THE FUEL CELL AS THE SOURCE OF ENERGY IN THE DRIVING SYSTEM OF AN ELECTRICAL GRASS MOWER – STUDIES OF THE FUNCTIONAL MODEL

Marek Adamiec¹, Marek Kuna-Broniowski^2
1 Department of Motor Vehicles, Lublin Technical University, Poland
² Department of Electrotechnics and Measuring Systems, University of Life Sciences in Lublin, Poland

ABSTRACT

The article present results of the studies on the functional model of an electrical grass mower, powered by an alkaline fuel cell in the aspect of exploitation properties and energetic efficiency. Variability of the load torque, tension and rotating speed while mowing were determined. The energetic efficiency of the system “fuel cell – mower” and the conditions of effective mowing (active length of the knife and the maximum progressive speed of the mower) were calculated.

Key words: fuel cell, driving system, renewable energy, hydrogen.

INTRODUCTION

Problems pertaining to environmental protection and increased prices of oil cause a search for ecological and renewable sources of energy. Contrary to considerable possibilities in case of stationary sources of energy, it is a difficult task, both technically and economically, to replace fuels based on crude oil and its derivatives for mobile devices such as means of transport, working and agricultural machines [2]. At present the best solution which can fulfill both the ecological and technical requirements set for the driving systems of self-propelled devices is to use fuel cells fed with hydrogen as the sources of energy. However, a widespread use of fuel cells meets a number of problems, the most important of which concerns the problems connected with storing hydrogen on a mobile object. A lot of companies, mainly producing cars, do intensive research on the use of fuel cells fed with hydrogen; however, the solutions are still at the initial stage and despite numerous announcements it cannot move beyond the limit enabling to start mass production.

The use of fuel cells to drive devices of a small range, operating near the source of hydrogen feeding makes it possible to avoid the main difficulties connected with storing hydrogen on the vehicle and fulfilling safety regulations. Such conditions of using fuel cells can be fulfilled in some kind of work connected with agriculture and horticulture, where the distances from the “base” station of hydrogen are not big and there is a possibility of supplementing the hydrogen fuel easily [1].

The fuel cell is a converter of the chemical energy of hydrogen fuel into electrical, and partly into heat energy. Hydrogen oxygenates in the anode of the cells, while on the cathode the reaction of oxygen reduction takes place. The fuel oxygenates in the electrochemical process, which makes it possible to transform the energy contained in it directly into electrical energy. That is why fuel cells have considerable efficiency, reaching 80%.

The fuel cell can be used in energetics and in electro-mechanical driving systems. An increased use of fuel cells is connected with the production and distribution of hydrogen fuel. There is a chance to use fuel cells locally in agricultural areas on condition of developing the technology of transforming renewable energy [3], which – apart from other goals – can also be used in hydrogen production (water electrolisers fed from photovoltaic installations, windpower stations and hydropower stations). These two ways of transforming energy (“renewable energy – hydrogen energy” and “hydrogen energy – electrical energy”) are interrelated and should complement each other.

The article presents studies on using the fuel cell in order to drive a grass mower, which is a device used in agriculture, in household and recreational objects. This kind of use, for the time being having only a niche character, is important, however, to work out the techniques connected with using hydrogen as a driving power and as the source of energy in agriculture.

In order to illustrate the phenomena taking place in the actual driving system using the fuel cell as the source of energy, studies were conducted on a model of an electrical rotating grass mower. The structure and studies of the mower model are only an example of a potential use of fuel cells in agriculture, where the demand for energy is huge and the cells can be used in transport devices and vehicles, agricultural machines, drying, heating industries and in computer devices [1,2,8].

A DESCRIPTION OF THE FUNCTIONAL MODEL

Rotating mowers with a disc or a drum cutting units are used to mow the lawns. In the former the cutting knives are placed on the circumference of the disc rotating in the horizontal plane, while in the drum unit the knives are placed on the circumference of the drum rotating in the vertical plane [4]. Cutting units in the form of a two-blade knife rotating in the horizontal plane are often used, like in the disc system.

While choosing the electrical engine to a mower, the course of its mechanical characterization should be taken into account. The specific character of the mower’s work causes that the load torque is irregular and can change within a broad range. In such a case an engine with stiff mechanical characterization, ensuring small changes of the rotating speed in the function of loading, should be used in its driving system. Among the traditional and inexpensive engines of direct current, this task is fulfilled by the engines of shunt type, where the winding of excitation is in a parallel manner connected with the winding of the armature. A more costly solution is to use an engine with a permanent magnet, with similar mechanical characteristics. In this case the winding of excitation does not take place, and the source of the magnetic field is a permanent magnet.

