Comparison of the energy conversion efficiency of a solar chimney and a solar PV-powered fan for ventilation applications

A study into the performance of a solar chimney and a solar photovoltaic (PV)-powered fan for ventilation applications was carried out using numerical simulations. The performance of the solar chimney was compared with that of a direct current (DC) fan powered by a solar PV panel. The comparison was carried out using the same area of the irradiated surface—the area of the solar absorber plate in the case of the solar chimney and the area of the solar panel in the case of the photovoltaic-powered fan. The two studied cases were compared under various solar radiation intensities of incident solar radiation. The results indicate that the PV-powered fans significantly outperform solar chimneys in terms of converting solar energy into the kinetic energy of air motion. Moreover, ventilation with PV-powered fans offers more flexibility in the arrangement of the ventilation system and also better control of the air flow rates in the case of battery storag

Utilization of an Air-PCM Heat Exchanger in Passive Cooling of Buildings: A Simulation Study on the Energy Saving Potential in Different European Climates

The energy saving potential (ESP) of passive cooling of buildings with the use of an air-PCM heat exchanger (cold storage unit) was investigated through numerical simulations. One of the goals of the study was to identify the phase change temperature of a PCM that would provide the highest energy saving potential under the specific climate and operating conditions. The considered air-PCM heat exchanger contained 100 aluminum panels filled with a PCM. The PCM had a thermal storage capacity of 200 kJ/kg in the phase change temperature range of 4 °C. The air-PCM heat exchanger was used to cool down the outdoor air supplied to a building during the day, and the heat accumulated in the PCM was rejected to the outdoors at night. The simulations were conducted for 16 locations in Europe with the investigated time period from 1 May–30 September. The outdoor temperature set point of 20 °C was used for the utilization of stored cold. In the case of the location with the highest ESP, the scenarios with the temperature set point and without the set point (which provides maximum theoretical ESP) were compared under various air flow rates. The average utilization rate of the heat of fusion did not exceed 50% in any of the investigated scenarios

Minimum Weight Triangulation (MWT)

For a long time, it has been neither known whether MWT is solvable in a polynomial time nor whether it belongs to NP. As we now, its status still remain unknown. We present severalknown approaches to MWT such as modifications of the problem with known time complexity or various heuristics and approximations which allow us to find an exact or at least an approximate solution in a reasonable time. We compare the approximations in some particular situations. The main part of the work is devoted to a description and implementation of an efficient heuristic with (almost?) linear expected complexity for points uniformly distributed in some convex shape. The algorithm is a modification of Drysdale's algorithm for finding GT candidates and Beurouti's computation of the modified LMT-skeleton, where we add some proofs of the correctness. We are able to complete MWT from the graph of candidate edges in O(n · d3 + n · d2+k), where d is the the maximum degree and k is the maximum number of inner components of some skeleton face. Further, we suggest a new approximation of MWT with polynomial complexity in the worst case and (almost?) linear expected complexity, which only rarely differs from the optimal triangulation and has O(1) approximation factor in the worst case. This approximation combines the LMT-skeleton..

Two-stage stochastic programming approach to a PDE-constrained steel production problem with the moving interface

summary:The paper is concerned with a parallel implementation of the progressive hedging algorithm (PHA) which is applicable for the solution of stochastic optimization problems. We utilized the Message Passing Interface (MPI) and the General Algebraic Modelling System (GAMS) to concurrently solve the scenario-related subproblems in parallel manner. The standalone application combining the PHA, MPI, and GAMS was programmed in C++. The created software was successfully applied to a steel production problem which is considered by means of the two-stage stochastic PDE-constrained program with a random failure. The numerical heat transfer model for the steel production was derived with the use of the control volume method and the phase changes were taken into account with the use of the effective heat capacity. Numerical experiments demonstrate that parallel computing facility has enabled a significant reduction of computational time. The quality of the stochastic solution was evaluated and discussed. The developed system seems computationally effective and sufficiently robust which makes it applicable in other applications as well

Seasonal COP of an Air-to-Water Heat Pump when Using Predictive Control Preferring Power Production from Renewable Sources in the Czech Republic

The paper presents a parametric study evaluating the effects of various predictive controls on the operating parameters of heat pumps. The heat pump represents a significant power appliance in the residential sector. Its connection to the heat accumulator creates a system with considerable potential to control electricity consumption according to the needs of the electricity grid. The air-water heat pump is considered in this study. A predictive control is used for priority operation of the heat pump at periods of peak power production from renewable sources. The following were tested as the parameters of predictive control: outdoor air temperature, photovoltaic power production and wind power production. The combination of photovoltaic and wind power production was also tested. A parametric analysis considering different sizes for the thermal accumulator and the heating capacity of the heat pump were proposed. The benefits of predictive control are evaluated based on historical records of meteorological data from 2015 to 2018 in the city of Brno, Czech Republic. The data on the historical development of the real electrical energy production from renewable sources in the Czech Republic are used for regulation control in a monitored period. The main comparison parameter is the heat pump seasonal coefficient of performance (SCOP). From the carried out study results, an increase in SCOP by 14% was identified for priority operation of heat pump (HP ) at periods with highest outdoor air temperature. Priority operation of HP at periods with peak photovoltaic (PV) production increased SCOP by 10.25%. A decrease in SCOP only occurred in case with priority operation of HP at peak production of wind power plants. Increasing the size of the accumulator contributes to an increase in SCOP in all assessed modifications of predictive control

Visual monitoring of the melting front propagation in a paraffin-based PCM

Experiments were carried out in an environmental chamber with the aim to monitor the melting front propagation in a rectangular cavity filled with a paraffin-based Phase Change Material (PCM). The PCM was contained in transparent containers with the heat flux introduced by means of an electric heating element. The stabilized power source was used to maintain the constant heat output of the heating elements. The experiments were performed for the heat flux introduced at the side wall of the container and at the upper surface of the PCM. The paraffin-based PCM RT28HC with the phase change temperature of 28 °C was used in the experiments. The temperature in the environmental chamber was maintained at the melting temperature of the PCM. The propagation of the melting front was monitored with a digital camera and temperatures at several locations were monitored with RTDs and thermocouples. Significant natural convection was observed for the heat flux introduced at the side wall of the container. As a result the melting front propagated much faster at the top of the container than at its bottom. The heat flux introduced at the upper-surface of the PCM resulted in almost one-dimensional propagation of the melting front. The acquired data are to be used for validation of an in-house developed numerical model based on the front-tracking method