Chiller seasonal energy consumption in historical buildings

A plenty of studies are devoted to estimation methods of seasonal energy consumption of cooling equipment. Several computing instrument give the ability to calculate or simulate chiller’s electricity consumption at variable conditions. Using European Seasonal energy efficiency ratio (ESEER), provided by equipment suppliers, engineers and project developers can make fast and rather rough calculations on annual cost in order to determine the economic feasibility for choosing the particular type of cooling equipment. And the question is, how precise those calculations could be for temperate and northern climates?

As a part of the research on evaporative cooling appliance in temperate climate of Baltic States, conducted in Latvia in 2011–2014 [1], an analytical research made in recently restored 19th century historical building, The Art Museum Riga Bourse.

Restoration of old buildings is a complex construction process, in which engineers and architects need to solve many atypical tasks concerning not only the structural stability of the building, but also the recovery of cultural—historical appearance of the building. Necessity of harmonious integration of modern HVAC devices in the historical interior also enforces limits to the equipment selection. The successful solving these and construction tasks results in a balanced and sustainable restoration of the building.

The analytical research overtakes the results of water cooling system operation for one year cooling period. The data of electricity and water consumption, chiller operation modes and cooling system temperature data were logged with one-minute interval. The duration of analysis period was chosen, based on the building cooling demand. The data obtained were converted to average hour values after the analysis of errors. The calculated cooling output depending on the outdoor air (OA) temperature is displayed on Figure 2 below.

Figure 2.The calculated cooling output depending on the outdoor air (OA) temperature.

The information on equipment efficiency that is necessary for seasonal energy consumption calculations is limited. Usually, energy efficiency ratio and the seasonal efficiency of the device are available. The industry has implemented ESEER (European Seasonal Efficiency Ratio), recently published regulations or those in the process of being published talk about seasonal coefficient of performance (SCOP) in heating mode, and its equivalents SEER and SEPR in cooling mode. Seasonal energy efficiency, in accordance with EN14511:3 – 2013 and [2] is calculated according to the Equation explained below:

ESEER = A ´ EERA + B ´ EERB + C ´ EERC + D ´ EERD,                (1)

Where EER = ratio of the total cooling capacity to the effective power input of the unit, Watt/Watt.

Table 1.ESEER components description.

In authors’ opinion ESEER introduction to the market was a great achievement within the equipment assessment principles, which is likely to reflect accurately the effectiveness of Central Europe or the Mediterranean part of Europe. On the other hand, when in Latvian climate conditions the building cooling load is traditionally calculated using an outdoor air temperature of +27°C (Riga), ESEER test parameters are quite distant from the real situation. Practice shows that the cooling demand in many objects occurs at much lower outdoor temperatures such as +15°C or even +10°C. Such objects may include: office premises with large amount of office equipment, facilities with high human density, retail spaces with great light intensity, and rooms with large windows surfaces and no shadows. The location of ventilation diffusers does not always provide cooling in the most comfortable way such as displacements ventilation, for people in the premises. In those cases the air cooling is carried out with chillers and the room terminal units like chilled beams or fan-coils. Sonderegger (1998), based on a number of energy efficiency project analysis pointed to a large inaccuracies in heating / cooling system energy savings estimates, if they are performed only based on the weather data [3].

The values of cooling energy produced at a certain temperature ranges with a step of 2.0°C are presented in Figure 3. The vast majority or about 67% of cooling energy during the cooling period was produced when the outside temperature was in the range from 10 to 20°C. According to the generally accepted practice, the cooling is required when ϑOA is higher than 18–19°C, (which is also generally accepted base temperature for cooling degree-hour calculation). Within the particular building non-weather dependent cooling accounts for more than half of the total annual cooling energy. This characterizes the objects with high heat gains, and / or high microclimate requirements.

Figure 3. Cooling energy produced at outdoor air temperature diapasons.

ESEER impact factors at the same outdoor (condenser inlet) temperature distribution for comparison are shown in Figure 4 below.

Figure 4.ESEER impact factors at the same outdoor (condenser inlet) temperature distribution for comparison.

The acquired data in art museum showed cooling system showed significant difference on cooling energy produced on site and ESEER weighing methodology. Due to that, the more precise coefficient distribution method is offered. The calculation principle can be expressed as follows:

(1)

Where:

Q_el.seas. =        electricity consumption during cooling season, kWh

 =            outdoor air temperature interval

Q_c.nom. =         nominal cooling load of the device in standard conditions, kW

EER =           energy efficiency ratio at a given interval

CL =              cooling load at a given temperature interval, % or part of 1, from nominal device capacity

GS =              the number of cooling degree hours at a given temperature interval, h

Q_el.st. =           standby electricity consumption, kWh

Using the proportional distribution method, seasonal energy consumption calculations are performed for combined compression cycle - evaporative chiller (KKCD). For comparison, one calculation is done for close to the ESEER temperature / load intervals. Cooling degree hours (CDH) was taken, using Latvian Typical Meteorological year data in temperature diapason ±2°C. We can see that it overtakes outdoor air temperatures from 18 to 37°C and the total electric energy consumed is 24 128 kWh for the chiller with maximum cooling capacity 320 kW (Table 2):

Table 2.Seasonal energy calculation using ESEER values and summarized CDH.

However, the actual yearly electricity consumption of the chiller, logged by electricity meters is 62 000 kWh. Using the above mentioned formula (2), proportional method calculation has been performed for 7 outdoor temperature / load intervals. EER data which was out of ESEER conditions was executed from laboratory experiments on the similar chiller. The portion of cooling energy at ϑOA less than 10°C, which was produced at the investigated site during the cooling season accounted for 5.9% of the total energy produced. This part of the calculation is added to the calculation due to the equipment operating efficiency drawbacks within this range (Table 3).

Table 3. Seasonal energy calculation using 7 intervals with summarized CDH.

The results showed much higher precision when using 7 interval proportional methods for the investigated building. It could be proposed to extend the ESEER part load test methods to wider TOA and cooling load ranges. Surely, further investigations on real cooling load should be made in different types of buildings to increase the amount of trustable data. These factors could help to increase the accuracy of economic and energy consumption calculations, taking into consideration peculiarities of various object types and conditions of northern climate.

References

[1]     Brahmanis A., Indirect evaporative cooling in air conditioning systems. Doctoral Thesis, Riga Technical University, Riga, 2014. – 119 p.

[2]     Marinhas S., Eurovent chiller certification key stones and future challenges. REHVA Journal – March 2013. – p. 31–33.

[3]     Sonderegger R. C., A Baseline Model for Utility Bill Analysis Using Both Weather and Non-Weather Related Variables. ASHRAE Summer Meeting, Toronto, Canada, June 18-25, 1998. – 10 p.

[4]     Zariņš M. Latvian climate data processing for air-conditioning systems optimization. Master Thesis. Latvian University of Agriculture, Jelgava, 2002. – 115 lpp.

[5]     Stankevica G., Varavs V., Kreslins A. Trends in Cooling Degree Days for Building Energy Estimation in Latvia. Construction Science 14, 2013. p. 89–94.