Category Archives: Solar

Greenhouse solar heating

The ultimate source of heat for my greenhouse is the sun.  Even if the outside temperature is less than 10C, inside, due to the energy input from the sun, I’m seeing up to 40C on sunny days and almost 30C on overcast days.  However, my water reservoir, which I pump through a geothermal bank 6 feet under the ground and through my soil bed, is only seeing a maximum of 15C on those hot days.  As nights have been dropping below freezing (-4C), it drops about 5C to 10C when I wake up.  That’s not acceptable.

The issue is that the air in the greenhouse can only heat the water if it comes in contact with the reservoir container.  Further, waiting for the air to heat up by the sun, and then the air to heat up the water isn’t ideal.  Air is a poor conductor of heat and has relatively low energy storage capacity.  To improve this situation, we need to increase the surface area of the water system.  We could put in a bigger reservoir but space is a scarce resource inside my greenhouse.  A larger reservoir will increase the surface area, but only a little bit of that will receive direct photon penetration from the sun.  What we really need is a solar heater.

I have a 4 ft x 6 ft area above my components.  I could mount a board of some sort up there are coil some water tubes to increase the surface area.  I also have 2 old CPU radiators with fans.  This will increase the effective surface area of the water’s contact with the air.

Over a couple weekends, I took the 200 ft of black poly tubing and made two spiral loops almost 2 feet in diameter on a 8 ft by 4 ft Styrofoam board I picked up at home depot.  In between the two spiral loops, I placed the radiators in series.  To hold down the tubes, I used automotive-grade 3M double-sided tape in an “X” pattern.  On the top, I used gorilla tape, but this wasn’t effective.  Right before installing, I used 10 gauge wire to lock it down in case the 3M tape fails.


I mounted the setup at a 30 degree angle near the top of my greenhouse facing south.  This is not ideal.  In my area, a 70 degree angle would be better, but space is limited, so this will have to do.


Before I installed it, I tested it on the ground at a 70 degree angle.  Over the space of an hour or two, the water heated up from 13C to 19C.  It was a sunny day.  This is about the same as pouring two good size pots of boiling water inside the reservoir.

I’ll continue tracking the performance, but today I’m optimistic this will help.  The running cost is pretty cheap too at only 28 Watts (12V @2.4A max).  I’ve coded up an algorithm in my automation controller to only turn on the pump if the air is hotter than the reservoir.  This is good for the winter time, but this rule probably won’t work will in the summer.  In fact, I may have to add a rule to turn on the solar heating system to help cool the greenhouse.  We’ll see.

What about the cost?  Well, this addition was relatively cheap.  If you want the cheapest solution, this might be it.

Cost breakdown:

Total: $104


Bonus Total: $119

Auto-irrigation system for raised garden using the Intel Edison

The Plan

I want a raised garden but I don’t want to have to manually water it like my lawn sprinkler system.  So I’ve been planning and gathering parts for an auto-irrigation system.  Here are the key parts:

  • Rainwater gathering system
  • Valve control to drip-water plants
  • Solar power (with solar tracking?)
  • Soil temperature and humidity sensors
  • Auto water-soluble fertilizer mixing

In this part, I’ll talk about the solar power system -specifically power storage.

Solar Power: Power Storage

I have a bunch of 350 farad super capacitors laying around.  The cool thing about super capacitors is that they can charge directly from the solar panel.  I picked up a balancer on ebay and connected six of them in series to give me about 16 volts.  I also have a spare 10W Instapark solar panel that I’ll use to charge the cells.  The Instapark solar panel is rated for 22V closed circuit.  I shouldn’t charge my super caps over 16 volts so I will need to reduce the voltage a bit.  The easiest way to drop the voltage is to use a resistor.  Using Ohm’s law we can calculate how much resistance we need:

R = V/I

My voltage drop (V) is 22V (the panel max) / 16V my super cap array max which is 6V.  The current (I) I expect to see is 600mA or 0.6A.  Plugging in my variables I get

36.66 ohms.  I want 10 watts to be safe (I figure, probably wrongly so, that a 10W resistor for a 10W panel will be fine).


I found some water resistant enclosures on amazon.  This was perfect size for my super cap bank.  I got an additional one to put the Intel Edison and related circuits in.  To keep it water tight, but also allow cables to get in and out I picked up 4 of these from adafruit along with matching water resistant cables.



I used a 5/8″ spade bit to create two holes for the cable glands for the super cap box.



Carefully I screwed in the glands and put some gasket sealer on the inside to seal some of the uneven spots from the drill.



I did the same thing with the “Edison box”, but on opposite sides.



I then stacked two power supplies on top of each other.  I got the power supplies from amazon.  They have adjustable output and a wide input range.  I have one set at 12V for the valve solenoid and the other at 4.2V for the Edison.


Finally, I attached a power button so I can turn on and off. This too needed to be water resistant.  The white LED color is a nice touch, IMHO:



Finished Power Enclosure

The enclosure works pretty well.  It took about 15 minutes to fully charge.  My hope is that it will power the Edison and friends for an entire day and most of the night.  If it turns off in the night, I can live with that.




Next part we’ll look at the 2nd Enclosure for the Edison and friends.  Stay tuned!