Refrigeration units can be complicated to operate, especially when they are coupled to each other and connected to one or more cooling towers. Their energy performances are closely linked to the efficiency of their subsystems (compressor, pumps, exchangers, evaporators, etc.), each of which in turn has a nominal efficiency and an optimum efficiency. They are also heavily dependent on their environment (cooling towers, distributor pumps, network, etc.). Let’s review some of the solutions for improving their energy performances.
Before you do anything, install instruments to measure the overall COefficient of Performance (COP)
The first and most obvious step towards optimisation, and a prerequisite for any corrective action, is to monitor the refrigeration units and cooling towers. This will establish the COP for each unit and tower by measuring the flow rate, the inlet and outlet temperature and the electricity consumption. More importantly, though, it enables you to construct an overall COP that includes the distributor pumps, the auxiliary pumps and the consumption of the cooling towers, and factors in the climatic conditions (temperature and hygrometry). Ideally, of course, you should compile a data history for an entire year in order to cover all of the seasons and as many production conditions as possible.
1- Invest in new refrigeration technology
The latest refrigeration units have magnetic bearings and are particularly efficient, achieving a COP of at least 7 or 8 (as against 3 or 4 for a standard compressor). These refrigeration units are highly modular and do not become less efficient, because they contain several small compressors that start up in a staggered sequence. They are, however, very expensive. If your budget doesn’t stretch that far, significant improvements in energy consumption can be achieved by adding variable power control units to the existing compressors and pumps.
Potential gains: between 20% and nearly 50%
2- Add buffer storage
Adding a cold storage tank at the bypass makes it possible to run the compressor mainly at its rated speed, thereby reducing the peak requirement. It is therefore an alternative to cascaded refrigeration units, that also ensures greater electrical stability for the plant as a whole. Bulk storage can even enable the plant to take advantage of lower-priced energy at certain times of day, depending on its energy contract.
Potential gains: 5% – 10%
3- Change the type of energy used
Absorption refrigeration units operate on the principle of “turning heat into cold”. If the factory needs large amounts of heat, it can be useful to install a trigeneration or combined cooling, heat and power plant: gas heating, cogeneration of electricity and recovery of some of the waste heat by absorption/adsorption chillers. The electricity that powered the compressor – and was traditional refrigeration units’ main consumption – is completely eliminated from the picture!
Potential gains: depend on the cogeneration operating contract
4- Add “free cooling”
When the climatic conditions allow it often enough, adding passive outside-air heat exchangers (generally on the factory’s rooftop) can eliminate the need for cooling towers and the associated consumptions.
Potential gains: 5% – 15%
5- NO CAPEX REQUIRED: Optimise start-up sequences
When several refrigeration units operate at the same time, the question arises as to the best combination (since their power ratings and performance may differ) and the right time to bring them online. By referring to their technical specifications (performance curves, power, flow rates, etc.) and the data history, it is possible to construct start-up rules based on objective criteria: flow rate in the bypass, atmospheric conditions, production plan, etc. These sequences also factor in the cooling towers’ performances and their technical constraints (number, speed, start-up lead times, etc.).
Potential gains: 10% – 20%
6- NO CAPEX REQUIRED: Modify the set points
Data analysis frequently yields counter-intuitive discoveries that can generally be linked to systemic effects: “The overall optimum is not the sum of the local optima”, explains the expert Jean Vielle in Factor.e (Issue #3: “The importance of a systems approach”). In practice, it often happens that, when we slightly increase a cooling tower’s consumption (by simply lowering the set temperature), we reduce the consumption of the refrigeration units’ compressors even further. It takes solid work on the data and full-scale testing to determine the right settings.
Potential gains: 5% – 10%
7- (Almost) NO CAPEX REQUIRED: Try floating head pressure
Benjamin Franklin is not known only for flying a kite in a thunderstorm. Before founding the USA, he performed an experiment that showed that, when atmospheric pressure is low, liquid boils at a lower temperature. This is known as the floating head principle: lowering the compressor’s high-pressure setting based on the outside temperature (especially in winter) can save unnecessary “bars”. However, a specific regulator module will have to be purchased.
Potential gains: 5% – 15%
So, either with or without investing, rapidly or with a data history and a few studies of the energy data, there are a variety of solutions for optimising your energy consumption on refrigeration. And the more your EIS (Energy Information System) is well-documented, the more effective these solutions will be.
 This is very clearly explained here: