indirect evaporative cooling: interaction between thermal performance and room moisture balance

Transcription

1 indirect evaporative cooling: interaction between thermal performance and room moisture balance M. Steeman 1, A. Janssens 1, M. De Paepe 2 1 Department of Architecture and Urban Planning, Ghent University, Ghent, Belgium 2 Department of Flow, Heat and Combustion Mechanics, Ghent University, Ghent, Belgium Marijke.Steeman@UGent.be Abstract In an indirect evaporative cooling (IEC) installation the extracted air is cooled by means of adiabatic humidification. By passing over an air/air heat exchanger this air cools down the supply air. A clear interaction can be observed between the relative humidity of the extracted air and the thermal comfort realized in the building. To be able to predict the performances of this technique well, a good knowledge of the indoor relative humidity is thus important. This paper presents the results of measurements carried out in the summer of 06 in a nonresidential building at the Belgian coast which makes use of indirect evaporative cooling. An evaluation of the indoor summer comfort is made and the interaction between the thermal performance and the indoor humidity is investigated. Furthermore, using the multizone building simulation program TRNSYS, simulations were performed to evaluate the parameters influencing the room moisture balance. 1 Introduction Different moisture sources exist in buildings e.g. people, plants, cooking and showering Apart from them, the relative humidity in a room is influenced by ventilation with outdoor air, infiltration and the exchange of moisture with walls and furniture. Hygroscopic materials such as wood and textile are able to dampen out relative humidity variations and provide therefore a healthier indoor climate [1]. Svennberg et al. [2] have shown that furniture plays an important role in the buffer capacity of a room. Nevertheless it is difficult to assess the buffer capacity of the indoor climate because of the uncertainty of many parameters: the material moisture properties vary with the relative humidity and for many materials there is a lack of available data [3] [4]. Furthermore, it is difficult to predict the exact surface of buffering material in the indoor climate. Due to this most building simulation programs, e.g. Trnsys, predict the relative humidity in a simplified way [5] [6]. One of the HVAC applications of which the thermal performances depend on the indoor humidity is indirect evaporative cooling (IEC), an interesting passive cooling method in which a clear interaction can be observed between the relative humidity of the return air and the thermal comfort realized in the building. This paper focuses on the interaction between the thermal performance of this cooling technique and the indoor relative humidity. The first part explains the principles of IEC, the second part discusses the measured performance of an existing installation in a non-residential building in Oostende. The third part presents some results from a parameter study performed with a simple simulation model.

2 2 Operation of indirect evaporative cooling In indirect evaporative cooling (Figure 1) the return air passes over the wet side of an air/air heat exchanger. By adiabatic humidification the air stream is cooled. At the same time fresh air flows over the other side of the heat exchanger and cools down. This method differs from direct evaporative cooling in which the supply air stream is directly humidified. By humidifying the return air the dry bulb temperature of the supply air can be lowered without increasing its humidity ratio. In this way a more comfortable indoor climate can be obtained. In figure 2 the states of both airstreams during the process are schematically presented making use of a Mollier chart. During humidification, the return air follows lines of equal wet bulb temperature until saturation, while the horizontal lines indicate the state of the supply air. moisture relative humidity 0% SUPPLY AIR indirect cooling 3 Figure 1: Indirect Evaporative Cooling [7] 2 1 RETURN AIR adiabatic humidification temperature Figure 2: State of the airstreams in IEC Because of his low cooling loads and his moderate humidity the Belgian climate is suitable for the use of this technique. Applications in new or retrofit buildings with low internal gains and without specifications of close temperature or humidity control are favourable. In winter the heat exchanger can be used for heat recovery thus preheating the outdoor air [8]. 3 Experimental determination of thermal performance 3.1 Indirect evaporative cooling installation in Oostende Measurements were carried out in the cafeteria of a holiday centre located at the Belgian Coast (Oostende) in the summer of 06. Temperature and relative humidity were measured in the installation at all stages of the process as well as in the cafeteria. Also the position of the valves, pumps and fans were registered. The cross flow heat exchanger which is used has a total width of 900mm, a length of 1950mm and is made out of polypropylene. Both the supply and return fan can work in two stages with a maximum air flow rate of 70m³/h. The installation can operate at different stages. When the outdoor air is fresh enough in summer, the air flow rate is increased and the air is used for free cooling. As soon as the outdoor air temperature is too high, the fresh air is adiabatically cooled by moistening the heat exchanger. If the desired indoor temperature is not yet reached, active cooling may contribute to lower the temperature of the supply air. 3.2 Discussion of the measured performance Thermal summer comfort Figure 3 presents the measured temperatures from 18 until July. Measurements were done every five minutes. The periods in which the IEC is working are indicated. Figure 1 shows where the temperature of the airstreams is measured.

