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Over the past months we have posted a few blogs on a new sustainable greenhouse prototype we are developing.  We have received comments from a number of traditional greenhouse users with concerns that this new prototype will not be capable of plant starts because of inadequate light levels.  The prototype does push the envelope of greenhouse design but through the use of careful daylighting strategies not typically implemented it is capable of effective plant starts.  The Research and Development greenhouse that is the basis for our prototype started over 1,000 tomato plants last spring without the use of supplemental light.

To understand how this works a typical all glass or plastic glazed greenhouse has no reflective surfaces and the glazing reduces light transmission into the greenhouse by up to 40%, or more.  This means that the light levels entering a traditional greenhouse are first significantly reduced by passing through the glazing and then any light that does not immediately hit a plant surface is either absorbed by the ground or passes back through the glass and out of the greenhouse.

The sustainable greenhouse prototype uses some of the clearest glazing available and a number of reflective surfaces to increase light levels inside the greenhouse.  First, reflective surfaces on the outside of the greenhouse, i.e. light shelves and reflective roof surface, bounce additional light into the greenhouse.  This reflected light allows more sunlight through each window than would direct light alone increasing the effective aperture size without more or larger windows.  Next, the clear glazing allows more of this light to pass into the greenhouse compared to a similar area of glazing on a typical greenhouse.  Finally, although the greenhouse is made up of a number of opaque walls and roofs each of these is covered with a white reflective surface.  The reflective surfaces inside the greenhouse mean that any light that does not immediately hit a plant or the ground is bounced around inside the greenhouse until it does.  Through a combination of thoughtful daylighting strategies our prototype greenhouse achieves adequate light level for plant growth and plant starts with fewer windows than a traditional greenhouse.

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Building a sustainable near zero energy greenhouse is not an insurmountable task.  Despite the basic strategies required, I am not aware of a manufactured greenhouse that combines all of the necessary elements to achieve high performance.  By combining a high R-value envelope with effective thermal mass and intelligent daylight harvesting, near zero energy greenhouses can be easily achieved.

Most greenhouses do not implement any of these strategies.  They are constructed primarily of glass, plastic glazing, or even plastic sheeting supported on metal frames with little or no insulation.  These materials tend to have low R-values, on the order of 0.5 to 2.  Although these materials are capable of letting in a lot of daylight, the daylight is not intelligently harvested.  Traditional greenhouses are over-glazed (contain too many windows) leading to overheating in the summer from too much solar heat gain and are difficult to keep warm in winter because there is little or no high R-value envelope to retain solar heat.  Well-designed greenhouses balance the amount of glazing with the amount of opaque insulated walls to allow the greenhouse to maintain comfortable temperatures year round while still allowing enough light for plant growth.

Greenhouses usually contain some mass that impacts the thermal performance of the building.  However, because of a lack of insulation and poor design the mass can actually contribute to maintaining cold temperatures in the winter and hot temperatures in the summer.  A well designed greenhouse uses thermal mass to reduce temperature swings throughout the course of the day (and year) by absorbing energy when it is available and releasing it when it is needed.

The first thing many proponents of traditional greenhouses notice when looking at the Hutton Architecture Studio/Synergistic Building Technology greenhouse is the greatly reduced area of windows.  The window area is carefully calculated to allow enough light in to heat the greenhouse in the winter and allow for productive plant growth, while reducing the area of low R-value envelope.  The area of the building that is not windows is highly insulated opaque wall assemblies that reduce heat loss.  Finally, to complete the envelope the reduced area of glazing is covered with automatic insulated shutters (built by SBT) that close on winter nights to greatly reduce heat loss.

In addition to this insulation above grade, the insulated envelope continues at least 3’ below grade.  This isolates a large volume of thermal mass that taps into the steady earth temperatures beneath the greenhouse, helping to stabilize the temperature inside.  By carefully controlling the daylight contribution, and hence, heat to the greenhouse, the thermal mass is able to help keep the greenhouse warm on cold winter nights and cool on hot summer days – exactly what thermal mass is intended to do!

By combining these strategies, the Research & Development greenhouse (as discussed in a previous blog) is able to maintain stable warm temperature without overheating throughout the summer by leaving the doors and insulating shutters open.  Evaporative coolers that were installed in the greenhouse were never turned on.  In the winter the greenhouse maintains warm growing temperatures, typically in the upper 50’s to lower 70’s with only solar heat by closing up the envelope and operating the shutters on automatic mode (open during daylight hours and closed at night).   Implementing all three of these simple strategies together has allowed the HAS/SBT team to develop the next generation of reliable near zero energy Green Greenhouses.

By Gardner Clute

3,000 sf Greenhouse

As the market for healthy fresh produce increases so also does the environmental and geopolitical impact of the food we consume.  Consumers are becoming more aware of these impacts leading to increased demand for locally grown food.  However, during the winter season, locally grown produce is not available in much of the United States.  Rather, it is shipped from southern states and Mexico or even farther away from South America, Australia, and beyond to feed US markets.  The small amount of produce that is grown locally in northern states is grown in heated greenhouses.  Unfortunately, this locally produced food is frequently more expensive than imported food and carries a high environmental cost.

To fully understand the impact of supplying fresh produce to the US in the winter it is important to understand the scope of the problem.  According to a USDA study, 44% of the fresh fruit and 16% of the fresh vegetables consumed in the US in 2006 were imported.  The same study showed that the total cost of imported food nearly doubled from 1990 to 2006 and that the trend towards imported produce is on the rise, increasing our reliance on other countries for basic needs such as food.  Additionally, some food produced outside the US is grown in countries with social and environmental standards not considered acceptable in the US.  Not to mention, that food is then shipped on planes and cargo ships to the US for distribution and consumption.

On the other side of the coin food grown in the northern United States in the winter is grown in greenhouses heated with fossil fuels to maintain minimum temperatures adequate for plant growth.  Frequently these greenhouses have easily avoidable design shortcomings that drastically increase energy use such as poorly designed envelopes, low solar heat gain glazing, and little or no thermal mass.  With little effort, greenhouses can be designed to take full advantage of the sunlight the plants need to grow to also heat the building.

The solution to both problems is greenhouses that store solar heat and therefore don’t use fossil fuels for heating.  If this wintertime performance can be achieved, then food can be grown locally in the northern US without fossil fuels, greatly reducing the environmental impact of produce while decreasing cost.

Hutton Architecture Studio in conjunction with Synergistic Building Technology (SBT) has designed a greenhouse that meets just these criteria, a near zero energy active solar greenhouse.

SBT constructed a research and development greenhouse funded by the Colorado Department of Agriculture in the fall of 2010 inBoulder, Colorado.  Its performance through the winter of 2010-2011 was outstanding.  The R&D greenhouse maintained temperatures above 50 degrees inside even when outside temperatures were 18 below zero with sunlight as the only heat source.

In 2011, Hutton Architecture Studio has been working with SBT to develop the next generation of greenhouses based on the R&D project.  We have developed concepts for a 500 sf attached residential greenhouse up to a 1 acre modular production greenhouse.  We are currently in construction on one greenhouse and will begin construction on a second early in 2012.  As the market for sustainably harvested locally grown food expands green greenhouses are key piece of the puzzle.

By Gardner Clute