WO1995003489A1

WO1995003489A1 - Heat engine

Info

Publication number: WO1995003489A1
Application number: PCT/GB1994/001599
Authority: WO
Inventors: John Varga
Original assignee: Carding Specialists (Canada) Limited
Priority date: 1993-07-23
Filing date: 1994-07-25
Publication date: 1995-02-02
Also published as: AU7232594A; GB9315326D0

Abstract

A heat engine (10) which is intended to receive thermal input energy from waste heat, low grade heat, a renewable energy source of any other convenient heat source, and which comprises an air compressor (11) arranged to receive an input of ambient air via input (12), and having an output (13) for delivering a supply of compressed air from the compressor to an air-driven motor (14) which provides a mechanical power output (15), a heat exchanger (16) arranged along the path of travel of compressed air from the compressor output (13) to the motor (14) and which is intended to be supplied with thermal input energy from any suitable heat source e.g. waste heat or solar energy heating a warm water supply (17) to the heat exchanger whereby the heat exchanger transfers thermal energy indirectly to the delivered supply of compressed air running between the compressor output and the motor, and a mechanical feed-back taking part of the power output of the motor to the compressor to operate the latter, leaving a net gain of power output to operate any required energy consumer e.g. an electricity generator (19).

Description

HEAT ENGINE

This invention relates to a heat engine which is intended to derive energy input from a low grade and / or waste heat source, or a suitable natural renewable source such as solar energy.

The concept of a heat engine operating on the Carnot cycle has been known for many years, but has largely been ^' a theoretical abstraction in view of the generally plentiful supplies of coal and hydrocarbon fuel at reasonable prices which has prevailed for most of this century, although there have been times of interruption in supply due to war and attempted organised control of supply by producer nations. When supply has been interrupted, or threatened, this increases the raw material supply cost, and at these times there has developed renewed interest in conservation of the natural ^•resources by making more efficient use of the resources, as well as interest in alternative sources of energy, and particularly in the area of so-called renewable energy sources e.g. wave power and solar power.

However, whenever supplies return to normal, 01 there is over capacity in relation to current demand (as prevails at the present time), interest in conservation and alternative energy sources inevitably wanes. Despite this, concerned observers, (taking the longer view that natural resources of fossil fuels and hydrocarbon fuels have a finite life, and also factors such as uncertainties over "global warming"), recognise a need to develop alternative technologies which can exploit naturally available and renewable sources of energy, and which also are capable of utilising low grade or waste heat as a thermal energy input source.

There are many applications of heat exchangers in industry, which are used to cool plant and equipment, and / or products by indirect transfer of heat energy through a cooling medium, and there are other applications of direct cooling of plant and equipment and products in which the cooling medium e.g. water is applied directly, such as water quenching of hot- rolled steel.

Regardless of whether heat transfer takes place directly, or indirectly, the end result is transfer of this energy as waste heat to atmosphere and to the surrounding environment.

This waste heat is a potentially valuable resource waiting to be utilised, subject to a suitable design of heat engine being made available to use this energy and convert this energy initially into mechanical form, and then if desired into electrical or other required forms.

The present invention seeks to provide an improved design of heat engine which is able to use waste heat of this type, or low grade heat, or renewable heat energy from naturally available sources e.g. solar heat, to provide energy input which, after conversion to mechanical energy to operate the engine, leaves a surplus of mechanical energy available for whatever use is required e.g. to drive an electrical generator.

According to one aspect of the invention there is provided a heat engine which is intended to receive thermal input energy from waste heat, low grade heat, a renewable energy source or any other convenient heat source and which comprises: an air compressor having an input arranged to receive ambient air, and having an output for delivering a supply of compressed air from the compressor; an air-driven motor arranged to be driven by the supply from the compressor output, and to provide a mechanical power output; a heat exchanger arranged along the path of travel of compressed air from the compressor output to the motor, said heat exchanger being intended to be supplied with thermal input energy from any suitable heat source, and to transfer thermal energy indirectly to the delivered supply of compressed air;' and, a mechanical feedback from the motor to the compressor to operate the latter.

