The main debating chamber is naturally ventilated by a funnel reaching the full height of the building.

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Green building: How can passive and active systems work together?

Kate Bode, Matthew Winter

The debating chamber building for the National Assembly of Wales - the Senedd - was officially inaugurated in March 2006. The design approach is an interesting example of how to create a building that naturally responds to the dynamics of the local environment on the one hand; while ensuring it's possible to use a selection of renewables - geothermal and biomass in this case - on the other.

Despite some adverse publicity over its eventual cost (even though building costs were on budget), the debating chamber building for the National Assembly of Wales - the Senedd - was hailed as a great achievement in sustainable building.

The design approach for the building was to create a building that naturally responded to the dynamics of the local environment. On the one hand, the goal where possible was to reduce the consumption of energy and water through the intelligent manipulation of the building form, fabric, and materials. And it was also important that renewable energy sources could be integrated where appropriate.

A four-step design approach was fundamental in achieving these objectives, sub-divided into managing energy demand and energy supply (see figure 1).

The environmental design for the building addressed both passive and active design features. These included a number of environmentally sound design strategies throughout the building, such as the extensive use of natural ventilation; night time cooling; maximising daylight availability and using daylight-linked lighting controls; and a range of water saving devices – which all contribute to a reduced demand for power and water.

To address the supply side of the energy/water equation, ground source heat exchangers and reversible heat pumps were installed to exploit geothermal energy; as well as a biomass boiler, rainwater harvesting tanks and intelligent controls.

Planning for renewable energy technologies

The renewable energy technologies investigated at the planning stage were categorised under two broad headings – electrical and thermal energy.

During the early stages of design a number of feasibility studies were conducted into the application of renewable technologies for the Senedd. In terms of renewable power generating technologies, feasibility studies were undertaken with regards to solar power, windpower and fuel cell technologies – the latter being highly attractive due to minimal architectural impact.

However, the fuel cell option was discounted early on in the studies due to the lack of maturity of this technology, particularly with regard to the commercial availability and attractiveness of hydrogen in terms of cost effectiveness, safety, storage, reliability and ecological generation of hydrogen.

Electrical energy – Solar PV

The application of Solar PV was considered first, as PV can be integrated into the fabric of a building and the public realm easier than other technologies such as wind turbines. In this case, with the availability of an extensive roof area (approx. 6,500m2), it was felt that the roof could be exploited for PV.

Hourly solar radiation data for the site was analysed, and it was determined that the best orientation was 25° south, with a tilt of 42°(+/−20°). The results of the study indicated that if monocrystalline cells (BP Solar's BP585L model) were used, a total of 3,500 modules would be required; this would necessitate a roof area of 2,170m2.

Owing to the complexity associated with simultaneously matching the electrical power demand and supply – and thus the need to both import/export power from/to the grid – dynamic electrical supply and demand profiles identified a total of 189MWh that would have needed to be purchased from the grid to supplement the power generated by the PV. This would have been offset by exporting a surplus of 190MWh generated through the PV, back to the grid.

The high capital cost and relatively low energy tariffs resulted in this solution being deemed uneconomical, with a payback period in excess of 75 years (even with the Government's more economically generous outlook on financial investment and returns).

Since this study, which was conducted in 2000, PV technology and especially inverters continue to mature with reduced capital costs; system efficiencies have increased and electrical tariffs have risen; albeit none of these positive developments would have generated a different outcome to the study today.

Electrical energy – windpower

The application of wind turbines as an alternative to PV was also investigated.

Small scale wind turbines that could be placed on the building were not considered, as they would have significantly impacted on both the architecture of the building and the public realm, whilst also not being able to offer the scale of power generation required to sensibly offset demand.

Power curves were examined for various wind turbine manufacturers, and used to correlate electrical power generation with wind speed to obtain a curve depicting annual power generation against wind turbine blade diameter.

Coupled with the building's annual demand profile, the developers determined that a turbine diameter of 28 m would have been required to cover 100% of the total annual electrical demand load.

Furthermore, during an average year, there would be periods in which the turbine would generate insufficient power to meet all of the building's energy demand at a given instant in time. In such cases the electrical power requirement would need to be supplemented with power from the grid; calculations suggested 197MWh of power would need to be imported from the grid in addition to the power generated by the wind turbine.

Nevertheless, for much of the year there would be a surplus of power generation such that there was potential to export 308MWh, yielding a net surplus of annual energy supply (i.e. a negative carbon footprint).

By increasing the blade diameter to 57m, the demands of both the Senedd and the existing Assembly building (Crickhowell House) could have been matched.

Considering the urban nature of the site, two effective locations for these large scale wind turbines were considered:

  •  Within Cardiff Bay – locating one or more wind turbines in Cardiff Bay itself, offered the advantage that the turbine could have been placed reasonably close to the Senedd (i.e. energy demand source); the potential factors of concern involved the effects of the turbine on local sailing boats; risk of noise; accessibility for maintenance and emergency work; plus the costs associated with marine infrastructure works;

  •  On the new Cardiff Bay barrier/barrage – mounting the wind turbine(s) on the barrier itself offered many advantages; the turbines would have been as far away from any buildings or boats as possible; noise generation would no longer be a problem; nor the risk of a turbine causing visual obstructions; simplified access and maintenance; as well as facilitating expansion of the system to cover future local electrical energy demand (a similar case study exists in Blyth Harbour, Northumberland). The only disadvantage of this alternative proposal was the relatively long distance between the building and the wind turbine(s), impacting on infrastructure costs. This solution was ultimately recommended to the NAW.

Whilst a comparatively attractive proposition, with a payback of less than 10 years, wind energy was ultimately not pursued further by the Client, owing to the need for a separate planning application.