The construction of an electrical mower model uses a commutative engine of direct current with permanent magnet in a stator with the nominal voltage of 6 V, belonging to a group of micro-machines [6]. A two-blade cutting knife is fixed on the rotor of the engine, and on the other side of the shaft there is an induction sensor, which makes the measurement of the rotating speed possible (Photo 1).

The mower’s engine was driven by a laboratory pile of alkaline fuel cells (with the maximum power of 30 W), consisting of eight elementary cells connected in a row and placed on the research site.

The alkaline fuel cell AFC belongs to a group of low-temperature fuel cells. Its work is based on the reaction of oxidation and reduction. Hydrogen is sued as fuel, and the oxidant can be pure oxygen or air oxygen, while KOH is the electrolyte [7]. The cell of 30 W used in the model has the weight of 0.9 kg and the dimensions (length×width×height) of 0.095×0.115×0.130 m. The same type of cell of 1200 W is characterized by the weight of 18 kg and the dimensions (length×width×height) of 0.4×0.21×0.25 m [9]. In case of the practical use in the mower, the weight of such a cell considerably increases the weight of the whole device; however, such a system is competitive to the mower using a battery, which is also of considerable weight.

Hydrogen and oxygen were obtained on the research site in the process of water electrolysis in the system of “photovoltaic cell – electroliser”. Gases were gathered in hydraulic containers, equipped with valves and connected by means of tube lines with the electroliser and the fuel cell.

The research site was equipped with a measurement unit and a computer with an analogue-digital converter, making it possible to record the electrical signals of voltage.

METHODS

The research was conducted on a laboratory stand (fuel cell, engine) and in the field (the model of an electrical grass mower). To this aim, an area which made it possible to do the experiment on a lawn belonging to the Department of Technology Basis of the Agricultural University of Lublin was separated. The area of grass chosen for model studies made it possible to mow by means of a small model of a mower and enabled considerable variability of the moment of loading, which constituted the basic condition of the experiment.

While mowing, the values of voltage, current and the rotary velocity of the engine were recorded, and then variability of the moment of loading was calculated on the shaft. The measurement of the rotating speed was realized by means of a unit consisting of an inductive sensor and the converter of frequency-voltage, obtaining a voltage signal proportional to the rotating speed of the engine. An analysis was made of the changes of voltage produced by the cell and the rotating speed of the engine in the function of the load torque, changing with big frequency and within wide ranges. The chart presenting the relation between the rotating speed of the mower’s engine and the load torque was compared with its mechanical characterization, which shows the relation between the rotating speed and the load torque with constant voltage of the engine power. Laboratory studies marked the energetic efficiency of the examined fuel cell and electrical engine, and next – on the basis of field studies of the mower’s model – energetic efficiency of the unit “fuel cell – mower” was calculated.

Energetic efficiency of the fuel cell was calculated as the relation of the obtained electrical energy to the chemical energy of the used hydrogen, on the basis of the following equation:

(1)

where:
U – voltage of the fuel cell, V;
I – current of the fuel cell, A;
t – time, s;
n – number of hydrogen moles used at time t, mol;
ΔH_T – change of enthalpy in the reaction of water synthesis at temperature T, J·mol^-1.

The number of moles of the used hydrogen n was calculated on the basis of Clapeyron’s law as the difference of the number of moles of gas contained in a container at the beginning n_p and at he end of the measurement n_k. The number of moles of hydrogen collected in the container before the measurement was calculated according to the following relation:

(2)

where:
p_p – hydrogen pressure at he beginning of the measurement, Pa;
p_atm – atmospheric pressure, Pa;
V_p – hydrogen volume at the beginning of the measurement, m³;
R – gas constant, 8.31 Pa · m³∙mol^-1∙K^-1;
T – hydrogen temperature, K.

The number of moles of hydrogen left in the container after time t was calculated according to the following:

(3)

where:
p_k – hydrogen pressure at he end of the measurement, Pa;
V_k – hydrogen volume at the end of the measurement, m³.

The value of hydrogen pressure in the container was marked on the basis of reading the water column putting pressure on the gas. The temperature was measured in the water contacting the gas.

In order to calculate the efficiency and the load torque of the mower’s engine, the resistance and inductivity of its winding and the mechanical and electrical losses of power were first established.

