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Citrus General Discussion / Eutectic Salts
« on: October 16, 2018, 02:48:19 PM »
A well-designed greenhouse can maintain temperatures in the night using just barrels of water as the thermal mass.
You can look at the temperature data for the SunCatcher design greenhouse here:
https://s3.amazonaws.com/sfc-dynamic-content/uploadfiles/212/Terry%20Carroll.pdf
The nighttime temperatures inside tend not to go below about 58 F, even thought the ambient outside temperatures are going down to between 2 to 15 degrees F.
The parts of the greenhouse not facing the angle of the winter sun to collect sunlight are well insulated. Both the inside and outside of the greenhouse is painted white, to reflect maximum light inside, and help avoid heat loss at night on the outside, and prevent overheating in Summer.
Another technology that exists is to use special eutectic salts instead of water as the heat mass. These eutectic salts can store 62 times the amount of heat by weight as water. The salts undergo a phase change from liquid to solid at a particular temperature.
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.
Like using water as a thermal mass, eutectic salts can be used to store the heat generated by light coming through the greenhouse during the daytime and allow it to be stored so that it can be released back at night. Unlike using water over its liquid range, however, the use of eutectic salts involve freezing or phase change to bank heat.
http://smallfarmcanada.ca/2016/chill-out-storing-heat-inside-a-greenhouse-using-salt/
Keep in mind though the night temperature inside the greenhouse will still likely be a few degrees lower than the temperature maintained by the phase change medium.
Dr. Maria Telkas, University of Delaware, proved that Glauber's Salt technology was sound and economical some 30 years ago. She had a couple of patents in that regard. She built a home based on same at the U. of Delaware, and it was a nice practical validation. This technology could have a major impact on how we deal with energy.
Thermal storage is a fundamental problem in solar designs and phase change materials make tremendous logical sense when you go through the numbers. For example, stone, brick and the like have about 0.2 cal./gram/deg C heat capacity. The different mixtures of Glauber's salts, for example, have about 50 to 80 cal./gram during the phase change. Water is 1 cal./gram/deg C. So if you cycle brick, for example, through about 5 degree Celsius change, you only have 1 cal./gram. In this case you have an advantage of 50 to 80 times the thermal storage with Glauber's salts as you do with brick.
As described on our web site, I can heat the north side of our home (passively) with these salts. Because of Dr. Telka's patented techniques, the salts cycle indefinitely. I have had mine for about 20 years.
My salt tubes are about 2 1/2 inches in diameter and 36 inches long and are black-heavy PVC -- sealed with caps on the ends. They work well. As you can see on our web site, I have them on the South side so that the winter sun gives them good exposure -- melting them during the day time, so that they will give off 90 degree F heat at night. If you had them in a circulating water bath, that would probably work as well.
The salts that I have melt at 90 degrees F and have a heat capacity of about 80 calories per gram.
As shown by Dr. Telkas, you cannot use straight Glauber's salt. She came up with a patented mixture to make them work properly.
She used borax to act as a seed to keep the salts from super cooling, and she added clay (I believe bentenite) to stabilize the mixture. The deca-hydrate water bond to the sodium sulfate is a week bond, and if it breaks then you loose the heat-of-fusion properties of about 80 calories per gram. The melting point is about 90 degrees F. The salts I have in our home came from England. Some companies in the states tried her idea, but bypassed the patents and they all failed.
We built our home in 1992 with no furnace and it often gets to -20 degrees F here. It did last us through the winter.
http://www.allanstime.com/SolarHome/Eutectic_Salt/
One such phase change salt material is NaCl·Na2SO4·10H2O, which has a melting point at 64 F, phase change heat of 286 kJ/kg
That's simply a mix of regular salt and sodium sulfate hydrate (Glaubers salt), in a ratio of 0.18 g of salt to 1g of Glaubers salt.
Greenhouses can get quite hot inside when the sun is out and the skies are clear, even though it may be cold outside, due to what has been termed the "Greenhouse Effect". However, during the night, temperatures are usually only a few degrees above the outside temperature without some sort of thermal mass. (Although being sheltered from the wind may have some additional benefit protecting the plants from wind chill, movement of air carrying away heat at a rapid rate)
You can look at the temperature data for the SunCatcher design greenhouse here:
https://s3.amazonaws.com/sfc-dynamic-content/uploadfiles/212/Terry%20Carroll.pdf
The nighttime temperatures inside tend not to go below about 58 F, even thought the ambient outside temperatures are going down to between 2 to 15 degrees F.
