Use of a latent heat thermal energy storage system for cooling a light-weight building: Experimentation and co-simulation

Accepted Manuscript Title: Use of a Latent Heat Thermal Energy Storage System for Cooling a Light-Weight Building: Experimentation and Co-simulation Author: Fabien Rouault Denis Bruneau Patrick Sebastian Jean-Pierre Nadeau PII: DOI: Reference:

S0378-7788(16)30469-8 http://dx.doi.org/doi:10.1016/j.enbuild.2016.05.082 ENB 6717

To appear in:

ENB

Received date: Revised date: Accepted date:

4-2-2016 22-4-2016 25-5-2016

Please cite this article as: Fabien Rouault, Denis Bruneau, Patrick Sebastian, JeanPierre Nadeau, Use of a Latent Heat Thermal Energy Storage System for Cooling a Light-Weight Building: Experimentation and Co-simulation, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2016.05.082 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Use of a Latent Heat Thermal Energy Storage System for Cooling a Light-Weight Building: Experimentation and Co-simulation Authors: Fabien Rouault*1, 2, Denis Bruneau1, Patrick Sebastian3, Jean-Pierre Nadeau1

1

Arts et Métiers ParisTech, I2M, UMR 5295, F-33400 Talence, France

2

Pontificia Universidad Católica de Chile, Escuela Construcción Civil, Santiago, Chile

3

Univervité de Bordeaux, I2M, UMR 5295, F-33400 Talence, France

*Corresponding author: Fabien Rouault, telephone: +56 2 2354 1283, e-mail: [email protected]

Highlights:   

We present an experimental pilot of Latent Heat Thermal Energy Storage (LHTES) system We couple a thermal model of the LHTES system with a BPS software package We compare experimental measurements and numerical simulations to validate cosimulation approach

Abstract: Air cooling systems that make use of the energy storage potential of the latent heat of Phase Change Materials (PCMs) are alternatives to conventional air-conditioning units for maintaining indoor comfort in summer in light-weight buildings. However, the functioning of such systems is closely linked to the ambient climatic conditions and to the spatial and energy specifications of the buildings to be cooled. For a better understanding of their performance in situ, a thermal co-simulation of a Latent Heat Thermal Energy Storage (LHTES) system and of an existing wooden building is proposed. The performance of this co-simulation is demonstrated by comparing results with experimental results from tests on a building which incorporates an LHTES system. This performance analysis, conducted using Normalised Mean Bias Error (NMBE) and Coefficient of Variation of the Root Mean Square Error (CV(RMSE)), demonstrates the viability of integrating co-simulation to facilitate the LHTES system design process. Keywords: Building Performance Simulation; Phase Change Materials

1

Nomenclature

A Area (m2) Cp Specific heat (J kg-1 K-1) CV(RMSE) Coefficient of variation of the root mean squared error (%) H Mass enthalpy (J kg-1) h Heat transfer coefficient (W.m-2.K-1) L Latent heat (J kg-1) n number of data points or periods in the baseline period (-) NMBE Normalised Mean Bias Error (%) p Number of parameters or terms in the baseline model, as developed by a mathematical analysis of the baseline data (-) PCM Phase Change Material q Volume air flow (m3.h-1) S Cross section (m2) T Temperature (°C) t time (s) U Velocity (m.s-1) y Dependent variable of some function of the independent variable(s) (-) Arithmetic mean of the sample of n observations (-) Regression model’s predicted value of y (-) Greek letters T κ λ ρ σ

Temperature difference Volume variation between solid-liquid during phase change (-) Thermal conductivity (W.m-1.K-1) Bulk density (kg m-3) Volume fraction (-)

Indices 0 a exp ext f in int out w

Initial condition air blown into the cooling system experimental air outside the house phase change entering the system air inside the house leaving the system PCM container wall

