The impact of the year-on-year variation in the intensity of solar radiation on the energy intensity of low-energy and passive houses

Solar radiation is a significant segment of heat gains in the operation of buildings. The importance of this segment is highlighted by lowering the energy performance of buildings. The current condition of assessment considers the standard values of solar radiation but these are often very different from the fair values. In the contribution it draws attention to not only to on-year variation in solar fluctuations in the intensity of solar radiation and its significant long-term deviation from the standard values but also to the impact to energy building in reliance to its energy intensity. The attention will be focused also to different values in standards valid in the Czech Republic. This specification of energy assessment of buildings is not only necessary to approximate calculations of real state, but mainly because we can expect more disputes about if a building has declared calculating the parameters of a building with nearly zero-energy or passive house. 1 Current condition in the Czech Republic Energy performance of buildings is assessed by calculations according to valid European standards taking into consideration the climatic data in the Czech Republic. Those are given by CSN 73 0540 (CSN = Czech Technical Standard) and TNI 73 0331 (TNI = Technical Standards Information) and they should be based on long-term average. Even though TSI is just a recommended information usually the values listed there are used for assessment of buildings. Although the information differs in both standards and mainly in global radiation perpendicular surface oriented to different cardinal points. Comparison for the winter season is shown in Table 1 where there is a noticeable difference both in individual months, and within different orientation to cardinal points. These different values lead to according TNI it is less necessary to consider the orientation of windows to different cardinal points than when evaluation by CSN. Another source of values of global solar radiation for the region of the Czech Republic can be obtained from the Internet, namely from the computational programs for calculation of electricity from photovoltaic power plants. * Corresponding author: roman@e-c.cz DOI: 10.1051/ 4 7 7 , 0 (2017) 79303008 93 matecconf/201 MATEC Web of Conferences Building Defects 2016 3008 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). In Table 2 shows the comparison of results of several houses and different assessment according to these standards. 2 Real climate data To determine the actual status of global radiation the data were purchased from ČHMÚ ( Czech Hydrometeorological Institute) which performs long-term measurements. Values for Hradec Králové are listed in Table 3 [1]. This table shows that global solar radiation over the past 10 years is higher than the long-term average which is given in CSN, although the global radiation does not correspond with average temperatures for that period. Table 1. Comparison of values listen in TNI 73 0331 and CSN 73 0540-3. source Monthly dose irradiation H [kWh/(m2•month)] January Fabruary March April October November December Total per year Horizon ČSN 23,06 36,75 76,12 110,53 52,74 25,53 18,62 343,35 TNI 20,8 37,0 72,2 113,8 56,5 25,2 14,9 340,4 difference 90 % 101 % 95 % 103 % 107 % 99 % 80 % 99 % South ČSN 41,94 53,31 89,73 88,42 71,57 41,07 30,95 416,99 TNI 34,2 51,1 74,4 84,7 74,4 45,4 29,0 393,2 difference 82 % 96 % 83 % 96 % 104 % 111 % 94 % 94 % Southeast, Southwest ČSN 32,2 41,17 76,16 84,33 57,61 31,99 23,86 347,32 TNI 26,8 41,0 64,7 86,4 60,3 33,8 23,1 336,1 difference 83 % 100 % 85 % 102 % 105 % 106 % 97 % 97 % East, West ČSN 15,01 22,21 48,89 65,84 32,23 15,87 11,18 211,23 TNI 14,1 25,5 46,9 74,2 38,7 18,0 11,2 228,6 difference 94 % 115 % 96 % 113 % 120 % 113 % 100 % 108 % Northeast, Northwest ČSN 6,42 9,55 23,25 38,3 14,06 6,98 5,09 103,65 TNI 8,2 14,8 29,8 50,4 21,6 9,4 6,0 140,2 difference 128 % 155 % 128 % 132 % 154 % 135 % 118 % 135 % North ČSN 5,21 7,26 15,6 24,04 10,36 5,52 4,03 72,02 TNI 8,2 13,4 25,3 36,0 18,6 9,4 6,0 116,9 difference 157 % 185 % 162 % 150 % 180 % 170 % 149 % 162 % DOI: 10.1051/ 4 7 7 , 0 (2017) 79303008 93 matecconf/201 MATEC Web of Conferences Building Defects 2016 3008


Current condition in the Czech Republic
Energy performance of buildings is assessed by calculations according to valid European standards taking into consideration the climatic data in the Czech Republic. Those are given by CSN 73 0540 (CSN = Czech Technical Standard) and TNI 73 0331 (TNI = Technical Standards Information) and they should be based on long-term average. Even though TSI is just a recommended information usually the values listed there are used for assessment of buildings. Although the information differs in both standards and mainly in global radiation perpendicular surface oriented to different cardinal points. Comparison for the winter season is shown in Table 1 where there is a noticeable difference both in individual months, and within different orientation to cardinal points. These different values lead to according TNI it is less necessary to consider the orientation of windows to different cardinal points than when evaluation by CSN. Another source of values of global solar radiation for the region of the Czech Republic can be obtained from the Internet, namely from the computational programs for calculation of electricity from photovoltaic power plants.
In Table 2 shows the comparison of results of several houses and different assessment according to these standards.

Real climate data
To determine the actual status of global radiation the data were purchased from ČHMÚ ( Czech Hydrometeorological Institute) which performs long-term measurements. Values for Hradec Králové are listed in Table 3 [1]. This table shows that global solar radiation over the past 10 years is higher than the long-term average which is given in CSN, although the global radiation does not correspond with average temperatures for that period.

Comparison of the calculated values
To determine the difference of the actual climate data was chosen reference building with dimensions of 10 x 10 m and 6 m in height with windows of 20 m 2 facing south, 4 m 2 north and after 8 m 2 on the east and west. This building was considered a total of 12 variants, according to storage capacity of construction:  Table 4, where ale also listed characteristic values of the glazing. Ventilation in the first two cases is considered natural, in the latter two cases is considered as a forced recovery.   Table 5 and later in graph 1 [3].
In Table 6 and graph 2 are then calculated values in percentage relative to the values calculated according to the standard. Effect of changes in the intensity of solar radiation on energy consumption for heating is documented in Table 7 and graph 3. There is calculated energy consumption for heating according to values listed in the standard and then is considered the same temperature, however the intensity of solar radiation is higher by 20%.

Conclusions
The results demonstrate clearly that the effect of fluctuations in solar radiation has a significant influence to energy performance of buildings for heating which increases especially with thermal insulation properties structure. In old buildings, the 20 % difference in the global radiation shows at model building the range of 3,5 to 4% depending on the heat capacity of the considered structure. For buildings with thermal insulation as recommended for passive houses, however, the difference is 11 to 12,7%. From this finding it implies knowledge that when assesing the measured energy consumption for heating is necessary, besides other things, také in account also global solar radiation, which is usually not performed, usually only takes into account the different temperatures in the heating season. Less significant impact on the energy performance of buildings have storage capabilites. Those at the model house ranged to 2,3%.