Numerical simulation of a solar cooling system with and without phase change materials in radiant walls of a building

Energy Conversion and Management 188 (2019) 40–53

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Numerical simulation of a solar cooling system with and without phase change materials in radiant walls of a building

T

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Maria T. Plytaria , Evangelos Bellos, Christos Tzivanidis, Kimon A. Antonopoulos Thermal Department, School of Mechanical Engineering, National Technical University of Athens, Greece

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar cooling Absorption chiller Radiant walls TRNSYS PCM Total cost

This work investigates energetically and financially a building solar cooling system with radiant walls which includes phase change materials (PCMs). The examined building has a floor area of 100 m2 and it is located in Athens (Greece). The solar cooling system includes evacuated tube collectors coupled to a single-effect absorption chiller for producing cold water for the building radiant walls. The use of PCM is examined in all the building’s outer walls and different scenarios with PCM in only some walls are also investigated in details. Different combinations of collecting areas and storage tank volumes are examined in order to determine the optimum design for every scenario. The study is performed with the commercial software TRNSYS which uses an active layer to simulate the radiant wall. The most important calculated parameters of this work are the auxiliary energy consumption, the solar coverage and the indoor temperature of the building. The results indicate that the best location of PCM layer is in south wall with a reduction of the auxiliary energy 30%, an increase of the solar coverage 3.8% and a reduction of the total system cost of about 3%.

1. Introduction It is well known that the building sector consumes a large amount of energy which is about 30%∼40% of the worldwide energy consumption. Nowadays, there are many energy problems such as the increase in the electricity price, the increased CO2 emissions and natural pollution [1]. The use of renewable energy sources, such as solar energy, can be an effective solution to many energy and environmental problems. Solar energy can be used in numerous applications for space heating and cooling in buildings in order to reduce the energy consumption [2]. Especially in the summertime, solar cooling is a technology which leads to the decrease of the electricity consumption [3]. Absorption chillers with LiBr/H2O are the main technology, which is usually used in solar cooling applications. The alternative choice is the use of H2O/NH3 but this working pair is usually used in refrigeration applications with refrigeration production at temperature levels lower than 0 °C. Moreover, the use of LiBr/H2O presents a bit higher performance than the H2O/NH3, while another limitation of the H2O/ NH3 working pair is the high toxicity of the NH3. Another choice is the use of LiCl/H2O, as the working pair, but it faces crucial crystallization problems and so it is not a proper choice. Aliane et al. [4] reviewed solar absorption cooling studies in order to lay emphasis on the operational aspects of absorption chillers which are fed by solar heat.

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

https://doi.org/10.1016/j.enconman.2019.03.042 Received 3 December 2018; Accepted 14 March 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

There are many investigations with solar-driven absorption chillers in the literature. Shirazi et al. [5] modeled five solar heating and cooling absorption systems and they investigated them for different climates. The same authors, in another study [6], analyzed energetically, environmentally and economically three systems with a single-effect, a double-effect and a triple-effect absorption chiller and found that the system with double-effect absorption chiller is the most attractive choice. Chen et al. [7] developed an air-cooled single effect absorption chiller and concluded that this system can increase the cooling capacity of about 65.5%. Bellos et al. [8] analyzed energetically, exergetically and economically a solar cooling system with a single effect absorption chiller coupled to evacuated tube collectors and they found a payback period of 15 years. Moreover, Bellos et al. [9] examined a solar-driven cascade absorption-compression refrigeration system and they found a payback period of 14 years. Bellos and Tzivanidis [10] investigated an ejector-absorption chiller system which was driven by solar parabolic trough collectors and found that this system presented higher performance compared to the conventional system. Lu [11] studied ten different absorption cooling/heating systems and he concluded that the system with a double-effect absorption chiller with a heat source of waste heat is the most optimized choice. Zhou et al. [12] investigated experimentally a single/double hybrid effect absorption cooling system driven by linear Fresnel solar collectors. Xu and Wang [13] investigated

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Nomenclature Ac b0 C cp cp,liquid cp,solid f FR GT Kel KO&M m mpcm N Q q1 q2 r T U UL V

aux c c,in c,out E el liquid loss max min N nom s S solid stored t th tot u W w,in w,out

Collecting area, m2 Incidence angle modifier constant, Cost, € Specific heat capacity, kJ/(kg K) Specific heat capacity at the liquid state, kJ/(kg K) Specific heat capacity at the solid state, kJ/(kg K) Solar coverage of the absorption chiller, Heat removal factor, Solar titled radiation, W/m2 Specific electricity cost, €/kJ Operating and Maintenance cost, € Mass flow rate, kg/s Mass of the PCM, kg Project life, years Heat rate, kW First logistic energy rate in TRNSYS, kW Second logistic energy rate in TRNSYS, kW Discount factor, % Temperature, °C Thermal transmittance, W/(m2 K) Thermal loss coefficient of the solar collector, W/(m2 K) Storage tank volume, m3

auxiliary collector collector inlet collector outlet east electrical liquid state thermal loss of the storage tank maximum minimum north nominal cooling solar south solid state storage tank tank thermal total useful west wall in wall out

