How will a load of balls containing water that freezes at 27°C heat and cool Bristol’s landmark science centre?
One of the more curious exhibits at the Bristol science museum due to open next year is a tall, cylindrical, transparent tank containing thousands of grapefruit-sized coloured balls.

Throughout the day, the balls slowly change colour. In summer, they start the day pink, slowly turn blue and end the day white; in winter, the process is reversed.

It is a fun exhibit, but there is more to it than that. The acrylic-walled tank is part of Explore@Bristol’s air-conditioning system, and the balls store energy that is used to heat and cool the building. This stored energy is distributed around the building in a pipe that feeds heat pumps blowing hot or cold air into the gallery spaces.

The scheme, designed by Ove Arup & Partners associate director Gregoir Chikaher, is a eutectic thermal storage system. It works on the principle that energy is stored or released from a compound as it freezes or melts.

Eutectic systems are rare outside large manufacturing facilities and refrigerated process plants. What makes the Bristol system even more unusual is that Chikaher has added a thermochromic resin to the balls’ skin that changes colour as their temperature rises and falls. External weather conditions, heat gain from the sun and visitor numbers will all affect the building’s cooling or heating load, and this will be reflected in the colour of the balls.

On show to the public

For Chikaher, the fact that the tank is transparent and on display to the public means that the performance of the building’s air-conditioning system is also on display.

If his calculations are wrong or the system performs badly, the result will be obvious.

If the 4000 m2 of exhibition space overheats, the balls will turn white.

No simple solution

Housed in a former railway shed, the Chris Wilkinson Architects-designed centre does not lend itself to a straightforward temperature regulation system. The building’s deep-plan form, the estimated high number of visitors and high heat loads from the exhibits meant that natural ventilation was not an option and that cooling had to be installed to keep the space comfortable in summer.

Chikaher looked at a variety of options, including using bore-hole water as a source of geothermal energy, before deciding on the eutectic thermal storage system. “Using a thermal store meant we could get rid of boilers and cooling towers,” he says.

This saved space on the tight site, but the thermal store still had to go somewhere. Chikaher’s decision to make a feature of the 7 m high, 3 m diameter tank by incorporating it into the exhibition was inspired.

The tank containing the 40 000 hollow polypropylene balls is filled with water that has been softened to keep it clear. Each 98 mm diameter ball is filled with water and a special type of salt, eutectic salt, to form a eutectic solution that freezes at 27°C – the temperature of a hot summer’s day. As the water melts, it absorbs heat – a property Chikaher exploits to cool the building.

The system comprises two separate piped circuits. A primary circuit transfers energy from the air outside to the eutectic tank through the water in the pipes, and a secondary circuit transfers energy from the tank around the building.

The tank provides the building’s main energy store for heating and cooling. On summer nights, two electric heat pumps mounted on the roof will freeze the eutectic solution using cheap-rate electricity. Using a solution that freezes at 27°C ensures that the heat pumps run at their most efficient operating temperature. The pumps work in much the same way as a domestic fridge, cooling the tank by taking heat from it.

In winter, the pumps work in the opposite direction, taking heat from the air and using it to melt the eutectic solution, so that heat energy is stored ready for the next day. This energy is transferred to the softened water inside the tank via a heat exchanger.

Working overtime

On exceptionally hot or cold days, the heat pumps will run during the day to supplement the energy stored in the tank. A sophisticated control system will ensure that all the energy in the tank is used before the pumps start using more costly daytime electricity.

Another series of heat pumps in the centre take heat from the tank to heat the space or expel heat into the tank to cool it. This is particularly economical in spring and autumn when the sun is low in the sky. Rooms on the sunny south side will need cooling, and spaces opening on to the shady north facade will need heating.

The eutectic tank is linked to the centre’s heat pumps via the heat exchanger. This allows heating or cooling energy to be transferred but stops the transfer of the eutectic salt if one of the balls leaks, which could cause corrosion.

Chikaher has not taken any chances with the system. A full pilot plant was constructed to demonstrate the effect of the temperature gradient on the colour of the balls and on the system’s thermodynamic properties.

He will not have long to wait before the real system is put through its paces. Testing begins this month, ready for the centre’s opening next Easter.