Thermal performance assessment of a cylindrical box solar cooker fitted with decahedron outer reflector

One of the important issues humankind globally faces in recent years is the scarcity of non-renewable energy resources. Solar energy is considered safe and renewable, which can fulfil the demand and supply chain requirements. Solar box cookers (SBCs) are popular in domestic cooking due to their ease of use and handling. The prime objective of the present work is to develop and test the performance of a cylindrical SBC fitted with decahedron-shaped reflector (CSBC-FDR). The CSBC is designed using minimum entropy generation (MEG) method. Through experiments, we observed that absorber plate attains peak temperature of about 138°C–150°C with the aid of decahedron reflector. The first figure of merit (F1) is found to be 0.13, indicating better optical efficiency and low heat loss coefficient for the SBC. The second figure of merit (F2) is obtained as 0.39, which indicates good heat exchange efficiency (F') and less heat capacity for cooker's interior. The average energy efficiency, exergy efficiency, and standardized cooking power values are 21.93%, 3.04%, and 25.28W, respectively. These results show that the present CSBC-FDR is able to cook food in a shorter period with better efficiency. The experimental and numerical values of overall heat loss coefficient of the developed SBC are in close agreement. The experimentally assessed performance parameters reveal superior performance of the present cylindrical SBC in comparison with many conventional rectangular and trapezoidal box solar cookers.

Thermal performance of a low and medium temperature flat plate solar collector when controlling the output-input temperature difference and the tilt angle

This work presents a sensitivity analysis of the overall heat loss coefficient UL and the thermal efficiency η in low and medium temperature encapsulated flat plate solar collectors when controlling the output-input temperature difference ΔT and the angle of inclination β. The UL and η were determined using heat flow calorimetry at indoor conditions, emulating the solar radiation by the Joule effect and a PID control. The angle of inclination β range was 0-90°, and the ΔT range was 5.0-25.0 K. The ambient temperature and the mass flow rate were preset for each test. The UL experimental uncertainty was ±0.85 W/m2K for the inclination range of 0-45° and ±0.27 W/m2K for the inclination range of 45-90°. The results matched previous outcomes with a difference of up to 0.3 W/m2K. The UL behaved exponentially as β increased from horizontal to vertical position and linearly with ΔT. It was also observed that the UL and the efficiency were sensitive to the confined airflow variations. This model shows a sensitivity of low and medium temperature flat plate solar collectors, as the efficiency increased 140% when β was raised and 40% with ΔT.

Influence of Initial and Boundary Conditions on the Accuracy of the QUB Method to Determine the Overall Heat Loss Coefficient of a Building

The quick U-building (QUB) method is used to measure the overall heat loss coefficient of buildings during one to two nights by applying heating power and by measuring the indoor and the outdoor temperatures. In this paper, the numerical model of a real house, previously validated on experimental data, is used to conduct several numerical QUB experiments. The results show that, to some extent, the accuracy of QUB method depends on the boundary conditions (solar radiation), initial conditions (initial power and temperature distribution in the walls) and on the design of QUB experiment (heating power and duration). QUB method shows robustness to variation in the value of the overall heat loss coefficient for which the experiment was designed and in the variation of optimum power for the QUB experiments. The variations in the QUB method results are smaller on cloudy than on sunny days, the error being reduced from about 10% to about 7%. A correction is proposed for the solar radiation absorbed by the wall that contributes to the evolution of air temperature during the heating phase.

Improving Building Energy Efficiency through Measurement of Building Physics Properties Using Dynamic Heating Tests

Buildings contribute to nearly 30% of global carbon dioxide emissions, making a significant impact on climate change. Despite advanced design methods, such as those based on dynamic simulation tools, a significant discrepancy exists between designed and actual performance. This so-called performance gap occurs as a result of many factors, including the discrepancies between theoretical properties of building materials and properties of the same materials in buildings in use, reflected in the physics properties of the entire building. There are several different ways in which building physics properties and the underlying properties of materials can be established: a co-heating test, which measures the overall heat loss coefficient of the building; a dynamic heating test, which, in addition to the overall heat loss coefficient, also measures the effective thermal capacitance and the time constant of the building; and a simulation of the dynamic heating test with a calibrated simulation model, which establishes the same three properties in a non-disruptive way in comparison with the actual physical tests. This article introduces a method of measuring building physics properties through actual and simulated dynamic heating tests. It gives insights into the properties of building materials in use and it documents significant discrepancies between theoretical and measured properties. It introduces a quality assurance method for building construction and retrofit projects, and it explains the application of results on energy efficiency improvements in building design and control. It calls for re-examination of material properties data and for increased safety margins in order to make significant improvements in building energy efficiency.

