CONSERVE your energy
15 Jan 2000
Air Combined Heat and Power is a unique and patented form of small-scale CHP, developed by Peterborough based Aircogen. The key feature of the Air CHP system is the direct heating of air by the waste products of power generation. The technology is similar to that of conventional CHP plants, but the equipment is packaged inside the heated air stream together with the integral heat exchange and air handling equipment. Reciprocating natural gas or fuel oil engines and small gas turbines are the prime systems used.
Aircogen's Air CHP technique is patented in the UK, Europe and the US. Other features of the system include an external circulating oil system and microprocessor-based control.
Conventionally, a small-scale water based CHP system replaces boiler capacity and achieves a financial return on it. However, claims Aircogen engineer Duncan Scott, that approach is not quite as efficient nor as flexible as the Air CHP system.
Already, there are more than 1000 small-scale (sub-megawatt) CHP installations in the UK. Thermal energy is used to produce low temperature hot water (LTHW) in most systems.
The typical CHP system achieves the following energy balance:
Fuel energy (100 per cent) = flue gas (15) + useful heat (50) + electrical (30) + mechanical loss (5).
With Air CHP, the technology is almost identical but it achieves a better balance, typically:
Fuel energy (100) = flue gas (5) + useful heat (65) + electrical (30), with no mechanical loss.
The industrial Air CHP system delivers heat and electricity with a peak system efficiency of 99 per cent. The maximum achievable air temperatures are 100 C, or120 C if combustion waste products are added to the air stream.
The packaged energy centre approach of Air CHP results in the displacement of a number of plant items, reducing overall capital costs. Aircogen also claims a 50 per cent reduction in CO2 emissions.
Hitherto, applications of Air CHP have been in superstores, supermarkets, warehouses, leisure complexes, airports and aircraft hangars. Aircogen is presently in discussions with with several chemical majors to provide plants with diverse energy sources and drying capabilities. 'Air CHP would be suitable for drying chemicals, bulk powders, fabrics and board products,' says Scott. 'Also for process preheating in burners, reactors and vessels, and paint drying processes.'
In one well established installation (a 440kW system at Aircogen's next door neighbour, Peterborough's Royal Mail sorting office) the financial saving is estimated to be at least £118 000 per year.
CHP engines are generally built specifically for that purpose. Several companies manufacture appropriate natural gas engines including MAN Rollo, Caterpillar and Perkins. Aircogen buys them to order. It is standard practice for the engines to run on gas.
Aside from the Government's enthusiasm in promoting CHP as a more efficient approach to managing energy, one key issue affecting CHP is the price relationship between gas and electricity.
Aircogen contends that at present the typical pricing relationship between the two energy sources is approximately 0.8p/kWh for gas and 3.5p/kWh for electricity a ratio of about 1:4.
'With small scale CHP systems, we can generate electricity at an efficiency of 33 per cent. On this basis the electricity is generated at a cost of three times that of gas , that is 2.4p/kWh typically, even after maintenance costs are considered. The electricity is cheaper than that from the grid and all the heat is free.' But Scott concedes : 'We are at the mercy of the price difference between gas and electricity.'
The Air CHP system's special circulating oil system extends maintenance period to 2000h; it also reduces the need to shut down the system for maintenance.The nature of the package, with its integral power generation, air handling and heat exchange systems, facilitates simple application.
