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When to get active with passive solar

When temperatures fall below 28 degrees (it is 27 in Fenton tonight) the huge solar battery that is the soil in our passive solar green houses just cannot keep up. It is then the the farmer must step in!


When we built our greenhouses, we chose not include those massive heaters that you sometimes see on the larger farms. Burning tons of fossil fuel to grow food for our customers just seemed counter intuitive to our mission... to nourish our community while we nourish the earth. We have made great progress with our carbon sequestration program (organic matter in our soils have gone from 2% or over 4% in one year!) and it would be a shame to see it all go up in flames... literally. In addition to the effect such active heating would have on our carbon footprint, it is simply not economically practical for us to regularly use active heating to heat the huge volume of air we have under plastic right now. So, instead, we rely on the sun in a passive solar system.


In a solar greenhouse, thermal mass is the name of the game. Thermal mass simply refers to the amount of mass available to store heat and in many passive solar systems designers use tricks, like full water barrels inside of the greenhouse to increase the amount of thermal mass that is heated during the day by the sun and radiates that heat back into the house when the temperatures drop. Some people bury their greenhouse partially, or even use passive geo thermal designs. For houses the size of ours, such strategies are less practical, but the amount of solar mass in the soil, particularly if that soil is moist, is, well, massive. There is enough there to keep our houses pretty cozy for our plants, until the temperature reaches around 28 degrees.


The enemy of passive solar is thermal loss. For those of you who have seen our houses you know that they are really just space under a giant plastic sheet, and, as you can guess, this thin plastic sheet is not great insulation. We do use some tricks. Our roofs, for example, are actually two layers of plastic with an inflated air pocket between. This increase our thermal gain during the day and reduces our thermal loss at night. These tricks help for sure, but the houses are not 100% air tight and they are, well, plastic so passive only gets us so far.


There are currently more than 500 tomato plants, thousands of onions, cucumbers, kale, etc. in our biggest hoop house (between what has already been planted there and in trays that have been moved from our garage propagation area into the house to make space for more propagation), and we simply cannot afford to take the chance of them freezing. These are plants that we have grown from seed (as are 99.9% of all plants on our farm), we cannot replace them with plants from Bordines or Meijers (they do not have the varieties we want, and they are not certified organic), and replanting would set us back six weeks or more (not a good scenario for our CSA members.) So, what do we do?


Well, we get active! The tradeoff for starting our plants when and how we do it is a risk (or certainty in Michigan) that we will get these lower-than-28 degrees scenarios, so we have 170,000 BTU portable heaters that we use to add just enough heat to get our little babies through the night. They do an amazing job, as long as they have liquid petroleum to gobble up. Oh.., and the farmer also usually stays awake and worries (thus a 3 AM blog entry.) More infrastructure here would result in a little less worry and a little more sleep, but this system works well for the handful of days we normally need it.


So, after just coming in from checking our propane levels and the temperature in our greenhouse, I am here to report that, so far, the crisis is averted and the forecast for some fantastic Wegener Farm heirlooms looks pretty good!


Warmest regards,


Rob (your personal... and tired... farmer)



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