How to Design a Passive Solar Greenhouse: Heating & Integrated Design— Part 4 of 4
This article is a continuation of article 1 , article 2 & article 3 about the design of passive solar greenhouses. In this article I am going to cover heating of passive solar greenhouses and how to use them for more than just growing food.
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Heating Your Greenhouse
If you have decided to grow plants in your greenhouse that requires supplemental/back up heat you are going to to want to understand a few things.
- What options exist to heat your greenhouse
- What the worst case scenario is for heating it
- How much is it going to cost per year.
There are many options that you can explore for heating your greenhouse, including wood, heat pump, electric, natural gas, oil and coal (if that is even still a thing).
I have a general-rule heating approach which is pragmatic and gives maximum flexibility. I believe that a heated greenhouse should have a primary and secondary heat source.
Here is why. A lot of our clients want to be able to leave on vacation or not be home for several days at a time and do not want to have to worry about the temperature in their greenhouse. For this reason there is nothing that beats having an automatic heat source that will kick on when you need it without having to start a fire.
Having two sources of heat means that during the time that you are on your property you can burn the least expensive fuel, such as pallets or coppiced wood from your property and then when you decide to leave your property for a while your back up source will kick in.
Let’s give an example of how this might work. Let’s assume that you had a greenhouse with a wood stove burning waste wood most of the time and a back up natural gas heater to kick in if needed. The wood would be the “cheap fuel” while the natural gas would act as the back up heat source. This is how the control philosophy would work for this scenario.
- While you were home you would keep your wood stove stoked to keep things at an appropriate temperature, let’s assume 10C (50f) or above
- The natural gas heater would have a set point of 5C (41f) and would kick in if the greenhouse temperature ever dipped below that threshold temperature. This threshold temperature would be set based on the least cold tolerant species in the greenhouse.
In the event that you left the greenhouse for a couple of weeks, the natural gas heater would keep things warm. While you were home you could keep things at a warmer, more optimal temperature.
To assess this…you guessed it…I built a tool. :)
The passive solar greenhouse design tool forms the backbone of our Comprehensive Passive Solar Greenhouse course.
This tool has the ability to select a primary and secondary fuel source just like the example above. The user can specify how much of the space heating will be done by the wood stove versus the natural gas heater, how much the fuels cost per cord, gigajoule, therm, liter, gallon or kWhr. The tool would the output a fuel cost per month and year based on the R-values and the climate the greenhouse is specified to be in.
The heat loss estimated will almost always be larger in the tool than in reality as the tool assumes a “worst case scenario” for exterior temperature, we call this an extreme minimum. This is handy, as it informs the user whether the greenhouse makes economic sense to heat before they go and build it. In addition, this part of the tool will allow the user to go back into the insulation section and optimize it based on the cost of heating either adding or subtracting insulation to see how it impacts the annual cost.
Integrated Design
I get a lot of fun out of designing one of these greenhouses, as we get to put unique elements into them that you would normally not find in typical greenhouses. Integration in design relies upon connecting yields from one element to the needs of another. In permaculture we call this needs and yields analysis. Learning needs and yields analysis completely changed how I design as an engineer an is one of the most important design tools presented in the permaculture design system. Here is a video that explains how needs and yields works in permaculture.
Elements that might be placed within a passive solar greenhouse might include a
- Wood fired hot tub
- Sauna
- Commercial kitchen
- Root cellar
- Solar dehydrator
There is really no limit to the types of things that you can integrate with these structures. The beauty with some of the elements above is that they can double as the space heat and or thermal mass.
In addition to these elements there are ways that you can integrate your greenhouse with your home. This can be challenging as you have to take considerable care when considering how vapor from the greenhouse interacts with the house, but it can be done. The greenhouse in the case study below is attached to a passive house which allows the greenhouse to get access to the “waste” resources from the house including, stale air from the bathroom fans, warm grey water, and exhaust from the heat recovery ventilator. The house gets access to the tempered space of the greenhouse as well as passive solar heat gain when it is warm inside the greenhouse. The residents have quick and easily accessible food right outside their kitchen, four seasons of the year.
Click on the video below to watch a webinar I recently did on how to design passive solar greenhouses you can watch it below. To learn more, see our 10-week passive solar greenhouse course which takes you through the process to ensure that you get a productive and profitable passive solar greenhouse at the end.
Passive Solar Greenhouse Case Studies
For more information on passive solar greenhouses: https://vergepermaculture.ca/passive-solar-greenhouse/
Rob Avis’s Bio:
In less than 10 years, Rob Avis left Calgary’s oil fields and retooled his engineering career to help clients and students design integrated systems for shelter, energy, water, waste and food, all while supporting local economy and regenerating the land. He’s now leading the next wave of permaculture education, teaching career-changing professionals to become eco-entrepreneurs with successful regenerative businesses. Learn more and connect with Rob at https://vergepermaculture.ca/contact/
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