GB2288460A - A combined heating, cooling and electrical power generating system - Google Patents

Abstract

A combined heating, cooling and electrical power generating system, consisting of a self-contained, compact unit to provide both electrical power, space heating and water heating in selectable proportions, and air conditioning, the system comprising an engine E drivingly connected to an electrical generator G, a vapour compression cycle heat pump C having a compressor driven directly or indirectly by the engine E, a plurality of heat exchangers H1 with H5, H2 and H4 connected to the engine E and heat pump C, and an evaporator H3 connected to heat pump C, and passages and adjustable dampers D therein to cause fan assisted air streams A to flow selectively across the heat exchangers to give off or absorb heat as required for cooling or heating the air streams coincidentally with electrical power generation. To heat a building, for example, airstream A1 is drawn from the buildings interior and airstreams A2, A3 are drawn from atmosphere. Chilled air is discharged at A6 to atmosphere, and heated air returned to the building at A5. <IMAGE>

Description

A COMBIND HEATING, COOLING AND ELECTRICAL POWER GENERATING SYShI THIS INVENTION concerns a system for generating electrical power coincidentally with heating or cooling of ambient air, for example in a building.

Conventional methods for generating electrical power are typically no more than 30 per cent efficient with the balance of the available energy being discarded as waste heat. It is known to combine heat and power systems in which the waste heat from the power generation is beneficially employed for heating of buildings.

Such schemes have been employed in domestic, commercial and industrial establishments and can provide energy and cost savings where the ratio of heat to power requirements matches that provided by the plant which typically may be 2:1.

One difficulty experienced with such schemes is that the heating requirements in a building will vary substantially according to the time of year so that the optimum heat to power ratio is achieved only for limited periods. Thus, there are only a few instances where the reduced running costs are sufficient to recover the additional capital cost of the plant.

An object of the present invention is to provide a selfcontained, combined system for power generation and heating or cooling as required, which can satisfy the entire heat and power requirements of a user, with adequate selection of operation thus to achieve optimum performance irrespective of ambient or climatic conditions.

Thus, according to the present invention there is provided a combined heating, cooling and electrical power generating system, comprising an engine drivingly connected to an electrical generator, a vapour compression cycle heat pump driven directly or indirectly by the engine, a plurality of heat exchangers connected respectively to the engine, generator and heat pump, and passage means and control means to cause air streams to flow selectively across the heat exchangers thus to give off or absorb heat as required for cooling or heating the airstreams coincidentally with electrical power generation.

By integrating into a single system the combined effects of power generation and heating and cooling, incorporating an engine driven heat pump, with an electronic controller, a system is provided which has greater capabilities and advantages than the known uncombined arrangements. In addition, the system provides a compact, self-contained and highly efficient unit to provide both electrical power, space heating and water heating in selectable proportions, and air conditioning, thus to accommodate regional and seasonal variations in climatic conditions and the particular requirements of different users.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of a system made in accordance with the invention and adapted to operate in a heating mode; Fig. 2 shows the same system but changed to operate in an air conditioning or cooling mode; and Fig. 3 is a data flow diagram illustrating the operation of an electrical control means for the system.

Referring now to Fig. 1, an internal combustion engine E is drivingly connected to an electrical generator G and to the compressor of a vapour compression cycle heat pump C. The choice of engine type will be determined by the available fuel options and relative costs. These may be for example mains gas, gas oil or biogas.

The engine E, generator G and heat pump compressor C are mounted in a thermally and acoustically insulated enclosure E2 which in turn is located within a larger enclosure El. Also within the outer enclosure El there is provided an arrangement of air ducts, fans and variable louvres or dampers which co-operate to transfer air streams between the interior of a serviced building and one or more heat exchangers as will be described. Connected to the cooling and exhaust circuits of engine E are heat exchangers H1 (with H5) and H2, respectively. Low grade waste heat from the engine enclosure E2 can be transferred as an air stream A4 by means of a fan F3 in a ducted outlet from the enclosure E2.

The vapour compression cycle heat pump C further consists of a refrigerant circuit with an air cooled condenser H4, an air heated evaporator H3 and an expansion valve V1. For the system to operate efficiently over a range of climatic conditions, and to enable adjustment of the heat pump performance to changing demands, several options are available to vary the capacity of the heat pump compressor C. For example there may be a variable speed drive connection between the engine E and the compressor C.

