New methodology and perspective for the air flow and thermal … · 2018-01-07 · 4th International Conference On Building Energy, Environment New methodology and perspective for

4th International Conference On Building Energy, Environment

New methodology and perspective for the air flow and thermal comfort characterization of personalized devices

A. Casalegno1, E. Monticelli2, M. Perino1 and F. Prosperi2

1Department of Energy – DENERG, Politecnico di Torino, C.so Duca degli Abruzzi n° 24, 10129, Torino, Italy 2Comfort R&D and Platform Innovation, De'Longhi Appliances, via L.Seitz, 47 - 31100 - Treviso - Italy

SUMMARY Personal Comfort Systems (PCS) are appliances available on the market and widely used since long time. Nevertheless, they were always considered just as a secondary equipment to be used for backing up or supporting centralized heating/cooling systems. In this paper a review of the increasing role that such devices may play in the future Zero Energy Buildings will be given. Moreover, the limitation of the typical current design procedures will be highlighted, providing suggestions on how the thermal comfort of the users should guide the conception and development of PCSs. Finally, a case study will be presented, where the current theories about the thermal comfort have been modified in order to establish a rationale for analyzing the performance of a personalized device for thermal comfort.

INTRODUCTION The pursuit of the energy efficiency in buildings is leading towards a future where the buildings will be ZEB or NZEB. This will have a number of consequences in the way the built environment is designed and operated. Indeed the first and most significant result will be a reduction of the energy consumption and of the environmental impact. Nevertheless, along with these benefits a likely change will also occur on the share of energy sources that will be used to satisfy the energy demand and on the type and structure of the mechanical systems adopted to provide a satisfactory indoor thermal comfort. On the one hand, the thermal loads that the HVAC systems will have to cope with will become quite small. On the other hand, the energy required to feed the mechanical systems (being small) could be covered, for a large quota, by means of a local and distributed PV production. Such features suggest that in a near future the use of large, centralized HVAC systems will not be so profitable, especially in case of residential buildings. Highly flexible and small personalized devices for thermal comfort, in fact, could control the indoor environment in a more efficient way. In order to better understand such statement, let’s consider a typical residential unit having a size of 10 m x 10 m x 3 m (height) with a window - to - wall ratio of 0.125 and let’s assume that the envelope components and the ventilation systems would satisfy the mandatory requirements set by the laws in effect at the time of construction. Under these hypotheses the design heat load in Torino (heating design temperature, Thd = - 8 °C, heating degree days HHD = 2894 °Cday) decreased during the time with the trend shown in figure 1. It switched from about 19 kW for buildings built in the fifties to less than 4.5 kW for the new constructions (present days regulations). Making a projection to the 2020 and beyond, and assuming a likely evolution of the laws enforcing the energy efficiency in buildings (that will also comprise

interventions on the heat recovery from the exhaust air ventilation) the expected design heat load will yet decrease to a mere 1 - 1.5 kW or less. From a technological point of view this means that the size of the installations will become smaller and smaller. For example, if today to heat a room having a size of 3.5 m by 4.5 m a heat radiator consisting of about 8 elements is required, for a NZEB just 3 elements are needed and after the year 2020 the size will further reduce to less than 2 elements. Besides, it has to be considered that new and more efficient energy conversion systems are strongly penetrating the market. If the classical hot water boiler has been, for decades, the typical heat generator for residential building, today reversible heat pumps, frequently coupled to ground heat exchanger or well water, are becoming a popular solution. As a consequence the energy demand of buildings is satisfied by a share of energy vectors that is also rapidly evolving and moving towards a wider use of electric energy. This, in turn, fosters an ever growing installation of PV systems at the building scale, to locally produce a significant quota of the energy demand. In addition, it has to be considered that centralized mechanical installations are designed to provide thermal environment as uniform as possible. However, as highlighted by Watanabe et al. (2010), “large individual differences in physiological and psychological responses, clothing insulation, activity, air temperature preference and air movement exist between individuals”. Therefore, a wider occupants satisfaction could be achieved only by using technological solutions that give the persons the possibility to adjust their local microenvironment based on the own subjective preferences.