The parts of the greenhouse not facing the angle of the winter sun to collect sunlight are well insulated. Both the inside and outside of the greenhouse is painted white, to reflect maximum light inside, and help avoid heat loss at night on the outside, and prevent overheating in Summer.
Another technology that exists is to use special eutectic salts instead of water as the heat mass. These eutectic salts can store 62 times the amount of heat by weight as water. The salts undergo a phase change from liquid to solid at a particular temperature.
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.
Like using water as a thermal mass, eutectic salts can be used to store the heat generated by light coming through the greenhouse during the daytime and allow it to be stored so that it can be released back at night. Unlike using water over its liquid range, however, the use of eutectic salts involve freezing or phase change to bank heat.
http://smallfarmcanada.ca/2016/chill-out-storing-heat-inside-a-greenhouse-using-salt/
Quote
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.Of course, water itself has a phase change temperature of 0 C, so barrels of water are very effective at keeping temperatures inside a greenhouse from going much below freezing, since the water will begin to freeze first before the temperature goes down.
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.,
I purchased the amount of phase change medium (PCM) product with a phase change temperature of 6C (43F) as recommended by the manufacturer.
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.
Keep in mind though the night temperature inside the greenhouse will still likely be a few degrees lower than the temperature maintained by the phase change medium.
Dr. Maria Telkas, University of Delaware, proved that Glauber's Salt technology was sound and economical some 30 years ago. She had a couple of patents in that regard. She built a home based on same at the U. of Delaware, and it was a nice practical validation. This technology could have a major impact on how we deal with energy.
Thermal storage is a fundamental problem in solar designs and phase change materials make tremendous logical sense when you go through the numbers. For example, stone, brick and the like have about 0.2 cal./gram/deg C heat capacity. The different mixtures of Glauber's salts, for example, have about 50 to 80 cal./gram during the phase change. Water is 1 cal./gram/deg C. So if you cycle brick, for example, through about 5 degree Celsius change, you only have 1 cal./gram. In this case you have an advantage of 50 to 80 times the thermal storage with Glauber's salts as you do with brick.
As described on our web site, I can heat the north side of our home (passively) with these salts. Because of Dr. Telka's patented techniques, the salts cycle indefinitely. I have had mine for about 20 years.
My salt tubes are about 2 1/2 inches in diameter and 36 inches long and are black-heavy PVC -- sealed with caps on the ends. They work well. As you can see on our web site, I have them on the South side so that the winter sun gives them good exposure -- melting them during the day time, so that they will give off 90 degree F heat at night. If you had them in a circulating water bath, that would probably work as well.
The salts that I have melt at 90 degrees F and have a heat capacity of about 80 calories per gram.
As shown by Dr. Telkas, you cannot use straight Glauber's salt. She came up with a patented mixture to make them work properly.
She used borax to act as a seed to keep the salts from super cooling, and she added clay (I believe bentenite) to stabilize the mixture. The deca-hydrate water bond to the sodium sulfate is a week bond, and if it breaks then you loose the heat-of-fusion properties of about 80 calories per gram. The melting point is about 90 degrees F. The salts I have in our home came from England. Some companies in the states tried her idea, but bypassed the patents and they all failed.
We built our home in 1992 with no furnace and it often gets to -20 degrees F here. It did last us through the winter.
http://www.allanstime.com/SolarHome/Eutectic_Salt/
One such phase change salt material is NaCl·Na2SO4·10H2O, which has a melting point at 64 F, phase change heat of 286 kJ/kg
That's simply a mix of regular salt and sodium sulfate hydrate (Glaubers salt), in a ratio of 0.18 g of salt to 1g of Glaubers salt.
Greenhouses can get quite hot inside when the sun is out and the skies are clear, even though it may be cold outside, due to what has been termed the "Greenhouse Effect". However, during the night, temperatures are usually only a few degrees above the outside temperature without some sort of thermal mass. (Although being sheltered from the wind may have some additional benefit protecting the plants from wind chill, movement of air carrying away heat at a rapid rate)
