Heat Loss Factor Research Articles

Experimental study of the energy and exergy performance for a pressurized volumetric solar receiver

This article presents an experimental investigation of the heat transfer characteristics as well as energy and exergy performance for a pressurized volumetric solar receiver under variable mass flow rate conditions. During a two-hour period of continuous operation in the morning, the solar irradiance is relatively stable and maintained at approximately 600W/m2, which is beneficial for analyzing the energy and exergy performance of the solar receiver. Experimental results show that the mass flow rate fluctuation has insignificant effect on the solar receiver outlet temperature, whereas the mass flow rate plays an important role in the solar receiver power, energy efficiency and exergy efficiency. The efficiency of the solar receiver is normally above 55% with the highest efficiency of 87%, and under steady state operating conditions the efficiency is maintained at approximately 60%. A very low value of the heat loss factor (0.014kW/K) could be achieved during the current steady state operating conditions. The highest exergy efficiency is approximately 36%. In addition, as the temperature difference increases, the impact of the exergy factor increases. The highest exergy factor is 0.41 during the entire test.

Electricity-heat response of energetic materials

ABSTRACTTransient pulse testing (TPT) is a kind of non-destructive testing technique that can be used to obtain the electro-thermal response of initiators by applying a considerable low current to them. During the testing progress, it is important to ensure that the mining detonators do not function. Thus, it is desirable to know the current-induced temperature increment of the bridgewire. In order to obtain the temperature increment of the bridgewire in the TPT, the simulation of this test is described in this paper. Based on the heat transfer theory, a three-dimensional model is built on the assumption that there is no chemical reaction. Considering the initial conditions and the boundary conditions, the influence of different heat loss factors, heat capacities of bridgewire and currents to the result is studied through the usage of software FlexPDE. Theoretical results are represented in figures. Those results show that this new model is better than model Rosenthal when considering the temperature distribution along the bridgewire.

A single-variate building energy signature approach for periods with substantial solar gain

The use of regression analysis for the identification of building performance parameters based on measurements is often difficult due to collinearity between the outdoor temperature and the global solar radiation (S). This study proposes a method to overcome this issue. The proposed method is based on using the seasonal symmetry of S to pair data from time-periods equidistant from the winter solstice. In addition, a method to utilize synthetic data to fine-tune the paired-data approach is presented. To evaluate the paired-data approach, two years data from a multifamily building in Umeå was used to estimate the heat loss factor (air-to-air transmission including air leakage). The results were compared with results obtained when S was very low (S≈0). It was found that, the fine-tuned paired-data approach resulted in a modest deviation in the heat loss factor with an average absolute deviation of 4.0%. The small deviation indicates that the paired-data approach can extend the use of single-variate regression models for accurate identification of heat loss factors to situations where the solar gain is substantial. The paired-data approach was also used to calibrate a commercial energy building simulation tool.

Adaptive thermal comfort opportunities for dwellings: Providing thermal comfort only when and where needed in dwellings in the Netherlands

Adaptive thermal comfort opportunities for dwellings: Providing thermal comfort only when and where needed in dwellings in the Netherlands

木造住宅の簡易な現場実測による熱損失係数推定に関する研究

木造住宅の簡易な現場実測による熱損失係数推定に関する研究

Experimental study on a coiled tube solar receiver under variable solar radiation condition

This article presents an experimental investigation of the heat transfer characteristics as well as energy and exergy performance for a coil type solar dish receiver under variable solar radiation conditions. During a 4.5h period of continuous operation in the morning, the solar irradiance fluctuates in the range between 160W/m2 and 650W/m2, and the concentrated solar flux is approximately 1000kW/m2 at aperture. Experimental results show that: (1) the efficiency of the solar receiver is found to be over 58% and the highest efficiency can be reached to 64%. Generally, the efficiency is maintained at around 60% under steady state condition. (2) A very low value of the heat loss factor (0.035kW/K) could be achieved during steady state operation. (3) The highest value of the exergy rate is approximately 7.3kW, whereas the maximum energy rate can reach 16.2kW. (4) The highest exergy efficiency is approximately 23%, whereas the highest energy efficiency is around 64%.

