1.1.2.Performance indices of cogeneration systems
The most important indices used to compare different cogeneration systems are the following:
mechanical efficiency of prime mover (heat engine):
(1)
where: is mechanical power of prime mover;
is heat flow produced by fuel combustion;
is fuel mass flow rate, kg/s or Nm3/s;
LHV is lower heating value (net calorific value) of fuel, kJ/kg or kJ/Nm3.
electrical efficiency:
(2)
where:
is electrical power generated by system (electrical power output);
thermal efficiency:
(3)
where
is the heat flow rate generated by the system (thermal power output).
overall efficiency or total energy efficiency:
(4)
Equation (4) is a subject of discussions because it is not appropriate to add heat with electricity since the heat quality is lower than that of electricity. It is very difficult to obtain 1 kW electric from 1 kW thermal due to the losses of heat conversion into electricity. We can use another index that uses the exergy, as a measure of energy quality, of system input and output, instead of the overall efficiency.
exergetic efficiency:
(5)
where
is exergy flow rate of generated heat;
is exergy flow rate associated to fuel:
(6)
ef is specific exergy of fuel, kJ/kg or kJ/Nm3.
power to heat ratio:
(7)
fuel energy savings ratio:
(8)
where
is fuel power input in individual generation system of heat (
)
or electricity (
);
is fuel power input in the system that cogenerates the same amount of
heat (
)
and electricity (
).
1.1.3.Types of cogeneration systems
The size of cogeneration systems can vary from small „micro” systems, which can serve the needs of a single building to large systems that can serve a town.
The cogeneration systems can be divided by the sequence of energy use and the operating schemes adopted into topping cycle systems and bottoming cycle systems (Fig. 3). In a topping cycle system, electricity or mechanical power is produced by a heat engine using fuel and the heat is recovered to meet heating demand. These systems are used in applications that do not require high process temperature. In bottoming cycle systems, the heat is produced directly from fuel combustion to meet high temperature heat demand of industrial process. The wasted heat is recovered and used as energy source to generate electricity or mechanical power.
In other way the cogeneration systems can be classified according to the type of prime mover or thermodynamic cycle used.
A. Rankine cycle (steam turbine) cogeneration systems
A steam turbine cogeneration system includes a heat source, a heat sink and a steam turbine driving an electrical generator. The system runs on improved Rankine cycle (steam reheat and regenerative preheating of feeding water) (Fig. 4). The heat source can be a boiler, a nuclear reactor or a waste incinerator. The boiler can use any type of fuel or combinations of fuels and solar radiation to produce superheated steam with high temperature and high pressure.
The power of steam turbine cogeneration system varies from 0.5 to over 100 MW. The generated steam can have up to 450°C and 100 bar in the commercial or industrial sector and about 540°C and over 100 bar in the utility sector.
The most widely used types of steam turbines are:
back pressure steam turbines;
condensing steam turbines;
bottoming cycle steam turbine systems;
bottoming Rankine cycle systems with organic fluids.
Figure 3. Types of cogeneration systems.
Figure 4. Rankine cycle with steam reheat.
The choice between types of steam turbines depends mainly on the quantities of power and heat, quality of heat, and economic factors.
A1. back pressure steam turbines
A backpressure steam turbine means a steam turbine from which the steam exits or is extracted at a pressure and temperature depending on the temperature level of heat required by the processes. Fig. 5 shows schematically a cogeneration system with backpressure steam turbine. the steam from turbine transfers the heat to technological/industrial process reaching the liquid state (condensation). The condensate is then returned to boiler.
Figure 5. cogeneration system with back pressure steam turbine and steam reheat.
These systems have the following advantages:
the construction is simple;
the high cost of low pressure turbine stages are avoided;
low capital cost due to the lack of cooling plant (condenser, cooling towers);
high overall efficiency as almost all entire generated heat in boiler is utilised (there is no heat transferred to the heat sink).
The disadvantages of the systems consist of the fact that the steam mass flow rate through the turbine depends on the thermal demand. This means that the electricity generated by turbine is controlled by thermal demand.
