| China is the second largest energy consuming country and one of the leading users of coal in the world. Following its successful bid for the 2008 Olympic Games, the Beijing City Government is trying to find a cleaner fuel source to mitigate the serious coal smoke pollution in urban areas.
In general a relatively clean energy source, natural gas has the potential to grow as the major energy source for power generation and for driving air-conditioning systems. In Beijing , the peak summer electricity load was 6,080 MW in 1999 and 6,480 MW in 2000. It is estimated that the air-conditioning load will take about 35 percent of the power consumption.
To support the mission of improving the Beijing environment, the gas company, with support from the Asia Development Bank, is carrying out a demonstration project of a trigeneration system for a newly constructed 31,800 m 2 (343,400 square feet) office building with 10 floors above ground and two basement levels (Figure 1) . The building will be the head office of the gas company and home to the natural gas network supervision and dispatching centre and the gas supply registration and maintenance centers. The building was completed in June 2002. At the time of writing, the system control testing was to be conducted in the winter and summer of 2004.
PB was appointed by the Contractor to develop the preliminary system design and interfacing, provide technical advice on the system design, configurations, control and integration, and audit the detail design, calculations, equipment selection and control system.
The Trigeneration System
A trigeneration system produces three different forms of energy from primary energy source, namely, hot water, cooling water and electricity. It is also referred as combined heating, cooling and power generation (CHCP). The trigeneration system in Beijing comprises the following major components:
- Gas engines and generators
- Direct fired absorption (DFA) chillers
- Radiators
- Cooling towers
- Chilled/heating water storage tanks.
The relationship of the basic system components is illustrated in Figure 2 .
Natural gas is the prime source of energy to drive the engine, which drives the generator and produces electricity for the entire building. Waste heat discharged through the gas engine/generator jacket cooling water system and heat extracted from the exhaust flue gas are used to drive the DFA chiller. Chilled water is produced by the absorption chiller and supplied to the building chilled water piping network. Hot water produced through the heat exchanger and the high-stage generator is provided to the heating water supply network. If the waste heat does not have sufficient thermal gradient to be used for the building heating or cooling load, natural gas can be used to drive the DFA chiller to supply heating or cooling water.
If the gas engine fails to operate, the automatic transfer switch (ATS) will transfer the load to the utility power supply grid automatically, thus ensuring the reliability of power supply. The burner of the DFA chiller can be fired to supply heating or cooling water to the entire building.
Gas Engine Generator. Two gas engine generators were adopted for this project. The key technical data of the engines are provided in Table 1
The gas engine and generator set is in an acoustic enclosure to ensure an acceptable noise level within the plant room, as it was planned to open this demonstration plant for the public to visit. The noise level within the plant will be controlled at 80 dBA (at 1 m around the noise enclosure).
DFA Chiller. The waste heat dissipated to the cooling water and exhaust chimney are used to drive the absorption chiller. If the building cooling or heating load is low, the excess waste heat is dissipated through the remote air-cooled radiators to ensure the engine can be operated continuously. A by-pass damper is also used to divert the hot exhaust gas from the engine to the atmosphere without it passing through the high stage generator of the DFA chiller. The key technical data of the DFA chillers are shown in Table 2.
The operation relationship of the DFA chiller and generator is illustrated in Figure 3 .
Radiators. Three radiators are used for the heat dissipation of the engines and generator jacket cooling water system and the lube oil system. The jacket cooling water radiators do not operate unless the heat discharged from the engine cannot be totally absorbed by the DFA for cooling or heating purposes.
Plant Layout. The gas engine, generator, absorption chiller and the control centre are located at the lower basement. Separate rooms are used for the remote radiators to prevent short circuiting. Cooling towers are located on the roof of the building to cool the DFA chiller during cooling mode.
System Operation. The system operation and the equipment selection depended on the building electricity, heating and cooling load. An optimum equipment combination was selected to suit the load pattern in different seasons and at different times. The estimated building power consumption, cooling and heating loads are shown in Figure 4. The relationships of fuel consumption and power output for the two generators are illustrated in Figure 5.
The summer and winter operation modes shown in Table 3 were determined to maximize the efficiency and the balance of gas fuel cost and the electricity cost from the utility grid. Neither heating nor cooling are required in Beijing during the autumn and spring seasons, so at those times the power supply to the building will be connected to the utility power supply grid and the gas generator will serve for standby purposes.
System Efficiency. The calculation of energy efficiency, which is quite complicated, is calculated by using an energy model. The flow chart of the modeling is illustrated in Figure 6 .
Based on the estimated power, heating and cooling load, the system annual average efficiency is above 85 percent when the system is in operation during the summer and winter seasons. In contrast, the power generation efficiency of a traditional gas engine is about 35 percent; i.e., 65 percent of heat energy is lost.
Conclusion
A trigeneration system using natural gas as fuel is becoming an increasingly attractive option for a clean and sustainable total energy solution. With the success in operation of this demonstration project and the implementation of natural gas supply network in Beijing , the on-site power generation with corresponding heating and cooling water supply may be the trend for the future energy business. In addition, the advance in the technologies development on the trigeneration system will make it more attractive to potential users and environmental-cautious business enterprises.
|
