INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 3, No 5, Copyright by the authors - Licensee IPA- Under Creative Commons license 3.

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1 INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 3, No 5, 2013 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN Determination of soil thermal conductivity using solar energy and soil temperature in Sokoto Gwani M 1, Abubakar G.A 1, Utah E.U 2 1- Department of Physics, Kebbi State University of Science and Technology Aliero. 2- Department of Physics, University of Jos. Gwani25@yahoo.com doi: /ijes ABSTRACT This study has investigated the thermal conductivity of sandy soil at a site in Usmanu Danfodiyo University Sokoto, Nigeria (13.10 o N,5.23 o E). The results are based on soil temperature, Surface temperature, insolation data collected from the meteorological Department of Sokoto Energy Research Center (SERC) between the months of March to June 2008 by sensor installed by the center for Basic Space Science (CBSS). The soil thermometer was buried at 5cm depth in the ground which measured the soil temperature. The mean value of the soil thermal conductivity is _0.06WM -2 k -1.The calculation of thermal conductivity assumed that the energy radiated from the surface of the earth is equal to that conducted from the soil. It was also observed that the soil thermal conductivity decreases as the temperature gradient is increasing. Keyword: Thermal conductivity, Soil temperature, Solar energy. 1. Introduction The amount of solar radiation reaching the surface of the earth is affected by such factors in the till of the earth on its axis, the sum earth distance, latitude of the location, attitude of the sum of the location, length of the day and the presence and density of clouds, aerosols, and gases in the atmosphere. Apart from direct measurement of any location on earth, the intensity of solar radiation could be computed by meteorological variable. The heat energy from solar radiation changes the states of water liquid to vapour at free water surfaces and rom the soil in a process called evaporation. On the textural,class of soil on the field (variation) sand, salt, clay, loams etc from a graduated sequence from soils that are in nature and easy to handle than the clay, which are very fine and difficult to manage, while these textural class names are determined by particles size distribution. They markedly affect other physical properties such as soil aeration and ease of tillage. The net radiation energy received at the land surface is partly in heating the sub medium. Soil thermal conductivity are useful for determining soil surface energy, soil temperature can be determined by the transport process of heat between the soil and atmosphere (Braver et al 1972). Solar energy has many connotations for individuals and society (Halacy, 1973), for society, the amount of solar radiation reaching the surface of the earth is affected by such factors in the tilt of the earth on its axis, the sun earth distance, latitude and altitude of sun of the location, length of the day, present of aerosols and gases in the atmosphere. Apart from the Received on December 2012 Published on March

2 direct measurements at any location on earth, the intensity of solar radiation can be computed by metrological variables which can be used to determine the soil thermal conductivity. Soil temperature at any time depends on the ratio of the energy absorbed to that being lost. The constant change in this relationship is reflected in the seasonal, monthly and daily temperatures. The temperature of soil in the field is dependent directly or indirectly on the net amount of heat the soil observed, the heat energy required to bring a given change in temperature of a soil and the energy required for changes such as evaporation, which are constantly occurring at or near the surface of the soil. (Utah et al). The net radiation received at the land surface is partly utilized in heating the submedium.soil thermal conductivity is useful in determining the soil surface energy, soil temperature can be determined by the transport process of heat between the soil and atmosphere (Baver et al, 1972). This transport of heat within the soil can occur by conduction (predominantly), convection and radiation (Koorevaar et al, 1983). 2. Materials and method Solar radiation and soil temperature measurement was carried out at the center for basic space science (C.B.S.S) under Sokoto energy research center in sokoto state Nigeria between March and June Sokoto energy research centre is situated in the Usmanu Danfodiyo University Sokoto at latitude of 13.1 o N which is 9km distance away from sokoto metropolis.the sensor is located around the meteorological center of the geography department Usmanu Danfodiyo University Sokoto nd the sensor is 2km away from the Sokoto energy research center where the data was collected. 2.1 Measurement techniques Solar radiation and soil temperature data are available based on several time scales hourly, daily, weekly and monthly basis, each of these data sets has its own specified utility depending upon the nature of the analysis (Kendal and Berdahl 1970), for this study, hourly values of soil temperature, solar energy, ambient temperature and soil temperature were obtained 2.2 Data collection method All measurement were obtain from the meteorology department of Sokoto Energy Research center by the use of Campbell PY56276X data logger which sample the data every hour for the period the experiment lasted, soil thermometer were buried in the ground at depth of 5Cm carefully so that the surrounding soil was left undisturbed and measurement of soil temperature in ( o C) are recorded on the data logger. While for the solar radiation measurement is based on the used of pyronometer which is place in horizontal recorded by the same data logger The logger records the total radiation after converting it using the below equation H h = B h + D h (1) (Kendal et al 1970). Where H h = Total solar radiation 1331

