The possibilities and limitations of using radiant wall cooling in new and retrofitted existing buildings

Applied Thermal Engineering 164 (2020) 114490

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The possibilities and limitations of using radiant wall cooling in new and retrofitted existing buildings

T

Michal Krajčíka, , Ondřej Šikulab ⁎

a b

Slovak University of Technology, Faculty of Civil Engineering, Radlinského 11, 81005 Bratislava, Slovakia Brno University of Technology, Faculty of Civil Engineering, Veveří 331/95, 60200 Brno, Czechia

HIGHLIGHTS

and limitations of four wall cooling systems were explored. • Possibilities novel indicator to quantify the efficiency of heat and cool transfer was defined. • ARadiant wall cooling is applicable in both new and retrofitted existing buildings. • Performance depends on requirements on installation, cool storage, thermal response. • In wall with pipes uncoupled from the core only pipe spacing affects the output. • ARTICLE INFO

ABSTRACT

Keywords: Radiant system Wall cooling Building retrofit Heat transfer Thermal dynamics Thermally activated building systems (TABS)

The use of radiant wall cooling presents a potentially feasible solution to cover the cooling demand of buildings due to its suitability for combination with renewable energy sources at relatively high sensible cooling capacity. We define and directly compare four types of wall cooling systems, from which three are potentially suitable for building retrofit. Besides using established performance indicators, an indicator called heat transfer efficiency is introduced to allow detecting differences in the thermal dynamics of various systems even in cases when their response time, defined as τ95, is alike. Systems with pipes underneath the surface provide higher cooling output and are sensitive to pipe spacing. Systems with pipes embedded in the core allow thermal storage and are sensitive to insulation thickness. Thermal conductivity of the core material proved to be an important parameter to consider except for the system with the pipes separated from the core by thermal insulation. The systeḿs suitability depends on the requirements such as avoiding interventions in the interior, exploiting thermal storage, or providing fast thermal dynamics. It is shown how various configurations of pipe location, material layers and thermal conductivity of the core allow compromising between the different performance indicators to design a system with the desired characteristics.

1. Introduction The growing trends in global temperature, population, urbanization, economic development and demands on thermal comfort have been causing a gradual increase in the energy consumption for space cooling of buildings. It is expected that the resulting increase in the number of space cooling systems and cooling capacity will put tremendous pressure on the energy infrastructure and severely increase the environmental impacts if the design of cooling systems is not optimized [1–4]. The installation of low-exergy water-based radiant systems can help alleviate these negative effects. This should be possible due to their suitability for combination with low-grade renewable energy sources such as ground-coupled heat pumps and solar collectors [5–8] at

⁎

relatively high sensible cooling capacity [9], and the possibility to use the same system both for heating and cooling. In a moderate and dry climate and well thermally insulated buildings like, e.g., in Europe, only a fragment of the surface may be enough to create thermal comfort throughout the whole year [10–12]. This makes radiant walls potentially feasible systems for new and retrofitted existing buildings, which could be preferable to the more common radiant floors and ceilings due to the following benefits:

• Suitability for retrofitted buildings. Additional installation of a radiant wall system does not reduce the story height. This can be useful in retrofitted buildings, where radiant floor and ceiling systems diminish the precious height, possibly beyond the acceptable limit.

Corresponding author. E-mail address: [email protected] (M. Krajčík).

https://doi.org/10.1016/j.applthermaleng.2019.114490 Received 27 April 2019; Received in revised form 5 September 2019; Accepted 3 October 2019 Available online 08 October 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

Nomenclature

qi,max qt qt,max ΔR

Abbreviations AC EPS HTE RC TABS TI

aerated concrete expanded polystyrene heat transfer efficiency reinforced concrete thermally activated building systems thermal insulation

S T Tamb Tsol-air Tw Tf Twater t ti, ti-1

Symbols c d dconc dTI h

specific heat capacity (J/(kg⋅K)) thickness (m) thickness of concrete (mm) thickness of thermal insulation (mm) overall heat transfer coefficient between radiant surface and environment (W/(m2⋅K)) he coefficient of heat transfer by long-wave radiation and convection at outer surface (W/(m2⋅K)) HTEstepdown heat transfer efficiency obtained by the step-down method (%) Ig total solar radiation incident on surface (W/m2) L ratio of cooling losses to cooling power (-) D characteristic dimension – inner diameter of the pipe (m) n index denoting a line perpendicular to surface Nu Nusselt number (-) Nu2300 Nusselt number at the Reynolds number equal to 2300 (-) Nu10000 Nusselt number at the Reynolds number equal to 10 000 (-) Pr Prandtl number at mean water temperature (-) q63 63% of the maximum cooling output (W/m2) q90 90% of the maximum cooling output (W/m2) qГ,i sum of all the heat fluxes entering and exiting the wall (W/ m2) IqГ,iI sum of absolute values of the heat fluxes entering and exiting the wall (W/m2) cooling output (W/m2) qi

w α αp

¯ n

ε θ95 i,

s, i ,

•

s, i 1

0 steady

λ λL

ρ τ τ95 ø

• Comfortable thermal environment [13–15]. Compared to radiant •

i 1

maximum cooling output (W/m2) cooling power (W/m2) maximum cooling power (W/m2) difference between long-wave radiation incident on surface from sky and surroundings and radiation emitted by blackbody at outdoor air temperature (W/m2) internal heat source (W/m3) temperature (K) ambient temperature (°C) sol-air temperature (°C) temperature of wall surface (°C) temperature of surrounding fluid (°C) temperature of water (°C) time (h) time that has elapsed since the heat flux was supplied in the wall through pipes (h) index denoting surface of an object absorptance of surface for solar radiation (-) convective heat transfer coefficient between water and pipe surface (W/(m2⋅K)) wall mean age of heat flux (h) nominal time constant (h) hemispherical emittance of surface (-) 95% of the difference between the final and initial values of the wall surface temperature (K) difference between the wall surface temperature at the time ti, ti-1 and the steady-state wall surface temperature (K) wall surface temperature at the time ti and ti-1 (K) wall surface temperature at the beginning (K) steady-state wall surface temperature (K) thermal conductivity of the material (W/(m⋅K)) thermal conductivity of the fluid (W/(m⋅K)) kinematic viscosity (m2/s) volumetric weight (kg/m3) time (h) response time (h) outer diameter of the pipe (mm)

