The Wärtsilä ‘Town Concept’ is a modular combined heat and power plant for decentralized energy production – designed to blend smoothly into the surrounding urban architecture. The concept is configured as functional modules surrounding the engine on two floors, resulting in a small footprint. Anders Ahnger reports.
Locating a power plant in a city or urban area can present challenges; not just in terms of emissions such as sulphur and nitrous oxides, or even noise, but also in terms of a design that blends in with the surrounding architecture while occupying a minimum footprint.
Wärtsilä’s ‘Town Concept’ is a plant that can be located very close to the point of consumption with the capability to provide hot water and district heating as well as electricity. With the Town Concept, the plant has not been modularized; the main focus is on the plant layout and building construction – offering customers the possibility of building a compact plant.
The Town Concept is the result of discussions with various customers and market research, which show that such a plant would be a viable solution in many neighbourhoods. In recent years, there have been discussions in terms of the appearance of the plant and whether it fits architecturally into the environment, especially in areas where it is surrounded by stylishly designed buildings.
An architectural study conducted a few years ago gave an idea of what a diesel or gas engine power plant could look like. It yielded a design with curved walls as opposed to the traditional ‘box’ design building. Certainly, plant design has become increasingly important to plant owners. When Wärtsilä delivered power plants to Denmark a number of years ago, every customer wanted a different architectural design. In many instances, customers like to handle the design of the building, although all of the interior equipment is often configured in the same way with the same dimensions.
The Town Concept enables them to utilize the same layout and dimensions, regardless of the outer building design.
UPSTAIRS AND DOWNSTAIRS
The Town Concept plant is configured over two floors, with radiators located on the roof for engine cooling. This is a departure from previous configurations, meeting the expectation that there would be a limited amount of land in an urban location. A heat recovery module for the district heating is located in front of the engine. Exhaust gases from the engine travel up to the exhaust gas boiler on the second floor.
The heat recovery module in front of the engine was a major determining factor in the overall plant layout. When district heating plants are normally delivered, loose equipment such as valves, pipes, heat exchangers and pre-heaters, arrive at the site to be arranged by the customer. The type of equipment delivered is also predominantly the same in each case. Wärtsilä therefore decided to include in the layout a module that has the heat exchangers and necessary piping, etc. to take care of the heat recovery and engine cooling. Another focus, also included in the module, was the control system – ensuring the plant operates correctly so that the customer receives the heat from the engine when it is needed.
Due to the resulting reduction in site work, these modules have proven to be popular in other plants delivered by Wärtsilä. On receipt of the module, the customer simply has to bolt it on to the front of the engine and connect the pipes. This is beneficial in this type of plant where the customer often takes care of the civil construction.
HIGH POWER, SMALL FOOTPRINT
Plants using the Town Concept typically have two to four engines, although a larger number of engines could theoretically be delivered in a single row.
The plant is based on the Wärtsilä 20V34SG (spark ignited gas) engines. This engine was chosen due to its popularity in the market, although the same plant layout could be achieved with the 16-cylinder version. The engine has an electrical output of 9 MWe plus a thermal output of about 8 MWth. Depending on the temperature of the district heating network, the plant can achieve an overall energy efficiency of about 85%.
Designed to be located in urban areas, Town Concept CHP plants can easily be expanded as the demand for power and heat grows
The Wärtsilä 20V34SG gas engines feature cylinder-wise admission of gas, pre-chamber with controlled gas flow, as well as individual cylinder control of gas charge and ignition timing. This concept, along with extensive research in combustion and combustion control has made it possible to increase the engine electrical efficiency from 44% to more than 47% in the bigger gas engine models.
The engine uses lean-burn combustion, where natural gas and air are pre-mixed before introduction into the cylinders. The lean-burn low fuel/air ratio reduces NOx emissions efficiently due to a lower combustion temperature. The lean-combustion concept is analogous to the dry low NOx concept used in modern gas turbines. The engine has low enough NOx emissions to comply with most national regulations, without other measures for NOx emission control.
A Town Concept plant with two engines will have a footprint of just 21×18 metres. The addition of a third engine adds just over 6 metres to the length of the installation, giving a footprint of 21×24 metres. This is significantly smaller than a standard single-level design where the engine house typically is 21 metres in length but the outdoor area for the exhaust gas boilers and chimneys requires an additional 25 metres (an overall footprint of about 47×30 metres).
