Kai Sirén

Approximate analytical model for solidification in a finite PCM storage with internal fins

Abstract In latent heat storage internal heat transfer enhancement techniques such as fins have to be used because of the low heat conductivity of the phase change material. Internal heat transfer enhancement is essential, especially in a solidification process where the main heat transfer mode is conduction. The aim of this paper is to present a simplified analytical model which predicts the solid–liquid interface location and temperature distribution of the fin in the solidification process with a constant end-wall temperature in the finned two-dimensional PCM storage. The analytical results are compared to numerical results and they show that the analytical model is more suited to the prediction of the solid–liquid interface location than the temperature distribution of the fin in the PCM storage. A new factor called the fraction of solidified PCM is also introduced. The fraction of solidified PCM easily gives a good picture of how much of the storage is solidified after a given time.

Performance investigation of plain and finned tube evaporatively cooled heat exchangers

The performance of two evaporatively cooled heat exchangers is investigated under similar operating conditions of air flow rates and inlet hot water temperatures. The heat exchangers are plain and plate-finned circular tube types which occupy the same volume. Spray water, which is circulated in a closed circuit, is injected onto the exposed surfaces of the tubes and fins. The contact between air and spray water results in evaporative heat transfer. The tubes are copper, 10 mm o.d. The finned configuration is constructed by introducing 0.5 mm thick copper plates between the tubes, with a total area ratio of four. A substantial increase in heat transfer takes place for the plate-finned tubes. The increase is 92–140% for air velocities from 1.66 to 3.57 m s � 1 . A model is used to calculate the thermal performance of the plain and finned tubes assuming a constant spray water temperature in the heat exchanger. The wet-finned surfaces show low fin efficiency compared with dry surfaces. An energy index defined as the ratio of volumetric thermal conductance to air pressure drop per unit length is found to be close for the two heat exchangers. This reveals higher thermal utilisation of the occupied volume by the finned tubes with the same energy index. 2002 Elsevier Science Ltd. All rights reserved.

Theoretical and computational analysis of closed wet cooling towers and its applications in cooling of buildings

Theoretical analysis and computational modelling of closed wet cooling towers (CWCTs) are presented. Experimental measurements for performance of a prototype tower are used to define the tower transfer coefficients. Tower flow rates and number of tubes and rows are optimised for the required cooling load and to achieve a high coefficient of performance (COP). The performance of a cooling system used to cool office buildings is simulated using the transient system (TRNSYS) simulation environment. The cooling system consisted of a CWCT, chilled ceilings, pumps and a fan. A control strategy, including night cooling, is introduced in the simulation. The results indicate efficient performance and high COP values for the system.

Technical dimensioning of a vertically upwards-blowing air curtain - part II

The technical dimensioning of an air curtain often rests on an insufficient basis when it comes to the influence of the building envelope and the ventilation system. The air curtain has a vital connection to both these elements and this has to be taken into account properly in the dimensioning procedure. The goal of the work was to bring new perspectives to such questions as well as to provide certain practical tools for the dimensioning process. This first part of the paper is focusing on simple fluid mechanical approaches. Several dimensioning methods based on the momentum balance are presented. The performance of the methods is compared. The breakthrough phenomenon of the jet is analysed and classified into three types. The factors determining each type are quantified. Equations for the breakthrough momentum flux are derived. The breakthrough ratio is introduced and its connection to the jet discharge angle demonstrated. The tightness, the vertical leakage distribution and the height of the building all have an influence on the dimensioning of jet parameters. The ventilation and process air flows should be in balance at all times to achieve working conditions for the air curtain. The most advanced dimensioning method is based on the moment-of-momentum principle enabling consideration of the jet impact point. Small discharge angles leading to a high jet momentum flux as well as a tight building envelop provide more tolerance against the wind-induced breakthrough.

The shadow band correction for diffuse irradiation based on a two-component sky radiance model

Abstract When diffuse radiation is measured with a pyranometer and the access of beam radiation to the measuring instrument is obstructed by using a shadow band, a band correction has to be made to obtain the correct result of measurement. The magnitude of the correction depends on the size and location of the shadow band and the sky radiance distribution. A two-component sky radiance model can be utilized when making the band correction. Measurements indicate that the band correction based on the isotropic approximation of the radiance distribution leads to an underestimation of the diffuse irradiation. By using the radiance model, this systematic error can be eliminated. In measurements of high quality, the effect of the radiation reflected from the inside of the band to the pyranometer should also be taken into account.

