Keep your cool - porous ceramic for temperature control
Dr Rosa Schiano-Phan, Research Fellow at the School of the Built Environment at the University of Nottingham, describes how porous ceramic can be used to cool buildings.
Traditionally, buildings in hot, dry regions of the world have achieved comfort using direct evaporative cooling coupled with high thermal mass, shading and other strategies to reduce the cooling loads of the building.
Porous ceramic media are often found in traditional examples of passive evaporative cooling systems integrated into building elements or components such as clay water jars in wooden windows or at the bottom of wind towers.
Air passing over the surface of the jars causes the water to evaporate and absorbs heat energy, thus cooling the air. This provides cooling, air movement and solar control in one system, just as a modern air-conditioning unit does. Whereas air-conditioning requires electrical energy and a control system, traditional systems rely only on sufficient air movement and a small quantity of water.
The Iranian wind towers or Baud-Geers are designed to provide natural circulation and cooling of the ambient air through the building. There are many wind tower types and designs vary according to the desired airflow rate, heat transfer area, heat storage capacity and evaporative cooling surfaces.
Similarly, courtyard houses in Iraq have wind-catchers on their roof to provide natural ventilation for a basement room where the residents normally take their summer afternoon siestas. Each catcher is connected to the basement by a duct contained between the two skins of a party-wall, which is cooled during the night by natural ventilation. Due to the lack of direct solar radiation and because of their thickness, during the day the surfaces of these internal walls remain at a lower temperature than the rest of the interior.
The incoming air is cooled by conduction when it comes into contact with the cold inner surfaces of the duct walls, and its absolute humidity is increased as it passes over porous water jugs before being discharged into the basement. The air then flows into the courtyard, helping to ventilate this area during the daytime. The same evaporative cooling system using porous water jars can be found in the design of wind-catchers of the same building typology in the Sind region of Pakistan and Hassan Fathi’s Egyptian architecture.
Conventional cooling solutions, such as refrigerant-based room air-conditioners and evaporative desert coolers, can be noisy and present a risk of microbiological contamination and low efficiency if poorly maintained. Furthermore, the environmental impact of refrigerant-based units, including those branded as ‘green’, is significant. Conventional desert coolers reduce energy consumption by 25% and do not use refrigerants, but are not building-integrated and are often unsightly, being attached to the outside of structures.
Research on passive evaporative cooling has concentrated on systems that cool large volumes of space by relying on high air exchange rates, however, these do not affect the building envelope and cannot be used in cellular spaces.
Modular porous ceramic evaporators are the modern transposition of the traditional concept of the water jar integrated into a building. They are an integral part of the building fabric which limits the risk of microbiological contamination as the water is contained in the ceramic evaporators, located in a diaphragm wall.
Research into the development of modular evaporators for cooling of non-domestic buildings was initiated in 2001 under an EC funded research project (Evapcool), which demonstrated the feasibility of a direct evaporative system using porous ceramic. The project did not reach the point of addressing installation, effective water supply and distribution, maintenance or control.
The performance analysis highlighted that the large area required to meet the typical cooling loads of a non-domestic building could compromise the cost effectiveness of the system and impact on its architectural integration. Additionally, the reduction of floor area can be a limiting factor in office buildings. Consequently, porous ceramic evaporators are better suited to residential cooling.
There are few known examples of porous ceramic applications in non-domestic buildings. Francisco Mongado’s Spanish Pavillion at the 2008 Zaragoza Expo in Spain featured an array of 750 clay columns, supporting the roof. The columns rise from shallow pools and are interrupted by a glazed exhibition space. The original design was intended to create a passive evaporative cooling effect in the semi-open space from the ceramic columns wetted by absorption and capillary action.
However, the process was interrupted by the sand and cement mortar used to join the ceramic cylinders. This could have been avoided if a lime-based mortar was used, maintaining the porous continuity of the tubes.
Porous ceramics for cooling are produced by casting. This process is based on the hygroscopical properties of the chalk. A liquid mixture of silica, kaolin and felspar is poured into a chalk mould. The water contained in this mixture is partly absorbed by the chalk and a thin coat of solid ceramic condenses on to the walls, taking the shape of the mould. When the required thickness of ceramic is reached, the mould is reversed to clear out the surplus of liquid ceramic.
