By: Anders Ahnger, Wärtsilä, Finland
The growing expectation of people in developed countries of a certain level of comfort in their lives is increasing the popularity of trigeneration. Most people enjoy comfortable living and working environments. The chances are that if people can afford air conditioning in the summer and heating in the winter, they will have it. This is the main driver behind trigeneration, which is gaining popularity, especially in places such as airports, offices and industries that have a joint need for power, heat and cooling.
There is a trend to build more efficient climate control systems for building complexes instead of having separate, smaller electrically driven units for heating and cooling. Office complexes and many industrial and public buildings would benefit from trigeneration. The electronics industry, shopping malls and airports are typical examples. A combined heat and power (CHP) plant can provide power, heating and cooling from one source to provide big operational and efficiency advantages over separate systems. Trigeneration produces power and multi-thermal energy. It adds an absorption chiller to a CHP system to supply cooling without using electrically driven chilling compressors. Its big advantage is that it can run all year round, useful in countries that shut down district heating plants in summer – the engines can be run to provide cold water, which results in a shorter payback period.
Trigeneration is cogeneration taken one step further, in other words, it is a system that provides power, heat and cooling. To maximize the total efficiency of a trigeneration plant, it must be close to the end users because it is more difficult and costly to distribute hot or chilled water than electricity. When installing air conditioning, 70 per cent of the plant’s peak cooling capacity is enough to meet most of the building’s annual cooling needs, while the remaining 30 per cent can be topped up using compressors. This way, the total investment cost for the chillers can be kept to a minimum. The chiller capacity can be further reduced by including cold water storage, which also gives more freedom to control capacities separately.
The 16V34SG and 20V34SG Wärtsilä engines in the engine hall of the Makuhari DHC plant in Japan
The running philosophy and control strategy are important and should be properly evaluated. The optimum solution is seldom based on one in which the entire chilled water capacity is produced by absorption chillers. For trigeneration, a single-effect absorption chiller connected to the hot water system is the most suitable solution. In winter, that part of the peak hot water demand that exceeds the recovered heat from the engine plant is covered by a stand-alone hot water boiler. In summer, the same boiler tops up the heat feeding the single-effect absorption chiller to produce cold water.
Absorption chillers are heat-driven cooling machines. Instead of a mechanical compressor, they use a thermal system that employs lithium bromide (LiBr) as an environmentally safe alternative to CFCs. They are used in a large number of petrochemical power and district cooling schemes today to provide custom-designed refrigeration solutions with capacities of up to 25 MW to produce chilled water at temperatures as low as 2 °C. Single-stage LiBr absorption chillers can use hot water at temperatures as low as 90 °C as the energy source. Two-stage LiBr-absorption chillers need hot water at about 170 °C. A single-stage LiBr absorption chiller that produces water at 6-8 °C has a coefficient of performance (COP) of about 0.7, and a two-stage chiller has a COP of about 1.2. This means they can produce a cooling capacity corresponding to 0.7 or 1.2 times the heat source capacity.
Trigeneration systems are typically based on reciprocating engines. Those fuelled by gas offer advantages for power and district heating and cooling (DHC) production. Their very high electrical efficiency is maintained at part loads and over long periods under severe operating conditions. The lean-burn engine is derated less than gas turbines by ambient temperature, altitude and back-pressure, nor does it suffer from reduced electrical efficiency or loss in rated output as it accumulates running hours. Lean-burn gas engines can easily handle varying load and frequent starts and stops. The gas engine is also insensitive to variations in operating conditions.
Both exhaust gas and engine cooling can be fully used in hot water production. Total efficiency can reach 93 per cent with return water of a low temperature. For higher return water temperatures when interconnected to absorption chillers, e.g. 75 °C, some of the cooling water of the LT circuit must be rejected. As the return water temperature fluctuates over the year, careful design is needed to optimize the plant for the most viable configuration.
The perfect application
Globally, Wärtsilä has been pushing trigeneration in airports, where there is a big need for cooling and heating in the terminals. Out of 120 projects per year, just under 10 per cent are trigeneration projects. Before 2003, there were just one or two projects, but when making airport installations, it was found that the solution was always the same – a typical trigeneration plant. Since then, the number of projects has grown by about 10 per cent a year.
Power and thermal energy needs are huge in new air terminal buildings, which often have a large floor area divided between ticketing, baggage handling and passenger embarkation. A trigeneration plant can supply all of the power, heating and cooling needs efficiently and securely.
In summer 2007, Wärtsilä and EuroPower SpA handed over a 24 MWe trigeneration power plant for use inside Linate airport in Milan, Italy. The plant employs three Wärtsilä 20V34SG gas fuelled gensets, plus ancillary equipment, exhaust heat recovery economizers and two gas fired boilers. It operates on baseload, but by providing both heating and air conditioning, it meets the variations in heat demand in summer and winter flexibly and economically. The heat output of the plant is 82 MW in winter and 72 MW in summer, with a year-round electrical power output of 24 MW.
Heat is delivered to the airport buildings and to a village near the airport as hot water at 125 °C and 70 °C. The plant also delivers electricity to the Italian grid. Normally, the gensets run parallel to the grid, but they also serve as emergency sets to maintain airport services if the grid supply stops.
