Analysis
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Energy indices

Increasingly closer attention is being paid towards the power consumption of air-conditioning equipment, both in Europe and elsewhere.
As regards air-cooled water chillers, the benchmark is the power draw at rated conditions: external air temperature of 35°C and water entering the evaporator at 12°C and leaving it at 7°C. These are the standard conditions used for air-conditioning purposes in the Mediterranean area.
In this case, the parameter used to indicate efficiency is the ratio between the cooling power delivered and the total power draw of the unit (compressors + fans).
This parameter is usually called EER (Energy Efficiency Ratio) or COP (Coefficient Of Performance). The first is generally connected with summer operation while the second is referred to winter operation (if the unit in question is a heat pump, therefore).

For many years in the United States, reference has not just been made to efficiency at rated conditions. A valuation index is also used which considers marginal operation of the unit at rated conditions as well as increased usage in part load conditions when the external air temperature is lower than the rated value and when the separation stages of the cooling compressors are used.

This takes into account the fact that, in most conditioning systems, the lower the external air temperature is, the lower the ambient load gets. This is due both to the reduced amount of heat transmitted through the walls of buildings and to the reduced amount of cooling power required to treat the intake air.

The valuation index adopted in the United States is called IPLV (Integrated Part Load Value) and is defined in the regulations issued by ARI (American Refrigeration Institute).

The most recent formulation of this index, issued in 1998, is:

ARI standards




where:

EER100%, EER75%, EER50% and EER25% are the efficiencies of the chiller in the various load conditions (100% - 75% - 50% and 25% respectively), calculated in the external air temperature conditions shown below.
The temperature of the water leaving the evaporator is considered constant at 6.7°C in all load conditions, with a delta of 5°C in the full load condition.
The multipliers 1, 42, 45 and 12 are respectively the percentage weights of the power produced at the various load conditions, referred to seasonal operation in the chiller mode.


IPLV ARI index

 
Load
100%
75%
50%
25%
Compressor inlet air temperature
35°C
26,7 °C
18,3°C
12,8°C

As can be seen, the part load distribution used by ARI supposes that the chiller in question works at a 25% part load with an external temperature of 12.8°C and at a 50% part load with an external temperature of 18.3 °C.

Such a wide-ranging external temperature field, 35°C to 12.8°C, allows us to deduce that the American regulations are based on a very large number of operating hours. It is therefore easy to see that the full-load operating time percentage of the chiller is extremely low and, consequently, its weight in the IPLV formula amounts to just 1%. The ARI standards also suppose that during the period in question the unit delivers 45% of its cooling power with an external temperature of 18.3 °C.

In normal European applications it is difficult to think of a chiller working at an external air temperature lower than 20°C. Air-conditioning is never used to this extent; design and legislative constraints tend to reduce power consumption and encourage the adoption of systems based on the direct or indirect use of external air wherever possible (FreeCooling).

There are at least two proposals on this issue, one promoted by Eurovent and another, more recent, proposed by Prof. ADNOT of ARMINES in the EECCAC (Energy Efficiency and Certification of Central Air Conditioners) European work programme. (1)

The formulations of the various indices are:

Eurovent proposal:


where:
EER100%, EER75%, EER50% and EER25% are the efficiencies of the chiller in the various load conditions (100% - 75% - 50% and 25% respectively), calculated in the external air temperature conditions shown below.
The temperature of the water leaving the evaporator is considered constant at 7°C in all load conditions, with a delta of 5°C in the full load condition.
The multipliers 10, 30, 40 and 20 are respectively the percentage weights of the power produced at the various load conditions, referred to seasonal operation in the chiller mode.


European IPLV index

Load
100%
75%
50%
25%
Compressor inlet air temperature
35°C
30°C
25°C
20°C

EECCAC proposal:



where:
EER100%, EER75%, EER50% and EER25% are the efficiencies of the chiller in the various load conditions considered by the method (100% - 75% - 50% and 25% respectively), calculated in the external air temperature conditions shown below. The temperature of the water leaving the evaporator is considered constant at 7°C in all load conditions, with a delta of 5°C in the full load condition.
The multipliers 3, 33, 41 and 23 are respectively the percentage weights of the power produced at the various load conditions, referred to seasonal operation in the chiller mode.

Load
100%
75%
50%
25%
Compressor inlet air temperature
35°C
30°C
25°C
20°C

Table 1 shows the formulations of the various part load efficiency indices in a different way:


Tab.1: parameters for calculating the ESEER, European IPLV and IPLV ARI indices


Index
ESEER
European IPLV
IPLV ARI
% part load
Air temperature
Weight coefficient
Air temperature
Weight coefficient
Air temperature
Weight
100
35°C
3%
35°C
10%
35°C
1%
75
30°C
33 %
30°C
30%
26,7°C
42%
50
25°C
41%
25°C
40%
18,3°C
45%
25
20°C
23%
20°C
20%
12,8°C
12%

The ESEER and European IPLV indices maintain the calculation “structure” of IPLV ARI but adopt different external air temperatures and weight coefficients. We should remember that the “weight” is the quantity of energy produced and therefore absorbed by the unit, at the various load conditions, referred to seasonal operation in the chiller mode.
In particular, the external air temperatures are identical for the ESEER and European IPLV indices and these are clearly higher than those used by the IPLV ARI. Being calculated in less heavy conditions, the IPLV ARI index underestimates power consumption compared with the other two European indices.
The purpose of the Eurovent and EECCAC formulations, in fact, is to make these values more consistent with the external air temperatures, construction criteria and typical uses of installations in Europe by proposing a more “targeted” benchmark index, especially as regards air-conditioning applications with air-water chillers.

Using the energy indices
After establishing which index to use and estimating the total power required by the system in the summer mode (in kWh), we can calculate seasonal electricity consumption (in kWh) using the following formula:

Power absorbed = Power requested / Index of efficiency

It is important to point out that an energy calculation obtained using the efficiency index at part load, regardless of which formula was used to obtain it, substantially represents a “static” situation based on four well-defined load situations, with which real operation is simulated to a fair degree of accuracy.
The real power calculation can be obtained more correctly in a “dynamic” form, that is, considering the load performance curve at different external temperatures, the location and the reference number of operating hours. It would therefore not be correct to compare the power consumption of two systems with different dynamic characteristics, just as it would not be correct to compare “dynamic” power calculations with “static” ones.

However, even if the calculations formulated with the efficiency indices are approximate, they are certainly more realistic than the pure comparison of power consumption based on full load operation in rated conditions using EER or COP parameters.

For this reason, in the international ISO TC 86 SC6 WG9 meeting, “Water chilling Packages using the Vapour Compression Cycle”, held in October 2003 at the ARI headquarters in Washington, it was decided to promote the publication by manufacturers of chillers of performance figures also at part loads.
These figures will allow plant consultants and designers to make their evaluations depending on the type of building, the place of installation and the type of heat load. etc.. They can also determine the energy index using the method that best reflects plant requirements and can make comparisons between similar or equivalent systems using the same reference unit.

(2) There are also other proposals that have not been given much attention up until now. A few years ago in Italy, AICARR proposed an index called EMPE (Average Weighted Summer Efficiency) which used the same weights as the Eurovent proposal but with slightly higher reference temperatures for the loads.



Climaveneta S.p.A