The heat engine according to the invention therefore derives energy input from any suitable waste, low grade or other thermal energy source, and converts this thermal energy into mechanical energy, part of which is fed back from the motor output to operate the compressor, and the remainder of which can be utilised as a source of net output mechanical power to drive e.g. an electrical generator.

The heat engine therefore may be used in an industrial environment, to utilise thermal energy which might otherwise be discharged to atmosphere, and provide a net source of power which can be fed back into the operating plant which is the source of the waste energy. Alternatively, the heat engine may be utilised as part of an installation which derives thermal energy input solely from renewable resources, such as solar power, in which case the heat engine may form part of an energy conversion power pack which can be used both in third world markets, and also in industrialised nations such as Canada and the United States. By way of example only, in a high altitude ^'country, such as Peru, solar energy is available, and the ambient air utilised as the input air to the heat engine is at a relatively low temperature, and the temperature gain achieved by heat transfer in the heat exchanger (by reason of the low input temperature) gives particularly advantageous improvement in the operating efficiency of the heat engine.

Preferably, the air compressor is a positive displacement type of compressor (though other suitable types of air compressor may be used including turbine type compressors), and the air motor may take any suitable form depending upon the output from the compressor. A linearly reciprocating piston type compressor may be provided, operating e.g. at 200 rp , and providing a relatively modest increase in pressure over ambient e.g. an increase of perhaps 4 to 6 psi above ambient pressure, in which case the motor will be designed to operate at this relatively modest pressure increase in order to provide mechanical power output. The motor may be of the fixed displacement type.

After passage through the motor, the air supply is then discharged to atmosphere.

The input of thermal energy to the heat exchanger is preferably by way of routing a circulating flow of hot water, which may be obtained directly from the heat source, or by indirect transfer depending upon the nature of heat source utilised.

According to a second aspect of the invention there is provided an energy conversion plant for converting solar energy into power and which comprises: a device for receiving an incident supply of solar energy, and for converting this energy into thermal energy for application to a fluid heat transfer medium; a heat exchanger arranged to receive the fluid heat transfer medium; an air compressor having an input arranged to receive ambient air, and having an output for delivering a supply of compressed air from the compressor, said compressor comprising a piston linearly reciprocatable within a cylinder, and a piston rod coupled rigidly with said piston and guided so that the piston does not require to make sliding contact with the wall of the cylinder; an air driven motor arranged to be driven by the supply from the compressor output, and to provide a mechanical power output, said heat exchanger being arranged along the path of travel from the compressor output to the motor and to transfer thermal energy indirectly to the delivered supply of compressed air; and, a mechanical feedback from the motor to the compressor to operate the latter, and leaving a surplus to provide a net power output from the motor.

Preferred embodiments of heat engine according to the invention will now be described in detail, by way of example only, with reference to the accompanying schematic drawing, in which:

Figure 1 is a schematic illustration of the essential components of a heat engine according to the invention, to illustrate the general principle of operation; and,

Figure 2 is a more detailed illustration of one example only of a way of carrying out the principle of operation disclosed in Figure 1.

A heat engine according to the invention is designed to utilise heat input energy from any suitable energy source, and preferably from waste heat, low grade heat, or from a renewable energy source.

The heat engine is designated generally by reference 10, and by way of example may use a supply of warm water derived from e.g. a heat exchanger (not shown) utilised as part of an industrial process, and comprising a source of low grade heat.

The engine 10 comprises an air compressor 11 arranged to receive an input of ambient air via input 12, and having a pressure line output 13 for delivering a supply of compressed air from the compressor. An air driven motor 14 is arranged to be driven by the supply from the compressor output 13, and to provide a mechanical power output 15.

A heat exchanger 16 is arranged along the path of travel of compressed air from the compressor output 13 to the motor 14, and which is supplied with thermal input energy from any suitable heat source, which in the arrangement shown in Figure 1 comprises a warm water circulating supply 17 which heats-up the compressed gas passing along output line 13, and then discharges via outlet 18 so as to receive further thermal input from the chosen heat source.