Figure 1. Time profile of the design heat load (Torino – Italy).

The possibility to control, to a certain extent, the local heating/cooling of the body not only increases the likelihood

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of fulfilling the expectancies of people as far as the thermal environment is concerned, but it also provides a further benefit. In fact, as demonstrated by Bauman et al. (1998), the satisfaction of people about their living space rises, independently of the thermohygrometric conditions, when they can interactively “play” with the environmental control. This picture gives rise to a fundamental question: in the future do we still need to design and install rather large centralized heating systems to assure a proper thermal comfort of occupants? The answer is “probably no”, especially in case of residential buildings, where the low thermal loads could be controlled in a more efficient way by means of smaller and more flexible devices directly located into the rooms. Such systems allow for a personal control of the microclimatic conditions and have been named in various ways. They were formerly called ‘”Task Ambient Conditioning” (TAC), making an analogy with task-ambient lighting, which has been generally understood for a long time. A rather comprehensive and rational classification was made by Zhang et al. (2015). The overall category of devices able to control the thermal and air quality conditions directly surrounding the occupant were defined as: PEC - Personal Environmental Control. PEC then divides into major sub-categories: PV - Personal (or Personalized) Ventilation, when the

device delivers outside fresh air to a person. PCS - Personal (or Personalized) Comfort System,

when room air is used to control or improve theperson's local thermal conditions and local air qualitywithout supplying an external air flow rate.

A desk fan or radiant heater falls in the PCS class, but not in the PV. PV systems need air ducts to collect fresh air from the outside and supply it at the person’s location. This paper focuses on PCS systems, in particular on fan heaters/coolers. These kind of electric appliances are available on the market since long time, but so far their use was mainly limited as a backup system when the centralized HVAC system had a fault or as a “reinforcement” (corrective power) when the user wanted an extra and localized heating/cooling. For such reasons the design procedures so far adopted for the development of these products were not particularly sophisticated and rarely the comfort performances have been used as a leading concept. Moreover, it has to be considered that standards, traditional design approaches and comfort theories have been, so far, focused just on centralized, general HVAC systems which provide uniform and steady-state thermohygrometric conditions. Instead, no or limited scientific research has been developed as far as the personalized devices are concerned. In the following sections a case study will be presented, where a rationale for analyzing the performance of personalized devices for thermal comfort is proposed and tested.

METHODS The analysis methods were developed starting from the investigation of the advantages and disadvantages that are typical of the fan heater/coolers. As far as the firsts are concerned, we may include:

- an air jet characterized by a long throw,- possibility to shape and direct the air jet in a way to

skim only certain part of the occupant’s body,- possibility to personalize the thermal environment

control,- air temperature at lower values compared to other

systems,- air distribution and speed controllable by the user,

- possibility to condition only that part of the room thatis actually occupied by people, with a resultingenergy saving.

- Possibility to deliver thermal energy only where andwhen it is really needed.

Among the disadvantages, it is possible to include: - The high air speed on certain part of the body, that

may cause complaints due to air draft,- Too high inhomogeneity of the thermal field around