Abbreviations Greek symbols α β τ

COP ETC HVAC PCM TABS TESS

Plate absorptance, Slope angle, ° Cover transmittance, -

Subscripts and superscripts abs

Coefficient of performance Evacuate tube collector Heating, ventilation and air conditioning Phase change material Thermally activated building systems Thermal Energy System Specialists

absorption chiller

of thermal comfort. Simko et al. [22] investigated, numerically and experimentally, an active wall with pipes which were arranged into channels. They found that this system could reduce heat loss. Jing et al. [23] designed a solar absorption-subcooled cooling system in buildings and concluded that the solar irradiation and the cooling demands influence on the system cooling capacity. Furthermore, the integration of the PCM into the building envelope, such as on the roof [24], on the floor [25] and in walls [26], is of great interest in recent years for both space heating and cooling. The review of Mengjie et al. [27] provided an update on recent developments in phase change materials into buildings’ envelope and into equipment. They concluded that for the buildings’ envelope the phase change temperature range of PCMs was changed between 10 °C and 39 °C and for the application into equipment between −15.4 °C and 77 °C. Zhu et al. [28] reviewed applications of shape-stabilized phase change materials which are embedded in buildings in recent ten years. They concluded that these PCMs reduce indoor temperature fluctuations and energy demands. Fateh et al. [29] examined the use of PCM in a lightweight building, and they found the optimum melting temperature of the PCMs to be 23 °C. Stritih et al. [30] examined the use of PCMs in the structural components of a building in order to make it a nearly zero energy building. They stated that the weather data are very important in building thermal behavior. Sun et al. [31] calculated the heat transfer of an office with PCM board, which had been installed on its inner face of the exterior wall and they found the optimal melting temperatures of the PCM board for different cities of China. Erlbeck et al. [32] found that by changing the design of a PCM, which is included in a concrete block, the thermal behavior optimizes without adding more PCM mass. Errebai et al. [33] carried out experiments in

theoretically a solar driving variable effect LiBr/H2O absorption cooling system and found that this system can be used for higher efficiency solar cooling in long working time and small collector area. Ibrahim et al. [14] investigated a solar absorption cooling system with ice-storage. Sokhansefat et al. [15] optimized a solar cooling system with a single effect absorption chiller and they found 28% performance enhancement and solar coverage up to 70%. Furthermore, Khan et al. [16] developed two different models of a solar cooling system with a single effect absorption chiller. Except for the absorption chillers, there are also other cooling techniques which reduced the energy consumption by increasing the energy efficiency in buildings. For example, the thermally activated building systems (TABS), which consist of pipes, can be integrated into the building envelope, such as walls, roofs and floors, for cooling applications by improving the thermal performance of a building. Many researchers have investigated these systems. Romaní et al. [17–18] studied experimentally the performance of a radiant wall cubicle coupled with a ground heat exchanger and found that there was a reduction of cooling loads of about 20% when the air indoor temperature was 26 °C. Liu et al. [19] proposed and studied experimentally an active solar thermoelectric radiant wall system which reduced the thermal loads of a building and had also a cooling efficiency of about 5%. Luo et al. [20] investigated a thermoelectric wall system with a photovoltaic façade in cooling climates and concluded that this system could save energy on average of 46%. Mikeska and Svendsen [21] carried out experiments in order to evaluate the influence of a radiant cooling system on the indoor thermal comfort of a building. They concluded that when the water temperature was 4 K lower than the operative temperature, then the indoor temperature’s levels were near the limits 41

