Hybrid greenhouses: Combining traditional greenhouse and vertical farming

The HybriGrowth project aims to demonstrate that this concept is a possibility that can provide sustainable, energy-efficient operation and increased productivity.

Global food crisis and self-sufficiency

World population growth and other factors require that food production must increase significantly by 2050. This outlook is  reinforced by the ongoing global food crisis affecting the world. Climate change, extreme weather, conflicts, and rising prices for food, fuel, and fertilisers have led to food shortages and hunger in some regions.

The COVID-19 pandemic and the war in Ukraine have highlighted the importance of food security and self-sufficiency worldwide. In Norway, these events have made it clear that improving the nation’s self-sufficiency rate is critical, with a national goal of raising it to 50%. To strengthen food security, the country must boost the production of cereals and crops suited to its growing conditions, alongside increasing the cultivation of fruits and vegetables. 

Traditional greenhouses

Expanding greenhouse production of fruit and vegetables can make a significant contribution to Norway’s self-sufficiency. Greenhouses provide both optimal growing conditions and protection against adverse weather and pests, which is very beneficial for ensuring a stable supply of locally produced food.

Traditional greenhouses typically feature transparent roofs and walls made of glass or plastic, where plants are grown in a horizontal layer – either in beds at ground level or on raised tables and benches. This design provides natural heat and light to the plants, enabling continuous production in a carefully controlled environment.

Modern greenhouses, also known as glasshouses, first emerged in Europe during the Renaissance and evolved in the 18th and 19th centuries. Initially, they were used to cultivate rare and exotic fruits like oranges and pineapples for the aristocracy, symbolising luxury and status at the time. Over the centuries, these structures became more advanced, eventually  laying the foundation for the design of the modern greenhouses we know today.

Norway’s cold and long winters make greenhouses particularly useful for cultivating crops that do not tolerate harsh climates well. They protect plants from frost and provide optimal light conditions during the dark season. Greenhouses also allow for temperature and humidity control, enabling extended growing seasons or production beyond the usual season. With favorable growing conditions, it is also possible to grow exotic vegetables that do not naturally occur in Norway, like eggplant, squash and okra.

However, optimal operation of greenhouses is generally energy-intensive, with energy costs accounting for up to 50% of the total production costs. Traditional greenhouses in Norway rely heavily on heating and lighting, typically powered by a combination of electricity and fossil fuels. Alternative heating options include district heating, waste heat, biogas, wood chips, pellets, and heat pumps.

Fossil fuels, such as heating oil, natural gas and propane, have long been used in Norway’s greenhouse industry. However, heating oil is being phased out and banned for standard use as of January 1^st, 2025. Still, the sector faces the challenge of transitioning away from fossil energy sources. The Norwegian Horticultural Federation’s Energy and Climate Strategy for 2021-2030 aims to make the greenhouse sector nearly 100% renewable by 2030.

One challenge in this transition is plants’ dependence on CO₂ for photosynthesis, a crucial element for their growth. Studies show that growth can increase by up to 50% with additional CO₂ supplied to the greenhouse environment, depending on factors like light levels and other growth conditions. Traditionally, natural gas and propane have been used for heating but also for supplying CO₂ to greenhouses. The flue gases from these fuels mainly contain CO₂ and water vapor, allowing the CO₂ to be directed straight into the greenhouse without the need for cleaning.

With the shift to renewable energy sources, growers must find alternative solutions to maintain CO₂ levels in greenhouses. One option is purchasing CO₂, although this can be relatively expensive. Another approach is to extract CO₂ directly from the air using advanced absorption technology.

A key element of CO₂ enrichment is maintaining elevated CO₂ levels for as long as possible without frequent ventilation. Roof ventilation is commonly used to remove humidity or cool greenhouses when temperatures rise, but this practice leads to unnecessary losses of both CO₂ and energy.

