The plate fin heat exchanger is a compact heat exchanger applied in many industries because of its high thermal performance. To enhance the heat transfer of plate fin heat exchanger further, three new kinds of wavy plate fins, namely perforated wavy fin, staggered wavy fin and discontinuous wavy fin, are proposed and investigated by computational fluid dynamics (CFD) simulations. The effects of key design parameters, including that of waviness aspect ratios, perforation diameters, staggered ratios and breaking distance are investigated, respectively, with Reynolds number changes from 500 to 4500. It is found that due to the swirl flow and efficient mixing of the fluid, the proposed heat transfer enhancement techniques all have advantages over the traditional wavy fin. At the same time, serration is beneficial to reduce the friction factor, and the breaking technique can reduce heat transfer area. Through the performance evaluation criteria, the staggered wavy fin has an advantage over the small waviness aspect ratio; with increasing waviness aspect ratio, this predominance is gradually surpassed by the perforated wavy fin, and the advantage of the discontinuous fin is the smallest and almost invariable. A maximum performance evaluation criteria (PEC) as high as 1.24 can be obtained for the perforated wavy fin at the waviness aspect ratio γ = 0.45.

To reduce the exergy loss that is caused by the high-grade extraction steam of traditional heating mode of combined heat and power (CHP) generating unit, a high back-pressure cascade heating technology for two jointly constructed large-scale steam turbine power generating units is proposed. The Unit 1 makes full use of the exhaust steam heat from high back-pressure turbine, and the Unit 2 uses the original heating mode of extracting steam condensation, which significantly reduces the flow rate of high-grade extraction steam. The typical 2 × 350 MW supercritical CHP units in northern China were selected as object. The boundary conditions for heating were determined based on the actual climatic conditions and heating demands. A model to analyze the performance of the high back-pressure cascade heating supply units for off-design operating conditions was developed. The load distributions between high back-pressure exhaust steam direct supply and extraction steam heating supply were described under various conditions, based on which, the heating efficiency of the CHP units with the high back-pressure cascade heating system was analyzed. The design heating load and maximum heating supply load were determined as well. The results indicate that the average coal consumption rate during the heating season is 205.46 g/kWh for the design heating load after the retrofit, which is about 51.99 g/kWh lower than that of the traditional heating mode. The coal consumption rate of 199.07 g/kWh can be achieved for the maximum heating load. Significant energy saving and CO₂ emission reduction are obtained.

High back-pressure technology is a promising method for the waste heat recovery of exhaust steams in combined heat and power systems. In this research, a 300 MW coal-fired subcritical combined heat and power system was selected as the reference system, and modeled in EBSILON professional. Then, energy-based and exergy-based performances of the high back-pressure system and traditional combined heat and power system were compared under full operating conditions. Moreover, a novel exergy-based evaluation method, which considers the energy level of the heating supply, was proposed and applied to evaluate the two systems. Results show that: In design conditions, both the heating capacity and power output of the high back-pressure system were higher than those of the extraction condensing system, which led to 17.67% and 33.21% increments of the gross thermal efficiency and generation efficiency, respectively. Compared with the extraction condensing system, the exergy efficiencies of the high back-pressure system were 7.04–8.21% higher. According to the novel exergy-based evaluation, the exergy efficiencies for the generation of the high back-pressure system and extraction condensing system were 46.48% and 41.22%, respectively. This paper provides references for the thermodynamic performance evaluation of the combined heat and power system.

Utilization of high-temperature energy in industrial production processes is often exhausted by huge low-temperature waste heat without recovery. Thus, energy efficiency is quite limited. Heat pumps are widely used as a high-efficiency waste heat recovery system and are divided into vapor compression cycle, driven by electricity, and absorption type, driven by steam or hot water. However, compression heat pumps are quite difficult to reach more than 100 °C due to the temperature and compression limits of compressors and the working medium. Meanwhile, the COP (coefficient of performance) of an absorption heat pump is quite low due to the thermodynamic cycle characteristics. In order to increase the outlet temperature and COP significantly, a new type of compression-absorption hybrid heat pump cycle is presented and simulated. Compared with traditional cycles, this heat pump can reach the heat sink temperature of 200 °C with a highly satisfactory COP. This heat pump could reach the optimal COP of 3.249 when the pressure ratio of the compressor is 6.5, the coupling temperature of the low-pressure stage is 55 °C and the coupling temperature of the high-pressure stage is 73 °C. Exergy analysis shows that evaporators and condensers show better efficiency. This heat pump could be promising in different kinds of heat recovery.

As an important research subject, district heating with combined heat and power (CHP) has significant potential for energy conservation. This paper utilised a 200 MW air-cooled unit as an actual case and presented a design scheme and energy consumption analysis of three typical CHP modes, including the low vacuum mode (LVM), the extraction condensing mode (ECM), and the absorbing heat pump mode (AHPM). The advantages and disadvantages of each mode (including their practical problems) were analysed, and suggestions for the best mode were proposed. The energy consumption of the three heating modes changed with the heating load. When the heating load was increased, the net power of the entire system decreased to different degrees. In this paper, the energy conservation effect of the LVM was the most ideal, followed by the ECM and the AHPM. Besides, the LVM and AHPM were able to supply larger heat loads than the ECM, which was limited by the minimum cooling flow of the low pressure cylinder. Furthermore, in order to get a more general conclusion, a similar case with an air-cooled 300 MW unit is studied, showing that the fuel consumption levels of ECM and AHPM have changed.

To improve the adaptability of direct air-cooled power generating units to the variations of both meteorological condition and power load, a piping-main arrangement of air-cooled condensers was proposed. The heat and mass transfer models of the air-side were established for the air cooling system of 2 × 600 MW thermal power generating units. The coupled model for both flow resistance loss and condensate flow rate distributions of exhaust steam inside air-cooled condensers were developed based on the temperature fields through numerical simulation. Calculation results, including the condensate flow rate, back pressure, and coal consumption rate, were acquired under different ambient temperatures and wind velocities. The results show that the proposed piping-main arrangement can weaken the ambient wind impacts and reduce the backpressure significantly in summer by adjusting the number of air-cooled condenser cells in operation. The steam flow rate can be uniformed effectively by adjusting the number of operating air-cooled condenser cells during winter. It can also avoid the freezing accident in winter while cooling the exhaust steam of two turbines by part air-cooled condenser cells.