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order to provide a technical solution for the improvement of the thermal behavior of PCM plasterboards. Xie et al. [34] studied five PCM wallboards of an air-conditioning room and presented their thermal performance. Plytaria et al. [35] examined the thermal behavior of a building with incorporated phase change materials in the south and the north wall and they found that PCM, as a wall layer, in combination with insulation reduces heating and cooling loads of the examined building and improves the occupant’s thermal comfort during summer when it works well. The same authors, Plytaria et al. [36] simulated and evaluated energetically three different solar assisted heat pump underfloor heating systems with and without the use of phase change material on the floor of a building. They found that the use of the PCM layer on the underfloor heating system reduces the heating load by about 40% and the electricity consumption between 42% and 67% using the solar-driven systems. Zhou and Pang [37] investigated the performance of a Trombe wall with the use of PCM and concluded that PCM keeps the indoor thermal comfort at good levels. Wang et al. [38] studied the performance of a PCM-wall during all the year and they found that the use of PCM reduces the heating loads on average of 15% and the cooling loads of about 24%. Chen et al. [39] developed an active-passive ventilation wall with phase change material which created a heat release capacity on average of 55% and a heat storage capacity on average of 41.5%. Moreover, the PCM can be incorporated into HVAC systems and in storage units such as storage tanks in solar heating and cooling applications. For example, Belmonte et al. [40] studied the performance of a solar cooling system where a dry cooler with PCM had replaced a wettower. Agyenim et al. [41] put a PCM into a tube heat exchanger. In this way, they tried to improve the coefficient of performance (COP) of the absorption chiller. Schweigler et al. [42] used a PCM in an absorption chiller and they found that the use of PCM improved the heat gain. Ponshanmugakumar et al. [43] studied a solar absorption air-conditioning system with the integration of a PCM into the generator and they concluded that PCM can decrease the auxiliary loads. Azzouz et al. [44] put a PCM on the back side of the evaporator and they found that the thermal load affected the increase of the COP. Sonnenrein et al. [45] studied the effect of different heat storage elements into a tube condenser and they concluded that the PCM decreased the temperature of the condenser and thus the energy consumption. Said and Hassan [46] studied experimentally a system of an air-conditioning unit coupled with a PCM heat exchanger and they found that with the use of PCM there was an increase of the COP of the air-conditioning unit. Zhou et al. [47] compared a traditional earth-air heat exchanger with a PCMfilled earth-air heat exchanger and found that with the PCM there was an improvement of 20.24% in cooling capacity. Hirmiz et al. [48] carried out numerical simulations in order to quantify the benefit of thermal energy storage with phase change materials on the solar absorption cooling system performance. Shabgard et al. [49] analyzed the thermal performance of a solar driven heating, cooling and hot water system for buildings integrated with latent heat thermal energy storage system and they found that it can reduce the auxiliary energy demand and it can also increase the exergy efficiency of about 80%. Aljehani et al. [50] developed a model of a system of an integrated air conditioning-thermal energy storage with phase change composite and they concluded that with the use of PCM there is an improvement concerning the compressor efficiency and size, and the electricity consumption. In the present study, a solar cooling system with evacuated tube collectors (ETC) coupled to a single-effect absorption chiller, which supplies four radiant walls with and without a PCM layer respectively, is investigated energetically and financially for a building of 100 m2 floor area in Athens. The innovation of this study is the combination of three technologies; a solar-assisted absorption chiller, the radiant walls and the incorporation of the PCM layer into radiant walls. The PCM layer is used into the radiant walls in order to increase the storage capacity and to reduce the operation of the absorption chiller. To our

Table 1 Layers of the building. Components

Layers

Values

Walls

Plaster Insulation Brick Active layer with pipes Brick Plaster Concrete Insulation Plaster Floor mortar Concrete Insulation Beton

0.02 m 0.06 m 0.06 m 0.002 m 0.06 m 0.02 m 0.24 m 0.07 m 0.01 m 0.01 m 0.06 m 0.05 m 0.20 m

Roof

Floor

knowledge, there is no other study which investigates the present configuration and so this work comes to present a novel design for producing the building cooling load with a renewable energy source and to combine an improved building envelope with increased storage capacity. This study is conducted with the commercial software TRNSYS [51] which uses an active layer to simulate the radiant wall. Type 1270, which is a component of TRNSYS from Thermal Energy System Specialists (TESS) Company [52], is used in order to simulate the PCM layer into the radiant walls. Finally, the examined building is simulated with Type 56 [53] of the TRNSYS library. 2. Methodology 2.1. The examined building In this study, the examined building is simulated and designed in TRNSYS 17 [51]. It is an office south-oriented of 100 m2 in Athens with the operation hours between 6:00 am and 18:00 pm for all the summer period. The building consists of four external same radiant walls. Table 1 gives analytically the layers of the walls, of the roof and of the floor. Moreover, in the south, in the west and in the east, there are doubled windows. The modeling of the radiant cooling system is performed using an “active layer” in walls. This layer contains fluid-filled pipes and its parameters are given in Table 2. The desired inlet mass flow rate is given and determines the segmentation of the area for the radiant wall system. In our case, the specific mass flow rate of 10 kg/hm2 separates the area of each wall into one segment. The convective heat transfer coefficient between the surface with the active layer and the zone air depends on the temperature of the active layer. It is important to state that the desired temperatures were set to 26 °C in summer, which is a temperature into the limits of thermal comfort. In these simulations, the component Type 1270 from TESS Company was used for the incorporation of PCM into the walls. Type 1270 has built-in values for a specific brand of PCM with the code name M91. In this study, where cool water enters into the tubes of walls, it is selected the smallest available melting temperature level which is the 23 °C. So, the selected PCM is the BioPCM Q23/M91 [54]. The layer of PCM is situated between the brick and the insulation of the walls as Fig. 1 shows. Also, Fig. 1 depicts the way that the radiant Table 2 Parameters of the active layer.

