STEFAN JENSEN, SCANTEC

All industry sectors across the world ranging from utilities, manufacturing, food preservation through to comfort cooling, food storage and transportation face unprecedented challenges concerning energy efficiency improvements and emissions reductions.

The International Energy Agency (IEA) has called for an improvement in the energy efficiency of air conditioners of 50% by 2030. The Global Cooling Prize targets an improvement in the energy efficiency of air conditioners by a factor of five.

Huge public and private sector investments are targeting the de-carbonization of the electricity grid. Private sector investments in photovoltaic panels have reached unprecedented growth rates in some nations.

Norway banned the use of fossil fuel for space heating in 2020. Several major vehicle manufacturers have signaled the discontinuation of the internal combustion engine by 2030.

The ammonia refrigeration industry will not be immune to these challenges. The Paris Climate Treaty, which targets carbon neutrality 2050, will force legislators and regulators to focus their efforts where the rubber hits the road – namely at minimum energy performance benchmarking.

What is minimum energy performance benchmarking? Minimum energy performance benchmarking is when legislators implement a regulatory framework that requires compliance with maximum energy consumption values of systems by all stakeholders.

Typical metrics are maximum energy consumption in kWh per ft² of lettable area for commercial buildings. For public housing in both the U.S. and the EU, maximum specific energy consumption values have already been set, partly to address the issue referred to as “energy poverty”. Energy poverty is a result of split incentives between developers and tenants.

Other typical metrics are kWhn ft3n year-1 for refrigerated warehouses, kWhn ton-1 for blast freezing operations and ice manufacture, and kWhn l -1 for liquid chilling operations and ice cream production.

It is also likely that Coefficient of Performance (COP) and/or EER compliance requirements for unitary equipment will be a thing of the past soon. Compliance will then require efficiency calculations at several operating points. This directs the focus of systems and equipment providers at energy efficiency at part load.

Examples of this are the EU major projects for the RACHP industry namely (EU) No. 2015/1095 and (EU) No. 2016/2281. Compliance with Tier 2 of the latter will be required effective January 2021 and will apply to heat pumps, chillers for air conditioning, and high-temperature process chillers.

For compliance, the Seasonal Energy Performance Ratio (SEPR) must be verified at four reference points representing the annual temperature profile.

Why is minimum energy performance benchmarking a regulatory development that the ammonia refrigeration industry must prepare for? It is because it is a matter of maintaining and growing market share for this industry segment.

Most practitioners within the ammonia refrigeration industry are probably of the view that ammonia refrigeration systems are very energy efficient. It is certainly correct that some are. However, there are published examples of ammonia refrigeration plants consuming ten times more energy than they need to.

A common metric used for energy consumption in refrigerated warehouses is Specific Energy Consumption (SEC) in kWhn m-3n year-1 (kWhn ft-3n year-1) where volume refers to refrigerated volume.

Figure 1 shows recorded SEC’s for a range of different plant concepts all originating from a single entity. The small green star represents a relatively small centralized, low charge NH3 system in the subtropics of the Southern hemisphere.

The introduction of a minimum energy performance benchmark represented by the equation:

where “V” is refrigerated volume in m³ would likely make >90% of the facilities shown in Figure 1 non-compliant. Unless the ammonia refrigeration industry understands the measures needed to ensure compliance, it will inevitably lose market share in a regulatory environment as described above.

How then may compliance with a minimum energy performance benchmark be assured?

The simple answer to this question is renewed focus on system part load energy efficiency.

Almost all refrigeration systems are oversized. This is a practice that has evolved because of the practical experiences gathered by industry stakeholders throughout the existence of mechanical refrigeration. This is not necessarily a bad thing, but the consequences of oversizing on energy performance may not be widely understood.

Most NH3 based liquid overfeed refrigeration systems include no means of regulating the capacities of the NH3 pump flow(s) as a function of load. If therefore, the system is designed for an overfeed rate of three to one at full load, the overfeed rate becomes 15 to 1 at 20% load.

Refrigerant pipeline pressure drops increase as a function of rising overfeed ratios. Wet return line and riser pressure drops can be up to sixty times greater than the pressure drop in equivalent pipelines conveying saturated vapour only.

What is more concerning is that the consequences of excess overfeed ratios and riser flow reversals in terms of overall system energy performance remain largely unidentified and unquantified.

The system design changes that are required to improve the energy efficiencies of new ammonia refrigeration systems challenge some very deeply entrenched design practices throughout the ammonia refrigeration industry globally.

The system modifications required within existing ammonia refrigeration plant to reduce the energy consumption of these are equally challenging but in a different way to new plants.

Many plant owners/operators are likely to question the relevance of “interfering” with a perfectly good system that has given no trouble in the past. Presenting a compelling business case in these scenarios will require dissemination and presentation of unbiased technical information.

Disseminating and presenting unbiased technical information is an area where IIAR has a long record of excellence. Improving the energy performance of new and existing ammonia refrigeration plant is an obvious area for IIAR to continue to excel.