Sustainable Greenhouse Heating

Can thermal batteries and passive solar greenhouse produce year round crops, even frost tender ones.

Abstract:

Currently in northern Wisconsin where this experiment will be carried out, production in Winter is all but impossible with temperatures reaching -30 in January.  Current commercial greenhouses are capable of producing cold tolerant plants year-round with a heat source, Frost tender plants in the winter would require a massive influx of energy in the form of heat with the current designs being used, either wood stove or propane heaters.   As a result, vegetables that are American Staples such as tomatoes must be imported from warmer climates or preserved in some manner. Either way, is energy intensive either in the form of shipping costs or energy costs in canning.  This has a direct impact on climate change as these heat sources and fuels are emitting greenhouse gases warming the atmosphere causing global weather instability among other things.   “The US agricultural sector contributes to 10% of greenhouse gas emissions annually” (https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions)

There are other things to be considered… the impacts of a globalized food system on food security with current geopolitical instability. The consideration must be made that perhaps total dependency on the global trade of food is ill-advised.  Current US debt is devaluing the US dollar creating a possible dollar collapse where food will be more valuable than dollars.  In such a scenario, the ability to produce as much food locally becomes an imperative.

Current commercial greenhouses use a metal aluminum frame or galvanized steel frame driven into the ground and covered with 2 heavy greenhouse films  This film is some form of plastic bringing up the issue of microplastic contamination in crops.  In addition, steel and aluminum are metals making them more conductive to energy, meaning they are acting as a radiator instead of an insulator, which is what we want in a cold climate.  Currently air is driven between these two sheets of greenhouse plastic to create an insulating layer of air between the two.  Taken as a whole. This design is monumentally inefficient at retaining heat. The curved design of most greenhouses also presents a problem in terms of refraction.  Richard Feynman found that light that hits at any other degree than perpendicular to the source it’s trying to penetrate it will simply bounce off in a parabolic rate of energy loss dependent how further the angle is off from 90° Here in Wisconsin the sun is 69° above the horizon at summer solstice and 22° above horizon at winter solstice. 

Taking all these factors into account this prototype greenhouse was built out of wood and glass. Both are insulators.  Furthermore, it was constructed with the stud brackets being  equilateral triangles.  This makes the south facing glass roughly perpendicular to the sun sun throughout late spring and early fall.  Because it has flat glass instead of a curved surface. This also allows for maximum energy penetration in the form of not only light but infrared radiation AKA heat.

The thermal battery itself consists of a sandpit dug in the center of the greenhouse with a simple heating element run inside the pit and the sand itself contained with in standard 8″ by 16″ masonry cinder blocks.  The pit itself goes down roughly 3 ft.  Sand, concrete and soil all have higher specific heats.  Specific heat relates to the ability of a material to retain energy. Once heated the higher the specific heat, the longer it retains energy. Conversely, the higher the specific heat, the longer it takes to be heated and the more resistant it is to heating.  The ground here freezes to a depth of 4 ft during the winter making any root penetration impossible.  Using the soil’s own properties against it, this battery will eventually be charged by a 500 w wind turbine directly to the north of the greenhouse, and every time the wind blows is heated, just a little more.  Theoretically, it should provide enough heat to keep the greenhouse soil and ambient temperature within the range of crop production.  Target temperature is between 40 and 100.

Note Feb 2025: 

I already had a wind turbine that I will use for the project, but it isn't working correctly at this time. Its up on a 15ft pole. It was a nightmare to get up there originally and due to personnel issues this year, and just not having anything tall enough to get up there, it actually presented a huge problem. So I decided to go with 4-100w solar panels and associated charge controller and battery. I dont like the idea of using solar just because its energy output is going to be lowest at the time of year I need the energy the most, but it sure beats shimmying up a frozen steel pole and taking my life in my hands. I intend on attempting to replace the turbine again come spring. That will give me double the original energy input, and theoretically double the heat.

 Methodology: 

Unfortunately there is no “control”, just does it work or not.  Three thermometers,  one at battery, one for grnhs ambient air & one for soil temperature 1′ from outer wall; daily.   In addition, a daily record of outside weather will be tabulated.  This includes whether there was sun or clouds, The outside ambient temperature & and then average windspeed.  

Genovese Basil will be the frost indicator plants, The cold hardy herbs of oregano, peppermint, chives and parsley will be the indicator of cold hardy growth during the winter.  Due to the design of this greenhouse, it reaches temperatures 60 to 70° above ambient without a heat source, meaning I cannot safely start this experiment until the inside midday temperature is regularly below 100°  that would mean daily highs could be no more than 40° before this can be started.

Data

6 months nov-april

Observational data – did plants survive and did they grow enough to produce a crop?

Results…

Conclusion…”