Monday, 31 October 2011

Detailed energy saving performance analyses on thermal mass walls.



The paper Detailed energy saving performance analyses on thermal mass walls
 is a study carried out on the effect on energy of mass walls compared to conventional wood-framed construction. The location for this study was in Las Vegas, Nevada, for two identically sized houses with contrasting types of wall designs; one using conventional methods and one with thermal mass walls. The thermal mass walled house (massive house) was designed to be a zero energy house (ZEH). Real-time data logging has taken place for over 2 years using a wide array of thermocouples and heat flux sensors and has reflected the excellent energy saving performance of the ZEH.

Wall framing in the timber frame home is based upon 2 x 4 construction. With Blanket and batt insulation. The R-value for this normal wall was estimated to be 2.15 (m2 8C)/W. The walls of the ZEH are a pre- cast mass sandwich panel construction. The R-value of the mass wall construction was found to be 2.06 (m2 8C)/W. 

For the lightweight wall construction of the baseline house, it is obvious that the internal wall temperature varies significantly with the external wall temperature, which changes in relation to the climate conditions. Comparatively, the internal temperature of the mass walls in the ZEH remains more stable in both the heating or cooling season. The external temperatures of the massive walls did not reach as high as those for the conventional construction, particularly where the sun-exposed walls are concerned.


Data was collected from April and October in 2006, during which periods both the heating and cooling systems are shut off. Las Vegas has a typical desert climate, in April the ambient temperature reaches 35.8C at midday. During these times, the indoor temperature in the ZEH was found to be more stable and more comfortable than the baseline house.

In comparison with the traditional wood framing walls, the ability of storing heat during peak time of the mass walls is favourable for peak electricity saving but the remaining heat at nights is not beneficial for the total cooling energy usage. Obviously the heating energy consumption is reduced by the mass walls in winter, but unfortunately the cooling usage is somewhat higher than the conventional house.
In the massive house it was found that heat is continuously transferred into the indoor space, which means that the heat stored during daily time cannot be completely released back during night time. In other words, the massive systems of the type evaluated here actually are not energy conservative for cooling requirements and not suitable for the typical desert climate areas, like Las Vegas.

The internal wall temperature of massive systems changes more slowly than the conventional wall constructions, leading to a more stable indoor temperature. The simulated heating energy use was much lower for the massive walls while the cooling load was a little higher. Further investigation on the heat flux indicates that the heat actually is transferred inside all day and night, which results in a higher cooling energy consumption.


The thermal mass wall does have the ability to store heat during the daytime and release it back at night, but in desert climates with high 24-h ambient temperature and intense sunlight, more heat will be stored than can be transferred back outside at night. As a result, an increased cooling energy will be required.

 

Here is a short video on how to create a zero energy home:





7 comments:

  1. From the results it is interesting to see that in the massive house the heat is continuously transferred into the indoor space, which means that the heat stored during the day,cannot be completely released back during night time which makes this wall type unsuitable in this area

    Also like my own study on the MHRV systems, these results are only suitable for this particular area which is Las Vegas, Nevada. These results couldn’t even be used in other areas of the country such as New York due to the temperature differences. It would be interesting if a broader study was carried out. Maybe if it carried out in different states or countries throughout the world.

    The data was collected from April to October; maybe to get a more accurate result they would have been better collecting data throughout the year.

    In this situation great care must be taken with these selection and construction of the walls for heating and cooling with the variation of temperature between 1celcius in winter to 41celcius in summer.

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  2. I was interested to read this blog about testing one construction type against another to determine which type was best suited to a particular area. Found it very interesting to see the pro’s and cons to both types of wall construction.
    I must say I agree with Kevin’s point about a more accurate study being formed if the measured time was over twelve months and if similar studies were carried out in varying climatic locations.
    Just to note, the article which I looked at might be of interest to compare. “CEPHEUS – measurement results from more than 100 dwelling units in passive houses” http://www.passiv.de/07_eng/news/CEPHEUS_ECEEE.pdf. In this paper detailed measurements for 11 passive house projects with different sites, planners and construction types from across Europe are presented.
    John

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  4. @ Kevin Cunningham and john.

    Just to add to your comments about collecting the data from april to october. Data was collected from April and October in 2006 because during this period both the heating and cooling systems are shut off. This was so they could analyse the buildings based purely on natural heating and cooling effects so in fairness it probably was a good time to do it.

    Your points about applying the study elsewhere is a valid one and well made.

    Thanks, Jan

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  6. The points raised in this article are similar to some of the points that were raised by Jelle the author of “Traditional, state-of-the-art and future thermal building insulation materials and solutions – Properties, requirements and possibilities”. He discussed the disadvantages if thick, sturdy traditional insulation as against thinner, lighter modern insulation techniques.

    Jelle uses the point that land can be expensive, and floor area needs to be maintained due to the real estate prices. Your study shows that there are more reasons for lightweight construction than economy. The mass walls need to be considered in terms of the whole energy performance of the house, and the investigation into whether they can contribute to the nocturnal heating of the house is a very useful study.

    Although the mass wall was not suitable for the Las Vegas climate, there may be instances where the thermal storage properties of it would be very beneficial. Perhaps where there is very low cloud cover at night, and temperatures drop dramatically like a “hot desert”. It would then be beneficial if the walls used their thermal mass to release the heat back to the building and reduce the night-time heating load.

    I like how you explained that the internal temperature varied according to the external temperature with the lightweight construction, and was more constant with the heavier construction. This makes the interaction of the walls with heat very clear and easy to understand. I was surprised to see that the mass walls continuously radiated heat back into the building, as I had assumed that during the day, they would only be absorbing it. It also raises the question of response times of walls to temperature changes. There would be instances where the slow response time of the mass walls can be of benefit, where a school is empty at night, but needs to maintain a warm temperature without any space heat demand.

    The main thing that I learned from your article summary is that the Passive House Standard has to be applied differently relative to local climates and customs. The mass walls are not suitable for Las Vegas, but certainly can be of huge benefit in other desert conditions. It is clear that the matrix of factors that need to be considered when choosing your wall construction is vast: economy, construction time, u-value, breathability, air-tightness, thermal mass, the list goes on. Thermal mass is considered of great importance in Passive House design, and can be calculated using the “summer” sheet in the PHPP planning package.

    I think that the situation would have to be studied more in terms of cooling energy demand versus heat energy demand at night, and the most economical and comfortable wall chosen this way.

    I came across the insulation material in the video below that was used in the deserts of America, it is lightweight and apparently cuts your cooling costs by half. It is only 1/8 of an inch thick, and is inexpensive (€5,000 for a 1400 square foot house), so definitely something to think about! (A problem with this is it isn’t fireproof, so this would have to be used on the external face of your conventional insulation)

    http://www.youtube.com/watch?v=GwGwytFkLIo

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  7. Eimear,

    Just to clarify, you said that the mass walls would be more suitable in a hot dessert climate.

    Las Vegas has a desert climate where temperatures reach up to 36 degrees celsius during the day. This study has showed that mass walls are un suitable for desert climates and that the timber frame construction is better in these conditions.

    In a desert climate temperatures still remain high at night even with no cloud cover, this is why the timber construction worked better as it released the heat back out of the building where the mass walls dont.

    Thanks for your comment, very interesting stuff especially the video link.

    Jan

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