Developing a Solar Drying Machine for Agricultural Products. RIRDC Publication No. 09/026. RIRDCInnovation for rural Australia

Transcription

1 Developing a Solar Drying Machine for Agricultural Products RIRDC Publication No. 09/026 RIRDCInnovation for rural Australia

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3 Developing a Solar Drying Machine for Agricultural Products by Kame Khouzam February 2009 RIRDC Publication No 09/026 RIRDC Project No QUT-9A (PRJ )

4 2009 Rural Industries Research and Development Corporation. All rights reserved. ISBN X ISSN Development of a desiccant solar drying system for agricultural products Publication No. 09/026 Project No. QUT-9A (PRJ ) The information contained in this publication is intended for general use to assist public knowledge and discussion and to help improve the development of sustainable regions. You must not rely on any information contained in this publication without taking specialist advice relevant to your particular circumstances. While reasonable care has been taken in preparing this publication to ensure that information is true and correct, the Commonwealth of Australia gives no assurance as to the accuracy of any information in this publication. The Commonwealth of Australia, the Rural Industries Research and Development Corporation (RIRDC), the authors or contributors expressly disclaim, to the maximum extent permitted by law, all responsibility and liability to any person, arising directly or indirectly from any act or omission, or for any consequences of any such act or omission, made in reliance on the contents of this publication, whether or not caused by any negligence on the part of the Commonwealth of Australia, RIRDC, the authors or contributors. The Commonwealth of Australia does not necessarily endorse the views in this publication. This publication is copyright. Apart from any use as permitted under the Copyright Act 1968, all other rights are reserved. However, wide dissemination is encouraged. Requests and inquiries concerning reproduction and rights should be addressed to the RIRDC Publications Manager on phone Researcher Contact Details Dr Kame Khouzam School of engineering Systems Queensland University of Technology George Street Brisbane QLD 4000 Phone: Fax: k.khouzam@qut.edu.au In submitting this report, the researcher has agreed to RIRDC publishing this material in its edited form. RIRDC Contact Details Rural Industries Research and Development Corporation Level 2, 15 National Circuit BARTON ACT 2600 PO Box 4776 KINGSTON ACT 2604 Phone: Fax: rirdc@rirdc.gov.au. Web: Published in February 2009 Printed by Union Offset Printing, Canberra ii

5 Foreword This report presents the technical development of an innovative method to dry agricultural products. The new liquid desiccant system uses dry (dehumidified) air rather than heated air to dry grain and seeds. Drying of agricultural products consumes significant amounts of energy usually by burning diesel or gas. Hot air drying can cause damage to seeds by breaking essential enzymes; affecting germination rates and rendering the product not suitable to use. Alternative drying techniques are being developed to reduce energy, cost and improve product quality. Replacing fossil fuel energy by solar or biomass energy has also been widely investigated. This project investigated the use of a liquid desiccant absorber and solar energy. It operates by drying with only a small increase in air temperature, protecting the product from damage caused by hot air drying. The prototype desiccant machine operates by drying ambient air in a dehumidifier module by spraying ambient air with a highly concentrated solution of lithium chloride (LiCl). The dried air is then blown on to the bin containing the wet product. The resulting diluted desiccant is reconcentrated using solar energy (or waste heat) via a liquid-air heat exchanger. The results suggest that desiccant dehumidification outperforms hot air drying techniques and that LiCl is an effective desiccant. It cuts drying time, energy and cost and replaces fossil fuels with solar energy. It also preserves seed quality. The dried air system would make drying grain and other products more viable in the tropics, where high ambient relative humidity makes heated-air dryers expensive to operate. The system would also have appeal in the colder parts of the country where high humidity and low temperatures also make drying expensive. The project is proceeding to commercialisation, with the liquid desiccant system offering to help create a new industry in crop drying and preservation which would benefit farmers and producers by reducing their energy costs. The possibility to broaden the use of solar energy applications into crop drying will be strengthened with the introduction of a carbon tax in 2010 under the proposed Australian Emissions Trading Scheme (ETS). The project was jointly funded from RIRDC core funds, which are provided by the Australian Government, the Queensland University of Technology, and industry partner Agridry RFM Pty Ltd. This report, an addition to RIRDC s diverse range of over 1800 research publications, forms part of our Environment and Farm Management R&D program, which aims to foster agri-industry systems that have sufficient diversity, flexibility and robustness to be resilient and respond to challenges and opportunities. Most of our publications are available for viewing, downloading or purchasing online through our website: downloads at purchases at Peter O Brien Managing Director Rural Industries Research and Development Corporation iii

6 About the Author Dr. Kame Khouzam received his B.Sc. in electrical engineering in 1977, a M.Sc. in solar thermal engineering in 1983 and a Ph.D. in photovoltaic system engineering in He has over 25 years experience in renewable energy systems and solar applications. He has authored or co-authored over 120 technical papers and reports. He received the Australian Sustainable Energy Industry Award for his photovoltaic salt-water chlorinator in Acknowledgments The author wishes to acknowledge the support received for this project through RIRDC and the assistance and cooperation given to the project by Dr. George Wilson. The technical support and enthusiasm of the staff at Agridry RFM greatly contributed to the timely completion of this project. Keywords Crop Drying Dehumidification Liquid Desiccant Regeneration Solar Energy iv

7 Contents Foreword...iii About the Author... iv Acknowledgments... iv Keywords... iv Executive Summary... vii 1. Introduction Objectives and methodology System description Air dehumidification Desiccant regeneration Benefits of desiccant drying Commercialisation pathway Conclusions Recommendations References v

8 Tables Table 1 Power rating of equipment used in drying system... 5 Table 2 Worldwide market for farm crop drying fans for grain, hay, and seed over 15,000 c.f.m Figures Figure 1 Schematic showing the main components of the desiccant system... 5 Figure 2 The desiccant air-dehumidification tower fabricated of fibre reinforced plastic... 6 Figure 3 Air parameters results versus time... 7 Figure 4 Air dehumidification results showing moisture content versus time... 8 Figure 5 Coupling the dehumidification module to the batch bin for drying... 9 Figure 6 Inlet and outlet air RH and temperature, desiccant concentration and grain MC Figure 7 Drying test showing air temperature and RH at the surface of the grain, MC and desiccant concentration Figure 8 Moisture content of grain in the drying bin Figure 9 Solar hot water system used for the regeneration of the desiccant Figure 10 A liquid desiccant drying machinery manufactured by Agridry in Toowoomba vi

