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[en] Highlights: • Mode 4 has the highest exergy efficiency. • Mode 2 has the largest exergy density. • Second heat exchanger has the largest exergy destruction. - Abstract: Advanced adiabatic compressed air energy storage system plays an important role in smoothing out the fluctuated power from renewable energy. Under different operation modes of charge-discharge process, thermodynamic behavior of system will vary. In order to optimize system performance, four operation modes of charge-discharge process are proposed in this paper. The performance difference of four modes is compared with each other based on energy analysis and exergy analysis. The results show that exergy efficiency of mode 4 is the highest, 55.71%, and exergy density of mode 2 is the largest, 8.09 × 106 J m−3, when design parameters of system are identical. The second heat exchanger has the most improvement potential in elevating system performance. In addition, a parametric analysis and multi-objective optimization are also carried out to assess the effects of several key parameters on system performance.
[en] Highlights: • The study focussed on the techno-economic assessment of thermal energy storage systems. • Data-intensive bottom-up models for each storage systems were developed. • Costs for sensible, thermo-chemical, and latent heat storage systems were developed. • The electricity cost from using these thermal energy storage systems is $0.02–$1.19/kWh. - Abstract: In this paper, a data-intensive cost model was developed for sensible heat, latent heat and thermochemical storage systems. In order to evaluate the economic feasibility of storage systems, five scenarios were developed depending on the method of storage. The five scenarios considered were indirect sensible heat, direct sensible heat using two tanks, direct sensible heat using one tank, latent heat and thermochemical storage. A Monte Carlo simulation was performed for all the scenarios to examine the uncertainty in the levelized cost of electricity when parameters such as solar multiple, plant capacity, storage duration, capacity factor, and discount rate are changed. The levelized cost of electricity ranges for individual scenarios are; 0.08–0.59 $/kWh for indirect sensible heat, 0.03–0.22 $/kWh for direct sensible heat using two tank, 0.02–0.16 $/kWh for direct sensible heat using one tank, 0.06–0.43 $/kWh for latent heat, and 0.22–1.19 $/kWh for thermochemical storage. The results indicate that when uncertainty is taken into account, the investment cost for thermochemical storage is clearly higher than other scenarios. This study will provide key information for industry and policy makers in decision making and in determining the economic viability of thermal energy storage systems.
[en] Highlights: • Heat transfer for PCHE in TEG was investigated in detail by 3D CFD analysis. • Experimental data for a 200-W TEG implemented with PCHEs are newly presented. • Power density of the TEG was sufficiently high at low temperature. • Reduction of TEG flow rate requirements from use of PCHEs is estimated. - Abstract: Printed circuit heat exchangers (PCHEs) are employed to improve the compactness of a thermoelectric generator (TEG). PCHEs allows miniaturization of the heat exchanger without excessive additional cost, and permit high temperature and pressure (up to 1100 K and 600 bar) of working fluid, which enable high thermoelectric conversion efficiency. To investigate the pressure loss and thermal resistance of a PCHE in detail, three-dimensional computational fluid dynamic (CFD) analysis is conducted. Experimental results of the proposed TEG with PCHEs are newly presented. The TEG provides power density of 233.1 kW/m3 at inlet temperatures of 448.15 K (hot side) and 293.15 K (cold side), which is the highest value in literature for a low-temperature TEG (<505.15 K hot side). Based on the models of friction and heat transfer in a PCHE validated by the experiment, it is noted that the flow rate required for the heat exchangers in a TEG producing a given amount of electrical power can be reduced by adaption of PCHEs. Such novel results on the TEG with PCHEs might be helpful for more compact design and expands the applicability of TEGs for waste heat recovery.
