Plant Bed Heating

Plant beds may be used to store excess greenhouse heat

By John Canivan

Conventional greenhouses use fossil fuel heating systems to prevent frostbite, prolong growing seasons and get seedlings off to an early start. BUT… the added expense of a greenhouse heating system is not always practical. Thermo-pane glazing helps keep the heat in, but all glazing materials are poor insulators. Passive solar greenhouses with thermal mass can moderate temperature swings but un-insulated, thermal-mass, heat losses are bothersome.

So where can we store excess greenhouse heat?
How about the plant beds?
Did you know that 9 out of 10 plants agree?

 “Happiness is a warm bed.”
Is your greenhouse a suitable candidate for a plant bed heating system? Answer these questions and find out:

1. Is your outside temperature frequently below freezing?
2. Is your greenhouse glazing pitched to maximize winter heat gain?
3. Is your glazing surface optimized to minimize heat loss?
4. Is your plant bed built against an insulated north facing wall?
5. Is your foundation wall insulated on the outside?
6. Are the walls and ceiling of your greenhouse well insulated?

An energy conserving solar greenhouse can provide a cost effective growing environment, but your plants could use additional help to get through a tough winter. To illustrate this concept a greenhouse with and without a plant bed heat storage system is compared. Notice how the green house air temperature and greenhouse bed temperature change during the day.

Comparative, Concept, Temperature Readings

AREA: Plattsburgh , NY

TIME OF YEAR:  January 15

  TIME            Outside                         Without                     With

                Temperature         Plant Bed Heating     Plant Bed Heating              

                                         AIR           BED              AIR            BED

Heat stored inside the plant beds will keep Jack Frost at bay for awhile and a clear plastic tarp draped over the plants also helps since much heat is lost during the process of transpiration. If we compare plant bed temperatures at 6AM we’ll notice that the air temperature in both greenhouses is quite low. As a matter of fact, we might find that the air temperature of the greenhouse with the plant bed heating system may be lower than the greenhouse without the plant bed heating system.

        Uh Oh! What can we do to keep Jack Frost away? I know it’s too late for the unheated plant bed, but how about the heated plant bed?

        If you think a plastic tarp is a good idea we’re on the same page. A simple .4mm clear plastic tarp could be used to retain plant bed heat when the going gets rough. By 2 PM the air temperature of our greenhouse without heat storage could reach 120⁰ and during a three hour period of solar heat gain about 30% is lost through the glazing and another 10% is lost through the walls and doors. Large temperature differences accelerate the heat loss process and make life difficult for delicate plants.

Thermal mass in the floor, walls and plant beds help moderate temperature swings, but the process of temperature moderation could be greatly improved by actively pumping excess greenhouse heat into the plant bed. Plants handle temperature variations better than people, but even plants have their limits.

The total heat gain for this 8’x16’ greenhouse from 3 hours of direct sunlight would be about 60,000 BTU, but almost half the heat gained during this period would be lost through glazing, without a plant bed heating system.

The rate of heat loss varies with temperature difference so a low temperature greenhouse loses less heat than a high temperature greenhouse. After 3 hours of direct sunlight a simple solar greenhouse could easily reach 120⁰ F, but most plants would not be impressed and a temperature difference of 100 F would loose heat at a rate close to 20,000 BTUs per hour through the glazing. Moderate temperature swings with a heated plant bed are all that’s needed to provide a cozy environment and extend the growing season.

OK, so much for concept. Now let’s see what’s involved with collecting and transferring the excess heat that accumulates on our greenhouse ceiling and transfer that heat into the plant bed. As you know, air is a poor conductor of heat so we’ll need a large surface area for heat transfer.

We’ll need to blow the hot air through something, but what?
Copper and steel are good heat conductors, but they’re also expensive.
How about plastic? Plastic won’t rust, but will it conduct heat?

Six inch plastic sewer pipe is being used in this illustration but four inch pipe will also work fine. The price is right. Plastic is not a good heat conductor, but air is much worse so our heat transfer rate depends more on surface area than the material we use. Every foot of 4” sewer pipe has a surface area of 1 ft², so seven 10 foot lengths of sewer pipe have a surface area of 70 ft².

At a temperature difference of 50⁰ F, ten 4” plastic pipes can exchange about 10,000 BTU’s worth of heat per hour with the help of a 300 cmf duct fan. This amount of heat could raise the plant bed temperature about 5⁰ F over the period of 3 hours. Additional heat could also be exchanged into the plant bed with a solar hot water system or other methods. Wet sand in the bottom of the plant bed can be used to transfer and store heat. Polystyrene foam can be used to insulate the plant bed.

The hot air distribution cavity is used to distribute hot air uniformly through the plant bed heating pipes. Your plant bed heating system may look a little different, but this is the basic idea.

GROSS HEAT GAIN:

Surface area of glazing = 6x16 = 96² ft = 9m²

Heat gain from one square meter of direct sunlight = 3,400 BTU/hr

Heat gain from 9m² for 3 hours = 3 x 9 x 3,400 = 61,200 BTU’s

Since there is only about 640 cubic feet of moist air heat available, 61,200 BTU’s worth of heat would raise the interior temperature of the greenhouse air about 2000⁰ F if there were no heat losses and no heat sinks.
Fortunately the glazing, the walls, and the floor lose heat and the plant bed gains heat to moderate temperature extremes.

GROSS HEAT LOSS (during 3 hr of sunlight)

For this calculation we will assume that the temperature inside the greenhouse goes from 40⁰ F to 100⁰ F during a three hour interval of heat gain. We will also assume that the average interior temperature inside the greenhouse during this interval of time would be 70⁰ F.

Heat loss from glazing = 96² ft x (70⁰ F - 10⁰ F) x 1 x 3  = 17,700 BTU

Heat loss  from sides and roof = 320² ft x 60 x .1 x 3    =   5,760 BTU

Heat loss through door              =   20² ft x 60 x .5 x 3    =      270 BTU

Heat loss through the floor      =   …………………….            =      800 BTU

TOTAL HEAT LOSS (FOR 3 hour interval)                     =  24,500 BTU

NET HEAT GAIN (during 3 hour interval)

After 3 hours of direct solar heat gain the net heat gain would be the difference between the gross heat gain and the gross heat loss or 61,200 – 24,200  =  37,000 BTU. This is all the heat we’ll have left to get us through the night. If we will assume that the greenhouse temperature at 1PM is 100⁰ F. The quantity of heat being stored in the greenhouse air relative to the outside temperature of 10⁰ F would be .5 x 50lb x 90 F or 2,200 BTU. The remaining heat must be stored in the interior walls of the greenhouse and the exposed masonry wall of the plant bed.

At a heat loss rate of 24,000 BTU/hr all the heat gained would be lost in 4.6 hours, but we should understand that the rate of heat loss will slow down as the inside temperature of the greenhouse drops. A solar greenhouse without heat storage could not be expected to keep plants above freezing throughout a cold winter’s night, so this is where a plant bed heating system comes to the rescue.

This cross section of the plant bed heat transfer system shows a network of seven plastic pipes imbedded in wet gravel. Notice the overflow outlet in the side of the plant bed wall used to drain excess water. The plant bed wall can be made from cement or wood or other materials but it must be waterproof at the level below the overflow outlet.

These calculations are based on theoretical models and experimental results that may vary from place to place, materials used and weather conditions. The only way to truly experience the value of plant bed heating is to build it yourself based on an understanding of solar thermal energy concepts. Feel free to visit www.JC-SolarHomes for additional information and clarification.

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