Voltaic cell activities of tree-crops and fruits

0360-5442/92 $5.00 + 0.H Copyright@ 1992 Pergamon Press Ltd

Energy Vol. 17, No. 10, pp. 993-995, 1992 Printedin Great Britain. All rightsreserved

NOTE VOLTAIC CELL ACTIVITIES OF TREE-CROPS FRUITS V. C.

SHARMA

and A.

AND

SHARMA

Department of Physics, University of Benin, Benin City, Nigeria (Received 29 July 1991; received for publication 23 February

Abstract-Some

commonly available tropical banana, mango, orange, lime, and pineapple renewable electrical energy.

1992)

tree-crops such as pawpaw, plantain, are identified as potential resources of

INTRODUCTION

the advent of microelectronics and smaller microdevices, progressively less electrical energy is required to operate such devices. For instance, only a few milliwatts of electric power are needed to control the colour and stability of some molecular-electronics materials such as liquid-crystals used in watches and calculators. In the human body, small electrical pulses of the order of few millivolts are all that is needed to sustain the heart-beat and perform vital neural functions. Clearly, small magnitudes of electrical energy have important roles to play. Although solar cells are currently the best known direct energy-conversion devices and are widely used in microelectronics, the possibility of obtaining electrical power from tree-crops has not been explored. The purpose of this study is to show that usable amounts of electrical energy can be obtained from some commonly available tropical tree-crops. Unlike solar cells, tree-crops can provide electrical energy at all times.

With

EXPERIMENTAL

STUDIES

Two electrodes, each of 20.8 cm2 surface area and 2 mm thickness, were cut from commercially available samples of copper and aluminium sheets. For ease of electrodeinsertion into the tree-crops and fruits, each of the two electrodes was cut into a triangular shape with a sharp pointed end (see Fig. 1). The electrodes were polished with sand paper and then washed with deionized water and dried. The dried electrodes were inserted into the tree-crop or fruit, the open-circuit current and potential of which were to be measured. After each use, the electrodes were cleaned, washed with deionized water and dried. In all measurements, the electrode distances were maintained at 3 cm. In order to detect possible directional variations in the open-circuit currents and potentials, the measurements were performed with electrodes first implanted longitudinally (i.e. electrodes inserted parallel to the direction of flow of the tree-sap) and then transversely (i.e. with electrodes perpendicular to the direction of flow of tree-sap).

RESULTS

AND CONCLUSIONS

We present in Table 1 the measured maximum values of the open-circuit current densities in PA/cm’ and the potentials in mV across the electrodes for the two directions of electrode993

994

Note

Fig. 1. Pointed-end

triangular electrode.

implantations. The measurements were repeated 10 times and the average values computed. The quoted errors are the standard deviations in the mean values. We observe from Table 1 that significant amounts of currents and potentials are generated from various tree-crops and fruits. The generation of electric current from tree-crops using aluminium and copper electrodes is analogous to the electrical activity in the Voltaic cell. The less electropositive copper acts as anode while the more electropositive aluminium acts as cathode. The maximum Table 1. Maximum open-circuit currents and potentials tree-crops and fruits.

Tree-crop

Location of electrodes/ nature of fruit

Open-circuit current density W/cm*)

from some tropical

Open-circuit potential across the electrodes (mv) f2

Direction of implanted electrodes

Pawpaw tree

Trunk

18f3 18 f 3

7 7

Longitudinal Transverse

Pawpaw fruit

Unripe

14f2 14f2

5

5

Longitudinal Transverse

Plantain

Trunk

17f3 18f3

5 6

Longitudinal Transverse

Banana

Trunk

18f3 20f3

4 7

Longitudinal Transverse

Mango tree

Trunk

18f3

5

Longitudinal

Orange fruit

we

13f2 14f3 13f2 15f3

4 5 4 5

Longitudinal Transverse Longitudinal Transverse

14f2 22f4 17f3 20f4

4 8 4 6

Longitudinal Transverse Longitudinal Transverse

5 5

Longitudinal Transverse

Unripe

Pineapple

Ripe Unripe

Lime

Ripe

loof loof

Note

995

values of the measured current densities ranged from 13 f 2 PA/cm* for oranges to 100 f 15 PA/cm* for lime. The corresponding values of the open-circuit voltages were 4 and 5 f 2 mV. We also observe from Table 1 that the maximum values of the currents and voltages measured in the transverse mode are generally higher than those in the longitudinal mode, which is probably due to the fact that in the transverse mode, a larger number of mineral ions are intercepted by the electrodes that participate in the conduction process than in the longitudinal mode. The amount of power generated depends on the nature and surface area of the electrodes and also on the distance between the two electrodes. From this preliminary study, we conclude that usable amounts of electrical energy can be produced from tree-crops and fruit. Further work is now in progress to study the effects of pH and concentrations of tree-sap on the electrical activity and also the decay currents with time for the measured data. Acknowledgementi-We express our thanks to S. Sharma and D. Shah for useful discussions. A research grant (No. 60/238) from the University of Benin is gratefully acknowledged.