• Energy Research
• 7. Clean energy close
• CN close
• Energy Conversion and Management close

Abstract This paper presented a novel method of dissipating solar photovoltaic heat based on the technology of micro-heat-pipe array and the utilization of photovoltaic-cell waste heat. This novel technology solved the problems of low PV electrical efficiency and thermal failure caused by high cell temperature, greatly increased the comprehensive utilization efficiency of solar energy, and extended the service life of photovoltaic modules. One-year experiments were conducted to investigate the electrical and thermal performance of a forced-circulation, household-type photovoltaic/thermal system based on micro-heat-pipe array in Beijing, China. Test results showed that on the four typical days in different seasons, the average electrical efficiencies were 13.76%, 11.92%, 13.71%, and 14.65%; the average thermal efficiencies were 31.62%, 33.07%, 24.99%, and 17.24%; and the average total efficiencies were 45.38%, 44.99%, 38.70%, 31.89%, respectively. The system met the demand of power supply on sunny days and the demand of hot water between March and November, except in cloudy days. These experimental results can provide basis and reference for practical applications of the system.

Abstract Global hydropower growth continues to accelerate with 25% of total capacity installed in just the last 10 years. This accelerating expansion and the important storage facility hydropower means it is increasingly important to understand the reasons for operational failures. This is a challenge because the major reason for failures involves the complex interaction of hydraulic, mechanical and electric subsystems. Historically, reliability modelling has been split in two directions, focusing on different sub-systems, and has not yet been unified. Here these approaches are unified with a novel expression of unbalanced forces. This model with operational data are validated and the important modes of oscillation in the shaft are identified. Finally, the mechanism of the first-order oscillation mode exciting a second-order mode is presented. This integrated and accurate mathematical model is a major advance in the diagnosis and prediction of failures in hydropower operation.

Abstract In this work, the combustion and emission fundamentals of high n-butanol/diesel ratio blend with 40% butanol (i.e., Bu40) in a heavy-duty diesel engine were investigated by experiment and simulation at constant engine speed of 1400 rpm and an IMEP of 1.0 MPa. Additionally, the impact of EGR was evaluated experimentally and compared with neat diesel fuel (i.e., Bu00). The results show that Bu40 has higher cylinder pressure, longer ignition delay, and faster burning rate than Bu00. Compared with Bu00, moreover, Bu40 has higher NOx due to wider combustion high-temperature region, lower soot due to local lower equivalence ratio distribution, and higher CO due to lower gas temperature in the late expansion process. For Bu40, EGR reduces NOx emissions dramatically with no obvious influence on soot. Meanwhile, there is no significant change in HC and CO emissions and indicated thermal efficiency (ITE) with EGR until EGR threshold is reached. When EGR rate exceeds the threshold level, HC and CO emissions increase dramatically, and ITE decreases markedly. Compared with Bu00, the threshold of Bu40 appears at lower EGR rate. Consequently, combining high butanol/diesel ratio blend with medium EGR has the potential to achieve ultra-low NOx and soot emissions simultaneously while maintaining high thermal efficiency level.

Abstract With the application of advanced information technology for the integration of electricity and natural gas systems, formulating an excellent energy conversion and management strategy has become an effective method to achieve established goals. Differing from previous works, this paper proposes a peak load shifting model to smooth the net load curve of an integrated electricity and natural gas system by coordinating the operations of the power-to-gas unit and generators. Moreover, the study aims to achieve multi-objective optimization while considering the economy of the system. A dynamic energy conversion and management strategy is proposed, which coordinates both the economic cost target and the peak load shifting target by adjusting an economic coefficient. To illustrate the complex energy conversion process, deep reinforcement learning is used to formulate the dynamic energy conversion and management problem as a discrete Markov decision process, and a deep deterministic policy gradient is adopted to solve the decision-making problem. By using the deep reinforcement learning method, the system operator can adaptively determine the conversion ratio of wind power, power-to-gas and gas turbine operations, and generator output through an online process, where the flexibility of wind power generation, wholesale gas price, and the uncertainties of energy demand are considered. Simulation results show that the proposed algorithm can increase the profit of the system operator, reduce wind power curtailment, and smooth the net load curves effectively in real time.

Abstract This study proposes a novel interconnector, termed beam and slot interconnector (BSI), for the anode-supported planar solid oxide fuel cell (SOFC). A detailed comparative investigation is conducted on various transport characteristics and electrical performance of the SOFC stacks with conventional straight channel interconnectors (SCIs) or with novel interconnectors. Results show that the peak power density of a SOFC stack with BSIs is 24.8% higher than it with SCIs at 700 °C and a fuel–air flow rate of 16–40 Nml/(min·cm2). Moreover, BSI can reduce the fuel–air feeding and enhance the fuel–air utilization while maintaining high output power density. Compared with SCI, BSI promotes the gas disturbance, significantly increases the gas velocity and vorticity, and leads the gas to flow in the direction perpendicular to the channel. BSI eliminates the limitation of SCIs on gas diffusion in the electrode and transfers sufficient reactant gas into the electrode function layers for electrochemical reactions. BSI shortens the charge transfer path in SOFC and almost avoids the adverse effects of the electrode-interconnector contact resistance. Compared with the conventional SOFC stack, the novel SOFC stack with BSIs significantly reduces various overpotentials. At 700 °C and 0.5 A/cm2, the activation, concentration, and contact overpotentials are reduced by 8.5%, 47.4%, and 96.4% respectively, and the total overpotential finally drops by 20.0%. Overall, the electrical performance of the SOFC stack with novel interconnectors significantly exceeds the one with conventional interconnectors under the same operating conditions.

