Energy calculations

Calculation method

The calculations were made on the basis of the data collected by the national energy experts. Several clarifications and consultations were needed in order to ensure the accuracy of the data and, eventually, of the energy calculations. There are slight differences among the SEARCH II countries in terms of the method used to calculate the overall energy performance of buildings. A common calculation method was therefore selected. In Hungary, the TNM Decree 7/2006 on the energy performance of buildings is harmonised with the Energy Performance of Buildings Directive and covers requirements, the design of the input data and the calculation method. The SEARCH II calculations were therefore made on the basis of this decree. Using a school-by-school approach, specific heat loss coefficients (W/m3K), primary energy for heating, DHW supply, air handling units, cooling and lighting (kWh/m2a) were calculated. Annual gas, district heating and oil consumption (kWh/a) were also calculated. The calculation was made initially based on the assumption that all the buildings were situated in the same location (Hungary) in order to be able to compare the results of the different schools. In a second step, the calculated heat consumption was corrected for each building with the heating degree day factor of the actual location. This latter calculation made it possible to compare the calculated energy consumption with the real energy consumption based on heating bills.

Specific heat loss coefficient

The specific heat loss coefficient was calculated school by school. This represents the total heat loss of the building structure, considering a 1°C difference between the indoor and outdoor temperature and a specified volume of heated air. In the initial calculation, Hungarian climate data were used for all buildings in order to be able to compare the results

(
Figure 40).

Distribution of specific heat loss coefficients



Most of the analysed schools had a specific heat loss coefficient of between 0.2 and 0.6 W/m3K. Within this range, the specific heat loss coefficient in the case of 39 schools was between 0.2 and 0.4 W/m3K, and in 32 schools it was between 0.4 and 0.6 W/m3K. In five schools, the specific heat loss coefficient was extremely high (over 1.0 W/m3K). An index had to be defined in order to compare the calculated specific heat loss coefficients: this is the building envelope index.

The building envelope index

In the TNM Decree 7/2006, the permitted value of the specific heat loss coefficient and the total primary energy for educational institutions are given as a function of the A/V ratio. A is the total of the external surfaces (walls+windows+roof+floor, around the heated air volume), and V is the heated air volume. These permitted values were used in SEARCH II as reference values. The building envelope index (qi) of a given school is the calculated value of the specific heat loss coefficient (q) divided by the reference value for the specific heat loss coefficient (qmax). If the qi is 1.0, then the calculated specific heat loss coefficient is equal to the reference value. If the qi is lower than 1.0, it means that the given building envelope has a better specific heat loss coefficient than the reference value, and that therefore the building envelope, in general terms, has a good level of energy efficiency. If the qi is higher than 1.0, it means that the given building structure has a worse specific heat loss coefficient than the reference value, and that therefore the building envelope, in general terms, has high specific heat loss and poor energy efficiency.

qi = q / qmax (2)

qmax is calculated using the following equations:

A/V ≤ 0.3 qmax = 0.2 [W/m3K] (3)

0.3 < A/V < 1.3 qmax = 0.38 (A/V) + 0.086 [W/m3K] (4)

A/V ≥ 1.3 qmax = 0.58 [W/m3K] (5)

A/V ≥ 1.3 qmax = 0.58 [W/m3K] (5)

In a school-by-school approach, the calculated values and reference values for the specific heat loss coefficient were compared in order to assign grades to the building structure.

Figure 41

Building envelope index by school

shows the distribution of the building envelope index. The minimum value for the building envelope index is 0.3 and the maximum is 3.98. The former building has 0.33 W/m2K outer walls, a 0.59 W/m2K roof, and 1.4 and 1.7 W/m2K windows; while the latter building has the far worse thermal characteristics of 1.51 W/m2K outer walls and roof, and 2.0 and 2.6 W/m2K windows. Only 15 percent of the buildings (14 buildings) have lower heat loss coefficients than the reference value. Nearly 50 percent have a heat loss coefficient over 1.5 times higher than the reference value. The average calculated specific heat loss coefficient is 1.64 times higher than the reference value, which means that most buildings have very poor thermal characteristics. The highest values for the building envelope index were found in schools in Tajikistan, Serbia and Bosnia and Herzegovina, and the lowest values in schools in Slovakia.

Total primary energy consumption

Total primary energy includes primary energy for heating, DHW supply, cooling, air-handling units and lighting. Primary energy was calculated for each school according to Hungarian weather conditions in order to make the results comparable. Primary energy needs were calculated according to the Hungarian TNM

Decree 7/2006. Schools in Slovakia were found to have the lowest primary energy consumption (105 to 147 kWh/m2a), with an average of 127 kWh/m2a. The Slovak schools had the best building structure out of all the analysed countries: schools had two-pipe heating systems with good control equipment; almost every school lowered the heating at night and/or at weekends; and all schools except one were connected to a district heating system that had a good primary energy conversion factor, meaning that the analysed schools in Slovakia had the lowest primary energy demand. The situation was very similar in the analysed schools in Belarus, where average primary energy consumption was only slightly higher. Total primary energy consumption in schools in Bosnia and Herzegovina, Italy, Hungary, Albania, Kazakhstan and Ukraine was between 169 and 209 kWh/m2a. Serbian schools had a higher calculated primary energy consumption (290 kWh/m2a), due to inappropriate building structure; the supply of DHW by electrical heaters; the use of oil-fired boilers in two schools; and the use of a coal-fired boiler in one school, with a low level of energy efficiency. Schools in Tajikistan had extremely high primary energy consumption due to high heat losses via the building structure and the use of electrical heaters (oil radiators) that have a high primary energy demand

(
Figure 42).

