Open Access Journal Journal of Power Technologies 91 (1) (2011) 23–36 journal homepage:papers.itc.pw.edu.pl Dynamic analysis of compressed air energy storage in the car Łukasz Szabłowskia,∗, Jarosław Milewskia aInstitute of Heat Engineering, Warsaw University of Technology 21/25 Nowowiejska Street, 00-665 Warsaw, Poland Abstract The article presents a dynamic analysis of the compressed air energy storage in the car. The analysis was used to determine those processes most relevant to achieving highest possible efficiency. A review of the state of the art is presented. Simple technical-economic analysis of usage those kind of cars is also performed and discussed by taking Polish local conditions from an electricity market. Main advantages as well as drawback of the compressed air cars are pointed out Keywords: Dynamic, Compressed Air Energy Storage, Vehicles 1. Introduction pean Union regulations are driving energy efficiency initiatives such as the gradual elimination of incan- In an era of intensive environmentally friendly ac- descent light bulbs in favor of low energy lighting tions in the face of a global economic crisis, in- alternatives. They also set down the rules for mark- creasing emphasis is placed on energy efficiency is- ing electrical equipment in terms of their energy effi- sues. Although mining technologies are opening up ciency class. the prospect of exploiting previously unusable fos- sil fuels, there is no doubt that the overall reserves Road vehicles present an ideal area for reducing of natural resources continue to shrink. From the energy consumption due to their abundance and the viewpoint of the Polish economy, dependent as it is interest governments of all persuasions show in new on gas and oil from abroad, a temporary solution ventures in this field, the latest of which being to ban might be found in the shape of either underground all petrol and diesel cars from cities by 2050. An coal gasification (which enables the recovery of nat- eco-car segment is slowly emerging, made up mainly ural resources from poor quality or otherwise uneco- of hybrids catering for different types of journeys. nomic seams) or shale gas, assuming gas-rich shale Most of these vehicles combine an internal combus- is found. Fossil fuel savings are both desirable for tion engine with an electric motor, which allows you self-evident reasons and achievable through recently to recover braking energy and reduce energy require- built high-efficiency power plants, but the solutions ments while stationary (also possible in ordinary cars are expensive and it will be quite some time before with a start / stop system). Another alternative is the Polish energy sector renews its capacity. Euro- to have electric vehicles powered by fuel cells or lithium-ion batteries. But there is one big flaw – these solutions are very expensive, putting them out ∗Corresponding author Email addresses: of reach of ordinary road users. One answer may be [email protected] (Łukasz Szabłowski a compressed air car which, in addition to lower pro- ), [email protected] (Jarosław Milewski) duction costs, will also reduce the car’s weight and Journal of Power Technologies 91 (1) (2011) 23–36 Figure 1: Armando Regusci’s car, which managed 100 km on Figure 2: Compressed air bike built in 1997. (source: [2]) one tank (source: [2]) hence its energy needs. Perhaps compressed air cars deserve a little more attention. The first vehicle powered by compressed air was constructed in France in 1870 by a Polish engineer – Louis Mekarski (more in [14, 16]). This invention was patented in 1872 and 1873 and then in 1876 was tested on the line TN in Paris. The principle of Mekarski’s engine was the use of Figure 3: Angelo Di Pietro’s motor (source: [4]) energy stored in compressed air, passed through a tank of hot water to increase gas enthalpy. Replenish- ment took place at purpose-built compressor points into torque via a chain dragged by the moving pis- at tram stops. Mekarski’s trams ran on lines in cities ton. After the up stroke the piston descends slowly across France at various times from 1879 onwards, to pull the chain again. All this is possible thanks to finally disappearing from the streets of La Rochelle the fact that the gear is rotated by a chain attached to in 1929. Compressed-air trams also appeared in New the wheels via a similar mechanism as in the moun- York, but boasting an engine built in 1882 by Robert tain bike and the chain has a stretcher. This solution Hardi (described in [6]). Since air-powered equip- enables a standalone start for the engine but unfor- ment generally does not sparks and heat it was put to tunately renders the vehicle incapable of travelling use in HK Porter compressed air locomotives (more at a steady speed. Regusci also developed a motor- in [6]), which were readily