•Outdoor air relative humidity affects frost accumulation limit temperatures.•Frost growth does not influence the Number of Transfer Units significantly.•Latent heat flux contributed to the local ...frost density should not be neglected.
This paper tackles the issue of frost formation in non-hygroscopic rotary heat exchangers used for energy recovery from exhaust air under high-speed rotor conditions by means of numerical simulations and experimental approaches. On the basis of some idealized assumptions, a frost growth submodel is presented to predict the behavior of the rotary heat exchanger under frosting conditions. Frost formation is modeled by considering the mass diffusion of water vapor through the frost layer, taking into account supersaturation phenomena. Calculations were carried out using a three-zone model based on the modified ε-NTU method. The local heat transfer coefficient and the NTU calculation method resulting from the influence of the heat exchanger entrance region were also applied. The obtained correlations for the temperature effectiveness agree with the simulation data within uncertainty bounds. The results of the numerical simulations allowed us to determine the outdoor air conditions that initiated the frost accumulation phenomenon inside the thermal wheel for two values of return air relative humidity: RH2i=20% and RH2i=40%. In both cases, the threshold temperature for unsafe operating conditions increases with increasing relative humidity of the outdoor air. Under ‘frost accumulation’ operating conditions, the frost growth rate is approximately five times higher at RH2i=40% than at RH2i=20%. In this regard, the need to implement frost protection techniques increases significantly with an increase in relative humidity of return airflow. Further analysis conducted for the operation of a thermal wheel’s operation under frosting conditions revealed that a latent heat flux contributed to the local frost density should not be neglected in a compact heat exchanger’s model. Interestingly, the operation time of the rotary heat exchanger, and hence the growth of the thickness of the frost layer, has a significant influence on a local heat transfer coefficient α, however, it does not affect the Number of Transfer Units (NTU) visibly.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
La Centrale de traitement d’Air par Dessiccation (CAD) offre un contrôle complet de la température et de l'humidité dans les locaux climatisés. Son élément clé est la roue dessicante qui permet la ...dessiccation de l’air et une régénération continue. A travers cette étude, nous nous intéressons au développement d’une méthodologie pour obtenir un modèle dynamique de la roue utilisable dans les algorithmes de contrôle avancés de la CAD. La roue dessicante peut être considérée comme un système de type multi-entrées/multi-sorties (MIMO). La seconde partie de ce mémoire concerne l'identification expérimentale des paramètres des modèles d’état de la roue dessicante pour deux types de modèles : boîte noire et boîte grise. Dans le cas de la boîte noire, tous les paramètres du modèle sont identifiés expérimentalement. Dans le cas de la boîte grise, certains paramètres sont dérivés de considérations physiques et les paramètres restants sont identifiés en utilisant les mesures expérimentales des entrées et des sorties. Les paramètres du modèle boîte grise ont une signification physique. En comparaison avec les modèles boîte noire, les modèles boîte grises sont moins précis sur le domaine sur lequel les paramètres ont été identifiés, mais beaucoup plus précis en dehors de ce domaine. Comme les paramètres ont une signification physique, leurs valeurs ne varient pas de manière significative avec le point de fonctionnement utilisé pour l’identification. Dans l’approche boîte grise, les valeurs des paramètres obtenues pour les modèles linéaires sont presque identiques pour tous les modèles locaux du coté dessiccation et pour tous les modèles locaux du coté régénération ; cela nous a permis de considérer qu’un modèle local est valable pour tout le domaine de variation des variables d’entrée. Le modèle final de la roue dessicante se compose de deux modèles globaux : un pour le côté de la dessiccation et l'autre pour le côté de la régénération. La troisième partie de ce travail consiste dans l'identification des coefficients de transfert de masse et de chaleur au sein de la roue dessicante en utilisant un modèle boîte grise. Le coefficient de transfert de masse, le coefficient de transfert convectif et le nombre de Nusselt ont été obtenus en écrivant les paramètres du modèle d’état en fonction d’une seule variable et en exprimant les paramètres en fonction des caractéristiques géométriques et des propriétés de matériaux de la roue. Ce travail contribue au développement d’un modèle d’état utilisable pour la synthèse des algorithmes de contrôle pour la roue dessicante.