the occupant.A prerequisite for a correct design of these appliances is therefore represented by a detailed knowledge of the thermofluidynamic field in the space located in front of them. For this sake an experimental campaign was developed adopting a two-sage procedure. Firstly, the PCS’s were located inside a thermostatic chamber having a size of about 3.8 m by 3.8 m by 2.5 m (height), whose air temperature was controlled by means of a radiant ceiling (in order to avoid as much as possible any additional air motion inside the chamber that could disturb the measurements of the jet air velocity). These tests were able to provide information about the behaviour of the device under conditions similar to those in a real domestic room. A second set of tests was performed in a large space, so as to determine the features of the air jets in an unconfined conditions. For the first set of experiments, hot wire anemometers and T thermocouples were used. A total of about 25 measurements points, suitably located on a plane perpendicular to the air jet axis, were investigated. Mean radiant temperature and relative humidity inside the test room were also monitored. The second series of tests was carried out using an automatic positioning system that allowed to do measurements on a Cartesian grid of evenly spaced points. A total of 1084 nodes were used. Air velocity was measured by means of a Gill ultrasonic anemometer, which provided the velocity vector and allowed to also analyse – to certain extent (the scan rate was of 4 Hz) – the turbulence intensity. Temperature was measured by means of J thermocouples. The experimental campaign was performed for two different operating modes of the PCS’s: cooling mode (e.g. isothermal conditions; the device just modifies the air velocity field) and heating mode (the device modifies both the air velocity and temperature fields). The results of these tests, besides providing information on the spatial distribution of the air velocity and temperature, allowed to characterize the air jets in terms of: throw (for a terminal velocity of 1.5 m/s), decay of the air velocity and temperature along the axis, entrainment flow rate. Finally, starting from the knowledge of the thermofluidynamic field, an analysis of the comfort conditions was developed for the PCS operating in cooling and heating mode. This last study was developed suitably modifying the existing theories/standards, developed for uniform environments, in order to take into account the non homogeneity of the velocity and temperature fields around the occupant body.

RESULTS AND DISCUSSION The next two sections resume and present the results related to the measurements performed inside the thermostatic chamber for two bladeless PCS available on the market and used, in the investigation, as case studies. The experiments were aimed at characterizing the velocity and temperature fields around a hypothetic person located in front of the device. The first section is related to the cooling mode, the second to the heating mode. In the third section, the analysis of the comfort conditions will be illustrated and analysed.



Figure 2. Entrainment and air velocity decay along the jets axis – Cooling mode – normalized quantities (Case study A on the left and B on the right).

Figure 3. Velocity field on two vertical planes (one longitudinal plane along the air jet axis, the other perpendicular to the jet axis and located 2.5 m away from the PCS) – The iso-velocity at 1.5 m/s is highlighted to allow the identification of the throw - Cooling mode (Top = Case study A, Bottom = Case study B).

Thermo fluid dynamic characterization – Cooling mode

First of all it must be explained what “cooling mode” means. In reality the cooling mode, from a purely physical point of view, should be named as “isothermal mode”, since these devices are not equipped with any appliance that can cool the air. The “cooling” effect is related not to the device but to the human body. As long as the indoor air temperature is lower than the cloth/skin temperature and the relative humidity is below 100 %, an increase of the air speed over the person increases the heat losses due to convection and evaporation, thus providing an enhanced cooling of the body

and hence a better thermal sensation during the summer time. Therefore, the air velocity field plays a fundamental role for the effective operation of the device. Figure 2 shows the relevant properties of the air jet, while figure 3 gives a general overview of the velocity fields and highlights the jet throw for a terminal velocity of 1.5 m/s. Compared to the inlet grill of centralized HVAC systems the air velocity is very high, even at relatively long distances from the device. Such feature, that – as previously discussed - is pursued by the manufacturers when designing these PCS, opens a number of questions. Usually, for centralized systems, the aim is to obtain a configuration in which the air velocity is almost imperceptible by the occupants, otherwise

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local discomfort due to air draft may occur. Therefore, for the users the best HVAC system is the one that cannot be “felt”. On the contrary, as it has been proved by interviewing the consumers, those people who buys a PCS want to sense a rather high air speed on their body and are quite disappointed if the feeling is of a too weak air draft. The point is to understand what is the limit of the air speed over the various parts of the body that separates a perception of a pleasant breeze form that of a disturbing draft. So far there are no references that can provide a conclusive answer to this question.

