As promised in last week’s Newsletter, I’m going to write this week about alternatives to concrete for thermal mass in buildings. To briefly recap, an astute reader named Ray, correctly pointed out that concrete is an environmental disaster. Estimates vary but somewhere between 6 and 10% of the world’s carbon emissions are due to concrete. That is an enormous environmental burden. BUT as I wrote last week…
‘…for Passive Solar Design to work effectively, a building needs to be able to store sufficient energy to keep the interior comfortable. In the case of the Greeny Flat we use the concrete floor to store the heat from the sun in winter to keep the house warm through the night. In summer we use the same thermal mass to help keep the house cool throughout the day.’
So, if concrete is such a problem, what options are there for providing the required thermal mass in a building and are there other methods of storing thermal energy?
Alternatives to Concrete for Thermal Mass
Thermal mass is vital for maintaining a comfortable temperature in a Passive Solar house. The key things about thermal mass are; 1) it has to be a heavy, massive material and 2) it has to INSIDE the Thermal Boundary of the building. The Thermal Boundary is the Insulation and Air-Sealing Layer that surrounds the ‘Conditioned Space‘ of the building. If the massive materials, like brick and concrete, are OUTSIDE the Thermal Boundary, they do absolutely no good at all in terms of enhancing the energy efficiency of the building. Consider the case of a brick veneer house with a concrete tile roof (probably still the most common configuration for homes in Australia)… neither the bricks nor the tiles are any help in terms of keeping the interior comfortable. In fact, in warm or hot weather, they tend to soak up heat during the day and radiate heat into the building until long into the night. This makes air-conditioners have to work harder for longer and leads to excessive energy use. Plus, both concrete and bricks require a great deal of energy to make which adds a LOT to the embodied energy of the building.
A concrete floor, especially if it is insulated as discussed last week, at least has the benefit of providing thermal mass inside the Thermal Boundary but this can also be achieved in other ways. One way is to use water which has the potential to store a LOT of thermal energy but the problems associated with storing large amounts of water inside buildings make this a method that I do not recommend.
Reverse Brick Veneer
A better way is to turn the brick veneer wall inside out. In other words, put the bricks on the inside and the timber-framed wall (with insulation of course) on the outside. This moves the thermal mass of the bricks INSIDE the Thermal Boundary where it will help with the energy efficiency but it doesn’t solve the problem of the high embodied energy of the bricks… unless or course you use recycled bricks or stone or mudbricks for the interior layer. Then you would have a system with low embodied energy as well as high energy efficiency.
One alternative to the ‘disaster’ that is regular concrete might be to use a Boral Product called ‘Envirocrete‘. This is a modified form of concrete in which up to 30% of the cement is replaced with industrial waste products PLUS the aggregate (gravel) is replaced with crushed, recycled concrete PLUS the water is either reclaimed or harvested rainwater. I haven’t seen a detailed analysis of the environmental benefits of Envirocrete over regular concrete but it seems that it must have a significantly smaller carbon footprint.
My friend, Daniel Jones, was one of Wollongong University’s ‘Illawarra Flame House‘ team that went to China in 2013 and won the International Solar Decathlon competition. One of the products used in this award winning project (which can be viewed at the Sustainable Buildings Research Centre in Wollongong) is called Descete. From a look at their website, Descrete appears to be an additive that allows for the use of high percentages of industrial by-products (like fly ash) to replace Portland Cement in concrete mixes.
Fly ash is a waste product from coal-fired power plants and is accumulating in vast quantities around the world. So any solution that allows Portland Cement to be replaced with fly ash has a double environmental benefit, it reduces the amount of cement PLUS uses up some of this waste.
This brings me to the mystery product which I alluded to last week and which is sitting right here on the desk in front of me.
100% Recycled Concrete
This lump of tan-coloured stuff might not look like much to you but, for me, it is a concrete reminder (pardon the pun) of a very exciting project that I had the great honour of being involved with back in 2008. At the time I was living in Missoula, Montana and working as a Project Manager for a very progressive architecture firm called MMW Architects. MMW was commissioned by the Missoula Federal Credit Union to design a new branch building that they hoped would set the standard for Environmentally Sustainable Design in Montana. I was given the role of LEED Accredited Professional on the project. LEED is a rigorous certification system for sustainable buildings. Two of the criteria it assesses are for Recycled Content and Local Content. So, in order to gain points for both criteria we came up with the idea of replacing all the concrete in the building with local, recycled material.
Two pressing problems in Montana at the time were 1) what to do with mountains of fly ash from the state’s many coal-fired power plants and 2) what to do with another mountain of crushed glass. The state had a program for collecting and crushing glass but had no way to recycle the stuff so the pile kept growing.
We learned that researchers from Montana State University had discovered a way to combine fly ash and crushed glass into a product that could be used as a substitute for concrete. After extensive (and at times disastrous testing) we managed to figure out a way to scale up their laboratory tests to use in concrete batching plants, concrete trucks and commercial building sites. Finally, with the help of a very brave engineer, we were able to build an entire bank building without any concrete. It was the first (and may still be the only) commericail building in the world in which all of the concrete has been replaced with 100% recycled material. And we’re not just talking about a few piers in the ground… this building uses this revolutionary material in all sorts of ways, from the foundations and polished floors to the precast wall panels, beams and counter tops.
