Build your own passive solar greenhouse for year-round food production in any climate

Extensively researched, written with personal experience, and full of essential facts and figures rendered simple and accessible.
― DARRELL E. FREY, author, The Bioshelter Market Garden

A practical, easy-to-read guide that enables anyone to design and build their own sustainable, year-round greenhouse. I highly recommend it for aquaponic growers, and any gardener looking to extend their season.
― SYLVIA BERNSTEIN, author, Aquaponic Gardening

FRESH, LOCAL, NUTRIENT-DENSE fruits and vegetables are hard to find in winter in cold climates. Growing warm-weather crops like tomatoes, bananas, avocados, or other perennials is nearly impossible using conventional structures. The solution for millions of backyard and small-scale commercial growers is self-heating solar greenhouses.

The Year-round Solar Greenhouse is the one-stop guide to designing and building greenhouses that harness and store energy from the sun to create naturally heated, lush growing environments even in the depths of winter. Topics include the principles of solar greenhouse design, siting, glazing material properties and selection, controlling heat loss, ventilation, and construction methods. Additionally, an in-depth section covers sustainable ways of heating the greenhouse without fossil fuels, including using thermal mass and storing heat underground with a ground-to-air heat exchanger.

Variations include attached solar greenhouses, earth-sheltered greenhouses, plus integrating hydroponics and aquaponics. More than a dozen case studies from across North America provide inspiration and demonstrate specific challenges and solutions for growing year-round in any climate. Grow your own food, anytime, anywhere using the power of the sun!

An important resource that will help farmers and greenhouse operators leverage innovation for sustainable and profitable food production. ...This book should be part of your tool kit.
― GAELAN BROWN, author, The Compost-Powered Water Heater

LINDSEY SCHILLER is a greenhouse designer and is, with co-author Marc Plinke, co-owner of Ceres Greenhouse Solutions. Lindsey has designed, toured, and helped build hundreds of energy-efficient greenhouses spanning small residential structures to acre-size commercial facilities.

MARC PLINKE is an inventor-innovator with a PhD in engineering. In recent years he has focused his engineering mindset on building innovative, energy-efficient, and smarter greenhouses, with the intention of enabling people to grow their own food sustainably and year-round.

Marc Plinke, an inventor-innovator with a PhD in engineering, started his career as a chemical and process engineer working for Gore Technologies. Marc has spent the past decade applying his expertise in green building and engineering mindset to building better greenhouses, with the intention of enabling people to grow their own food sustainably, year-round. Marc lives in Boulder, Colorado.

Lindsey Schiller studied conventional greenhouse design and management at the University of Arizona's Controlled Environmental Agriculture Center before delving deeply into solar greenhouse design. With co-author Marc Plinke, she founded Ceres Greenhouse Solutions to research, design and build energy-efficient year-round greenhouses. Lindsey lives in Boulder, Colorado.

What Is a Solar Greenhouse?

"Don't all greenhouses use the sun?"
"You mean a greenhouse with solar panels?"

Stand in front of a sign that says "Solar Greenhouses" at a green products trade show, and you'll frequently be asked these questions. Few people are familiar with the concept of a solar greenhouse. Simply put, it's a greenhouse that uses the sun's energy not only for growth, but also for passive heating; thus, it is able to maintain suitable growing temperatures without reliance on fossil fuels.

Indeed, all greenhouses use the sun for heat and growth during the day. At night, most greenhouses quickly lose all that heat due to the poor insulating quality of their materials. On a winter morning, a standard unheated greenhouse usually is only a few degrees warmer (if at all) than the outdoor temperature. Moreover, unless it is ventilated or artificially cooled, a standard greenhouse traps so much heat during the day that it will drastically overheat.

Energy author Dan Chiras once used an excellent analogy to give a quick picture of traditional greenhouses: Imagine living in a tent.1 When it's 90°F (32°C) outside, sitting in a closed tent is the last place you want to be. When it's 32°F (0°C), sitting unprotected in a closed tent is also uncomfortable. A tent offers very limited protection and insulation. Traditional greenhouses work similarly for plants; they overheat during the day if uncontrolled, and then they let all that heat out at night. The result is wild temperature swings that stress or kill plants. To compensate, greenhouse growers often blast the greenhouse with heating and cooling systems in order to grow year-round.

The reason for these inefficiencies has to do with some basic principles of design. Traditional greenhouse design focuses on maximizing light by maximizing glazing. (Glazing is a term for any light-transmitting material, like glass or clear plastic.) Traditional greenhouses are normally "100% glazed," meaning all surfaces are made of clear or translucent materials. While they are good at letting in light, glazing materials are extremely poor at retaining heat. You've experienced this first-hand if you've ever sat next to a window on a cold night - it's a chilly spot. Now imagine an entire building made out of windows. It naturally gets very cold if not heated through the winter.