heating output. When operated as space heating, embedding the pipes in thermal insulation reduced the heating capacity by 50% as compared to systems with pipes arranged in a concrete core and by 63% for pipes arranged in a layer underneath the surface. For the wall with the pipes embedded in thermal insulation, the thickness of the thermal insulation, the spacing of the pipes, and the supply water temperature had a substantial effect on the heating capacity. These findings are consistent with those of Ning et al. [32,33], who investigated the thermal response of various types of radiant systems as defined in EN ISO 11855 [34]. They concluded that for thermally active building systems (TABS), concrete thickness, pipe spacing, and concrete type have a significant impact on the response time. In a numerical study of cooling panels subjected to heat fluxes, Mosa et al. [35] have shown that a dendritic architecture of the flow channels and a more compact design of the panels allow greater cooling capacity at less pumping power as compared to a serpentine flow architecture and a less compact design. Mikeska and Svendsen [36] analysed the heat transfer in a radiant wall with capillary mats embedded underneath the wall’s surface and operated either as space heating or cooling. At suitable configuration, spacing and tubes diameter, the capillary system was theoretically capable of supplying up to about 50 W/m2 when operated as space heating and about 70 W/m2 when operated as space cooling. Subsequent experimental measurements [37] have demonstrated a fast reaction of this system on the changes in temperature of the water

floors, wall cooling creates a more homogeneous thermal environment and reduces the risk of thermal discomfort due to cold floors in spaces like residential rooms and cellular offices [16,17]. Higher heating and cooling capacity. The cooling capacity is higher for radiant walls (70 W/m2) than floors (40 W/m2), though lower than for chilled ceilings (100 W/m2). In heating mode, the maximum capacity is 160 W/m2, superior to that of radiant floors (100 W/m2) and ceilings (40–50 W/m2) [9]. Possibility of operation as a thermal barrier to reduce heat transmission through walls. This is possible when the water temperature is very close to the room temperature, thus preventing heat losses in winter [18–21], and absorbing external heat gains in summer [22–26].

Radiant walls have certain specifics pertaining to their construction and operation that need to be considered. They can be installed on the outer side of existing buildings in the form of, e.g., insulation panels attached to their facades [18,27], in which case they are subject to daily and seasonal weather variations. Especially in summer, these variations may be complex because of the fluctuating solar radiation which affects the surface temperature of the facades [28–31]. Few studies have addressed in detail the heat transfer within radiant walls and the various factors vital for their correct design. Šimko et al. [18] report that the position of the pipes within the wall is crucial for its 2

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

because of its low heat capacity. The fast reaction can be considered a general advantage of the systems with short response time over TABS because it makes them applicable even in spaces with considerable variations in thermal load. On the other hand, the thermally active systems can provide benefits such as lower required peak output [38], and the ability to store energy [39,40] and shift the thermal loads [41,42]. However, their use is more limited as compared to the fast-response systems, because their large thermally active mass prevents efficient control of operative temperature in rooms with frequently changing thermal loads [42–44]. Even though the existing body of knowledge indicates the potential benefits related to the use of radiant wall systems, the research focuses primarily on radiant floors and ceilings. The potential applications and specifics of radiant walls have not been fully considered. The focus is on new buildings, and the research regarding the use of radiant wall systems in existing buildings as a part of their retrofit is lacking. Moreover, the existing studies that involve details of the heat transfer in radiant walls usually pertain to their operation as space heating. Less attention is paid to wall cooling systems. The aim of the present study is therefore to explore the possibilities and limitations of using radiant wall cooling in new and retrofitted existing buildings. We define and directly compare four types of radiant walls that differ from each other by the configuration of their material layers, pipe location, and their suitability for installation in existing buildings. The comparison of the wall systems is based on their evaluation by several performance indicators, obtained from dynamic and stationary numerical simulations of heat transfer. For this purpose, a novel indicator is introduced to quantify the efficiency of heat or cool distribution from pipes to the inner surface of the wall. The effect of various parameters of the wall systems is also investigated to provide practical guidance to the system design.

(a) Wall A has pipes embedded in a reinforced plaster between concrete and thermal insulation. It can be used for new buildings or attached to facades of existing buildings as a part of their retrofit, without any significant intervention on the interior side. (b) Wall B has pipes embedded in the concrete core, which makes its installation realistic only in new buildings. (c) Wall C has pipes embedded underneath the surface in a plaster. The pipes are thermally coupled to the concrete core. This system can be used for buildings retrofit because the active layer containing the pipes can be easily attached to an existing wall structure. (d) Wall D corresponds to Wall C, but it has pipes thermally uncoupled from the concrete core by a layer of thermal insulation. Like Wall C, it is potentially suitable for buildings retrofit. 3. Physical model and calculation methods The results were obtained by solving the equations of two-dimensional heat transfer by conduction, using a dedicated CalA software [45,46], which has been verified in accordance with EN ISO 10211 [47]. The heat flux and temperature distribution were calculated for a characteristic fragment of a radiant wall (Fig. 2). Dynamic and stationary simulations were used to compute key performance indicators and study the effect of selected parameters on the system performance (Table 2). The key indicators used were the heat transfer efficiency (see Section 3.3), thermal dynamics, thermal output and losses, and cooling power. The parameters studied were the water temperature, wall material, wall location, thickness of the wall, and spacing of the pipes. The computational model of Wall A was previously validated by Šimko et al. [18] for winter conditions and by Šimko et al. [48] for summer conditions. The climatic conditions, simulation setup and solver in Ref. [48] were identical with those used in the present study. The geometry and thermo-physical properties of the material layers were slightly adjusted in the present study to make the model more realistic and the system better suited for practical use. In the computational models of the other wall systems (B, C, and D), the thermo-physical properties of the material layers, simulation setup, calculation procedure, and boundary conditions were identical with those for Wall A. The differences between the systems were in the location of the pipes within the wall and configuration of the material layers, which were unique for each of the four systems.

