Schiavon, Stefano (2008) Energy saving with personalized ventilation and cooling fan. [Tesi di dottorato]
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Indoor environmental quality substantially influences health, comfort and productivity. The cost related to a poor indoor environment is high. Numerous field studies have documented substantial rates of dissatisfaction with the indoor environment in many buildings, therefore an increment of the actual indoor environmental quality is necessary. Global warming of the climate system is now unequivocal and it has had a discernible influence on many physical and biological systems, therefore, it is needed to reduce the greenhouse gases emission. On this challenge, an important role is played by the building sector. Technological solutions able to improve the indoor environment and to reduce the energy consumption simultaneously should be developed.
In warm environments elevated air movement is a widely used strategy for cooling of occupants. Increasing the air movement let the opportunity to set the maximum permissible room temperatures to higher values. According to many authors this solution leads to substantial energy savings. In the present international indoor climate standards a relationship is present between the air speed and the allowed increment in operative temperature. The air movement increase can be produced by several devices as cooling fans (ceiling, floor standing, tower and table fans) or Personalized Ventilation (PV) systems.
The cooling fans ability to cool the human body is limited because they operate under isothermal conditions. Cooling fans may save energy but they do not improve the indoor environmental quality. Appearance, power consumption and price are the main parameters considered when purchasing cooling fans while their cooling capacity and efficiency of energy use are unknown. Comparison of the performance of cooling fans regarding cooling capacity and energy consumption is important for their application in practice.
The personalized ventilation is an individually controlled micro-environmental system that provides clean air close to occupants. Numerous studies show that PV in comparison with traditional mechanical ventilation system may improve health, inhaled air quality, thermal comfort, and self-estimated productivity and it may decrease the risk of airborne transmission of infectious agents. Little is known about its energy consumption. Personalized ventilation systems have better performance than cooling fans with regard to thermal comfort since they may operate under non-isothermal conditions, i.e. the supplied air can be cooled below the room air temperature in addition to increased velocity. The PV system affects the pollutant concentration and the thermal conditions mostly in the microenvironment at the workstation. Therefore, occupant’s exposure to pollutant and his/her thermal comfort depend on the ratio of time occupant stays at the workstation over total time he/she stays in the room.
The main objectives of the present work were to study, by means of computer simulation, the energy saving when providing occupants with thermal comfort with increased air movement at elevated room temperature, the energy consumption of a personalized ventilation system and energy saving strategies which can be used to control a PV system, and to develop and to test in laboratory, an index for evaluating the cooling fan efficiency. An additional objective of the study was to develop and to test an index for assessing the air quality improvements in rooms with non-homogeneous contaminant distributions (e.g. with personalized ventilation) taking into account the occupant location pattern.
The potential saving of cooling energy by elevated air speed, which can offset the impact of increased room air temperature on occupants’ thermal comfort, was quantified by means of simulations using EnergyPlus software. Fifty-four cases covering six cities (Helsinki, Berlin, Bordeaux, Rome, Jerusalem, Athens), three indoor environment categories and three air velocities (<0.2, 0.5 and 0.8 m/s) were simulated. Cooling energy savings in the range of 17-48% and a reduction of the maximum cooling power in the range 10-28% have been obtained. The results reveal that the required power input of the fan is a critical factor for achieving energy saving at elevated room temperature. Under the assumptions of this work, energy saving may not be achieved with the methods for air speed increase, such as ceiling, standing, tower and desk fans widely used today when the power consumption of the fan is higher than 20 W. From the results of the simulation it can be deduced that knowing the cooling capacity and the energy consumption of the fan is important.
A new index has been developed, named “cooling fan efficiency index”, defined as the ratio between the cooling effect generated by a fan and its power consumption. The cooling effect is calculated as the difference of manikin-based equivalent temperature measured with and without the fan in operation. The cooling fan efficiency can be a useful index for comparison performance of fans, for costumers, fan designers/manufacturers, policymakers, HVAC designers and building managers. The index was determined experimentally for a ceiling fan, a desk fan, a standing fan and a tower fan in a real office at three room air temperatures and at different fan velocity levels. The results revealed that the index is sensitive enough to identify differences in the performance of the cooling devices. The cooling fan efficiency index of the four fans differed substantially. The whole-body cooling effect and the local cooling effect for body segments caused by the fans also differed and were strongly non-uniform. The desk fan had a significantly (p-value<0.01) higher efficiency than the other three fans tested. A standard method for measuring the cooling fan efficiency index should be developed.
The cooling fans generate a non-uniform velocity field around occupants which cannot be described with a single value. This makes the recommendation in the standards for elevated velocity in warm environments difficult to use in practice. The present thermal comfort standards need to be revised to better address the issue of thermal comfort in warm environments.
The energy consumption of a PV system installed in a high quality standard Scandinavian building located in a cold climate have been studied by means of simulations with IDA-ICE software. An optimization algorithm was used to determine the optimal supply air temperature. The effectiveness of the following energy saving strategies have been studied: reducing the outdoor airflow rate due to the higher ventilation effectiveness of PV, expanding the room temperature comfort limits by taking advantage of PV’s ability to create a controlled thermal microenvironment and supplying the personalized air only when the occupant is present at the desk. The results showed that the control strategy of the supplied personalized air temperature has a marked influence on energy consumption. The energy consumption with personalized ventilation may increase substantially (between 60% and 270%) compared to mixing ventilation alone if energy-saving strategies are not applied. Among the studied energy-saving strategies the most effective way of saving energy is to increase the maximum permissible room temperature (saving up to 60% compared to the mixing ventilation may be achieved) but it can be applied only in offices where occupants spend most of their time at the desk. Reducing the airflow rate does not always imply a reduction of energy consumption because the outdoor air may have a free cooling effect. Supplying the personalized air only when the occupant is at the desk is not an effective energy-saving strategy. The best supply air temperature control strategy is to provide air constantly at 20°C, i.e. the minimum permissible supply temperature.
A further index has been developed in this work, named “occupant normalized concentration”, which makes it possible to assess more realistically occupant’s exposure in a room characterized by a non-uniform pollution distribution. The index can be used to evaluate the average pollutant exposure as function of the pollutant distribution in a space and of the occupant activity, and it can be used to compare and quantify the variation in terms of inhaled pollution by occupant in a room with PV in conjunction with a total-volume ventilation system. The results of the application of the index to data collected during full-scale room measurements showed that it can be used at the design stage for assessment the benefit of PV when applied in practice for office buildings with different occupation patterns. It is demonstrated that displacement ventilation alone was able to provide the occupant with better inhaled air quality than displacement ventilation in conjunction with PV when the occupant stay less than 50% of the office time at the desk. These analyses are performed under steady state conditions, i.e. without disturbance of the displacement pattern due to occupants’ walking.
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