42

Parameters

Values

Specific heat capacity of the water Pipe spacing Pipe outside diameter Pipe wall thickness Pipe wall thermal conductivity

4.19 kJ/kgK 0.2 m 0.02 m 0.002 m 0.35 W/mK

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Fig. 1. (a) Radiant wall layers of the examined system (b) Structure of the tubes into bricks.

calculated using a zero efficient term of 0.82 and a first efficiency term (slope term) at 2.19 W/m2K, as Table 4 shows. The optical analysis of the collector is performed by using an incidence angle modifier constant of 0.2 [55], as Table 4 indicates. The operation of the solar collectors is regulated with a proper controller which checks if the global solar irradiation is over 100 W/m2. In the cases with the lower solar potential, the circulating pump of the solar field stops to operate because there is no margin for useful heat production. This limit of 100 W/m2 has been determined by a simple sensitivity analysis. Practically, lower solar irradiation values lead to negative collector thermal efficiency (the thermal losses are more than the useful production) and so the solar field has not to operate. Moreover, there is a cut-off temperature at 120 °C in the solar system. The working fluid in the solar field is pressurized water at 8 bar.

wall has been constructed. It is observed that the water tubes are between the two brick layers. Finally, the following Table 3 gives the values of the parameters of the building, which are typical in Greece, and the parameters of the selected PCM. 2.2. The examined cooling system In this section, the examined cooling system is presented, which are simulated in TRNSYS 17. Solar cooling system, without and with a PCM layer into the radiant walls respectively, are presented and compared energetically and financially. The calculations were performed when there is cooling demand for Athens namely for the summer period (May – September). After a sensitivity analysis, the time step was selected to be 5 min. More specifically, this time step makes the yearly auxiliary energy consumption of the system to be converged. The parametric analysis was made by changing the storage tank volume and the collecting area of the ETC. Lastly, it has to be said that the used weather data regards the location of Athens (Greece) and they have taken from Type 109 which includes weather data for the typical meteorological year.

Table 3 Parameters of building and PCM in TRNSYS [53–54].

2.2.1. Solar cooling system with radiant walls The total configuration of the solar cooling system with radiant walls is presented in Fig. 2. Solar energy is utilized from the evacuated tube collector in order to heat water, which is stored in the storage tank. A single-effect absorption chiller, which operates with the LiBr-H2O working pair, is fed by the hot water from the storage tank. Moreover, when the temperature of the water is lower than the desired, then an auxiliary heater after the storage tank heats the water up to 85 °C [8]. Then, the absorption chiller produces chilled water and this water flows through the tubes into the radiant walls of the building when there are cooling needs. The indoor desired temperature of the building is set to be 26 °C. The nominal COP of the chiller is selected at 0.7 and the rated capacity is 20 kW. The COP is not constant in this work but it varies according to the inlet temperature by using the catalogs from the TRNSYS libraries which are included in Type 107. The collecting area of ETC is ranged from 50 m2 to 110 m2 and the storage tank volume from 1 m3 to 7 m3. Every collector module is equal to 2 m2 and every series has 5 modules. In this work, the parallel series is ranged from 5 to 11 in order to achieve a total collecting area from 50 m2 to 110 m2. The thermal efficiency of the solar collector is

Components

Parameters

Values

Building (Type 56)

Height Floor area South double window East double window West double window Wall U-value Roof U-value Floor U-value Window U-Value Structural material absorptance Window g-value Light power Shading coefficient Persons in the building Air changes per hour Specific gains from the equipment Melting temperature Thickness Width Weight per area Latent heat storage capacity Density Thermal conductivity Specific heat capacity

3 m2 100 m2 6 m2 3 m2 3 m2 0.545 W/m2K 0.467 W/m2K 0.648 W/m2K 1.4 W/m2K 80%

BioPCM Q23/M91 (Type 1270)

43

60% 500 W 60% 7 2 ach 350 W 23 °C 14 mm 419.1 mm 6.20 kg/m2 180 kJ/kg 850–1400 kg/m3 0.15–2.5 W/mK 2200–4500 J/kgK

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Fig. 2. The examined solar cooling system with radiant walls.

2.3. The utilized components of the system

Table 4 Parameters of the utilized components in TRNSYS. Components

Parameters

Values

ETC (Type 71)

Collector area (Ac) Optical efficiency factor [FR (τα)] Slope angle of the collector (β) Thermal loss coefficient [FR UL] Water mass flow rate (mc) Fluid specific heat capacity (cp) Incidence angle modifier constant (b0) Rated cooling capacity Rated COP Cooling water inlet temperature Cooling water flow rate Hot water inlet temperature Maximum heating rate Set-point temperature Tank volume Mixing Zones Tank loss coefficient

50–110 m2 0.82 25o 2.19 W/m2K 1–2 kg/s 4.19 kJ/kgK 0.2

Absorption chiller (Type 107)

Auxiliary heater (Type 6) Storage Tank (Type 4b)

The default values of each utilized component in TRNSYS are given in Table 4. According to the literature, the COP of the absorption chiller is selected to be 0.7 [5] which is a typical value for the single-stage machine. Moreover, there are extra components such as water pumps (Type 3d), an on/off differential controller (Type 2b), a heating controller (Type 970), a cooling controller (Type 971) and their parameters have been generally selected. More specifically, the heating controller checks the inlet temperature to the absorption chiller in order to be at least 85 °C, while the cooling controller checks the indoor space temperature in order to be up to 26 °C. The differential controller is used in the solar system in order to regulate the circulation pump operation. This differential controller gives the operating signal if the global solar irradiation is over 100 W/m2 and the temperature in the tank is up to 120 °C.