2. Wall cooling systems investigated The four types of radiant wall systems investigated (Fig. 1) are constructed either of aerated concrete with low thermal conductivity, or a thermally conductive reinforced concrete. These two materials are representative of the broad scale of materials used for the construction of walls. The thermo-physical properties of the aerated concrete are similar to those of porous ceramic bricks and ceramic hollow bricks, which makes the results applicable to a wide range of buildings. Three out of the four systems in Fig. 1 have the active layer coupled to the concrete core, thus represent TABS. The radiant wall systems studied are described as follows:

3.1. Thermo-physical properties of the wall The thickness and thermo-physical properties of the material layers are described in Table 1. The total heat transfer coefficient of all wall types is equal to or less than 0.15 W/(m2⋅K), which corresponds to a

Fig. 1. Wall cooling systems investigated. 3

Applied Thermal Engineering 164 (2020) 114490

exterior

interior

M. Krajčík and O. Šikula

adiabatic boundary

cooling losses

cooling output cooling power

adiabatic boundary plaster

Newton´s law

Newton´s law

cooling pipes

concrete

thermal insulation

Fig. 2. Boundary conditions defining specific heat flux on a wall surface.

wall of a nearly zero-energy building in the region of Central Europe as required by European Directives [11,12,49].

resulting Reynolds number equals to 4598, indicating a transitional flow. The Nusselt number was, therefore, calculated by interpolation between Nu2300 and Nu10000 following a procedure as defined in Ref. [52]. For all the simulations, the temperature of the inner surface of the pipe was variable within the range of 20.1–26.3 °C. The effect of this temperature on p was relatively small, and the mean value of 23.2 °C was used in the calculations. The resulting Nu was 28.87. Assuming L equal to 0.599 W/(m⋅K), αp was determined to be 1218 W/(m2⋅K). In reality, p varies to some extent depending on the boundary conditions. To observe the effect of changing p , simulations were performed for p in the range of 1218 ± 300 W/(m2⋅K) for all the wall systems as defined in Fig. 1 and for the boundary conditions as defined in Section 4. Such variation in p led to changes in the cooling power, output, and losses (Fig. 2) of less than one percent. A representative value of p equal to 1218 W/(m2⋅K) was therefore used in all the simulations. The boundary conditions defining the specific heat flux on the surface of a computational domain were calculated according to Newton's law of cooling (Eq. (4)), assuming adiabatic wall fragment boundaries (Eq. (5)) as shown in Fig. 2:

3.2. Calculation of heat transfer The calculation was based on a detailed numerical solution of a twodimension stationary temperature field by the method of rectangleshaped control volumes, each representing a single temperature [50]. The distribution of temperature in the Cartesian coordinate system was described by the Fourier equation of thermal diffusion [51]:

T x

x

+

y

T y

+ S = . c.

T (1)

The resulting system of linear equations was solved by the Gauss elimination method and Gauss-Seidel iterative method. The successive over-relaxation method was used in the dynamic simulations as a variant of the Gauss-Seidel iteration method. The thermo-physical properties of materials were constant, isotropic, and temperature-independent in all the simulations. The computational mesh was carefully refined so that it fulfilled the criteria on the cell size as defined in EN ISO 10211 [47]. The calculation was considered converged when it complied with the convergence criterion defined as:

.

.

k q ,i i=1 k |q , i | i=1

0.001

(2)

= Nu.

L

D

(W/(m2 ·K))

w

T n

w

= h. (Tw

Tf )

(4)

=0

(5)

The temperature and heat flux distribution over time was calculated using the Robin-Newton boundary condition. The simulated fragment represented a section of the radiant wall as shown in Fig. 2. The green arrows indicate the direction of the cool transfer. The pipes in the radiant wall were spaced regularly, and the temperature of the water in the pipes and material properties were considered homogeneous along the wall. If not stated otherwise, the spacing of the pipes was 150 mm.

The heat transfer within the pipes is governed by forced convection. The calculation of the heat transfer coefficient is based on the boundary layer theory, in which the ratio of convective to conductive heat transfer is expressed by the Nusselt number [51]: p

T n

(3)

3.3. Calculation principle of heat transfer efficiency (HTE)

The value of p was calculated for a pipe with a length of 50 m and an inner diameter of 14.2 mm. The mean velocity of the water at the characteristic cross-section was 0.32 m/s. The mean water temperature was 20 °C. The corresponding = 0.981.10-6 m2/s and Pr = 7.05. The

In previous studies, the response time of radiant systems, defined as “the time it takes for the surface temperature of a radiant system to reach

Table 1 Thermo-physical properties of material layers. No.

(1) (2) (3) (4) (5) (6)

Material

Inner plaster Insulation - EPS F (only in Wall D) Aerated concrete or reinforced concrete Plaster containing the pipes (only in Wall A) Insulation - mineral wool Outer plaster Plastic pipe ø 20

Thickness

Volumetric weight

Thermal conductivity

Specific heat capacity

d

ρ

λ

c

m

kg/m3

W/(m⋅K)

J/(kg.K)

0.01–0.03 0.03 0.25 0.25 0.02 0.2 0.01

1300 17 600 2400 1300 20 1600 1200

0.7 0.04 0.19 1.58 0.7 0.04 0.8 0.35

840 1020 1000 1020 840 940 840 1000

4

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

63% or 95% of the difference between its final and initial values when a step change in control of the system is applied as input,” has proven as a useful indicator of thermal dynamics [32,33]. However, preliminary tests showed that response time was not capable of fully expressing the differences in the thermal dynamics of the various wall systems. Specifically, some of the systems had similar response time despite clear differences in their thermal dynamics caused by differences in configuration, thickness, and properties of the material layers, as illustrated in Fig. 3. To account for the differences in thermal dynamics, we have introduced an alternative indicator, called heat transfer efficiency. The heat transfer efficiency helps detect differences in the thermal dynamics of two systems even when their response time defined as τ95 is similar (Fig. 3). Heat transfer efficiency is an analogy to air change efficiency which is used to characterize ventilation effectiveness [16,53,54]. It is a measure of how efficiently the heat or cool supplied from pipes is transferred to the inner surface of the wall where it is emitted to the conditioned space. The heat transfer efficiency can be calculated by the step-down method from a declining curve, or by the step-up method from an ascending curve. In the present tests, the temperature of water at the beginning was 26 °C, equal to the operative temperature on both sides of the wall. Subsequently, the cooling was turned on, meaning that a step-change in water temperature from 26 °C to 20 °C was applied. The heat transfer efficiency was calculated from the decline in the surface temperature of the wall (Fig. 4) by the step-down method as follows: n

HTEstepdown = 100

2. ¯

·100

(%)

default, is realistic for radiant cooling systems operated under design weather conditions in temperate climates. 4.1. Weather data The combined effect of ambient temperature and solar radiation incident on the wall was replaced by sol-air temperature (Tsol-air). The sol-air temperature can be interpreted as the outside air temperature which, in the absence of solar radiation, would give the same temperature distribution and rate of heat transfer through a wall as exists due to the combined effects of the actual outdoor temperature distribution plus the incident solar radiation [57]. It is calculated by [51]:

Tsol

=

i= n i= 1

i+ i 1

2

(6)

. (ti

ti 1)

(h)

i+ i 1

2 i=n i=1

. (ti

i+ i 1

2

ti 1). . (ti

ti + t i 1 2

ti 1)

. R he

(9)

