Diary Entry No.12 - Carbon Neutral

Carbon Neutral Construction

Zero carbon construction guidelines require new buildings to take into account their emissions from space heating, ventilation, hot water and fixed lighting,

The CREST pavilion is one of few non domestic buildings in the UK and Ireland that is carbon neutral – the building will provide enough electricity to heat and illuminate itself throughout the year (regulated energy use)

The definition of zero carbon has been under review and discussion since the initial proposal of the standard. The intention has always been that it would mean 100 percent reduction in carbon produced, this was where any emissions created by the building and its users were offset by those ‘saved’ using on-site renewable capacity.

Carbon emissions have been separated into what are now termed ‘regulated’ and ‘unregulated’ carbon emissions. Regulated emissions are those from fixed building services, i.e. heating, ventilation and lighting; unregulated emissions are those relating to energy used by the building occupants, e.g. from cooking or electrical appliances. The CREST pavilion is carbon neutral from its fixed building services (i.e. regulated energy use), the PV panels generate enough electricity for the mechanical ventilation and heat recovery system and hot water requirements that the users have.

During the design period, we were aware that the efficiency of all the services is of paramount importance.  To build a carbon neutral structure, first you need to design and construct a super insulated envelope with triple glazing and mechanical ventilation with heat recovery. The Passive House rules are quite prescriptive and although they are not necessary for a carbon neutral building if you adhere closely to them your building will be much more efficient and require fewer PV panels etc to offset the carbon you used in heating and running the building.

The Passive House principals are as follows: -

  1. Triple-glazed window technology. A type of window providing thermal insulation by using three panes of glass hermetically sealed with two internal air gaps.
  2. Whole house mechanical ventilation with heat recovery.
  3. Air-tightness. Air leakage through unsealed joints must be less than 0.6 ACH
  4. Super Insulation
  5. Thermal Bridging - Enhanced Construction

After the building was made as efficient as possible the strategy was to provide renewable power to cover the anticipated demand for space heating, hot water, lighting (regulated energy use). A heat loss calculation, using the government SBEM rating scheme, is used to work out what this figure would be.  This figure is then used to calculate the amount of renewable technologies that are required to offset the carbon generated by the fixed services of the building.

Off-site low and zero carbon energy generation technologies are not suitable to ensure that the CREST Pavilion building is zero carbon, because the ‘clean’ energy generated could be lost in the overall electricity generation.  This led to the decision being made to use PV panels installed at the site.  The calculation for the CREST Pavilion was that 12kw of PV power would be required to run all the fixed services.

PV Panels

There are two types of PV panels at the CREST centre, conventional fixed panels on the south elevation of the building and secondly PV panels that employ a tracking system to allow the panels to follow the suns path.  The use of Solar PV cells for generating electricity is more predictable and perhaps a more benign method of generating electricity albeit on a smaller scale than wind.  The CREST team are monitoring all PV panels to allow them to analyse the electricity produced by the fixed panels versus the movable panels.

Exterior showing extent of PV panels

Exterior showing extent of PV panels

The image above illustrates the fixed PV panels mounted on the south elevation of the building and the ground mounted panels controlled by robotics.

How do solar panels (PV) cells work?

PV cells are made from layers of semi-conducting material, usually silicon. When light shines on the cell it creates an electric field across the layers. The stronger the sunshine, the more electricity is produced. Groups of cells are mounted together in panels or modules that can either be mounted on a roof or on the ground.

These cells don't need direct sunlight to work – they can still generate some electricity on a cloudy day. The cells convert the sunlight into electricity, which can be used to run household appliances and lighting.

The power of a PV cell is measured in kilowatts peak (kWp). That's the rate at which it generates energy at peak performance in full direct sunlight during the summer. PV cells come in a variety of shapes and sizes.  At the CREST Pavilion, there are two different sizes of PV panels, wall mounted and ground mounted on the tracking system.  The combination of these panels and their quantity generate more electricity than the pavilion requires, making it zero carbon.  The calculation carried out at design stage determined that the PV panels will generate approx 50kW of power which is far in excess of the required load.

The installation of the solar PV panels provides benefits to the operators of the CREST pavilion.

  1. It allows them to cut their electricity bills. Sunlight is free, so once the initial installation cost has been met, their electricity costs will be reduced or eliminated.
  2. The quantities of panels that have been installed allow the CREST Pavilion to cut their carbon footprint. Solar electricity is green renewable energy and doesn't release any harmful carbon dioxide or other pollutants.

The images above illustrate the large amount of PV panels that are installed at the CREST Pavilion.  The track used by the robot is visible at the base of the posts that the ground mounted panels are located on.

The ground mounted PV panels at the CREST pavilion employ a robotic tracking system to allow them to follow the suns path, increasing the amount of time that they are able to generate power.

South facing louvres to shade sun and overheating

South facing louvres to shade sun and overheating

The image above illustrates the ground mounted PV panels with the support track for the robot that is used to change the panel’s orientation.  The Robotic Tracking System uses a pair of autonomous, mobile robots to control the PV panels with high accuracy and reliability. The robots travel on a rail and adjust each panel periodically to optimally face the sun.  Using robots to move the panel’s means that each panel does not need individual motors and controllers found on conventional tracking systems.

In our next article, we hope to be able to talk about the final stages of the construction process and the internal finishes of the scheme.