2

Introduction

Ensuring indoor comfort in summer has become an increasing problem in the residential sector and the service industries, where buildings have become increasingly insulated and airtight, and therefore sensitive to internal and external energy supplies. Experimental studies in the literature have shown that Latent Heat Thermal Energy Storage Systems (LHTES) are able to cool air for several hours under controlled conditions [1]-[7], i.e. with constant incoming air flow and temperature. Air-cooling systems based on LHTES, which are composed of Phase Change Materials (PCMs), should now be tested in terms of their adaptability to different spatial layouts of buildings as well as in terms of their capacity to respond to the cooling problem in summer, to see if they can offer a reasonable technological solution with potentially interesting performance coefficients. The thermal co-simulation of the building with the LHTES system that it contains is a possible alternative to tests in situ; under ideal circumstances, it is a suitable way to develop decision support tools for the use and validation of the performance of such phase change systems (Sebastian et al.[8]). Yanbing et al. [9] present the modelling and experimental study of an LHTES system for maintaining thermal comfort in summer. This system, which contains 150 kg of PCM in the form of flat-plate capsules, is incorporated in a false ceiling. It extracts air from the inside (air cooling mode) or from the outside (regeneration mode) and blows air into the premises. The authors have developed a calculation tool that couples the thermal model of an LHTES system with a simplified thermal model of a mono-zone building. Although the mathematical model they developed disregards perceptible heat compared with latent heat and underestimates transfers within the molten PCM, overall, the behaviour of the simulation and the experimental results are similar. Furthermore, the authors demonstrate the contribution of the system in terms of comfort by comparing results obtained with and without the LHTES system. In Japan, Takeda et al. [10] studied the appropriateness of using an LHTES system based on PCMs for air cooling . The LHTES system studied was composed of a packed bed of granules with a diameter of 1 to 3 mm containing 35% paraffin. Contrary to Yanbing et al., the system developed by Takeda et al. was incorporated into a hygienic ventilation system: only air coming from the outside enters the LHTES system. This choice of design means that the LHTES system operates free from all connection with the building, which greatly simplifies the associated modelling. Based on climate data from different towns in Japan and a very simplified LHTES model (the localised temperature of the encapsulated PCM granules was assumed to be uniform and equal to the local air temperature), the authors assessed the cooling potential of the LHTES system by calculating the reduction in need for mechanical ventilation for cooling compared to a conventional ventilation system that carries out the same function. They conclude that it is thus possible to reduce the need for ventilation by 62.8% by using an LHTES system. The prototype presented by Arkav et al. [11] is composed of two cylindrical LHTES devices, whose encapsulated spheres are filled with paraffin RT21. In order to maintain

a comfortable temperature of 26°C during the day, these authors combined mechanical and/or natural night ventilation cooling with daytime cooling using an LHTES system. With the aid of a building model coupled with an approximate LHTES model (transfers within the PCMs were purely conductive) in a TRNSYS® environment, they assessed numerically the potential of such combinations for different commonly occurring scenarios in terms of energy efficiency. The use of co-simulation for automated optimisation of air-PCM systems has not been studied very extensively. Chiu et al. [12] used TRNSYS software (the block function TYPE 842 developed by the University of Graz, Austria), adapting it to the case of an LHTES-Air-PCM system (the block function TYPE 842 in its original version, only allows simulation of the behaviour of LHTES of the water-type PCM system). Simulating the LHTES system coupled with a building thus enables them to determine the lack of comfort in the building and the energy use of the fan required to make the system work. By using multi-objective optimisation algorithms, these authors have determined the Pareto front by seeking to minimise the remaining cooling requirements and the cost over the life cycle of the LHTES system. It is important to note that co-simulation is used for Building Performance Simulation (BPS) in order to assess the performance of various systems that interact with the building. Wetter [13] has developed the BCVTB® (Building Controls Virtual Test Beds) platform, which ensures the exchange of information between different calculation and simulation software packages, thus providing the possibility of testing innovative control systems or algorithms. For example, Novakovic and Cvetkovic [14] propose temperature regulation by way of slatted blinds, where the rising/falling and the angles of the slats depend on the amount of solar input, while Zhao et al. [15] propose a system of predictive technical building management using MATLAB/SIMULINK and EnergyPlus. The work presented here forms part of a research project to develop a design support tool for buildings from the preliminary design stage by permitting an optimal dimension pre-calculation of air-cooling systems using PCMs. A thermal behaviour simulation approach to the system in its environment has been chosen as a basis for a multi-criteria optimisation tool. First, a dynamic simulation model of an LHTES system was developed on the MATLAB® software platform and subsequently validated by Rouault et al. [7], [16]. The objective here is to couple this thermal model of an LHTES system with a BPS software package and to validate this coupling by comparing the simulation results with results from experimental tests in situ. With this aim, a prototype of an individual one-storey house was fitted with an LHTES system (used for maintaining indoor comfort in summer) and a thermal model of this prototype was developed on a BPS software package and considered with the help of experimental results from a period of measurements without the use of the LHTES system. The suitability of a building-LHTES co-simulation was then assessed by comparing the simulation results with experimental results from a measurement period during which the LHTES system was used.

