How to Design a Passive Solar Greenhouse: Light, Insulation and Geothermal Heating and Cooling Systems — Part 3 of 4
This article is a continuation of article 1 and article 2 about the design of passive solar greenhouses. In this article I am going to discuss artificial light, insulation, thermal mass and subterranean heating and cooling systems.
Artificial light
If you live in a cold and cloudy environment or in the extreme north where daylight is very short, you might be looking to add artificial light. Generally this is a situation that I try and avoid as the cost of lighting a greenhouse can be very expensive. This is because plants have evolved with the sun, which is not an easy element to replicate on earth. Here is why…
“The amount of energy the earth receives from the sun in 2 minutes is enough to the current energy usage of humanity for an entire year.” — Gulf News
In other words, the sun is very powerful and trying to replace its function is very expensive. My approach to artificial light is to explore if your greenhouse can operate without it first; if not, the next step is to quantify what it is going to cost to operate the lighting apparatus per month and year to make sure it makes sense.
Here are a couple of scenarios where artificial light is not needed.
- The greenhouse only operates three seasons of the year. The greenhouse is used as a hang out space in the fourth, dark season. I don’t consider this a failure as, in my climate this means that I am growing for 200–250 days instead of the measly 100 days I typically get outside.
- The greenhouse is growing tropical plants that typically would live in the understory of the rainforest so low light is something that can be tolerated.
- You are growing microgreens in your greenhouse which does not require additional light in most scenarios.
If none of those apply you will want to design your light apparatus carefully. For this I built a passive solar greenhouse design tool which has a light calculator inside of it to make estimating power consumption and cost EASY.
The light calculator has different technologies built into it with their respective power draw per square foot, the pros and cons of the the light technology and the ability to estimate the number of hours the lights will operate per day per month. Once the user enters a lit area and a cost of power they will be able to estimate how much the lighting in the greenhouse will cost to operate.
The passive solar greenhouse design tool forms the backbone of our10 week Passive Solar Greenhouse Design Course. Click the link to learn more about it!
If you don’t have the tool, estimate the area you need lit, the technology and its respective wattage and figure out how many hours you need to light your greenhouse per month.
Cost of light = Cost/kWhr*Power consumption (kW) * hours of opperation
From here you can make an informed decisions about whether your greenhouse should be artificially lit.
Insulation
Correctly placed insulation is what makes a passive solar greenhouse different then a conventional gable style greenhouse.
Insulation is important but it has an interesting diminishing return in passive solar greenhouses. Because the southern glazing surface (south facing glass) has such a low R-Value relative to the insulated walls there is a very important point in which there are massive diminishing returns in adding any additional insulation into the North, East and West walls. Basically adding more insulation into the walls has no effect on overall heat loss which translates into, you spend more money on adding insulation and get no benefit. For this reason I built an easy calculator that does not require you to be an engineer to use. The tools allows to sort out what the optimal insulation values are for each of the walls and glazing surfaces in your greenhouse. It does this by asking the user to enter in surface areas and proposed R value for every surface and then spits out a data visualization pie chart which shows you the breakdown of heat loss in the greenhouse space. This means the user does not need to understand what a BTU is, just how much of the “pie” is leaking through which surfaces. The user can then go back into the tool and try different R-values and strategies until they land on an insulation design that works for them.
In my ecosystem the R value I start with is around R-20 for walls, R-15 for the foundation and R 1.8 for my glazing.
You will note in the image above that 58% of the energy is leaking from the glazing while only 10%, 6% and 15% and 11% from the footing, infiltration, roof and walls respectively. It should be apparent that doubling the R value of the walls might only reduce the heat loss by ~6% which is hard to justify. Instead we need another approach.
It turns out that most of the heat loss from the greenhouse occurs through the glazing at night. This means if we want to reduce the heat loss dramatically we need to add a thermal curtain at night. Adding a thermal curtain with an R value as little as R-2 can reduce the building heat loss as much as~25% for a fraction of the cost of doubling the R value in the walls.
The passive solar greenhouse design tool forms the backbone of our 10-week Passive Solar Greenhouse Design Course. Click the link to find out more.
