Make waste history
September 1st 2007

Danfoss' variable speed drives help reduce energy at the Natural History Museum

Initially, the South Kensington Museum Estate Centralised Boiler House, installed in 1952 in the basement of the Natural History Museum, used to distribute heat to a number of adjacent museums and colleges and formed a very early district heating system. In recent years, especially when the Science Museum and Imperial College installed CHP systems, it became clear that the NHM's boiler house was over-large, inefficient and in need of refurbishment.

Now, Vital Energi of Bolton are to guarantee to save the Natural History Museum (NHM) £500,000 every year over the next fifeteen years with their first Energy Performance Contract. This £12 million project will involve the finance, design, supply, installation and commissioning of the necessary plant and equipment to provide tri-generation of electrical power and heating services to both the Natural History Museum and the Victoria and Albert Museum next door.

Through improved energy efficiency, the NHM will achieve a reduction of 1800 tonnes of CO2 per annum.

The work involved the removal of two of the boilers and in their place the installation of a 1.8MWe gas fired CHP engine, two 750KW absorption chillers to use CHP waste heat for heating and cooling purposes and the installation of two new cooling towers. Vital Energi will also be responsible for the operation and maintenance of the energy centre over the next fifteen years.

According to Helge Wonsbek of Vital, "It's all too common for systems such as this to be over-engineered 'to be on the safe side'. We don't regard that as engineering and we take great care to size every aspect of the system to meet the exact needs of the building" In a complex the size of the NHM, it's inevitable that the various air and water circuits and sub-circuits will require to be balanced and some method of control provided to maintain that balance, otherwise the hot water and cooled air would not be delivered to the appropriate parts of the building. Traditionally, proportional balancing was achieved using valves in the water circuits and dampers in the air ducts but this form of control is, at best, sloppy and inefficient.

Today, the use of valves has increasingly given way to accurate, more efficient flow control by varying the speeds of the fans and pumps delivering the air and water throughout the building. As energy costs have risen, so the cost per kW of VSDs has steadily reduced and the economic case for VSD control has become undeniable.

It's for that reason that nowhere on this system has Vital Energi used regulating valves but instead has installed Grundfos variable speed pumps and Danfoss VSDs on all the primary and secondary water circuits. As Mr Wonsbek explained, "there's little point in refurbishing a system to improve its efficiency then to install control valves which would merely drag the overall efficiency down again. Even where there is no need for continuous variable flow control, we have used variable speed pumps to tune the system initially and these will then run fixed at that reduced speed set point. We didn't do any specific cost justification calculations on the benefits of VSD on this system. We proved the economic benefits to ourselves long ago and now VSDs are standard fitments on all our new CHP systems; they're much more cost effective than control valves. The only time we fit a valve today is for isolation purposes. Those parts of the system that demand control, for example the main circulation pumps, distributing hot water throughout the complex and the CHP inlet and outlet vent fans, are also speed controlled, along with the fans in the cooling tower." Ease of interconnection with Building Management Systems is another benefit, simplifying control and enabling energy monitoring at a central location and based on their past experience, Vital Energi selected a total of 16 Danfoss VLT 6000 series drives of between 2kW to 30kW for these functions since they tied in readily to the NHM's existing BMS.

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