3 3.1

Presentation of a case study Presentation of the prototype and the house

Napevomo (cf. Figure 1) is an individual house with a living area of 47 m² able to accommodate two people. This house was built within the remit of the international inter-university competition Solar Decathlon Europe 2010 (SDE 2010) [17], and was developed by students of the Arts et Métiers ParisTech engineering school with technical support from a business consortium and scientific support from the Institut de Mécanique et d’Ingénierie de Bordeaux (I2M). After this competition, which took place in Madrid in June 2010, the Napevomo house was rebuilt on the site of BordeauxTalence d’Arts et Métiers ParisTech (France), where it was instrumented and monitored for a duration of two years. The Napevomo house is light in structure (wooden frame) in order to meet the criterion of being transportable and very lightly tied to the ground, which was a requirement of the SDE 2010 competition rules. The composition of the walls retained for the building model developed in this work, is that described by Bruneau et al. [18] with certain simplifications. In fact, water loss from the roof and from the green wall and the contribution to resistance of the earthen bricks in the west wall were disregarded. Furthermore, the heterogeneous transverse character of the walls of the Napevomo envelope (vertical structural supports of solid wood placed every 60 cm, inter-support filling with wood-wool, outer overlays of insulating material limiting the thermal bridges, interior facing and exterior weather boarding) was not taken into account. An LHTES air-cooling system (see Figure 2) was installed in such a way as to actively compensate for the lightweight construction of the Napevomo house. This air-cooling system is composed of two identical air-PCM heat exchangers, with the PCMs packed in aluminium tubes. It was designed and produced by Ekomy Ango [19] and works over a daily cycle. During the day, hot air is extracted from the interior of the building and goes through the LHTES system, thus leaving the heat in the PCM. When cooled, the air is then blown back inside the building; as the PCM is molten over a narrow temperature range, it is thus possible to regulate the inner temperature of the house over a similar temperature range, assuming that the power delivered by the LHTES system is adequate for the immediate cooling requirements of the building concerned. During the night, the PCM is regenerated (solidification of the PCM) with the aid of cool air coming from outside the building and passing through the LHTES system. This reheated air by the PCM is then expelled to the outside of the building. It should be noted here that during the diurnal working of the LHTES system (cooling of the air), there is a very strong interaction between the building and the system. In fact, the temperature of the air inside the building influences the working of the LHTES that the air passes across, and conversely the air coming from the LHTES influences the performance of the building by the flow of cool air that it provides. 3.2

Modelling

EnergyPlus® is a calculation vehicle for BPS backed by the United States Government Department of Energy [20] and equipped with numerous third-party software packages used by agencies of thermal studies such as DesignBuilder®, Symergy®, gEnergy®, OpenStudio®, and CYPE®. Unlike TRNSYS®, EnergyPlus® does not allow easy development and the integration of internal modules. However, it is equipped with an external interface allowing the use of third party software packages. The model of the LHTES system developed by Rouault et al. [7] was kept in the MATLAB software during the writing of the work, while the MLE+ toolbox [21] was used to facilitate communication between MATLAB and EnergyPlus. The connection between Napevomo and the LHTES system having been established by way of the air temperature inside the building, this instance of dialogue between the two software packages was then used to calculate the following variables at each time step: the temperature inside the building, and the cooling power supplied to the building by the LHTES system, according to the control data and the solid/liquid state of the PCM. 3.2.1 Thermal model of the Napevomo house To carry out the thermal modelling of the building, EnergyPlus was used. A thermal model of the Napevomo house was created using the OpenStudio® software package [22], a graphics interface of EnergyPlus (see Figure 3). This software allows 3-D geometric information on the building to be provided, including the composition of the outer and inner walls, the awnings, the different thermal zones, and scenarios of common usage (e.g. temperature set point, occupation, ventilation). With EnergyPlus software, information about the composition of walls can only be provided as a superposition of homogenous layers, and so conductivity and a specific heat equivalent are calculated for each wall with the aid of the ratio of the total surface of each component over the total surface of the wall considered. Furthermore, the thermal bridges, which are very low, are not taken into account. During cooling periods, the simulation model of the LHTES system saves the air temperature value inside the (mono-zone) building, calculated during the previous time step by the BPS programme, to be used as the entering air temperature for the LHTES system. From the solid-liquid state of the PCM in the LHTES up to this instant and this ̅̅̅̅̅̅̅ value of the entering air temperature, the value of the air exit temperature (𝑇 𝑎,𝑜𝑢𝑡 ) of the LHTES is calculated over a ten-minute period, which corresponds to the time step of the BPS programme. So the cooling power (Pcool) is calculated from the mean of the exit temperatures over this ten-minute period, 𝑃𝑐𝑜𝑜𝑙 = −𝜌𝑎 𝐶𝑝𝑎 𝑞𝑎 (𝑇𝑖𝑛𝑡 − ̅̅̅̅̅̅̅ 𝑇𝑎,𝑜𝑢𝑡 )