3 For the first two days the temperature of the supply air downstream from the heat exchanger section did not differ much from that of the outdoor air. The fresh air was clearly bypassed allowing free cooling. The temperature measured behind the evaporator was always lower than before, showing active cooling has been working the whole measured period. temperature 40 max 13.9 C with IEC 35 1 free cooling /jul/06 19/jul/06 /jul/06 /jul/06 /jul/06 fresh outdoor air supply air behind heatexch supply air behind evaporator cafetaria IEC Figure 3: Measurements of IEC in Oostende (18/07/06 - /07/06) During the measured days the outdoor temperature varied from C to 35 C. The average temperature measured in the cafeteria was C. Making use of evaporative cooling, a maximum temperature drop of 13.9 C was noticed on 19/07 when the outdoor temperature reached 34.8 C. On average a cooling of 3.3 C could be achieved with this method within the measuring period. At night free cooling was used for some hours to cool down the cafeteria. The indoor summer comfort was evaluated using the Adaptive Temperature Limits Indicator (ATG) developed in The Netherlands which is based on the adaptive comfort theory of de Dear and Brager [9] - [11]. They state that the degree in which people accept the indoor climate depends on their expectations and thus is partly influenced by the outdoor climate. In this method, two types of buildings are distinguished, depending on the availability of operable windows and the possibility to adjust the indoor climate and clothing. The studied building was determined to be a so-called beta-building as it is top cooled by a mechanical cooling installation and every two occupants do not have an operable window available. Thermal comfort is divided into three levels. The standard level B corresponds to 80% thermal acceptability and thus means a good indoor thermal comfort. Level A (90% acceptability) is applied in buildings with high performance requirements to thermal comfort and level C (65% acceptability) should only be applied for temporary situations in existing buildings. In figure 4 the evaluation of the indoor climate is given for July, August and September 06. On the horizontal axis the running mean outdoor temperature is presented, which is derived from the external temperature from the current day and that of the three preceding days. The vertical axis shows the indoor operative temperature. In the graphs only the occupancy hours are taken into account (08-h). From the measurements of the outdoor temperature could be derived that July 06 was the warmest month [12]. During 79.3% of the occupancy hours in July, a good indoor climate was obtained. In August and September 06 this was

4 respectively 98% and 88%. Thus, although active cooling was working most of the time, the cafeteria did not always meet the criterion for good indoor thermal comfort. T i,o Tmax 65% acceptability Tmax 80% acceptability Tmax 90% acceptability Tmin 90% acceptability Tmin 80% acceptability Tmin 65% acceptability T e,ref T i,o Tmax 65% acceptability Tmax 80% acceptability Tmax 90% acceptability Tmin 90% acceptability Tmin 80% acceptability Tmin 65% acceptability T e,ref T i,o Tmax 65% acceptability Tmax 80% acceptability Tmax 90% acceptability Tmin 90% acceptability Tmin 80% acceptability Tmin 65% acceptability T e,ref Figure 4: Indoor summer comfort during respectively July, August and September 06 in Oostende. Table1: Percentage of occupancy hours the indoor thermal comfort is situated in class A, B and C Class A Class B Class C July % 79.3% 93.1% Aug % 98.0% 99.4% Sept % 88.0% 96.7% Regarding humidity comfort criteria the indoor air humidity should be, according to recommendations in EN between and 70% [13]. Measurements have shown that this requirement was satisfied in the cafeteria during the whole measurement period (July- September) Hygrothermal interaction The relative humidity of the return air plays an important role in the performance of IEC because it is moistened in the installation. As moisture buffering in hygroscopic materials is able to reduce humidity peaks in the indoor climate a better performance of this technique is expected. In that case a larger amount of moisture can be evaporated in the return air before saturation is reached. Figure 5 shows that there is a linear relation between the amount of cooling of the supply air and the difference between the dry bulb temperature of the supply air entering and the wet bulb temperature of the return air leaving the heat exchanger. The smaller this latter temperature difference, the less the fresh air can be cooled because saturation is reached faster