Numerical study of flat plate solar collector with novel heat collecting components

In this paper, a novel design of heat collecting component for a flat plate solar collector is presented and a numerical study on this solar collector collection efficiency by computational fluid dynamics (CFD) method is conducted. The new heat collecting component consists of a corrugated upward surface and a flat downward surface. The significant influences of the tilt of solar collector, the mass flow rate of inlet water and the distance of air gap on the solar collector collection efficiency are investigated by CFD-simulation. Simulation results reveal that the optimum values of the three factors are the tilt of 10° for April to September, 30° to 35° for the rest months (in Guangzhou of China), the mass flow rate of inlet water 0.15kg/s and the distance of air gap 20mm, respectively. Finally, the maximum instantaneous efficiency of 85.1% and the total heat loss factor of 3.127 are obtained by fitting the instantaneous efficiency curve of this solar collector.

Bonding Performance of Adhesives with Lamina in Structural Glulam Manufactured by High Frequency Heating System

The bonding performance of two types of wood adhesives, namely phenol-resorcinol-formaldehyde (PRF) resin and melamine-urea-formaldehyde (MUF) resin for glued laminated timber manufactured by high frequency (HF) heating was evaluated. The HF heating system consists of HF oscillator with dielectric heating system for curing adhesives, and hydraulic press system for clamping glued laminated timber. The designed frequency and output power of the HF system was as 5 MHz and 60 kW, respectively. To verify dielectric heating mechanism under HF oscillation, the heat loss factors of laminae and adhesives were measured. The results show that it is possible to selectively heat adhesives for their curing due to the remarkably higher loss factor of the adhesives than those of wood laminae. The temperature of adhesive in the bonding line reached up to the set temperature within a few seconds by high frequency oscillating, which advanced the curing of adhesive afterwards. The bonding performance, such as shear strength of bonding line, water soaking delamination, and boiling water soaking delamination of PRF resin met the requirement of Korean Standard (KS), however the MUF resin did not meet the KS requirement of boiling water soaking delamination. These results indicate that the HF heating system is successful to manufacture glued laminated timbers with PRF resins to meet the bonding requirements.

Experimental investigation on the energy and exergy performance of a coiled tube solar receiver

In this article, an experimental investigation is carried out to examine the heat transfer characteristics of a coil type solar dish receiver under actual concentrate solar radiation conditions. During the test, the concentrated solar flux is approximately 1000kW/m2 at aperture. The solar irradiance is almost unchanged (650W/m2) for continuous two hours in the afternoon, which is used to analyze the energy and exergy performance of the solar receiver. Experimental results show that, the efficiency of the solar receiver is normally above 70% with the highest efficiency of 82%, whereas at steady state, the efficiency is maintained at around 80%. A very low value of the heat loss factor (0.02kW/K) could be achieved during the current steady state operating conditions. The highest value of the exergy rate is around 8.8kW, whereas the maximum energy rate can reach 21.3kW. In addition, the highest exergy efficiency is approximately 28%, and the highest energy efficiency is around 82%.

The Effect of Measurement Uncertainty and Environment on Domestic Solar Water Heating Systems’ Energy Efficiency Grades

Abstract In China, two important national standards were implemented on Aug 1st, 2012, one is ‘Specification of domestic solar water heating systems’ (GB/T 19141-2011), the other is ‘Minimum allowable values of energy efficiency and energy efficiency grades for domestic solar water heating systems’ (GB 26969-2011). According to these two standards, the energy efficiency grades indicator CTP (coefficient of thermal performance) should be tested and calculated by daily useful energy and average heat loss factor of domestic solar heating system. Through experiment and calculation, the effect of measurement uncertainty and daily average ambient temperature on the migration of domestic solar water heating systems’ (DSWHS) energy efficiency grades was discussed. The calculation and analysis methods may be helpful for data consistency co-verfication between different inspection organizations.

Research on Annual Thermal Performance of Solar Water Heating Balcony System

A project has been conducted on an indirect natural circulation solar water heating system which is hung on a simulated balcony wall in Lianyungang City in the east of China. In the solar water heating system, the flat plate collector has an inclined surface of 70°. Tests and analysis have been carried out to obtain solar heat gain of the system and the heat loss coefficient of the storage tank under sunny weather conditions in 2013. The results show that annual solar heat gain of the system is mainly affected by the irradiance and the ambient temperature, and the storage water temperature drops less when the ambient temperature rises, but the average heat loss factor is nearly unchanged when the ambient temperature changes. In addition, there is almost no influence on the heat gain of the system if there is a reverse slope in the circulation pipeline.

