The controller has a 32-bit ARM processor that can be interfaced and simultaneously communicate with 8.0 driver boards via Ethernet. The control signals can be generated by the crop models algorithms coded into the processor or cloud-based streaming systems. At the same time, environmental sensors can collect measurements, store data on an SD card, and transmit data directly to a web server or via wireless communication to a gateway using LoRa modulation. A detailed description of this framework for greenhouse tomatoes is explained in Rezvani et al.. Some of the specific features of the IoT automation system shown in Fig. 13 include simultaneous switching control of up to 33 actuators from any location, multiple and independent voltage lines for DC and AC actuators, open-source programming, and waterproof IP66 enclosure, and external charging circuits. The wireless sensor nodes and the controller boards are shown in Fig. 13 were used with a conventional PID temperature controller to maintain an experimental hydroponic greenhouse. The air temperature responses were collected at ten samples per minute using LoRa sensors with different sensor probes and plotted on the IoT dashboard, as shown in Fig. 14. ach data point was assigned a unique ID that represented the collection time and location and stored on a private cloud database with a secured API key address to be accessed and used by the IoT controller as the feedback of the control algorithm. Several data collection samples were carried out, and all samples were analyzed to ensure the reliability of IoT control. Results showed that no single data point was lost during the tests as long as the WiFi network was available.

The system’s reliability can be significantly improved by increasing the number of WiFi access points so that the controller can switch between the networks. The response of the controller demonstrates the robust performance of the IoT-based automation with a high spatiotemporal resolution and excellent stability in data transfer at 10.0 readings per minute within 1.0 km distance from the wireless controller, hydroponic net pots which can be considered a reliable approach for adjusting growth parameters for CEFP. It can be concluded that the integration of wireless communication, distributed data analysis, and a web-based data monitoring dashboard provides flexible automation and an online assessment tool to investigate the effects of structure design, covering materials, cooling and heating techniques, and growing seasons on the optimality and comfortability of growth parameters for CEFP. The limitation in wireless communication due to the high-density plants and other disturbances can be overcome by increasing the number of the sensors and repeater nodes which reduces the loss of connection and data interruptions even in mesh applications or using antennas with cable for higher positions, higher mesh density, multiple gateway nodes, and higher output power. Hydroponic fodder production is likely to play an increasingly important role in livestock production with the current dwindling supply of grazing land, water, and increased food demand. As fodder could be grown hydroponically in CE, most grains are still not feasible to produce indoors. So, the effective use of agricultural land for grain production would be beneficial in fighting against world malnutrition problems by transforming the fodder and pasturing fields into crop production fields. Also, controlled environment hydroponic fodder farmings could contribute to climate mitigation objectives if complemented with effective energy management and land use policies.

A study showed that incorporating hydroponic barley production in CE can reduce GHG emissions with increased seed-to-fodder output, efficient energy management, and the integration of renewable energy for operation. Fig. 15 shows the sensitivity in GHG emissions with the seed-to-fodder output ratio in a hydroponic fodder production system powered by renewable energy in Canada. The results indicate hydroponic fodder production outperforms conventional open-field production only when the seed-to-fodder ratio is more than 5.5.In general, the feasibility of fodder production in CE is being challenged by some criticism regarding nutritional benefits for livestock, high capital and operational costs, and social difficulties for using grains as animal feed. In the coming decades, food demand is expected to grow rapidly, while agricultural land is being degraded as a consequence of climate change and unsustainable agricultural practices . Greenhouses are used as one of the strategies to mitigate these problems and their area has been growing rapidly. Greenhouses can have high food productivity in limited space, while being less dependent on environmental conditions. Greenhouses with soilless forms of agriculture, also known as hydroponics, have significantly higher efficiencies for water and nutrients use than conventional agriculture. However, soilless forms of agriculture generally rely heavily on synthetic fertilizers, as organic fertilizers still result in significantly lower yields . For the creation of nitrogen fertilizers fossil fuels are used, while phosphate fertilizers are manufactured from non-renewable sources. Their discharge into the environment causes severe damage to aquatic ecosystems through eutrophication . Consequently, the production of synthetic fertilizers has significant negative effects on the environment.

