Effect of humidity and small air movement on thermal comfort under a radiant cooling ceiling by subjective experiments

doi:10.1016/S0378-7788(98)00086-3

Energy and Buildings

Volume 30, Issue 2, June 1999, Pages 185-193

https://doi.org/10.1016/S0378-7788(98)00086-3 Get rights and content

Abstract

Radiant air-conditioning systems are expected to be more comfortable and superior energy-saving systems than convective air conditioning ones generally used. There are some studies on radiant cooling systems. However, they were seldom put to practical use because of dew point problem in Japan. The objective of this study is to investigate the thermal comfort of local parts of the body and the whole body, in particular, including the effects of humidity (45% rh, 65% rh, 85% rh) and small air movements, by subjective experiments under a radiant cooling system. The experiments have been performed by using radiant cooling panels in a climate chamber. Subjects were seated on a chair under the radiant cooling panels, and voted their thermal sensation and comfortable sensation. The following results were obtained. Even in the radiant cooling system, the influence of humidity and small air movement on thermal sensation votes of the whole body could be correctly estimated by using a standard new effective temperature (SET*) within one scale error of thermal sensation. Small air movement with the radiant cooling system had a possibility of improving the comfortable sensation votes in the radiant cooling.

Introduction

Radiant air conditioning systems are expected to be more comfortable and superior energy-saving systems than convective air conditioning systems generally used [1]. This is because radiant air conditioning systems can create an indoor environment, which has smaller vertical temperature differences and almost no air movement field, to be able to prevent local thermal discomfort. In addition, radiant air conditioning systems can be operated under smaller temperature differences between the heat source temperature and the room air temperature, which leads to superior energy-saving performance in heat pump systems.

Electric floor carpets and electric heaters are widely used as radiant heating systems in Japan. In recent years, floor heating systems using electrical heaters and circulating hot water systems are also in practical use. On the other hand, some studies have been carried out in the radiant cooling systems. However, radiant cooling systems are seldom in practical use in Japan because of the technical problem of condensation and lack of design information concerning thermal comfort. In particular, radiant cooling systems are seldom applied to Japanese housing.

Some studies have discussed thermal comfort and energy-saving performance in the radiant cooling systems. Fanger et al. [2]examined a comfort limit for radiant asymmetry, which is caused by cool or warm ceilings and a cool or warm vertical wall, by subjective experiments and using a thermal manikin. As a result, they obtained comfort limit lines for radiant asymmetry conditions. The ISO standard 7730 [3]used this result as a basis on the thermal comfort limit condition caused by radiant asymmetry.

Fitzner [4]reported that the combined system with radiant ceiling cooling and displacement ventilation created successfully comfortable thermal environment, whose characteristics were draftless and small temperature distributions in the occupied zone, by experimental studies. Feustel [5]examined several office air conditioning systems applied in European and American countries from a view of air conditioning load and he reported that the radiant cooling systems were expected to be superior with regard to energy-saving performance.

In Japan, a ceiling cooling and heating system for office air conditioning was reported to have been reduced 10% and 30% energy consumption in cooling and heating, respectively. Satoh and Murakami [6]studied the thermal environment created by a convective system with a radiant cooling panel, and they reported that the combined system reduced draft in the zone occupied by their experiments. Takahashi and Murakami [7]also reported that boundary conditions of radiant panels and windows were important factors in the numerical analysis. Murakami et al. [8]discussed the radiant effect on the ceiling cooled or heated by detached air flow.

In these studies, effects of air temperature and radiant panel temperature on thermal comfort were discussed. However, effects of humidity and air velocity were seldom discussed. Radiant air conditioning systems usually utilize building thermal capacity as thermal storage and they are continuously used. However, in Japan, air conditioning systems are generally operated only while the occupants are in the room. This custom needs large-capacity air conditioning systems for rapid start-up. Radiant air conditioning systems have disadvantages in this start-up performance and the dehumidifying problem, when compared to convective air conditioning systems. The air conditioning system, which composes both radiant and convective systems, improves this performance in the start-up conditions, and the system is a candidate of the most suitable air conditioning systems for housing.

We studied thermal comfort in the radiant cooling ceiling panels by using experimental panels similar to those of Fanger et al. [2], however, taking reciprocal radiant effects of floor into consideration [9]. Fanger et al. [2]discussed radiant asymmetry effects on thermal comfort and they carried out their subjective experiments for several panel temperatures by keeping thermal neutral conditions. That is, panel temperatures were gradually lowered and room air temperatures were set up gradually higher during the experiment. On the other hand, we were interested in conditions for thermal comfort in the real operating conditions for radiant cooling ceiling systems. Subjective experiments were carried out to evaluate thermal comfort under the radiant cooling system which had effects of reciprocal radiant heat exchanges. In this study, the radiant system cooled the air temperature and the floor below the panels near the subjects. We discussed the effects of humidity and small air movement on thermal comfort in the radiant cooling system, by using a similar experimental facility 2, 9.