20 kW 0.7 35 °C 1 kg/s 85 °C 5 kW 85 °C 1–7 m3 10 0.8 W/m2K

2.4. Mathematical formulation In this section, the basic mathematical modeling of the examined system is presented. The useful heat production (Qu) of the evacuated tube collector (ETC) is given below:

The storage tank volume is examined parametrically from 1 m3 up to 7 m3 for all the solar collecting area cases. Moreover, the storage tank is modeled using the mixing zones modeling and, in this work, we selected 10 mixing zones after conducting a simple sensitivity analysis about this issue. More specifically, the yearly auxiliary energy consumption has been calculated for different scenarios of mixing zones and with the 10 zones, the results converge with reasonable computation time. The thermal losses of the storage tank are important in this work because there are temperature levels around 100 °C and they have taken into account by the use of the thermal loss coefficient equal to 0.8 W/m2K.

Qu = mc ·cp·(Tc, out − Tc, in )

(1)

The available solar energy (Qs) in the solar collector is calculated as below:

Qs = Ac ·GT

(2)

The energy balance of the system (solar field – storage tank and absorption chiller) can be written as below:

Qu + Qaux = Qabs + Qstored + Qloss

(3)

This previous equation indicates that the energy that the useful heat input from the solar collectors (Qu) and of the auxiliary heater (Qaux) is separated into the heat in the absorption chiller inlet (Qabs), to the stored energy in the storage tank (Qstored) and to the thermal losses from the tank to the ambient (Qloss). Furthermore, Eq. (4) shows the COP of the absorption chiller, which is the ratio of the produced cooling (Qcool) to the heat load to absorption chiller (Qabs).

2.2.2. Solar cooling system with PCM into radiant walls The next case is a configuration with a PCM layer into the four radiant walls, as Fig. 3 shows. The PCM layer is used in order to increase the storage capacity and to avoid the operation of the absorption chiller. The calculations were performed for the application of the PCM layer in each wall and then in combination with more than one wall.

44

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Fig. 3. The examined solar cooling system with the PCM layer into radiant walls.

Qcool Qabs

COP =

where q 1 and q2 are the quantities of energy entering the PCM from the adjacent wall layers, mPCM is the mass of the PCM, cp,solid is the specific heat capacity at the solid state of PCM and cp,liquid is the specific heat capacity at the liquid state of PCM [52]. During the transition of the PCM, the initial and the final temperatures are equal, while the stored energy is calculated by the Type 1270. Furthermore, it is essential to state that the validity of this model has been checked in References [57,58] and so the Type1270 can be adopted as valid for this work. It is important to state that the previously mentioned parameters of Eq. (8) are calculated by the TRNSYS software in every time step. The use of the total cost of the investments (Ctotal) gives the financial evaluation of the examined system with and without PCM. Equation (9) gives the analytical definition of this parameter, where Ccapital is the capital cost of the system and Cvariable is the variable cost for all the investment years, which are selected to be 25 years.

(4)

At this point, it has to be said that Eq. (4) shows the COP only for the absorption chiller and not for the total system. This is the reason that we put the heat input of the absorption chiller in the denominator of Eq. (4). In this work, the solar coverage for the heat input in the absorption chiller (f) is a critical parameter which is calculated indirectly according to Eq. (5). This parameter shows the contribution of solar energy in the absorption chiller energy need. Practically, this parameter is calculated using the energy of the auxiliary heater (Qaux) and the heat load to absorption chiller (Qabs).

f=1−

Qaux Qabs

(5)

Ctotal = Ccapital + Cvar iable

At this point, it has to be said that the pumping work in the system is generally very low and thus it has been neglected from the calculations [56]. The cooling load (Qcool) of the building is approximately given from (6):

More analytically, Eq. (10) defines the capital cost of the investment with and without PCM while Eq. (11) defines the variable cost, which is mainly the cost of the auxiliary electricity consumption for all the investment years.

