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Comparing like with like

07 December 2011

Assessing the effectiveness of energy efficiency technology isn’t always as simple as it seems. Tony Willis of Sabien Technology provides some pointers

Clearly, when making any sort of meaningful comparison it’s important to compare like with like – and the exceptionally cold weather of December 2010 was a very good illustration of how this isn’t always straightforward for energy consumption comparisons.

For example, if an organisation retrofitted energy efficiency controls to gas-fired boiler plant during 2010 they would expect a reduction in gas consumption. But a straight comparison between figures for December 2009 and December 2010 would be meaningless because of the extra heating in December 2010. So the results need to be ‘weather-corrected’ using ‘degree day data’ generated by the Meteorological Office. This enables accurate comparisons, irrespective of weather variation.

In fact, weather is only one variable that needs to take into account. Staying with heating, increased staff densities in workspaces mean more heat from people and computers, so there is less demand for heating. Similarly, if a building’s insulation is improved, less heat is lost through walls and windows and heating demand is reduced. These factors contribute to the boilers becoming effectively oversized, which can lead to inefficiencies.

Standing losses and dry cycling

Ironically, reducing demand on the heating system can increase the energy wasted ‘boiler dry cycling’. When a boiler – even a well-insulated one - isn’t firing to heat the building, it gradually loses heat to its surroundings. However, most boilers have internal thermostats that require them to maintain the water in the boiler at a certain temperature (the boiler set-point).

When the temperature of the water in the boiler falls below the set-point, the boiler often fires to restore the temperature, even though it isn’t delivering heat to the building. This is boiler dry cycling and, typically, isn’t controlled by building management systems (BMS) or other existing controls.

Many organisations are now seeking to eliminate dry cycling by retrofitting controls specifically designed to prevent boiler dry cycling, but some of these can have a negative impact on comfort levels. They may delay the boilers’ firing, or artificially reduce the boiler set points, allowing temperatures to fall in the workspace. They may also interfere with the operation of the BMS, leading to even more energy wastage.

Because of this, an evaluation of the success of an energy-saving project should also take account of factors such as comfort. There’s no overall benefit to saving energy if it leads to a shivering, complaining workforce with low morale and low productivity.

The latest intelligent boiler load optimisation technologies take a different approach to boiler dry cycling, analysing each boiler’s temperature profile in real-time to ensure the boilers only fire in response to a genuine demand for heat from the building. These devices are easily retrofitted and work in harmony with a BMS and – crucially – do not alter set-points or compromise room temperatures. Therefore any such project can be deemed successful on counts of both energy saving and maintaining occupancy comfort.

Although we have used boiler dry cycling as an example here, the same principles apply to the introduction of any energy-saving technology, so it’s worthwhile having a check list of key criteria before shortlisting appropriate products. Such a check list should include checking that plant is being controlled effectively by existing controls, that the plant has been properly maintained and that temperature set-points are correct.

In parallel, it’s vital to understand the technology you are being offered, check its track record and talk to other people that have used it. And when evaluating technologies, be sure to compare like with like. Then you can be sure that the selection process is based on the true merit of the product and not just what the salesman wants you to hear!

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