Trigeneration solutions are making inroads into Scandinavia and southern Europe. Already there are district cooling systems in large cities such as Helsinki and Stockholm that cool buildings such as office complexes that house large numbers of computers. Elsewhere, trigeneration systems are being installed in countries such as South Korea and Japan.
Wärtsilä won a contract in May 2007 from JB Entech Co Ltd of South Korea to deliver engineering and equipment for a 25.3 MWe CHP plant for the Cheong Soo community in Cheon Ahn City in South Korea. This is Wärtsilä’s first CHP project in that country under the Community Energy System (CES) concept. The CES is based on South Korean government legislation to encourage decentralized energy production.
Wärtsilä will deliver the equipment for the Cheong Soo plant in two phases. The first, due to be operational in the second half of 2009, requires two Wärtsilä 20V34SG gas-engine generating sets with a combined output of 17 MWe. Waste heat recovery from these engines will provide 14 MWth for district heating and cooling. In the second phase, Wärtsilä will deliver a third identical generating set to raise the total electrical output to 25 MWe and 21 MWth. This will be operational around mid-2012.
Another interesting project, which began operation this year, is a plant in Makuhari, Japan, owned by Energy Advance (ENAC), an energy services company that is a subsidiary of Tokyo Gas, Japan’s largest gas company. The project was launched in 1989 as an energy facility for Makahuri new city – a futuristic business city between Tokyo and Narita International Airport. It is home to office complexes, shopping centres, hotels, schools, housing and parks. The energy facility is part of the Makuhari Business Centre.
The energy facility supplies heat to nine customers, including a convention centre. In conventional air conditioning systems, heat sources must be installed in each building. This requires room for installation and attention to operation and maintenance. But in trigeneration systems, thermal energy comes from a single heat source facility in the district and is piped to buildings. Because the heat source is high-temperature steam, it can also be used for other purposes, for example cleaning in kitchens and bathrooms.
The Makuhari Business Centre, which contains the recently extended Makuhari DHC plant
Under Makahuri is an 8 km network of large-diameter pipes for chilled water at 6.5 °C and small diameter pipes for steam at 170 °C. Five boilers at the plant burn a form of natural gas to create steam. There are two types of boilers – three furnace flue boilers and two water tube boilers. The first type creates HP steam at 4 MPa, which is sent to a steam turbo-chiller. The steam turbo-chiller unit has a freezing capacity of 35.2 MW. The furnace flue boilers generate LP steam at 0.8 MPa as a heat source for the steam absorption chillers to produce chilled water. Some of the steam (at 170 °C) from the furnace boilers goes directly to the customer for heating, etc.
The chillers are in a pyramid-shaped part of the building on the second floor. Cooling towers here that are 20 metres tall allow heat to radiate from the chillers. Towers like these usually need a lot of space to contain them, but the multi-layered pyramid shape saves about 30 per cent of space.
Since the installation, fuel consumption in the business district has been cut by 25 per cent, so there has also been a huge cut in carbon dioxide emissions.
ENAC is now also using the trigeneration system as a way to contribute to the achievement of a microgrid that will link power sources in the district. As the result of an extension to the Makuhari DHC, ENAC has since April this year been delivering electricity from a gas engine-based CHP system. The extension consists of two Wärtsilä gas engines, two exhaust gas boilers rated at 4.5 tonnes/h and 5.6 tonnes/h of steam, and two chillers – one electrical turbo-chiller and one absorption chiller. The chillers and exhaust gas boilers were arranged locally by ENAC. Wärtsilä supplied the gas engine generators, auxiliaries and control systems. The engines are the Wärtsilä 16V34SG, rated at 6970 kW, and one Wärtsilä 20V34SG, rated at 8730 kW, each with an electrical efficiency of 45.6 per cent. Some 80 per cent of the electricity generated at the plant is for external use, 20 per cent is for internal use in the district heating and cooling centre. Power from the engine also drives the electric turbo-chiller to produce chilled water. Heat from each engine is recovered as steam in the associated heat recovery boiler, which is used as a heat source for the steam absorption chiller. In addition, hot water is recovered from the engine cooling water. This is fed into the hot water absorption cooler to make chilled water.
The engines, delivered in 2007, are the first of their kind to be used in Japan and have now accumulated 2000 hours of running. ENAC is using the plant as a test bed for verifying the reliability and performance of these newer 16V and 20V gas engines for cogeneration and DHC systems.
The main challenge facing the growth of trigeneration in DHC must be investment. Piping can be substantial and its cost far exceeds that of the generating system. The customer must have an existing network for delivery of the steam and chilled water. It is an extensive infrastructure undertaking to get the network and consumption points and buildings correctly designed.
But such infrastructure is becoming more commonplace in certain parts of the world. For example, Vattenfall is building up its chilled water network in Stockholm. But these are conscious, long-term decisions that need to be made by local governments and utilities. Nevertheless, the drivers for trigeneration remain sound. In industry, it is just a case of installing the plant – an extensive piping infrastructure does not already have to be in place.
On a political level, the growth of this type of system in Europe will continue to be driven by the European CHP Directive, which promotes the efficient use of primary fuel. Environmental drivers like the Kyoto Protocol will also continue to have a positive impact on the future of trigeneration.