The Coconut As a Renewable Energy Source

THE COCONUT AS A RENEWABLE ENERGY SOURCE

Julian A. Banzon[*]

© Philippine Journal of Coconut Studies. June 1980.

ABSTRACT

The coconut in the Philippines is shown to be a renewable energy source of large magnitude. The energy contained in the 1978 harvest of over 12 billion nuts is calculated to equal 31 x 1012 kilocalories considering only the energy in the husk and shell; this is equivalent of 3.8 billion liters of gasoline. Each of the existing 377 million trees, whether bearing or not, sheds one leaf per month; the petiole of each leaf weighs 2.17 kg dry. Energy from these petioles is calculated to be 39 x 1012kilocalories, a value even larger than that from husk and shell because of so many non-bearing trees. Considering a “standard” coconut plantation of 150 trees/hectare and bearing 10,000 nuts/year, the calculated energy from the shell, husk and petiole amounts to 54.5 million kilocalories. For other sizes of nut harvests, E = 2.6 N + 15.6 where N is in thousand nuts and E is in million kilocalories. If only husk and shell are considered, E = 2.6 N. Shell, husk and petiole may be converted to charcoal, but with an energy loss of about 50 percent; the latter is partly recoverable by using as fuel the non-condensible gasesevolved during charcoaling.

In these days of energy shortage, the worth of the coconut as energy source deserves consideration. Sugar cane, cassava, forest trees and oil-bearing plants have all been carefully considered as possible sources of calories. The coconut has been largely ignored probably because of a preconceived notion that it will not compare well with other crops. In oil yield per unit area of land, the coconut falls below that of the oil palm for example; in fact the oil production per tree and per hectare is so awfully low in the Philippines that there has been calls for replacement with crops of greater productivity. While there has been much enthusiasm in the potential use of coconut oil as diesel fuel substitute, it should be realized that only 10% of the weight of the nut is coconutoil. By many counts it appears that the coconut is not a very good commercial crop. Not only is yield per hectare and per tree low, but also there are other reasons seemingly unfavorable to the coconut. Soybeans for example are known to contain 18% oil, a figure considered low compared to copra with oil content 60-70%; but it should be noted that coconut oil is only 10% of the weight of the nut. Again soybeans are well ahead in their 40% protein content compared to coconut with 1% by weight of the whole nut (It is about 4% of the weight of the coconut meat). The nutsare bulky and heavy and pose problems in transport. One-fifth of the nut is water, a “dead weight”. The percentage occurrence of the principal components of the coconut is given in Figure 1.

The coconut however has its own good points. It is a permanent crop. Once planted there is no need to reprepare the land with consequent expenditure of energy. It is non-seasonal ad provides a continuous supply of nuts practically month after month. This is quite an advantage in maintaining a processing plant in full operation throughout the year.

Energy-rich components of the coconut. The oil of the coconut is the item first remembered as the energy-rich component. Several studies have been made in the past regarding energy from coconut oil (Balce and Moreno, 1936; Banzon, 1953). But if entire dependence is made on the oil (as for diesel fuel substitute), the yield is rather low, being only 10% of the weight of the nut; the soybean is a better performer, at 18% oil. The coconut however is unique in having other components of the nut that are potentially large energy sources. These are the husk, the shell and the leaf. The coconut trunk is not considered here being harvestable only once and only after several decades. The energy potential of the components of the coconut is given in Table 1.

Table 1. Energy from Components of an Average Coconut
Component / Kg / Kcal/kg / Energy
Kcal / Percent of Total Energy
Coconut oil / .12 / 9000 / 1080 / 27.7
Carbohydrates and proteins / .06 / 4000 / 225 / 5.7
Shell / .18 / 5500 / 990 / 25.4
Husk / .40 / 4000 / 1600 / 41.1
.76 / 3895 / 99.9

The energy value of coconut oil is taken at 9,000 kcal/kg and was adopted after considering the value of 9,288 kcal/kg, gross, given by Child (1974) and a value of 8,944 kcal/kg calculated for trilaurin from the equation of Kharasch cited in Markeley (1947) and 9,020 kcal/kg in Bailey’s treatise on fats and oils. Coconut oil has a slightly higher molecular weight of 645.6 (Banzon & Resurreccion, 1979) than trilaurin, which is 638. Since kcal/kg increases with molecular weight, the rounded figure of 9,000 kcal/kg may be taken as the “best value”.

Table 2. Energy from Philippine Harvest of Coconuts, 1978.
Energy / Calculated Gasoline equivalent
Item / Number x 1010 / kcal x 1012 / for the Year x 109 liters / for one day x 106 liters
Shell / 1.2 / 11.88 1.47 / 4.0
Husk / 1.2 / 19.20 2.38 / 6.5
Coco Meat / 1.2 / 15.65 1.94 / 5.3
Total / - / 46.73 5.79 / 15.8
* Gasoline equivalent: 8,050 kcal/liter
Shell + husk, total 31.08 x 1012 kcal or 3.85 x 109 liters gasoline – equivalent or 10.5 x 1061/day.

