Erkko Fontell, Wärtsilä, Finland
This autumn, a new cogeneration project is scheduled to begin operation in western Finland. The installation, powered by a solid oxide fuel cell (SOFC), will not only supply electricity and district heating to a new energy efficient housing community, but will also dispose of landfill gas from a nearby disused waste dump.
The roots of the project date back to around 2000, when an old landfill site located close to the city of Vaasa was closed. According to Finnish environmental laws, when a waste dump is closed there is a requirement to collect the gas emitted from the site.
When it was learnt that this year’s Finnish Housing Fair would take place in Vaasa, just 3 km away from the waste site, Vaasa City saw an opportunity to develop an energy project as part of the fair.
Green energy scheme
The Vaasa Housing Fair site is located in Suvilahti, approximately 3 km east of the city centre. The fair took place between 11 July and 10 August, and about 500 residents, living in the 48 showcased single-family houses and three blocks of flats, now inhabit the 15 hectare housing area. The area is pioneering the implementation of energy production processes for a restricted area, with both electricity and heating being produced and consumed entirely within the housing area itself.
The Housing Fair is Finland’s largest summer event, with approximately 150 000 visitors
Wärtsilä came up with the unique solution of supplying a fuel cell unit that uses the landfill gas to produce electricity and heat for the area with close to zero emissions. As the project took shape, more participants came on board. With an existing wastewater pumping station in the middle of the fair area, Vaasa Water decided to come onboard, as did others such as Vaasa Electricity, who needed somewhere to locate a distribution transformer.
Vaasa District Heating, which is part of Vaasa Electricity, also participated as it had a need for hot water to supply the district heating system. In addition to the fuel cell plant delivery, Wärtsilä was responsible for construction development and project-time financing for the power plant facility. Other partners in the project include Sarlin Oy, Mateve Oy, Suomen Làƒ¤mpàƒ¶pumpputekniikka Oy, Sonera, as well as the City of Vaasa, Vaasa District Heating, Vaasa Electricity and Vaasa Water.
The fuel cell at the Vaasa Housing Fair is now part of a wider energy scheme. As well as being a test bed for further development of fuel cell technology, the entire energy project will be used to demonstrate other green energy production processes for a limited area.
In addition to the heat and power produced by the fuel cell power and heat are also produced by two micro-turbines. These micro-turbines were used instead of a boiler that Sarlin had planned to install at the waste dumpsite.
In another of the site’s innovative developments, low-temperature heat is collected from piping installed in the seabed. This heat is then distributed using a carrying liquid, to all the houses on the estate, each of which has its own individual heat pump.
The fuel cell will deliver electricity to the heat pumps, with any surplus electricity being fed to the city network. It will also produce hot water for the district heating system.
Heart of the matter
At the heart of the cogeneration project is a fuel cell based on planar SOFC technology. The plant will produce approximately 20 kW of electricity and between 14-17 kW of heat for the fair site’s needs. The main benefit of employing a fuel cell, compared to an engine or another prime mover, is that there are virtually no emissions because there is no direct combustion. In addition, it has a high electrical efficiency. The electrical (AC) efficiency at this power size is about 40-45 per cent, while direct current (DC) efficiency is around 50 per cent.
Fuel cells are electrochemical energy conversion devices that can combine hydrogen, carbon monoxide (CO), and oxygen to produce electricity, with water and heat as by-products. Unlike batteries, which store electrical energy chemically within a closed system, fuel cells require an externally supplied fuel that has to be replenished.
The Wärtsilä SOFC is clean, quiet and vibration-free
The fuel cell comprises an anode, electrolyte and a cathode. Oxygen is supplied to the cathode side. SOFCs differ from other fuel cell technologies in several ways. Firstly, they are composed of all-solid-state materials à‚— with the anode, cathode and electrolyte all being made from ceramic substances.
The use of ceramics means that the cells can typically operate at temperatures in the region of 750 à‚ºC, which is significantly hotter than any other type of fuel cell. The high temperature operation of a SOFC, together with its ability to operate at high pressures, makes it well suited to CHP applications. Also, these high temperature fuel cells are able to convert CO and hydrogen to electricity.
At the Vaasa fair site, landfill gas is first cleaned before entering the fuel cell power unit, where it is then humidified and heated before being fed to a reformer. The reformed gas is then heated further before being fed to the anode side as a mixture of steam, methane, hydrogen and CO. At the high operating temperatures, oxygen ions are formed from the air around the ceramic plates at the air electrode (the cathode).
When the fuel gas containing hydrogen is passed over the outside of the plates in contact with the ‘fuel electrode’ (the anode), the oxygen ions migrate through the electrolyte to oxidize the hydrogen and CO. There is also a nickel catalyst at the anode to promote the reaction. Electrons generated at the anode move out through an external circuit, creating power. Any unused gas at the anode side is burned in an after-burner. A district heating heat exchanger is located behind the after-burner for recovering the heat from the fuel cell air-venting pipe. The only by-products are water and CO2.
The Wärtsilä fuel cell at the fair consists of 24 stacks, each generating 1 kW DC of current at close to 60 V DC. Power electronics convert the low DC-voltage up to 440 V AC for distribution.
In addition to their high efficiency, fuel cells have the benefit of no moving parts, which means they are very quiet and vibration-free. The only audible noise comes from the air supply and from the electronics cooling equipment. Furthermore, because there is no combustion, there is almost no formation of nitrogen oxides.
This fuel cell power plant at the housing fair site is the first field application of this technology, and the use of landfill gas will present challenges since its composition depends both on weather conditions, and the composition of the landfill itself. This means that its quality constantly fluctuates. It also has impurities that have to be cleaned to prevent damage to the fuel cell.
The biogas pumping station at the completed Suvilahti landfill site collects methane from 16 gas wells
The methane content in natural gas is usually more than 90 per cent, while in landfill gas it is around 35-60 per cent only. While the technology is suitable for running on this lean gas, there are limitations in how lean the gas can be.
Controlling the system will also be a challenge, particularly in obtaining the right input quality of the gas.
The new energy plant is a validation centre where long time operation of the technology will be carried out, and during the plant’s operation some key aspects will be observed. The design itself will be studied to assess its suitability for biogas operation, as will the performance of the gas cleaning equipment. Controlling the fluctuating gas composition will also be important.
However, while operating on landfill gas is more difficult than with hydrogen, it has the advantage of being widely available à‚— at landfill sites, wastewater treatment plants, farms, etc à‚— and is essentially a free fuel. By burning methane, which is a more potent greenhouse gas than CO2, it is also a greenhouse gas reduction technology.
Technology for the future
The Vaasa fair site plant is an important turnkey project for Wärtsilä, and will be operated by the company’s power plants division, enabling the operators to gain experience in operating and developing this promising technology.
While reciprocating engines, turbines, and other prime movers are the technology at the heart of today’s cogeneration plants, fuel cells represent the technology of tomorrow. Wärtsilä’s fuel cell development programme aims to extend its portfolio of sustainable power generation technologies.
Although currently high, the cost of fuel cell-based power plants will reduce as they become more widespread. It is envisioned that their applications will include hotels, supermarkets, service stations and data centres.
As more units are manufactured, costs will fall and fuel cell technology is expected to achieve commercial feasibility levels sometime between 2010-2015.