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Fia. 16.L Pictorial cross-section of the JP1 Stirling laboratory research engine (after

Hochn 1978).

^Crankcasc explored systematically by General Motors in the 1960s for underwater power systems. This work is discussed in Chapters 13 and 18. Although primarily concerned with marine applications, the program, in 1964, embraced the installation in an automobile of a 22 k\V (30 hp) engine driven from a heated alumina thermal-energy source. This was probably the first Stirling engine in a vehicle as well as the first vehicle to be driven from a thermal energy source but was never intended as a serious alternative propulsion system for automobiles. It did serve to demonstrate that a Stirling engine driven from a thermal energy source can be a fraction of the weight of an electric motor/lead-acid battery combination.

Later Meijer (1970a) presented a milestone paper containing the results of extensive studies carried out at Philips concerned with vehicular applications of Stirling engines. Part of the paper was concerned with the combination of a Stirling engine and thermal-storage systems in vehicles.

Fig. 16.2 is a schematic representation of the combination of Stirling engine and thermal storage system discussed by Meijer (1970a). The thermal-storage medium was lithium fluoride stored in thin-walled sealed bottles charged with argon to maintain a slight internal positive pressure to prevent collapse of the bottles. Lithium fluoride melts at 848 "C (1558 F) and has a high latent heat of fusion with reasonable volume, a combination which makes it the preferred thermal storage medium for use with Stirling engines.

The thermal-energy content (measured from a datum of 550 °C (1022°F)) of lithium fluoride is shown in Fig. 16.3 as a function of temperature. The thermal capacity, O of solid lithium fluoride increases steadily with temperature until, at 848 °C (1558 °F), the solid begins to

Fit;. lf>.2. Schematic representation of a Stirling engine with lithium fluoride thermal storage system coupled by sodium vapour heat pipe (niter Meijer l*^70al.
energy potentially available from ¿1 Stirling engine driven by thermal storage over the temperature range 850 so 550 "C (after Meijer L970a>.

melt. The latent heat of fusion (heat to melt) is 250 W'h/kg (0.15 hph/lbj and so the heat capacity increases by this amount at constant temperature until all the solid has melted.

It was reasonable to anticipate that the heat stored in this way could be used to drive a Stirling engine with a reducing temperature of the storage media down to a temperature as low as, say, 550 C (1022"F). The efficiency of the engine was of course a function of the temperature and therefore decreased as I he storage temperature decreased. The possible variations in efficiency with temperature is shown in Fig. 16.3. The lower curve shown in the same figure is the potential engine work available. This is the product of the energy stored. Q, times the thermal efficiency, t). It can be seen from Fig. 16.3 that as many as 200 Wh (0.27 hph) of work could be obtained per kg of lithium fluoride contained in the thermal store ranging in temperature from 550 °C to 850 °C (1022 aF to 1562 °F).

The thermal storage system was coupled to the engine by a sodium vapour heat pipe shown in l ie. 16.2. Liquid sodium boiled on the hot bottles of lithium lluoride salts and condensed on the heater tubes and heater head of the engine so that heat was effectively transferred from the thermal store to the engine. The storage unit and connecting ducts were all thermally insulated so that condensation of sodium vapour occurred only at the engine and thermal loss by conduction was minimized.

Heat could be transferred very effectively by heat pipe systems involving boiling and condensing of the transfer fluid. There was virtually no difference between the supply and delivery temperature and the rates of heal transfer could be orders of magnitude higher than by conventional conduction. Another advantage when used with Stirling engines was that the vapour condensing on the heater tubes and cylinder head ensured a uniform temperature. There could be no 'hot spots' which were virtually unavoidable in direct heating combustion systems. As a consequence, the mean heater temperature could be elevated to the metallurgical limit of the heater-tube material. This gain was often as much as 75 °C (167 °F) (Meijcr 1970a) with consequent improvement to the power and efficiency of the engine.

Moreover, the heat transfer rates of condensing sodium vapour were very high, so the size of the heater tube could be reduced to the limit dictated by thermal conduction capacity through the walls of the heater tubes and by internal heat transfer in the tubes to the working fluid. This resulted in short heater tubes giving appreciable savings in the dead volume with further beneficial consequences lo the engine power and efficiency.

This system of indirect heating by heat pipes was not limited to Stirling engines with thermal storage systems. It was equally beneficial when used on Stirling engines with iossil-fuel combustion systems. Meijer (1970a) gave the data reproduced in Fig. 16.4 as a comparison of engine performance with direct and indirect heating for engines with helium and hydrogen working fluids. It can be seen that the power and efficiency of indirectly heated engines were markedly superior at higher speeds for both hydrogen and helium. Lia and Lagerqvist (1973) have described similar work with indirectly healed Stirling engines at United Stirling.

Heat pipes have been extensively studied by Philips not only for Stirling engines but also for other applications. Asselman and Green (1973a and b) gave excellent reviews of the basic technology and applications.

Returning again to the Stirling engine with thermal storage system and sodium heat pipe as illustrated in Fig. 16.2, the system could be recharged with heat using the sodium boiler shown at point 3 in Fig. 16.2. An electric heater is shown but a combustion heater could also be used. The thermal battery is charged periodically by heat supply to the boiler. The

FlG. 16.4. A comparison ol the power output and thermal efficiency ol Stirling engines with direct and indirect heating and with livdroeen .ind helium working fluid (after Meijer

FlG. 16.4. A comparison ol the power output and thermal efficiency ol Stirling engines with direct and indirect heating and with livdroeen .ind helium working fluid (after Meijer

sodium boils and condenses on the lithium fluoride bottles thereby reheating them. II electric heating is used, the thermal battery may simply be recharged overnight or when the vehicle is not in use. If on the other hand combustion heating is used, there are two possibilities. One is that a portable external combustion system be employed when the vehicle is not in use. The other possibility is that the combustion system be self-contained, on board the vehicle, and is used intermittently when convenient or as required. A unit similar to this latter type was described by Agarwal et al. (1969) in reporting the General Motors 'Stirloc* hybrid vehicle. This was an electric car with electric rather than thermal-storage battery capacity and an electric generator driven by a Stirling engine (General Motors Type GPU-3, 7.3 kW (10 hp) engine).

Meijer (1970a) presented the results of calculations made at Philips for six types ol vehicles with Stirling-engine/thermal-battery propulsion systems. The calculations were made with the assumption that the battery was charged once only every day and that the vehicle radius of action was the same as that of a gasoline driven car, circa 1.968. The basic vehicle data was similar to that used elsewhere lor » study of electric propulsion of vehicles and is reproduced in Table 16.1. With this basic information, calculations were made to establish the principal parameters for the vehicles equipped with a Stirling engine/thermal battery.

The results oí the calculations are summarized in Table 16.2. reproduced from Meijer (1970a). Comparison of the results given in the two

Table 16.1. Some data concerning the principal types of cars at present in use.


American European family car (com- Utility Delivery City City car muter car) car van taxi bus

1. range of operation km

2. maximum speed km/h

3. acceleration to km/h in s

4. maximum power output kW

5. loaded weight kg

6. total weight assignable to new propulsion system a. conventional construction kg b. lightweight construction kg

7. energy delivered kWh

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