(1)

and is then sent back to the BPS programme, which integrates it into a dedicated schedule as a contribution of negative heat. During the regeneration periods the LHTES simulation programme calculates, for each time step, the solid/liquid state of the PCM by taking as the LHTES entry temperature

the air temperature outside the building (Text) and sends back a cooling power value of nil to the BPS programme. Communication between MATLAB and EnergyPlus is established by the intermediary of the BCVTB software. The MLE+ toolbox offers MATLAB functions that facilitate the organisation and management of co-simulation from MATLAB. Figure 4 shows a diagram of the interactions between the different software packages used for this work. 3.2.2 Modelling the LHTES system Figure 5 shows the equations, initial conditions and boundary conditions for the LHTES model, which is precisely the one developed for the integrated LHTES system at Napevomo by Rouault et al. [7, 16] except for the type of PCM: the PCM in the Napevomo LHTES is Paraffin RT21 while in their model, Rouault et al. used Paraffin RT28HC, commercialised by Rubitherm® GmbH. The expression of the exchange coefficients ha-w, hext-a and hw-PCM shown in Figure 5 was described by Rouault et al. [7]. The relations in this figure show the parameters Tw0, T0 and Tin(t=0), which define the initial conditions of the LHTES system. The behaviour of the PCM has been taken into account in this thermal model of the LHTES system by means of a description of the enthalpy-temperature function of the PCM in the following form: 𝜌𝑀𝐶𝑃 𝐻𝑀𝐶𝑃 = [𝜌𝑠

1−𝜎𝑙 1+𝜅

𝐶𝑝𝑠 + 𝜌𝑙 𝜎𝑙 𝐶𝑝𝑙 ] 𝑇 + 𝜌𝑙 𝜎𝑙 𝐿𝑓 .

(2)

With this type of description it is easy to adapt to the PCM considered here (RT21) simply by changing the enthalpy function HMCP, the heat capacity values Cps and Cpl, and the latent heat value Lf. The volumetric variation of the PCM in the course of its phase change , which appears in Equation 1, is generally disregarded in the literature [23]–[25]; the volumetric enthalpy then becomes: 𝜌𝑀𝐶𝑃 𝐻𝑀𝐶𝑃 = [𝜌𝑠 (1 − 𝜎𝑙 )𝐶𝑝𝑠 + 𝜌𝑙 𝜎𝑙 𝐶𝑝𝑙 ]𝑇 + 𝜌𝑙 𝜎𝑙 𝐿𝑓 ,

(3)

Paraffin RT21 is a mixture of two or more alkanes developed in order to provide a sample group of PCMs with different phase change temperatures. According to Rakotosaona [26], who has studied the phase diagrams of mixtures of n-alkanes, during its rise in temperature a mixture goes through solid-solid transitions (ordered phase → disordered phase) followed by a solid-liquid phase change extended over a temperature range (a stable solid-liquid mixture due to the solid-liquid separation). Stankovic and Kyriacou [27] have recently characterised the enthalpy-temperature function of Paraffin RT21 experimentally, using the T-history method. Figure 6 shows their results and an approximation of the results obtained using the function configured from the following volumetric liquid PCM fraction σl: 𝜎𝑙 = 0,5 [1 − 𝑡𝑎𝑛ℎ (

𝑇𝑓 −𝑇 ∆𝑇2

) ].