Alternatively, several separate compressors of equal or unequal capacity may be provided with the capability to seiect any combination of the compressors by mechanical means such as clutches or suction vents. Yet again, a multi-cylinder reciprocating compressor may be employed and which may be unloaded in one or more of the cylinders. As a further alternative there may be provided a reciprocating compressor with a variable clearance volume at the top of the stroke. The above are some examples only of the options available to vary the performance of the compressor.

In the example illustrated in Fig. 1, the compressor C is driven directly by a physical driving connection to the engine E.

Alternatively, the compressor may be driven by an electric motor supplied with electricity from the generator G. Although such an arrangement would entail a small reduction in efficiency, there are compensating benefits including ease of selection and de-selection of the compressor, for example by electrical switching, and the ability to use a hermetic or semi-hermetic type compressor with improvements in refrigerant sealing integrity and increased flexibility of use. For example the heat pump could be operated from the mains electrical supply, for example, at low tariff or off-peak rates, or when the engine and generator are not available during maintenance periods.

The expansion valve Vl is adapted to meter the quantity of hot, high pressure liquid refrigerant flowing from the condenser H4 to the evaporator H3 such that the fluid is fully vaporised at the evaporator outlet but with minimum superheat. To achieve this condition for a wide range of evaporating temperatures, an electrically operated expansion valve is preferred to the more commonly adopted thermostatic expansion valve. Regulation of the expansion valve V1 must be a function of an electrical control device for the system.

Again as illustrated in Fig. 1 where the system is operating to provide space heating for the building, air stream Al is drawn from the interior of the building and divides into airstream A7 and A8. Airstream A8 represents the larger proportion of Al since it is drawn through fully open damper D1 by fan F1 to mix with a smaller proportion of fresh external air as airstream A2 which is drawn through the partially open damper D2. The combined airstream A2 plus A8 is drawn into the heating system by fan F1 and passes in sequence over the heat pump condenser H4, the engine coolant heat exchanger H1 and the engine exhaust heat exchanger H2.With the heat pump operating, the condenser H4 and both heat exchangers H1 and H2 serve to increase the temperature of the air stream which is then returned, via a fully open damper D3, as heated air to the building at A5.

Also in this mode of operation, fan F2 draws an ambient air stream A3 through a fully open damper D7, and building ventilation air stream A7 through partially open damper D8 together with air stream A4 through a fully open damper D10 from engine enclosure E2.

The combined air stream from fan F2 passes over and is chilled by the heat pump evaporator H3 and is discharged at A6 to atmosphere through a fully open damper D6. The heat extracted from this air stream when added to the mechanical work input to compressor C provides a part of the building heating and is transferred to the warm air stream by the heat pump condenser H4 as described previously.

To accommodate change in plant operating conditions it is preferable that fan F2 should have a variable capacity and may have a multi-speed drive motor. Alternatively this may be a centrifugal type of fan a characteristic of which is that of reduced power consumption as the flow rate is reduced by increasing the pressure drop using dampers D7, D8 and D10. If required, all or part of the heat energy from the engine coolant system may be diverted via valve V2 to heat exchanger H5 of a water heater T to heat water within the serviced building.

The heating process as described empioys two primary energy sources to satisfy the electrical power and heating demands of the serviced building, i.e. high grade chemical energy from the engine fuel and low grade thermal energy from ambient air. Heat extracted from the'ambient air may include both sensible heat and the latent heat of any water vapour content.

In order to maximise efficiency the system as described has a number of features to derive maximum benefit from the thermo-dynamic availability of the primary and subsidiary energy sources. These include: (a) the choice of direct warm air space heating in preference to a circulating water system which therefore avoids an additional heat transfer stage and reduces the required temperature of the heated fluid; (b) heat from higher grade sources i.e. engine coolant and exhaust waste heat is transferred to the fluid to be heated with the minimum number of heat transfer stages.

(c) the heat pump recovers heat only from low grade sources which could not be otherwise usefully employed. This includes the ambient air heat source, waste heat from the building ventilation exhaust, and low grade waste heat from the insulated engine enclosure E2; (d) the sequence of heat exchangers H4, H1 and H2 is chosen to make maximum use of the availability of each source and in particular by placing the heat pump condenser at the first stage, the difference between the heat pump evaporating and condensing temperatures is minimised and the co-efficient of performance maximised.

If the requirement for heat should be reduced then the heat transferred by the heat pump C is also reduced by means of its variable performance. If the heat pump compressor power is reduced to and beyond a certain level, the efficiency of the plant may be enhanced by partially or fully closing damper D7 to reduce or discontinue ambient air stream A3. In this case, the heat pump is used only to recover the low grade waste heat from the ventilation exhaust air stream A7 and engine enclosure outlet air stream A4. By closing the damper D7 in these circumstances the refrigerant evaporating temperature increases and consequently so will the heat pump coefficient of performance. The point at which it becomes advantageous to close damper D7 will be determined by the electronic control system thus to maximise the overall efficiency of the plant.

