Sustainable agriculture: 2 examples of drying with renewable energy

The drying process is essential for a sustainable agriculture. This process aims to reduce the moisture content of products grown after harvest. Reducing moisture is crucial to prevent the development of fungi, molds and bacteria, and to ensure the quality and durability of agricultural products.

Crops are harvested at the right time of maturity to ensure their quality. After harvesting, a selection is made to separate good quality products from those that may be damaged and a cleaning to remove impurities.

Some crops, such as grains, herbs, and fruits, can be dried directly in the sun. They are placed in a location such as fields, patios, shelves or special trays, depending on the type of crop and the climatic conditions of the area. They are then spread out in a thin layer to allow uniform exposure. During the drying process, it is common to turn crops over to ensure a uniform dry.

Products requiring precise temperature and humidity control use mechanical dryers. These can be fluid bed dryers, drum dryers, or convection dryers.

During the drying process, the product moisture is continuously monitored to determine when it has reached the desired moisture level. After drying, some products may require a cooling period before being stored. Dry products are stored in suitable conditions to maintain their quality until use or sale. This includes controlling temperature, humidity and protecting against pests.

The drying method may vary according to the type of crop, the region and the resources available. The main objective is to reduce humidity in an efficient and controlled manner to ensure the quality and food safety of agricultural products.

"Dried" fruits (almonds, hazelnuts, nuts, etc.) are natural products that have gone through a process of dehydration to eliminate much of their water content.

Other fruits (figs, grapes, dates, plums, etc.) are also dried to prolong their shelf life by removing most of their water content. Not all fruits are suitable for drying, as some may not retain their organoleptic properties (taste, texture, aroma) well after the process.

Drying technologies for sustainable agriculture

Various drying technologies exist in sustainable agriculture. The choice of technology depends on the type of crop, the scale of production, available resources and environmental conditions.

Some common drying technologies are:

According to the Airflow:

• Solar Beds: Crops spread on thin beds and are directly exposed to solar radiation. This method is simple and widely used, but depends on weather conditions.
• Convection Dryers: Hot air circulates through crops in a convection current system, eliminating moisture:
• Tray Dryers: Crops are placed on overlapping trays and exposed to hot air currents.
• Solar Tunnels: Closed structures that capture solar radiation and create controlled temperature and humidity conditions for drying.
• Drying using air turbines:
  • Ventilation Drying: High speed air is used to remove moisture from crops. Additional heating systems may be required.
  • Fluidized Bed Dryers: Crops are placed on a bed of solid particles through which hot air is passed to remove moisture.
  • Drum Dryers: Crops are placed in a rotating drum and exposed to hot air. The rotation of the drum helps ensure uniform drying.

According to the Heat Source:

• Solar Air Dryers:
  • Direct: The air is heated directly by solar radiation and directed to crops.
  • Indirect: The air is heated by solar collectors and then directed to crops.
• Drying with a heat pump. Heat pump drying is an innovative technology that improves energy efficiency by using ambient heat to dry agricultural products. It works by extracting heat from the surrounding air and transferring it to the drying chamber. Heat pumps consume less energy compared to conventional drying methods, which reduces operating costs.
• Microwave Dryers: Exposure to microwaves helps heat crops and remove moisture. This method can be faster than others and is useful for certain products.
• Hybrid Dryers: Combines various drying technologies to improve drying efficiency and quality.

Source: academia- prodel

The choice of drying technology will depend on various factors, such as the amount and type of crop, the availability of energy, the local climate and the infrastructure available.

Drying in sustainable agriculture refers to the application of practices and technologies that seek to reduce environmental impact and improve efficiency in the drying process of agricultural products. To do this, we must minimize the consumption of resources such as energy and water, incorporate renewable energy sources, select low-emission drying technologies, and take advantage of the heat generated during the drying process for better energy efficiency.

Case 1: Solar convection tunnels for producers in Ecuador.

Solar convection tunnels are a practical and sustainable alternative for thousands of cocoa, coffee and herbs producers in Ecuador and other countries. Their installation allows the adoption of solutions based on clean and low-cost energy in collection centers and small-scale farmers' farms in rural areas.

