Maybe you’ve heard the term TEWI (pronounced “teewee,”) an acronym for Total Equivalent Warming Impact. But what exactly is TEWI and why do we use it?
TEWI is a measure of the greenhouse gas emissions (i.e. the global warming impact) associated with the use and disposal of a particular piece of refrigeration equipment. This is important for understanding the lifecycle impact of your equipment and ultimately may help save you save on costs later down the line.
So let’s break this down.TEWI is comprised of two parts:
- The warming impact from direct refrigerant emissions (from leaks, servicing losses and end-of-life disposal)
- The warming impact of CO2 emissions from burning of fossil fuels to generate the electricity needed to run the equipment.
We can calculate TEWI to compare existing pieces of equipment or to predict the warming impact of newly designed equipment. In the case of the later, leak rate and electricity usage will be assumed and not based on actual data.
TEWI = Direct refrigeration emissions (in MT of CO2-equivilant) + Indirect CO2 emissions from electricity generation (in MT of CO2)
We know that HCFC and HFC refrigerants are greenhouse gases (GHG) that have really high global warming potentials (GWP). We also know that we want to reduce leaks as much as possible, not only because its expensive to replace refrigerant and because an under-charged system is less efficient, but because emissions of greenhouse gases contribute to global warming. So if you want to determine the environmental impact of running your equipment, it’s important to account for the GWP of the refrigerant, and the amount of refrigerant leaked over time. The direct emissions component of TEWI is a way to capture the climate impact of all of the refrigerant emissions over the lifetime of the system.
We also know that big refrigeration systems, like a centralized DX system that does all of the cooling for a typical supermarket, use a lot of electricity. Take your standard DX centralized system in a supermarket, maybe it runs on R-404a, or R-22, or perhaps it’s a recent retrofit to R-407a. Regardless of the refrigerant, that system uses quite a bit of electricity. In most cases that electricity is generated from burning fossil fuels (e.g. natural gas), which means that CO2 was emitted to the atmosphere in order to provide that electricity. Those CO2 emissions are what we call “indirect emissions”—they aren’t coming directly from your DX system, but we have to account for them to accurately capture the emissions associated with running that DX system.
In order to capture the indirect emissions associated with electricity generation, you need to know (or have a regional baseline) for what energy source your electric utility uses. That way you can figure out how many kWh of electricity are generated for every MT of CO2 released into the atmosphere. (MT stands for metric ton, which is equal to just over 2,200 pounds.) The amount of CO2 emitted per kWh generated, times the number of kWhs consumed each year, times the expected lifetime of the system will give you the indirect emissions that are part of your TEWI equation (shown in the equation below as (Eannual x ß x n)).
Figure 1: TEWI calculation from the Best Practices Guidelines published by the Australian Institute of Refrigeration, Air Conditioning & Heating
Of course, the devil is in the details. Determining accurate leak rates, electricity usage and the indirect emissions factor based on energy source(s) is not always easy. The Australian Institute of Refrigeration, Air Conditioning & Heating has a great guide to TEWI and best practices guidelines, available here, if you want to read more and determine how to do TEWI calculations for your own equipment and systems. End-users can always ask their OEM to provide TEWI estimates for any new piece of equipment.
In the words of Geoff Amos, of NASRC member NRMS Global (based in the UK): “Increasingly carbon (or CO2 equivalent) is a metric in other markets outside the US. With the increasing regulatory burden and future price hikes and availability issues that come with using HFC’s TEWI is another tool that supports the transition to natural refrigerants.” NASRC certainly hopes so!