The Case for a (Mostly) Passive Solar Greenhouse
Defining Terms - What is a Passive Solar Greenhouse?
Let's start with a definition. What is a meant by a passive solar greenhouse?
Not every source I see on the internet is clear with regards to the definition of a passive solar greenhouse. One source mentioned that a passive solar greenhouse is one that does not need to be heated. The intentions of the article are good, but may provide the mistaken impression that no "heating" is happening.
Another source, referring in general to passive solar design, puts it succinctly: "Passive solar design refers to the use of the sun's energy for the heating and cooling of living spaces by exposure to the sun.”
In a greenhouse structure, this means that we passively collect the sun's warmth during the day and it gets "reused" at night through conduction and radiation from the warmed surfaces and materials. It's not that there's no heat source; rather, that we design the greenhouse itself to be the source of heat. We don't actively do anything other than to set up the system for success. We know the patterns of the sun and how to best capture it, and we know the thermal properties of the materials that are doing the capturing of the heat. So once the system is setup, it works passively, without our active involvement.
In the passive solar greenhouse world, there seems to be a desire for a completely self-powered system. I certainly applaud this sort of effort, especially if it simplifies greenhouse designs and makes them more resilient.
For the purist, this may mean that no electricity or other external energy sources, such as fossil fuels, are utilized. I hope to make the case through this article that a little added electricity can go a long way toward a more functional and effective use of solar energy.
Passive Solar Greenhouse - Why is it needed?
But wait. Aren’t all greenhouses, to some degree, a passive solar greenhouse? Yes and no. For sure, greenhouses are designed to passively capture sunlight through their glazing (or “covering”).
However, most greenhouses are pretty terrible at retaining that captured heat. Here’s why:
Greenhouses have really poor insulation values, especially compared to modern houses. Most of the heat you produce to heat your home is slowed significantly from escaping due to insulation in our foundations, walls, and ceilings. At the very least, most modern houses are insulated to R20 (R being a measure of resistance to heat transfer).
Contrast that with greenhouses, whose outside glazing has an R value of somewhere between 1 and 2, and you’ll quickly begin to see the problem. Greenhouses are excellent at capturing heat, but they’re also experts at losing heat. That means that greenhouses heat up really well during the day, but at night the temperature in the greenhouse plummets and is roughly equal to the outside temperature by the morning. This is sometimes referred to as the “freeze or fry” tendency of a typical greenhouse.
However, there are measures employed in greenhouses to combat this tendency. Energy curtains or blankets, row covers, special coatings on glazing materials, and heavier insulated walls all can slow the migration of heat while not significantly impacting the ability of the greenhouse to capture heat. In fact, many of these options are relied upon in a passive solar greenhouse. With that, let’s begin the discussion of the benefits of the passive solar greenhouse.
Passive Solar Greenhouse Benefits
Solar Energy Capture
Passive solar greenhouses are oriented in such a way as to most efficiently capture energy from the sun. For us in the northern latitudes of the northern hemisphere, we orient our greenhouses lengthwise east to west and have our south-facing side open to the sun to make the best use of the sun in the winter as well as the shoulder seasons of late fall and early spring. This is typically as true for passive solar designs as it is for the “normal” greenhouse: capture light as efficiently as possible, especially during times when the sun is the weakest.
Where does all of that solar energy go? In a greenhouse structure, some of the heat warms the air and plants, some warms the ground, and some warms the structure itself. Generally a passive solar greenhouse provides greater opportunities to capture that heat passively through thermal mass: items strategically used in the greenhouse for the purpose of heating up on a sunny day, then ideally slowly radiating that heat through the cold night. Items typically used for this purpose are barrels of water (often painted black to absorb the sun’s rays) and rocks and rock walls within the structure.
Threefold Farm greenhouses setup to maximize solar capture in the winter
Source: Fair Share Farms. Barrels of water absorb heat during the day and release it at night.
Solar Energy Retention
Energy retention is where passive solar greenhouses really shine. Most all designs utilize an insulated, north-facing wall and also heavily insulated east and west walls. Why? Because in the northern hemisphere, most sunlight enters through the south-facing glazing. It doesn’t make a ton of sense to leave the north-facing side open if its primary contribution is to allow a small fraction of the sun through and to lose heat. The same goes with the east and west walls. The sun is weakest when it’s rising or setting, so blocking off the east and west walls is really only blocking the weakest portion of the sunlight for any given day. Insulating these three walls greatly reduces the heat loss of a greenhouse structure at the cost of somewhat reduced sunlight entering the greenhouse.
That said, there’s still a significant source of heat loss through the remaining low insulation value surface: the south facing glazing. Some of the best passive solar designs, like the Chinese solar greenhouse, make up for this through energy curtains or energy blankets. These blankets can be rolled to cover the greenhouse area (either inside or outside of it depending on climate conditions) at night when the majority of the heat loss occurs. Think of it as “tucking in” your greenhouse for the night. Though they’re often not super-insulating in and of themselves, the blankets can greatly improve the performance of south-facing glazing. Remember, even a jump from an insulated R value of 2 to an R value of 4 is effectively doubling your insulation, thereby halving your heat loss.
As a side note, most modern greenhouse glazings now block infrared radiation (IR), or radiant heat, helping to further prevent heat from escaping your structure.
Passive Solar Greenhouse Drawbacks
High Cost Per Square Foot
Many passive solar greenhouse designs are expensive on a per-square-foot-basis compared to other greenhouse structures. While a reasonably heavy duty and well-equipped high tunnel structure can be completed for around $4 per square foot, the materials alone for the Deep Winter Greenhouse plans run around $25 per square foot. Certainly the Deep Winter Greenhouse is better insulated, but at some point it becomes nearly impossible for the farmer to have the structure pay for itself, even when considering fuel savings.