(4)

The corresponding thermo-physical data for RT21 are shown in Table 1 and are used in the Enthalpy-Temperature model of RT21, which is shown here, in order to take into account precisely the behaviour of the PCM present in the Napevomo house.

4

Calibration of the thermal model of the house

The thermal model was calibrated with the aid of experimental data from the monitoring system permanently installed in the Napevomo house. In particular, to measure the ambiance (occupation, temperature, natural and artificial lighting) the house was equipped with TEHOR® sensors [28] positioned 2.4 meters above floor level. Calibration was carried out in two stages: from the 1st to the 102nd day of 2013 inclusive, the air temperature in the house was set at 21°C; here the system used for the temperature regulation was a 3 kW heat pump working only as a heater and coupled to a double flux Controlled Mechanical Ventilation system. Then from the 103rd to the 119th day inclusive, the experiments were interrupted as the building was not being monitored during this period. From the 120th to the 141st day of 2013, the house was left in free-floating conditions (without setting the temperature). In order to assess the performance of a thermal model of a building, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) [29] advises using the Normalised Mean Bias Error (NMBE) and the Coefficient of Variation of the Root Mean Square Error (CV(RMSE)), which are defined as follows: 𝑁𝑀𝐵𝐸 =

∑𝑛 ̂𝑖) 𝑖=0(𝑦𝑖 −𝑦 (𝑛−𝑝)𝑦̅

100

(5)

∑𝑛 ̂ 𝑖 )2 100 𝑖=0(𝑦𝑖 −𝑦

𝐶𝑉(𝑅𝑀𝑆𝐸) = √

(𝑛−𝑝)

𝑦̅

(6)

According to ASHRAE, the NMBE and the CV(RMSE) must not exceed 10% and 30% respectively, for a calibration sampling process based on hourly data. In order to carry out the calibration, a meteorological file was created in EPW (EnergyPlus Weather Data File) format from the Meteonorm® software and from weather data (global horizontal irradiance and air temperature) provided by a meteorological station located 50 m from the place where the Napevomo house was set up. Figure 7 shows (a) the global horizontal irradiance and (b) the outside air temperature data, measured over the first semester of 2013 together with the average seasonal values of the air temperature during the period 1971-2000. Figure 8a demonstrates the comparison of the air temperature inside the Napevomo house, measured by the TEHOR® sensor [28], and the inside air temperature simulated by the EnergyPlus software package during the period from 1st January 2013 (day 0) to 13th April 2013 (day 102). These experimental results show that in the great majority of days monitored, the 3-kW heat pump carries out its function of maintaining the inside air temperature at around 21°C. These experimental results reveal regular overheating

on sunny days when air temperature values occasionally exceed 25°C, and the overheating demonstrates the ability of the Napevomo house to take advantage of the winter sunshine. With regard to the modelling results obtained, although the simulated overheating values corresponded well with those observed, in most cases they were too low, particularly where the overheating lasted only one day. In principle, this was caused by an overvaluation of the thermal mass of the house. In fact, on the one hand, during a period of several days (day 45 to day 54 inclusive, see Figure 8b, when daytime overheating took place, the simulated night-time temperature fell less rapidly than the measured temperature, and on the other hand, over the same period, the simulated mean temperature increased constantly during this period of overheating. Nevertheless, as the NMBE and the CV(RMSE) were the same, i.e. –1.06% and 14.6% respectively, the model is considered well calibrated over this simulation period, regarding the values required by ASHRAE. The comparison of the measured inside air temperature and that simulated from the 120th to the 141st day of 2013, when the Napevomo house was floating freely (without setting the air temperature), is shown in Figure 9. The simulated air temperature follows the pattern of the daily variations in the measured air temperature but they are only very rarely exactly superimposed on each other. Nevertheless, the NMBE is estimated at 0.7 % and the CV(RMSE) is 4.5%, which lies well within the range of values required by ASHRAE. Given the capabilities of the model developed with EnergyPlus, the air temperature measurements taken in the Napevomo house under floating conditions and using a set thermostat can be used to describe the thermal behaviour of this building when unoccupied and with controlled internal heat load (which is either nil or imposed to comply with a low air temperature setting). These choices of simple scenarios in terms of contributions have enabled us to eliminate damaging uncertainties from the assessment of the thermal building model, as these internal contributions are parameters that generate great uncertainty about determining the needs for cooling and heating a building (Liu and Henze [30]).