3 B h = Beam solar radiation D h = Diffuse solar radiation, The computation of thermal conductivity was based on the Stefan s Boltzmann equation R Earth = K (2) R Earth = Rate of conduction of heat per unit area. = Temperature gradient. K = Thermal conductivity of soil in question. T is obtained from the two temperatures the mean ambient temperature and mean soil temperature. Z is the depth of the soil thermometers = 5cm = 5*10-10 M R Earth = δt A (3) (Stefan s Boltzmann equation) δ = 5.67 * 10-8 MW -2 K Soil Thermal Wave From the heat conduction, expressed in a partial differential form as (4) Where D = Soil thermal conductivity, this parameter is indicative of the speed at which thermal waves move in the soil type and depth of thermal influence of its active surface. Generally D increase in value with soil moisture until it reaches maximum about 20% by volume and beyond it drops off. For shallow soil layers, it can be assumed that D is constant (as in equation (4) above. 2.4 Soil thermal conductivity Conduction is important for heat transfer in soil. However, if the temperature gradient in a solid or motionless fluid is the rate of conduction of heat per unit areas is proportional to the gradient and the constant of proportionality is called the thermal concuctivity of the materials k i.e G = -K (5) where the negative sign is reminder that heat move in direction of decreasing temperature. Table 1: Monthly average Values of soil temperature, solar radiation and ambient temperature at 12:00 noon Month Soil Temperature Solar Energy Ambient T S - T A ( o C) T S ( o C) Mjm -2 Temperature ( o C) March April May June

4 For the month of March, T o A C T s T A o C :. TA = =309k T 4 A = R earth ( Stefan Botlzman Equation) where = 5.67 x 10-8 Mw -2 K -4 Rearth = 5.67 x 10-8 x (309) 4 T A 4 = 5.23 x 10 2 wm -2 = Rearth Rearth = K 5.23 x 10 2 wm x 10 2 = K x = 1.39wm -1 K -1 Where k is the thermal conductivity, Depth = 5cm or 5 x 10-2 or 0.05m T A mean ambient temperature, = Temperature gradient From R Earth = K = = From Table 1for the month of April T A = o C : = K Rearth = K 5.23 x 10 2 wm x 10 2 = K x = 1.19wm -1 K -1 = Temperature gradient = = 441 From table 1 for the month of May T A = that is T A = = k = 0.05m Rearth = K 5.23 x 10 2 wm -2 T S - T A = O C 5.23 x 10 2 = K x = 1.32wm -1 K

5 = Temperature gradient = = From table 1 for the month of June T A = that is T A = = k = 0.05m Rearth = K 5.23 x 10 2 wm -2 T S - T A = O C 5.23 x 10 2 = K x = 1.30wm -1 K -1 = Temperature gradient = = Table 5: table of result for the month the experiment lasted Month Earth Radiation (Rearth) Thermal Temperature wm -2 conductivity (K) wm -1 k -1 Gradient March 5.23 x April 5.25 x May 5.06 x June 5.04 x Mean Thermal Conductivity K = = 1.30wm -2. K = w -1 mk -1 Table 6: Values of solar energy in (wm -2 ) at the local time on 9 th March 2008 in Sokoto Local 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 Time Energy MJm -2 Table 7: Values of solar energy in (wm -2 ) at the local time on 2 nd April 2008 in Sokoto Local 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 Time Energy MJm Results 1334

