Since I started writing Common Science in April of 2011, I have averaged 50 columns a year.  Most weeks I have my topic selected by Monday, a first draft done by Wednesday and am ready to submit to my editor (she’s not hard to find) by Friday.  This routine keeps me on track to publish on Sunday afternoons.  Once or twice a year something goes awry during the week, Friday morning rolls around and it’s clear the column is not going to come together. Well, it’s Friday morning now and this is one of those weeks.  Maybe I can take a shot at fixing this week’s first effort in the future, but in the meantime, let me tell you about a project I am working on.  It should give you a little insight into how engineers spend their spare time, a question that I am sure has been plaguing you.

As I have mentioned in previous columns, I have a “farmette” west of Carrboro where I grow vegetables, generate my own electricity, raise chickens and bees, and am attempting to create a local pollinator refuge.  Designing and building projects and systems for the farm is a relaxing and enjoyable hobby for me.  Lately I have been working on improved systems for growing tomatoes.  I love tomatoes and want to grow my own all year long.  However, like most gardeners I know, I find the task of staking and trellising them to be tedious and the solutions offered for this by the local garden shop to be inadequate.
tomatoesThe staking problem can be resolved by growing tomatoes upside down.  You can buy expensive planters for this approach if you like.  I use inexpensive two gallon buckets hung from a pole with a hole drilled in the bottom.  Here is a picture of some tomatoes hanging upside down in my greenhouse (more about the greenhouse below).   At this early stage, the plants grow out to the sides of the buckets and then upwards towards the sun.  Once the tomato fruits get large enough, the plants are weighed down and the vines hang below the buckets.  It’s a wonderful way to grow tomatoes, no staking and no weeding.
greenhouseThe primary flaw in the upside down tomato approach is that the buckets, being suspended in the air, subject the plant roots to much larger temperatures swings that they would experience if planted in the ground.   This can quickly kill the plants either by freezing or frying the roots.   In order to try to provide an environment with better temperature control, I built the green house pictured here.  To keep costs down, it’s built from some 2” by 3”s, a couple of shed wall panels, and the roof and top third of the walls are made from corrugated clear plastic panels.  The cost for all of the materials was a little less than $250.

I started my first batch of tomatoes in the greenhouse last April.   For a couple of months they looked beautiful and I was feeling like a gardening genius.   Then we had a hot spell in June. The temperature in the greenhouse shot up to over 100 °F and the plants began to wither.  I bought a thermostat and two exhaust fans which would run on the 12 VDC power coming from my solar electric station (1) to blow the hot air from the top of the greenhouse out while sucking in somewhat cooler air from vents near the ground.  This helped a little bit, but by mid July I had lost all of the plants. Next summer I intend to purchase a reflective mesh to put over the roof to see if I can keep temperature below 90 °F while still letting in sufficient sunlight.

Meanwhile, I started a new batch in the green house in August and now have some promising-looking ¾” diameter, green tomatoes. (These are the ones pictured above.) My current concern is that before they have a chance to ripen, we will have a cold snap and the plants will freeze.  To prepare for that possibility I started on a new project this week, a solar water-heating system for the greenhouse.

solar collectorMy first step was building the solar collector pictured here.  The box is built from treated lumber.  On the bottom is a 1” thick sheet of foam board insulation covered with aluminum which I painted black to absorb sunlight. (2) Next I constructed and installed the copper serpentine you see in the picture.  Soon after this picture was taken I painted the copper piping black as well.

The next steps are to cover the collector with a sheet of glass – I am using an old window I bought at the Habitat for Humanity Restore (love that place) – face the collector to the south, tilt it to approximately 30° from verticle, and fill the serpentine with a water/antifreeze solution.  When the sun warms the liquid in the tubes it will start to rise due to reduced density.  The liquid coming out of the collector will flow through an insulated hose to a small tank located high within the greenhouse.  From the tank the water will fall by gravity through another serpentine which will be embedded within a concrete block located inside greenhouse.  As the liquid flows through the block it will warm the concrete.  After passing through the concrete the liquid goes back to the bottom of the solar collector to be warmed again and the cycle continues.  This sort of temperature-driven liquid flow is called a thermosyphon.  Here is a simple schematic of the system.

If my calculations are correct, the heat stored in the concrete block should be sufficient to keep the air inside the greenhouse above freezing on even a cold Chapelboro night, and we’ll be having fresh homegrown tomatoes for my birthday in January.  I’ll let you know how it works out.

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(1) Consistent readers will have noticed that I have been using more footnotes of late.  As I write these columns a large number of tangential topics come to mind.  I used to weave these into the narrative of the column which created for a fair bit of clutter.  So I am trying to put more of those thoughts down here.  In this case, I wanted to remark on the absolute joy of running equipment from electricity you make yourself.  This is not difficult to do, and if you like tinkering around with things, I highly recommended it.  Unfortunately, there is not a supplier of solar electric equipment in the area, nor is it easy to find equipment, thermostats, pumps, etc. which run on DC power.

(2) If you are installing a solar water collector to heat water for your house, you want to use expensive solar absorbing paint rather than just black spray paint.