300 Temp. of inner surface (K)

i= n i= 1

he

The radiant heat transfer coefficient on the inner walĺs surface is relatively constant within the range of surface temperatures investigated [9]. The convective heat transfer coefficient will change depending on the air velocity and temperature difference between surface and room air [61,62]. Koca et al. [63] measured a total heat transfer coefficient of 8.20 at a surface temperature equal to 24.58 °C and an air temperature equal to 20.06 °C. In the present study, the design value of the overall heat transfer coefficient on the inner wall’s surface (h) of 8 W/(m2⋅K) [64] was used in all the simulations to avoid bias by factors that were not the subject of our investigations. The heat transfer coefficient of 25 W/(m2⋅K) as defined in EN ISO 6946 [65] for external walls corresponds to winter rather than summer conditions. A heat transfer coefficient of 15 W/(m2⋅K) was used as recommended by the national standard [66] because it better represents summer weather conditions. This heat transfer coefficient corresponds

where ti is the time that has elapsed since the heat flux was supplied in the wall through pipes (h); i is the difference between the wall surface temperature at the time ti and the steady-state wall surface temperature, i.e. s,i steady (K); i 1 is the difference between the wall surface temperature at the time ti-1 and the steady-state wall surface temperature, i.e. s,i 1 steady (K); 0 is the temperature of the inner surface of the wall at the beginning (K). The wall mean age of heat flux is calculated by the equation:

¯ =

. Ig

4.2. Heat transfer coefficients on the inner and outer wall’s surface

(7)

0

= Tamb +

The correction term for the vertical surfaces ε.ΔR is 0 °C under the assumption that the long-wave radiation emitted by the warm surfaces of terrestrial objects at high solar radiation compensates to some extent for the sky’s low emittance [58]. The absorptance of the external wall surface (α) is assumed 0.5, typical of, e.g., sandstone paint [59]. The course of sol-air temperature for the month of July and a southern wall is shown in Fig. 5 together with the input data. The values represent design conditions that are realistic for the temperate climate of Central Europe [60]. To account for the dampening effect of thermal inertia on the heat transfer through the wall, the inputs were averaged over eight hours. This period represents the time, in which a thermal impulse on one side of the wall is expressed with maximum intensity on the other side of the wall, based on tests with Wall A as defined in Fig. 1. The resulting sol-air temperature used in the simulations was 41 °C.

where n is the nominal time constant (h) and ¯ is the wall mean age of heat flux (h). The age of heat flux means the time passed until the heat flux is stored in the wall structure after entering the structure through pipes. For a point within the wall, the local mean age of heat flux is the time it takes for the heat flux to be stored in the actual point after entering the wall. The structure mean age of heat flux is the mean age of all the heat flux in the structure. The heat transfer efficiency is defined through the ratio of the lowest possible mean age of heat flux n/2 and the actual mean age of heat flux ¯ . The lower the ratio, the less efficient the heat storage and the more efficient the heat transfer from the pipe to the inner surface of the wall. High heat transfer efficiency therefore indicates efficient heat or cool transfer from the pipe to the inner surface of the wall. In the step-down method, the nominal time constant is obtained from the following equation (Fig. 4): n

air

(h) (8)

4. Boundary conditions The room temperature of 26 °C used in the simulations is interpreted as the operative temperature. It represents the upper temperature limit in conditioned buildings as defined in relevant standards [55,56]. The mean temperature of cooling water of 20 °C, used in the simulations as a

299

HTE1 = 50.7

θ95,1 298 297 θ95,2 296

HTE2 = 90.4

τ95,1 = τ95,2 Time elapsed, t (h)

Fig. 3. Two radiant wall systems with the same response time τ95 but different HTE. 5

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

on the wall’s performance was studied, numbered (5) to (9). Depending on the performance indicator or parameter, dynamic, stationary or both types of simulation were performed. 6. Results 6.1. Heat transfer efficiency For each of the wall systems, the heat transfer efficiency was tested for a matrix of 3 × 3 realistic boundary conditions (Fig. 6). These involve combinations of the thickness of the concrete equal to 200 mm, 300 mm, and 400 mm and spacing of the pipes equal to 150 mm, 200 mm, and 250 mm. The thickness of thermal insulation was always 200 mm. The box plots in Fig. 6 represent the minimum, median and maximum values of the heat transfer efficiency. The heat transfer efficiency was substantially lower for Walls A and B with pipes further from the wall’s inner surface than for Walls C and D with pipes underneath the surface. For Wall A the heat transfer efficiency was always lower than 50%, and it could be as low as 24%. The heat transfer efficiency has improved by moving the pipes from thermal insulation to the concrete core (Wall B), although the maximum was only about 50%. Moving the pipes closer to the interior (Wall C) improved the heat transfer efficiency considerably. In this case, the values were notably lower for the thermally conductive reinforced concrete than for aerated concrete. The heat transfer efficiency was best when the pipes were located underneath the surface and insulated from the core (Wall D). The heat transfer is visualized by the temperature and heat flux distribution in Fig. 7. The yellow arrows indicate the general direction of the cool transfer. The heat transfer efficiency of Wall A is low because of the high losses and the cool being accumulated in the concrete core. In Wall B the concrete properties have little effect on the heat flux distribution, although the cooling output is better for the conductive reinforced concrete. The heat transfer efficiency of Wall C is lower in the case with reinforced concrete because the cool is distributed more evenly. Consequently, more cool is being accumulated in the structure and its transfer to the interior is less efficient.

Fig. 4. Decline in temperature of inner surface of the wall after turning the cooling system on.

Fig. 5. The course of ambient temperature, sol-air temperature and incident radiation on a southern wall in July [60], and their averages over eight hours.

to the wind speed of about 0.6 m/s for an exposed surface and about 1–1.5 m/s for a sheltered surface [67–69], representing a calm to light air on the Beaufort scale [70]. The heat transfer coefficients were kept constant throughout the study.

6.2. Thermal dynamics and the effect of water temperature

5. Performance indicators and parameters investigated

The tests of thermal dynamics were done for the sol-air temperature of 41 °C as determined in 4.1. To retain consistency, the same 8-hour interval was used in the test of thermal dynamics as in the calculation of sol-air temperature, i.e. the cooling system was powered on at 900, and

Four performance indicators were used to evaluate the wall systems, numbered (1) to (4) in Table 2. Moreover, the effect of five parameters Table 2 Parameters investigated and simulation methods used. Wall

Performance indicators (1 – 4) (1) HTE

(2) Thermal dynamics

(%)

Parameters of the wall system (5 – 9)

(3) Output, losses

(4) Cooling power

(5) Water temp.