After going through a drying-chamber, the ceramic body is ready to be extracted and dryed, fired, and finished. The process provides high plasticity and a high level of porosity. The casting technique is therefore regarded as the most suitable for developing passive evaporative cooling systems employing porous ceramic materials.
Large-scale casting involves the high costs related to establishing automation. Such impetus came in Italy in the early 1950s when increased urbanisation necessitated improved control of indoor humidity levels.
A simple solution was to use small water containers hung on radiators. The market potential was soon spottedby ceramic manufacturers such as Il Coccio Umidificatori srl (one of the Evapcool research partners). In 1986, in co-operation with the Department of Chemistry of the University of Florence, Italy, the company defined an improved ceramic mixture, that offered a very high level of porosity, while preventing droplet formation. Since then, the Italian humidifiers have been produced by casting this ceramic mixture in moulds and firing the biscuit at low temperature. The manufacturing plant was modified for mass ceramic production using a highly automated production cycle.
In the fully automated cycle, a precise thickness of the ceramic is obtained using a first drying chamber set along the production chain. The water content of the ceramic coat before the drying process is ~20%. On reaching the required thickness, the mould is taken from the chamber to be reversed and cleared out. After the drying-chamber, the water content of the ceramic coat falls to 12-15% and the ceramic body is extracted from the mould. The extraction is quite a complex operation because of the ceramic’s soft consistency. The mechanical structure of the ceramic can be seriously affected by the mechanical stress connected to this operation.
The stress also depends on the characteristics of the mould. Depending on the desired shape, a mould can be made of one or more pieces of chalk. If it consists of only one piece (as for any ceramic body presenting no undercuts, such as a vase or an empty cylinder) the extraction is relatively easy. But if the mould consists of more than one piece (as for the ceramic prototypes of the Evapcool research project) the mould has to be opened before extraction.
Passive evaporative cooling using porous ceramic evaporators can be integrated into any building where there is a perimeter wall or a roof.
To employ dry and hot air from outdoors, the system needs to be placed adjacent to the perimeter walls or in contact with the roof (see images over page). This is generally possible in shallow plan buildings, but in deep plan buildings, only the rooms with direct access to external walls can use this technique. Mitigation techniques and alternative cooling solutions must be adopted for core areas such as corridors, lobbies or general circulation spaces.
In apartment blocks, the floor level also has an impact on the sizing of the porous ceramic system (PCS). The ground floors can greatly benefit from a PCS. The ingress of ‘fresh’ air through a wet cavity wall system induces particles and dust to be filtered and noise to be attenuated.
In the intermediate floors, the reduced availability of perimeter space is compensated by reduced envelope gains (solar and conductive) and hence less cooling is needed. In top floor apartments, the roof can be used as an additional surface for the integration of the PCS.
Mitigation techniques, such as night-time ventilation, solar control and improvement of the building envelope, considerably reducing the cooling loads – which can drop by as much as 23kWh/m2 (corresponding to a B rating in the Spanish Energy Performance Building Classification and close to the Spanish Passivhaus performance review of about 22kWh/m2). The wall integrated ceramic system is capable of fully meeting this residual cooling load and can keep indoor temperatures below 28°C for 80% of the occupied time when outdoor conditions exceed 32°C.
Despite the extensive research there are not any built examples to date,
and it has proven difficult to identify alternative and competitive ceramic manufacturers employing the casting technique that could replace Il Coccio Umidificatori (since their departure from the project due to bankruptcy).
A lack of confidence, both from the building owner and the builders/installers, in new energy technologies may influence market response and consumer choice between a conventional cooling system and an innovative passive version.
The lifespan of buildings and the long gaps between refurbishment cycles hinder application, as the proposed system and energy efficient measures are only cost effective or feasible to the building’s owner if installed during initial construction or refurbishment.
However, lifecycle cost analysis, comparing the proposed system with that of a conventional room air-conditioning unit of 2kW cooling power, showed that the cost of the air-conditioning unit over a period of 25 years is substantially greater than that of the porous ceramic system. This demonstrates that the system is an economically viable cooling solution compared with equivalent conventional air-conditioning.