Thermo fluid dynamic characterization – heating mode

The same analysis done for the “cooling mode” was repeated in heating mode. In figure 4, the two upper charts show the temperature field on 6 vertical planes perpendicular to the jet axis, highlighting the air dilution and the consequent temperature decrease along the flow path and the rise of the air trajectory due to buoyancy effects. The air temperature decay (normalized values) along the jet axis is shown in figure 4, together with the best fit curve (lower plots). Finally figure 5 gives a general overview of the combined velocity

Figure 4. Temperature field on six vertical planes perpendicular to the air jet axis and temperature decay curves (lower plots) – Left side = Case study A, Right side = Case study B.

and temperature fields. The iso-temperature at 24 °C is highlighted to allow a better analysis of the device performance in terms of comfort. For case study “A” (figures 4 and 5) a more concentrated jet is realized. In the region approximately located in the first 175 cm from the appliance, the jet momentum is very high, the air tends to dilute moderately and the temperature assumes quite high values. In this zone the jet trajectory is not notably influenced by the buoyancy and tends to stay horizontal. After this distance the air looses quite suddenly its momentum and the jet rises significantly. In comparison, Case study “B” shows more spread and homogeneous temperature and velocity fields. As a result, the buoyancy forces exert their influence at an early stage of the jet development and the jet trajectory starts to rise soon after leaving the front surface of the device.

Though the temperature values in the near field are a little lower than in the previous case, their distribution is far more homogeneous. The appliance is able to influence the temperature field practically from the floor level to an height corresponding to the head of a sitting person. For case study “A”, instead, as it is possible to see from figure 4 (upper left chart) and 5 (upper plots), the room volume located in the first 35 cm from the floor is practically unaffected by the PCS. This temperature field is, more likely, prone to cause discomfort due to too high non homogeneity of the indoor environmental conditions.

Thermal comfort – Cooling mode

As it has been discussed in the introduction, one of the main problems that one has to face today in designing a PCS is

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Figure 5. Temperature field on two vertical planes; one longitudinal plane along the air jet axis, the other perpendicular to the jet axis and located 2.5 m away from the PCS. For this last plane also the air speed is plotted (right most plots) – The iso-temp at 24 °C is highlighted – Heating mode (Top = Case study A, Bottom = Case study B).

the lack of specific comfort theories able to provide rational results for non uniform thermal environments; that is the case of appliances aimed at modifying only locally the thermo hygrometric parameters (Vesely & Zeiler, 2014, Zhang et al., 2015). Indeed the analysis of the thermal and velocity fields, as discussed in the previous sections, can provide some useful information. A more uniform thermal field, that assure a homogenous local sensation from the feet to the head, is for sure better then a configuration where the occupants have the feet in a colder region and the head at an upper temperature. Nevertheless, even in this case there are few studies that supply information (see e.g. Zhang et al., 2010a, 2010b, 2010c) and an established and generally recognized procedure for a rational and quantitative evaluation of these comfort/discomfort levels is still lacking. Even worst is the case in “cooling mode”. For this situation the standardized procedures, set up for centralized HVAC systems, show all their limitation. Should the Italian technical standard related to air conditioning system be used, all the PCS would be non-compliant. In fact, a maximum air speed of 0.15 m/s and 0.25 m/s are prescribed in the occupied zone for the heating and cooling seasons respectively. Consequently, in the present study, in order to analyse the

performances of PCS in cooling mode a procedure was proposed, that takes its basis from the energy balance of the human body, as specified by the Fanger theory and the ISO standard 7730, and recalls - to some extent - the concepts of corrective power (CP) introduced by Zhang et al. (2015) (it is worth mentioning that the CP is not a power, but a temperature difference). The underling idea is as follow. When operated in “cooling mode” the fan heaters do not decrease the indoor air temperature, but achieve the goal of producing a thermal sensation perceived as “cooler” just by improving the air velocity (Personalized Convective Cooling, Vesley & Zeiler, 2014). As a result, an equivalent operative temperature may be defined that relates the real environment with the PCS to a virtual environment with still air. The equivalent operative temperature is assessed assuming for the calculation the average air speed produced by the PCS over the occupant (weighted average) and imposing the same thermal load on the thermoregulation system of the body, L, in still air conditions. Various weighted averages for the air speed may be adopted, as well as, different standards (e.g. ISO 7730 or ASHRAE st. 55, ISO 14505). The results of these

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Figure 6. Curves relating the equivalent operative temperature with the average air speed over the person body.

