Case Study 2

When applying new technology to reduce energy doesn’t make sense

Following on from the success of applying Variable Speed Drive technology to the condenser pumps on the air conditioning system, see case study 1, it was a logical progression to continue researching ways of making further energy savings on this system (one of the largest consumers of electrical energy in the building).

The opportunity

The cooling towers, which are used in the final stage of the heat removal process from the building, rely on 2 speed fans to increase cooling capacity. The obvious question to ask was ‘could the VSD technology be applied to this equipment and achieve a similar level of savings?’


The cooling load of the building is variable based on several factors;

  * External weather conditions
  * How the building is being used
  * Occupancy levels

The cooling system is also variable so that the supply demand can be matched to minimise energy use while meeting the comfort levels required. To achieve this the building and therefore the system is divided in to zones; east and west. Each zone has two cooling towers providing heat rejection from the air conditioning of individual floors. Each cooling tower has 3 stages of cooling; free cooling, fan speed low and high. This provides 6 levels of cooling for each zone. Replacing the 2 speed control with variable speed technology would provide the ability to match fan speed precisely to demand, this in turn would save energy with the secondary benefit of reducing wear and tear on equipment due to elimination of the starting and stopping of the 2 stage motors.

The solution

By applying variable speed drive technology we would able to vary the fan speed based on demand using the existing fans. The financial evaluation for the project was produced the following results

  * The project cost was £11,502.50
  * The savings achieved would have been £2,126 per annum
  * The payback period would be 5.4 years

The results

As the client was already using ‘green energy’ and the reduction in respect to the total building load would only be 0.3% this was not considered a priority investment. This project was rejected.


Consideration for any project is not just based on the savings it makes but also the potential waste it creates from the disposal of the old materials that are made obsolete.

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