9 Executive Summary What this report is about This report presents a new technique for drying agricultural products (such as cereal grains and seeds). The project developed and trialled a prototype drying machine based on dry (dehumidified) air rather than heated air and using a liquid desiccant (drying agent) and solar energy. Who is the report targeted at? The report is targeted at farmers, grain dryers and aeration industries as well as fruit and vegetable drying companies and policy makers involved in alternative energy sources, particularly the use of solar thermal energy. Background Drying of crops is critical for preserving product quality and achieving a storage life of 1-3 years, but is one of the most energy-intensive processes associated with agricultural production. Drying by heated air not only consumes a considerable amount of fossil fuel, but can also cause damage to seeds by breaking essential enzymes; affecting germination rates and rendering the product unsuitable for use. In humid climates such as in Queensland, crops require higher temperatures to dry, and this increase in temperature required substantially affects the quality of the product. The limitations of hot air drying led Agridry RFM Pty Ltd to undertake a collaborative work with Queensland University of Technology (QUT), with support from RIRDC to develop an innovative crop dryer different from fossil fuel dryers commonly used in Australia and many other countries. The new dryer uses a desiccant solution as a drying medium to absorb moisture from the air. Once diluted, the desiccant is reconcentrated by heating the solution and evaporating absorbed water. This technique reduces the humidity of the air without significantly increasing air temperature. A number of techniques to optimise the energy requirement for grain drying have been widely researched. The quality of dried product and the overall drying costs have been equally important considerations. Approaches range from modifications of existing dryer systems to development of new designs and concepts. This project recognised the need for new drying equipment in Australia which: is more energy efficient and relies on renewable energy as the main source, and addresses the problem of damage associated with hot air drying, particularly in humid climates. Desiccants are used for air dehumidification (and for storage in cool rooms) to allow drying at low temperatures, thus preserving the natural enzymes and enhancing the quality of produce. When moist air is passed over the surface or through a mist of liquid desiccant, a dehumidifier process occurs which lowers the absolute humidity compared with a hot air only drying air stream. As a result the drying effect when applied to grain is accelerated. The extent of undesirable effects of heat on the grain can be reduced since the liquid desiccant process only raises grain temperature by a few degrees as compared with using direct heated air streams. vii

10 Aims and objectives The objectives of this project were: to demonstrate the technical viability of solar drying using liquid desiccant air dehumidification, by developing a prototype drying system applicable to a number of agriculture products (such as peanuts and sorghum) the system would utilise solar energy as the main energy source with auxiliary electricity for air blowing and desiccant circulation. Methods A prototype system based on liquid desiccant was designed and developed. The system comprises: a drying bin which holds the product a dehumidification module for drying ambient air using concentrated solution of lithium chloride a regeneration module for reconcentrating the lithium chloride solution to its original state, and a solar hot water collector (or waste heat) to provide thermal energy for desiccant regeneration. In addition to the main modules, the system includes air fans, desiccant pumps and heat exchangers. The electricity required to operate the fans and pumps is relatively minor compared to the energy required for regeneration, which is provided by solar energy at air temperature as low as 50 o C. The packed bed construction was used in both the desiccant regenerator module and the air dehumidification module. The system was trialled over 12 months in different climatic conditions and with different products including sorghum, corn, grass seeds, and bokashi (mixture of rice husks and bran). Using a strong desiccant solution, a substantial reduction in relative humidity was noted and values below 12% were reached irrespective of the air intake. This was accompanied by a small increase in air temperature due to an exothermic reaction of the desiccant. Results The work carried out suggests that desiccant dehumidification outperforms hot air drying techniques. Results showed that the desiccant dehumidification system has the potential to cut drying time, energy and cost. Besides essentially preserving seed quality, the main benefit of the system is to replace large amounts of fossil fuel with solar energy. In addition, grain drying with desiccant is not hindered by the air humidity as it can operate successfully 24 hours a day in virtually all ambient conditions, including days with very high humidity (rain, mist or fog). This will lead to great reductions in time and in fuel usage. As a result, it is estimated that over 50% savings in energy cost is achievable. It concluded that LiCl is a very effective desiccant. At high concentration of 75%, LiCl has the ability to lower air relative humidity to as low as 10% regardless of ambient condition. Following the trials of the pilot plant a detailed design was undertaken to manufacture a commercial unit suitable for small scale application. A machine of desiccant capacity 500-litre with 1000 litres/sec air was manufactured for the purpose of demonstration at the Agriculture Show in Toowoomba in September The machine is currently being field used and will be monitored to evaluate its long term performance. This will provide important feedback to establish the viability and lifetime of the desiccant solution and the entire system operation. viii