Qcool = m1·cp·(Tw1, out − Tw1, in) + m2 ·cp·(Tw2, out − Tw2, in) + m3·cp·(Tw3, out − Tw3, in ) + m4 ·cp·(Tw 4, out − Tw 4, in )

(6)

Ccapital

where the index w symbolizes the four active walls. This cooling load is practically used by the absorption chiller component in order to calculate the heat input by using Eq. (4). The temperature levels of the Eq. (6) are calculated by the TRNSYS components in every time step. Type1270 for the PCM is quite simplistic according to TESSLibs3Mathematical Reference [52]. The following Eqs. (7) and (8) give the proper definition of its performance. When the PCM material is fully frozen, the temperature at the end of a time step is given by:

Tfinal = Tinitial +

(q1̇ + q2̇ ) mPCM ·Cp, solid

(q1̇ + q2̇ ) mPCM ·Cp, liquid

⎧ Ac ·Cc + Vt ·Ct + Caux + Qnom·Cabs + Cact . walls1,2,3,4, ⎪ = Ac ·Cc + Vt ·Ct + Caux + Qnom·Cabs + Cact . walls1,2,3,4 ⎨ ⎪ + CPCM , walls1,2,3,4 ⎩

Cvar iable = Eaux ·K el·R + K O & M

(10) (11)

where Kel is the specific electricity cost (Kel [€/kJ]) and it has been selected to have a value of 0.000056 €/kJ as the most representative in Greece [59]. The operation and maintenance cost (KO&M) is calculated as the 1% of the capital cost (Ccapital) and it is defined in Eq. (12). At this point, it has to be said that the cost for the pump work is assumed to be included in this value.

(7)

K O & M = 0.01·Ccapital

When the PCM material is fully thawed, the temperature at the end of a time step is given by:

Tfinal = Tinitial +

(9)

(12)

The yearly auxiliary consumption is defined according to Eq. (13):

Eaux =

(8) 45

September

∫May

Qaux ·dt

(13)

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building. More analytically, when the PCM is situated in the south wall, the solar coverage is increased by 3.8%, in the east by 3.2%, in the west by 2.8% and in the north wall by 0.6%. So, the results indicate that the best location of the PCM layer is on the south wall. Fig. 8 depicts the auxiliary energy consumption and Fig. 9 the solar coverage of the system without and with PCM in walls combinations. It is observed that the highest reduction of energy consumption (on average 63%) is achieved when the PCM layer is placed in all walls. The difference in energy reduction among the PCM in all walls, in three walls (south-west-east) and in two walls (south-east) is about 2.6% − 4.0% respectively. As concerns the solar coverage, we observed that the highest increase (on average 10%) is achieved when the PCM layer is placed in all walls. The difference of solar coverage among the PCM in all walls, in three walls (south-west-east) and in two walls (south-east) is imperceptible and about 0.9–1.2% respectively. It is important to say that the curves in Figs. 8 and 9 show that after the 80 m2 the percentage of decrease and of increase respectively is smaller. So, the best collecting area is under 80 m2. The collecting area of 60 m2 achieves the highest increase (about 9.7%) of solar coverage.

Moreover, the discount factor (r) and the effective years of the project (N) give, with the following equation (14), the parameter R.

R=

(1 + r ) N − 1 r·(1 + r ) N

(14)

In the end, Table 5 presents analytically the financial data of this study. 3. Results The energetic and financial evaluation of the examined system is presented in this section. Moreover, the indoor temperature for all cases is presented for all the summer season. 3.1. Energetic evaluation of the examined system Fig. 4 illustrates the auxiliary energy consumption of the system without PCM as a function of seven different collecting areas of ETC and for seven different storage tank volumes. The purpose of this analysis is to find out the optimum storage tank volume of each collecting area, which leads to the minimization of the auxiliary energy. The analysis shows that for collecting area AC = 50 m2, the optimum storage tank volume (V) is 1 m3. For Ac = 60 m2 and 70 m2, the optimum storage tank is V = 2 m3, for Ac = 80 m2 and 90 m2 is V = 4 m3 and for Ac = 100 m2 and 110 m2 is V = 5 m3. It is obvious that lower storage tank volumes are more suitable for low collecting areas, while higher storage tank volumes are suitable for higher collecting areas. Practically, higher collecting area produces more useful heat production and so higher storage capacity is needed in order to keep the working fluid at the desired temperature levels. Moreover, it is observed a lower auxiliary energy consumption for higher collecting areas due to the higher useful energy production by the solar field. Fig. 5 illustrates the auxiliary energy consumption of the system with PCM in the south wall as a function of seven different collecting areas of ETC and for seven different storage tank volumes. The south wall is selected for the first analysis with PCM because the examined building is south-oriented and the effect of the sun is greater in this side. The results show that with the use of the PCM layer, there is a reduction of the auxiliary energy consumption for each collecting area which is about 17%. In this case, the best storage tank volumes for each collecting area are the same as the previous system without PCM. The next step regards the energetic comparison of the system without and with PCM in each wall separately. The different building envelopes are studied for different collecting areas, while the storage tank volumes have the previously respective optimum values (see Figs. 4 and 5). Fig. 6 shows the auxiliary energy consumption of the case without PCM and the case with PCM in each wall [south (S) – east (E) – west (W) – north (N)] for various collecting areas of ETC. It is observed that there is a reduction of the auxiliary energy with the use of the PCM layer. The higher reduction, on average 30%, is when the PCM is situated in the south wall and this is reasonable because in this case, the effect of the sun is greater. When the PCM is situated in the east wall, there is a reduction on average 25% and when the PCM is situated on the west wall, there is a reduction on average 24%. The lowest reduction, on average 5%, is when the PCM is situated in the north wall. Practically, the PCM makes the wall temperature not to be so affected by the incident solar irradiation in its outer surface. Moreover, the use of PCM aids the radiant walls to keep a relatively low temperature due to the cooling water which flows inside them. So, there is “cooling storage” in the walls due to the PCM which reduces the fluctuations in the indoor temperature during the day and the cooling loads of the building are reduced. Fig. 7 shows the solar coverage for various collecting areas without and with PCM in each wall. It is observed that for higher collecting areas, the solar coverage is increased. Moreover, the use of the PCM layer increases the solar coverage due to the lower cooling needs of the