If the requirement for heating reduces further then the heat pump compressor may be stopped so that the heat is supplied only from the engine coolant and exhaust systems.

By appropriate selection of the positions of the various dampers controlling the airstreams, together with the heat pump capacity and the operation of fan F2, it is possible to operate the plant in a variety of different states providing, at one extreme, as described above, maximum heating, and at the other extreme, full cooling or air conditioning with heat extraction and de-humidification of the building interior. Where maximum heating is required this is achieved with a high degree of efficiency by exploiting the available sources of waste and ambient heat whereas when cooling is required waste heat is discarded to the atmosphere and so the energy efficiency of the plant is reduced accordingly.

For cooling or air conditioning, the plant is adjusted as illustrated in Fig. 2 where damper D4 is partially open to allow the air which is passing over the heat exchangers to be partly discarded to atmosphere, whilst the chilled air stream A6 is directed via open damper D5 into the main outlet air stream A5, and damper D6 is closed to prevent the chilled air from escaping to atmosphere.

Similarly, damper D7 is closed to prevent warm external air from entering the system. Damper D10 is closed to prevent warmed air from the engine enclosure E2 from being mixed with the air to be chilled, and damper D9 is opened to allow the warm air to escape to the atmosphere. Damper D8 is fully open and damper D1 partially open so that the majority of airstream Al extracted from the building is directed to evaporator H3 as airstream A7 for cooling. The arrangement of Fig. 2 shows cooling with de-humidification since airstream A7 will be cooled by evaporator H3 to a temperature below the required return temperature in order to extract more moisture.

By selective mixing with a smaller proportion of the warm airstreams A8 + A2 the desired return temperature is restored in the outlet airstream A5. By this means the relative humidity of the return airstream A5 is reduced to less than 100%.

A common problem with air source heat pumps in certain conditions is ice information on the evaporator surfaces which causes a reduction in the heat transfer efficiency as well as obstruction of the air flow. Conventional means for overcoming this problem involve a defrost cycle during which time ice formation is removed by electrical heating elements, or by the redirection of hot refrigerant.

Both methods represent a loss of efficiency and it is the intention to ensure that the system continuously avoids excessive ice formation.

The frequency of occurrence of icing is reduced in the system as illustrated in Fig. 1, firstly by mixing the ambient air stream over the evaporator with the warmer air streams A7 and A4 from the building ventilation outlet and engine enclosure E2 respectively.

Again, prevention of icing will be a function of the electronic controller responding to sensed evaporator air inlet temperature and humidity and refrigerant evaporating temperature. The controller will adjust the operation of the plant to prevent excessive ice formation by regulation of the proportion of ambient air admitted at D7, the evaporating temperature by means of the refrigerant expansion valve and the heat extracted from the evaporator by means of the compressor capacity. The electronic controller will perform a calculation to determine damper settings which enable maximum heat extraction without unacceptable ice formation.

For any particular application, the size of the evaporator will be selected such that sufficient heat can be extracted by the ice prevention procedure described above, within the normally expected range of climatic variations. In the event of abnormal conditions when the prevention of freezing would otherwise preclude the supply of sufficient heat, supplemental electrical resistance heating can be employed.

A disadvantage of air source heat pumps is that as the ambient air temperature reduces, both the capacity and performance of the system reduces whereas the required heat output is likely to increase. To avoid the need for a larger and more expensive plant to accommodate exceptional climatic extremes, the system includes a supplemental electrical resistance heater RH which can be energised when required under extreme climatic conditions. Although this would be less efficient in operation than the larger capacity heat pump, it is likely to provide a more economical overall solution.

Fig. 3 illustrates the main data flows of the electronic, computer based controller for the system. The control process algorithm will respond to four categories of input data namely, (a) user demands such as the required air and water temperatures within the serviced building. These will usually be pre-programmed but may also be enhanced by a short-term override command. The instantaneous electrical power required must also match the demand of the user.Required temperatures may be set as preferred (minimum and maximum) values, rather than a single value, in order to optimise the plant operation; (b) system data which is instantaneous measured data by way of feedback from the plant and the building; (c) external data, i.e. instantaneous measured external climatic data and certain pre-programmed external data, in particular fuel and alternative electricity supply cost data; (d) historic data which is stored by the controller and pre-set according to the particular application, including local climatic trends. This data store will be updated continuously, and the operator will be alerted to any marked change thus indicating a fault.

