Makerere unveils a Hybrid Solar Dryer (HSD) for perishable agricultural produce

Makerere University researchers in collaboration with pineapple farmers in Kangulumira sub county and a team of experts from Green and Gold Uganda Ltd, Badaye Technologies Ltd and the Ministry of Energy and Mineral Development have designed, constructed and promoted a Hybrid (sensor -controlled dual heat source solar) dryer to ensure consistent drying of large volumes of perishable agricultural produce.

Dr. Simon Savio Kizito in the dryer

The hybrid solar dryer has a loading capacity of 300 kg of fresh and sliced fruits (approximately 450 pineapples each weighing 1 kg on average) per drying batch. A drying rate of 20kg/h and drying time of 10 hr of continuous drying was recorded during dryer testing. Hence the dryer produces 20 kg of quality dried pineapple product per batch within a day.

This technology was unveiled by Makerere reseachers to the different stakeholders at Kisega Horticulture Association based in Kangulumira sub county in Kayunga district on 3rd March 2021.

The Research work leading to the development of the HSD prototype was part of the Mak-RIF COVID-19 Special project titled, “Developing an automatically controlled solar dryer and efficient resource recovery innovations for sustained market responsive fruit production in Uganda”. The estimated cost of the prototype hybrid solar dryer is UGX 25,000,000 approximately $ 6,870 USD.

The research and technology development was spearheaded by   Assoc. Prof. Ahamada Zziwa as the Principal Investigator, Dr. Isa Kabenge, Ms. Filda Aya, and Mr. Sam Cherotich from the Department of Agricultural and Bio Systems Engineering. Other team members are Dr. Simon Savio Kzito from the Department of Forestry, Biodiversity and Tourism and Mr. Amos Kambagambira Tamusuza from the Renewable Energy Department of the Ministry of Energy and Mineral Development. Mr. Henry Kayondo and Mr. Sudo Paul supported the project as research assistants.

The pilot commercial hybrid sensor-controlled dryer can be used to dry any produce for which it is economically viable to dry without significant loss in the nutritional value. This is possible because the recommended drying conditions for the different fruits can be pre-set and maintained in the drying chamber.

Furthermore, a backup heat source (biomass burner) is attached to the drying chamber, to supply indirect heating using distributed hot air especially during cloudy/rain days. This shortens the drying times and improves quality of the final product. Additionally, the drying chamber is optimized with a loading capacity of about 300kg of fresh fruit per drying batch, which is appropriate for commercial enterprises.

Dr. Ahamada Zziwa and the consultant Sudo Paul at the facility

The Principal Investigator, Assoc. Prof. Ahamada Zziwa said the existing solar dyer designs such as the cabinet and tunnel dryers have been successful in concentrating heat onto the drying items but do not provide for controlling drying conditions thereby failed to ensure consistent drying and attainment of recommended nutritional quality of dried products.

Moreover, the existing designs according to the PI only rely on presence of the solar radiation as a single energy source which limits their performance to only day time while drying efficiency is highly affected by unpredictable rain and cloud weather conditions.

Dr. Zziwa also explained that the efficient heat distribution is another shortcoming of existing designs noting that,  in addition to poor heat distribution that affects drying quality and dryable capacity, products directly receive the fierce heat and end up being over baked while those in colder regions tend to ferment.

Consequently, he explained that poor drying affects nutritional and sensory quality of the end products and limits farmers competitive advantage for high market prices.

The PI observed that due to over dependence on solar radiation energy, the existing dryers have small drying capacities per batch thus limiting wide adoption by commercial dried food entrepreneurs.

 Dr. Zziwa said the Mak hybrid dryer comes with distinctive innovative features. “It is powered by two energy sources namely solar radiation and biomass heat energy enabling uninterrupted drying even during times of low solar radiation. Integration of heat distribution fans and automated sensor controls means that drying conditions (temperature and relative humidity) can be predetermined to match the drying properties and desired quality of targeted product and better uniformity of drying conditions within the drier.

In addition, the sensor based controlled subsystem enables regulation of drying rates leading to nutritional quality control during the drying process. This feature makes the hybrid dryer multipurpose with the ability to dry a wider range of agricultural produce.

The hybrid dyer has relatively large loading capacity of up to 300 kg of fresh fruit per batch over drying period of days making it suitable for commercial entrepreneurs interested in drying large volumes of agricultural produce and; it is made from 95% of locally available and affordable materials meaning that maintenance is easy and has high potential for replicability,” The don explained

Dr. Ahamada Zziwa and Dr. Simon Kizito explaining how the alternative energy source of charcoal chamber works

Explaining the technology design and construction process, the Co-PI Dr. Simon Savio Kizito said the arched vent 8mx 4mx 2m solar dryer was constructed at Kisega Horticulture Association based at Kangulumira  in  Kayunga district because of farmers’ long term experience and commitment towards drying of  fruits  and vegetables.

