Energy Efficiency at GreenTech

Remember GreenTech? As you can imagine it was cancelled this summer, but there was an online GreenTech last week. A lot of great themes came up. In particular, I was impressed by how circularity was being talked about more than I remember last year. We’ll cover that in another article.

Energy efficiency is a big theme in greenhouse horticulture. I’ve called it the elephant in the room for vertical farming, but greenhouses have the advantage that most of their energy is free: solar energy. Still, a lot is being done on making the most of that solar energy and minimising fossil fuel consumption. A lot of that is in line with the circular economy as well. This is a summary of what five speakers at the energy session at GreenTech brought up.

Stijn Jochems – Delphy

Delphy‘s goal is to build a carbon-neutral greenhouse by 2040, and reduce carbon emissions and natural gas consumption by 50% by 2030. Stijn mentioned three principles to achieve this.

The first principle was to reduce the energy input required in the first place, through insulation and using LED lighting instead of conventional lamps. Another tip he had was to use vertical fans to distribute heat, rather than increasing the temperature of heating pipes.

The second principle was using energy from sustainable sources: biomass, geothermal and active dehumidification (for more on that, read this). In Delphy’s trial greenhouse, active dehumidification was able to cover 75% of their energy requirements. A challenge here is making sure the temperature profile is uniform – but Delphy is monitoring this and looking at the data to avoid harming the crop.

The last principle Stijn mentioned was being in sync with the crop. It’s no use heating a greenhouse if the crop isn’t going to put that extra heat to use. Through a better understanding of the crop, growers can make sure they are only adding energy or ventilating it away when it is actually worth doing so.

Frank Kempkes – Wageningen University & Research

Frank’s presentation was focused on the goals for Greenhouse 2030. Greenhouse 2030 aims to make it possible to grow free of fossil fuels, water emissions and pesticide use. It is the first research programme that focuses on all three together. During the presentation, he brought up loads of practical ways to save energy.

The first was insulation, mainly through screening. Some greenhouses could benefit from multi-layer coverings too – but this comes at a cost, since you lose out on light (and as they say, 1% more light is 1% more yield). It also means snow is more likely to stay on the roof.

Another challenge of better insulation is that humidity control becomes harder. Usually, humidity is regulated in part by condensation on the covering, which is the cheapest way to do it. If the covering is thicker, so not as cold compared to indoors, humidity will have to be controlled in another way. That’s not necessarily a bad thing, since humidity itself contains lots of energy, as discussed above.

The second big factor is the grower’s approach. Instead of growing against nature, the grower should grow in harmony with it, following outdoor conditions. For example, the grower should try to get a constant ratio of daily light sum to daily temperature. They should also reduce their use of heating. One of the ways to do this is to use low-quality heat, i.e. using residual heat which is usually a lower temperature than the typical pipe heating system. This requires a heating system with a higher surface area. We’ll see an example of this being used in the UK in a bit.

Another strategy Frank brought up was making the most of natural resources, namely the sun. He mentioned the ‘light greenhouse’, which is constructed in such a way that it lets 10% more sunlight in than a standard greenhouse would. Of course, this contradicts the first strategy of improving insulation, but that’s where every greenhouse should be tailored to its local context. Another possible approach is the use of agrovoltaics, transparent solar panels which absorb UV radiation but let in all photosynthetically-active radiation (PAR).

If you want to increase yield per unit of energy in the same area, either you reduce energy consumption or increase yield. Increasing plant density is a fairly straightforward way to increase yield. By using moveable platforms, they were able to increase the plant density in strawberries by 25%. Climate control won’t remain identical, because of extra transpiration, but the surface area of the greenhouse will remain the same, meaning similar energy losses but far higher yields.

Frank also said how to make greenhouses ‘fossil-free’, they would have to become all-electric. In times of peak energy demand, they could use biobased fuels like methane from biodigesters. However, by going all-electric, the grower loses a big source of CO2 for the plants. This could be obtained from industry in a satisfyingly circular way.

Marco de Bruijne – VB Climate

VB is a climate-focused company building greenhouse projects worldwide. Their new headquarters are energy-neutral, year-round. Marco mentioned some of the principles from Frank’s presentation being applied: all-electric, so no natural gas, and better use of existing resources through solar panels. Their building also ventilates based on temperature and carbon dioxide concentration, which means less ventilation occurs when there are fewer people in the building, for example.

Martin van Zeijl – Bom Group

Bom Group is a turnkey greenhouse construction company. Companies like theirs used to only do greenhouse-related components like screens – but now they do everything, from land-levelling to marketing.

Martin’s presentation was focused on two greenhouse projects in the UK, both for Low Carbon Farming Ltd. Before we get to the energy aspect, another big focus of GreenTech this year was local production. Because of the pandemic, countries that would usually outsource their agriculture to Africa or Ukraine are changing their minds and building greenhouses. This concern has been around for a bit longer in the UK because of Brexit, so this project has been in the works since 2016. LCF’s 29 hectares will be able to produce 12% of the UK’s tomatoes. The other factor behind this project is decarbonising production. LCF’s website claims a 75% reduction in carbon footprint from their new facilities.

The Crown Point Estate, Norwich. Courtesy Low Carbon Farming Ltd.

Now back to energy. One of the greenhouses was built near a sewage facility. A huge amount of wastewater leaves the facility – 1000 litres per second – and all this water is 25 ºC. This energy is to be used for the greenhouse. Someone spotted this opportunity by the steam coming off the effluent on a cold day. This is a good example of low-grade heat. Both facilities have separate tanks for storing low- and high-grade heat.

At the moment, these greenhouses rely on subsidies. However, Martin expects them to be economically viable once natural gas becomes less available.

Vincent Stauffer – Hortinergy

Insulate your greenhouse, or let in as much light as possible? Greenhouse construction depends on local context, but how do you make the best design choices? Hortinergy can help inform greenhouse design through simulating hourly energy consumption. They even simulate crop transpiration. Vincent showed us some of the things their software can do.

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