The calorific value of coconut shell is taken as 5,500 kcal/kg based on the value given by Paddon and Parker (1979). The heat of combustion of coconut husk is given by Festin (1976) as 3,515 kcal/kg and by Wilson (1930) at 4,192 kcal/kg. A rounded value of 4,000 kcal/kg is used in this paper. The non-oil components of coconut meat are largely carbohydrates. The heats of combustion of starch and of cellulose are 4,177 and 4,179 kcal/kg respectively (International Critical Tables 1929, cited in Hougen et al., 1954). For purposes of computations the rounded figure used here is 4,000 kcal/kg as the calorific value of the carbohydrates and proteins in the coconut meat.

Implications of the energy available from coconut. Considering the 1978 annual harvest given at 12 billion nuts, the energy obtainable from the Philippine crop is given in Table 2; the salient figures are the following:

from coconut shell:990 kcal/nut x 1.2 x 1010 nuts

= 11.88 x 1012 kcal

from coconut husk:1,600 kcal/nut x 1.2 x 1010nuts

= 19.20 x 1012 kcal

from coconut meat:1,305 kcal/nut x 1.2 x 1010 nuts

= 15.65 x 1012 kcal

These figures are better appreciated in terms of gasoline equivalent (liters). Taking the heat of combustion of gasoline at 0.70kg/l, the heat of combustion comes out to be 8,050 kcal/l gasoline. Hence from 12 billion nuts, the gasoline equivalent of the shell is 1.47 x 109 liters and from the husks, 2.38 x 109 liters. Hence, if all the shell and husk of the Philippine coconut crop of 1978 were collected, the energy contained in the shell and husk is 31.08 x 1012 kcal or 3.85 x 109 liters gasoline equivalent. On a daily basis, the volume is 10.5 x 106 liters gasoline equivalent.

The coconut oil is not considered here since it has its own important use as food; but should it become necessary to utilize it as fuel, the potential from this energy source is 1,296 x 1010 kcal or 1.6 x 109 liters of gasoline – an average of 0.134 liter per nut. While these amounts of energy are very large, there are problems in their full utilization. What is endeavored to be shown here is the existence of a very large source of energy in the coconut. It is shown that the coconut crop is as much an energy crop as it is as a food crop.

A coconut plantation can rival the so-called energy forests which are grown solely for the wood fuel that these forests can produce. The coconut plantation has the advantage of producing for besides energy.

Shell and husk are solid fuels and hence have the peculiarities and problems inherent in this kind of fuel. Research is needed to solve these problems. The point is: the amount of energy associated with the shell and husk is so large as to merit the large expense and effort needed by studies to fully exploit these energy sources. It is apparent that even if only one % of the shell and husk is made use of, the energy which may be supplied is still in the order of 38.5 million liters of gasoline equivalent/year on 10,500 liters gasoline equivalent per day.

Comparison of coconut and cassava as energy sources. High hopes are being placed nowadays on cassava as energy source, the ultimate product is to be fuel alcohol. For the present, assume a yield of 20 MT of cassava roots per hectare. At 33% carbohydrate yield, it will give 6.7 MT of carbohydrate/hectare. The calorific value of starch is 4,177 kcal/kg. Hence, the energy expected per hectare of cassava is:

6,700 kg x 4,177 kcal/kg = 28 x 106 kcal

or 28 x 106 kcal divided by .085 x 104kcal/l =

3.48 x 103 liters gasoline equivalent.

For the coconut, assume a harvest of 10,000 nuts/hectare which is equivalent to 67 nuts/tree in a hectare of 150 trees. The total energy potential (oil + shell + husk) of such a hectare of coconut is 36.7 x 106 kcal (Table 3) or 4,550 liters gasoline equivalent. This amount of energy is slightly larger than that from cassava which is 3,480 liters (28 x 106 kcal). If only the shell and the husk are considered, the energy amounts to 25.9 x 106 kcal or 3,210 liters gasoline equivalent. Thus, the energy in the shell and the husk from a 10,000 nut-hectare of coconuts is almost as much as can be furnished by a 20-ton/hectare of cassava. The coconut plantation has the edge over the cassava due to the 1,200 kg oil which can also be extracted from the coconuts.

