Add Thermal Mass Materials to Your Off-Grid Greenhouse

Install a greenhouse system that can store daytime heat and release it at night to keep the temperature regulated 24/7.

| February/March 2018

All greenhouses use the sun for heat during the day. At night, most greenhouses quickly lose that heat because of the poor insulating quality of their materials. The reason for this inefficiency has to do with some basic principles of traditional greenhouse design that focus on maximizing light. Glazing materials, such as glass or clear plastic, are good at letting in light, but they’re terrible at retaining heat.

Solar greenhouse design takes a different approach, finding a balance between glazing and insulation to create a structure that naturally resists overheating and overcooling without reliance on fossil fuels. Instead, the sun provides the energy, and the greenhouse collects and stores that energy to provide its own heating when required. The use of thermal mass materials is the oldest and simplest strategy for storing heat and naturally mitigating temperature swings.

How Thermal Mass Works

When light hits a material, some of it is absorbed and converted to heat. Thermal mass materials absorb this heat via conduction. Heat slowly conducts from the surface to the center of the mass, allowing the entire volume to heat up by a few degrees in a day. When the air temperature in the greenhouse drops at night, the mass slowly starts radiating this heat through conduction and by releasing short-wave infrared radiation. In this way, thermal mass regulates the temperature of the greenhouse. It absorbs energy from the sun during the day and slowly re-radiates it as heat at night (or whenever the air temperature drops below the temperature of the mass), which evens out daily temperature swings.

Because mass materials store heat, they’re often referred to as “heat sinks,” “thermal banks,” or “thermal batteries.” The idea is the same: to store as much heat as possible when it’s oversupplied, and slowly distribute it when it’s in demand. To make this possible, you must maximize the materials’ exposure to light in winter (or whenever greenhouse heating is needed). This will allow the sinks to charge as much as possible during the day so they can radiate heat later. Even if the materials aren’t directly illuminated, they’ll still absorb some heat from the surrounding air. This means they can have some effect on overcast days; however, the predominant driver of a sink’s ability to store heat is direct light absorption.

During summer, the goal of thermal mass is to help keep a greenhouse cool. To that end, the mass should be shaded during summer months. On summer days, it will still absorb some heat from the greenhouse as hot air moves across it. It will then radiate this heat at night, given a sufficient drop in temperature, and resume cooling the next day.

Choosing a Thermal Mass Material

All materials absorb and store heat to some extent, but some are much more effective than others. The amount of heat a material can store is called its “heat capacity” and is determined by two factors. First, the material’s specific heat is a property of the material, defined as how much energy is required to raise its temperature 1 degree Fahrenheit.
5/27/2018 10:09:56 PM

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