11 Implications for relevant stakeholders The design and construction of the desiccant system is relatively simple and solves a number of problems associated with hot air drying. However, the set up cost is rather high particularly when a solar collector is employed. Their running costs are much lower though and a payback period of 5 to 7 years can be achieved. The niche application of the desiccant technique is realised when dealing with high value seeds (such as parent seeds) and other heat sensitive crops. The dry air system would also make drying grain and other agricultural products more viable in the tropics (e.g. Atherton Tableland, Qld), where high ambient relative humidity makes heated-air dryers expensive to operate. The system would also have appeal in the colder parts of the country (Victoria and Tasmania) where high humidity and low temperatures also make drying expensive. As a result of this research investigation and by taking the results of the project to commercialisation, the liquid desiccant system offers to help create a new industry in crop drying and preservation which would benefit farmers and producers by reducing their energy costs. In addition, the use of solar energy has the potential to replace fossil fuels for drying purposes and to enhance the quality of produce in a reasonable processing time. The market potential for desiccant and solar powered driers is likely to improve drastically with the introduction of a carbon tax in 2010 under the proposed Australian Emissions Trading Scheme (ETS). A market assessment showed that the worldwide latent demand for liquid desiccant air and gas dryers was valued at US$175 million in Although the demand for driers in Australia is relatively small, the demand for desiccant driers is expected to grow in many parts of the world where environmental legislations, fuel costs, and other climatic conditions make conventional driers expensive to run. Other incentives to switch to solar power for low and medium heat applications will reinforce the market pull for the desiccant system; which will help reduce the carbon footprint associated with drying. This work presented the technical viability of the new technology; and would provide a market push for desiccant driers. Agridry is now able to manufacture small, manually-operated driers. A machine of air capacity 1000 L/sec was demonstrated at the Agriculture Show in Toowoomba. To assist its introduction into the market, a combination of solar and backup power source was implemented to maintain continuity of operation on cloudy days. Agridry RFM and QUT are currently planning to scale up the desiccant dryer to larger sizes, incorporate desiccant and hot water storage, and fully automate the system to optimise its operation with minimal attention by the operator. Conclusions In concluding this project the research team believes that the use of solar thermal energy in Australia has been neglected. Applications such as grain drying, heating water for domestic use, water heating in hospitals (and resorts) for washing and disinfection, pre-heating in food processing and beverage industries and other chemical industries are just some examples of several requiring just heat which can be obtained economically by solar collectors. Our results of the experimentation suggest that the desiccant drying technology can be expanded to crop storage, grain preservation and other greenhouse plantation. To improve acceptance into the market, a combination of solar and auxiliary energy source should be available to maintain operation during cloudy periods. A tank for strong desiccant should be provided to permit for the entire drying batch and between desiccant regeneration. Steps are being taken to promote the new desiccant technology and these are expected to yield positive results. QUT and Agridry have collaborated successfully and benefited from this partnership; research and development efforts complemented with practical industrial experience and marketing strategies. Recommendations Further investments are needed to support commercialization of the solar powered desiccant drying. Tasks include ix

12 Conducting field demonstrations to attract early sales as well as market penetration. Controlling growth until the product proves itself to avoid dissatisfaction. Ensuring sufficient demand from high value applications to support initial manufacturing at low volume. Selling equipment for a small profit during early stage of commercialization Emphasizing the inherent benefits of solar drying in early sales as customers willing to switch to solar will feel personal gratification and satisfaction. Leasing the system instead of selling to permit early trials by prospective clients. x

13 1. Introduction Crop drying is the most energy consuming process in all processes on the farm [1]. The purpose of drying is to remove moisture from the agricultural produce so that it can be processed and/or safely stored for increased periods of time. Crops are also dried before storage or, during storage, by forced circulation of air, to prevent spontaneous combustion by inhibiting fermentation. It is estimated that 20% of the world s grain production is lost after harvest because of inefficient handling and poor implementation of post-harvest technology [2]. Most grain driers operate by heating ambient air using diesel or LPG burners. Ambient air (without heating) driers use less energy compared with hot air driers; but they are known to cause damage to seeds (and grain) due to prolonged exposure to humid air. Solar drying has many advantages over the previous methods; but relies heavily on weather conditions. Hot air drying increases the temperature of the air (and product) and lowers the air relative humidity (RH) and thus allows the air to carry moisture from the product. Forced air ensures continuous supply of air to replace saturated air. Although this is adequate in relatively dry and less humid weather, it is not possible to reduce the actual moisture level (absolute humidity) in the air in humid climates. In tropical climates, high air temperatures and humidity provide a very narrow temperature range in which fuel-fired seed dryers can operate. As a result, drying by heated air becomes costly, slow and less effective. As well as using large amounts of fossil fuels, conventional hot air dryers are subject to significant shortcomings. High temperature drying can cause breakdown of enzymes, which render the produce unsuitable for consumption. Fumes from burning diesel and other fuels can impart adverse odour or taste, and hot cinders can set fire to the easily-ignited dried product causing loss of the product and destruction of the dryer. Grain and seeds are normally harvested at a moisture level between 18% and 40% (moisture content; MC) depending on the nature of the crop. These must then be dried to a level of 7% to 11% depending on application and market need. The energy requirements for evaporating water from grain range from 4 to 8 MJ/kg of water removed depending on type of product, temperature and operating conditions. Evaporating water requires heat and, for crop containing 40% moisture, an average of 1,200 MJ of heat energy is needed to for each tonne of product. To reduce the MC from 40% to about 10% requires at least 30 litres of diesel fuel or 50 litres of LPG to provide the heat requirement for each tonne of dried product. The total gaseous emission (mostly CO 2 ) associated with drying is typically 100 kg per tonne of dried product. Research work in industrial drying has intensified in recent years to reduce energy use and operating costs. The approach has changed from modifications of existing dryer systems to development of new designs and concepts [3, 4]. Replacing fossil fuel energy by solar, waste heat or biomass energy has also been investigated [5]. Some significant developments in grain drying are dry-aeration, multistage drying, a combination of high and low temperature drying, layer drying, drying with intermittent rest periods, recirculating the exhaust air, stir drying and use of grain preservatives. Desiccant dehumidification was initially investigated for use in air-conditioning in order to reduce energy consumption and improve efficiency of vapour-compression systems [6, 7]. The advancements made in desiccant technology led to its expansion into other fields such as crop protection [8], aeration and cooling of stored grain [9-11], food production [12] and grain drying [13, 14]. Solid desiccant using silica gel has been investigated for use in air-conditioning applications and air dehumidification systems especially in food processing and beverages [15, 16]. The main drawback of solid desiccant is the relatively high temperature requirement for regeneration compared with liquid desiccant. Theoretical and practical work have been carried out by researchers on drying using lithium 1

