CO2 efficient vegetable production in greenhouses with Air-to-Air heat pumps

Dr. Agris Auce, University of Latvia, Institute of Chemical Physics

It is believed that the consumption of fresh vegetables promotes health and rarely anyone argues against it. Unless, when discussing with sworn vegans and vegetarians, it can be argued that, unlike animals, vegetables cannot escape from attackers who try to eat them, and therefore they protect themselves chemically - by developing various poisons against their attackers. Those vegetables, to which people have become accustomed during the course of evolution, are considered the basis of a healthy diet. Fresh vegetables are also an important source of vitamins and other active substances, which are also important for our health.

In the Latvian climate, to ensure fresh vegetables all year round, they must either be imported or grown in covered, heated areas, popularly called greenhouses. There is a reasonable opinion that locally grown vegetables are better because they require less transportation and do not need to be stored for as long as those imported from far away. However, growing vegetables in greenhouses requires significant energy consumption and, among other things, also creates significant CO2 emissions, which all countries, including Latvia, must limit in order to reduce the risk of climate change.

The Latvian University of Agriculture, together with the University of Latvia and one of the most successful and largest tomato producers in covered areas, SIA Rītausma, has implemented a research project supported by the Latvian Agricultural Society and the European Union (project 17-00-A01620-000013, Development of heat energy efficiency improvement technology for extending the vegetation period of covered areas) to develop technologies to reduce greenhouse gas emissions in covered areas using an Air-to-Air heat pump.

As part of the project “Development of heat energy efficiency improvement technology for extending the vegetation period of covered areas”, a small experimental greenhouse was created. It was created by separating a small corner of 34m2 from the large production greenhouse, installing a heating system based on Air-to-Air heat pumps, and installing a remotely controlled sensor system to continuously monitor the 3D temperature distribution in the greenhouse and compare it with the production control greenhouse. LLU specialists, together with SIA Rītausma, monitored the growth of tomatoes in the experimentally heated greenhouse.

When heating a greenhouse with an Air-to-Air heat pump and comparing the electricity consumption with the electricity consumption of a control greenhouse, where the control greenhouse was heated with gas, it was found that in terms of energy consumption, heat pumps consumed approximately 8 times less energy than gas heating. Taking into account the fact that electricity, especially after all OIK components and transmission costs, is much more expensive than gas, the net economic effect is not large - in the 2020 season, which had a warm winter and cool spring - therefore a very suitable regime for the efficiency of heat pumps, the obtained economic effect was approximately 30% (costs of 4.11 EUR/m2 for a heat pump compared to 5.94 EUR/m2 for gas).

However, the reduction in CO2 emissions is very significant. Here, the reduction is 4 times if we assume that electricity is generated only in the cogeneration mode of the Riga TEC, or 10 times if we assume that over 50% of electricity is generated in Finnish or Russian NPPs or hydroelectric power plants.

Thus, the project demonstrated that using modern technologies – Air-to-Air heat pumps – it is possible to provide Latvian residents with fresh vegetables and herbs all year round and at the same time, almost tenfold, reduce greenhouse gas emissions in greenhouse farming. This is very good news – both for the producers of these healthy products and for the Latvian economy as a whole, as it allows a large agricultural sector to significantly reduce CO2 emissions.

During the project, possible complications that may arise when using Air-to-Air heat pumps for greenhouse heating were also analyzed. The complications are related to maintaining the exact heat regime and preventing air temperature fluctuations caused by the cyclical nature of heat pump operation, because typically, both air conditioners and Air-to-Air heat pumps operate in a direct or modified on/off mode, but plants prefer a steady temperature regime. Attention should also be paid to ensuring that heat pumps maintain the temperature with an external temperature sensor that regulates the temperature in the greenhouse, but not the slightly cheaper internal temperature sensor that regulates the operation of the heat pump by measuring the temperature of the air entering the heat pump. The resulting temperature differences between these regimes can be small – 0.5-2 degrees, but monitoring of tomato growth showed that even such seemingly small deviations from the optimal plant growth temperature have a strong impact on the speed of tomato development. On the other hand, if heat pumps are precisely calibrated, they maintain a much more accurate temperature than conventional greenhouse heating with hot water (radiators). This is due to the much smaller thermal inertia of an Air-to-Air heat pump compared to a water heating system. When weather conditions change – when the Sun shines or clouds appear, the inflow of solar energy to the greenhouse changes rapidly. An Air-to-Air heat pump can react to these temperature changes in the greenhouse caused by solar radiation very quickly – within a minute and reduce heating or start it. On the other hand, the thermal inertia of a hot water heating system is much greater – these are tens of minutes, during which the greenhouse temperature can exceed the optimum by about a degree if the Sun suddenly shines, or fall below the optimum if the Sun suddenly disappears.

It is important to note another aspect: Air-to-Air heat pumps can help provide Latvia with fresh vegetables and herbs with a low CO2 emission “footprint”. The construction of an air-to-air heat pump heating system requires significantly lower capital investments than traditional heating with hot water. The air-to-air heat pump system is modular – the greenhouse complex can be increased by connecting new heat pumps, each with a relatively small capacity. If the greenhouse is used not all year round, but to extend the vegetation season in spring and autumn, then the advantages are even greater – an even better efficiency factor advantage, less installed heat capacity per square meter of greenhouse is required, because the installed heat capacity is determined by the lowest temperatures on winter nights, when maximum capacity is needed. It should also be noted that no special preparation of greenhouse equipment for the winter frost period is required, because heat pumps do not freeze in the frost. Among the shortcomings, it is important to note that guarantees for uninterrupted electricity supply are needed. This means either two independent electricity supply lines for large producers, or their own generators for smaller producers. However, heat pump heating solutions could be particularly advantageous for producers who have their own cogeneration capacities - this provides significantly cheaper electricity prices without the existing additional payments (such as OIK, transmission payments, etc.), and also does not require two electricity supply lines, and the heat generated in cogeneration can also be utilized. So-called "renewable" energy sources such as wind and solar will not be suitable for local electricity generation for heating covered areas, because they have absolutely no capacity stability and are not suitable for systems where a guaranteed electricity supply is required.

All these technological and economic advantages of Air-to-Air heat pumps make them particularly suitable for vegetable and greens producers who want to build new production capacities with the lowest possible capital investment and who want the possibility of flexibly increasing their covered area production capacities over time. Also for small producers who want to obtain products from their existing unheated greenhouse for a longer period, this will be a very suitable solution, as it requires relatively small investments and is easy to use.

Overall, the study has shown that Air-to-Air heat pump technology in covered areas makes it possible to economically provide Latvian residents with healthy, locally produced fresh vegetables and herbs, and in doing so, significantly reducing CO2 emissions from vegetable cultivation in heated areas by 4 to 10 times.

Collaboration project17-00-A01620-000013 name: Development of thermal energy efficiency improvement technology to extend the vegetation period of covered areas

Cooperation partners: University of Latvia; Latvia University of Agriculture; Rītausma SIA; Baltic Open Solution Center SIA; A.Punduras Consulting Center SIA;

Article published here http://www.vesels.lv/raksti-par-veselibu/cilveks/co2-efektiva-darzenu-razosana-siltumnicas-ar-gaiss-gaiss-siltumsukniem.html