More advanced thermal storage systems can use additional stores such as rock beds2, earth ducts3 or water walls to provide greater thermal storage capacity. With these systems, sophisticated control strategies can be adopted to maximise the cooling potential by de-coupling the store from the space served.
This also means that the thermal store can be bypassed during the morning, so retaining maximum cooling for use during peak temperature hours in the afternoon. Furthermore, with these systems storing 'coolth' for periods of longer than a few hours, daily or even seasonal storage becomes theoretically possible.
Benefits of passive cooling
Passive cooling possesses environmental, economic, and human comfort benefits.
The energy required for heating and cooling buildings is approximately 6·7% of total world energy consumption. By proper environmental design, at least 2·35% of the world energy output could be saved. Coupled to sensible design measures to limit solar gains, passive cooling could make comfortable conditions achievable within buildings, even with average peak temperatures as high as 31°C.
In economic terms, the structural shell of a commercial building typically has a design life of 50 years, and represents 45% of the initial construction cost. The air conditioning plant makes up 20% of the cost of construction, but will have to be replaced twice during the life of the building.
Thermal storage systems, which form part of the building shell and therefore have the same design life of 50 years, may impose an additional cost on the initial construction of around 10%, but over the life of the building may yield substantial savings.
In human comfort terms, passive cooling systems offer greater thermal stability. In addition, radiative cooling from a thermal store is associated with higher levels of occupant satisfaction than air-based comfort cooling systems.
The Earth Centre design study
A model of the labyrinth proposed for the Earth Centre Galleries was constructed using the dynamic thermal modelling program Tas. A series of studies was undertaken to: test the empirical decisions, illustrate the effects of adjusting key variables, and understand the performance of the system over time.
These studies involved:
- varying the air volume;
- adjusting the surface area;
- adjusting surface weight and material type;
- changing air path geometry;
- monitoring annual energy consumption;
- using different weather data.
For the purposes of analysis a model building was created – formed from a slice through one cell of the labyrinth. The Gallery itself was modelled by taking a series of six horizontal slices to represent the stratification effect of displacement ventilation.
The labyrinth at the Earth Centre Galleries is composed of a combination of block and concrete walls that serve to support the Gallery floor. The thicknesses and properties of the labyrinth floor (300 mm), ceiling (200 mm) and external wall (300 mm) were therefore input to the model. For reinforced concrete:
The blockwork dividing walls were modelled as:
The first key variable to be validated by the model was the supply air volume to the Gallery space. Data from the diffuser manufacturers indicated that a volume of between 0·4 kg/s and 0·6 kg/s would satisfy these loadings, with a supply air temperature between 2-3°C.
The simulations showed that, for a maximum temperature differential of 2·5°C in the occupied zone, a supply air flow rate of 0·5 kg/s is required. This was equivalent to a room air change rate of 2 ac/h.
A conventional air distribution system, such as air supply from high level grilles with full mixing above the occupied zone, would typically require three to four times more air to achieve the same comfort conditions.
Further calculations explored changing the exposed surface area, the properties of materials, and the air path geometry. Annual energy consumption was also compared with an equivalent air conditioning system.
Results from dynamic analysis
To allow the temperatures to be evaluated at different points through the labyrinth, it was zoned into sections. The cumulative effect of additional exposed thermal mass was assessed by monitoring the temperature of the air as it passes along the labyrinth.
Each cell of the labyrinth contains some 50 m2 of surface (wall, floor and ceiling), all adding to cooling capacity. The analysis is based on a peak summer day, with a target supply temperature of 19°C for a room condition of 22°C.
The cooling capacity of the labyrinth was modelled by studying the temperature of the room and low level inlets.
For the conditions chosen, a supply temperature of 21·5°C (corresponding to a room temperature of 24°C in the Gallery) can be achieved with an exposed area of approximately 150 m2 of concrete. The labyrinth was designed with an exposed area of 380 m2 – considered to provide a supply temperature of less than 19°C if the full storage potential is exploited.
This corresponds to a room temperature of 22°C, which is comparable to a full air conditioning system using mechanical refrigeration.
An early scheme for the labyrinth had included additional thermal storage: low resistance cones built off the floor of the labyrinth. However, the modelling work concluded that their contribution would have been negligible.
The effect of different materials
Simulations assessed the virtues of different construction materials in the performance of the labyrinth. Materials to be tested included concrete walls, floor slab and ceilings, a rammed earth construction with concrete floor slab, and water storage walls with concrete slabs.
The difference in performance between the materials was small, and performance was found to be proportional to the input density and specific heat capacity. The water walls gave slightly better performance thanks to water's specific heat capacity, but the benefits did not justify the extra costs. However, if a designer were really trying to work the thermal store hard, then this approach would merit closer analysis.
The Tas simulation tool only considers the effects of material properties. It estimates the surface and boundary layer conditions which are so crucial to effective heat transfer. Further research into the effect of surface roughness would help to develop these systems further.
The effect of air path geometry
The most significant heat transfer between the air and the labyrinth take place on the bends within the system. This is due to the more turbulent flow in these locations. The Tas program does not intuitively allow for this, however, and a series of simulations was attempted to demonstrate the effect.
The effect was modelled by running a simulation using the heat transfer coefficient for turbulent air flow in the region of the bends, and that for laminar air flow in the straight sections. Results were then compared to a purely laminar air flow heat transfer coefficient.
The results showed that a system with bends delivers air approximately 1·5°C cooler at the minimum condition than a system without bends. This corresponds to a cooling capacity increase of 25% for the system with bends, over one without. This is clearly a significant benefit to the design performance of a thermal labyrinth.
The model created to test the thermo-labyrinth was necessarily crude, and reduced the heat transfer affects to simple coefficients. That said, it is important to note that benefits will result from keeping air velocities low as resistance pressure losses will also be lower.
Source
Building Sustainable Design
Reference
1Holdsworth B, 'Unlocking Earth's energy', Building Services Journal, 6/89. 2Anon, 'School of hard rocks', Building Services Journal, 2/99. 3Anon, 'The future world of advanced houses' (shared equity houses), Building Services Journal, 7/94 (page 27).