A more efficient way to manage the greenhouse climate is by installing an integrated heat pump system. Heat pumps are energy-efficient systems that provide both heating and cooling as needed, while also helping to regulate both humidity and CO₂ levels effectively. This reduces the need for ventilation through roof vents, which in turn minimises energy and CO₂ losses.

Heat pumps can utilise heat from various sources like air, water, geothermal energy, or solar energy. The choice of heat source depends on availability and local conditions, with installation costs being a key factor. In greenhouses, heat pumps combined with thermal energy storage can be an excellent solution. Thermal energy storage can store surplus heat, such as from summer to winter through seasonal storage in boreholes, or on a smaller scale in a heat storage tank that takes advantage of temperature variations between day and night. The type of thermal energy storage chosen depends on local conditions and cost-effectiveness for the specific greenhouse.

Vertical farming

In recent years, vertical farming has emerged as an alternative to traditional greenhouses and conventional agriculture, both globally and in Norway. This method involves growing plants in vertical layers, using LED grow lights instead of natural sunlight, and relying on advanced technologies to create optimal growing conditions.

Vertical farming systems typically stack plants in multiple layers or use other vertical structures. The plants are grown in controlled environments, either in nutrient-rich soil with recycled water or directly in a nutrient solution (hydroponics). Another method, aeroponics, involves plant roots hanging freely and being regularly misted with a nutrient solution.

The benefits of vertical farming include rapid growth, low water usage, and minimal use of pesticides. The method is also space-efficient and allows for year-round production, protected from extreme weather and independent of sunlight. The space efficiency enables optimal use of available land, which is particularly beneficial in urban areas where space is limited and property costs are high. As a result, transport distances for fresh products to consumers are shorter, reducing both costs and the carbon footprint of transportation.

The disadvantages of vertical farming include its high costs and energy-intensive nature compared to traditional greenhouse production. Greenhouses naturally benefit from freely available sunlight, supplemented by additional grow lights, making them efficient collectors of free solar energy. Although LED lamps are highly efficient, their use in vertical farming still leads to significant energy consumption and heat generation. This excess heat must be managed through efficient cooling and ventilation systems to maintain an optimal growing environment for the plants. Consequently, the energy consumption per plant in vertical farming can be substantial, potentially resulting in a higher carbon footprint, depending on whether the electricity used comes from renewable or fossil sources.

Hybrid greenhouses

By integrating the advantages of both traditional and vertical greenhouse farming, can a combination of these two methods enhance efficiency and reduce the overall carbon footprint?

The purpose of the HybriGrowth project is to demonstrate that hybrid greenhouses can  provide energy-efficient operation, increased productivity, and a reduced carbon footprint through less reliance on fossil fuels for heating.

Two greenhouses in Norway, Viken Gartneri and Snarum Gartneri, have recently installed vertical systems alongside traditional greenhouse production to develop this symbiosis and increase productivity and profitability. These facilities serve as case studies for HybriGrowth.

A key challenge is efficiently utilising the heat generated by the LED lights (light-emitting diodes) in the vertical section to support heating in the traditional greenhouse, which has larger areas and significant heating demands. This requires both effective cooling of the vertical section and an optimal heat recovery system, managed through an integrated system that balances the energy flows.

Another important factor is ensuring optimal growing conditions for the plants during both the early seedling phase and the later main growth phase, when growth accelerates exponentially. In the hybrid concept, the young plants grow for a few weeks in the vertical system before being moved to the traditional greenhouse for the main growth and final maturation. This approach offers the advantage of a carefully controlled growing environment when the plants are young and most vulnerable, while the traditional section provides more space and access to both artificial and natural light. Utilising sunlight is both energy-efficient and highly beneficial for the plants, as they can tolerate more solar energy than artificial light without damage. The traditional greenhouse also provides more air and better air circulation, which can be challenging and costly to achieve in a vertical system.