3.2. Indoor temperature profiles The indoor temperature of the building in summer has to be lower than the ambient temperature in order to achieve the desired thermal comfort conditions. Fig. 10 illustrates the indoor temperature’s fluctuation during the summer period (May – September) in Athens for the system without PCM in comparison with ambient temperature for the same period. It is obvious that with the use of the cooling system, the indoor temperature is lower than the ambient temperature and near the limits of thermal comfort. Fig. 11 shows the indoor temperature’s fluctuation for the cooling systems without and with a PCM layer in each wall during one week (17th − 23rd July) of the summer. The lowest indoor temperature is achieved when the PCM layer is situated in the south wall. In Fig. 12, the differences between the cases of PCM in each wall are more visible. So, this figure depicts the indoor temperature’s fluctuation during one day (20th July) of the summer. It is observed that the differences in temperatures are imperceptible. None the less, the application of the PCM layer in the south wall gives a decrease of the indoor temperature on average 0.3 °C. Generally, it can be said that the deviations of the indoor temperature levels between the examined cases are not so high but the respective cooling load deviations are significant. Fig. 13 depicts the temperature’s fluctuation during one week (17th − 23rd July) for the systems without and with PCM in walls combinations. The observation of the results shows that the lowest indoor temperature is achieved when the PCM layer is situated in all walls and the higher indoor temperature is achieved when there is not a PCM layer. The indoor temperature’s fluctuation for all these cases is clearer in Fig. 14 where it depicts the temperature during one day (20th July). The application of the PCM layer in three walls (south-west-east) and in all walls gives a higher decrease in the indoor temperature on average Table 5 Financial data for this study [59–63].

46

Parameter

Value

Specific cost of ETC (Cc) Specific cost of Storage Tank (Ct) PCM cost (Cpcm) Auxiliary heater cost (Caux) Active wall cost (Cact.wall) Absorption chiller cost (Cabs) Discount factor (r) Project life (N) Specific electricity cost (Kel) Operating and Maintenance cost (KO&M)

250 €/m2 500 €/m3 30 €/m2 100 € 30 €/m2 300 €/kW 3% 25 years 0.000056 €/kJ 1% Ccapital

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Fig. 4. Auxiliary energy consumption as a function of the storage tank volume and the collecting area for the cooling system without PCM (The black points shows the optimum storage tank value for every collecting area).

Fig. 5. Auxiliary energy consumption as a function of the storage tank volume and the collecting area for the cooling system with PCM in south wall (The black points shows the optimum storage tank value for every collecting area).

Fig. 6. The auxiliary energy consumption for various collecting areas of the system without PCM and of the systems with PCM in each wall.

47

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Fig. 7. The solar coverage for various collecting areas of the system without PCM and of the systems with PCM in each wall.

Fig. 8. The auxiliary energy consumption for various collecting areas of the system without PCM and of the systems with PCM in walls combinations.

Fig. 9. The solar coverage for various collecting areas without PCM and of the systems with PCM in walls combinations.

48

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Fig. 10. Indoor operative temperature for the system without PCM and ambient temperature during the summer period in Athens.

Fig. 11. Temperature’s fluctuation during one week (17th-23rd July) for the system without PCM and for systems with PCM in each wall.

Fig. 12. Temperature’s fluctuation during one day (20th July) for the system without PCM and for systems with PCM in each wall.

3.3. Financial evaluation of the examined system

0.6 °C. More analytically, Fig. 15 shows the minimum, the maximum and the average values of the indoor temperature during the summer period in Athens for all the examined cases. Finally, it can be said that all the examined cases give acceptable indoor temperature into the limits of thermal comfort.

The financial evaluation of all the examined cases is given in this section. Fig. 16 depicts the total cost (Ctotal) of the system without and with PCM in each wall. It is observed that the PCM layer in the north 49

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Fig. 13. Temperature’s fluctuation during one week (17th-23rd July) for the system without PCM and for systems with PCM in walls combinations.

