Passive Solar Greenhouse Grows Sustainable Farming

In today’s high-tech society, what would it look like to rediscover the simple but ancient wisdom of using only the sun for our heating needs? Knowledge and use of passive solar design to heat buildings has been around since ancient times. Greek, Roman, and Native American structures, as well as many others, were designed to maximize the use of the sun for heating. With the advent of technology and modern eras of cheap energy, the importance of passive solar design became obsolete and the knowledge was all but lost in U.S. building design. However, with present energy crises and uncertainty of future energy sources, passive solar design is being revived as a way to reduce carbon dioxide emissions and dependence on foreign oil.

Specifically, local food production is one area that stands much to gain from passive solar design. Especially in areas with more extreme winters, greenhouses serve to lengthen both ends (spring and fall) of the growing season, as well as offering a greater measure of control over growing conditions. This can dramatically increase yields and provide more food and profit for growers. The community also benefits from the availability of more fresh, locally grown, and possibly organic food. However, the only way conventional greenhouses can offer these benefits is with the input of expensive fossil fuel energy. Conventional greenhouses use the sun’s light while ignoring its heat contributions and often require additional heating on cold nights and on cloudy winter days or a mechanism of cooling on sunny winter days. This extra energy is typically supplied by using fossil fuels such as natural gas or propane and electricity. While this may have been cost effective in times of cheap energy, rising fuel prices are rendering most conventional greenhouses uneconomical to operate, and virtually eliminating the possibility of year-round use in some regions. Enter: a resurrection of this ancient knowledge of passive solar design to help combat the modern world’s problems of fuel, carbon dioxide emissions, and food production through the work of SunCatcher Design Group.

The Problems: Food, Fuel, CO2

Supporting local growers is a simple answer to all three of these problems. Food production and distribution represent large ecological impacts, including high energy use. For every dollar spent on food, only 19 cents goes to the farmer. As much as 58 cents is spent on expenses associated with packaging and transport. One 2003 Iowa State University study reveals that much of the food we consume travels an average of almost 1500 miles from farm to table. This further exacerbates fuel and CO2 problems, and makes most people dangerously dependent on far away, less wholesome food. The sustainability movement is encouraging more locally grown food to reduce energy use and improve food quality. This is often difficult in regions where the climate provides an abbreviated growing season, making greenhouses essential for many crops that might be grown locally. For many locations, growers must start seedlings indoors in early spring. Conventional greenhouses meet this need, but are costly due to high energy use for heating, cooling, and venting the structure. With the use of passive solar heated greenhouses, greater growth can be achieved at less than 10 percent of the ongoing energy costs. The term passive solar greenhouse generally refers to greenhouses whose light and heat requirements are largely provided by the sun. All greenhouses receive most of their light from the sun, but conventional greenhouses have no method of retaining the heat they collect during the day and that heat is usually vented as waste energy. The conventional greenhouse must then be reheated at night and on cloudy winter days.

SunCatcher Design Group has specially designed energy efficient passive solar greenhouses, or as we like to call them, SunCatchers, to help farmers and growers in the same ways conventional greenhouses do, but without extra heating. SunCatchers have the ability to directly harness the sun’s thermal energy and make it available for heating the greenhouse at night and on overcast winter days. This is accomplished through the use of thermal mass, insulation, glazing, and an east-west orientation of the long axis of the SunCatcher to collect, convert, and store the low-angle winter sun’s energy for heating the SunCatcher 24 hours a day.

The passive solar process minimizes the SunCatcher’s interior temperature fluctuations without venting the excess solar heat during the day, but instead storing that heat to be used at night. These characteristics separate SunCatchers from conventional greenhouses, providing as much as 90 percent savings or more in fuel use for growers.

Because many local growers use organic principles, increasing their productivity will enable them to be more competitive with non-sustainable growers, without the large costs associated with transportation and packaging. As a very important part of sustainable agriculture, the energy-efficient SunCatcher allows local organic growers to grow year round and still remain competitive in a market that traditionally favors large-scale, fossil fuel intensive agriculture. Local farmers can start growing seedlings indoors long before winter weather typically allows, and crop production can be extended well into the fall without the major expense of heating a conventional greenhouse. If increased local food markets are the answer to our modern problems of food, fuel, and CO2 emissions, the SunCatcher is a key player in fostering their growth and prosperity.