5 when humidifying the return air. This can as also be seen on figure 3. In figure 5 only the period during the measurements (from 18/07 to /07) in which the evaporative cooling was working is taken into account. The effectiveness ε of an indirect evaporative cooling system describes its thermal performance and is given by: ε θ θ s,1 s,2 = (1) θ s θ ',1 r,1 In Eq. (1) θ s,1 and θ s,2 are respectively the temperature of the supply air entering and leaving the heat exchanger. θ' r,1 is the wet bulb temperature of the return air entering the heat exchanger. The effectiveness thus compares the actual temperature difference realized by IEC with the maximum possible difference. Calculated from the measurements (the slope in figure 5) the effectiveness was averagely 82.5%. temperature difference of supply air realized with IEC y = 0.8x R 2 = effectiveness ε temp difference dry bulb supply & wet bulb return air Figure 5: Interaction between humidity level and thermal performance in Oostende. (18/07/06 - /07/06) 4 Simulations 4.1 Parameter study The definition of effectiveness ε in Eq. (1) can be used as a first step in evaluating the performance of an IEC system having a constant effectiveness. As the wet bulb temperature of the return air depends on the moisture balance indoor, the influence of moisture production, moisture buffering and ventilation rate can be easily studied. Therefore in Trnsys [6] a simple model was built for calculating the temperature of the supply air in order to evaluate the indoor temperature in every time step. For the indoor environment, the BESTEST building was used [14]. The indoor climate consisted of a volume of 1.6m³ of which all walls are built from 15cm aerated concrete. Assuming that all boundaries are adiabatic, it is possible to study only the effect of the IEC installation on the indoor conditions. The outdoor climate is stationary C and 50% relative humidity. There is no infiltration, no radiation and no cooling or heating device.

6 Every day from 09-17h a moisture production of 0.35kg/h and a heat production of 600W due to five persons occupying the room (seated, light work) existed in the building [6][15]. The air is assumed to be well mixed and there was no other ventilation present then the IEC. To model the moisture buffering the Simple Capacitance Ratio model from Trnsys, assuming that the capacity of the air is fictively increased, was used. Runs were carried out with hourly values until stationary conditions were reached. It is easy to understand that a better effectiveness of the system will result in a lower supply temperature and therefore in a more comfortable indoor climate. This model assumed a constant effectiveness of 85% Moisture production Several simulations were run in which the indoor moisture production was varied. First, from the basic case A, only the moisture production was increased (due to e.g. extra activities: cases B and C), later also the heat production was enlarged (due to e.g. larger occupancy: cases D and E). As the indoor moisture production increases, the IEC system cools the supply air less, resulting in a less comfortable indoor climate. Furthermore with higher moisture productions the variations in indoor temperature enlarge due to larger variations in the supply temperature. From figure 7 the interaction between heat and moisture production is demonstrated: a larger occupancy of the room causes a higher cooling demand due to internal gains, but at the same time the supply temperature increases due to the larger moisture production. Table 2: Moisture production Occupation Moisture & activities production Extra activities (kg/h) Heat production (W) (kg/h) A 5persons 5*0.07 / 5*1 B 5pers+extra 5* *1 C 5pers+extra 5* *1 D pers *0.07 / *1 E pers *0.07 / *1 temperature relative humidity (%) prod Tin A Tin B Tin C A B C average temperature increasing moisture & heat production A A D D E E B B indoor air indoor air supply air supply air increasing moisture production moisture production (kg/hr) Figure 6 and 7: Influence of indoor moisture production on thermal performance of IEC (ε 85% / n 2h -1 / Ratio5) Moisture buffering and ventilation rate Simulations were performed with and without taking into account moisture buffering in the indoor climate. A capacitance ratio of five equals with three walls (+/- 52.5m²) finished with C C

