A novel solar dryer for drying woodchip has been developed in Scotland. In this paper, designs and performance of both solar collector and dryer have been presented separately. Woodchip drying performance has been analysed for a range of temperatures (10ºC to 51ºC) and flow rates (70m³/h to 280m³/h). Page model has been used for modelling the drying curves as a function of temperatures and drying velocities.The thermal solar system considered consisted of a solar collector based on the transpired plate type and a small 10We PV panel unit employed to run a 5We fan. The performance of the system is presented in terms of air flow rate and temperature increments as a function of irradiance levels.Keywords: Solar air heating, solar dryer, woodchi

Clemente, Andres

Grassie, Tom

Henderson, Douglas

Kubie, Jorge

Repository@Napier

Solar application for drying woodchip in Scotland A. Clemente, T. Grassie, D. Henderson and J. KubieNapier University, 190 Colinton Road, Edinburgh, EH10  5DT Scotland, UK AbstractA novel solar dryer for drying woodchip has been developed in Scotland. In this paper, designs and performance of both solar collector and dryer have been presented separately. Woodchip drying performance has been analysed for a range of temperatures (10ºC to 51ºC) and flow rates (70m³/h to 280m³/h). Page model has been used for modelling the drying curves as a function of temperatures and drying velocities.The thermal solar system considered consisted of a solar collector based on the transpired plate type and a small 10We PV panel unit employed to run a 5We fan. The performance of the system is presented in terms of air flow rate and temperature increments as a function of irradiance levels.Keywords: Solar air heating, solar dryer, woodchip1. IntroductionWoodfuel is a clean energy resource that reduces the dependency on imports of fossil fuels and contributes to the reduction of CO2 emissions that cause climate change. The main production of woodfuel in Scotland comes from forestry and timber industries. Changes in the energy policy and high production of forestry mass give a significant role to the wood fuels in the Scottish heat power market. [1]. Researchers have predicted that wood fuel production will be equivalent to 4.5 TWh or  11 % of the heat demand in Scotland [2].Woodchip for burning is a bulky fuel characterized by the size and the shape of the chip and its heating value, highly dependant on the moisture content, MC. The percentage of water in a fresh cut Sitka Spruce wood sample can be up to 65 % MC on wet basis. Thus removing water from the woodchip is a necessary step in the wood fuel chain supply in order to improve the quality of the product: reduce storage and haulage costs and enhance the burning performance [3]. Drying wood requires time and energy. As an alternative to natural drying or fuel heated dryers, solar thermal systems can be used as a cheap and sustainable method to reduce the drying times, suitable for small scale producers [4].  Scotland is located at high latitudes (between 50°N and 60°N) and it has a moderate maritime climate. Despite the low average temperatures, there is a long period of daylight during the spring and summer time that makes solar energy an important power resource for preheating air applications. Previous works on solar ventilation have been accomplished in Scotland as a solar slate system by Odeh [5] and solar heater for pebble bed stores by Grassie [6].A novel solar dryer has been designed in order to assess the capacity of drying woodchip using exclusively solar energy. The solar thermal system consists of a solar collector that increases the temperature of air that has been delivered by a fan connected to a PV-panel. This warm air passes through the wet wood chip located on a tray.The design and operation of the present system is considered in respect of drying woodchip in a small scale. Although woodchip is commonly dried in high volume rates in forestry factories, the decentralization of wood fuel production in Scotland leads to its use in medium and small size installations where users look for minimizing production costs [2] Woodchip usually is stored outdoors drying in natural air ventilation. So a solar thermal system can be used as a backup to reduce the drying times for a small woodchip production.For effective woodchip drying it is necessary to supply the maximum flow rate at higher temperatures. The solar thermal system when run at high flow rates yields lower flow temperatures and vice-verse. The optimum system design and operation are a compromise between the performance of the woodchip dryer and the solar collector.The solar dryer tests were taken in the workshops of Napier University in Edinburgh. The project consisted of two independents parts that were studied separately: the dryer and solar collector. The dryer was designed and built on basis of the outlet flow from a solar collector described in the paper. After the study of the woodchip drying performance, the dryer was connected to the solar collector for the study of the solar collector and solar dryer. 