5 5.1

Validation of the co-simulation Experimental set-up

The experimental plan consists of an LHTES system installed in the Napevomo house and a 1,080 W electric convector (Pheat), which provides a heat load. The LHTES system consists of (1) a fan with an output of 300 m3 h-1 and power consumption of 170 W, (2) four air valves with the option of indoor or outdoor air, and (3) two LHTES units filled with paraffin RT21 (see Figure 10). The air vent (outlet) is situated in the only thermal

area of Napevomo at 2.4 m above floor level, and the median height of the air inlet is 0.2 m above the floor. When the first tests of the experimental plan were set up, a stratification phenomenon of the air inside the house was observed, generated by the electric convector. This stratification results in an uncertainty about the temperature measurement provided by the TEHOR sensor located 2.4 m above the floor of the house. As a result, three type K thermocouple sensors, measuring the indoor air temperature (TTC) at three different heights h1, h2 and h3 at 0.12 m, 1.44 m and 2.43 m respectively, were installed in the Napevomo house, specifically to validate the co-simulation model. The thermocouples were positioned along an inner wall and protected against the sun in order to avoid measuring undesirable heat transfers by conduction or radiation. In what follows, the experimental indoor air temperature (Tint) is considered equal to the mean of the three temperature measurements from the three thermocouple sensors. The LHTES system is fitted with two PT100 temperature sensors at the entrance (Ta,in) and exit (Ta,out) of the LHTES units. It should be noted that these sensors are positioned at the junction of the two LHTES units. Values for the set of temperature measurements were obtained each minute. 5.2

Experimental results

The experiment was conducted over two separate periods, taking advantage of the Napevomo house being unoccupied, i.e. from 6th to 12th June 2013 (days 156 to 162) and from 20th to 25th June 2013 (days 171 to 176). During these periods, control of the heating and the LHTES systems was carried out manually by alternating the cooling and regeneration modes of the LHTES. The air temperatures inside and outside the Napevomo house, as well as the entering and leaving air temperatures for the LHTES system are shown in Figure 11a. This figure is accompanied by a chronogram (Figure 11b) showing the working of the electric convector (light grey) and of the LHTES system in its two operational modes: regeneration (black) and cooling (dark grey). Depending on the working mode of the LHTES system, the air extracted by the fan comes either from inside the house (cooling mode) or from outside (regeneration mode). The temperature of the air entering the LHTES system (dark grey) therefore alternates between the temperatures inside and outside the house. In cooling mode, the differences between the indoor temperature and that of the air entering the LHTES system are small. In regeneration mode, however, frequent and significant differences were recorded between the outdoor temperature and the temperature of the air entering the LHTES. These differences, of 0.61°C on average, are mainly due to the different positions of the meteorological station and the air inlet of the LHTES system: the former is situated on the roof of a two-storey building exposed to the wind and at a distance of 50 m from the Napevomo house, whereas the latter is placed at a height of 2.5 m on the façade of the Napevomo house. Despite this observed difference, the likely

source of which has been explained above, the experimental results have subsequently been considered valid and usable to carry out the validation of the co-simulation. 5.3

Results of the co-simulation

The co-simulation was carried out over the same periods as the experimental tests. Regarding the initial conditions of the building (internal and external temperatures of the walls and the inside air), these were determined in advance of the co-simulation (carried out from the end of the 156th day of 2013, see Figure 11.a and 11.b) by EnergyPlus Warmup convergence procedure [31], the LHTES system being not activated. Regarding the initial conditions of the LHTES for the co-simulation, they were determined by setting the values of parameters Tw0, ∆T0 and Tin(t=0) (cf. Figure 5); the initial values selected were, respectively, the estimated temperature when the LHTES system was first switched on: 15°C (PCM totally solid), a difference of 0.01°C (difference which avoids any problem of divergence) and 26°C (temperature of the inside air measured when the LHTES system was first switched on). The experimental results and results from the co-simulation are compared in terms of air temperature inside the building (Figure 12), and temperatures of the air entering (Figure 13a) and leaving the LHTES system (Figure 13b). These figures are accompanied by the same chronogram as presented previously. Overall, a good agreement is achieved between the measurements and the results obtained from the co-simulation, as shown in Figure 12. Nevertheless, it should be noted here that the differences between measurement and simulation observed in Figure 12a always correspond to periods when the electric convector was switched on and/or the LHTES system was in cooling mode. This observation points out the difficulty in taking into account experimentally the vertical stratification of the indoor air temperature for obtaining a spatial mean value of this air temperature that is representative of reality (here, the mean of three air temperature measurements located at three heights in the main room of Napevomo was taken). In order to be able to validate the co-simulation model quantitatively, the NMBE and the CV(RSME) of the indoor air temperature are calculated over three different periods of the co-simulation (see Table 2): 1) when the LHTES system is in cooling mode, 2) when the electric convector is switched on, and 3) over the complete co-simulation period. Table 2 presents a summary of the calculations of the NMBE and the CV(RMSE) coefficients. It can be seen that the values of the NMBE and CV(RSME) are of the same order of magnitude as the values observed during the calibration of the thermal model of the house. Furthermore, the NMBE and CV(RMSE) calculated for the cooling and heating periods remain of the same order of magnitude as those for the total period of the co-simulation.