Total calculated primary energy consumption (country averages)

The energy index

The energy index (ei) is the calculated value for total primary energy consumption in the given school (EP, measured in kWh/m2a) divided by the reference value for total primary energy (EPmax). The reference value is based on the TNM Decree 7/2006 of Hungary and is given for different building types, including educational buildings, offices and residential buildings. The reference value for total primary energy is given as a function of the A/V of a building.

An ei of 1.0 means that the calculated total primary energy is equal to the reference value. If the ei is lower than 1.0, it means that the given building and its combined HVAC systems have a total primary energy consumption lower than the reference value, and that therefore the building, in general terms, has a good level of energy efficiency. If the ei is higher than 1.0, it means that the given building has a total primary energy consumption higher than the reference value, and that therefore the building, in general terms, has high annual energy consumption and poor energy efficiency.

ei = EP/EPmax (6)

The EPmax is given using these equations with respect to educational buildings:

A/V ≤ 0.3 EPmax = 90 (kWh/m2a) (7)

0.3 < A/V < 1.3 EPmax = 164 (A/V) + 40.8 (kWh/m2a) (8)

A/V ≥ 1.3 EPmax = 254 (kWh/m2a) (9)

The calculated total primary energy is the sum of the primary energy for heating, DHW, lighting, air handling units and cooling. The average primary energy consumption was 220.9 kWh/m2a in the analysed schools.

Figure 43

Energy index by school

shows the distribution of the energy index. The calculated primary energy consumption is generally 1.7 times higher than the reference value, suggesting that the modernisation of building structures and HVAC systems offers very large energy-saving potential.

Comparison between real and calculated energy consumption

In the second calculation, energy consumption in the buildings was calculated according to the actual location. The heating degree days for all analysed locations were provided by the national energy experts. In this calculation, the heating energy consumption of the given building was corrected (multiplied) with the heating degree day factor, which is the given heating degree day of the location divided by the Hungarian (Budapest) heating degree day.

Following the correction, the calculated and real (based on the school’s energy bills) heating energy consumption were compared. Some schools were connected to a district heating system, where the heating energy consumption shown on the bill is not a measured value but is calculated based on the heated air volume in the building. In other schools, consumption data were not available. In a third group of schools, the given data were not precise. The calculated heating energy consumption and the energy consumption based on bills can therefore differ widely. The behaviour of building occupants can also have an impact on heating energy consumption:

  • the indoor temperature can be adjusted to any value, although in the calculation we consider a value of 20°C;
  • the actual air exchange rate may differ from the value used in the calculation: in some schools there are higher air exchange rates because windows are regularly opened, while in other schools the air exchange rate is lower because windows are rarely opened; and
  • the calculated heat transfer coefficients and other input parameters may differ from the real values.

Bearing in mind that actual energy consumption usually differs from the calculated values, the correlation coefficient R2 = 0.77 for the calculated consumption and actual energy consumption is an appreciated value. The simulation models of the buildings were therefore useful for further analysis and could be considered as a basic tool for the modernisation of the building structures and HVAC systems.

Analysis of measured temperatures

Four KIMO KT 110 temperature data loggers were placed in each school: three of the data loggers were placed in classrooms to measure indoor temperatures; and one was placed outside the building to measure outdoor temperature. The data loggers recorded the temperature every 10 minutes over 7 to 10 days. The data loggers were configured by the experts, who downloaded the recorded figures that were then sent to the REC and Comfort Consulting Ltd. for further analysis.

The measurements showed how the temperature changed in the analysed classrooms during the week and weekend, and during the day and night. The measured data showed:

  • whether the temperature was low, high or appropriate;
  • the temperature difference between the analysed classrooms; and
  • whether there was any temporary heating reduction at night and/or during the weekend.

In several of the analysed schools the measured air temperature was very low, far lower than even the minimum requirement. The lowest temperatures in classrooms during school hours were found in some schools in Albania and Tajikistan. In one extreme example, temperatures in the three analysed classrooms were 8 to 16°C, 7.8 to 14.7°C and 7.5 to 16.1°C in February 2012. Daytime temperatures were typically between 10 and 15°C, which is very cold.

In other schools, temperatures were found to be high. In one school, minimum and maximum temperatures were 22.6 and 32°C, and the temperatures in the three analysed classrooms were very different: 23.5 to 24.5°C, 26 to 27°C, and 28 to 30°C. This school used a one-pipe district heating system and it is possible that control over the heating system was not good (leading to high temperatures), and that the system was not balanced (leading to big differences in temperature between classrooms).

 
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