used in explosive atmo- cycle and a moped (Fig. 2) powered by compressed spheres (e.g. mines). Another interesting exam- air, before widening his intereste to engines running ple of the use of compressed air to drive vehicles on ethanol from sugar cane, and on ways to obtain comes from Uruguay, where since 1984 Armando hydrogen. Regusci, Ph.D. has been involved in constructing One of the most interesting aspects of the pneu- these unusual machines (more on [2, 7]). One of his matic motor came courtesy of Angelo Di Pietro – an most successful constructions was a four-wheel ve- Italian designer currently working in Australia where hicle with pneumatic engine which managed (Fig. 1) he founded the company Energineair (fully described 100 km on a single tank. The event took place in in [4]). His invention does not have a conventional Maldonado in 1992. crankshaft and the pistons and cylinders and their Regusci’s engine does not have a rod and the force structure is more like a Wankel’s engine due to a sin- the compressed air exerts on the piston is converted gle rotating piston (Figs. 3 and 4). — 24 — Journal of Power Technologies 91 (1) (2011) 23–36 Figure 5: PHEV power scheme (source: [3]) Figure 4: Working principle of Angelo Di Pietro’s motor (source: [4]) The concept is based on a simple cylindrical ro- tary piston which rolls, without any friction, inside the cylindrical stator. The space between stator and piston is divided into 6 expansion chambers by piv- oting dividers (shown in Figure 3, right side). These dividers follow the motion of the piston as it rolls around the stator wall. The piston, forced by the air pressure on its outer wall, moves eccentrically, so driving the motor shaft by means of two rolling ele- ments shaft mounted bearings (Fig. 4). In the cham- bers there is a cyclical process of expansion which causes piston movement around the stator. This so- lution makes it possible to self-start the engine. The Di Pietro engine weighs only 13 kg and does Figure 6: Piston, connecting rod and crankshaft in Nègre’s en- gine (source: [1]) not require a gearbox, because it is controlled by the pressure difference which allows smooth control of torque and rotational speed. This device can start hicle) - is driven by both compressed air and elec- with a pressure difference of 1 psi (6.8 kPa) by us- tricity with a 48-volt battery. During starting and ac- ing air film technology, which significantly reduces celeration, when you need serious power, the car is friction between engine parts. driven by compressed air whereas during normal op- With torque simply controlled by throttling the air eration its requirements are met by a small electric pressure the Di Pietro motor gives instant torque at motor. zero RPM and can give soft start and good acceler- The choice of power source is controlled by a com- ation control. Angelo Di Pietro vehicles performed puter unit. Additionally the car has a small pres- well in supermarkets and on fruit and vegetable mar- sure container accumulating air (Fig. 5), which en- kets, and there are boat applications too. A different ables the car to run at a constant pressure at the inlet approach to power cars is presented by the Korean (10 bar) without a significant drop in power during Cheol-Seung Cho – a constructor at Energie com- discharge, and a main tank with a maximum pres- pany (more on this in [3]). sure of 300 bar. This combination gives the vehicle a His car - the PHEV (pneumatic hybrid electric ve- top speed of 120 km/h. Cars running on compressed — 25 — Journal of Power Technologies 91 (1) (2011) 23–36 amount not exceeding 2 liters per 100 kilometers. To minimize energy consumption MDI cars use an au- tomatic, computer-controlled gearbox, changing the gear ratio depending on the speed of the car. Guy Nègre vehicles are equipped with a moto- alternator connecting the engine with the gearbox, whose task it is to fill up the tank with compressed air using electricity. This device works as an alternator, it also helps when starting and at any moment when you need more power to drive the vehicle. MDI cars are equipped with an engine that stops momentarily when the car is stationary. They also have a pneu- matic system that can recover 13% of the energy used during braking. The compressed air tanks mounted in these cars are made of lightweight but very durable composite material containing among others carbon fiber. While working at a pressure of 350 bar, for safety they are tested at a pressure of 700 bar. The Figure 7: Air distribution system in MDI cars (source: [1]) body of the car is also built of composite attached to an aluminum frame, assembled using glue, as in aircraft structures, so construction is fast and the re- air have also been designed by retired F1 engineer – sulting product lightweight and robust.