| Combined cooling heating and power(CCHP) systems based on internal combustion engines have been widely accepted in building energy supply for its demonstrated performance of energy saving, cost saving and part load operation. However, there are still problems limiting the full function and further improvement of system performance. Current researches on CCHP systems have paid too much attention to system optimization and evaluation, while the internal mechanism and thermodynamic characteristics are not enough emphasized. As far as now, the waste heat has not been sufficiently and efficiently used, some thermal matching problems are not solved, and on-grid optimal operation issues are not identified. In sight of these problems, a kind of CCHP system based on waste heat cascade utilization is proposed and investigated.Firstly, the system principle is illustrated and the energy output characteristics are explained. Basing on the waste heat cascade utilization, the potential of energy conversion is analyzed and compared. The mechanisms of single-dimensional and double-dimensional energy output characteristics are illustrated. The extended energy output area under on-grid operation is also discussed.Secondly, some typical thermal matching problems in CCHP systems are investigated. The exhaust gas deep-recovery process is studied. Both the thermal characteristic curve and outlet hot water temperature range are discussed. The thermodynamic principles of various absorption refrigeration cycle are explained, and the formula of lower limit desorption temperature is deduced. This formula supplies theoretical basis for waste heat cascade utilization by absorption chiller. The single effect and double effect absorption refrigeration are suitable for hot water and exhaust gas waste heat recovery from ICE, respectively. If they are combined together, it is mixed effect absorption refrigeration. The principles and thermodynamic constraints for the application of mixed effect AC in a CCHP system are investigated.Thirdly, the whole condition performance of CCHP system in both heating mode and cooling mode is studied by simulation. In heating mode, the exhaust gas deep-recovery and thermoelectric generator are cascaded together to heat a water flow. Results show that this kind of integration can improve the thermal output by 16% and generate another 1.18 k W electricity at most. A simulation model for mixed effect AC is built and calculation algrithms are developed. Considering the thermal characteristics of waste heat from ICE, the design principles for mixed effect AC are discussed. The off-design performance for refrigeration is simulated, and results show that this kind of AC is adaptive to the whole condition waste heat characteristics of ICE, and the refrigeration COP varies between 0.77 and 0.96. Evaluation shows that, this kind of CCHP system can get a much higher ESR and CSR than a general CCHP system.Fourthly, an experimental study is conducted based on a 200 k W CCHP system. Five different working conditions are tested in steady condition. Results show that the maximum waste heat recovery efficiency and electric efficiency are 0.77 and 0.31, respectively. In the condition of 157.5k W electric output, the refrigeration output and COP reach 188.76 KW and 0.911, respectively. In order to study the dynamic performance of mixed effect AC, a typical working condition is tested for hours. Results show that it takes about 2 hours to make the performance steady, and some phenomena during system start-up to steady state are explained.Finally, the optimal operation of an on-grid CCHP system is studied based on above research results. A single objective non-linear optimization model is built based on the performance equations fitted from the simulation results. A calculation method is developed based on the genetic algorithm. By studying the optimal energy saving operation strategy and optimal cost saving operation strategy under a certain range of load structure, it can be found that following the thermal load operation strategy as well as the full load operation strategy are not necessarily the optimal choice. The optimal performance distribution of on-grid operation is compared with that of on-grid but not up-grid operation. Results show that the former operation mode not only enhances the adaptability of CCHP system under low load structure, but also largely improves the optimal performance. The influences of energy price ratio and on-grid price ratio on the optimal operation strategy are also discussed, in order to supply theoretical basis for system operation and on-grid policy-setting. |
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