A2. Condensing steam turbine systems
In these systems, the steam is extracted from one or more intermediate stages at the pressure and temperature suitable for the heating process (Fig. 6). The main part of the steam exits the turbine at the condenser pressure. A lower condenser pressure (implicit temperature) means a higher thermal efficiency of cycle. The condenser temperature is limited by the temperature of a cooling agent (water, air).
Figure 6. Cogeneration system with condensing steam turbine.
The cogeneration systems with condensing steam turbine have a higher capital cost and lower overall efficiency than backpressure systems. The electricity load can be adjusted independently on the thermal load by controlling steam flow rate through the turbine.
A 3. Bottoming cycle steam turbine systems
Evacuated gases from many industrial thermal processes have high temperature (500-600°C). The sensible heat contained in gasses can be recovered to generate steam in a Rankine based cogeneration system. The steam turbine can be a condensing turbine (Fig. 7) or a backpressure one.
Figure 7. bottoming cycle cogeneration system with condensing steam turbine.
A4. Bottoming Rankine cycle systems with organic fluids
For effective use of low temperature level heat sources (80-300°C) have been developed plants that operate on Rankine cycle with organic working fluid. Organic Rankine cycle is similar to the conventional steam turbine cycle, but uses as working fluid a high molecular weight substance and low boiling point, which is suitable for recovery of heat available at low temperature.
An Organic Rankine Cycle (ORC) has the following advantages [6]:
organic fluids require less heat than water during the evaporation process. Thus the organic working fluids follow better the heat source fluid reducing the temperature differences and therefore the irreversibilities at evaporator (Fig. 8). The heat source can be cooled to a lower temperature by using organic fluids and thus more electric power can be generated from a given heat source;
the evaporation process takes place at lower pressure and temperature;
the expansion process ends in the vapour region and hence the risk of blades erosion is avoided;
turbines with organic fluids can provide higher efficiency at part loads;
systems have great flexibility, high safety and low maintenance;
the smaller temperature difference between evaporation and condensation means that the pressure drop/ratio is much smaller and thus simple single stage turbines can be used. The turbine cost is therefore lower.
Figure 8. Temperature profile of water and an organic fluid at heat addition.
Substances that can be used in ORC are: hydrocarbons (HC); hydrofluorocarbons (HFC); hydrochlorofluorocarbons (HCFC); chlorofluorocarbons (CFC); perfluorocarbons (PFC); siloxanes; alcohols; aldehydes; ethers; hydrofluoroethers (HFE); amines; fluids mixtures (azeotropic and non-azeotropic).
The working fluid in organic Rankine cycle plant determines the energy and economic performance. In selection of working fluid for different application, the following characteristics are taken into account:
good thermodynamic properties (vapour saturation curve with zero or positive slope (ds/dT) (isentropic or dry fluids); high latent heat of vaporization; high density (liquid/vapour phase); high specific heat; moderate critical parameters; acceptable condensing and evaporating pressures);
high thermodynamic performance (high energetic/exergetic efficiency);
good thermal and chemical stability during plant operation;
low environmental impact (ozone depletion potential, global warming potential, atmospheric lifetime);
good safety characteristics (non toxic, non corrosive; low flammability and auto-ignition properties).
good heat transfer properties (low viscosity, high thermal conductivity);
low cost and good availability.
The heat losses through irreversible heat transfer from the heat source and to sink influence the overall efficiency of a thermodynamic conversion cycle. These losses depend mainly on matching heat capacity of working fluid, heat source and sink. When there is divergence in the thermal behaviour between a working fluid and a heat source or sink, a complex configuration of heat absorption from the source (evaporation at multiple pressure levels) and/or heat released to sink (condensation at 2 or 3 decreasing temperatures) are used. In order to achieve the matching of working fluid and source heat capacities, the multi-component, non-azeotropic working fluids featuring non-isothermal heat addition are used (Fig. 9).
The examples of multi-component non-azeotropic working fluids are: mixtures of simple molecules (ammonia, methane, light hydrocarbons) and mixtures of complex molecules (propane, etherscyclic and aromatic hydrocarbons and siloxanes).
The electric power generated by cogeneration systems with organic Rankine cycle is in the range from 2 kW to 2.5 MW. The electric efficiency is about (10-30)%.
