Energy performance analysis of STILE house at the Solar Decathlon 2015: Lessons learned

Author’s Accepted Manuscript Energy performance analysis of STILE house at the Solar Decathlon 2015: lessons learned Cristina Cornaro, Stefania Rossi, Stefano Cordiner, Vincenzo Mulone, Luigi Ramazzotti, Zila Rinaldi www.elsevier.com/locate/jobe

PII: DOI: Reference:

S2352-7102(17)30211-5 http://dx.doi.org/10.1016/j.jobe.2017.06.015 JOBE291

To appear in: Journal of Building Engineering Received date: 13 April 2017 Revised date: 20 June 2017 Accepted date: 26 June 2017 Cite this article as: Cristina Cornaro, Stefania Rossi, Stefano Cordiner, Vincenzo Mulone, Luigi Ramazzotti and Zila Rinaldi, Energy performance analysis of STILE house at the Solar Decathlon 2015: lessons learned, Journal of Building Engineering, http://dx.doi.org/10.1016/j.jobe.2017.06.015 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 galley proof before it is published in its final citable 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.

Energy performance analysis of STILE house at the Solar Decathlon 2015: lessons learned

Cristina Cornaroa*, Stefania Rossia, Stefano Cordinerb, Vincenzo Muloneb, Luigi Ramazzottic, Zila Rinaldic

a

University of Rome Tor Vergata, Department of Enterprise Engineering, Via del Politecnico, 1, 00133 Rome, Italy b University of Rome Tor Vergata, Department of Industrial Engineering, Via del Politecnico, 1, 00133 Rome, Italy c University of Rome Tor Vergata, Department of Civil Engineering and Computer Science Engineering, Via del Politecnico, 1, 00133 Rome, Italy *

Corresponding author. Cristina Cornaro University of Rome Tor Vergata, Department of Enterprise Engineering Via del Politecnico, 1, 00133 Rome, Italy. [email protected]

Abstract West Virginia University and University of Rome Tor Vergata have partnered for the Solar Decathlon 2015 to present STILE (Sustainable Technologies Integrated in a Learning Experience), a house that merges Italian and Appalachian design concepts with innovative energy techniques. The aim of the work is the construction of a reliable and accurate dynamic building simulation model to perform a posteriori critical analysis of energy performance of the STILE house during the competition and to investigate the capability of the same house during a typical year in different locations. This was possible through the calibration of a dynamic simulation model of the house, using data gathered during the contest in Irvine, USA in October 2015. The work helped us to better understand the problems faced and to find optimized solutions that did not alter the original architectural concept. A 17% saving in the HVAC energy consumption for the period of competition was obtained by acting on shading, windows materials, and adequate floor insulation. The same model was also used to evaluate STILE behavior in different climatic conditions for a typical year. The study shows that the house has satisfactory thermal performance with mild and temperate climates, or whenever a benefit can be obtained by the use of the solar chimney.

Keywords: Solar Decathlon, microclimate monitoring, building dynamic simulation, IDA ICE, model calibration, thermal comfort, energy efficiency. 1

1. INTRODUCTION Nowadays, there is a growing concern on the environmental impact of energy we consume daily. However, instead of reducing energy consumption, homeowners use more energy each year. Since 1990, the worldwide residential electricity consumption has grown at an average rate of 3.4% per year [1]. This number represents mostly the increase of electricity consumption by individual dwellers. Decisions affecting the way people use their buildings leading to over-consumption are due mostly to lack of information and living habits, practices, and norms [2]. Within this scenario, the Solar Decathlon (SD) student contest was created as an international competition among universities by the U.S. Department of Energy with the objective of disseminating knowledge and good practices, thus promoting energy efficiency and renewable energy exploitation in the building sector, and creating awareness on responsible energy consumption among generations of international students. The first SD edition was held in 2002 in Washington DC and it has occurred every other year since then. It promotes research and development of efficient solar houses, allowing university teams to implement strategies toward electricity autonomy and self-consumption [3]. The objective of the participating teams is to design and build houses consuming the least possible natural resources and producing minimum waste products during their life cycle. Moreover, the SD promotes testing of new technical solutions outlining scientific challenges and future needs. Sustainable design is the practice of designing buildings with low environmental impact. These design practices include the choice of energy and water-efficient appliances, reducing consumption of non-renewable energy, and increasing indoor environment quality. These practices minimize the overall environmental impact of the building. Now more than ever, this aspect assumes great importance as national and international institutions recognize the threat from human-induced climate change as a major concern. Commercial and residential buildings consume almost 40% of the primary energy in the United States and Europe, and nearly 30% in China so that the building sector has a great potential for energy saving and for the use of renewable energy sources [4, 5]. Many countries have adopted regulations to reduce energy consumption in buildings. In Europe, the Energy Performance of Buildings Directives (EPBD) [6,7] are reference points that all European countries, including Italy, are recognizing to define building energy regulation codes [8]. The main objective is then to push the building sector 2

toward the Nearly Zero Energy Building (NZEB) concept [9]. Metrics have been introduced to define NZEB even if a certain debate on the most appropriate one to use is still open [10, 11]. In addition, many studies on cost optimality of NZEB have been performed to make these buildings more affordable [12, 13]. Since 2002, Solar Decathlon has spread these concepts all over the world starting from Solar Decathlon Europe and ending up more recently with Solar Decathlon China and Solar Decathlon Middle East that will be held in Dubai in 2018. Some works can be found in the literature describing experiences gained from different editions of the SD; for example, the Andalusia team presented the project Patio 2.12 for Solar Decathlon Europe (SDE) held in Madrid 2010 [14]. Cronemberger et al. [15] published a paper where the BIPV solutions adopted in SDE 2010 were presented and highlighted. Most of the scientific publications on the subject arose from the SDE 2012 competition, also held in Madrid. Navarro [16] reports a review on the experience and methodologies achieved during SDE 2012 while the work of Matallanas et al. [17] from the same group at the University of Madrid describes the design and development of specific rules of SDE 2012 for the evaluation of the house’s self-electrical efficiency. Other papers are devoted to the description of various houses attending the competition such us Astonyshine house presented by the French – Italian team [18], EKO house from the Brazilian team [19], Eki House by the Basque University [20], Omotenashi house by the Japanese team [21] and Med in Italy by the University of Roma Tre, Italy [22]. Among them, the Astonyshine team did a review of possible ways to improve energy performance obtained during the competition. In 2013, SD was also held in China, as already highlighted, and some works were presented to the scientific community about the performance of the houses during the competitions. A team from Australia won SD China 2013 using a retrofit strategy of an existing building instead of a completely novel design [23]. The Chinese team presented their Solark I house energy performance [24]. Wermager and Baur [25] evaluated the energy consumption of a house of traditional construction compared with the 2013 Missouri University of Science and Technology Solar House known as the Chameleon House and presented at the SD in Irvine, USA. Rome for Density was the Italian house winner of SDE 2014 in Paris. Its energy performance and thermal comfort was presented in a scientific work [26]. Most of the works reviewed here about SD house performance are focused on the presentation of the house’s features and innovation aspects while some present the competition results. However, to the authors’ knowledge, few critical analyses of the building have been reported in the literature. Moreover, to the authors’ knowledge, there is a lack in the literature of SD houses 3