14 chloride [17], calcium chloride [18], mixtures of both [19], and triethylene glycol as desiccants [20]. The properties of starch and agriculture waste as desiccants have been recently investigated [21, 22]. The basic concept of these studies was to directly reduce the moisture and warm the air, which would be used for drying only a few degrees above the ambient temperature. It was concluded that dry air generator systems require only 20% to 30% of the energy required by hot air driers [23, 24]. This project has been motivated by a desire to investigate an innovative grain drier that can operate on solar power as the main energy supply and thus can save on fuel costs and associated emissions. The author approached Agridry RFM in Toowoomba, seeking collaboration into the development of the new drier using liquid desiccant to achieve the desired air dehumidification. The discussion led the Queensland University of Technology and Agridry RFM, with support from RIRDC, to undertake a collaborative work to trial the technology with the ultimate goal of commercialisation. The developmental work started with the construction of a small scale prototype machine capable of about 250 l/s of air flow. Various tests were carried out to determine the optimum operating range for the concentration of the desiccant and the required range of temperature for desiccant regeneration. In addition, a number of desiccants were trialled in order to identify the most effective and practical desiccant. The following sections give detailed information on the system design, operation and performance. 2

15 2. Objectives and methodology The objectives of this project were: to demonstrate the technical viability of solar drying using liquid desiccant air dehumidification, by developing a prototype drying system applicable to a number of agriculture products (such as peanuts and sorghum) the system would utilise solar energy as the main energy source with auxiliary electricity for air blowing and desiccant circulation. The methodology required the design and development of a prototype air dehumidification system. The system is based on a liquid desiccant to absorb the moisture from the air before blowing on to the drying bin. As a result, a significant reduction in Relative Humidity is achieved which reduces the drying time considerably and preserves the quality of produce (as little increase in air temperature occurs). While drying, little energy, except for circulating pumps and air blowers, is used. Solar energy is then used to reconcentrate the desiccant (having absorbed moisture) to its optimal condition. A backup source such as off-peak electricity or fossil fuel may be used to allow for greater reliability. Many products were tested including sorghum, corn, grass seeds, and bokashi (mixture of rice husks and bran). The prototype plant was operated continuously in various conditions including days of high humidity, rain and fog, and day-night. Various tests were also conducted in which the liquid desiccant concentration, flow rate, regeneration temperature, and airflow rate in the drying system were altered. 3

16 3. System description Drying with dehumidified air is accomplished by reducing the moisture of the air until the partial pressure of water vapour in the air is less than the partial pressure of water in the grain. Lithium chloride (LiCl: specific gravity 2.0) was chosen because of its favourable properties; very stable and has low vapour pressure. In its pure form LiCl is a white crystalline powder, is very susceptible to setting into hard lumps owing to its extreme affinity for water. The dissolution of LiCl in water is an exothermic reaction (generates heat). The liquid desiccant machine consists of the following components: a drying bin (containing grain or seeds, through which dried air was passed) an air drying module (using concentrated desiccant solution to absorb moisture from the air) a regenerator module (to evaporate water from heated desiccant solution), and a solar water heater. In addition to the main modules, desiccant pumps, heat exchangers and air blowers are used to supply and control air parameters. Electricity required to operate the fans and pumps is relatively minor compared to the heat energy required for evaporation, which is provided by solar energy. The power ratings of fans and pumps used are given in Table 1. Figure 1 shows a complete schematic of the system. The air drying (dehumidification) system provides low temperature drying with only a small increase in air temperature. This consists of a packed-bed tower dehumidifier, desiccant droplet separator and an air fan. As air is pumped in the tower, it is dried by the spraying desiccant before being blown on to the crop. In the dehumidification process, both the solution and air temperature increases due to the liberation of latent heat of vaporisation of water. Various tests were conducted in which the liquid desiccant concentration, flow rate, regeneration temperature, and airflow rate in the drying system may be altered. Strong solution absorbs water from air in the dehumidification tower and the resulting weak solution is collected in the tank. The regeneration system consists of a packed column dehumidification tower, solution tank, pumps and a liquid to -air heat exchanger. In this process, the system heats the incoming air using the heat exchanger supplied via the solar water heating system, which is then blown on to the tower where the weak desiccant is sprayed. This process causes the weak liquid desiccant in the regeneration tower to lose moisture and reconcentrate. The packed bed structure was chosen in order to give greater flexibility to run the unit either as a dehumidifier or as a regenerator [25, 26]. The difference between the dehumidification unit and the regenerator unit is that the first is used to dehumidify the air by spraying the desiccant on to the air (opposite flow) while the latter is used for drying the desiccant by blowing heated air (desiccant may also be heated). By varying the airflow and temperature of the desiccant, each module can serve as either a dryer or regenerator. 4

17 Table 1 Power rating of equipment used in drying system Equipment Power-kW Solar hot water circulating pump 3 speed 0.09 Cold water circulating pump for desiccant cooling (auxiliary) 0.18 CP25 Main desiccant pump used on large unit (U1) 0.37 CP11 Auxiliary desiccant pump used on small unit (U2) 0.18 Fan on Dehumidifier (U1) 1.50 Fan on Regenerator (U2) 0.40 Figure 1 Schematic showing the main components of the desiccant system: (a) air dehumidification module and (b) desiccant regenerator module 5

18 4. Air dehumidification The objective of this testing was to establish the conditions of the air to be used for drying as it exits the dehumidification system. The desiccant must be able to dehumidify the air significantly to be used for drying. To form a correlation between the desiccant concentration and level of dehumidification, first trials were conducted without regeneration and in order to determine the rate at which the concentration decreases with time and ambient air conditions. Several dehumidification tests were conducted using a machine constructed of fibre reinforced plastic (FRP) as shown in Figure 2. The desiccant concentration ranged between 48% and 72% (W/V). A conductivity meter was used to determine the concentration after being laboratory calibrated. The desiccant flow rate was adjusted between 2 and 6 litres/min. The airflow rate was measured at 280 l/s. Pumps and fans were equipped with voltage regulators. A controller allowed reduction of the flow in order to obtain specimens for testing. Figure 2 The desiccant air-dehumidification tower fabricated of fibre reinforced plastic. A liquid-air heat exchanger is used for heating the air for regeneration of desiccant Testing was done using around 45 litres at concentration of 72% (W/V). A delay of about 20 minutes was observed between switching on the system and the effect it had on the process air. This may be reduced if the pump was switched on prior to switching the air blower. Air is passed in the system and both air temperature and humidity were recorded before and after exposure. The effect of the desiccant on air continued for about 1 hour after stopping the pump. An explanation is that the air path across the surface of the desiccant may provide exposure to facilitate moisture absorption at that interface. Traces of desiccant suspended in the plastic material within the dehumidification tower are likely another cause. It is postulated that desiccant flow could be reduced by a significant amount thus 6