Figure 9. Temperature profile of a multi-component, non-azeotropic working fluid at heat addition.
B. Gas turbine cogeneration systems
The gas turbine can operate in a simple Brayton (called Joule cycle when irreversibilities are ignored) thermodynamic cycle or in a combined Brayton-Rankine cycle. The gas turbine systems have been developed initially for industrial and utility applications and as aircraft engines. Later, these turbines have been modified in order to be used in stationary applications. The modified turbines are called aeroderivative turbines. The main advantages of gas turbine cogeneration systems are the following: low capital cost; low-cost maintenance; low installation cost; fast start-ups; rapid response to changing load; fuel-switching capabilities, high efficiency of larger plants; high temperature level of heat (450-600°C) which can be recovered; good environmental performance. The main disadvantage is in low heat to power conversion efficiency. Due to their advantages, the gas turbine systems are the most frequently used technology in cogeneration systems. Their electric power output ranges from few kilowatts (micro turbine systems) to 250 MW.
The most gas turbine cogeneration systems operate on the open Brayton cycle (Fig. 10). The atmospheric air is compressed in compressor at a pressure ratio up to 30:1 and then introduced in combustion chamber where fuel combustion takes place. The generated flue gas exits the combustion chamber at high temperature and pressure and with high oxygen concentration (15-16%). The higher flue gas temperature is at the turbine inlet, the higher cycle efficiency is. The upper temperature limit at which turbine can operate is determined by the resistance of turbine materials and by the cooling efficiency of blades. The current higher cycle temperature is 1400°C. The sensible heat contained in flue gas from turbine outlet is recovered in a recovery steam generator with single pressure or double pressure. The exhausted gases from turbine can be also used directly in heating or drying processes (Fig. 11). The heat content of exhausted gases from turbine can be increased by using a burner inside the heat recovery boiler. The necessary air for additional fuel combustion is taken from the exhaust gases passing through the boiler.
The gas turbine cogeneration systems can use a variety of fuels: natural gas, gas oil, Diesel oil, biogas, landfill gas and syngas. The life cycle of systems depends on fuel quality and varies between 15 and 20 years.
Figure 10. Cogeneration system with open-cycle gas turbine.
Figure 11. Cogeneration system with closed-cycle gas turbine.
A reduces number of gas turbine cogeneration systems are running on closed Brayton cycle. The source heat (nuclear energy, solar energy, heat from a solid waste combustor) is used to heat the working fluid (helium, air) which circulates in a closed circuit. The heat contained in working fluid after expansion in turbine is released to a process fluid.
The electric efficiency of gas turbine systems is in the range of (25-35)% for small and medium systems and in the range of (40-42)% for larger systems. This lower electric efficiency is due to the power consumption of air compressor, which is about 50% of generated power by turbine. To reduce the power consumption of air compressor, the following techniques are used: precooling of the compressor inlet air; cooling of air at intermediate stage of compression and regenerative air preheating before the combustion chamber inlet.
The overall efficiency of gas turbine cogeneration systems is about (60-80)% and the power to heat ratio is in the range 0.5-0.8.
C. Reciprocating engine cogeneration systems
For cogeneration applications in institutional, commercial and residential sectors are preferred the reciprocating engine based cogeneration systems. The engines drive an electrical generator and heat contained in the exhaust gases and cooling systems, which represent (60–70)% of the inlet fuel energy, is recovered to generate hot water or steam. Most of the waste heat is available in the exhaust gases and jacket coolant, while smaller amounts can be recovered from the lubricating oil cooler and the turbocharger's intercooler [8]. the exhaust gases can also be used for drying or other direct heat processes.
the main advantages of reciprocating engine cogeneration systems are:
low capital cost;
fast start-ups;
good operating reliability;
high efficiency at partial load operation.
The main disadvantages are:
high vibrations and high acoustic noise;
high maintenance costs;
high pollutant emissions (especially NOx);
the waste heat is available at different temperature levels making difficult full utilisation of heat.
The reciprocating engine cogeneration systems may be classified by the engine cycle in Otto cycle and Diesel cycle. The Diesel engines are classified as high speed, medium speed and low speed engines.