Comparing the measured and simulated temperatures of air entering (Ta,in) and leaving (Ta,out) the LHTES system (Figure 13), it will be recalled here that the entering air temperature is either a piece of entry data of the model (a measured value of the ambient outdoor temperature) in regeneration mode or a value calculated by the model (simulated indoor air temperature (Tint.)) in cooling mode. It is therefore in the cooling mode that the differences observed between experimental and simulation results are greatest (the simulation does not take a possible stratification effect of the indoor air into account, which has an influence on the temperature of the air entering the LHTES); in regeneration mode the recorded differences are small. The recorded differences in cooling mode are nevertheless of the same order of magnitude as those observed between the outside air temperature measured by the meteorological station and the measured temperature of the air entering the LHTES system in regeneration mode (see Figure 11a); thus, in general they are not due to an inaccuracy of the co-simulation model but rather to an uncertainty in some of the entry data (the value of the outdoor air temperature). The values of the NMBE and the CV(RMSE) for the temperatures of air entering and leaving the LHTES when it is working in cooling or in regeneration mode are presented in Table 3, which also shows the NMBE and CV(RMSE) values over the complete cosimulation period. Despite the occasional differences mentioned above, the NMBE and CV(RMSE) values for the entering air temperature in both cooling and regeneration modes fall within the 10% and 30% limits recommended by ASHRAE, respectively.

Regarding the temperature of the air leaving the LHTES system, Figure 13b shows that there is a constant and significant difference between the experimental results and the results derived from the co-simulation in regeneration mode, i.e. the simulated value of the temperature of air leaving the LHTES is always shown to be lower than the measured value, regardless of the difference observed between the simulation and the measured temperature of air entering the LHTES (see Figure 13a). This indicates that the modelling of thermal transfers in purely conductive PCMs in the LHTES in regeneration mode overestimates its capacity to regenerate, a fact also observed by Rouault et al. [32] in the case of the LHTES system installed in the Napevomo house. However, the NMBE and CV(RSME) values for the temperature presented in Table 3 remain below those recommended by ASHRAE. Thus, although there are differences recorded between the experimental results and the co-simulation, the calculations of the NMBE and CV(RSME) for each variable demonstrate that the co-simulation is a viable solution for studying the performance of an LHTES system paired up with a building.

6

Conclusion

The purpose of the co-simulation of the thermal behaviour of an LHTES system and a building is to assess the energy performance of this system in all the complexity of its interactions with its habitual thermal environment. What has been presented here is an LHTES simulation model paired up with a BPS software package. In this specific case, the co-simulation was carried out in the Napevomo house, which integrates the first prototype of an LHTES system. After initial calibration tests on the thermal model of the Napevomo house, the coupling of the LHTES system with the house was assessed experimentally. The responses that were obtained in relation to temperature are indeed close and correlated, and therefore the measurements validated the model to a certain extent. Nevertheless, certain physical phenomena, which are difficult to master, in particular the stratification of the air and the low thermal mass of the house, explain the differences between experimentation and co-simulation. It was also necessary to reduce the number of sources of uncertainty by taking into account the presence of users in the house and the automatic control of the LHTES system. The uncertainties inherent in the hypotheses simplifying the model are nevertheless sufficiently low for the results of the simulations to be taken into account in a design process for an LHTES system; achieving this design goal is the aim at this stage. The co-simulation will be used as a tool to aid sizing, to assess the performance of an LHTES system, and to validate decisions for cooling houses. In the next stage of this research project, a new LHTES system prototype, which has been set up in the Sumbiosi house, will be monitored. This house has been installed at the Université de Bordeaux after competing in the Solar Decathlon Europe in 2012. The current activities are aimed at validating the behaviour of an entirely automatically controlled LHTES system coupled with the house.

