National Gallery of Ireland – the challenges of preservation
12 September 2017
Baroque Room, National Gallery of Ireland (photo: Marie-Louise Halpenny)
This is the second in a two-part feature on the National Gallery of Ireland refurbishment. You can read the first article here.
The provision of a close control environment for the protection of priceless artwork is difficult in a new, purpose-constructed building. But achieving it within dramatic historic spaces, such as that shown in Fig 1, represents an extreme engineering challenge.
The National Gallery of Ireland building, and all spaces in it, are listed so there was a limited scope to add services distribution routes or air terminals. In addition, the appearance of the completed spaces had to be be almost identical to their pre-contract condition.
The historic glazed roof had to be retained, despite the 300kW of heat gains through the roof that made close control and the protection of art from daylight a challenge. All main plant had to be be located outside the galleries and no water services were allowed to pass through or near the gallery spaces.
There was a tiny existing space on the roof that could be used for air handling plant and this was surrounded by glass, making it difficult to get ductwork in or out of the plant area. The completed mechanical and electrical systems had to control the environment to at least the same quality as a recently completed, bespoke new-build art gallery and the energy performance had to be better.
The historic wings of the National Gallery of Ireland comprise the Dargan Wing (completed in 1864) and the Milltown Wing (completed in 1903). A network of cast-iron heating pipes were provided to achieve background heating to the gallery.
Quality natural light was a key feature of the original design, although some of the original windows had been blocked up over the gallery’s history. The missing windows were inevitably re-discovered by the project architect and restored to return the gallery to the naturally lit beauty that was originally intended.
After some debate, it was agreed that the term ‘compromise’ can justifiably have unilateral connotations in the context of historic restoration – and it was ‘agreed’ that the windows would be treated as a welcome addition to the challenge of providing close control of conditions within the gallery spaces!
Taming the glazing and hiding the plant
The large glazed roof of the Dargan Wing (Fig 1) had to remain, but its daylight and heat-gain properties had to be tamed to prevent damage to paintings. The large areas of glazing within the gallery roofs were replaced with a modern, high performance glazing that incorporated micro louvers within the glazing panels.
The micro louvers diffuse light to reduce peak solar radiation on paintings and have a UV transmittance below 1%. They produce a glazing g-value of 0.14, which reduces solar gains by 80% compared with the historic glazing. The new glazing provided also reduces heat loss by almost 70%.
The pop-up pod glazing of the Milltown galleries required additional consideration, as the lower floor-to-ceiling height made the solar gain more critical. The glazing was divided into two separated layers to form a twin roof construction, with controlled natural ventilation provided between the glazing layers to naturally remove heat build-up captured within the glass.
A blind is also fitted between the layers of glass and can automatically close if the total cumulative lux level measured within the gallery reaches a point at which paintings may be damaged. The building management system can also close these blinds during extremely warm conditions to reduce system loads.
A new plant location was required for the heating plant, combined heat and power (CHP) unit, ice banks misting system and electrical switchgear, and the only possible location where a plant room could be concealed was below the ground.
The ‘invisible’ plant room is located below the grass courtyard in front of the building, as shown in Figs 2a and 2b. While it was possible to conceal some air vents discreetly in the vertical steps between the grass and the paving, the flues needed to make a 50m horizontal journey below the building to pick up a historic shaft that leads to the roof.
The roof air-handling plant required a virtual game of ‘Twister’ to be played repeatedly within the BIM (building information modelling) model until a convincing plant room arrangement was achieved. While the roof plant room available was roughly half the size that would normally be considered comfortable, there was also a significant change of level in the middle of the plant area, and the main duct riser appeared in the centre, creating a comical set of boundary conditions for the game.
Distribution and return air
Horizontal distribution of services within the galleries was not possible, but a number of small vertical distribution options were identified into which supply ducts could squeeze, such as a leftover void behind the stairs (Fig 4) and a small recess where a window was once installed.
Whilst it was acceptable to form a new, albeit very compact, riser from the plant room to ground level, it was not permissible to exit this riser at any point within the building.
The only logical option for distributing ductwork horizontally within the building was to bury the new supply air ducts under the building’s basement. This allowed the services to unfold and route directly below each of the galleries that they served, a section of which is shown in the BIM model in Fig. 5 and an installation photo Fig. 6.
The significant length of duct runs raised potential issues for the provision of close control, as this generates a time lag between the control action at the plant room and the air outlets. This challenge required some fine tuning of the building management systems’ PID (proportional-integral-derivative controller) loops to obtain an optimum balance between stability and response.