[en] Highlights: • Triplex loop heat pump system for ventilation heat recover is proposed. • Mass flow rate in heat pump system can be improved by triplex loop system. • COP of triplex loop is increase with the decrease of outdoor temperature. • The performance of triplex system is higher than traditional system in most cases. - Abstract: Ventilation heat recovery is an important means of effectively reducing the energy consumption of buildings. To improve the performance of a heat pump heat recovery system under large temperature difference conditions in winter, a triplex loop heat pump system, which contains three independent heat pump cycles, is proposed in place of its single loop counterpart. Operating characteristics and system performance were analyzed while indoor temperature was constant at 20 °C and as outdoor temperature dropped from 15 °C to −20 °C. Results showed that with the decrease of the outdoor temperature, the mass flow rate and temperature effectiveness of the triplex loop heat recovery system decreased whereas the heating capacity and the coefficient of performance (COP) increased. Under the experimental conditions, the COP of the triplex loop system had an advantage over the traditional heat pump system when the outdoor temperature was below 2.5 °C. When the outdoor temperature was −20 °C, the COP of the triplex system could reach 9.33, which was 23.1% higher than that of the traditional system.
[en] Highlights: • A novel integrated system of solar energy and air source heat pump was proposed. • The novel system was compared with other two typical existing systems. • I-T diagram was proposed to divide the optimal working condition ranges. • Effect of different factors on the dividing lines in the I-T diagram was analyzed. - Abstract: Integrated systems consisted of solar energy and air source heat pump have been a hot research topic in recent decades due to their high efficiency and low environmental pollution. Recently, much attention has been paid to the performance characteristics of these systems, however, scarce of attention has been paid to indicate which kind of integrated systems have the optimal performance under different working conditions. For the integrated system of solar heating independently, the solar collectors have high heat-collecting temperature and high heat loss to ambient, so it has advantages in high solar radiation. For the integrated system of solar energy used for the low temperature heat source of heat pumps, the solar collectors have low heat-collecting temperature and low heat loss, so it has advantages in low solar radiation. But for the condition of medium solar radiation, both the above existing two types of systems may not be the optimal ones. Therefore, in this study, a novel integrated system was put forward for this condition. The characteristics and optimal working condition range of this novel system were comparatively studied by the simulation method. By comparison with the above existing two types of systems, the results proved that for most conditions of medium solar radiation, the novel system has the optimal performance, and its COP can be about 55% higher than that of the two types of existing systems when the outdoor temperature is −25 °C. An I-T diagram was proposed in this study to quantificationally divide the optimal working condition ranges of the three types of systems. This study can effectively guide the selection of optimal systems under different working conditions.
[en] Lorentz gauge theory of gravity was recently introduced. We study the homogeneous and isotropic universe of this theory. It is shown that some time after the matter in the universe is diluted enough, at , the decelerating expansion shifts spontaneously to an accelerating one without a dark energy. We discuss that Lorentz gauge theory puts no constraint on the total energy content of the universe at present time and therefore the magnitude of vacuum energy predicted by field theory is not contradictory anymore. It is demonstrated that in this theory the limit on the number of relativistic particles in the universe is much looser than in GR. An inflationary mechanism is discussed as well. We show that the theory, unlike GR, does not require the slow-roll or similar conditions to drive the inflation at the beginning of the universe. (paper)
[en] Highlights: • A solar-driven Kalina cycle is investigated by advanced exergoeconomic analysis. • The highest exergy efficiencies are related to the separator and turbine with the values. • Rotary machinery have more than 83% avoidable share of exergy destruction rate. - Abstract: A Kalina cycle driven by solar energy resource is evaluated by conventional exergy and exergoeconomic analysis methods. Because conventional exergy analyses isn’t able to give information about costs of the irreversibilities and investment, advanced exergy is investigated. Based on the conventional exergy analyses, the most exergy destruction occurs in a heater with a value of 94.44 kW. Also the highest exergy efficiencies are related to the separator and turbine with the values of 99.67% and 89.81%, respectively. Advanced exergy analyses demonstrates absorber (1.3 $/h) and one of the pumps (0.009 $/h) have the highest and lowest exergy destruction cost rate, respectively. Also the results show turbine (85.88%) and separator (1.105%) have the highest and lowest exergoeconomic factor, respectively. Finally, in order to determine optimum point of the inlet temperatures and pressure ratio of the pumps and turbine (rotary machines), a parametric study is applied at different stages.