Today, many farms continue to use fossil fuels such as propane gas, diesel or coal, being costly and highly polluting practices. Its use favors a production of lower quality, with an uneven drying and in some cases contaminated with toxic waste. On the other hand, the coverage by tendales with plastics constitutes an inefficient drying model that favors the development of fungi and due to its large size producers are forced to work inside these covers under conditions not favorable to health.

In recent years, numerous proposals have been developed for the development of dryers alternative to natural drying, many with advantages but mostly with disadvantages in terms of efficiency and applicability.

whereas solar energy varies according to time, time and radiation intensity, this project proposes an innovative solution with a simple solar dryer and an auxiliary energy source to store energy and make the drying process uniform, efficient and constant even during the night.

Solar convection tunnels use technologies at hand in developing countries and at reasonable cost, making it possible to disseminate this technology to thousands of producers locally and subsequently to other regions and countries. In Ecuador, it is possible to reach more than 5,000 local producers for adoption.

Technically the dryers are composed of several elements, including:

• A drying tunnel with a platform on which the raw material is deposited and a thermal plastic cover mounted on reed arches to achieve a low-height dome shape. The structure is constructed with cane in order to take advantage of the insulating properties of this material.
• A convection chamber, a cube-shaped structure with a raised platform and a black surface covered with transparent glass or polycarbonate, has the function of heating air in a proportion equal to the air volume of the drying tunnel to accelerate the heating process evenly.
• A ventilation system, composed of efficient fans and a thermostat that controls the temperature for the activation of the fans.
• A power supply with a photovoltaic panel and a 12 volt battery charged to have electricity permanently.
• Additionally a circulation system can be installed, with a storage tank and a pump to recirculate hot water through the drying platform. The system is closed and takes advantage of the convection chamber.

The drying chamber is tunnel type and is adapted so that the hot air coming from the convection chamber circulates continuously and efficiently, thus reducing the moisture of the raw material.

The drying tunnel acts as a device that accumulates heat by increasing the ambient temperature by about 10 to 15ºC, resulting in a 10-22% reduction of the original relative humidity. The heat conversion efficiency is optimal and ranges from 50 to 75 %. 

Case 2: Prototype hybrid system for cassava drying

The drying technique used for the dehydration of fruits, grains, vegetables, meat and fish has been used for hundreds of years in a poorly technified way, and the best known method is open solar drying or natural solar dryer. 

This drying has limitations such as lack of control over the drying process, which can cause excessive drying, loss of germination grains and nutritional changes; lack of uniformity of drying, contamination by fungi, bacteria, rodents, birds or insects, weather conditions and because the speed of drying is slow and the products cannot be dried overnight must be collected and redistributed, causing loss of quality and price of the product 

This project proposes a hybrid system of heating air by biomass combustion and solar radiation, for the drying of cassava. This increases the speed of drying by the use of complementary thermal sources and continuity also through the use of paraffin (phase change material) as thermal energy storage.

The prototype was built in the city of Montería (Colombia) combining a solar collector with paraffin and a rice husk biomass burner, to take advantage of the waste from the rice industry. Rice husk obviously has environmental advantages over other fuels, particularly in reducing sulphur and particulate emissions, and a neutral CO2 cycle, without contributing to the greenhouse effect.

• The solar collector was designed to heat air to dry 5 kg of cassava, with 33 cl aluminum cans filled with white paraffin, in order to increase the efficiency of the collectors.
• The biomass burner had a heat exchanger with an exchange area to keep the drying temperature at 70°C.
• A data acquisition system captures temperatures in the collectors and the dryer, as well as humidity relative to the dryer’s inlet and outlet. 

With a drying temperature of 70 °, a yucca with initial humidity of 62% approx. During the first 5 hours, the drying speed was constant and decreased to 15 hrs. where the surface film of the liquid has completely evaporated.

The construction of the hybrid system used a centrifugal fan of 0.5 Kw that provides drying air with the engine to meet a maximum air flow in the system of 3200 m3/h. The fan was attached to the collectors through a PVC pipe.

Each collector was built with a box of oak painted black with a thickness of 2 cm and area of just over 1 m2, an aluminum absorbing plate of 0.7 mm thick black color (to absorb the most solar radiation) and a transparent glass cover 77x137.1 mm and 4 mm thick. 

Inside each of the solar collectors are located 33 cl aluminum cans filled with white paraffin.