performance evaluated in other periods of the year than the period of competition and in other locations using a calibrated dynamic building simulation model. The aim of the present work is to build a reliable and accurate dynamic building simulation model of STILE house (Sustainable Technologies Integrated in a Learning Experience) presented by West Virginia University and University of Rome Tor Vergata at Solar Decathlon 2015. Reliability of the model was guaranteed by a robust calibration and validation using data gathered during the SD contest in Irvine (CA, USA). In such way, the model allowed to reach two goals: to critically analyze some of the design and building choices and to evaluate the house thermal behavior in different locations to explore the possibility of using the house in different climates. The critical analysis allowed extending the learning experience of SD facing the critical issues encountered during the competition. The analysis of the energy performance and comfort behavior in different climate allowed analyzing the capability of the designed house to fulfill its requirements not only during the competition, in a particular period of the year but also during the full year at various locations. In the next sections a brief description of SD competition is presented (section two) then the main features of STILE house are described (section three). In section four the house performance during the competition are illustrated while section five shows how the dynamic building simulation model was built and calibrated. Section six and seven present the results of the critical analysis and of the energy and comfort performance evaluation under different climatic conditions.

2. SOLAR DECATHLON The SD project aims at making students more responsible toward the environment while teaching them a more efficient way of powering the world up through sustainably converted energy. In fact, as the U.S. DOE [27] explains, the Solar Decathlon has the following three main purposes: - To educate students and the public about the money-saving opportunities and environmental benefits presented by clean energy products and design solutions; - To demonstrate to the public the comfort and affordability of homes that combine energy-efficient construction and appliances with renewable energy systems available today; - To provide participating students with unique training that prepares them to enter our nation’s clean-energy workforce.

2.1. The competition

4

The Solar Decathlon competition is divided into different steps, each corresponding to a progressive stage of design completion. The first preliminary concept draft is delivered as a proposal to be selected as one of the 20 teams to compete two years later [28]. Collegiate teams spend almost two years designing and building energy-efficient houses powered by solar radiation. Teams can earn points according to three mechanisms: Task completion: Teams complete household tasks such as cooking, washing dishes, and making laundry. Monitored performance: Team houses should perform according to predetermined requirements, such as maintaining a comfortable (21.5-24.5°C) indoor temperature and humidity range. Jury evaluation: Jurors who are experts in their field (such as architecture, engineering, and communications) award points for qualitative features (such as aesthetics and design inspiration). Contests based on task completion or monitored performance are called “measured contests”; contests based on jury evaluation are call “juried contests”. Similarly to the Olympic decathlon, the Solar Decathlon competition consists of 10 different contests including Energy Balance and Comfort zone which are discussed thoroughly in this paper. These contests are designed to gauge how well the houses perform and how livable and affordable they are. Each contest is worth a maximum of 100 points, for a competition total of 1,000 points [27]. The Solar Decathlon 2015 edition took place in Irvine (California) in the Orange County Great Park from 8th to 18th of October 2015. Out of 20 teams selected just 14 teams were able to finish the design process adequately to complete the project and participate in the competition. University of Stevens gained the first place with SURE house while STILE got the twelfth place.

3. STILE HOUSE 3.1 West Virginia University and University of Rome Tor Vergata collaboration West Virginia University (WVU) and the University of Rome Tor Vergata (UTV) have partnered together for the Solar Decathlon 2015 competition to present STILE to demonstrate that residents need not sacrifice comfort for solar power. STILE is the acronym for Sustainable Technologies in a Learning Experience. Indeed, WVU and UTV chose to collaborate for the competition to allow students to learn more about energy efficient houses and international team work. The two universities have developed a strong partnership that allowed the two groups to become one team that built the house together in Morgantown [29]. 5

Being one of the twenty schools selected for the competition out of hundreds of schools that applied, the STILE team strived to meet the challenge. Not only more than 60 students have been involved in the project but also students from different backgrounds, highlighting the importance of a multi-disciplinary approach. STILE brings Italian architecture to the West Virginia borders. From exterior to interior there are many characteristics of these two rather distant worlds, but within the envelope of using clean and sustainable energy. By integrating two different cultures together, team STILE presented a one-ofa-kind solution to meet the competition challenge. 3.2 Architectural concept The overall design of STILE is linear and modern (fig. 1), using steel materials and neutral tones. The concept of the project is inspired by the bicultural nature or the team STILE, in order to be in coherence with both of the cultures, Roman and Appalachian, is designed to proficiently match some features from both architectural philosophies. The concept is based on the formal opposition between the two elements that forms the house: the box and the arch, as shown in fig. 2 [28].

Fig 1. STILE house built in Irvine.

The living box is a clear volume composed of a large daylight open space, connected to the outside thanks to panoramic windows. The functional arch is a covering structure developed from the archetypal form of the arch, a basic element of classical Roman architecture. The smooth angle of the arch's face reiterates the sleek and modern design, while also providing a structure to set STILE's solar panels, thus providing energy to the house.

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Fig 2. Architectural concept as formal opposition of box and arch.

The arch produces a patio acting as a shading barrier to the south side reducing the sun exposure. Figure 3 shows the house orientation and the shadows diagrams in Irvine.

Fig 3. Stile orientation and shadows evaluation for Irvine.

The house itself, also referred to as the Box, reflects more of the American component of the dualculture concept. The structure itself is (12.2 m × 7.3 m) and is partially made of three shipping containers. These containers have been deconstructed and modified to create a fully functioning, aesthetically pleasing, and energy efficient house.

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Following with American practical design considering space, the interior is split with one container as the "core", and the other two creating the central living space (Fig. 4). By using recycled shipping container structures as storage during the transportation process, the modularity of the house is increased, and a more efficient and cost effective means of shipping is obtained [29].

Fig 4. Functionality of the containers that made STILE house.

The floor plan of STILE is shown in figure 5.

Fig 5. Floor plan of STILE house.

Located above the canter of the main living area, is another key component in the energy efficiency of STILE, the solar chimney. The solar chimney passively ventilates and cools the house by using the pressure differences induced by the temperature gradient. While the Chimney heats up on the roof, cool air from under the house is pulled up through vents in the floors. The cool air slowly 8

warms and rises, and then reaches the top of the chimney (Figure 6). This change in temperature also causes a change in pressure, which causes more cool air to be pulled up from under the house and constantly replaces the warmer air inside, in a positive feedback loop. The solar chimney passively cools down the house using only natural air taking advantage of the convection mechanism and without the support of any heat exchanger.

Fig 6. Solar chimney. Ventilation paths.