In response to data inputs from these sources the controller will generate two groups of output commands or information namely, plant controller outputs, and operator information displayed on a panel or screen. Such information will include selfdiagnostic readings and notice of imminent maintenance requirements.

The main function of the control process algorithm as indicated in Fig. 3 is to determine the plant controller output settings to achieve the demanded conditions at minimum cost. The major task can be considered to be "open loop" such that it is necessary to determine which of the possible operating states is most appropriate and economical in response to a larger number of input considerations.

Having determined the most efficient state, the controller will be required continuously to perform certain closed loop tasks including regulation of the expansion valve V1 to maintain the desired evaporating temperature and/or superheat; regulation of engine power to maintain the correct electrical frequency; and regulation of heat pump capacity or progressive variation of damper positions to control building temperature within the required limits.

The above subsidiary control functions may be accomplished either by the overall computer controller or by dedicated subsidiary controllers responding to a demand input defined by the main controller.

For the optimal selection of plant states, any one or a combination of the following techniques may be included in the process control algorithm namely, (a) selection of plant state by determining rules which map all possible combinations of input data and define a single "best response" for any given input condition; (b) selection of plant state by the fuzzy logic. rules. This is a known technique based on the mathematics of fuzzy sets in which the decision making rules are defined in subjective linguistic terms; (c) selection of plant state by reference to a mathematical model of the system behaviour. For a given set of input data, the most efficient operating condition is determined by comparison of options using the model.Parameters of the model may be fixed and represent known characteristics of the plant, or they may be "learnt" and represent characteristics of a particular application.

To illustrate the task to be formed by the process controller, the following are three examples of required plant settings and a statement of some of the considerations involved in determining the optimum settings.

1. SELECT WATER HEATING OR SPACE HEATING - this is determined by actual and required air and water temperatures. If either are below a minimum permissible level the heat is supplied as necessary. If both are within the allowable band but below the maximum, a decision will be made with reference to predicted changes in power demand, outside air temperature and typical air and water heat loss. For example, in temperate weather the system may learn that space heating demand falls during the day. By maintaining minimum water temperature initially, the opportunity will arise to increase water temperature at a later time without supplemental heating from the heat pump.

(2) SELECT HEAT PUMP CAPACITY - this is selected with consideration of actual and demanded temperatures, current and predicted heat loss rates, current and predicted power production and predicted climatic trends.

(3) SELECT SELF-GENERATION OR MAINS - This is determined by a consideration of current and predicted electrical power demand, actual and demanded air and water temperatures, current and predicted heat loss rates, predicted climatic changes and a database of self-operating costs and mains electrical supply costs. For example, if operating at low total power at night, particularly if hours-related maintenance costs and reduced mains supply tariffs are considered, switching to mains supply might be more economical. The requirement for reversion to mains operation applies only to electrically (rather than direct) driven compressors. It is possible also that in some cases it will be preferable to operate from mains power at certain times of the day to minimise noise nuisance.This can be defined as a user instruction to the controller either as a rigid rule or as a preference within defined economic cost limits. In the case of a fuzzy logic implementation of the control. algorithm, the degree of preference can be defined in fuzzy linguistic terms. A deterministic controller implementation requires numerate cost limits.

Certain advantages accrue from a system made in accordance with the invention such as a high degree of inherent flexibility in the manner of heat production and the capability to select, by means of the electronic controller, the most efficient manner of operation for the prevailing climatic conditions and heat and power demands of the premises served.

Compared to an engine driven heat pump without combined generation of electricity the arrangement provides a greater return on investment costs since the heat recovery arrangements serve also to enhance the total efficiency of the electrical power generation process, and a cost saving benefit is obtained from running the plant throughout the year. Also, since ambient air is used for the heat pump source the system can be employed at almost any location.

Compared to the known combined heat and power (or cogeneration) scheme, the arrangement as described adds the capability to produce heat and power in variable ratios and quantities to satisfy the demands of the user. The known combined heat and power scheme typically makes provision for recovery of waste heat from the engine coolant and exhaust systems. The arrangement as described further enhances the overall energy efficiency by recovering heat from the following additional sources: (a) Radiated heat from the engine and electrical generator. From measurements of a typical small scale combined heat and power plant, utilisation of this heat source alone would increase the waste heat recovered by more than 50%.

(b) Recovery of waste heat normally discarded in the building ventilation.

(c) Extraction of heat from the atmosphere.

Where it is possible to drive the compressor using an electric motor the heat pump may be powered from mains power during off peak, low cost periods.