2. Drying woodchip characteristics2.1. Dryer setupThe woodchip drying process has been assessed experimentally in the lab. The range of flow rates and heat inputs covers the values expected from the solar thermal system, thus the dryer works at low drying velocities and low temperatures.The dryer consisted of the supply unit and drying chamber where the woodchip was located. The supply unit consisted of the apparatus and instrumentation necessary to deliver the air flow and control its quality. The fan used was a PASPT 10W 12DC that blows a volume of air up to 280m³/h. The air temperature was regulated by an electrical resistor that provided heat up to 1200W.Fig 1: Dryer set upThe drying chamber was an isolated wooden box that contained the tray where the woodchip was held. The volume of the chamber was defined by the height, 65 cm and the area of the tray, a square area of 51 cm side. At the bottom of one of the sides, there was a square aperture (28 cm x 28 cm) that adapted the flow input getting into the box. Once the air crosses the tray of woodchip, the air was exhausted through a cardboard chimney located on the top of the drying chamber. The tray that holds the woodchip was made of aluminium sheet on the sides and a plastic mesh (3 mm square holes) at the base that allows the air pass through.The woodchip treated for the study was shredded wood coming from Sitka Spruce trees grown in the Scottish forests. The woodchip employed had a normal distribution of sizes and shapes. The product was characterized by the length of the blade used to chip the wood; hence 2 cm was the characteristic dimension of the woodchips. The average of the initial moisture content in the sample, MC0, for all the tests was 53% MC.2.2. Drying experimentsIn order to describe the woodchip drying curves and study how the drying factors affect, a series of tests were conducted at different temperatures and velocities.Drying is the process of removal water from a product. As a thermal process, the performance of any dryer mainly depends on the construction, design parameters as well as the operating conditions: flow rate and heat input [7]. During the tests, the weight of the sample, flow rate and psychrometric parameters: temperature and humidity were measured periodically using the appropriate instrumentation. Sets of tests were undertaken at different temperatures, between 10°C and 50°C, for different drying velocities. The experimental drying velocities tested on the tray (area 0,25 m²) were up to 0.25 m/s that correspond to an air flow rate of 280 m³/h. For each test, 3 kg of woodchip at 52% MC were spread over the tray forming a thin layer of 3 cm thickness. The woodchip was stirred to facilitate uniform drying. The sample weight was measured at 2 minutes intervals by a precision scale. Moisture content decreased until it reaches the equilibrium with the ambient. At the end the sample was dried up in an oven at 105°C. The ambient lab conditions were steady during the tests; temperature and relative humidity remained initial levels through the test. The performance of the dryer was expressed the rate of water content with the product weight in the drying curves. Figure 2 shows the experimental variation of MC with time for selected tests at different drying conditions described in Table 1. As it is expected, MC decreases with increasing drying time describing an exponential curve. Drying time decreases as temperature and flow rate increase.Table 1: characteristics of the tests used for the drying modelTest	Tamb, ºC	RHamb, %	Flow, m³/h	Vdrying, m/s	Tdrying, ºC	RHdrying, %	Power, W1	8	70	210	0.1875	8	70	**2	21.5	29	140	0.125	21.5	29	**3	20	28	70	0.0625	32	15.5	2464	21.7	30	280	0.25	31.4	13.5	6435	23	23	210	0.1875	45	8	1210Fig 2:  shows the evolution of MC with time for 5 tests2.3. ModellizationThe main purpose of the drying tests is to obtain a mathematical model that describes the performance of the dryer under any given condition. The drying curves can be modelled using theoretical or semi-theoretical equations [8]. The model expressions concern the simultaneous heat and mass transfer equations that describes the process. Moisture ratio equation is the common theoretical expression to model the drying process, described in Equation 1:						(1)Where MC is the moisture content at any time, MC0 is the initial moisture content and MCeq is the equilibrium moisture content obtained as the asymptotic value of the weight of the sample when it remains constant [9].Many researchers have described the solar drying process for common products like crops, fruit, leaves, etc. using different mathematical expression, all based on equation 1. The Page model equation, equation 2, resulted in a simple expression, similar to the theoretical expression that employs two constants: k and n, to describe with high degree of precision the woodchip drying performance [9]. Thus each drying test is described by two constants as it is shown in the table 2.								(2)Table 2 shows the values of the constants k, n for 5 tests, described before in Table1, selected to build a global model that describes the drying process for woodchip. The statistical values, correlation coefficient, R, and mean square of deviations, X², shows the good agreement between data and modelled values for the Page model.Table 2: Constant values k and n for 5 selected tests.	