Data Measurement and Analysis of a Solar Heating System with Seasonal Storage

The technologies for seasonal thermal energy storage are of great significance in balancing the energy utilization and in enabling the renewable sources, especially solar energy, to be more widely used. The solar heating system with seasonal storage discussed in this paper is located in LuoYang City (Approx. E112’, N34.5’), in HeNan Province of China. The working data of the whole system, including temperatures of different parts, water flow, thermal energy, and solar irradiation, etc. are measured continuously and recorded using the technology of internet remote data transmission. The efficiency of the solar heating system is considered to be good when operating in general weather conditions, for example, 45% on sunny days. The average heat loss factor of the heat storage tank is only 0.50W/(K·m3),the ratio between surface and volume is about1.08 1/m.

Experimental energy and exergy performance of a solar receiver for a domestic parabolic dish concentrator for teaching purposes

An experimental setup to investigate the thermal performance of a cylindrical cavity receiver for an SK-14 parabolic dish concentrator is presented in this technical note. The thermal performance is evaluated using energy and exergy analyses. The receiver exergy rates and efficiencies are found to be appreciably smaller than the receiver energy rates and efficiencies. The exergy factor parameter is also proposed for quantifying the thermal performance. The exergy factor is found to be high under conditions of high solar radiation and under high operating temperatures. The heat loss factor of the receiver is determined to be around 4.6W/K. An optical efficiency of around 52% for parabolic dish system is determined under high solar radiation conditions. This experimental setup can be used as teaching tool for people with little or no knowledge about solar dish concentrators due its simplicity and the basic mathematical formulations applied. Different types of receivers and different types of deep focal region parabolic dishes can also be tested with the experimental setup.

Integrated Thermoelectric–Thermal System Resistance Optimization to Maximize Power Output in Thermoelectric Energy Recovery Systems

ABSTRACTThermoelectric energy recovery is an important technology for recovering waste thermal energy in high-temperature industrial, transportation and military energy systems. Thermoelectric (TE) power systems in these applications require high performance hot-side and cold-side heat exchangers to provide the critical temperature differential and transfer the required thermal energy to create the power output. Hot-side and cold-side heat exchanger performance is typically characterized by hot-side and cold-side thermal resistances, Rh,th and Rc,th, respectively. Heat exchanger performance determines the hot-side temperature, Th, and cold-side temperature, Tc, conditions when operating in energy recovery environments with available temperature differentials characterized by exhaust temperatures, Texh, and ambient temperature, Tamb. This work analytically defined a crucially important design relationship between (P/Pmax) and (Rh,th / Rc,th) in TE power generation systems to determine the optimum ratio of (Rh,th / Rc,th) maximizing TE system power. A sophisticated integrated TE device / heat exchanger analysis was used, which simultaneously integrates hot- and cold-side heat exchanger models with TE device optimization models incorporating temperature-dependent TE material properties for p-type and n-type materials, thermal and electrical contact resistances, and hot side and cold side heat loss factors. This work examined the (P/Pmax) - (Rh,th / Rc,th) relationship for system designs employing single-material and segmented-material TE couple legs with various TE material combinations, including bismuth telluride alloys, skutterudite compounds, and skutterudite / bismuth telluride segmented combinations. This work defined the non-dimensional functional relationships and found the optimum thermal resistance condition: (Rh,th / Rc,th)opt > 10 to 30 created the maximum power output in TE optimized designs for various TE material combinations investigated. The non-dimensional relationships were investigated for various electrical contact resistances, differing thermal loss factors, and at various hot-side/cold-side temperature conditions. This work showed that the non-dimensional functional relationships were invariant under these differing conditions. It was determined that a condition of (Rh,th / Rc,th) = 1 creates power output far below maximum power conditions. The (P/Pmax) - (Rh,th / Rc,th) relationship also dictated certain temperature profile conditions, defined by the parameter, (Th – Tc) / (Texh – Tamb), which were directly associated with design points in this relationship including maximum power points. The value of (Th – Tc) / (Texh – Tamb) was generally less than 0.5 at maximum power conditions in TE energy recovery designs using TE materials investigated here. The wide-ranging ramifications on TE energy recovery systems and their design optimization for industrial and transportation-related applications are discussed.