The engine’s efficiency was determined as the relation of its mechanical useful power to electrical power:

(4)

The useful instant on the engine’s shaft, equal to the load torque, was calculated as the relation of the useful instant of the engine to its angular speed:

(5)

The energetic efficiency of the unit “fuel cell – mower” was marked as the relation of the obtained mechanical energy to the chemical energy of the used hydrogen fuel, on the basis of the following relation:

(6)

where:
– mean value of the mechanical useful power of the engine while mowing, W;
t – time of mowing, s;
n – number of hydrogen moles used at time t, mol;
ΔH_T – change of enthalpy in the reaction of water synthesis at temperature T, J·mol^-1.

In case of using a cutting mechanism without the counter-cutting surface, the mowing efficiency is determined by the knife’s speed, which should be higher than the critical speed for a given kind of the plant that is cut. In case of grasses, this critical speed is within the range from 8 to 16 m∙s^-1 [5]. On this basis the active length of the mower’s knife was calculated for which this condition is fulfilled.

For the mowing to take place without omitting a part of the lawn’s area, the proper relation between the linear speed of the knife and the progressive speed of the mower V_n∙V_k^-1 must be kept. For a disc cutting unit with two knives on the disc’s circumference (or a two-blade knife), this condition has the following form [5]:

(7)

where:
R – distance between the rotation axis and the knife’s edge, m;
h – knife’s active length, m.

After considering the calculated active length of the knife and the critical speed required for it (8 m·s^-1 for grasses), the maximum progressive speed of the mower, with which the latter should move to ensure effective mowing (without omitting a part of the mowed area) was calculated.

RESULTS

The studies conducted with the use of a mower’s model recorded variability of the cell voltage and the speed of the engine in the function of load torque while cutting the grass. The measurements were performed with the frequency of 1 Hz in the time of 60 s. Different values of voltage and corresponding rotating speed were observed for the same load torque (Figs. 1, 2). The phenomenon is caused by a certain delay in reproducing the cell voltage, which with considerable loads requires the proper time to get stabilized.

While driving the engine by way of a fuel cell with the nominal voltage of 6 V, because of a rather considerable reduction of the cell’s voltage in the function of loading, we also observe a considerable decrease of the rotating speed of the engines with increased load torque. On the other hand, the mechanical characteristics of the engine, marked with constant voltage of 6 V, is linear and shows smaller speed depending on the load (Fig. 3).

The load torque of the engine, which changes with considerable frequency and within broad limits, was calculated on the basis of the measurements of voltage, current and rotating speed of the rotor (Fig. 4).

The energetic efficiency of the examined fuel cell depends in the smallest degree on its current load and it changes within the ranges from 30 to 51% for the current intensity changing within the limits from 4.5 to 0.5 A. The examined engine, used in the driving system of a mower’s model, takes the current of the intensity from 1.2 to 4.5 A, and it reaches the maximum efficiency of 53% with the current of 3.4 A. The energetic efficiency of the unit “fuel cell – mower” was calculated on the basis of relation (6) during a test of 60 s. This efficiency is equal to 19.03%.

Next the active length of the knife (width of mowing) for a given model of mower was calculated so that the linear speed of the mower’s knife would be higher than the lower limit of the interval of critical speeds for grasses (8-16 m·s^-1). In this case the established active length of the knife is 0.015 m.

It was calculated on the basis of relation (7) that the maximum progressive speed of the mower with which it should move to ensure effective mowing (without omitting a part of the area) should be according to the following equation:

CONCLUSIONS

The theoretical analysis and experimental studies conducted on its basis in the field point out that at the present state of development fuel cells can be an alternative source of power in agricultural work. The fuel cell is in practice the current source of electrical energy, which means that its intensity lowers considerably in the function of the current load and for the maximum current it reaches the value by about 50% lower than the value of voltage produced in the unproductive state. The power units requiring small rotating variability in the function of loading (a grass mower) should be designed in such a way that he cell should work in within the range of stabile voltage. The studies showed that the unit “cell – electrical engine – cutting device” fulfills the requirements necessary to mow the grass in an effective way. What attracts attention is the high energetic efficiency of the whole process, which is 19% and this makes it possible to utilize the energy for instance from renewable sources. Fuel cells can already at the present state of development constitute an alternative for the devices powered by batteries, also including grass mowers. Their advantages, besides the enumerated above high efficiency and protection of the environment as well as utilization of renewable energies, include the possible of charging the containers with hydrogen fast and a lower weight of the cells as compared to the battery. Hydrogen containers can – as opposed to batteries – be filled quickly, which is of importance in case of field work taking place near a household. This is the direction which further research on the kinds of driving system should take.

REFERENCES

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Accepted for print: 1.04.2008

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