k(min−1)	n	R²	X²test1	0.068747	1.285319	0.999008	0.00008690test2	0.176961	1.158643	0.995203	0.00036985test3	0.123004	1.323546	0.99714	0.00024686test4	0.516446	1.13697	0.995961	0.00021541test5	0.572723	1.045855	0.998157	0.00008684The effects of temperature and drying velocity on the moisture ratio were investigated using a multiple regression analysis to account for the drying variables on the Page model constants. The values of constants k and n were regressed against those of drying air temperature and air velocity using multiple regression analysis. All possible combinations of drying variables were included and tested in the regression analysis [10]. The multiple combinations of different parameters which gave the highest R2 were finally included in the model. The model equation was as follows:    (3)Validation of the Page model was confirmed by comparing the estimated or predicted moisture ratio at any other particular drying condition. The validation of the Page model at different air temperatures and air velocities is shown in Figure 3, where the experimental data of 4 tests is compared with the predicted values obtained from the model giving a good fitness.Fig 3: Comparison of experimental and predicted MR for 4 new tests.3. Solar collector performance3.1. System design and operationThe solar thermal system considered consisted of a fan connected to a PV panel unit and the solar collector that warmed up the air flow. Small increments of temperature were expected from the thermal performance so this fact implied simplicity of the prototype. The solar collector was based on the transpired plate type: the collector was a wooden box that comprised a perforated absorber plate made of 1.6 mm thick aluminium. The area of the collector is 1.80 m2 has been drilled forming a distribution of 2 mm diameter holes spaced 20mm apart. The lower section of the collector frame had 35 holes of 20 mm diameter for air inflow. At the rear backing plate on the top, the fan has been mounted to deliver the air into a 150 mm flexible duct. The gap between the bottom of the collector and the absorber plate was 110 mm. In addition, considering the operating temperatures predicted at higher levels of irradiance, the channelled transparent polycarbonate cover is held 4 cm from the absorber plate to minimise convective heat loss. [5]3.3. Solar collector flow rates and temperaturesThe system employed a small capacity fan, 12 DC 5We PAPST that required an external energy supply. The power needed was supplied by a small PV-panel unit of 10 We. Thus the performance of the fan was not continuous and therefore the flow rate was mainly depended on the irradiance levels. Thus for the highest irradiance level, the collector delivered the maximum flow rate and for lower irradiance levels, there was a limit where the fan did not operate.The study of the solar collector performance requires the analysis of the flow rate dependence with the irradiance but also the pneumatic system characteristics. The fan was powered by the PV-panel that pumped the flow through the system to cross the woodchip. In order to find the relationship between flow rate and irradiance, it was necessary to study the pneumatic characteristics of both fan and system, and also the electrical connection between fan and PV-panel separately.The electrical connection between PV-panel and fan can be analysed in the I-V curves that describes the electrical characteristics of both 10We PV-panel and 5We fan, Figure 5 shows operational voltages at two irradiances levels. It may be noted that not all the power generated by the PV-panel is supply to the fan as operational points don’t coincide with the maximum power curve.  During the periods of high irradiance, the fan will be operated over capacity, although the fan motor showed no signs of fatigue or damage. [6]Fig 5:  PV module and fan electrical characteristics with systems operating voltageThe final drying flow depended on the pneumatic characteristics curves of the system. The flow blown by the fan needs to overcome the pressure drop associated to the air resistance to pass through the solar dryer. The head losses of the solar dryer were located in three differenced parts: the transpired plate, the flexible duct and the woodchip layer. Therefore the drying flow depended on the type of plate used and on the thickness of the woodchip layer. For all the tests, the thickness of the woodchip layer was 3cm that corresponds to the 3kg of product employed before in the drying tests.Figure 6 shows a plot of measured flow rate as a function of irradiance for a single day period. The test was taken on the 4th of August in 2008 when the sky was overcast and occasionally sunny. The flow rate increases reaching its maximum value for the higher irradiance, so for 1310 W/m² the air flow was 248 m³/h. The connection of the PV-panel with the low power fan is reflected on a low threshold irradiance of 135 W/m² that corresponds to a flow of 57 m³/h. This enhanced the system potential to work even on days with low light.Fig 6: Measured flow rate vs. irradianceFigure 