This system allows for a passive cooling of the Box and it is able to reduce the power consumption of the house by working alongside STILE's high efficiency HVAC system. Used in conjunction, the solar chimney eliminates spikes in the temperature of the house, while the HVAC system will keep a constant, comfortable temperature inside the house. This solar chimney uses no electrical power in order to cool down the building, increasing the efficiency of the STILE house while decreasing the total power consumption. The solar chimney also adds aesthetic appeal by acting as a skylight through which natural light can enter, and further opens up the main living area. The southern half of the Box contains floor to ceiling windows. These windows open up the house’s interior to the outdoors and create a smooth transition between the Box and the deck.

3.3 Materials The projected energy behavior of a building is a fundamental parameter to measure the future impact of that building on the surrounding environment as well as its impact on the people who live inside it. This foundational concept of building design, based upon the energy needs, motivated the team to closely investigate the energy performance of the house, carefully choosing the materials and all technological features while paying close attention to relevant details.

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The specifications of materials used in the construction are listed in Table 1. Figure 7 shows the stratigraphy of each wall and window of the house and its U value. Insulation materials have been properly chosen according to the energy usage of the HVAC system to keep the indoor temperature optimal, avoiding the waste of energy not needed to heat or cool the space. Three types of insulation are used in the STILE house. The exterior walls contain polyiso foam board insulation, which is moisture resistant, durable, and maintains insulation properties over time. Recycled denim insulation is used in the interior walls. In addition to its high thermal benefits, this insulation has high sound absorption properties. Then, fiberglass insulation is used in the roof. The insulation layers have been wrapped with Oriented Strand Board (OSB), on which all the exterior finishes have been fixed: Fiberglass Reinforced Plastic (FRP) for the exterior, oak plywood, and bamboo for the interior.

Table 1. List of materials used in STILE construction. MATERIAL OSB

Thermal Conductivity [W/mK] 0.13

650

Specific Heat [J/kgK] 1700

Density [kg/m³]

Oak Plywood

0.15

660

1700

FRP Fiberglass insulation Denim insulation POLYISO foam board insulation Bamboo Oak window frame Air

0.067 0.056 0.051

800 30 25

1170 1030 1000

0.029

35

1400

0.058 0.5 0.026

900 660 1.2

1700 1700 1006

10

Fig 7. Material composition of walls and windows of STILE house. The windows are double paned and include a layer of argon and an insulating layer in between the glass panes; this further improves the energy efficiency of the house (figure 7). The windows have been made in Italy by Askeen and can shield some of the radiating energy coming into the house. However, in order to have a significant impact on energy efficiency, solar shades were implemented for STILE’s resource conservation. These shades, made by high density polyethylene with UV block and U=0.4 W/mK, are located along the southern side on the exterior of the house, and wrap partially around the sides. Thermal bridges are mainly present in proximity of the main joints, for example corners, external walls-internal walls, and joints between the roof and walls and the floor and walls.

3.4 Description of Equipment STILE uniquely stylish solar panel array has been oriented at the optimum direction and angle for maximum light absorption and is supported by the house’s overarching design, as showed in Figure 8. Each of the SolarWorld mono crystalline PV panels is chosen to provide a peak power of 285 W. The arch, the energy system housing, has the spacious capability to install 36 solar panels above the 11

roof for a total peak power of 10 kW. The wiring of the panels consists of two branches of 18 panels. The two branches are controlled by a Solar Edge Inverter [29].

Fig 8. View of the PV system mounted on the arch of STILE house.

The Solar Edge technology is new to the market and is quickly outpacing the old methods of DC/AC conversion. Typically, micro-inverters are attached to every panel in the array limiting the amount of panels on one string. The HVAC system chosen in the house is a Carrier multi-zone ductless system, made of three internal units (two in the living room, one in the bedroom) and one external unit. A schematic of the system is shown in Figure 9. Each unit has 2.7 kW of power, 10.2 HSPF (Heating Seasonal Performance Factor) and 22 SEER (Seasonal Energy Efficiency Ratio). The system was designed according to the European norms (UNI 10339/1995 [30] and UNI EN 13779:2008 [31]) and ASHRAE Standards 62.1/2013 [32]. The total energy consumption had to be calculated considering the usage of an HVAC system, heat pump, laundry machine, TV set and several other electrical devices that are typical furnishings for a common house. In addition to powering the internal equipment up, the contest requires that the team must charge an all-electric car using the power produced by the photovoltaic panels. Thus, the energy analysis has been one of the main steps in addressing the project goals, given the restrictions that are imposed by the contest's rules.

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To heat the water, the Energy Star®-rated AO Voltex Hybrid Electric water heater, which is more than twice as efficient as a standard water heater, is chosen. It has an energy factor of 3.06 and it is wrapped by about 5 cm of foam insulation. This water heater reduces water heating costs by up to 71% by pulling heat from the environment, while simultaneously cooling and dehumidifying ambient air. STILE is also equipped with a filtered greywater system, which along with rainwater collection, is used to water the plants on the deck. In addition, the house provides a drinking water purification system. STILE house automation system electronically manipulates many of STILE’s assets while also cutting costs on energy consumption. To accomplish this, the team has implemented a software and set several measurement probes in and around the house. One of the ways the system helps the homeowner save energy is by controlling lights and doors. Another energy saving feature involves using smart sockets equipped within the house. These measure energy usages of anything plugged directly into them. The HVAC system is designed from the ground up to limit energy usage. Using several temperature sensors inside and outside, the system can determine whenever it would be better turning off the air conditioning and opening vents and solar chimney. STILE has a Nest thermostat, which automatically learns the homeowner’s schedule and turns down the HVAC system when no one is home.

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Fig 9. Schematic of the HVAC system of STILE house.

4. COMPETITION PERFORMANCE In this section, after a brief overview of Irvine climate and a summary of the weather conditions during the competition period, STILE house performance will be analyzed and discussed. The competition phase took place in Orange County Great Park from October 8th to October 18th. During this period, a strict schedule to give public tours, to complete home life tasks and to maintain comfortable indoor temperature had to be respected to earn points in the measured contests and also to meet the juries to explain to them the aesthetics and the innovative characteristics of the house, beyond the affordability, market appeal and the educative aspect of the project.

4.1. Irvine climate conditions Irvine, like most of coastal Southern California, generally has a Mediterranean climate characterized by warm-to-hot summers and cool-to-warm winters, rarely falling below freezing. Precipitation occurs predominantly during the winter months. Irvine has an average temperature of 17.2 °C and 293 mm is the value of average annual rainfall. With an average temperature of 22.2 °C, August is the hottest month of the year and 13.0 °C is the average temperature of January that is 14

the lowest average temperature during the year. During the year, the average temperatures vary by 9.2 °C. July precipitation, which is the driest month, is 1 mm and, with an average of 63 mm, the month of January is the month with more precipitation. There is a difference of 62 mm precipitation between the driest and the wettest month of the year [29]. Figure 10 shows the trend of global, direct and diffuse irradiance and dry bulb temperature during construction and competition phases. These hourly data have been collected by DOE and NREL employees at Orange County Great Park.

