Exploiting the benefits of part-load efficiency
Modern air-conditioning systems operate more efficiently at part load than at full load, so what do building operators need to be aware of to ensure they are getting the most out of their building services? Sebastien Desmottes of Mitsubishi Electric Air Conditioning Systems looks at the issues around part-load efficiencies.
The efficiency of a building’s services is under scrutiny as never before as further legislation changes come into effect to ensure that our buildings are heated, cooled and ventilated in the most efficient ways possible.
Equipment needs not only to be designed with this in mind, but that energy efficiency also needs to remain a key factor throughout its operational life.
Part L of the Building Regulations and the Energy Performance of Buildings Directive both place an increased focus on energy efficiency within our buildings, and realising the benefits of part-load efficiencies can play a major part in achieving a reduction in carbon emissions, energy use and running costs.
Part-load efficiency refers to the ability of a system to handle part-load energy use and should be taken into consideration when specifying an HVAC system — so that it can deliver the exact load requirement with the minimum power input.
Air-conditioning systems are traditionally designed and sized to operate at their peak efficiency when they are working at their maximum capacity.
However, with very few exceptions (such as server rooms) most systems are sized to meet heating and cooling conditions that actually occur less than 3% of the time.
For the vast majority of situations, therefore, the efficiency of the equipment on paper bears little or no resemblance to the actual efficiency because too much capacity is being delivered to match the required power input.
Planning for this scenario and utilising part-load operation as a part of the overall control of the system is much more efficient. For the vast majority of HVAC applications, the operating hours at reduced load can be significant.
For split air conditioning systems under 12 kW, the requirements of the Energy Related Products (ErP) Lot 10 Directive which came into force at the beginning of 2013 recognises this and now requires more detailed labelling and performance criteria, which takes into account operation at part-load.
The EU-wide ErP rules, which focus on performance in three distinct climate zones across Europe in the heating season (cold, moderate and warm) are designed to help meet the urgent need to reduce energy use, running costs and emissions— as well as the requirement to reduce equipment sound levels.
With these rules now in place, it is more important than ever to understand that annual efficiency should be the measurement of effectiveness, not a single test measured in laboratory conditions. This enables a better understanding of real-life running costs and CO2 emissions.
Mitsubishi Electric has long championed the use of seasonal efficiency as the best way of delivering a truer picture of performance for end users, rather than coefficient of performance (COP) and energy efficiency ratio (EER) at nominal conditions.
All air-conditioning units affected must have a data sheet detailing seasonal coefficient of performance (SCOP) and seasonal energy efficiency ratio (SEER), as well as COP and EER. It also shows data on the estimated annual energy consumption and sound power levels.
Manufacturers have had to redesign split systems to make them more efficient, and now they calculate the SCOP and the SEER. These figures more accurately reflect performance at part-load; although they are not yet applicable to air conditioning over 12 kW, it is a sign of the way things are moving.
To maximise the overall efficiency levels of systems and prepare for the way SCOP/SEER that is coming, we need people to look beyond the current standards and towards the longer term.
Manufacturers are already working on this. For our own City Multi YKM, for example, we have already designed for part-load efficiency delivering a performance that is an average 20% more efficient annually than the previous model at part load.
However, there is a limit to what manufacturers can do, and this is where proper sizing and specification of an HVAC system becomes important in helping to maximise part-load efficiencies.
Selecting the appropriately sized system requires an understanding of the peak heating and cooling loads and the system’s overall load profile to determine how often the system will be running at part load.
If running at part load is known to be a frequent occurrence, then specifiers and installers should look for a system that will be efficient for those part-load conditions, rather than one which will provide maximum efficiency for only a tiny part of the time the system operates.
Modern controls have a major role to play here, especially in larger buildings, where multi-unit systems can operate together and offset heating and cooling to match demand.
Such cascade systems can determine which individual units operate at any given time, which are heating and which cooling, and which are at part-load and which are at full. This not only increases overall system efficiency, it will also help prolong equipment life by rotating which unit operates as main and which as back-up, extending the overall life-span considerably without compromising on performance within the building.
Another concern is sizing equipment correctly. There are system components and modular components that can be chosen to improve efficiency. These include components that can operate efficiently at part-load such as variable-volume fan systems and variable-speed drive controls for fan motors, along with other elements of the building services covering hot water and ventilation.
Modern air conditioning can work with traditional forms of heating and ventilation in a hybrid system that can benefit from the best of both worlds. Key to making these work best together is adopting a control system that can cope with the variable demands and communicate effectively with not only with dual manufacturers’ systems but also with the overall BEMS equipment so that systems are not fighting each other. It is also important that the performance and energy use is carefully monitored and managed to deliver year-round efficiencies.
Recognising and increasing the ability of a system to operate at part-load will optimise the overall energy efficiency. It will also reduce initial capital and installation costs, as well as reduce the overall energy bills for the building occupier.
Mitsubishi has produced a comprehensive range of CPD-accredited guides on key legislation such as Part L, the Energy Performance of Buildings Directive, and the ErP; they are available in the digital library of the our website (URL below).
Sebastien Desmottes is controls manager for Mitsubishi Electric Air Conditioning Systems.
