Optimisation of Combined Heat and Power Systems (CHP).
Thermal power systems such as gas turbines inevitably reject heat into the environment as required by the Second Law of Thermodynamics which effectively states that the thermal efficiency of a heat engine cannot be 100%. This means that some of the heat released during combustion in a gas turbine is lost into the environment. The thermal efficiency of gas turbines varies from 20% to about 40%. Thus, 60% to 80% of the heat supplied into a gas turbine by burning of the fuel is wasted.
CHP systems also known as Cogeneration utilises this waste heat when heating requirements are present resulting in an appreciable increase in the overall efficiency of the power and heating system. Figure 1.1 shows the increase in overall efficiency of a CHP system with increasing heat to power ratio for a series of gas turbine thermal efficiencies. The overall efficiency in the region of 90% is possible with CHP resulting in a significant reduction in fuel and consequently operating costs. The reduction in fuel consumption inevitably results in a reduction in CO2 emissions, which is thought to be responsible for global warming. Hence, CHP systems are not only beneficial in reducing life cycle costs but they are also friendlier to the environment.
Figure 1.1 shows the variation of overall efficiency with heat power ratio of a CHP system.
CHP or Cogeneration may also be considered when significant cooling loads are required as in refrigeration and air conditioning requirements. In this instance, a vapour absorption system is used to produce the refrigeration loads utilising the heat rejected by the gas turbine. Although vapour absorption systems are less efficient than the more common vapour compression systems, the impact is minimal, as a source of waste heat from the exhaust of the gas turbine is readily available.
1.1 Cogeneration Optimisation System (COOPS)
CHP or Cogeneration systems usually consists of more than one gas turbine exhausting into a single waste heat boiler which recovers the exhaust heat by producing hot water or steam for process and space heating. When a CHP system consists of more than one gas turbine the optimisation of a CHP system is possible resulting in reduced fuel costs and lower CO2 emissions. This is achieved by selecting suitable loads for each gas turbine such that the fuel requirements are a minimum whilst still producing the necessary power and heat loads.
GPAL has developed an optimisation system (COOPS) for CHP systems which can be integrated into the engine control system. COOPS uses GPAL technology developed for the monitoring of gas turbine performance, working in conjunction with an optimiser similar to that employed by GPAL GasComp product for optimising gas compression systems. COOPS optimises the CHP system taking into account performance deterioration when present, resulting in a model based control system. The amount of fuel saving depends on the power and heating requirements for a given ambient condition. At low power requirements and ambient temperature, greater the potential saving in fuel consumption. This applies not only to the application of COOPS for CHP units, but is equally applicable for optimising a bank of simple cycle gas turbines or combined cycle plants.
Figure 1.2 Outputs of the optimised CHP system (with the use of COOPS).
An example of the optimisation using a gas turbine simulator in conjunction with COOPS is given below: The CHP system consists of two units where each CHP unit consists of two gas turbines operating with a power turbine exhausting into a single waste heat recovery boiler. It is assumed that the CHP system has a 25% standby capability built into the system at ISO operating conditions. The example considers the case when the ambient temperature and pressure are 5 degrees Celsius and 1.013 Bar respectively. Power output and heat loads required are 19,200 kW and 60,000 kW respectively.
The outputs generated from COOPS after optimisation is shown in Figure 1.2. The total fuel flow required and the overall efficiency are 2.074 kg/s 79.17% respectively. We note that COOPS have shutdown one gas turbine and two of the remaining gas turbines are operating on the gas generator speed limit. (At high ambient temperatures, it is the exhaust gas temperature (EGT), which limits the power output of the gas turbines). If we choose to operate all four gas turbines each operating at equal powers then the fuel flow required and overall efficiency are 2.169 kg/s and 75.70%, which is shown in Figure 1.3 (un-optimised case). This represents about 4.4 percentage in fuel consumption, which is a significant reduction of fuel consumption. Larger reductions up to about 15% are possible when operating at lower powers and heat load.
Figure 1.3 Outputs when the CHP system is using all engines at equal loads.
1.2 Optimised load shedding
COOPS also calculates the maximum power output from the gas turbines. In our example we have a surplus capacity of about 7,463 kW. The maximum power output from the gas turbines is calculated using accurate modelling techniques (component matching) accounting for ambient conditions and engine degradation. This therefore helps prevent frequency shifts due to excessive power demand from the gas turbine. Thus, the operator is now in a position to sell this surplus capacity to the grid, particularly if the price of electricity is attractive, which occurs at certain times of the day. Such optimised load shedding not only improves the revenue for the operators but their own power and heat loads are generated more efficiently resulting in lower fuel costs and CO2 emissions in real terms.
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