[en] Highlights: • Relative importance of the determinants of FDI in wind and solar energy are clarified. • Expert opinions are analyzed using analytical hierarchy process. • Renewable policies hold strong influence when compared to traditional determinants. • Renewable policies need to be designed to reduce exchange rate volatility risk. • Creating one-stop agency is recommended for efficient administrative procedure. - Abstract: The importance of foreign direct investment (FDI) for the development of renewable energy in developing countries has been increasingly recognized. Numerous countries have created various measures to attract FDI in the renewable energy sector. This paper uses the analytical hierarch process to clarify the relative significance of the determinants in the location decisions of foreign wind and solar energy investors. A total of 18 determinants that are categorized into the macroeconomic environment, institutional environment, natural conditions, and renewable energy policy categories are used for the analysis. The results show that adding to the traditional determinants of FDI, including the macroeconomic environment, the institutional environment, and natural conditions, renewable energy support policies have the same or stronger influence as location determinants of FDI. The paper also points out that some of the traditional determinants, such as exchange rate volatility, access to land, and an efficient and transparent administrative procedure, are also very important as determinants of FDI in wind and solar energy. Policy implications focus on the determinants of FDI in wind and solar energy. The relative significance of the determinants clarified through this study offers criteria for prioritizing policies and actions for policy makers.
[en] Highlights: • Three novel schemes about integrating solar energy into the boiler are proposed. • Solar-coal hybrid system under fuel saving mode and power boosting mode are studied. • Sankey diagram is used to analyze the exergy destruction of key components. - Abstract: A Solar Tower Aided Coal-fired Power (STACP) system utilizes a solar tower coupled to a conventional coal-fired power system to reduce pollutants, greenhouse gas emissions and the investment of solar energy facilities. This paper examines three different schemes for integrating solar energy into a conventional boiler. For each scheme, an energy and exergy analysis of a 600 MWe supercritical coal-fired power system is combined with 53 MWth of solar energy in both a fuel saving mode and a power boosting mode. The results show that, for all these integration schemes, the boiler’s efficiency and system’s efficiency are reduced. However, the standard coal consumption rate is lower in comparison to conventional power plants and the standard coal consumption rate in the fuel saving mode is lower than that in the power boosting mode for all three schemes. Comprehensively considering both the standard coal consumption rate and efficiency, the scheme that uses solar energy to heat superheat steam and subcooled feed-water is the best integration option. Compared with a coal-fired only system, the saved standard coal consumption rate of the above mentioned scheme in fuel saving mode and power boosting mode can reach up to 11.15 g/kWh and 11.11 g/kWh, respectively. Exergy analysis shows, for STACP system, exergy losses of boiler and solar field contribute over 88% of whole system’s exergy loss.
[en] Highlights: • Afield test was carried out for deep boreholes with 2000 m depth. • The performance of deep borehole ground source heat pump was evaluated. • A 3D numerical model according to geological structure was developed. • The simulation results showed a good agreement with the field test data. • The impacts of inlet velocity, inlet temperature and flow pattern were examined. - Abstract: Deep borehole ground source heat pump (DBGSHP) is a new type of heat pump heating system which extracts deep geothermal energy through heat exchange and can be applied for space heating in winter. To date, the development of deep borehole heat exchangers (BHEs) is limited to the cognized structure design and there is a lack of the experimental studies. This paper presents the investigation of the heat transfer characteristics of the heat exchanger of a DBGSHP heating system through both field test and numerical simulation. A field test was first carried out based on the DBGSHP implemented in a demonstration project. A numerical model was then developed to facilitate the evaluation of the heat extraction capacity and the outlet temperature of the coaxial deep BHEs. Based on the numerical model developed, a sensitivity study was further performed to examine the effect of the primary parameters including the inlet velocity, inlet temperature, flow pattern (one was that the circulating fluid flowed from the inner pipe to the annular space and the other was that the circulating fluid flowed from the annular space to the inner pipe) and pipe diameter on the performance of deep BHE. The results from the field test indicated that the average heat transfer capacity of each single borehole, the average COP of the heat pump unit and the DBGSHP heating system COP were 286.4 kW, 6.4 and 4.6, respectively. The simulation results matched well with the field test data, and showed that the inlet fluid velocity between 0.3 m/s and 0.7 m/s as well as the circulating fluid flowed from the annular space to the inner pipe can result in a better performance for the system of concern. The results from this study could be used as a reference basis for optimal design of coaxial deep BHE and to promote the utilization of deep geothermal energy.