Andy Pye looks at how variable speed drives help keep things like food and medicine cool in industrial refrigeration applications.
For around 200 years, refrigeration technologies have progressively aided the provision of essential goods and services, such as:
- Food preservation
- Medical processes
- District heating and cooling
Many refrigeration and air conditioning systems require reliable processes which are more efficient, compact, environmental friendly, easy to install and to maintain. The cooling requirements vary over a wide range during the day and over the year due to such factors as ambient conditions, occupancy and use, and lighting.
There may also be the need for a stable and accurate temperature and humidity control in areas such as hospitals, IT and telecoms. In applications such as schools, restaurants and office buildings, it is important that the cooling system is able to adapt to wide daily shifts in load.
In process cooling applications such as fermentation, growing tunnels and industrial processes, accurate temperature settings are required to secure production quality.
There are several ways to modulate the cooling capacity in refrigeration or air conditioning and heating systems. The most common in air conditioning are: simple on-off cycling, hot gas bypass, use (or not) of liquid injection, configurations of multiple compressors and inverter technology.
Capacity modulation is a way to match cooling capacity to cooling demand to application requirements. The compressor is specifically designed to run at different motor speeds to modulate cooling output. An inverter controls the compressor motor speed to modulate cooling capacity.
Variable speed technology can be implemented in HVACR, close control and process cooling applications and in packaged or split air-conditioning units, rooftops, chillers, precision cooling, VRF and condensing units.
Using high performance drives with permanent magnet motors maximises energy efficiency in industrial refrigeration products. Typical installations can see large increases in Coefficient of Performance (COP) of around 15%, resulting in huge energy savings and lower life cycle costs, with the return on investment often in less than 12 months.
The compressor and drive need to be qualified to work together and for dedicated applications. The drive modulates the compressor speed and also prevents it from operating out of the compressor operating limits. The inverter frequency drives need to use algorithms developed specifically for heating, ventilation and air conditioning (HVAC) or for refrigeration – these ensure that the system will run within the application constraints.
Variable Refrigerant Flow (VRF) units are very popular cooling or reversible systems for heating and cooling. They combine flexibility for building owners and occupants alike, with energy efficiency, high comfort, and ease of installation, without compromising on reliability. VRF systems extensively use inverter technology.
Examples of energy savings using variable speed drives
The installation of a large free-standing variable speed AC drive from the Unidrive SP range has improved temperature control in the chilled rooms of Portadown (Northern Ireland) food manufacturer Henry Denny, and is well on the way to delivering a massive saving of around £23,400 in their annual electricity bill – a cut of 50% of the plant’s total power usage!
The company manufactures a wide range of pies, sausage rolls, pasties and cottage pies and a complex of 20 chilled rooms stores a huge amount of not only the company’s own production output, but, as the site acts as a regional distribution depot for Northern Ireland, it also holds many thousands of cheeses, butter, hams and bacon from other companies in the Kerry Group.
Elsewhere, in Calgary, Canada, energy savings of 35% on a refrigeration plant are being achieved at a grocery business. Here, a new cooling system comprised of two new compressors and condenser coolers and the company needed to minimise the operating costs. One variable speed Commander SK AC drive per cooler controls all the fans simultaneously. The drives are integrated into a custom-designed panel.
For most of the year, the condenser runs at a fraction of its full capacity. In the old system, condenser cooling control was achieved using contactors to switch the fixed-speed fans off and on in pairs. This resulted in the fans constantly cycling and failing to provide the fine temperature control that is required.
Because the pressure set-point was constantly being overshot, excessive expansion and contraction of the refrigerant resulted in mechanical fatigue of the lines. In time, this could lead to failure of the piping or connectors.
Four motors running at half speed use much less energy than two motors at full power. In addition, smooth changes to fan speed use less energy than repeated start-ups. In this VFD-controlled system, not only is there a significant saving in energy, but pressure and temperature are regulated very accurately resulting in less stress on the entire system.
Applicable legislation
The standard and legislation which apply are ISO 50001 (the Energy Management standard) and the Seasonal Performance Part Load Efficiency Legislation (SEER).
ISO 50001:2011 specifies requirements for establishing, implementing, maintaining and improving an energy management system. Its purpose is to enable an organisation to follow a systematic approach in achieving continual improvement of energy performance, including energy efficiency, energy use and consumption. ISO 50001:2011 applies to all variables affecting energy performance that can be monitored and influenced by the organization. It specifies requirements applicable to measurement, documentation and reporting, design and procurement practices for equipment, systems, processes and personnel that contribute to energy performance.
In Europe, the seasonal efficiency of refrigeration equipment, chillers and air conditioners is often rated by the European Seasonal Energy Efficiency Ratio (ESEER) which is defined by the Eurovent Certification Company. In the United Kingdom, a Seasonal Energy Efficiency Ratio (SEER) for refrigeration and air conditioning products, similar to the ESEER but with different load profile weighting factors, is used for part of the Building Regulations Part L calculations within the Simplified Building Energy Model (SBEM) software, and are used in the production of Energy Performance Certificates (EPC) for new buildings within the UK and the European Union; both as part of the European Directive on the energy performance of buildings (EPBD).
The formula for SEER can be presented as follows:
SEER = a (EER@25% load) + b (EER@50% load) + c (EER@75% load) + d (EER@100% load)
where a,b,c,& d are the load profile weighting factors relevant to the proposed application.
A similar standard to ESEER in the United States is the integrated energy efficiency ratio (IEER).
The ESEER is calculated by combining full and part load operating Energy Efficiency Ratios (EER), for different seasonal air or water temperatures, and including for appropriate weighting factors.