Sunday, May 20, 2012

Herb Drying: Take 2

Last August/September I was sun-drying herbs, and started by putting them in my car in the sun while I worked.  I gave a hint about another idea; that is, using reflection of sunlight and 6 mil poly to retain some of the heat.  This way, you don't have the car intermittently occupied by silent, aromatic, green passengers all herb harvesting season.  What I didn't tell you yet, is that I have already begun implementing such a plan and it works very well.  It's actually not very expensive to build.

Before I show you the pictures I should tell you that you don't have to make a reflector as interesting as mine to make the plan work.  Truthfully, I'm not making optimal use of my parabolic cylinder reflector.  (I would need to track the sun to do so, which I have often wanted to do, but haven't made the time to research and source the needed parts for any of my unwritten, half-baked designs.)  A three part reflector is probably more than adequate:  orient the three parts so that you get (1) the mid to late morning sun, (2) the afternoon sun, and (3) the evening sun.

Options for reflectors include:

  1. shiny metal - excellent durability
  2. aluminium foil - inexpensive and easily obtainable, a little crinkly which results in poorer focus
  3. emergency blankets - slightly more expensive than foil, but smoother and shinier
I use option 3.  My local Heritage Co-op grocery store (in Brandon, MB) often has them in stock along with a limited selection of other camping/survival supplies.

The last time I dried herbs, I took a few pictures.  Here are some chives freshly picked and washed:  

The more surface area you allow to be fully exposed, the better, which is why I am using three dishes when just one would do.

This is the unattractive monster which dries my herbs:

If I were picky, I would replace the hideous stained 6 mil poly.  I could also use thinner poly which would be more translucent.  In a way, this low translucence is good as it reduces the likelihood of UV reflection into people's eyes.  From the solar cooking perspective, the low translucence is a limitation to your peak temperature and that is bad.  But for drying herbs, you don't want it too warm anyway.

Here is my reflector and screen box.

The reflector is a parabolic cylinder covered with part of an "emergency blanket" (also called "space blankets").  It reflects a high percentage and broad spectrum of solar radiation.  To stabilize the reflector, I drove a wood stake about a foot behind the reflector and screwed another scrap both to the back of the reflector and to the stake.  The screen box is nothing special; I made it out of 2×2s (ripped 2×4s) and nylon screen.  It simply allows air to flow through.  My original idea was to put some of the derelict glass I have at my disposal over the screen box to protect from rain, bird droppings, and wind-blown debris organic or otherwise.  But the reflector construction is susceptible to rain damage (the glue and Masonite in particular) and I wanted to benefit from a greenhouse effect as well as reflection.  So, I poly over it.  Here is the vegetation placed in my box:

If I have a lot of vegetation to deal with, I just spread it out as best I can, even without dishes inside the box.  I usually give it a daily stir if I have time.  This batch was almost done in one day, easily in two.  Much larger batches have been done in three days.  (If that sounds long, bear in mind that that's three days of paying hardly any attention to it.)  Here I've tucked the poly under the box:

Note that the sides are somewhat open to allow moisture to escape.  We actually have a few things in our favour to get the moisture out of those herbs:

  1. We are adding heat to the system which satisfies the energy requirements for evaporation.
  2. The temperature of the air inside the poly is increased which increases its capacity to hold moisture.  By increasing the capacity of the air, it changes the equilibrium distribution of moisture (between the air inside the poly and the herbs).  It's a bit like increasing the voltage of a circuit.  The potential rate of evaporation is increased.  (This, of course, would be largely useless without a means for the increased moisture content to exit the system.)
  3. As the air in the poly gets more moist, it wants to migrate.  The moisture wants to go from moist to dry.  It's like the tea I like to drink so much of.  The contents of the tea leaves want to distribute themselves as evenly as possible.  The same is true of moisture.  (Although there's a sticky point with the relative capacities of the air inside and outside of the poly to hold moisture.  But don't worry about that.  Bottom line: we have vapour pressure taking moisture out through any opening it can find, provided the outdoor climate provides a suitable drying environment.)
  4. The ventilation can be increased by making sure there is an opening near the top, but in any case, higher than the herbs.  (Determining the optimum opening sizes is probably non-trivial.)  The hotter air inside the poly is less dense than that outside and so it rises up through the relatively cooler air outside.  If there is no opening near the top, the effect of heat migration will be less effective for creating ventilation.  Although the heat will still move from hot to cold and the hot air will still rise, the hot air at the top will transfer heat to the poly which will give off the heat on the outside air.  So, if the openings were exclusively at the bottom of the system, the heat would still be transferred (at some rate) outside of the system, but the hot air itself (which contains our unwanted moisture) would not be exiting the system as a direct result of heating (although it may exit due to other factors or effects, such as the wind effect which creates pressure differentials).
Here is the finished product:

I often put a bunch of herb in a sandwich bag, leaving it open, and scrunch it up inside the bag.  This reduces loss of material and increases hygiene.  Seal the bag when done, or transfer to another container.

Removing the dehydrated herbs from the dryer is best done prior to the setting in of dew in the evening - late afternoon or early evening.  If you wait too long, equilibrium will demand the dry herbs to take on moisture which will require them to be re-dried.  Just check how crinkly they are.

Before your next fill of the screen box it may be wise to vacuum it, especially if you are changing herbs.  I find I have to vacuum occasionally since the plant material likes to hook on to the screen in places.

This would make an excellent project for someone looking to get their feet wet in the pursuit of solar cooking, especially if they have a garden and like to have summer herbs for free all winter long!
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