19 minimising parasitic power required for pumping. A pump may be then operated on a duty cycle depending on air RH and desiccant concentration. In the realisation of this, it may be possible to design a system, which dramatically reduces desiccant pumping, apart from circulation required to achieve initial solution. At high concentration the flow rate would drop. At low temperature (8 o C) and when the concentration was high (74% W/V), the flow rate dropped to approximately 1.2 l/min and partial crystallisation was observed. The filter and spray nozzles had to be cleaned to restore the desiccant flow. This gave an indication not to increase the concentration beyond 72% when the ambient temperature is low. Tests were conducted to verify the results and to reach a concentration level too low to provide any effective dehumidification. A representation of data is shown in Figures 3 and 4 indicating changes in concentration, air RH and temperature over time. Figure 4 gives the values of absolute humidity. The decrease in RH can not be considered in isolation to an increase in outlet air temperature, which occurred over the same period. On the other hand the air temperature increases as it gains some heat in the dissolution process. The decrease in concentration over time can be clearly seen in the graph. Figure 3 Air parameters results versus time When drying using simply a heat source, reduced moisture is achieved by virtue of the reduced RH that results from increased temperature. However the total water content defined in terms of grams of water per kg of dry air doesn t change. This is not the case with desiccant dehumidification which alters the absolute humidity values. While the inlet air data was inherently varied, the outlet data was very smooth and showed much less fluctuations. An interesting point to note was the way in which the inlet RH varied drastically over the course of the testing and none of these variations were visible in the outlet data. The same is not entirely true for the temperatures, which showed some parallel trends although greatly reduced. The change in the outlet RH would appear to be mainly a function of the concentration of the desiccant, at least in the range experienced during testing. 7

20 Figure 4 Air dehumidification results showing moisture content versus time From the results, it may be reasonable to assume that the gradient of the outlet RH may be a useful method for predicting concentration. Another observation was the difference (ΔT) noted between the inlet air temperature and outlet air temperature. There was strong correlation between desiccant concentration and temperature difference. At strong concentration (66% 68%), ΔT was 10 o C to 12 o C, whereas when the concentration dropped (52% 54%) ΔT was 4 o C 6 o C. The ΔT may therefore be a crude indicator of the desiccant concentration. The desiccant machine was then connected to a drying bin as shown in Figure 5. The initial moisture content (MC) of the grain (sorghum) was 18%. Other data obtained were: Inlet and outlet air parameters: RH and T; recorded every 5 minutes during the day. Air out of drying bin: RH and T; recorded every 5 minutes during the day. Desiccant concentration: recorded every 30 minutes. Moisture content: three readings were recorded every 60 minutes. Figures 6 and 7 show the parameters at the inlet air, outlet air, and the air at the bin versus time. The concentration of the desiccant and moisture content are also shown. The air flow dropped to around 180 l/sec when the dehumidification tower was connected to the drying bin. Observations were made regarding the temperature difference between inlet air and outlet air. 8

21 The output relative humidity RH (out of the tower) was nearly constant between 8.5% and 9% when the desiccant concentration ranged from 68% down to 65%. This condition was observed for about 4 hours. The results showed that over the 24-hour period the grain had dried from a 17.7% MC to 13.3% average MC. The grain was further subjected to dehumidified air overnight. Despite the high RH experienced throughout the night, the dehumidified air had on average 50% less humidity than the incoming air. The difference in RH and in temperature between the inlet air and dry air are shown in Figure 7. Figure 8 shows the moisture content of the grain after 18 and 28 hours of operation. Figure 5 Coupling the dehumidification module to the batch bin for drying 9

22 Figure 6 Inlet and outlet air RH and temperature, desiccant concentration and grain MC The temperature and humidity at the drying bin were recorded (Figure 7). The temperature at the surface of the grain was rather cooler at 16 o C to 19 o C compared with 20 o C to 23 o C at the inlet of the bin. The RH at the surface of the bin varied between 39.8% and 73.2%. These numbers suggest that a closed loop drying is not suitable until the RH (out of the bin) had dropped to below that of ambient air. 10

23 Figure 7 Drying test showing air temperature and RH at the surface of the grain, MC and desiccant concentration. Also shown are the differences in temperature and in RH between inlet and dried air Figure 8 Moisture content of grain in the drying bin (expected to reach 12%) 11

24 5. Desiccant regeneration As described, as moisture condenses in the desiccant solution, the solution gradually becomes diluted. Once the concentration of the desiccant falls to about 48%, its ability to absorb moisture is greatly reduced and it needs to be regenerated. The desiccant is restored to its optimal concentration in the regenerator module. The packed-bed structure was used for the regenerator because of its simplicity and also because it allowed the same machine to run in either mode. A liquid-air heat exchanger was used to supply the thermal energy requirement of the air via a closed loop circulating hot water system. The solar collector used is shown in Fig. 9. First the desiccant solution is heated to about 45 o C by hot water from a solar water heater. Then, the hot desiccant is sprayed and drains through the bed of packing material, creating a large surface area in contact with air passing upwards through the regenerator. This causes water to evaporate from the desiccant solution, cooling the solution and increasing its concentration. The exhaust air is generally more moist and warmer than ambient air. This suggests that a technique may be used to re-claim the water from the exhaust air. Figure 9 Solar hot water system used for the regeneration of the desiccant. As the desiccant concentration increases the minimum temperature of the process air should also be increased. For example, when the concentration reaches 60% the air temperature should be close to 60 o C. Failure to observe the minimum heating temperature will result in the desiccant operation reversing to dehumidification. 12

