GB2295858A

GB2295858A - Liquid hydrogen fuelled powerplant

Info

Publication number: GB2295858A
Application number: GB9425044A
Authority: GB
Inventors: Graham Alfred Reynolds
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1994-12-09
Filing date: 1994-12-09
Publication date: 1996-06-12
Also published as: GB9425044D0

Abstract

An airbreathing hydrogen fuelled combustion turbine engine 10 comprises an air intake, an air compressor 12, a combustor 14, a turbine 16 for driving the compressor 12 and a power turbine 18 for extracting mechanical power from the combustion products. A precooler 24 cools intake air A down to about 77 DEG K by heat exchange with liquid hydrogen (LH2) before the intake air enters the compressor 12, the liquid hydrogen thereby being gasified for feeding to the combustor with the compressed air (CA). Using the fact that the temperature (T3) of the air after compression is less than ambient air temperature (T1), the thermodynamic efficiency of the powerplant 10 is enhanced by provision of a recuperator 26 which cools the intake air (A) by heat exchange with the compressed air CA from the compressor, before the intake air (A) enters the precooler 24. It is stated that similar efficiency improvement features can be applied to reciprocating or rotary internal combustion engines. <IMAGE>

Description

ENHANCED EFFICIENCY POWERPLANT CYCLE The present invention relates to improvements in the efficiency of powerplants which use cryogenically stored hydrogen as a fuel.

In view of the finite nature of fossil fuel resources, worries about global warming due to excessive carbon dioxide production, the apparent limited ability of renewable energy sources to supply the large amounts of power needed by present and future industrial economies, and doubts about the long term future of nuclear energy as a major source of power, research work is underway to investigate the viability of a so-called "liquid hydrogen economy". In an industrial economy which relies to any significant extent on the use of non-polluting cryogenically stored hydrogen as a source of energy, hydrogen fuelled combustion powerplants must make the most efficient use of the fuel due to its high production, storage and transport costs.

Cryogenically stored hydrogen fuelled powerplants are already known in the aerospace industry. Various rocket motors use a mixture of hydrogen and oxygen in their combustion chambers, the fuel and oxidant being stored in liquid form to minimise size of the storage tanks. During operation, the cryogenic liquid propellants are gasified before injection to the combustion chambers by passing them through heat exchangers. The usual way of doing this is to let the liquid propellants exchange heat with the exhaust gases by circulating them through passages built into the walls of the combustion chambers.

Known designs of air-breathing propulsion engines, proposed for single-stage-to-orbit vehicles, utilise more efficient thermodynamic cycles in which the engine's intake air is precooled by liquid hydrogen in a heat exchanger situated before the first stage of air compression. This also gasifies the hydrogen for feeding to the combustor at high pressure, while the intake air, having been made very cold before compression, can be compressed to high pressures at moderate temperatures for combustion with the fuel, the high pressure combustion maximising the power available from the engine.

The above principle, of using cryogenically stored hydrogen fuel to precool air before it is compressed for high pressure combustion with the gasified fuel, is potentially useful for primary industrial powerplants suitable for producing large amounts of power. For example, when the principle is applied to an industrial gas turbine engine, a liquid hydrogen fuel cooled precooler can be used to cool the intake air down to, say, 770K before entry to the compressor. The compressor can then operate at, say, a 200:1 compression ratio and yet the compressed air delivered from the exit of the compressor to the combustor will still be appreciably below ambient temperature. This fact allows relatively cheap materials to be utilised at the high pressure end of the compressor, and facilitates effective cooling of the combustor and the turbine, if required, using bleeds of the compressed air.After passing the combustion products through a first turbine to drive the compressor, a power turbine can extract large amounts of energy from the exhaust. This type of powerplant is liable to require up to 30% more hydrogen to cool the compressor intake air than is needed by the combustor. Any surplus gasified hydrogen is therefore passed on for burning in a secondary powerplant, which of necessity is less fuel efficient than the primary powerplant.

It is a major object of the present invention to maximise the thermodynamic efficiency of airbreathing powerplants which use cryogenically stored hydrogen as fuel.

A further object is to reduce the amount of surplus gasified hydrogen passed on for use in less efficient secondary powerplants.