This is certainly not an attempt to discredit the efforts of the Deep Winter Greenhouse folks or anyone building passive solar greenhouses. Certainly some great work has been done and needs to be done with energy savings in greenhouses. This is only to say that commercial viability is important in order for a technology to gain traction.
Interior Thermal Mass = Inefficient Use of Space
Many passive solar greenhouse designs rely on thermal mass like rock walls, water-filled barrels, and other features to gather the sun’s energy and slowly release it back into the structure. This is admirable, but it diminishes the amount of usable space within an already expensive structure (as seen above).
Truly Passive = No Electricity
To be truly passive, no external energy besides that of the sun can be used in a passive solar greenhouse. To be sure, this is an admirable goal, but most modern greenhouses require or would at least greatly benefit from fans circulating air, electronically controlled vents, and other devices to automate some aspects of the greenhouse environment.
Lack of Ventilation
Passive solar greenhouses are built to be tight, well-insulated structures. Once again this is an admirable goal. However, for the majority of months out of the year, at least in most of the continental US, the problem isn’t a lack of heat generated by the sun, but rather too much heat concentrated during the daylight hours.
For us in south-central Pennsylvania, we receive more solar gain than we can use 8-9 months out of 12. The problem then becomes ventilating that excess heat to avoid baking or otherwise damaging our plants. Passive solar greenhouses are often great at capturing heat, but their well-insulated nature often means that adequate ventilation is overlooked. If the ability to grow through the late spring into the summer months is important (and it should be given the cost of these spaces), it’s critical to include adequate ventilation to deal with the excess solar gain during the summer months.
Making the Case for a (Mostly) Passive Solar Greenhouse
The value of passive solar greenhouses is in their efficiency. They’re built to be very good at collecting and retaining solar energy. Their primary downside, in our opinion, is their high relative cost per unit of growing area.
The Benefit of a Climate Battery
A climate battery inside of a passive solar greenhouse makes for an excellent pairing. Rather than deploying thermal mass objects above ground, the ground within the footprint of the greenhouse itself serves as the thermal mass (the climate battery). However, unlike any thermal mass above ground which relies on passive heat exchange (radiation and conduction into the air within the greenhouse), a climate battery provides an active means of transfer. This means that you can extract heat when you need it, and leave it underground when you don’t.
Inside a passive solar greenhouse, the heating demands (BTU’s/hour, or however you measure it) are lower when compared to a conventional greenhouse structure, meaning you’re making more efficient use of the heat you’re able to retain. Coupled with the lower heat loss due to a well-insulated structure, the benefit of increased heat capture enabled by the climate battery compounds, leading to a faster “spin-up” of the thermal flywheel effect. This virtuous cycle leads to a longer and more productive growing season.
In addition, because a climate battery can capture excess heat rather than relying on passive heat storage, less heat is wasted due to venting it outside of the greenhouse. This is especially helpful during shoulder seasons when the sun is strong, but heating demands may be high as well, thus creating a need for greater heat storage than is possible through strictly passive means.
Threefold Farm’s Blue House climate battery during construction.
Climate Battery - How It Works
Brief Segway… Curious About Climate Batteries and Their Benefits?
The Benefit of a Thermal Curtain
A thermal curtain just makes sense. Greenhouses are typically so poorly insulated that it only makes sense to cover them, internally or externally, with a blanket at night. A night, the transparency of greenhouse glazing isn’t important as there is no light to capture, and it’s the greatest source of heat loss in a growing structure (unless you have large openings). It only makes sense, then, to cover the structure at night when the potential for heat loss is the greatest.
This is certainly an area for greater exploration and innovation. Inexpensive yet effective covering solutions in addition to a climate battery could lead to a greatly increased ability to extend growing seasons, shift growing zones, and offset the limiting factor to a lack of light during the winter months.
The Benefit of a Little Electricity
It goes without saying that both of the aforementioned technologies require some degree of external energy. A thermal curtain could be deployed manually, and is (or was) in Chinese solar greenhouses. Most modern structures utilize motors to deploy curtain materials.
A climate battery requires a relatively small amount of electricity to run the fans required to push air through the tubing. For example, in a 30’x96’ greenhouse, 3-4 climate battery fans each drawing 400-500 watts may be required, leading to an overall energy consumption of 1,200 to 2,000 watts. For comparison’s sake, that’s about the draw of a typical home space heater. That little bit of electricity provides heating for the greenhouse structure on cold nights and some degree of structure cooling on sunny days. Check out our grower benefits page for more information and a comparison between a climate battery and propane heat.
In addition, most greenhouse structures rely on fans to perform some degree of air movement. This air movement helps reduce condensation on leaf surfaces, leading to reduced disease and stratification of air temperatures within the greenhouse environment.
Wrapping It Up
Passive solar greenhouses make an admirable effort toward reducing the overall energy footprint of greenhouse spaces while attempting to improve on the growing environment within a greenhouse structure. I hope I’ve made an adequate case for adding a little bit of external energy in the form of fans, curtains, and (perhaps most importantly) a climate battery, which have a compounding payoff.
What’s your experience with passive solar greenhouses?
Where have you seen the biggest payoff in terms of adding a bit of electric usage or automation?
Do you have any insights into alternative means of insulating greenhouses, especially at night?
We’d love to hear from you in the comments!
Credit Where It’s Due
Also, credit where credit is due. We’ve benefited greatly from Shannon Mutschelknaus and his excellent work at his farm, Wayward Springs Acres in South Dakota, to create and thoroughly document a (mostly) passive solar greenhouse through SARE Project FNC19-1185. In one of his presentations, Shannon used the phrase “(Mostly) Passive Solar Greenhouse” and that became the inspiration for this post. Thank you Shannon!
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