25 6. Benefits of desiccant drying Tests showed that lithium chloride is a very effective desiccant. At high concentration of 75% LiCl has the ability to lower air relative humidity (RH) to as low as 12% regardless of air intake. This is achieved with only a small increase in air temperature. The advantages of low temperature drying can be summarised in the following: By not using heated air to lower the RH of the drying air, a method of lowering the RH by dehumidifying the incoming air removes the risk of causing heat damage or rapid deterioration in seed viability during the drying process. There is a clear and significant correlation between the temperature that seed is exposed to during the drying stage and the resulting seed viability and storage life potential. [e.g. soybean seed dried to 8% moisture at 37 o C had an initial germination of 91% and a germ after 3 months of 87%. Contrast this with seed dried at 48 o C and after 3 months storage was only 5% germination [23]. Minimizing temperatures during the drying process is extremely important to help maximize the initial and long term viability of both high value hybrid seed and their inbred parent lines (Pacific Seeds, Toowoomba). For long term storage or storage under hot tropical conditions it is necessary to dry seed to low MCs to reduce the seed deterioration rate. The equilibrium relative MCs of seed is below 40%. Such low RH levels in the drying air are difficult to achieve economically or consistently when relying on heat to lower ambient RH. Dehumidified air dryers are capable of consistently achieving these low RH values regardless of ambient weather conditions. Drying can be maintained during the night by using hot water produced during the day, or by using desiccant solution that has been regenerated during the day. Storage tanks for desiccant solution and hot water will enable maximum flexibility to utilise available solar energy, and to allow heat gained when the desiccant absorbs moisture to be naturally dissipated to the environment. Dehumidified air systems have the following benefits: Seed drying can continue 24 hours per day under all weather conditions, avoiding costly delays in harvesting, transport, processing and shipping schedules. The drying rate is easily and accurately controlled, regardless of the ambient air temperature and relative humidity conditions. Drying can be done at a predetermined rate. Drying rate can be constant or varied according to predetermined drying schedules. The system is relatively easy and quick to retrofit into an existing drying infrastructure making use of the existing plenum chamber, fans, ducting, bins and handling equipment. Heating systems in existing drying facilities can also be utilised as part of the desiccant regeneration system. The system can be modular, allowing for relatively easy increases in capacity to be achieved without requiring additional staffing. 13

26 7. Commercialisation pathway The worldwide market for desiccant compressed air and gas dryers was valued at US$175 million in 2007 and had a forecast compound annual growth rate of 2.18% for the period The market in Asia and Oceania was valued at US$64 million for 2007 making it 36.4% of the worldwide market. The Australian market was valued at US$1.91 million equating to 3% of the market for Asia and Oceania [27]. The worldwide demand for farm crop drying fans (over 15,000 cf, at 1 inch pressure) is expected to increase from US$89 million in 2007 to US$127.4 million in 2012, or 5.95% per annum. Table 2 gives a breakdown of the data for 2007 by regions. Asia was the region with the highest latent demand of US$31 million, equating to 34.9% of demand for the globe. Europe and the Middle East was the second largest region with a demand of US$26 million, followed by North America and the Caribbean with a demand of US$21 million. When combined Asia, Europe and the Middle East and North America and the Caribbean equate to 87.9% of the global demand. The Oceania market was valued at US$1 million with the Australian market for crop drying fans being US$0.96 million. Table 2 Worldwide market for farm crop drying fans for grain, hay, and seed over 15,000 c.f.m. (at approximately 1 inch pressure) [AIC, April 2007] Region Latent Demand US$ Million % of Globe Asia Europe & the Middle East North America & the Caribbean Latin America Africa Oceana Total The market for dryer sales in Australia has been predominantly in QLD, NSW and WA, with several in SA and a small number in Victoria. The dry-air system would make drying grain and other agricultural products more viable in the tropics (such as the Atherton Tableland), where high ambient relative humidity (RH) makes heated-air dryers ineffective. The system would also have appeal in the colder parts of Australia (Victoria and Tasmania) and New Zealand where humidity and low temperatures also make drying expensive. Research has shown that there is a strong need for more energy efficient drying equipment, which relies on renewable energy. In addition to its cost savings; demand exists for the technology because it preserves the quality of the produce and dramatically cuts drying time, which are significant factors for potential buyers. The system can also be modular, allowing for relatively easy increases in capacity to be achieved. Presently there is no direct competitor for manufacturing the system in Australia, although R&D has been conducted overseas. It is believed that there is a huge potential for the system in Australia and overseas and Agridry is in a good position to capture a large percentage of this market. The close proximity of Agridry and QUT to the high demand in the Asian region is advantageous in the development and sale of the drying machinery. It is worth noting that desiccant dehumidification can be adapted to aeration of stored grains, to control temperature and moisture levels particularly for high value seeds. New applications for desiccants are also being investigated such as in car air-conditioning and water desalination. 14

27 The design and construction of the desiccant system is relatively simple and solves a number of problems associated with hot air drying. However, the set up cost is rather high particularly when a solar collector is employed. Research showed that a payback period of usually 5 to 7 years can be achieved. The niche application of the desiccant technique is realised when dealing with high value seeds (such as parent seeds) and other heat sensitive crops. A business plan was undertaken to formulate the strengths, weaknesses and opportunities associated with the new technology. These are summarised as follows: The strengths of the technology high efficiency improved yield low risk system reduced energy use reduced heat damage saves time and money can be easily integrated to conventional systems. The weaknesses high initial cost desiccant cost and life need for alternative back up existing prior art may affect licensing new technology requires time to accept long payback period especially with solar system. The threats facing the technology protection of IP conventional systems are cheaper technical issues to overcome (e.g. pressure drop, desiccant leaks) new technology requires market acceptance (require demonstration). The opportunities associated with the system first move advantage no large companies in the market strong demand for grain and seed dryers 15

28 public acceptance of solar power applications opportunity as energy cost increases with carbon tax in A number of activities were taken to publicise and promote the new liquid desiccant technology by Agridry and QUT. A prototype machine was manufactured and since then has been used for testing and demonstration. The machine was tested for a number of months using different products obtained from various companies. Several types of desiccants and packed bed material were used. A list of contact companies and their representatives can be obtained from the author. A number of Agridry clients have already expressed interest in the new air drying system especially for low temperature application. Following the trials of the prototype a larger machine (Figure 10) was also developed and demonstrated at the Agriculture Show in Toowoomba in September Recommendations for improvement to simplify construction were implemented. The desiccant machine was featured on Channel 9 (Win TV) on the 6 o clock news on 14 February A digital copy of the recording can be obtained directly from Win TV. Agridry also submitted an entry into the 2007/08 DuPont Australian and New Zealand Innovation Award competition and was a finalist. Further demonstration of the drying machine will be done at various opportunities. A large new drying system is now being developed by Agridry and QUT for a company in Pittsworth. This is likely to be the world s first commercial desiccant drier. The system will process six tons of grass seeds per day from 20% moisture content (MC) down to 2%. 16