Desiccant Air Unit (DAU) offers a complete control of air temperature and humidity in the conditioned space. Its key component is the desiccant wheel which provides the functions of air desiccation and regeneration. The aim of this study is to develop a methodology for obtaining a dynamic model of the desiccant wheel which can be used for the model-based control algorithms of DAU. The desiccant wheel can be regarded as a multi-input/multi-output (MIMO) system. The first part of the thesis is devoted to the modeling of the desiccant wheel based on energy and mass balance equations. The resulting set of equations is formulated as a second order state-space system without delay. The second part of this thesis concerns the experimental identification of the parameters of the state-space model of the desiccant wheel by using a black-box and a gray-box approach. In the case of the black-box, all the parameters of the model are identified experimentally. The identified parameters have values which minimize the difference between the output of the model and the experimental values. The parameters of the black-box model do not have physical significance. Although precise in the range of variation of the inputs in which the parameters were identified, this model gives significant errors in other domains of variation of the inputs. The parameters of the gray-box model are physically significant. Compared with the black-box models, the gray-box model was less accurate for the domains for which the parameters were identified, but it was notably more robust when applied to other ranges of the inputs. Since the parameters are related to physical properties, their values do not vary significantly with changes of the operating point used for identification. For the gray-box approach, the parameter values obtained for the linear models are almost identical for all local models on the desiccation side and all the local models on the regeneration side, suggesting that a local model may be valid for all the complete range of input variables. Using the above results, a final model of the desiccant wheel was developed, comprising two global models: one for the desiccation side and another for the regeneration side. The third part of the thesis deals with the identification of mass and heat transfer coefficients of the air within the desiccant wheel using a gray-box model. The mass transfer coefficient, the convective heat transfer coefficient and the Nusselt number were obtained by defining the variable parameters of the model as a function of a single variable and by expressing the constant parameters as a function of the geometric and material properties of the wheel. This work contributes to the development of a state-space model used for the synthesis of control algorithms for the desiccant wheel.
•New insight into annual performance and net energy savings of heat wheel in demand-controlled ventilation.•The first study to quantify the most influential design parameters of heat recovery and ...their interaction effects using GSA.•The optimal design of rotary heat recovery maximises the annual net energy saving amount in heat recovery ventilation.•The methods and findings contribute to highly energy-efficient ventilation and hence zero emission buildings.
Energy-efficient building services are necessary to realise zero-emission buildings while maintaining adequate indoor environmental quality. As the share of ventilation heating needs grow in well-insulated and airtight buildings, heat recovery in mechanical ventilation systems is increasingly common. Ventilation heat recovery is one of the most efficient and viable means to reduce ventilation heat losses and save energy. Highly efficient heat exchangers are being developed or applied to maximise the energy-saving potential of heat recovery ventilation. Nevertheless, the effects of practical operating conditions and the constraints of heat recovery – such as variations in ventilation rates, frost protection, and the prevention of an overheated air supply over a long-term period, which may significantly influence realistic recovery rates – have been less considered in efforts to maximise the energy savings. It is unclear which design parameters for heat recovery devices have the greatest effect on the annual energy savings from ventilation.
This study proposes annual efficiency and annual net energy saving models for heat recovery ventilation that consider ventilation rate variations, the longitudinal heat conduction effect and operating controls. We use a global sensitivity analysis to quantify the contributions of various design input parameters to the variation in annual recovery efficiency and annual net energy savings. We identify the most influential parameters and their significant interaction effects for the annual energy performance of heat recovery ventilation. More attention should be paid to these most influential parameters during the design process. Furthermore, the optimal designs for rotary heat exchangers (as identified by a pattern-search optimisation algorithm) can improve annual net energy savings in demand-controlled ventilation by 33–48%, depending on the building areas. In combination with the reference year analysis presented in this study, heat recovery and demand-controlled ventilation can help to meet the need for highly efficient ventilation systems and zero-emission buildings.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•A new construction of rotary heat exchanger with bypass dampers.•Thermal behaviour of rotary heat exchanger with bypass dampers.•The effect of bypass opening on pressure loss of heat ...exchanger.•Energy savings potential of rotary heat exchanger with bypass dampers.•Economic benefit – return on investment.