7

Acknowledgments

This research was carried out under the Project QUALITAIRBAT in the framework of Inef4 (www.inef4.fr). The authors would like to acknowledge the financial support of Nobatek (www.nobatek.com) and ANRT (www.anrt.asso.fr).

8

References

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Figure 1: The Napevomo house

Figure 2: LHTES system designed and manufactured by Ekomy Ango [19]

Figure 3: Three-D modelling of the NAPEVOMO house using the OpenStudio Sketchup plugin

Figure 4: Diagram representing the interactions between the software packages used for the co-simulation

Figure 5: LHTES Air, Wall and PCM equations, initial values, and boundary conditions

Figure 6: Characterisation of Paraffin RT21 – parametric curve (continuous black line) and measurements taken by the T-history method carried out by Stankovic and Kyriacou in 2013 (broken lines): rise in temperature (black broken line) and fall in temperature (grey broken line)

Figure 7: Meteorological data measured at Talence (France) during the period 1st January 2013 to 30th June 2013: (a) Global horizontal irradiance against number of days and (b) outside air temperature against number of days (black), and maximum, mean and minimum outside air temperatures (dashed, dotted, and solid line, respectively) during the period 1971-2000.

Figure 8: (a) Air temperature inside the NAPEVOMO house from the 1st to the 102nd day of 2013 measured by the TEHOR interior sensor (black dotted line) and simulated using the EnergyPlus software package (grey continuous line), and (b) Zoom of days 45 to 54.

Figure 9: Air temperature inside the Napevomo house from 1st (day 120) to 21st May (day 141) 2013, measured by the TEHOR interior sensor (black dotted line) and simulated using the EnergyPlus software package (grey continuous line)

Figure 10: Schematic diagram of the experimental set-up

Figure 11: (a) Experimental air temperatures inside (broken line) and outside (solid line) the Napevomo house and air entering (dark grey dotted line) and leaving (light grey dotted line) the LHTES system measured from 5 th June (day 155) to 25th June (day 176) 2013; (b) Chronogram of the working of the electric convector (light grey line) and the LHTES in cooling mode (dark grey line) and regeneration mode (black line).

Figure 12: (a) Comparison of the results of the co-simulation (grey solid line) and the mean experimental measurements (black broken line) of the air temperature inside the NAPEVOMO house; (b) Chronogram of the electric convector in operation (light grey line) and of the LHTES in cooling mode (dark grey line) and regeneration mode (black line).

Figure 13: Comparison of the results of the co-simulation (continuous lines) and the air temperature measurements (dotted lines) inside the NAPEVOMO house: (a) entering air temperature and (b) leaving air temperature, and (c) chronogram of the working of the electric convector (light grey), and of the cooling system in regeneration (black) and in cooling (dark grey) mode.

Table 1: Thermo-physical properties of Paraffin RT21 determined by Stankovic and Kyriacou [27]

Property

RT21

Fusion/solidification temperature (°C)

19

Range of phase change T (°C)

2

Latent heat (kJ kg-1)

134

Heat capacity of the solid state (J kg-1 K-1)

4 500

Heat capacity of the liquid state (J kg-1 K-1)

1 800

Volumetric mass of the solid state (kg m-3)

880

Volumetric mass of the liquid state (kg m-3)

768

Table 2: Summary of the NMBE and the CV(RMSE). Values for the indoor temperature over three different periods

Period of calculation

Tint NMBE (%)

CV(RMSE) (%)

Cooling

0.93

3.28

Electric convector switched on

1.20

3.82

Total

0.1

3.21

Table 3: Summary of the NMBE and the CV(RMSE). Values for the entering and leaving temperatures of the LHTES (Ta,in and Ta,out)

Ta,in

Ta,out

NMBE

CV(RMSE)

NMBE

CV(RMSE)

(%)

(%)

(%)

(%)

Cooling

-2.97

4.41

-10.53

14.97

Regeneration

-3.86

8.82

-9.13

11.34

Total

-1.20

7.85

-9.40

12.44