Solar greenhouse designs takes a different approach. Instead of creating a fully glazed structure, it finds a balance between glazing and insulation in order to create a more thermally stable structure (one that naturally resists overheating and overcooling). Designers use glazing strategically, placing and angling it to maximize light while reducing the glazing area as much as possible to minimize heat loss. Furthermore, solar greenhouse design emphasizes storing the excess heat of the greenhouse during the day and using it for heating at night. Instead of ventilating excess heat outside, only to have to re-heat the structure at night, solar greenhouses rely on the simple greenhouse effect for heating - using the heat from the sun that is collected and trapped in the greenhouse during the day. Instead of fossil fuels, the sun provides the energy; the greenhouse collects and stores that energy, providing its own heating when it's required.

The Many Meanings of Solar

The word "solar" is an incredibly broad term - meaning relating to the sun - but it conjures up some specific images. When most people hear "solar," they picture a building with solar photovoltaic (PV) panels. Greenhouses can include solar panels to generate renewable electricity; however, a much wiser use of the sun's energy for heating is through passive solar design: the practice of using solar energy for heating without relying on any electrical or mechanical devices. Specifically, it advocates carefully enhancing solar gain and minimizing heat loss in order to reduce or eliminate the dependence on fossil-fuel- based heating/cooling.

Though passive solar heating does not use electricity, in can be applied to buildings that do. Today in the building industry, a passive solar home generally refers to a house that utilizes passive solar design. These homes usually still have electrical appliances, like a refrigerator or washing machine. Similarly, solar greenhouses rely on passive solar heating, but they often have some electrical components. Many of these electrical systems transfer heat from the greenhouse to a storage medium, like the soil or water, allowing the greenhouse to take full advantage of the powerful greenhouse effect. The term passive solar greenhouse is often used to more explicitly describe a greenhouse that uses passive solar heating and has no electrical components at all - so it uses no electricity.

As you can see, there are some overlapping terms, so we should clarify: In this book we use the word passive on its own to describe systems that don't use electricity. Active is shorthand for systems that require electricity, like fans or pumps. For us, "solar greenhouses" are those that rely on passive solar design, and can be electrical or nonelectrical structures.

The Seven Principles of Solar Greenhouse Design

Solar greenhouses vary in almost every way - their shapes, styles, sizes, building methods, and technologies. However, there are a few unifying elements that apply to them all. To put them in a nutshell (because every book needs a nutshell), we've distilled them into these seven best practices:

1. Orient the greenhouse toward the sun. In the Northern Hemisphere, the majority of the glazing should face south to maximize exposure to light and solar energy.
2. Insulate areas that don't collect a lot of light. In the Northern Hemisphere, the north wall of the greenhouse plays a minor role in light collection. It should be insulated in order to reduce heat loss, creating a more thermally stable structure.
3. Insulate underground. Insulating around the perimeter of the greenhouse allows the soil underneath it to stay warmer, creating a "thermal bubble" underneath the structure that helps stabilize temperature swings.
4. Maximize light and heat in the winter. To grow year-round without dependence on artificial lights or heaters, it is crucial to maximize naturally occurring light and heat during the colder months. This is done by using proper glazing materials and angling the glazing for winter light collection - in general, using the glazing area strategically.
5. Reduce light and heat in the summer. Growing during the warmer months can create problems with overheating. Strategic shading, glazing placement and angles reduce unnecessary light and heat in the summer.
6. Use thermal mass (or other thermal storage techniques). Thermal mass materials are materials that store the excess heat in the greenhouse during the day and slowly radiate it at night or when needed. This evens out temperature swings, creating a more controlled environment for growing. Almost all solar greenhouses have some mechanism to store heat, broadly called thermal storage.
7. Ensure sufficient ventilation. Natural ventilation ensures a healthy plant environment and controls overheating.

The Case for Solar Greenhouses

Fig. 1.1 shows the temperatures in two unheated greenhouses over a few cold, winter days in Boulder, Colorado. The first is an uninsulated greenhouse, made out of a PVC frame and polyethylene plastic. The second is an insulated solar greenhouse designed with the principles listed above.

The standard greenhouse drops to a low of 2°F (-17°C); the solar greenhouse stays above freezing.

Solar greenhouses are often described using nebulous terms like high-performance or energy-efficient, but this is what it simply comes down to: they are able to stay much warmer year-round, and thereby grow much more than conventional greenhouses - without relying on fossil-fuel heating. They also overheat less, because they do not have excessive areas of glazing. Hence, they maintain a more stable growing environment, conducive for plants and able to grow year-round, even in harsh environments.

We've addressed the top two most common questions about solar greenhouses, now let's address a third: Do they get enough light? People often note that solar greenhouses look more like sunrooms or sheds than greenhouses. Indeed, they usually have less glazing because they work by balancing the glazed area with insulation for efficiency. However, contrary to what you might expect, they still receive roughly equivalent or even greater light levels than conventional structures. This has to do with the directional nature of sunlight and the placement of glazing, a topic discussed in Chapter 5. When light enters a solar greenhouse, rather than being transmitted through the north wall, it is reflected back inside by an insulated north wall (usually painted white).