Fig. 14. Temperature’s fluctuation during one day (20th July) for the system without PCM and for systems with PCM in walls combinations.

examined systems with PCM in comparison with the system without PCM. Finally, the reduction of the total cost with the use of the PCM layer is reasonable because it reduces the energy consumption.

wall leads to higher total cost while the PCM layer in the south wall leads to the minimization of the total cost. So, when the PCM is situated in the north wall, there is an increase in the total cost in comparison with the system without PCM of about 2%. When the PCM is situated on other walls, there is a decrease in the total cost. The highest reduction is when the PCM is on the south wall and it is on average 3%. It is also important to state that in this case, for collecting area 60 m2, there is the highest reduction in total cost of about 3.9%. The reason for the cost increase, when the PCM is used in the north wall, is based on the small decrease of the auxiliary energy consumption which is not able to counterbalance the higher investment cost due to the use of PCM. Fig. 17 shows the total cost (Ctotal) of the system without and with PCM in walls combinations. Over the collecting area of 90 m2, the total cost is increasing and thus, the investment in a system with PCM is unprofitable. Under the collecting area of 90 m2, there is a reduction in the total cost. The highest reduction (on average 6.4%) is achieved when the PCM is situated in the south and in east walls. Furthermore, it is observed that in this case, for the collecting area of 60 m2, there is the highest reduction of the total cost (on average 6.7%) for all the

4. Discussion of the results The results clearly show that the application of the PCM layer into radiant walls leads to lower auxiliary energy consumption. More specifically, the results indicate that the best location of the PCM layer is in the south wall in comparison with the other walls. Moreover, the solar coverage curves indicate that the collecting area of 60 m2 achieves the highest increase (about 9.7%) with the use of the PCM layer in all the examined wall’s combination. The financial evaluation of the systems indicates that for the collecting area of 60 m2, there is the highest reduction of the total cost (on average 6.7%) for all the examined systems with PCM in comparison with the system without PCM. Table 6 summarizes the results about cases with 60 m2 collecting area. Moreover, the results of subsection 3.2 show that the use of the PCM layer gives an acceptable indoor temperature into the limits of thermal comfort. 50

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Fig. 15. Minimum, maximum and average values of the indoor temperature during the summer period in Athens for all the examined cases.

Fig. 16. Total cost (Ctotal) for various collecting areas of the system without PCM and of the systems with PCM in each wall.

Fig. 17. Total cost (Ctotal) for various collecting areas of the system without PCM and of the systems with PCM in walls combinations.

5. Conclusions

separately and then in combinations. The examined building has a yearly cooling demand of about 1.87 105 kJ/m2 according to the calculations with the TRNSYS software. The different cases are examined by changing the area of the collectors and the storage tank volumes. The main conclusions are the following:

This study investigates energetically and financially a solar cooling system with and without incorporated PCM into the radiant walls of a building. The examined building has a floor area of 100 m2 and it is located in Athens (Greece). The PCM-layer is placed in each wall 51

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Table 6 Summary of the results about cases with 60 m2 collecting area. [12] Cases System System System System System System System System System System

without PCM with PCM_S with PCM_W with PCM_E with PCM_N with PCM_S, W with PCM_S, E with PCM_W, E with PCM_S, W, E with PCM_S, W, E, N

Qaux (kJ)

f

Ctotal (€)

1.26 107 1.05 107 1.11 107 1.08 107 1.23 107 0.884 107 0.739 107 0.926 107 0.715 107 0.682 107

70.50% 73.60% 72.80% 73.30% 71.00% 76.20% 79.10% 75.70% 79.50% 80.20%

37,289 35,848 36,549 36,340 37,869 35,003 33,596 35,561 34,120 34,696

[13] [14]

[15]

[16]

[17] [18]

- The incorporation of PCM into the radiant walls reduces the energy consumption of the building. - The incorporation of the PCM in the south wall is the best choice among the four external walls. In this case, the reduction of the auxiliary energy is 30%, the increase in solar coverage is 3.8% and the reduction of the total cost of about 3%. - The collecting area of 60 m2 achieves the highest increase of solar coverage with the use of PCM which is approximately 9.7%. Furthermore, for the same collecting area, there is the highest reduction of the total cost (on average 6.7%) for all the examined cases with PCM in comparison with the system without PCM. - The application of the PCM layer in the south wall gives a decrease in the indoor temperature on average 0.3 °C. The application of the PCM layer in three walls (south-west-east) and in all walls gives a higher decrease in the indoor temperature on average 0.6 °C.

[19] [20]

[21]

[22]

[23]

[24]

[25]

Conflict of interest

[26]

There is no conflict of interest. [27]

Acknowledgments [28]

The first author would like to thank the Special Account Research Funds of the National Technical University of Athens for its financial support. Moreover, the second author would like to thank “Bodossaki Foundation” for its financial support.

[29]

[30]

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