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A better way to calculate energy efficiency for polyvalent units

There are several energy efficiency indexes used in the refrigeration business. However, not all of them are applicable to all types of units – specifically, there are some problems with using existing indexes for polyvalent units. But first, let us go through some of the different indexes out there.

Traditionally, the energy efficiency for chillers and heat pumps has been expressed in two indexes – EER for chillers and COP for heat pumps. Simply put, EER (Energy Efficiency Ratio) is the amount of cooling energy produced for each kW of electrical energy used by the chiller, and COP (Coefficient Of Performance) is the amount of heating energy produced for each kW of electrical energy used by the heat pump.

Both these indexes are highly theoretical, since they are calculated for maximum capacity at arbitrarily chosen “standard” conditions (for example, at 35°C temperature of the outdoor air for the chillers). These conditions rarely, if ever, occur.

Two new indexes for energy consumption
To achieve a more realistic view of the total energy consumption, the European Commission introduced two new indexes which give a better idea of the actual energy consumption during year-round operations: SEER (Seasonal EER) and SCOP (Seasonal COP). The energy efficiency in these indexes is weighted for different outside conditions over the year. SEER and SCOP minimum levels are mandatory for chiller and heat pump units sold in EU countries.

However, neither SEER nor SCOP is really adapted for another type of unit: the multi-purpose polyvalent unit. Polyvalent units can function as both chillers and heat pumps, making them eminently flexible and adaptable for a wide variety of uses. Not only that, polyvalent units can transport heat within a building – for example taking heat from a warmer part of the building to heat up a cooler part – bypassing the need to heat or cool outside air altogether.

The polyvalent units can thus not be optimized for either chilling or heating, but have to be pretty good at both. This penalizes polyvalent units when SEER and SCOP are used, especially since these indexes do not take the heat recovery functionality in polyvalent units into account at all.

New index STER - Seasonal TER
A better energy efficiency index for polyvalent units is TER (Total Efficiency Ratio), used by several companies, including Swegon. TER is defined as the total energy produced for both heating and chilling for each kW of electrical energy used by the unit. But like EER and COP, TER is a highly theoretical index, and polyvalent units almost never work in the conditions shown by TER.

Which brings us to STER – Seasonal TER. This index, like SEER and SCOP, is weighted for year-round conditions and has been developed by Swegon together with Clivet-Midea. STER has been proposed to the Eurovent association, and the proposal has been accepted. This means that Eurovent will form a dedicated workgroup consisting of members from several producers, with the goal to create a common standard for calculating STER, and include STER in the guidelines from Eurovent.

In the long run, we hope that STER will become a European standard on the level of SEER and SCOP.