End uses - introduction

Motor systems deliver services such as the storage of foodstuffs (refrigeration systems), occupant comfort (air-conditioning systems) and the transfer of materials (conveyor systems). These services are the end uses of the motor systems.

When investigating energy use in motor systems it is important to take into account the big picture, that is, the end use delivered by the motor system. Having an efficient motor system delivering a poorly managed end use is not energy efficient or cost effective. For example, a highly efficient fan system delivering cooling air to a building is not best practice if the temperature set point is too low. In fact, a surprising number of motor systems could simply be turned off without adversely affecting the delivery of the end use required.

Therefore, before you analyse the details of the motor and motor system, it is useful to ask yourself:

Is the motor system performing a service that is actually useful?
Could the service be provided more efficiently by a different means?
Can the system used for delivering the service be more efficient?

After you have answered these questions, you can analyse the motor and motor system.

For successful motor system analysis, it is essential to be aware of the assumptions that you make and to measure unknowns where possible. For example, Energy Efficiency Best Practice has been working with Bakers Delight to build the most efficient bakery in Australia. It was widely reported that oven energy use was likely to be 80 per cent of overall bakery electricity use. After extensive monitoring was undertaken, the actual oven energy use was found to be only around 55 per cent.

Starting downstream and improving energy performance delivers more than just lower electricity bills. Typically safety and equipment reliability and life is improved, downtime is reduced and production improvements often result.

Investigating the difference in energy consumption between two refrigeration systems in a hotel

Introduction

This case study investigated the difference in energy consumption between motors driving two meat freezers in the same hotel. The study found that one freezer was consuming more than twice as much power as the other due to a number of system factors.

This case study demonstrates that:

Motors are a small but significant part of complex systems. The biggest savings can be found by starting downstream and working towards the motor.
Understanding the role of the motor in the whole system will enhance your ability to make energy and cost savings.
There are usually a large number of hidden system factors that impact on motor energy consumption for any given end use.

The characteristics of the systems

The investigation considered two motors operating refrigerant compressors for separate meat freezer systems. The freezers are of the same make and model with seemingly identical components, apart from the motors.

Freezer 1 is on level 8 of the hotel and incorporates a 4kW premium efficiency motor.

Freezer 2 is on level 7 of the hotel and incorporates a 5.5 hp standard efficiency motor.

What was investigated?

The power consumption, current and power factor of the motors were logged over a total of eight days and it was found that:

Freezer 1 drew 0.78 kW on average with a measured power factor of 0.59.
Freezer 2 drew 1.64 kW on average with a measured power factor of 0.78.

Extrapolating from the results of data logging, Freezer 2 would consume more than twice as much electricity as Freezer 1 (14 300kWh compared with 6 800kWh) every year. Assuming an electricity tariff of 12c/kWh and an emission factor of 1.08 kg CO₂/kWh, this equates to a difference of 7 500 kWh, $900 and 8 tonnes of CO₂ each year.

Although the actual dollar savings are not large, the key observation here is that one system is costing twice as much to run as the other. This large differential is often also observed in large industrial applications where the dollar savings are relatively much larger. For example, by starting with the end use Energy Efficiency Best Practice assisted Carlton United Brewing identify 35 per cent energy savings in their plant's refrigeration system, worth around $500 000 a year.

What were the causes of the differences in efficiency?

Differences in the motors

It is likely that the motors differ in efficiency. According to the motors catalogue the difference between the most and least efficient currently available 4kW motors at half load is 4.5 per cent. As the 5.5hp (equivalent to 4kW) motor in Freezer 2 is old, the efficiency differential may be even higher. Overall, the difference in the motors probably accounts for about 10 percent of the difference in the performance between Freezer 1 and Freezer 2. The low power factors, particularly in Freezer 1, indicate that the motors are oversized for their application.

The oil refinery and biotechnology case studies contain detailed analysis of the impacts of motor loading on power factor and efficiency, and a comparison of energy use between different motors.

Differences in the motor systems and end uses

While the freezers appear to be identical, it is likely they have very different performance characteristics and actual end uses. The difference in power consumption observed can be explained by a multitude of system factors not immediately obvious to the naked eye.

