The system is explained in great detail on John Cruickshank’s website, SHCS explained.  The system works by taking warm humid air in the greenhouse, and pumping it through a set of perforated drain tubes, which are buried in the subsoil, eventually exiting back into the greenhouse minus some heat and moisture.  It is based on the phase change of water, from water to vapor, back to water and the cycle starts again.  This amazing technology is fueled by the power of the sun.  The sun heats up the greenhouse causing evaporation and evapotranspiration from plants.  When the heat of the greenhouse reaches a certain temperature that is set by the manager of the greenhouse, a thermostat will turn on the fan.  The air and water vapor are pumped into the cooler soil where the water vapor condenses.  The condensed water holds much of the energy that it took to lift it into the air as vapor.  This technology allows us to use the thermal mass of the ground as a storage tank for surplus energy.  At the same time the SHCS is storing heat in the soil it is also cooling the greenhouse, the daytime cooling aspect.  There is another thermostat that will turn on the fan at a low temperature chosen by the manager.  This will take the stored heat in the soil and transfer it to the greenhouse, the nighttime heating aspect. The system needs a summer of heat storage to run optimally.  It allows us to run a greenhouse at a relatively constant climate throughout the year. Having been tested mostly in cold winter regions, it has been tried and true for a heating and cooling system in cooler regions. But for using in a warmer climate where the cooling capacity of the system may be more important there has been little experimentation.  John Cruickshank has suggested the idea of burying the tubes outside of the greenhouse to use the SHCS strictly as a cooling technology.  The site where I am working with this system is more similar to where it has been tested but in very close proximity to warmer climates, I hope to be able to experiment with the same technology in adjacent climates, testing it in colder climates as well as warmer climates. John Cruickshank has been kind enough to make the knowledge of this technology available on his website, The Land of Hobbit House and Company.  He highlights the physics of the system and how it works.  He explains where the heat storage is, how it accomplishes daytime cooling and nighttime space heating. He not only makes the knowledge available, he has created a calculator to help people figure out how much tubing is needed to install, the size of the fan needed to run the system, how much it will cost and how long it will take to pay you back in terms of energy stored.

            When I returned to California to resume my studies at UC Davis I knew that I eventually wanted to work with the SHCS technology.  At the intentional community that I was living in there was a greenhouse that was in need of a cooling system.  I had proposed the idea to the community, but alas with little firsthand experience and the arrogance of some that were engineers the technology was never adopted and a swamp cooler installed instead.  This only reaffirmed that I wanted to eventually have a SHCS so I could demonstrate the technology.  After graduating from UC Davis in 2007 I moved back home to Love Creek where I continued working on a permaculture demonstration site and started my business, the Love Creek Center for Permaculture.  I planned to eventually install the system there.  One of my main intentions for adding this technology to the Love Creek Center For Permaculture is to increase the amount of sustainable technology models.  It compliments Dave Holmgren’s Permaculture Principle #2; Catch and store energy, as well as others.  I’ve seen it in action and I know it is important for people to see it.  I would like to see others adopt it for their own use.  I also want to increase our own food production capabilities on site. 

            There is one greenhouse on site that has been used since 2004; it does not have an SHCS as of February 2011.  It is not uncommon to see the temperatures dip into the 20s in that greenhouse on very cold nights.  This limits the ability to grow some plants, only frost hardy greens and herbs and some over wintering of other perennials.  I want to be able to do more in the cold winter months. With the inspiration of seeing the technology first hand it was finally time to install my own SHCS.  In 2009 we constructed a small greenhouse attached to our woodshop, we’ll call it greenhouse 1(GH1).  This was to be the future home of the first SHCS at the Love Creek Center For Permaculture, it was a retrofit as the greenhouse was already in use.  In the fall of 2010 we built another greenhouse on site that had the SHCS in place before the greenhouse was in place.  I will now show the progression up to date of each greenhouse and then show a chart that shows the differences and similarities.

Greenhouse 1 is attached to our woodworking shop and has been in use for over a year.  

I have used it to over winter tender perennials and start seedlings for the market garden.  It has worked very well in the past year, allowing me to start tomatoes in January and every other garden vegetable, so I can have them ready to transplant in the ground once the timing is right.  Even though we have experienced no frost in this greenhouse I still wanted to see the SHCS in action in it.

            The first thing I had to do was make sure I was doing everything right.  John Cruickshank’s website, The Land of Hobbit House and Company, has several links to all of the details needed.  Before I knew how much tubing I needed to install and how big of a fan required I needed to run some numbers.  I needed to find out the square footage and the volume of the greenhouse.  I came up with 108.6 ft² for the floor and 648.6 ft³ for the air space.  So I plugged these numbers into the calculator that John Cruickshank provides on his website.  Step 1 gets you to calculate the greenhouse volume and all of its dimensions.  When I enter the numbers in the correct spot, things begin to change on the calculator.  Entering square footage gives an estimation of how many feet of tubing I’ll need.  Entering the volume of air space gives me a ratio of cubic feet to square feet and comes up with a recommendation for the size of the fan, measured in cubic feet per minute (cfm).  Step 2 entails entering either the calculated fan size or a fan size of your choice.  The recommended fan was 54 cfm, but since we had one on site that was 50 cfm, we decided to use it.  I plugged in the cfm of the fan used and was on to step 3, where we calculate the underground air circulation tubing (UACT).  Here I began to run into some problems with the calculator.  The estimation of tubing was 144 feet.  However when I plugged those numbers in, the tubing air speed of feet/second didn’t fall into the recommended range of 2-4 ft/second.  I wasn’t sure what to do.  When I changed the number to half of the recommended tubing, the air speed was in the correct range.  I tried to contact John Cruickshank through the email on his website, but no reply.  I contacted a colleague, who recently installed the system, to see if he could help me.  He didn’t know.  I’m thinking the small size of the greenhouse might have influenced the numbers.  So, the learning is the doing and I just went with the numbers that looked right.  I went within the recommended range of airflow and buried 4 tubes at two levels in the ground. Now we go to step 4 where we plug in the costs for the system hardware.  My total will be around $230 after I order the thermostat controls.  Then on to step 5, which is the payback calculation.  I changed the numbers for the cost of energy and for the number of days I expect to get a 1ºF rise in soil temperature.  Since it is working so well in midwinter I expect at least 200 days where the air temperature will reach at least 90ºF.  After I plugged in all of the numbers I am very pleased.  It is saying that it will only take about 1 year to gain the energy equivalent of the cost of the hardware, very nice.  And from then on I will be gaining the energy equivalent of my investment every year.

            I bought the perforated tubing and plastic culverts used as the plenum and the ADS tubing at local hardware stores.  I was able to use a fan that we had around that pumps 50 Cubic Feet per Minute, which was close enough to the recommended size from Hobbit House and Company. In the fall of 2010, after almost a year of using GH1, my intern, Jack Smith, and myself began digging the trench for the tubing.  The tubing and fan were added soon after.  I built the permanent bed that runs the length of the greenhouse with lumber we had on site.  Soon after finishing the bed-frame I planted it with starts that were already on shelves in the greenhouse.  I planted dill, bunching onions, arugula, mustard greens, peppers, galangal, lemongrass, and prickly pear.