Suncatcher Origins

Passive solar greenhouse research has been ongoing by several scientists and educators in the rural mountain college town of Boone, in western North Carolina, where long and unpredictable winters can pose an extra menace to local farmers. Early snows and late frosts are not uncommon and, along with periods of excessive rain and high winds, can quickly slash profits to make for a frustrating growing season and disappointing harvest. Greenhouses offer protection against dangers like weather extremes, pests, cross pollination, and weeds. A protected growing environment increases food security in a time when global climate change will bring more extreme weather conditions to more regions of the planet. Enabling access to locally grown food also contributes to community stability and is important for the development of sustainable agriculture. Higher yields help farmers meet the rapidly growing year-round demands from regional restaurants, grocers, and farmers markets for fresh organic produce. This is a great boost to economic development in rural areas such as western North Carolina.

In 1996, a friend’s request prompted Dr. Terry Carroll to design a passive solar greenhouse in which tomatoes could be grown year round. Carroll had experience with passive solar design and energy conservation, and had studied a number of passive solar applications in the southwestern U.S. With very little specific information to go on, Carroll designed a passive solar greenhouse tailored to North Carolina. The structure functioned well, and tomatoes were grown and harvested in the greenhouse throughout the winters.

Carroll’s involvement with passive solar greenhouse design did not stop there. In fact, it soon took off. In the spring of 2000, Dr. Jeff Boyer, then director of the Goodnight Family Sustainable Development Program at Appalachian State University (ASU) suggested that Carroll build a research and demonstration passive solar greenhouse. At the same time, Ms. Mary Jo Pritchard, then a teacher at Parkway Elementary School wanted to teach science in a hands-on experiential fashion. Her eighth grade students wrote grant proposals and received funding for various projects at the school that were collectively known as the Parkway Ecology Project. Carroll was the Appalachian State University science advisor for the Parkway Ecology Project. He and Pritchard determined that building a passive solar greenhouse at the school would be an excellent addition to the project. The first energy efficient SunCatcher was built on the Parkway school campus as a true community effort. The students used Dr.Carroll’s expertise, funds from Parkway PTSA and the ASU Goodnight Family Sustainable Development program, and the help of many community volunteers to get the job done.

During the following winter, Carroll and his sons Josh and Jon, both students at Parkway, searched for the best site for the greenhouse on the school property. They took pictures of snowmelt patterns and shadows at different times during the day in order to determine which site would optimize the sun’s low-angle rays during the winter.

Construction commenced through the 2001-2002 school year. After delays from a very rainy summer, construction was completed in the fall of 2002, at a cost of about $7,000 for materials. The SunCatcher was constructed on-site in order to give students, parents and other community members the opportunity to learn principles of passive solar design, and then apply their knowledge to build and operate an energy efficient SunCatcher.

The three main purposes of the SunCatcher included public education through demonstrations and tours to exhibit the use of passive solar design to students and local growers. This onsite resource also enhanced Parkway Elementary School’s science education opportunities and programs. Finally, the Parkway SunCatcher has served as a setting for research on the most effective ways to utilize this type of greenhouse in western North Carolina.

Data taken in the Parkway SunCatcher demonstrates its superior performance over conventional greenhouses without the ongoing excessive fuel expense. The lowest temperature registered inside the SunCatcher during the 2002-05 winters was 42 degrees Fahrenheit. This occurred at the end of a very unusual ten day period of overcast weather. Normally, the interior low temperature rarely dips below 50 degrees. During January 2003, when the average outdoor temperature was 28 degrees, the SunCatcher maintained an average interior temperature of slightly more than 61 degrees. This represents approximately a 33 degree temperature difference between the interior of the greenhouse and the windy, cold conditions outside–without the use of any heat source other than the sun itself.

Parkway Classic Greenhouse Data from the Winter of 2002-2003

Data Set / Date Outside* Greenhouse* Thermal Mass* % light**
I. 12/6-12/17 33.5° 59.1° 62.0° 92%
II. 12/17-1/9 34.6° 58.9° 63.6° 74%
III.1/10-1/20 21.9° 63.7° 69.3° 100%
IV.1/20-1/30 25.4° 63.7° 67.6° 84%

*Average temperature in degrees Fahrenheit.
**Percent sunlight entering greenhouse compared to percent recorded during the third data set (1/10 – 1/20).