Fig 10. Irradiance and ambient temperature trends during the SD2015 competition in Irvine, CA. Over the period October 4th to October 16th, Irvine was characterized by different weather conditions but, almost always, temperatures hotter than average were experienced. Temperature reached a peak of almost 40 °C on October 10th. Paying attention to the irradiance trends, the last days have been rather cloudy, however temperatures were still high, with high relative humidity.

4.2 STILE performance To analyze the energy performance and the temperature trend inside the house it has been necessary to retrace all the activities carried out. Since the schedule of activities was different day by day, a different schedule was considered for each day of competition. For the sake of brevity, only the schedules of the two most significant days are shown in Table 2. 15

Table 2. Schedules of STILE for two days of competition. 11-oct Mechanical Room

Living Room Contes People t Closed

Time People

Contest

0:00-7:00

Bathroom Lights People

HVAC

Windows

Solar Chimney

On

Closed

Closed

Open

Open

Closed

Closed

Windows

Solar Chimney

Contest

7:00-7:30 7:30-8:00

1

Electr.

3

Cook. + Dishw. +Electr .

1

Dishw.

8:00-8:30 8:30-9:00 9:00-9:30

2

Dryer+Washer

9:30-10:00 10:00-10:30

Off

10:30-11:00 11:00-19:00

Off

Tour

19:00-23:00

On

23:00-24:00

Closed

Off

On

12-oct Time 0:00-7:00 7:00-8:00 8:00-9:30 9:30-10:30 10:3011:30 11:3012:00 12:0012:30 12:3013:00 13:0013:30 13:30-1830 18:3019:00 19:0023:00 23:0024:00

Mechanical Room People

Contest

Living Room People Contest Closed

Bathroom People

Contest

Lights

HVAC

Closed

Market Appeal Jury Engineering Jury

Open

Social Media Recording Video Recording

Off On

2

Washer + Dryer

Closed

Closed 1

Electr.

On Closed

Off

These two schedules have been chosen since they represent an example of the various activities occurring inside STILE, including the visitor tour (11 October) and the presence of the jury for the performance evaluation (12 October). The contests are scheduled in order to simulate the real use of a house. For example, hosting a dinner party for neighbors, operating television and computer in the morning or in the evening, turning all lights after sunset until bed time, producing hot water to 16

simulate a shower, boiling water to simulate cooking and operate appliances like dishwasher, washing machine and dryer. As for the solar chimney, it was operated according to the needs of the house thermal comfort and scheduled in the model as it was run during the competition. Usually it was open in the morning to cool down the house, taking advantage of the fresh air without using the HVAC system, and closed in the hottest hours, except during the visitors’ tour hours when it was mostly open. Environmental data have been collected during the two contests named “comfort zone” and “energy balance”. As for the comfort zone, temperature and relative humidity sensors provided and monitored by the DOE are placed in the living room (two sensors) and in the bedroom (one sensor), respectively, and data are collected following the schedule summarized in Table 3. The sensors collected the data during the whole period of the competition except when the public exhibit was going on and the house was open for tours.

Table 3. Time schedule of Comfort zone measurements. Time/Day 0:00-7:30 7:30-10:30 10:30-19:30 19:30-24:00

08-oct

09-oct

Comfort zone schedule 10-oct 11-oct 12-oct

Tour

13-oct

14-oct

15-oct

16-oct

Comfort zone measurement

During the measurements, inside the house, contests and tasks to be completed were also occurring simultaneously; so the team had to be careful to not compromise the environment by doing inappropriate actions like opening the windows during the hottest hours. Indeed, as for the energy balance contest, continuous measurements have been collected. There was no energy contest going on during the public tours; so, almost no energy was consumed in that specific period. On the contrary, the photovoltaic system produced energy continuously and the house was connected to the site grid; so, all the energy produced and not consumed was given to the grid but calculated as “stored” by the house. Figure 11 shows the average temperature trend measured inside the house during the competition period together with the outside dry bulb temperature. To obtain full points, the houses had to maintain temperatures between 22.2°C and 24.4°C. 17

Peaks of about 35°C were reached on October 9th to 11th from around 12 pm to 6 pm that coincides not only with the hottest hours but also with the public tours. In particular, comfort conditions were not guaranteed due to high occupancy during the public tours and for the particularly heavy climate conditions experienced especially in October 11th and October 12th.

Fig 11. Temperature trends recorded inside the bedroom and living room during the competition.

For energy production, the teams received full points for producing at least as much energy as its house needed, thus achieving a net energy consumption of zero during the competition. Reduced points were earned for a net electrical energy balance between -50 kWh and 0 kWh. As can be seen in Figure 12, the STILE house was able to stay above net zero during the overall competition. The peaks were reached during the sunniest days when the PV system produced the most.

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Fig 12. Net energy produced by STILE house during the competition.

For the energy consumption contest, the teams received full points if using 175 kWh of energy or less during the whole competition period. Reduced points were earned for consumption between 175 kWh and 300 kWh. Team STILE, as it can be seen from Figure 13, consumed 332 kWh and produced 333 kWh during the competition period. For this reason, the team got 0 points for the energy consumption and 50 (maximum score) for the energy production because above net zero.

Fig 13. Cumulative energy consumption of STILE house during the competition.

Table 4 shows the energy balance of STILE during the competition summarizing the energy production and consumption of each day. The house consumed the highest amount of energy on October 9th, considering that the HVAC was on for half of the time of the other days. This was due 19

to an inaccuracy during the construction phase that brought us to have a bad performance from the HVAC system for the first 2 days. On October 12th, 13th and 14th, when the HVAC was always turned on, high consumption values were also registered.

Table 4. Energy balance of STILE during the competition. Day Production Consumption Net Energy

08-oct 09-oct 10-oct 35.38 50.38 50.71 22.27 43.46 37.44 13.11 6.92 13.27

Energy Balance (kWh) 11-oct 12-oct 13-oct 14-oct 49.98 31.11 47.53 40.89 34.38 51.48 60.28 40.13 15.59 -20.37 -12.75 0.76

15-oct 16-oct TOT 21.56 6.38 333.92 28.52 14.09 332.06 -6.96 -7.72 1.86

4.3 STILE total energy consumption and design criticality Figure 14 shows the total energy consumption of the house related to each of the equipment used. As can be seen, the main contributions are due to the HVAC, the electric car, and the refrigerator.

