The value of validation
Not only is it important to validate the results of energy-saving projects, it is also essential to ensure that the validation is meaningful, says Tony Willis of Sabien Technology.
Very often when a retrofit energy-saving project has been completed the end user will require some validation that the project has been successful and will want to know what savings have been achieved. In the case of heating systems this can be more complex than end clients realise, so the building services engineer can play an important explanatory role.
For example, we have had instances of users comparing gas consumption in the severe winter of 2010/11 with that for the preceding, milder winter and wondering why gas consumption has increased after fitting our M2G technology. Yet clearly this is a meaningless comparison because of the very different heating loads. So end clients need to be made aware of the importance of ‘weather correction’ using degree day data – then they can make accurate comparisons, irrespective of weather variation. Where more precise evaluation is required, degree days for the region can be replaced by an onsite temperature data-logger.
In fact, weather is only one variable that needs to be taken into account. Increased staff densities, changes in occupancy patterns and improvements to the building fabric will all alter heating loads and impact on the performance of the heating system. Typically, a reduced heating demand will contribute to the boilers becoming effectively oversized, which can lead to inefficiencies through increased boiler dry cycling.
Given all of these variables, there are a number of things the specifier can do to facilitate accurate measurement of energy consumption and, therefore, meaningful comparisons. For instance, it’s important that the measurement of energy consumption is accurate. Many buildings now use Automatic Meter Reading (AMR) but this generally only measures the total volume of fuel being used. It doesn’t differentiate between, say, the gas being used for heating and that used for catering. So an element of sub-metering is desirable, but can be expensive to implement.
To overcome this approach, Sabien has developed a ‘toggling’ technique for our M2G boiler optimisation technology that tackles the issue of boiler dry cycling mentioned above. The ‘toggling’ switches between running the M2G one day and then bypassing it the next, usually over a period of one month. The result is a comparison of ‘with and without’ – thus taking account of demand variations within the building. This general principle can also be applied to many other types of energy-saving projects.
In addition, the changes in energy consumption over longer periods of time (pre, during and post project) can be established by CUSUM (cumulative sum) analysis, a Carbon Trust-approved tool that examines trends for sequential events, such as energy consumption, over time.
In parallel with accurate measurement, it’s important to ensure that energy reduction initiatives are carried out one at a time. If there are two projects running simultaneously for the same energy source, for instance, it will be virtually impossible to determine which is responsible for the results by using CUSUM.
The majority of factors mentioned here can also be used with other retrofit technologies. The important thing is to end up with meaningful data that hasn't been compromised by other variables.