Internationally, the costs of providing electrical power, heating and air conditioning for large public buildings and industrial complexes are becoming progressively more expensive as public utilities increase their charges in line with higher fuel and network costs.

In addition, many traditional methods of heating and cooling can have significant environmental impacts due to inefficient outdated technologies. Air-conditioning, in particular, is growing in prevalence.

Fortunately, the trend in larger industrial and commercial facilities is towards more efficient and technically advanced combined climate control systems for heating and cooling rather than many smaller electrically driven units.

Cogeneration (combined heat and power) systems which provide electrical power as well as heating through a single on-site energy source, offer significant operational and efficiency advantages. Furthermore the thermal energy produced by such units can be utilised for chilling as well as heating, replacing electrically driven chilling compressors with an absorption chiller. This is known as ‘trigeneration’ – the simultaneous conversion of a fuel into three useful energy products.

Industrial facilities and public buildings such as shopping centres, hospitals and airports, have a suitable mix of heating and cooling needs to benefit from trigeneration power plants. In Issue 39 of EcoGeneration we profiled the 160 kW GridX district trigeneration system at a housing estate in Glenfield, Western Sydney, the first system of its kind in Australia. Trigeneration, however, is more common overseas, where such buildings and complexes are more often of a sufficient scale to make these technologies cost-competitive with the conventional alternatives as well as a lower emissions-intensive option.

Since it is more difficult and costly to distribute hot or cold water than electricity, trigeneration plant must also be located in close proximity to the heat and cold water consumers. Close proximity also minimises electricity distribution losses. An investment in trigeneration starts to pay off when there are seasonal periods where the power plant’s waste heat can be fully utilised. The flexibility to provide heating during winter and cooling during summer, or for heating one area and cooling another, allows maximisation of the running time of the plant at high efficiency - which is both economically and environmentally beneficial. If all the heat recovered can be used effectively for heating only (cogeneration) during all the plant running hours then there is no advantage to making an extra investment in a chiller.

The Barajas Airport in Madrid is an example of a typical trigeneration project. It has recently undergone a major extension, including the addition of two terminals and new runways. It now has a floor area up to 760,000m2.

In 2003, Spanish Engineering Company, Sampol Ingenieria y Obras S.A. in partner with Wartsila won the contract with their trigeneration plant proposal to supply thermal and electrical energy over a 20 year power purchase agreement.

The requirement was for a cost-efficient and environmentally friendly plant that would be able to deliver an extremely high level of reliability. Power must be available at all times of the year, independent of external factors such as weather or gas supply. The plant, which was commissioned in January 2005, will provide a continuous source of electricity as well as heating during winter and cooling during summer.

Technology and heat recovery

Under the Engineering, Procurement and Construction (EPC) contract for the facility, Wartsila was responsible for the engineering, equipment, installation, commissioning and start-up of six Wartsila 18V32DF dual fuel engines and their auxiliaries, the heat-recovery boilers at the engine’s exhaust, the engine hall ventilation systems and also the control system.

Reciprocating engines have good electrical efficiencies which are maintained when operating at part loads, excellent load-following characteristics and generally have high reliabilities and fast start-up. Modern lean burn reciprocating engines also have better efficiency than simple cycle gas turbines at high ambient temperatures. Reciprocating engines also do not suffer from reduced energy efficiency or loss in rated output as running hours accumulate.

The 18V32DF is a high-efficiency, turbo-charged, four stroke engine with an output of 5.5MWe. The engines burn natural gas as the primary fuel with light fuel oil (LFO) as back-up and security of power supply is maximised by the multiunit installation which instantly switches over to LFO if the gas feed to the airport is interrupted.

Superheated water

Both exhaust gas and engine cooling can be fully used in superheated water production. Each of the six engines has a heat exchanger for the cylinder-liner-water, a heat exchanger for the second turbo-compressor stage and a heat recovery boiler for the flue gas heat exchanger. One conventional boiler is common to all six heat-recovery boilers and supplements the superheated water supply during periods of high demand. The conventional boiler brings the total thermal power up to 30.9 MWth. The flue gas collected at the outlet of the turbocharger can be sent to the heat recovery boiler or released outside the plant depending upon the required heat recovery.

Chilled water

Water is chilled by six single-stage absorption coolers using lithium bromide salt as absorbent. The superheated water released by the recovery boilers is the main energy source for the water chilling process. There are also six compressor chillers utilising electricity which supplement the capacity of the absorption chiller, bringing the total chilling to 37.4 MWch.

The plant’s control system is based on Siemens programmable automatic devices, which integrate the main engine control system developed by Wartsila and other plant control systems developed by Sampol. This controls and coordinates that heat-recovery and hot and cold water generating systems.

Energy supply and environmental impact

The plant provides electricity on a continuous basis with a net output of 33MWe as well as heating during the winter and cooling during the summer. The trigeneration plant is connected to both the airport’s internal grid and the public grid. The plant is connected to the grid at two locations very distant from each other. A single-mode optic fibre communication line sends critical speed and voltage adjustment signals to enable resynchronising the generating sets with the grid.

By using embedded generation solutions such as cogeneration and trigeneration, transmission and distribution losses are kept at a minimum. The plant has a low emissions-intensity of 250 kg CO2-equivalent per MWh of used energy.