(W/m2)

(W/m2)

(°C)

(6) Wall material

(7) Wall location

(8) Thickn. of wall

(9) Spacing of pipes

(mm)

(mm)

A

d

d

s/d

s/d

d

s/d

s

s

s

B

d

d

s/d

s/d

d

s/d

s

s

s

C

d

d

s/d

s/d

–

s/d

s

s

s

D

d

d

s/d

s/d

–

s/d

s

s

s

Key: d – dynamic simulations, s – stationary simulations, s/d – both stationary and dynamic simulations. 6

Applied Thermal Engineering 164 (2020) 114490

95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20

Wall B

Wall A

max

Heat transfer efficiency (%)

Heat transfer efficiency (%)

M. Krajčík and O. Šikula

median

min

95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20

Wall C

Wall D

Fig. 6. Heat transfer efficiency of the four wall types. AC – aerated concrete, RC – reinforced concrete.

turned off at 1700 o’clock. To study the thermal dynamics, a control strategy was created where the cooling output was kept between 63% (q63) and 90% (q90) of its maximum value by turning the cooling system on and off. Although simplified as compared to real operating conditions, this control strategy allows evaluating thermal dynamics and controllability of the wall systems. Fig. 8 shows the thermal dynamics of the two systems with pipes located in the core, Walls A and B. Regardless of the water temperature and concrete properties, at the end of the test interval the cooling output reached only a fraction of its maximum value (Table 3). The negative cooling output in the beginning, at about 900 o’clock, is interpreted as a heat gain from outside to the interior. The cool transfer within the wall was too slow to cover the heat gain. The only potentially feasible cooling output was attained by Wall B combined with reinforced concrete at the water temperature of 15 °C. Fig. 9 shows that concrete properties are crucial for the thermal dynamics of Wall C. Despite the cooling output being relatively high regardless of the core properties (Table 3), the combination with reinforced concrete resulted in slow thermal dynamics. This contrasts with the results for Walls A and B, in which using reinforced concrete enhanced the thermal dynamics and cooling output. Adding thermal insulation between the thermally active layer and the concrete core in Wall D resulted in a fast reaction of the system and

Wall

Aerated concrete (AC),

40°C B

C

D

6.3. Cooling output vs. Cooling losses Fig. 11 shows the maximum cooling output and losses for the cooling systems located on an external wall, i.e. exposed to weather conditions. The results can be understood by looking at the visualisation of thermal fields and heat flux distributions in Fig. 7. Walls A and B have the lowest cooling output, are most sensitive to the concrete properties, and their ratio of cooling loses to power (L) is highest. The maximum cooling output of the two systems with pipes underneath the surface, C and D, is higher than that for Walls A and B, and it is less dependent on concrete properties. Especially in Wall D, which has the active layer insulated from the concrete core, the effect of concrete properties is negligible. The cooling output of Wall C is superior to that of Wall D because the thermal coupling of the active layer with the concrete core allows a more even distribution of the cool throughout the surface layer (Fig. 7).

= 0.19 W/(m.K)

Thermal field Lowest output

A

high cooling output (Table 3). In this case, the number of operation cycles was highest among all of the wall systems because of the low heat capacity of the active layer. Consequently, even during a day with substantial variations in the cooling load, Wall D can keep the room temperature in a comfortable range, although it may require a high number of operation cycles (Fig. 10).

Heat flux

20°C 0 W/m2

20°C 0 W/m2

36 W/m2 40°C

Heat flux

20°C 0 W/m2

120 W/m2

20°C 0 W/m2

120 W/m2

20°C 0 W/m2

120 W/m2

20°C 0 W/m2

120 W/m2

Lower output than AC 20°C 0 W/m2

120 W/m2 40°C Same output as AC

Lower output ; than “C” 40°C

= 1.58 W/(m.K)

Higher output than AC

Higher output than “D” 40°C

Thermal field Higher outputthan AC

36 W/m2 40°C

Higher output than “A” 40°C

Reinforced concrete (RC),

20°C 0 W/m2

120 W/m2 40°C

Fig. 7. Temperature and heat flux distribution for the cooling system located on an external wall. 7

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

Wall A 6

Reinforced c. =1.58 W/(m.K) Twater = 20°C

4

Aerated c. =0.19 W/(m.K) Twater = 15°C

2 0

Aerated c. =0.19 W/(m.K) Twater = 20°C

-2 -4

Reinforced c. =1.58 W/(m.K) Twater = 15°C

29 Cooling output, qi (W/m2)

8 Cooling output, qi (W/m2)

Wall B

Reinforced c. =1.58 W/(m.K) Twater = 15°C

25

Reinforced c. =1.58 W/(m.K) Twater = 20°C

21 17 13

Aerated c. =0.19 W/(m.K) Twater = 15°C

9 5

Aerated c. =0.19 W/(m.K) Twater = 20°C

1 -3

Time of operation (hh:mm)

Time of operation (hh:mm)

Fig. 8. Effect of core material and water temperature on thermal dynamics of Wall A and Wall B.

6.4. The effect of wall location

also elaborated for the concrete thickness of 300 mm and 400 mm, but the trend was similar regardless of the thickness. Adding thermal insulation has minor effect on the cooling output when the core is made of aerated concrete. The negative cooling output for Wall A with no thermal insulation means that the wall system does not cool the room, but it absorbs heat from the exterior and transfers it to the inside. On the other hand, the effect is crucial when the core is made of the conductive reinforced concrete. The first centimetres of thermal insulation are critical. Increasing the thickness beyond 50 mm does not provide any considerable benefits. The exception is Wall D when the cooling output is high even without any insulation on the outer side of the wall. Fig. 15 shows the relationship between the cooling output and the spacing of the pipes. The results refer to the concrete thickness (dconc) of 250 mm and insulation thickness (dTI) of 200 mm. The spacing of the pipes has an important effect on the cooling output of Walls C and D. The effect is much smaller for Walls A and B, and it is more pronounced when the core is made of the conductive reinforced concrete as compared to aerated concrete.

For a cooling system located on an internal wall, the losses depend especially on the type of wall, concrete properties, and room temperature on the outer side of the wall (Fig. 12). It is not advisable to use Walls A and B without any thermal insulation. Additional simulations have shown that for Walls A and B the ratio of losses (L) can be significantly reduced by adding a layer of thermal insulation on the outer side of the wall. The cooling output is always lowest, and the losses are highest for Wall A (Fig. 12) because the concrete insulates the pipes from the interior (Fig. 13). It is advisable to consider using this system only in cases when the installation of the cooling pipes on the inner side is not possible. In Wall B the heat flux distribution is similar for both types of concrete (Fig. 13), but the aerated concrete acts as thermal insulation and reduces the cooling output significantly (Fig. 12). Wall C can be used without any thermal insulation when combined with aerated concrete, which serves as thermal insulation and directs the cool transfer to the interior (Figs. 12 and 13). Wall D can be safely used without any thermal insulation on the outer side. Additional simulations have shown that adding outer thermal insulation on Walls C and D reduces cooling losses but has little effect on the cooling output. The impact of thermal insulation for these systems is lesser in combination with aerated concrete.

