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Heat your home with passive solar energy

Posted by Tim Pullen on 26 February 2013 at 11:01 am

It is quite possible to benefit from solar energy without the need for external mechanical means such as solar panels. It is closely related to architecture and was well understood in the ancient world. The Chinese, Greeks, Romans and North Africans have been building houses that use solar energy for more than 2,000 years. 

All buildings are affected by solar energy in some way and it is a matter of design and planning to ensure it is used well and effectively. It requires four basic elements: 

  1. A means of collecting heat
  2. A means of storing heat
  3. A means of distributing heat 
  4. A means of keeping the heat in the house. 

In this case insulation is a bit of a two-edged sword. It is crucial to keeping heat in the house, but it keeps heat out of the house as well. If you think of old stone houses, those 500mm thick walls had no insulation but lots of thermal mass. The sun would warm the stone which in turn helped to keep the house warm. Put some insulation on that wall and it stops working as thermal mass and stops heating the house. 

But insulate we must; to comply with building regulations and because, on balance, it is more effective to retain the heat we inject in the house within the house. 

1. Collecting heat

As sunlight passes through glass the short wave infrared radiation gets refracted to long wave infrared, which cannot get back through the glass. So the radiation bounces around inside the building and warms anything able to absorb the heat. To collect solar heat we need glazing and the obvious options are patio doors, conservatory or the like. But any glazing on a south-facing elevation is good (it doesn’t have to be due south but the more south in it the better). 

2. Storing heat

Equally important is something to store the heat. What we want is for that infrared heat to be absorbed for later release when the sun sets. So we need something with high specific heat capacity and high density. We think in terms of brick, stone, concrete etc. Ideally this would be the floor but equally can be walls, chimney breast or similar. In a remodelling we could be thinking of replacing wallboards and in that case we might think of replacing them with a phase change material wallboard (ThermalCore, Racus, Micronal or similar – these are expensive but effective). 

3. Distributing heat

If the solar heat is locked into a small space it will tend to overheat that space. To overcome that we need a means of distribution and typically that will be a ventilation system. That can be passive (with no electrically driven fans) or it can be mechanical. Installing ducting can be tricky in a remodelling project and passive ventilation, using uni-directional inlets and outlets can be the answer. 

Bear in mind that hot air rises in the same way that oil rises in water – floating may be more accurate. Careful consideration needs to be given to positioning the inlets and outlets to ensure the warm air doesn't rise and stagnate in the loft, but circulates around the house. There are one or two companies that will do this for you but realistically it is a matter for research and self-teaching. 

Think about the conservatory

Conservatories are often taken as the default means of collecting solar heat but is not necessarily the case. As mentioned above, any south(ish)-facing glazing will work. Careful thought and design are critical to a useful conservatory. Consider how many conservatories you know that are ice-boxes in winter and ovens in summer. The principal of passive solar energy works because they do get hot in summer, but the design is failing. They get hot but don’t circulate that heat to the rest of the house. Conservatories with glass or poly-carbide roofs tend to be a poor idea (in this context) because they are poorly insulated. A conservatory with a standard roof with the same u-value as the main roof is a far better solar energy collector. 

Avoid overheating

And finally: how to avoid summer overheating. Essentially shading is the answer and in terms of effectiveness it does not matter too much if it is a brise soleil, an awning or deciduous trees planted at an appropriate distance. Internal shading – venetian blinds or similar – do not work as the light has already passed through the glass and refracted to long-wave infrared before it gets to the blinds.  

Passive solar energy – heat and light – is relatively simple and surprisingly poorly understood. It is only in the last three years that overheating potential has featured in the building regulations. And it has to be recognised that most designs – new build and remodelling – completely ignore passive solar energy. As a consultant advising on sustainable house design I am still regularly told that there is not enough sunshine in the UK to warrant installing solar thermal systems. Often by people who really should know better. So it is perhaps disappointing rather than surprising that passive solar is ignored.

Photo Credits: Lori Greig via Compfight cc, Nicole McMurray

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

If you have a question about anything in the above blog, please ask it in the comments section below.

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