Fig 14. Total energy consumption considering the equipments used in the house during the competition. Energy efficient products were chosen as appliances during the design phase; however the refrigerator was consuming much more energy than expected (115 kWh instead of an expected quota of 56 kWh) probably due to malfunctioning and giving a penalization in the energy contest. In the design phase, the idea was to shadow the windows using high quality blinds, but that was not possible due to budget problems and so shaded windows were used. Unfortunately, the shading used in the competition was not as effective as expected. Probably, by increasing the efficiency of 20

the blinds and their performance in resisting wind action, a higher share of solar radiation would have been avoided. Moreover, the arch was supposed to be a shading element, but it did not really help to block the heat gains. Indeed, the simulation done during the early design stage demonstrated that the best option for the arch was to have it as the support for solar panels and brise soleil, to shade as much as possible, without preventing the daylight going inside through the windows and the skylight. However, during the construction phase, different elements had to be cut down due to time and budget reasons and also the brise soleil was eliminated. Furthermore, since the sun had a low elevation angle, more brise soleil put in the lower part of the arch would have helped to shade the patio in the south side of the house. Moreover, some criticisms were offered by from the jury related to the arch position. In fact, the architecture jury commented that it would probably be better to place it overhanging the west side of the house instead of the east because of the heat from the sun during sunset hours. One more problem was the solar chimney, specifically, the element that was used for natural ventilation. The solar chimney did not perform as expected since the air was rarely fresh and the air passing through the vents was actually warmer than expected. The system, however, performed correctly from the standpoint of the elaborated flow. Thus, the system was well designed but the real weather conditions and ambient temperature did not help in taking full advantage of the chimney’s operation. Another problem related to the construction phase was due to the use of a poor floor insulation, which did not help in maintaining the required temperature range continuously and efficiently.

5. DYNAMIC SIMULATION MODEL CONSTRUCTION AND CALIBRATION The a posteriori energy analysis was done using the software IDA Indoor Climate and Energy (ICE) 4.6 distributed by the Swedish company EQUA Simulation AB. IDA ICE is an innovative and trusted whole-year detailed and dynamic multi-zone simulation application for the study of thermal indoor climate as well as the energy consumption of the entire building [33]. To define climate conditions IDA ICE uses .prn files that contain hourly data of a typical year. To be able to recreate the same weather conditions of the competition, a data file with hourly average outdoor temperature, relative humidity and horizontal solar radiation, wind speed and wind direction, collected on site at the Orange County Great Park, from October 4th to 16th was retrieved. From horizontal solar radiation data, it was possible to calculate the direct normal radiation and the diffuse horizontal radiation necessary as input to the software. 21

Because of the complex geometry due to the solar chimney shape included in the ceiling and the arch element, the house envelope was first modeled using Sketch Up and then imported in the software. STILE was divided into 7 zones: kitchen-living room, bedroom, bathroom, hallway, mechanical room, west abutment and east abutment. The last two are not real “rooms” but they are separated from the other spaces so it was necessary to consider them as zones. In the STILE model, wall stratigraphy has been modeled according to Figure 7 and Table 1, thermal bridges values were assigned depending on the assembly effectiveness obtained in the construction phase. A low value of thermal bridges for the windows was chosen, 0.008 W/K/(m perim), because of their good insulation properties, medium value for the connection between external walls and internal walls, 0.098 W/K/(m perim), and high values for the joints of the external walls with roof, 0.174 W/K/(m joint), and floor, 0.268 W/K/(m joint). On the other hand, infiltrations, extra energy, and losses concern very technical specifications, such as air infiltration due to wind through the windows and leaking components of the systems in the building. Thus, the default values were maintained for all the parameters except for the distribution system losses where values of 0.2 W/m2 and 0.08 W/m2 were assigned for the domestic hot water circuit and for the supply air ducted losses, respectively. Equipment, occupants, and lights were simulated representing the same time schedules experienced during the competition, as reported in Table 2. This could be done, thanks to the accurate monitoring and recording by the team of all the activities carried on in the house. That includes schedules of equipments, occupants (team member, jurors and visitors), windows, solar chimney, and lights. In order to include the PV system in the simulation, the ESBO Plant was imported providing the total area (53 m2), angle (an average of 15°), and efficiency (0.17) values experienced by the real PV plant. To simulate the ductless HVAC system, air-to-air non-ducted air conditioner units were inserted in the model, two in the living room and one in the bedroom, with proper values of power (12 kW total) and efficiency (COP=4). 5.1 Calibration using temperature profiles Figures 15 and 16 show the temperature trends measured and calculated during the three consecutive days when the HVAC system was always turned on, night and day, and the house was closed to the public; so it was possible to keep track of what happened inside easier and more accurately. The first step consisted in the calibration of the HVAC system, and later, in the 22

verification of the accuracy of the envelope definition comparing parts of the temperature profile collected when the air conditioning system was switched off. The good agreement between the two data sets could be obtained mainly by finding the optimum set point temperature that was able to reproduce the real conditions. The same process was used for both the living room (Fig. 15) and the bedroom (Fig. 16). To check the envelope construction accuracy the temperature profiles calculated and measured during the night of 15th of October were compared since in that period the HVAC system was turned off. As observed from Figures 15 and 16 and as expected, the envelope modeled with the chosen materials reflects the real measurements showing a very good agreement between the two profiles.

30

TEMPERATURE [°C]

Mean air Temp_IDA

LR Temp_Real

28 26 24 22 20 11-Oct

12-Oct

13-Oct

14-Oct

15-Oct

16-Oct

TIME [dd-mmm] Fig 15. Measured and simulated temperature profiles for the living room of STILE house.

23

28 BR Temp_Real

Mean air Temp_IDA

TEMPERATURE [°C]

26 24 22 20 18 11-Oct

12-Oct

13-Oct

14-Oct

15-Oct

16-Oct

TIME [dd-mmm] Fig 16. Measured and simulated temperature profiles in the bedroom of STILE house during the competition. Root mean square error (RMSE) and normalized root mean square error (NMRSE) were used as indicators of the accuracy of the model. RMSE, as shown in Equation 1, is a measure of the difference between values predicted by the model and the values actually observed and recorded. √

∑

(

) (1)

NRMSE (Equation 2) is the RMSE normalized by the maximum variable difference that occurred in the time of observation and it is useful to compare data with different scales. It gives the percentage of the error related to the chosen period.

(2)

Table 5 summarizes the RMSE and the NRMSE obtained for the living room and the bedroom. The average NRMSE of 6.2% can be considered very satisfactory.

24

Table 5. RMSE and NRMSE for the STILE house model. Living room Bedroom RMSE [°C] 0.53 0.40 NRMSE [%] 6.48 5.79

5.2 Validation through the energy balance The model, calibrated using inside temperature measurements, was then used to check the energy balance of the house during the entire period of the competition. The first step was to estimate the energy consumption of the appliances and equipment in the house. Then, knowing that on October 16th the HVAC system would be turned off, only the appliances involved in the contests going on that day would be consuming energy. Having knowledge of the power of each appliance and the hours that they were functioning it was possible to estimate the energy consumption and the average power used by each unit. Then, knowing the total energy consumption of the house during the overall period of the competition it was possible to calculate the amount of energy used by the HVAC system. Table 6 shows the results obtained. A difference of 0.9% between IDA results and the measured consumptions was found. The electric car’s contribution was calculated separately because it was not inserted in the model.