According to the present invention, an airbreathing hydrogen fuelled combustion powerplant having ambient air intake means, air compression means, combustion means and combustion product expansion means for extracting power from the combustion products, further comprises air precooling heat exchanger means for cooling intake air by heat exchange with liquid hydrogen before the intake air enters the compression means, the liquid hydrogen thereby being gasified for feeding to the combustion means with compressed air output from the compressor means, wherein the temperature of the air after compression is less than ambient air temperature; the thermodynamic efficiency of the powerplant being increased by providing recuperating heat exchanger means for cooling the intake air before it enters the air precooling heat exchanger means, by heat exchange with compressed air from the compressor means.

Further aspects of the invention will be apparent from the following description and claims.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a simplified block diagram illustrating the main parts of a hydrogen-fuelled airbreathing combustion turbine engine according to the prior art and according to the present invention, with fuel, air and exhaust flows also shown; Figure 2 is a graphical plot of absolute temperature T against entropy S for the thermodynamic cycle performed by the engine shown in Figure 1; and Figure 3 is a graphical plot of the percentage thermal efficiency of the thermodynamic cycle against the pressure ratio of the compressor in the engine of Figure 1.

Referring now to Figures 1, a known design of airbreathing combustion turbine engine 10 for power generation or other industrial use is shown by the full lines. The engine arrangement 10 has main sections comprising a compressor 12, a combustor 14, a turbine 16 and a free power turbine 18 with a power output shaft 20.

The power output shaft 20 may be connected to an electrical power generation set (not shown) or other load.

The compressor 12 operates at a high pressure ratio, say in excess of 100:1, preferably at least 150:1, so that it can take in air A at or near atmospheric pressure and output high pressure compressed air CA before passing it to the combustor 14 for burning with hydrogen gas fuel GH2 to produce combustion gases CG at a temperature preferably not exceeding about 18000K to avoid production of oxides of nitrogen (NOx).

A shaft 22 directly connects the turbine 16 to the compressor 12 so that as the turbine is driven by the expanding combustion gases CG, the turbine in turn drives the compressor. The combustion gases CG continue to expand through the power turbine 18, which extracts most of the remaining energy from the gases to drive the load on the output shaft 20. The exhaust gases E, consisting mostly of nitrogen and water vapour or steam, are passed to atmosphere, possibly after passing through an exhaust heat recuperator (not shown) and a condenser (not shown) to recover any energy remaining in the gases after passage through the power turbine 18.

The known engine arrangement 10 further comprises a so-called "intake air precooler" 24 situated before the compressor 12, in flow series with it. The precooler 24 is a heat exchanger which greatly cools the engine's intake air A by heat exchange with liquid hydrogen LH2 pumped from a suitable cryogenic storage facility (not shown).

Referring now also to Figure 2, cooling of the intake air A by the precooler 24 is from a temperature TI, representing an ambient atmospheric temperature of about 2880K , down to a temperature T2, say about 770K. While passing through the precooler 24, the liquid hydrogen LH2 is gasified as its temperature is raised towards that of the precooled air. The gaseous hydrogen GH2 is then fed to the combustor 14 for burning with the compressed air, which is output from the compressor 12 at a temperature T3.

Immediately after combustion, the temperature of the combustion gases CG has been raised to a maximum of T4 and thereafter the temperature falls to T5 as the gases expand through the turbines 16,18. Temperature T5 may not greatly exceed ambient temperature T1.

To cool the intake air A down to about 770K by means of the precooler 24 takes more hydrogen than is burnt in the combustor 14. Consequently, about one third of the total amount of liquid hydrogen LH2 input to the precooler 24 is output from the engine 10 AT 23 as surplus gaseous hydrogen, having bypassed the combustor 14. The surplus hydrogen is passed for burning to one or more further powerplants having somewhat less efficient thermodynamic cycles.

Even after compression in the compressor 12 at a pressure ratio in the range 150-200:1, the temperature T3 of the compressed air CA before entry to the combustor 14 is about 200-2300K, well below normal ambient air temperatures. In the present invention, this fact is utilised to improve the thermodynamic efficiency of the engine arrangement 10 by incorporating a recuperating heat exchanger 26 in the engine between the compressor 12 and the combustor 14. This novel addition is shown in Figure 1 by the dashed lines.