Furthermore, the system will achieve considerable building ventilation performance since heat is recovered from the ventilated air.

Analysis of performance has revealed a number of notable operating parameters.

The conclusions drawn are that the system will efficiently provide a ratio of power to heat ranging from 0% (all heat) to 60%, with total engine power output ranging from 20% to 100% of the engine rated output. At power to heat ratios greater than 60% it would be necessary to discard surplus heat. With varying power to heat ratios, energy utilisation efficiency is maximum in the heating only mode, but the level of cost saving relative to conventional energy sources is substantially constant. The energy cost is typically 50% of the conventional cost at maximum total engine power output, increasing to 60% cost at 25% rated power irrespective of power to heat ratio within the stated range.

Claims (22)

1. A combined heating, cooling and electrical power generating system comprising an engine drivingly connected to an electrical generator, a vapour compression cycle heat pump having a compressor driven directly or indirectly by the engine, a plurality of heat exchangers connected respectively to the engine and heat pump, and passage means and control means to cause airstreams to flow selectively across the heat exchangers thus to give off or absorb heat as required for cooling or heating the airstreams coincidentally with electrical power generation.

2. A system according to Claim 1, wherein the engine is an internal combustion engine directly connected to the generator and to the heat pump compressor.

3. A system according to Claim 1 or Claim 2, wherein the engine, generator and heat pump compressor are mounted in a thermally insulated enclosure which is located within a larger enclosure.

4. A system according to Claim 3, wherein the larger enclosure also contains an arrangement of air ducts, fans and variable louvres or dampers which co-operate to transfer airstreams between a zone to be heated or cooled, and one or more of said heat exchangers.

5. A system according to Claim 4, wherein at least one of the air ducts is connected to a building interior to receive low grade waste heat therefrom, and contains a heat exchanger connected to the heat pump, and wherein at least one other of the air ducts is connected to the thermo-insulated enclosure to receive therefrom low grade radiated heat from the engine, generator and heat pump compressor.

6. A system according to any preceding claim, wherein one of the heat exchangers is connected to the cooling system of the engine and another is connected to the exhaust circuits thereof.

7. A system according to any preceding claim, wherein one of the heat exchangers is directly connected to a further heat exchanger for water heating.

8. A system according to Claim 3, wherein a fan is provided to extract low grade waste heat from the thermally insulated enclosure.

9. A system according to any preceding claim, wherein the vapour compression cycle heat pump consists of a refrigerant circuit with an air cooled condenser, an air heated evaporator, and a variable expansion valve.

10. A system according to Claim 9, wherein the variable expansion valve of the heat pump is adapted to meter the quantity of hot, high pressure liquid refrigerant flowing from the condenser to the evaporator such that the fluid is fully vaporised at the evaporator outlet but with minimum superheat.

11. A system according to Claim 9 or Claim 10, wherein the variable expansion valve is electrically operated.

12. A system according to any preceding claim, including means to enable adjustment of the performance of the vapour compression cycle heat pump to suit changing demands.

13. A system according to Claim 12, wherein said adjustment means includes a variable speed drive mechanism connection between the engine and the heat pump compressor.

14. A system according to any preceding claim, when the compressor of the vapour compression cycle heat pump is driven by an electric motor supplied with electricity from the generator.

15. A system according to Claim 1, wherein ducted air is adapted to pass in sequence over several heat exchangers such that it passes first over a heat pump condenser and thereafter over a heat exchanger connected to the engine cooling system, and finally over a heat exchanger connected to the engine exhaust circuit.

16. A system according to Claim 1, wherein the passage means and control means are adjustable selectively to provide a variety of different operating states between maximum heating and maximum cooling.

17. A system according to Claim 9, including means to prevent the formation of ice on the evaporator of the heat pump.

18. A system according to Claim 17, wherein the means for prevention of ice is the provision of passage means and control means to enable mixing of an ambient airstream with warmer airstreams generated by the system.

19. A system according to any preceding claim, including an electronic controller adapted to calculate and determine variable settings of the passage means and control means thus to cause optimum airstreams to flow within the system to produce the desired heating or cooling performance.

20. A system according to Claim 19, wherein the electronic controller is adapted to control said settings in such a way as to enable maximum heat extraction without unacceptable ice formation on the heat pump 'evaporator.

21. A system according to any preceding claim, so arranged and adapted as to provide a ratio of power to heat selectable between 0% (all heat) to 60%, with total engine power output ranging from 20% to 100% of its rated output.

22. A combined heating, cooling and electrical power generating system, substantially as hereinbefore described with