Table 6. Calculated and measured energy consumption of STILE house during the entire period of the competition

Tot Equipment Energy consumption HVAC system consumption Electric car Tot

IDA model kWh/m2

STILE kWh/m2

2.65

2.69

1.94

1.94 0.73

5.31

5.36

25

6. OPTIMIZATION OF PERFORMANCE DURING COMPETITION The dynamic simulation model of STILE was then used to evaluate interventions that could possibly improve the energy performance of the house without undermining its design by altering the architectural concept in order to maintain the design of STILE substantially unchanged. The optimizations were done starting from a critical analysis of the problems observed during the competition. It has to be pointed out that the majority of optimizations would increase the budget at the disposal of the team. All the results were evaluated in terms of the HVAC energy consumption since it was assumed that the other equipment remained the same.

6.1 Arch influence One of the comments received from the jury was that the design and placement of shading structure, the arch, was ineffective; moving it to the west would have provided more benefits to the structure below. Starting from this comment, the behavior of the house with an overhanging arch on the west side instead of the east side was simulated. (Figure 17). According to the simulation results, this location change would not lead to a substantial decrease of the energy consumption of the cooling system; i.e., the house with the arch placed more toward the west would have had more or less the same performance shown during competition.

Fig 17. Model with the arch overhanging the west side.

This behavior can be explained by the slope of the sunrays to the ground surface in October. In fact, during that period the sun is rather low, and the height of the arch, relative to the house, does not 26

produce efficient shading as the light and heat due to solar radiation pass substantially unaffected under the arch during the hottest hours. In fact, the arch, if placed on the east, produces minimal shielding of the east side during the early hours of the morning and of the south side during the day due to its height; anyway, sunrays pass between the frontal columns, which were initially destined to house the brise soleil. However, above all, it completely leaves uncovered the west side during the hours of the day when temperatures are higher, allowing the sunlight to penetrate inside the living room and bringing the house to very high temperatures. If the arch would have been placed on the west side, indeed, it would probably have resulted in a benefit if the competition would have taken place during a different period of year when the sun path is higher, for example in the summer. To double check these results and observations, the behavior of the house without the arch was analyzed and the results obtained indicate that HVAC consumption is increased by only 5%, thus confirming that the benefit of the arch to energy saving during the period of competition is low for what discussed earlier.

6.2 Influence of Shadings As explained earlier, due to budget and time reasons, brise soleil were not used. Since the sun was very low, brise soleil would have helped to shade the patio in the south side of the house. Thus, the house was simulated assuming that shading elements were placed in the south side, among the columns of the arch. As Figure 18 shows, the shading element has been modeled as a continuous surface since the distance between the brise soleil is not significant relative to the distance between the base of the arch and the house.

Fig 18. Model with the shading elements among the columns of the arch. IDA ICE screenshot. 27

Results show that this solution would have decreased the energy consumption of the cooling system by 2%. The value is not remarkable as the arch columns are almost 5 meters away from the windows; so, the solar radiation can still pass through the east and the west side. However, this can still be considered a partial solution contributing toward energy savings. Summing up, the solution to this problem would be the setup of a shielding structure, not only to solve the south side issues, but also the east side and, with special focus, the west side, to limit radiation during the hottest hours and throughout the whole year. Fixed shields or reflective films may also be a solution to fix this issue. In fact, as confirmed by the experiments discussed below, increasing the blind shading efficiency would lead to a 6% decrease of the energy used by the cooling system. So, more efficient blinds would have almost completely blocked the sunrays; however, they would have also compromised interior lighting of the house and thus leading to a worse visual comfort and less natural light inside the living room.

6.3 Windows influence Since most of the external wall surface is covered by glass, a numerical test was done to understand the impact of window performance parameters on energy consumption during the competition. A high solar heat gain coefficient (SHGC) and a low value of solar transmittance characterized the windows and that allowed the solar radiation to enter the house and then hardly let the heat generated be dissipated in the ambient. A simulation was done changing the SHGC from 0.63 to 0.40, observing an 8.3% reduction of energy consumed by the HVAC system, which is the most significant result obtained. In fact, it is thus demonstrated that fine tuning this parameter can facilitate better adaptation of the house design to different locations.

6.4 Overall influences and saving The energy saving percentage of the HVAC system of the original STILE house is shown in Figure 19. By optimizing the design, according to the strategies described implementing in the model all the interventions that led to an improvement, a 17% reduction of energy consumed by the HVAC system can be observed, during the competition period. As already highlighted, the optimized house would have implied a significant increase in the overall budget.

28

Fig 19. HVAC energy consumption improvement due to the various optimization solutions tested. Finally, considering a regularly operating refrigerator on top of the design optimization, STILE house would have consumed about 150 kWh less during the competition. Assuming this energy load, team STILE would have earned about 50 points in the energy consumption contest instead of 0.

7. THERMAL PERFORMANCE OF STILE AT DIFFERENT LOCATIONS The simulation model of the house as it was built for the competition was then used to compare the thermal performance of the house at different locations during the whole year particularly studying the effects of the cooling load, the heating load, the energy production through the photovoltaic system, the indoor comfort, as well as the influence of the solar chimney on ventilation. The cities chosen for the analysis are: Irvine (CA), Cartersville (GA), Rome (IT), Palermo (IT), Madrid (SP), Paris (FR) and Copenhagen (DK). The reasons of these choices are the followings: - Irvine: Southern American city located on the West coast where the competition took place. - Cartersville City located in the southeastern United States: this is the location where the house is being rebuilt currently (Tellus Museum). - Rome: Italian city located in the center of the region, seat of the University of Rome “Tor Vergata”. - Palermo: Italian city located in Sicily: it is one of the hottest cities in Europe. 29

- Madrid: European city with warm and temperate Mediterranean climate. Here the SDEurope 2012 took place. - Paris: Western European city with warm and temperate Oceanic climate. Here the SDEurope 2014 took place. - Copenhagen: Northern European city with cold and temperate Oceanic climate. To evaluate the energy performance, the model was set simulating a normal use of the house of a working couple. Schedules of the blinds, windows, lights, and solar chimney were set according to the occupational habits of the householders. The simulation model orientation has been maintained as it was during the competition. Climate files used for the simulation were Typical Reference Years (TRY) available in the software for each location. Due to the different climate conditions of the sites chosen, therefore different length of cooling and heating periods and minimum and maximum temperature settings, it was decided to analyze the HVAC energy consumption of STILE house during the common months of the different places selected. Table 7 summarizes schedules of the house considering an occupancy of 2 people. For all the locations, the heating period was assumed from December 1st to February 29th.The months of June, July and August were considered for the cooling period. Table 7. Schedules used for heating and cooling seasons at different locations. Heating season PERIOD LIGHTS PEOPLE

