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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] France is aiming to reduce the share of nuclear power in the country's electricity mix, and increase the share of wind and solar power. However, given the intermittency of these renewable energy sources, massive storage systems are required when their share exceeds a certain threshold. That generates additional costs on top of the subsidies already granted to these energies. This article attempts to quantify that additional cost, which varies between 6.3 Euros and 31.6 billion Euros depending on various scenarios. In fact, the cost of renewables storage varies according to our calculations between 108.3 Euros and 251.4 Euros per MWh. That additional cost could be reduced if storage yields were improved. One of the lessons to be drawn from this analysis is that it would be preferable to finance R and D in storage technologies rather than using costly feed-in tariffs or feed-in premiums to finance an increase in renewables. The greatest challenge lies with respect to inter-seasonal storage rather than very short-term battery-based storage. The prospects for power-to-gas and gas-to-power are also promising, but have yet to be proven in practice. (authors)
[en] In simulation of fluid injection in fractured geothermal reservoirs, the characteristics of the physical processes are severely affected by the local occurence of connected fractures. To resolve these structurally dominated processes, there is a need to develop discretization strategies that also limit computational effort. In this paper, we present an upscaling methodology for geothermal heat transport with fractures represented explicitly in the computational grid. The heat transport is modeled by an advection-conduction equation for the temperature, and solved on a highly irregular coarse grid that preserves the fracture heterogeneity. The upscaling is based on different strategies for the advective term and the conductive term. The coarse scale advective term is constructed from sums of fine scale fluxes, whereas the coarse scale conductive term is constructed based on numerically computed basis functions. The method naturally incorporates the coupling between solution variables in the matrix and in the fractures, respectively, via the discretization. In this way, explicit transfer terms that couple fracture and matrix solution variables are avoided. Numerical results show that the upscaling methodology performs well, in particular for large upscaling ratios, and that it is applicable also to highly complex fracture networks.
[en] The issue of power quality is largely absent from discussions on energy, including those related to multi-annual energy programming, where the focus remains on security of supply, admittedly of critical importance. Clearly, blackouts or power outages, such as those that crippled Paris' Montparnasse railway station and its surrounds in late July 2018, tend to make the headlines. And a deep dive into this subject rapidly leads to complex considerations on the physics of electricity and network codes that the non-specialist may find daunting. Nevertheless, the growing dependence of our economies on digital technology and the emergence of intermittent renewable energies in the electricity mix are creating new challenges, with the question of power quality being a prime example. (authors)
[en] Biodiversity is threatened by the expansion of human activities. In this context, the nuclear industry, which is essentially concentrated in terms of material flows and industrial sites, has a number of advantages over wind and solar energy: much lower consumption of land and raw materials, and the ability to recycle. In de-carbonised energy scenarios, these differences argue in favour of maintaining a significant part of nuclear power rather than 100% renewable energy production
[fr]La biodiversite est menacee par l'expansion des activites humaines. Dans ce contexte, la filiere nucleaire, concentree par essence en termes de flux de matieres et de sites industriels, presente quelques avantages en comparaison des energies eolienne et solaire: consommations bien plus faibles de surfaces et de matieres premieres, aptitude au recyclage. Dans les scenarios d'energie decarbonee, ces differences plaident pour le maintien d'une part significative de nucleaire plutot qu'une production assuree a 100 % par les energies renouvelables
[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] The past fifty years have witnessed the advent of a new energy source and the beginning of yet another in the series of energy-use transitions that have marked our history since the start of our technological development. Each of these transitions has been accompanied by adaptive challenges. Each unique set of challenges has been met. Today the world faces the need for another transition. This paper outlines some of the associated challenges that lie ahead of us all, as we adapt to this new and exciting environment. The first step in defining the challenges ahead is to make some form of prediction of the future energy supply and demand during the period. Herein, the future up to 2010 is presumed to include two major events – first, a decline in the availability and a rise in price of petroleum, and second a need to reduce greenhouse gases in our atmosphere. Both of these events are taken to be imminent. Added to these expected events is the assumption that the total of wind, solar, and other such energy sources will be able to contribute, but only in a relatively small way, to the provision of needed energy to our ever-expanding human population. (author)
[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] Large electricity systems require flexibility to both balance consumption and production, and to manage flows on the grid. Renewable energies increase the need for flexibility in both areas. New technologies such as batteries can address those needs, often in competition with other solutions and at times enjoying a particular advantage due to their speed and ease of location. This article provides an overview of flexibility in the electricity system, and an in-depth analysis of the flexibility required for forecast balancing. (authors)