How Do These Suncatchers Work?

Unlike conventional greenhouses, the SunCatcher uses mother nature’s laws to create an ideal growing environment. It uses the sun itself for most if not all of the plants’ two primary energy needs: light and heat. Because it works with these natural occurrences instead of outside of them, fossil fuel input is far less. For example, in a conventional greenhouse, the sun’s sole purpose is to provide light for the plants. The SunCatcher design makes use of the sun’s energy to meet needs for heat, as well as light. The SunCatcher itself is situated on an east-west axis, with glazing (windows) only on the south side.

This allows it to absorb direct sunlight for at least six hours of the day. The design also takes advantage of the sun’s seasonal paths across the sky to customize winter and summer heating needs. A specifically angled south facing glazing ensures that the maximum light is collected during the sun’s low southern orientation in winter, but with minimal direct sunlight absorbed in summer to reduce the need for cooling.

During sunny days, the sunlight is collected with much of it being converted to heat energy as it strikes the flat black surface of the SunCatcher’s thermal mass. This thermal energy is stored in the thermal mass using 55-gallon, metal, water-filled barrels. Water is used because of its ability to hold a tremendous amount of heat energy.

As temperatures drop at night, the energy collected in the thermal mass is radiated back into the SunCatcher, effectively keeping inside temperatures an average of 20 to 30 degrees warmer than outside temperatures, even in the middle of winter. Heavy insulation on the roof and north side of the SunCatcher ensures that very little of this heat escapes. A creative venting arrangement uses natural convection to cool the SunCatcher.

There are several variations on the SunCatcher design that are adapted to certain types of climates. In areas with mild winters, conventional hoop greenhouses can often be modified to incorporate elements of passive solar design such as thermal mass and insulation to significantly reduce fossil fuel energy costs in both winter and summer. These modified hoop greenhouses should be located with their long axis running east-west.

Economic Benefits and Markets

Economic impacts from the simple change from conventional greenhouses to SunCatchers are direct, as well as vast. The low operating costs make it economical for more people to grow their own food year round. Increasing interest in sustainability and food security create a niche that a SunCatcher fills perfectly. With more growers, there is an increase in the local food market, lending to food security, better quality food, and transparency of food practices. An increase in local food production can bind communities together and increase quality of life in both urban and rural settings, helping to ease poverty. What’s more, local economies are more prosperous, as a dollar spent locally goes three times further than a dollar spent on foreign corporations. Plus, more of the food dollar goes directly to the farmer. Energy is also saved in food production through reduced transportation and packaging, as well as the adoption of more sustainable methods. Local food production can reach into other realms such as education and healthcare. The abundance of higher quality local food with fewer chemicals and fewer steps between plant and plate can serve as prevention for many of today’s rising health concerns. The SunCatcher can be incorporated into the education system from the elementary to community college level, serving to teach growing methods and how to most efficiently operate the greenhouse.

The versatility and adaptability of the SunCatcher to most climates ensures that its effects can be both local and widespread. It is one key (and mostly unexplored) component to building our future green economy.

The SunCatcher Design Group is now helping growers with design and construction of new passive solar greenhouses, as well as retrofitting existing hoop greenhouses. Ongoing research fuels new innovations and designs. A commercial size SunCatcher has been completed at Newton-Conover High School in North Carolina, and a custom SunCatcher is being added to a home in Madison County, NC.

The SunCatcher Design Group is also working on hobby-sized greenhouses for non-commercial growers, as well as customizing various designs for extreme climates like Alaska. Visit their website for the latest developments.

Dr. Carroll is now retired from ASU to run SunCatcher workshops and research at Laurel Ridge Moravian Camp & Conference Center Grounds in the mountains of western North Carolina and to further develop the SunCatcher Design Group business. Engineered blueprints for SunCatcher’s new hobby design will be available online summer 2010. Workshops and conferences will begin the first day of spring 2011 at Laurel Ridge.