Chillers as the key to reducing the energy consumption of buildings

ThermOzone, chiller, air conditioning, chilled water
Remote monitoring and control over the Internet, such as that provided by ThermOzone’s RODEM service, contributes to continuing operational efficiency and reliability of chillers.

With users of larger buildings encouraged to reduce energy consumption by initiatives such as ESOS (Energy Savings Opportunities Scheme), Trevor Dann of ThermOzone looks at the opportunities for improving the performance of chillers.

Reducing energy consumption makes business sense, especially for water chillers as they are a significant load on many buildings. Yet we are seeing an increasing number of highly viable projects left on the back burner due to lack of motivation to invest in energy-saving projects, even with payback periods return on investment consistently less than three years and often less than two.

The part-load efficiency of inverter-driven screw compressors are now similar to those of turbo-style compressors, but without the narrow operating envelope that so often restricts the application of turbos. One manufacturer we work with has developed its own inverter, which has enabled them to tune the inverter and motor stator design to work in harmony and, hence, even more efficiently.

Screw compressors have an inherent disadvantage at very low ambient operating conditions because they have to maintain a higher discharge pressure to keep the oil flow through the compressor sufficient. However, above about 6 to 8°C ambient we are finding this difference of little consequence. Average running conditions for most chillers sees a typical ambient of about 15°C — and higher in central London due to micro-climate. However, at such an average ambient temperature the machines are generally operating nowhere near their peak-load capacity, so optimising energy efficient operation is critical.

Even now with the emphasis so much on energy costs we still see the major emphasis at the design stage being focused on peak-load capacity and Energy Efficiency Ratio (EER) at this point; even reference to the SEER/ESEER figures (seasonal energy efficiency ratio and European energy efficiency ratio) do not take due account of likely real-time operating conditions.

For a building with a chiller, a large part of the total energy consumption is due to the chiller — so much so that a 30% energy reduction in the energy used by chiller plant means a 10% reduction at the energy meter for the whole building.

Few manufacturers offer much information on life expectancy, and clearly the use and application, plus maintenance regime, is beyond their control, but much equipment is built to price, where life expectancy beyond warranty is a low marketing priority. In this regard all that glitters certainly is not gold, and for any new design we believe the starting point for any new chiller should be the 20-year marker. Such a life expectancy is the starting point for our NuChill bespoke design chillers, with base modular designs allowing straightforward longer-term repairs and the ability to apply newer technology during the chiller’s extended life-cycle, which could be 25 to 40 years).

The performance of existing chillers can also be improved substantially. The average age of chillers we tackle under our ReChill concept is around 6 to 8 years, and we normally only get involved where operational problems have arisen. Yet we know there are thousands of chillers in service operating reliably, but with appalling energy efficiency.

ThermOzone, chiller, air conditioning, chilled water
This ThermOzone Re-Chill chiller refurbishment project for HBOS in the City of London included the replacement of reciprocating compressors using R22 with Hanbell screw compressors using R407C.

This situation is not helped by the key legislative driver — air-conditioning inspections. Assessments for chillers carried out using CIBSE’s TM22 energy assessment and reporting methodology indicate a couple of basic checks that have no impact nor follow up, yet the power cable supplying the chillers is often the largest in the building. Chillers in most office buildings account for about a third of that building’s electrical load. So a 25% reduction in the energy use of the chiller is a significant 8% reduction at the energy meter for the whole building.

Even for chillers being supplied new today, significant improvements can be applied, but the payback is difficult to estimate for plant that is already the subject of a capital-costing assessment. The upshot is that users get a cheap unreliable chiller that uses too much energy for its first few years. The manufacturer may work very hard to get the chiller safely past warranty, but after that the user is on his own.

So why not simply take account of the best features of chiller design at the selection stage. Bespoke chillers such as our NuChill are not expensive for most installations, and being bespoke will take account of the idiosyncrasies of the specific site for which it is designed.

Such an approach to designing a chiller can include all current design advantages and be engineered to last for 20 years at the optimum energy performance. Remote monitoring and control over the Internet (our RODEM service) contributes to continuing operational efficiency and reliability.

Trevor Dann is technical and sales director with ThermOzone.

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