The article deals with the possibilities of using a bypass damper in a rotary regenerative heat exchanger for heat recovery in ventilation and air conditioning system and calculates the energy savings related to overcoming the pressure loss of the exchanger. For the purposes of the analysis, a prototype heat exchanger with a wheel diameter of 1000 mm and bypass damper with a diameter of 200 mm was designed and constructed. The exchanger was subjected to an experimental investigation in order to determine its behaviour. The pressure loss of the exchanger, depending on the air flow rate, as well as the pressure loss for three nominal air flow rates, depending on the opening of the bypass damper, were measured. The measurement showed that the new exchanger design enables a reduction in the pressure loss of the exchanger by up to 49% compared to the original value. Based on the measured values, an analysis of the potential savings of electrical energy for driving fans in various European cities was carried out. The energy savings depend on the given climatic location where the exchanger will be used and on the operating time of the air handling unit. From the analyses carried out, it is clear that, by including the bypass damper, 18 to 44% of the electrical energy of the fan can be saved, which overcomes the pressure losses of the heat exchanger. Based on the current electricity prices, the return on the investment for the bypass damper was calculated based on the price of electricity at the given time and place (Prague - Czech Republic, 2022), which is from 0.5 to 3 years, depending on the type and period of operation of the air handling unit equipment, at common air velocities.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Today's buildings are becoming more insulated and airtight to reduce transmission heat losses. Energy use for ventilation can represent up to half of these buildings' total energy use. Heat recovery ...in ventilation and demand-controlled ventilation (DCV) are energy-efficient measures to reduce ventilation energy use, especially when combined. However, this study revealed that the often-overlooked longitudinal heat conduction (LHC) in aluminium rotary heat exchangers might yield less efficient heat exchangers, particularly for intended high-efficiency heat recovery at low ventilation rates in DCV. This study presents a theoretical method to assess the effect of LHC on the amount of energy used to heat ventilation air for several ventilation strategies. The method is demonstrated in a case study for a virtual office building in a cold climate (Oslo, Norway). When neglecting the LHC effect, the energy used to heat the supplied air using DCV with a rotary heat exchanger is about three times smaller than when considering LHC. Unlike earlier studies, we find that DCV may consume more ventilation heating energy than constant air volume (CAV) ventilation when the selected wheel is deep and oversized due to LHC. This study highlights the need to design rotary heat exchangers carefully in order to account for the LHC effect, particularly when targeting zero emission buildings (ZEB).
•Understanding the role of longitudinal heat conduction of heat wheel in ventilation.•A deep and oversized rotary heat exchanger yields degraded energy performance.•New insights into energy inefficient combination of heat recovery and DCV.•Contribute to closing the energy performance gap and facilitating zero emission.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This paper presents exergo-economic and enviro-economic assessment of a novel building integrated photovoltaic thermal-compound sensible rotary heat exchanger (BIPVT-SRHX) system. The innovative ...BIPVT-SRHX system preheats/precools the outdoor air in winter/summer and generates electric power. The performance of the system are analyzed from the energy/exergy viewpoints for Kermanshah, Iran climatic conditions. Then, the multi-objective genetic algorithm (MOGA) optimization is used to optimize to determine the optimum values of geometric and operating parameters in order to maximize the annual average exergo-economic and enviro-economic aspects of the system. The considered geometric and operating parameters include the length, width and depth of the air channel located underneath the PV modules, the air mass flow rate, and the diameter, rotational velocity and length of the SRHX. Moreover, the annual performance of the optimized and non-optimized BIPVT-SRHX systems are compared. The results showed that the annual average exergo-economic and enviro-economic aspects of the optimized BIPVT-SRHX system are 0.0076 $/annum and 246.9 kWh/$, respectively. Furthermore, it was found that the annual average enviro-economic aspect, annual average exergo-economic aspect, and yearly sum of CO2 mitigation of the optimized BIPVT-SRHX system are respectively 36.8%, 23.1% and 37.7% higher than the non-optimized system.
•An innovative exhaust air energy recovery system consisting of a BIPVT and a SRHX.•Pre-heating/pre-cooling ambient air in winter/summer and producing electricity.•Genetic algorithm-based multi-objective optimization approach for BIPVT-SRHX system.•Optimization leads to a 36.8% increase in annual average enviroeconomic parameter.•Optimization leads to a 23.1% increase in annual average exergoeconomic parameter.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•For single-room ventilation, a novel rotary heat exchanger uses a low-cost plastic honeycomb.•Thermal design theory predicts 84% temperature efficiency for 8.2L/s with 6% bypass leakage.•Measured ...and modelled temperature efficiencies agree for 8L/s and 13L/s and disagree for 4L/s.•All leakages are slightly excessive and should be reduced with proper sealing.