Cooling season

Dec-Jan-Feb Jun-Jul-Aug 19:00-22:30 20:00-23:00 2 People - working schedule

SOLAR CHIMNEY

Always closed

Open 0:00-11:00 / 20:00-24:00

WINDOWS

7:30-8:00

8:00-8:30 / 19:00-19:30

DOORS HVAC SETPOINT

BEDROOM Always open BATHROOM Never open 11:00-22:30 11:00-20:00 MIN 21 - MAX 25

Moreover, the energy production from the PV system is strongly dependent on orientation and slope, on top of the location and the weather conditions. 30

The PV system was considered at the best orientation, which is south, and with the optimum slope angle referred to each location to ensure the greatest power production. Indoor thermal comfort is evaluated by IDA ICE in dynamic conditions referring to the temperature values limits reported by the European standards (EN 15251/2007) [34] combined with the Fanger theory (UNI EN ISO 7730/2006) [35], providing also PMV and PPD indexes. For this study the acceptability of indoor temperature values according to EN 15251:2007 classes was evaluated for the period of test. Activity corresponding to 1.2 met was considered and clothing resistance variable between 0.5 clo and 1 clo, depending on the season (cooling or heating), was applied.

7.1 Cooling season Figure 20 shows energy production and consumption of STILE at the various locations for the cooling season. It is evident that the PV system is very efficient during the summer in all the cities analyzed, producing enough energy to cover the HVAC system consumption used to cool the house down.

CONSUMPTION

PRODUCTION

COPENHAGEN PARIS

CITY

MADRID PALERMO ROME CARTERSVILLE IRVINE 0

10

20

30

40

50

60

70

80

90

100

ENERGY [kWh/m2]

Fig 20. Energy consumption and production of STILE house at the various locations during the cooling season. Comparing the results of the house energy consumption in each city, during each month, as shown in Figure 21, it can be noted that in the cities with Mediterranean climate, the house needs more energy to be cooled. In Paris and in Copenhagen, as they are characterized by cooler climates, the

31

house needs lower energy to operate the HVAC system. Irvine weather conditions are the best considering the overall period.

Fig 21. Cooling energy consumption for the various locations during the months of June, July and August.

Figure 22 shows the comfort levels reached inside the house at the different locations considering the HVAC system with the same power size as during the competition in Irvine. It can be noted that, even in Copenhagen where the house needs the less energy for cooling, acceptable comfort levels are almost never guaranteed and thus it is unacceptable for most of the time considered. It can be further observed that the house performs better in climatic conditions similar to Rome, Irvine and Cartersville, maintaining for most of the period an acceptable comfort level. These cities are in fact characterized by warm and temperate climates.

32

60

50

BEST

GOOD

ACCEPTABLE

UNACCEPTABLE

TIME [%]

40

30

20

10

0 IRVINE

CARTERSVILLE

ROME

PALERMO

MARDID

PARIS

COPENHAGEN

CITY

COOLING COMFORT LEVELS 23.5-25.5 °C 23-23.5 °C Good /25.5-26 °C 22-23 °C Acceptable /26-27 °C >27 Unacceptable <22

Fig 22. Comfort levels reached at the various locations during the cooling season. 7.1.1 Influence of solar chimney The ventilation contribution due to the solar chimney during the cooling season was also analyzed. Two different simulations were done to check the influence of the passive cooling system during a typical hot day. The first scenario had the solar chimney always closed (NO SOLAR CHIMNEY case), while in the second scenario the solar chimney was opened from sunset to midmorning and the HVAC system was turned on from 11 am to 8 pm (SOLAR CHIMNEY case). Four cities were considered: the hottest ones, where it was expected that the solar chimney could benefit more: Irvine, Cartersville, Rome and Palermo. Figure 23 shows the temperature trends in the two configurations tested. A benefit in the temperature trend in each city can be observed, giving a decrease of the energy amount required by the HVAC system during the day. 33

It can be noted that the best results are obtained in Irvine and in Rome, both in terms of comfort and energy consumption. The solar chimney, in fact, cools the house down during the night decreasing up to 8% the amount of energy required by the mechanical cooling system. Another consideration may be done on the HVAC maximum power: in fact, while in Irvine it can almost fulfill the cooling demand keeping temperature to the comfort level most of the time, for the other cities it is definitely downsized.

IRVINE, CA - ENERGY BALANCE Solar NO Solar Chimney Chimney CONSUMPTION 901 982 (kWh)

ROME, IT - ENERGY BALANCE Solar NO Solar Chimney Chimney CONSUMPTION 1210 1295 (kWh)

CARTERSVILLE, GA - ENERGY BALANCE Solar NO Solar Chimney Chimney CONSUMPTION 1350 1400 (kWh)

PALERMO, IT - ENERGY BALANCE Solar NO Solar Chimney Chimney CONSUMPTION 1340 1382 (kWh)

Fig 23. Temperature trends obtained for the living room of STILE located in four of the locations studied considering the effect of solar chimney. 7.2 Heating season Figure 24 presents the energy production and consumption of the house for the locations considered during the heating season. It can be noted that during this period the energy production by the photovoltaic system decreases compared to the cooling season; it can be also said that the heating requirement is very low. This can be explained as the house can be compared to a greenhouse, thus being able to maintain a high temperature inside. It has to be pointed out that in Copenhagen the energy demand would not be covered by the PV production. 34

COPENHAGEN

CONSUMPTION

PRODUCTION

PARIS

CITY

MADRID PALERMO ROME CARTERSVILLE IRVINE 0

10

20

ENERGY

30

40

50

[kWh/m2]

Fig 24. Energy consumption and production of STILE house at the various locations during the heating season.

Fig 25. Heating energy consumption for the various locations during the months of December, January and February. Considering the heating consumption (Figure 25), it can be noted that, as expected, the cities characterized by colder climates need more energy to heat the house up. Focusing on cities with warm climates, it is clear that the energy demand is very low, a confirmation of the greenhouse behavior of the house. As seen in Figure 26, all the cities, except Palermo, are characterized by an unacceptable comfort level, for most of the daily hours. For cities with cold winters, most of the unacceptable comfort levels are mainly due to the significant temperature drop during the night, when the heating system is switched off. For the cities characterized by milder winters, however, most of the unacceptable 35

hours are partly due to overheating during the day. In this situation, the effect of solar chimney could be of some benefit. Indeed, it may be reiterated that the solar chimney has been always maintained closed during the winter. Simulations with the solar chimney opened are discussed in the next section. 70 BEST

GOOD

ACCEPTABLE

UNACCEPTABLE

60

TIME [%]

50 40 30 20 10 0

IRVINE

CARTERSVILLE

ROME

PALERMO

MARDID

PARIS

COPENHAGEN

CITY

HEATING COMFORT LEVELS 21-25 °C 20-21 °C Good /25-26 °C 18-20 °C Acceptable /26-27 °C Unacceptable

>27 <18

Fig 26. Comfort levels reached at the various locations during the heating season. 7.2.2 Weather and solar chimney influence The effect of the solar chimney was also evaluated during the heating season especially for the city of Rome, considering that most of the discomfort registered for warm climate was related to overheating due to solar radiation. First, the difference, in terms of inner temperature, between a cloudy day and a sunny day was analyzed. As shown in Figure 27, on a cloudy day the house is able

36

to reach the desired temperature. On a sunny day, the house acts as a greenhouse for the giant panoramic windows with high SHGC and low solar transmittance. Then, the solar chimney was considered open in winter from 12:00 to 15:00 hours and, as can be seen in Figure 28, this allowed the temperature to drop, while still being comfortable. Under these assumptions, the house, in hot climates, hardly needed heating and indeed, the solar chimney, can provide a remarkable benefit also during the winter.