The effectiveness and production potential of solar greenhouses has been documented in research trials - and thousands of backyards - for decades. Notably, in the early 1970s, The Brace Institute at McGill University conducted a unique side-by- side study comparing a conventional greenhouse with one built according to solar design principles. Made out of double-layer polyethylene plastic on all sides, the conventional greenhouse served as the control. The experimental solar greenhouse, called the Brace greenhouse, featured an insulated north wall, a double-layer plastic south wall and several other efficiency features. Both operated over a few seasons, and key data - temperatures, light levels and yields - were recorded. The Brace study found that light levels inside the solar greenhouse during the winter were comparable to the fully glazed structures. They were high enough to grow as much or more than conventional structures.

Here are some of their key findings:

• "The new design has yielded significant savings in energy requirements, of up to 1/3, compared to the conventional greenhouse."
• "Total weight of fruit produced in the Brace greenhouse was three times that produced in the control greenhouse."
• "Frost does not occur in the Brace greenhouse until one month after frost had destroyed the crops in the standard greenhouse."2

The Need for Solar Greenhouses

The greenhouses referred to in Fig. 1.1 were both residential structures; however, commercial greenhouses encounter the same problems. Typically, energy costs are the third largest expense for commercial greenhouse growers in the US (behind labor and plant materials). As of 2011, 70% to 80% of energy costs went to heating the greenhouse through cold North American winters.3 Moreover, because of the inherent inefficiency of most greenhouses, these energy costs are vastly greater than for other types of buildings, making it challenging to grow yearround profitably. For instance, currently, the heating/cooling costs for commercial year-round greenhouses in Colorado are $3–4 per sq. ft.4 In comparison, the heating/cooling costs for an average Colorado home are between $0.10 to $0.50 per sq. ft.5

As a backdrop to this situation, our agricultural system is precariously dependent on fossil fuels. For every calorie of food on your table, it took an average of ten calories of fossil-fuel energy to produce it. Every step of the food production chain relies on fossil fuels, from growing (pesticides and fertilizers), to processing (emulsifiers, additives, preservatives), packaging (plastic containers), and transportation. For many fruits and vegetables, shipping increases the 10:1 ratio of "energy in" to "energy out." For example, "97 calories of transport energy are needed to import one calorie of asparagus by plane from Chile [to the UK], and 66 units of energy are consumed when flying one unit of carrot energy from South Africa."6

Combined with volatile oil prices, finite oil supplies, and a warming planet, these statistics present a grim picture. Greenhouses are just one of many solutions that reduce the energy dependence of our food supply and re-localize food production. However, the current design of greenhouses has the potential to only shift the problem, not solve it. Though many greenhouses provide local crops, the inefficiency of the structures can undermine the effort. For example, a study conducted by Cornell University compared the total energy needed for growing tomatoes in greenhouses in New York for local markets versus growing tomatoes in fields in Florida and shipping them to New York. Taking into account production and transportation, tomatoes grown in standard greenhouses used about six times more energy than the shipped tomatoes. Though greenhouses created a local food supply, they increased the total demand for fossil fuels.7

"Many farmers are interested in greenhouses; what scares them the most are the heating bills". -Steve Newman, Colorado State University, Greenhouse Extension

Solar greenhouses hold tremendous potential as a way to reduce both food miles and fossil-fuel use, for commercial and home growers alike. The nature of solar greenhouses as warm year-round structures enables backyard gardeners to grow crops (like bananas, mangoes, avocados and vanilla) that are normally shipped thousands of miles across oceans. Unlike conventional greenhouses, which often struggle to stay above freezing, solar greenhouses greatly expand what we can grow, in any climate, by harnessing the sun.

Endnotes

1. Dan Chiras, "Off-Grid Aquaponic Greenhouses," DVD. www.evergreen institute. org
2. T. A. Lawand, "Solar Energy Greenhouses: Operating Experiences," Brace Research Institute, Macdonald College of McGill University, July 1976.
3. Scott Sanford, "Reducing Greenhouse Energy Consumption: An Overview," University of Wisconsin-Madison College of Agricultural and Life Sciences, 2011, articles.extension.org
4. Personal communication with Steve Newman, Colorado State University.
5. US Energy Information Administration, "Household Energy Use in Colorado," data from 2009, eia.gov
6. Norman J. Church, "Why Our Food Is So Dependent on Oil," published by Powerswitch (UK), April, 1, 2005. Available at resilience.org
7. D. S. de Villiers, et al., "Energy Use and Yields in Tomato Production: Field, High Tunnel and Greenhouse Compared for the Northern Tier of the USA (Upstate New York)," Cornell University, Ithaca, NY, goo.gl/OLCBu9