Combined cycle power plants are particularly effective in cogeneration and district heating applications. Cogeneration or Combined Heat and Power (CHP) is an extremely efficient way to deliver the maximum benefits of heat, power and cooling to industry and commerce, as well as the public sector and private homes.

CHP, the simultaneous production of heat and power, is an important path for economies with a high growth rate to reduce emissions and at the same time meet the energy needs for the foreseeable future. It is also the favourable solution in terms of economical and environmental efficiency.

A CHP plant generates usable heat and power in a single process. It consists basically of one or more prime movers (usually gas and/or steam turbine) frequently driving an electrical generator, where the heat generated in the process is utilised via suitable heat recovery equipment for such purposes as industrial processes, community heating and space heating. Indeed, the cost-effective adaptation to district heating and industrial heat customers is the primary design focus of such plants.

In terms of total efficiency (the sum of electricity and useful heat divided by fuel input) all cogeneration technologies can reach very high per centages of up to 90 per cent and more, provided the boundary conditions like heat level, return condensate temperature and heat load curve are appropriate for the respective technology.

Generally it is boundary conditions such as fuel availability (eg coal, natural gas, waste gases, oil), fuel prices, power and heat demand curve which define the power generation technology used.

If natural gas is available for power generation, the most efficient CHP solution in terms of environmental efficiency is a combined cycle power plant, which is very flexible in terms of the ratio of power and heat output. The heat to power ratio, which describes how much electrical power a plant produces in relation to the heat output, is one of the main differentiators for environmental efficiency of a plant.

The benefit of CHP plants can be seen if the CO2 emission of a CHP plant is compared to separate generation of electricity and heat via best available technology. If natural gas is available, the best available technology for power generation is a large-scale combined cycle power plant with a net efficiency of 57.5 per cent. The best available technology for heat generation is a gas-fired boiler with 90 per cent efficiency. In order to produce the same amount of electricity and heat, a typical modern combined cycle CHP plant (50 per cent electrical efficiency and 85 per cent total efficiency) uses 20 per cent less fuel and consequently emits 20 per cent less CO2 than separate power and heat generation plants.

Siemens offers turnkey solutions for combined cycle power plants in all power ranges. If used as CHP plant, they can generate heat levels from 50 up to 500¢ªC, with a high flexibility of power and heat generation. The gas turbine exhaust is packed with thermal energy, which is recovered in a heat exchanger generating either high quality steam or hot water. These sources of heat are then used to meet local domestic and industrial needs or to generate additional electricity through a steam turbine.

The smaller combined cycle power plants are very often designed as CHP plants to provide heat for district heating or industrial purposes. The main customer groups here are the oil and gas industry, and the chemical, petrochemical and process industries, as well as the power generation industry. Other smaller or specialised industries, such as food and beverage, pulp and paper, ceramics, textiles, building complexes, automotive, are also discovering the advantages of a grid-independent captive CHP plant to furnish all their power and process needs.

This was recently done at the whisky distillery of William Grant & Sons, in Girvan, Scotland, representing one of their largest and most important investments in new technology in recent years. External supply had begun to form a major and rapidly-increasing percentage of the company’s total production costs. The integration into the facility of a SGT-100 gas-turbine driven CHP solution from Siemens now gives Grant complete control over their process and their power costs. In the four years since the CHP plant was installed the initial capital investment has been completely recovered through savings in fuel and operating costs.

No less than a quarter of the generating capacity owned and operated by Essent Energie, the Dutch power utility, is centred on CHP installations. One of these is a recent Siemens project in Antwerp, Belgium, based on a SCC-800 combined cycle power plant. Essent opted for a Siemens CHP (a ‘combicogen’, a combined cycle plant used in cogeneration application) for both environmental and business reasons, as it is company policy to utilise high efficiency energy technologies.

The site hosts eight third-party chemical manufacturers, around half of which are supplied with steam by INEOS. The plant will supply these industrial customers with power, steam of different pressure levels and hot water; plant operation will be optimised according to the power prices on the local power exchange market.

It is a prerequisite for CHP plants that there are users for the waste heat, as heat cannot be stored or transported over distances, which makes this type of concentrated facility ideal for the technology.

Lynne Anderson is with Siemens Industrial Turbomachinery AB, Finspong, Sweden. www.siemens.com/powergeneration