Section snippets

Experimental facilities

The experiments took place in an environmental chamber (dimensions 4.0×4.0×2.5 m) at the Air-Conditioners and Appliances Engineering Laboratory of Toshiba as shown in Fig. 1. The environmental chamber consisted of walls whose inner wall temperature was controlled by using a circulating water system and panel heaters. The radiant panel, which was shown in Fig. 2, was situated in the center of the chamber above the subjects to examine thermal comfort in the radiant cooling. The radiant panel form

Analysis concerning the influence of humidity

First of all, the effect on humidity was discussed from the results obtained in Experiment I. Three different humidity conditions, approximately 85, 65, 45% rh, (each called by high humidity, medium humidity and low humidity condition) were tested. The relationships between the thermal sensation vote and operative temperature (OT) and SET* were examined. The following measured values: air temperature, mean radiant temperature, air velocity and humidity at one point (in front of the subject, 0.6

Effects of small air movement on computing SET*

The relation between the metabolic rate and thermal sensation vote will be discussed here. In the process of calculating SET*, the convective heat transfer coefficient h_c is evaluated by , , . The first one, h₁, is calculated from the Eq. (1)using the metabolic rate M, and the second one, h₂, is calculated from Eq. (2)using the air velocity v_a. Then, the convective heat transfer coefficient of the human body (h_c) is evaluated as the larger value of the two values, but the minimum value of h_c is

Conclusion

The influence of humidity and small air movement on the thermal sensation vote and the comfortable sensation vote in the radiant cooling system were evaluated by subjective experiments, and the following results were obtained.

(1) In radiant cooling, the higher the humidity, the warmer the thermal sensation was for the same level of OT, but SET* could more correctly estimate the thermal sensation vote of the whole body within ±1 scale error of thermal sensation.

(2) Small air movement in the

Acknowledgements

This paper is concerned with a part of a study that was carried out with the financial support for development in making practicable energy usage rationalization from the Agency of Industrial Science and Technology, the Ministry of International Trade and Industry of the Japanese Government.

References (11)

P.O Fanger et al.
Comfort limits for asymmetric thermal radiation
Energy and Buildings
(1985)
H.E Feustel
Hydraulic radiant cooling
Energy and Buildings
(1995)
Y. Seshimo et al., Study on control for indoor radiant environment (4), Proceedings of the 26th Japanese Conference on...
ISO Standard 7730, Moderate thermal environments—determination of the PMV and PPD indices and specification of the...
K. Fitzner, Quelluftung-Schichtstromung, SONDERDRUCK 44, Heizung Luftung/Klima Haustechnik,...

There are more references available in the full text version of this article.

Cited by (74)