Fig 27. Temperature profiles in the living room for a sunny day and a cloudy day in Rome during the heating season.

Fig 28. Temperature profiles in the living room for a sunny day in Rome during the heating season considering the solar chimney closed (red line) or opened (blue line).

During winter, in the sunniest days, the solar chimney should always be kept open and modulated to have the right control of the flow rate of fresh air to maintain the thermal comfort without the need of extra heating.

37

7.3 Overall Influence of climate conditions and location on STILE performance In the previous sections, it was pointed out how the weather conditions in Irvine during the SD 2015 competition were considerably different from the typical average reference conditions used for the design of STILE house. Figure 29 shows the carpet plot of temperature and direct solar radiation in Irvine. The graph named “Irvine” describes temperature and solar radiation for the TRY throughout the day; on the other hand, the highlighted zone in the graph named “2015” describes temperature and solar radiation during the competition period according to the data collected onsite. It can be noted that there is a rather large difference between temperature and solar radiation intensities expected (according to the typical climate conditions) and those registered during October 2015. Although all the teams clearly were faced with the same climatic conditions during the competition and the best ones built houses efficient also under those conditions, a test was performed to evaluate the HVAC energy consumption, considering weather input data characterizing a typical average year in Irvine. From this verification, it became evident that the consumption would have been 70% less than the one STILE actually had.

Fig 29. Carpet plots that evidenced temperature and direct solar irradiance differences between the TRY for Irvine and the weather experienced in October 2015. Regarding the evaluation of the STILE performance at different locations, as far as cooling and heating energy consumptions are concerned, Table 8 shows that the difference between the energy required by the house in the different climatic conditions is relatively small. However, it must be considered that the same peak power of the HVAC system and the same time schedule were used at the various locations.

38

Table 8. Heating and cooling energy consumption for STILE house at different locations. Energy Consumption (kWh/m2) IRVINE CARTERSVILLE ROME PALERMO MADRID PARIS COPENHAGEN 14.5 21.8 19.6 21.6 20.2 12.3 10.4 Cooling 0.1 2.5 1.8 0.2 1.8 8.2 13.0 Heating 14.7 24.2 21.5 21.8 22.0 20.6 23.4 Tot

The house, in conclusion, might have easily presented satisfactory thermal performances with mild and temperate climates, or whenever a benefit can be obtained by the use of the solar chimney. In fact, since the house behaves as a solar greenhouse, the energy demand for heating in winter is reduced by a great amount and the extra heat can be controlled, thanks to the solar chimney flow. Moreover, STILE may present good performance at locations characterized by cold climates, as the greenhouse effect, in conjunction with the insulation material used, would decrease the heating requirement during winter. In this case though, the PV system may not be able to produce enough energy to have a neutral balance over the year, as shown in the Copenhagen case. On the other hand, at locations identified by hot climates, STILE performance is not satisfactory because it leads to unacceptable internal comfort conditions. As regards the cities analyzed, the best performances were observed in Irvine and Rome. This result actually reflects the aim of the project and the unexpected weather conditions during the competition did not allow to benefit for the solar chimney not demonstrating the optimal behavior of the house. 8. CONCLUSIONS An a posteriori critical analysis of the energy performance of STILE house for the Solar Decathlon 2015 was done to better understand some of the issues faced during the competition [33]. This was possible due to the accurate monitoring of environmental variables and energy and occupancy schedules during the competition. The data collected allowed for accurately calibrating and validating a dynamic simulation model of STILE that was used to critically evaluate some of the architectural choices adopted during the design phase. It was demonstrated, by using the model, that some design improvements may have helped to decrease the energy consumption by 17%. Considering the fact that a non-optimal refrigerator was used during the competition, it is likely that 39

the house would have probably earned full points in the energy contest with the aforementioned design improvements. This analysis led to an awareness of the practical impact of design choices that is essential for the architectural functionalities and to get a high energy efficiency of a building. Moreover the reliable model obtained allowed investigating STILE behavior in different climates. Indeed, Solar Decathlon houses are specifically designed to perform under the anticipated climatic conditions during the competition. As a result, their performance at other times of the year and at other locations is typically unknown. The primary contribution of the paper is to address anticipated energy performance in varied world locations over the course of a typical year. This was possible using building simulation obtaining great confidence in the results using monitoring data of the actual building collected during the competition. The results highlighted that the house could have good performance with mild and temperate climates where it is possible to benefit from the solar chimney both in winter and summer.

ACKNOWLEDGMENTS The authors would like to acknowledge: -the Italian team from UTV: Giovanni Consiglio, Tiziana Costanzo, Andrea Di Nezio, Ambra Guglietti, Pierfrancesco Italia, Flavio Martella, Marco Napolitano, Damiano Raparelli, Elisa Roncaccia, Alessandro Zonfrilli, Nico Agnoli, Lucrezia Alfonsi, Dario Atzori, Lorenzo Bartolucci, Iacopo Carinci, Daniele Lanza, Giorgio Milita, Simone Pretolani. -the American team from WVU: Sharrafti Kuzmar, Lauren Hogan, Andrew Strand, Amanda Summers, Stephen Cavanaugh, Joanna Ridgeway, Molly Banfield, Todd Funkhouser, Alex Hatch, Hudson Barrett, Nick Spinello, Jordan Lockett, Michelle Jamshidi, Beau Eddy, Brandon Lee, Alex Credo, Branden Bellanca, Daniel Bolich, Morgan Southall, Syihan Muhammad, Timothy Scott. -Mario Grimaudo for his great support especially during the final design stage. -Paul Norton and Greg Barker for helping during the data collection and analysis. -all the sponsors for their support and for making STILE’s realization possible (solar.wvu.edu.) Additionally the authors thank the organizers and sponsors of the Solar Decathlon 2015.

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Highlights     

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A posteriori critical analysis of STILE house at Solar Decathlon 2015 is presented Dynamic simulation with IDA ICE software and on-site ambient measurements were used Calibration was made comparing monitored and simulated indoor air temperatures Improvement of 17% in energy consumption could be reached with design revision Energy demand and comfort analysis of STILE house in various city is discussed STILE house overall design is suitable for warm and moderate climate

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