Thermal Mass
Thermal mass is very important in any passive solar building. Thermal mass helps to cool the building in the summer and keep it warm at night in the winter. There are several different thermal mass options that you can choose from including, rock, concrete, cob, water, water + glycol, metal and basically any cheap material that stores a lot of heat in a small space.
We chose a rocket mass heater in our greenhouse (Cob) but there are an endless number of elements you can use for thermal mass.
To help design thermal mass we built a specific calculator to help determine how much thermal mass a greenhouse should have. It calculates it based on the glazing surface area (solar collection surface) and type of material that you choose to use (rock, concrete, cob, water, water + glycol or metal).
The passive solar greenhouse design tool forms the backbone of our 10 week Passive Solar Greenhouse Design Course. Click the link to find out more.
Generally speaking more thermal mass is better than less, however there comes a point when adding more starts to diminish the production area in the greenhouse, so it is a fine balance. There are pros and cons to each thermal mass which you need to be aware of, the materials and their respective pros and con can be delinted into solid non freezable materials and freezable materials.
Non Solid Material Pros and Cons
Water can freeze which can make using it challenge to use, however it has a high thermal capacitance (it holds a lot of energy) and its cheap, which makes it attractive.
Solid Material Pros and Cons
Solid materials hold roughly 4 times less energy than water but they don’t freeze and can be easily set up inside of a greenhouse and never touched again.
Subterranean Heating and Cooling Systems
Subterranean Heating and Cooling Systems (SHCS) or “Climate Batteries” as some people call them are a heat storage technology that stores heat underground for later use. These systems capture hot air from the top of the greenhouse where the hottest air accumulates and a fan pumps the air below grade through a pipe network similar to the one shown below. Hot air in a greenhouse typically has a high humidity, which translates to moisture stored in the air. The more humidity in air, the more energy air can hold. As warm humid air is pumped through the cold ground, the moisture drops out of the air through condensation which releases large amounts of energy into the soil. This is due to the fact that condensation releases energy while evaporation consumes energy. Water then infiltrates through the perforated pipes in the SHCS irrigating the soil and the energy is stored.
These systems store surplus energy from the greenhouse during the day when temperatures rise above 21 C (72f) by turning on a fan. The fan does not operate between 12C (53f) and 20C (68f), however it will turn on when the temperature in the greenhouse goes below 11C or (51.9f) quickly extracting the thermal energy from the ground to keep it above freezing.
To design a SHCS properly you need to be able to size the right duct sizes and fan sizes so that you are not using more power to push the air then you are storing in the ground. If this occurs, it is cheaper to just add energy into the greenhouse with direct resistance electric elements.
I size these systems manually with a “ductulator” a calculator for mechanical engineers (we are total nerds) who want to design ducts. However, I realized that no one that was not an engineer would want to use one of these so I built a SHCS design tool so that non-engineers could do this for themselves EASILY.
The passive solar greenhouse design tool forms the backbone of our 10-week Passive Solar Greenhouse Design Course. Click the link to find out more about our course.
This tool allows the user to choose duct diameters and through a little iteration choose a fan size. I always recommend that these systems have variable speed fans so that they increase the variability of the system. Basically after the system is installed the user can play with the fan speed to find an optimal air flow rate for the greenhouse and or potentially the season of use. Generally speaking we want our SHCS to change the air over in the greenhouse 2–6 times per hour, with the ability to increase or decrease this if required.
Thermal Dynamic Modeling
The tool and process will work for any size of greenhouse however, if the greenhouse goes beyond 1500 sqft the construction costs start to escalate quickly. Buildings of this magnitude are worth performing thermodynamic modeling on which is where I typically get involved from a consultancy perspective. This tool and process can be used as a feasibility tool before getting me engaged but I would not build a large greenhouse without doing some additional optimization and design.
In the last article we will talk about heating your greenhouse and how to add in components like hot tubs, saunas, commercial kitchens and root cellars.
If you want to see the webinar I recently did on how to design passive solar greenhouses you can watch it below. We offer a 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.
For more information on greenhouses go to:
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/
PS. If you see any typos, please let me know.
PPS. Please hit the clap button if you found this helpful.