The thermal energy transferred indirectly by the heat exchanger 16 to the compressed air output passing along output line 13 adds energy to the overall system, so as to increase the power input to the motor 14, over and above what would otherwise be supplied by unheated delivery of compressed air from compressor 11, and this additional energy is utilised in the heat engine to provide a net gain of power to be utilised in any required manner.

The motor output 15 may lead to a generator 19, or any other preferred utilisation of the net energy gain by the heat engine, and a mechanical feedback 20 from the output 15 leads back to the compressor 11 in order to operate the latter.

After doing mechanical work by adiabatic expansion within the motor 14, the air is then discharged to atmosphere. Atmospheric air will be the preferred gaseous medium utilised in the operation of the heat engine, although evidently in a closed loop system any other suitable working fluid may be utilised.

Figure 1 is a schematic illustration of the operating principles of the heat engine 10, and Figure 2 shows one example of a way in which this could be carried out in practice. Corresponding parts are designated by the same reference numerals, but with the addition of the letter a.

Ambient air is supplied at input 12a to compressor 11a, but the compressor output is routed along output line 13a, and passes internally through heat exchanger 16a, before being fed as a heated-up compressed air supply to air motor 14a. A common drive member 15a runs between the motor 14a and compressor 12a in order to drive the latter in any required manner. A crank or other suitable mechanical output mechanism 21 converts the motion of drive member 15a into rotary form suitable to drive e.g. a generator.

Air compressor 12a preferably comprises a positive displacement type compressor adding relatively modest pressure to the ambient pressure supply e.g. 4 to 6 psi, and after energy gain from the heat exchanger, the heated compressed air supply operates the motor 14a which is designed accordingly.

Figure 1 shows use of low grade heat to operate the heat engine, but evidently any other suitable heat source may be used, including thermal energy in a cooling fluid which would otherwise be discharged to atmosphere e.g. cooling water which normally passes to a cooling tower in a power station. Also, in the application of the invention to operation by renewable energy sources, such as solar energy, a suitable circulating system of a working fluid may pass to and from the heat exchanger, and during its circulating path will be exposed to the input energy of the renewable source e.g. solar energy.

Figure 2 is a somewhat schematic illustration of practical embodiment of the invention, and particularly advantageous development of the schematic construction of Figure 2 will now be described. Figure 2 shows schematically drive and driven double acting pistons 22 and 23 of the compressor 11a and motor 14a respectively, and these are "floating" type pistons. Therefore, the energy feed-back fro the motor 14a to the compressor 11a is cycled directly, and without an intermediate rotating mechanism, and therefore in a way such as to minimise mechanical energy loss.

The pistons are coupled together for joint linear reciprocation by linearly interconnected piston rods. The piston rods are mounted in bearings which guide the reciprocation of the rods, and therefore the pistons 22, 23 do not need to engage the internal walls of the cylinders for guidance. The piston rods guide the reciprocation of the pistons, and therefore piston -rings are not required. This provides a wear-free piston / cylinder arrangement with low maintenance requirements.

In view of the indirect heating of the compressed air which is supplied to the motor 14a, which is at a higher temperature than the ambient air entering the compressor 11a, it is necessary for the diameter of the cylinder of motor 14a to be larger than the diameter of the cylinder of compressor 11a.

Although not shown, the motor 14a has power operated valves of any suitable known type, to direct the compressed air to alternative sides of the piston as the latter reciprocates.

To be meaningful, both the compressor 11a and the motor 14a should be large in size, and, by way of example, an air throughput of 10,000 cubic feet per minute is envisaged, and with the compressor working to provide a pressure gain of 6 pounds per square inch, ambient input air at 25°C, and heated compressed air supply to the motor at 90°C, will allow generation of about 35 kilowatts.

If it is desired to use solar radiation to provide the thermal input to the heat engine i.e. to heat the heat exchanger 16a, this will require very large areas of radiation receptors. For convenience^'and storage purposes, the preferred heat transfer medium from the radiation receptors to the heat exchanger is water. It will usually be desirable to avoid use of a pressurised container, in which case the maximum water temperature will be kept below 100°C. However, this is not essential to the operation of the invention, and for certain applications a pressurised circulation of heat transfer medium (water) may be suitable, in which case the circulating water temperature can be maintained above 100°C.

