Greenhouse films and energy savings
SUBSCRIBE SOURCE: Interview with the president of Geo Polska, Bartosz Kłyk, conducted by Ms Aleksandra Andrzejewska, published in in May 2022 in the monthly magazine Warzywa
For many years, there has been a systematic increase in energy costs in every aspect of horticultural production.
In the current situation, the possibility of finding savings in this area can ultimately determine the profitability of horticultural production.
Bartosz Kłyk of Geo Polska explains that solutions currently available on the cover crops market not only increase rentability by reducing costs but also improve quality and speed up yield.
Aleksandra Andrzejewska (AA): Should saving on production under covers only be a concern of producers who grow crops in heated facilities? What affects the energy consumption?
Bartosz Kłyk (BK): Due to the specific nature of horticultural production under covers, every energy-saving solution should be carefully analysed by the greenhouse owner, regardless of whether the facilities have a heating system or don’t have one.
The energy consumption of a film-covered greenhouse depends on the following factors:
*the volume of the facility (a larger facility has a lower energy requirement per m2 of cultivation);
*the airtightness of the facility, including the effectiveness of ventilation solutions;
*technical solutions (thermal insulation curtains, etc.);
*type and quality of greenhouse films.
On a horticultural farm, the first three factors are difficult to change or may involve much money. Nonetheless, the right choice of greenhouse films and the awareness of the critical parameters offer opportunities for quick savings or even additional revenue at a relatively low cost.
Films must be replaced periodically. The differences in the prices of the different materials are insignificant (especially considering the total production expenses), so the right decision should be made mainly based on technical utility and the impact on the future economics of production.
AA: How does the way of installation and greenhouse film quality affect the facility's energy efficiency?
BK: We can identify 3 factors of the quality and installation method of greenhouse films affecting the energy efficiency of the facility as well as the yield and the quality of the crop:
* thermic effect;
* Anti-Drip effect (with particular reference to the permanent effect);
* Double-inflated film installation.
AA: Could you explain what the thermic effect is?
BK: The thermic effect - EVA films - the key to additional savings.
A horticultural tunnel should ensure that the correct temperature can be maintained and that adverse temperature variations are minimised. This includes, above all, a reduction in daily temperature fluctuations. The greenhouse film itself has very little thermal resistance. Research even suggests that the thickness of greenhouse films (regardless of type) is so tiny that there is no temperature gradient. Therefore, to improve the energy efficiency of their products, film manufacturers focus primarily on the transmission of long-wave infrared radiation. A standard film can absorb 50-55% of infrared radiation.
Modern films are created by adding ethylene-vinyl acetate copolymer (EVA) and mineral additives.
Depending on the film manufacturer's technological skills and the film type, infrared losses of less than 20% can be reduced through this process.
Thermic films (often additionally marked EVA) allow several per cent energy savings. At the same time, they significantly reduce the daily temperature fluctuations in the building. Thermal films have been proven to offer earlier harvests, higher yields and better crop quality, even when low temperatures are not a big problem. The benefits of thermal films can, therefore, be enjoyed not only by greenhouse owners who actively heat their facilities but also (and perhaps especially) by those gardeners who start growing in, for example, April and do not heat their facilities.
The economic effects of covering the tunnels with EVA film will be visible in our climate especially during spring and autumn season.
In the case of heated facilities, the benefits are mainly on the cost side, providing a fuel saving of several per cent. In this case, temperature fluctuations are eliminated by the heating system.
The benefits for unheated facilities are mainly on the revenue side—thanks to a reduction in daily temperature fluctuations, we can count on better quality/productivity. The benefits on the cost side for unheated facilities are mainly a significant reduction in the risk of frost losses and a reduction in expenses for plant protection products (resulting from a better crop condition less exposed to temperature shocks).
AA: Please explain what the Anti-Drip effect consists of.
BK: Apart from the sheer loss of light (and energy) as it passes through the film, a significant problem is the secondary loss caused by condensation on the inner surface of the film. The air inside greenhouse objects very often has a high relative humidity. Significant temperature differences on both sides of the cover cause condensation on the inner surface of the film. The problem is particularly substantial with low insulation levels (more condensation observed with a single film, less with double-inflated film) and poor ventilation.
Polyethylene is hydrophobic, which causes the formation of large droplets that refract light (resulting in a partial loss of energy, including heat energy) and—when falling on plants—increase their susceptibility to diseases. For this reason, manufacturers add hydrophilic additives to greenhouse films to reduce surface tension (Anti-Drip additive).
As a result, the condensate does not form droplets but runs off very quickly in a thin layer. With conventional films, Anti-Drip additives disappear during the use of the facilities. You can often see areas where droplets collect adjacent to places where this phenomenon has not yet been observed.
The latest technology, available in Plastika Kritis' 7-layer greenhouse films, makes producing films with a permanent Anti-Drip effect possible.
