A Green School for Gaza: design and thermal ... - PLEA 2014

A Green School for Gaza: design and thermal performance evaluation. Brian Ford

Giulia Pentella, DiplArch

Rosa Schiano-Phan, PhD

University of Nottingham. [email protected]

MCA Architects [email protected]

University of Westminster. [email protected]

Juan Vallejo, MSc

Wei Zhu

Naturalcooling Ltd. [email protected]

University of Nottingham [email protected]

Figure 1: (left) Map of Gaza Strip and (right) image from the refugee camp in Gaza.

Concerned about the current situation in Gaza, the Kuwait School designed by Mario Cucinella Architects (MCA) and UNRWA aims to develop an off-grid building relying on passive design strategies and only locally available and renewable resources (figure 2). The design process was continuously informed by dynamic thermal simulations and computational fluid dynamic simulations in order to achieve thermal comfort in classrooms and enhance the performance of natural stack and cross ventilation.

ABSTRACT

This paper describes the design and thermal performance evaluation of the first low carbon school building in Palestine. The project is a response to the mounting environmental, energy and infrastructure crisis within the Gaza Strip. The ambition is to provide a better learning environment for the children of Gaza, while also being self reliant in terms of electricity and water requirements. The project has been developed in close collaboration with UNRWA. The school, located in the Khan Younis refugee camp, will host 32 classrooms distributed on three floors for a total capacity of 2050 children divided in two shifts. It relies almost entirely on passive strategies to achieve thermal comfort within the classrooms through the year. The roof is formed from a lightweight external ‘parasol’ shading a concrete deck below. Cylindrical concrete columns contain vertical ventilation shafts leading into solar chimneys at roof level. The classrooms are arranged around a large central shaded courtyard, which provides diffuse daylight and fresh air. The classrooms are naturally ventilated by a combination of cross and stack ventilation via the solar chimneys. The façade presents a high window-to-wall ratio and a series of large Mashrabiyas to reduce the incidence of direct solar radiation in the classrooms. Dynamic thermal performance analysis evaluated the effect of the proposed strategies on temperatures in the occupied classrooms during summer and winter. The study concludes that mechanical cooling of classrooms can be eliminated, and that space heating demand is reduced to below 10kWh/sqm.yr. The residual electricity demand for lighting and power can all be met from the PV installation on the roof of the building. The project is currently under construction and the UNRWA intends to measure temperatures and energy consumption within the building and will report in a future paper on performance in-use. INTRODUCTION

Gaza Strip is located in the Eastern coast of the Mediterranean Sea, bordering Egypt in the South-West and Israel in the East and North (figure 1). Since 2005, the area has been essentially isolated and the combined effect of growing population, polluted environment and unsustainable construction is questioning Gaza as a liveable place in the near future (United Nations, 2012). The expected increment in the population requires a review of the infrastructure required for electricity, water and sanitation to meet the future demand. Moreover, Gaza Strip is recognised as one of the youngest populations worldwide and currently presents a shortage of schools which barely responds to the needs of the population. This situation requires quick action and the efforts in education must be accelerated in order to maintain the quality of the education.

30th INTERNATIONAL PLEA CONFERENCE 16-18 December 2014, CEPT University, Ahmedabad

Figure 2: (left) Bioclimatic section during the warm period and (right) façade model of Kuwait School.

The first part of this paper will describe the different passive solutions adapted, including the building layout, solar shading, thermal inertia and natural ventilation; and will provide detailed information about the impact of each strategy on the building thermal performance through different parametric studies. The second part of the paper will evaluate the overall building thermal performance during the year and will compare it with the standard constructions in Gaza. The paper will conclude with a critical comment on the inmediate reaction required in Gaza and the potential of designing passive buildings in order to provide better environments and reduce carbon emmisions. DESIGN APPROACH

The strategies adopted in this project are influenced by the weather conditions in Gaza, the current situation in the country and the building typology. An early climate analysis provided essential information about the risk of overheating, which is the result of an unobstructed building exposed to high global horizontal radiation, maximum average temperatures of 30C in June and September and high density occupied spaces (figure 3).

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Figure 3: Mean monthly dry bulb temperatures and Global Horizontal radiation in Gaza. Source: Meteornorm. Figure 5: NE Classroom (2F) – Resultant temperatures and first roof conduction gains (Mon 4th – Sun 10th June). The majority of the classrooms are 52sqm with 32 students organised in two shifts from 8-2 and 3 to 6. This becomes the main source of internal gains in the classrooms (46 Wh/sqm), still below the average in schools in Gaza (58 Wh/sqm) and the benchmarks suggested by CIBSE, 2006 (53 Wh/sqm). Building layout

The classrooms are arranged in the perimeter of the building and around a large central shaded courtyard (figure 4). Under this layout the classroom can benefit from cross ventilation, diffuse daylight and fresh air from the courtyard.

Thermal inertia and building materials

An exposed heavyweight construction coupled with a night ventilation strategy was also adopted in order to absorb part of the high internal gains in the classrooms during the occupancy time and release the heat during the night. The façade is formed by compressed rammed earth blocks (density: 1800 kg/cbm; specific heat: 0.26 Wh/kgK) and the ceiling is an exposed concrete slab (density: 2400 kg/cbm; specific heat: 0.26 Wh/kgK). Natural ventilation

Figure 4: (left) Building model – classroom layout and (right) shaded courtyard.

