Effect of humidity and small air movement on thermal comfort under a radiant cooling ceiling by subjective experiments
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 hc 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 va. Then, the convective heat transfer coefficient of the human body (hc) is evaluated as the larger value of the two values, but the minimum value of hc 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.
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