The Assembly did not commit to either PV or wind turbines and opted instead for the purchase of green electricity from the grid.

Thermal energy – geothermal

When considering the need for heat in any building, it is always important to distinguish the grade of heat required – a balance needs to be struck between expanding more energy to increase the hot water temperature, versus the size and cost of heat emitting appliances (including their operational efficiencies when running on lower grade heat).

As many of the surfaces within the Senedd were finished in slate and concrete, underfloor heating was specified in most areas where there were no raised floors.

The primary source of low grade heat for such a system was sourced from renewable geothermal energy, culminating in the application of reversible geothermal heat pumps coupled to ground source – heat exchangers used for both low grade heat extraction in winter and heat rejection in summer cooling mode. In total, 27 closed loop ground source heat exchangers (polyethylene pipework grouted in 100m deep 40mm diameter bentonite piles) were installed and connected to three heat pump units. Having minimised the demand for cooling substantially – through the integration of various passive design features into the building design – this system was also able to cover the total net active cooling demand for the building.

Although ground source heat exchangers abstracting geothermal energy can be classified as a renewable technology, the heat pumps require electricity to drive the system, and as such this technology was defined as a low carbon technology rather than a 100% renewable technology. However, the application of this technology not only helps to reduce the overall power demand (and thereby minimises the residual electricity demand required from renewable power generating technologies) but can also be coupled to green electricity to render it effectively carbon neutral.

Thermal energy – biomass

In the drive to minimise the carbon footprint of the development still further, the use of biomass as an alternative source of fuel to efficiently generate high grade heat was appraised.

A single 360kW dual fuel (wood chip/pellet) fully modulating biomass boiler (Binder GmbH) – with automatic ignition and exhaust gas cyclone dust eliminators – was installed. To enhance the environmental credentials of the biomass boiler, the Binder boiler has a combustion temperature of around 1,000°c, well above the 850°c threshold needed to destroy dioxins (complex chlorine-bearing organic compounds) which are typically experienced in more conventional low-temperature combustion conditions.

Although the boiler can accept either wood pellets or wood chips, driftwood collected from Cardiff Bay by the Harbour Authority can also be used as an alternative fuel source, albeit less efficiently due to the higher moisture content and unitary calorific value. This system also includes an underground 25m3 fuel storage silo, linked to the boiler via an automatic auger.

When considering biomass as an alternative green source of energy, attention must be given to the actual source of fuel with regards to transportation emissions – wood pellets/chips for the Senedd are being sourced from Welsh Biofuels, which processes the pellets in Bridgend – 24 miles from Cardiff.

Commissioning and post completion activities; environmental performance simulation and energy monitoring

To monitor, control and evaluate the environmental performance of the Senedd, the building is fitted with a comprehensive number of metres and sensors, whose primary role is to monitor the building's energy consumption by resource and functional area.

This extensive data logging network can also be used as an educational platform for teaching the public in general terms – and academic institutions in more specific detail – about the different aspects of energy conservation and environmental design.

Extensive computer simulations covering a wide range of fields including thermal, lighting, air flow, energy consumption predictions, etc. were undertaken throughout the various design stages of the project to inform the design development.

A number of these simulations have been repeated post-completion of the project, as part of an over arching exercise in determining project-specific environmental performance benchmark figures (based on the as-built design and accommodating a range of design changes that have occurred during construction) and alternative end-user occupancy profiles/activities implemented during the first year of occupation.

These revised predictive figures/results will be used as benchmarks to compare against actual monitored data on site, and through this feedback loop provide guidance to the actual energy efficient operation and management of the building. This process also offers a platform of communication and understanding of the building's environmental behaviour, to both Facility Managers and end users alike, as they have an equally responsible role in achieving the theoretical environmental performance targets.


By adopting a holistic design approach where architecture and engineering are inseparable, one is able to first minimise demand before addressing the supply equation (which includes renewable technologies).

Through this approach it was possible to substantially reduce the energy demand for the Senedd. This process made the application of renewable technologies more feasible, and in so doing achieve some of the primary goals of sustainable development, as endorsed by the National Assembly's constitutional obligation to sustainability.

To demonstrate this achievement, a bespoke BREEAM Assessment was independently conducted by the Building Research Establishment (BRE) for the Assembly building, and the building design was certified as BREEAM Excellent, the highest awarded in Wales at that time.


  •  The brief for the Senedd demanded it achieve an excellent BREEAM rating. This was achieved by minimising energy use and waste, as well as using renewable energy sources (passive solar, geothermal and biomass);
  •  Fundamental to the design was the use of natural ventilation and daylight. A funnel rising from the depths of the debating chamber and covering the full height of the building is the most visual result of this strategy. Air is supplied to the chamber through diffusers at floor level, and rises by passive stack effect out through the funnel. The funnel also provides daylight into the chamber, and mirrors on the underside of a roof cowl reflect daylight back into the building;
  •  In the committee chambers lining either side of the building, roof lights provide outlets for the air plus daylight. These have automatic blinds controlled by sensors that calculate the angle of the sun, and are linked to fully-dimmable lights, adjusting as needed to maximise daylight use while minimising glare;
  •  Concrete structures were used as thermal mass;
  •  A wood-chip boiler supplies the heating system, and the building also has a ground source heat pump system, which can be used simultaneously for heating as well as cooling. 27 boreholes, 100 m deep were drilled underneath the building.

Project details

  •  Construction value – £41.5 million;
  •  M&E services value – £7 million;
  •  Building area – 5000m2;
  •  Client – National Assembly for Wales.

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About the authors:
Klaus Bode and Matthew Winter work for the BDSP Partnership (Engineering) in London, UK


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Energy efficiency  •  Green building