Greenhouses

Solar Energy

for Greenhouses

Energy represents 15% of total costs in greenhouses, making it the 3rd largest expense after labor and plant materials. The typical energy budget of a greenhouse is broken down as follows:

  • Space heating represents 70-80%
  • General electricity consumption represents 10-15%
  • Transportation accounts for the rest of energy consumption

An energy efficient greenhouse depends on two main elements: an efficient heating system, and a high-performance envelope. The operating costs and environmental footprint can be further reduced with solar trackers and battery systems to provide clean energy 24/7 for operations. According to the University of Connecticut, many greenhouse improvements can achieve a payback period of less than two years.

Since heating is the largest energy expense in a greenhouse, heat loss prevention is key for energy efficiency. There are four main ways in which a greenhouse can lose heat: conduction, convection, radiation and infiltration.

solar greenhouses

The following are some of the energy efficiency measures that have been analyzed by the University of Wisconsin:

Greenhouse Energy Efficiency Measure Typical Savings
Infrared and anti-condensation treated films 15-20%
Insulated side walls 558 therms of natural gas per year, for a 28ftx100ft greenhouse
Night curtains 20% to 70% when closed
Air sealing 3-10% of heating costs in a typical greenhouse
Double polyethylene cover for glass greenhouse Up to 50% heat loss reduction
Windbreaks 5-10% heat loss reduction

The glazing material used is a very important decision when building a greenhouse. Single-pane glass has a service life of over 25 years, but the structure is expensive, and its typical heat loss is 1.1 BTU/sf-hr.-°F (BTUs per square feet, per hour, per °F of temperature difference). On the other hand, double polyethylene film only loses 0.5-0.7 BTU/sf-hr.-°F while having a lower installation cost, but the typical service life is only 3-4 years.

Wind breaks are also important since heat loss doubles with 15 mph wind. One option is shielding the greenhouse with 4 or 5 rows of trees, mixing coniferous and deciduous species or consider installation of dual axis solar trackers.

Greenhouse Heating

The greenhouse heaters also have an impact on energy efficiency, and they can be vented or unvented. The following table compares the efficiency of the four main types of vented unit heaters, considering both thermal efficiency (TE) and seasonal efficiency (SE):

Vented Heater Type TE SE
Gravity vented 80% 65%
Power vented 80% 78%
Separated combustion (SC) 80% 80%
High efficiency condensing with SC 93% 93%

Thermal efficiency refers to the performance of the unit heater only, while seasonal efficiency describes the overall system performance under changing weather conditions. The installed price depends on project conditions, but it generally increases along with the SE. In greenhouses with gravity-vented heaters, a conversion to power-vented normally has a payback period of less than two seasons.

There are also non-vented unit heaters, with a typical TE of 99% and SE of 80%. They increase both carbon dioxide and humidity levels – while the CO2 increase is beneficial for plants, the moisture increase can cause condensation issues and disease. This type of heater must be used with air intakes, to compensate for the lack of venting.

The heat distribution method also affects efficiency, and the UW has found that under-bench heating can lower costs by 20-25%. Adequate maintenance is also also important, achieving up to 20% fuel savings.

Hydronic heating is also an option, using a high-efficiency boiler and hot water circulation. This configuration can achieve an efficiency of over 90%, especially when using a modern condensing boiler. However, this configuration has a higher cost than unit heaters.

Solar Energy

Powered Greenhouses

Additional savings can be achieved with environmental controls, which ensure that all systems in the greenhouse are used optimally.

The typical energy budget of a greenhouse is broken down as follows:

  • Space heating represents 70-80%
  • General electricity consumption represents 10-15%
  • Transportation accounts for the rest of energy consumption

This includes battery backup and off-grid capability to ensure 24 x 7 uninterrupted operations for spray systems, pumps, heating and lighting systems.

With respect to ventilation, fans with a larger diameter are generally more efficient, especially when controlled with variable frequency drives (VFD). The motors should be totally enclosed and with suitable bearings for the application.

Additional savings are possible with a passive solar design for the greenhouse, and by using solar LEDs for supplemental lighting.

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