Energy Effect Analysis And Comparable Methodology Study On CCHP Based On Distributed Energy System

The current situation of energy consumption and air pollution of urban district heating (UDH) systems in China constraints our urban development and improvement of quality of human life. China has been vigorously developing natural-gas-based UDH to reduce the severe impact of emissions from coal-based UDH. However, the majority of the exergy of natural gas is destroyed when this high-value energy source is used for gas-fired heating boilers to provide low-value energy. This paper conducted energy and exergy analysis on various methods that are used to expand heating and peak-shaving capabilities of natural gas-based distributed energy systems, and improve their system performances. Furthermore, the characteristics of traditional energy-effect analysis methodologies and the issues encountered in the application of these traditional methodologies in combined cooling, heating and power (CCHP) systems were analyzed. This paper proposed a distributed energy system with heat pump heating (DES+HPH) and power-based comparable coefficient of performance (CCOP) methodology on CCHP systems based on the distributed energy system.Effects of different ambient temperatures and load percentages of prime movers on performances of various UDH systems were investigated. The optimization of the peak shaving ratio of the UDH systems can improve the average load rates of the prime movers, and significantly reduce the exergy losses of the prime movers and the UDH systems. Meanwhile, natural gas consumption for peak heating hours is considerably lowered. The DES+on-demand HPH can reach the highest average-exergy-efficiency among the UDH systems. The DES+HPHs in the six representative UDH cities in China are studied with energy and exergy analysis. The primary energy ratio, exergy efficiency and heat-to-power ratio of the DES+HPHs are significantly influenced by the peak shaving ratio. The average exergy efficiencies of the DES+HPH in China can be optimized when the peak shaving ratio is in the range of0.3to0.4.Various cooling/heating energy sources provided by the DES were investigated based on energy grade. When low-pressure steam and hot water are provided by the GTCC for cooling/heating, the power-generation capacity of the GTCC is reduced compared with that under the pure-power-generation working condition. The power-generation capacity reduction of the GTCC is the direct energy cost of the low-pressure steam and hot water consumed for cooling/heating. This study proposes a power-based CCOP methodology to resolve the issues of reasonability and comparability of performance evaluation between DES-based CCHP systems and separate production systems. The proposed CCOP methodology uses power-generation capacity as benchmark to convert different cooling/heating energy consumptions of various cooling/heating systems based on the DES into equivalent power consumptions. The cooling/heating performance can be directly evaluated according to the CCOP (the ratio of cooling/heating capacity to the sum of the equivalent power consumption and direct power consumption) without reference to external power-generation efficiency. DES-based steam and hot-water heating systems can save comparable energy cost (a sum of the equivalent and the direct power consumptions) by approximately72%and98%, respectively, compared with natural-gas-fired boilers. Hot-water-driven and steam-driven absorption cooling system based on the DES reach the CCOPs of approximately11.4and5.7, respectively, which are higher than those of high performance vapor-compression chillers.China’s on-grid distributed combined heating and power systems were studied with the proposed CCOP methodology. Various heating energy consumption (i.e., electricity, hot water, and steam) used in the DES+HPH were converted to the energy grade of exergy (power). The heating comparable energy cost was defined and calculated as the sum of electricity consumption of HPH and the equivalent power consumption of the low-pressure steam and hot water. Effects of different peak-shaving ratios on the heating comparable energy costs and heating CCOPs of the various DES+HPH systems were investigated. A comparative study was also conducted to contrast the proposed CCOP method with conventional methodologies. After the natural gas consumption corresponding to the equivalent power consumption of the low-pressure steam and hot water is deducted from the total natural gas consumption of the DES, the equivalent electrical efficiencies of the DES+HPHs with various peak-shaving ratios are numerically equivalent to the electrical efficiency of the DES under the pure-power-generation mode. The power-generation performance of various CHP and separate power-generation systems can be directly evaluated without reference to the heating efficiency of any separate production system.