My favourite thing about the finished building is the way it sparkles in the sun. For years after it was completed I used to ride my bike past it just to see the way the glass in the wall panels reflected the sunlight and made the building come to life. And you should see the polished floor in the lobby, trust me, it’s beautiful. This ended up being just the second building in Montana to achieve LEED Platinum certification and is one project that I feel proud and privileged to have been involved with.
Phase Change Materials
All the above alternatives to concrete are good but what do you do if your building doesn’t have, and can’t accommodate such heavy materials?
The example that Ray suggested in last week’s Newsletter was to use a refrigerated container. The steel in a structure like this would only provided a small amount of thermal mass and most light-weight timber buildings with wood floors have almost no thermal mass. This can be a good thing in a hot, humid climate where you have to rely on shading and ventilation to keep a house cool without air conditioning. But in a cool climate like Mittagong we need to be able to store thermal energy. So how can you do that without thermal mass?
One answer is to use Phase Change Materials or PCMs. I don’t have a Master’s Degree in Thermodynamics but I’ll have a crack at explaining how this works… Most people are familiar with the three common phases of matter… solid, liquid and gas. In the case of water, the solid phase is ice, the liquid is water and the gas is water vapour. When water melts it changes phase from solid to liquid. When it boils it changes phase from liquid to gas. When it condenses it goes from gas to liquid. And when it freezes it goes from liquid to solid. So melting, boiling (evaporating), condensing and freezing (solidifying) are the four common Phase Changes.
During the process of changing phase a material can absorb or release an enormous amount of energy with little or no change in temperature. For example, when you boil a pot of water on the stove, the temperature will stay at about 100degC even though you continue to add heat to it, until all of the water has evaporated. Where does all that heat go? It goes into the water in the form of ‘latent energy’. When the water vapour cools down and condenses it will release all that latent energy again at 100degC until all of the vapour has turned back to liquid then the temperature will start to drop again. A similar thing happens when water changes phase from solid to liquid and back. As ice melts it absorbs a great deal of energy from its surroundings at a constant temperature of about 0degC and as it freezes it releases all of that latent energy again without significantly changing temperature.
Refrigerators, air-conditioners and ground source heat pumps all use the latent heat of phase changes to transfer energy from one location to another. In the case of a fridge, it takes heat from inside the box and releases it outside the box. An esky uses the latent heat of ice to keep its contents cool. As the ice melts, it absorbs heat from its surroundings, including the food in the esky.
In a similar way, PCMs in buildings use the latent energy of a material as it changes from solid to liquid and back as a way to store energy at a constant temperature. Imagine that the inside of a room is lined with a PCM that has a melting point of 25degC and there is a north facing window in the room. As the sun comes in through the window, the temperature inside the room will rise until it gets to 25degC then it will stabilise as the PCM melts and absorbs all of the extra energy. Only after all of the PCM has melted will the temperature go above 25degC so, if you have enough PCM in the room, it won’t go above 25, no matter how much sun is shining in.
After the sun goes down and the temperature outside starts to drop, the PCM will start to solidify again, releasing its latent heat as it does and keeping the temperature inside the room at a comfortable 25degC. Only after all of the PCM has solidified will the temperature go below 25. This can be a fantastic way to store energy in a Passive Solar house, especially if it has insufficient thermal mass, and PCMs can be purchased with a variety of melting temperatures so they can be tuned to a particular situation.
This concept was incorporated into the Illawarra Flame House which uses a special system to draw hot air from under its solar panels during the day, blow it through a box full of PCM under the house, and thereby store that energy so that it can be directed into the house after the sun goes down. Because the Flame House had to be taken apart and transported all the way to China and back for the Solar Decathlon competition, it could not include a great deal of thermal mass or it would have been way too heavy. PCM’s provided a way to achieve a similar sort of passive heat storage without all of the extra weight.
This is very much an oversimplification of how Phase Change Materials work and how they can be used but hopefully you get the idea. Now you might be wondering why we don’t use PCMs in all buildings. I think the day may come when we do. In the meantime they are still fairly rare, expensive and under-appreciated but keep your eyes out… the PCMs are coming!
Other Good Stuff This Week
Japanese home appliance for turning plastic waste back into oil. This has been around for a while but it’s a good one.
University of NSW sets new record for solar cell conversion efficiency. ‘Engineers at the University of New South Wales (UNSW) achieved 34.5 percent sunlight-to-electricity conversion efficiency, a new mark that also comes closer than ever to the theoretical limits of such a system’.
The entire country of Portugal runs for four days straight on renewable energy alone. The thing I find most interesting about this story from The Guardian is the news there have been times recently in Germany when there has been so much renewable energy pumping power into the grid that energy prices have actually turned negative. This highlights the reason why smart grids and energy storage are so vital to our transition to a low-carbon world. In a smart grid situation, there can be lots of buildings, devices, vehicles, appliances and equipment all communicating with the grid so that any time there is excess power a bunch of stuff can turn on to use that energy. Or it can be stored in batteries, flywheels, heating devices or gravity systems (such as this railway based concept) for use later when the sun isn’t shining and the wind isn’t blowing.
Global temperatures are spiraling out of control. It’s worth watching the animation in this article which shows the rise in global temperatures of the last 165 years. It’s depressing and terrifying, but also an important reminder of the urgency of our current situation.
Completely Off The Topic
Are you, like me, sick and tired of those reminders to Update to Windows 10? Well I’ve found a neat solution called Never10. It’s quick and painless and seems to solve the problem. Plus it allows you to choose to upgrade to Windows 10 if and when YOU want to.