Wild exotic oils play an important role in the societies in Africa and globally, from a flavour, fragrance, and aromatherapy perspective. South Africa, a country with a strong history of traditional healing, hosts a variety of more than 30,000 plant species which account for almost 10% of the world’s higher plants habitant in the country. Widely known as essential oils, these liquids containing volatile aroma compounds have now found applications in industry to produce perfumes, cosmetics, soap, shampoos, and cleaning gels. To some extent, application of these oils in the agro-food industry, where they are used to make beverages and flavour foods is rapidly becoming popular. Because of their anti-inflammatory, anti-oxidant, and anti-microbial properties, essential oils can be highly valued when blended to pharmaceutical and cosmetic products.
Since the oils are in high demand and yields are significantly low, new technologies for processing these herbs and producing essential oils for the cosmetics and pharmaceutical industries is of paramount importance. A century-old technique namely distillation consists of capturing essential oil from flowers or plants using saturated steam is still applied to date. The method is proven to be energy intensive thus linking it to sustainability and climate change mitigation process is critical that the industry operates within safe ecological limits. In future, aspects of energy efficiency and sustainable energy sources, such as concentrating solar thermal (CST) technologies, have the potential to support and improve the extraction process while reducing the carbon footprint. Although, the CST technologies have a great potential in the agro-processes and industrial sector, they have not yet been deployed extensively. Various types of CSTs consisting of several solar thermal collector models have been constructed, although most of these installations are of pilot type for the study of the operating conditions.
Solar thermal collectors include flat plate collector (FPC) and evacuated tube collector (ETC) which operate below 100oC and are mostly used for heating water, buildings, and swimming pools. There is research towards increasing the temperature of FPC and ETC systems, however these systems still cannot operate efficiently at high temperature despite effort to improve them. Parabolic trough collector (PTC) is one of the CST technologies and it is the most mature system capable of achieving temperature of up to 400oC. New smaller PTCs have been manufactured to suit process heat applications at temperatures below 300°C. These collectors may provide industrial process heat (IPH) to several applications, enabling them to eliminate the use of fossil fuels.
The ARC-Agricultural Engineering (ARC-AE) has initiated a research project with an aim to investigate the potential use of CST technology for essential oil extraction. The project resulted in the construction, testing and commissioning of a solar thermal system, comprising of parabolic trough mirrors, evacuated tube receivers, heat exchanger and solar tracking system. Satellite imagery and aerial photos show the PTC aligning on a true north-south line as installed at the ARC-AE. The PTC system was used to power a steam distillation process for essential oil extraction of orange, lemon, and mandarin peels. During the extraction process, important data such as the solar thermal output, efficiency of solar distillation, temperatures, essential oil yields, essential oil compositions and energy consumption of the distillation process were recorded and analysed.
Benefits of the CST in essential oils extraction:
- The operation is of the low pressure (< 1bar)and reach high temperatures of 295oC, The system generated a net thermal output of about 12 kW, with the collector inlet and outlet temperatures, as well as volumetric flow rate being about 50oC, 295oC and 100 L/h, respectively,
- Several plant-bearing oils including oranges, lemons and mandarins could be extracted at around temperatures of 96oC. These temperatures compare well with temperatures usually expected from a steam distillation operating under atmospheric conditions,
- The average yields of orange, lemon and mandarin essential oils were found to be 0.67, 0.53 and 1.09 %, respectively and these compare well with gas-powered extractions, the advantage here is that solar thermal technology was applied unlike LPG which is costly.
- With regards to energy efficiency, the overall system efficiency of the PTC-powered steam distillation was found to be 54.99 %. Although the overall system efficiency is relatively small, it still compares very well with other similar research studies on CST technologies and in some cases a little bigger.
Drawbacks of the CST in essential oils extraction:
- As of 2021, it is more expensive to procure and install solar energy than buy a traditional fossil fuels powered distiller. To a small-holder processor, start-up costs of solar thermal energy devices are generally higher because of the greater expense per unit of PTC system, however, thermal energy is cheaper over time.
- CST technologies cannot generate solar thermal energy with the consistency of most fossil fuels. For example, they cannot produce enough solar energy on cloudy days or after dark. This makes it an unreliable source of energy if a continuous operation must be conducted, otherwise one must have back-up power sources.
- Unlike fossil fuels, energy from the PTC cannot easily store either as thermal energy or hot water. While further research into electricity generated by addition of turbines to PTC can be stored in batteries, the storage is currently costly.
While there is no doubt solar energy can be an important solution for essential oils extraction, it was also observed that yields from PTC could compare well with fuel gas-powered system, thus rendering the PTC system suited for use in agro-based industrial applications. Additionally, the PTC offered much more benefits, such as continuous steady supply of energy if there was sufficient sunlight, lower operation costs and reduced carbon footprint. This technology is mature and has seen significant deployment around the world, yet to be tested in adverse weather conditions in South Africa. Additional benefits can be derived through integration of electricity production turbines to eliminate grid power and significantly reduce carbon emissions.