Opportunities for improvement

To improve the performance of the meat freezers, the company would be well advised to start downstream. This means starting with some big picture questions, such as:

Are the freezers performing a useful function? For example, in this case, it may be that one or both freezers are seldom or never used and can therefore be turned off.
Are meat freezers the best way of providing this service? Perhaps the meat could be delivered cost-effectively on a daily basis as required so that freezing could be avoided. Alternatively, if meat is delivered frozen, it could possibly be defrosted slowly in a refrigerator, rather than being kept in a freezer.
Are the freezers fully utilised? For example, perhaps one freezer could be used instead of two, or the two large freezers replaced by two smaller freezers (one on each floor).
Is it cost-effective to replace the freezers with more efficient equipment? When equipment running-costs are high in comparison with the capital cost of new equipment, it may be cost-effective to replace existing equipment with new high-efficiency equipment.

The questions above are may appear simplistic, but they are of critical importance. Often the big picture questions are not asked, leading to poor overall performance.

Once you have asked these types of questions and selected the appropriate equipment for the service, then you can focus on the details of the system. System factors relevant to freezer operation and suggestions for improvement are investigated in Table 1.

Table 1 - System factors and improvements to freezer systems energy usage.
Factor	Affect on energy use Suggestions for improvement
Usage patterns and loading	The more frequently a freezer is used, the more the load on the system will increase, both to freeze the arriving goods and to compensate for air transfer. If items are being frozen, rather than being stored at below freezing temperature, significant amounts of energy will be used during freezing. Is the freezer door left open for long periods? If so, and if this is necessary, would plastic strips; flexible plastic swing-doors or other options reduce heat leakage? Otherwise can self-closing doors or door alarms be installed, or staff trained to take greater care in their use of the freezer? Can you avoid placing unfrozen goods in the freezer?
Siting	Solar radiation can significantly increase the load on a freezer. For example, a freezer located close to a west-facing exterior wall may experience a greater load due to solar radiation gains. Avoid placing freezers close to west-facing exterior walls where possible.
Door seals	The cleanliness and condition of the door seals influences air transfer between the freezer and ambient air. Clean and maintain door seals regularly as part of an overall maintenance plan.
Thermal bridging	Thermal bridging is the conduction of coolth (the opposite of warmth) to the outside of the freezer, bypassing insulation. Thermal bridging is often identified by the presence of condensation or mould growth on parts of the external surface that are colder than the rest of the cabinet surface. Purchasing high-efficiency, well-built equipment is the best way to avoid thermal bridging.
Damage to cabinet and insulation	Is there any visible damage to the cabinet? Higher heat flow through areas of insulation that no longer perform as designed due to damage leads to wasted energy. Are there cold spots or signs of icing up that may indicate excessive entry of outside air into the cabinet? If so, can these paths be sealed?
Ambient air temperature	The larger the differential between the freezer temperature and ambient air temperature outside of the freezer, the greater the load on the system. Try to locate freezers in cool environments, and avoid placing near heat producing equipment such as ovens.
Condenser fan type, number, controls, evaporator fan selection, compressor efficiency, refrigerant type and charge	The more efficient and well controlled the system components, the better the energy performance of the freezer itself. Too much or too little refrigerant can have a major impact on performance. Are you sure the refrigerant charge is correct and that there are no leaks? A possible refrigerant leak should be checked. Are the evaporator and condenser clean and fans operating effectively? Have the operation of your refrigeration equipment checked at regular intervals by contractors. Clean condenser coils regularly as part of an overall maintenance plan.
Internal loading due to equipment type, not usage	The internal load on the freezers will be influenced by the efficiency and total power consumption of the internal lighting and evaporator fans - the more efficient the lighting and fans, the less heat they will generate inside the freezer. Purchase high efficiency equipment.
Temperature set-point and dead band	The colder the set point, the larger the load on the motors. This is due to the larger difference between the freezer temperature and ambient temperature. In addition, the size of the dead band affects efficiency - with very narrow dead bands the compressors cycle on and off too much. Don't set the freezer any colder than it needs to be. Ensure the dead band is not too narrow.
Motors	The efficiency and power factor of the freezers could be improved by matching the motors to their respective loads - small motors have poor efficiency and power factor at low load. Match motors to the load.

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