Greenhouse Solar Heater

One season-extending upgrade I've added to the new seedling greenhouse is a solar heat absorber.  The idea is to replace part of the south wall with something that will primarily heat the inside air. I used a completely passive design - this system requires no power to run, other than direct solar heat gain from the sun. This basic design was popularized during the energy crisis in the 70s, so it is easy to find reference material to help design it. 

I started out by building the south knee wall "inside out" - that is, I tacked the plywood on to the inside of the framing, instead of the outside. This creates a cavity where the absorber will be built. In the picture below, you can see the cavity as well as some circular ducts I drilled out for cold air in (at the bottom) and warm air out (at the top).

Next, I added two layers of black aluminum window screen, separated  by a small spacer. It is held in place with an additional spacer strip. The screen is attached to the outside of the framing on the top, and to the inside below, so that the screen is actually held at an angle in the cavity. The purpose of the angled fastening is to force the air though the screen as it flows through the system. Cold air enters through the lower ducts (outside of the screen) and is forced back through the screen on its way up towards the upper ducts. Since the black window screen heats up when the sun shines on it, natural convection keeps the air flowing during the day. When the sun goes down, the convection shuts down and the collector acts much more like an insulated wall than like a window.

Finally, the cavity is glazed with corrugated pvc. I chose this material for a couple of reasons. FIrst of all, it is just about the cheapest solid glazing available. The corrugations create more turbulent flow in the absorber which is supposed to increase efficiency. Lastly, I was not sure about using the brittler, more expensive polycarbonate where it might be exposed to huge temperature variations. 

Here is the view from the inside. I've just covered the ducts with scraps of screening. Mainly this is to provide a backstop for the backdraft dampers, but I also felt like it might be necessary to keep the mice and other varmints out of the space.

And here is the system in action! The backdraft dampers are thin pieces of polyethylene plastic taped up above the ducts. At night (or on a very cloudy day), there is not enough airflow to push the plastic out so reverse flow is prevented (in which air would be cooled by the collector).

The day I took this photo was nice and sunny but with weak winter sun. Still, a steady stream of hot air pushed the dampers open. The temperature was probably in the 120 degree F range (although I didn't measure exactly). 

There are many more sophisticated designs out there, but I put this together without really adding any new pieces to the plan with the exception of window screen and glazing. I'm pretty happy with the results so far - we'll see how it does in the early spring when I really need the extra heat. Next up: a solution for how to store heat in the structure overnight.

Greenhouse Progress

The new greenhouse is progressing nicely. Two layers of plastic are up on the hoops. This is a change from the last one which only uses a single layer. The advantage of doubling the plastic is that the space between the layers can be inflated with a small fan. This creates an insulating air space and also maintains tension on the plastic which (in theory) reduces stress on it from wind and other weather.

The south end wall is framed and is now glazed with triple layer polycarbonate, which has decent insulating properties while still letting a good amount of light through. Here we are working on a ventilation window which is now covered with polycarbonate. Overheating is a significant concern and I've decided to skip the roll up sides for this one so it's important to be able to move air through the ends

The north end wall is framed and sheathed with plywood. I'll probably add some rigid foam insulation on the inside here. It's quicker to cover the end with clear plastic but very little light comes in from the north so a semi-insulated wall will provide more frost protection without much of a downside.

I hooked up a fan through a length of dryer ducting to test out the system. Because I'm planning on using a solar panel to power everything, I used a 12 volt computer fan as the inflator instead of a specialized greenhouse fan. It's not perfectly suited to the job but it seems to work just fine. I was able to measure the current draw during my test (about 280 milliamps, or about 3.5 watts) as well, which will help me size the solar system. 

Here's the view from inside with the fan running and the polycarbonate in place. The inner layer balloons inwards a bit and the outer layer curves out. It took a minute to inflate enough to be noticeable so at first I thought it wasn't working but once I let it run for a bit it looked great.

Stay tuned in the weeks to come for more updates about the solar panel setup, a passive solar heat absorber, and my thermal battery seedling bench.

Greenhouse 2

Another fall, another offseason building project! After the success of our first year with our greenhouse, I've decided to build a second one. The new one will be smaller and dedicated to seedlings so that I don't end up trying to juggle space between seedling benches and young plants in soil. This way I can have a separate space just for seedling benches and I don't have to worry about when to move them aside for bed space.

I'm also making the new greenhouse warmer than the other one in a few different ways. One is to make the walls a double instead of single layer of plastic. Another is to replace the roll up sides (used for ventilation on hot summer days) with non-movable insulation to retain heat.

Other designs for added warmth are still in progress - like a water tank for the seedlings trays to sit in that will circulate water warmed by the daytime sun overnight to keep temperatures steady. I'll post more updates as this project moves forward!