7 gypsum board, while a ratio of ten equals with all walls and ceiling (+/- 1.6m²) finished with gypsum. From figure 8 can be seen that the average indoor temperature does not change much for different buffer capacities. Obviously this can be explained by the fact that moisture buffering does not have a large effect on the average relative humidity, but rather dampens out the variations. This was also shown by [2] by experiments. In other words with moisture buffering the amount of cooling is enlarged during periods where moisture is buffered while when moisture is released the supply air will be less cooled. On average the thermal performance achieved with the IEC system will be similar. As a last parameter the ventilation air change rate was varied from 1 to 5h -1. High air flow rates cause more moisture to be removed from the room providing a lower supply temperature and therefore a better indoor climate. On the other hand the variations in relative humidity and thus in supply air temperature are dampened out (figure 9). temperature relative humidity (%) 0 Tin no 90 buffer 80 Tin buff5 70 Tin 60 buff 50 prod 40 no buffer buff5 0 buff temperature relative humidity (%) 32 0 Tin n Tin n2 Tin n5 50 prod 40 n1 n n Figure 8: Influence of moisture buffering on thermal Figure 9: Influence of air flow rates on thermal performance of IEC performance of IEC (ε 85% / n 1h -1 / prod. 0.35kg/hr moisture & 600W) (ε 85% / Ratio5 / prod. 0.35kg/hr & 600W) 5 Future work The measurements have only been the first step in studying indirect evaporative cooling. A simulation model for a cross flow wet surface heat exchanger is recently being developed and will be validated against the measurements. The aim is to use the model in the Trnsys environment to be able to simulate buildings where IEC is applied. Further research will also be done on calculating the system s effectiveness. In such a way the thermal performance of an installation could be easily assessed without calculating the whole model thus permitting a much smaller calculation time, e.g. in the design faze of an IEC installation. 6 Conclusions Indirect evaporative cooling is an interesting passive cooling technique in which the thermal performance depends mainly on the indoor humidity. Measurements carried out in July 06 show that by using this technique the supply temperature can be cooled up to 14 C. Despite the active cooling present to further cool the supply air, an evaluation by the adaptive temperature limits indicator showed that the indoor thermal comfort was not satisfied during the whole measured period.

8 Some first simulations with a simple model based on the system s effectiveness showed that the knowledge of the inner moisture balance is essential to predict the performance of IEC well. The indoor moisture production plays the most important role in lowering the indoor temperature, while the amount of moisture buffering material rather influences the stability of the climate. Higher ventilation rates also help to lower the indoor temperature. Further research will be carried out using a wet surface heat exchanger model allowing a better understanding of the parameters influencing this evaporative cooling technique. 7 Acknowledgements This PhD research is established with financial support of the Flemish Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT). 8 References [1] Simonson, C.J., Salonvaara, M., Ojanen, T. 02. The effects of structures on indoor humidity - possibility to improve comfort and perceived air quality. Indoor Air 12, p.3-1. [2] Svennberg, K.,Hedegaard, L., Rode, C. 04. Moisture Buffer Performance of a Fully Furnished Room. ASHRAE Special Publications, Proceedings of Buildings IX- Conference. [3] IEA Annex Condensation and energy. Volume 3. Catalogue of material properties. Belgium, K.U.Leuven. [4] Nordtest. Moisture Buffer Capacity Initial review of definitions and methods. 04. Meeting may 14, 04, ETH Zürich. [5] Janssens, A., De Paepe, M. 05. Effect of moisture inertia models on the predicted indoor humidity in a room. Proceedings of the th AIVC Conference, Brussels. [6] SEL, TRANSSOLAR, CSTB, TESS. 04. Trnsys 16: A Transient System Simulation Programme. University of Wisconsin, Madison, USA. [7] Menerga Klimatechnologie. [8] IEA Annex. 01. Subtask 2: Design tools for low energy cooling. Technology selection and early design guidance. [9] ISSO. 04. Thermische behaaglijkheid. Eisen voor binnentemperatuur in gebouwen (in Dutch). ISSO-publicatie 74. Rotterdam, The Netherlands. ISBN [] van der Linden, A.C., Boerstra, A.C., Raue, A.K., Kurvers, S.R., de Dear, R.J. 06. Adaptive temperature limits: A new guideline in The Netherlands. A new approach for the assessment of building performance with respect to thermal indoor climate. Energy and Buildings 38. p [11] van der Linden, A.C., Kerssemakers, M., Boerstra, A.C., Raue, A.K. 00. Thermisch binnenklimaat als gebouwprestatie. Bouwfysica 11(4). p [12] Koninklijk Meteorologisch Instituut van België. [13] CEN. 04. EN Ventilation for non-residential buildings Performance requirements for ventilation and room-conditioning systems. Brussels, Belgium. [14] Rode, C., Peuhkuri, R. 04. Annex41 Subtask1: Common Excercise1 0A &0B revised (Whole building heat and moisture analysis). [15] Harriman, L., Brundrett, G., Kittler, R. 01. Humidity Control Design Guide for Commercial and Institutional Buildings. ASHRAE. ISBN