The supply air to the Dargan Wing terminates largely behind the historic iron grilles that were placed under perimeter benching that originally allowed heat from the traditional exposed cast-iron pipe heating system to enter the space. Discrete displacement grilles are fitted behind the historic grilles and the air-conditioning system becomes invisible to the visitor.
Finding routes for the air returning to the plant room was equally as complex, as all the vertical risers had been filled with supply ductwork. Return air from the large galleries on the ground floor was allowed to wander up the stairwell (seen in Fig 7) and through the upper galleries. While this avoided return air ducts, it introduces complex air-flow patterns in the upper gallery that had to be considered as part of a detailed CFD (computational fluid dynamics) study.
Return air exits through existing openings in the ceiling decoration of the upper-floor galleries and avoids the need for visible return grilles in most locations.
‘Invisible’ services and enhanced control
Throughout history, many an exasperated engineer has suggested to an appropriately uncompromising architect that the only way to meet the criteria for invisible services distribution is to construct the services from glass. It was only a matter of time before such a suggestion was interpreted as a realistic offer to an otherwise unsolvable problem.
The main return air ductwork from the Dargan Wing was completely isolated from the plant room by a large glazed ceiling. Even a hint of a ductwork shadow crossing above the ceiling glazing would be completely unacceptable. After some debate about the impossibility of the situation, the inevitable glass duct comment slipped out.
The installed glass duct is completely invisible from within the gallery spaces below and allows all of the return air to be transferred back to the plant room without intrusion.
Galleries are traditionally controlled through the use of a single sensor in each space, which can lead to local variations in conditions within spaces during certain conditions. These local variations are normally minimised by continuously providing air-flow rates that are much larger than required for the majority of the year.
In the refurbished galleries, an array of sensors was provided in each space and a bespoke control routine was developed to ensure that all sensors are kept within range while minimising the air volumes required. This dramatically reduces the fan energy required.
The control routine works by adjusting the temperature and humidity to maintain the centre point condition (the midpoint of the highest and lowest sensor reading, not the average) at the required set point and then adjusting the air volume to control the bandwidth between these two points at the condition limits. This method ensures that the minimum possible air flow is used to ensure all points are maintained within the acceptable band.
Sensors were also placed at high level to monitor stratification within the galleries and ensure the paintings are not subjected to significant variations in condition with height.
A ‘hot swap’ solution was used that allows a sensor to be unplugged for re-calibration or replacement at any time. The controls system will automatically detect that the sensor has been removed and adjust to compensate for the missing sensor.
It was important that the sensors used were not visually intrusive and that the sensor face plates did not contain visible fixings. A bespoke solution was adopted that used magnets to secure the face plate invisibly.
Protecting our history and future
While the use of specialist glazing, VAV (variable air volume) air control and an enhanced enthalpy-based, free-cooling routine were implemented, the provision of 24-hour/365-day close control remains an energy-intensive process. It was therefore important that the generation of heating and cooling was carefully selected.
The close control of humidity and temperature within a historic gallery results in a requirement for year-round heating and cooling loads, many of which are either simultaneous or alternating within a relatively short time-period.
A four-pipe chiller allows the waste heat from the cooling process to be used to reduce heating demand, but also allows the chiller to operate in air-source heating mode when serving a heat load alone.
Combining the chiller with an ice bank allows the generation of ice during the night and the associated heat to be used when it is more likely to be required (due to cooler temperatures at night). The ice bank also assists with the smoothing of the electrical demand on the grid.
A 150kWe CHP system runs continuously and provides the majority of heat for the gallery, with excess heat transferred to the adjacent Millennium Wing galleries. The system has not been provided with a heat dump because, while the use of heat dumps on CHP units does improve their economic performance, they guarantee a negative environmental performance. The system is instead sized to ensure that all of the heat from the CHP unit is used.
Over time, the efficiency of the electrical grid in Ireland is improving and, as it does, the environmental benefits of the CHP unit will decrease, but the environmental benefits of energy from the buildings heat pump will increase. The systems in the gallery are designed to gradually change priority away from the CHP system to the air -ource heat pump as this transition occurs.
A visitor to the galleries will be almost completely unaware of the intricate servicing system that is now providing close control to the galleries, with the majority of equipment buried below the building and threaded invisibly through its structure.
Monitoring of the galleries has shown that an excellent level of both temperature and humidity control has been achieved in all of the historic galleries and the artwork has been safely returned for the public to view.
Chartered engineer Chris Croly is environmental engineering director with BDP. He has led the design and delivery of a range of innovative low-energy new build and retrofit/renovation projects in the commercial and public buildings sectors.