Table 3. Energy from Components of the Coconut for Various Nut yields per Hectare.
Nut Yield / Energy, kcal x 106
per tree / per ha
x 1000 / oil / shell / husk / total
40 / 6 / 6.5 / 5.9 / 9.6 / 22.0
60 / 9 / 9.8 / 8.9 / 14.4 / 33.1
67 / 10 / 10.8 / 9.9 / 16.0 / 36.7
80 / 12 / 13.0 / 11.9 / 19.2 / 44.1
100 / 15 / 16.2 / 14.8 / 24.0 / 55.0
120 / 18 / 19.4 / 17.8 / 28.8 / 66.0
150 / 22.5 / 24.3 / 22.3 / 36.0 / 82.6

The energy potential from (the husks and shell) a hectare of 150 coconut trees giving increasingly larger harvests of nuts is given in Table 3. Likewise, the energy obtainable from cassava furnishing different amounts of roots/ha is givenin Table 4 from Table 3, the energy E in 106 kcal obtainable from shell+ husk of N nuts (in thousands), may be obtained from the equation: E= 2.59 N and N=0.3862 E. Thus, the number of nuts needed to give 28 x 106kcal (from 1 ha cassava) is N= 0.3862 x 28 or 10.8 thousands (10,800 nuts).

Table 4. Energy from One Hectare of Cassava at Various Root Yields.
Yield, MT/ha / Energy
Roots / carbohydrates / kcal x 106
10 / 3.3 / 13.9
15 / 5.0 / 21.0
20 / 6.7 / 28.0
25 / 8.3 / 34.9
30 / 10.0 / 42.0
35 / 11.7 / 49.0
40 / 13.4 / 56.0
45 / 15.0 / 62.9
50 / 16.6 / 69.8

Coconuts charcoals.Charcoal has desirable characteristics as a fuel. It is non-smoky when burned and is not subject to organic decay, and is a concentrated form of energy; thus while coconut shell has a heating value of 5,535 kcal/kg, the charcoal made from it has a heating value of 7,200 kcal/kg (Paddon & Parker, 1979). Other charcoals produced from coconut materials are very promising. Festin and his students (1976) have been producing coir dust charcoal thus providing another potential commercial outlet for this industrial nuisance. A recent innovation is the development of coconut husk charcoal by Lozada (1978). Coconut husk is so loose and friable a material that it has escaped attention in its utilization as charcoal. The energy inventory of the coconut is given in Table 1 shows that the husk, not only constitutes the largest component of the nut (33.3%) but also the greatest contributor of energy which is over 40% of the total energy value of the coconut.

In the carbonization of coconut shell, Wells (1917) obtained a yield of 32.5% charcoal. The non-condensible gases, which may be used as gaseous fuel, amounted to 16.2% of the weight of the shell; as diagrammed below:

Tamolang (1976) reports that carbonization of the shell in a retort at 3150C gave a charcoal with a heating value of 7,860 kcal/kg and fixed carbon of 83.9% while kilnproduced charcoal at 3240 C gave fixed carbon of 69.5% and kcal/kg of 6,784. Lozada (1978) gives 6,540 kcal/kg as the heat of combustion of his shell charcoal which constituted 28% of the shell or 4.2% of the whole nut. This amounts to 0.0504 kg charcoal/nut which agrees exactly with the figure given by Montenegro (1976).

Coconut husk charcoal. There is probably little difference in heating value between coconut husk charcoal and coir dust charcoal, hence they will be discussed together as one. Coir dust charcoal has a heating value of 5,965 kcal/kg while the fiber, 6,584 kcal/kg(Festin, 1976). It may be noted that the heat of combustion of carbon is 94,050 kcal/mole or 7,837kcal/kg carbon. Festin (1976) gives a value of 5,688 kcal/kg for coir dust charcoal while Lozada (1978) used the higher figure 6,320kcal/kg. For comparison, the heat of combustion of cellulose is 4,179 kcal/kg (Hougen et al., 1954) while for coir dust itself, it is 3,515 kcal/kg (Festin, 1976). The process of charcoaling therefore concentrates the energy by removing extraneous materials leaving the fixed carbon as the final product. The temperature of charcoaling has a large effect on the relative quantities of the products. For a specific example of charcoaling at 5500C, the following was obtained:

Lozada (1978) who has been producing coconut husk charcoal quite extensively for his newly invented general purpose drier, gets a yield of charcoal at only 22% of the weight of the husk.

The energy potentially available from coconut shell charcoal and coconut husk of one nut may be summarized as follows:

Total energy potential from 12 billion nuts:

From shell char 1.2 x 1010 x 362 kcal = 4.34 x 1012 kcal

From husk char 1.2 x 1010 x 548 kcal = 6.57x 1012 kcal

Total 10.91 x 1012 kcal

Energy recovery as charcoal. During the process of charcoaling, non-condensible gases and liquid distillates are removed and solid charcoal with very little volatile matter is left. The total heating value of the charcoal is less than that of the original material, either shell or husk. The extent of such reduction in calories from one coconut is as follows:

Heating value in kcal: % energy recovered

Material of material of charcoal as charcoal

shell 990 362 36.5

husk 1600 548 34.3

These heating values show that as much as possible the shell and the husk should preferably be used direct as fuel to avoid energy “loss”. Part of the lost energy is recoverable by using the gases generated during charcoaling, as fuel. Pilot plant studies on this topic have been reported by Cruz (1978) and by Festin (1976). The calculated energies of shell charcoal and husk charcoal at various nut yields/hectare are given in Table 5.