Integrated Thermoelectric–Thermal System Resistance Optimization to Maximize Power Output in Thermoelectric Energy Recovery Systems– CORRIGENDUM

Thermoelectric energy recovery is an important technology for recovering waste thermal energy in high-temperature industrial, transportation and military energy systems. Thermoelectric (TE) power systems in these applications require high performance hot-side and cold-side heat exchangers to provide the critical temperature differential and transfer the required thermal energy to create the power output. Hot-side and cold-side heat exchanger performance is typically characterized by hot-side and cold-side thermal resistances, Rh,th and Rc,th, respectively. Heat exchanger performance determines the hot-side temperature, Th, and cold-side temperature, Tc, conditions when operating in energy recovery environments with available temperature differentials characterized by exhaust temperatures, Texh, and ambient temperature, Tamb. This work analytically defined a crucially important design relationship between (P/Pmax) and (Rh,th / Rc,th) in TE power generation systems to determine the optimum ratio of (Rh,th / Rc,th) maximizing TE system power. A sophisticated integrated TE device / heat exchanger analysis was used, which simultaneously integrates hot- and cold-side heat exchanger models with TE device optimization models incorporating temperature-dependent TE material properties for p-type and n-type materials, thermal and electrical contact resistances, and hot side and cold side heat loss factors. This work examined the (P/Pmax) - (Rh,th / Rc,th) relationship for system designs employing single-material and segmented-material TE couple legs with various TE material combinations, including bismuth telluride alloys, skutterudite compounds, and skutterudite / bismuth telluride segmented combinations. This work defined the non-dimensional functional relationships and found the optimum thermal resistance condition: (Rh,th / Rc,th)opt > 10 to 30 created the maximum power output in TE optimized designs for various TE material combinations investigated. The non-dimensional relationships were investigated for various electrical contact resistances, differing thermal loss factors, and at various hot-side/cold-side temperature conditions. This work showed that the non-dimensional functional relationships were invariant under these differing conditions. It was determined that a condition of (Rh,th / Rc,th) = 1 creates power output far below maximum power conditions. The (P/Pmax) - (Rh,th / Rc,th) relationship also dictated certain temperature profile conditions, defined by the parameter, (Th – Tc) / (Texh – Tamb), which were directly associated with design points in this relationship including maximum power points. The value of (Th – Tc) / (Texh – Tamb) was generally less than 0.5 at maximum power conditions in TE energy recovery designs using TE materials investigated here. The wide-ranging ramifications on TE energy recovery systems and their design optimization for industrial and transportation-related applications are discussed.

Experimental and simulated thermal stratification evaluation of an oil storage tank subjected to heat losses during charging

Experimental and simulated quantitative thermal stratification evaluations of a small un-insulated oil storage tank subjected to heat losses during charging are presented. A model for simulating the thermal profile in the storage is developed. Simulated results using the model are found to be in reasonably good agreement with experimental results. The thermal gradients along the height of the storage are evaluated as function of the charging time. The thermal gradients are also evaluated as a function of the difference between the average temperature of the storage tank and the ambient surrounding temperature which signifies heat losses. The thermal gradient is seen to rise to a maximum value at some instance of time and then start to drop as thermal de-stratification in the storage tank starts. Parametric results using the model are also presented. The effect of the ambient temperature and the heat loss factor on the thermal gradient of the storage tank is investigated. Lower ambient temperatures and lower heat loss factors are found to increase the thermal gradient of the storage tank during the period when thermal stratification is increasing.

Performance experiments for flat plate solar water heater based on micro heat pipe array