The introduction of the aquaponics concept, the combined cultivation of fish and crops, is an attempt to reduce the use of synthetic fertilizer and to further improve nutrient and water use efficiencies of hydroponics . Nowadays, an industrial aquaponics system consists of a recirculating aquaculture system and hydroponic system , with water flowing between the two to exchange nutrients . In the RAS, fish are reared in tanks and a screen filter and nitrification reactor are used to remove solids and to convert ammonia to nitrates, respectively. In permanently coupled aquaponics , the nutrient-rich RAS water is used as a hydroponic solution to grow plants in the HPS and the water from the HPS is returned to the RAS after the plants have taken up a fraction of the nutrients . However, this single-loop system inherently creates sub-optimal conditions, as the plants and fish have different needs for nutrient levels and pH . To eliminate this problem, an on-demand coupled aquaponics system has been developed , where the water flow from the HPS to the RAS becomes optional, allowing more control over both systems . To increase the nutrient use efficiency in on-demand coupled aquaponics systems a remineralization loop was added, in which an anaerobic digester converts the filtered-out solids from the RAS into plant-available nutrients for the HPS, leading to a multi-loop system . In addition to the remineralization loop, a desalination loop, in which nutrients in the discharged RAS water are concentrated and clean water returned to the RAS through reverse osmosis filtration, was also proposed . The concentrated nutrient solution is subsequently transferred to the hydroponic solution. Besides concerns regarding fertilization, highly productive greenhouses are also known for their high energy consumption. In northern latitudes, the majority of greenhouse energy demand is for heating, resulting in a heavy reliance on fossil fuel . For a standard greenhouse in the Netherlands, Kempkes et al. reported that heating can account for up to 90% of the energy use and 25% of the production costs. Reducing the energy consumption for heating is crucial for reducing greenhouse gas emissions and reducing the dependency on fossil fuels.

In the past decades, there has been plenty of innovation and research on saving energy in greenhouses, driven by environmental regulation and cost reduction . To achieve this, a mix of technologies and operational strategies can be used. For example, dehumidification and heat recovery can be used to control the indoor climate, which decreases the need for ventilation . In terms of operational strategies, changes in the temperature and humidity ranges can decrease the need for heating in a dehumidification process . With increasing latitude, seasonal differences generally increase, while average temperatures decrease. In addition to an increase in heating demand, Goddek & Körner showed in a modeling study that this increased seasonal variation makes an aquaponics system more difficult to manage and less efficient. This is because the main water transfer in an on-demand coupled aquaponics system is driven by transpiration, which in turn is mainly related to global radiation. Thus, greenhouses at higher latitudes have much potential for energy use reduction. Several studies already cover the effects that different energy-saving strategies have on crop production . However, its effect on nutrient use efficiencies in aquaponics systems is largely unknown and complex due to dynamics and interactions in such coupled systems. This study aims to investigate and explore what effects various operational strategies and design parameters have on the optimal design and operation of an on-demand multi-loop aquaponics system in a cold and varied climate. For this, a model was used to simulate a multi-loop aquaponics system, growing lettuce and Nile tilapia , using weather data from the Netherlands. Subsequently, several design and operation parameters were varied to find their influence on water and nutrient use efficiency,energy use and the growing environment. Special interest was paid to the variation in crop transpiration, which was used for the comparison of different scenarios.

To better support the theory proposed in this article, the production of tomatoes was explored, as well. Additionally, two new buffering strategies were investigated, intending to further increase the performance of the aquaponics system in varied climates.The aquaponics system in this study mostly follows the design of Dijkgraaf et al. and consists of three compartments: the recirculating aquaculture system , anaerobic digester and hydroponic system.These elements are shown in Figure 1 and explained in the following sub-sections.The main component of a RAS is a set of fish tanks, blueberry grow pot where fish is produced in staggered cycles at high densities. Nutrients are added to the system in the form of fish feed, which is either consumed or wasted by the fish. From the consumed fraction, nutrients partly contribute to fish growth and leave the system with the harvest of fish, whereas the other part is excreted as feces and urea. The urea dissolves in the water, increasing the nitrogen concentration, while the solids remain in suspension. In addition to the fish tanks a RAS has a treatment train with a drum filter to remove these solids and a moving bed bioreactor to prevent the build-up of ammonia, as shown by Goddek & Keesman . The discharge of RAS water is mainly directed to the HPS, while a small fraction of the water remains with the separated solids.The solids form a sludge that is transported to the AD. To maintain a constant water volume in the RAS, fresh water from tap, rain or groundwater is added to compensate for the discharge. The dilution of RAS water takes place when the ammonia or nitrate concentration surpasses a prespecified limit.To prevent, to a large extent, the loss of nutrients in the fish sludge,it is treated in an anaerobic digester system.This system consists of two sequential UASB reactors with different pH levels for optimal performance, as in Goddek & Körner . The chemical oxygen demand present in the fish sludge is digested in the first reactor, resulting in the production of biogas which can be utilized for heating the reactor, fish tanks or greenhouse. In the second reactor, as a result of the remineralization process, a large fraction of the nutrients in the fish sludge becomes soluble . Combined with the direct RAS discharge, this highly concentrated effluent flow makes up the nutrient source from fish to crops. The whole AD system is explained in great detail by Delaide et al..The hydroponic system is similar to conventional soilless, closed-loop cultivation systems.Several irrigation techniques can be used, such as Deep Water Culture,Nutrient Film Technique or drip irrigation, each with its own range of suitable crops.The advantages and disadvantages of many soilless systems are described by Maucieri et al. . For this study, lettuce is grown, either in DWC or NFT. The main difference concerning this study is the overall water volume of the nutrient solution,with DWC having a far greater volume than NFT.For both technologies the nutrients not taken up by the plant is returned to the sump for recirculation, forming closed loop systems. In this study, the preparation of the nutrient solution is different from standard hydroponics, as it is largely based on a mix of RAS water and digestor effluent, topped up with synthetic fertilizers.