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Heating a Greenhouse

by Ron Kurtus

A greenhouse is a building that is heated with solar radiation and insulated to prevent loss from convection, conduction and radiation, such that it can stay warm without external heating even during cold days of winter. Such a building is used to grow plants during the winter.

Light from the Sun passes through the glass roof of the greenhouse to heat the plants and ground inside. Objects heated by sunlight emit infrared radiation.

These objects then emit infrared radiation that is absorbed or reflected by the glass roof, thus trapping the thermal energy in the greenhouse instead of letting it escape. This helps keep the building warm.

Questions you may have include:

What effect does the glass roof have on the solar radiation?
How does the glass roof affect infrared radiation?
How does the greenhouse stay warm at night?

This lesson will answer those questions. Useful tool: Units Conversion

Glass lets in solar radiation

A greenhouse is built with glass walls and a glass roof. Sometimes clear plastic is used instead of glass. One purpose is to provide light to the plants to help them grow and another purpose is to help keep the greenhouse warm.

Solar radiation or light from the Sun or passes through the glass and warms the plants, soil, and other things inside the building. Light is almost completely absorbed into the dark soil in the pots holding the plants, increasing the temperature of the materials.

Greenhouse heated by solar radiation

Besides letting the light energy in, the glass walls and roof act as a thermal insulator to protect the inside from cold air and winds outside the greenhouse.

Warmed air warmed is retained in the building by the roof and walls. There is no heat transferred to the outside by air convection. The glass allows only a little heat loss due to conduction of heat through its material.

(See Heat Transfer for more information on that subject.)

Glass prevents infrared from escaping

Although the glass used for a greenhouse allows visible light and short wavelength infrared radiation to pass through it, it does not transmit the longer infrared wavelengths. This means the radiation is prevented from escaping, causing a loss of heat.

Greenhouse remains warm

Typically, the greenhouse glass will transmit solar radiation of wavelengths between 280 nm and 2500 nm, while absorbing infrared or thermal radiation in the 5000 nm to 35000 nm region.

Note: nm stands for nanometer or one 10 millionth of a meter (10^-9 meter). Some books denote wavelength in microns. A micron is one millionth of a meter (10^-6 meter). Thus 2500 nm is the same wavelength as 2.5 microns.

Infrared radiation that does not pass through the greenhouse glass walls and roof is absorbed in the material. The glass then re-radiates the infrared back to the material inside the greenhouse, thus trapping the thermal energy inside the building and keeping it at a warmer temperature.

(See Infrared Radiation and Infrared, Gases and the Greenhouse Effect for information on those subjects.)

Ground slowly releases energy

Besides using the soil in the pots or on the ground of the greenhouse as a source of storing thermal energy, some greenhouses add materials such as containers of water or bins of sand and rock to absorb and store even more of that energy during the day. They help maintain a steady temperature in the greenhouse.

Since heated materials give off infrared radiation, the warm greenhouse soil, water, sand and other materials emanate this thermal radiation in longer wavelengths than the radiation that heated the materials. This energy is slowly released, even during the night.

Summary

A greenhouse is a building that is heated with solar radiation, such that it can stay warm even during cold days of winter. Light from the Sun passes through the glass roof to heat plants and the ground inside the greenhouse. These objects then emit infrared radiation, which is absorbed in the glass roof. Thermal energy is trapped in the greenhouse, keeping the building warm.

Be kind to your environment