29 Figure 10 A liquid desiccant drying machinery manufactured by Agridry in Toowoomba Agridry has gained adequate experience with the desiccant technology to enable the manufacturing of modular units of different capacities. Drawings have been prepared to manufacture a machine with a maximum desiccant capacity of 500 L and with different air flow (up to 2,000 L/s). Although this model is not automated, it is quite simple to operate and would offer a low cost drying option in the case of heat sensitive products (such as parent seeds). A rotating drum is currently being investigated and promises to be more efficient to adapt to the desiccant technology. 17

30 8. Conclusions Many forms of nuts, seeds and grains often undergo a drying procedure, which can be improved using a desiccant assisted system. Although conventional hot air drying methods are already in place to achieve the required drying, a system using solar power for the regeneration of the desiccant material would significantly improve product quality, replace large amounts of fossil fuel and reduce running costs. Many problems associated with hot air drying such as heat damage to seeds and machinery can thus be avoided. This report presents the development of a working prototype solar grain drying system based on liquid desiccant. When moist air is passed through a mist of liquid desiccant, a dehumidifier process occurs which lowers the absolute humidity compared with hot air only drying air stream. As a result, the drying effect when applied to grain is accelerated. The extent of undesirable effects of heat on the grain can be reduced since the liquid desiccant process only raises grain temperature by few degrees as compared with using direct heated air streams. Comprehensive testing was done in different climatic conditions (in Toowoomba) and using a variety of products. Different types of liquid desiccants were trialled and lithium chloride was found to be the most effective. At high concentration of 75% LiCl has the ability to lower air RH to as low as 10% regardless of ambient condition. Two types of drying bins were used to find a system which will reduce back pressure. The desiccant system was trialled in two configurations: open air stream and closed loop. It was found that halfway during the drying process (when the air RH exiting the drying bin is lower than the ambient air RH) the air loop may be closed. This would save up to 25% on the drying time. This feature can only be effective with desiccant technology. One of the benefits of using liquid desiccant is that it can be adapted to using solar energy (or waste heat) for the regeneration of the weak solution. The liquid desiccant drying system has the following advantages: 1. It improves crop yield and offers a more efficient means for crop preservation. 2. Can be adapted to using solar energy or waste heat for the regeneration of the desiccant. 3. It allows much lower air inlet absolute humidity, while keeping the air temperature below harmful levels for the seeds and has the potential to lower drying time drastically.. 4. Expected to lower drying cost and reduce pollution associated with conventional drying methods. 5. It can be designed for optimum air conditions required for the agricultural product regardless of ambient conditions. 6. The lower inlet (drying) air temperature offers major advantages in that there is much less risk of damaging the product qualities, e.g. germination rates and milling qualities. 7. The lack of a naked flame as used in most hot air dryers means that the risk of fire in the drying vessel is eliminated. Whilst the market identified for crop drying machinery was small in Australia, the Asian region was the biggest market, comprising 34.9% of the worldwide market with a total value of US$31 million. The market potential for desiccant driers is expected to grow dramatically in many parts of the world especially with the continued escalation in fuel costs. Public pressure to reduce harmful emissions of carbon based fuels and the introduction of carbon tax will stimulate more demand for solar powered applications such as in grain drying and heating. 18

31 Agridry has gained experience in desiccant drying and a new type of machinery has been added to its line of manufacturing capabilities. The next phase is to produce a fully automated version equipped with additional storage tanks for liquid desiccant. The developmental work suggests that the desiccant drying technology can be expanded to grain storage (in silos) and crop preservation. Although aeration fans are used to help protect the stored grain, damage still occurs, which forces farmers to use chemicals to control the spread of insects. One of the immediate uses of the desiccant system is in long-term storage of crops; where normal aeration fans would not be effective. The collaborative work between QUT and Agridry was beneficial; research and development efforts have been complemented with practical industrial experience, networking, marketing tools and media coverage. Steps have been undertaken to promote the liquid desiccant machine and these are expected to yield positive results. Solar energy is expected to play a greater role in many everyday applications: domestic, commercial and industrial. Solar thermal energy can be used directly in many applications requiring medium and low heat temperature (without electric conversion). Applications such as grain drying, pre-heating of water for industrial and commercial use, and air heating are some examples of several requiring just heat, which can be obtained economically by solar collectors. Developmental projects such as the solar desiccant drying provide the technology push for product commercialisation while financial incentives and government policies provide the necessary market pull. Legislations such as the proposed carbon tax in 2010 and the escalation in fuel prices (diesel and LPG) will be strong drivers to promote the use of solar energy into the agriculture sector. To enable its development policies and incentives are needed to promote the spread of solar powered applications particularly suited to thermal energy. In promoting the technology recognise that Automation as a key feature in any new development of the machine to reduce labour interference and produce consistent results. The ability to operate the machine in either mode (regeneration and dehumidification) would be advantageous to improve performance, reduce cost and offer modularity to expand. A backup power supply for operation on cloudy days would be an essential component of the system to assure clients of energy supply. Further investigations are needed in the use of the desiccant system in long-term storage and preservation, particularly given its potential in regions with very high humidity where normal aeration fans would not be effective. 19

32 9. Recommendations Further investments are needed to support commercialization of the solar powered desiccant drying. Tasks include Conducting field demonstrations to attract early sales as well as market penetration. Controlling growth until the product proves itself to avoid dissatisfaction. Ensuring sufficient demand from high value applications to support initial manufacturing at low volume. Selling equipment for a small profit during early stage of commercialization Emphasizing the inherent benefits of solar drying in early sales as customers willing to switch to solar will feel personal gratification and satisfaction. Leasing the system instead of selling to permit early trials by prospective clients. 20

