment.  I boiled some water and poured it into the reservoir.  This brought the temperature to about 21C (from about 14C).  Over the space of a couple hours, I logged the temperatures from both over the space of 3 hours.  Here’s a scatter plot of the reservoir and soil temperatures:


The chart shows a negative correlation between dropping reservoir temperatures falling and rising soil temperatures.  We have energy transfer!  It also shows about how much pumping time is required to raise the temperature of the soil by a few degrees.  This could certainly explain how there could be a difference between the reservoir temperatures and the soil.  If the reservoir heats up quickly during peak temperature, it could be several hours before the soil will rise.

During this test, the control soil bed (unregulated) remained around 13.11C and eventually fell to 12.9C after 3hrs.  The air temperature dropped from 10.1C to 9.7C.  We lost some energy to the air, but not enough to change the air temperature upward.  We also likely lost a significant amount of energy to the geothermal bank which as about 4 or 5 time the contact area as the soil bed.