The thermal mass properties of Waterhogs were investigated and compared to concrete.  Using computational fluid dynamics (CFD) techniques, the daily temperature variations for specific conditions were ascertained.  The results indicated that Waterhogs are superior to concrete, and offer improved thermal mass as well as improved embodied energy and flexibility of use. 

1  Introduction

Thermal mass plays a very important role in the temperature regulation of buildings.  This can be observed in British architecture, with buildings constructed during the little ice age (approximately mid sixteenth to mid nineteenth century) having  very high thermal mass to avoid the more extreme colds that occurred.  Thermal mass was particularly important as today's insulation materials were largely not available.

Thermal mass is again becoming very important to British architecture, but now also to regulate high temperatures due to the high energy efficiency standards of some high end EcoHomes.  Using thermal mass to regulate the temperature avoids excessively high temperatures, and moderates the need for day-time heat removal.  It also allows for higher night-time temperatures and avoids unnecessary night time heating.  It is particularly important for today's EcoHome trend towards lightweight timber frame construction, but thermal mass solutions have so far relied upon high embodied energy cement based products, such as concrete.

Our partners Green Carbon wanted to know the efficacy of an alternative solution to the thermal mass of lightweight structures that would improve the environmental impact of materials used.  The solution involved the use of a novel Australian product which is currently used for grey water storage and for adding thermal mass: The Waterhog.  The Waterhog has the great advantage of being sized to fit discreetly into walls or between floor joists.

Sequoien CIC as thermal solution partner to Green Carbon performed a numerical analysis of an idealised EcoHome containing Waterhogs (full) to see how they compared with an EcoHome with the same volume of concrete.  The objective of the analysis was to make a comparative judgement regarding efficacy.  Suitability for any particular EcoBuilding design would depend highly on the geometry and construction details of the specific application.

2  Analysis:  Model & Data

2.1  General

The model of the EcoHome can be seen in Figure 1 as an animation of the analysed daily temperature variation.  It is an idealised two storey EcoHome with a footprint of 10.4m width by 5.8m depth, and a height of 5.4m.

With the analyses performed, the central floor consists either entirely of full Waterhogs or the same volume of concrete.  An entirely lightweight structure with no thermal mass in the central floor was also analysed for comparison.  In each case the solar irradiation is taken to be that of the 21^st June, with no cloud cover, at a longitude and latitude of -1.075 degrees and 51.583 degrees respectively (corresponding to Green Carbon's Oxfordshire construction yard). 

The south face is entirely 6mm single-glazing (reflectivity 0.1; absorptivity 0.1; transmissivity 0.8); all other walls consist of 200mm of mineral wool insulation.  The building is idealised as completely airtight and there is no other cladding.

2.2  Material Properties

The material properties used were as per Table 1.

2.3  External Conditions

In order to obtain some form of temperature equilibrium, external conditions need to be applied to the model to allow heat to be removed.  An emmisivity of 0.8 was applied to all external walls, as a typical value for wood and/or minerals and glasses, radiation could also be transmitted through the South facing glazed wall.  Additionally a convection heat transfer coefficient of 4.2 W/m²K was implemented corresponding to an external wind of approximately 5 m/s.

The external temperature and its daily variation can be seen in Figure 2.

3  Computational Fluid Dynamic (CFD) Simulations

3.1  General

From an initial temperature estimate the model was analysed for a 24 hour period, and the temperature recorded at four key points in the EcoHome corresponding to the center of the EcoHome on the North-South axis, but at 0.25L and 0.75L in the East-West axis where L corresponds to the EcoHome width.  Two points are on the top floor and two on the bottom floor.  Temperatures reported are an average of these values.   The analysis has to be repeated, with each solution used as a better temperature estimate for the next analysis, until such time as the starting condition is within a sufficient tolerance of the ending condition, i.e. the temperature asymptotically approaches an equilibrium solution over repeated 24 hour cycles with the same conditions.

3.2  The Surface to Surface (S2S) Radiation Model

The surface-to-surface radiation model can be used to account for the radiation exchange in an enclosure of gray-diffuse surfaces.  The energy exchange between two surfaces depends in part on their size, separation distance, and orientation.  These parameters are accounted for by a geometric function called a "view factor''.

The main assumption of the S2S model is that any absorption, emission, or scattering of radiation can be ignored; therefore, only "surface-to-surface'' radiation need be considered for analysis, i.e. the media (in this case air) does not significantly participate in the radiation exchanges.

3.3  Gray-Diffuse Radiation

The S2S radiation model assumes the surfaces to be gray and diffuse. Emissivity and absorptivity of a gray surface are independent of the wavelength. Also, by Kirchoff's law, the emissivity equals the absorptivity (ε=α).  For a diffuse surface, the reflectivity is independent of the outgoing (or incoming) directions.

Also, as stated earlier, for applications of interest, the exchange of radiative energy between surfaces is virtually unaffected by the medium that separates them. Thus, according to the gray-body model, if a certain amount of radiant energy E is incident on a surface, a fraction ρE is reflected, a fraction αE is absorbed, and a fraction τE is transmitted. Since for most applications the surfaces in question are opaque to thermal radiation (in the infrared spectrum), the surfaces can be considered opaque.  The transmissivity, therefore, can be neglected. It follows, from conservation of energy, that α+ρ=1, since α=ε (emissivity), and ρ=1-ε.

3.4  Natural Convection

In order to significantly ease the computational burden, natural convection was ignored for all analyses.  The inclusion of natural convection would have resulted in slightly more effective heat transfer to and from the thermal mass but would not have changed the comparative result.

4  Results

Figure 3 below shows the internal daily air temperature variation averaged across the four points previously described. 

The rather wild variation in temperature for a lightweight structure with no thermal mass can be seen with an almost 18 degree daily variation.  The addition of thermal mass improves things significantly.  The concrete shows only a 4 degree temperature variation while the Waterhogs show less than 3.7 degrees.  This further indicates the superiority of the Waterhogs which moderate the temperature by 10% more than the concrete in this application.  It is expected that the percentage variation is dependent on a number of issues so it might not be fair to extend this result to any other situation, however the thermal mass should always be superior to some extent over concrete.

5  Conclusions

While the EcoHome analysis is not based on a specific design and the conditions and construction are idealised, a comparison is still possible between the thermal mass properties of concrete and Waterhogs.  As was seen the Waterhogs outperformed concrete by around 10% in the analysis and it can be concluded that they would do so to some extent under any circumstances.

This is great news due to the low embodied energy of the full Waterhogs, and leads to a great option for EcoBuilders to improve the environmental credentials of their design with a ready-made water tank solution that can be fitted discreetly in any number of locations.