However, in the case of use of non-pressurised water as the heat transfer medium below 100°C, although the temperature difference between ambient air temperature and the temperature of the heated compressed gas will be relatively low (65°C in the set of figures given above), and only a small percentage of that can be extracted as useful heat energy according to the formula: [ (T,-T,) /T1 ]x100, the use of simple reciprocating circular pistons in circular cylinders of the compressor and motor in a preferred embodiment of the invention enable a ^•practical system to be obtained. Further, when as is preferred the compressor and the motor are "area sealed" types, in which the piston peripheries do not touch the insides of the cylinders, the effectiveness of the system is further improved.

The further preferred development of the heat engine (shown schematically only in Figure 2) and as described above provides a particularly advantageous and practical means of utilising low grade heat, or heat derived from renewable energy sources, to obtain a net power gain in a compressor / heat exchanger / air motor type of heat engine which has practical application. This compares very favourably with existing attempts at using low grade heat, which has been attempted in the past by trying to extract heat from waste steam by using large low velocity turbines or other large gas engines by direct transfer, namely the gas is fed into the engine to make it move. However, the only practical solution to a compressor / motor type heat engine to date has always used direct combustion to heat up the compressed gas.

By contrast, the invention first compresses the cold gas in the compressor, which cold air is then heated by the compression if this is not isothermal, then the compressed gas output is further heated indirectly (in the heat exchanger) to comparatively low temperatures, and only then passes it to the motor which provides the power to operate the engine overall, and leaves a surplus of power for whatever utilisation is required e.g. to drive a generator.

Temperature rises of as little as 25°C above the compressed gas temperature may be sufficient to make the system economical. All forms of low grade waste heat i.e. from a generating station, or water heated by sunlight (which can be stored in simple dug-out containers), can be converted into useful energy e.g. electricity. This latter use will be very economical in countries which enjoy strong sunshine. It will be particularly useful because of the possibility of having smaller decentralised power stations.

Therefore, in countries in which there are plentiful and reliable sources of solar energy, relatively unsophisticated and inexpensive devices may be used, within this invention, for receiving an incident supply of solar energy, and for converting this energy into thermal energy for application to a fluid heat transfer medium, which is routed in a closed loop to the heat exchanger. The use of relatively low capital cost solar energy gathering means, such as dug-out containers, or brick or black painted tanks, in conjunction with the reliability of using substantially maintenance free "floating piston" types of compressor, provides a very advantageous design of energy conversion plant for use in third world countries, and also in countries in which there is plentiful and reliable supply of solar energy. Most importantly there is no CO, emission.

The solar energy conversion plant can be utilised to provide "local" power generating facilities, and as indeed is possible when other sources of thermal input energy are utilised e.g. waste heat, low grade heat etc.

Capital costs are envisaged as being competitive with coal fired stations approximately £1 per watt generated, including the cost of the water heater. Running costs are negligible, as designs of compressor can be obtained which are virtually maintenance free and entirely non-polluting, and particularly if linearly reciprocating piston types of compressors with "floating pistons" are used.

Solar radiation energy amounts to about 1.^'5kw per square meter, and with a conservative assumption of 5% of that being convertible into mechanical power, will give a viable technical and economic system. Furthermore, use of solar energy on plateaus, in countries such as Mexico, Kenya, Ethiopia and Sinkiang may enable improved conversion rates to be obtained (conversion of solar energy to mechanical power) of up to 10%, because of the low temperature of the air drawn into the compressor.

Therefore, for use in environments in which there is plentiful and reliable supply of solar energy, a suitable design of device for receiving incident solar energy, and converting it into thermal energy applied to a heat transfer medium, may be manufactured and installed relatively cheaply. By way of example, a receptacle of suitable heat transfer fluid e.g. water, may comprise a shallow fluid-proof tray of a dark colour, and covered by glass or other transparent material, with no need to provide means to withstand fluid vapour pressure, and as a result can be a very cheap construction, whereby capital cost of about £1 sterling per installed watt can be maintained.

