3 Methods for Heating Greenhouses for Free

| 11/14/2014 10:34:00 AM

Greenhouses can be interesting environments to grow in. This is because standard greenhouse materials like glass and plastic (“glazing”) are extremely good at letting in light and heat in, and extremely good at letting heat out. With so much glazed surface area, greenhouses usually overheat during the day if uncontrolled. And because glass and plastic provide no insulation, at night they lose all that heat, causing them to freeze. Take this October day in Boulder, Colorado for instance: An all-glass greenhouse fluctuated from a high of 110 F to a low of 30 F in one day. Plants, like people, do not like this.

The primary challenge with greenhouse growing is stabilizing these temperature swings. Conventionally, people do this by blasting energy via heating or cooling systems into the greenhouse. But the smarter, more sustainable way of creating a stable greenhouse environment is to harness the excess solar energy coming in during the day, store it and use it at night. Or, if working with an existing greenhouse, to add an efficient heater that uses cheap and renewable fuels. These strategies all take understanding and research, and have some upfront cost, but the pay-back in terms of added growing and long-term savings is well worth it.

Also, remember there’s no cheaper energy than the energy you don’t have to use, so if designing a new greenhouse, build it so that it does not require much heating and cooling in the first place. This means using building a air-tight, insulated structure, using proper roofing materials, and orienting the greenhouse with the glazing facing South — where all our light in the Northern hemisphere comes from. If growing in an existing greenhouse, you can insulate your greenhouse and weather-strip air leaks among other things. Reducing your energy requirements to a minimum is always the first step, then incorporate the strategies below.

1) Store solar energy in thermal mass

The easiest and most common way to even out the temperature of your greenhouse is utilize thermal mass, also called a heat sink.  Thermal mass is any material that stores thermal energy. Most materials do this to some extent, but some do it much better than others. Water for instance, holds about 2 times as much heat as concrete, and about 4 times as much as soil.

Incorporating mass does two things. First, it absorbs excess energy during the day, creating a cooling effect. When the temperature drops at night, it starts releasing that energy, thereby ‘heating’ the greenhouse. Note: though I say ‘cooling and heating’, the thermal mass is not actually providing the energy, it’s simply storing it and releasing it later, like a battery. The size of the battery (or how much energy you can store) depends on the heat capacity of the material and how much mass you have. Below is a table comparisons a few different sources of thermal mass and their heat capacities.

5/28/2018 1:04:46 PM

I'd like to bury several 50 gallon drums under the floor and install solar water heater panels (not PV). Install a pump to aid passive water movement. Any thoughts advise on doing that?

5/28/2018 1:01:56 PM

I'd like to bury several 50 gallon barrels under a greenhouse floor, and install non-PV solar water heater panels. Run a pump to aid passive flow. Any comments, advice, formula? I live at 6200 feet in the San Bernardino Mountains So Cal. Can get down to zero but is rare.

5/28/2018 1:01:54 PM

I'd like to bury several 50 gallon drums under the floor and install solar water heater panels (not PV). Install a pump to aid passive water movement. Any thoughts advise on doing that?

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