Chill: out! Storing heat inside a greenhouse using salt

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Farmers are by nature optimists. We believe that we can take tiny seeds and add the right amounts of heat, water, light and nutrients to get food on our table and hopefully a living as well. In December we pore over seed catalogues, and what we envision our mature gardens to be is circumscribed by our climates.

But sometimes we allow ourselves the what-ifs. What if we could keep our greenhouse running all winter so we could sell more than root vegetables? What if we could grow warm weather crops like citrus? What if food security means that we don’t need to rely on fuel supplies to have a year round food supply? What if we could use a resource as abundant as, say, salt to do all of these things and stop burning the fossil fuels that are heating up our planet?

As the owner of Carmenia Farm, in Duncan, B.C., I regularly indulge in the what ifs and was determined to make these visions a reality. The secret to doing all of the above is in invoking the phase-change properties of a special formulation of salt — eutectic salts.

Eutectic salts are salt solutions specifically formulated to change phase (freeze/melt) and therefore release/store heat at a specifically desired temperature. Road salt and table salt are forms of eutectic salts, but their phase change temperatures are not suitable for food production.

 Eutectic salts can be used to store the heat generated by light coming through the glazing materials by day in a way that allows  the stored heat to be released back into the greenhouse at a given freezing or phase change temperature.

Phase change principles explained

Using phase change principles inside greenhouses is not new. All matter has a temperature at which it freezes; above this temperature, matter absorbs heat energy as molecules expand, and below this temperature the matter releases the heat as the molecules huddle closer and expel heat.

Farmers have long been using barrels of water, often painted black, to warm a greenhouse. Water does an excellent job of storing and releasing heat, but it can’t match the capabilities of eutectic salts.

The salt formulation used inside Carmenia's greenhouse is capable of storing 62 times the amount of heat by weight as water. In this trial the salt based PCM stored 260 kilojoules of heat per kg of PCM whereas water stores 4.19 kj per kg. In order to realize the 6-8 degree Fahrenheit increase in nighttime temperature gain achieved in the trial, Carmenia Farm would have had to place 33 45-gallon barrels of water inside its 200 square foot greenhouse. This would have been impossible as 33 barrels would not even fit. A major benefit of the salt-based phase change medium PCM was that it presented a minimal intrusion into growing space.

Materials such as rocks, concrete and brick also store and release heat. Most of us have experienced this principle by sitting on a warm rock just after sunset. The heat absorption capacity of many products has been measured, and it turns out that salt outperforms them all

The principles of heat generation and storage in off-grid greenhouses are laid out in easy to understand terms by James McCullagh in The Solar Greenhouse Book.  Written in 1978, McCullagh mentioned the up-and-coming technology of eutectic salts. Now 37 years later, there are only two companies with salt-based energy storage products on the market. I first reached out to one of them, RGEES, in the U.S., and received funding from the federal-provincial-territorial Growing Forward 2 initiative to pilot the technology in my greenhouse as part of their Agri-Innovation program for energy management projects. 

Greenhouse demonstration project undertaken

It seemed simple enough. I purchased the amount of phase change medium (PCM) product with a phase change temperature of 6C as recommended by the manufacturer. The selection of a phase change temperature of 6C (43F) was predicated on a combination of factors: greenhouse size, glazing material and thickness, latitude, average minimum temperature during winter months and average daylight hours. The greenhouse would need to generate a temperature over 43F for at least two hours a day in order to produce enough heat to melt the salts required for reloading them with more heat to release again the next time the temperature went below 43F.  A higher phase change temperature was intended to keep the air inside at or above 32F throughout the night. (This was written into the funding agreement as the key determinant of a successful trial.)

The salt solution arrived pre-packaged in 89 1-litre bottles that were placed side-by-side in wire racks along the outside of raised beds. The 200 square foot greenhouse houses eight lemon trees, a lime tree and an avocado tree that had been wrapped with incandescent Christmas tree lights to get it through the previous winter. I disconnected the lights, waited for the temperature drop and . . . . watched in horror as the temperature plummeted to 25F (-3.88C) and my citrus to frost burn. For some reason the PCM had failed to recharge.