Window to wall ratio & mashrabiyas

The window design tended to maximise the effective aperture area to enhance the natural ventilation in the classrooms. The façade window to wall ratio is 0.38 whereas in courtyard is 0.43. Along the façade a set of mashrabiyas from the ground floor to the top floor reduce incident solar radiation penetrating the classrooms facing NE and SW by 35%.

The classroom layout, with openings facing the courtyard and façade, allows cross ventilation within the space. The typical air flow path is from the courtyard, across the classrooms and out through the façade. In order to enhance the average flow rate and provide the space with an additional natural ventilation scenario, a series of stack chimneys were designed to extract air. In rainy/noisy days, the classrooms can benefit from stack-cross ventilation with the façade openings closed. The grilles were dimensioned in order to extract the minimum required ventilation to ensure fresh air for 34 students. Each stack chimney extracts air from two classrooms in each floor and acoustic insulation inside the chimneys and noise attenuators in classrooms were applied. The top of the stack chimneys were designed with a high conductivity material which was proved to enhance the air extraction when it is exposed to incident solar radiation. The apertures for each natural ventilation scenario were also designed according to the seasonal period and moment of the day (figure 7). During occupied hours in the warm period, the openings are considered with the maximum aperture, achieving an average of 42 ach in the classrooms. The non-occupied hours will be also ventilated and coupled with the high thermal inertia of the spaces by taking fresh air from the courtyard and extracting the released heat from the building elements through the grilles. During occupied hours in the cold period, an effective aperture of 20% of the total opening area will provide the minimum fresh air required to the space.

Roof shading

A lightweight external parasol was designed to reduce the incident solar radiation in the top floor and at the same time provide shade in the courtyard. Dynamic thermal simulations helped to quantify the solar gains in the top floor classrooms for a shaded roof and an exposed roof. The excessive gains in the classroom through the exposed roof increases the internal temperatures achieved during a typical hot week in Gaza. The top classrooms can benefit by a 2ºC drop during the occupancy time by having a “second roof” (figure 5).


Figure 7: Natural ventilation scenarios and mean flow rates achieved in classrooms.

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The air flow rates achieved in the classrooms will provide also a comfortable breeze that will create a cooling effect on the occupants. 2D computational fluid dynamics simulations were done in order to visualise the air flow path and velocities achieved in a section of the classroom. The cross/stack ventilation creates a minimum air velocity of 0.3 m/s, which crates a mean cooling effect of 1ºC according to CIBSE, 2005. This factor will increase the number of hours within comfort in the classrooms during the warm period.

Figure 10: (left) Annual heating loads and (right) NE Classroom (1F) resultant temperatures – heating loads - internal gains for a typical cold week. Figure 8: (left) Air velocity in classrooms - scenario I and (right) air movement cooling effect. Source: CIBSE, 2005. Common practice in Gaza OVERALL PERFORMANCE Kuwait School

A final dynamic thermal simulation evaluates the overall building performance after the design process. A predicted comfort band using the European standard EN 15251 was used to evaluate the comfort conditions in classrooms during the year. The comfort band limits adopted for the cold period are 20-26ºC, and for the warm period 26-30ºC. The percentage of hours during the occupancy time above and below these limits suggest that a comfortable learning environment could be achieved during the warm season (figure 9). The flow rates achieved during the warm period keeps the resultant temperature in the classrooms very similar to the external dry bulb temperature. A reduced temperature considering the cooling effect of the air velocity for each hour suggest that pupils could remain in comfort for most of the occupied hours.

The current construction practice in Gaza is characterized by the lack of passive strategies adopted in buildings and a poor construction quality. This creates poor thermal conditions in classrooms and becomes the main reason for poor attendance in schools during the warm period. A generic model with no passive strategies adopted was tested. An increase of 70% in the number of occupied hours within the predicted comfort band during the warm period was achieved with the final Kuwait School design, compared with the ‘common practice’ design (figure 11).

Figure 11: Frequency of occupied hours in classrooms within comfort during the warm period.

CONCLUSIONS

Figure 9: (left) Cumulative frequency of resultant temperatures in the warm period and (right) SW Classroom (1F) resultant temperatures - internal gains for a typical hot week.

The environment, the climate and culture must be experienced first-hand in order to design bottom-up solutions for Gaza. Better buildings can be designed by taking advantage of local resources, but we all need to raise the awareness and organize information campaigns on the benefits of climate responsive buildings. Natural ventilation can create a considerable benefit when coupled with the occupied space and adopting other passive strategies from the desing stage was proved to have an enormous potencial to reduce energy demand and carbon emisions and provide at the same time a better learning environment for the pupils. REFERENCES

The classrooms also present a good performance during the cold period mainly due to the high occupancy gains and the reduced ventilation flow rates. The equivalent heating loads for the classrooms remains below 10 kWh/sqm.yr after setting the thermostat set point to 20ºC. Mechanical heating is only required for the first 1-2 hours of occupancy (figure 10), same time when there is no incident solar radiation penetrating the classrooms due to the sun position and building orientation.


CIBSE, 2005. Natural Ventilation in Non-Domestic Buildings. Applications Manual AM10. Chartered Institution of Building Services Engineers, London. CIBSE, 2006. Environmental Design. Guide A, 7th Edition. Chartered Institution of Building Services Engineers, London. United Nations, 2012. Gaza in 2020: A liveable place? UNSCO. Office of the United Nations Special Coordinator.

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