To save space and reduce the footprint, the Town Concept has two floors
There is adequate ventilation of the building in the Town Concept plant so that there is no reduction in plant efficiency. In any event, gas engines are not very sensitive to ambient temperatures; derating only occurs when temperatures reach 35°C (95°F). Since district heating plants are usually in Europe, this is rarely an issue.
The plant offers the customer a high degree of flexibility. Although it is optimized for heat generation, it is configured so that it can be run without the heat recovery system. Therefore, if the customer has no heat load, they can produce power only. The heat recovery system also has no effect on the electrical output of the engine or its fuel consumption.
While the gas engine meets most environmental requirements (such as German’s TA Luft regulations on air pollution) it is possible to include a catalyst in the plant layout on the second floor. While the catalyst would have a small effect on the exhaust gas boiler, it would have no effect on the performance of the engine. In fact, the second floor is left quite open to provide space to accommodate equipment such as different types of catalyzers or boilers. This is important since the boiler design will vary depending on the sub-supplier or needs of the customer. For example, there is sufficient room if the customer has a requirement to provide steam.
The Town Concept is being aimed at markets such as Europe, Eastern Europe and Russia. This type of equipment, including the heat recovery module, has been sold in other parts of the world as well. However, in these cases, there has been no discussion on building design or recommendations on dimensions. In Asia and the Middle East, there are requirements for chilling to provide, for example, district cooling. This is common in many countries where cooling is needed in the summer and heating in the winter. This is possible, but results in a more complex plant and would require a new layout to accommodate the additional necessary equipment.
The trigeneration plant delivered to the Linate airport, Milan, Italy, is in many ways a Town Concept. The contractor handled the building design but utilized Wärtsilä’s plant layout and dimensions. The trigeneration power plant is equipped with three Wärtsilä 20V34SG gas-fuelled generating sets, together with their ancillary equipment, exhaust heat recovery economizers and two gas-fired boilers. The plant is located inside Linate airport.
The plant operates on baseload, but, by providing both heating and air conditioning, it is designed to flexibly and economically meet the variations in heat demand in summer and winter. The heat output of the plant is 82 MWth in winter and 72 MWth in summer, with a year-round electrical power output of 24 MWe.
The heat is delivered as hot water at 125°C (257°F) and at 70°C (158°F) to the airport buildings and a small village close to the airport. The plant also delivers electricity to the Italian national grid. Normally, the generating sets run in parallel with the grid but they also serve as emergency sets to maintain airport services in the event of a break in the grid supply.
The three gas engines at the airport are located on the ground floor with a module in front of the engine; the exhaust gas boilers are located on the next floor. Some additional equipment, such as chillers, were installed in an annexe, but little else was changed.
Nevertheless, the widespread introduction of the Town Concept will not be without challenges. In terms of markets, district heating is not used everywhere and Wärtsilä has to be flexible to customers’ needs and whether there is a need for heat recovery.
While the Town Concept is easy to build, the architecturally designed buildings have their own challenges. Noise issues have to be addressed when plants are located in the middle of cities. The ventilation and suction air inlet systems for the engines in such a plant would result in a noise level of about 50 dB at 100 metres. One building design option has a glass wall. Where there are tight restrictions on noise, such an option is not possible since it would increase noise levels to about 60 dB. This means the glass wall would have to replaced with solid panels. Stylish building designs also come at a cost – bolts are expensive to make and curved walls are more expensive to build than straight walls.
Installing equipment on the second floor of the building can also be a challenge in building the plant, and the weight of equipment also has to be considered when building the floors and structure. For example, a boiler is a large piece of equipment which can typically weigh 35 tonnes when filled with water. The building structure has take into account the size and weight of the equipment as it is completed before the equipment arrives.
Equipment to be installed on the second floor usually arrives quite late to the site, since there is limited space for storing large pieces of equipment. This means that when it arrives, it has to be slid into position (since the roof is already in place). Good construction logistics are therefore important, and equipment delivery has to be carefully scheduled. Construction time of a Town Concept plant is typically 12–18 months, depending on delivery time of the major equipment. Ultimately, the Town Concept is an option for power plant operators needing a plant that can meet their energy needs at the point of consumption. It is a plant that can deliver both style and practicality.
Anders Ahnger is with Wärtsilä Corporation, Helsinki, Finland.