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, in some areas of the country that can be as much as 10 months of the year. Traditionally, greenhouses in the North Island were usually powered by natural gas and in the South Island by coal fired boilers. Those using natural gas still 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 (ETS) 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 and there was an abundance in supply across the whole country. In 2026, one of the major recycled oil providers took their supply off the market and many greenhouse growers were left without a supply, just ahead of the colder winter months.

For natural gas users, while supply for now might still be guaranteed, growers are facing price surges, albeit from frequent increases in lines charges, 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 (TNZ) and Vegetables NZ (VNZI) have been working for a number of years with the Energy Efficiency Conservation Authority (EECA) to provide a range of tools and materials to help greenhouse growers decrease their energy requirements and what decarbonisation options are available. The work that we have done together is captured on EECA's website here, including a series of case studies of what other growers have done. In addition to working with EECA, TNZ and VNZI have also worked with other partners and consultants. This page is an attempt to collate all of this information together in one place for growers to access.

Know your energy use

Before you can look for energy efficiency savings, you need to know how much energy you have used over the course of at least 12 months. Have your energy bills  to hand through your supplier's invoices, however they send these to you. Download the energy calculator below and look at the 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. Ideally, if you're looking for an alternative fuel supply, you'll have reduced your energy use as much as possible first so that you aren't quoted for an oversized heating system that has an output which is more than you require.

Energy Efficiencies

Once you know how much energy you consume a year, you can look at ways to reduce this. Elly Nederhoff produced a series of articles which was collated into an e-book for download here covering a variety of topics from humidity, screens and fans. There is also useful information on how temperature effects plants and climate physics. Elly is a Dutch consultant and used her vast knowledge of the greenhouse industry in the Netherlands to write these articles.  

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, a cucumber grower based in Canterbury, talks a little about his energy and screens here (around 1 minute 22) and Albert Shih, who grows tomatoes and eggplants in Canterbury, 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

This is a vast area and will depend on each operation, the location, age of the greenhouse etc. However, Rob Lindsay 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

It is thought that up to 40% of heating in a greenhouse typically dries out the atmosphere, before 60% then goes on to heat the air. Watch this presentation on how Southern Belle Orchards decreased their use of Recycled 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

Energy Transition Audits

In 2022-23, 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. Keep in mind that most growers need a way to heat water which might not require a boiler.

The EECA website also has extra information for businesses here which includes a search function. This includes a page of providers who can either assist with financing and/or provide a 'Heating as a Service' model whereby the energy system is installed and maintained by the provider, and only the heat is paid for. Have a look here for businesses who partake in this service. 

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 cost a lot. Talk to a wood supplier (see link below) to estimate how much storage space you might need, keeping in mind any plans that you might have for your site in terms of expansion or contraction. There are also options for converting a recycled oil burner to using wood as well. Watch 2 growers who use wood pellets talk here (Bay of Plenty grower) and here (Nelson grower) about their experiences and learnings and 2 very different storage options.

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 electric 2 heat pumps to heat the water that flows through their hydroponic system in one of their greenhouses to support their leaf green growing businesses. 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.

Low temperature Geoheat

When most people think about geothermal heat, they usually think about the steam producing, high temperature water in areas around Taupo and Rotorua. In fact, many areas of New Zealand sit on low temperature geoheat that can be used either alone or with a small extra boost to achieve an ambient temperature for a greenhouse. Geoheat is further explained here. While the cost of drilling can be expensive, the pay off is that operating costs can be literally the cost of electricity to pump the warm water to the surface or much reduced heating costs to bring the warm water up to the required temperature.

TomatoesNZ, Vegetables NZ and Earth Sciences NZ worked together to build a tool for growers based in the top of the North Island, which pulls together known data of geoheat and where available, flow rates. This can be accessed here, all you need is the site address. If you also have your energy bills available, you can input this data and it will give you an idea of costs involved, as well as the processes you will need to think about.

As a side project to the mapping tool, TNZ and VNZI worked with GeoExchange to produce a case study of the process and costings involved to access geoheat at a greenhouse operation in the Bay of Plenty. The case study below looks at different options for accessing different temperatures of geoheat and the cost of boosting this to the required temperature through a variety of options including installing extra heating pipes. The case study also includes the costs to drill further for a higher temperature geoheat which would generate heat at the required temperature and require no further heating boost.

Want to talk to a expert about this? Have a look at the GeoExchange website as they have conducted numerous geoheat projects across New Zealand and Australia.