What's up with the sun this year!
The sun was very shy this winter, especially for about 3 weeks from 29 Dec to 18 Jan. It was hard on our walipini and our solar production. Read on to learn more about how our walipini faired over its first winter and how our winter solar production compared to last year.
Last year our solar production was great, so we had high hopes for our walipini this year, which relies on passive solar to keep it toasty. We dreamed of having loads of fresh peppers and tomatoes right through the winter. Alas, Jack Frost and Murphy got in the way. We still managed to harvest tomatoes and peppers right into January, but not at the levels we were hoping. Things are picking back up and we still have one happy tomato and other thriving plants, so it wasn't all bad.
Solar data
Production this winter (Dec-Feb) was only 75% of last year with the first three weeks of Jan only producing about 10% of the power from last year!
We only produced 353 kWh in Dec 2019, while we achieved 476 kWh in Dec 2019. January 2020 production was almost half that of 2019, which was a huge surprise! February was better, but still fell below last year's numbers. As you can see in the graphs below, the first 20 days of January were especially miserable due to a combination of cloudy days and snow-covered panels.
It takes 1-2 sunny days for the panels to clear after a good snow storm. We have a snow rake with a very long handle that we can use to clear some panels, but we can't get to them all. We are also very careful to be gentle and not damage any panels in the process. Regardless, the sun just wasn't shining for about three weeks. This hampered our solar PV production, passive-solar heating of our house, and passive-solar heating of the walipini. A triple threat to our homestead!
Walipini performance
The Walipini performed well this winter but the lack of sun at the beginning of January was very hard on the plants. Despite this headwind, we only used about $30 worth of power (~200 kWh) to try keep our plants happy. In hindsight, we may have been better off using a bit more power and some grow lights. Next year we will probably push it a bit again, but reserve more space for cold-season veggies, which will be happy regardless.
The images above show our happy walipini after Hurricane Dorian where the plants (and some monarch caterpillar refugees) took shelter. The walipini didn't even groan in the high winds.
As we will illustrate in more detail in another blog post, the walipini uses a sophisticated design that tries to make the best of passive solar heating. This includes:
Daytime heating with large south-facing, high-pitch windows
Thermal mass to store heat in the floor (assisted by fans), concrete walls, and water barrels
Nighttime recovery of heat from the thermal mass (assisted by fans) along with insulation in the walls and around the foundation to slow down heat loss
This design worked for us right up until Christmas, but with the lack of sunshine it wasn't a big surprise that plants struggled in early January. Until then, we had happy tomato and pepper plants, which continued to flower and fruit well beyond their normal season. All this despite some really cold nights. Then we got hit with a big solar downturn and noticed the impact almost immediately.
The tomatoes quickly withered, but our herbs and cold-season veggies continued to thrive and we continued to harvest a few tomatoes and peppers into January.
The graphs below try to tell the story of what happened. The first image shows the temperature inside the greenhouse itself (in-air), while the yellow plot shows light levels, and the purple line at the bottom shows the temp in the piping coming from the in-floor thermal mass. I will get into more detail on how these data are gathered later, but for now just look at the trends:
You can see the in-air temp's overnight lows dropping from a cozy 15°C at the beginning of October to about 6°C by 18 December. Temps have been creeping back up and staying above 10°C most nights now (late Feb).
Over the same period, the floor exhaust temp dropped from almost 20°C to about 6°C. It's also making its way back up again.
Peak light levels (an indication of sun and passive solar heating) were dropping off too, but levelled off around mid-Nov and didn't make a comeback until late Feb.
Up to 18 December, we were only running the floor exhaust fans at night to help keep the air warm in the greenhouse. Then we chickened out and started adding a bit of 'smart' heat as soon as the temperature hit 6°C until it was back over 8°C. The plants weren't overly happy as the temperatures were cool and humidity was high, but they were still making out OK.
On Christmas Day our smart switch lost connection and the heat didn't come on. Joe noticed the problem early in the morning and managed to get the heat going, but we hit a low of 3.9°C and the tomatoes and peppers were not very happy about that. A number of plants started to wilt and lose some leaves.
Shortly after that, the sun went away and didn't really come back for almost 3 weeks, as further illustrated by the next image. On 7 Jan, we decided to add more heat, bumping the heating cycles to start at 8°C until it was back over 10°C. It was too late for most of the tender plants, while others didn't seem to mind at all. Here is a quick summary:
We managed to save one green-zebra tomato that is starting to renew itself.
All of the pepper plants have lost their leaves, but we are hoping that at least some of them will bud out again.
Our lemon and orange tree remained happy, while our fig tree lost its leaves but is budding out again and looks happy.
Our spinach, beets, and herbs remained happy through the whole ordeal. (A reminder that we should have culled more tomatoes and peppers to make space for cold-season veggies.)
The greenhouse has been steadily warming up with a number of over-30 °C days over the past few weeks and the it has gone a number of nights without any heating at all! We planted spinach, strawberry spinach, kale, onions, peas, and radishes, all of which are poking up their heads, so things are looking up. We already have tomatoes, peppers, and a few more cold-season veggies growing in the house and we are itching to fill up the walipini for the summer. It was a challenging winter, but we aren't going to let that get us down!
A few more details on the data
For those that might be interested, here is some more detail on our Greenhouse monitoring. We have the following sensors, all of which are logging data every 5 minutes:
Temp, light, and humidity near the ceiling; temp and light shown above. This tends to be the hottest spot, which is why we pull air from here into the floor on sunny days
Temp and humidity for the floor exhaust; temp shown above. This sensor only measures the in-floor temp accurately when the fans are running.
Air temp just above the floor (in the cold well formed at the base of the raised beds)
Air temp, light, and humidity near the far wall (by the window) at plant height
Water temp in one of our water barrels
These sensors allow us to get a very good understanding of where the hot and cold spots are in the greenhouse and how our thermal masses react to sunlight and cool nights.
A note on the light sensors - they are not placed in direct sunlight so that the temperature sensors are not biased by direct sun. Thus, the absolute light levels are not that useful. However, they provide a good relative measure of the difference between sunny and cloudy days.
If anyone is interested in more detailed data, please feel free to contact us and we can probably work something out.