7. Discussion The past researches report a significant impact of the concrete thickness, pipe spacing, and concrete properties on the response time of radiant floors and ceilings [32,33]. Moreover, it was found that the thickness of thermal insulation, the spacing of the pipes, and water temperature have a substantial effect on the heating capacity of Wall A [18]. The present study confirms the effect of insulation thickness, pipe spacing and concrete properties for Wall A operated as space cooling. In general, insulation thickness is especially important for the walls with a

6.5. Thickness of wall and spacing of pipes Fig. 14 shows the effect of wall thickness on the cooling output. The results refer to the concrete thickness (dconc) of 200 mm, pipe spacing of 150 mm, and variable thickness of thermal insulation. The results were

Table 3 Maximum cooling output and cooling power, and the number of operation cycles (Twater = 20 °C). Wall

Concrete

Max. cooling output

Max. cooling power

qi,max (W/m2)

qt,max (W/m2)

Number of operation cycles

A

Aerated, λ = 0.19 W/(m⋅K) Reinforced, λ = 1.58 W/(m⋅K)

3.6 15.8

8 20

0 0

B

Aerated, λ = 0.19 W/(m⋅K) Reinforced, λ = 1.58 W/(m⋅K)

6.0 23.1

10 27

0 0

C

Aerated, λ = 0.19 W/(m⋅K) Reinforced, λ = 1.58 W/(m⋅K)

26.2 29.0

29 33

3 0

D

Aerated, λ = 0.19 W/(m⋅K) Reinforced, λ = 1.58 W/(m⋅K)

25.4 25.2

28 29

6 5

8

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

120 100

q63

80

15

60

10

40

Cooling power

5

20

0

0

25 20

140

q90 Cooling output

120 100

q63

80

15

60

10

Cooling power

40

5

20

0

0

Time of operation (hh:mm)

Cooling power, qt (W/m2)

20

140 Cooling output

Cooling output, qi (W/m2)

q90

25

Wall C, reinforced conc. =1.58 W/(m.K) Cooling power, qt (W/m2)

Cooling output, qi (W/m2)

Wall C, aerated conc. =0.19 W/(m.K)

Time of operation (hh:mm)

Fig. 9. Effect of core material on thermal dynamics of Wall C.

thermally conductive core. Insulation thickness and concrete properties have little effect on the cooling output of Wall D which has the active layer insulated from the concrete core.

spacing the pipes closer to each other, especially for Walls C and D with the pipes located underneath the surface (Fig. 15). This is consistent with the findings of a previous study regarding wall systems with capillary mats located underneath the surface, where the heating and cooling output was enhanced by spacing the tubes closer to each other [36]. Several studies are available regarding the thermal output of radiant wall heating [13,18,63,71] and cooling [6,17,36,48,72,73,74] systems. A summary of the research studies that report the cooling output of waterbased wall cooling systems is shown in Table 4. The cooling output of Wall A in Fig. 11 (reinforced concrete) can be compared with Ref. [48]. The 16 W/m2 obtained in this study is higher than the 11 W/m2 reported in Ref. [48] for a system with no metal fin between pipe and wall, despite the h being lower in this study. This is because in the present study the pipes were located in plaster between thermal insulation and the concrete core instead of in channels in thermal insulation, which led to a more efficient cool transfer from pipes to the interior. The cooling output of Wall C was 26 W/m2 for AC and 29 W/m2 for RC (Fig. 11), thus slightly less than the 31 to 35 W/m2 as reported in Refs. [6,17]. In Ref. [6] this was likely partially caused by the pipes directly exposed to the room air, i.e. not embedded in plaster. In Ref. [17] the cause is difficult to determine due to a lack of information. On the other hand, Romaní et al. [72] obtained a much lower cooling output of 17 W/m2. The factors contributing to the relatively low output were likely cooling losses due to the high daily outdoor temperatures and solar irradiation, the h possibly being lower than in the present study, and the uncertainty in the area of the active wall surface which we estimated from a figure.

The heat transfer depends on concrete properties, which affect the heat flux distribution (Figs. 7 and 13). A thermally conductive concrete core improves the heat transfer efficiency and cooling output of Walls A and B. Conversely, using aerated concrete in Walls A and B may diminish the cooling output to a value so small that it cannot be compensated for by decreasing the water temperature (Fig. 8). The maximum cooling output of Walls C and D is always higher than that of Walls A and B. A thermally insulating core is preferable for Wall C to decrease the losses and direct the cool transfer to the interior. For Wall D the cooling output is almost independent of the concrete properties (Figs. 7 and 11). For a well-insulated external wall, the absolute value of cooling losses is similar regardless of the wall system (Fig. 11). For the location on an internal wall, using systems A and B is meaningful only when the outer side of the wall is thermally insulated (Figs. 12 and 13). This is true also for system C if the core is made of a highly conductive material. The cooling output increases with moving the pipes closer to the interior. This extends the previous findings for wall systems operated as space heating [18] also to their operation as space cooling. From the range of wall cooling systems investigated, the cooling output was highest for Wall C (29 W/m2). The cooling output can be enhanced by

20 15

q90

140 Cooling output

120 100

q63

80 60

10

40

5 Cooling power 0

20 0

Cooling output, qi (W/m2)

25

Wall D, reinforced conc. =1.58 W/(m.K) Cooling power, qt (W/m2)

Cooling output, qi (W/m2)

Wall D, aerated conc. =0.19 W/(m.K)

Time of operation (hh:mm)

25 20 15

q90

140 Cooling output

100

q63

80 60

10

40

5 Cooling power 0

Time of operation (hh:mm)

Fig. 10. Effect of core material on thermal dynamics of Wall D. 9

120

20 0

Cooling power, qt (W/m2)

7.1. Heat transfer efficiency, cooling output, and cooling losses

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

Aerated concrete = 0.19 W/(m.K)

Output

35

L = 0.1

30

Losses Cooling output & losses (W/m2)

Cooling output & losses (W/m2)

Losses

L = 0.1

25 20 15

L = 0.4

L = 0.5

10 5 0

Wall A

Wall B

Wall C

Reinforced concrete = 1.58 W/(m.K)

Output

35

L = 0.1

30

L = 0.1

L = 0.1

25

L = 0.2

20 15 10 5 0

Wall A

Wall D

Wall B

Wall C

Wall D

Fig. 11. Cooling output and losses for the cooling system located on an external wall. L = losses/power.

