Sources of Free Heat in a Climate Battery Greenhouse
“Free” Heat? What do you mean?
Let’s define what we mean by “free” heat in a climate battery greenhouse. For the purposes of this post, “free” heat is that which we don’t have to spend any energy to store and in one case below, the thermosiphon, one that we don’t have to spend any energy to retrieve.
Normally in a climate battery greenhouse, the excess greenhouse heat is stored within the soil beneath it, then extracted as needed. This heat isn’t free in the sense that it requires us to run fans to “pump” the heat underground. While this is a pretty efficient operation, the fans require electricity. Therefore the purpose of the article is to describe other means by which heat enters the ground.
Let’s get started!
Gradual Soil Warming
tl;dr, Bottom Line: The sun gradually warms the soil during the summer for use in the fall, all without running climate battery fans.
In most every climate, there is excess heat in a greenhouse in the summer. This heat warms the soil surface, and gradually works its way into the ground to a certain depth. The interesting, and perhaps very useful, part of this cycle is that the temperature at deeper and deeper depths is delayed in reaching its peak. So the temperature at, say, 3’ below grade reaches its temperature peak before the ground at 6’ depth. The useful part of this is that the high temperature at the deeper depths seems to occur around the same time we’d require some extra greenhouse heating in the fall. Therefore the sun warms the soil for free all summer, then that soil heat can be drawn out in the fall when needed. This is illustrated well by the folks over at builtitsolar.com that originally came from Virginia Tech: Ground Temperatures as a Function of Location, Season, and Depth (wayback machine cached copy: https://web.archive.org/web/20120211203828/http://www.geo4va.vt.edu/A1/A1.htm)
Ground temperature as a function of depth and season for a site in Virginia. Your prevailing ground temperature will be different (this one was 62°F), but the general seasonal pattern will be the similar.
Source: https://web.archive.org/web/20120211203828/http://www.geo4va.vt.edu/A1/A1.htm
Temperature Buffering
tl;dr, Bottom Line: The warmer soil surrounding the climate battery can help bump up the climate battery soil temperature, primarily during winter months.
Winter can be tough, especially during cold spells. Not much solar gain, but a potentially big need for heating in the greenhouse. If the temperature of the soil drops too low, the ability of the climate battery to heat the greenhouse is diminished. Thankfully, the soil surrounding the climate battery thermal mass can help. If the soil in the climate battery is colder than the prevailing surrounding soil temperature (almost a constant 52°F in our area of the country), the climate battery will draw heat away from the surrounding soil and gradually work its way back toward the prevailing ground temperature for your area. This can work to your benefit if you avoid constantly drawing out heat from the climate battery over the course of a few days or a week.
Conversely, this can work against you during certain times of the year, too. In spring, a series of sunny days followed by a series of cloudy days can mean that the heat stored over the sunny days can be gradually lost over the following cloudy days through heat loss to the surrounding colder soil.
A map of the prevailing ground temperature by region of the country. This is the temperature that your soil tends toward if not actively heated or cooled which can be useful in the winter season.
Source: https://web.archive.org/web/20120211203828/http://www.geo4va.vt.edu/A1/A1.htm
Thermosiphons
Bottom Line: Large differences between air and ground temperature create a convective loop to “pump” heat into the greenhouse without the use of fans.
This source of free heat refers to the method of heat delivery rather than free stored heat described in the other two examples.
A thermosiphon works off of the principles of natural convection: in a cold greenhouse with a warm climate battery, the coldest air falls into the risers of the greenhouse, where it is warmed by heat transfer and rises back into the greenhouse warmer. Normally, fans would assist this process along, but in the instance where the difference between the air temperature and ground temperature is greater, this happens at a greater rate. We haven’t yet quantified this effect but it’s neat to see it working as we transition into the fall. We see this most often in the early fall where the ground is very warm from the summer (> 70°F, but the air temperatures in the greenhouse have dropped to < 50°F).
Wrapping It Up
We hope you found this article helpful to your understanding climate battery greenhouses a little better.
Are there other “free” source of heat we’re overlooking? Did you have a question on any of these three? Let us know in the comments!