1、2009 ASHRAE 563ABSTRACTRadiant systems became in the last years very common as heating and cooling system. Originally used in the slabs in multi-storey buildings, during the 70s floor heating systems increased the number of applications and nowadays they represent a big part of the market all over t
2、he world.Afterwards several other systems have been introduced and recently radiant wall systems became very popular as well. Although radiant systems are usually sold and designed, it does not appear very clear what behind the pipes level happens. Standards in fact take in consideration only the he
3、at-ing capacity output on the room side, but the feeling is that something on the back side is not taken correctly into account. Moreover, when radiant wall systems are installed on external walls, sometimes no insulation is taken into account: many times radiant systems are put in without taking in
4、to account the U-value of substratum structure.The goal of this paper is to compare a radiant wall system and a floor system under the same conditions in terms of insu-lation volume; also the position of the insulation layer in radi-ant wall has been analyzed. A detailed model for radiant systems (n
5、amed DigiThon) has been used for this purpose, for determining both the heating capacity of the systems in winter design conditions and the seasonal energy demand of the radi-ant systems during winter and summer period. Results show that the capacity of the systems is the same in heating but not in
6、cooling season.INTRODUCTIONRadiant systems became very popular in the last decades. They are commonly used as radiant floor systems, although from the 1990s radiant ceilings have been increasingly installed. In the last years several producers propose radiant wall systems: these systems have some in
7、convenient when hanging pictures, as well as problems with furniture. Wall systems have an overall heat exchange coefficient equal to 8 W/(m2K) (1.41 Btu/hft2F) both in heating and cooling conditions (CEN 2008). Radiant floor systems have an overall heat exchange coefficient equal to 11 W/(m2K) (1.9
8、4 Btu/(hft2F) in heating and equal to 7 W/(m2K) (1.23 Btu/(hft2F) in cooling conditions.Many times manufacturers of radiant wall systems propose their application directly on external walls. In this case insulation behind pipes has to be applied, since, other-wise, losses become very high.Since it d
9、oes not seem clear the entity of the losses when applying a radiant floor or a radiant wall heating, in this work a comparison between those systems has been carried out; moreover the influence of position of insulation layer in radi-ant wall systems has been analyzed.A comparison between different
10、systems is almost impos-sible to carry out by means of measurements, since many uncertainties appear when measuring energy performance in buildings. Therefore the recourse to a dynamic simulation with detailed models seems inevitable when comparing differ-ent systems under the same conditions.A suit
11、able model should take into account the internal loads, solar radiation, as well as thermal conduction in tran-sient conditions inside structures where pipes are embedded. Many models have been proposed in the last years. In this work the model DigiThon (Brunello et al. 2001) has been used. Such a m
12、odel is based on transfer function method (Kusuda 1969, De Carli 2002), and it allows simulating which-Comparison Between a Radiant Floor and Two Radiant Walls on Heating and Cooling Energy DemandMichele De Carli, PhD Angelo Zarrella, PhD Roberto ZecchinMichele De Carli is an assistant professor, An
13、gelo Zarrella is a research fellow, and Roberto Zecchin is a full professor with the Dipartimento di Fisica Tecnica at the University of Padova, Padova, Italy.LO-09-053 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transac
14、tions 2009, vol 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.564 ASHRAE Transactionsever radiant system, since transfer functions can be calculated via two-dimensio
15、n detailed model (Blomberg 1999).In this way it is possible to evaluate hour by hour the energy demand of the radiant wall, both as useful thermal energy for the house and losses behind pipes.CASE STUDYIn Figure 1 the plan of a floor of the case study is shown. It is a residential house, with two le
16、vels and the same room distribution. On the lower side, the ground floor is adjacent to no heated rooms where 10C (50F) air temperature has been considered for each considered climate.All rooms have a radiant system (floor or wall depending on the considered case) except bathrooms and corridors with