The hybrid greenhouses at Viken Gartneri and Snarum Gartneri are unique in Norway, and the concept has been tested and analysed to a limited extent around the world. In the United States, the hybrid concept has been developed for decentralised lettuce production of lettuce in states like California, Georgia, and Montana, resulting in a significant increase in the number of harvests and higher profitability. Similarly, in China, a new demonstration facility for combined vertical and horizontal lettuce cultivation was opened in 2023.

The main challenge in hybrid greenhouses is achieving a balanced and efficient use of the surplus heat from the vertical growth sections. This heat is only available when the LED lights are on. When the lights are off and the plants in the vertical system are resting, no surplus heat is generated for transfer to the traditional section. During these periods, alternative heating sources, such as gas or electric boilers, may be needed to meet the heating demand in the greenhouse.

At other times, the surplus heat exceeds the heat demand and must be released to prevent excessive temperatures in both the vertical and traditional sections. The traditional greenhouse also acts as an effective solar energy collector, often eliminating the need for additional heating, especially on hot summer days. However, venting the warm air leads to the loss of valuable CO₂, halting CO₂ enrichment for the plants.

A possible solution to these challenges is thermal energy storage. Surplus heat from the vertical system, or even from the traditional greenhouse itself when it becomes too hot, can be stored for later use. A practical approach is to install buffer tanks or heat storage tanks to store this surplus heat. Such water tanks provide a flexible solution and allow the heat to be used in the traditional section when the LED lights in the vertical section are turned off, that is, when no surplus heat is generated. An effective method to achieve this is through an integrated heat pump system that cools and extracts heat from the vertical section, regulates the temperature in the traditional section as needed, and delivers or retrieves heat from the buffer tanks to optimise the heat distribution in the greenhouse.

In HybriGrowth, the operational systems for the hybrid greenhouses in Norway will be thoroughly investigated to enhance energy efficiency and optimise operations. This includes analysing and modelling operational data to maximise the utilisation of surplus heat from the vertical growth sections, as well as designing and optimising heat pumps and buffer tanks. The energy balance in the greenhouses will also be assessed by examining the replacement of HPS lamps (high-pressure sodium) with LED lights in the traditional greenhouse sections. Since HPS lamps produce significantly more heat than LED lights, it is important to study how this replacement affects the heating and cooling demands in these sections.

The results from HybriGrowth will provide general guidelines for more energy-efficient and climate-friendly greenhouse operations in colder climates like in Norway, as well as in other countries or areas with similar conditions. The project will assess whether the hybrid greenhouse concept can be implemented on a broader scale in Norwegian horticulture and evaluate its overall potential. The benefits include increased competitiveness for Norwegian growers through reduced energy consumption, higher productivity, and improved profitability. This also entails lower CO₂ emissions by reducing the use of fossil fuels for heating, which aligns with the horticulture sector’s goal of becoming nearly 100% emission-free by 2030.

By contributing to sustainable food production in colder climates, hybrid greenhouses can play an important role in addressing climate change, self-sufficiency, and global food security.

HybriGrowth, a collaborative project aimed at addressing challenges in society and industry, is supported by the Research Funds for Agriculture and Food Industry (FFL/JA) and by Grofondet, as well as with administrative support from the Research Council of Norway.  The project is coordinated and led by SINTEF Energy Research, with SINTEF Ocean, SINTEF Industry, and the Norwegian Horticultural Federation as research partners, and Viken Gartneri and Snarum Gartneri as industry partners. Additionally, Bama and Gartnerhallen are part of an advisory group and play a key  role in communicating with the horticultural industry.

Gemini Centre on Vertical Farming

In addition to HybriGrowth, SINTEF is also engaged in research on indoor vertical farming through the VertiGrow Centre. Established in collaboration with NTNU and NTNU Social Research AS, the Centre unites expertise in plant science, technology, and sustainable food production.

VertiGrow’s mission is to develop sustainable vertical farming systems, integrating biology and technology to optimise production processes and drive agricultural innovation in Norway.