Palomar Mountain Geothermal Greenhouse Construction Project

I've built solar (off-grid home, solar-powered treehouse), wind, biogas, and hydro "green" power systems, and it was time to try geothermal. I'll likely skip nuclear power, so I think this completes the "green" power options for me. The application? A geothermal greenhouse for growing more of our own food. It is fully-automated, in order to preserve the plants when we are not there or when the weather beomces extremely harsh. More on that later.

Lots of calculations are needed to make this work in below-freezing temperatures without needing extra heat/power. We have an ideal location, about 50 feet from the home and in direct view. It took approximately four months to complete the greenhouse. The greenhouse is off-grid, solar-powered, fully-automatic, and geothermal - it has a novel "climate battery" to permit us to grow even in the deep of winter and snow without heating it. Title-24 compliant, Platinum LEED-certified, of course.

The greenhouse is geothermal, using the earth’s stored heat to our advantage. Five feet below ground, the temperature is always stable at around 55 degrees, summer or winter, regardless of location in the US, even at our 5,000+ feet altitude. The way we harness this stable underground temperature is to circulate the air from the greenhouse through 160 feet of underground pipe loops. During the day, it transfers heat from the greenhouse to below ground; at night, we transfer heat from below ground to the greenhouse. The energy spent is a fan that circulates the air through the tubes, and it is, of course, solar-powered.

The greenhouse is surprisingly efficient at capturing solar energy. In its first week of operation in 32-degree weather outside, during the daytime, the greenhouse reached over 100 degrees when the vents were closed! In the summer, I needed to add solar cloth over the entire roof to keep the temperature below 100 degrees, otherwise it would reach 140 degrees or more and cook the vegetables (and baby trees) prematurely.

The severe weather (freezing in winter, snow, hail) require extraordinary construction techniques, not unlike any building in harsh climates. Construction is done with redwood for rot and insect resistance. Not a single solitary nail, all done with wood screws and/or bolts.


Literally dozens of hours in design work, one of many early design drawings at right. And lots of calculations, including:
- solar radiation patterns over 12 months
- shadow profiles of nearby trees and buildings to ensure direct sunlight
- calculation of BTU's (and equivalent watt-hours) of solar radiation
- calculation of air velocities through the manifolds

After doing solar calculations by hand, I discovered this online web application that does precise solar position and shadow calculations for any location on earth. At right: The black rectangle shows the garage's maximum shadow length is on December 21. I want to ensure that the greenhouse is not located in the shadow of any nearby tree or structure. This permitted me to precisely locate the greenhouse. The app is at
The construction starts with a rough layout or ground plot with the four corners identified. The orientation and location are critical; hence the year-round solar position calculations are essential. The greenhouse size will be eleven feet by sixteen feet, three inches.
Next, I dug out the "cellar," which will permit the underground tubing to be buried for the geothermal "climate battery." About 25 cubic yards are dug up, some by hand, tough work. That is about 25 tons of earth or 50,000 pounds.
These gray plywood boxes are geothermal manifolds that will be buried underground for the climate battery.

Here are the manifolds in place with six of the eight 4" tubes that will be buried underneath the soil.

A geothermal "climate battery" with a fan circulates the solar-warmed air during the day through the underground tubes, heating up the soil and cooling the greenhouse air. At night, the air is circulated again to extract heat from the ground and warm the greehouse.

I did about 100 feet of manual ditch digging to locate water and connect to the greenhouse for watering and misting. This was tough work.
Electrical conduit from the solar room in the garage to the greenhouse.
Water pipe and electrical conduit in place prior to foundation and floor preparation.
Foundation floor and stem wall ready for a single pour.
Getting ready to pour-pump five tons of concrete.
The pour: The concrete pumper adds cost but greatly simplifies delivery and control of the pour.
Pour finished, concrete surface-finished, and concrete setting up. The floor drops four inches over sixteen feet to ensure there is no water puddling inside when wet. There is a large drain at the end.

The assembled greenhouse shell at night. It has six vents, two on each side and two on the roof. On sunny days, the greenhouse is hot, so venting is necessary, even in winter. On the first day of 32-degree temperature with full sun and the vents closed, the greenhouse exceeded 100 degrees. In the summer, I add solar shade cloth over the entire roof to keep the temperatures down.

For those of you interested, the solar energy directly heating the greenhouse in full sunlight exceeds 17,000 watts or about 60,000 BTUs. Solar cells would only capture about 20% of that energy; the greenhouse captures most of it.

Roof vents automatically open at a preset temperature using a mechanical vent opener that converts sunlight heat into mechanical motion. The hotter it gets, the wider the vent opens. There is also a thermostatically controlled vent fan to help exhaust air. That fan kicks on at 90 degrees and above. Solar powered, of course.
When the roof vents open far enough, a microswitch delivers power to open the four side vents via an electromechanical assembly I built (one of two, at right).
At right: Two of the four side vents that open automatically when the internal greenhouse temperature reaches a critical temperature. There are six vents in total, two in the roof, four in the sides.

There are a lot of electronics in this greenhouse, at least seven thermostats, one humidistat, etc. These, along with misters and drip systems, make the greenhouse fully automatic.

At right: Two of the thermostats that turn on the manifold fan to circulate air through the geothermal pipes. The fan runs when the greenhouse exceeds a certain temperature during the daytime, thus warming the soil beneath; and warming the greenhouse at night, returning the heat from below.

A bit difficult to see, but the (blue) misters in action automatically turn on when the humidity falls below a certain level.
Far right: A humidistat which controls the misters to provide a constant humidity level. Temporarily surrounded by cardboard so I can see the displays inside in bright sunlight. To its left is an automatic sprinkler control to automate the drip system.
I barely completed the greenhouse before the first snow flew! Cold outside, warm inside.
All this work to enable us to grow food year-round without wildlife consuming it. Of course, it will be ideal for raising young trees from seed as well. At right, our first bean seedling.
More about controls and automation of the greenhouse:

Humidity: There is a humidity probe that automatically turns on the greenhouse misters anytime the humidity falls below a certain level. When the temperature drops at night, the humidity goes way up since cold air cannot hold much water. As the sun rises and the greenhouse air warms, the humidity level drops, and the misters turn on.

Temperature (1): During the day, as the greenhouse temperature rises, says above 80 degrees, the manifold fan turns on to move warm air underground to heat the soil. At night, when the temperature drops below, say, 40 degrees, the fan runs again, bringing air warmed by the underground into the greenhouse. So, the earth acts as a thermal battery, storing heat during the day and releasing it at night. Separate thermometers inside each manifold ensure that the temperature differential is correct between intake and exhaust air from the underground.

Temperature (2): If the temperature rises about, say, 80 degrees, the two vents in the roof open, followed by the four vents in the side walls, as needed. When the temperature rises above 90 degrees, an exhaust fan in the roof vent runs to force warm air out.

Drip irrigation: A drip system with an automatic timer and automatic fertilizer dispenser feeds the plants during the daytime. There are seven separate "drip routes" or dispense routes, so we can precisely control the drip to specific plants. In addition to the drip zones, we can control the drip to most plants individually.

There is a careful balance between all of the set points of the controls in the greenhouse. This makes such a system ripe for artifical intelligence software, but a human (me) must figure it out first through trial and error.

>T1 80 ON       Cooling greenhouse by transferring heat to subterranian
>T2 85     OPEN   Cooling greenhouse by opening roof vents
>T3 90   ON   OPEN Added cooling greenhouse by opening side vents and forcing roof vent exhaust
<T4 40 ON       Warming greenhouse at night by transferring heat from subterranian
<H1 30%         Misters on