17、 convective systems. In Figure 2 the thermal properties of the structures of the building are reported. The solar factor (ratio of the total solar energy flux entering through the glass to the incident solar energy flux) of the glazed surfaces is 0.35.A mechanical ventilation system (0.5 Vol/h of ai
18、r change per hour (ach) with 50% sensible heat recovery has been considered: it runs always in no stop mode.The considered radiant systems are shown in Figure 3. The radiant floor shown in Figure 3.I is the one used when the lower adjacent room is not heated, i.e. with additional insula-Figure 1 Pla
19、n of the floor of the considered building (dimensions in meter).Figure 2 U-value of structures.ASHRAE Transactions 565tion, as required by the Standard 1264 part 4 (CEN 2001), in order to reduce losses. In the slab between the first and second level, the insulation layer thickness becomes 3 cm (0.09
20、84 ft).Two radiant walls have been considered (Figure 3.II):case a: insulation layer is put on the inside and also on the outside part of the wall;case b: whole insulation layer is put on the outside part of the wall.Three cities have been taken into account: Wuerzburg, Venice and Naples. In Figure
21、4 mean monthly days outdoor temperatures are shown and in Figure 5 beam and diffuse solar radiation are shown.The radiant system is installed on the whole surface of floor in the case of radiant floor but when it is installed on the external walls the radiant surface is about 40% of floor surface: i
22、t depends on the zone load and difference between supply water temperature and room air. Only in Wuerzburg radiant walls have a greater surface, about 50% of the floor surface, since the outdoor air temperature is lower than other cities.WINTER DESIGN DAYIn this study two different types of simulati
23、ons have been carried out: winter design day simulations using data in Table 1 and simulations on the long period based on the test refer-ence year (TRY). In winter design day simulations, solar radi-ation and internal loads have not been considered but in the long period analysis they have been tak
24、en into account.Winter design day simulations allow to evaluate the required power and also supply water temperature to radiant systems compatible with comfort conditions. The power demand on the building side does not take into account convective systems of bathrooms and corridors: their power is a
25、lmost equal for each case. Both with radiant floor and with radiant wall, operative temperature in the rooms is about 20C (68F).The supply water temperature in the radiant system is shown in Table 2 for the three different climates. In winter design day, radiant systems run without control for the w
26、hole day (continuous running): these simulations are in steady state conditions. As it can be seen, design water supply temperature is rather high (10C to almost 20C 50F to almost 68F) for radiant wall in comparison with radiant floor system. In Table 3 the total power and losses (ratio of radiant u
27、seful energy on the inside surface and overall water energy in the pipes) are presented for the three cities: the volume of insula-tion material is the same for radiant floor and radiant wall, therefore the power demand is almost the same. In Wuerzburg with the same water energy, the radiant floor e
28、nergy is higher than radiant wall; in the other climates the two radiant systems are nearly the same. In steady state conditions, case a and case b radiant walls present no differences.As for the power installed, Wuerzburg has the same values for both two systems, while in warmer climates the heatin
29、g capacity of radiant floor is slightly lower than radiant wall.Figure 3 Radiant systems (dimensions in millimeter).566 ASHRAE TransactionsFigure 4 Mean monthly days outdoor temperatures.Figure 5 Mean monthly outdoor radiation on horizontal plane.Table 1. Winter Design TemperaturesLocationT Latitude
30、 AltitudeC (F) m (ft)Wuerzburg -15 (5) 49.0 182 (597)Venice -5 (23) 45.0 10 (33)Naples 2 (35.6) 40.0 17 (56)ASHRAE Transactions 567ANNUAL HEATING ENERGY DEMANDIn TRY simulations an on/off control on the indoor air temperature with set-point of 20C (68F) has been considered.In Figure 6 the specific y
31、early need of heating energy of the considered building is reported. In this case air to water heat pump has been taken into account, by means of an appro-priate simulation tool (Scarpa 2007). In Table 4 mean COP and the amount of working hours of air to water heat pump in heat-ing season are shown.