Reinventing hybrid office design through a people-centric adaptive approach
2024, Building and Environment
The indoor environment in office spaces has a significant impact on occupants' health, satisfaction, well-being and productivity. The use of co-working space is rising as a potential alternative workspace post pandemic to respond to the preference for hybrid and flexible working. Hence, it is imperative to understand how the space planning and design of co-working space impacts the occupants. A year-long longitudinal questionnaire survey, along with in-situ field measurement, is conducted in a co-working space in London, to assess the environmental comfort, perception and preference in the space. The relationship between environmental factors like lighting, ventilation, privacy, thermal comfort and access to amenities and occupants' seat preference is explored. The results indicate a general desire for more daylight and air flow suggested by the occupants, while the factors like ‘good daylight’, ‘good ventilation’ and ‘near power sockets’ contribute to occupants' decision-making on seat selection significantly, while privacy and noise are not seen as significant factors in this environment. The findings offer insights for the environmental design for future workspaces in the hybrid working setup.
Human thermal comfort under lateral radiant asymmetries
2023, Energy and Built Environment
Citation Excerpt :
The first issue is the exposure duration. In previous studies, each asymmetric condition was often tested in a short exposure duration, normally within 1 h [5–8,10–13,16]. In reality, the time that typical office occupants stay indoors may last 3∼4 h, far longer than these durations.
Occupants’ thermal comfort in buildings may be affected by the cool wall and warm wall, which is attributed to the effect of asymmetric radiation. However, the previous majority of the researches on asymmetric radiation were mainly about the comfort limits under thermally neutral condition within 1∼1.5 h but had not considered the effect of exposure duration and the condition beyond neutral. To investigate the human thermal comfort under an asymmetric environment caused by the cool wall and warm wall, forty-four subjects were exposed to neutral air temperature with lateral radiant asymmetries in winter and summer for 3 h. The results indicated that the cool wall caused thermal discomfort easier than the warm wall because the thermal sensation decreased and deviated from neutral with time. Subjects' sensitivity of local parts to asymmetric radiation was affected in the conditions beyond neutral, thus their acceptability to asymmetric radiation decreased. The currently used limits of radiant temperature asymmetry tended to underestimate the local discomfort due to the walls. For the conditions tested, The limits of 5 % dissatisfaction in radiant temperature asymmetry were 4.4 °C (180 min) and 1.8 °C (60 min and 120 min) for the warm wall, and 1.8 °C at 60 min for the cool wall.
Performance optimization of a sidewall radiant heating system integrated with diffuse ceiling ventilation
2022, Building and Environment
Citation Excerpt :
Researchers found that the influence of fresh air on thermal environment could not be ignored when studying the influence of radiation asymmetry on human thermal comfort. Kitagawa studied the influence of humidity and airflow on thermal comfort of participants exposed to a radiant cooling ceiling system [18]. The results showed that occupant thermal comfort could be improved by adjusting air velocity when participants were thermally neutral.
Radiant heating systems typically created a non-uniform thermal environment. The effect of radiant asymmetry on human thermal comfort was the main consideration in its application. However, previous studies did not emphasize the method of supplying fresh air and its impact on the asymmetric thermal environment. In this study, flow field distribution in a radiant heating room with diffuse ceiling ventilation system which had different porous ceiling opening rates was studied by numerical simulations. Meanwhile, subjective experiments were conducted to evaluate the influence on human thermal comfort. The results revealed that under different porous ceiling opening rates, non-uniform opening on the porous ceiling could optimize the indoor asymmetric thermal environment and reduce local draft discomfort. Furthermore, the neutral level in the room could be maintained at the same height by adjusting ceiling opening rates. The method of supplying fresh air into the radiant heating room had a certain impact on the asymmetric thermal environment. This paper served as a reference for the future design of the radiant system combined with a fresh air system.
A comparative review on the application of radiant low-temperature heating and high-temperature cooling for energy, thermal comfort, indoor air quality, design and control
2022, Sustainable Energy Technologies and Assessments
Radiant low-temperature heating (LTH) and high-temperature cooling (HTC) has become popular due to their high energy efficiency, thermal comfort, and improving indoor air quality. This system has been investigated in many studies from theory to practical applications. In this review article, LTH/HTC systems based on their results on energy usage, thermal comfort, indoor air quality, design and control are analysed and discussed. Furthermore, the radiant system with all-air systems are compared and the application of a hybrid system in different climate conditions is also presented. The outcome of this study revealed that in many studies radiant LTH/HTC systems can save between 10 and 30% energy and provide better thermal comfort compared to the all-air system. Moreover, combining a radiant system with a small-sized air system has a positive impact on indoor air quality and thermal comfort as required ventilation air is introduced and the latent load is removed. Overall, more studies are needed to monitor long-term performance of the building in use with radiant LTH/HTC to optimize the overall system performance and system design, and to extend its application in different climates and wide ranges of building types.
Thermal comfort in residential buildings using bimetal radiator heating vs. floor heating terminals
2022, Journal of Building Engineering
Bimetal radiator heating terminals (BRH) have gradually replaced the traditional cast-iron radiator and have been widely used in urban residential buildings in cold areas of China. Meanwhile, floor heating (FH) terminals are increasing rapidly in recent years. Therefore, two common heating terminals in urban residential buildings exist in cold areas of China, namely, BRH and FH. The comfort of the two heating terminals has been a focus of controversy. To compare the thermal comfort of BRH and FH terminals, field tests and subjective questionnaires were made on the residential buildings during heating seasons in cold areas in China. A total of 71 residences and 136 occupants were investigated in this field study, where 294 and 299 valid data sets were obtained from FH and BRH terminals, respectively. The results showed that FH has significantly higher mean radiant temperature (MRT) and operative temperature, although no significant differences in indoor air temperature, relative humidity and air velocity between the two heating terminals are observed. FH terminals were significantly less dressed compared with those from BRH terminals at the same temperature. The actual lower limit of 80% acceptable temperature for FH and BRH in winter were 15.9 °C and 17.0 °C, respectively. The use of the FH terminals increased people's acceptance of the low-temperature environment and reduced their expectation for high-temperature environment. PMV significantly underestimate the thermal sensation of the occupants. This paper can be a reference for the selection of heating terminals and may lead to more efficient energy use for heating systems.
Subjective assessment of thermal comfort by radiant cooling in a tropical hot humid climate
2022, Energy and Buildings
This paper investigated thermal comfort of radiant cooling with subjects who are accustomed to a tropical hot humid climate. Thermal sensation, thermal acceptability, and thermal preference were assessed from observations of a large group of the subjects in a radiant cooling experimental room refurbished as an office-like environment. The analysis results showed that the operative temperature for the thermal neutrality was about 25.0 °C, and the acceptable temperature ranged from 22.5 °C to 26.5 °C. Air movement played a key factor influencing on the subject preference in the radiant cooling, while the room relative humidity had less significance. In this study, a field comfort survey of the conventional mixed-air system was also carried out in real occupied spaces by using the same questionnaire and measuring devices from the radiant comfort experiments. The comparison showed that the thermal responses of the subjects were different between the two air-conditioning systems.

View all citing articles on Scopus

View full text