Natural History Museum

Museum drives down energy use and ups protection for collections

Fitting Trend variable speed drives on the air conditioning plant at the Natural History Museum’s Wandsworth storage facility has made it possible to cut energy consumption at the site by over 30%. It has also enabled much improved control of environmental conditions, thereby aiding preservation of the large collections of specimens that are held within the building. The high level of energy savings has meant that the £100,000 spent on installing the drives and making major changes to plant control strategy was recouped in just 16 months.

The Wandsworth site, which has a floor area of 13,500m2, currently provides storage for a variety of life and earth science collections, including insects, animal skins and fossils. The forty-year old building had previously been a stationery warehouse and when the museum acquired it in the mid nineteen-nineties a major refurbishment was carried out. This included the installation of nine air-handling units to serve the individual stores, plus a Trend building management system to control and monitor them and the central boilers and chillers.

Unfortunately, the air handlers were oversized (the largest units have 22kW and 18.5 kW fan motors), making them more suited to an office building rather than an environment such as the Wandsworth store with its very low staffing levels. The air handlers were also without speed control they used to run flat out 24hrs a day and, owing to their design, it was impossible to control the fresh air intake. Even with the dampers only slightly open the power of the supply fans ensured that fresh air made up some 20% of what the AHUs delivered to the space and this of course had to be conditioned to the right temperature and humidity. To achieve balance, a similar volume of conditioned air had to be exhausted from the building, which was naturally highly wasteful of energy.

It was also difficult to maintain stable conditions within the stores; meaning the collections were put at risk as the temperature has to be maintained below a certain level to prevent pest infestation of the organic specimens. Similarly, if the relative humidity gets too high, problems such as mould growth or pyrite decay of fossil specimens may occur. Control was especially problematic in the height of summer, when it was often impossible to adequately cool and dehumidify the large volumes of air being treated. The only option in such cases was to turn the plant off.

In a bid to eliminate energy waste and provide the closely controlled environment needed to protect the collections, the museum’s Head of Engineering Simon Tilleard devised a new operating strategy for the AHUs’ Trend IQ controllers. For the change in strategy to be implemented it was necessary to fit variable speed drives on the supply and extract fan motors of all nine air handlers. The drives, all of which are from Trend’s extensive NX range, were supplied and installed by Energy Efficient Controls (EEC) Ltd, who also reconfigured the controllers’ firmware.

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 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.

In practice, the extract fans now hardly run at all, only 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 an 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 dehum 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 – equivalent to a financial saving of £74,175. The figures would have been even more impressive had it not been for some 60 staff moving into the site’s previously empty office area at the start of this period.

This meant that the offices had to be heated (to 21ºC, rather than 12ºC) and also resulted in a significant increase in the building’s lighting load. The storage areas are now controlled to 15-17ºC and 35-50% RH, all year round. Even during last summer, when the outside air temperature reached 36ºC, conditions were always kept within these limits. In the past this would have been completely unachievable. According to Simon Tilleard: “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.”

The Trend NX variable speed drives that are at the heart of the solution connect directly to the BEMS communications network, also linked to which are the IQ controllers that automatically regulate their output. Because they are connected to the network, the drives can all be managed and monitored from the BEMS’s supervisor, rather than this having to be done 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.

Simon Tilleard is able to remotely access the Wandsworth system and drives via the Natural History Museum’s intranet. He is based at the museum’s main site in South Kensington, which also features a Trend BEMS. Much of this very large system was supplied and engineered by EEC.