How to make a Zero Carbon home

In the 1960’s when the environmental movement begun and the concept of energy conservation took root.  The idea of constructing  Zero Carbon homes gained momentum in the wake of Kyoto Protocol.  The definition of Zero Carbon homes can be confusing but the fundamental idea is a dwelling that makes the best use of building elements and minimizes the energy usage. To unpack the idea , the utilization of energy inside the house has been broken into two parts, namely regulated and un regulated energy.  The regulated portion of the energy includes heating, hot water, lighting and ventilation.  The unregulated portion of the energy comes from using appliances (Laptop, TV, Cookers etc).  It is the former segment of the energy that has to be either completely removed or minimized to a certain threshold (depending upon the type of house). More details of Zero Carbon homes can be accessed from this link.

In the UK, the emissions by housing  amounts to almost a third of the overall greenhouse emissions. The gradual tightening of the building regulations has improved the energy performance of  most newly constructed buildings . There are now dwellings that require very limited heating and some are so well insulated that often heat from the human body is enough to get the temperature in a comfortable range. A video clip showing the energy performance of these high energy performance houses can be accessed from this link.

In the UK, water and space heating makes up more than 50% of the energy used inside a building. Therefore targeting this area is essential for developing Zero carbon home. There are  variety of ways in which energy can be harnessed by the building fabric using renewables technology. Most prominent example of this is BIPV (Building Integrated Photo Voltaics).  However, energy conservation should always be the prioritized before energy generation. Considering renewables before energy conservation options is akin to filling a bucket with holes. Thermal insulation and heat capacity of buildings should be at the forefront when designing a zero carbon home.

Earth Ship

Radically Sustainable Housing: The Earth ship project

A laudable effort for developing radically sustainable buildings came in the form of Earth ship project. These building are not only sustainable in terms of energy usage but also sustainable based on the material sourced for construction. The Earth ship project teaches  that making dwellings out of high specific heat capacity elements, reduces the energy usage drastically. The idea is to increase the thermal inertia of the building. Often the walls in an earth ship dwelling are worn out tyres packed with earth (sand, clay).  Modern building materials incorporate large amount of metal. The use of metal reduces the specific heat capacity of the building. Metals also create thermal bridges which allow an easy passage for energy to escape. Consequently such buildings require more heating/cooling for maintaining a comfortable range of temperature. On the other hand, buildings made out of mud,  in most cases achieve Zero Carbon status. Mud is both a good insulator and has high specific heat value. Once the building reaches a comfortable temperature range, it tends to stay at that temperature for a longer duration. By contrasting a mud house with a tin shed, one can truly appreciate the importance of thermal inertia.

Traditionally, Cavity wall insulation,  Solar PV, Solar hot water heaters and Ground source heat pumps have been used for improving the energy performance of buildings. Some of the less known ways in which high energy performance can be achieved are listed below:

Trombe Wall

Trombe wall is a passive solar building design feature that works on the same principle as a greenhouse. It is essentially a glass wall with a material of high specific heat placed behind it.  The high specific heat material could be a brick wall with surface treatment to improve absorptivity. The rays of sun pass through the glass, and are absorbed by the wall which in turn raises the wall temperature. The wall re-radiates this heat in electromagnetic waves of shorter wave length, which  cannot pass through the glass. Instead this radiation is reflected back and heats the building. Furthermore, the air adjacent to the wall gets heated and starts to rise. This starts a convention current and warm air is passed on to the room.

Trombe Wall

Trombe Wall: Passive Solar Design Element

 The concept of Trombe wall is very old. It was first patented by an American Professor Edward S Morse in 1881. However, it wasn’t until the 1960 that idea was fully materialized by a French engineer Felix Trombe.  One objection levelled against a Trombe wall is that it can block the view. However half height Trombe walls can also be created, leaving ample room for windows. An overhang over the glass on the outside shades the glass when the sun is high. This minimizes the overheating during the summer months.

If the Trombe wall is integrated in the building fabric it can essentially provide a significant portion of the space heating during the cold winter month.  The direction of Trombe wall is south facing in the northern hemisphere.

Air Sourced Heat Pumps

Ground sourced heat pumps are an efficient means of transferring geothermal heat into the building envelope. If however,  the building is such that installing GSHP is not a possibility (such as in a flat) or the cost of ground work is prohibitive, than there is still an alternative. The Air Source Heat Pumps work on the same principle and can transfer heat from the ambient in to the home.   Generally, Air source heat pumps have a lower value of CHP when compared to GSHP. A well installed ASHP can still achieve COP of 3. This means for every single unit of electricity, 3 units of heat are furnished. However this value can decrease during the cold winter months when heating is most required.  Unlike the GSHP, the ASHP do not have access to an energy source, that remains at almost the same temperature throughout the year.

Light Pipes

Light pipes are a great way to illuminate the indoors with natural light from the outside.  They are particularly useful in lighting up building spaces where windows cannot be installed, such as in the basement. Light from Incandescent lamps is costly while from fluorescent lamps is unnatural.  Light pipes on the other hand have no running costs and provide natural light to the indoors.   Their disadvantage is obvious i.e. they don’t work when its dark outside. As the do not require electricity, they can be deployed in wet indoor spaces such as bathrooms or pools.

Air to Air Heaters

In the old days, buildings were designed to breathe in more  air and  ventilate more air compared to modern buildings, which are more air tight. Ventilation is a requirement for maintaining  good air quality indoors.  However, ventilation can take away precious heat with it. Therefore in modern houses, air to air heat exchangers can provide an ideal solution.  They are sometimes also called Air plates. The warm air that leaves the house heats up the incoming cold air. As a result, very little heat is lost in the ventilation process.

Air to Air heat exchanger

Air to Air heat exchanger

Energy recovery can be 50-85% depending upon the conditions. They  are solid state device and come in a variety of shapes and require little to no maintenance. They are a passive device and consume no electricity.

Solar Shingle/ Roof Slates

In certain areas, it is difficult to obtain planning permission for installing solar hot water panels or solar PV panels. This usually happens in touristy areas where the council seeks to block any developments that make a property standout compared to the neighbouring properties. In such instances, Solar Shingles can provide a good alternate. They are not obtrusive and can seamlessly integrate into the building fabric. The cost per watt capacity of solar shingle is higher than normal PV panels. The cost ranges from $4 to $12 per watt.

Solar Tiles

Solar Shingles: Available in the market or can be made using flexible thin film modules

In addition to the above mentioned measures,  the option of Inter seasonal energy storage can also be explored if there is space enough to accommodate a pool of water (or oil). Inter seasonal energy storage allows the heating up of a working fluid during the times when excess solar energy is available. This energy is then stored in large underground pools that are super insulated.  The combination of both the insulation and the ground heat keeps the water  or antifreeze warm. During cold winter months the energy stored in this heated fluid can be utilized by heat exchangers or GSHPs.

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