Pengaruh Overall Heat Loss Coefficient Terhadap Hasil Output solar still

Penelitian ini bertujuan untuk mengetahui pengaruh koefisien kehilangan panas keseluruhan terhadap hasil output yang terjadi pada solar still. Penelitian ini menggunakan material stainless still tebal 1,6 mm dan kaca penutup tebal 3mm dengan kemiringan terhadap alat solar still 30 ̊. Pengujian dilakukan mulai pukul 08.00 WIB sampai 17.00 WIB selama 3 hari, dengan beberapa parameter yang di ukur seperti suhu kaca bawah (Tgi), suhu air (Tw), kecepatan angin (v), intensitas radiasi matahari (I(t)s) yang terdapat dalam sistem alat solar still. Dari hasil pengujian yang dilakukan overall heat loss coefficient tertinggi yaitu sebesar 50,7 W/m2.K.. Semakin tinggi coefficient top heat loss sangat mempengaruhi coefficient heat loss overall sehingga hasil output tidak mengalami kenaikan.

Heat Removal Factor in Flat Plate Solar Collectors: Indoor Test Method

This paper presents a couple of methods to evaluate the heat removal factor FR of flat plate solar collectors, as well as a parametric study of the FR against the tilt angle β, and (Ti − Ta)/G, and its effects on the a0-factor (FRτα) and the a1-factor (FRULmin). The proposed methods were based on indoor flow calorimetry. The first method considers the ratio of the actual useful heat to the maximum useful heat. The second takes into account the slopes of the family of efficiency curves (FRULmin) according to ANSI/ASHRAE 93-2010, and the minimum overall heat loss coefficient, ULmin. In both methods, a feedback temperature control at collector inclinations from horizontal to vertical allows the inlet temperature and the emulating of the solar radiation to be established by electrical heating. The performance of the methods was determined in terms of the uncertainty of the FR. Method 1 allowed a three-fold improved precision compared to Method 2; however, this implied a more detailed experimental setup. According to the first method, the effects of the tilt angle β, and the (Ti − Ta)/G, on the a0-factor were considerable, since FR is directly proportional to the a0-factor. The changes in (Ti − Ta)/G caused an average change in FR of 32% The FR shows almost linear behavior for inclinations from horizontal to vertical with a 14.5% change. The effects of β on the a1-factor were not considerable, due to the compensation between the increase in FR and the decrease in ULmin as β increased.

Consideration of Operative Temperature in Design and Operation

In order to provide appropriate thermal conditions current national regulations prescribe operative temperature as the base of design and operation. In simplified calculation procedure prescribed operative temperature can be provided using a corrected air temperature. Interrelation of operative and indoor air temperature has been investigated in function of overall heat loss coefficient and glazed ratio. Based on regression analysis necessary corrections in function of the above parameters are investigated, the consequences of neglected Mean Radiant Temperature are analysed. Operative temperature represents a control problem, too: disregarding the sensor itself its position in the room, the uneven distribution of radiant field in one room and in the rooms of a flat requires compromises. The possible solutions, their pros and cons are presented.

Performance Testing of a Parabolic Solar Concentrator for Solar Cooking

The objective of the present work is to conduct a worthwhile experimental study of the performance of a parabolic solar concentrator for solar cooking. The literature survey briefly highlights the standard performance tests of solar cookers and gives the experimental studies obtained by some authors. Our experimental device, made from simple means using local materials, consists of a parabolic concentrator having a 0.80 m diameter and 0.08 m depth as well as a cylindrical absorber with a 0.10 m diameter and is 0.20 m long. The testing period started on April 24th, 2014 and continued till July 10th of the same year, in Rabat (33°53′ N, 6°59′ W), Morocco. The average ambient temperature is 24 °C. The results show that using synthetic oil as the heat transfer medium has achieved a maximum temperature of 153 °C against 97 °C with water. The overall heat loss coefficient is estimated to be 17.6 W m−2 °C−1. The energy and exergy efficiencies are, respectively, 29.0–2.4% and 0.1–0.5%. Adding a glass cover on the front face of the absorber improved the maximum temperature by 15 °C. Automatic two-axis sun tracking system also increased the maximum temperature by 13 °C compared to manual tracking system.