There has been increasing concern on the environmental impact of the products and equipment that we use in our lives. In particular about HVAC&R products, there are two commonly used (and widely accepted) terms to calculate the greenhouse impact of a product/equipment over its lifetime. This is given in terms of its equivalent CO2 emissions, in kg or Tons depending upon its absolute value.
The terms used are:
i) Total Equivalent Warming Impact (TEWI), and
ii) Life Cycle Climate Performance (LCCP).
The second term is an extension of the first, in that it includes even the warming impact due to the manufacturing of the product including the energy consumed during the
manufacture of the raw materials. For example take a window AC. The coils are made of copper tubes and aluminium fins. If the copper tube and/or aluminium finstock manufacturing process is highly energy inefficient, it will reflect adversely on the LCCP of the window unit. Though it sounds far-fetched, it has its merit in evaluating the overall impact. But now let us talk about the main TEWI calculation.
The following equation gives the expression for calculating TEWI for a particular product (mind you, it cannot be generalized; it is application/product specific):
TEWI = GWP x L x n + GWP x m x (1-α) + n x E x β (1)
GWP - Refrigerant Global Warming Potential (kg of CO2/kg of refrigerant)
L - Annual leakage rate (kg/year)
n - System operating life time (years)
m - Refrigerant charge (kg)
α - Recycling factor (%)
E - Annual energy consumption (kWh/year)
β - CO2 emissions on energy generation (kg CO2/kWh)
The above equation is fine considering that it takes into account both the direct (due to refrigerant leakage/non recycling) and the indirect (due to energy consumption) effect.
However I feel it has a shortcoming and hence I would like to propose a new term, viz. LCWI (Life Cycle Warming Impact). The main difference between my proposed equation (see below) and TEWI calculation is in the 3rd term i.e. energy consumption.
Equation (1) above uses only one figure, E, for calculating total energy consumption. This is rather simplistic, because over the AC’s lifetime its energy consumption can never be the same. More often than not the consumption increases because of some inherent inefficient happenings. For example, with an air cooled condenser, the fins get choked with dust and dirt, which reduces the air flow and the compressor discharge pressure increases. In case of a water cooled system, scaling and corrosion due to poor water quality affect heat transfer in the condenser, which again results into rise of condensing temperature. On the other hand, there are many new
I am therefore proposing a modified equation that will include a weighting factor η for energy efficiency improvement/ degradation over a product’s lifetime.
The new equation will look like this:
LCWI = GWP x L x n + GWP x m x (1-α) + n x E x β x η (2)
All the terms in Equation (2) are the same as (1), except η. The value of η can be more or less than 1 depending upon how the product/equipment is used/maintained. Rigorously speaking, there should be multiple η values (η1, η2 , .....etc.) for each year. But that will make the computation complex and hence, to start with, I am proposing a single weighted average value. This approach can help the customers to select the right product for an application. The objective of writing this is to get some responses from ISHRAE members on my proposed equation and take it forward.