The sidelining of such basic energy facts reveals a dangerously irrational approach to energy policy among normally intelligent minds that, one suspects, are strangely disorientated by vested interests. Forget a rational or coherent strategy, it is the already powerful industries, such as coal, that are being allowed to set the agenda.
This is not the approach needed to put an end to resource wastage and to deliver an electricity system fit for the profound challenges of the 21st century. Clear thinking is needed on how to reduce emissions rapidly on both the demand and supply sides, and how to support technological advances in low-carbon technologies. The potential to reduce the overall demand for electricity through energy efficiency measures has been endlessly analysed but weakly pursued by government. Less well rehearsed, and still less acted upon, is the potential to reduce energy wastage and carbon dioxide by remodelling our electricity system – the subject of recent Greenpeace report Decentralising Power.1 Decentralizing our electricity system presents an opportunity to deliver on the commendable UK energy policy goals set out in the 2003 Energy White Paper, which are: to reduce carbon dioxide by 60% by 2050; to maintain security of supply; to promote competitive markets in the UK and beyond; and to tackle fuel poverty. By enabling the capture of waste heat, and establishing an infrastructure and regulatory regime responsive to the characteristics of renewables and other decentralized energy technologies, a decentralized model has the potential over the coming decades to put our electricity system on the steep downward curve that climate change demands.
While Greenpeace accepts gas as a transition fuel, ultimately the goal must be a fully renewable system. A decentralized pathway plays to the economic advantages of renewables; the geographical ubiquity of renewable energy means renewable DE offers relatively little dependency on the expensive fuel supply and wires infrastructure that fossil fuels can demand.
In the long run, a decentralized pathway may also be cheaper, not least because it obviates the need for the current level of investment in hugely expensive highvoltage transmission (and some distribution) networks. It also offers to stimulate massive innovation through capturing the benefits of technological progress, offering immediate practical application rather than the grant dependency that accompanies trying to shoehorn DE technologies into a centralized infrastructure. At the same time it delivers a more secure network configuration and improves security of supply by massively reducing primary energy demand.
The vulnerability of brittle, centralized electricity systems has been amply demonstrated around the world in recent years and is set to increase as a result of climate change impacts and our growing dependence on energy imports. Moreover, our society’s increasing reliance on electronic technology makes the potential consequences of centralized-system vulnerability all the more expensive and catastrophic.
how to maximize it
Gas turbines form the base of many cogeneration systems, so the efficiency of the turbine is fundamental to the efficiency of the overall plant. Here, David Flin goes back to basics with a look at how the turbines work and how their design affects performance.
A gas turbine functions by allowing the passage of expanding combustion gases through a series of turbine blades (see Figures 1 and 2). The efficiency of the turbine is measured by comparing power input to power output as measured by mechanical energy in the output shaft. Gas turbine efficiencies are usually given for ISO conditions at 15°C, 60% relative humidity and an atmospheric pressure equivalent to average sea level conditions. Variations in temperatures and relative humidities during operation of the turbine will result in changes to its efficiency.
The efficiency of a gas turbine – more correctly called the overall thermal efficiency – is the ratio of work done to the heat supplied. Efficiency is defined as:
Efficiency = 100 x K x (Tmax – Tmin)/Tmax
Tmax is the temperature of the gas at the inlet to the gas turbine, Tmin is the ambient temperature and K is internal losses.
As a result, there are three theoretical methods of increasing efficiency: increasing inlet temperature, decreasing ambient temperature and reducing internal losses.
Theoretically, a gas turbine could achieve efficiencies of up to 65%. At present, simple open-cycle turbines achieve efficiencies of about 40%. In addition, it is possible to use waste heat from the outlet of the gas turbine to improve efficiency of use. This is where the very high overall efficiencies from cogeneration come from.
The basic gas turbine cycle is shown in Figure 3. Air is compressed from point 1 to point 2. This increases the pressure as the volume of space occupied by the air is reduced. The air is then heated at constant pressure from point 2 to point 3. This heat is added by injecting fuel into the combustor and continuously igniting it. The hot compressed air at point 3 is then allowed to expand (point 3 to point 4), reducing the pressure and temperature and increasing the volume. This represents flow through the turbine to point 3’ and then flow through the power turbine to point 4. The combustion cycle is completed by decreasing the volume of air (point 4 to point 1) through decreasing the temperature, with heat being absorbed into the atmosphere.