Significant differences between facilities covered with a film with a traditional Anti-Drip effect and a permanent (non-fading) Anti-Drip effect can be seen after just 2 years of facility use. An additional advantage of the film with a permanent Anti-Drip effect is that it significantly reduces the risk of fogging in the facility (Anti-Mist effect).
All these advantages result in significant savings and enable additional benefits. For heated facilities, we reduce energy costs by increasing the absorption of solar energy (through no additional light loss on the droplets). In addition, we make much more efficient use of heating inputs - fewer droplets mean more light available to plants, thus reducing the risk of a light deficit bottleneck. We, therefore, reduce the risk that we are heating and incurring costs, and the result is little because the plants are underlit.
With unheated facilities, measurable benefits occur on both the cost and revenue side. Without a heating system, we often cannot defend ourselves against excessive foil shading (resulting in less light available to the plants and, therefore, delayed production/lower yield) or the risk of fog (entailing expenses to fight disease).
For both heated and unheated facilities, the use of films with a permanent Anti-Drip effect (or a high-end film with a traditional effect combined with an Anti-Mist effect) makes it possible to find significant savings (directly on heating or indirectly on crop protection products) and additional income, resulting mainly from better light for the crop.
AA: What are the benefits of using double-inflated film?
BK: A large group of horticultural farms use double-inflated film in their facilities.
Depending on the facility, how the crop is grown and how the retrofit work involved in changing from single to double-inflated film is carried out, reported energy savings range from 25-40%.
An interesting study was carried out at the University of Belgrade, where the authors sought to identify the most optimal thickness of air cushion between films in horticultural facilities. This research showed that, considering the thermal conductivity coefficient of the air and the behaviour of the insulating layer between the films, the optimum thickness of the cushion in the coldest months should be in the range of 9-13 cm.
Considering the temperature expansion of polyethylene films, this thickness should be significantly greater in summer (after all, the most significant savings are at the most remarkable temperature differences).
Interestingly, an air cushion of less than 9 cm reduced its insulating efficiency more or less proportionally. In comparison, an air cushion of more than 13 cm reduced its insulating efficiency partly exponentially, especially with significant temperature differences. The explanation is convection inside the cushion: the greater the insulating width, the more 'room' the air has to start moving inside the cushion, and the calmed system is turned into a dynamic one.
For obvious reasons, the values given should be considered approximate. After all, different cushion thicknesses can be achieved with a pillow width of 9 m (e.g. the roof and ventilator section in a multi span greenhouse) and completely different with a pillow width of 3 m (e.g. the section under the ventilator in a foil block). However, it is worth noting the general rule: more does not necessarily mean better. Double-inflated film is worth mentioning one risk involving additional light loss. For very early and, at the same time, light-sensitive crops, an extra layer of film can mean a slowdown in growth.
It is, therefore, very important to use products with the highest possible light transmission as double-inflated film.
Manufacturers and distributors should indicate to the greenhouse owner such a combination of materials, the combination of which allows them to achieve the best results. It is worth asking for appropriate certificates confirming the declared parameters; unfortunately, the practice in this respect in Poland is far from ideal.
AA: Are the differences significant?
BK: You can see how significant the differences in the amount of light can be by looking at the following combinations:
*two 7-layer films of the highest quality, with a certified transmission of 92%: the total amount of light reaching the plant is about 85%;
*two medium-quality films, with a transmission of 88-89%: the total amount of light is 77-79% (loss of 6-8% light compared to the highest-quality film).
Considering the above, covering an object with a double film in our climate is one of the best ways to find energy and financial savings.
In addition, we get one of the warmest facilities for horticultural production if we use EVA films.
With heated facilities, we reduce energy costs in a very significant way by reducing energy losses.
In addition, we make much more efficient use of our heating costs - a much warmer facility shortens the heating period. On the other hand, in the case of unheated objects, the significant benefits of double-film roofing are mainly on the revenue side.
Double film means much more stable climatic conditions inside the building. The differences can be so significant compared to a single film that they determine the profitability of the crop production. In addition, a much warmer facility reduces the risk of frost (and therefore the cost of potential losses). It can lead to a reduction in the price of plant protection products (caused by an overall better plant condition).
AA: What would you advise farmers who are facing the choice of a new film for their tunnels?
BK: The current period is a time of difficult decision-making. The massive increase in production costs and market volatility make it increasingly necessary to analyse every aspect of running a horticultural farm in depth.
It is necessary to move away from simple expenditure analysis and start considering costs over the longer term. Above all, one must be aware that choosing to spend more today could mean much less cost tomorrow. Some energy-saving solutions involve significant investments.
A large proportion of horticultural businesses may not opt for these under uncertain conditions. However, decisions regarding the choice of film or installation technology and the possibility of using modern materials and technical solutions mean relatively small additional costs. They can contribute significantly to maintaining or improving profitability in the long term.
Thank you for the interview
Aleksandra Andrzejewska