The recuperator's function is to cool the engine's intake air A, before it enters the precooler 24, by heat exchange with the compressed air CA output by the compressor 12. Hence, in Figure 2, the recuperator 26 cools the intake air from ambient temperature T1 down to a temperature T1', which of course is about the same as temperature T3. Duo his cooling of the intake air in the recuperator 26, the precooler 24 only has to cool the intake air from temperature T1' to T2, instead of all the way from T1 to T2. The amount of liquid hydrogen required to cool the intake air is therefore reduced, so using the hydrogen more effectively by reducing the amount of surplus gaseous hydrogen passed on for use in other, less efficient, powerplants.Furthermore, because the the intake recuperator 26 warms the compressor delivery air CA before it is passed to the the combustor 14, the amount of fuel needed by the combustor to raise the temperature of the combustion products to T4 (say, 18000K) is reduced.

Referring now to Figure 3, it will be seen how the pressure ratio of the compressor 12 influences the overall thermal efficiency of the improved cycle. At low pressure ratios, up to about 50:1, there is a steep rise in thermal efficiency with pressure ratio, progressively becoming less steep. From a pressure ratio of about 50:1 upwards, thermal efficiency increases level off, until for pressure ratios greater than about 100 or 150:1, thermal efficiency is relatively insensitive to increases in pressure ratio.

For example, the graph shows there would be little thermal efficiency gain by increasing the compressor's pressure ratio from 200:1 to 250:1. The significance of this is that the thermal efficiency gain resulting from the inclusion of the recuperator 26 in the cycle at a compressor pressure ratio of 200:1 may be greater than would be obtainable merely by increasing the compressor's pressure ratio from 200:1 to 250:1.

Also shown on Figure 3 is the effect on thermal efficiency of varying the combustion temperature. The main plot shows efficiencies for a combustion temperature of 18000K. Lower and higher combustion temperatures of 17000K and 19000K, respectively, result in lower and higher thermal efficiencies, but the efficiency gain obtained by increasing combustion temperature from 1800 to 19000K is less than that obtained by increasing it from 1700 to 18000K. In practice, a combustion temperature of 19000K would probably not be used for environmental reasons, since at this temperature some NOx would be produced.

In the above description, a combustion turbine engine is used to put the invention into effect. However, the principle is applicable to other types of powerplant, such as reciprocating or rotary internal combustion engines having pistons and cylinders or vanes, etc., which would replace the compressor 12 and turbines 16,18 of Figure 1.

In this case, cooling of the engine's intake air before entering a liquid hydrogen cooled precooler would be by heat exchange with compressed air, or a compressed air/hydrogen mixture, at the end of each compression stroke of the piston.

Claims

1. An airbreathing hydrogen fuelled combustion powerplant has air intake means, air compression means, combustion means and combustion product expansion means for extracting power from the combustion products, and further comprises a precooling heat exchanger for cooling intake air by heat exchange with liquid hydrogen before the intake air enters the compression means, the liquid hydrogen thereby being gasified for feeding to the combustion means with compressed air output from the compressor means, wherein the temperature of the air after compression is less than ambient air temperature; the thermodynamic efficiency of the powerplant being enhanced by provision of a recuperating heat exchanger for cooling the intake air by heat exchange with compressed air from the compressor means, before the intake air enters the precooling heat exchanger.

2. An airbreathing hydrogen fuelled combustion turbine engine having an air intake, an air compressor, a combustor, a turbine for driving the compressor and a power turbine for extracting mechanical power from the combustion products, and further comprising a precooling heat exchanger for cooling intake air by heat exchange with liquid hydrogen before the intake air enters the compressor, the liquid hydrogen thereby being gasified for feeding to the combustor with compressed air output from the compressor, wherein the temperature of the air after compression is less than ambient air temperature; the thermodynamic efficiency of the powerplant being enhanced by provision of a recuperating heat exchanger for cooling the intake air by heat exchange with compressed air from the compressor, before the intake air enters the precooling heat exchanger.

3. An airbreathing hydrogen fuelled powerplant substantially as described herein with reference to the accompanying drawings.

4. A thermodynamic cycle substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.