The existing building stock will likely undergo widespread energy renovations to meet future emissions targets. Single-room ventilation may enable the process due to its simple installation, low fan power, and potential for local heat recovery. A short plastic rotary heat exchanger is developed for single-room ventilation based on thermal design theory. Performance is predicted from correlations of dimensionless groups for regenerative heat exchangers, and this guides the selection of a polycarbonate honeycomb with small circular channels. Experiments quantify flows and determine temperature efficiencies at several ventilation rates while accounting for heat gains from motors and air leakage. The measured and modelled temperature efficiencies show adequate agreement and exceed 80% for a balanced nominal ventilation rate of 28m3/h. This result meets the development criteria but cannot validate the model due to the presence of unmeasurable bypass leakage. All leakages are slightly excessive and should be reduced with proper sealing. Experimental results demonstrate the potential to reduce heat recovery by slowing rotational speed, which is required to prevent frost accumulation. Overall, the development meets objectives and provides a novel and efficient option for ventilation heat recovery.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Summary
In this study, a multicriteria optimization is developed for a hybrid building integrated photovoltaic/thermal (BIPVT) system with a sensible rotary heat exchanger (SRHX) for both heating and ...cooling purposes. The BIPVT system is considered with and without considering the glass cover. Both the heat and electricity produced by the BIPVT‐SRHX system are determined according to the energy and exergy phenomena. The objective functions aimed to be maximized are the annual total amount of generated energy and exergy. The length and depth of SRHX, geometrical parameters of PV modules, and the air mass flow rate are the parameters considered in the optimization. The results reveal a high amount of produced energy for the optimized design; however, the total amount of generated exergy is low. The system with the glass cover shows a higher amount of gained heat and less generated electricity compared with the system without the glass cover. The total energy produced by the system with and without the glass cover is about 227 and 220 MWh, respectively, and the total produced exergy by the systems with and without the glass cover is 1207 and 1585 kWh for the optimized design, respectively.
The effect of setting a glass cover on the photovoltaic panels belonging to a building integrated photovoltaic thermal system on the energy and exergy performances of a hybrid system consisting of a building integrated photovoltaic/thermal system and a sensible rotary heat exchanger is investigated numerically. The outcomes are compared with those of the hybrid system incorporating an uncovered building integrated photovoltaic/thermal system. For this purpose, the multi‐objective optimization is used to determine the optimal energy and exergy performances of the covered and uncovered hybrid systems, then the performances of optimal units are compared for climate of Kermanshah, Iran. Glass cover has a considerable effect on the energy and exergy performances of hybrid system, and when designing the hybrid system, its impact must definitely be considered.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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•Pressure differences across the façade negatively affect room ventilation units.•Units with axial fans are more sensitive compared to units with centrifugal fans.•The resulting ...imbalance between supply and exhaust is higher at lower fan speeds.•A humidity-controlled damper can be used in the intake of a natural ventilation shaft.•This can mitigate the pressure differences across the façade due to stack effect.
Room ventilation units (RVUs) with heat recovery represent an alternative ventilation solution for renovated residential buildings, allowing simple installation through the façade. However, wind pressure on the façade can have a substantial impact on fan performance and thus affect the airflows through the RVU. Airflow imbalances between supply and exhaust negatively affect total heat recovery and supply air temperatures. Moreover, RVUs installed in apartments on the ground floor of multi-story buildings with a natural ventilation shaft are highly affected by negative pressures due to stack effect during winter periods. The issues that characterize these units were examined in this study in relation to the fan type and fan operation, aiming to establish why the performance is varying for different units. The study simulated the performance of RVUs in a typical Danish test apartment under various wind pressure conditions. To investigate the combined impacts of wind and stack effect, the simulations assumed the test apartment being located on the ground floor of a 4-storey building with an 11-meter natural ventilation shaft. The authors performed dynamic annual simulations with different fan characteristic curves and fan operating points for centrifugal and axial fans. Simulations tested the additional impact of adding a humidity-controlled damper in the intake of the natural ventilation shaft to reduce pressure differences across the building envelope due to stack effect. Results showed that centrifugal fans were less sensitive to pressure differences than axial fans due to their steeper fan curves and generally higher operating pressures. When taking into account wind pressure and disregarding stack effect, the sensible heat recovery decreased by up to 8% in RVUs with centrifugal fans and up to 22% in RVUs with axial fans at the same airflow settings. After including stack effect, the decrease in sensible heat recovery was instead up to 20% for RVUs with centrifugal fans and up to 52% for RVUs with axial fans. The inclusion of a humidity-controlled damper reduced the pressure differences due to stack effect and halved the negative impact on sensible heat recovery on average. As a result, the RVUs were able to provide considerably higher supply air temperatures. The study showed that a humidity-controlled damper could reduce the negative impact of stack effect on the performance of an RVU.
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