British Museum drives down energy use for collections
Excellent BMS in cooperation by Trend and Vacon
In museums, environmental conditions, air quality and temperature are some of the most critical issues for preserving the large collections. To cut down energy consumption and to improve environmental conditions, the Natural History Museum in London, UK, decided to install variable speed AC drives and to make major changes to their plant control strategy at their storage facility in Wandsworth. It was worth it. The investment was recouped in just 16 months. The variable speed AC drives supplied represent the Vacon NX range of drives under the Trend brand.
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The Natural History Museum’s storage facilities in Wandsworth are now controlled to 15-17 oC and 35-50% RH, all year round. Conditions can always be kept within these limits, even during hot summer days. In the past this would have been completely unachievable.
Problem and its solution
The Wandsworth site, which has a floor area of 13,500 m2, currently provides storage for a variety of life and earth science collections, including insects, animal skins and fossils. When the museum acquired the forty-year old building in mid 1990’s, a major refurbishment was carried out, including the installation of nine air-handling units (AHU) to serve the individual stores, plus a Trend building management system (BMS) to control and monitor them and the central boilers and chillers.
Unfortunately, it was soon discovered that the refurbishment had not been a total success. The air handlers were oversized, and without speed control, they used to run flat out 24 hours a day leading to a very high consumption of energy. Also, owing to their design it was impossible to control the fresh air intake, and it was impossible to maintain stable conditions (temperature, relative humidity) within the stores, which meant that the collections were put at risk.
To eliminate energy waste and provide closely controlled environment, the museum’s Head of Engineering, Mr Simon Tilleard devised a new operating strategy of the AHUs’ Trend IQ controllers. It was necessary to fit variable speed AC drives on the supply and extract fan motors of all nine air handlers.
Now, supply fan speed is controlled to maintain a slightly positive pressure (2Pa) in the storage spaces. Crucially, the AHUs’ export ports have been permanently closed off; as a consequence, the volume of fresh air supplied is only what is needed to compensate for air leakage through the building fabric and hold the differential pressure. However, it is still more than enough for the few staff working in the storage areas. If temperature and/or relative humidity (RH) levels start to go out of tolerance, the recirculation dampers are modulated open, causing supply fan speed to increase and more conditioned air to be delivered to the space. Should this not restore conditions then the extract fan is brought on to boost recirculation.
Clear savings
In practice the extract fans are now really being needed during the summer months. Most of the supply fans generally operate at 30 - 50% of full speed, which owing to the cube law relationship between motor speed and power consumption is equivalent to electricity saving of around 90% or more. Those fans serving the smaller stores will often tick over at just 10% of maximum.
There have also been appreciable power savings on the chillers, less chilled water being needed for cooling and dehumidification owing to the much lower fresh air volumes passing through the air handlers. Since the latter’s demand for hot water has fallen significantly as well – both for heating in the winter and dehumidification in summer – there has been a major reduction too in boiler gas consumption.
In the 12 months following the drives’ installation, electricity and gas usage fell by 30% (689 MWh) and 38% (754 MWh) respectively.
“Previously it was the air handling units that posed the greatest risk to the collections. We knew the solution lay in being able to limit fresh air volumes. The particular way we have chosen to achieve this has not only resulted in very stable control and allowed us to set strict environmental conditions that are conducive to preservation, but has also led to a substantial reduction in running costs,” says Mr Tilleard.
Variable speed AC drives in the heart of the solution
The NX variable speed AC drives that are at the heart of the solution connect directly to the BMS communications network, to which also the IQ controllers that automatically regulate their output are linked. Because they are connected to the network, the drives can all be managed and monitored from the BMS’s supervisor instead of through their local displays. This makes life a lot easier for the site’s maintenance technician. Up to 30 variables can be monitored, including motor status, speed, power, current, torque and hours run.
Based at the museum’s main site in South Kensington, Mr Simon Tilleard is able to remotely access the Wandsworth system and drives via the Natural History Museum’s intranet.
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02844 613 782