The high costs of fuel and the need to cut carbon dioxide emissions are leading utilities worldwide to seek new ways to extract ever more value out of each hydrocarbon molecule they burn. Lighter components, tighter seals, water injection systems and chillers improve turbine efficiency and output. Heat recovery steam generators have become standard equipment, converting even more of the calories into kilowatts.
Vuosaari A and B natural gas cogeneration plant credit: helsingin energia
The best option, though, is to site the generators near cities and industrial facilities which have a need for otherwise wasted heat. While no-one would want to live downwind of the coal-fired plants of 50 years ago, that is not an issue with the current combustion turbines and clean coal plants. The US Environmental Protection Agency’s Combined Heat and Power Partnership is targeting four markets à‚— dry mill ethanol production plants, hotels and casinos, municipal wastewater treatment facilities, and utilities à‚— for increased use of CHP.
In northern Europe, on the other hand, higher population density and lower average temperatures mean that district heating is a key opportunity. The European Union Directive on cogeneration (2004/8/EC) estimates that development of cogeneration could reduce carbon dioxide emissions by 258 million tonnes in 2020.
Finland, however, did not wait for orders from Brussels before embarking on its district heating implementations. Plans for CHP in Helsinki started before the First World War, though they were not brought to fruition until after the Second. Now, according to the International Energy Agency’s CHP/DHC Collaborative, CHP produces 74% of Finland’s district heating needs.
Figure 1. Schematic of the Vuosaari plant
‘This combined power and heat production is the cornerstone of our operations, providing efficiency and environmental benefits as well as economic ones,’ says Taneli Lampinen, an engineer for Helsingen Energia. ‘Our overall annual efficiency, depending on the weather and the price of electricity, averages 80%, but in 2007 it was close to 90%.’
This efficiency led the European Parliament in November 2008 to award Helsingin Energia with a European Regional Champion Award in the Energy Champion category, stating that ‘Helsingin Energia possesses unique capabilities and know-how, which make it the world leader in cogeneration and energy efficiency.’
Finland in hot water
Helsingen Energia is the utility provider for the city of Helsinki and the surrounding area. Summer afternoons there only reach an average of 21à‚°C and for half the year daytime temperatures are less than 10à‚°C. Heating the buildings, therefore, is close to a year-round activity. This makes the city, like the rest of the country, an ideal location for CHP. In 2007, CHP provided 29.7% (26.8 TWh) of the electricity demand, more than half of that (14.4 TWh) from district heating networks.
This approach also helps the country reduce its dependency on imported fuel. Thirty years ago Finland derived 90% of its heat in district heating and CHP plants from coal and oil. Today those two account for less than 40% of the fuel à‚— having largely been replaced by peat, wood and natural gas. A total of 57% of the country’s energy is now imported, including more than 20% of its electricity.
Overall, one-fifth of the country’s power goes in to heating buildings. In the cities, about 80% of the buildings are connected to district heating. In Helsinki, that figure is approaching saturation point, with 93% connected to the hot water network.
Heat pumps located underneath Katri Valan park, used for district heating and cooling credit: juhani eskelinen, helsingin energia
This commitment to cogeneration has a long history. The first local power plant was built in 1884 and by the start of the next century there were more than two dozen power producing companies providing electricity to light the city. In 1907, the city built its own steam plant and two years later all the independent producers were merged into the city utility. In 1913, the utility’s managing director, Bernhard Wuolle, proposed using CHP to provide district heating. The proposal sat on the shelf for more than three decades until it was revived by another managing director, Unto Rytkàƒ¶nen.
The first customers started receiving steam heat in January 1957. That same year, with the opening of the coal-fired Salmisaari A plant, the city switched to providing heated water. Several other coal plants were added over the years. The Hanassari B power plant was completed in 1974 and provides 220 MW of electricity and 445 MW of district heat. Though the city added desulphurization, new burners and electrostatic precipitators to its coal-fired plants (bringing emissions below the legal requirements), it still wanted its emissions lower. It built its first natural gas-fired plant in the suburb of Vuosaari. This facility began operations in 1991 and the majority of the city’s power now comes from natural gas imported from Russia.
The Vuosaari plant
The Vuosaari plant contains two combined cycle units. Vuosaari A has two Siemens KWU V64.3 gas turbines. Those units are ISO rated at 53 MW each, but in Helsinki’s winters, that amount rises to 63 MW each. The 500à‚ºC flue gas boils water to drive a 40 MW single cylinder backpressure extraction turbine from MAN-GHH coupled to an ABB generator, boosting the total electrical output to 165 MW. In winter, the units also provide up to 162 MW of district heating. The system has a maximum design efficiency of 91% when used for CHP.
The second unit à‚— Vuosaari B à‚— went on-line in 1997, using the newer and much larger Siemens KWU V94.3 gas turbines, which are rated at 163 MW at an ambient temperature of 0à‚ºC, and higher during winter. The exhaust goes to a Foster Wheeler Energia Oy boiler, which drives an ABB Stal steam (145 MW backpressure turbine only; 172 MW when backpressure and condensing turbine both operating) connected to a 200MVA GEC Alstom generator. In winter, the plant can operate at up to 92% efficiency when generating 470 MW of electricity and 400 MW of district heat.
Hansaari B coal fired plant credit: helsingin energia
To give the plant operators greater flexibility, ABB-Stal Utility Division ordered a Size 260FT Spacer Clutch from SSS Gears Ltd for Vuosaari B. When the demand for electricity is low at night, the steam can be redirected from the LP turbine and used to heat water which is stored in a hot water accumulator (HWA) for use when needed. When demand rises, steam is redirected to the LP turbine and the 48 MW LP turbine is brought back on-line.