The cooling output of Wall D (Fig. 11) is best comparable with that obtained by Mikeska and Svendsen [36]. The 28 W/(m2⋅K) reported in Ref. [36] was slightly higher than the 25 W/(m2⋅K) in Fig. 11. Two of the causes were the dense spacing of the tubes in the capillary mats and the higher thermal conductivity of the surface layer in the study by Mikeska and Svendsen [36]. A substantially higher cooling output of 36 W/m2 was measured by Vangtook and Chirattananon [73]. This value refers to a difference in the room and surface temperature of 5 K, thus greater than the 3.2 K in this study. Another contributing factor was likely the high efficiency of the cool transfer between the copper pipe and the emitting element (copper plate). From all the studies presented in Table 4, the cooling output was greatest in Ref [74]. In this case, cooling was provided by grooved vertical cylinders that were freestanding and directly exposed to the room all over their diameter, with the surface temperature well below the dew point.

require higher peak cooling power than Walls A and B (Table 3) and a corresponding energy source that can cover the peaks (Figs. 9 and 10). This might be a potential disadvantage of the systems with fast thermal dynamics as compared to the thermally active systems which may be charged over nights and weekends by energy sources with a relatively low design cooling load. In a previous study including three radiant systems with various levels of thermal dynamics [38], the peak cooling power depended on the heat gains. The results were consistent with the present study in cases with high internal heat gains and solar radiation through the windows. In these cases, the peak cooling power was highest for ceiling panels with a fast thermal response, followed by lightweight floor cooling and TABS. At lower cooling power the differences between the systems were smaller. 7.3. Recommended operation strategies and the potential for energy storage

7.2. Cooling power

Walls A, B, and C in the present study correspond to radiant heating and cooling systems type E as defined in EN ISO 11855-2 [64]. Previously it has been shown that this type of radiant system has the

L=0

L = 0.2

L = 0.1

L = 0.1

30

L = 0.7

L = 0.8

L = 0.5

L = 0.3

40

L = 0.5

50

Reinforced concrete λ = 1.58 W/(m.K)

L = 0.2

L = 0.1

L=0

L = 0.1

L = 0.2

L = 0.1 L=0

0

60

Output

L = 0.7

Cooling output & losses (W/m2)

10

L = 0.2

20

L = 0.7

30

L = 0.5

40

L = 0.7

50

Losses

Aerated concrete λ = 0.19 W/(m.K)

L=1

60

Output

L = 0.9

Cooling output & losses (W/m2)

Losses

L = 0.3

Walls C and D are capable of a quick adjustment of the cooling output due to their fast thermal dynamics (Fig. 8). However, they

20 10 0

Fig. 12. Cooling output and losses for the cooling system located on an internal wall with no thermal insulation, at three different room temperatures on the outer side of the wall. L = losses/power. 10

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

Aerated concrete (AC),

Wall

A 26°C B 26°C C

= 0.19 W/(m.K)

Reinforced concrete (RC),

Thermal field Heat flux Not applicable without TI 144 W/m2 0 W/m2 20°C 26°C Not applicable without TI 2 44 W/m2 26°C 20°C 0 W/m Applicable without TI 20°C 0 W/m

26°C

2

140 W/m2 26°C

20°C Applicable without TI

Applicable without TI

D 26°C

20°C

140 W/m2

0 W/m2

= 1.58 W/(m.K)

Thermal field Heat flux Not applicable without TI 0 W/m2 168 W/m2 20°C Not applicable without TI 0 W/m2 168 W/m2 20°C TI recommended

26°C

20°C

0 W/m2

168 W/m2

0 W/m2

144 W/m2

Fig. 13. Temperature and heat flux distribution for the cooling system located on an internal wall. TI – thermal insulation.

Aerated concrete, = 0.19 W/(m.K) C

25

Cooling output (W/m2)

Cooling output (W/m2)

30

Reinforced concrete, = 1.58 W/(m.K)

D

20 15 10

B

5 A

0 -5

0

50

100

150

C

30

200

25 D

B

20 15

A

10 5 0 0

-5

Thickness of thermal insulation (mm)

50

100

150

200

Thickness of thermal insulation (mm)

Fig. 14. The effect of insulation thickness on cooling output, dconc = 200 mm.

Reinforced concrete, = 1.58 W/(m.K) 30

25

25

20

Cooling output (W/m2)

Cooling output (W/m2)

Aerated concrete, = 0.19 W/(m.K) 30

C

15 D

10

B

5 A

0 150

C

20

B D

15 10

A

5 0

200

250

300

150

Spacing of pipes (mm)

200

250

300

Spacing of pipes (mm)

Fig. 15. The effect of spacing of pipes on cooling output, dconc = 250 mm, dTI = 200 mm.

longest thermal response among all the types defined in the standard [32]. This corresponds with the slow thermal dynamics of Walls A, B, and C obtained in the present study (Fig. 8). An exception is the combination of Wall C with aerated concrete. In this case, the concrete core acts as thermal insulation and directs the cool transfer to the interior which results in faster thermal dynamics (Fig. 9).

The advantage of the thermally active Walls A, B, and C may be their potential for energy storage. The slow thermal dynamics of these systems is to a high extent caused by energy storage in the wall structure (Fig. 7). Especially Wall B is suitable for applications that involve energy storage. In contrast, the energy storage potential of Wall D is very limited, but its fast thermal dynamics allows fast adjustments of 11

12

Experimental room exposed to weather conditions

Climate chamber Nine parallel vertical cylinders. Each cylinder is a doublewall pipe and the cooling medium flows between the walls. The device has a condensate pan at the bottom.

Haiwen et al. [74]

Wall element composed of two concrete plates, with thermal insulation between them. Capillary matts located in the middle of 30 mm concrete plate. Radiant cooling panel constructed from copper pipe bonded to copper plate.

Vangtook & Chirarattananon [73]

Pipes embedded in a brick wall, located close to wall surface.

Radiant terminals embedded close to the surface.

Generic wall fragment

Simulated office

Le Dréau & Heiselberg [17]

Mikeska & Svendsen [36]

Four rooms in experimental house

Akbulut et al. [6]

Pipes attached to outer side of concrete core. The pipes are arranged in milled channels in thermal insulation. Pipes attached to wall surface, directly exposed to room air.

Experimental room exposed to weather conditions

Generic wall fragment

Šimko et al. [48]

Description of the wall cooling system

Romaní et al. [72]

Location of cooling wall

Study

Wall D

Wall D

Wall D

Wall C

Wall C

Wall C

Wall A

Similar to

Cylinders are not thermally coupled with the main thermal mass.