32、The efficiency of the electricity production mix, to convert electricity into primary energy consumption, 37% has been considered. In Figure 7 the annual primary energy demand per net surface (259 m22788 ft2) is shown (the area of bathrooms and corridors has not been considered): it takes into accou
33、nt also pumping energy, on the building side, with pump efficiency of 0.4. In Table 5 the seasonal mean supply and return water temperatures are shown. Radiant wall supply temperature is always higher than radiant floor: this is due to less surface and heat exchange efficiency. It is interesting to
34、see that, in case b, the water supply temperature is slightly lower than case a.In Table 6 the ratio of radiant useful energy on the inside surface and overall water energy is reported. In Naples the radiant wall behaves better than radiant floor: this is probably due to solar radiation (for this ci
35、ty is higher than the other loca-tions), which increases the temperature on external surfaces, thus reducing losses.In order to obtain in case b the same indoor conditions of case a (same operative temperature) the decrease of the T between supply and return water temperatures is necessary, due to t
36、he effect of the thermal inertia in transient conditions; in this way the water energy is the same in cases a and case b: part of this energy is stored in the structure and released later in case b, while in case a the structure responds immediately. Consequently the supply water temperature for cas
37、e b is lower than case a (Figure 8).For each room hourly PMV (Predicted Mean Vote) as well as PPD (Percentage of Persons Dissatisfied) (Fanger 1970, ISO 2004) have been calculated: thermal resistance of clothing of 1 clo in heating and 0.5 clo in cooling period has been considered. In this work a re
38、sidential house has been analyzed, therefore the hypothesis of a range for comfort conditions of -0.7PMV+0.7 has been made; only hours when persons are in the room have been considered (De Carli and Olesen 2002) according to EN 15251 (CEN 2007). During heating period, in all rooms the comfort condit
39、ions are the same for the three radiant systems. In Figure 9 the cold, hot and comfort hours in Room 1, at the second floor, are shown: the result is the same for all analyzed climatesfor each hour, PMV index is in the considered comfort rangeANNUAL COOLING ENERGY DEMANDDuring the cooling period, an
40、 on/off control on the indoor temperature with set-point of 26C (78.8F) has been consid-ered. As it can be seen in Table 7, the supply water temperature is almost the same in each considered city and also for each radiant system.Table 2. Supply Water to Radiant Systems in Winter Design DayLocationRa
41、diant Floor Radiant WallSupply T water T water Supply T water T waterC (F) C (F) C (F) C (F)Wuerzburg 27 (80.6) 4 (7.2) 45 (113) 4 (7.2)Venice 25 (77.0) 4 (7.2) 45 (113) 4 (7.2)Naples 24 (75.2) 4 (7.2) 35 (95) 4 (7.2)Table 3. Power and Losses For Radiant Systems in Winter Design DayLocationRadiant F
42、loor Radiant Wall Radiant Floor Radiant WallPower1Power1Losses2Losses2case a case bkW (Btu/h) kW (Btu/h) - - -Wuerzburg 9.1 (31047) 9.1 (31047) 0.87 0.76 0.76Venice 6.3 (21494) 6.6 (22518) 0.84 0.85 0.85Naples 5.0 (17059) 5.5 (18765) 0.81 0.85 0.851without taking into account convective power of bat
43、hrooms and corridors.2ratio between radiant useful energy on the inside surface and water energy at pipes level.568 ASHRAE TransactionsTable 4. Mean COP and Working Hours of Air to Water Heat Pump in Heating.LocationRadiant Floor Radiant Wallcase a case bCOP Working hours COP Working hours COP Worki
44、ng hours- - -Wuerzburg 3.12 2797 2.84 4056 2.87 3998Venice 3.23 2451 2.88 3427 2.98 3281Naples 3.39 1545 2.89 2112 2.95 1977Figure 6 Specific yearly heating energy (building side).Figure 7 Specific yearly primary energy demand for heating with air to water heat pump.ASHRAE Transactions 569In Figure
45、10 the specific yearly cooling energy need of the building is reported (in absolute value); in Table 8 mean COP and the amount of working hours of air to water heat pump in cooling season are shown.In Figure 11 the specific annual primary energy demand for cooling can be seen (the area of bathrooms
46、and corridors has not been considered). In Table 9 the ratio between energy removed by inside surface and overall water energy is reported. It is possible to see that the losses of radiant floor, with the same water energy, are lower than radiant wall; the difference decreases in warmer climates. Th
47、e ratio greater than 1 is due to the contribution of the back side of the struc-ture.About the relative humidity in the rooms, in Wuerzburg it is lower than 70%; in the other analyzed cities it exceeds the 70% for a few hours, when outdoor conditions are severe.During cooling period the indoor condi
48、tions are different: in the case of radiant walls the percentage of unsatisfied persons (PPD index) is higher than radiant floor. In Figure 12 the cold, hot and comfort hours in Room 1 (Figure 1) at the first Figure 8 Difference in supply water temperatures between cases a and b during heating perio
49、d.Table 5. Seasonal Mean Supply and Return Water Temperatures for Radiant Systems in HeatingLocationRadiant Floor Radiant Wallcase a case bT input T output T input T output T input T outputC (F) C (F) C (F) C (F) C (F) C (F)Wuerzburg 25.9 (78.6) 23.2 (73.8) 28.8 (83.8) 26.5 (79.7) 28.3 (82.9) 25.9 (78.6)Venice 24.7 (76.5) 22.5 (72.5) 29.8 (85.6) 27.3 (81.1) 29.4 (84.9) 26.8 (80.2)Naples 24.5 (76.1) 22.3 (72.1) 27.8 (82.0) 25.7 (78.3) 27.5 (81.5) 25.2 (77.4)Table 6. Losses1of Radiant Syste
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