The Otto engines can use beside the gasoline many gaseous fuels such as propane, biogas, and landfill gas. When they use gaseous fuel they are called “gas engines”. The Diesel engines can operate on Diesel oil, petrol, bio-oils, natural gas, biogas, landfill gas, syngas, alcohols and residual fuel oil.
Another classification may be made on the size of the engine as follows:
small units with gas engine (15-1000kW) or Diesel engine (75-1000kW);
medium power systems (1-6MW) with gas engine or Diesel engine;
high power systems (> 6MW) with Diesel engine.
The water passing through the cooling system is heated up to (75-80)°C before entering the heat recovery boiler. When there is no heat demand the cooling water is guided to the auxiliary cooler. Medium power systems generate saturated steam with (180-200)°C while high power systems generate superheated steam at pressure of (15-20) bar and temperature of (250-350)°C by using an auxiliary boiler. The minimum temperature of exhaust gases at boiler outlet depends on the fuel type. For fuels containing sulphur the minimum temperature is (90-100)°C and for fuels with no sulphur content is (160-170)°C.
Closed-loop cooling is the most common method of heat recovery from reciprocating engines (Fig. 12). In these systems, the engine is cooled by forced circulation of a cooling agent through the coolers and when there is no heat demand through an auxiliary cooler (cooling tower or radiator).
Another cooling method of reciprocating engines is ebullient cooling. This method consists of a natural circulation of a boiling coolant through the engine. The cooling water at its boiling point is placed to the bottom of the engine where it is heated and starts to boil generating bubbles. The bubbles generation leads to a lower coolant density, causing natural circulation to the top of engine. As the heat transfer from the engine to the coolant occurs at constant temperature, the engine thermal stress is lower. The mixture of liquid and vapour after exiting the engine is introduced in a steam separator for steam separation. The saturated steam is used in thermal process.
Figure 12. cogeneration system with reciprocating engine.
The overall efficiency of reciprocating cogeneration systems is about (70-85)% and the power to heat ratio is in the range of 0.8-2.4. The electric efficiency is in the range of (35-45)% for small and medium size engines and about 50% for large engines. The system performances are not so dependent on ambient conditions or load.
D. stirling engine cogeneration systems
The Stirling engines are not so well developed, but they present high interest due to their advantages: high efficiency, good performance at partial load, low temperature operation, low pollutant emission level and low vibration and noise level due to the continuous combustion process [2]. besides these, the Stirling engines can use a large variety of fuels and different energy sources (combustion gases, solar and nuclear energy). The fuels can be changed during operation without stopping the engine.
The main disadvantages of the Stirling engines are:
high capital cost (2000-50000$/kW);
long start-ups;
short lifetime of certain parts (shaft seal, piston rings, bearings leakage).
Unlike the reciprocating engines, at the Stirling engines the fuel combustion occurs outside the engine and the heat is transferred to a working fluid (usually helium or air) through a heater.
There are three configurations of Stirling engines: Alpha, Beta and Gamma (Fig. 13). Alpha configuration consists of two pistons in separate cylinders, which are connected in series by a heater, regenerator and cooler. Beta configuration uses the displacer-piston arrangement inside the same cylinder connected to a heater, regenerator and cooler, and Gamma configuration uses the displacer-piston arrangement placed in separate cylinders connected to a heater, regenerator and cooler.
Figure 13. Stirling engine configurations [9].
The ideal Stirling cycle consists of the following processes (Fig. 14):
isothermal compression (1-2), during which the heat is removed from the engine at the cold sink temperature;
constant volume heat addition (2-3), in which both pistons move simultaneously (compression piston towards regenerator and expansion piston away from regenerator), so that the volume between pistons remains constant and working fluid is transferred from compression volume to expansion volume through porous media regenerator;
isothermal expansion (3-4) during which heat is added to the engine at the hot source temperature (heater);
constant volume heat rejection (4-1), in which both pistons move simultaneously to transfer working fluid from expansion space to compression space through regenerator at constant volume
The cylinder volume connected to heater is expansion volume and connected to cooler is compression volume. The regenerator between the heater and cooler absorbs and releases heat alternatively. The temperature difference between cylinder volumes (Tmax-Tmin) is maintained.
