Optimal Configuration of Power-to-Cool Technology in District Cooling Systems

In a multienergy framework, power-to-heat technology is becoming increasingly attractive. This interest is mainly due to the possibility of exploiting excesses and unbalances of electricity, which are becoming more and more common with the increasing capacity of the renewable sources. An interesting option consists in using heat pumps to convert excess of electricity produced by photovoltaic systems (especially in the midday hours) into cold to be provided to district heating and district cooling networks. This article aims to propose a methodology to select the best heat pump location in district cooling system. The analysis is performed with the aim of minimizing the cost of network construction and pumping. The procedure includes the best heat pump location and the design of the pipeline. Results show that distributed heat pumps allow one reducing both the costs and the average pipeline diameters by about 50% with respect to concentrated production. Furthermore, the optimal location of distributed heat pumps allows reducing costs of about 7% with respect to a uniformly distributed production

Compact model of latent heat thermal storage for its integration in multi-energy systems

Nowadays, flexibility through energy storage constitutes a key feature for the optimal management of energy systems. Concerning thermal energy, Latent Heat Thermal Storage (LHTS) units are characterized by a significantly higher energy density with respect to sensible storage systems. For this reason, they represent an interesting solution where limited space is available. Nevertheless, their market development is limited by engineering issues and, most importantly, by scarce knowledge about LHTS integration in existing energy systems. This study presents a new modeling approach to quickly characterize the dynamic behavior of an LHTS unit. The thermal power released or absorbed by a LHTS module is expressed only as a function of the current and the initial state of charge. The proposed model allows simulating even partial charge and discharge processes. Results are fairly accurate when compared to a 2D finite volume model, although the computational effort is considerably lower. Summarizing, the proposed model could be used to investigate optimal LHTS control strategies at the system level. In this paper, two relevant case studies are presented: (a) the reduction of the morning thermal power peak in District Heating systems; and (b) the optimal energy supply schedule in multi-energy systems

Analytical approach for entropy generation and heat transfer in CNT-nanofluid dynamics through a ciliated porous medium

The transportation of biological and industrial nanofluids by natural propulsion like cilia movement and self-generated contraction-relaxation of flexible walls has significant applications in numerous emerging technologies. Inspired by multi-disciplinary progress and innovation in this direction, a thermo-fluid mechanical model is proposed to study the entropy generation and convective heat transfer of nanofluids fabricated by the dispersion of single-wall carbon nanotubes (SWCNT) nanoparticles in water as the base fluid. The regime studied comprises heat transfer and steady, viscous, incompressible flow, induced by metachronal wave propulsion due to beating cilia, through a cylindrical tube containing a sparse (i.e. high permeability) homogenous porous medium. The flow is of the creeping type and is restricted under the low Reynolds number and long wavelength approximations. Slip effects at the wall are incorporated and the generalized Darcy drag-force model is utilized to mimic porous media effects. Cilia boundary conditions for velocity components are employed to determine analytical solutions to the resulting non-dimensionalized boundary value problem. The influence of pertinent physical parameters on temperature, axial velocity, pressure rise and pressure gradient, entropy generation function, Bejan number and stream-line distributions are computed numerically. A comparative study between SWCNT nanofluids and pure water is also computed. The computations demonstrate that axial flow is accelerated with increasing slip parameter and Darcy number and is greater for SWCNT- nanofluids than for pure water. Furthermore the size of the bolus for SWCNT-nanofluids is larger than that of the pure water. The study is applicable in designing and fabricating nanoscale and microfluidics devices, artificial cilia and biomimetic micro-pump

Second law optimization of Y-shape fins for the solidification process in a PCM based storage system

The aim of this paper is to perform a thermodynamic optimization of a Y shaped fin design used to improve thermal performance of a cylindrical latent heat thermal energy storage (LHTES) unit. The investigation is performed by means of a CFD model that takes into account of the thermal behavior of the system. Temperature and phase fields are obtained to characterize the heat transfer phenomenon and to compute entropy generation rate within the system. Global entropy generation and energy flux are then adopted as objective functions in order to perform a shape optimization of the Y shaped fins with angles and branch lengths that can vary freely. The optimization results indicate that higher energy transfer is achieved by fin configuration with long secondary branch with an orientation angle of 30°; this design allows to increase PCM solidification rate of about 30% with respect to radial fins. Furthermore that Y-shaped fins allow also to increase exergy flux released by the PCM thus Second-law efficiency is not affected although entropy generation increases. In the authors’ knowledge this work represents a first detailed thermodynamic optimization of a system involving an unsteady process. This aspect is particularly important since a clear tendency of many energy systems is toward transient operation, thus design optimization methods should evolve accordingly

PUMPING COST MINIMIZATION IN AN EXISTING DISTRICT HEATING NETWORK

District heating is expected to significantly contribute to the reduction of primary energy needs for heating in urban areas. This result is obtained through use of such as CHP systems, residual heat from industries or waste-to-energy plants, as well as the integration of renewable energies. The pumping system plays a crucial role and may significantly affect its performances. In this paper a large district heating system is considered. Various operating conditions corresponding with partial load operation are analyzed through a thermo-fluid dynamic model of the network. For each condition, the optimal set point of the various pumps is obtained. The set of optimal operating conditions is finally used to obtain a control strategy for the network. Results show that with respect to conventional control strategy significant reductions in primary energy consumption can be achieved

Building Efficiency Models and the Optimization of the District Heating Network for Low-Carbon Transition Cities

Nowadays, greenhouse gas emissions continue to increase with the consequent climate changes. Energy consumption of buildings strongly affects atmospheric pollution, therefore for a sustainable development it is necessary to adopt energy efficiency policies combined with low-carbon technologies. In particular, the use of district heating (DH) has environmental and economic advantages in energy production and distribution for space heating consumption. In this paper, the combined effect of DH expansion with different buildings retrofit scenarios using a GIS-based model is proposed for a more sustainable city. This methodology is applied to the DH network of the city of Torino and, energy savings hypotheses were analyzed, evaluating different energy saving trends starting from the current one with existing policies. A GIS-based methodology has been developed with bottom-up and top-down approaches; then two future energy savings scenarios have been hypothesized. Energy retrofit measures have been applied to the most critical areas with low potential of heat distribution; in a second phase, to the whole area connected to the DH network. The results showed that intervening in the critical areas only +5% of potential buildings can be connected to the existing DH network (standard retrofit) while this percentage could grow up to +25% with advanced buildings retrofit. On the other hand, intervening on the whole city, there is a considerable reduction of consumptions and the connectable quota of buildings to the DH network reaches +42% with standard retrofit and +82% with advanced retrofit scenario with an optimization of energy distribution as well

Towards future infrastructures for sustainable multi-energy systems: A review

Integration of different energy infrastructures (heat, electricity and gas vectors) offers great potential for better managing energy sources, reducing consumption and waste as well as enabling a higher share of renewables, lower environmental impact and lower costs. This paper aims at reviewing the state-of-the-art energy system infrastructures in order to provide a comprehensive overview of technologies, operational strategies, modelling aspects and the trends towards integration of heat, electricity and gas infrastructures. Various technological domains are taken into account, ranging from energy distribution networks (thermal, electric and gas), components for the energy vector conversion (e.g. combined heat and power, power to heat, power to gas, etc.) and energy storage. Furthermore, the aspects related to smart management in energy systems are investigated, such as integration of renewable energy sources and energy recovery systems