TEWI might sound like a funny word, a bit like kiwi, the New Zealand bird or the fruit.
Actually, the word has some specific uses in Japan, for a comic character relating to rabbits (!), and as a slang term for a good-looking baby-faced boy. 
Despite it’s nice sound, the most common meaning of TEWI (Total Equivalent Warming Impact) is not as well-known as it should be. The reason is quite simple: it’s pretty complicated!

Total Equivalent Environmental Impact

TEWI is used in the HVAC/R sector, and is described by a formula. That’s the first problem: formulas are not that popular, with very few exceptions, e.g. E=MC². In the TEWI formula, the impact of a refrigeration system is expressed by the sum of its direct effect and its indirect effect on global warming.

The direct effect is quite obviously related to emissions of the refrigerant used in the system, in other words its better-known GWP (Global Warming Potential). The indirect effect is the “equivalent direct impact” of the tonnes of CO₂ that are emitted to generate the amount of electricity consumed by the refrigeration system over its lifetime. Is this confusing? Yes! Intriguing? Maybe.

Certainly, this way of calculating the impact of a supermarket, for example, is fairer and more complete. It takes into account the amount of refrigerant and electricity used over the lifetime of the store, and thus is affected by the way in which it has been designed, installed and serviced. Not just by the GWP of the refrigerant. 

Despite this fact, all international directives and regulations that affect the HVAC/R sector are based on the GWP of refrigerants. Indeed, this value is based on a simple measurement of the different molecules and provides an easy way to define a rule (e.g. “all refrigerants with a GWP higher than 150 cannot be used starting from 20XX…”)

I personally think that the use of GWP has had an important positive effect on our sector. Without it, there would have been no rules addressing the huge impact of HVAC/R applications on our environment.

However, it can be shown that a well-designed system can have a low TEWI, irrespective the GWP of its refrigerant.

A case study on supermarkets

This study is focused on a supermarket installation in Portugal that follows the current market trends in the HVAC/R industry. The main need of the retailer involved was to comply with EU legislation to control F-gases emissions, without compromising the quality of products and maintaining the high level of energy efficiency that defines their installations.

The installer proposed a refrigeration system based on semi plug-in cabinets connected to a waterloop, integrated with the air conditioning system. 

The cabinets were water-cooled and equipped with DC inverter compressors and electronic expansion valves to allow precise cabinet/cold room temperature control with the highest energy efficiency. Each cabinet works independently from the others.

Compared to compressor rack-based systems, semi plug-in cabinets with modulating capacity have the capability to keep a constant air/product temperature within a range of +/-0.1K around the given set point and without the typical intermittent start-stop behaviour of fixed capacity/flow solutions. Moreover, independent semi plug-in cabinets and cold rooms are able to work with continuous control on their evaporation temperature, instead of all being forced to work at the same value due to their being connected to a compressor rack. This provides higher efficiency without unnecessary waste and better food and beverage quality due to the almost complete absence of temperature fluctuations.

The main reasons for adopting this kind of solution were related to a streamlined and flexible store layout, a reduction in the area usually required by technical equipment such as compressor racks, a reduced refrigerant charge and full recovery of the heat from cooling the display cabinets, used by the building’s heating system.  
 

Refrigeration and air conditioning system diagram

Here is a quick summary of the store’s characteristics: 

• Sales area: about 1,200 m²
• Refrigerated units: 17 medium temperature cabinets with 5 cold rooms and 1 low temperature cabinet with 1 cold room.
• Cooling capacity: about 61 kW, of which 52.5 kW for MT and 8.5 for LT

The air conditioning system, integrated into the waterloop, has a reverse-cycle air/water chiller, a water/water chiller for the low temperature cabinet loop, air handing units and fan coils for indoor climate control. This system adjusts its configuration according to the seasons. There are several operating modes: in heating mode, the heat produced by the cabinets is used for store space heating. In cooling mode, the air conditioning system provides cold water to cool the cabinets, while in the mid seasons the unnecessary heat produced by the cabinets is handled by dry coolers outside of the store (so-called “free cooling”).

Let’s then compare the TEWI of three stores

The store described above has been compared with two stores in the same area that use a centralised compressor rack cascade refrigeration system (R134a/CO₂) with rooftop/split air conditioning units and have a similar store area:

 Compared store data

Due to the different system layouts and technologies, including the refrigerant, the only way to compare stores is to calculate their TEWI.

The following table shows the TEWI parameters for the different stores and related calculation formula:

 
TEWI calculation parameters

The Perafita store, thanks to the semi plug-in cabinet layout with integrated air conditioning system, has a 65% lower TEWI, including a reduction in both direct and indirect emissions, as shown in the following figure.

 
TEWI comparison between waterloop store and stores with centralised system (average)

In terms of direct emissions, this result was achieved despite the higher GWP of the gas used in the Perafita store (2088) compared to the others (1022, calculated as the weighted average between the different refrigerants). In fact, both refrigerant charge and leakage rate are definitely lower in a semi plug-in layout compared to the compressor rack cascade, which requires long liquid lines with a higher refrigerant charge and plenty of potentially-weak braze welded joints, leading to a 12.5% annual leakage rate.
In terms of indirect emissions, the energy consumption of the 3 stores has been compared after being normalised for each store’s nominal cooling capacity. This includes both refrigeration and air conditioning.

Specific (normalised) energy consumption of the 3 stores.

Going back to the preamble, I understand perfectly that TEWI is not suitable to be used in international regulations to address the global warming impact of the HVAC/R sector. We needed months of data to calculate the TEWI values of these stores. Nonetheless, good design and environmental sustainability are not necessarily driven only by regulations, and luckily the awareness of stakeholders in our sector can do even more for our planet.
 

Related Posts
 

Five advantages of the water loop system for refrigeration in supermarkets

One more step towards a “greener” planet