Figure 14. Stirling cycle [10].
The conceptual diagram of a cogeneration system with Stirling engine is shown in Fig. 15 where the guideline values of the fluid temperatures are also given. The Stirling engine drives an electrical generator and the waste heat from the heat source is used to heat up the water. To obtain a high electrical efficiency of cogeneration system, the temperature of heating source should be as high as possible. This is achieved by combustion air preheating using the heat contained combustion gases leaving the heater. The combustion air temperature usually increases up to (500-600)°C.
To increase the system thermal efficiency, the engine cooling water (heat sink) is used to recover the waste heat in an economizer (heat exchanger).
Figure 15. Conceptual diagram of a Stirling cogeneration system
Stirling engines are usually installed horizontally downstream the combustion chamber. The air preheater and economizer are placed above the combustion chamber to obtain a more compact structure.
The Stirling engines have initially developed as car engines with power in the range of 3kW to 100 kW. Now the engines are built for cogeneration systems with the power up to 1.5MW especially for micro-cogeneration systems. The electric efficiency is up to 50%, the overall efficiency lies within the range of (65-85)% and the power to heat ratio is 1.2-1.7 [2].
E. fuel cell cogeneration systems
Fuel cells are still at an early stage of development. They may generate power in the range of 0.1kW to 50MW. A fuel cell is an electrochemical cell which can continuously convert the chemical energy of a fuel and an oxidant to electrical energy by a process involving an essentially invariant electrode-electrolyte system [11].
The structure of a simplified fuel cell consists of an electrolyte layer in contact with two gas permeable electrodes coated with a catalyst (Fig. 16). The electrodes are connected to a device that completes an electric circuit. The hydrogen fuel is fed continuously to one electrode and the oxidant (oxygen from air) is fed continuously to the opposite electrode. The hydrogen fuel is oxidised into hydrogen protons loosing its electrons to the electrode, which becomes in this way anode. The electrolyte membrane permits only the positive ions to flow from anode to cathode. Therefore, the electrons move to the cathode through the external electrical circuit and electrical current begins to flow. The electrons react with oxidant and hydrogen protons at the cathode forming water and producing heat.
Figure 16. Fuel cell working diagram.
The chemical reactions involved at electrodes are:
anode reaction:
cathode reaction:
overall reaction:
The efficiency of a fuel cell as the ratio of electrical energy produced and the chemical energy of the fuel is about (35-55)%. The working temperature is in the range of ambient temperature to 1000°C. By using the generated heat, the overall efficiency of a fuel cell reaches up to (75-90)%, the highest efficiency amongst all conversion systems.
The main advantages of fuel cells are:
high efficiency;
size flexibility (single cells can be stacked to provide the appropriate voltage for any application);
high reliability due to the lack of moving parts;
low pollutant missions.
The main disadvantages of fuel cells are:
high cost;
large size and weight relative to generated power;
large start-up time;
fuel availability;
some fuel cells require expensive catalysts;
some fuel cells are susceptible to poisoning;
some fuel cells suffer from corrosion and breakdown.
As the fuel cell voltage is too low (about 1.2V) it is necessary to stack many cells to increase the power output. Besides the stack, a fuel cell system includes other subsystems and component such as (Fig. 17):
fuel preparation unit. When other fuels than pure fuels (hydrogen) are used, fuel preparation (impurities removal, thermal conditioning) or fuel reforming is required;
air compressor or blower for air supply;
temperature control system;
water management system;
electric power conditioning system.
Figure 17. Fuel cell cogeneration system.
The fuel cells are classified according to the type of electrolyte and fuel as follows [12]:
proton exchange membrane fuel cell (PEMFC):
direct formic acid fuel cell (DFAFC);
direct Ethanol Fuel Cell (DEFC).
alkaline fuel cell (AFC):
proton ceramic fuel cell (PCFC);
direct borohydride fuel cell (DBFC).
phosphoric acid fuel cell (PAFC);
molten carbonate fuel cell (MCFC);
solid oxide fuel cell (SOFC);
direct methanol fuel cell (DMFC).
A comparative presentation of all fuel cells is given in Table I.