A new approach using the Pierce two-node model for different body parts

This paper presents a new approach, in applying the Pierce two-node model, to predict local skin temperatures of individual body parts with good accuracy. In this study, local skin temperature measurements at 24 sites on the bodies of 11 human subjects were carried out in a controlled environment under three different indoor conditions (i.e. neutral, warm and cold). The neutral condition measurements were used to adjust the local skin set-points in the model for each body part. Additional modifications to the calculation algorithm were introduced corresponding to different body parts. The local core set-points were then calculated, using a line search method, as the input values that allow the model to predict the skin temperatures with maximum deviation of ±0.1°C for the neutral condition. The model predictability was verified for the other two indoor conditions, and the results show that the modified model predicts local skin temperatures with average deviation of ±0.3°C.

A thermal manikin with human thermoregulatory control: Implementation and validation

Tens of different sorts of thermal manikins are employed worldwide, mainly in the evaluation of clothing thermal insulation and thermal environments. They are regulated thermally using simplified control modes. This paper reports on the implementation and validation of a new thermoregulatory control mode for thermal manikins. The new control mode is based on a multi-segmental Pierce (MSP) model. In this study, the MSP control mode was implemented, using the LabVIEW platform, onto the control system of the thermal manikin ‘Therminator’. The MSP mode was then used to estimate the segmental equivalent temperature (teq) along with constant surface temperature (CST) mode under two asymmetric thermal conditions. Furthermore, subjective tests under the same two conditions were carried out using 17 human subjects. The estimated segmental teq from the experiments with the two modes and from the subjective assessment were compared in order to validate the use of the MSP mode for the estimation of teq. The results showed that the teq values estimated by the MSP mode were closer to the subjective mean votes under the two test conditions for most body segments and compared favourably with values estimated by the CST mode.

Influence of energy demand response actions on thermal comfort and energy cost in electrically heated residential houses

This study has two aims to investigate the energy demand response (DR) actions on thermal comfort and energy cost in detached residential houses (1960, 2010 and passive) in a cold climate. The first one is to find out the acceptable range of indoor air and operative temperatures complying with the recommended thermal comfort categories in accordance with the EN 15251 standard. The second one is to minimize the energy cost of electric heating system by means of the DR control strategy, without sacrificing thermal comfort of the occupants. This research was carried out with the validated dynamic building simulation tool IDA Indoor Climate and Energy. Three different control strategies were studied: A) a strategy based on real-time hourly electricity price, B) new DR control strategy based on previous hourly electricity prices and C) new predictive DR control strategy based on future hourly electricity prices. The results show that the lowest acceptable indoor air and operative temperatures can be reduced to 19.4℃ and 19.6℃, respectively. The maximum annual saving in total energy cost is about 10% by using the control algorithm C.

A multi-aid optimization scheme for large-scale investigation of cost-optimality and energy performance of buildings

According to the European Energy Performance of Buildings Directive (EPBD-2010/31/EU), all EU-Member states are obliged to continuously apply analysis on cost-optimal levels of minimum energy performance requirements towards nearly/net zero energy buildings. To perform such techno-economic analysis, a large number of technical/financial assumptions should be covered and possibly billions of design/operation options should be explored. This is computationally expensive. This study introduces a novel multi-aid optimization scheme (MAOS) for supporting robust cost-optimal decisions on energy-performance levels of buildings. The schemes feature is reduction of the computational cost by avoiding time-consuming simulations through the use of post-processing and/or simplified models (when possible), while holistic optimization is adopted for considering multivariate interactions between possible design/operation options and financial/technical assumptions. The effectiveness of MAOS is demonstrated by optimizing a single-family house under 108-financial scenarios, where more than 1.610 solutions would be possible. The results show significant (∼95%) time reduction compared with those of the usual simulation-based optimization approach.

The protection ability of the building shell against sudden outdoor air contamination

Abstract A sudden contamination of the outdoor air by some toxic gas can have several causes. To find out the protection afforded by sheltering indoors was the primary goal of the investigation. The object of the computational approach was a single family house with two floors. Three different models were utilized to calculate the infiltration air flows, the contaminant transport inside the building and the temperature decay of the building. The variation in the weather parameters was treated using the two-dimensional distribution of the outdoor air temperature and wind speed and a statistical approach. The results show the cumulative distribution functions of the relative doses inside the building for different tightness levels and exposure times.