Dr. Kizito said the size of the dryer was informed by the target capacity of fruits per batch.

He said Mathematical modelling and simulation of the heat distribution and airflow in the drying chamber was done using ANSYS CFD software to inform the solar dryer design (sizing, geometry, heat, airflow and humidity distribution).

“The solar drying chamber is composed of a concrete foundation, metal frame work and visqueen covering. The solar dryer foundation was constructed using cement, sand and bricks to give the structure the desired structural strength.

 Additionally, black oxide, hard core and gravel were used as admixtures in the concrete to improve heat properties of the drying chamber. A solar panel and battery are used to run fans to ensure forced air circulation in the drying chamber and adjustable drying rate of the product,” The don explained

The Co-PI further said,  this  hybrid solar dryer has a loading capacity of 300 kg of fresh and sliced fruits (approximately 450 pineapples each weighing 1 kg on average) per drying batch adding that  a  drying rate of 20kg/h and drying time of 10 hr of continuous drying was recorded during dryer testing.

“Hence the dryer produces 20 kg of quality dried pineapple product per batch within a day. The achievable drying temperatures in the dryer house range from 45-60 degrees centigrade on clear sunny days and 30-38 degrees Centigrade on cloudy days.

Conversely, when running on biomass, the average temperature ranges from 48-68 degrees centigrade which are comparably equal or higher on sunny days. During the biomass burner testing, it was noted that temperatures in the drying chamber starts to increase with time upon setting the fire in the burner and starts to fall as biomass burns out in the biomass heating chamber.”, Dr. Kizito explained.

Some of the dried pineapple chips in the dryer

The automation consultant Mr. Sudo Paul explained that for stable drying temperatures, the burner should be fed with biomass occasionally during operation. Additionally, he said, the suction fan on the burner should maintain a stable speed to ensure continuous heated air circulation in the drying chamber

“Food grade moisture content sensors, temperature and humidity sensors were installed in communication with the fans, within the drying chambers to control drying parameters.

Depending on temperature and relative humidity readings as compared to pretest value ranges, the sensors triggered signals to control the fan speeds hence regulating the drying process. The moisture content sensors are used to track moisture loss from the drying food product hence, the sensors inform key actions depending on whether slow or quick drying is required.

Both slow and quick drying have a negative impact on the nutritional quality of the drying product. Hence appropriate drying rate is achieved by turning on the second fan and automatically increasing fan speed so as to bring down drying temperatures and maintaining a steady recommended drying rate.

The supplemental heat source was attached to the drying during varying weather conditions, hence slow drying is boosted by pre heating the air before pushing it into the drying chamber. The backup burner is used during cloudy weather to raise temperatures in the drying chamber with the use of charcoal/briquettes as the fuel source,” The consultants stated.

Project Co-PI Dr. Simon Kizito and PI Ahamadda Zziwa and the Automation consultant Soddo Paul at the facility

Amos Kambagambira Tamusuza from the Renewable Energy Department in the Ministry of Energy and Mineral Development said the ministry’s role was to provide technical support to the project especially the construction right from the beginning as partners to ensure it fits within the ministry’s priorities.

“We as a department prioritize and promote the use of productive renewable energy resources and solar enegy is one of those resources we are promoting to benefit communities in terms of  accesses sing alternative energy sources  to help them uplift their livelihoods”, He said.

Mr. Tamusuza said this initiative is part of the government efforts to implement its manifesto to uplift the standards of living, alleviate poverty and creating jobs.

Mr. Tamusuza noted that most of Ugandan farmers and in this case Kayunga district produces a lot of Agricultural products in excess beyond the local demand leading to wastage and losses.

“ Farmers in Kayunga produce  a lot of  pineapples and at time, ten of them go for one thousand shillings while many rot because they are perishable and due to lack of storage facilities. This technology comes in handy to add value, promote shelf life and save wastage” Tamusuza said.

One of the farmers delivering the pineapples at the facility

Success stories to inform scaling up a number of farmers, farmer groups and entrepreneurs dealing in dried agricultural produce have expressed interest in adopting the hybrid solar dryer.  The HSD can be and has been adopted to other produce by entrepreneurs involved in cassava, amaranth seeds, ginger and vegetable drying.

The project implementers have partnered with Lutheran World Relief (LWF) an international NGO which has adapted the HSD to Arabica coffee drying in Kasese areas. This partnership has come in at a right time to ease and contribute to the commercialization and adoption of the HSD through the pilot construction and testing of the HSDs among Arabica coffee farmers in Kasese. The MakRIF researchers want to involve the LWF in the dissemination process since LWF has high potential to scale out and popularize the innovation and scale out the components of the research project

Report Compiled by;
Jane Anyango
Principal Communication Officer, CAES



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