Abstract Water fuel emulsion has been widely studied with the advantages of saving energy, enhancing engine torque, improving engine performance, and reducing the pollutant emissions. However, it has unfavorable disadvantages such as phase separation and long ignition delay. Water fuel microemulsion with rhamnolipid as the surfactant was formed in this study and characterized in comparison to water fuel emulsion. Water fuel microemulsion was thermodynamically stable without phase separation after 90 days vs. the milky-white emulsion fuel, separated within 2 days. In the thermogravimetric analysis, the TG and DTG curves were shifted to higher temperatures as the increment of heating rate. However, the shift for emulsion at 40 K min −1 was inconspicuous, which implies the reduction in heat transfer, mass transfer, and vaporization rates and further the lengthened ignition delay upon combustion in diesel engine. The activation energies ( E a ) predicted by Ozawa–Flynn–Wall (OFW), Kissinger–Akhira–Sunose (KAS), and Starink’s methods indicate that the formation of microemulsion could decrease the activation energy of the fuel by about 5 kJ mol −1 , while the formation of emulsion would increase by 15 kJ mol −1 . The lower activation energy of microemulsion fuel is an indication of easy ignition or shortened ignition delay. Thus, microemulsification may be a more competitive technique for fuel upgrading compared to emulsification.

Abstract This paper presents an ultra-low frequency vibration energy harvester using a zigzag piezoelectric spring oscillator, which consists of two piezoelectric zigzag springs and a rolling metal ball. The metal ball rolls and drives the piezoelectric springs to deform to harvest energy when a slight vibration occurs in the external environment. The natural frequency of zigzag spring oscillator piezoelectric energy harvester (ZSO-PEH) is related to the length of the spring and the weight of the ball, correlation analysis is carried out by theoretical derivation and ANSYS simulation. It is found experimentally that the proposed device offers efficient energy output in ultra-low frequency excitation. A maximum output power of 5.68 mW is achieved under the best matching resistance of 5.1 k Ω at the excitation frequency of 3 Hz. The performance of energy harvester can be optimized by adjusting the length of the spring and the mass of the ball. The results show that the proposed piezoelectric energy harvester has the potential to power low-power electronic devices and wireless sensor nodes.

Thermally integrated pumped-thermal electricity storage (TI-PTES) offers the opportunity to store electricity as thermal exergy at a large scale, and existing studies are primarily focused on TI-PTES systems based on high-temperature thermal energy storage. This paper presents a thermo-economic analysis of a “cold TI-PTES” system which converts electricity into cold energy using a vapor compression refrigeration (VCR) unit and stores it at sub-ambient temperatures during the charging process, and generates electricity by using an organic Rankine cycle (ORC) working between the sub-ambient temperature and an external low-grade heat source during the discharging process. The effects of key parameters, i.e., mass flowrate and temperature of the storage medium, ORC evaporation temperature, component efficiencies, and pinch-point temperature differences, on the system performance are evaluated based on a whole-system thermo-economic model. The results reveal that the roundtrip efficiency and levelized cost of storage (LCOS) of the system increases while the electrical energy storage capacity decreases as the temperatures of the two cold storage tanks approach each other. When the temperature of the cold storage tank 1 rises from 1 °C to 8 °C while the cold storage tank 2 remains as 13 °C, there is an increase of 25% and 20% in the roundtrip efficiency and LCOS respectively while the energy storage capacity decreases by 69%. A roundtrip efficiency of 0.74 and LCOS of 0.32 $/kWh are achieved with a heat source temperature of 85 °C, using a mass flowrate and temperature of the cold storage medium of 50 kg/s and 1 °C. Furthermore, any change in cold storage medium mass flowrate changes both electrical energy storage capacity and power output by the same proportions. With a continuous high-flowrate external heat source, the LCOS can be as low as 0.17 $/kWh. By providing sufficient heat from an external heat source, the proposed system possesses a high potential for medium-to-large scale energy storage with a unique hybrid ...

Abstract A new air-cooled gas-steam combined cycle cogeneration system with absorption heat pump for recovering waste heat from exhausted steam of the steam turbine to achieve double effects of waste heat recovery and water saving is proposed based on a conventional water-cooled gas-steam combined cycle cogeneration system in the paper. The property criteria variation is analyzed before and after modification. In addition, the exergy analyses of primary equipments are carried out based upon the exergy analysis theory. The results demonstrate that the net generating power is approximately increased by 11,082 kW, equivalent coal consumption is reduced by 2.71 g/kWh, the net overall thermal efficiency is improved by 0.91% with 334,245 kW heating load at 100% load of the gas turbine in the modified system. Besides, the overall exergy loss is decreased by 6448 kW and exergy efficiency is improved by 0.98%. The overall property of the whole system is improved. The results show that the property reduction caused by air-cooling modification can be made up by the property improvement due to waste heat recovery. Moreover, the cooling circulating water can be saved by 1196.34 kg/s. The presented measure can not only improve performance of the system but also simultaneously achieve energy and water saving on the premise of satisfying user needs, which has a wide application potential in the water-shortage regions.