Greenhouse transition

As the season progresses towards summer, the greenhouse is steadily transitioning to summer mode. First, the seedling benches filled up with transplants:

On the warmer days, the roll up sides are open for ventilation:

Then, with enough transplants planted out in the field, the centre bed is freed up for some more tomatoes:

The tomatoes now have their extra row cover off, and are starting to grow up their string trellis:

A few seedlings are still hanging around in the back. In a week or two, I'll have it down to one small bench for late season transplants so that I can fill in the side beds with peppers and eggplants:

This small structure (built just last year) has been a hub of activity for its first spring.  I'm looking forward to seeing how everything does in the summer ahead. It won't be changing so quickly once the peppers, eggplants and tomatoes are all in place, but it should still be a very productive area.

New Greenhouse

A big offseason project in anticipation of next year has been this brand new greenhouse. The most important purpose for this area is to start seedlings for transplants, but the structure is sized large enough to do some season extension as well, which I'm really looking forward to trying.

The greenhouse

I had hoped to slowly work on this as a late fall project, but an early cold snap froze the ground and I was worried that it wouldn't happen for another year. Fortunately, a brief thaw gave us just enough time to quickly lay out and set the ground posts for this structure. It was a tough couple of days getting it laid out (thanks for the help, Dad!) but we got the job done. 

I managed to get all of the hoops bent and bolted in place a couple of days later:

Hoops in place, end walls framed


The metal hoops are made from chain link fence top-rail. It's a widely available, reasonably economical way of getting tubing. I made a bending jig (basically some curved blocks of wood screwed to a big sheet of plywood) to shape the hoops. The only tricky part was accounting for the spring back of the metal - after a bit of trial and error, I managed to get the hoops to relatively repeatably bend to a finished 6 foot radius.

One of the many items lost in our 2011 house fire and forgotten about was apparently my plumb bob, so I made do caveman-style:

Stone age plumb bob?

In this photo you can see all the main structural elements coming together. The purlin on the top is centered and the two wires on either side are suspended from every hoop, giving me three rows to do overhead trellising after the bulk of the seedlings are out in the spring. It would have been much harder to put these wires in after the plastic was on so I put in the time and effort up front to lay them out the way I think I want them - I hope they get used! I'm also considering ways I can use the wires to hold a layer of row cover fabric for extra insulation in the coldest weather. You can also see the hip boards that will form the upper point for the roll up sides (for warm weather ventilation).

Structure in place

Here is the end wall finished with a door and plastic. I've just rolled up the excess so I can adjust the tightness when the top plastic goes on - it gets trimmed at the end.

End wall

The plastic is held to the hip boards and the end bows with this "poly lock" system - an aluminum channel with a springy wire that just barely fits in. It is a bit expensive, but it makes it much, much easier to get the plastic on tightly compared to squeezing it down with a wood furring strip (as I've done in the past).

Poly lock system

Finally, the plastic goes on over the hoops. We went out as a three person team early in the morning (usually the least windy part of the day) and got it on without incident. With such a large sheet of plastic even the slightest breeze would pull it fairly hard but once we were over the peak it went pretty well.

The view from inside

Two curious investigators

Draped over and secured

The last step was to trim the sides and set up the roll up sides. The bottom edges of the plastic are rolled around some metal tubing (half inch metal electrical conduit) and secured with clips. Then, parachute cord lacing goes on to hold the rolled plastic against the hoops. To roll up the sides, you rotate the end of the tubing (with a t-fitting) and the plastic rolls around it as it goes up to the hip board. I was a little worried it would be hard to do alone but the process is very smooth - important because rolling the sides will probably be a twice daily routine for six to eight weeks in the spring.


Now I just need to build some seedling benches, figure out an emergency heat source, and run some irrigation lines - easy!

A Cooler And Some Self-Reliance

Having a cooler greatly increases farm efficiency as a whole. It allows for me to harvest and pack produce the day before delivery and keep it cool and fresh. Being able to refrigerate the boxes themselves is also a big advantage as this helps keep the veggies cool when they are on the road.

The cooler was specifically sized to accommodate the pre-packed delivery oxes.

In the future farm-of-my-dreams I would like to have a full size walk-in cooler. Time constraints this year meant compromise and so I ended up building a large cooler in the basement of the house. It's more than enough for this season as I'm starting out small but in a few years I'll have to re-evaluate. Fortunately, all the equipment in the current cooler can be reused someday when I build the walk-in version.

The other end of the 8 x 4 x 2 ft cooler features the air conditioning unit below and the Coolbot above.

The design for this cooler came from Ron Khosla, a fellow electrical engineer turned farmer. The basic idea is to force a compatible air conditioning unit to continue to cool below it's standard range. It's called the Coolbot and it's connected it to a simple window air conditioning unit as pictured above. The two devices, along with a wood frame and some polystyrene make up a very effective and energy efficient cooler. If I were to buy a pre-made cooler of this size I would have easily spent 10-20 times as much. I feel like there some harmony between the engineer's need to tinker and re-build and the farmer's desire for self-reliance and independence. In other words, farming is a good way to support my DIY habit.