Based on advice from the manufacturer and other project partners, I tried a number of fixes, until at last the PCM product worked as intended by early January of 2015. From early January through the end of April, when the data collection ended, the PCM kept the inside of the greenhouse continually above 32F (O Celsius), even when the outside temperature went below freezing on over 20F (-6C) nights, sometimes as low as 26F (-3C). Even more significantly, the soil temperature never went below 50F (10C) throughout the entire trial period.  Despite the early setbacks, all of the citrus, as well as the avocado tree, made it through the winter and are flourishing. Winter production with an air temperature above freezing and a 50F (10C) soil temperature offers year round farming.

   Although this study focussed on winter production, different PCM formulations with different phase change temperatures can be used to moderate heat in the summer.

Lessons learned in the greenhouse trial

All of the fixes addressed mitigating heat loss from the greenhouse and slowing the release of the heat. It was important to have some heat left for the plants in the early hours of the morning rather than dump it all at once at the beginning of the night. Lessons learned included the following:

1.  Start with a properly insulated greenhouse with no air leaks. Three of the four walls of my greenhouse are twin wall polycarbonate, which provides an insulated airspace, but the roof was single wall polycarbonate that frosted over like a car windshield on cold nights. Adding a sublayer of twinwall polycarbonate to the ceiling and insulating the non-glazed wall with pink insulation batts added 5 degrees Fahrenheit to the inside temperature.

2.  The PCM bottles are better stacked than set side-by-side. Following a cold night, the outer

bottles of the stack were frozen while those inside the stack still had heat left inside to release. Covering the plants, with the PCM bottles at their feet, with remay crop cover overhead, also kept the heat close to the plants. Air movement helped as well. The PCM worked better in parts of the greenhouse with lower temperatures but better convection.

3.  A single barrel of water placed inside the

greenhouse went a long way to correct the issues around recharging the PCM. Because the water inside the barrel was not freezing, it was not serving the same phase change function of the salt based PCM. The water did however present a reserve of thermal energy for the PCM to draw from to recharge, as the water remained in liquid form.

Is salt-based heat storage part of your greenhouse’s future?

I’ve kept detailed weather records of my greenhouse and outside weather since a year before the trial, so I was able to compare the average monthly data with the PCM storage to the prior year data without it. Without heat storage, the greenhouse was not very effective at night and sometimes worse than just not having one. Once the issues were resolved, the salt based heat storage yielded an average 6-8 degree F heat gain over the minimum temperature without the storage media, making year round farming possible.

Cost-wise, the salt based storage is tied with hydroelectric, if your greenhouse is already wired. The cost of electricity is continually escalating, however, while the cost of the salt-based PCM has been dropping, tilting the scales in favour of the salt based storage. The savings are amplified if you are considering a new greenhouse that is not already connected to the grid. The PCM product has an expected life of 3,000-5,000 cycles of freezing and recharging, and given the average number of subfreezing nights in coastal Vancouver Island, the PCM product has a useful life of 100 years. The salt based storage cost $1,850 CAD with shipping and exchange rate differences, or about $9/square foot, while the citrus is expected to bring in revenues of $8/square foot annually.

The beauty of this technology is that different formulations of the salts produce different phase change temperatures, so farmers in colder climates are able to also use the technology, simply with a phase change temperature that is appropriate for the local climate. The manufacturer produces salt based storage products that change phase at a number of different temperatures, as low as -26C.  I had concluded that my greenhouse would have been better served by using the 0C product, as it would release the heat just as the plants needed it instead of sooner.

I am now planning activities and seeking funding to make this technology available to the general public. Anyone interested in serving as an early adopter of the technology is encouraged to contact me at carmeniafarm@telus.net.  Early adopters will be expected to pay for the cost of the PCM product but will receive substantial support in terms of product selection, procurement and use. Hopefully, sufficient interest will allow for bulk procurement to lower costs. Reports, graphs and data from my trial are available online at www.carmeniafarm.ca/pcm-trial.html.

- Becky Mason