As well as being an architectural and artistic feature, the tank forms the core of the building's air conditioning system. The balls are filled with a phase changing material which is the basis of a eutectic thermal storage system designed by consulting engineer Ove Arup & Partners' associate director, Gregoir Chikaher.
The exhibition is housed in the shell of a listed 1903 Great Western Railway depot. The services design brief required year round thermal comfort conditions, with a focus on energy and sustainability. One other major factor in the design was budget, as limited funds were available for the project.
With over 4500 visitors expected daily, in addition to the exhibits, lighting and solar gains, significant ventilation and thermal loads are required to maintain comfort conditions. The deep plan of the structure made it implausible to use purely natural ventilation and daylighting, so the design team investigated other methods of conditioning the area.
The exhibition will have an irregular energy use profile, peaks coinciding with the most popular times of day and year. Use of a traditional boiler and chiller system would therefore require periods of part-load operation. With the low energy requirements in mind various solutions were considered, including using the adjacent harbour as an energy sink.
The building's orientation promotes a natural flow of energy from North to South, and Chikaher intended to utilise this with heat pumps – a thermal store using a solution with a 27°C phase change temperature being incorporated for maximum efficiency.
The eutectic salt solution selected matches this phase change criteria and consists of a six part water, one part calcium chloride mix. The quantity required was determined on expected energy loads of 119 W/m² cooling and 87 W/m² heating. For maximum surface area, hence heat transfer, this was transferred into balls.
The balls are a carefully calculated 98 mm in diameter and provide an energy capacity of 50 kWh/m3. They have a 1·2 mm thick polypropylene skin and are ultrasonically sealed to prevent leakage. An air pocket enables expansion.
A total of 61 200 balls – carefully positioned in a 3 m-diameter, 7 m-high acrylic tank –make up the thermal store. This store was initially intended to be out of public view, however, with great foresight, Chikaher decided to make it an exhibition piece – the services system being on display for scrutiny and learning.
The simplest way to assist visitors' understanding was to make the system visually responsive. A thermally-responsive coloured thermochromic resin was added to the polypropylene skin. Changes in colour reflect the change in temperature and state of the solution, hence the system's response to ambient and internal conditions. The system operation therefore creates the artistic value of the piece.
In heating mode the balls turn pink as the eutectic salts change from liquid to solid. In cooling mode they will turn white, with a phase change from solid to liquid. They are blue at the actual point of change ie the fusion temperature. It is estimated that full tank change from white to pink will take around two hours, with a mix of colours being evident in the intermediate times due to the depth of the tank.
The tank was constructed in Colorado by Reynolds, the company which supplied the city's Sea World aquarium, then shipped to site in November 1998. Acrylic was chosen for its transparency and relatively low weight. Although the system is not pressurised, the volume of balls and water create a service pressure of over 8 bar. This necessitated a tank wall thickness of 63 mm – impractical for glass due to the immense foundation support that would require.
An ultraviolet absorbing material was added to the tank walls to prevent sunlight affecting internal temperatures, hence system efficiency. This is also intended to prevent algae growth. With the system on display, the last thing the designers want is anything obscuring the view of the balls or floating in the water. The only opening, a hinged manhole on top of the tank, has a special insect cover.
The system works on the thermodynamic principle of energy gain through phase change. The water surrounding the balls acts as an energy transfer medium. This is softened and treated with uv filters and is circulated via diffusers at the inlet and outlet points of the tank – the inlet during charge being in the lower position to ensure natural stratification.
The tank is connected to Versatemp air-to-water heat pumps by a two-pipe reverse return system. It is charged by central heat pumps out of hours and acts as an energy store for the reverse cycle heat pumps to condition the building as required.
Two modes of operation are used: in winter, the salts are melted overnight using cheap-rate electricity. This provides a heating store for the following day. In summer, heat is absorbed during the day and released at night, the balls being solidified to ensure a store of cooling energy for the next day. Heat rejection and injection are provided by the primary heat pumps.
In cooling mode, the water is increased above the phase change temperature to between 29-35ºC. In heating mode it is reduced below 27ºC. The surrounding conditions are constantly monitored and the balls will change phase and colour as the system load varies. The peak cooling and heating loads are approximately 755 kW and 550 kW respectively.
Conventional central ahus provide filtered and heated fresh air via low velocity displacement ventilation units. Thermal wheels recover the sensible heat from the exhaust air to preheat the fresh air during the winter.
A significant worry was balls leaking – with a calcium chloride solution base this could cause severe corrosion. To counter this potential problem a stainless steel Alfa Laval plate heat exchanger separates the tank from the main system loop. This operates in series with the central air-to-water heat pumps when heating or cooling is required. Full plastics pipework is used on the tank side.
With no need for large plant, the entire system uses only 5% of the building's floor space. The high ceiling in the North Gallery enabled the 50 m3 volume to be obtained by height, giving it a relatively small footprint. The main pipework is routed underfloor directly from the tank and the remaining plantwork is sited on a mezzanine floor and at roof level. Initially it was calculated that a tank of 60 m3 would be required, however this was reduced to 50 m3 simply by careful placing of the balls rather than loose pouring – another space-saving practice.
This is the first system of its kind to be installed so intense testing was carried out to ensure that the theory worked in practice and the balls could withstand the constantly recurring changes in state of the eutectic salts. A one metre high pilot system was built in ball manufacturer Cristopia's factory in the South of France. This is included in the exhibition, complete with a transparent heat pump, so that visitors can fully understand the system and try it out for themselves.
Fatigue testing to 10 000 cycles was carried out on uncoloured versions of the balls. Further tests were carried out on coloured versions to ensure the pigment did not affect the process. The only uncertainty remaining from the tests is the lifetime of the colour pigment, however this has been guaranteed for a minimum of two years by the manufacturers and the uv filters should minimise any fading problems.
The system is controlled via a bems which predicts temperatures two days in advance and charges the tank accordingly. Linked to sensors throughout the building, the programme is tied to a database of annual ambient temperatures. From these two sources it can then predict daily temperatures. Actual figures are stored to increase future accuracy and a log of visitor numbers can be included.
The tank of balls can be seen from the exhibition approach road – providing one final use. Instead of looking for the car park full signal to know the exhibition is very busy, the balls tell the story – a full white tank means full cooling mode therefore many visitors, so anyone wanting more time or space to look around should come back later.
Source
Building Sustainable Design
