Energy for greenhouses

Energy for greenhouses

Commercial greenhouses throughout the world rely on climate control for disease control, heating and cooling. Here in New Zealand, most commercial greenhouses are heated for at least part of the year. Traditionally, greenhouses in the North Island were often powered by natural gas and in the South Island by coal fired boilers. Those using natural gas, have the benefit of being able to syphon off the CO2 to reinject it back into the greenhouse during the day to boost yield by up to 20%.

With the introduction of the Emissions Trading Scheme for the covered crop industry in 2010, some coal users turned to recycled oil as the ETS had already been paid, the first time the oil was used. In 2026, one of the major recycled oil users took their supply out of the market and many greenhouse growers were left without a supply just ahead of the colder winter months. Natural gas users are finding that lines charges are increasing even if the supply charges are controlled under contract. Once a contract runs out, the terms of a new contract are likely to have substantial increases in the supply of the gas.

TomatoesNZ and Vegetables NZ have been working for a number of years with the Energy Efficiency Climate Authority (EECA) to provide a range of tools and materials to help greenhouse growers decrease their energy requirements and ultimately move to decarbonise. The work that we have done together is captured on their website here, this includes a series of case studies of what other growers have done.

Know your energy use

Before you can look for energy efficiency savings, you need to know how much energy you used over the course of at least 12 months. Download the energy calculator below and look at these instructions to calculate your current energy use. After you've completed this step, download the decision tree below to work out what steps you could take to reduce your energy use.

A number of years ago, EECA part funded some energy transition reports for a number of greenhouse growers across both the North and South Islands. These growers had a variety of crops and were different sizes and using different heat sources. The data from these reports was then collated, anonymised and a report created (see below) so that all growers could benefit from this information. While the capex and opex will change over time, there is still much that can be learned from this report.

Thermal Screens

While it might be easier to install thermal screens at the time of building, it is definitely possible to retrofit them as well, even with a crop growing. Rob Lindsay talks a little about his energy and screens here (around 1 minute 22) and Albert Shih talks about his thermal screens here. The energy saving will depend on many factors including location of your site and the type of screen you install.

Quick wins with energy efficiency

There is a vast area and will depend on each operation, the location, age of the greenhouse etc. However, Rob, a Canterbury based cucumber grower, talks about climate control via a sensor in milk bottles that helps to control his boiler and controlling the vents when a hot dry air (the NorWester) is blowing here

Dehumidifiers to reduce heating requirements

Watch this presentation on how Southern Belle Orchards decreased their use of Waste Oil when they installed 2 10kW DryGair dehumidifiers. Other benefits of dehumidifying include:

• no longer using heating to dry the air so a decrease in energy used to heat (for Southern Belle up to 25% less)
• reduced disease pressures due to the drier atmosphere in the greenhouse.
• better use of heat and CO2 absorption as the vents are closed more often
• likely payback could be 4 - 5 years due to increased marketable yield and lower energy use

Wood pellets and Biomass

The main difference between using wood and coal is that while coal storage can be open to the weather, wood needs to be kept dry. Weather tight storage can be the most expensive part of a conversion from coal to using wood but it doesn't need to be. There are also options for converting a recycled oil burner to using wood as well. Watch 2 growers who use wood pellets talk here and here about their experiences and learnings.

The Bioenergy Association website also has a lot of useful information about using wood as a fuel. There is a list of suppliers across the country here.

Hot Water Heatpumps

Clearwater Gardens installed 2 heat pumps to heat their water in one of their greenhouses. This video captures all the main info here. Compared to coal or diesel, heatpumps are very clean and require little maintenance, but there are also some drawbacks including potentially needing to upgrade the electricity network which can be a significant cost.