33 10. References [1] S. Gunasekaran, Optimal energy management in grain drying, CRC Critical Reviews in Food Science and Nutrition, Vol. 25 Issue 1, pp. 1-48, [2] M. D. Lucia and D. Assennato, Agricultural engineering in development Post-harvest operations and management of food grains, Agricultural Services Bulletins, [3] A. S. Mujumdar, Research and development in drying: Recent trends and future prospects, Drying Technology, Vol. 22, [4] A. S. Mujumdar, An overview of innovation in industrial drying: current status and R&D needs, Transport in Porous Media, Vol. 66, pp.3-18, [5] K. J. Chua and S. K. Chou, Low-cost drying methods for developing countries, Trends in Food Science & Technology, Volume 14, Issue 12, December 2003, pp [6] Gershon Grossman and Alec Johannsen, Solar cooling and air conditioning, Progress in Energy and Combustion Science, Vol. 7, Issue 3, 1981, pp [7] Y. J. Dai, R. Z. Wang, H. F. Zhang and J. D. Yu, Use of liquid desiccant cooling to improve the performance of vapour compression air conditioning, Applied Thermal Engineering, Vol. 21, Issue 12, August 2001, pp [8] R. O. Clements and C. A. Jackson, Use of chemical desiccants pre-ploughing to enhance establishment of reseeded pastures, Crop Protection, Vol. 8, Issue 6, December 1989, pp [9] Y. J. Dai, R. Z. Wang and Y. X. Xu, Study of a solar powered solid adsorption desiccant cooling system used for grain storage, Renewable Energy, Vol. 25, Issue 3, March 2002, pp [10] T. F. N. Thoruwa, A. D. Grant, J. E. Smith and C. M. Johnstone, A solar-regenerated desiccant dehumidifier for the aeration of stored grain in the humid tropics, Journal of Agricultural Engineering Research, Vol. 71, Issue 3, November 1998, pp [11] G. R. Thorpe, The modelling and potential applications of a simple solar regenerated grain cooling device, Postharvest Biology and Technology, Vol. 13, Issue 2, April 1998, pp [12] P.A. Davies, A solar cooling system for greenhouse food production in hot climates, Solar Energy, Vol. 79, Issue 6, December 2005, pp [13] Riyad Hodali and Jacques Bougard, Integration of a desiccant unit in crops solar drying installation: optimization by numerical simulation, Energy Conversion and Management, Vol. 42, Issue 13, September 2001, pp [14] Kamel G. Mahmoud and Herbert D. Ball, Solar desiccant systems for grain drying, Energy Conversion and Management, Volume 31, Issue 6, 1991, pp [15] S. Murali Krishna and S. Srinivasa Murthy, Experiments on a silica gel rotary dehumidifier, Heat Recovery Systems and CHP, Vol. 9, Issue 5, 1989, pp [16] M. H. Ahmed, N. M. Kattab and M. Fouad, Evaluation and optimization of solar desiccant wheel performance, Renewable Energy, Vol. 30, Issue 3, March 2005, pp [17] Nelson Fumo and D. Y. Goswami, Study of an aqueous lithium chloride desiccant system: air dehumidification and desiccant regeneration, Solar Energy, Vol. 72, Issue 4, April 2002, pp [18] T. F. N. Thoruwa, C. M. Johnstone, A. D. Grant and J. E. Smith, Novel, low cost CaCl2 based desiccants for solar crop drying applications, Renewable Energy, Vol. 19, Issue 4, April 2000, pp [19] A. Ertas, E. E. Anderson and I. Kiris, Properties of a new liquid desiccant solution Lithium chloride and calcium chloride mixture, Solar Energy, Vol. 49, Issue 3, September 1992, pp [20] Esam Elsarrag, Moisture removal rate for air dehumidification by triethylene glycol in a structured packed column, Energy Conversion and Management, Vol. 48, Issue 1, January 2007, pp [21] Kyle E. Beery and Michael R. Ladisch, Chemistry and properties of starch based desiccants, Enzyme and Microbial Technology, Vol. 28, Issues 7-8, 7 May 2001, pp

34 [22] J. Khedari, R. Rawangkul, W. Chimchavee, J. Hirunlabh and A. Watanasungsuit, Feasibility study of using agriculture waste as desiccant for air conditioning system, Renewable Energy, Vol. 28, Issue 10, August 2003, pp [23] Murray Hill, The drying and storage of grain and herbage Seeds, Foundation for Arable Research, ISBN , [24] D. V. Merrifield and J. W. Fletcher, Analysis and development of regenerated desiccant systems for industrial and agricultural drying. Final Report, April - December 1977, Department of Energy, Ed., pp [25] Esam Elsarrag, Performance study on a structured packed liquid desiccant regenerator, Solar Energy, Vol. 80, Issue 12, December 2006, pp [26] P. Gandhidasan, Prediction of pressure drop in a packed bed dehumidifier operating with liquid desiccant, Applied Thermal Engineering, Vol. 22, Issue 10, July 2002, pp [27] Mark Davie, Liquid desiccant solar crop drying system - Market assessment, Australian Institute for Commercialisation, April

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36 Developing a Solar Drying Machine for Agricultural RIRDC Publication No. INSERT PUB NO. Products HERE bu Kame Khouzam RIRDC Publication No. 09/026 This report presents a new technique for drying agricultural products such as cereal grains and seeds. The project developed and trialled a prototype drying machine based on dry (dehumidified) air rather than heated air and using a liquid desiccant (drying agent) and solar energy. Our business is about developing a more profitable, dynamic and sustainable rural sector. Most of the information we produce can be downloaded for free from our website: Books can be purchased by phoning or online at: gov.au. The report is targeted at farmers, grain dryers and aeration industries as well as fruit and vegetable drying companies and policy makers involved in alternative energy sources, particularly the use of solar thermal energy. The Rural Industries Research and Development Corporation (RIRDC) manages and funds priority research and translates results into practical outcomes for industry. This publication can be viewed at our website All RIRDC books can be purchased from:. Contact RIRDC: Level 2 15 National Circuit Barton ACT 2600 PO Box 4776 Kingston ACT 2604 Ph: Fax: rirdc@rirdc.gov.au web: RIRDCInnovation for rural Australia