6 Figure 1: Variation of Solar energy against time March 2008 Figure 2: Variation of solar energy against time 2008 April. 1335

7 Figure 3: Variaiton of soil temperature and ambient temperature March Discussion Figure 4: Variation of soil temperature and ambient temperature April The continuous measurement of soil temperature and solar energy have been made from 08 March to 10 th June The data in table 1 shows the monthly average values of the soil temperature and solar radiation and ambient temperature computed for the experimental site during the period of the observation. From the calculation made on the thermal conductivity, it was observed that the thermal conductivity varies from month to month as shown in the table above. In the month of March when the experiment started, the thermal conductivity was calculated to be 1.39wm -2 and the 1336

8 temperature gradient was calculated to be In the month of April it was found that the thermal conductivity decrease by a factor of 0.2 with a value of 1.19wm -2 and it was observed that the temperature gradient increases to The thermal conductivity further increases from 1.19 wm -2 to 1.32wm -2 in the month of May and the temperature gradient increases from 441 to in the month of May. Finally the month of June also experience an increase in temperature gradient to and a decrease in thermal conductivity to 1.30wm -2. The Stefan Boltzmann equation used gives an estimated value of radiation to the earth (Rearth ) for the soil in question it has been shown that the steady state equation evaluated the temperature gradient which increases the thermal conductivity of the soil. Generally it was observed that the solar earth radiation was calculated to be constant throughout the four month under study with a value of 5.23 x 10 2 wm -2 where as it was observed that the thermal conductivity and the temperature gradient changes, as the soil thermal conductivity is decreasing,the temperature gradient is increasing this may be because steady state are rare where a soil surface is exposed to annual and seasonal cycles of radiation and partly because changes in the water control or compaction of a soil may change its thermal properties profoundly. The mean thermal conductivity was calculated to be or wm -1 k -1. Figure 1 above shows the variation of the solar radiation as a function of time, from the figure, it was observed that the solar radiation increases with time of the day and the peak solar energy was obtained by 12 noon before it decreases again through the afternoon in the month of March. Figure 2 shows the variation of the solar radiation as a function of time for the month of April, from the figure it was observed that the Solar radiation increases in the morning hour attain it peak at 11:00am and fluctuate a little before it start decreasing gradually in the late hour of the day. Figure 3 and Figure 4 shows the plot of the soil temperature against ambient temperature and both plot shows a similar trend in the month of March and April. 4. Conclusion The analysis of heat conduction in soil is more complex partly because steady state are rare when a soil surface is exposed to annual and seasonal cycles of radiation and partly because changes in the water control or compaction of a soil may change its thermal properties profoundly. The conclusion that can be drawn from this research is that the thermal conductivity decreases as the soil gets wetter and the temperature gradient increases, also it can be concluded that the calculation of thermal conductivity assumed that the energy radiated from the surface of the earth is equal to that conducted form the soil. The Stefan Boltzmann equation used gives an estimated value of radiation to the (earth R Earth ) for the soil in questioned, it has been shown that the steady equation evaluated the temperature gradient which increases the thermal conductivity of the soil. Reference 1337

9 1. Braver L.O.,Gerdner W.H. and Garner N.R., (1972), Soil Physics 4 th Edition J Wiley and Son s Inc, New York. 2. Halacy, O.S., (1973), The coming Age of Solar Energy, Hapars and Row, New York. 3. Kendahl J.M. and Berdahl,.C.M., (1970), Two Black Body Radiometers of High Accuracy New York. 4. Koovaar;,l,B.,Martin, J.,Xiaoyan,J., (1983), Introduction to soil Physics 2 nd edition, Wile and son s. 5. Utah E.U, Ahonsi G.E.C and Makama.E.K., (2006), Effect of solar radiation on evaporation Nigeria journal of space research, (2), pp 55-62, 1338