A novel flat plate solar water heater based on the micro heat pipe array (MHPA) was invented, and the experiments were carried out on the MHPA heat collector and water heater to test their thermal performance. Owing to its distinctive structure, the MHPA has good heat transfer ability, high reliability, high compressive strength and low cost. The structure and character of the novel collector with MHPA as the high-efficiency heat transfer element are as follows. First, the contact surface between the absorber and the MHPA is so large that the heat resistance is smaller than the traditional one, thus its thermal transfer capability is greatly improved. Second, the condenser section of the MHPA is connected closely to the heat exchanger with dry type. The heat pipes do not contact with water, which precludes scaling and leakage in the collector. Third, the MHPA uses low freezing points of the refrigerants, which makes the collector more suitable for extremely cold areas. Fourth, the MHPA is made of aluminum instead of copper, which could reduce the fabrication cost significantly. Fifth, the unique heat exchanger could exclude water easily, which could prevent the frozen cracking of the collector in winter. Consequently, compared with other conventional evacuated glass tube solar water heaters, the novel flat plate solar water heater offers the additional benefits of high compressive strength, light weight, high reliability and durability, easy integration into buildings, and resistance to freezing during the winter months. Compared with traditional sheet-and-tube structure flat plate solar water heaters, this novel solar water heater also demonstrates many advantages: lower cost, absence of welding, resistance to freezing. The experiments focused on the novel collector's instantaneous efficiency. The test results for the collector's instantaneous efficiency show that the slope and intercept of the instantaneous efficiency curve are 4.7 and 0.80 respectively, which are 11.0% and 22.3% superior to the technical data of the Chinese national standard. The high maximum instantaneous efficiency can be attributed to high transmittance of the glass cover and good absorption of the absorber, as well as great performance of micro heat pipe array. Due to the excellent isothermal characteristic of the MHPA, the absorber have uniform temperature field so that the heat loss by natural convection in the collector is reduced, thus the novel flat plate solar collector have relatively low heat loss factor. The low emissivity of absorber film also reduces the radioactive heat transfer between the absorber and glass cover. Finally, a detailed experimental study focusing on the thermal performance for the novel flat plate solar water heater based on the MHPA is conducted during daytime over several days. The test results show that the effective heat gain per contour aperture area for the novel flat plate solar water heater are higher than that of the Chinese national standard, and all of the average daily thermal efficiency are higher than 60%. Therefore, the flat plate solar water heater based on the MHPA has the broad application prospect because of its excellent performance.

Experimental and numerical study of furniture fires in an ISO-room for use in a fire forecasting framework

The proposed method of forecasting a single-room fire growth based on inverse modelling relies upon the assimilation of temperature measurements at the doorway level from floor to soffit height. These measurements are processed in the context of a two-zone model to obtain transient profiles of neutral plane height, XN, and upper layer temperature, Tu. An additional correlation is used to deduce the smoke layer height, h, within the fire room. The obtained profiles of Tu, XN and h over a given period of time (the ‘assimilation window’) are used to estimate the fire growth factor, α, the time delay, t0, and the heat loss factor, λc, with a two-zone inverse modelling procedure. Instantaneous displays of the future fire development are then delivered. Experimental data of four furniture fires in an ISO-room are analysed to illustrate the proposed forecasting methodology, which provides positive lead times between 20 and 235 s, depending on the fire growth rate.

An experiment to evaluate the thermal performance of an oil-heating copper spiral coil

An experimental setup to evaluate the thermal performance of an oil-heating copper spiral coil heat exchanger is presented. The thermal performance is evaluated in terms of energy and exergy analyses. Results obtained while increasing the flow-rate of the fluid through the coil show an increased heat transfer rate, resulting in a reduction in the temperature difference between the outlet and inlet temperatures of the coil. An increase in the flow-rate is found to increase the rate of heat gained by the oil for a limited range of the flow-rate since heat losses are also increased. The average rate of heat gained by the oil is found to be around 350 W, while the average thermal efficiency of the coil is found to be around 0.25. The rate of exergy delivery and exergy factor results show that lower flow-rates result in a greater rate for the exergy delivered to the oil during the initial stages of the experiments. The heat loss factor is found to increase with an increase in the flow-rate. Due to the simplicity of the experimental setup, it can be used to introduce experimentally the concepts of energetic and exergetic performance of a thermal process or device to senior undergraduate students taking courses in applied thermal physics or applied thermodynamics.

A Simplified Mathematical Model for Predicting the Vertical Temperature Profiles in Enclosure Fires Without Vertical Opening

This paper presents a mathematical model to predict the instantaneous temperature profiles in sealed or ceiling vented compartment fires. It has been observed in the existing research that in compartments without vertical opening, smoke fills the volume very soon indicating that the so-called one-zone type distribution forms quickly, and the gas temperature inclines linearly with height above the fire source. These characteristics are different from the smoke filling properties in enclosure fires with vertical openings. An assumption of linear distribution for temperature was introduced and a modified one-zone model was subsequently proposed in order to predict the transient smoke temperature profiles after the smoke fills the enclosure. With the knowledge of the heat release rate, the prediction model was established based on unsteady energy conservation by changing the heat loss factor using the semi-empirical models for fire plume and ceiling jet. Experiments including sealed and ceiling vented conditions were conducted to validate the model and the comparisons between measurements and predictions suggested the model can give fairly satisfactory estimations for the transient temperature profiles for both tests.