While both the Vuosaari units are designed for greater than 90% efficiency, operating the plant requires balancing the fuel prices, electrical demand, weather, and the prices of carbon credits. The electrical market was deregulated in June 1995 under the Energy Market Act, and sometimes it is cheaper to purchase power than generate it locally. At other times it is better to generate extra electricity to sell to the grid.
The Kyoto Protocol has also had an impact. The Vuosaari plant operates on natural gas and so has lower carbon dioxide emissions than Helsingin Energia’s coal-fired plants. The utility now generates 48% of its electricity with gas, compared to 36% for coal. As a result, the average amount of carbon dioxide emissions has dropped from 400 g/kWh, in the Kyoto reference year to 1990, to 290 g/kWh currently. Nevertheless, due to rising power demand, the utility still emits 20% more carbon dioxide (4.1 Mt) than it did in 1990 (3.4 Mt). The utility does, therefore, have to factor in the carbon costs in making its generation decisions.
Finally, temperature plays a role à‚— since the Vuosaari plants achieve their efficiency through CHP, and heat is not needed in the summer months, it may be more efficient to shut the plant down and just buy the electricity from other sources.
Heating and cooling
The district heating network has grown to cover the entire city, and continues to expand. There are already more than 1200 km of district heating pipes buried in the area, with another 27 km being added annually. The company has its own business unit, HelenTunnels, to construct and maintain the tunnels that contain the district heating network, as well as the 110 kV and medium-voltage cables, fibre-optic cables and the water mains. This includes the 30 km tunnel connecting Vuosaari plant to the city centre à‚— the longest district heating tunnel in Europe. The heating network is built in a loop so there are alternate routes to the customers in the event there is an interruption along one branch. The network is monitored 24 hours a day, and as a result, the average customer experienced a heating outage of just three hours in 2007.
As part of its efficiency plan, the Vuosaari plant creates hot water overnight and stores it in a 25,000 m3 storage tank at close to 100à‚ºC. At capacity, this represents about 1400 MW of energy. When a high volume of heat is needed à‚— i.e. when people wake up in the morning and return home in the evening à‚— there is an adequate supply of hot water available. At night when demand is low, the power plants can recharge the heating network. This arrangement works best in the spring and autumn. Summer months do not require heating and in the winter the heat is needed even at night.
Salmisaari coal-fired plant credit: juhani eskelinen: helsingin energia
To further expand its services to customers, and to reduce operating costs and emissions, Helsingen has added district cooling to its offerings; 80% of the district cooling comes from free or otherwise wasted energy sources, including cold seawater and absorption cooling units using the steam from the power plants. Cooling also comes from the world’s largest heat pump, located in a rock cave 25 metres below a park. It can produce 90 MW of district heat and 60 MW of cooling, transferring the heat to/from treated waste water or seawater as needed. The Salmisaari power plant has a 10 MW absorption chiller which opened in 2001.
By the end of 2007, Helsingen Energia was providing cooling to 75 customers connected through 26 km of piping. Those customers had a total cooling load of 53.1 MW, and used 37 GWh hours of cooling that year. About a third of that cooling was used in the six coolest months of the year.
As it stands, there is not much more that Helsinki can do to expand its use of district heating. With more than 90% of buildings using district heating, and fuel efficiencies running in the 80à‚—90% range, any improvements will have to be small. It can, however, serve as a model for other utilities on how to establish a comprehensive CHP system.
Drew Robb writes on energy matters for COSPP magazine.e-mail: email@example.com
The European Union is engaged in several initiatives to increase the use of CHP. As outlined in its 2000 Green Paper, Towards a European Strategy for the Security of Energy Supply: ‘The stability of consumption between 1985 and 1998 was due to the introduction of combined heat and power generation and greater technological efficiency…’
Household and industrial heating were identified in the report as the largest final users of energy, consuming about one-third of the total. After looking at three different scenarios for CHP à‚— coal-fuelled boiler, oil-fuelled boiler and gas turbine combined cycle (GTCC) à‚— the report concluded that CHP had an ‘often overwhelming advantage’ over steam-only boiler systems in all countries, ‘due to the very high overall efficiencies that characterize CHP systems and their very competitive costs.’
This was further backed up by the European Parliament and Council of Europe Directive 2004/8/EC (the CHP Directive) which stated that: ‘Promotion of high-efficiency cogeneration based on a useful heat demand is a community priority given the potential benefits of cogeneration with regard to saving primary energy, avoiding network losses and reducing emissions à‚— in particular of greenhouse gases.’ The EU estimated that, since CHP installations operate at around 90% efficiency, increased use of cogeneration could avoid the emission of 127 million tonnes of CO2 in 2010 and 258 million tonnes in 2020. The CHP Directive calls for doubling the share of electricity produced by CHP to 18% by 2010.
The October 2006 Commission Communication Action Plan for Energy Efficiency: Realizing the Potential, listed proposed measures to further increase the use of CHP and included minimum performance requirements for district heating and micro-CHP, and stricter requirements for market regulators to promote CHP.
On 20 January 2008, the European Commission issued the EU climate and energy package (’20/20/20 by 2020′) which calls for reducing greenhouse gases by at least 20% below 1990 levels, increasing renewables to 20% of total energy consumption, and cutting projected energy consumption by 20% à‚— all by 2020.