This cheapness of heat collection area is very important, as the area per watt needed is about six times the rate of insolation needed to produce at the rate of one watt (this is because of the need to collect heat also for night use, which obviously doubles the area needed, and because full insolation is only obtained for about four hours per day (2 hours either side of mid-day), which adds another threefold need of area. Half of the heated fluid then has to be stored, and this again is very cheap if no pressurisation is needed. For example, a swimming pool type of tank, dug-out and tiled, and provided with some suitable closure lid is all that is necessary, to provide storage of heated water, supplied by a number of separate solar-energy receiving receptacles.

Claims

1. A heat engine which is intended to receive thermal input energy from waste heat, low grade heat, a renewable energy source or any other convenient heat source and which comprises: an air compressor having an input arranged to receive ambient air, and having an output for delivering a supply of compressed air from the compressor; an air-driven motor arranged to be driven by the supply from the compressor output, and to provide a mechanical power output; a heat exchanger arranged along the path of travel of compressed air from the compressor output to the motor, said heat exchanger being intended to be supplied with thermal input energy from any suitable heat source, and to transfer thermal energy indirectly to the delivered supply of compressed air; and, a mechanical feedback from the motor to the compressor to operate the latter.

2. A heat engine according to Claim 1 , including an energy consumer device arranged to be driven by the mechanical power output from said air-driven motor.

3. A heat engine according to Claim 2, in which said device comprises an electrical generator.

4. A heat engine according to Claim 1 , and adapted to receive waste energy from an operating plant, and to provide a net source of power.

5. A heat engine according to Claim 1 , including means for receiving thermal input energy from a renewable energy source, to operate the heat engine.

6. A heat engine according to Claim 5, in which said means is adapted to receive input of solar energy.

7. A heat engine according to any one of Claims 1 to 6, in which the air compressor is a positive displacement type of compressor.

8. A heat engine according to Claim 7, in which the positive displacement compressor is a linearly reciprocating piston type compressor.

9. A heat engine according to Claim 8, in which the compressor is a floating piston type of compressor.

10. A heat engine according to Claim 9, in which the compressor and the motor are directly coupled vnth each other via respective linearly interconnected piston rods.

11. A heat engine according to any ^"one of Claims 1 to 10, in which the heat exchanger is arranged to receive input of thermal energy by routing a circulating flow of hot water, obtained directly from the heat source, or by indirect transler from the heat source utilised.

12. An energy conversion plant for converting solar energy into power and which comprises: a device for receiving an incident supply of solar energy, and for converting this energy into thermal energy for •application to a fluid heat transfer medium; a heat exchanger arranged to receive the fluid heat transfer medium; an air compressor having an input arranged to receive ambient air, and having an output for delivering a supply of compressed air from the compressor; an air driven motor arranged to be driven by the supply from the compressor output, and to provide a mechanical power output, said heat exchanger being arranged along the path of travel from the compressor output to the motor and to transfer thermal energy indirectly to the delivered supply of compressed air; and, a mechanical feedback from the motor to the compressor to operate the latter, and leaving a surplus to provide a net power output from the motor.

13. An energy conversion plant according to Claim 12, in which said device for receiving an' incident supply of solar energy comprises a non-pressurised liquid-containing receptacle.

14. An energy conversion plant according to Claim 13, 14, in which the receptacle comprises a fluid-proof tray covered by glass, transparent plastics or other transparent material through which the incident rays can pass.

15. An energy conversion plant according to Claim 14, in which the tray is darkly coloured to absorb solar radiation energy.

16. An energy conversion plant according to Claims 13 to

15, including a storage tank connected to a plurality of said receptacles .

17. An energy conversion plant according to Claims 12 to

16, in which said compressor comprises a piston linearly reciprocable within a cylinder, and a piston rod coupled rigidly with said piston and guided so that the piston does not require to make sliding contact with the wall of the cylinder.