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Part 1: Greenhouse location and design

Pointers on greenhouse heating and energy conservation

Autumn has just arrived and with it we are noticing that day length is shortening.

This is the time of the year where our free energy source, sunlight, becomes limited. Sunlight is essential to maintain the global temperature in the ranges appropriate for plant growth. In winter, plant production is challenging because temperature and solar radiation decline to their minimum values. In addition, there are several cloudy days which further diminish photosynthesis. Nevertheless, greenhouses are used to minimize the negative effects of those environmental constraints. As winter approaches, the need for heating increases. The energy consumed to heat a greenhouse can be as high as 80% of the total energy required to run a greenhouse business.

The heat loss or gain in a greenhouse occurs as conduction, convection, radiation and infiltration. Conduction occurs when heat is transferred between two bodies in direct contact. Convection occurs when the heat is transferred via a fluid like water or air to the surface of an object. Radiation occurs when the heat is transferred without any physical contact or with the help of a fluid i.e. solar radiation. The last category is infiltration, this occurs when there is an exchange or leakage of air between the inside and the outside. In order to fully understand the different ways to save energy for heating, it is essential to study how to conserve energy. Here are some strategies:

Greenhouse location. Starting in summer, the day length decreases as latitude increases. For example, on December 21st the day length in New York (Latitude: 40˚N) is 9 hours and 15 minutes and the solar radiation for a sunny day would be 1555 J/cm2 during this day. On the other hand, in Queretaro, Mexico (Latitude: 20˚ 35’ N), for the same day, the day length is 10 hours 53 minutes, solar radiation for a sunny day would be 2783 J/cm2 during this day. There is almost twice the amount of solar radiation energy received in Queretaro, Mexico, than in New York, and as a result we can expect lower temperatures and greater heating costs in New York than in Queretaro, Mexico.

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