7 shows the collector outlet and collector temperature rise against the irradiance for the same day as previous. Considering that air flow rate increases with irradiance, the graph shows the tendency of temperatures to increase with irradiance as well. For low irradiances the average temperature rise was 10ºC and for high irradiance levels it was 20ºC. The degree of scatter in the temperature rise is mainly attributed to variances in the ambient temperature and fluctuations in the collector flow. Also the transitory time to the steady state and wind effect may affect in the collector air and absorber plate temperatures [11].Fig 7: Steady state temperatures in the collector and temperature rise vs. irradiance3. Conclusions and future workThe solar dryer presented is an alternative for drying woodchip in Scotland. Dryer and solar collector are parts of the solar dryer and have been presented and analysed separately. Woodchip drying performance has been described for certain conditions in a range of low temperatures, relative humidity and low drying velocities. Concluding that increments in either flow rate or drying temperature reduces the drying times. Page model has been found to be a simple and accurate model that describes with good agreement the moisture content curves as a function of drying velocity and temperature.The solar thermal system presented is a particular prototype based on the transpired plate collector. The connection between PV-panel and fan comes to a low threshold irradiance that allows the system works in poor light conditions. The thermal system increases considerably the ambient temperature in all the range of irradiance and delivers flow at high temperatures with high efficiency at high irradiance levels. In future work, in order to optimize the solar collector performance, the configuration of the collector may be modify: hole distribution of the absorber, material employed, the using of glazing cover. Once dryer and thermal systems are completely assessed, it will be possible to draw main conclusions about using solar energy for drying woodchip in Scotland.References1.	FREDS, Forum for Renewable Energy Development in Scotland. Promoting and Accelerating the Market Penetration of Biomass Technology in Scotland, B.E. Group, Editor. 2005: Scotland.2.	Commission, S.D., Wood Fuel for Warmth. June 2005, Scottish Executive: Scotland.3.	Timo Tahvanainen, A.A., Storing of energy wood and wood fuel quality management, F. (Metla), Editor. 10 11 2006: Soria. Finland.4.	Healion, K., Wood as a Renewable Source of Energy. 2002, COFORD Connects, Socio-Economic Aspects of Forestry.5.	Odeh, N., et al., Modelling of flow rate in a photovoltaic-driven roof slate-based solar ventilation air preheating system. Energy Conversion and Management, 2006. 47(7-8): p. 909-925.6.	Grassie, T., Modelling the outlet temperature of a PV driven transpired solar air heater, in SOLARIS New Delhi, India.7.	Brenndorfer, B., Solar Dryers: Their Role in Post-harvest Processing. 1985: Commonwealth Secretariat.8.	Midilli, A. and H. Kucuk, Mathematical modeling of thin layer drying of pistachio by using solar energy. Energy Conversion and Management, 2003. 44(7): p. 1111-1122.9.	Ekechukwu, O.V., Review of solar-energy drying systems I: an overview of drying principles and theory. Energy Conversion and Management, 1999. 40(6): p. 593-613.10.	Sharma, G.P., R.C. Verma, and P. Pathare, Mathematical modeling of infrared radiation thin layer drying of onion slices. Journal of Food Engineering, 2005. 71(3): p. 282-286.11.	Duffie, J.A. and W.A. Beckman, Solar Engineering of Thermal Processes. 1991: Wiley-IEEE.2.	Commission, S.D., Wood Fuel for Warmth. June 2005, Scottish Executive: Scotland.3.	Timo Tahvanainen, A.A., Storing of energy wood and wood fuel quality management, F. (Metla), Editor. 10 11 2006: Soria. Finland.4.	Healion, K., Wood as a Renewable Source of Energy. 2002, COFORD Connects, Socio-Economic Aspects of Forestry.5.	Odeh, N., et al., Modelling of flow rate in a photovoltaic-driven roof slate-based solar ventilation air preheating system. Energy Conversion and Management, 2006. 47(7-8): p. 909-925.6.	Grassie, T., Modelling the outlet temperature of a PV driven transpired solar air heater, in SOLARIS New Delhi, India.7.	Brenndorfer, B., Solar Dryers: Their Role in Post-harvest Processing. 1985: Commonwealth Secretariat.8.	Midilli, A. and H. Kucuk, Mathematical modeling of thin layer drying of pistachio by using solar energy. Energy Conversion and Management, 2003. 44(7): p. 1111-1122.9.	Ekechukwu, O.V., Review of solar-energy drying systems I: an overview of drying principles and theory. Energy Conversion and Management, 1999. 40(6): p. 593-613.10.	Sharma, G.P., R.C. Verma, and P. Pathare, Mathematical modeling of infrared radiation thin layer drying of onion slices. Journal of Food Engineering, 2005. 71(3): p. 282-286.11.	Duffie, J.A. and W.A. Beckman, Solar Engineering of Thermal Processes. 1991: Wiley-IEEE.

Solar application for Drying Woodchip in Scotland.

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Solar application for Drying Woodchip in Scotland.

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