The pipe is not thermally coupled with the main thermal mass.

Pipes located close to surface, thermally coupled with the main thermal mass. Pipes embedded in surface layer, thermally uncoupled from the rest of thermal mass.

Pipes located close to surface, thermally coupled with the main thermal mass.

Pipes located on surface, thermally coupled with the main thermal mass.

Pipes attached to outer side of concrete core.

Similarity with present study

Table 4 Summary of research studies that report the cooling output of water-based wall cooling systems.

26

Close to water temp.

17.7 (avg. of inlet and outlet temp.)

No report

7.2 (estimate) No report

26

7.45

5 to 7 (estimate)

No report

No report

No report

66.8

36

28 (linear interpolation)

14.5 to 17 (depends on control strategy)

35

31

13.5 to 16.5 (depends on fin) 32.6

10.8

(W/m2)

(W/(m2⋅K)) 12.5

Cooling output

h

31

26

No report

26

22.7 No report

26

No report

26

(°C)

Room temp.

22.3

No report

> 24

20

20 to 21

21.13 (avg. of inlet and outlet temp.) No report

No report

(°C)

(°C) 20

Surface temp.

Mean water temp.

The estimate of h was based on the boundary conditions. Surface temp. of Wall D in Fig. 11 was 22.8 °C. Cooling output refers to 2times the area of the freestanding test rig. Surface temp. of Wall D in Fig. 11 was 22.8 °C.

Surface temp. corresponds to reinforced concrete, Wall C (Fig. 11) Surface temp. corresponds to aerated concrete, Wall C (Fig. 11) The active surface area was estimated; h estimated for room temp. of 24 °C and surface temp. of 21 °C. Cooling output refers to tubes with ø 4.5 spaced by 100 mm.

Without metal fin between pipe and wall. With metal fin between pipe and wall.

Note

M. Krajčík and O. Šikula

Applied Thermal Engineering 164 (2020) 114490

Applied Thermal Engineering 164 (2020) 114490

M. Krajčík and O. Šikula

Table 5 Recommended operation strategies for the wall cooling systems. Category

Description

Wall system

I II III

Insufficient cooling output, the change in cooling output is slow Enough cooling output, the change in cooling output is slow Enough cooling output, the change in cooling output is fast

All combinations of Wall A; Wall B + aerated concrete Wall B + reinforced concrete; Wall C + reinforced concrete Wall C + aerated concrete; All combinations of Wall D

Table 6 Performance of the wall cooling systems. Wall

Performance indicator HTE

Thermal dynamics

Output & losses

Cooling power

A

Lowest (24–47%)

The change in cooling output is slow. Possibility of energy storage.

Lowest power. Much lower for AC than for RC.

B

Higher than for Wall A (42–52%)

The change in cooling output is slow. Possibility of energy storage.

C

Higher than for Wall B. Higher for AC (87–93%) than RC (75–84%)

D

Highest, regardless of material (≥93%)

RC: change in cooling output is slow. Limited possibility of energy storage. AC: change in cooling output is fast. Not suitable for energy storage. The change in cooling output is fast. Not suitable for energy storage.

Lowest output, highest losses, especially for AC. TI needed to prevent losses. Reasonable output only for RC at lower Twater. TI needed to prevent losses. Higher than Wall B. For RC, TI is recommended to prevent losses. Higher than Wall B. Losses are low, even without any TI on the outer side of the wall.

Similar to Wall C with TI. Lower than for Wall C without any TI.

Higher than for Wall A. Much lower for AC than for RC. Higher than for Wall B. For RC higher than for AC.

Key: AC – aerated concrete, RC – reinforced concrete, TI – thermal insulation, Twater – temperature of water

the cooling output and more accurate control of the room temperature. Table 5 summarizes the recommended operation strategies for the various combinations of wall systems and core materials.

•

7.4. Overall evaluation Table 6 summarizes the performance of the wall cooling systems. This table can serve as a tool to quickly acquire complex information about a wall cooling system and thereby support the decision-making process to select the system that is most convenient for the specific situation.

•

8. Conclusions and recommendations The present study adds to the existing body of knowledge regarding low-exergy wall cooling systems by considering their specifics as compared to floors and ceilings and their potential for application in existing buildings as a part of their retrofit. To accomplish this, we defined and directly compared four types of wall cooling systems from which three are potentially suitable for installation in retrofitted existing buildings. The effect of the key design parameters on the system characteristics was studied by using four performance indicators. For this purpose, an indicator called heat transfer efficiency was introduced to help detect the differences in thermal dynamics of various systems even in cases when their response time, defined as τ95, is similar. The results facilitate the selection of the most suitable wall cooling system and provide practical guidance to the system design. The main conclusions that can be drawn from this study are as follows:

•

Acknowledgement

• Walls C and D with the pipes located underneath the surface have a

•

helps avoid major interventions in the interiors of retrofitted buildings but has the lowest cooling output. The low heat transfer efficiency and slow thermal dynamics of Walls A and B with the pipes embedded in the core indicate the ability of these systems to store energy. This reduces the required cooling power and the number of operation cycles of the cool source, and allows shifting its operation to times when the electricity is more affordable. Wall D with high heat transfer efficiency and fast thermal dynamics requires continuous operation of the cool source or additional thermal storage. Wall C represents a compromise between these extremes. The appropriate operation strategy of the wall system depends on the location of the pipes, configuration of the material layers and material of the core. Wall C combined with a core with low thermal conductivity and Wall D allow fast changes in the cooling output and thereby good controllability of the room temperature. The other systems investigated require more sophisticated control strategies because of their slow thermal dynamics and lower heat transfer efficiency. The cooling output of Walls A and B with the pipes embedded in the core is sensitive to insulation thickness, whereas the cooling output of Walls C and D with the pipes underneath the surface is sensitive to pipe spacing. The sensitivity of Wall C to insulation thickness depends on the thermal conductivity of its core. Wall D functions well even without any thermal insulation on the outer side of the wall.

This research was supported by the Slovak Research and Development Agency under contract No. APVV-16-0126, Ministry of Education, Science, Research and Sport grants VEGA 1/0807/17 and 1/ 0847/18, and by TAČR NCK CAMEB project TN01000056/06.

higher cooling output than Walls A and B with the pipes embedded in the core. They, therefore, require a smaller area of active surface at the same temperature of cooling water, or a higher temperature of cooling water at the same area of active surface which improves the efficiency of the cool source. Wall A with the pipes located on the outer side of the thermally active core and Wall C with the pipes underneath the surface provide the potential for thermal storage in retrofitted buildings. Wall A

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