Power Sources for Robotic Systems

Power Sources for Robotic Systems

1. Fuel Cells

   • Hydrogen Fuel Cell - Unlike a battery which stores energy, a fuel cell relies on fuel being oxidised within an electrolyte in the cell to generate electricity. The fuel and oxidant are supplied externally and so the fuel cell can continuously produce electricity so long as there is a supply of reactants. The terminal voltage of a fuel cell is 0.6 – 0.7V and they operate with an efficiency of around 50%. Hydrogen fuel cells have been around for many years and provided the electrical energy for the Apollo space missions. When used within a fuel cell, Hydrogen has an energy density of approximately 1.5MJ/kg. Hydrogen is difficult to transport and can be explosive when ignited with oxygen. All fuel cells have the advantage that they are very reliable, since there are no moving parts to wear out
   • Methanol Fuel cell = These fuel cells are an emerging solution to overcome the problems of using Hydrogen as a fuel. The fuel does not have to be stored at very low temperatures and high pressures and the fuel cell is capable of having energy densities up to 15MJ/kg. A current limitation is the low efficiency of the conversion process (15 – 25%) but this is likely to improve over the coming years. These fuel cells are currently finding use in Laptop computers and mobile phones

2. Electrical Storage

   • Capacitor – Electrochemical Double Layer Capacitors (ECDLC) with a capacitance as high as 5000F and energy densities of up to 100kJ/kg. Advantage - can be charged significantly faster than batteries. Disadvantage – ECDLCs only operate at low voltages (2-5V).
   • Lead-acid battery – Similar density to that of ECDLC Disadvantage – they take a long while to charge and has a limited life. Cell voltage of 2.2V, usually connected in series to supply 13.2V.
   • Nickel metal hydride battery – Advantages – they contain no toxic poisonous materials. They have a much higher energy density, 300kJ/kg and they do not have to be completely discharged before recharging. Cell voltage of 1.2V, usually connected in series to supply 6 and 12V batteries.
   • Lithium ion battery – advantages Much higher energy density than other types of battery, 500kJ/kg and have a cell voltage of 3.6V. They are used in consumer electronics which require high power consumption e.g. laptops, mobile phones. Disadvantages – safety – can result in fire and exploding.
   • Lithium polymer battery - A variant of lithium ion batteries, were introduced to reduce the risk of fires and explosions. Cells have a voltage of 3.6V but have an increased energy density of 1MJ/kg.

3. Chemical

   • Petrol - To produce large amounts of power on demand and ease of use, there is currently little to beat a petrol engine. Petrol has an energy density of around 40MJ/kg and even when used within an internal combustion engine operating at only 25% efficiency, it still gives a much higher energy output per kg than any of the previous mentioned systems. However, fuel cell technology is continuing to improve and will get to a state where they can compete with burning hydrocarbon fuels systems.