Table 1. Equivalent operative temperatures for the two test case devices.

analysis are shown in figure 6, in which the two graphs refer to ISO 7730 and to ISO 14505 standards respectively. Table 1 resumes the results obtained for the two test cases. As it is possible to see, the PCSs allow to improve the operative temperature, providing a satisfactory thermal comfort, of about 1 °C up to about 3 – 3.5 °C. These results are coherent with the findings of Zhang et al. (2015) relative to the CP.

CONCLUSIONS Personal Comfort Systems (PCS) are appliances whose role is likely to significantly increase in the near future, especially in case of Zero Energy Buildings. Nevertheless, despite their diffusion and importance, the design procedures adopted so far overlook the comfort implications. Besides, few researches have been done as far as the thermal comfort for non uniform and variable indoor environment conditions are concerned. In the present paper a procedure for the overall assessment of the performance of PCS is described and tested on two case study bladeless devices. An equivalent operative temperature was also defined to rate the capabilities of the appliances in providing a “cooling effect” in terms of thermal sensation.

ACKNOWLEDGEMENT This study has been done in the frame of a research contract between Politecnico di Torino – DENER and De’Longhi Appliances S.r.l.

REFERENCES UNI EN ISO 7730, 2006, Ergonomics of the thermal

environment. Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria, International Organization for Standardization.

UNI EN ISO 14505, 2007, Ergonomics of the thermal environment. Evaluation of thermal environments in vehicles, International Organization for Standardization.

ANSI/ASHRAE Standard 55, 2013, Thermal environmental conditions for human occupancy, American Society of

Heating, Refrigerating and Air-Conditioning Engineers, Atlanta

Bauman F.S., Carter T.G., Baughman A.V., Arens E.A., 1998, Field study of the impact of a desktop task/ambient conditioning system in office buildings. ASHRAE Transactions 1998;104:1153–71.

Deng Q., Wang R., Li Y., Miao Y., Zhao J., 2017, Human thermal sensation and comfort in a non-uniform environment with personalized heating, Science of The Total Environment, Volume 578, 1 February 2017, Pages 242-248

Veselý M., Zeiler W., 2014,Personalized conditioning and its impact on thermal comfort and energy performance – A review, Renewable and Sustainable Energy Reviews, Volume 34, June 2014, Pages 401-408, https://doi.org/10.1016/j.rser.2014.03.024.

Veselý M., Molenaar P., Vos M., Li R., Zeiler W., 2017, Personalized heating – Comparison of heaters and control modes, Building and Environment, Volume 112, 1 February 2017, Pages 223-232

Watanabe S., Melikov A.K., Knudsen G,L., 2010, Design of an individually controlled system for an optimal thermal microenvironment, Building and Environment, vol 45, no. 3, pp. 549-558. DOI: 10.1016/j.buildenv.2009.07.009

Zhang H, Arens E., Huizenga C., Han T., 2010a, Thermal sensation and comfort models for non-uniform and transient environments: Part I: Local sensation of individual body parts, Building and Environment, Volume 45, Issue 2, February 2010, Pages 380-388.

Zhang H, Arens E., Huizenga C., Han T., 2010b,Thermal sensation and comfort models for non-uniform and transient environments, part III: Whole-body sensation and comfort, Building and Environment, Volume 45, Issue 2, February 2010, Pages 399-410.

Zhang H, Arens E., Huizenga C., Han T., 2010c, Thermal sensation and comfort models for non-uniform and transient environments, part II: Local comfort of individual body parts,Building and Environment, Volume 45, Issue 2, February 2010, Pages 389-398.

Zhang H., Arens E., Zhai Y., 2015, A review of the corrective power of personal comfort systems in non-neutral ambient environments, Build. Environ. 91, pp 15 - 41, http://dx.doi.org/10.1016/j.buildenv.2015.03.013.

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New methodology and perspective for the air flow and thermal … · 2018-01-07 · 4th International Conference On Building Energy, Environment New methodology and perspective for