The leaves of the coconut. One of the largest leaves of the plant world is that of the coconut. It averages 6.1 m (20 ft) long and weighs 2.65 kg air-dry (Zuniga et al. 1965). The average number of fallen leaves/hectare is reported at 2,507/year. Ninety-one percent of the leaf is the petiole which is often used as fuel for cooking in the villages. The leafblade constitutes 7%, and the midribs, 2% of the leaf.

To assess the fuel potential of the coconut leaf, only the petiole will be considered here, hence in one leaf, the petiole will weigh 0.91 x 2.65 kg x 0.90 (10% moisture) or 2.17 kg dry. There are at present (1979) 376.9 million trees (Anon, 1978) each producing at least 12 leaves a year or a total of 45 x 108 leaves with a total weight of 4,000 kcal/kg makes the energy available from the coconut petiole equal to 39 x 1012 kcal. Summarily, the 12 billion nuts excluding he oil can yield the following energy potential:

energy fro shell :11.88 x 1012 kcal

energy from husk:19.20 x 1012 kcal

energy from leaves:39.00x 1012 kcal

or a total of 70.00 x 1012 kcal equivalent to 8.6 billion liters of gasoline.

Probably about a fraction of this energy is already being used for many purposes like drying of copra, cooking fuel, production of shell charcoal etc.

The (coconut) energy plantation. There is so much talk nowadays about establishing energy plantations:

Table 5. Energy from Shell-Charcoal, Husk-Charcoal and from Uncarbonized Shell and Husk, per Hectare of Various Nut Yields
Energy in kcal x 106
nuts/ha
X 103 / shell / husk / total / uncarbonized
shell + husk
6 / 2.2 / 3.3 / 5.5 / 15.5
9 / 3.2 / 4.9 / 8.1 / 23.3
10 / 3.6 / 5.5 / 9.1 / 25.9
12 / 4.4 / 6.6 / 11.0 / 31.1
15 / 5.4 / 8.2 / 13.6 / 38.8
18 / 6.5 / 9.9 / 16.4 / 46.2
22.5 / 8.1 / 12.3 / 20.4 / 58.3

quick-growing tress that either can be cut back periodically or can be replanted vegetatively. It is shown here in this study, that the coconut plantation is not only an excellent energy plantation but also contains elements of more advanced concepts:

1. The tree is not cut down as has not be done in the case of forest trees. The energy harvest is in the form of husk, shell and leaf-petioles.

2. The energy harvest (of shell, husk and petioles) is more or less regular throughout the year; the supply of energy is practically uniform, month by month.

3. The plantation is permanent and needs no replanting.

4. The energy-rich material is only a by-product and hence low cost; the main commercial product is the coconut meat from which oil, copra meal, desiccated coconut, etc., are commercially prepared and marketed.

5. The coconut (as well as some palms) have leaves with petioles heavy enough and woody as to be harvestable for commercial fuel purposes. The production of these leaves by the tree is very regular, once a month; their formation is certain and independent of fruit production.

While planners are still calculating and designing their energy plantations, the coconut industry has these plantations already existing to the extent of 2.7 million hectares; 377 million trees producing 11.8 billion nuts (Philippine Coconut Authority, 1978); 15% of the nut is shell and 33% is husk. Each coconut tree, whether bearing or not sheds 12 leaves per year, each leaf averaging 2.17 kg dry. For a hypothetical one hectare containing 150 coconut trees bearing 10,000 nuts and 1,800 leaf petioles per year, the energy yield to be expected is given in Table 6.

CONCLUSION

At, present only meat of the coconut whichcomprises 30% of the nut is utilized commercially. The component considered most valuable which is the oil comprises only 10% of the nut. In these days of energy shortage, advantage should be taken of the shell, the husk and the leaves as fuel. It is not a matter of coconut yields per hectare or per tree; the real situation is that the Philippines harvested more than 10 billion nuts in 1977 and is increasing with the years. This is a virtual energy plantation already existing. The shell and husk of this plantation has already calculated energy value of 31 x 1012 kilocalories equivalent to about 3,850 million liters of gasoline. Research should now be directed towards realization of the uses of this energy potential. Some of the problems are: cost of collection of shell, husk and leaves; and the inefficient use of these solid fuels.